Effective Sintering Aids for Low-temperature Sintering of AlN Ceramics
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Effective sintering aids for low-temperature sintering of AlN ceramics Koji Watari,a) Hae J. Hwang, Motohiro Toriyama, and Shuzo Kanzaki National Industrial Research Institute of Nagoya, Hirate-cho, Kita-ku, Nagoya 462 Japan (Received 5 January 1998; accepted 5 September 1998)
A disappearing sintering aid was used to promote densification during the initial and middle stages of sintering and to be removed in gaseous form from the specimens during the final stage of sintering. From thermodynamic consideration such as assessment of Gibbs free energy change of formation of Al2 O3 compounds including metal-oxide and evaluation of the vapor pressure of metal-oxide, Li2 O is expected to become a disappearing sintering aid for AlN sintering. Doping with Li2 O resulted in densification of AlN ceramics with Y2 O3 and CaO additives by sintering at a firing temperature of 1600 ±C. The amount of Li2 O in the specimens decreased by volatilization at temperatures higher than 1300 ±C, and its amount was at a level of several ppm after firing at 1600 ±C for 6 h. Low-temperature densification of AlN specimens by addition of Li2 O also caused the improvement of thermal conductivity and mechanical strength of sintered specimens. Present results indicate that a Li2 O addition is effective for AlN sintering. Furthermore, LiYO2 was also used as a new sintering aid instead of Li2 O and Y2 O3 , and the results of thermal conductivity and mechanical strength are shown.
I. INTRODUCTION
Since the type and amount of sintering aids strongly relates to densification rate, microstructural development, and thermal, mechanical, and electrical properties of ceramic materials, there is considerable effort to find and optimize sintering aids for high performance ceramic materials. After sintering, the distribution of sintering additives in ceramic materials is divided into three categories, as seen in Fig. 1: sintering aids (a) exist at grain boundaries, (b) are incorporated into the ceramic grains, or (c) vaporize. Most sintering aids remain as an intergranular crystalline or glassy phase in the sintered materials, as shown in Fig. 1(a). The chemical composition and amount of grain boundary phase frequently control mechanical,1,2 electrical,3 thermal, and chemical properties4 of ceramic materials. On the other hand, certain sintering aids promote densification at the initial and middle stages of sintering, and subsequently are incorporated into ceramic grains [see Fig. 1(b)]. In this case, it is possible to decrease the amount of the grain boundary phase. Significant decrease in the quantity of the grain boundary phase in sintered materials has been found in sintering of silicon nitride solid solutions (sialon), which exhibited excellent mechanical properties at elevated temperatures.5,6 In this work, we concentrate on using a disappearing sintering aid to enhance densification during the initial and middle stages of sintering. This sintering aid differs from the previous two examples in that it can be
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