Abstract
New sintering aids based on ytterbia-alumina system were developed for the sintering of silicon nitride with 28GHz millimeter-wave (mm-wave) heating. Sinterability was examined in silicon nitrides with 5wt%Yb2O3-3wt%Al2O3 (5Yb-3Al) and 5wt%Y2O3-3wt%Al2O3 (5Y-3Al). It was found that fully densified silicon nitride with 5Yb-3Al aid was obtained at 1600°C in mm-wave heating, and 1850°C in conventional heating. On the other hand, nearly same densified silicon nitride with 5Y-3Al aid was obtained at 1700°C in mm-wave heating, and 1750°C in conventional heating. For the silicon nitride with ytterbia-alumina system, it was indicated that their densification temperatures strongly depended on the ratio of Yb2O3 to Al2O3. In the mm-wave sintering, the temperature necessary for densifying up to 96% theoretical density showed a quite similar tendency to the liquidus temperature in the ytterbia-alumina system. While, the densification temperatures for the same silicon nitrides in the conventional sintering were rather higher than those in the mm-wave sintering and monotonously decreased with increasing the content of alumina in the aid. Because of lower sintering temperatures in the mm-wave heating, finer microstructures were obtained in the mm-wave sintered silicon nitrides, compared with those sintered by the conventional heating.
