Study On Synthesis Of Doped CaMnO₃ Based Materials And Their Thermoelectric Properties

Thermoelectric material is a kind of functional material which can convert heat energy to electric energy directly. The current development of thermoelectric oxides material were reviewed and discussed in this dissertation. Currently, the thermoelectric oxides have been recognized as possessing more potential in the research and more valuable in the application. In the present work, the synthesis and thermoelectric properties about CaMnO₃ based thermoelectric materials were studied. The influences of reaction process, the doped quantum on their properties and the possible mechanism were discussed.Modified glycine-nitrate process (MGNP) and citric acid method (auto-ignition method) were adopted to synthesize N-type Sm_0.1Ca_0.9MnO₃. While The powders of SDC (Sm_0.2Ce_0.8O_1.9) and SDB((Sm_0.2Bi_0.8)₂O₃) which were synthesized by glycine-nitrate process and solid state reaction(SSR) respectively. The sintering temperature dependence of the thermoelectric properties for Sm_0.1Ca_0.9MnO₃ was discussed. A new thermoelectric composite was prepared by mechanical mixing SDC or SDB powder to Sm_0.1Ca_0.9MnO₃ thermoelectric powder. The doped quantum of SDC or SDB powder on their properties was discussed too.The main achievements of the work are listed as followings:1. The powder of Sm_0.1Ca_0.9MnO₃ with single pure phase was synthesized by MGNP. According to its XRD diagram, the calculated average grain size of Sm_0.1Ca_0.9MnO₃ was about 43nm. Compared to traditional GNP, modified method that can produce large amounts of powder in a short time and have high yields effectively reduced the powder synthesizing temperature, which could prevent Sm_0.1Ca_0.9MnO₃ powders'decomposition.2. Sm_0.1Ca_0.9MnO₃ powders were better followed the Huang-peiyun's pressure equation. It has larger m value 4.5445 which indicates the larger trend of rigidification. The small repression modulus M (1.1247) shows that the synthesized Sm_0.1Ca_0.9MnO₃ powders have high repressive properties which are favorable to prepare high-performance thermoelectric ceramics. 3. Sm_0.1Ca_0.9MnO₃ bulks were prepared by using normal atmosphere sintering in air and their thermoelectric performance was studied. The results showed that the semiconductor-metal transition was taken place during the Sm_0.1Ca_0.9MnO₃ at a certain temperatures owing to the Sm³⁺doping. Meanwhile, Sm3+ doping effectively increased the carrier concentration and reduced the electrical resistivities of the materials. The sample sintered at 1573K had maximum power factor 6.0×10^-4 Wm^-1K^-2 at 473K.4. In present study , the Sm_0.1Ca_0.9MnO₃,SDC(Sm_0.2Ce_0.8O_1.9) and SDB((Sm_0.2Bi_0.8)₂O₃) powders were fabricated by three different chemical methods. The Sm_0.1Ca_0.9MnO₃ powders was synthesized by self-combustion method by using citric acid as chelate while the SDC(Sm_0.2Ce_0.8O_1.9)powder was fabricated by conventional glycine-nitrate method and the SDB((Sm_0.2Bi_0.8)₂O₃) was prepared by solid state reaction(SSR). Sm_0.1Ca_0.9MnO₃/SDC and Sm_0.1Ca_0.9MnO₃/SDB compound materials were obtained by mechanical blending Sm_0.1Ca_0.9MnO₃,SDC(Sm_0.2Ce_0.8O_1.9) and SDB((Sm_0.2Bi_0.8)₂O₃) powders.5. Compared to the SCM system, the composite materials of Sm_0.1Ca_0.9MnO₃/Ce_0.8Sm_0.2O_1.9 had less Seebeck coefficients, but the reduced extent of their electrical resistivities was larger than that of their Seebeck coefficients. Therefore, the obtained power factors of system were still larger on the whole. The sample sintered at 1573K of SCM+5%SDC(wt) had minimum electrical resistivity 9.0×10^-5Ω·m and maximum power factor 1.2×10^-3 Wm^-1K^-2 at 423K. Presence of the second phase (SDC and SDB) promoted effectively the sintering process of the composite materials and improved availably the thermoelectricity property of materials. On the one hand, its resistivity had decreased remarkably which was due to the present of SDC and SDB. SDC or SDB could increase the carrier concentration of Mn'Mn; On the other hand, SDC or SDB could increase the carrier concentration of VO .. too. The present of SDC and SDB was helpful to increase the crystal lattice and phonon dispersion, which could decrease greatly the resistivity of material and increased the thermoelectric properties.6. Sm_0.1Ca_0.9MnO₃/(Sm_0.2Bi_0.8)₂O₃ had larger Seebeck coefficients whose decreased extent was lower than that of electrical resistivities. The sample sintered at 1193K of SCM+6%SDB(wt) had minimum electrical resistivity 8.44×10^-5Ω·m and maximum power factor 0.86×10^-3 Wm^-1K^-2 at 723K.Sm_0.1Ca_0.9MnO₃ fine powder materials with high thermoelectric performance were prepared by modified glycine-nitrate process (MGNP). It is reasonable that Sm_0.1Ca_0.9MnO₃ powders were better followed the Huang-peiyun's pressure equation. What's more, composite materials with excellent thermoelectric performance were obtained by mechanically blending Sm_0.1Ca_0.9MnO₃ and SDC(Sm_0.2Ce_0.8O_1.9) and SDB((Sm_0.2Bi_0.8)₂O₃) powders, the reason was explored by the defect chemistry theory, which would provide new and more extensive researching ideas for the others in improving the properties of thermoelectric materials.