| With the rapid development of grobal economy, the energy consumption and environmental deterioration caused by the utilization of fossil fuels are getting more and more serious in recent years. Thermoelectric (TE) materials can harvest electrical energy directly from waste heat and have attracted tremendous attention because of their potential in relieving the energy crisis and environmental pollution.Traditional TE materials like Bi2Te3, CoSb3 and PbTe have already been well researched, and their thermoelectric performance have been improved step by step in recent years. At the same time, some new concept TE materials also provide new ideas for researchers. With the improvement of TE theory and experimental technique, new TE materials show enormous potential for further research. Phase transformation materials are one of the very important parts of new TE materials, such as Cu2Se (the melting of sublattice) and SnSe. Therefore in this thesis, two typical compounds, CuCrSe2 and InSb, which have the transformation of ionic migrationand solid-liquid transition respectively, have been selected and studied. The composition, phase and microstructure were characterized by X-ray diffraction analysis (XRD), microscope, field emission electron microscopy characterization (FESEM), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), differential scanning calorimetry (DSC) and differential thermal analysis (DTA). The Seebeck coefficient and resistivity at different temperature were measured by a home-made Namicro-? thermoelectric measurement system. Hall coefficient test system was employed to measure the carrier concentration and mobility, and the thermal diffusion coefficient was measured bv laser thermal conductance instrument.For the ionic migration phase transformation TE material of CuCrSe2, the main experiments and results are as follow:1) By incorporation of mechanical alloying and heat treatment, the fabrication process of CuCrSe2 has been greatly simplified, and the preparation duration has been shortened from five days to only 6 hour.2) Through analyzing the DTA results and the composition after heat treatment, four reactions have been proposed and the final reaction has been confirmed as:CuCrSe4+ Cu2-xSe ? CuCrSe2.3) The Cu ions migrate from one type of the tetrahedral sites to the another type of tetrahedral sites when the ionic migration phase transformation of CuCrSe2 is happened at 373K. It induces strong carrier and phonon scattering, and the thermal conductivity and electrical resistivity decrease rapidly at this temperature. Futhermore, the TE performance of CuCrSe2 can be enhanced by indium doping. When the content of indium is lower than 0.25 at.%, In can enter into CuCrSe2 lattice and form the In-Se bond, which has weaker electronic localization than the Cr-Se bond, and get high carrier concentration. When the content of indium is higher than 0.25 at.% and exceed the solubility limit, it can form CuInSe2, and lead to higher resistivity, Seebeck coefficient and lower thermal conductivity. As a result, a maximum ZT of 0.65 was achieved in the 1.0 at.% In doped sample at 673K, which is enhanced almost 54% in compared with the In-free sample.For the solid-liquid transition which is represented by InSb, the main experiments and results are as follow:1) Vacuum melting and quenching have been employed to synthesis the InSbx samples with excess Sb. The second phase is elemental Sb which was confirmed by many analysis methods, and the content of second phase increases with the excess Sb.2) Through analyzing the DTA results and phase diagram, a divorced eutectic structure has been observed, and the melting of the eutectic at 765K was confirmed.3) The melting of divorced eutectic at high temperature exerts a huge influence on TE properties of the samples. It can improve the average carrier concentration, and reduce the resistivity rapidly. At the same time, the liquid phase inhibits the propagation of transverse acoustic phonons, and decreases the thermal conductivity dramatically. As a result, the thermoelectric performance of the eutectic included InSb-based compounds has been enhanced anomalously, and the ZT value for InSb1.04 increases from 0.47 at 723K (before melting) to 1.21 at 773K (after melting). At the same time, the sample shows no evident strength loss after a melting-solidification cycle of the separate eutectic structure. |
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