Enhanced crystallization and phase transformation of amorphous silicon nitride under high pressure
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Xian-Feng Ma and Suo-Jing Cui Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 159 Renming Street, Changchun 130022, People’s Republic of China
Liling Sun Institute of Physics, Chinese Academy of Sciences, Bejing 100080, People’s Republic of China (Received 14 April 2000; accepted 11 September 2000)
The crystallization and phase transformation of amorphous Si3N4 ceramics under high pressure (1.0–5.0 GPa) between 800 and 1700 °C were investigated. A greatly enhanced crystallization and ␣– transformation of the amorphous Si3N4 ceramics were evident under the high pressure, as characterized by that, at 5.0 GPa, the amorphous Si3N4 began to crystallize at a temperature as low as 1000 °C (to transform to ␣ modification). The subsequent ␣– transformation occurred completed between 1350 and 1420 °C after only 20 min of pressing at 5.0 GPa. In contrast, under 0.1 MPa N2, the identical amorphous materials were stable up to 1400 °C without detectable crystallization, and only a small amount of ␣ phase was detected at 1500 °C. The crystallization temperature and the ␣– transformation temperatures are reduced by 200–350 °C compared to that at normal pressure. The enhanced phase transformations of the amorphous Si3N4 were discussed on the basis of thermodynamic and kinetic consideration of the effects of pressure on nucleation and growth.
I. INTRODUCTION
Si3N4 materials have been known for long appearing in two kinds of modifications: the low-temperature type ␣ structure (space group P31c) and the high-temperature form  phase (P63/m).1– 4 The two phases generally coexist at ambient condition in synthesized ceramics (sintered ceramics or powders). At high temperature, generally above 1600–1700 °C, the ␣ phase transforms to the  modification (which is usually assisted by impurities),5,6 but the reverse transformation from  to ␣ has never been observed. There was some controversy if the ␣ phase is thermodynamically metastable with respect to the  phase at low temperature7 or has an oxygen stabilized and defected structure (proposed structure, Si11.5N15O0.5; oxygen, 0.9–1.48 wt%).8–11 Since ␣/ transformation is involved during liquidphase sintering of ␣–Si3N4 which contributes greatly to the densification and microstructure of the final ceramics,12,13 considerable work has been done in the past on a)
Present address: National Institute for Research in Inorganic Materials, Research Center for Advanced Materials, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 1, Jan 2001
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the ␣/ transformations, in particular, in the system containing oxides (which are generally used as sintering aids; e.g., Y2O3, Al2O3, Yb2O3, ZrO2, etc.). It is generally agreed that ␣/ transformation of Si3N4 proceeds by a solution-precipitation process which is assisted by the liquid phase that presents in the systems.12–15 Under such conditions, ␣/ transformation occurred between 1500 and 1800 °C depending on the type
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