Multicomponent ceramics oxides

Sintering with a liquid phase. Solid stale sintering is characteristic particularly for modern technical ceramics most theoretical studies are based on results from sintering pure oxides. Traditional multicomponent ceramics, however, contain liquid phase at sintering temperatures the main problem is then the distribution of the melt in a porous system. 

Since complicated multicomponent ceramic materials with precise levels of doping are required for some of the new materials that show promise in solid oxide fuel cells, we aim to use the relatively low-cost, simple steric entrapment method to synthesize several solid oxide fuel cell materials, and evaluate some of their characteristics. We have selected lanthanum gallates doped with strontium and magnesium, samarium doped ceria and a promising non-stoichoimetric composition composed of strontium iron and cobalt. 

Lao 75Cao,25) 1.01 Fe03 is formed at the seal. Since the third multicomponent metalUc oxide is identical to the composition of the ceramics to be joined, the thermal expansion coefficient of the joint material closely matches the thermal expansion coefficient of the (Lao.75Cao,25)i,oiFe03 parts that were joined. The joint thus has about the same thermal cycUng stability as the parts themselves. The joint also has the same chemical properties as the membrane material and therefore, the seal will be as resistant to the process atmospheres as the membrane material itself There are also no concerns with reactivity or chemical incompatibility of the seal material and the membrane material since the membrane and the joint material are chemically identical after the joint has been made. The invention of Butt et al. [33, 34] thus produces a ceramic-to-ceramic seal that has the very attractive properties of being expansion matched to the membrane material chemically identical to the membrane material just as chemically compatible with the process gas atmospheres as the membrane material just as thermodynamically... 

When multicomponent alkoxide solutions, or a single alkoxide and a soluble inorganic salt, are mixed, a multicomponent alkoxide may result. In this way, such complex oxides such as the YBCO superconductor (cf. Section 6.1.2.4) can be formed. Sol-gel processing can also be used to coat fibers for composites and to form ceramics with very fine pore sizes called xerogels. A xerogel commonly contains 50-70% porosity, a pore size of 1-50 nm, and a specific surface area exceeding 100 m /g. 

Hydroxycarboxylic acids, which include citric acid, malic acid, lactic acid, etc., are benign to the environment and very convenient for the solution processing. Moreover, since these reagents can form stable complexes with other cations, they rarely yield a precipitate. For several complexes single crystals of well-defined composition suitable for the X-ray structural analysis were isolated. Thus, these water-soluble titanium complexes of hydroxycarboxylic acids are promising precursors for the synthesis of ceramics from an aqueous solution and their industrial utilization is expected in the future. In this chapter we decribe the method of synthesis, structural analysis, and stability of these complexes. The examples of multicomponent oxide materials preparation using these compounds are presented. 

Electroanalytical methods have been used repeatedly in HTSC studies for the quantitative determination of the chemical composition of ceramics and films, their precursors, and also the degradation products. To analyze a multicomponent non-stoichiometric oxide it is necessary to determine independently with sufficient accuracy, the content of individual components that are simultaneously present in the samples [282]. The independent quantitative determination of oxygen is most essential (difference analysis introduces noticeable errors in the values of the important parameter 6). Also important is the determination of the valence of copper. Certain theories of superconductivity of cuprate systems consider Cu " as the principal essential component of HTSCs [9,10], which attracts special attention to this problem. 

In addition to the use of heterometal alkoxides, metal alkoxides are often associated with more easily available precursors such as acetates for the SG route to multicomponent oxides. A number of such alkoxide acetate precursors [e.g., MNb2(/i-OAc)2(/i-OR)4(OR)6 (M = Cd or Mg), PbZr3(/t4-0)(/i-0Ac)2(/i-OR)5(OR)5, and Gd2Zr6(/i4-O)2(pi-OAc)6(/t-OR)l0(OR)i0 (with R = i-Pr)] were characterized (564) by X-ray crystallography. Their hydrolytic studies indicate their potential use as precursors for the synthesis of electrooptical materials, for example, Pb(ScNb)03 (PSN), and dielectric ceramics, for example, [PbMg1/3Nb2/303] (PNM). 

Agglomeration reactions. Agglomeration reactions among multicomponents to prepare special composites could be conducted under hydrothermal or solvothermal conditions. Examples include the preparation of multi-oxide composite materials and ceramic materials containing volatile OH, F, and S2- components. 

A. Douy, Polyacrylamide gel an efficient tool for easy synthesis of multicomponent oxide precursors of ceramics and glasses. Int. J. Inorg. Mater. 3(7), 699-707 (2001). 

The hydrolysis reaction usually occurs at room temperature and dehydration occurs below 600°C, resulting in the formation of very fine ceramic particles, that is, 2-5 nm [22]. This method has been successfully used to make high-purity submicrometer-sized oxides from several metal alkoxides [23,24]. Focus on multicomponent oxide powder synthesis through the two-step hydrolysis and dehydration of metal alkoxides constitutes the remainder of this chapter. 

A.I. Kingdon, R. F. Davis, and M. M. Thackeray, Engineering properties of multicomponent and multiphase oxides, in Engineered Materials Handbook, Vol. 4, Ceramics and Glasses, ASM international. Metals City, OH (1991). p. 758-774. 

L. Bonhomme-Coury, M. Najman, F. Babonneau and P. Boch, Multicomponent Oxide Coatings via Sol-Gel Process, in Ceramic Microstructures Control at the Atomic Level. A. P. Tomsiaand A. Glaesereds., Plenum Press, New York (1998) 513-525. 

The sol-gel technique has been used to prepare sub-micrometer metal oxide powders [5] with a narrow particle size distribution and unique particle shapes (e.g. AljOj, TiOj, ZrOj, Fe Oj). Uniform SiO spheres have been grown from aqueous solutions of colloidal SiO [6]. Metal-ceramic composites (e.g. Ni-Al Oj, Pt-ZrO ) can also be prepared in this manner [7]. Organic-inorganic composites have been prepared by the sot-gel route. By employing several variants of the basic sol-gel technique, a number of multicomponent oxide systems have been prepared. Some typical examples are SiO -B Oj, SiO -TiO, SiO -ZrO, SiO -Al Oj and ThO -UO. A variety of ternary and still more complex oxides such as PbTiOj, PbTij r 3 and NASICON have been prepared by this technique [1-3, 8]. 

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