Chemical composition of the bleached spicules measured by means of XRF.
\r\n\tAnimal food additives are products used in animal nutrition for purposes of improving the quality of feed or to improve the animal’s performance and health. Other additives can be used to enhance digestibility or even flavour of feed materials. In addition, feed additives are known which improve the quality of compound feed production; consequently e.g. they improve the quality of the granulated mixed diet.
\r\n\r\n\tGenerally feed additives could be divided into five groups:
\r\n\t1.Technological additives which influence the technological aspects of the diet to improve its handling or hygiene characteristics.
\r\n\t2. Sensory additives which improve the palatability of a diet by stimulating appetite, usually through the effect these products have on the flavour or colour.
\r\n\t3. Nutritional additives, such additives are specific nutrient(s) required by the animal for optimal production.
\r\n\t4.Zootechnical additives which improve the nutrient status of the animal, not by providing specific nutrients, but by enabling more efficient use of the nutrients present in the diet, in other words, it increases the efficiency of production.
\r\n\t5. In poultry nutrition: Coccidiostats and Histomonostats which widely used to control intestinal health of poultry through direct effects on the parasitic organism concerned.
\r\n\tThe aim of the book is to present the impact of the most important feed additives on the animal production, to demonstrate their mode of action, to show their effect on intermediate metabolism and heath status of livestock and to suggest how to use the different feed additives in animal nutrition to produce high quality and safety animal origin foodstuffs for human consumer.
",isbn:"978-1-83969-404-2",printIsbn:"978-1-83969-403-5",pdfIsbn:"978-1-83969-405-9",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!0,hash:"8ffe43a82ac48b309abc3632bbf3efd0",bookSignature:"Prof. László Babinszky",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/10496.jpg",keywords:"Technological Feed Additives, Feed Industry, Quality of Compound Feed, Non-Antibiotic Growth Promoter, Product Quality, Additive Enzymes, Digestibility of Nutrients, NSP Enzymes, Farm Animals, Livestock, Immunity, Microbiome",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:null,numberOfDimensionsCitations:null,numberOfTotalCitations:null,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"November 24th 2020",dateEndSecondStepPublish:"December 22nd 2020",dateEndThirdStepPublish:"February 20th 2021",dateEndFourthStepPublish:"May 11th 2021",dateEndFifthStepPublish:"July 10th 2021",remainingDaysToSecondStep:"a month",secondStepPassed:!0,currentStepOfPublishingProcess:3,editedByType:null,kuFlag:!1,biosketch:"Professor Emeritus from the University of Debrecen, Hungary who authored 297 publications (papers, book chapters) and edited 3 books. Member of various committees and chairman of the World Conference of Innovative Animal Nutrition and Feeding (WIANF).",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"53998",title:"Prof.",name:"László",middleName:null,surname:"Babinszky",slug:"laszlo-babinszky",fullName:"László Babinszky",profilePictureURL:"https://mts.intechopen.com/storage/users/53998/images/system/53998.jpg",biography:"László Babinszky is Professor Emeritus of animal nutrition at the University of Debrecen, Hungary. From 1984 to 1985 he worked at the Agricultural University in Wageningen and in the Institute for Livestock Feeding and Nutrition in Lelystad (the Netherlands). He also worked at the Agricultural University of Vienna in the Institute for Animal Breeding and Nutrition (Austria) and in the Oscar Kellner Research Institute in Rostock (Germany). From 1988 to 1992, he worked in the Department of Animal Nutrition (Agricultural University in Wageningen). In 1992 he obtained a PhD degree in animal nutrition from the University of Wageningen.He has authored 297 publications (papers, book chapters). He edited 3 books and 14 international conference proceedings. His total number of citation is 407. \r\nHe is member of various committees e.g.: American Society of Animal Science (ASAS, USA); the editorial board of the Acta Agriculturae Scandinavica, Section A- Animal Science (Norway); KRMIVA, Journal of Animal Nutrition (Croatia), Austin Food Sciences (NJ, USA), E-Cronicon Nutrition (UK), SciTz Nutrition and Food Science (DE, USA), Journal of Medical Chemistry and Toxicology (NJ, USA), Current Research in Food Technology and Nutritional Sciences (USA). From 2015 he has been appointed chairman of World Conference of Innovative Animal Nutrition and Feeding (WIANF).\r\nHis main research areas are related to pig and poultry nutrition: elimination of harmful effects of heat stress by nutrition tools, energy- amino acid metabolism in livestock, relationship between animal nutrition and quality of animal food products (meat).",institutionString:"University of Debrecen",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"1",totalChapterViews:"0",totalEditedBooks:"0",institution:{name:"University of Debrecen",institutionURL:null,country:{name:"Hungary"}}}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"25",title:"Veterinary Medicine and Science",slug:"veterinary-medicine-and-science"}],chapters:null,productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"185543",firstName:"Maja",lastName:"Bozicevic",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/185543/images/4748_n.jpeg",email:"maja.b@intechopen.com",biography:"As an Author Service Manager my responsibilities include monitoring and facilitating all publishing activities for authors and editors. From chapter submission and review, to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. Whether that be identifying an exceptional author and proposing an editorship collaboration, or contacting researchers who would like the opportunity to work with IntechOpen, I establish and help manage author and editor acquisition and contact."}},relatedBooks:[{type:"book",id:"7144",title:"Veterinary Anatomy and Physiology",subtitle:null,isOpenForSubmission:!1,hash:"75cdacb570e0e6d15a5f6e69640d87c9",slug:"veterinary-anatomy-and-physiology",bookSignature:"Catrin Sian Rutland and Valentina Kubale",coverURL:"https://cdn.intechopen.com/books/images_new/7144.jpg",editedByType:"Edited by",editors:[{id:"202192",title:"Dr.",name:"Catrin",surname:"Rutland",slug:"catrin-rutland",fullName:"Catrin Rutland"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"1591",title:"Infrared Spectroscopy",subtitle:"Materials Science, Engineering and Technology",isOpenForSubmission:!1,hash:"99b4b7b71a8caeb693ed762b40b017f4",slug:"infrared-spectroscopy-materials-science-engineering-and-technology",bookSignature:"Theophile Theophanides",coverURL:"https://cdn.intechopen.com/books/images_new/1591.jpg",editedByType:"Edited by",editors:[{id:"37194",title:"Dr.",name:"Theophanides",surname:"Theophile",slug:"theophanides-theophile",fullName:"Theophanides Theophile"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3092",title:"Anopheles mosquitoes",subtitle:"New insights into malaria vectors",isOpenForSubmission:!1,hash:"c9e622485316d5e296288bf24d2b0d64",slug:"anopheles-mosquitoes-new-insights-into-malaria-vectors",bookSignature:"Sylvie Manguin",coverURL:"https://cdn.intechopen.com/books/images_new/3092.jpg",editedByType:"Edited by",editors:[{id:"50017",title:"Prof.",name:"Sylvie",surname:"Manguin",slug:"sylvie-manguin",fullName:"Sylvie Manguin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3161",title:"Frontiers in Guided Wave Optics and Optoelectronics",subtitle:null,isOpenForSubmission:!1,hash:"deb44e9c99f82bbce1083abea743146c",slug:"frontiers-in-guided-wave-optics-and-optoelectronics",bookSignature:"Bishnu Pal",coverURL:"https://cdn.intechopen.com/books/images_new/3161.jpg",editedByType:"Edited by",editors:[{id:"4782",title:"Prof.",name:"Bishnu",surname:"Pal",slug:"bishnu-pal",fullName:"Bishnu Pal"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"72",title:"Ionic Liquids",subtitle:"Theory, Properties, New Approaches",isOpenForSubmission:!1,hash:"d94ffa3cfa10505e3b1d676d46fcd3f5",slug:"ionic-liquids-theory-properties-new-approaches",bookSignature:"Alexander Kokorin",coverURL:"https://cdn.intechopen.com/books/images_new/72.jpg",editedByType:"Edited by",editors:[{id:"19816",title:"Prof.",name:"Alexander",surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"1373",title:"Ionic Liquids",subtitle:"Applications and Perspectives",isOpenForSubmission:!1,hash:"5e9ae5ae9167cde4b344e499a792c41c",slug:"ionic-liquids-applications-and-perspectives",bookSignature:"Alexander Kokorin",coverURL:"https://cdn.intechopen.com/books/images_new/1373.jpg",editedByType:"Edited by",editors:[{id:"19816",title:"Prof.",name:"Alexander",surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"57",title:"Physics and Applications of Graphene",subtitle:"Experiments",isOpenForSubmission:!1,hash:"0e6622a71cf4f02f45bfdd5691e1189a",slug:"physics-and-applications-of-graphene-experiments",bookSignature:"Sergey Mikhailov",coverURL:"https://cdn.intechopen.com/books/images_new/57.jpg",editedByType:"Edited by",editors:[{id:"16042",title:"Dr.",name:"Sergey",surname:"Mikhailov",slug:"sergey-mikhailov",fullName:"Sergey Mikhailov"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"371",title:"Abiotic Stress in Plants",subtitle:"Mechanisms and Adaptations",isOpenForSubmission:!1,hash:"588466f487e307619849d72389178a74",slug:"abiotic-stress-in-plants-mechanisms-and-adaptations",bookSignature:"Arun Shanker and B. Venkateswarlu",coverURL:"https://cdn.intechopen.com/books/images_new/371.jpg",editedByType:"Edited by",editors:[{id:"58592",title:"Dr.",name:"Arun",surname:"Shanker",slug:"arun-shanker",fullName:"Arun Shanker"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"878",title:"Phytochemicals",subtitle:"A Global Perspective of Their Role in Nutrition and Health",isOpenForSubmission:!1,hash:"ec77671f63975ef2d16192897deb6835",slug:"phytochemicals-a-global-perspective-of-their-role-in-nutrition-and-health",bookSignature:"Venketeshwer Rao",coverURL:"https://cdn.intechopen.com/books/images_new/878.jpg",editedByType:"Edited by",editors:[{id:"82663",title:"Dr.",name:"Venketeshwer",surname:"Rao",slug:"venketeshwer-rao",fullName:"Venketeshwer Rao"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"4816",title:"Face Recognition",subtitle:null,isOpenForSubmission:!1,hash:"146063b5359146b7718ea86bad47c8eb",slug:"face_recognition",bookSignature:"Kresimir Delac and Mislav Grgic",coverURL:"https://cdn.intechopen.com/books/images_new/4816.jpg",editedByType:"Edited by",editors:[{id:"528",title:"Dr.",name:"Kresimir",surname:"Delac",slug:"kresimir-delac",fullName:"Kresimir Delac"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"18176",title:"Microscopic Features of Biologically Formed Amorphous Silica",doi:"10.5772/24951",slug:"microscopic-features-of-biologically-formed-amorphous-silica",body:'Many animals base their skeleton on calcium containing compounds and in particular calcium phosphates are prevalent as the main bone constituents. There are, however, animals that rely on different compounds for their skeleton. An example of such a class of animals is sponges, which have a skeleton constituted of silica (Müller et al, 2007). The fact that silica is used as skeleton provides the sponges with the ability to live in calcium-poor and acidic environments. Despite the characteristic composition of the skeleton, sponges have managed to spread to various living environments such as seawater (Bavestrello et al., 1995; Croce et al., 2003; W. E. G. Müller et al., 2005; Pisera, 2003; Schwab & Shore, 1971; Uriz et al., 2000) and lakes (Kaluzhnaya et al., 2005; Schröder et al., 2003). Furthermore, the prevalence of sponge across the entire planet witnesses that they can grow at different temperatures. Due to the unique features of sponges, they have been intensively studied by scientists through the last decades. Many of the studies focused on the evolutionary and biological aspect of sponges (Barthel 1986; Barthel, 1995; Calcinai et al., 2007). After clarification of the structure of the sponges, it is known that they possess unique features. As a consequence of the unique features of sponges, these creatures have attracted interest from material scientists (Wang et al., 2010) since study of sponges could lead to new materials or facilitate the production of already existing materials through biomimetic approaches.
The composition and structure considerably vary from one type of sponge to another, and therefore it is not possible to address all kinds of sponges in one chapter. In this chapter, we focus on the sponge called Cauxi (Porifera, Demospongiae). From its habitat in the lower Rio Negro in the eastern Amazon basin, the natives have known and used the sponge for reinforcement of their pottery through centuries (Costa et al., 2004). Besides the fact that the sponge is known to be amorphous to X-ray diffraction (XRD), the mesoporous feature of the sponge remained unrevealed until recently (Jensen et al., 2009). In this work, we present a detailed description of the fascinating features of the Cauxi sponge and highlight differences between Cauxi and other species. This work will also shed light on the potential of biological formation of amorphous mesoporous materials at ambient conditions.
The freshwater sponge Cauxi was collected at the river bank of the Rio Negro at Praia Grande in the Amazon basin 60 km west of Manaus, Brazil (3°03’22.5S and 60°30’33.56W). After extraction, the sponge was stored in a plastic bag under dry conditions at room temperature. The sponge belongs to the group of Demospongiae, Porifera.
To separate the inorganic part from the organic part of the sponge, the organic part of the sponge was removed by means of bleaching. A selected amount of the sponge was added to a Teflon container and mixed with 35 % H2O2, 69 % HNO3, and water in the volume ratio 8:7:1. The Teflon container was placed in a 90 °C water bath for 20 min as the heat accelerates the breakdown of organic material. The solvents were then removed and fresh solvents added. The container was replaced in the water bath for 20 min. The procedure was repeated until a white material without any colorants appeared in the Teflon container. The solvents were then removed and the material rinsed once with water. The material was heated at 160 °C for 2h in order to evaporate any remaining bleaching agent without changing the properties of the bleached material.
Calorimetric and gravimetric responses of the sponge to temperature were measured using a simultaneous thermal analyzer (NETZSCH STA 449C Jupiter (Selb, Germany)), which provides functions of both differential scanning calorimeter (DSC) and a thermogravimeter. For the measurements, a platinum crucible containing the sample and an empty reference platinum crucible were placed on the sample carrier at room temperature. Initially both crucibles were held 5 minutes at an initial temperature of 333 K. Thereafter an upscan to 1543 K and a subsequent downscan were performed at 10 K/min. The samples underwent a second up- and downscan with 10 K/min in order to achieve uniform thermal histories (Yue, 2008). The purge gas was air with a flow of 40 mL/min. Before measuring each sample, a baseline was measured by using two empty crucibles according to the above-stated heating procedure, which was used for correcting the DSC signal of the samples. The isobaric heat capacity (Cp) of the samples was determined by comparing the heat flow data of the samples with those of a reference sample (sapphire) measured using the above-stated heating procedure, since the temperature dependence of the Cp values of the sapphire is known.
To remove the organic material from the inorganic part by another approach than bleaching, a part of the sponge was put in a porcelain crucible and heat-treated in a muffle furnace (Scandiaovnen A/S, Denmark) in air at 823 K for 7 h. This temperature was selected in such manner that all organic substances could be completely removed.
Another sample was prepared by heating untreated sponge in an alumina crucible in air to 1723 K for 17h in an Entech SF6/17 electric furnace (Ängelholm, Sweden) in order for the material to crystallise.
SEM measurements were done on a Zeiss 1540 XB scanning electron microscope (Oberkochen, Germany) using uncoated samples. The secondary electrons were recorded using an acceleration voltage of 1 kV. Images of untreated, bleached, and heat treated samples were acquired.
Since it was not possible to obtain any images with sufficient contrast on the untreated sponge, heat treated samples were used for transmission electron microscope (TEM) imaging. After annealing the untreated sponge in an electric furnace at 1723 K for 17 h it was finely ground using an agate mortar. The obtained powder was dispersed in isopropanol and after thorough dispersing, a droplet of the dispersion was allowed to dry on a carbon-coated copper grid. The ground, heat-treated sponge fragments kept on the grid were imaged in a high-resolution TEM (JEOL JEM 4010, acceleration voltage 400 keV, point-to-point resolution: 0.155 nm).
The fibres were investigated on a Zeiss Axioskop microscope. The length of the sponge spicules was measured employing a 10x objective. The width was measured as the broadest point of the spicule with a 100x objective in oil immersion. Both width and length measurements were done on 50 spicules.
Bleached sponge powder of about 2 grams was mixed with 12 g Li2B4O7 per gram powder. The mixture was melted at 1573 K and made into a tablet. The tablet was placed in a sample holder, and after calibration, the measurement was initiated. The measurement was conducted on a S4-Pioneer X-ray spectrometer (Bruker-AXS, Karlsruhe, Germany).
The water content of the bleached fibres was measured by mass spectrometry coupled with vacuum hot extraction. Prior to the measurement, the sample was kept at 473 K to remove adsorbed water. The sample was cooled to room temperature in the device. Subsequently, the degassing rate was recorded upon heating of the sample with a rate of 10 or 20 K/min to 1773 K.
The small angle X-ray scattering (SAXS) measurements were done on a pinhole instrument. The instrument was a modified version of a commercially available small-angle X-ray camera (NanoStar, Bruker AXS). It employs a rotating anode X-ray source (CuKα) (0.1 x 1.0 mm2 source size with a power of 1 kW) and a set of cross-coupled Göbel multilayer mirrors for monochromatising the radiation and for converting the divergent beam from the source to an essentially parallel beam. To reduce the background, the instrument has an integrated vacuum. The instrument covers a range of scattering vectors q between 0.010 and 0.35 Å-1 (q = (4π/λ) sin θ, where 2θ is the scattering angle and λ = 1.542 Å, the X-ray wavelength). All measurements were made at room temperature. The SAXS data were azimuthally averaged, corrected for variations in detector efficiency and for spatial distortions. After the background subtraction, the data is converted to an approximate absolute scale using the scattering from pure water as a primary standard.
For the experiments, a bunch of the thin spicules were collected using a Scotch® tape in such a way that they were randomly oriented. The scattering data of a clean tape was used as background for the data treatment.
The sample heat-treated at 1723 K was measured with a Seifert FPM HZG4 diffractometer (Freiburg, Germany) with Fe Kα radiation. The scan was conducted in the range 5° < 2θ < 65°.
The sponge itself has a hard, but somewhat brittle and spherical structure with a brownish colour (Fig. 1). The brown colour arises from the organic part of the organic and from humic acids from the river that have precipitated on the skeleton of the sponge (Keding et al., 2010). The initial key findings of the structure of the sponge are reported in Jensen et al., (2009).
The Cauxi sponge. In the lower right corner, the branch that the sponge was attached to appears.
SEM images of the untreated sponge.
To examine the macroscopic structure of the sponge, scanning electron microscopy (SEM) images are acquired. The skeleton consists of needle shaped spicules that are bundled together (Fig. 2). There are branches on the bundles of spicules which give the sponge its porous structures. Due to the porous structure, the density of the sponge is only about 15 g/L.
When examining each individual spicule, it is seen that spicules are cemented through junctions (Fig. 3a) and that an axial cavity in each spicule exists (Fig 3b).
SEM images of untreated sponge showing junction between two spicules (A) and a cross section of a spicule (B).
The presence of an axial channel in the spicules is a common feature for sponges (Aizenberg et al., 2005; Cha et al., 1999; W. E. G. Müller et al., 2005; Schwab & Shore, 1971, Uriz et al., 2000). The axial channel has been found to harbour an axial filament containing the catalytic protein silicatein (Cha et al., 1999; Shimizu et al., 1998). Furthermore, the cross section of the spicules in Fig. 4B shows a solid structure around the channel, whereas other sponges have a layered structure (Aizenberg et al., 2004; Levi et al., 1989; Shore, 1972; Uriz et al., 2000).
To investigate the chemical nature of the junctions, spicules subjected to bleaching and heat-treatment at 823 K are considered. After the two treatments, the sponge loses its macroscopic structure and appears as a powder. The loss of the macroscopic structure is confirmed by SEM, since only isolated spicules are found (Fig. 4).
SEM image of spicules subjected to bleaching.
From Fig. 4 it can be observed that the junctions that cement the spicules have been removed. As a consequence of the removal of the junctions, the macroscopic structure of the sponge collapses and isolated fibres are left. The length and width of the spicules have by optical microscopy been measured to 305 and 15.6 µm, respectively. Since the macroscopic structure collapses after both bleaching and heat treatment at 823 K, it can be concluded that the junction consists of organic material. This is in contrast to spicules in other sponges that are cemented by a silica deposition (Aizenberg et al., 2005). Due to the brittle nature of the spicules, sponges that apply silica cemented spicules, obtain mechanical stability by a hierarchical arrangement (Aizenberg et al., 2005). As Cauxi lives in a river with a strong current, Cauxi requires a larger flexibility than the sea sponges. In the sponges that cement their spicules by silica depositions, a layered coaxial structure of the spicule is found. Between each of the layer, organic material is present (Aizenberg et al., 2005). Resistance to bending stresses in this inorganic skeleton is obtained since a stress that leads to fracture only breaks a single layer of the spicules. Organic junctions provide a better flexibility than the silica depositions and therefore, the spicules in Cauxi can be solid without deteriorating the mechanical properties of the skeleton. Thus by combining the strength of an inorganic skeleton and the flexibility of an organic binder to cement the junctions, Cauxi obtains a remarkable macroscopic and microscopic strength. In other words, the brittleness of each individual spicule is compensated by the organic binder that cements the spicules into the sponge skeleton. The importance of combining microscopic and macroscopic features to obtain a strong and stable structure in sponges is described for another sponge elsewhere (Aizenberg et al., 2005). Studies of the structure of sponges can provide inspiration for the development of stronger man-made materials.
The composition of the bleached sponge has been examined using the vacuum hot extraction (VHE) method up to 1780 K for volatile components such as water and by X-ray fluorescence (XRF) for the remaining inorganic components. During the hot extraction experiments, volatile species liberated from the sample upon heating are detected by means of a mass spectrometer. Prior to the VHE experiments, the sample is kept at 473 K to remove water absorbed to the surface of the spicules since the surface of silica is known to be hygroscopic. The release of water vapour from bleached spicules at two different heating rates is shown in Fig. 5.
The water release occurs at similar temperatures at both 10 and 20 K/min. with a maximum release rate around 900 K. The spectrometer measures roughly an ion current that at 20 K/min is twice of that measured at 10 K/min. The increased ion current is caused by the higher heating rate, i.e., the experiment at 20 K/min only has half of the time to release the water compared to a heating rate of 10 K/min and consequently, the water release is twice as intense. Since the water is released at the same temperatures at different heating rates, the water release is thermodynamically and not kinetically controlled. Through integration of the degassing curve in Fig. 5 and the measurement of a standard, it is possible to determine the water content (R. Müller et al., 2005). From this approach a water content of 5.5 wt % is determined.
The XRF measurement shows that the chemical composition of the bleached spicules is dominated by SiO2 (Table 1).In addition to the 99.7 wt% SiO2, the spicules contain impurities. The impurities are mainly CaO and Al2O3 and could arise from sample handling and preparation. After the clarification that the spicules mainly consist of SiO2, it is investigated
Ion current as a function of temperature measured at 10 and 20 K/min on bleached spicules. The plotted ion current from the spectrometer is from mass to charge ratio (m/z) 18, i.e., water. The curves have a shoulder at Tg.
SiO2 | CaO | SO3 | Al2O3 | Fe2O3 | TiO2 | K2O | P2O5 | |
Content (wt%) | 99.7 | 0.070 | 0.041 | 0.07 | 0.04 | 0.034 | 0.010 | 0.005 |
Chemical composition of the bleached spicules measured by means of XRF.
whether the 5.5 wt% water determined by means of VHE, reside in the glass structure or are present in pores. To do so the glass transition temperature (Tg) is investigated since Tg is affected by water in the glass structure but not water residing in pores isolated from the glassy phase.
The glass transition temperature (Tg) measured by means of DSC is plotted together with the thermogravimetry signal in Fig. 6. There is an endothermic peak at around 450 K, which is attributed to initial vaporization of superficial water. This is verified by a mass loss in the same temperature region (see the grey curve in Fig. 6). In addition, a broad weak endothermic response is observed at around 1400 K which is a typical feature of the glass transition of the glassy silica. The DSC pattern of untreated sponge is similar to that of the bleached one except for an exothermic peak between 500 and 900 K due to combustion of organic material. The inset in Fig. 6 shows the endothermic event at around 1400 K, where an increase in the heat capacity is clearly seen. This endotherm must be associated with the glass transition of silica (Richet et al., 1982). This implies that the Cauxi skeleton is amorphous, which is confirmed by XRD reported elsewhere (Costa et al., 2004). By applying a previously suggested method (Yue, 2008), the Tg of the spicules of the bleached sponge is found to be 1414 K. Tg of silica is very sensitive to hydroxyl groups incorporated in the glass network structure, especially at low concentrations of hydroxyl groups (Deubeneret al., 2003).
DSC and TG upscan in air on the bleached sponge. The DSC upscan reveals an initial endothermic peak until around 450 K where the TG curve demonstrates a mass loss. Another endothermic peak occurs in the DSC patterns around 1400 K. Inset: DSC upscan in the range 1300 – 1500 K. The dashed lines in the inset are used to determine Tg.
To determine the water content in the network structure of the Cauxi spicules, it is necessary to know the relationship between Tg and the hydroxyl content in the glass structure. To establish this relationship, the Tg of commercial silica glasses with 1, 150, and 1000 ppm water has been measured and found to be 1438, 1425 and 1375 K, respectively. From the Tg ~ hydroxyl content relationship, it can be interpolated that the spicules of Cauxi contain approximately 400 ppm hydroxyl species, i.e., structural water. The large discrepancy between the 400 ppm hydroxyl species determined by means of DSC and the 5.5 wt % determined by VHE, implies that the majority of the water must reside in pores isolated from the glassy network. The exclusion of water from the glass structure is in contrast to other Demospongiae sponges (Sanford, 2003). Since the SEM images in Fig. 2-4 do not reveal the presence of such pores, the water must reside in nanopores or mesopores.
To reveal the presence of possible mesopores, transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) are employed. Due to pronounced decomposition, TEM images of untreated spicules could not be obtained. Although the bleached spicules underwent massive radiation damage upon TEM inspection, the amorphous nature of the spicules was doubtlessly disclosed. Therefore focus is turned to the sample heat-treated at 1723 K.
The TEM image in Fig. 7 reveal mesoscale crystalline phases (dark areas) embedded in the amorphous phase. The crystalline phases possess a width of ~ 15 nm and a length of up to 200 nm and seem to be randomly orientated in the amorphous phase. To study the crystalline phase a high resolution TEM (HRTEM) image is recorded (Fig. 8).
The HRTEM image in Fig. 8, shows that the crystalline phase indeed is embedded in the amorphous phase. Hence, upon heat treatment at 1723 K, initially causes a removal from some type of mesopores (Fig. 5 and Fig. 6 TG curve) where after a crystallisation of the
Bright-field transmission electron micrograph of a spicule after heat-treatment at 1723 K for 17 hours.
HRTEM image showing the single channel (scale bar, 20 nm). Inset: Close-up high-resolution transmission electron micrograph (scale bar, 5 nm).
surface layer of the mesopores occurs. The presence of mesopores explains that the water is released from the spicules at very high temperatures (Fig. 5). Thus, we can infer that the Cauxi skeleton indeed contains mesopores. Due to surface nucleation effects, the crystallisation occurs more easily at the wall of mesopores than at other places in the bulk part of the spicules. Therefore, only the mesoporous channels and not the bulk of the spicules crystallise. During heat treatment at 1723 K, not only the mesopores, but also the surface of the spicules crystallises (Fig. 9a).
SEM images of the surface of spicules heat treated at 1723 K (A) and Fe Kα XRD pattern of spicules heat treated at 1723 K (B).
On the surface of the spicules, round crystalline structures are found. A comparison with the TEM images in Figs. 7 and 8 shows that only the surface of the fibres and the mesoporous channels crystallise during the heat treatment. X-ray diffraction (XRD) measurements of the heat-treated spicules reveal the presence of cristobalite (Fig. 9B). The presence of cristobalite is expected as it is the stable crystalline form of SiO2 at high temperatures.
To obtain a fit for the SAXS measurements (Eq. 1) of the untreated spicules, both cylindrical and polymer structures are used (Pedersen, 2000; Pedersen & Gerstenberg, 1996).
Where Sc1 and Sc2 are scale factors for the cylinder and polymer scattering contribution, respectively, σ is the polydispersity of the cylinders, Pcs and PROD are form factors for cross section and length, respectively, and Rg is the radius of gyration for the polymer structure. The Schultz-Zimm distribution is used to calculate the polydispersity. The SAXS measurements and the data fit are given in Fig. 10.
Small angle X-ray scattering pattern of untreated spicules and the model fit based on Eq. 1.
From the fitting, cylindrical channels with a length and width of 110 and 23 nm, respectively are found. These dimensions correspond well the size of the channels observed on the TEM images in Figs. 7 and 8. Thus the channel like structures detected by SAXS are the mesoporous channels. In addition to the channels, polymer structures with a radius of gyration of 3.45 nm are determined. This polymer structure is expected to be the protein catalysing the polymerisation of silica, i.e, Silicatein. Silicatein has been reported to be similar to Cathepsin S (Shimizu et al., 1998), which has a diameter of 3.0 nm. Since Silicatein contains 50 % more amino acids than Cathepsin, it is likely that Silicatein or an analogue catalytic protein has a radius of gyration of 3.45 nm. Since Cauxi excludes water from its siliceous network, Cauxi might apply a catalytic protein analogue to Silicatein since sponges with this catalytic protein incorporates water into the silica network.
The growth mechanism of Cauxi (Jensen et al., 2009) and a review of the growth mechanisms of sponges in general have been described elsewhere (Wang et al., 2010). Here we focus on discussion of the chemical aspects of the formation of pure, mesoporous, amorphous silica and its applications. In order to polymerise silica for its skeleton, Cauxi must live in water, which is supersaturated with silica. The supersaturation can arise from pH changes since the lowest solubility of silica in water is around pH 7-8 (Iler, 1979). Hence, if the silica is dissolved in the water (as silicic acids) at a pH with high solubility and a subsequent pH change causes lower silica solubility, the water becomes supersaturated in silica. Despite the supersaturation, silica does not polymerise spontaneously due to an activation energy of 76.6 kJ/mol (Hurd & Marotta, 1940). The catalytical proteins of sponges overcome this activation energy and enable the polymerisation of silica for their skeletons. Through sol-gel synthesis, it is already possible to polymerise silica in water and at ambient temperatures. However, these sol-gel silica glasses contain water in the per cent level in the silica network, which is similar to that of most sponges. Thus, a biomimetic approach of silica must lead to a silica glass with low content of hydroxyl groups in the silica structure. Since Cauxi unlike other sponges has been shown to exclude water from the siliceous network of its spicules, studies on Cauxi are from this perspective more relevant than studies on sponges with a large content of hydroxyl groups in their silica network. From other sponges, the catalytic proteins have been isolated and expressed in E. coli, which is capable of polymerising silica In vitro (Cha et al., 1999). A similar approach on Cauxi could lead to the production of mesoporous amorphous silica without the incorporation of water into the glass structure since water incorporation weakens the strength and thermal properties of the silica glass.
The technological and scientific importance of glassy silica is demonstrated by its wide applications such as membranes (de Vos & Verweij, 1998), columns (Dai et al., 2003), heat-proof materials (Saito et al., 2000), optical communication fibres (Tong et al., 2003) and catalysts in organic synthesis (Minakata et al., 2004). At present, silica of high purity is often produced by means of fusing (Brückner, 1970; Tohmon et al., 1989) or argon plasma methods (Tohmon et al., 1989). Due to the costly and advanced methods, a biomimetic approach based on the catalytic proteins in Cauxi has the potential to revolutionise the production of high purity silica.
Cauxi consists of amorphous silica spicules that are cemented by an organic binder. A detailed study of the spicules reveals that the spicules contain 400 ppm structural water (hydroxyl groups) in the silica network. In mesopores within the spicules, 5.5 wt % of water is found, i.e., unlike other sponges Cauxi excludes water from its silica network that build up its skeleton. Isolation of the proteins catalysing the silica polymerisation in Cauxi followed by expression in e.g. E. Coli seems to be a feasible route for biomimetic production of high-purity amorphous silica. If In vitro production of the mesoporous amorphous silica by host organisms such as E. Coli is feasible, the next step is to explore whether the pore size or the pore volume fraction in the spicules can be tailored by varying the synthesis environment or the amino acid sequence of the catalytic proteins. Complete control of the pore structure provides an avenue for the production of highly selective membrane materials.
The reasons for the interest in actively controlled journal bearings (alternatively called sleeve bearings or plain bearings for radial load) lie in demand for the introduction of high-speed cutting or machining as a technology for the future. Increasing the machining speed was required to get beyond the limits of the interval, where unwanted temperature increases. Some researchers define high-speed machining as machining whereby conventional cutting speeds are exceeded by a factor of 5–10. Increased machining speed has advantages. The ability to benefit the advantages of high-speed cutting in steel, cast iron, and nickel-based alloys can be obtained with spindle speeds in the range of 8k to 12k rpm. High-speed cutting of nonferrous materials such as brass, aluminum, and engineered plastics demands a significantly higher rpm capability. For these materials, we must focus on milling equipment capable of operating at high-speed spindle speeds of 25k to 50k rpm or more. High-speed machining can also include grinding and turning.
\nLet us now notice the machine tool spindles. Roller bearings support these spindles. Prestressed ball or tapered roller bearings are used to eliminate play. This chapter focuses on plain radial bearings, namely, hydrodynamic bearings. Plain bearings of this type require a clearance for their function, which is selected in the range of 0.1–0.3% of the journal diameter.
\nJournal hydrodynamic bearings are a standard solution to support rotors. Their advantage is a possibility to carry the high radial load and to operate at high rotational speeds. The disadvantage of the journal bearings is the excitation of unwanted rotor vibrations by whirling of the journal in the bearing bushing. The bearing journal becomes unstable as the journal axis begins to perform a circular motion that is bounded only by the walls of the bearing bushing. When the speed threshold is exceeded, the axis of the bearing journal starts to circulate, causing the rotor to vibrate. These vibrations are called whirl. A passive way of how to suppress vibrations consists in adjusting the shape of the bearing bushing, such as lemon or elliptical bore of the bushing, or the use of tilting pads. Even though there are several solutions based on mentioned passive improvements, this article deals with the use of active vibration control (AVC) with piezo-actuators as a measure to prevent instability.
\nThe disadvantage of bearings of this type is whirl instability, which can cause machine tool vibrations. The following chapter describes the possible operating range of the spindle speed.
\nSpecial oil for high-speed spindle bearing of the OL-P03 type was used for testing (VG 10 grade, viscosity \n
Side-view technical drawing and a photo of bearing housing.
The operating conditions of the hydrodynamic bearing are described by the Sommerfeld number [1]:
\nwhere \n
The value of the Sommerfeld number for the given bearing size and the rotor mass of 0.83 kg is as follows: \n
The magnitude of friction coefficient in the plain bearings was analysed in the past by the McKee brothers [2]. It has been found that bearing friction is dependent on a dimensionless bearing characteristic given by a ratio \n
An example of a gradual change of position of the bearing journal centre during an increase in speed up to 7k rpm at the constant increase rate is shown in Figure 2. The lubrication is of the boundary type in the range up to about 1.2k rpm and is accompanied by oscillations.
\nThe run-up of a journal bearing.
The reason for the oscillations is the step change of speed to about 300 rpm after switching on because it is not possible to increase the rotational speed continuously from zero. Hydrodynamic lubrication at stable motion is produced for rotational speed up to 5k rpm. Motion instability of the whirl type occurs when this speed of 5k rpm is exceeded. Fluid force makes sense to be modeled just for stable motion and hydrodynamic lubrication. It is almost impossible to determine the initial conditions for boundary lubrication. Notice that the centre of the journal rises to the level of the centre of the bearing bore and gradually approaches this centre so that the small eccentricity gradually decreases to zero as is shown on the right panel of Figure 2. The data for this orbit was approximated by the five-degree polynomial in the time interval which begins at the 3rd second and ends at the 12th second. The difference between thin and thick film lubrication is also evident on the right panel of Figure 2, which depicts an orbit plot for the entire measurement time up to sixteenth-second.
\nThe threshold of the instability of the journal movement in the bearing is given by the clearance and viscosity of the oil, which depends on the temperature.
\nFor developing a new design of the actively controlled bearing, a test rig was built; see Figure 3. This figure provides different views of the test rig. An inductive motor of 400 Hz drives the rotor, and therefore the maximum rotational speed is 23k rpm. The engine is connected to the rotor via the Huco diaphragm coupling. The bearing diameter is 30 mm, and the length-to-diameter ratio is equal to about 0.77. The span of bearing pedestals is of 200 mm. The results of the experiments presented in this article are for the radial clearance of 45 μm. Also, the journals of the other clearance are available for testing. The performance of the actively controlled bearing was tested on the test bench (Rotorkit) of the TECHLAB design [3, 4]. Additionally, it should be emphasised that research was focused at rigid rotors and the journal bushing of the cylindrical bore, where the journal motion is measured at the location closest to the bearing bushing. The research work resulted in putting into operation of the active vibration control system, which became the first functional bearing prototype known up to now [5].
\nActively controlled journal bearings.
The mechanical arrangement of the actively controlled bearing is shown on the right of Figure 1. Oil leakage from the volume between the bearing body and the loose bushing and the piezoactuator rod is sealed with rubber O-rings. As it was stated before, vibrations of the rotor is suppressed using the system for active vibration control with piezoelectric actuators enabling to move the non-rotating loose bushing. The motion of the bearing bushing is controlled by the controller, which responds to the change in position of the bearing journal related to the bearing housing. Two stacked linear piezoactuators are used to actuate the position of the bearing journal via the position of the bearing bushing. The bearing uses a cylindrical bushing which did not require unique technology of production and assembly. This new bearing enables not only to damp vibrations and to prevent instability but also enables to maintain the desired bearing journal position with an accuracy of micrometers.
\nThe bearing journal can be considered as a rigid body rotating within the bearing housing at an angular velocity \n
A cross-section of the hydrodynamic bearing.
where \n
In addition to force components in the horizontal and vertical directions, the force balance will be solved in other possible directions. Force in the direction of the line of the centers is denoted as a direct force \n
The system is described by two motion equations, and therefore the total order of the system is four. This system may become unstable even for positive parameter values such as stiffness and damping.
\nThe motion equation of the rotor with the journal bearing in coordinates \n
The derivation of the motion equation is described, for example, in the article [5].
\nThe theory of hydrodynamic bearing is based on a differential equation derived by Osborne Reynolds. Reynolds equation is based on the following assumptions: The lubricant obeys Newton’s law of viscosity and is incompressible. The inertia forces of the oil film are negligible. The viscosity \n
Furthermore, it is assumed that the thickness \n
There is no analytical solution for the Reynolds equation.
\nDuring operation, the journal axis shifts from the centre of the bearing bushing to the distance of \n
The oil film moves in adjacent parallel layers at different speeds, and shear stress results between them. The oil layer at the surface of the journal moves at the peripheral velocity of the journal, while the oil layers at the surface of the bearing bushing do not move (at zero velocity). The surface of the journal moves at a velocity of \n
On double integrating, see [7], we get
\nwhere \n
The solution must meet the boundary condition:
\nwhich gives
\nOn simplifying, we get a formula for calculating the first integration constant \n
Extreme oil pressure values as a function of the attitude angle \n
The first integration constant is related to the thickness of the oil film at the perimeter of the journal, where the maximum and minimum oil pressure is achieved:
\nThe attitude angle where the maximum and minimum pressure occur is given by
\nThe result of double integration is as follows:
\nThe first integration constant was selected to meet the boundary condition \n
Pressure distribution along the angular coordinate.
The forces acting on the journal in the centre of gravity along the bearing length of \n
where the force \n
Note that according to Eq. (14) the pressure on the part of the journal surface is negative, which is, in fact, a relative negative pressure. Since the pressure distribution is antisymmetric with respect to \n
Dependence of the direct and quadrature force on the eccentricity ratio.
The nominal force that multiplies the dimensionless functions \n
where \n
For speeds ranging from 1.25k to 5k rpm, the force factor \n
The presence of direct force can be explained, e.g., by the cavitation or the inability to achieve high vacuum, but the mathematical model is more complicated (Ferfecki) [8]. The lubricant flows through the bearing, but in the part of the bearing journal circumference where the pressure is below the barometric pressure, the lubricant can also be sucked. The magnitude of the negative pressure for \n
The effect of negative pressure reduction is demonstrated in the right panel of Figure 6. Negative pressure is limited to 1% of the magnitude of positive pressure for the angle interval of \n
where \n
The stiffness in the directions of the Cartesian coordinates \n
The vector of the direct and quadrature forces depends on the coordinates \n
The cross-coupled stiffness \n
The damping matrix can be derived based on its relationship to the stiffness matrix according to the model that was designed by Muszynska:
\nAs is \n
The sum of direct and quadrature forces must compensate for the gravitational force that does not depend on the speed of rotation. The suitability of this model is confirmed by Mendes [9].
\nIf the attitude angle approaches the angle of \n
The coordinate system in the complex plane for the bearing journal position is shown in Figure 7. A variable u is a control variable, and a variable r is a controlled variable. The controlled variable is a two-component coordinate of the bearing journal axis, while the control variable is a two-component coordinates of the bushing axis as is shown in Figure 4. Because both the variables indicate coordinates in the plane, then they can be considered as two-component vectors. The same meaning as the vector has a complex variables. The real part of this variable has the meaning of the x coordinate, while the imaginary part has the meaning of the y coordinate, therefore it is possible to denote it as \n
Coordinate system in the complex plane.
There are many ways how to model journal bearings, but this paper prefers a lumped parameter model, which is based on the concept developed by Muszynska [10]. This concept assumes that the oil film acts as a combination of the spring and damper, which rotates at an angular speed \n
where M is the total rotor mass, Ω is the rotor angular velocity, K and D are specifying proportionality of stiffness and damping to the relative position of the journal center displacement vector, λ is a dimensionless parameter which is slightly less than 0.5, and FP is an oscillating disturbing force defined by \n
Force action of the oil film on the bearing journal can be modeled by Reynolds partial differential equations. The good accuracy of the Muszynska approximate model confirms Mendes and Cavalca [9]. Eq. (2) can be rewritten in matrix form
\nThe entries of the stiffness and damping matrices according to Muszynska model and the calculation of these matrix entries using Reynolds equation agree except for very low rotor speed. Some entries are constants, and others are linear function of the rotational speed. Even entries of the damping matrix are similar. Coordinates of the bearing journal axis for the force of gravity \n
where \n
Movement of the bearing journal inside the bearing bushing may be unstable as is apparent from Eq. (26). This phenomenon is called a whirl. The threshold of stability in angular velocity of the rotor can be calculated by the Muszynska’s formula:
\nThe equation of motion in the complex form for the rigid rotor operating in a small, localised region in the journal bearing with the movable bushing (\n
The Laplace transform specifies the transfer functions relating the displacement of the bushing to the displacement of the shaft \n
where the mentioned transfer functions are as follows:
\nThe active vibration control of journal bearings uses the bushing position as the control variable \n
Closed control loop.
Substituting \n
The transfer functions relating the set point of the closed-loop system to the displacement of the shaft \n
The stability margin can be calculated under assumption that the open-loop frequency transfer function \n
If the proportional controller is disconnected, i.e. KP\n = 0, then the critical angular velocity Ω\nMAX\n coincides with the critical frequency Ω\nCRIT\n of the closed loop. Increasing of the stability margin for the rotational speed of the rotor is possible by introducing an additional feedback.
\nThe reciprocal value of the transfer function (Eq. (35)) has the meaning of dynamic stiffness for radial force acting at the rotor:
\nIf a static force is applied to a rotating journal, then the stiffness of the journal bearing is given by
\nwhich means that the stiffness is \n
Analysis of the effect of active vibration control on the stiffness of the bearing journal assumes a linear mathematical model. Practical calculation of matrix entries of stiffness \n
shows that the linear model does not differ substantially. The dependence of the matrix entries for the journal bearing of the test rig on the rotor rpm is given by the graphs in Figure 9. Pertinent stiffness and damping coefficients are obtained by solving Reynolds equation providing that the journal performs small harmonic motion in neighborhood of its equilibrium position.
\nReal stiffness and damping matrices according to the Reynolds model.
The range of the manipulated variable, which is the position of bearing bushing and at the same time, the controller gain, determines the way to install piezoactuators. The equivalent circuit of the mechanical branch of the control loop for the horizontal direction is shown in Figure 10. Parameters that indicate stiffness in the scheme in Figure 10 are associated to the individual elements of the control loop as follows: \n
Mechanical branch of the control loop.
The force produced by the piezoactuator balances the force effect of the oil film. Virtual motion of the unloaded piezoactuator is proportional to its control supply voltage \n
Piezoactuator operation graphs.
Operation graph with all the limitations for the piezoactuator of the P-844.60 type is shown in the left panel of Figure 11. If the stiffness of sealing rings is taken into account and the stiffness of the support is assumed to be infinite, then the original range of motion of the bearing bushing is reduced to the size as follows:
\nThe effect of the stiffness of the O-ring seal is shown in the middle panel of Figure 11. Displacement of the bearing bushing is reduced from 90 to 77 μm for the given parameters of the control loop. The ultimate stiffness of the support affects the virtual stiffness of the piezoactuator as follows:
\nwhere \n
First experiments with an imperfect support showed displacement of the bearing bushing of about 20 microns at maximum electrical voltage to supply the piezoactuators. This happened at the beginning of the development when the support arrangement was provisionally extended due to the use of longer piezoactuators. The ideal solution is to install piezoactuators into the bearing housing.
\nExperiments with the active vibration control run for several years, while the hardware and software of the control system was upgraded step by step. We have improved design of the piezoactuator support, found suitable sensors for measuring the position of the bearing journal, upgraded the lubrication system, and improved the control algorithm. Properties of the active vibration system have previously been described in the paper [11], and now the main results will be described only, which relate to control the position of the bearing journal.
\nThe instability onset of the bearing journal motion inside the bushing arises when crossing the threshold value of rotational speed Eq. (5). This phenomenon means that the steady-state rotation of the journal is not stable and the journal axis starts to whirl at the frequency, which is 0.42–0.48 multiple of the frequency of rotational speed of the rotor. Measurements in this article were carried out on the shaft with the radial clearance of 45 μm. Rotor speed increases according to a ramp function as it is shown in the left panel of Figure 12. The time history of the axis coordinates of the bearing journal is shown in the other panels of Figure 12. The x coordinate corresponds to horizontal direction, and the y coordinate is for vertical direction. Active vibration control is switched off for the time histories in the second panel. Instability occurs at about 2k rpm. This threshold of instability depends on the viscosity of the oil and the bearing clearance. Oscillations of the bearing journal position are limited by the journal clearance within the bushing.
\nrpm and the journal position as a function of time.
If the active vibration control is switched on and rates of increase of rotational speed are identical for both measurements, the instability of the bearing occurs at the rotational speed about 12k rpm as is shown in the third and fourth panel of Figure 12 from the left. Vibrations during the instable motion of the journal are also limited by the journal clearance within the inner gap of the bearing bushing.
\nThe transient of the journal seems to be reverse for the vertical motion (y-axis) of the bearing journal in the second and third panel of Figure 12. The scale for the vertical motion is reverse in these figures, meaning upside-down. The relationship between horizontal and vertical motion of the journal shows the orbit of the journal axis in the rightmost fourth panel of Figure 12. The shape of the orbit is approximately circular when instability occurs.
\nThreshold of instability is increased six times now using a proportional feedback. According to Eq. (12) this multiple corresponds to the open-loop gain, which is equal to 35. Years ago, we achieved an increase in the threshold of instability only about by 70% for a piezoactuator support with insufficient stiffness. Such an increase of the instability threshold corresponds to the open-loop gain equal to 2.
\nThe active vibration control is not turned on at 0 rpm of the rotor but after finishing a transient process, which ends by lifting the journal to approximately the middle position in the vertical direction which takes approximately 15 seconds for the given rate of the increase of speed.
\nThrough the experiment under specific conditions, the observed onset of instability was at 8450 rpm for control only in the x-axis direction and at 7100 rpm for control only in the y-axis direction [12]. It confirms the rule that static load delays the onset of instability at higher speeds. Control in both directions is required if the direction of the radial force may change or if the rotor has a vertical axis, i.e., the radial force is missing.
\nThe linear proportional controller was used for active vibration control for measurements presented in Figure 13. Parametric excitation means that at least one parameter of the system varies periodically in time according to a sinusoidal function, as was suggested by Tondl and Dohnal [13, 14]. The gain of the proportional controller was selected as this varying parameter. The system becomes nonlinear and nonstationary. The gain of the proportional controller is given as follows:
\nThe journal position as a function of time for tests with active vibration control on.
where \n
Dohnal [14] has solved a similar problem for magnetic bearings. Our experiments on the test bench were conducted for the following amplitudes of excitation \n
Power losses in the journal bearings were estimated from the electric power which is consumed by frequency convertor and motor. Dependence of electrical power upon rotational speed of the motor was measured with and without active control as it is shown in Figure 14. Basic power consumption of the motor and frequency convertor was measured with the disconnected clutch between the motor and rotor; it means that the bearings were inoperative. The friction loss of a pair of bearings at 7k rpm is 66 W in an unstable operation, and if the active vibration control is on, then the friction loss is of only 48 W. The active vibration control reduces the friction losses of journal bearings by 27%. The bearing clearance amounts to 90 μm for the bearing journal of the diameter 30 mm. As a lubricant the hydraulic oil of the OL-P03 type (VG 10 grade, kinematic viscosity 2.5 to 4 mm2/s at 40°C) was used. All tests were undertaken at ambient temperature about 20°C. For small power loss by friction in the bearings, the actively controlled bearings can be used in systems for storing the kinetic energy as they are flywheels that spin at high speed. Longer life compared with roller bearings is another advantage of this type of bearings [11].
\nThe electric power consumed by the frequency converter, motor, and bearings.
The bearing bushing is suspended on a pair of piezoactuators, and the bearing journal is supported by an oil wedge. According to catalogue data, we used a linear piezoactuator, which is able to generate force of 3 kN in pressure or 700 N in tension on the track in the range of 90 μm. These parameters correspond to the piezoactuator stiffness of 33 MN/m. Stiffness of the force transducer is 2000 MN/m, which is two orders of magnitude higher than the stiffness of piezoactuators. Stiffness of the O-ring seal is 5.5 MN/m which increases the stiffness of the piezoactuator by this value. Force is transmitted to the bearing journal through the oil film. Based on the simulations, it can be estimated that the direct stiffness (\n
Stiffness of precision rolling bearings ranges from 100 to 200 kN/m, regardless of the load, while the stiffness of hydrodynamic bearings in neighborhood of central position (low load) is of the order of several kN/m. However, with active control, the stiffness can increase as much as 35 times, i.e., it can achieve values around 100 kN/m, which is comparable to that of precision ball bearing.
\nExperiments prove the correctness of the theoretical prediction which refers to the extending of the operating range of plain bearings when active vibration control is used. The performance of the actively controlled bearing was tested on the test rig. The bearing diameter is 30 mm, and the length-to-diameter ratio is equal to about 0.77. The radial clearance of the journal is 45 μm and the very low viscosity oil is used. This combination causes instability of the oil whirl type from the rotational speed of 2k rpm. The active vibration control extends stable operating rotational speed up to 12k rpm, i.e., six times. Also the stiffness of the bearing journal increases significantly during a displacement from equilibrium position. The friction loss of a pair of bearings at 7k rpm is 66 W in an unstable operation, and if the active vibration control is switched ON, then the friction loss is of only 48 W. The active vibration control reduces the friction losses by 27%. The linear proportional controller was used for the active vibration control. The quality of control has been enhanced with the use of periodic changes of the controller gain, which is known as a parametric excitation. The effect of this way of control reduces the journal residual oscillation to the limits which does not exceed 8 μm. This amplitude is comparable with the radial clearance of the ball bearings of the deep groove type. The experiments with the time-periodic changes of the controller gain confirm the positive effect on the vibration response.
\nThis work was supported by the European Regional Development Fund in the Research Centre of Advanced Mechatronic Systems project, CZ.02.1.01/0.0/0.0/16_019/0000867 within the Operational Programme Research, Development and Education and the project SP2020/57 Research and Development of Advanced Methods in the Area of Machines and Process Control supported by the Ministry of Education, Youth and Sports. This publication was issued thanks to supporting within the operational programme research and innovation for the project: “New generation of freight railway wagons” (project code in ITMS2014+: 313010P922) co-financed from the resources of the European Regional Development Fund.
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