Densification kinetics of vacuum hot-pressed sodium beta-aluminas

Abstract

Abstract Hot pressing of two sodium beta-alumina powders of nominal composition Na 2 O · 6.9Al 2 O 3 and Na 2 O · 8.5Al 2 O 3 at 1100 – 1600°C and 150 – 350 kg/cm 2 in inductively heated graphite dies under vacuum, proved an easy method for obtaining fully dense materials in very short pressing times. Densification rates and microstructures developed are strongly dependent on the characteristics of the starting powders. The microstructure of the materials obtained from the first powder consisted of approx. equiaxed grains of the β and β″ phases; the intermediate and final stages of densification (ϱ > ∼ 85%) obeyed first-order kinetics and showed stress exponents in the steady state creep equation ( e ∼ σ n ) from 0.74 to 1.76. The apparent volume diffusion coefficients at 90% relative density for which n values close to unity were found, calculated on the assumption of the Nabarro-Herring diffusional creep model were of the order of 10 −9 cm 2 /sec. The apparent activation energy for densification was 55 kcal/mole. A duplex microstructure was observed in the materials hot-pressed from the second powder consisting of nearly equiaxed grains and strongly anisotropic elongated crystals grown perpendicularly to the hot-pressing direction. First-order densification kinetics were observed also for this powder but with much lower densification rates. The apparent activation energy for densification was 130 kcal/mole. A strong texture was observed in the materials hot-pressed from both powders with most of the c axes oriented parallel to the hot-pressing direction.