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Abstract
The synthesis of beta-alumina powder from kaolin-derived aluminium sulphate solution is described. A homogeneous mixture of AI2(S04)3·18H20 and Na2SO4·10H2O was precipitated by dropping the mixed solution of the kaolin-derived aluminium sulphate and sodium hydroxide into ethanol with agitation by stirring. The preparation conditions - mixing mole ratio of Na20/Al203 in the solution and calcination temperature of the resultant precipitate - were experimentally determined for obtaining the beta-alumina powder with a structure consisting of b- and b"-alumina and a well crystallized state.
1. Introduction
Beta-alumina ceramic is well known as an efficient sodium ion conductor that can be used as the solid electrolyte of sodium-sulphur batteries. Derivatives known as beta-alumina are generally found in materials of the type (3·Al203 with the empirical formula Na20·11A1203, and in materials of the type b"-A1203 identified in the form Na20·5Al203. The fabrication of beta-alumina ceramic can be separated into three distinct stages. The first stage is powder preparation, in which the chemical constituents are thoroughly mixed. The powder is then formed into the required shape and the resultant green body is sintered to a sufficient density to produce the required properties
One aspect of this processing that has received a significant amount of attention is powder preparation. Numerous methods of preparing beta-alumina powder are reported in the literature. The powder is usually prepared by mechanically mixing ?-alumina with sodium and dopant salts in variable proportions, followed by calcining the mixture at around 1600°C. But following calcination, a significant amount of ball-milling is required to achieve a sinterable powder; this is because the high calcination temperature produces a coarse particle size. Other synthetic techniques which have been reported include sol-gel processing and oxalate coprecipitation. All of these have been aimed at attaining a more intimate mixing of the various reactants than is possible with the mechanical mixing technique. However, an approach to the synthesis of high quality powder by the two techniques is the use of alkoxides on chemically pure reagents. From the viewpoint of purity, homogeneous mixing of starting materials and low calcination temperature, both processes have advantages. But the high purity of their starting materials makes these processes expensive and their costs need to be improved.
In this study, beta-alumina powder with a structure consisting of b- and b"-alumina was obtained from a mixed solution of aluminium sulphate and sodium hydroxide by a homogeneous precipitation method using ethanol as precipitation agent. Kaolin and sodium hydroxide were used as A1203 and Na20 sources, respectively. Beta-alumina powder was synthesized by homogeneous precipitation and calcination. The effects of heat treatment were also investigated.
2. Experimental procedure.
Kaolin and sodium hydroxide were used as the starting materials for the preparation of beta-alumina powder. The chemical analysis of kaolin is presented in Table I. The kaolin powder was heat treated at 800°Ñ in an electric muffle furnace for 3 h, cooled and ground to below a size of 0.074 mm in an agate mortar.
The general procedure for the synthesis of beta-alumina powder with a phase consisting of b- and b"-alumina is shown as a flow chart in Fig. 1.
The beta-alumina powder was synthesized by the reaction of kaolin-derived aluminium sulphate solution and sodium hydroxide. The aluminium sulphate solution was prepared in a Pyrex glass reactor in a water bath fitted with a thermometer and a reflex condenser. First, 600 ml of 1.0 M H2S04 solution was added to the reactor; 18 g of the kaolin samples was then charged in the reaction flask, which was maintained at 80°C. The solution was continuously stirred at about 450 r.p.m. with an electric stirrer for reaction for 3 h. then cooled to room temperature. After reaction, leach residue was separated at room temperature by filtering the solution to obtain aluminium sulphate solution, as described in detail earlier.
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