Preparation And Thermoelectric Properties Of CaMnO₃ Based Oxide Thermoelectric Materials

Under the background of the current energy shortage and environmental pollution problem,the thermoelectric materials using the Seebeck effect and the Peltier effect to realize the waste thermoelectric generation and thermoelectric cooling technology have gradually become the research hotspot of the functional materials.In recent years,alloy thermoelectric materials like PbTe,Bi₂Te₃ and GeTe have attracted much attention because of their high thermoelectric properties,and they also show great potential for application.However,they have problems such as easy oxidation,volatility,and high cost.And oxide thermoelectric materials are of low cost and high temperature stability,and can be used in air atmosphere.This paper first choose sol-gel method to prepare CaMnO₃ oxides powders,then using argon atmosphere plasma sintering and atmospheric pressure sintering to obtain CaMnO₃ bulk samples,and choose the best preparation method of CaMnO₃ bulks.Finally we use the optimized CaMnO₃preparation process to prepare(Ca_1-xD_x)MnO₃?D=Sm,Sr,Rb,In,0.04?x?0.14?bulk samples,and systematically analyze its phase composition,microstructure and thermoelectric properties.The main conclusions are as follows:In the preparation of CaMnO₃ powder by sol-gel method,the continuous heating step for various drying temperatures is beneficial to obtain dry gel precursor with complete dehydration.The drying process of 90oC+120oC+140oC+160oC is the best among the four processes.The CaMnO₃ polycrystalline powder with good crystallinity,uniform particles and regular morphology was obtained after calcining the dry gel at 950?,the CaMnO₃ polycrystalline particles were composed by single-crystals with different orientations.In the preparation of CaMnO₃ bulk samples,the CaMnO₃ bulk samples prepared by spark plasma sintering in argon atmosphere have occurred phase decomposition,which is due to the SPS poor oxygen sintering environment.High quality CaMnO₃ bulk samples with single phase can be prepared by atmospheric sintering at ordinary pressure.In the(Ca_1-xSm_x)MnO₃?0.04?x?0.14?bulk samples,the trivalent Sm³⁺instead of the two valence Ca²⁺will produce Mn³⁺,and a large number of electronic carriers are introduced in the system,thus the number of carriers increases on the migration path Mn³⁺?O?Mn⁴⁺,leading to the resistivity of Ca_1-xSm_xMnO₃ decreases by 2 orders of magnitude.Meanwhile,the absolute merit of Seebeck coefficient of Sm doped samples is also reduced to 1/3 of un-doped sample.The thermal conductivity of all doped samples is lower than that of un-doped sample at high temperature?T>700K?.The highest dimensionless figure of merit ZT reaches 0.09 at 873K for x=0.04 sample.The(Ca_1-xRb_x)MnO₃?0.04?x?0.14?bulk samples have accured second phase Ca₂Mn₂O₅,and have a low density.The electrical properties of Rb doped samples are also low,but their thermal conductivity is much lower than that of un-doped sample.This owns to that the scattering center introduced by the second phase enhances the scattering of phonons.The x=0.14 sample has the lowest thermal conductivity of 0.910Wm^-1K^-1 at 773K.The highest dimensionless figure of merit ZT reaches 0.062 at 773K for x=0.04 sample.The resistivity of(Ca_1-xSr_x)MnO₃?0.04?x?.14?samples are significantly decreased,this is due to the electronegativity of Sr less than Ca,Sr instead of Ca will produce a large number of electrons.Meanwhile,the doped sample still maintains a relatively high Seebeck coefficient,which leads to an effective enhancement in the electrical properties of the material.The maximum of PF can reach 2.75?Wcm^-1K^-22 at 873K for x=0.04sample,and its value is 1.78 times higher than that of un-doped sample(0.99 Wcm^-1K^-2).The highest dimensionless figure of merit ZT of 0.14 has been acquired at 873K for x=0.10 sample.(Ca_1-xIn_x)MnO₃?0.04?x?0.14?samples are made up of CaMnO₃,Ca₂Mn₂O₅ and CaIn₂O₄.Compared with the un-doped sample,the resistivity of In doped samples decreased and the Seebeck coefficient did not decrease.But the Seebeck coefficient decreased linearly with the increase of temperature at low temperature,and gradually increased with the increase of temperature at high temperature?T>773K?.The thermal conductivity of In doped samples is significantly lower than that of un-doped sample,which is due to that the occurred second phase and the increased boundaries enhanced phonon scattering.The minimum of thermal conductivity can reach 0.837 Wm^-1K^-1 at873K for x=0.04 sample,the highest dimensionless figure of merit ZT of 0.12 has been acquired at 873K for x=0.04 sample.