The key principles of sol-gel process and its characteristics are outlined and its major control parameters are summarized. Different samples of functional powders and films with magnetic, optical, and dielectric properties prepared by the sol-gel method are described. To determine the relationship between microstructure and properties, the effects of preparation conditions on the size and microstructure and electric properties, dielectric properties, optical properties, and magnetic properties are analyzed.
- optical properties
- hexagonal ferrite
Sol-gel processing is a well-recognized, cost-competitive, bottom-up synthesis technique, which is used in the field of material science and ceramic engineering for the generation of oxide nanopowder and composite NPs from a sol followed by a gel formation . Functionality refers to typical chemicophysical properties owned by materials, which makes them possible to be exploited in technological applications, as in electrical conductors, thermoelectrics, ferroelectrics, magnets, and transparent optical devices. The sol-gel process of functionality opens promising applications in many areas, such as optics, electronics, ionics, mechanics, energy, environment, biology, medicine, for example as membranes and separation devices, fuel and solar cells, catalysts, sensors, and functional smart coatings. Sol-gel-derived coatings provide a wide range of applications, such as corrosion protective, hydrophilic, hydrophobic, antireflective, and antifog coatings; migration barriers against liquid and volatile compounds; antibacterial modification of textiles and water-repellent antistatic textiles; and self-cleaning coatings .
The sol-gel method initial study can be traced back to 1846, when Ebelmen find SiCl4 mixed with ethanol in the wet air hydrolysis and formation of gel. The process of making tofu in ancient China may be the first method to use sol-gel technology in a purposeful and effective way; the soybean protein solution was flocculated into a solid bean curd (gel). Until the end of 1930s, W. Geffeken can conduct oxide film preparation through metal alkoxide hydrolysis and gelation. In 1971 in Germany, H. Dislieh reported sol by the hydrolysis of metal alkoxides, after gelation and retreatment in 923–973 K and high pressure resulting in the formation of SiO2-B2O3-Al2O3-Na2O-K2O multicomponent glass, which aroused great interest and attention in the field of material science. In 1975, B. E. Yoldas and M. Yamane dried the gel to prepare monolithic ceramic materials and porous transparent alumina films. Since 1980s, sol-gel technology has been successfully applied in glass, oxide coatings, and functional ceramic powders, especially composite oxide such as high critical temperature superconducting oxide material synthesis which is difficult to prepare by traditional method.
2. Key principles of sol-gel process and its characteristics
Sol is a system of tiny solid particles suspended in the liquid phase and constantly showing Brownian movement. Because the Gibbs free energy of the interface atoms is higher than that of the internal atoms, the sol is thermodynamically unstable. The particles tend to agglomerate spontaneously, reaching a low surface energy state. If the process is reversible, it is called flocculation; if not reversible, it is called gelation. The gel is a kind of solid feature of the colloidal system composed of fine particles in three-dimensional network structure and a continuous medium consisting of dispersed phase. The transition process to the gel can be briefly described as the formation of the polymer or the particle aggregates formed by the condensation reaction into a small particle cluster and gradually connected to a three-dimensional network structure. Therefore, the gelation process can be regarded as a small particle cluster connected to each other as a continuous solid network.
Sol-gel processes are mainly based on hydrolysis and condensation reactions of metal alkoxides, the molecular precursors that develop an oxide network in a liquid medium .
Accurate control of the sol-gel reaction parameters can be designed to produce many forms of certain properties of the materials in the form of glass, fiber, ceramic powder, and thin film (Figure 1).
According to the types of precursors, the sol-gel process can be divided into two types: organic and inorganic. The organic route is that the sol-gel is prepared by metal alkoxides which can be considered as a two-step inorganic polymerization. The first step is the hydrolysis of alkoxide ligands to yield hydroxylated metal centers :
The second step is the condensation of the hydroxylated species to form oxypolymers, involving an oxylation reaction which creates oxygen bridges and expels XOH species as follows :
3. Major control parameters of sol-gel process
The macroscopic properties of the material are determined by its microstructure, and the microstructure is determined by the conditions of the material.
Generally speaking, the sintering properties of the powder vary in different preparation methods. While using large surface area and high surface activity of monodisperse ultrafine ceramic powder, due to short diffusion distance and only need lower sintering temperature and activation energy.
Factors that influence the properties of final products, acquired by sol-gel technique, include hydrolysis ratio, acidity of the hydrolyzing agent, gelation condition, drying condition and procedures and types of solvent.
3.1. Chemical compositions of the precursors
The nature of the precursor can significantly affect the sol-gel reaction kinetics and final products.
3.2. Hydrolysis ratio
where [H2O] and [M(OR)
The lower amount of water added is easy to form the product of low crosslinking, and the viscosity of the sol is increased; the higher amount of water added is easy to form a highly cross-linked product, and the viscosity is decreased. Therefore, the amount of water has an important effect on the structure of the product and the viscosity and gelation time. When
Both acids and bases can catalyze sol-gel reactions. The hydrolysis and condensation reactions can be explained by SN2 nucleophilic substitution reaction mechanism. Take alcohol as an example, it is not easy to hydrolyze silicon alkoxide, and its hydrolysis and polymerization are usually catalyzed by acid or alkali. The inorganic acid can make the partially negatively charged alkoxy matrix, so that it is easy to break away from the silicon atom. Alkali provides nucleophilic hydroxyl OH in alkali-catalyzed hydrolysis, and Si—OH loss of protons, thereby accelerating polymerization. In the presence of excess water, because of the acid-catalyzed hydrolysis reaction, Si(OH) can be formed. The rate of polymerization is faster than that of hydrolysis under the condition of alkali catalysis. Acid or base catalysis is not the only catalyst. Some nucleophiles, such as NaF or 2-methylpyridine, also have the ability to improve the reaction rate.
3.4. pH value
Zinc oxide (ZnO) nanoparticles were prepared and synthesized via sol-gel method, using citric acid as a precursor. The annealing temperature was fixed at 600°C. The impact of pH on the particle size was investigated. Three different pH (3.0, 5.0, and 1.01) for the precursor were chosen, and it was revealed that the Zn NPs size increased with the alkalinity of the precursor . The pH value of the sol-gel solution will change when added with acids and alkalis to catalyze the reactions. Experimental results show that the pH value and hydrolysis ratio have a synergistic effect on the morphologies of the products.
4. Advantages and shortages of sol-gel processing
The sol-gel process provides samples with high purity, homogeneity, and structure of easy control. The most important advantages of the sol-gel process in the preparation of functional materials are as follows:
In all stages, the temperatures required are low and close to room temperature. Then, thermal decomposition of organic material and any entrapped species is minimized leading to high purity and stoichiometry .
As the organometallic precursors for different metals are miscible, the homogeneous sol solutions are easily achieved . Since the sol-gel is initiated by the reaction of the solution, the materials are very uniform and easy to modify, which is crucial for controlling the physical and chemical properties of the material.
Precursors such as metal alkoxides and mixed alkyl/alkoxides are easily purified by common techniques (e.g., distillation or sublimation), which lead to high-purity products .
Highly porous and nanocrystalline materials can be synthesized by this method .
Colloid particle size and pore size, porosity, and chemistry of the final product can be optimized by chemical treatment of the precursors, controlled rates of hydrolysis, and by condensation .
Incorporating several components in a single step or in two steps .
Production of samples in different physical forms [2, 10]. Starting from the same raw material, changing the process can get different products, such as fiber, powder, or film and composite materials.
Treating temperature is low, the inorganic materials can be synthesized under the condition of low temperature at about 600°C, the composition and the structure of the product are uniform, the grain size is small, the activity of the material particles is increased, and the performance of the material is expanded.
Suitable for large-scale industrial production.
The negative factors of sol-gel processing include high precursor cost, overall high cost, and environmental problems associated with the disposal of large quantities of organic by-products. A common theme out of the negative comments was wet chemical processing using alkoxides as precursors .
The sol-gel process is more complex in terms of the difficulty in phase control, which is the different chemical and crystal morphology formation at different temperatures. The morphology is relatively simple, generally, spherical particles.
Gelation, drying, and heat treatment take a lot of time to prepare the sample. In the stage of drying and heat treatment, the sample has a great weight loss and residual stress, film prone to cracking and objectively restricting the thickness of thin film and result in film residual porosity.
5. Sol-gel processing of functional powder materials
5.1. Multiferroic properties of powder materials
First, Y(NO3)3 ·5H2O and Fe(NO3)3 ·9H2O were dissolved in deionized water in the molar ratio of
Figure 2 shows the room temperature hysteresis loop and magnetization temperature curve of h-YFO powder. As can be seen from Figure 2, h-YFO has a weak magnetism, and the hysteresis loop is narrow and small, which belongs to the soft magnet. The remanent magnetization (Mr) and coercive field (H) are 0.11 memu g-1 and 0.19 kOe, respectively. The coercivity and saturation magnetization of possible impurities Y3Fe5O12 and γ-Fe2O3 are about 100 Oe and 26.0 memu g-1 and 150 Oe and 70 memu g-1 at room temperature, respectively [12, 13].
The magnetization of h-YFO is very small and the coercive field is much larger, so it is possible to exclude the influence of impurities on the magnetic properties of the samples, which is characterizing the magnetic properties of h-YFO.
Figure 3 shows the h-YFO magnetization-temperature relationship curve (M-T) and its first derivative curve (dM/dT-T), measured at the external magnetic field strength of H = 1000 Oe using temperature mode. It can be seen from the figure that the magnetization of h-YFO is gradually reduced from 0.72 memu g-1 to about zero in the temperature range of 750 K to 877 K, which indicates that h-YFO has a complex magnetic phase transition in this temperature range.
Figure 4 shows the hysteresis loop and the magnetization temperature curve of o-YFO powder at room temperature. Compared to h-YFO, the hysteresis loop of o-YFO is wide which belongs to hard magnet, and the magnetization and coercive field of o-YFO is 0.72 memu g-1 and 24.5 kOe respectively which is much larger than h-YFO. It is also shown that the influence of impurities on the magnetic properties is small. From the extension direction of the loop line, o-YFO has a certain antiferromagnetic property.
It is worth noting that Figure 4 llustrations for the hysteresis curve of o-YFO on a small magnetic field (<10 kOe) that shows a peculiar shape with a sudden change magnetization in H = 0 kOe, which is considered to be the inherent characteristic of o-YFO. The causes of the abnormal YFeO3 hysteresis loop are produced by the interaction of the same iron ions, the interaction with the external magnetic field, the DM antisymmetric interaction, and the anisotropy effect of the magnetic crystal. The adjacent two iron ions in YFeO3 is of a slight distortion, rather than a strict spin antiparallelly which reveals overall magnetic anisotropy. When the external magnetic field is too small to overcome the anisotropic magnetic field of the magnetic material, the magnetic behavior will be different.
Figure 5 shows the magnetization-temperature curve (M-T) and its first derivative curve (dM/dT-T) for o-YFO, measured in the same way those for h-YFO. The magnetic properties of o-YFO become smaller in the range of 540 K till 645 K, the magnetization is 0, which is similar to the Nair temperature TN = 640 K of the orthorhombic phase YFO, which indicates that the magnetic phase transition process is from antiferromagnetic to paramagnetic.
Figures 6 and 7 show the
Figures 8 and 9 show the dielectric constant and dielectric loss tan
5.2. Optical properties of powder materials
For semiconductor nanomaterials, the optical absorption is closely related to the band gap. The short wavelength cutoff in the transmission is due to the fundamental band gap excitation from the valence band to the conduction band. Energy band gap of samples can be evaluated from the absorption spectra.
The optical absorption spectra as a function of wavelength of (La(1−
The plots of (
With the increase in the concentration of doping, the donor states push the Fermi level higher in energy, which shift close to the conduction band above the occupied donor state. In the degenerate semiconductor, only an electron can be excited from the valence band into conduction band above the Fermi level. Based on the Pauli's exclusion principle this excitation into occupied states is prohibited, resulting in the increase in the optical band gap .
6. Sol-gel processes of functional film
Sol-gel-derived coating film is suitable for substrates with large areas. Preferred crystal orientation can be given depending on the conditions of preparation. The techniques can be applied to accomplishing a large variety of functional films with optical, electronic, magnetic, chemical, and mechanical functions.
In sol-gel process, the key film-forming method mainly includes the dip-coating method, the rotating method, the sputtering method, and so on. The dip-coating method is elevating impregnated substrate from uniform solution, after gelling process, to become gel, and the film with nanoparticles can be obtained after a certain temperature, and the film thickness can be controlled by the number of pulling speed or spin coating. The rotation method is a method of liquid film coating on substrate, the film thickness reduces with the increase in the substrate rotation speed. Sputtering is a method of forming film by spraying the solution from one to several stationary sputtering guns onto a preheated substrate which runs at a certain speed.
6.1. Dielectric properties of the perovskite-related oxide CCTO film
Perovskite-related oxide CaCu3Ti4O12 (CCTO) ceramic has an extremely high dielectric constant (~10,000 for bulk and ~105 for single crystals) and moderately lower dielectric loss (~0.1) at low frequencies with centrosymmetric bcc structure (space group (no. 204), lattice parameter
Calcium nitrate (Ca(NO3)2 ·4H2O), copper nitrate (Cu(NO3)2 ·4H2O), and tetrabutyl titanate (Ti(OC4H9)4) were selected as solutes; ethylene glycol monomethyl ether (CH3OCH2CH2OH) as solvent; dilute nitric acid as stabilizer; acetic acid as chelating agent; and citric acid as colloidal complexing agent. Zirconium nitrate (Zr(NO3)4) was used to introduce Zr elements as dopingagents in the sol. Stoichiometric calcium nitrate (Ca(NO3)2 ·4H2O) and copper nitrate(Ca(NO3)2 ·4H2O) were dissolved in methoxyethanol. Subsequently, the solution was heated and stirred to be fully clarified and was cooled to room temperature (select appropriate amount of zirconium nitrate (Zr(NO3)4) as the introduction of Zr doping agents for the preparation of Zr-doped CaCu3Ti4O12) with a certain dose of dilute nitric acid, and then a stoichiometric amount of tetrabutyl titanate was added. Citric acid (C6H8O7 ·H2O) in the ratio of 1:1.5 of sum of the metal ions to the number of moles of citric acid was placed into the solution as complexing agent. By mixing completely, the transparent homogeneous solution of concentration 0.15 mol/L was formed. The solution was aged for 24 h to prepare the powder. Then it was calcined at 450°C for pyrogenic decomposition, and a brown, black powder was obtained. As-prepared powder was made into bulk samples, 10 mm in diameter and 2 mm in thickness at 20 MPa pressure. Samples were sintered at 1000°C in silicon carbide furnace for 2 h and dense polycrystalline ceramic flakes were formed. The furnace heating rate was 100°C/h and the cooling rate was 150°C/h.
The densities of samples were measured by Archimedes method. The crystalline phases were identified by powder X-ray diffraction analysis installation of a spectrum analyzer with CuKα radiation (
The CaCu3Zr0.05Ti3.95O12 was confirmed by X-ray diffraction. The X-ray diffraction patterns of pure CaCu3Ti4O12 and CaCu3Zr0.05Ti3.95O12 are shown in Figure 12. It shows the room temperature XRD patterns obtained for the CaCu3Zr0.05Ti3.95O12 nanocrystalline powder compared well with the ICDD data (01-075-1149). The samples reveal quite similar diffraction profiles and diffraction peaks corresponding to oxide CaCu3Ti4O12 phase (JCPDS No. 75-2188) with bcc cubic perovskite-related structure. All the XRD patterns of the main diffraction peaks can be attributed to the CCTO phase. It can determine the crystalline CCTO phase generated. Two curves can be found similar from the figure, which indicate that the content of zirconium has almost no effect on the crystal phase. From the point of view of the ionic radius of Ca2+, Cu2+, and Ti4+, the ionic radius of Ca2+, Cu2+, and Ti4+ are l00 pm, 73 pm, and 60.5 pm, respectively, whereas the ionic radius of Zr4+ is 79 pm. Through the solution rule of thumb, zirconium is likely to replace titanium ions at B site belonging to the perovskite ABO3 structural ceramics in case of the occurrence of solid solution.
The temperature dependence of dielectric properties of CaCu3Ti4O12 and CaCu3Zr0.05Ti3.95O12 were studied and are illustrated in Figure 13. The dielectric temperature spectra of CaCu3Ti4O12 and CaCu3Zr0.05Ti3.95O12 ceramics were measured at 100 kHz. Dielectric properties' variation with temperature is relatively flat in the range of 100–350 K. As the temperature increases, the thermal stabilities of CaCu3Ti4O12 and CaCu3Zr0.05Ti3.95O12 deteriorate. The dielectric property stable temperature zone of CaCu3Zr0.05Ti3.95O12 is narrower than that of CaCu3Ti4O12, whereas the peak width of CaCu3Zr0.05Ti3.95O12 is larger than that of CaCu3Ti4O12. In the low frequency regime, the dielectric permittivity of CaCu3Zr0.05Ti3.95O12 increased from 50,000 to 500,000 when the temperature was increased from 420 K to 450 K. This sudden increase in dielectric permittivity observed at 100 kHz may be assigned to the space charge/interfacial polarization effects.
Figures 14 and 15 show the frequency dependence of dielectric permittivity and loss at room temperature for pure CaCu3Ti4O12 and CaCu3Zr0.05Ti3.95O12, respectively. As the frequency increases in the range of 1 KHz to 1000 KHz, the dielectric constant of CCTO-based ceramics decreased. However, the dielectric loss increased significantly with increasing frequency. The dielectric loss of the test samples in the lower frequency at room temperature are relatively closed and nearly keep constant. With increasing frequency, the dielectric constants of pure CaCu3Ti4O12 and CaCu3Zr0.05Ti3.95O12 decrease rapidly at low-frequency ranges. Both the CCTO-based ceramics possess large dielectric constant in low-frequency ranges, which is attributed to the interfacial polarization at the interfaces between the two components.
As shown in Figures 3 and 4, each sample has similar dielectric behavior at room temperature. The dielectric constant displays a steep decrease at a certain frequency, and at the same time the dielectric loss shows a relaxation peak. There is a clear relaxation characteristic and the relaxation is closely related to the measurement frequency, which shows a typical Debye relaxation.
It is obvious that all of these samples exhibit the giant permittivity at low frequency. As the frequency increases, permittivity has a sudden drop in the frequency range below 100 kHz; and then becomes flat. The permittivity increased with decreasing frequency which signaled the possibility of the presence of interfacial polarization. It is probably that the charge carriers may be blocked at an electrode interface when migrated under the influence of an electric field, which causes interfacial polarization. At the same time, the frequency response of dielectric properties of CCTO-based ceramics is closely linked to the polycrystalline microstructure. CCTO ceramics are of very high permittivity, the performance is more obvious especially in the condition of lower frequency, which happens on account of the famous Maxwell-Wagner effect.
It can be seen from the SEM photographs in Figure 16 that the powder particles are homogeneous and dense, the particle sizes are less than 100 nm estimated from the figure. Crystalline phase was obtained which can be confirmed from XRD and SEM photographs. The powder prepared by sol-gel process helped in the uniform mixing of all the elements in nanoscale, and corresponding nanocrystals were formed. From the SEM photographs, it is evident that prepared samples of CaCu3Ti4O12 and CaCu3Zr0.05Ti3.95O12 are composed of many grain, the grain boundary effect can make a major contribution to the high dielectric constant of polycrystalline CCTO-based ceramics.
6.2. Optical properties of magnetic films
Iron-based semiconducting oxide magnetic nanoparticles, for instance spinel ferrites with general formula M2+(Fe3+)2O4 (where M2+ = Mn2+, Co2+, Ni2+, Cu2+,Zn2+, etc.), have become a prominent family in the magnetic materials due to their high electrical resistivity, high value of magnetic permeability, and use in advanced technologies like microwave devices, magnetic information storage devices, and biomedical applications . Ferrites are one of the most effective photocatalysts for the ejection of toxic dyes from the industrial sewage.
Analytical grade Fe(NO3)3 ·9H2O, (Co(NO3)2 ·6H2O, (Mg(NO3)2 ·6H2O, and citric acid (C6H8O7) were used as starting materials for the preparation of Co(1−
UV-visible spectra were utilized to determine the band gap energies (
The preparation methods of the thin films or powders and the heat treatment conditions have a crucial influence on the grain growth of the film. It affects not only the grain size of the film but also the surface morphology of the film, which affects the dielectric properties of the material. Microstructure has effects on the dielectric properties, optical properties, and magnetic properties of samples. Sol-gel method is a reasonable way to prepare function materials. Polycrystalline and nanocrystalline materials can be prepared by this process.
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