Hot Isostatic Pressing to Increase Thermal Conductivity of Si 3 N 4 Ceramics
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Hot isostatic pressing to increase thermal conductivity of Si3 N4 ceramics Koji Watari, Kiyoshi Hirao, Manuel E. Brito, Motohiro Toriyama, and Shuzo Kanzaki National Industrial Research Institute of Nagoya, Hirate-cho 1-1, Kita-ku, Nagoya 462, Japan (Received 5 February 1998; accepted 21 August 1998)
Highly anisotropic Si3 N4 ceramics were successfully fabricated by tape-casting of raw a –Si3 N4 powders with b –Si3 N4 single-crystal particles as seed particles and Y2 O3 as an effective sintering aid, followed by hot isostatic pressing at a temperature of 2773 K for 2 h under a nitrogen gas pressure of 200 MPa. The microstructure consists of very large elongated grains (diameter ,10 mm; length of ,200 mm), highly oriented in the tape-casting direction. The thermal conductivity along this direction reaches 155 W m21 K21 at room temperature, but varies significantly between room temperature and 1273 K. This thermal conductivity is closely related to (1) formation of extremely large elongated b –Si3 N4 grains with a reduced amount of crystal defects due to the high-temperature firing and to (2) orientation of b –Si3 N4 grains due to addition of seed particles and to tape-casting.
I. INTRODUCTION
Thermal conductivity is an important physical property of ceramic materials. The theoretical thermal conductivity (k) of nonmetallic crystals is described using the following equation: k B?M
? d ? uD3 ysT
?g d, 2
(1)
where B is some constant, M the average mass of atoms in the crystal, d the average volume occupied by one atom of the crystal, uD the Debye temperature, T the absolute temperature, and g Gr¨uneisen’s constant.1 Slack et al. investigated the thermal conductivity of nonmetallic solids, and they clarified that diamond, BN, SiC, BeO, BP, AlN, Si, GaN, and GaP possess intrinsic thermal conductivities higher than 100 W m21 K21 .1 Slack also reported a proportionality relation between the intrinsic thermal conductivity and value of M ? d ? uD3 for a nonmetallic crystal.1 Assuming that the intrinsic thermal conductivity of Si3 N4 obeys Slack’s relation, we calculated the value of M ? d ? uD3 . The values of M, d, and uD were estimated to be 20.04 g, 1.04 3 10223 cm3 , and 1100 K, respectively.1–3 Quasi-intrinsic thermal conductivity of Si3 N4 was about 400 W m21 K21 at room temperature.4 This value, almost equal to that of BeO,1 indicates that Si3 N4 is a high thermal conductivity material. Experimentally, the thermal conductivity of Si3 N4 ceramics at room temperature is reported to be in the range of 10 –120 W m21 K21 .2,4–12 These values are strongly influenced by the quality of raw powders,2,5,7,8,12 type of sintering aids,2,5,11 forming technique,12 sintering method,2,5,6,9,10 and sintering conditions.2,4,9 But the experimental values reported are much less than the 1538
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J. Mater. Res., Vol. 14, No. 4, Apr 1999
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calculated intrinsic thermal conductivity of Si3 N4 (about 400 W m21 K21 ). The purpose of the pr
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