HPHT Synthesis And TE Properties Of Na-filling Type And Fe Substituting Type CoSb₃ Skutterudites TE Materials

Thermoelectric (TE) materials are a functional material which can convert heat and electricity directly. TE devices based on TE materials could convert heat to electricity or use electricity to pump heat from cold to hot, both without any moving parts or bulk fluids. They are lightweight, small, portable, inexpensive, quiet performance and the ability for localized'spot'cooling. Thus the current research on TE materials is focused to find new materials or to improve the properties of TE materials.The efficiency of a TE device depends on the TE material. The efficiency of a TE materials can be defined by the dimensionless thermoelectric figure of merit, ZT, where T is the absolute temperature and (S is the thermopower,σis the electrical conductivity andκis the thermal conductivity). In theory, the ZT value can attain infinite. However, the best TE materials available today such as Bi2Te3,PbTe,SiGe for devices have a ZT of about 1, which is just about 10% of Carnot efficiency. 30% of Carnot efficiency (comparable to home refrigeration) could be reached by a device with a ZT of 4. ZT value of 3 or 4 is likely necessary for practical thermoelectric generators, although more modest increases in ZT would still have a substantial impact on applications.Doped materials can improve the performance of TE materials through introducing extra phonon scatter. Skutterudite compounds exhibit a wide range of electrical and thermal transport phenomenon by the phonon-glass and electron-crystal (PGEC) approach, they have high thermo-power and high charge-carrier mobility, so they become fascinating family of materials continuously. CoSb₃ and its related skutterudite compounds are expected to be the most promising TE materials.About high pressure and high temperature synthesized TE materials, Doctor Zhu first prepared TE materials PbTe used cubic multi-anvil high pressure apparatus. PbTe samples which have NaCl construction were synthesized under pressure ranging 3.0-5.0GPa and temperature 900-1000℃and time ranging 10-30 minute. His results show that the thermoelectric properties of PbTe prepared by HPHT are higher dramatically than that synthesis at normal press. He contributed the enhancing to the ETT for PbTe which changing at high press. Doctor Guozhong Ren validated the resultthrough the In-situ measurement at HPHT. In this work, we tried to study the performance of Skutterudite compounds. Firstly, we prepare (NaN₃)_χCo₄Sb₁₂ samples filled with NaN3 using the HPHT method, at P=1.5-3 GPa, and 900K. All the samples are single phase with Im3 strcture through the test of X-Ray, and the filled CoSb₃ Skutterudite compounds are n-type semiconductors.We test the electrical resistivity and Seebeck coefficient of (NaN₃)_χCo₄Sb₁₂ samples at room temperature. The result shows that the absolute value of Seebeck coefficient and electrical resistiance of (NaN₃)_χCo₄Sb₁₂ samples decrease with increasing Na content. The power factor of samples increase firstly and then decrease with increasing Na content, and the obtained maximum power factor reaches 14.28·Wcm^-1κ^-2 when Na content is 40%.On the base of above research, we select Fe as impurity element to study Fe doped CoSb₃ Skutterudite compounds Fe_χCo_4-χSb₁₂ at P=1.5GPa and 900K. All the samples are single phase with Im3 strcture through the test of X-Ray, and the doped CoSb₃ Skutterudite compounds are p-type semiconductors. The results of electrical resistivity and Seebeck coefficient test indicate that with temperature increasing the electrical resistivity increases, the Seebeck coefficient firstly increases and then decreases. Compared with the samples at ambient pressure, HPHT method could improve the Seebeck coefficient and electrical resistivity effectively.The thermal conductivities of Fe_0.6Co_3.4Sb₁₂ and CoSb₃ decrease with an increase of temperature, which can be contributed to the intensified phonon scattering as the temperature increasing. The room temperature thermal conductivity of Fe_0.6Co_3.4Sb₁₂ (6.15 W/mK) is lower than that of CoSb₃ (6.8 W/mK), however, the high temperature (>500 K) thermal conductivity of Fe_0.6Co_3.4Sb₁₂ is higher than that of CoSb₃, which indicates that the thermal conductivity can not be effectively reduced only by Fe substituting doping.We calculates the figure of merit(ZT) for doped Fe samples Fe_0.6Co_3.4Sb₁₂ and CoSb₃ from the measured quantities, The ZT values for both the samples firstly increase and then decrease with increasing temperature. At about 550K, Fe_0.6Co_3.4Sb₁₂ sample obtains the maximal ZT value (0.09), which is smaller than that of undoped CoSb₃ (0.17). It is related to the large electrical resistivity for Fe0.6Co3.4Sb12 compound.In conclusion, the HPHT method can effectively enhance the electrical transport properties of CoSb₃-based Skutterudite compounds, but can not improve the thermal conductivity effectively. The double-filled will be an effective way to improve the figure of merit of CoSb₃.