Thermal Effects During Microwave Plasma Sintering of Ceramics
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THERMAL EFFECTS DURING MICROWAVE PLASMA SINTERING OF CERAMICS MWRY P. SWEENEY, AND D. LYNN JOHNSON MATTHEW HSU, of Materials Science Northwestern University, Dept. Evanston, IL 60208-3108
and
Engineering,
ABSTRACT A variety of ceramic materials have been heated in various microwave excited plasmas at reduced pressures. Temperatures were measured by Specimen heating was accomplished by direct optical fiber thermometry. contact with the plasma, as well as direct coupling with the microwave Higher energy, although the latter was screened heavily by the plasma. specimen temperatures were achieved in polyatomic gases which have greater reactional enthalpy. In general, insulating materials achieved higher temperatures than semiconducting or covalently bonded materials.
INTRODUCTION The history of microwave plasma sintering began more than twenty years ago when Bennett et al. sintered A12 0 3 and a variety of other oxides in a 2.45 GHz system.(l,2] Their apparatus consisted simply of a quartz or alumina tube inserted through the broad wall of a rectangular waveguide fed by a coaxial line and terminated by a tuning stub. They observed a remarkable enhancement in the densification kinetics of Linde A when compared with conventional sintering at the same temperatures.i[] They further characterized microwave plasma sintering by partially sintering specimens in a plasma, and continuing the sintering in a conventional furnace at the same temperature. For instance, a specimen sintered in the plasma for 20 min at 1300 C reached 74.4% of theoretical density. Upon further heating at that temperature for 100 min in a conventional furnace, this density increased to 74.7%. Returning the specimen to the plasma for an additional 100 min at the same temperature caused the density to increase to 83.1%, which was only slightly above the density achieved on other specimens sintered for 120 min without interruption in the plasma. A significant enhancement in densification rate in the plasma was thus clearly demonstrated. After successfully sintering 0'-alumina and a-alumina in an induction coupled plasma (ICP), we turned our attention to microwave plasma sintering of alumina. [3-5] An 11 or 15 mm diameter quartz tube was inserted through a 12 mm high waveguide at a position 1/4 wavelength from a terminal short circuit. Rod specimens 3 and 4 mm diameter were translated at rates ranging from 5 to 109 mm/min. Some specimens were also sintered statically for times up to 10 min. Most of the effort was directed towards sintering pure and magnesiadoped alumina in nitrogen. In addition, magnesia was sintered in nitrogen, CO2 and H2 0 plasmas.[6] High density, high densification rates, and small grain size were observed, particularly in the case of MgO-doped alumina. The density of 3 mm diameter MgO-doped alumina rods was essentially independent of translation rate from 5 to 109 mm/mmn at ?99% (see Fig. 1). Average linear shrinkage rates at the highest velocities approached 3%/s; maximum rates, although not measured, would be greater. Grain sizes
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