Bi Department Of Co-oxide-based Of Thermoelectric Ceramics And Film Preparation

With the decreasing of developable energy resources and the environment pollution problem become more and more serious, the thermoelectric materials, a kind of functional materials which can achieve the direct transform between thermal energy and electrical energy, is attracting more and more attention. As one of the important class of thermoelectric materials, it has many advantages such as high temperature resistance, anti-oxidant, good stability, pollution-free and low costs as compared with other thermoelectric materials, that make it become one of the hot researches. Bismuth Cobalt oxide thermoelectric materials is an important kind of misfit-layered oxide thermoelectric materials. The single crystal Bi2Sr2Co2Oδhas great thermoelectric property with ZT value higher than 1.1 at the temperature of 1000K. But it's very hard to achieve a big enough single crystal, and therefore to enhance the thermoelectric property of polycrystalline is the focus of the study. The thermoelectric property can be effectively enhanced by improving fabricating techniques, doping and low-dimension. In this thesis, the synthesis process of Bi2SrCaCo2Oδand Bi2Ca2Co2Oδoxide thermoelectric materials were optimized, and the structure characteristics and electrical properties of the materials were studied. The Bi2Ca2Co2Oδthin films were prepared and the LIV signals were also analyzed. The main studies are as following.The Bi2SrCaCo2Oδand Bi2Ca2Co2Oδpolycrystalline ceramics materials were prepared by solid state reaction with Bi2O3,Co2O3,SrCO3 and CaCO3. A series of polycrystalline ceramics with different sintering processes including different pre-sintering temperature, sintering temperature and heating preservation time were synthesized, and then selected the best preparation processes from the angle of phase and micro-organization through x-ray diffraction(XRD) analysis and metallurgical analysis. The best preparation processes are:Bi2SrCaCo2Oδwith 850℃pre-sintering temperature heating preservation 6h,870℃sintering temperature heating preservation 8h; Bi2Ca2Co2Oδwith 860℃pre-sintering temperature heating preservation 6h,880℃sintering temperature heating preservation 8h. The rate of temperature rise of both materials are 150℃/h. The phase and crystal structure were characterized by XRD and electron diffraction. The microstructure and surface element content were determined by SEM and EDS. The electrical properties were measured by four-point probe method. The results show that the unit cell parameters of Bi2Co2Oδare c=1.469nm, a=0.520nm, bRS=0.502nm, bH=0.295nm, mismatch degree bRS/bH=1.702; the unit cell parameters of Bi2Ca2Co2Oδare c=1.466nm, a=0.513nm, bRS=0.491nm, bH=0.291nm, mismatch degree bRs/bH=1.687. The Ca substitution for Sr reduced the unit cell parameters, and the ceramics were still rock salt layered structure with apparent c-axis preferred orientation. The resistivity-temperature diagrams show that the ceramics were typical semiconductor.The Bi2SrCaCo2Oδthin films were prepared by pulsed laser deposition (PLD) on Al2O3 single crystal substrates, in order to study the influences of substrate temperature, laser intensity and oxygen pressure to the growth of thin films. The best thin films preparation conditions were selected by XRD:substrate temperature 780℃, deposition oxygen pressure 40Pa, deposition time 8min, annealing oxygen pressure 3000Pa and laser intensity 280mj, with fast cooling rate. The LIV signals of Bi2SrCaCo2Oδthin films deposited on 15°vicinal-cut and two different orientations 10°vicinal-cut Al2O3 single crystal substrates were measured. The results show that the thin films with the same angle of inclination and different orientations have different LIV signals; and the LIV signals of the same angle of inclination thin films increased as the angle of inclination rise. The influence of laser intensity to LIV signal was also studied.