Study On Liquid Phase Sintering And Dielectric Property Of α-Si₃N₄ Ceramic

As one of the novel techniques for fabricating materials, Spark Plasma Sintering (SPS) could finish rapid sintering for diverse materials and preserve the characteristic to the best extent. In light of this advantage, in this work, a series of α-Si₃N₄ ceramics were prepared below 1500℃ with MgO, MgO&SiO₂, MgO&Al₂O₃, MgO&AlPO₄ as the sintering aid respectively. Phase composition, microstructure, mechanical property and dielectric performance were explored, and the emphasis was focused mainly on the liquid sintering process and densification mechanism of α-Si₃N₄, and the dependence of dielectric property on density.It is found that MgO can effectively help to the densification of α-Si₃N₄ at 1500 ℃, with 4wt% of MgO, the relative density reaches 96%, but the starting sintering temperature is as high as 1350℃ with MgO&SiO₂ as mixed sintering aid, the starting sintering temperature is reduced to 1150℃; when using MgO& Al₂O₃ as the sintering aid, at 1450℃ , the relative density could be above 96% and the starting sintering temperature is around 1200℃. the densification process finishes between 1400-1450℃. the mixed sintering aid MgO&AlPO₄ not only brings down the starting sintering temperature (¹100℃), but also enables sintering to be conducted in a wider temperature range, and the relative density could be tuned by sintering condition and content of AlPO₄.XRD(X-Ray Diffraction) and SEM(Scanning Electron Microscopy) indicate that the major phase is α-Si₃N₄ and grain fails to grow any larger, which prove that SPS helps to prevent the grain from growing, preserve the main phase and realize the downsize of grain.In SPS process, the densification of Si₃N₄ is caused by the realigning of grains, which could be cut into the following steps: the initial realigning, the extensive realigning and elimination of closed pores. The liquid viscosity flowing, the quick decrease of liquid viscosity, the slip of grain boundary and vanish of pores are main contributors to densification, whereas the role of each of them differs in each step.The mechanical property relies on the relative density. The mixed sintering aid MgO&AlPO₄ can enhance the strength of sintered samples with a low density. Although the relative density is as low as 63.61%, the bending strength stays above 89MPa.The dielectric constant ranges 4.5⁸.5. As long as the density is high to certain extent(e.g. 93%), the dielectric constant tends to certain value and the species of sintering aid and its content difference influences little on the dielectric constant. The dielectric loss varies from 0.001 to 0.03, which is sensitive to the relative density, sintering aid and its content. The addition of AlPO₄ remarkably reduces the dielectric loss of the sintered sample.