Crystallite growth of a sodium beta-alumina powder determined by X-ray line broadening

Abstract

growth of a sodium beta - alumina powder (nominal composition Na2O · 8.5Al2O3; impurity content < 0.3w/0 is reported. The powder was obtained from an aqueous solution of reagent grade NaOH and γ-Al2O3∗ calcined at 1200°C after dewatering at 50°C in a rotary drier and then ground for 8 h in an alumina ball-mill. X - ray diffraction and SEM and TEM observations made on the synthesized powder showed pseudo-amorphous forms and plate-like crystallites of the β and β″ phases having an average diameter of 0.02 μm and a BET surface area of 26 m2/g. The specific gravity of the powder, determined by the liquid displacement technique, was 3.10 g/cm3 against a value of 3.27 g/cm3 measured on a sample of the same powder hot pressed to essentially full density. This could indicate the existence of a pore substructure in the interior of the aggregates of crystallites resembling the hydroxide or carbonate derived MgO porous pseudomorphs observed by Tagal et al.1. Isothermal crystallization heat treatments of loose powders were performed in a Pt crucible rapidly introduced in a laboratory electric kiln pre-heated at the crystallization temperature. No appreciable Na2O losses were observed even at the higher temperatures and longer treating times. Crystallization runs were performed in the temperature range from 800°C to 1400°C and from 30 min. to approx. 50 h. The growth of the crystallites of the β phase was determined by the broadening of the (004) diffraction peak at 2θ = 14° 40′ using a scanning speed of 0.5 Deg./min.