The Properties of Alumina Sintered in A 2.45 GHz Microwave Field
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THE PROPERTIES OF ALUMINA SINTERED IN A 2.45 GHz MICROWAVE FIELD. MARK.C.L.PATTERSON,
ROBERT M. KIMBER AND PRASAD S.APTt.
Alcan International Ltd., Kingston Reasearch Princess Street, Kingston, ONTARIO, K7L 5L9.
and Development
Centre,
945
ABSTRACT A method has been developed to reproducibly sinter high purity alumina Sintering times of (>99.8%) to densities in excess of 98% of theoretical. between 6 and 120 minutes have been investigated and a uniform grain structure is only obtained with sintering times greater than 30 minutes. Microwave sintering produces a product with a smaller, more uniform grain structure with a more narrow size distribution. The alumina is also tougher than the conventionally sintered material. When scaled-up to sinter batches of 400 grams, an energy consumption of 3.8 kWh/kg is required, which is estimated to be a 90% savings over conventional sintering in an electric resistance furnace. INTRODUCTION The sintering of alumina has traditionally been carried out using either gas fired or electrically heated resistance furnaces, both of which primarily heat only the surface of the component, relying on thermal conduction to bring the bulk of the material to the sintering temperature. Microwave heating offers the advantage of being able to deliver heat more uniformly to the bulk of the component thereby substantially reducing the thermal gradients and allowing comparitively rapid sintering times which in return, results in different material properties over the conventional processing routes. Recent work has drawn from the experiences of Schubring[l] and others[251, who showed the novelty of fast heating using microwaves and demonstrated that high sintered densities could be achieved in short processing times, resulting in a fine microstructure and an improved energy utilisation. More recently improved uniformity[6J and microstructural properties[7J have been claimed and the merits of higher frequencies such as 28 GHz[8J and 60 GHz(9J have been investigated, which have been shown to offer certain advantages. At 2.45 GHz, the experimental work on alumina reported in the literature [1,3,5,10,111, deals with the heating of low grade (92% - 98%) alumina, which contain impurities that have a high dielectric loss factor and readily absorb microwave energy. High purity alumina has only been sintered using high frequencies (28 GHz[121 and 60 GHz[9]), or by using a combination of conventional and microwave heating at 2.45 GHz[131. This combination technique, utilises the increase in dielectric loss of alumina with increasing temperature, allowing the high purity alumina once "hot", to directly couple to the microwave energy. The increase in dielectric loss with increasing temperature has been found to result in non-uniform sintering and thermal runaway[8,11,131. EXPERIMENTAL PROCEDURE Experiments were carried out on three types of high characteristics of which are shown in table I.
Mat. Res. Soc. Symp. Proc. Vol. 189. ©1991 Materials Research Society
purity aluminas,
the
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Table
I
Charac
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