Investigation Of Oxide Thermoelectric Properties Of CaMnO₃ And Ca₃Co₄O₉ By Doping

Thermoelectric materials and thermoelectric effects,which are responsible for the direct conversion of heat into electrical energy and vice versa,oxide thermoelectric materials are more suitable for high temperature applications because of their structural and chemical stabilities,oxidation resistance,easy manufacture and low-cost.In this paper,two kinds of typical high performance oxide thermoelectric materials,CaMnO₃and Ca₃Co₄O₉,were doped with different elements and contents by the via sol-gel method.Then the structure,morphology,electrical transport,thermal transport and thermoelectric properties of the samples were studied and the main contents were as follows:The preparation process of doped CaMnO₃ and Ca₃Co₄O₉ samples were studied,and the optimum preparation conditions and process were determined.The XRD results and SEM images revealed that prepared samples are uniform crystal particles with high purity,the cell parameters of the prepared samples were changed by the solid solution of the appropriate amount of doped ions into the crystal,which changes the cell volume of the dual doping materials but does not change the crystal symmetric structure.All the synthesized samples could exist stably at 300 K¹273 K.When the calcination temperature was 1273 K and 1073 K,the samples had the highest ZT value.For the CaMnO₃ system,the single doping of Er optimized the electrical transport performance of the system,making the power factor of 173?W/?K²m?.Then the dual doping of Ca sites of the system was studied and the doping elements included La,Sm,Yb and Er.The XRD and XPS results showed that,compared with the single doped Ca_0.98Er_0.02MnO₃,all the prepared samples were Pnma space group.With the increase doping amount,the lattice parameters increased due to more Mn³⁺cations in the CaMnO₃ system caused by dual doping.From the XRD data after GSAS software processed,it showed that the A-site average ionic radius<r_A>were all increased with the increasing doping content and the expansion of the ionic radius,the increasing of doping amount also led to the disordered degree of?².Such increase of<r_A>and?²resulted in the La bond angle of Mn-O-Mn closer to 180 ^o,and the more deviation of Sm and Yb cations from 180^o.On the other side,the bond length of with Mn-O decreased with the smaller ion radius of doped elements,and the change of the final bond angle and bond length led to the different distortion of MnO₈ octahedral in CaMnO₃ system.The results of electric transport characterization showed that the electrical transport performance of the system was improved when the doping amount of Er was 0.02 and Yb was 0.08.The highest power factor of Ca_0.9Er_0.02Yb_0.08MnO₃sample could reach 208?W/?K²m?.The results of thermal conductivity characterization showed that Ca_0.9Er_0.02Yb_0.08MnO₃ had the lowest thermal conductivity value of 1.68W/mK,and it also had the highest ZT value of 0.11 under 973 K,which was higher than that of single-doped and pure phase CaMnO₃ samples.The results indicated that dual doping was a good method to optimize thermoelectric properties.Furthermore,the dual doping of Ca and Mn sites of the system were done,and it was found that the doping of Er and Ta showed a higher ZT value of 0.13 than that of Er and Yb doped samples,and the dual doping of Ca and Mn sites is an effective method to improve the thermoelectric properties of CaMnO₃ system.The Ca₃Co₄O₉ system was single doped with Cu and Fe.The results showed that the ZT value of the system reached to 0.22 when the doping content of Fe was 2%.The Ca and Co sites of Ca₃Co₄O₉ system were doped with Ag,Sr,Yb and Fe,and the results of resistivity and Hall characterization indicated that the carrier concentration was increased by Ag doping.The Sr²⁺doped samples showed a higher migration rate with the increasing doping content,the doping of Yb³⁺resulted in the decreasing carrier concentration and a higher resistivity value.Seebeck results showed that Yb and Cu doping showed the maximum Seebeck value as well as a low power factor due to the high resistivity value,while for the dual doping of Ag and Fe led to the great power factor improvement of the system,and finally,the Ca_2.7Ag_0.3Co_3.9Fe_0.1O₉ sample showed the highest power factor of to 733?W/?K²m?.The results of thermal conductivity characterization showed that Ca_2.7Ag_0.3Co_3.9Fe_0.1O₉ had the smallest thermal conductivity of 2.02 W/m K,which was much smaller than that of pure phase samples?3.5 W/mK?and single Fe-doped sample?2.39 W/mK?.According to the above results,the excellent thermoelectric value was calculated,and it was found that,thanks to the improvement of the electrical transport properties of the system due to the doping of Ag and Fe,the ZT value of the material could reach 0.37 at 973 K.Finally,p-type semiconductor and n-type semiconductor with the best thermoelectric performance were used to form a simple thermoelectric device and the current and voltage were tested,the result shown that the device accords with the principle of thermoelectric effect.In this dissertation,the via sol-gel method was used to study the effect of doping on the thermoelectric properties of the two systems through single doping and dual doping methods.The results show that dual doping is a good way to improve thermoelectric performance.At the same time,the influence of doping of the second phase on the thermoelectric performance of the system was systematically studied,and the content of rare metal doping was successfully reduced.This is very beneficial to the industrial production of thermoelectric materials in the future,and also improves the thermoelectric performance of the system.The study provides a certain reference value.