Fiber bundle surface parameters of three processing methods [27].
\r\n\tTopics covered include but are not limited to: Hydrologic Cycle (Precipitation, Runoff, Infiltration and their Measurement, Land surface interaction); Hydrologic Analysis (Hydrograph, Wave routing, Hydrologic statistics, Frequency Analysis); Applied Hydrology (Applications in Engineering, Sciences and Agriculture, Design storms, Risk analysis, Case studies); Computational Hydrology (Numerical modeling, Hydrologic modeling and forecasting, Flow visualization, Model validation, Parameter estimation); Interdisciplinary Hydrology (Hydrometeorology, Impact of Climate Change, Precipitation data analysis, Mathematical concepts, Natural hazards); Radar Hydrology (Precipitation estimation techniques, Promise and Challenges in Radar technology, Uncertainty in radar precipitation estimates).
\r\n\r\n\tThe contents covered in this book will serve as a valuable reference guide to students, researchers, government agencies and practicing engineers who work in hydrology and related areas. We hope that this book will open new directions in basic and applied research in hydrological science.
",isbn:"978-1-83962-330-1",printIsbn:"978-1-83962-329-5",pdfIsbn:"978-1-83962-331-8",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"02925c63436d12e839008c793a253310",bookSignature:"Dr. Theodore Hromadka and Dr. Prasada Rao",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/9864.jpg",keywords:"Runoff, Land Surface Interaction, Hydrograph, Wave Routing, Design Storms, Risk Analysis, Numerical Modeling, Hydrologic Modeling and Forecasting, Flow Visualization, Hydrometeorology, Precipitation Data Analysis, Radar Hydrology",numberOfDownloads:338,numberOfWosCitations:0,numberOfCrossrefCitations:0,numberOfDimensionsCitations:0,numberOfTotalCitations:0,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"June 8th 2020",dateEndSecondStepPublish:"September 11th 2020",dateEndThirdStepPublish:"November 10th 2020",dateEndFourthStepPublish:"January 29th 2021",dateEndFifthStepPublish:"March 30th 2021",remainingDaysToSecondStep:"4 months",secondStepPassed:!0,currentStepOfPublishingProcess:4,editedByType:null,kuFlag:!1,biosketch:"Principal and founder of Hromadka & Associates, professor United States Military Academy, Professor Emeritus at the California State University, and a Member of Board of Directors and an Adjunct Professor at Wessex Institute of Technology.",coeditorOneBiosketch:"Prasada Rao, Ph.D. is a professor in the Civil and Environmental Engineering Department at California State University, Fullerton. 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There is considerable consensus that a qualified IL must melt below 100 °C. Ionic liquids are salt, existing in the liquid phase at and around 298 K. A typical IL with a bulky organic cation (e.g., N-alkylpyridinium, N-N-dialkylimidazolium, alkylimidazolium, alkylphosphonium, and alkylammonium) is weakly coordinated to an organic or inorganic anion, such as BF4−, Cl−, I−, AlCl4−, PF6-, NO3-, CH3COO-, CF3SO3-, [(CF3SO2)2N-], etc. to constitute a series of low melting ILs, as shown in Figure 1.
Common cations and anions for room temperature ionic liquids
Ionic liquids have attracted much attention as a replacement for traditional organic solvents as they possess many attractive properties. Among these properties, intrinsic ion conductivity, low volatility, high chemical and thermal stability, low vapor pressure, low combustibility, hydrophobicity, wide electrochemical windows, and high heat capacity and cohesive energy density are few. Compared with commonly used organic compounds, they have low toxicity, are non-flammable, and have negligible or nonzero volatility. Furthermore, alteration of the anions or the length of the alkyl groups allows fine-tuning of the physicochemical properties of ILs, such as viscosity, conductivity, solvation, catalytic activity, hydrophobicity, melting points, etc. Thus, ILs can be strategically designed for different applications [1,2]. The application of ILs can primarily be traced to the pioneering work in the beginning of the 1980s on pyridinium-based [3] and imidazolium-based [4] ILs for electrochemical studies. Over recent years, the concept of “green chemistry” has become well known among scientists worldwide. In particular, exploration of environmentally friendly green solvents as alternatives to volatile organic compounds in synthesis, catalysis, extraction and separation, and electrolytic processes has been persistently pursued. IL, possessing many novel properties, is the most competitive candidate that caters to all trades and professions. Properties such as nonflammability, high ionic conductivity, and electrochemical and thermal stability of ILs make them ideal electrolytes in electrochemical devices like in batteries [5-8], capacitors [9-11], fuel cells [12], photovoltaics [13-18], actuators [19], and electrochemical sensors. In addition, ILs have been widely used in electrodeposition, electrosynthesis, electrocatalysis, electrochemical capacitor, lubricants, plasticizers, solvent, lithium batteries, solvents to manufacture nanomaterials, extraction, gas absorption agents, and so forth. ILs can improve separation of complex mixtures of both polar and nonpolar compounds when used either as stationary phase or as additives in gas-liquid chromatography [20-23], liquid chromatography [22], and capillary electrophoresis [24]. They are also used in optical sensors [25,26] and also to enhance the analytical performance of the matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) [27]. The use of ILs in different applications is determined by their intrinsic properties.
To date, there have been many feature reviews dealing with different aspects of ILs, including catalysis, extraction, synthesis, nanomaterials, biosensing, energy applications, etc [28-44].
The advantages of ILs for the synthesis of conducting polymer and nanoparticle when compared to conventional media and also their electrochemical sensors and biosensors based on IL/composite modified macro-disk electrodes are the major purpose of Singh et al. in [45]. These compounds have become a novel solution to problems encountered with organic solvents and these molecules are a prospective solution to the limitations encountered in electrochemical systems [46,47]. This new chemical group can reduce the use of hazardous and polluting organic solvents due to their unique characteristics as well as taking part in various new syntheses. Due to these unique properties, ionic liquids have been widely used in different field of applications, see Figure 2.
Applications of ILs
Considering the structural aspects of ILs, especially their surface structure on electrodes, is usually helpful for the rationalization of physical and chemical processes in ILs. The electrochemical reactions in principle are the processes at the ILs/electrode interface, including the diffusion of electroactive species, transport, capacitance at the interface, electron transfer processes etc., and which will dominate the performances in the electrochemical applications of ILs. On the other hand, the electrochemical properties in ILs are strongly dependent on the role of charge, size, polarization, and intermolecular interactions of ILs on the electrode surface. As a result, the understanding of the surface electrochemistry in ILs will benefit the performance enhancement of their application, and some novel application directions will be explored. These physical properties can be varied by selecting different combinations of ions [28,48]. Since the electrochemical window of the pure ILs depends on the electrochemical stability of the cation and/or anion, understanding the ion behavior at the electrode surface leads to improvement and implementation of the IL to the desired system [49]. The presence of an abundance of charge carriers means that when ILs are used as solvents, no supporting electrolyte is required for electrochemical experiments and this minimizes waste towards greener site [48].
Supercapacitors (also called electric double-layer capacitors or ultracapacitors) are electrochemical capacitors that store energy through reversible ion adsorption onto active materials that have high specific surface area [50]. Because of their many advantageous properties, such as high power density, high capacitance, and long cycle life (>100000 cycles), these systems play an important role in electrical energy storage. To generate a high specific capacitance, the specific surface area of the electrode materials needs to be as high as possible to accommodate a large number of electrolyte ions at the electrode/electrolyte interface thereby promoting the electrical double-layer capacitance [50-52]. The latter property greatly exceeds that of conventional electrochemical energy storage devices, e.g., batteries and fuel cells. Furthermore, supercapacitors can store a much greater charge per unit volume of mass than conventional dielectric-based capacitors. An electrochemical supercapacitor is based on the electrochemical double layer resulting from the electrostatic adsorption of ionic species at the electrode-solution interface, i.e., no actual redox reaction is supposed to take place during the charging-discharging of these devices. To obtain the maximum possible capacitance, supercapacitor electrodes must have a high surface area; the standard material used in these devices is typically high surface area carbon. Because these devices are based on the electrosorption of ionic species, the region between the electrodes of the capacitor must contain an electrolyte with mobile ions. To obtain the maximum operating voltage without solvent decomposition, it is necessary to use aprotic solvents such as acetonitrile. Carbon-based supercapacitors based on conventional aprotic electrolytes are commercially available.
Not surprisingly, because of the numerous favorable properties described above, ionic liquids are considered promising electrolytes for electrochemical supercapacitors. [C2mim]BF4 and [C2mim]NTf2 dissolved in alkyl carbonate solvents were among the first ILs to be investigated [53]. More recently, an electrochemical supercapacitor based on N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate was shown to have superior properties compared to supercapacitors based on conventional aprotic electrolytes such as mixtures of Et4NBF4 in propylene carbonate [10].
An IL-based supercapacitor has even been prepared from carbon nanotubes. This device utilizes carbon gel electrodes fabricated by combining [C2mim]NTf2 with pulverized single-walled carbon nanotubes [54]. Hybrid supercapacitors obtain energy storage from the electrostatic double layer capacitance obtained at a high surface area carbon electrode and from a rapid, reversible charge-transfer process that occurs at a dopable conjugated polymer, e.g., poly(3-methylthiophene) [55]. This charge-transfer process is designated as a pseudocapacitance. Not surprisingly, ionic liquids have also found their way into hybrid supercapacitors. In fact, a hybrid supercapacitor based on activated carbon, poly(3-methylthiophene), and n-BuMePyrNTf2 (Pyr = pyrrolidinium) may be the first viable supercapacitor based on a IL, producing 24 Wh kg-1 and 14 kW kg-1 [56].
Graphene, a monolayer of sp2-hybridized carbon atoms arranged in a two-dimensional lattice, has attracted tremendous attention in recent years owing to its exceptional thermal, mechanical, and electrical properties. One of the most promising applications of this material is in polymer nano-composites, polymer matrix composites incorporating nano-scale filler materials [57]. In addition, graphene, as an atom-thick 2D nanostructure [58,59], is a promising material for supercapacitor electrodes owing to its low mass density, excellent electronic conductivity, and high surface area (~2630 m2/g, theoretical) [60-62]. Reduced graphene oxide, RG-O, a composition closely related to graphene, is a promising material for supercapacitor applications, as specific capacitance values of 135 and 99 F/g based on RG-O-based electrodes in aqueous and organic electrolytes, respectively, have been obtained [60]. However, dispersed graphene oxide, G-O, platelets can agglomerate during reduction by, for example, hydrazine in a solvent system such as water, resulting in the possible decrease of effective surface area, resulting in a lower specific capacitance than might be expected for an ideal graphene-based supercapacitor [60]. Moreover, current supercapacitors have energy densities well below the values required to provide power assists in various applications including hybrid electric vehicles or other high energy uses [51,63]. Hence, recent efforts have been focused on the development of supercapacitors with high energy densities, which may be achieved both by enhancing the operating voltage of the devices and by improving the accessibility of the ions from the electrolyte to the active regions of electrode materials.
Toward this goal, graphene-based electrodes combined with ionic liquid electrolytes can provide an attractive alternative for supercapacitors since such combinations result in an optimal pairing of high specific surface area electrodes and wider operating potentials that may be afforded by some IL electrolytes. Generally, ILs feature moderately high ion conductivity, nonvolatility, high decomposition temperatures, and wide electrochemical stability windows, and many ILs are being considered as electrolytes to increase supercapacitor operating voltages [10,44,64,102,65]. Despite the potential of ILs as electrolytes, further work is needed to explore their potential for supercapacitors assembled with graphene-based electrodes. One of the challenges is achieving graphene-based electrode materials capable of being well wetted by the chosen ILs [51], which may be attainable by the surface modification of graphene.
Kim et al. have reported their progress toward high performance supercapacitors based on poly(ionic liquid)-modified RG-O electrodes and an IL electrolyte, [C2mim]NTf2. Poly(ionic liquid), PIL, polymers formed from IL monomers can be prepared by the polymerization of unsaturated salts. Specifically, the use of poly(1-vinyl-3-ethylimidazolium) salts bearing NTf2- or CF3SO2-N-SO2CF3 anion has been reported to effectively stabilize hydrazine-reduced graphene oxide (RG-O) platelets via electrostatic and cation-π interactions, resulting in the formation of PIL-modified RG-O materials [66]. The PIL is likely physisorbed to surface of RG-O platelet and not covalently linked. These PIL-modified reduced G-O materials, PIL:RG-O, are expected to offer advantages for supercapacitor applications in that they should provide enhanced compatibility with certain IL electrolytes and improved accessibility of IL electrolyte ions into the graphene electrodes.
To investigate whether surface modification with PIL can be extended into other types of carbon electrodes, comparative experiments with the same PIL were applied to activated carbons (ACs) without RG-O platelets present. The result showed that the PIL is blocking the pores of the ACs to such an extent that the IL electrolyte cannot penetrate. For example, the PIL-modified AC electrodes exhibited far lower specific areas as measured. Given the apparent blocking of mesopores by the PIL, it would seem that good options for electrode materials include those having relatively high aspect ratios such as conductive platelets and conductive nanorods. For example, it is reasonably likely that a good configuration for SWNTs can be found where they would be PIL-coated and combined with an appropriate IL. Other options could include 3D solids with significantly larger pore size distributions than are present in the typical activated carbon currently used in supercapacitors. The use of [C2mim]NTf2 takes advantage of its larger electrochemical stability window, allowing for operation at 3.5 V, which in turn increased both the energy density and power density of the device. To further evaluate the device performance, the frequency response of the supercapacitor incorporating the PIL: RG-O electrodes and [C2mim]NTf2 electrolyte was analyzed using electrical impedance spectroscopy (EIS). The general concept of a supercapacitor design based on PIL-modified RG-O electrodes and a compatible IL electrolyte holds potential as an electrical energy storage device indicating stable electrochemical performance and a specific capacitance as high as 187 F/g. This relatively high capacitance is presumably due to improved wettability of the chosen IL electrolyte on the PIL-modified RG-O materials which, synergistically, enhanced the effective surface area of the electrode/electrolyte interfaces.
Lithium-ion batteries are now ubiquitous in society and serve as the power sources in almost all portable electronic devices that are marketed to today’s consumer. With such widespread use and in view of the safety issue of lithium-ion batteries, there are considerable ongoing efforts by battery manufacturers to improve the performance of these devices. As a result of the many attractive aspects of ILs, there is a modest but continuing interest in using them as electrolytes for these cells. Until several years ago, the IL electrolyte of choice was some variety of chloroaluminate [67], but interest in the use of nonchloroaluminate ILs has gradually increased. It is difficult to pinpoint the first instance where nonchloroaluminate ILs were used in lithium-ion batteries. An early report describes a successful Li/LiMn2O4 cell prepared with 1,2-dimethyl-4-fluoropyrazolium tetrafluoroborate + LiBF4 or LiAsF6 [68]. Li/LiCoO2 cells utilizing n-PrMePipNTf2 (Pip = piperidinium) show good cyclic efficiency [69,70], and it is clear that ILs based on those anions that offer good anodic stability, e.g., NTf2- or N(SO2F)2-, give the best performance [70,71]. At the present time, the main problem is the incompatibility of the anode, e.g., Li metal, and the ILs. That is, the solid electrolyte interphase film that is produced on the anode during the charge/discharge process is less stable than that obtained in conventional organic solvents. This incompatibility problem limits the cycling efficiency of the cell. MacFarlane et al. [72] have succeeded in elucidating the mechanism of film formation on Li in ILs based on N-alkyl-N-methylpyrrolidinium ions and NTf2-. Perhaps, future research of this nature will lead to resolution of this problem, enabling the practical use of ILs as electrolytes in Li batteries.
By comparison of the background electrochemical behavior of an IL at both GC and platinum electrodes, one can approximately determine the amounts of protonic impurities present. The microstructure and capacitance of the electrical double layers (EDLs) at the interface of ILs and electrodes play an essential role in determining the system performance. Compared with simpler electrolytes, such as aqueous electrolytes and high-temperature molten salts, the ions in ILs are larger and often feature a complex shape. In addition, their charges are typically delocalized among many atoms [73]. Considering the solvent-free nature and the complex shape of ILs, it is expected that the classical theories for the EDLs in dilute aqueous electrolytes and high-temperature molten salts cannot accurately describe the structure and properties of the EDLs at the interface of ILs and electrified surfaces. Therefore, it is necessary to rediscover the IL/electrode interface and renew the model of IL/electrode EDLs.
Although ILs have always being investigated from the first finding in 1941 for a long period time, only a few works [74,75] concentrated on their intrinsic capacitive behaviors. A small capacitance (compared to smooth electrodes) was achieved in a practical capacitor comprising high-surface-area carbon cloth electrodes. To understand whether the observed capacitance might be due to the microporosity of the carbon cloth electrode or to the practical limitation of the device itself, the differential capacitance of 1-ethyl-3-methyl imidazolium imide IL was determined by measuring the potential of electrocapillary maximum or the point of zero charge (PZC) at an Hg electrode. The obtained capacitance at the cathodic potential is mainly determined by the cation, rather than the anion, as expected. The rate of charge, discharge, and the accessibility of electrolyte to the electrode surface ultimately determine the realizable capacitance in a practical device employing high-surface-area carbon electrodes.
The intrinsic capacitance of imidazolium-based ILs at carbon paste electrodes was investigated by using an electrochemical impedance technique [76]. The large capacitance was accounted for by intramolecular hydrogen bonding interactions that created a third charge layer between the IL and electrode EDL, which might make the interface rougher and hold more charge. The importance of ion chemistry and structure for the capacitive response of carbonaceous electrodes in ILs was stressed [77]. The double-layer capacitance of negatively charged carbon electrodes was strongly determined by the cation polarizability, which affected the dielectric constant in the double-layer, as well as double-layer thickness, which in turn also depended on the preferred orientation of the cations under the applied electric field. This suggests that the EDL constitutes a monolayer of cations up against the negatively charged carbon surface. Differential capacitance–potential curves were measured at IL/Hg, GC and Au electrode interfaces [78]. Unlike in aqueous or conventional organic solvents, capacitance–potential curves were found to vary significantly with the electrode substrates in ILs. It could be due to the absence of the inner Helmholtz layer of the molecular solvent between the electrode and ionic species, which usually works as a dominant factor in shaping the capacitance curves. The use of an electrochemical gate and ILs can reduce the Debye ionic screen length to a few angstroms, which makes the measured capacitance nearly equal to the real quantum capacitance. The unique properties allow the direct measurement of the quantum capacitance of graphene as a function of gate potentials using a three-electrode electrochemical configuration in ILs electrolytes [79].
However, different types of ILs consist of various ions and ionic pairs structured in the bulk or IL/electrode interfaces. The bulk and interfacial behavior is governed by Columbic, van der Waals, dipole-dipole, hydrogen-bonding, and solvophobic forces [80]. The molecular organization of ILs is more complex than traditional solvents, and ILs cannot be considered as unstructured molten salts [81]. Many investigations have shown ionic liquids are nanostructured, which helps to explain their solvating power and some other unusual physical properties [82]. Spectroscopy and molecular dynamics simulation studies revealed distinct alkyl and ionic clusters in alkylimidazolium ILs [83-86]. Self-assembled IL nanostructures have also been elucidated using small-angle neutron scattering (SANS) for ILs [87], in which electrostatic contributions to solvophobicity are enhanced by the extensive hydrogen-bond network of the liquids. IL solvation layers were first detected for the ethylammonium nitrate/mica interface using surface force apparatus (SFA) [88], and subsequently detected for both protic and aprotic ILs confined between atomic force microscope (AFM) tips and mica, silica, graphite, and Au(111) [89-91]. It was found that IL nanostructures are the consequence of alkyl tail aggregation, driven by solvophobic forces inducing alkyl chains to segregate from the charged cation group and the anion, forming ionic domains. The increasing alkyl chain length leads to larger, more regular domains. The self-organized IL/substrate interfaces were observed to compose of three to seven solvation layers depending on the IL species [92]. Surface frequency generation data indicated that the interfacial cations exhibited orientational ordering and their orientation depended not only on the electrical potential of the electrode but also on the type of anions in the ILs [93]. Some experimental data suggest that the IL/electrode interface is one ion layer thick (typically 3-5 Å), which supports the idea that the EDLs in ILs are essentially Helmholtz layers [94]. Despite the lack of sufficient experimental data, two major approaches have been proposed to model the IL/electrode interface.
A molecular dynamics (MD) simulation model for an IL/metallic electrode in which the metallic electrode is maintained at a preset electrical potential is described in [95]. The model uses variable charges whose magnitudes are adjusted on the fly according to a variational procedure to maintain the constant potential condition. As such, the model also allows for the polarization of the electrode by the electrolyte, sometimes described by the introduction of image charges. The model has been implemented in a description of an electrochemical cell as a pair of parallel planar electrodes separated by the electrolyte using a two-dimensional Ewald summation method. The method has been applied to examine the interfacial structure in two ILs, consisting of binary mixtures of molten salts, chosen to exemplify the influences of dissimilar cation size and charge. The stronger coordination of the smaller and more highly charged cations by the anions prevents them from closely approaching even the negatively charged electrode. This has consequences for the capacitance of the electrode and will also have an impact on the rates of electron transfer processes. The calculated capacitances qualitatively exhibit the same dependence on the applied potential as have been observed in experimental studies.
The MD simulation study of the EDL structure near electrodes with different surface charge densities indicates that a Helmholtz-like interfacial counterion layer exists when the electrode charge density is negative or strongly positive but becomes poorly defined when the electrode charge density is weakly positive [96]. However, regardless of the presence of a distinct Helmholtz layer, the charge separation and orientational ordering of the ions persist up to a depth of 1.1 nm into the bulk ILs. The structure of the EDL is affected strongly by the liquid nature of the IL and the short-range ion-electrode and ion-ion interactions, especially at low electrode charge densities. In addition, the charge delocalization is found to affect the mean force experienced by the bulky ions near the electrode and, thus, can play an important role in shaping the EDL structure. An investigation of the electrified Au(100) surfaces in ILs by combined in situ scanning tunneling microscopy (STM) and differential capacitance measurements has revealed that the differential capacitance curves of the IL/Au interfaces have a bell-shaped feature, and ion adsorption at Au single-crystal electrodes depends critically on the structure of the surfaces [97].
The capacitance-potential (C-V) curve is virtually bell-shaped with metal electrodes in ILs composed of cations and anions of comparable size [98-101]. However, U-like C-V curves are observed at IL/nonmetallic electrode interfaces in contrast to those observed at IL/metallic electrodes [64]. The degree of curvature of the ‘‘U-like’’ curve measured at the GC electrode decreases in the ILs with low inherent ionic concentrations. The capacitance at the GC electrode exhibits a complex potential dependence, being different from those at highly oriented pyrolytic graphite (HOPG) and metal electrodes. However, in most real IL systems they are not hard and are not spheres. Therefore, long and challenging investigations to better understand IL/electrode interfaces are still ahead.
Besides the above application, ionic liquids are also used in some other electrochemical applications, such as electrochemical biosensing, electrochemical capacitors, lithium batteries, etc. Precisely, controlling the interface between the electrode and IL solvents allows scientists to alter the electron transfer and storage ability in the devices, and thus improve their performances. The details in this field have been well reviewed by other authors in references [102-104] and here is just glance at the use of ILs in these fields.
Enzyme electrodes are one of the most intensively researched biosensors because enzymes are highly selective and respond quickly to a specific substrate [105]. A new type of amperometric biosensor based on IL sol-gel material using the hydrolysis of tetraethyl orthosilicate in [C4mim]BF4 solution was reported [106]. The IL sol-gel enzyme electrodes retained the high activity of horseradish peroxidase (HRP) and provided long-term stability of HRP in storage. The uniform porous structure of the IL sol-gel matrix resulted in a fast mass transport, which provided a unique microenvironment around the enzyme, resulting in high sensitivity and excellent stability of the enzyme. An interesting electrochemical biosensor towards electrocatalysis of H2O2 was designed by entrapping HRP in IL doped DNA network [107], in which DNA with a specific double-helix structure and stacked base pairs may afford both biocompatible microenvironments around enzymes entrapped in the membranes and an effective pathway for electron transfer. Recently, IL-based sensors have been used for trace explosive detection by integrated electrochemical and colorimetric methods [108]. The explosives are pre-concentrated in ILs first.
A recent report showed that the use of [C4mPy]TFSI in combination with a graphite and silicon electrode could maintain good lithium cycle performance in the presence of film-forming additives, such as vinylene carbonate (VC), that served as the only source of reactive chemical for an effective SEI formation at the electrolyte/electrode interface [109]. Silicon electrodes displayed higher compatibility with [C4mPy]TFSI compared with graphite.
The latest researches of ILs in surface electrochemistry, including the IL/electrode interface and electron transfer in ILs has been highlighted in a review by Liu et al. [110]. As a result, the absorption, mass transport, and electron transfer at the interfaces of ILs/electrodes are complicated and quite different to that in traditional solvents. Some experimental data suggest that the IL/electrode interface is one ion layer thick (typically 3-5 Å) showing that they are essentially Helmholtz layers. Through spectroscopic techniques, e.g. SFA, AFM, SFG, and SANS, ILs are recently illustrated to be nanostructured as a consequence of alkyl tail aggregation driven by solvophobic forces. In the IL medium, the mobile carrier densities are shown to be able to be determined using a simplified capacitor model in a three-electrode electrochemical system, in which the electron transport characteristics are interpreted in terms of charged impurity induced scattering [79].
These strategies give ILs great promise for applications of ultrahigh frequency electronics and for studying the intrinsic transport properties of the Dirac fermions in graphene devices. It is believed that some new application directions beyond those mentioned above will be explored with the deep understanding of the structure and composition information at the ILs/electrode interface in the near future. There is no doubt that ILs will play more important roles in the electrochemical applications in both science and technology because of their unique properties.
Ionic liquids are a good choice as binder in carbon paste electrodes due to their interesting properties, mentioned previously. Recently, ionic liquids have been widely used as efficient pasting binders instead of non-conductive organic binders in preparation of carbon composite electrodes [111].
Carbon paste electrode based on MWCNTs and ILs types of electrodes show superior performance over traditional carbon paste electrodes. Solution studies shows a selective interaction between cerium acetylacetonate complex (CAA) and monohydrogen phosphate respect to a number of anions tested, therefore, the complex was used as sensing material in construction of a HPO42- nano-composite carbon paste sensor based on MWCNTs and ILs. A new HPO42- nano-composite carbon paste electrode was introduced [111]. From the other side of view, the purity of the nanotubes affects the gelation.
Ionic liquids have shown great promise for application in heterogeneous systems, such as lubricants, heterogeneous reactions, heterogeneous catalysis, electrochemistry, and fuel cells [15,29,31,112-116]. However, for these particular applications, a more detailed understanding of the molecular structure of the interface between IL and solid is essential.
Recently, the structures of the gas-liquid and liquid/liquid interfaces of ILs have been investigated computationally [117-121] and experimentally [122-137]. Some groups have studied the structure and dynamics of ILs by analytical techniques, such as X-ray diffraction [136-137], sum-frequency generation technique, SFG, [126,127] IR and Raman spectroscopies [134,135], direct recoil spectroscopy [122], neutron reflectometry [129], etc. They have suggested that the cations lie normal to the liquid surface and a density oscillation near the surface is also evident at the vapor-liquid interface. These studies, as well as several molecular dynamics simulations [117-121], have indicated that both cations and anions are populated at the liquid surface via a specific orientation.
Compared to gas-liquid and liquid-liquid properties of ILs, efforts to investigate solid-liquid surfaces or interface structures of ILs have so far been limited [93,132,138]. Owing to possible strong interactions between ILs and solid surfaces, a transition of IL from liquid to solid may take place. Evidence [116,139,140], has already demonstrated the coexistence of liquid and solid phases of IL [C4mim]PF6 on mica surfaces at room temperature by using atomic force microscopy, AFM. Although it has previously been found that some liquids, e.g., water become ordered or solid-like in the layers adjacent to the surface of crystallized solid substrates [141], a solid layer of [C4mim]PF6 formed on mica surfaces is much more stable. Since the melting point of [C4mim]PF6 is ~7 °C, it is a good model for the study of liquid/solid interactions and the possible phase transformation. Moreover, it has also been observed that, when confined to multiwalled carbon nanotubes, [C4mim]PF6 can be transformed into a crystal with a high melting point [142]. Some other groups have also studied the interactions between solid surface and IL. For example, by using sum-frequency vibrational spectroscopy, Fitchett and Conboy [143] have reported that the alkyl chains of the imidazolium cations are nearly normal to SiO2 surfaces for a series of hydrophobic ILs. Romero and Baldelli [144] have studied [C4mim]PF6 and [C4mim]BF4 with hydrophobic and hydrophilic properties, respectively, and shown that the imidazolium ring lies on the plane of the quartz surface with the methyl group pointing 43 °C-47 °C from normal, displaying resonances from the alkyl chain as well as the aromatic ring by using SFG. Atkin and Warr et al. [89] have measured the solvation force profiles for several ILs on different solid surfaces by AFM. However, there is still a lack of detailed understanding of the ordered or solid-like molecular structures of ILs at solid/liquid interfaces.
ILs have been demonstrated to be interactive toward a number of solid materials, such as single-walled carbon nanotubes (SWNTs), graphite, silica, mica, and kaolinite, through interaction mechanisms such as H-bonding, π–π stacking, Van der waals forces, electrostatic forces, and so on [145-147]. For instance, Fukushima et al. [146] discovered uniform SWNTs bucky gels by grinding SWNTs in ILs, since the cation–π interaction exists between the imidazolium ion of IL and the π-electrons of the fullerene of SWNTs. In contrast, common organic solvents such as dichlorobenzene, ethanol, N, N-dimethylformamide, and 1-methylimidazole (a precursor for ionic liquids) did not form gels, even upon prolonged grinding with SWNTs. Likewise, no gelation of ionic liquids took place with other carbon allotropes such as graphite (1 to 2 μm) and C60 [148]. With all the above features in mind, rheological properties of the bucky gels of ionic liquids have been investigated in reference. SWNTs can orient imidazolium ions on their π-electronic surfaces by way of a possible “cation-π” interaction. Such a molecular ordering may trigger clustering of the surrounding imidazolium ions columbically and consequently interconnect neighboring SWNT bundles.
Lungwitz et al. [147] suggested that, due to the anion-silica interactions, both physisorption and chemisorptions occurred in the phase boundary of N-methylimidazolium chloride/silica. In addition, the adsorption of IL on other silicates or soils surfaces are mostly spontaneous; many adsorption mechanisms have been proposed, such as ion exchange, ion pairing, dispersive force, and so on [149]. Adsorption of IL on these solid materials indicates that these ILs could modify the surface of clay and eventually leads to significantly changed properties of both the clay and IL, such as the thermal stability, supermolecular structures [140], and crystallization behavior [142]. Halloysite nanotubes (HNTs), mined from natural deposits, are a kind of clay aluminosilicate mineral with hollow nanotubular structure and a high aspect ratio. The length of HNTs is usually in the range of 1-15 μm and the inner diameter and outer diameter of HNTs are 10-30 and 50-70 nm, respectively [150]. Halloysite, also named metahalloysite and chemically similar to kaolin, has the molecular formula Al2Si2O5(OH)4.nH2O. In the pH range of 2-12, the inner and outer surfaces of HNTs are negatively charged [151] so it may absorb imidazolium cations of IL via an electrostatic effect. In addition, it was found that the outer surfaces of HNTs are mainly composed of siloxane and have only a few silanols and aluminols, which indicates that HNTs possess potential ability for the formation of hydrogen bonding with imidazolium-based ILs.
Guo and his coworkers confirmed [107], by thermogravimetric analysis, the retention of [C4mim]PF6 IL on HNTs in a tetrahydrofuran-water mixture and the formation of IL-coated HNTs (m-HNTs). In addition, the interaction was confirmed and the hydrogen bonding was proposed to as a possible mechanism. HNTs have been utilized as effective reinforcements for various polymers [152], therefore, the curing and performance of rubber compounds with m-HNTs were examined. The unique changes in the rubber compounds were correlated to the changes in filler dispersion and interaction between IL and HNTs. The IL absorbed on HNTs surface to form a mesostructure, which is different from that of the neat crystallized IL. The interaction between IL and HNTs was proposed to be hydrogen bonding and verified by the spectral results. Because of the interaction, the crystallization behavior of IL in the presence of HNTs was found to be changed. Compared with the compounds with HNTs, the uncured compounds with HNTs coated with IL showed significantly faster curing and the resulting vulcanizates showed substantially higher tensile strength and much lower hardness.
Interest in carbon nanotubes (CNTs) dispersed in ILs is rapidly growing [44,51]. One of the main reasons is the extraordinary electrochemical and bioelectrochemical properties of ILs [153] as well as of carbon nanotube composite materials. Such, CNTs/ILs have promising supercapacitor applications.
It has been shown in several experimental [89,91,92,117,154] and theoretical [98,155] studies that the molecular structure of IL ions strongly influences the IL interface properties at different charged and uncharged interfaces. For instance, Lockett et al. [154] showed that asymmetry in the size and shape of molecular ions results in unequal distribution of molecular cations and anions in direction normal to the IL-vacuum interface. These results suggest that the molecular structure of IL ions should make significant effects at the CNT/IL interface. However, still, there is a lack of molecular level information on the mechanisms of the IL interactions with nanocarbon electrodes.
Endres et al. [156] quoted an ‘‘undoubted’’ formation of at least three solvation layers of [C2mim]TFSI on metal electrodes detected by atomic force microscopy (AFM). Recently, Hayes et al. [157] investigated the influence of the electric potential on the interface solvation layers in [C2mim]FAP and [C4mPyrr]FAP, by AFM, at the charged Au(111) electrode the support the experimental observations. They showed that IL layering is more pronounced at charged Au(111) surface compared to the neutral surface and increase of the potential leads to flattening of the tightly bound cation layer, indicating possible reorientation of cations ([C2mim] and [Py]) to lay flat on the surface. The similar effects can be observed: increase of the potential at the CNT cathode significantly increases the tendency of [C2mim] cation to lay flat on the surface. Applying an electric potential on the CNT electrode and/or varying the chemical structure of IL molecular ions, it is possible to change ion orientations and thus the structure of the CNT-IL interface shell.
Applying an electric potential, they found that the innermost layer changes its structure and becomes more strongly bound to the surface. At the cathode, for “the first time an interfacial (innermost) anionic layer at a solid interface has been detected by AFM”. Atkin et al. [91] published an AFM study of the gold interface solvated in [C2mim]TFSI and reported results which coincide well with the observed layered structure in MD simulations of neat [C2mim]TFSI at the CNT surface performed by Frolov et al. [158]. They attributed the ‘‘weaker’’ layering pattern in simulations to the differences in temperatures: temperature in the simulations was about 70 °C, while “all force curves were acquired continuously at room temperature (22 °C)” for the AFM measurements.
There are some experimental studies on the orientation of IL molecules on the liquid-vacuum interface, see for example references [154,159,160]. Nakajima et al. [159] investigated the liquid-vacuum interface of different 1-alkyl-3-methylimidazolium-TFSI ionic liquids using high-resolution Rutherford backscattering spectroscopy. They showed that, due to their solvophobic nature, long alkyl chains of cations point away from the bulk liquid to vacuum and therefore stimulate the imidazolium ring to stay perpendicular to the surface. Simulations on the IL-carbon interface were observed an opposite effect: increase of alkyl chains increases the tendency for the imidazolium ring to lay parallel on the surface. The differences between IL-vacuum and IL-carbon nanotube interfaces to the strong Van der Waals attraction between the non-polar alkyl chains and carbon nanotube surface can be attributed. Contrary to the IL-vacuum interface, at the IL-CNT interface the alkyl chains of imidazolium-based cations tend to lay parallel on the CNT surface and force the imidazolium rings also to lay flat on the carbon nanotube surface.
One of the main reasons on rapidly growing interest in carbon nanotubes, CNTs, dispersed in ILs is the extraordinary electrochemical and bioelectrochemical properties of ILs as well as of carbon nanotube composite materials. Such, carbon nanotubes (nanotube forests) with ILs have promising supercapacitor applications. It has been shown in several experimental and theoretical studies that the molecular structure of IL ions strongly influences the IL interface properties at different charged and uncharged interfaces. For instance, Lockett et al. [161] showed that asymmetry in the size and shape of molecular ions results in unequal distribution of molecular cations and anions in a direction normal to the IL-vacuum interface. These results suggest that the molecular structure of IL ions should make significant effects at the CNT-IL interface.
Molecular simulations can provide complimentary information to the experimental data that should help to obtain a detailed picture of the interface phenomena in IL systems. Therefore, fully atomistic simulation for studying basic mechanisms of the IL interactions with the CNT surface has been applied [158]. The results have shown that ILs based on combination of imidazolium-based cations with hydrophobic anions (e.g. BF4- or TFSI) are moisture stable and have very promising electrochemical applications. Ion conductivity of [C2mim]TFSI is comparable to the best of organic electrolyte solutions, and this liquid is stable up to 300-400 °C. The TFSI-based ionic liquids are practically not miscible with water but they are well miscible with several organic solvents, e.g., acetonitrile, AN [162]. The effects of the cation molecular geometry on the properties of the interface structure in the IL systems was investigated by a set of three ILs with the same anion (TFSI) but with different cations, namely, [C2mim], [C4mim], and [C8mim] [158]. The cations had identical charged methylimidazolium ‘heads’ but different nonpolar alkyl ‘tails’ where the length of the tail increases from ethyl to octyl and the focus was concentrated on a set of the following questions:
What is the interfacial structure of IL-AN mixture at the neutral CNT surface?
How does the interfacial structure change at the positively charged CNT surface?
How does the interfacial structure change at the negatively charged CNT surface?
Does the length of the cation alkyl tail affect the interfacial IL-AN structure and preferred orientation of the IL ions at the CNT surface?
What is the role of acetonitrile solvent in these interfacial effects?
Taking into account the molecular volume of the investigated ions, the probability of finding an ion inside the CNT pore was assumed to be low.
The analysis of the simulation data results in the following conclusions:
There is an enrichment of all molecular components of ILs under study at the CNT surface with formation of several distinct layers even at the non-charged CNT surface.
Mixing IL with acetonitrile decreases ion-counterion correlations in the electric double layer.
Increase of the length of the non-polar cation ‘tail’ increases propensity of imidazolium-based cations to lay parallel to the CNT surface.
At the CNT cathode TFSI anions and molecular cations are preferentially oriented parallel to the surface.
At the CNT anode the TFSI anions are oriented parallel to the surface, however the preferred orientations of cations depend on the length of non-polar tail: [C2mim]+ cations are oriented perpendicular to the surface, [C4mim]+ are in both parallel and perpendicular orientations, [C8mim]+ are oriented parallel to the surface.
By applying electric potential on the CNT electrode or/and varying the chemical structure of IL molecular ions it is possible to change the interfacial orientation of IL ions and, consequently, the structure of the CNT-IL interface shell.
As an effective computation technique, molecular dynamics program has been widely used for simulating interfacial phenomena. The interfacial molecular structure of [C4mim]PF6 in contact with the graphite surface has been studied for the first time at 2008 [163]. Maolin and his worker used [C4mim]PF6 as the model because it is hydrophobic and widely investigated. They investigated how the hydrophobic graphite surface affects the structure and orientation of hydrophobic [C4mim]PF6 at the interface. The MD calculation indicated the formation of a stable bottom layer, as well as possible single, double, or triple layer of [C4mim]PF6 on the graphite surface. The orientation calculations showed that the alkyl chains and imidazolium ring of cations both lie in the plane of the hydrophobic graphite surface.
Molecular dynamics simulations were performed to understand the microscopic structure of the IL [C4mim]PF6 on a graphite interface. In addition, MD simulations showed the existence of a solid-like IL bottom layer of about 6 Å thickness on the graphite surface. Compared to the bulk IL, the mass density peak of the bottom layer is 90% higher and its electron density peak is 80% higher. The butyl group and imidazolium ring of the cation of the IL bottom layer are parallel to the graphite surface. Due to the strong interactions between the cations and the graphite surface, ILs possessing longer alkyl tails and more imidazolium ring or aromatic ring may be applied to form more stable and well-regulated layers at graphite surfaces. This finding is important for the understanding of modification or lubrication mechanisms of ILs on solid surfaces, especially on the surfaces of carbon nanotubes and carbon black.
Several experimental and computational articles suggest that a possible layering of molecules in IL occur at the vapor-IL interfaces [119-121]. The influence of the vapor-IL interface on single, double, and triple layers of [C4mim]PF6 on the graphite surfaces was shown that the mass density profiles of different surface layered films are similar, and the vapor-IL interface appears to have little effect on the layered IL formation. Moreover, it is also shown that the thickness of the monolayer or the bottom layer away from the graphite surface is nearly identical, i.e., 6.0 Å. Very early simulations of a short alkyl chain IL, dimethylimidazolium chloride, have indicated a layered surface with a clear oscillatory density profile akin to that observed for liquid metals [117]. It was also found that [C4mim]PF6 exhibit density oscillations at the liquid-vapor interface [119-121]. However, the density oscillation of dimethylimidazolium chloride is more obvious than that of [C4mim]PF6. From MD simulation, vapor-IL interface appears to have little effect on the layered IL formation. In the monolayer, the interaction energy between per cation and per anion is −175.29 kJ/mol. The interaction energy between per ion and the graphite also achieves a comparable quantity of -80.54 kJ/mol. This may be used for the explanation that the strong configurational effect and the interactions between the graphite surface and IL molecules induce the ordering of IL and stretching into a couple of IL layers. Furthermore, the above results imply that the configurational effect and the interactions between the IL layers and the graphite collectively induce different degrees of layered distribution. The vapor-IL interface may disorder to some extent the ordering of the outmost layer and weaken the interactions between the underlayers, resulting in a lower mass density peak of the outmost layer and higher mass density peak of the underlayer. This finding is important for the understanding of modification or lubrication mechanisms of ILs on solid surfaces, especially on the surfaces of carbon nanotubes and carbon black.
The behavior of a model IL, [C1mim]Cl, confined between two parallel walls have been studied at various inter-wall distances, focusing on confinement effects on the structure and dynamics of the ions, and its impact on the charge-transport capacity [164]. The results focus both on structural and dynamical properties. Mass and charge density along the confinement axis reveal a structure of layers parallel to the walls that lead to an oscillatory profile in the electrostatic potential. Orientational correlation functions indicate that cations at the interface orient tilted with respect to the surface and that any other orientational order is lost thereafter. The diffusion coefficients of the ions exhibit a maximum as a function of the confinement distance, a behavior that results from a combination of the structure of the liquid as a whole and a faster molecular motion in the vicinity of the walls. Density profiles perpendicular to the walls confirmed an interfacial liquid layer twice as dense as the bulk followed by oscillations that decay toward the center of the cell; a major part of the layering is due to the distribution of anions. The number of layers changes with the interwall distance and so does the concentration of ions at the interface, showing a maximum at 28 Å. The observed structural effects result from the response of the liquid to the boundary conditions imposed and are detected also in the pressure on the walls. The pressure is determined by the number of particles in the first density layer, a result of the short-range interactions between the atoms and the walls. This effect depends on the details of the ion-wall interactions and may be significantly different in the case of charged surfaces. This is a result that, in light of recent experimental findings [143], may be independent of the type of confining walls used in simulations.
The distribution of charge perpendicular to the surface was determined by the arrangement of the ions and, in particular, by the alignment of the cations. As a result, any test charge entering the liquid encounters a first layer of positive charge followed by a second layer of opposite sign. The electrostatic potential drop between a point inside the walls and the middle of the liquid slab was around -0.5 V, although the potential oscillates strongly into the liquid.
An interesting result is that, under confinement, ionic diffusion is faster than in the bulk, at least in the presence of noncorrugated walls. Ions close to the surfaces diffuse faster than those in the middle of the slab and the diffusion coefficients reflect the changes in the density and proportion of ions near the walls. The reasons for the faster diffusion near the walls may be related to their lack of corrugation, their solvophobic nature, or a combination of both. However, it must also be kept in mind that the local mobility depends on the local density and orientation of the ions, which are the result of collective structural effects.
If the observed trends in ionic mobility apply also to more realistic surfaces, then the higher diffusivity of the ions will surely have an impact on the electrical conductivity and response of the IL to internal electric field changes. In fact, it has been proposed that the ability of the liquid to screen, fast and efficiently, an external field controls the rate of charge-carrier percolation across the nanocrystalline film.
There are several aspects of ILs that need further attention in connection with their uses in solar cells. First, a more realistic modeling of the walls would be necessary as well as the consideration of more sophisticated ILs. Second, it is important to understand the dynamical response of the semiconductor/IL interface to changes in the surface charge.
A recent comprehensive computer simulation of ILs at the SiO2 surface clearly revealed that the interfacial structure is sensitive to polar or apolar surface as well as hydrophobic or hydrophilic IL components. The study nicely corroborates the experimental sum frequency generation vibrational spectroscopy (SFG) studies of ILs at SiO2 substrate. Despite the detailed interfacial structure depending on the nature of substrate and ILs, a common feature shared by these simulations is the well-ordered structure at the IL/solid interface. Such ordered interfacial structure was also found in both simulation and experimental SFG study of the IL/TiO2 interface, as well as the simulations of the IL/graphite interface by Wu et al. [163] and by Kislenko et al. [165]. Wu’s latter study also highlights the solidation of ILs at graphite, as analogue to the confined ILs in carbon nanotube. Recently, Reichert and co-workers [166] investigated ILs interfacial ordering mechanism at the charged Al2O3 substrate via high-energy X-ray reflectivity and observed strong interfacial layering which decays exponentially into the bulk region. Such interfacial layering was expected to be a generic trait of ILs at charged walls. Understanding the interfacial structure, especially the electric double layer (EDL), is crucial in exploring the applications of ILs in electrochemical devices. Recently, Kornyshev [167] proposed a mean-field theory in which a compressibility parameter γ is incorporated. It was shown that the differential capacitance (DC) is bell-shaped when γ > 1/3, and it is came l-shaped otherwise, while the U-shaped DC, predicted by the classical Couy-Chapman theory, is recovered for the γ>0 limit for the infinite dilute electrolyte solution. The bell-shaped DC is supported by Oldham’s modification of the Couy-Chapman model for ionic liquid interface in a specific case with γ = 1. The bell-shaped DC was observed experimentally on an IL/metal electrode (platinum and gold), with similar IL ion sizes, by Ohsaka and co-workers [101]. The study highlights the compressibility of ILs and the different sizes of cations and anions. The focus was on liquid-solid systems and reported molecular dynamics studies at the liquid-solid interface between imidazolium-based ILs, [C4mim]PF6 and [C8mim]PF6, and an apolar uncharged graphite surface. The main aim of the study was to investigate the influence of different alkyl-side chain lengths of imidazolium on the interfacial structures. The density of IL was much enhanced at the interfacial region and the density oscillation extended to ∼15 Å into the bulk with three layers. The results also demonstrated the polar groups tend to aggregate forming a polar network while the nonpolar groups fill up the rest of the vacancy. The imidazolium rings and the side chains preferentially lie flat at the graphite surface with the alkyl side chains of the cations elongated at the interfacial region, and the cations are closer to the graphite surface (ca. 3.6-3.7 Å) than the anions. The surface potential drop across the interface is more profound for [C8mim]PF6 than for [C4mim]PF6 due to relatively larger local density of the anions for [C8mim]PF6 near the graphite surface.
While receiving much attention due to ILs importance in a broad range of applications, yet little is understood about their microstructure and phase transition in confined systems. Understanding the microstructure and freezing processes of ILs in confined systems is of practical importance in lubrication, adhesion, and the fabrication of solar cells or IL/nanomaterial composites, in which ILs are in contact with solid surfaces or under confinement. In general, the reduction of the liquid film thickness to fewer than 4-6 molecular layers will promote solidification. This results from the characteristic transverse density profile of thin films, which can induce lateral ordering and lead to freezing. Some evidence has indicated a possible liquid-solid phase transition for ILs in confined systems. Several reports have also revealed the astonishing property of melting point depression of 1,3-dialkylimidazolium-based ILs confined to nanospaces, which was discovered utilizing differential scanning thermal calorimetry. However, the phase behavior of ILs confined to nanospaces remains largely unexplored. The first simulation results of a liquid-solid freezing transition of [C1mim]Cl IL between two parallel graphite walls has been reported by Sha et al. [168]. Their result is of importance to understand the interfacial interactions between ILs and carbon nanotubes because of dispersing uniformly on the surface of single-walled carbon nanotubes via simple mulling. The molecular dynamics simulations were utilized to investigate the freezing of a [C1mim]Cl monolayer. The simulations predicted a first-order freezing transition from a liquid monolayer to a solid monolayer induced by varying the distance between the parallel graphite walls. The resulting monolayer solid consisting of a hydrogen-bonded network structure is very different from bulk crystalline [C1mim]Cl. The phase transition can be induced only at a molecular surface density of ρ=1.9/nm2. It is important to note that [C1mim]Cl IL confined between hydrophobic walls indicated no “hardening” or transition to a solid like phase structure, but only down to a wall distance of 2.5 nm. The wall distance plays a crucial role in the phase transition. Like an ice monolayer confined between walls, the molecule area density (ρ=N/lxly) of [C1mim]Cl is another important factor. During the simulations, the molecule number of the system was decreased in steps and simulated for time periods equal to the first set of simulations. The change in N reflects the change in surface density since the area of the graphite wall remains unchanged in the simulation. As the molecule number is decreased, the lateral pressure of the monolayer liquid decreases to a lower value. If the surface density exceeds this value, a solid monolayer will not be formed. Meanwhile, the interaction energy between the IL and the walls is about -62 kJ/mol at a distance of 0.7 nm. Hence, the results imply that the configurational effect and the attraction interaction between the IL and the walls collectively induce the solid monolayer. The distance dependences of the diffusion coefficient and the potential energy indicate a strong first-order phase transition in confined [C1mim]Cl. These results are helpful for the understanding of microstructures of ILs in nanostructured confinement as well as the thermodynamic mechanism of liquid-to-solid phase transition.
For the applications of electrolyte membranes and catalysts, there is a need to immobilize ILs on solid supports or within a solid matrix.
The microstructure of the IL bilayer was studied by varying the graphite wall distance. A liquid-to-solid phase transition of bilayer [C1mim]Cl was observed at 425 K in this confined system, whereas the melting point of the bulk [C1mim]Cl crystal is 399 K. Further calculation indicates a high melting point of the confined IL: melting point ∼825-850 K. The imidazolium ring of the solid bilayer forms a strong π…π stacking structure in which each cation is surrounded by the three nearest-neighbor anions. The bilayer solid is a new phase of [C1mim]Cl differing from the monolayer solid [168] under identical confinement conditions or the bulk crystal.
The simulation on the [C4mim]NO3/rutile (110) system shows that the adsorbed NO3 anions at the interface organized themselves into a highly ordered manner, while changing the anion to PF6- does not present such an ordered interfacial structure. On the other hand, Grimes and co-workers have conducted experiments to study the high rate photocatalytic conversion of CO2 to hydrocarbon production on the high surface area TiO2 nanotube arrays.
An operating efficiency of electrochemical devices is greatly influenced by the molecular structure of the electrode/electrolyte interface. For example, the electron lifetime and open-circuit voltage in dye-sensitized solar cells depend on the structure of the electrical double layer (EDL). The specific capacitance of supercapacitors also depends on the EDL structure and is capable of being varied on change of the ions or even molecular ion fragments in IL. However, a double layer in ILs has not yet been adequately explored.
The dye-sensitized solar cells (DSSCs) have been extensively investigated since their applications in the production of high light-to-electricity conversion efficiency. Traditionally, the devices are immersed in electrolytes, which are usually composed of an I−/I3− redox couple and organic solvents such as acetonitrile. IL crystal system (C12mim+/I−/I2) has been used as electrolyte of dye-sensitized TiO2 solar cells; it has been found that though large viscosity of the IL, the diffusion coefficient is still high since the exchange reactions facilitate the charge transport process. To acquire detailed information of DSSCs, Aliaga and Baldelli have performed a sum frequency generation study on one component of the solar devices, the liquid/solid interface between [C4mim]DCA and [C4mim]MS and TiO2. The results pointed out that both the ions are present at the interface for [C4mim]DCA while only the cations are detected for [C4mim]MS. The spectra also emphasized on a stronger charge adsorption of the DCA anions than the MS anions. In addition to experimental researches, it has been previously shown that the interface between [C4mim]NO3 and rutile (110) surface by means of molecular dynamics simulation. The results indicated that the NO3 anions prefer to aggregate at the interface and arrange themselves in a highly ordered manner. As for the [C4mim]+ cations, occupying the region adjacent to the NO3- layer, they tend to stand vertically on the TiO2 (110) surface.
Shu et al. [169] presented a simulation on the interface between [C4mim]PF6 and rutile TiO2 (110) surface, considering the other polymorphs of TiO2 are less stable at room temperature. The main objective of their work was to model the IL/semiconductor interface, where the recombination process occurs, and study the structural behaviors of the confined IL. The results revealed that both ions are gathered on the surface, forming ionic double layers with regularity. It is also interesting to see that the cations lie flat on the surface, with alkyls stretched out in the bottom region. In addition, the ions are assembled at the interface, forming several enhanced layers at each side of the slab and are self-organized into alternate double ionic layers. The adsorbed cations are inclined to lie flat on the rutile (110) surfaceThis MD simulation casts new light on the microscopic structure features on the IL/solid interface and provides insights on the development of DSSCs. They have performed a molecular dynamics simulation on the [C4mim]PF6/ rutile (110) system.
Thanks to their low vapor pressure, ILs are ideal extraction solvents or reaction media because simple evaporation methods can be used to separate solutes from ILs. In addition, ILs can be custom-made with targeted functions. Because of these advantages, ionic liquids have been engineered as extraction solvents, reaction media and drug delivery materials. In most IL applications – such as extraction, lubrication, and IL super capacitors – the core function of the IL occurs at the ionic liquid–solid interfaces. Ionic liquids are different from conventional molecular liquids because no individual molecule exists in the liquid. Moreover, they are not diluted electrolyte solutions either. Hence, no existing theory and model can precisely describe the behavior of ILs, especially at the IL interfaces. Therefore, studies of the IL interfacial properties are necessary for further developments of IL-based applications. Furthermore, new applications – such as IL reactor, IL-circuit, and surface pattern visualization – require the precise control over the position of the IL drop on surface. The chemical pattern-directed assembly of IL on surface has been under investigation by Zhang et al. [170]. The chemical patterns can control the shape, size, and position of the IL on surface. Furthermore, IL drops on surface can be coated with a layer of silane film, forming an IL capsule.
However, experimental data are still lack, and none of the existing theories can completely explain the much diversity. Thus, first, the challenge for surface chemists, electrochemists, and theoreticians is to understand the detailed interface structure, including the adsorption, configuration, distribution, and orientation of ILs on the electrode surface. Some new models, new experimental techniques, and the details of this important system should be explored in the future to further realize the IL/electrode interface structures. Moreover, up to now, only quite a few number of ILs species were studied as models. However, the interface properties, structure and functions of the ILs with different cations and anions on the electrodes varies distinguishably. Thus, there is a need to have systematic studies on a wide range of ionic liquids so that more meaningful and comparable results can be obtained.
In an IL system, the heterogeneous electron transfer kinetics is highly dependent on the nature of both cation and anion in ILs. In addition, it should be pointed out that the physicochemical properties of ILs are largely dependent on the temperature and impurities such as water and some organic solvents, which will remarkably affect the transport and heterogeneous electron transfer ability. Though vast works have been carried out in this aspect, there is a huge data gap with respect to the thermodynamic and transport properties over a wide range of temperatures, as well as the effect of solutes and impurities on these properties. It is worth noting that the classical Marcus theory for outer sphere electron transfer, which is based on the reorganization of solvent dipoles, is not suitable to the entire ion systems of ILs. Developing a novel theory and electron transfer model system for the entire ion systems to explain the electron transfer process in ILs is also an important task in the future.
To date, the applications of ILs have involved nearly all aspects of electrochemistry, such as electrodeposition, electrosynthesis, electrochemical capacitors, electrochemical biosensors, and lithium batteries. Besides those stated above, it was recently shown that ILs can be used as a simple and effective way to reduce the scattering of carriers by charged impurities in graphene transistors supported on solid substrates [171].
Confinement can induce unusual behavior in the properties of matter. Using molecular dynamics simulations, a liquid-to-solid transition of a bilayer of [C1mim]Cl confined between graphite walls was studied in order to mimic the phase transition of an IL confined to hydrophobic nanospace. It was found that the IL bilayer undergoes a clear and drastic phase transition at a wall distance of about 1.1 nm, forming a new high-melting-point solid phase with different hydrogen bonding networks. In the new phase, each cation is surrounded by the three nearestneighbor anions, and each anion is also encircled by the three nearest-neighbor cations. Strong π-π stacking interactions are found between the cations of the bilayer solid. The anions can be formed into a hexagonal ring around the cations. The new bilayer solid is a high-melting-point crystal possessing a melting point of 825-850 K, which is higher than that of the bulk crystal by more than 400 K.
For imidazolium-based ILs, the effect of the alkyl-chain length on the differential capacitance of EDL, electrocapillary curves, and the potential of zero charge (PZC) were experimentally investigated [8,9]. Based on the results obtained, a qualitative pattern of the Hg/[C8mim]BF4 interface at the electrode potentials close to PZC was suggested [9]. A quantitative information on the EDL structure is given in a number of papers wherein the imidazolium cations’ orientation at the platinum and quartz surfaces has been determined using the sum frequency generation vibrational spectroscopy [10–13]. Since the electrode/electrolyte interface affects appreciably the relevant electrochemical processes, the development of adequate theoretical models of EDL in ILs takes on great significance. Owing to high ion concentration in ILs, the Gouy-Chapman classical theory is no longer valid and the double layer model must take into account a finite size of ions [14]. Investigating the double layer in IL [C4mim]PF6 at the graphite surface by use of the molecular dynamics simulation was the major goal for Kislenko, Samoylov, and Amirov [165].
Account was also taken of a large body of experimental and theoretical data available for comparison with the simulation results. The basal plane of graphite emulates the surface of an activated carbon used as an electrode material for supercapacitors. The calculations were performed both for an uncharged graphite surface and for positively and negatively charged ones. It is found that near an uncharged surface the IL structure differs from its bulk structure and represents a well-ordered region, extending over 20 Å from the surface. Three dense layers of ca 5 Å thick are clearly observed at the interface, composed of negative ions and positively charged rings. It is established that in the first adsorption layer the imidazolium ring in the [C4mim]+ cation tends to be arranged in parallel to the graphite surface at a distance of 3.5 Å. The PF6- anion is oriented in such a way that the phosphorus atom is at a distance of 4.1 Å from the surface and triplets of fluorine atoms form two planes parallel to the graphite surface. Ions adsorbed at the uncharged surface are arranged in a highly defective 2D hexagonal lattice and the corresponding lattice spacing is approximately four times larger than that of the graphene substrate. The influence of the electrode potential on the distribution of electrolyte ions and their orientation has also been investigated. Increase in the electrode potential induces broadening of the angle distribution of adsorbed rings and a shift of the most probable tilt angle towards bigger values. It was shown that there are no adsorbed anions on the negatively charged surface, but the surface concentration of adsorbed cations on the positively charged surface has a nonzero value. In addition, the influence of the surface charge on the volume charge density and electric potential profiles in an electrolyte was studied. The differences in the cation and anion structure result in the fact that the integral capacitance of the electrical double layer depends on the electrode polarity. The phenomena confirmed experimentally such interfacial layers formation and increase of an average tilt angle of adsorbed rings with increasing the electrode potential. In addition, the calculated values of the EDL capacitance coincide in the order of magnitude with the experimentally measured specific capacitance at the electrode/IL interface for activated carbons as electrode material and imidazolium-based ILs.
Fiber woven composites (FWCs) are a kind of new fiber composites. The fibers inside are woven to form a preform, and then the matrix grows on the preform to generate the final composites. The woven fibers can heighten the reinforcing effect of the fibers and improve the mechanical property of the composite. Therefore, FWCs could perform better than other fiber composites.
\nAmong FWCs, woven ceramic matrix composites (WCMCs) are star materials, widely used in aerospace, military, national defense, and some other advanced fields [1, 2, 3, 4], because of their high specific strength and rigidity, corrosion and wear resistance, and other excellent characteristics [5, 6]. For the industrial application of WCMCs, it is of vital importance to objectively evaluate the surface processing quality and, on this basis, judge the type and degree of processing damage. To do so, it is essential to measure the surface topography both accurately and efficiently and select proper indexes to evaluate the surface process quality. However, WCMCs are far more complicated than traditional materials. On the one hand, their surfaces are anisotropic and inhomogeneous and have obvious directionality and complex structures, which means that there are difficulties and challenges in measuring and evaluating their surfaces. Traditional surface measurement and evaluation approaches for isotropic materials are no longer suitable to WCMCs [7, 8, 9]. On the other hand, the surfaces of an WCMC present more types of processing damage than isotropic materials, including fiber pullout, debonding, and matrix cracking [10]. Each shows a different influence on the composite application, and thus judging the type and degree of processing damage to a WCMC is a new but difficult task.
\nTo date, there have been no uniform measurement standards to ensure that undistorted WCMC surface features are obtained or proper evaluation approaches accurately assess the surface damage [11, 12]. It is widely believed that only 3D measurements can obtain the complete surface information [7, 13, 14]. However, a traditional evaluation method used to assess the isotropic materials is limited to a quantitative description of the entire surface through some typical surface topography parameters, which ignores the subtle details of the surface. Such judgment standards are brief with respect to the direct relation to the surface damage. Moreover, to date, a majority of composite surface evaluations still use the profile arithmetic mean error Ra as the only evaluation parameter [15, 16, 17, 18, 19, 20], which is fairly incomplete.
\nAs such, the complexity of a WCMC surface calls for a newer and more targeted methodology that is tightly connected with the topography characteristic. When we look into the WCMC surface, it is obvious that its composition sequence is as follows: fiber -fiber bundle -cell body -whole surface [21]. Here, a fiber is the smallest composing unit, a fiber bundle is the smallest structural unit, and a cell body is the smallest repeatable unit. A cell body is made up of fiber bundles and matrix and has a nearly fixed surface microstructure. The material surface is formed through its repeating copy and translation [22, 23]. Thus, fiber damage influences the fiber bundle surface, damage to the fiber bundle surface influences the cell body surface, and damage to the cell body surface influences the whole surface property. Merely depicting the entire surface at one time without considering the surface structure composition of a WCMC is inadvisable.
\nIn this chapter, it is proposed that the measurement and evaluation of a WCMC surface should adopt a grading evaluation system based on its complex surface structure, which includes the four levels: fiber, fiber bundle, cell body, and the whole surface. On the fiber level, the typical forms of fiber damage and their effects on the surface morphology of WCMCs are analyzed, which lays a foundation for the measurement and evaluation methods on the next three levels. On each subsequent level, the system proposes a set of surface measurement sampling parameter determination methods and surface quality evaluation methods based on the principle of statistics.
\nAccording to the common fabrication process of WCMCs (shown in Figure 1), single fibers are surface modified to improve the bonding strength between fibers and matrix, and then several fibers are twined to form a fiber bundle. Multiple fiber bundles are woven in a certain way to a preform. The preform is then immersed into an environment with the elements or components of the matrix. The matrix can grow on the preform to generate the final WCMCs.
\nThe fabrication process of an WCMC.
The schematics of the WCMC surfaces of different woven methods (shown in Figure 2(a–c)) and process angles (shown in Figure 2(d,e)) indicate that there exists a minimum repeatable unit, which is marked with a red block in the figures. The unit is composed of the fiber bundles of every directions and the ceramic matrix. The whole surface can be formed through its repeating copy and translation. The unit is defined as the “cell body” in this chapter. It is obvious that the shape of a cell body is not uniform for a WCMC. In fact, the appearance of a cell body can change with different woven methods and process angles.
\nSchematic diagram of different woven patterns of WCMC. (a) 2D, (b) 2.5D, (c) 3D, and (d) 2D woven style with a processing angle of 90° and (e) 2D woven style with a processing angle of 45°.
In summary, fiber is the minimum characteristic of a WCMC, which forms fiber bundle. Fiber bundles are woven to different directions. The cell body consists of fiber bundles of every directions and the matrix. Eventually, the whole surface is generated by copy and translation of the cell body. Therefore, fiber is the minimum evaluable unit of WCMCs. Its damage form can influence the surface state of fiber bundles. Fiber bundles build a bridge between the “microscale” of the fiber and the “macroscale” of the cell body. Their surfaces include the information of the fibers and impact the surface quality of the cell body. Cell body is a key feature. On the one hand, its components are as complicated as the whole surface, which means that the evaluation results of a cell body can be used to represent and estimate a certain range of a whole surface. On the other hand, it is obviously affected by the fiber bundles and matrix inside; thus the analysis of fiber and fiber bundle can be used to evaluate the cell body.
\nIn this chapter, it is believed that the measurement and evaluation of WCMCs should employ a grading system. The evaluation of fiber, fiber bundle, cell body, and the whole surface should be separately researched and then integrated. On the fiber level, the typical damage forms of fibers should be identified and classified, and how the damage influence the using properties should be research. On the subsequent levels, two tasks should be accomplished. The first one is to select proper sampling methods to acquire the undistorted surface information. And the second one is to propose and test reliable evaluation indexes to quantitatively estimate the main damage type and degree of the surfaces. With all the work above, a grading measurement and evaluation system for the surface of a WCMC can be eventually built. The following parts of this chapter introduce the methodologies on each level.
\nWhen processed by machining tools, the surfaces of WCMCs interact with the cutting edges, leading to fiber damage. The damage of fiber on the one hand causes the removal of fibers and, on the other hand, turns into the machining defects on the surfaces. The multifarious types of the damage of WCMCs are the main feature of difference to the traditional homogeneous materials and, meanwhile, are the main source of technological difficulty of the evaluation of WCMCs.
\nThe typical forms of damage on the surfaces of WCMCs are the following:
\nFiber fracture, which is caused by the cutting edges directly cutting the fibers off, often happens when the fibers at the cutting area are tightly fixed by the matrix or by other fibers nearby. The cutting section of fiber fracture is V-shape, and the bottom of the V often appears plastic deformation, which is caused by the friction and squeezing between the fibers and the cutting edges. Fiber fracture is the most common material removal form of WCMCs. If the materials are mainly removed by fiber fracture, the finished surfaces are always of good quality and with low roughness.
\nFiber pulled-off is caused by the cutting edges breaking the matrix without cutting off the fibers or the cutting edges pulling the fibers out of matrix without cutting them off. This type of damage can leave fibers exposed or form holes on the finished surface, which decreases the surface quality. When the finished surface is assembled with another part, the raised fiber can act as a tiny cutting edge, harming the counterpart surface. When the finished surface performs as a friction surface, the holes may help contain lubricating oil and wear debris, alleviating three-body wear, thus improving its tribological performances.
\nFiber debonding, which is caused by the cutting edges removing the entire layer of fibers, often occurs when the fibers at cutting area are poorly connected with the matrix. Fiber debonding can result in collapses of large areas on the surface and greatly reduce the surface quality.
\nFrom the analysis above, it is clear that the machining process can cause manifold types of damage on the surface of WCMCs. Each kind of damage can affect the quality and performance of the surface in its own way. Therefore, the evaluation technology of WCMCs is required to recognize the main type of damage on a surface and quantitatively estimate the degree of the damage.
\nMoreover, fiber bundles, cell bodies, and the whole surface are, basically speaking, made up of fibers. Because of the directional arrangements of fibers, the height of the surface of WCMCs could fluctuate with the period of fiber diameter. The fiber diameter acts as an obvious fundamental frequency on the surface of WCMCs. It can influence the sampling parameters, such as sampling step, sampling length, and sampling area, on other levels. Meanwhile, the direction of fibers determines the direction of the fundamental frequency and can eventually influence the sampling direction. Thus, the direction and diameter of fibers are tightly connected with the surface measurement technology of WCMCs.
\nIn conclusion, fiber, as the minimum evaluable unit of a WCMC surface, significantly influences the grading surface measurement and evaluation system of WCMCs. On the one hand, when considering whether an evaluation index is appropriate for WCMC surface, it should be checked whether this index can help recognize and estimate the type and degree of damage. On the other hand, when determining the proper sampling parameters, the directionality and the diameter of fibers must be taken into consideration.
\nThe measurement of a surface is a process that obtains the height information of the surface. Surface measurement methods are divided into two classes: contact measurement, which uses a probe to measure the height data of points on the surface, and non-contact measurement, which uses light to measure. It was always believed that contact measurement could achieve higher measurement accuracy, although its efficiency was quite low and the measurement process was time-consuming. However, thanks to the development of optical theories and technologies, the non-contact measurement technology based on white light interferometry can get extremely high level of accuracy now as well. NANOVEA ST400 (shown in Figure 3), an optical non-contact measurement system, is used to measure the surface micro-topography in the research of this chapter.
\nThree-dimensional non-contact surface morphometer.
Because the measurement of a surface is in fact the measurement of the points on the surface, it has to be determined which points are chosen to be measured. This topic is related to the sampling strategy, which means how to choose a proper set of points to measure, in order to make the measurement results of these samples able to reflect the information of the entire surface. The first question is to use 2D measurement or 3D measurement.
\nIf a 2D measurement is adopted, points in a line are measured and calculated as one data set. In this term, sampling step (the length between two adjacent sampled points), sampling length (the length of the entire sampling line), and sampling direction (the angle between the sampling line and the structure of the surface) should be determined. In most instances of 2D measurement, one line of sampling is not able to reflect the entire surface because of the measuring error and the random surface damage. Several lines should be selected and measured in order to improve the stability of the measurement results. Therefore, sampling number (the number of the sampling lines) is also to be determined.
\nIf a 3D measurement is adopted, an array of points inside the entire surface are measured and calculated as one data set. That’s to say, the sampling area is the area of the entire surface. When sampling step is determined, the points to be measured are selected. It was believed that 3D measurement was more adaptable for complex surfaces because it could get more information of the surfaces. However, the research of this chapter proved that, if the surface to be measured is obviously directional, 3D measurement will lose its advantages and 2D measurement should be adopted. On the other side, 3D measurement always means long sampling time, huge data processing work, and, thus, low efficiency.
\nSelecting proper sampling parameters, including sampling step, length, direction, number of 2D measurement, and sampling step of 3D measurement, is a complex work. Small step and large length and number are always related to higher measuring accuracy but low efficiency and vice versa. Proper sampling parameters balance both two sides, maintain undistorted sampling, and, on this basis, reduce sampling points.
\nThe task of surface evaluation technology is to select proper statistical characteristics (defined as evaluation indexes in this chapter), which can be calculated from the measurement data of the heights of the sampling points. The following are the indexes adopted in the research of the chapter:
\nFor 2D measurement data:
\nFor each sampling profile j, its average is defined as μ0j\n, the standard deviation is σ0j\n, and the normalized height of every sampling point is Z0ij\n, and thus
\nwhere the height of every sampling point is defined as Zij\n, j is the jth profile on the fiber bundle surface, and the number of sampling points within the sampling profile is M, where M = the sampling length/sampling step.
\nBased on the above, the four 2D evaluation indexes used in the chapter are profile arithmetic mean error Ra, profile square root deviation Rq, profile skewness Rsk, and profile kurtosis Rku, which can be calculated as follows:
\nHere, Ra and Rq are quite similar in reflecting the surface roughness, although Rq is in general more sensitive than Ra to the degree of surface roughness.
\nThe normalized R\n\nsk0 and R\n\nku0 of the jth profile can be obtained as follows:
\nAccording to the definition of Rsk, the closer it is to 0, the more approximate the sampling profile is to a Gaussian distribution. When Rsk > 0, the profile presents a positive peak. This indicates that the profile has more crests or the crest height is larger than the trough height. In contrast, if a profile has more troughs, or the trough height is larger than the crest height, it presents a negative peak.
\nOn the other hand, Rku is compared with 3. The closer it is to 3, the more approximate the sampling profile is to a Gaussian distribution. That is, the degree of dispersion of the profile data is similar to a Gaussian distribution profile. The more Rku is greater than 3, the smaller the degree of data dispersion is, and in contrast, the more Rku is less than 3, the larger the degree of data dispersion is.
\nFor 3D measurement data:
\nThe four 3D evaluation indexes used in the chapter are surface arithmetic mean deviation Sa, surface square root deviation Sq, surface skewness Ssk, and surface kurtosis Sku, which can be calculated as follows:
\nwhere the height of every sampling point is defined as Zij\n and the number of sampling points within the sampling area is M and N, where M and N = the sampling length (of X and Y direction, respectively)/sampling step.
\nIn order to illustrate the measurement and evaluation method, several materials were measured and evaluated as examples in this chapter. The information of the materials is shown as follows:
\nThe carbon fiber-reinforced silicon carbide ceramic matrix composite (Cf/SiC) was fabricated through chemical vapor infiltration (CVI) combined with a liquid melt infiltration process (LMI) [24]. The preform was prepared using a 3D needling method and densified using CVI to form a porous carbon/carbon (C/C) composite. Next, the porous C/C composite was converted into Cf/SiC during LMI, in which silicon carbide (SiC) matrix was formed through a reaction with carbon and melted silicon [25]. The density of the Cf/SiC composite is 1.85 g/cm3.
\nThe fiber diameter of the material is about 7 μm, and size of the cell body is about 1.6 mm × 1.6 mm.
\nThe Cf/SiC specimens were ground with four different processing angles. For 90° processing angle, the fiber bundles are divided into side fiber bundles and end fiber bundles, according to their directions (shown in Figure 4).
\nDefinition of the processing angle of Cf/SiC [26].
When measuring fiber bundle surfaces, 2D measurement should be adopted because of the obvious directionality of the surfaces. Here we take the surface of Cf/SiC with a processing angle of 90° as an example. Owing to the directionality of the fiber bundles, different sampling directions often result in different numerical characteristics, as shown in Figures 5 and 6. We can see that no matter what type of measurement direction is applied, there is no influence on the 3D surface topography. That is, a 3D sampling and evaluation method may not be able to reflect the surface details of the fiber bundle. The use of one or a group of 3D evaluation indexes based on a 3D sampled data fails to reflect the damage types related to the fiber orientation and machining direction.
\nSide surface topography of a fiber bundle with a scanning track perpendicular and parallel to the fiber direction [27]. (a) Surface topography-scanning track perpendicular to the fiber direction. (b) Surface topography-scanning track parallel to the fiber direction. (c) Original profile 1, scanning track perpendicular to the fiber direction. (d) Original profile 1, scanning track parallel to the fiber direction. (e) Original profile 2, scanning track perpendicular to the fiber direction. (f) Original profile 2, scanning track parallel to the fiber direction.
End surface topography of a fiber bundle with a scanning track perpendicular and parallel to the machining direction [27]. (a) Surface topography-scanning track perpendicular to the machining direction. (b) Surface topography-scanning track parallel to the machining direction. (c) Original profile 1, scanning track perpendicular to the machining direction. (d) Original profile 1, scanning track parallel to the machining direction. (e) Original profile 2, scanning track perpendicular to the machining direction. (f) Original profile 2, scanning track parallel to the machining direction.
On the side surface of a fiber bundle, the bonding strength between the fiber and matrix is weaker than that of the end surface. The most direct reflection of the machining direction is fiber damage such as fiber debonding, fiber fractures and delamination. The fiber direction scale is more notable than the machining direction scale, and the directionality of the surface topography mainly depends on the fiber orientation. On the end surface of a fiber bundle, the fiber is mainly subjected to a shear force. The main fiber damage is fiber shearing and fiber pullout. The machining direction scale is more notable than the fiber orientation scale, and the directionality of the surface topography mainly depends on the machining direction.
\nOn the side surface of a fiber bundle, as shown in Figure 5(c,e), when the scanning track is perpendicular to the fiber direction, the profile shows damage between fibers, whereas the profiles only show single fiber damage when the scanning track is parallel to the fiber direction (Figure 5(d,f)), which means that the profiles cannot reflect the machining effect on the whole fiber bundle surface. The same phenomenon occurs in the end surface of a fiber bundle. When the scanning track is perpendicular to the machining direction, the profile shows the integrated influence of the processing (Figure 6(c,e)); however, when the scanning track is parallel to the machining direction, the profiles simply show the effect of a single grain on the surface (Figure 6(d,f)).
\nFrom the analysis above, it can be seen that the 2D sampling and evaluation method is more suitable for a fiber bundle scale measurement. To guarantee measurement accuracy and consider the influence of the fiber orientation and machining direction on the surface topography, the scanning track should be perpendicular to the fiber orientation on the side surface of a fiber bundle and perpendicular to the machining direction on the end surface.
\nAccording to our research, it may be reasonably inferred that, for planes which are not truly along the fibers, the influence of the fiber orientation and machining direction should be considered. When the surface is full of processing traces and the fiber orientation is so obscure, the sampling direction should be perpendicular to the machining direction. In other cases, the sampling direction should still be perpendicular to the projection direction on the vertical plane along the fiber axis. However, a definite conclusion in this regard still requires further research.
\nThe side surface of Cf/SiC with processing angle of 90° is taken as an example to illustrate the determination method of sampling length when measuring a fiber bundle surface. Since the diameters of the fibers are approximately 7 μm, a set of candidate sampling length are chosen as the integral multiple of 7 μm, namely, 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, 91, 98, 105, 112, 119, 126, 133, 140, and 147 μm. In each sampling length, 1500 surface profiles are measured with a constant sampling step of 0.1 μm. And 2D surface roughness Ra of each profile is obtained.
\nAccording to the numerical values of 1500 Ra, a frequency histogram is made. It can be seen that the distribution of 2D surface roughness Ra in every sampling length is almost of its normal distribution. The result shown in Figure 7 is the one using normal distribution function to fit the frequency histogram. With the growth of sampling length (Figure 8(a)), the curves are thinner and higher. Their shapes do not change any more in the case that sampling length is more than a certain value (Figure 8(b)). It is known to all that normal distribution has two parameters: the mean value \n
The distribution of 2D surface roughness Ra under different sampling length [24]. (a) With the sampling lengths of 7,42 84 125 and 147. (b) With the sampling lengths of 119,126 133 140 and 147.
The changing trends of \n\nσ\n\n and Raave under different sampling length [24].
\nFigure 8 clearly shows the changing trends of standard deviation \n
Based on the results obtained above, a speculation is proposed that the mean value of a few number of Ra can steadily estimate the entire surface roughness of fiber bundle. The average value of Ra under the changing sampling number from 10 to 150 and the constant sampling length 120 μm could be obtained, which is represented by Raavg. Under every sampling number, the measurement process repeats 50 times independently. Then the maximum relative error of every sampling number, calculated by Eq. (13), is demonstrated in Figure 9. It is shown that with the rise of sampling numbers, the maximum relative error would decrease dramatically. Once the number of Ra reaches to 70 or above, it is stable under 2%, which is acceptable in terms of accuracy.
\nThe changing trend of the maximum relative error under different number of Ra [24].
According to the analysis given above, the conclusion can be made that as long as extracting surface profiles averagely distributed on the side surface of Cf/SiC composite fiber bundle with the appropriate sampling length and sampling number, the mean value of Ra is steady and can estimate the whole surface roughness. For Cf/SiC composite used in the present work, the critical sampling length is 119 μm, which is about 17 times of the fiber diameter, and sampling number is 70.
\nIn this section, the side surface and end surface of Cf/SiC with processing angle of 90° are taken as an example to illustrate the determination method of sampling step, under the condition that the critical sampling length is 150 μm and sampling number is 200. It is clear that using a smaller sampling step can achieve more accurate surface data, whereas a too small step may cause an unnecessary sampling time and data processing cost. A method is proposed to determine the maximum sampling step (MaxSS) that can minimize the data size under the premise of undistorted surface sampling.
\nWe start from setting the data measured at the step of 0.05 μm as a certain type of real value μ of surface topography parameters. The data measured using larger steps are to be compared with the real values μ to determine whether they are acceptable in terms of accuracy. To set a range of acceptance, the idea of a confidence interval in probability theory is used. If the real value is μ, a measurement result that is acceptable based on confidence level of 1-α must fall into a computable interval. Based on the probability theory, when the mean value of the overall sample μ is known and the standard deviation σ is unknown, the confidence interval of the mean value μ with the confidence level (1 − α) is
\nwhere S is the standard deviation of the samples and n is the number of the samples.
\nBy looking up the table-α quantile of the t-distribution, \n
Here, Raave\n, Rqave\n, Rskave\n, and Rkuave\n on a Cf/SiC fiber bundle surface are taken as the evaluation standards. For each index, the real value is determined as the value measured based on a sampling length of 150 μm, sampling number of 200, and sampling step of 0.05 μm. In addition, \n
Changing trends of four evaluation indexes with increasing sampling steps on fiber bundle side surface [27]. (a) Raave, (b) Rqave, (c) Rskave, and (d) Rkuave.
Changing trends of four evaluation indexes with increasing sampling steps on fiber bundle end surface [27]. (a) Raave, (b) Rqave, (c) Rskave, and (d) Rkuave.
What needs to be mentioned is that the data used in Figures 10 and 11 are acquired using a sampling direction perpendicular to the fiber orientation on the side surface and the machining direction on the end surface. For each index, the upper and lower limits express the acceptance range. The measurement result that first falls out of range is marked with a red box, and the last sampling step before it is the MaxSS of this index. Combining all four indexes on each surface, the global MaxSS is 0.75 μm on a side surface and 1 μm on an end surface, which are approximately 1/10 of the fiber diameter.
\nThis chapter proposes four indexes for evaluating a fiber bundle surface, namely, Ra, Rq, Rsk, and Rku. To illustrate how the four indexes can estimate the main type and degree of damage of a fiber bundle surface, the side surface and end surface of Cf/SiC with processing angle of 90° are taken as a demonstration. The surfaces of Cf/SiC were process with three machining methods: a) ground using a grinding wheel with a wheel speed of 15 m/s, grinding depth of 0.15 mm, feed rate of 4 m/min, and grain mesh size of 80#; b) polished using a 1200# sandpaper under a constant force of 5 N, spindle speed of 0.1 m/s, and sliding time of 60s; and c) friction against a ZrO2 disk under a constant force of 30 N, spindle speed of 0.5 m/s, and sliding time of 3600 s. The three different methods caused different surface topographies and damages. Therefore, a proper set of indexes should be able to reflect the difference of the six kinds of surfaces.
\nAfter measuring the six surfaces with the sampling length of 280 μm and sampling number of 200, the scanning track perpendicular to the fiber orientation and machining direction, and the sampling step of 0.5 μm, four indexes can be calculated for every surface. The results are shown in Table 1.
\nFiber bundle surface | \n\nRaave\n/μm | \n\nRqave\n/μm | \n\nRskave\n\n | \n\nRkuave\n\n | \n
---|---|---|---|---|
Side surface, ground | \n1.97 ± 0.03 | \n2.44 ± 0.03 | \n−0.55 ± 0.05 | \n3.29 ± 0.10 | \n
Side surface, sandpaper polished | \n0.98 ± 0.02 | \n1.41 ± 0.05 | \n−2.15 ± 0.09 | \n6.48 ± 0.59 | \n
Side surface, friction | \n0.56 ± 0.02 | \n0.78 ± 0.02 | \n−0.84 ± 0.07 | \n7.96 ± 0.44 | \n
End surface, ground | \n1.23 ± 0.02 | \n1.56 ± 0.03 | \n−0.31 ± 0.04 | \n3.60 ± 0.12 | \n
End surface, sandpaper polished | \n0.26 ± 0.01 | \n0.36 ± 0.01 | \n−1.39 ± 0.35 | \n12.80 ± 0.55 | \n
End surface, friction | \n0.20 ± 0.01 | \n0.24 ± 0.01 | \n−0.34 ± 0.19 | \n21.14 ± 2.88 | \n
Fiber bundle surface parameters of three processing methods [27].
The data in Table 1 reflect that, regardless of the surface processing method used, the side surfaces are rougher than the end surfaces, which can be indicated by all side surfaces having larger Raave\n and Rqave\n than the end surfaces. This phenomenon can be explained as follows. During the machining process, the anti-shear strength of the end surface is stronger; it is thus not easy for the fibers to be pulled-out and form surface damage, and the end surface becomes smoother than the side surface. However, fiber debonding and fiber delamination are more likely to appear, leading to more damage and a rougher surface on the side surface. In addition, for both fiber orientation surfaces, Raave\n and Rqave\n of the friction-applied surfaces are the smallest, followed by sandpaper-polished surfaces, and finally ground surfaces. A conclusion can be made that Raave\n and Rqave\n are both valid in evaluating the degree of surface roughness. The rougher the surface is, the larger Raave\n and Rqave\n are.
\nAs a comparison, Rkuave\n of either a ground side surface or a ground end surface is roughly equal to 3. That is, the height distribution approximately obeys a Gaussian distribution on both surfaces. Polishing with sandpaper or sliding against a ZrO2 disk can make the surfaces flat, thus decreasing the amount of surface damage. Meanwhile, they show larger Rkuave\n values. It is clear that the surfaces after friction turn out to have the fewest numbers of surface defects, and Rkuave\n of the friction surfaces has the biggest value among the three processing methods on both the side and end surfaces. It can be seen that Rkuave\n is related to the amount of surface damage. The less damage a surface has, the larger Rkuave\n is.
\nFor both ground surfaces, Rskave\n is close to 0, which is consistent with their Gaussian distribution characteristic. For the surfaces polished by a sandpaper, the crests are chipped off during this processing, with the original troughs remaining, and thus it is reasonable for these two surfaces to have a larger negative Rskave\n. After friction is applied, however, their Rskave\n values reach closer to 0 again, which can be explained by the wear debris embedded into the troughs during the friction process decreasing the height of the troughs. Thus, it can be inferred that Rskave\n is able to reflect the damage type or degree of the surface. A larger negative Rskave\n value is caused by a trough-dominant surface, and a larger positive Rskave\n value is caused by a crest-dominant surface. The surface state can be inferred by combining Rskave\n and Rkuave\n.
\n\nFigure 12 shows the fiber bundle surface topographies of three processing methods. Figure 13 shows the microscopic surface topography of six surfaces. It is clear that the values of the four proposed indexes have a strong and direct connection with the surface damage and, thus, have a good feasibility and interpretability for a surface evaluation.
\nTopographies of the six surfaces [27].
SEM images of the six surfaces [27].
Cell body contains different fiber bundle orientations, and the whole surface is composed of cell bodies; thus standard procedures designated to 2D profile sampling at fiber bundle are in general not applicable for 3D topography measurement for cell body and the whole surfaces, because 2D measurement is of directionality which mainly reflects the damage between fibers, the fiber, and the matrix. Whereas for cell body and the whole surfaces, the scales are bigger, thus the measurement and evaluation mainly reflect the damage between fiber bundles and the matrix, which cannot consider the integrated effect of the fiber orientation and the processing direction simultaneously. Therefore, a 3D surface measurement and evaluation method should be adopted at these two grades.
\nA proper sampling step can make the surface information of a cell body extracted accurately and meanwhile save the cost of data collection and processing. MaxSS refers to the balance point of the accuracy and sampling cost. If a sampling step larger than the MaxSS is adopted, the information of a surface is distorted; if a sampling step larger than the MaxSS is adopted, unnecessary data sampling cost is spent. Therefore, how to determine the MaxSS on a cell body surface is very important. This section proposes a method for this topic, based on the principle of residual estimation.
\nFor a cell body surface, the following steps can be executed to determine the MaxSS:
\nSample the surface using a small sampling step of the measurement device which is at least one third of the WCMC fiber diameter.
\nCalculate Sa, Sq, Ssk, and Sku based on the sampling results, and set them as surface standard values θ (the standard value here does not refer to the ideal measurement result which has no error, rather, it means a standard which can be used to check whether other measurement results are acceptable).
\nGenerally speaking, this standard value θ, containing measurement error, can be regarded as a random variable, which obeys Normal distribution, so, θ ∼ N (μ\n1, σ\n1\n2). A measurement result \n
Set e as the difference between \n
Since \n
Combining Eqs. (16) and (17) together,
\nBased on the analysis above, for a set of measurement results obtained from different sampling steps, the Residual Errors (REs) between each of them and the standard value θ are IID to Normal distribution, so
\nFor every actual engineering question, it is reasonable to find an acceptable range of RE according to the actual requirements of measurement. For example, if the ±15% smallest REs are acceptable, the acceptable measurement results fall into the range of
\nWhen the sampling step is small enough, the measurement result is in the range above. However, if the sampling step grows larger, the measurement result will go out of the range sooner or later. The largest sampling step that holds the measurement result within the range of Eq. (20) can be defined as the MaxSS for cell body surface measurement.
\nHere we take the measurement of a cell body of the Cf/SiC with processing angle of 90° as an example. The sampling steps of 1–45 μm were adopted to measure the cell body. The measurement results of 1 μm were set as the standard values. The rest of the results were compared with the standard values to calculate the REs. The acceptable ranges of measurement results are available through Eq. (20).
\nThe changing trends of the measurement results of the four evaluation indexes under different sampling steps are illustrated in Figure 14. The red and blue lines refer to the boundaries of the acceptable ranges calculated from Eq. (20). It is clear that when the sampling steps exceed a certain value (in red blocks), the corresponding results begin to go out of the ranges. Then the MaxSS can be determined for each index. Combining the four MaxSSs together, the MaxSS is available and for this material, it is 7 μm.
\nThe changing trends of (a) Sa (b) Sq (c) Ssk and (d) Sku with sampling steps on cell body surface [28].
It can also be proved that the MaxSS of a cell body is approximately equal to the diameter of its reinforcing fiber.
\nThe whole surface of a WCMC consists of many cell bodies. Some cell bodies nearby each other faced the same fabrication and machining process and may perform similar surface quality. Therefore, the measurement and evaluation of a cell body can be used to estimate the surface quality state of a certain area nearby it. It has been proved that, for the exampled Cf/SiC with processing angles of 0°, 30°, 45°, and 90°, the measurement results of the four indexes of one cell body have the similar values with the results of the nearby four cell bodies (shown in Figure 15).
\nEvaluation parameters with processing angles on cell body surface of Cf/SiC (a) Sa, (b) Sq, (c) Ssk, and (d) Sku.
This chapter aims at providing a grading surface measurement and evaluation system for woven ceramic matrix composites. The system contains four grading of fiber, fiber bundle, cell body, and the whole surface. The main conclusions are as follows:
The type and degree of the damage on fibers influence the processing quality and property of the surface. The diameter and the direction of the fibers determine the measurement parameters when sampling fiber bundle or cell body surfaces.
2D measurement should be adopted on fiber bundle surfaces. Sampling parameters, including sampling length, number, step, and direction should be determined carefully to balance the accuracy and the efficiency. Four evaluation indexes, namely, Ra, Rq, Rsk, and Rku, are usable for fiber bundle surface evaluation.
3D measurement should be adopted on cell body surfaces. Maximum sampling step can be determined with the principle of residual estimate. Sa, Sq, Ssk, and Sku are usable on this grade.
The whole surface is consist of many cell bodies. Therefore, a small number of cell bodies can be used to represent a larger area nearby. This idea can help reduce the workload when measuring and evaluating a large area of WCMC surface.
Special thanks the National Natural Science Foundation of China (Nos. 51375333 and 51805366) for financial assistance.
\nAll the authors listed have approved the manuscript, and no interest of any third parties is infringed.
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I have served as the editor for many books, been a member of the editorial board in science journals, have published many papers and hold many patents.",institutionString:null,institution:{name:"Sheffield Hallam University",country:{name:"United Kingdom"}}},{id:"54525",title:"Prof.",name:"Abdul Latif",middleName:null,surname:"Ahmad",slug:"abdul-latif-ahmad",fullName:"Abdul Latif Ahmad",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:null},{id:"20567",title:"Prof.",name:"Ado",middleName:null,surname:"Jorio",slug:"ado-jorio",fullName:"Ado Jorio",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"Universidade Federal de Minas Gerais",country:{name:"Brazil"}}},{id:"47940",title:"Dr.",name:"Alberto",middleName:null,surname:"Mantovani",slug:"alberto-mantovani",fullName:"Alberto Mantovani",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:null},{id:"12392",title:"Mr.",name:"Alex",middleName:null,surname:"Lazinica",slug:"alex-lazinica",fullName:"Alex Lazinica",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/12392/images/7282_n.png",biography:"Alex Lazinica is the founder and CEO of IntechOpen. After obtaining a Master's degree in Mechanical Engineering, he continued his PhD studies in Robotics at the Vienna University of Technology. Here he worked as a robotic researcher with the university's Intelligent Manufacturing Systems Group as well as a guest researcher at various European universities, including the Swiss Federal Institute of Technology Lausanne (EPFL). During this time he published more than 20 scientific papers, gave presentations, served as a reviewer for major robotic journals and conferences and most importantly he co-founded and built the International Journal of Advanced Robotic Systems- world's first Open Access journal in the field of robotics. Starting this journal was a pivotal point in his career, since it was a pathway to founding IntechOpen - Open Access publisher focused on addressing academic researchers needs. Alex is a personification of IntechOpen key values being trusted, open and entrepreneurial. 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He is an expert in structural, absorptive, catalytic and photocatalytic properties, in structural organization and dynamic features of ionic liquids, in magnetic interactions between paramagnetic centers. The author or co-author of 3 books, over 200 articles and reviews in scientific journals and books. He is an actual member of the International EPR/ESR Society, European Society on Quantum Solar Energy Conversion, Moscow House of Scientists, of the Board of Moscow Physical Society.",institutionString:null,institution:{name:"Semenov Institute of Chemical Physics",country:{name:"Russia"}}},{id:"62389",title:"PhD.",name:"Ali Demir",middleName:null,surname:"Sezer",slug:"ali-demir-sezer",fullName:"Ali Demir Sezer",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/62389/images/3413_n.jpg",biography:"Dr. Ali Demir Sezer has a Ph.D. from Pharmaceutical Biotechnology at the Faculty of Pharmacy, University of Marmara (Turkey). He is the member of many Pharmaceutical Associations and acts as a reviewer of scientific journals and European projects under different research areas such as: drug delivery systems, nanotechnology and pharmaceutical biotechnology. Dr. Sezer is the author of many scientific publications in peer-reviewed journals and poster communications. Focus of his research activity is drug delivery, physico-chemical characterization and biological evaluation of biopolymers micro and nanoparticles as modified drug delivery system, and colloidal drug carriers (liposomes, nanoparticles etc.).",institutionString:null,institution:{name:"Marmara University",country:{name:"Turkey"}}},{id:"61051",title:"Prof.",name:"Andrea",middleName:null,surname:"Natale",slug:"andrea-natale",fullName:"Andrea Natale",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:null},{id:"100762",title:"Prof.",name:"Andrea",middleName:null,surname:"Natale",slug:"andrea-natale",fullName:"Andrea Natale",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"St David's Medical Center",country:{name:"United States of America"}}},{id:"107416",title:"Dr.",name:"Andrea",middleName:null,surname:"Natale",slug:"andrea-natale",fullName:"Andrea Natale",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"Texas Cardiac Arrhythmia",country:{name:"United States of America"}}},{id:"64434",title:"Dr.",name:"Angkoon",middleName:null,surname:"Phinyomark",slug:"angkoon-phinyomark",fullName:"Angkoon Phinyomark",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/64434/images/2619_n.jpg",biography:"My name is Angkoon Phinyomark. I received a B.Eng. degree in Computer Engineering with First Class Honors in 2008 from Prince of Songkla University, Songkhla, Thailand, where I received a Ph.D. degree in Electrical Engineering. My research interests are primarily in the area of biomedical signal processing and classification notably EMG (electromyography signal), EOG (electrooculography signal), and EEG (electroencephalography signal), image analysis notably breast cancer analysis and optical coherence tomography, and rehabilitation engineering. I became a student member of IEEE in 2008. During October 2011-March 2012, I had worked at School of Computer Science and Electronic Engineering, University of Essex, Colchester, Essex, United Kingdom. In addition, during a B.Eng. I had been a visiting research student at Faculty of Computer Science, University of Murcia, Murcia, Spain for three months.\n\nI have published over 40 papers during 5 years in refereed journals, books, and conference proceedings in the areas of electro-physiological signals processing and classification, notably EMG and EOG signals, fractal analysis, wavelet analysis, texture analysis, feature extraction and machine learning algorithms, and assistive and rehabilitative devices. I have several computer programming language certificates, i.e. Sun Certified Programmer for the Java 2 Platform 1.4 (SCJP), Microsoft Certified Professional Developer, Web Developer (MCPD), Microsoft Certified Technology Specialist, .NET Framework 2.0 Web (MCTS). 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