2019 Volume 127 Issue 3 Pages 136-142
The effects of the size of Si powder on the thermal conductivity and strength of sintered reaction-bonded silicon nitride (SRBSN) were investigated. Si powders with various sizes were prepared by controlling the high-energy milling duration of the starting Si powder, where the median Si size varied from 3.97 to 0.62 µm, corresponding to an oxygen content of 0.54 to 2.57 wt %. The thermal conductivity increased gradually up to the 4 h milling (D50 = 1.10 µm or 1.60 wt % oxygen) owing to a gain in the relative density, but decreased by 25% for the 12 h milling (D50 = 0.62 µm or 2.57 wt % oxygen). On the other hand, the flexural strength showed almost no change until the 4 h milling, but increased by 20% for 12 h milling. This opposite tendency was elucidated by the difference in grain size distribution and thickness of intergranular grain boundary phase (IGP); Clear bimodal size distribution of grains contributed to high strength, and the nearly doubled thickness of IGP was observed for SRBSN from the finest Si powder (D50 = 0.62 µm or 2.57 wt % oxygen). In order to meet the required thermal properties for the substrate usages by sintering at 1900°C, the size of silicon powder of D50 = 1.10 µm (1.60 wt % oxygen) is allowed for 6 h sintering, but that of D50 = 0.62 µm (2.57 wt % oxygen) needs prolonged sintering time over 6 h (12 h in the current experiment) to reach required grain growth.