Two kinds of roles of MgO in the densification and grain growth of alumina under various atmospheres: Sensitive and inse
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Two kinds of roles of MgO in the densification and grain growth of alumina under various atmospheres: Sensitive and insensitive roles to the experimental procedures Takayasu Ikegami National Institute for Research in Inorganic Materials 1-1, Namiki, Tsukuba-shi, Ibaraki, 305 Japan
Katsuya Eguchi Dow Corning Asia Ltd., Research Center, 603, Kishi, Yamakita-machi Ashigarakami-gun, Kanagawa 258-01 Japan (Received 20 February 1998; accepted 14 May 1998)
Two kinds of high-purity Al2 O3 powder were studied with respect to the effects of not only MgO doping but also atmosphere on both densification and grain growth. Controversial results for MgO doping are explained in terms of two roles that MgO can play during these processes: sensitive and insensitive to the experimental procedures. Atmospheres, whether dry or wet, had little influence on densification or grain growth in the early stage. After closed pores appreciably formed, however, both N2 and Ar atmospheres quickly reduced the densification rate.
I. INTRODUCTION
The first translucent ceramic, named LUCALOX, was developed in 1959 by Coble,1 who sintered highpurity Al2 O3 with a small amount of MgO in an H2 atmosphere and in vacuum. This new material introduced a succession of studies2–6 on the role of MgO in the densification of Al2 O3 . Fundamental information on the fabrication of transparent ceramics7–9 has been derived from those studies.1–6 Reported data usually have shown that the density of the sintered body is larger for MgOdoped Al2 O3 than for pure Al2 O3 ; that is, the dopant effect of MgO10 is insensitive to such experimental procedures as the fabrication history of the Al2 O3 powder, the amount of MgO, or the sintering temperature. Kinetic studies,2–6 however, have caused serious controversy11 over whether the effect of MgO on a densification rate or a grain growth rate in Al2 O3 depends on its fabrication history. A previous study10 suggested two kinds of roles for MgO, that is, sensitive and insensitive to the experimental procedures. The present study was made to verify qualitatively those roles, especially the latter one. II. EXPERIMENTAL PROCEDURES A. Specimen preparation
Two high-purity Al2 O3 powders, designated S-1 and S-2 (S-2: AKP-30, S-2: AKP-15, Sumitomo Chemical Industries, Ltd., Osaka, Japan), were provided for the present study. Characterization of these powders by the supplier yielded the following results: major impurities (in ppm by weight) of Si 18, Na 2, Mg 3, Cu , 1, Fe 11 for S-1 and Si 22, Na 2, J. Mater. Res., Vol. 14, No. 2, Feb 1999
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Mg 4, Cu 1, Fe 11 for S-2; light and heavy tapping densities for S-1 and S-2 of 1.0 and 1.2 gycc, respectively. Figure 1 shows the particle size distributions of both S-1 and S-2 as determined by water sedimentation, which gave the average radii, rs , as 0.40 and 0.68 mm, respectively. The specific surface areas, calculated by the BET method using nitrogen at 2196 ±C, were 7.1
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