Diffusion-Controlled Processes in Microwave-Fired Oxide Ceramics
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DIFFUSION-CONTROLLED CERAMICS
PROCESSES IN MICROWAVE-FIRED
OXIDE
MARK A.JANNEY AND HAL D. KIMREY Oak Ridge National Laboratory P.O. Box 2008 Oak Ridge, TN 37831
ABSTRACT Processing oxide-based ceramics using microwave heating leads to a number of unexpected results, which can only be interpreted in terms of enhanced diffusion. Enhanced sintering has been observed in alumina and zirconia. Accelerated grain growth in dense, hot-pressed alumina has been demonstrated. Increased diffusion coefficients have been observed for diffusion of oxygen in sapphire. As yet, a satisfactory theory to account for these phenomena has not been developed. This paper reviews the experimental work conducted at the Oak Ridge National Laboratory during the past four years on the processing of oxides in both 2.45 and 28 GHz microwave furnaces.
INTRODUCTION Microwave processing enhances numerous kinetic processes in oxide ceramics. Enhanced sintering has been demonstrated in high-purity alumina [1,2,31, 94% alumina [4], zirconia [5], and alumina-zirconia composites [6]. The rate of densification is generally enhanced over that of coarsening during sintering such that a finer grain size after sintering is obtained in microwave-fired ceramics as compared to conventionally-fired ones [2,5]. Accelerated grain growth in dense, hot-pressed, high-purity alumina has been demonstrated [7,8]. As will be reported in this paper, diffusion of oxygen in single crystal sapphire is also enhanced in the microwave field; to the best of our knowledge, this is the first report of direct evidence for enhanced diffusion in a ceramic single crystal by a microwave field. The phenomenology of microwave processing of ceramics has been well documented in terms of enhanced sintering and annealing effects. However, at present there does not exist a satisfactory theory of why the microwave field acts to enhance these diffusioncontrolled processes. Current indications are that the electric field is important; furthermore, there is some limited evidence for the existence of an effect of frequency on some kinetic processes.
Mat. Res. Soc. Symp. Proc. Vol. 189. 01991 Materials Research Society
216
EXPERIMENTS IN SINTERING, GRAIN GROWTH, AND DIFFUSION Enhanced sintering in alumina was the first phenomenon that was observed in our studies of microwave processing of oxide ceramics. This means either sintering at a lower temperature, or sintering faster at the same temperature, for microwave processing as compared to conventional processing. Fig. 1 shows a typical sintering curve for microwave (28 GHz) and conventional sinterng of high-purity alumina in terms of sintered density vs temperature (50"C/min heating rate, with a 1 h hold)[1]. There is an obvious acceleration of densification for the microwave case. Depending on where in the sintering cycle one looks, there is a 300-400"C differential in the sintering curves. Furthermore, there is a significant difference in the shape of the sintering curves; in the microwave case, density increases much faster with temperatu
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