Defect Properties Of Thermoelectric Materials Studied By Positron Annihilation

Thermoelectic materials can convert heat into electric energy and vice versa. It can convert waste heat into clean energy. Furthermore, the long wavelength part of the solar energy can be also utilized bu using thermoelectric conversion technique, thus the whole band of the solar energy can be used. This is expected to relieve the growing crisis of energy in the present time.How to improve the thermoelectric vonversion efficiency has become an important subject in this research area. The thermoelectric conversion efficiency depends on various factors, such as the electric conductivity, thermal conductivity, Seebeck coefficient and absolute temperature. More and more research have aimed at the influence of the microstructure of the materials on the thermoelectric properties. By substituting guest atoms on the host lattice site, it can enhance the scattering of phonons and lower the thermal conductivity. The vacancy defects will be effective phonon scattering centers, thus improve the thermoelectric behavior. Therefore, it is a very important to study the defects in thermoelectric materials and their effects on the thermoelectric properties by using an appropriate method which is sensitive to the atomic scaled defects.Skutterudite compound is one of the most promising thermoelectric materials owing to large carrier mobility, high electrical conductivity and relatively great Seebeck coefficient. Binary skutterudites are compounds with the general formula MX3 having a crystal structure with a bcc lattice. Six X4 rings give rise to two icosahedral voids. Skutterudite crystal structure contains two large interstitial voids in a unit cell that can be filled with various impurities, which can dramatically reduce the lattice thermal conductivity due to the impurities act as a rattling local vibration mode. ?-Zri4Sb3 has very low thermal conductivity due to its structural disorder and Sb dimmers rattling and has been considered one of the best thermoelectric materials in the intermediate temperature of 473K-673K. The occupancy of the Zn lattice site is about 90%, and each Zn vacancy with two or three interstitial Zn atoms around. The vacancies and interstitials can scatter phonons, such make the low lattice thermal conductivity and a high ZT value. These two kinds of thermoelctrical materials satisfies the definition of phonon-glass and electron-crystal thermoelectric material. In this thesis, a combination of positron annihilation spectroscopy and X-ray diffraction were used to study the effect of single-atom-filling on CoSb3 crystal structure. The effect of different time annealing on P-Zn4Sb3 structure and thermal properties is also investigated. The main results are as follows:1. BaxCo4Sb12(x=0,0.1,0.2,0.3,0.4) were prepared by traditional melt method, long time annealing and spark plasma sintering. XRD measurements confirm that these samples are simple phase. It indicates that foreign atoms have filled into icosahedron cages. The positron lifetime spectra are well decomposed into two lifetime components. The caculated positron lifetime for perfect lattice is close to the experimental lifetime component ?1 for CoSb3.The positrons are well localized in the intrinsic voids. The lifetime component ?1 increases with increasing content of Ba filled in CoSb3, which reflects changes of the phase of BaxCo4Sb12 throughout the regions of Ba content of 0< x< 0.5. By comparing the calculated positron lifetimes with the experimental results, order-disorder phase transitions of the filler species in the voids.2. Zn4Sb3 were synthesized by melt method and spark plasma sintering. Each side of the SPS sample has different phases, ZnSb-side and Zn4Sb3-side. The reversible structural transformation between ?-Zn4Sb3 and ZnSb phases has been confirmed by XRD results. The structural transformation starts at 250 ? and completes at 300?. Positron annihilation results indicate the formation of large amounts of vacancy clusters in the ZnSb-side with the structural transition from Zrn4Sb3 to ZnSb. The vacancy clusters are sufficiently removed after the ZnSb phase recovers back to the ?-Zn4Sb3 phase. Vacancy clusters existed in the Zn4Sb3-side have lower concentration and remain stable during the annealing process.3. Thermoelectric ?-Zn4Sb3 samples prepared by spark plasma sintering method have been studied by resistivity measurement, X-ray diffraction (XRD) and positron annihilation spectroscopy (PAS). The existence of Zn vacancies in ?-Zn4Sb3 structure is confirmed by PAS, which contribute a positron lifetime component of 212±3 ps. By comparing the calculated lifetime values with the experimental result, we find that each Zn vacancy must have at least two close interstitial Zn atoms in the crystal structure of P-Zn4Sb3. Two separate phase transitions observed by resistivity measurement occur at about 232 K and 252 K, whereas the positron annihilation measurements indicate they are about 225 K and 240 K. Positron lifetime and Doppler broadening results reveal a'phase is a transition phase between ? and ? phases. The results also indicate the disordering of interstitial clusters into interstitial Zn sites during the ?' to ? phase transition. The interstitial Zn clusters are found to act as shallow positron traps which apparently capture positrons below 200 K.