Formation of Al 2 O 3 grains with different sizes and morphologies during the pulse electric current sintering process
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T. Hirai Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Jingkun Guo Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China (Received 7 April 2001; accepted 25 September 2001)
Commercial micrometer Al2O3 powder was sintered at 1550 °C under a mechanical pressure of 30 MPa by pulse electric current sintering (PECS). Microstructure observation was performed on polished, thermal-etched cross sections parallel to the direction of mechanical pressure. Platelike Al2O3 grains formed when the powder was heated at a heating rate of 5 °C/min, while a heating rate of 200 °C/min resulted in equiaxed grains. These results indicated that PECS is an effective approach to hinder grain growth by application of a higher heating rate. However, Al2O3 grains at the upper edge were larger than those at the side edge of the samples in both cases. It implied that there were different temperatures at the upper edge and the side edge of the Al2O3 powder compacts during the PECS process.
Recently, interest has been growing in the use of a pulse electric current sintering (PECS) technique to sinter ceramics and ceramic composites.1–10 The apparatuses used for this technique are a plasma-activated sintering (PAS) system and a spark plasma sintering (SPS) system, which were developed in 1988 and 1990, respectively.11 PECS is similar to the conventional hotpressing, but the graphite mold and punches are themselves heating elements because a pulse electric current and/or a direct current are directly conducted to the mold. Details of PECS apparatus can be found elsewhere.12 It was reported that additive-free AlN (aluminum nitride) powder with a particle size of 0.44 m was sintered to a near-theoretical density and an average grain size of 0.77 m in 5 min (min) by PAS.1 Similarly, with 15 min by SPS, fabrication of silicon nitride nanoceramics4 and nanograined ZrO2-based composites10 was achieved. A nanostructure in alumina oxide was also retained by PAS.13 The potential of PECS for rapid sintering (by application of a higher heating rate) was clearly illustrated by the above studies. Furthermore, its use has advantage in the fabrication of large-sized products14 or those with complicated shapes.15 However, attention has not been given to the extent to which a homogeneous microstructure can be resulted in samples prepared by PAS or SPS. In our previous reports,16,17 microstructure inhomogeneity appeared in the Al2O3 sintered bodies by 3514
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J. Mater. Res., Vol. 16, No. 12, Dec 2001 Downloaded: 03 Mar 2015
PAS and/or SPS. Namely, the edge was denser than the inside of the samples. The aim of the present research is to clarify the homogeneity issue by examining grain size and morphology in Al2O3 ceramics heated at different heating rates. Al2O3 powder (CB-A05S, Showa Denko, Japan) with an average particle size of 3.46 m and a narrow size distribution was used as a starting material. The powder contains 0.37 wt% SiO2, 0.03 wt% Na2O and 0.02 wt% Fe2O3. Six g
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