【作者】 姜一平；

【导师】 贾晓鹏；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2010， 博士

【摘要】 热电材料是一种能够实现电能和热能直接转换的功能材料。由热电材料制作的温差发电和制冷器件具有无污染、无噪声、安全可靠等优点,具有广泛的应用前景。而具有“电子晶体–声子玻璃”输运特征的填充式方钴矿笼状结构化合物较传统热电材料具有潜在的更高的热电性能。压力可以改变晶体内部的电子结构及其相关的各种参数。在压力的作用下,热电材料的热电性质也会随之发生改变,因此我们通过改变压力来借此获得高性能的热电材料。而且高压合成方法具有合成时间短,调制参数连续变化等优点。因此高温高压合成是一个寻找新材料,提高材料热电性能的一个极为有效地手段。在本论文中,我们采用高温高压方法合成CoSb₃基方钴矿热电材料并对其热电性质进行了研究。利用高压合成方法在1.5⁵.0GPa不同压力条件下成功制备出CoSb₃方钴矿热电材料,XRD衍射图谱显示合成的为单相CoSb₃。在室温下对不同压力下合成的CoSb₃的电学特性进行测试,结果表明样品的电阻率和Seebeck系数随着合成压力的升高而增大。利用高压合成方法成功制备了置换掺杂型CoSb₃基方钴矿化合物CoSb_2.750TexGe_0.250-x （x=0.125, 0.175, 0.200）并对其性质进行了研究,发现样品的功率因子随着合成压力的升高逐渐降低,随着Te掺杂量的增加逐渐增加。虽然样品的Seebeck系数的绝对值随着压力的升高呈增加的趋势,但由于电阻率随着压力的升高增加的更为显著,所以导致合成样品的功率因子随合成压力的升高逐渐降低。压力为2.0GPa时, CoSb_2.750Te0.200Ge0.050的功率因子最大,达到7.59μW cm-1 K-2。研究结果表明高压合成方法制备的样品电输运性能有了明显的提高。采用高压合成方法在1.5⁴.5GPa压力条件下合成出了系列不同填充量的La填充型方钴矿热电材料LaxCo4Sb12（x=0.1, 0.3, 0.5, 0.8, 1.0）,并研究了合成压力和不同填充量对其电输运性能的影响。从制备的样品的XRD衍射图可以看出LaxCo4Sb12具有CoSb₃立方结构,与常压下的CoSb₃的衍射图谱相吻合。随着La填充量的增加,晶格常数逐渐增加。LaxCo4Sb12样品的电阻率随着合成压力的升高而增大。在合成压力是1.5GPa时,研究了Seebeck系数、电阻率及功率因子随着La填充量的变化关系。研究结果表明,随着La填充量的增加,其电阻率和Seebeck系数均呈现出先增加后逐渐降低的趋势。当La的填充量为0.5时,CoSb₃的功率因子达到最大值5.16μW·cm-1·K-2。在1.5GPa压力条件下制备了系列不同填充量的Sm填充型方钴矿SmxCo4Sb12（0<x≤1.0）。随着Sm填充分数x的增加,SmxCo4Sb12化合物的Seebeck系数的绝对值呈现降低的趋势, SmxCo4Sb12的电阻率先上升,在填充量为0.2时电阻率达到最大。随着填充量的增加,方钴矿的电阻率大幅下降,在填充量为0.5时电阻率达到最小值。样品经过对比选择,进行变温的电学输运性能和热学输运性能测试。样品的晶格热导率都随着温度的升高而减小,Sm填充样品的热导率明显小于未填充样品的热导率,表明Sm填充能够有效降低CoSb₃方钴矿的晶格热导率。在测试温度范围内,Sm填充方钴矿热电材料的ZT值随着温度的升高逐渐增加。样品Sm0.5Co4Sb12在723K时,得到最大ZT值为0.81。更多还原

【Abstract】 Thermoelectric material is one of kind of function materials that can directly convert electricity and heat. Thermoelectric devices based on thermoelectric materials could convert heat to electricity, transferred through or pumped by the charge carriers in the thermoelectric materials, and rejected at the heat sink. The advantages of solid-state thermoelectric devices are lightweight, small, portable, inexpensive, quiet performance and the ability for localized‘spot’cooling. Thus, there is now a renewed interest in research on improved for thermoelectric applications.The use of TE device is limited by their low efficiencies. A prerequisite for an efficient energy conversion in thermoelectric devices is indexed by the dimensionless figure of merit, ZT=TS2σ/κ, where T is the absolute temperature, S is the Seebeck coefficient,σis the electrical conductivity, andκis the thermal conductivity. Because of the low energy converting efficiency （<10%）, several categories of thermoelectric materials used currently have some limitations in applications. Therefore, the development of novel thermoelectric materials with high thermoelectric properties has been the principal concerns of the related researches.The recent interest in semiconducting skutterudite materials is due to their potential as effective thermoelectric materials. The structure of binary skutterudite compound belongs to the body-centred cubic space group Im3 and has two relatively large voids at positions in the crystal lattice. Filling these voids with lanthanide, actinide, and alkaline-earth ions to form filled skutterudite had been proved an effective method to reduce the lattice thermal conductivity. The void-filling atoms“rattle”in their voids and substantially affect the phonon propagation through the lattice. In this paper, we present data on the thermoelectric properties of Sm filled skutterudites to discuss the dimensionless figure of merit ZT.Up to date, the hot pressing （HP）, spark plasma sintering （SPS）, mechanical alloying （MA） and high temperature and high pressure （HTHP） have been used to synthesized the thermoelectric materials.Recently, high-pressure technology as a potentially tool to increase the rate of discovery of useful materials has spread all over the world. Comparing to other preparation methods for thermoelectric materials, the method of high-pressure and high-temperature （HPHT） has many advantages, including the ability to tune rapidly and cleanly, restraining the disorder phase separation and other complicating factor during the preparation of materials.In this work, we tries to study the performance of CoSb₃ based skutterudite compounds. Firstly, we synthesized the pure CoSb₃ skutterudite compounds by HPHT technique and studied the electrical properties of CoSb₃. The results indicated that the electrical resistivity increased with an increase of synthetic pressure. The calculation of the band structure studies show that the band gap increases with an increase of pressure, which may be source of decreasing of carrier concentration.Skutterudites compound CoSb_2.750Ge_0.250-xTex （x=0.125, 0.150,0.175, 0.200） were studied by HPHT. The electrical resistivity of CoSb_2.750Ge_0.250-xTex decreases with increasing Te concentration. Because of the compensating effect of Ge. Seebeck coefficient measurements revealed that all the samples were n-type.The measurement of transport properties show that the Seebeck coefficient and resistivity increase with an increase of synthetic pressure. The largest power factor at room temperature reaches to 9.0μW cm-1 k2 for the CoSb_2.750 Ge0.100Te0.150 at 2.0 GPa.We synthesized LaxCo4Sb12 samples with different La filling content by HPHT method, at 1.5-4.5 GPa, and 900 K. all the samples are single phase with Im3 structure from the test of XRD. With increasing of La filling fraction, the lattice spacing increase linearly.We test the electrical resistivity and the Seebeck coefficient of LaxCo4Sb12 samples at room temperature. The results indicate that the LaxCo4Sb12 skutterudite compounds show p-type conduction. The filling La atoms may contribute some electron to the conduction band. The value of power factor 5.16μW·cm-1·K-2 was obtained at x= 0.5.The skutterudite compounds of SmxCo4Sb12 were synthesized by HPHT technique. The XRD patterns indicate that the major phase is CoSb₃ phase, which is crystallized in a cubic CoAs3-type structure with Im3 space group. In addition, compared with the samples prepared by a traditional method, the processing time of HPHT method is reduced from a few days to less than an hour, and the Sm filling fraction limit increases. The magnitude of Seebeck coefficients of these n-type compounds increases with increasing temperature, the values that are dependent on the doping level. The electrical resistivity of SmxCo4Sb12 samples increases and then decreases with an increasing Sm filling fraction. For the samples with filling fraction x=0.5, electrical resistivity increases with increasing temperature monotonously, indicating the character of a degenerated or heavily doped semiconductor. The power factor increases significantly and the maximum of power factor shifts toward higher temperature with the increasing the filling fraction of Sm. A maximum value of 25.1μW cm-1K-2 is obtained in Sm0.5Co4Sb12 sample at 665K, which is much higher than pure CoSb₃ prepared by the traditional method at normal pressure.The thermal conductivity of the Sm-filled skutterudites is lower as compared to that of CoSb₃. ZT increases almost linearly with increasing temperature for SmxCo4Sb12 compounds. The maximum ZT value is 0.81 for Sm0.5Co4Sb12 at 723K.In conclusion, the results obtained in our work show that HPHT is an effective technique for synthesizing the CoSb₃-based skutterudite TE materials. The performance for the TE materials with the structure of CoAs3 could be improved by high pressure according to the results by now.更多还原