Study On Doped CaMnO 3 Based Thermoelectric Ceramics

Nowadays,the economy in human society is developing rapidly and living standard of human beings has been improved much than ever before.Unfortunately,we humans are also faced with such dilemmas as serious energy shortage and environmental problems.As the fossil fuels like coal,gas and petroleum are still the major energetic resources for the next century,it is of much urgency to exploit highly effective energy producing or saving methods.Throughout industrial field and our daily automobiles,a large amount of energy is missed in waste heat.If we can change the wasted heat into usable electric energy,it will be a great compensation for electric power supply.Therefore it is a compelling need for excellent thermoelectric?TE?materials that can directly and reversibly convert heat to electrical energy.Thermoelectric ceramics based on oxides have many advantages for they are non-toxic,stable under high temperature and suitable for atmospheric condition.However,n-type and p-type semi-conductors materials are required to compose thermoelectric devices while we still have not found n-type materials with properties better than p-type materials.So,this thesis focuses on the structure and thermoelectric properties of n-type Ca MnO₃ ceramics and tries to improve their thermoelectric performances via doping.The band structures of undoped and doped CaMnO₃ are obtained using the first principle density functional theory calculation based on a plane wave basis and pseudo potentials.According to the band structure results,the combination of Mnd and Op orbitals exhibited enhanced covalent nature.The bands near Fermi level in the system doped with R?R stands for a kind of metal element?or Ti experienced a significant distortion.The band gaps between valence band and conductive band in R0.05Ca0.95MnO₃ and R0.1Ca0.9MnO₃ were 0.42 eV and 0.39 eV respectively and those for Ca Ti0.05Mn0.95 O and CaTi0.1Mn0.9O₃ were 0.502 eV and 0.48 eV respectively.All of them were lower than the band gap in CaMnO₃?0.6eV?.It is estimated that the thermopower and carrier conduction capability of doped samples should be enhanced.The synthesis procedures of traditional solid-state reaction method?SSR?were studied to gain optimized preparation parameters.The major results are as follows:?1?By comparing the contents and DSC/TG results,it has been found that using CaO in raw materials would lead to much second phase CaMn2O4 while replacing MnO2 with MnCO₃ leaded to a lower calcining temperature of powder as well as modest conductivity of disk samples;?2?Preparing samples under 300 MPa was suitable because the sample pieces were condensed and showed lower resistivity;?3?Sintering the samples above 1175?can ensure a single phase and also better properties.Finally,CaCO₃ and MnCO₃ were used as the raw materials in this thesis and all the samples were pressed with a pressure around 300 MPa and sintered under 1200?.Ca1-xRxMnO₃?x=0.02,0.05,0.08,0.1?and CaMn1-xTixO₃?x= 0.02,0.05,0.1?samples were made by modified SSR method.All of the samples belonged to n-type semiconductor materials and the principles of influences resulted from R doping at Ca sites and Ti doping at Mn sites were studied.By evaluating and analyzing the TE properties of doped samples,several results are got:?1?Doping with R or Ti was an effective way to improve properties of CaMnO₃ ceramics and it brought down the resistivity obviously.Metal R was better than Ti element.?2?Among the four samples containing R element,sample with a composition of Ca0.98R0.02MnO₃ showed the largest Seebeck coefficient-210?V/K under 750 K.It also showed a large power factor of 1.43?W/?cmK2?and ZT value of 0.0668 under1000 K.?3?As for the CaMnO₃ ceramics doped with Ti at Mn sites,the largest power factor and ZT value were obtained in CaMn0.98Ti0.02O₃,which were 0.52704?W/?cmK2?and 0.0105 respectively.Graphite-added CaMnO₃ ceramics were synthesized with modified solid state reaction method.The influence of adding graphite on the thermoelectric properties of CaMnO₃ ceramics has been investigated between 450 K and 800 K.The results showed that adding graphite powder into CaMnO₃ ceramics enhanced the Seebeck coefficients and lowered the resistivity.The largest absolute value of Seebeck coefficient 540?V/K was obtained in the samples with 2wt% graphite under 450 K.And the largest power factor 4.03?W/?cmK2?was obtained in the samples with 1wt% graphite under 800 K.