Preparation of fine multicomponent oxide ceramic powder by a combustion synthesis process
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An important, rather novel procedure for the synthesis of submicron crystalline multicomponent oxide ceramic powders has been studied. The synthesis of powder, a ferrite material, has been used as a model system for understanding the synthesis process. The effect of the fuel content, powder packing, and surface heat loss has been investigated in terms of the maximum reaction temperature and reaction period, phase formation, and particle size and morphology. It has been shown that the maximum temperature and reaction period can be tailored to produce different phases. The submicron features of the synthesized powders are indicated by the large surface area values obtained from BET measurement.
I. INTRODUCTION Very fine crystalline unagglomerated multicomponent oxide ceramic powders are often difficult to produce by existing synthesis routes. Several conventional synthesis techniques are available, including solid state synthesis, freeze-drying, co-precipitation, spray-drying, and spray roasting.1'2 Each method has its own characteristic degree of purity, compositional homogeneity, powder morphology, and degree of agglomeration. Combustion synthesis is also an important powder preparation process, by which several hundred compounds might be prepared.3 The process is, in the most simple sense, the exploitation of an exothermic and usually very rapid chemical reaction to form a useful material. The form of the combustion synthesis-derived material is often a powder. A key feature of the process is that the heat required to drive the chemical reaction is supplied from the reaction itself and not from an external (and therefore expensive) source. Materials that have been successfully synthesized include oxides, carbides, borides, intermetallics, and rather complex composites.3'4 The goal of the present study was to conduct an investigation of a rather novel method for the synthesis of ceramic oxide powder by a specific variant of the combustion synthesis process, where metal nitrates react with carbonaceous reductive fuel materials in an exothermic, rapid, and self-sustaining manner. The process results in the synthesis of fine multicomponent oxide ceramic powder. The metal nitrates are the cation sources for the metal oxide in the target ceramic material. By simple calcination, the metal nitrates can, of course, be decomposed into metal oxides upon heating to or above the phase transformation temperature. A constant external heat supply is necessary in this case to maintain the system at a high temperature for the appropriate J. Mater. Res., Vol. 9, No. 8, Aug 1994
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phase transitions to be accomplished. With the presence of the reductive fuel materials, however, the internal (chemical) energy released from the exothermic reaction between the nitrates and the fuel can rapidly heat the system to a high temperature and sustain that elevated temperature for a certain period even in the absence of an external heat supply. The exothermic reaction is usually ignited at a
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