Coprecipitation synthesis of doped lanthanum chromite
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S. E. Dorris, R. B. Poeppel, S. Morissette, and U. Balachandran Materials and Components Technology Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received 4 February 1993; accepted 23 April 1993)
Two coprecipitation methods were used to synthesize powder precursors of doped lanthanum chromite (La, Ca)(Cr, Co)O 3. The effects of synthesis method and calcination temperature on the composition, sintered density, and microstructure of pressed compacts of (La, Ca)(Cr, Co)O3 were studied by differential thermal analysis/thermogravimetric analysis, x-ray diffraction, scanning electron microscopy, and density measurement. The cation ratios in the precipitated solids were, with few exceptions, within experimental error of the desired compositions for all four components. Powders obtained by both techniques could be sintered to densities exceeding 93% at 1400 °C. The highest densities were obtained with calcining temperatures from 450 to 700 °C. The sintered microstructures exhibited uniform grain sizes averaging 3 - 5 fim. The Cr(vi) compounds, CaCrO4 and La 2 Cr0 6 , were observed in all of the calcined powders. The possible role of these phases on chromite densification is discussed.
I. INTRODUCTION A. Background Multicomponent oxides for electronic applications are frequently synthesized by the Pechini or liquid-mix process.1'2 This technique starts with the addition of citric acid and ethylene glycol to an aqueous solution of salts of the desired metal cations. The solution gels during heating, helping to maintain intimate mixing of the metals during the subsequent driving off of water and the organic components. The resulting powders are typically very fine and uniform in size and composition. However, the heating step can be slow and energyintensive, and it generates large amounts of gaseous combustion products. Coprecipitation synthesis3'4 can also generate fine multicomponent powders. As in the liquid-mix route, the process starts with a solution of soluble metal salts. This is combined with a precipitating agent, typically an ammonium- or alkali-carbonate, oxalate, or hydroxide. The metals combine with the anionic group of the precipitating agent and settle out as insoluble salts, which can be calcined to form the desired metal oxide. The supernatant, containing the residual reagents, is removed from the powder by filtering or centrifuging and then rinsing. However, any of the metals that remain in the supernatant will be removed as well, thus altering the composition of the precipitate. Process variables such as temperature, pH, solution concentrations, choice of reagents, and means of mixing the solutions
will significantly affect not only the composition of the precipitate, but also its homogeneity and particle size. Less traditional precipitating agents that have been used in the synthesis of ceramic precursors include organic amines, such as triethylammonium carbonate5 or triethylammonium oxalate.6 In the present work, guanidine carbonate, [(NH2)2C=NH] 2 H 2 CO3, was used as a precipitatin
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