Preparation And Thermoelectric Properties Of P-type Filled-skutterudites/GaSb Nanocomposites

In this dissertation we concentrate on the p-type filled skutt erudites. First, polycrystalline nanocomposites CeFe4Sb12/xGaSb with the in situ formed nanostructured GaSb second phase have been prepared by a melt-quench-anneal-spark plasma sintering method.Thermoelectric properties of the CeFe4Sb12/mGaSb compounds were greatly enhanced due to the carrier energy filtering effect and phonon energy filtering effect introduced by the GaSb nano-grains, which is evenly distributed on the boundaries of the skutterudite matrix according to the FESEM results. On the basis of this work, to obtain nanostructured skutterudites matrix, CeyFe4-xCoxSb12+z/mGaSb (0≤x≤1,0.6≤y≤1,0≤z≤0.6; m=0.2,0.4,0.6) have been prepared by melt-spinning (MS) method combined with spark plasma sintering (SPS). According to the results of the phase composition, microstructure and thermoelectric properties, we discus the status and distribution of the Ga element, analyse the characteristics of the microstructure and investivigate the correlation between the microstructue and thermoelectric properties and their dependence on the GaSb content. The main results are listed as follows:(1)In the whole process of the preparation the specimens, the Ga element mainly existis in the form of GaSb compound. The average scale of the GaSb nano-grains gradually increased from 100 nm to 500 nm with the increasing GaSb content.Compared to CeFe4Sb12, the power factor of CeFe4Sb12/0.2GaSb was remarkably improved,which was attributed to the energy filtering effect. Meanwhile, the lattice thermal conductivity of the CeFe4Sb12/0.2GaSb is significantly suppressed due to the phonon filtering caused by the in situ GaSb nano-phase. Hence, the thermoelectric performance of CeFe4Sb12/0.2GaSb is greatly improved, and the figure of merit reach about 0.96 at 800K.(2)The XRD results showed that the lattice constant of MS-SPS CeFe4-xCoxSb12 decreased linearly with the increase of Co content. Electrical conductivity and thermal conductivity of p-type CeFe4-xCoxSb12 compounds also decreased with the increasing Co content. With moderate electrical conductivity, Seebeck coefficient and low thermal conductivity, CeFe3.5Co0.5Sb12 possess the largest ZT value between 725K-800K.(3)The high angle XRD peaks CeyFe3.5Co0.5Sb12 cubic lattice shift to high angle with the decreasing of Ce concentration. CeyFe3.5Co0.5Sb12 compounds showed metallic conduction characteristics and electrical conductivity increases with the decreasing of Ce filling fraction. The Seebeck coefficients of CeyFe3.5Coo.5Sbi2 are positive, indicating the main carriers are hole. The predominant mass fluctuation scattering is not between Fe and Co but between Ce and□, which is a very strong effect since the mass difference is 100%. Ce0.9Fe3.5Co0.5Sb12 reaches the largest figure of merit 0.87 at 750K.(4)The electrical conductivities of the MS-SPS Ce0.9Fe3.5Co0.5Sb12+z (0≤z≤0.6) decreased with the increasing of the temperature, indicating a metallic conduction behavior. Although to some extent the electrical conductivities of the Ceo.9Fe3.5Coo.5Sb12+z samples can be increased by additional antimony, the Seebeck coefficients were deteriorated due to the mixed conduction mechanismand the thermal conductivities increased.(5)On the CeFe3.5Coo.5Sb12/mGaSb (m=0.2,0.4,0.6) samples, the microstructure analysis indicates that GaSb nano-dots with the grain size of 10-20 nm were embedded in the CeFe3.5Co0.5Sb12 matrix. Thermal performance tests show that: good electrical transport properties were maitained and the thermal conductivity was significantly reduced at a reasonable GaSb doping concentration.m=0.2 sample has the best integrated thermoelectric properties, with the dimensionless figure of merit ZT reaching 0.96 at 725K.