Relation between crystal structure and lattice oxygen content of sintered reaction‐bonded silicon nitride

Abstract

When reaction‐bonded silicon nitride containing MgO/Y2O3 additives is sintered at three different temperatures to form sintered reaction‐bonded silicon nitride (SRBSN), the thermal conductivity increases with sintering temperature. The β‐Si3N4 (silicon nitride) crystals of SRBSN ceramics were synthesized and characterized to investigate the relation between the crystal structure and the lattice oxygen content. The hot‐gas extraction measurement result and the crystal structure obtained using Rietveld analysis suggested that the unit cell size of the β‐Si3N4 crystal increases with the decrease in the lattice oxygen content. This result is reasonable considering that the lattice oxygen with the smaller covalent radius substitutes nitrogen with the larger one in the β‐Si3N4 crystals. The lattice oxygen content decreased with increasing sintering temperature which also correlated with increase in thermal conductivity. Moreover, it is noteworthy from the viewpoint that it may be possible to apply the lattice constant analysis for the nondestructive and simple measurement of the lattice oxygen content that deteriorates the thermal conductivity of the β‐Si3N4 ceramics.

Effects of nitridation temperature on properties of sintered reaction‐bonded silicon nitride

“…The thermal conductivity increased with increasing the nitridation temperature. Thermal conductivity is not only affected by microstructures such as porosity and amounts, distribution and crystalline state of grain boundary, but also the lattice oxygen in Si 3 N 4 grain because an oxygen impurity dissolved in the β‐Si 3 N 4 grain creates phonon‐scattering silicon vacancies that decrease the thermal conductivity of Si 3 N 4 ceramics 8,12,14,28,32,33 . As the microstructures were composed of almost the same morphologies regardless of the nitridation temperature, the increase of thermal conductivity should be due to the influence of the amount of lattice oxygen in Si 3 N 4 .…”

“…Thermal conductivity is not only affected by microstructures such as porosity and amounts, distribution and crystalline state of grain boundary, but also the lattice oxygen in Si 3 N 4 grain because an oxygen impurity dissolved in the β-Si 3 N 4 grain creates phonon-scattering silicon vacancies that decrease the thermal conductivity of Si 3 N 4 ceramics. 8,12,14,28,32,33 As the microstructures were composed of almost the same morphologies regardless of the nitridation temperature, the increase of thermal conductivity should be due to the influence of the amount of lattice oxygen in Si 3 N 4 . Kitayama et al 32 reported that the thermal conductivity of Si 3 N 4 ceramics increased from 58.8 to 99.1 W/(m K) with decreasing the oxygen contents from .400 to .058 wt.% by increasing the amount of Y 2 O 3 additives.…”

“…One of the preparation methods for Si 3 N 4 ceramics is a sintered reaction‐bonded silicon nitride (SRBSN) technique, which consists of nitridation of high purity silicon and post‐sintering of the nitride compact 5 . Much effort has been made to improve the thermal conductivity of Si 3 N 4 ceramics, and the effects of sintering temperature, sintering time, particle size of raw materials, and sintering additives have been intensively investigated 6–13 . Zhou et al 14 .…”

“…5 Much effort has been made to improve the thermal conductivity of Si 3 N 4 ceramics, and the effects of sintering temperature, sintering time, particle size of raw materials, and sintering additives have been intensively investigated. [6][7][8][9][10][11][12][13] Zhou et al 14 prepared Si 3 N 4 ceramics with the highest thermal conductivity ever reported of 177 W/(m K) by the SRBSN method in which a silicon compact was nitrided at 1400 • С for 4 h and sintered at 1900 • С for 60 h, followed by cooling at a very low rate (.2 • С/min). As this sintering procedure could reduce the lattice oxygen content in β-Si 3 N 4 grain and the amount of grain boundary and promote the crystallization of the boundary phase, the prepared Si 3 N 4 ceramics showed the previously stated high thermal conductivity and high fracture toughness (11.2 MPa m 1/2 ).…”