Thermal Transport And Mechanical Properties Of The Monazite-type Rare Earth Phosphate And Their Composites

Monazite-type rare earth phosphate (LnPO4(Ln=La-Gd)), as a class of refractory (insoluble) materials,have many excellent properties such as high temperature stability, chemical stability and low thermal conductivity. They are expected to have widely applications in thermal insulation applications, chemical protection and other fields. The investigations on the thermal transport and mechanical properties are undoubtedly of importance for their applications. However, there are few systematic investigations on these properties of the monazite-type LnPO4. Otherwise, LaPO4/Al2O3 composites, which are representative LnPO4 composites, are expected to have many applications including machinable ceramics. Comparing with traditional sintering process such as pressureless sintering and hot pressing sintering, spark plasma sintering (SPS), as a new sintering technique, may be used to prepare high-density LaPO4/Al2O3 composites with excellent properties. Base on the above analysis, the main investigations in this thesis are as follows.The thermal transport properties of the monazite-type LnPO4 (Ln=La-Gd) are systematically investigated. The results reveal that they have extraordinarily low thermal conductivities. With altering from LaPO4 to GdPO4, the thermal conductivities of all the LnPO4 ceramics decreases nearly at first, reach a minimum value, and then rise at relatively lower temperature (≤400oC). As the temperature increasing, the conductivity trends become obscure. As the temperature further increasing to above 800oC,the conductivities approach their minimum thermal conductivities. The variation mechanisms are also explained. Phonon scattering theorys are used to fit the experimental values of the conductivities, and a semiempirical equation of the intrinsic thermal conductivity is obtained for the monazite-type LnPO4.The mechanical properties of the monazite-type LnPO4 (Ln=La-Gd) are systematically investigated including the elastic constants, hardness, fracture toughness and bending strength. The results reveal that they are a class of ceramic materials with weak mechanical properties. The monazite-type LnPO4 are expected to have thermal conductivity anisotropies because of their asymmetric crystal structure. LaPO4, as a representative LnPO4, is investigated in this thesis. The samples with different texture are obtained by changing the powder synthesis processes and using hot-pressing sintering and SPS techniques. The results reveal that there are good relations between the thermal conductivity and the texture. The formation mechanisms of the texture and the thermal conductivity anisotropy are explained.The high-density LaPO4/Al2O3 composites are prepared by SPS techniques. Within the temperature range of 25-1000oC, the thermal conductivities of the composites continue decreasing with increasing LaPO4 content due to the lower thermal conductivity of LaPO4. Despite of the weak mechanical properties of LaPO4, the fracture toughness and the bending strength of the composites are successfully retained with small quantity addition of LaPO4. The composites can be machined using a tungsten carbide drill when the LaPO4 content is higher than 34.4vol.%.