Solution Combustion Reaction Research Articles

Yttrium aluminum garnet powders by nitrate decomposition and nitrate–urea solution combustion reactions—a comparative study

Precursors for yttrium aluminum garnet (Y 3Al 5O 12—YAG) were synthesized by simple decomposition of concentrated aqueous solution of nitrates and combustion of concentrated aqueous solution of nitrates with urea on a heater. The precursor formed by the former reaction was granules of agglomerated powder while that from the latter reaction was a voluminous and porous sponge-like mass. Both precursors were ground to powders and subjected to detailed thermogravimetric–differential thermal analysis and X-ray diffraction studies. The precursor from the simple decomposition of nitrates exhibited a total loss in weight of about 18% in stages (25 to 300 °C and 300 to 600 °C) accompanied by endotherms—characterized as processes of dehydration of absorbed moisture and decomposition of residual nitrates, respectively. The as formed precursor and that heated to 820 °C were amorphous. Crystallization to YAG phase occurred from an amorphous oxide characterized by an exotherm above 820 °C with no loss of weight. The precursor from nitrate–urea combustion reaction was found to exhibit a weight loss of 2.5% accompanied by a shallow endotherm in the range of 25 to 300 °C—characterized as the process of dehydration of absorbed moisture. No further weight loss or heat effect was noticed, confirming it to be chemically pure YAG. This as formed precursor was found to be crystalline YAG. The difference in chemical composition of the precursors formed by these two reactions is attributed to the difference in the actual reaction temperatures during their formation—lower reaction temperature for the endothermic decomposition of nitrates and higher reaction temperature for the exothermic combustion associated with the formation of a bright flame. The morphology of the precursor powder formed by the former reaction exhibited only cracks while that of the precursor from the latter reaction exhibited pores and voids. The precursor from the former reaction was calcined at 1100 °C to form into chemically pure YAG. Zeta potential variation with pH for the aqueous suspensions of the crystalline YAG powders from both the reactions exhibited a maximum value in the range of 40 to 50 mV around a pH of 4, indicating stability of these dispersions towards coagulation at this pH. Particle size distribution of wet ground powders (slurries with 20%, v/v, solid at a pH of 4) showed that the powder from combustion reaction could be formed into a finer size than that from simple nitrate decomposition, indicating the agglomerates of combustion reaction were softer.

Characteristics of La 0.7 Ca 0.3 MnO 3 Powders Prepared by the Solution Combustion and Solid State Reaction Methods for Colossal Magnetoresistance Applications

La0.7Ca0.3MnO3 powders were prepared by both the solution combustion method and the solid state reaction method and were calcinated at various calcination temperatures and time intervals in air atmosphere. In the solid state reaction method, single-phase La0.7Ca0.3MnO3 was obtained after heat treatment of the powder at 1000°C for 24 hr. In the solution combustion method, however, single-phase La0.7Ca0.3MnO3 powder could be obtained easily when the powder was heat-treated at 650°C for only 30 min. Polycrystalline La0.7Ca0.3MnO3 powder, using the solution combustion method, showed good powder characteristics, such as an average grain size of 50 nm and a specific surface area of 92 m2/g. The resistance as a function of temperature and the magnetoresistance ratio in La0.7Ca0.3MnO3 thin films were attempted to examine the colossal magnetoresistance characteristics. These thin films also showed excellent colossal magnetoresistance properties in that 96% of the maximum magnetoresistance ratio was obtained at 97K.

Solution Combustion, Characterization and Sintering Behaviour of 9ZnCr2O4.2LiZnVO4

A systematic study of the synthesis and sintering of a composition, namely, 9ZnCr2O4.2LiZnVO4 in the zinc chromate-lithium zinc vanadate (ZnCr2O4-LiZnVO4) system was carried out. The powders were prepared by both solution combustion and solid state reaction methods. In solution combustion technique, stoichiometric mixture of nitrates of desired cations and urea in aqueous solutions was heated rapidly to ensue the reaction. Ammonium vanadate was used as the diluent. In solid state reaction, oxides of zinc, chromium and vanadium and lithium carbonate were homogenized and calcined at 800°C. The compound formation was confirmed by XRD. Compacts of both powders were prepared and sintering studies were carried out at 1000° and 1100°C. Sintering behaviour of both the powders were similar. Porosity of sintered specimens could be varied between 0 and 20% by controlled sintering.