Combustion synthesis and characterisation of lanthanum hexa-aluminate

Abstract

Lanthanum hexa-aluminate (LHA) is considered a promising material for thermal barrier coatings for gas turbine applications as well as oxidation resistant coatings for fibres in oxide based ceramic composites. Combustion synthesis has attractive advantages such as simple experimental setup, low cost, short reaction time, energy savings due to exothermic reaction, better control of stoichiometry and fine powder size. The present study involves combustion synthesis of LHA using lanthanum and aluminium nitrates as oxidising precursors, and urea as fuel. The precursor salts were dissolved in a minimum amount of deionised water and the solution was heated in a mantle heater to temperatures of around 300–350°C, at which combustion takes place with rapid evolution of gas. The LHA powder was also prepared by a precipitation route for comparative study. The as synthesised powders were calcined at up to 1450°C and subjected to X-ray diffraction for phase evolution studies. Product powders were then characterised. The influence of the lanthanum/aluminium molar ratio in the starting composition and the effect of temperature on the formation sequences of lanthanum monoaluminate (LMA) and LHA and on the conversion of LMA to LHA are discussed.