Optimization Of The Properties Of S-filled CoSb₃-based Thermoelectric Materials By High-temperature And High-pressure Preparation

Globalization has accelerated in the 21^st century,causing a rapid increase in demand for traditional fossil fuels,making it urgent to find alternative green energy and conversion technologies.Thermoelectric conversion is a green and pollution-free energy conversion technology that can improve the secondary utilization rate of energy and alleviate the worldwide energy crisis.Thermoelectric devices based on thermoelectric materials are small,lightweight,and generate no noise or pollution.Thermoelectric materials are widely used in aviation and ocean exploration,wearable flexible electronic devices,semiconductor refrigeration chips,and other fields.Thermoelectric materials are new clean energy materials that can recycle waste heat.Skutterudite thermoelectric materials have a high Seebeck coefficient,high conductivity,and high mechanical stability,making them some of the most promising thermoelectric materials in the middle-temperature region.However,the high thermal conductivity of skutterudite compounds leads to a low quality factor（）.The special crystal structure of skutterudite makes it possible to reduce its thermal conductivity,via strategies such as introducing guest atoms into the intrinsic cavity of skutterudite to enhance phonon scattering;replacing Co and Sb sites in the skutterudite framework with homogeneous or heterogeneous atoms,introducing point defects;introducing a second phase to enhance grain boundary scattering.The above methods can reduce the thermal conductivity of skutterudite and improve its quality factor.Research has shown that a high pressure can improve the Seebeck coefficient,power factor,and other electrical properties of thermoelectric materials.However,the excellent properties at high-pressure environment after pressure relief cannot be permanently retained.In this paper,the excellent thermoelectric properties of skutterudite thermoelectric materials at high pressure were intercepted to normal pressure by combining a high temperature and high pressure.A series of S-filled skutterudite thermoelectric materials were synthesized.The high-temperature and high-pressure（HPHT）method formed a closed reaction environment to prevent the volatilization of raw materials.The introduction of a high pressure accelerated the reaction process and improved the filling limit of guest atoms.S was used as the basic filling element to optimize the thermoelectric properties of skutterudite by using different synthesis pressures,types,contents,and compositions of fillers and substitution elements.The effects of synthesis pressure,S and Ba filling,and Ni,Te,and Se substitution on the microstructure and thermoelectric properties of skutterudite thermoelectric materials were systematically studied.The main innovative achievements of this paper are as follows:1.A series of S-filled S_xCo₄Sb₁₂（x=0,0.05,0.10,and 0.20）and S_xCo₄Sb_11.6Te_0.4（x=0.05,0.10,and 0.20）samples was synthesized by an HPHT method at 3.0 GPa,900 K,and 30 min.The phase structure,and room-temperature electrical properties of the samples were systematically studied.The diffraction peaks of S-filled and S-filled Te substituted skutterudite compounds synthesized at high pressure were skutterite phase.The Seebeck coefficient of S single-filled skutterudite was positive,indicating that skutterudite in this system was a P-type semiconductor.The Seebeck coefficient of S-filled Te-substituted skutterudite was negative,indicating that skutterudite in this system was an N-type semiconductor.S_0.05Co₄Sb₁₂ and S_0.05Co₄Sb_11.6Te_0.4 samples synthesized at high pressure had good room-temperature power factors of 4.24×10^-4and 14.64×10^-4 Wm^-1K^-2,respectively.2.A series of S-filled Ni-substituted S_xCo_3.6Ni_0.4Sb₁₂（x=0,0.05,0.10,and 0.20）samples was synthesized by the HPHT method at pressures in the range of 1.0–3.0 GPa.The phase structure,filling limit,microstructure,thermoelectric properties,and thermal stability of the samples were systematically studied.The experimental results showed that the XRD patterns of the samples synthesized at high pressure were well-matched with the standard card of skutterudite（PDF#78-0976）.A combination of high pressure and Ni substitution increased the filling limit of S in the intrinsic vacancy of skutterudite.There were many lattice stripes in different directions in the sample synthesized at high pressure.The sample also contained significant lattice distortions and dislocation defects.Within the studied pressure range,when the synthetic pressure increased,the absolute value of the Seebeck coefficient and room-temperature conductivity of S_xCo_3.6Ni_0.4Sb₁₂ series samples increased and decreased,respectively.The maximum power factor at room temperature of S_0.05Co_3.6Ni_0.4Sb₁₂ synthesized at 1.0 GPa was7.98×10^-4 Wm^-1K^-2.The room-temperature electrical properties of S_0.05Co_3.6Ni_0.4Sb₁₂did not change significantly after being stored for 6 months without any protective measures.The thermoelectric properties of S_0.05Co_3.6Ni_0.4Sb₁₂ sample synthesized at high pressure and S_0.05Co_3.6Ni_0.4Sb₁₂ sample after several high-pressure thermal cycles were characterized at variable temperature.It was found that the thermoelectric properties did not change significantly,and the maximum z T values were 0.46 and 0.43,respectively.The above results preliminarily show that the skutterudite compound synthesized at high pressure had stable properties and could be stored for a long time.It also maintained its thermal stability after high-pressure annealing.3.A series of S-filled Te-Se double-substitution S_xCo₄Sb_11.9-yTe_ySe_0.1（x=0 and0.05;y=0.40,0.50,0.60,and 0.70）samples was synthesized by the HPHT method in the pressure range of 2.0–3.5 GPa.Changes in the synthesis pressure on the phase structure,micromorphology,and thermoelectric properties of S-filled Te-Se double-substituted skutterudite compounds were systematically studied.The experimental results showed that the main peak（130）of skutterudite shifted to a higher angle when the synthetic pressure increased within the studied pressure range.The Rietveld refinement showed that S filling did not change the lattice constant of skutterudite.At high pressure,the filling of S and the double substitution of Te-Se at Sb sites in skutterudite inhibited grain growth.Within the studied pressure range,when the synthesis pressure increased,the conductivity of the sample decreased,and the absolute value of the Seebeck coefficient and power factor increased.Therefore,the maximum power factor of S_0.05Co₄Sb_11.3Te_0.6Se_0.1 sample synthesized at 3.0 GPa pressure was23.85×10^-4 Wm^-1K^-2 at 773 K,the minimum thermal conductivity was 1.42 Wm^-1K^-1,and the maximum z T value is 1.30.The maximum z T value of Co₄Sb_11.3Te_0.6Se_0.1sample synthesized at the same pressure was 1.07,and the z T value of S_0.05Co₄Sb_11.3Te_0.6Se_0.1sample was about 21.5%higher than that of Co₄Sb_11.3Te_0.6Se_0.1sample.The above results show that combining a high pressure with multi-element filling and substitution can reduce the thermal conductivity of skutterudite and optimize the thermoelectric performance.4.A series of Ba-S double-filled Te-substituted Ba_0.27S_0.05Co₄Sb_11.6Te_0.4compounds were synthesized by the HPHT method within the pressure range of 2.0–3.5 GPa.The effects of phase structure,microstructure,thermoelectric properties,and pores on the thermal transport of the samples were systematically studied.The XRD peaks of all samples showed a typical skutterudite structure without impurity phases.When the synthesis pressure increased,the lattice constant of the sample decreased.There were many nanoscale and microscale pores in the sample,and relatively uniformly-distributed Ba and S elements were detected by EDS.There were also many special structures in the samples synthesized at high pressure,such as lattice stripes with different directions,nanocrystalline and amorphous regions,lattice distortion,and dislocation defects.Pores and special microstructures formed many phonon scattering centers and hindered phonon transmission.Within the pressure range studied,the power factor of the sample increased with the synthesis pressure.The porosity of the samples synthesized at 2.0–3.5 GPa were 10.52–9.98%,and the thermal conductivity of the sample decreased by about 15.00–14.26%due to the presence of pores.Therefore,the minimum thermal conductivity of Ba_0.27S_0.05Co₄Sb_11.6Te_0.4 sample synthesized at 3.5GPa was 1.10 Wm^-1K^-1 at 773 K.The maximum power factor of Ba_0.27S_0.05Co₄Sb_11.6Te_0.4 sample synthesized at 3.0 GPa was 20.16×10^-4 Wm^-1K^-2 at 773K,and the maximum z T was 1.39.The above results show that Ba-S double-filled skutterudite compounds that cannot be synthesized by conventional methods can be synthesized by the HPHT method.The existence of pores during high-pressure synthesis greatly reduced the thermal conductivity and optimized the thermoelectric properties.5.A series of S-filled Te-substituted C_x-S_0.05Co₄Sb_11.6Te_0.4 composites with different graphene contents（x=0,0.05,0.10,and 0.20）was synthesized by the HPHT method within the pressure range of 1.0–3.5 GPa.The phase structure,micromorphology,room-temperature,and variable-temperature thermoelectric properties of skutterudite composites were systematically studied.The sample XRD pattern matched well with the standard card of skutterudite（PDF#78-0976）,and the second phase of graphene was not detected.When the content of graphene increased,the grain growth of the sample was inhibited,and graphene was randomly attached to different grains.The EDS analysis of the C_0.10-S_0.05Co₄Sb_11.6Te_0.4 composite showed that the ratio of atoms in the sample was basically consistent with the nominal stoichiometric ratio.There were many lattice defects and distortions in the samples synthesized at high pressure.The room-temperature power factor of C_0.10-S_0.05Co₄Sb_11.6Te_0.4 composites increased first and then decreased upon increasing the synthesis pressure.The maximum power factor at room temperature of the C_0.10-S_0.05Co₄Sb_11.6Te_0.4 composite synthesized at 1.5 GPa was 20.18×10^-4 Wm^-1K^-2.The influence of different graphene contents on the thermoelectric properties of skutterudite at 1.5 GPa was discussed.When the content of graphene increased,the absolute value of the Seebeck coefficient increased,and the conductivity first increased and then decreased.A combination of high pressure and graphene further reduced the thermal conductivity of the composite.Therefore,the C_0.10-S_0.05Co₄Sb_11.6Te_0.4 composite with a graphene content x=0.10 had a relatively large power factor of 29.36×10^-4 Wm^-1K^-2at 773 K,relatively low lattice thermal conductivity of 0.99 Wm^-1K^-1and the maximum z T value of 1.25.Compared with the S_0.05Co₄Sb_11.6Te_0.4 sample,z T was increased by about 25%.The above results show that a skutterudite composite with a random graphene distribution was prepared by the HPHT method.In this paper,a series of S-filled skutterudite thermoelectric materials was synthesized by the HPHT method,and relevant experiments were carried out.The results show that the properties of skutterudite thermoelectric materials were optimized by introducing pressure parameters combined with filling,substitution,and compositing methods.Introducing pressure further expanded the control parameters of skutterudite,making it possible to further optimize the thermoelectric properties of skutterudite compounds.