Oxide Powders and Ceramics

Nanopowders are agglomerates of extremely small particles, namely, nanoparticles (smaller than 100 nm), and they are the result of a fabrication process conceived to obtain superior products [1]. 

Interest in the synthesis of monodispersed oxide nanopowders increased essentially in the past few years due to their specific properties such as chemical, thermal, electrical, optical, mechanical, magnetic, and morphological properties [2,3]. Such properties recommend the oxide nanopowders for applications in very different areas such as pigments, catalysts, supports in chromatography, or as raw materials for the preparation of advanced polycrystalline ceramics [4-6]. 

Fine colloidal particles can be made by different methods from the vapor or the liquid phase [7]. 

The most promising advantage of the sol-gel method is that it offers the possibility of preparing solids with predetermined structure and shape (films, fibers, powders, and bulk) by varying the experimental conditions. 

Alkoxides are mainly used as reagents in the sol-gel method, but inorganic or organic salts are also used. In this way, two types of sol-gel processes are developed in an alcoholic (organic) or an aqueous medium. 

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C. Hyde and W. H. Duckworth, Investigation of Sinterable Oxide Powders and Ceramics Made from Them, WADD Technical Report 61-262 (1961). 

Beryllium Sulfate. BeiyUium sulfate tetiahydiate [7787-56-6], BeSO TH O, is produced commeicially in a highly purified state by fiactional crystallization from a berylhum sulfate solution obtained by the reaction of berylhum hydroxide and sulfuric acid. The salt is used primarily for the production of berylhum oxide powder for ceramics. Berylhum sulfate chhydrate [14215-00-0], is obtained by heating the tetrahydrate at 92°C. Anhydrous berylhum sulfate [13510-49-1] results on heating the chbydrate in air to 400°C. Decomposition to BeO starts at about 650°C, the rate is accelerated by heating up to 1450°C. At 750°C the vapor pressure of SO over BeSO is 48.7 kPa (365 mm Hg). 

Beryllium sulfate, [CAS 13510-49-1], BeSO 4H2O, is an important salt of beryllium used as an intermediate of high purity for calcination to beryllium oxide powder for ceramic applications. A saturated aqueous solution of beryllium sulfate contains 30.5% BeSC>4 by weight at 303C and 65.2% at 111"C. 

Inorganic precursors are much cheaper and easier to handle than metal alkoxides. Therefore the industrial production of oxide powders for ceramics and catalysts is mainly based on the precipitation or coprecipitation of inorganic salts from aqueous solutions. Gibbsite, Al(OH)3, (see Aluminum Inorganic Chemistry) is precipitated from aluminate solutions. Ti02 powders are made via the controlled hydrolysis of titanium salts. Stabilized zirconia is coprecipitated from aqueous solutions of zirconium oxychloride, ZrOC, and yttrium nitrate, YlKOsjs. 

The important properties of aluminum oxide ceramics are their high temperature stability (melting point of AI2O3 2050°C), their good thermal conductivity, their high electrical resistivity and their high chemical resistance. Their mediocre thermal shock resistance is a disadvantage. All these properties are dependent upon the chemical purity and particle size distribution of the oxide powder and the density, structure and pore size di.stribution of the ceramic. 

Figure 1.23 The fracture strength of Ce02)o.go LnOi 5)0.20 (Ln=Y, Cd, and Sm) ceramics prepared by solid-state reaction ofCe02 and the respective rare earth oxide powder and by an oxalate coprecipitation method [272],...

Syskakis, E., Jungen, W., and Naoumidis, A. (1993) Properties of per-ovskite powders and ceramics for solid oxide fuel cell application. In International Conference on Materials by Powder Technology PTM 93, Dresden, 23-26 March 1993 (ed. F. Aldinger), DGM Informationsgesellschaft, Oberursel, pp. 707-715. 

Williams P.A, Jones A.C., Bickley J.F., Steiner A., Davies H.O., Leedham T.J., Impey SA., Garcia J., Allen S., Rougier A., Blyr A. Synthesis and crystal structures ofdimethylaminoethanol adducts of Ni(ll) acetate and Ni(II) acetylacetonate. Precursors for the sol-gel deposition of electrochromic nickel oxide thin films, J. Mater. Chem. 2001, 11 2329 Yanovskaya M.L, Kotova N.M., Golubko N.V., Turova N.Ya. Reactions of magnesium and titanium alkoxides. Preparation and characterization of alkoxy-derived magnesium titanate powders and ceramics, J. Sol-Gel Sci. Tech. 1998 11 23... 

Besides the sol-gel processing, there are many aqueous routes to synthesize ceramic powders, fibers, films and bulks. Niesen and DeGuire reviewed these low temperature and non-electrochemical processes (Niesen and DeGuire, 2001). According to them, the processes include a chemical-bath deposition (CBD), successive ionic-layer adsorption and reaction (SILAR), liquid-phase deposition (LPD), electroless deposition (ED), and film deposition on organic self-assembled monolayers (SAMs). Of course, an electrochemical route is an important process. Another non-sol-gel route is a spray pyrolysis of solution or sol, and is applied to the direct preparation of oxide powders and films. Since these processes do not form the gel phase, they are not described here. 

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