Nano-alumina by gel combustion, its thermal characterization and slurry-based coating on stainless steel surface

Gel combustion method was used to prepare nano-alumina from aluminum nitrate and stoichiometric amount of glycine as fuel. The TG-DTA pattern of the as-prepared powder (combustion product) exhibited exotherms with peaks around 500 and 900 A degrees C accompanied with loss of weight of 25 and 5 % attributed to burning away of carbon left behind and decomposition of residual reaction intermediates left behind, respectively. Even though mass stability is attained above 900 A degrees C, the DTA exhibited an exotherm around 1,150 A degrees C attributed to transformation of gamma to alpha form of alumina. The XRD studies revealed that the powder heated to 900 A degrees C was chemically pure nano-crystalline alumina while that heated above 1,150 A degrees C was crystalline alpha form. As nano-crystalline powders are sinter-active, the nano-crystalline alumina formed by calcination at 900 A degrees C was used to form the coating. A morphological feature of the agglomerates of nano-alumina powders were evaluated using SEM. The powder was de-agglomerated by wet grinding method. The dispersion conditions to form slurry using 900 A degrees C calcined powder for slurry-based coating was optimized using zeta-potential studies, and it was found to exhibit a maximum value of -45 mV at a pH of 9. After 8 h of grinding, the median agglomerate size reduced to 2 mu m. Rheological studies exhibited desired pseudoplastic behavior in the range of 10-20 vol.% of solid while the slurry with 15 vol.% only form crack free, dense, and adherent coating after heat treatment at 1,150 A degrees C. The morphology of the coating was found to be uniform and dense.