| As a new energy material, thermoelectric (TE) oxides attract more attention due to their low cost and environmental friendliness. Among all the TE oxides, CaMnO3 could be synthesized and used in atmospheric environment, which make it a promising candidate as TE material. This thesis is focused on the properties of the electrical and thermal transport of CaMnO3. Optimization of the preparation process and chemical compositions are adopted to modify the TE properties and study the mechanism of transport in CaMnO3, and both their temperature stability and time stability are evaluated. By these researches mentioned above, the following major innovations have been achieved:1. The preparation process of traditional solid state method is studied to determine the optimal process condition. The major results include:1) Adjust the milling time (6-24 hours) to study their influence on TE properties of CaMnO3. By comparing the TE properties of samples, it is found that 12 h ball-milling is most effective in achieving higher relative density and TE performance. This makes 12 h the best milling time.2) Adjust the sintering temperature (1473-1673 K) to study their effects on TE properties of CaMnO3. Highest power factor,387 μW(K2m), is achieved in the sample sintered at 1573 K. This indicates that 1573 K is most effective in achieving higher TE performance and the best sintering temperature is 1573 K.2. Single doping at Ca and Mn sites is adopted to reduce the resistivity of CaMnO3 ceramics, and the law of single doping is studied. By evaluating and analyzing the TE properties of doped samples, several results are got:1) Using Dy as the doping element, TE properties of samples with different doping concentration (2-15%) are evaluated. The highest power factor,387 μW/(K2m), is achieved in 10% doped sample. This indicates 10% is the most effective doping concentration.2) A series of elements (La, Sm, Nd,..., Lu) are adopted in doping CaMnO3 to improve their TE properties. It is found that lanthanides are effective in reducing the resistivity. The highest ZT is 0.19 obtained by Dy doping, which is 4 times higher than that of undoped CaMnO3.3) Nb, Ta, W and Mo are chosen as doping elements at Mn site of CaMnO3 in order to improve the TE properties. From XRD patterns, we find that only Ta doped sample shows single phase of CaMnO3, while other three elements doped samples show a second phase. This result suggests that it is not easy to realize the doping at Mn site in CaMnO3 system. The highest power factor is 236 μW/(K2m), obtained in Ta doped sample, however, which is still lower than that of Ca-site-doped samples.4) These results all show that better electric conductivity would be achieved in samples with higher relative density, which is effective in improving their TE properties.3. Dual doping at Ca site is employed to reduce the thermal conductivity, and the law of dual doping at Ca site is studied. Dy and another element, such as Bi, lanthanides, are adopted as doping elements. Some major results are obtained:1) Dual doping at Ca site could modify both the electrical and thermal conductivity. It not only accomplishes higher power factor than single doping does, but also reduces the thermal conductivity further.2) Effects of doping concentration on TE performance are studied in Dy/Bi and Dy/Yb dual doped CaMnO3. The most optimized doping concentration is 2% in both dual doping systems, in which concentration achieves the highest power factor and the lowest thermal conductivity. And the highest ZT is obtained 0.27 in Dy/Yb dual doped CaMnO3.3) Systematic study of 2% dual doping is carried out with the whole lanthanides. Different elements lead to different thermal conductivity, and Dy/Yb dual doping sample shows the lowest thermal conductivity 1.4 W/(Km). Within the dual doping, ZT values increase with the increasing mass of doping element, which implies that heavier element is more effective in dual doping.4. Dual doping at Ca/Mn sites is carried out to reduce the thermal conductivity, and the law of dual doping at Ca/Mn sites is studied. Dy is doped at Ca site, while at Mn site Nb, Ta, W are chosen. The structure and TE properties are characterized, and the following results are achieved:1) Among all the Ca/Mn dual doping, Dy/Ta doped samples show XRD patterns without the second phase, and the second phase in Dy/Nb and Dy/W doped samples leads to a rise in resistivity.2) It is found dual doping at Ca/Mn sites is effective in reducing resistivity while keeping a moderate Seebeck coefficient.2% Dy/Ta doping also results in the lowest thermal conductivity as well as the highest ZT=0.23@1023K.3) All dual dopings at Ca/Mn sites show that 2% is the optimized concentration. If the concentration is higher than 2%, Seebeck coefficient would be sharply reduced, which leads to the decrease of ZT.5. The aging properties are studied in Ca0.96Dy0.02Yb0.02MnO3 sample. Both the temperature aging and time aging are evaluated, and the results are obtained:1) Many heating and cooling processes are carried out with Cao.96Dyo.o2Ybo.o2Mn03 ceramics to exam its temperature aging stability. After 10 times of heating and cooling processes, the sample still keeps excellent TE performance, which implies a good temperature aging stability in CaMnO3-based ceramics.2) Within different time TE properties of Cao.96Dyo.o2Ybo.o2Mn03 ceramics are characterized. The TE performance stays time independent in 12 months, which implies a good time aging stability.In this thesis, several methods, such as optimization of technical conditions, single doping, dual doping, are carried out to enhance the TE properties of CaMnO3. Thermoelectric transport properties of CaMnO3-based ceramics are studied systemically, and dual doping with lanthanides is effective in improving power factor and reducing thermal conductivity. The TE properties of CaMnO3-based ceramics are improved by dual doping with heavy lanthanides. This thesis shows that modified CaMnO3 is a promising material with high value in research and application. The research approach in this thesis is of great reference value to similar TE material systems. The study of aging stability in this thesis shows CaMnO3-based ceramics are stable in the processes of temperature aging and time aging, which is much helpful to their application in TE modules. |
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