【作者】 范鑫；

【导师】 贾晓鹏；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2023， 博士

【摘要】 本论文通过高温高压(high-temperature and high-pressure,HPHT)合成一系列硫(S)元素填充方钴矿体系热电材料。探究了合成压力、填充、置换和复合等多种方法协同优化对方钴矿化合物热电性能的影响以及调控规律。对高压合成的方钴矿体系热电材料表征了其相结构、微观结构、元素分布以及热电性能等。实验结果表明,高压环境下以压力作为反应驱动力可以降低反应温度加快合成过程;高压可以提高方钴矿化合物本征孔洞中客体原子的填充极限,填充极限的提升有利于在更大维度调控材料的热电性能;同时,高压密闭反应环境有效的防止合成过程中原料的挥发等优点。通过合成压力、填充和置换等多种方法协同优化了载流子浓度、能带结构和改善材料的电学性能。本论文主要创新性成果如下:(1)通过HPHT合成了Sx Co4Sb12、Sx Co3.6Ni0.4Sb12和Sx Co4Sb11.6Te0.4(x=0、0.05、0.10和0.20)样品,探究了合成压力对方钴矿化合物的填充极限、热稳定性和力学性能等规律的影响。实验结果表明:高温高压合成了S单填充方钴矿化合物。对S单填充和S填充Te置换方钴矿化合物表征了其室温电学性能。S填充Ni置换方钴矿化合物的实验结果表明:高压和Ni置换相结合提高了S在方钴矿本征孔洞中的填充量。与其它压力合成的S填充Ni置换方钴矿化合物相比,1.0 GPa合成S填充量为x=0.05的S填充Ni置换方钴矿室温功率因子最大为7.98×10-4 Wm-1K-2。对样品进行6个月无任何保护措施储存和多次高温高压下退火,初步验证了高压合成的方钴矿化合物具有良好的热稳定性。在773 K时,1.0 GPa合成的S0.05Co3.6Ni0.4Sb12样品z T值为0.46,经多次高温高压下退火的样品z T值最大为0.43。通过HPHT合成S填充Ni置换方钴矿化合物的最大维氏硬度为7.00 GPa,与其它方法合成的方钴矿硬度相比提升了约15%。(2)通过HPHT合成了Sx Co4Sb11.9-y Tey Se0.1(x=0和0.05;y=0.40、0.50、0.60和0.70)样品,探究合成压力、S填充以及Sb位Te-Se双元素置换对方钴矿化合物热电性能的影响。实验结果表明:当合成压力增大时,样品晶格常数变小,(130)主峰向大角度略微偏移。高压和多元素的填充和置换有效的抑制了样品晶粒的生长。当合成压力增大时,样品电导率呈现减小趋势,Seebeck系数的绝对值和功率因子呈现增大趋势。高压、S填充和Te-Se双置换增加了低频声子的散射,降低了样品的热导率。因此,在773 K时,3.0 GPa合成S0.05Co4Sb11.3Te0.6Se0.1功率因子最大为23.85×10-4 Wm-1K-2,热导率最小为1.42Wm-1K-1和z T值最大为1.30。(3)通过HPHT合成了Ba0.27S0.05Co4Sb11.6Te0.4,探究合成压力、Ba-S双填充以及孔隙对方钴矿化合物微观结构和热电性能的影响。实验结果表明:高压合成的Ba-S双填充方钴矿化合物存在大量孔隙。通过EDS检测到分布相对均匀的Ba元素和S元素,表明压力作为反应驱动力能够合成常规方法难以填充或合成的方钴矿化合物。样品中晶格条纹的取向差异巨大,存在大量位错和晶界缺陷。在2.0-3.5 GPa下合成样品的孔隙率为10.52-9.98%,孔隙引起样品的热导率降低了15.00-14.26%。高压、多元素的填充和置换以及孔隙对样品的热导率起到协同优化的作用。因此,在773 K时,3.5 GPa合成样品热导率最小为1.10 Wm-1K-1,3.0 GPa合成样品功率因子最大为20.16×10-4 Wm-1K-2和z T值最大为1.39。(4)通过HPHT合成了引入石墨烯第二相的Cx-S0.05Co4Sb11.6Te0.4(x=0、0.05、0.10和0.20)样品,讨论了合成压力和石墨烯含量对方钴矿复合材料热电性能的调控规律。实验结果表明:石墨烯第二相的引入抑制了晶粒的生长。同时,石墨烯随机分布在不同晶粒之间。当合成压力由1.0 GPa增加到3.5 GPa时,样品电导率减小和Seebeck系数增大。同时,1.5 GPa合成石墨烯含量为x=0.10样品室温功率因子最大为20.18×10-4 Wm-1K-2。因此,选择1.5 GPa下讨论了石墨烯含量对方钴矿复合材料热电性能的调控规律。在方钴矿复合材料中石墨烯含量的增大导致样品的电导率先增大后减小,Seebeck系数的绝对值增大。HPHT合成的随机分布在晶粒之间的石墨烯,有效的增加了样品的晶界,使声子散射增强,热导率降低。因此,在773 K时,石墨烯含量为x=0.10样品功率因子最大为29.36×10-4 Wm-1K-2,晶格热导率为0.99 Wm-1K-1和z T值最大为1.25,与未复合样品相比z T值提升了约25%。综上所述,通过合成压力、填充、置换、复合等多种方法协同优化,调控方钴矿化合物的载流子浓度和内部微观结构,降低热导率,实现方钴矿体系热电材料性能优化。更多还原

【Abstract】 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.更多还原