Combustion synthesis and properties of fine particles spinel, Perovskite and K2NiF4 Type oxides

Abstract

Technologically important fine?particle, mixed?metal, transition?metal oxides such as spinels (MCr?O?), perovskites (LnCrO?, La(Sr)MnO?, LaCoO? and LaNiO?) and K?NiF??type oxides (La?CoO?, La?NiO?, La?CuO? and La?.?Sr?.?CuO?) have been prepared by a novel combustion process in a few minutes under ambient conditions. The process involves rapid decomposition of an aqueous solution containing stoichiometric amounts of corresponding metal nitrates (as oxidizers) and hydrazine derivatives—carbohydrazide (CH), tetraformal trisazine (TFTA), triazole—or urea as fuels at 350°C / 550°C. The solution initially boils with foaming and ignites to burn with a flame temperature above 1000°C to yield foamy, voluminous, fine?particle oxides. Preparation of metal chromites, MCr?O? (where M = Mg, Ca, Mn, Fe, Co, Ni, Cu and Zn), has been achieved using urea and TFTA as fuels. The particulate properties such as particle size and surface area of the chromites vary with the choice of fuel. Chromites prepared by the TFTA process have surface areas in the range of 15–30 m²/g, whereas those prepared by the urea process vary from 5–20 m²/g. This is attributed to the nature of combustion of the redox mixtures and the amount of gases evolved during combustion. Sintering studies of CaCr?O? and MgCr?O? reveal uniform grain growth at low temperatures, demonstrating the effect of fine?particle nature and narrow size distribution of the oxides. Nearly 95% theoretical density has been achieved for MgCr?O? when sintered at 1500°C for 2 hours in air. Rare?earth orthochromites, LnCrO? (where Ln = La, Pr, Nd, Sm, Gd, Tb, Dy and Y), have been prepared using both CH and TFTA as fuels. Both processes yield high?surface?area powders (8–65 m²/g) compared to LnCrO? prepared by conventional methods. LaCrO? sintered at 1500°C for 2 hours in air achieved ~85% theoretical density. Other perovskites such as La(Sr)MnO?, LaCoO? and LaNiO? have been prepared similarly. La?.?Sr?.?CuO? prepared by the combustion process shows the expected superconducting transition at 33 K. In applied magnetic fields of 2 and 5 Tesla, the low?temperature side of the transition is broadened up to 16 K and 13 K, respectively. The catalytic activity of the combustion?derived oxides such as CuCr?O? / LaCrO?, LaMnO? and La?NiO? has been tested for the decomposition of N?O. All these oxides show a 10–100?fold increase in activity compared to oxides prepared by ceramic or oxalate?precursor routes. The reactivity of these oxides follows the order: La?NiO? > LaMnO? > CuCr?O? > LaCrO?