Silicon carbide (SiC) has very wide applications (e.g. heat exchangers, mechanical seals, cladding material, etc.) due to its high thermal conductivity, excellent high temperature strength, good oxidation resistance, and high radiation resistance. For those applications, thermal conductivity of SiC ceramics is an important property which however varies from 32 W/m/K to 490 W/m/K depending on microstructure. To further understand the effects of fabrication conditions on microstructure of SiC ceramics and dependence of thermal conductivity on microstructure, SiC ceramics were sintered by spark plasma sintering (SPS) with two different liquid phase sintering additives. Aside from that, SiC ceramics without sintering additive were also fabricated by SPS and subsequently irradiated by proton at 340 degree Celsius and different damage levels. Microstructure evolution and thermal conductivity degradation of the irradiated SiC were investigated and correlated. In the SiC sintered with 3-10 wt.% Al2O3-Y2O3, an increase in the sintering additive content results in decrease in grain size and thermal conductivity of the SiC. Lower thermal conductivity of the SiC ceramic with higher sintering additive content is mainly due to smaller grain size rather than low intrinsic thermal conductivity of secondary phase. For the SiC sintered with same content of sintering additive and different holding time, increase in holding time has little influence on grain size but results in formation of continuous network of sintering additive in the SiC. Such continuous network of the sintering additive leads to increase of interfacial thermal resistance and thus decreases thermal conductivity. In the SiC sintered with Y2O3-Sc2O3 at 1750-1850 degree Celsius, the dominant grain growth mechanism changes from interface reaction at 1750 degree Celsius to atom diffusion at 1850 degree Celsius. Moreover, grain growth of the SiC sintered with Y2O3-Sc2O3 not only reduces the number of grain boundary per unit volume but also results in lattice purification originating from removing oxygen impurity of starting SiC powders by Y2O3-Sc2O3, which is suggested to be responsible for higher thermal conductivity of SiC ceramics with a larger grain size. In the SiC sintered without sintering additive, the unit cell volume expansion and significant thermal conductivity reduction are observed after receiving proton irradiation at different damage level, which have been correlated with point defects and interstitial clusters induced by proton irradiation. It is suggested that interstitial-type defects make dominant contribution to unit cell volume expansion while vacancy-type defects and interstitial clusters are responsible for significant thermal conductivity degradation. Furthermore, higher damage level leads to higher volume expansion and lower thermal conductivity but variation extent of volume expansion and thermal conductivity at high damage level are smaller than that at low damage level, indicating more contribution from interstitial clusters at high damage level.
|Date of Award||1 Aug 2021|
- The University of Manchester
|Supervisor||Philip Withers (Supervisor) & Ping Xiao (Supervisor)|