Redox Effects in Self-Sustainiing Combustion Synthesis of Oxide Ceramic Powders
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REDOX EFFECTS IN SELF-SUSTAINIING COMBUSTION SYNTHESIS OF OXIDE CERAMIC POWDERS
L.A. Chick, G.D. Maupin, G.L( Graff, L.R. Pederson, D.E. McCready and J.L. Bates Pacific Northwest Laboratory, Richland, Washington 99352 ABSTRACT The glycine/nitrate process (GNP) is a combustion synthesis method that is particularly useful for synthesizing ultra-fine, multi-component oxide powders. During the self-sustaining combustion, the precursor solution is rapidly converted into an oxide product with glycine serving as the fuel and nitrates providing the oxidant. This paper examines the effects of altering the fuel/oxidant ratio on the characteristics of the product. Examples discussed include La(Sr)CrO 3 and La(Sr)FeO 3 perovskites and NiO. INTRODUCTION
Over the last few decades, a number of oxide ceramic powder synthesis techniques have been developed that begin with preparation of a precursor solution, in which the ions are well-mixed on a molecular scale. These techniques then employ a variety of methods for obtaining solid phases from the precursor solution, the objective being to maintain, as much as possible, the intimate mixing that was present in the precursor. Most often, the first solid product is a physical mixture of crystalline solid intermediate phases that must be thermally reacted to convert to the desired phase(s). The temperature and time required -for this final conversion are controlled by the chemistry and crystallite size of the intermediates. Time and temperature, in turn, dictate the crystallite size of the final product. Thus, for examples, spray drying, freeze drying, solgel, and coprecipitation often produce physical mixtures of the individual oxides, hydroxides, carbonates, oxalates, or cyanides, which are then heated to cause decomposition and reaction to the desired multi-component oxide phase [1]. Another group of synthesis techniques seeks to form an amorphous solid intermediate from the liquid precursor' Ideally, the desired crystalline phase is formed directly from the intimately-mixed amorphous solid by local rearrangement of the ions without the need for interdiffusion of species between adjacent solid crystallites. The Pechini [2] or amorphous citrate [3-6] processes form intermediate amorphous solids that are subsequently decomposed and thermally reacted to form complex oxide phases. However, as discussed below, it is often the case that undesirable intermediate crystalline phases form during the extensive thermal treatment required to obtain the final phase. Recently, several variants of self-sustaining combustion synthesis [7-12] have been developed. Unlike the Pechini or amorphous citrate processes, these self-sustaining combustion processes (including the GNP) are rapid and may closely approach direct conversion from the molecular mixture of the precursor solution to the final oxide product, avoiding formation of intermediate crystalline phases that require interdiffusion for complete reaction. This paper presents a description of the GNP and of several powder products that illustrate
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