In situ monitoring of the sintering behavior of microcomposite particles using a laser scanning micrometer
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In situ monitoring of the sintering behavior of microcomposite particles using a laser scanning micrometer Z. Chen, S-F. Chen, R. A. Overfelt, and M. F. Rose Space Power Institute, 231 Leach Science Center, Auburn, Alabama 36849 (Received 30 May 1997; accepted 13 November 1997)
The densification behavior of silica-coated alumina particles was investigated during sintering using a laser scanning micrometer. Traditional dilatometric techniques require contact between a push-rod and the sample under study and thus place the sample under stress during the test. However, the utilization of a noncontact laser micrometer to measure dimensional changes during sintering enabled the densification behavior to be very accurately characterized under a stress-free condition. Thus higher temperature experiments, where densification rates are particularly temperature sensitive and the samples are especially soft, can be reliably investigated without the disturbing influence of an external force. The present paper describes an application of the technique to evaluate the densification behavior from 900–1300 ±C of silica-coated alumina microcomposite particles used for the fabrication of mullite.
I. INTRODUCTION
Isothermal studies of the sintering behavior of ceramics exhibit serious limitations, such as being unable to monitor the sintering behavior before thermal equilibrium is established. This has resulted in the development of constant rate of heating (CRH) studies.1,2 A constant rate of heating allows monitoring the sintering behavior of the material through the entire thermal cycle3 and approaches the industrial sintering procedures utilized most often in production kilns. Silica-coated alumina microcomposite particles are used for the fabrication of mullite due to the easy densification of the material at relatively low temperatures (1200–1300 ±C).4 The densification rate of silica-coated alumina particles changes rapidly when the temperature approaches 1200 ±C. The standard dilatometric techniques require mechanical contact with the specimens in order to measure the sample shrinkage. These mechanical contacts induce small but finite stresses that are particularly deleterious due to the viscous flow that can occur in the samples at temperatures above 1200 ±C. For example, when a glass powder compact sample is sintered at its glass flow point, the glass viscosity is about 1022 MPa s. Thus, when a 0.1 g force is applied to a cylindrical sample surface 10 mm in diameter, the calculated longitudinal deformation rate is 0.0004 s21 (assuming viscous flow occurring at constant volume). For samples 10 mm long and loaded as above for 2 min, the longitudinal deformation caused by the mechanical contact will be approximately 0.48 mm. A small contact force on a sample during liquid phase sintering can be a significant factor which influences the measured sintering data. Analysis of the sintering kinetics is desirable in this temperature range 2202
J. Mater. Res., Vol. 13, No. 8, Aug 1998
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