Sol gel technique

J. W. Fleming, "Sol-Gel Techniques for Lightwave AppUcations," paper no. MH-1 in Technica/ Digest of Optica/ Fiber Communications Conference, Reno, Nev., Optical Society of America, Washington, D.C., 1987. 

The sol—gel technique has been used mosdy to prepare alumina membranes. Figure 18 shows a cross section of a composite alumina membrane made by sHp coating successive sols with different particle sizes onto a porous ceramic support. SiUca or titanium membranes could also be made by the same principles. Unsupported titanium dioxide membranes with pore sizes of 5 nm or less have been made by the sol—gel process (57). 

The gels precipitated as described above are not useful in ion-exchange systems because their fine size impedes fluid flow and allows particulate entrainment. Controlled larger-sized particles of zirconium phosphate are obtained by first producing the desired particle size zirconium hydrous oxide by sol—gel techniques or by controlled precipitation of zirconium basic sulfate. These active, very slightly soluble compounds are then slurried in phosphoric acid to produce zirconium bis (monohydrogen phosphate) and subsequently sodium zirconium hydrogen phosphate pentahydrate with the desired hydrauhc characteristics (213,214). 

Sol-Gel Techniques. Sol-gel powders (2,13,15,17) are produced as a suspension or sol of coUoidal particles or polymer molecules mixed with a Hquid that polymerizes to form a gel (see Colloids SoL-GELtechnology). Typically, formation of a sol is foUowed by hydrolysis, polymerization, nucleation, and growth. Drying, low temperature calciaation, and light milling are subsequently required to produce a powder. Sol-gel synthesis yields fine, reactive, pseudo-crystalline powders that can be siatered at temperatures hundreds of degrees below conventionally prepared, crystalline powders. 

A new generation of mesoporous silica (SG) materials obtained by sol-gel technique where polymers and ionic or non-ionic surfactant act as stmcture - directed templates is widely developed during last year s. Final materials can be synthesized as thin films and used as sensitive elements of optical and electrochemical sensors. 

In the present work it was studied the dependence of analytical characteristics of the composite SG - polyelectrolyte films obtained by sol-gel technique on the content of non-ionic surfactant in initial sol. Triton X-100 and Tween 20 were examined as surfactants polystyrene sulfonate (PSS), polyvinyl-sulfonic acid (PVSA) or polydimethyl-ammonium chloride (PDMDA) were used as polyelectrolytes. The final films were applied as modificators of glass slides and pyrolytic graphite (PG) electrode surfaces. 

In this case study, a zirconia-alumina membrane has been developed using the sol-gel technique with and without support.6-7 The porous ceramic was prepared to fabricate the membrane support. A thin film of aluminum and zirconium were formed on the porous ceramic support. Unsupported membrane was also prepared. The unsupported membrane was not strong enough to hold a high-pressure gradient it was very fragile and not useful... 

Once the membrane was successfully produced, it was analysed for characterisation and scanning. The sol-gel technique was successfully used to obtain a crack-free unsupported membrane, which was expected to have pore size of 1-2 nm. The development of the crack-free membrane may not have the same strength without strong, solid support. The next stage of this work was to characterise the fabricated membrane. Hie objectives of this study were to develop a zirconia-coated 7-alumina membrane with inorganic porous support by the sol-gel method and to characterise the surface morphology of the membrane and ceramic support. 

The vast increase in the application of membranes has expanded our knowledge of fabrication of various types of membrane, such as organic and inorganic membranes. The inorganic membrane is frequently called a ceramic membrane. To fulfil the need of the market, ceramic membranes represent a distinct class of inorganic membrane. There are a few important parameters involved in ceramic membrane materials, in terms of porous structure, chemical composition and shape of the filter in use. In this research, zirconia-coated y-alumina membranes have been developed using the sol-gel technique. 

We have successfully developed a new inorganic ceramic membrane coated with zirconium and alumina. A thin film of alumina and zirconia unsupported membrane was also fabricated. The successful method developed was the sol-gel technique. 

The use of sol-gel techniques to prepare ceramic powders has recently attracted much interest in academia and industry. Sol-gel techniques involve dissolving a ceramic precursor (e g., tetramethyl orthosilicate) in a solvent and... 

The formation of ordered two- and three-dimensional microstructuies in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug dehvery, for the contaimnent of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil recovery, ore beneficiation, and lubrication. Ceramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the production of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and microciystallites. 

Wen and Wilkes [12] enlisted the names and types of most of the polymers employed for the preparation of hybrid composites using sol-gel technique. In the field of mbber, wide-scale research... 

The sol-gel technique was also used to prepare solid electrolytes containing MEEP, triethoxysilane (TEOS) and lithium triflate. Homogeneous, transparent and mechanicaUy stable materials have been obtained by Gughelmi [611] from a partially hydroxylated MEEP and TEOS, which after doping with LiSOjCFj exhibited a conductivity in the range 3x10 S cm at 60 °C. 

Sol-gel techniques have been widely used to prepare ceramic or glass materials with controlled microstructures. Applications of the sol-gel method in fabrication of high-temperature fuel cells are steadily reported. Modification of electrodes, electrolytes or electrolyte/electrode interface of the fuel cell has been also performed to produce components with improved microstructures. Recently, the sol-gel method has expanded into inorganic-organic hybrid membranes for low-temperature fuel cells. This paper presents an overview concerning current applications of sol-gel techniques in fabrication of fuel cell components. 

Sol-gel technique has been used to deposit solid electrolyte layers within the LSM cathode. The layer deposited near the cathode/electrolyte interface can provide ionic path for oxide ions, spreading reaction sites into the electrode. Deposition of YSZ or samaria-doped ceria (SDC, Smo.2Ceo.8O2) films in the pore surface of the cathode increased the area of TPB, resulting in a decrease of cathode polarization and increase of cell performance [15],... 

Sol-gel technique has also been applied to modify the anode/electrolyte interface for SOFC running on hydrocarbon fuel [16]. ANiA SZ cermet anode was modified by coating with SDC sol within the pores of the anode. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the TPB area, which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode leads to higher stability of the cell in long-term operation due to the reduction of carbon deposition and nickel sintering. 

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

For this micro reactor version, the catalyst was coated on the AlMg3 platelet as a thin silver layer by sputtering [43,44], A further set of platelets was covered with an a-alumina layer by sol-gel technique and impregnated by a three-step procedure with silver lactate. 

A Pt catalyst was applied by dry and wet techniques. By means of sputtering using a mask process protecting parts of the microstructure, the micro channel bottom was coated selectively. In addition, an y-alumina layer was applied by the sol-gel technique. Initially, the whole micro structure was covered by such a layer. Then, photoresist was applied and patterned so that only the channel part remained covered. After removal of the exposed photoresist and unprotected y-alumina, only the channel bottom was coated with y-alumina. 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

Prepared by nitrate precipitation 2Prepared by a sol-gel technique (Zr propoxide) 3Prepared by ball milling of oxides 4Total OSC (value close to OSCC)... 

The route also provides a satisfactory path to some alkyl silicates for which no other satisfactory routes are available. Some of these alkyl silicates may be of interest in the synthesis of ceramics by the sol-gel technique. 

The combination of inorganic-organic polymers on a molecular level opens an interesting possibility of synthesizing new materials. Organic polymer synthesis and sol-gel techniques seem to be suitable techniques for this. The field is just at the beginning of its development. 

Thammachart M, Meeyoo V, Risksomboon T, Osuwan S (2001) Catalytic activity of Ce02-Zr02 mixed oxide catalysts prepared via sol-gel technique CO oxidation. Catal Today 68 (1) 53—61... 

The next publication appeared in 1984. Venton et al. [67] entrapped antiprogesterone antiserum into a sol-gel derived silica. It retained about 56 % of progesterone binding capacity. The authors mentioned the considerable promise of the sol-gel technique for biopolymer immobilization, but their study was not further continued. 

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