Effect of Green Microstructure and Processing Variables on the Microwave Sintering of Alumina
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EFFECT OF GREEN MICROSTRUCTURE AND PROCESSING VARIABLES ON THE MICROWAVE SINTERING OF ALUMINA Arindam D6. Iftikhar Ahmad, E. Dow Whitney and David E. Clark Dept. of Materials Science and Engineering. University of Florida, Gainesville. FL-3261 1.
ABSTRACT The concept qf 'hybrid heating with microwave (MW) energy at 2.45 0Hz.' for ultra rapid sintering qf alumina is being introduced. This technique is a combination of MW - materual interaction as well as conventional radiant/ conduction mechanisms, and facilitates the attainment of perhaps, the highest possible heating rates in a multimode MW cavity at 2.45 0 GJZz. (1500 C in 120 sees.). Rapid sintering of pure. undoped alumina with this novel techniQue culminates in uniform, homogeneous microstructures and mechanical property enhancements vis-a-vis conventional fast firing. The role of green microstructure (particle size) on MW(hybrid) heating and processing variables (temperature, time) on the MW (hybrid) heating phenomena vs. conventional fast firing were investigated. Hybrid heated samples showed accelerated densification with comparable grain sizes when compared with the conventionally fast fired samples. The effect of particle size on the microwave (hybrid) heating phenomena was found to be analogous to conventional sintering.
INTRODUCTION Microstructural benefits accruing from the rapid firing of ceramics in terms of higher densities and smaller grain sizes have been extensively studied[1-31 The higher densities and smaller grnin sizes were attributed to the rapid transition to higher temperatures which caused a suppression of surface diffusion and other 'coarsening without densiflcation' mechanisms that exist at lower temperatures[31. At higher temperatures the predominance of the mechanisms of grain boundary and lattice diffusion that cause densification. leads to rapid sintering and densification with little or no coarsening. MW sintering Is a technique that offers enormous potential for the fabrication of ceramics and ceramic composites with improved microstructures. Work in this area by a number of researchersý4-8] demonstrate the efficacy of MW sintering from the standpoint of lower sintering temperatures and smaller grain sizes compared to conventional sintering. This has been attributed to higher diffusion and a lower activation energy for sintering that is characteristic of MW energy[4.61. However. although the use of MW energy for sintering serves to accelerate the process. preheating the material to the critical temperature at which it starts to couple efficiently with the microwaves is still a problem, especially at the low frequency of 2.45 GHz. The longer time spent in heating up the sample to the critical temperature range above which MW-material Interaction and consequently, rapid heating occurs readily, as well as the higher diffusion rates that is associated with MW heating may be the reason for the appreciably higher grain sizes reported in a recent study on the 'comparison of conventionally fast fired and MW fired alumina'[ 91 In order t
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