Crystallization and Porosity Evolution of AlOOH–Fe(NO3)3 Gels

Monolithic planar boehmite gels of thickness ∼0.15 mm seeded by Fe(NO3)3 were examined by quantitative DTA, dilatometry, SEM, surface area and pore size distribution and on-line optical transmittance from RT to 1270°C. Dispersed Fe3+ ions resulted in 5wt% Fe2O3, seeds in monolithic boehmite gels which affected the crystallization of corundum phase and the associated evolution of porosity (the change of inter-colloidal to inter-grain porosity) during heat treatment. Transformation of γ to α-(Al, Fe)2O3 took place within individual grains of 100–120 nm size and was similar to unseeded boehmite gels except the a significantly higher frequency of homogeneous nucleation of corundum phase occurred. Gradual elimination of inter-grain pores during sintering was associated with the increase of light transmittance. Ceramics became transparent after sintering at temperatures over 1270°C.