Effect of rare-earth oxide addition on the thermal conductivity of sintered aluminium nitride

Abstract

The densification of A1N ceramics and the improvement of its thermal conductivity are typically achieved by the addition of a small amount of oxides. The effects of oxide addition on the density and thermal conductivity of sintered A1N were investigated by Tsuge and co-workers [1, 2]. They found that the alkaliand rare-earth oxides (Re203) were useful additives for the fabrication of fully densified and highly thermally conductive A1N; Y203 is an especially good sintering additive. In the A1N sintering process, Y203 additive reacts with the surface oxide layer of A1N particles, forms the liquid phase of the Y A I O N system, causes the precipitation of yttrium-aluminium-oxide phases such as Y4A1209 (2Y203.A1203) in grain boundaries and lowers the oxygen content dissolved in A1N grains, and then the thermal conductivity of sintered A1N increases [2]. However, the effects of Re203 addition other than Y203 on the thermal conductivity of A1N have not been studied in depth. Recently A1N sintering with Y203 additive under reducing nitrogen gas atmosphere was reported to be a useful method to obtain sintered bodies with high thermal conductivity (>150 W m -a K -1) [3-7]. In this study the effects of Re203 addition on the thermal conductivity of A1N specimens sintered in reducing nitrogen atmosphere were investigated. A commercial A1N powder (Tokuyama Soda Co., Tokyo) with an average particle size of 0.6/xm and a specific surface area of 3.2 m 2 g-1 was used in this research. According to the manufacturer, this powder contains 1 wt % oxygen, 220 p.p.m, carbon and trace amounts of iron, calcium and silicon. In this work ND203, Sm203, Gd203, Dy203, Er203 and Yb20 3 powders (Hokko Chemicals Co., Tokyo) were used as the Re203 additives. I mol % of each of the rare-earth oxide powders as sintering aids were added, respectively, to the raw A1N powder. These powders were mixed in ethanol in a ball mill. After drying, the mixed powder was formed into pellets 14 mm in diameter and 8 mm high using a stainless steel die, and then cold isostatically pressed (CIPed) under 200 MPa pressure for 60 s. The CIPed pellets were placed in a graphite crucible, and then sintered using a graphite heater furnance at temperatures of 1773, 1873, 1973, 2073 and 2173 K for 1 and 3 h under 0.1 MPa nitrogen atmosphere. Furthermore, the CIPed samples were sintered in a graphite crucible with A1N packing powder in order to investigate the effect of the sintering atmosphere. The heating rate was 0.17 Ks Gd203 > Er203 > Yb203 >Nd203 >Sm203 addition. On the other hand, in sintering with packing powder, the thermal conductivity of A1N specimens with Re203 were about 95 W m -1K -1. The maximum thermal conductivity of A1N ceramics obtained in this work was 195 W m -1K -a for Dy203 addition. The results for the oxygen content of sintered A1N with the Re203 are shown in Fig. 2. The oxygen content of A1N specimens sintered without packing