Microwave sintering of ZnO at ultra high heating rates
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Isabel K. Lloyd Department of Materials and Nuclear Engineering, and Institute for Plasma Research, University of Maryland, College Park, Maryland 20742
Yuval Carmel and Tayo Olorunyolemi Institute for Plasma Research, University of Maryland, College Park, Maryland 20742
Otto C. Wilson, Jr. Department of Materials and Nuclear Engineering, and Institute for Plasma Research, University of Maryland, College Park, Maryland 20742 (Received 12 January 2001; accepted 22 July 2001)
In this paper, a unique processing approach for producing a tailored, externally controlled microstructure in zinc oxide using very high heating rates (to 4900 °C/min) in a microwave environment is discussed. Detailed data on the densification, grain growth, and grain size uniformity as a function of heating rate are presented. With increasing heating rate, the grain size decreased while grain size uniformity increased. At extremely high heating rates, high density can be achieved with almost complete suppression of grain growth. Ultrarapid microwave heating of ZnO also enhanced densification rates by up to 4 orders of magnitude compared to slow microwave heating. The results indicate that the densification mechanisms are different for slow and rapid heating rates. Since the mechanical, thermal, dielectric, and optical properties of ceramics depend on microstructure, ultrarapid heating may lead to advanced ceramics with tailored microstructure and enhanced properties.
I. INTRODUCTION
Microstructure is one of the most important factors affecting the properties of ceramics. Density, grain size, pore size, and defects are microstructural characteristics that typically influence behavior. It is generally accepted that a critical issue in microstructure development is the interplay between densification and coarsening. To control microstructure development, parameters such as sintering temperature, sintering time, and heating rate must be optimized and the relationship among densification, grain growth, and pore removal must be understood. Heating rate influences microstructure evolution and densification and thus acts as an important parameter in microstructure control. Rapid thermal heating, also called fast firing, has been reported to produce beneficial effects such as maintaining relatively fine microstructures compared to slow heating for similar densities.1–3 It is usually performed by the insertion and soaking of a specimen in a preheated high temperature furnace. Brook1 attributed the superior densification associated with fast firing to rapidly reaching the high-temperature region where densification mechanisms predominate over coarsening mechanisms. Kim et al.4 pointed out that the advantages of fast firing 2850
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J. Mater. Res., Vol. 16, No. 10, Oct 2001 Downloaded: 05 Oct 2015
include not only finer grain size for a given sintering temperature but also smaller pore size which results from restricted pore growth. Johnson5 modeled the effects of heating rate on initial stage sintering of silver and found
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