Sintering of nanopowders of amorphous silicon nitride under ultrahigh pressure
- PDF / 369,790 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 13 Downloads / 217 Views
Xian-Feng Ma and Suo-Jing Cui Laboratory of Ultrahigh Pressure, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 159 Renming Street, Changchun 130022, People’s Republic of China (Received 17 May 1999; accepted 31 January 2000)
Nanopowders of amorphous silicon nitride were densified and sintered without additives under ultrahigh pressure (1.0–5.0 GPa) between room temperature and 1600 °C. The powders had a mean diameter of 18 nm and contained ∼5.0 wt% oxygen that came from air-exposure oxidation. Sintering results at different temperatures were characterized in terms of sintering density, hardness, phase structure, and grain size. It was observed that the nanopowders can be pressed to a high density (87%) even at room temperature under the high pressure. Bulk Si3N4 amorphous and crystalline ceramics (relative density: 95–98%) were obtained at temperatures slightly below the onset of crystallization (1000–1100 °C) and above 1420 °C, respectively. Rapid grain growth occurred during the crystallization leading to a grain size (>160 nm) almost 1 order of magnitude greater than the starting particulate diameters. With the rise of sintering temperature, a final density was reached between 1350 and 1420 °C, which seemed to be independent of the pressure applied (1.0–5.0 GPa). The densification temperature observed under the high pressure is lower by 580 °C than that by hot isostatic pressing sintering, suggesting a significantly enhanced low-temperature sintering of the nanopowders under a high external pressure.
I. INTRODUCTION
Silicon nitride ceramics (Si3N4) have superior physical, chemical, and high-temperature mechanical properties and have become one of the most studied advanced ceramics, as an important candidate for a variety of technical and engineering applications. Common dense Si3N4 polycrystalline materials are generally fabricated by liquid-phase sintering of powders with the addition of various sintering aids (such as Al2O3, Y2O3, Yb2O3, ZrO2, etc.) at high temperature (1750–2000 °C). The existence of metallic glass phases at grain boundaries resulting from the additives greatly impairs the hightemperature mechanical properties of the materials.1,2 Thus, the sintering of Si3N4 ceramics without additives is an important approach to reducing impurity phases in the sintered bodies and hence achieving Si3N4 ceramics with the intrinsic properties of the materials. Studies have
a)
Address all correspondence to this author. Present address: National Institute for Research in Inorganic Materials, Research Center for Advanced Materials, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. e-mail: [email protected]
988
http://journals.cambridge.org
J. Mater. Res., Vol. 15, No. 4, Apr 2000 Downloaded: 18 Mar 2015
shown that Si3N4 ceramics free of additives that were densified under high pressure exhibited improved mechanical and high temperature compared to those sintered with additives.3 Si3N4 ceramics cannot be densified in their “pure” state using traditional sintering routes, owing to their s
Data Loading...
