Synthesis, Characterization, and Processing of Monosized Ceramic Powders

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Bruce Fegley, Jr.* and Eric A. Barringer*'** Ceramics Processing Research Laboratory and Materials Processing Center, Dept. of Materials Science and Engineering, MIT, Cambridge, MA 02139

ABSTRACT Controlled alkoxide hydrolysis reactions for the synthesis of monodispersed oxide powders are described. The chemical and physical properties of representative monodispersed powders of TiO2 , doped Ti0 , 2 ZrO2 , doped Zr0 2 , Si0 2 , doped Si0 2 , and ZrO2 -A1 2 0 3 are described. The importance of surface chemistry for control of powder dispersion and packing is discussed and related to the control of sintered microstructures. CONTROLLED CHEMICAL SYNTHESIS OF OXIDE POWDERS Thermal decomposition and hydrolysis of metal alkoxides have been used to prepare a variety of high-purity oxide powders [1]. However, the major objective of this work was not to control the size distribution and shape of the resulting powders, which were generally very fine (< 100A) and highly agglomerated. Thus, the subsequent processing of the powders to uniform fine-grained, low porosity ceramic bodies was generally not achieved and the full advantages of the alkoxide synthesis techniques for microstructure control were not realized. Controlled hydrolysis reactions of metal alkoxides have been utilized to prepare monodispersed oxide powders of controlled size, shape, and composition, e.g., Ti0 2 , doped TiO2 , Zr0 2 , doped ZrO2 , Si02 , doped SiO , and 2 ZrO2 -Al 2 0. The synthetic methods used to prepare these materials involve fairly simple solution chemistry, but give a high degree of control and reproducibility. Basically, a dilute solution (- 0.2 to 0.4 M) of the respective metal alkoxide in 200 proof anhydrous ethanol is hydrolyzed by adding an equal volume of a solution of deionized water in anhydrous ethanol. The second solution is poured into the first solution with stirring. The hydrolysis reactions are conducted in a glove box under a dry N2 atmosphere and generally at room temperature (- 25*C); precipitation of a white powder occurs in several seconds to several minutes. The precipitation time increases as the concentration of either the alkoxide solution or the water solution is decreased. The hydrolysis reaction may be schematically represented by the general equation M(OR)

+ -x- H2 0 = MOx 2 + xROH


where x is a function of the valence of the metal cation. After precipitation, the powder is washed by centrifuging and redispersing in distilled water (or other solvent); this cycle is repeated two to three times. The powder is then further processed by adding cation dopants or is sedimented into compacts for sintering studies. The synthesis and processing of the various oxide powders are briefly reviewed in the following sections. Pure TiO, Barringer and Bowen [2] described the synthesis of monodispersed, spheroidal TiO2 by the controlled hydrolysis of dilute solutions of titanium tetraethoxide, Ti(OC2 H5 ) 4 . In a typical experiment, a 0.30 M solution