Effect Of Preparation And Doping On The Thermoelectric Transport Properties Of Ca₃Co₄O₉ Thermoelectric Material

With the increase of global population and economic development,energy crisis and environmental pollution have become key problems,which will hinder global sustainable development.As one of the ways to solve these problems,thermoelectric materials have become a hot topic in recent years.Thermoelectric material can convert heat energy and electric energy directly without conversion medium.There are no other emissions during the conversion process.The layered cobalt-base oxide thermoelectric materials are suitable for medium and high temperature fields,such as waste heat utilization of automobile exhaust.Compared with alloy thermoelectric materials,the layered cobalt-base oxide thermoelectric materials are more stable,but the thermoelectric conversion efficiency is lower at present.Therefore,how to improve the thermoelectric transport performance of the layered cobalt-base oxide thermoelectric materials is the goal pursued for researchers.The layered cobalt-base oxide thermoelectric material Ca₃Co₄O₉ was the research object of this paper.The precursor sheets were prepared by sol-gel integrated preparation technology（SGIT:sol-gel method,PEG800,self-propagating combustion and cold isostatic pressing）,and the Ca₃Co₄O₉ based thermoelectric materials were prepared after sintering.Ca_3-xM_xCo₄O₉（M=Ag,Al;x=0.05-0.5）materials were prepared with Ag and Al doping at Ca site and Ca₃Co_4-xM_xO₉（M=Ni,Y;x=0.05-0.5）materials were prepared with Ni and Y doping at Co site.The crystal structure,microstructure and thermoelectric transport properties of the samples were measured by means of DSC,XRD,SEM,ZEM-3 and laser flash point thermal conductivity meter.The thermoelectric properties of Ca_2.8Ag_0.2Co₄O₉ were the best under the experiment conditions.Based on the design of chemical formula Ca_2.8Ag_0.2Co₄O₉,Ca_2.8Ag_0.2Co_4-xM_xO₉（M=Ni;x=0.05-0.3）materials were prepared with different amounts of Ni doping.The structure,microstructure and thermoelectric transport properties of Ca_2.8Ag_0.2Co_4-xM_xO₉ materials were studied.The major research content and results were as follows:1. Ca₃Co₄O₉ thermoelectric materials were fabricated by SGIT successfully.When the molar ratio of citrate to nitrate was 0.8 and the mass fraction of PEG800 was 4%,the samples had the smallest grain size and the lowest thermal conductivity.The ZT value of Ca₃Co₄O₉material reached a maximum of 0.05 at 673 K,which was 9.3%higher than that obtained by the traditional sol-gel method,and was 3%higher than that of the sample prepared by SGIT without PEG800.2. Compared with the unidirectional static pressure,cold isostatic pressing could inhibit the original c-axis preferred alignment and obtained higher particle density,which were beneficial to improve the electrical conductivity of Ca₃Co₄O₉ thermoelectric materials,but reduced the thermal conductivity at the same time.3. With a proper amount of Ag doping,Ag entered completely the lattice of Ca₃Co₄O₉.The cell parameters of Ca₃Co₄O₉ increased with the increase of Ag doping content.The diffraction peaks of Ag were observed in the XRD patterns when the content of Ag was excessive,which showed that Ag had a certain solid solubility in Ca₃Co₄O₉.With the increase of Ag doping content,the electrical conductivity of the material increased gradually,the Seebeck coefficient increased first and then decreased and the thermal conductivity showed an opposite to Seebeck coefficient.When the Ag doping amount was 0.2,Ca_2.8Ag_0.2Co₄O₉showed the maximum ZT value with 0.170 at 673 K.The cell parameters of Ca₃Co₄O₉decreased with the increase of Al doping amount.With the increase of Al doping amount,the electrical conductivity decreased,Seebeck coefficient increased,and the thermal conductivity first decreased and then increased.The preliminary estimation results showed that the change of carrier concentration was consistent with the Pisarenko relationship.The mobility and average free time increased with the increase of Al doping.The best doping amount of Al was0.3,and the maximum ZT value showed 0.149 at 673 K.4. The lattice parameters and the particle size of Ca₃Co₄O₉ decreased with the increase of Ni doping amount,and the uniformity of particle size became worse.When the content of Ni was 0.5,the impurity phases of Co Ni O₂ and Ca O₄ appeared.The electrical conductivity and thermal conductivity of the all sampls decreased with the increase of Ni doping amount the same tendency,while Seebeck coefficient increased first and then decreased.When the doping amount of Ni was 0.2,the maximum ZT value was 0.072 at 673 K.The cell parameters and the particle size of Ca₃Co₄O₉ increased gradually with the increase of Y doping amount.The electrical conductivity increased first and then decreased with the increase of Y doping amount,and the thermal conductivity showed the opposite trend.The Seebeck coefficient increased monotonously with Y doping content increasing.When the doping amount of Y was0.3,the maximum ZT value was 0.070 at 673 K.5. Based on Ca_2.8Ag_0.2Co₄O₉ had the best the thermoelectric properties for Ag doping sampls,different content of Ni was doped for the Ca_2.8Ag_0.2Co₄O₉.With the increase of Ni content,the diffraction peak shifted to a large angle,and the particle size of the sample decreased gradually and the particle size uniformity became worse.The electrical conductivity and Seebeck coefficient increased first and then decreased with the increase of Ni doping content,and the thermal conductivity decreased monotonously.The maximum ZT value of Ca_2.8Ag_0.2Co_3.8Ni_0.2O₉ sample is 0.212 at 673 K,which was more than 4 times higher than the pristine Ca₃Co₄O₉.6. The Seebeck coefficients of all pristine and doped samples were positive,which indicated that the Ca₃Co₄O₉ based materials were p-type semiconductors and doping did not change the electrical transport characteristics and hole transport mechanism of the material.All samples conformed to the conduction mechanism of the small polaron adjacent hopping model in the whole temperature range.The thermal conductivity depended on the lattice thermal conductivity,and the electronic thermal conductivity had little effect on the total thermal conductivity.