Investigation Of Thermoelectric Properties On Perovskite-type Oxides

Thermoelectric materials are semiconduting functional materials, which can be used to achieve a direct interconversion between thermal energy and electric energy by the movements of carriers. Thermoelectricity generation and cooling will be extensive applied in future. As the worldwide environment deterioration and the energy crisis pose more and more threats to the daily life of mankind, there is a pressing pursuit for a new green environmental energy, making thermoelectric materials a hot spot. Equipments using thermoelectric materials for power generation or refrigeration are characterized with quite a few merits, including compact structures, great reliability, noise-free operation, long operating life and non-pollution. Thus, they can be used in a great many fields, such as, civil use, military, aerospace, etc. Oxide thermoelectric materials, with the advantages of low costs for raw materials, stability under high temperature and pollution-free, have become a relatively new research field. Hence, oxide thermoelectric materials, which are regarded as one of the few kinds of clean and green inexpensive thermoelectric materials for new energy, have a bright future in the daily life of mankind in the future. Designing and developing high performance thermoelectric materials have been a object for researcher. The performance of thermoelectric materials depends greatly on the dimcnsionless figure of merit ZT. The higher ZT value gives higher efficiency of energy conversion. ZT=S2σ/κ, where S,σandκ, are Seebeck coefficient, electrical conductivity and thermal conductivity, respectively. The S2σnamed power factor (PF), which is a judging parameter for thermoelectricity output power. Therefore, high performance thermoelectric materials need to have a high figure of merit, and a high power factor.In this thesis, environmental friendly oxide thermoelectric materials as the object are chosen to research. The thermoelectric properties of several typical oxides have done deeply research, and we tentative explore the idea and fabrication of thermoelectric generation module with whole ceramics. LaFeO₃,SrTiO₃, CaMnO₃-based thermoelectric materials are respectively prepared using conventional solid state reaction techniques. The microstructures of these thermoelectric materials are observed with XRD and SEM. The electrical resistivity and Seebeck coefficients are measured respectively with self-made equipments or ZEM-3. Their thermal conductivity is measured by TC-7000 or LFA-427. The main research contents of this thesis includes modifying the thermoelectric transport behavior by optimizing chemical composition and technological conditions, final improving the thermoelectric properties and searching for the physical mechanism of the electric heat transport for the oxide materials. On this basis, aÏ€-shaped thermoelectric power module has been fabricated using P-typed Ca3Co4O9-based units and N-typed SrTiO₃-based units, and investigated the property of this module. Through these researches mentioned above, this thesis has achieved the following major innovations:1. With LaFeO₃-based materials the object of study, we design methods including Sr, Cu and other with heavy doping density and optimizing preparation conditions to investigate the influence of the doping density, doping ionic radius and technological conditions on the thermoelectric properties. The results show that:(1) Heavy Sr doped A-site can efficiently reduce the electrical resistivity and maintain high Seebeck coefficients and low thermal conductivity. So the thermoelectric properties can be improved by heavy Sr doped. The highest dimensionless figure of merit ZT=0.031 is obtained, and first reported in LaFeO₃-based materials. Adiabatic polaron hopping mechanism is found responsible for the conductivity in La1-xSrxFeO₃ by fitting the electrical resistivity and Seebeck coefficients. (2) Heavy Cu doped B-site optimizes the electrical properties of LaFeO₃ materials, and obtains higher power factor among that material system. The average grain size is effectively reduced by Cu doping, and improve the compactness of the samples. The decrease of electrical resistivity of heavy Cu samples has been explained with the compensation theories of valence states. (3) For samples of La0.9A0.1FeO₃ with different A-site elements, it is found that the electrical resistivity and Seebeck coefficient of A-site doped lanthanum ferrite decrease nearly with the increase of the atomic number of the doped element, except La0.9Ba0.1FeO₃. This is mainly caused by the degree of lattice distortion of larger ionic radius. In our work, doping of ionic with larger radius ionic can improve the electrical properties of these materials at some extent. (4) The increase of the sintering temperature can efficiently increase the compactness, the mobility of carrier, and reduce the electrical resistivity. So the properties of electrical transportation of La0.9Sr0.1FeO₃ are improved. The highest power factor 90μW/K2m is obtained for sample sintered at 1350℃.2. This thesis proposed the idea of N-typed SrTiO₃-based thermoelectric materials, which improves the thermoelectric properties of SrTiO₃ as a result of dual or multi-doping and nano-second phase including. The physical mechanism for the enhancement of the thermoelectric transportation is systematic investigated. The results are:(1) compared with the preparation environment of the air and argon, the reducing atmosphere to sinter of SrTiO₃-based thermoelectric materials can reduce the sintering temperature, the energy consumption, and obtain higher thermoelectric figure of merit for the deficiency of oxygen. (2) The thermal conductivity of SrTiO₃-based thermoelectric materials is reduced at some extent by La, Nb (Ta) doping. This result is in consistent with our initial idea of reducing thermal conductivity. However, these doping cannot improve significantly the properties of electrical transportation, and final result that the performance of thermoelectric is not optimized. Therefore, the maximum figure of merit ZT=0.29 is achieved for the sample of only La doping. (3) The thermoelectric performance of SrTiO₃-based ceramics is effectively enhanced by dual doping of Rare Earth Element at A-site. It is found that the maximum figure of merit ZT=0.36 is obtained at the doping amount of 20% of La and Dy, the value of figure of merit is the largest value among n-type oxide thermoelectric ceramics; while dual doping of La and Yb, the thermal conductivity can be obviously decreased at a small amount of Yb, and improve the thermoelectric properties to some extent. When the Yb doping amount is more, the thermoelectric properties cannot be greatly influenced as Yb cannot further enter the lattice; the dual doping of Y and Dy can reduce thermal conductivity and further improve the power factor, achieving thus the aim of coordination between thermal conductivity and power factor. The maximum thermoelectric figure of merit in this research ZT=0.22 is obtained. (4) The nano-second phase is introduced by the dual doping of rare earth elements at the A-site. It is found that nano-second phase occurring is easy to reduce thermmal conductivity, improve power factor, and final optimize its thermoelectric properties. Thus, the maximum thermoelectric figure of merit reaches 0.36, and the grain size of second phase is 700 nm, observed by SEM.3. In consideration of the effects of dual doping in SrTiO₃, we attempts to use the dual doping method of heavy elements, including Yb, Dy, Nb, further reduce the thermal conductivity, and optimize figure of merit for of CaMnO₃-based materials. The influence of dual doping of heavy elements on thermoelectric transportation and and physical changes is investigated. (1) Ca0.9Yb0.1Mn1-xNbxO₃ ceramics with the relative density of 97% is successfully prepared. With the increase of Nb, the MnO6, octahedral is distorted. Thus, the electrical resistivity is not effectively decreased. So the sample without Nb doping has received the high power factor of 297μW/K2m, and showed excellent stability in temperature. (2) The Ca0.9-xDyxYb0.1MnO₃ sample has a single phase with orthorhombic structure and a compact microstructure. The electrical resistivity is efficiently reduced with Dy doping and not significant changed in the whole measured temperature range. This behavior is beneficial to useing in the thermoelectric module. The thermal conductivity is successfully decreased, because of large mass difference between Dy3+ and Ca2+, which meets the original idea. In the end, the highest figure of merit ZT=0.11 is achieved for Ca0.88Dy0.02Yb0.1MnO₃ sample.4. On the basis of the researches of the properties of thermoelectric materials, an idea of a thermoelectric generation module with whole ceramics is proposed. The characteristics of output voltage and the internal resistance are studied. The feasibility of the thermoelectric generation module is checked. AÏ€-shaped ceramic thermoelectric power module with P-type Bi:CCO and n-type Y:STO has been fabricated, and the feasibility of this creative idea is proved. From the measurement of module, the highest open voltage is 37 mV with output power about 1.03μW.In this thesis, environmentally friendly oxide thermoelectric materials LaFeO₃, SrTiO₃, and CaMnO₃-based ceramics have been systematic investigated. The thermoelectric transportation is improved in different degrees by several methods, such as heavy elements doping, dual doping, nano-second phase, and optimization of preparation conditions. These research results show that wide band gap oxide thermoelectric materials are great potentiality material system, and bear much research value and application prospects. Meanwhile, these results have provided useful theoretical and technological references for the research and development of oxide thermoelectric materials with high properties. The exploratory research of thermoelectric generation module with whole ceramics is point of departure of environmental friendly and low cost module. As a result of time and length of this article, there is much work to be carried out in a deep-going way. However, it is believed that after the measures including, optimization of craftsmanship, changes of the controlling and regulating methods, larger breakthroughs will come up in the future for oxide thermoelectric materials.