Thermoelectric Materials Of CoSb₃ And The Ba-filled System:High Pressure And High Temperature Preparation And Properties

Energy is the cornerstone of social development and sufficient energy can promote the progress of human civilization.At present,the energy utilization efficiency does not exceed 40%^[1].Excessive energy waste leads to the emergence of two oil crises,which makes human know the limitation of energy.At the same time,the exhaust gases emitted during the energy utilization process can pollute the ecological environment.China has begun to limit the emission of polluting gases,but the goal set by the Paris Climate Agreement is still far from enough.Improving energy efficiency and developing new energy are the strategic priorities for the country to achieve sustainable development.As a new type of functional material,thermoelectric materials rely on internal carriers and phonons^[2,3]to enable direct conversion of electrical energy and thermal energy.Applications can improve energy efficiency and mitigate environmental pollution.Compared with traditional power generation technology,solid-state energy conversion has the advantages of no internal motion,light weight,small size and high mobility.Thermoelectric devices can be used in the secondary utilization of low-grade waste heat^[4,5],space detection^[6,7],precision equipment,medical^[8,9]and other fields.With the advancement of basic theory and preparation technology,the performance of thermoelectric materials has been broken,and thermoelectric devices are accelerating into people's production and life.But its conversion efficiency is still less than 10%^[10,11].Bi₂Te₃ thermoelectric material is currently used in commercial applications^[12].Its low melting point and inferior heat stability^[13]make it suitable for reuse in low temperature areas.High temperature and low grade waste heat such as automobile exhaust pipes and factories need device that is resistant to high temperatures and has a high ZT value.PbTe has excellent ZT value and high temperature resistance.The disadvantages are poor mechanical properties and toxicity.Therefore,it is of great significance to study the low-temperature,non-polluting and excellent mechanical properties of high-temperature thermoelectric materials such as CoSb₃.Based on the domestic six-face press,the thermoelectric materials of CoSb₃ and Ba-filled system were successfully prepared by high temperature and high pressure method.The mechanism of synthesis pressure and element doping on the microstructure and thermoelectric properties of CoSb₃ were studied in detail.The specific research contents are as follows:1.Through the analysis of Co-Sb phase diagram,CoSb₃ was successfully prepared by high temperature and high pressure method in the pressure range of1.5-3.5 GPa.The regulation mechanism of the synthesis pressure on the thermoelectric properties of CoSb₃ was studied.The research results are as follows:XRD and EDS test results show that CoSb₃ can be synthesized by high temperature and high pressure in half an hour,which greatly shortens the preparation cycle of CoSb₃.The results of scanning electron microscopy show that the sample crystal size is decreased with the increase of synthesis pressure in the pressure range of 1.5-3.5GPa.The grain size is gradually reduced and the grain boundaries are more abundant.The resistivity of CoSb₃ and the Seebeck coefficient increase with the synthesis pressure,indicating that the microstructure is closely related to the electrical properties of the crystal.At room temperature,the sample synthesized under 2 GPa has the best power factor 1.28?Wcm^-1K^-2.The thermal conductivity of the sample synthesized under 2 GPa was calculated.It was found that CoSb₃ prepared by high temperature and high pressure method has lower thermal conductivity than samples synthesized under normal pressure,and the lowest thermal conductivity in the test temperature range is 2.78 Wm^-1K^-1@623 K.2.Ba_0.2Co₄Sb_11.5Te_0.5.5 was successfully prepared by high temperature and high pressure method in the pressure range of 1.5-3.5 GPa.The optimization of the thermoelectric properties of Ba_0.2Co₄Sb_11.5Te_0.5.5 was studied.The research results are as follows:XRD and EDS test results show that the target compound can be synthesized by high temperature and high pressure method for half an hour.Ba and Te doping can effectively improve the thermoelectric properties of CoSb₃,making conversion of the sample from P type to N type,the power factor at room temperature increased more than 10 times,and the thermal conductivity of the sample synthesized under 2 GPa decreased.For Ba_0.2Co₄Sb_11.5Te_0.5,the results of scanning electron microscopy show that the sample grain size decreased gradually with the increase of synthesis pressure in the pressure range of 1.5-3.5 GPa.With the increase of synthetic pressure,the grain size become small,the grain boundary is more abundant.As the microstructure can change the electrical properties of the sample,the resistivity and absolute value of Seebeck coefficient increase with the pressure.The optimal power factor of the sample synthesized under 3 GPa in the test range is 20.6?Wcm^-1K^-2@723 K.The thermal conductivity of Ba_0.2Co₄Sb_11.5Te_0.5.5 gradually decreases with the increase of synthesis pressure,and the lowest thermal conductivity of the sample synthesized under 3 GPa in the test range is 2.35 Wm^-1K^-1@623 K.Within the synthetic pressure range,the Ba_0.2Co₄Sb_11.5Te_0.5.5 synthesized under 3 GPa has the maximum ZT value of 0.61@723 K.