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nji Furuya and Kazuo Matsuo Materials Research Laboratory, Nissan Motor Company, Ltd., 1 Natsushima-cho, Yokosuka 237-8593, Japan
Takeo Okamura Research Department, Nissan ARC Ltd., 1 Natsushima-cho, Yokosuka 237-0061, Japan
Fumio Munakata and Yoshio Akimune Materials Research Laboratory, Nissan Motor Company, Ltd., 1 Natsushima-cho, Yokosuka 237-8593, Japan (Received 18 May 1999; accepted 25 May 2000)
Two kinds of obviously different-sized –Si3N4 whiskers were grown from silicon melt with different pretreatment vacuum conditions. Their growth interface structures were studied in a cross-section view from micro-areas to macro-areas by combination of micro-area state analysis with chemical shift mapping of Si K bands using electron probe microanalysis. The one pretreated under the lower vacuum condition with a rotary pump was 10–20 m in diameter and hundreds of micrometers in length, and another pretreated under the higher vacuum condition with a diffusion pump was 0.1–0.2 mm in diameter and 2–5 mm in length. The small Si3N4 whiskers were grown from the surface of the SiC particles within the Si melt. The large Si3N4 whiskers were grown from the surface of Si3N4 crucible. On the basis of these results, their growth mechanisms are discussed from the view of the nucleation sites, impurity source, and thermodynamic stability of the SiC particles. Compared with the Si3N4 grains, the SiC particles influenced the nucleation deeply and caused the process to grow small-sized crystals. Preventing the carbon impurities into the Si melt from forming the SiC particles in the pretreatment process was one effective way to grow the large-sized –Si3N4 single crystals.
I. INTRODUCTION
–Si3N4 whiskers are commonly used for reinforcements in ceramic–matrix composites to control microstructures of ceramic materials in a sintering process.1 Using longer whiskers can make orientations of grains more favorable for enhancement of fracture toughness2 and improvement of thermal conductivity.3 Recently, we have obtained a new Si3N4 ceramic material with thermal conductivity of 162 W/(m K) at room temperature by using commercial –Si3N4 whiskers in its sintering process.4 To promote the –Si3N4-whisker reinforcements, it is necessary to grow long whiskers instead of small commercially available ones. However, few studies on a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 15, No. 8, Aug 2000
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growth of large –Si3N4 single crystals have been reported.4,5 To date the growth processes of large –Si3N4 single crystals have not been well established, and the growth mechanisms have not been understood. It is necessary to investigate the growth characteristics of various Si 3 N 4 -whisker/Si growth interfaces under different growth processes. In this study, two kinds of –Si3N4 whiskers with obviously different sizes are grown from an Si melt by using two different vacuum conditions in the pretreatment step of the growth process, which is p
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