层状化合物材料热输运性质的第一性原理研究

发布时间：2018-04-08 08:19

本文选题：第一性原理　切入点：BiCuOSe　出处：《南京大学》2016年博士论文

【摘要】：近年来,实验发现了许多层状化合物材料。由于它们具有独特而丰富的光、电、磁、热、催化等性能,这些材料在热电、超导、铁电等很多领域具有广阔的应用前景,因此得到了研究者广泛的关注。晶格热导率及其它相关热输运性质,对于各种材料的实际应用是一个非常重要的性质。基于密度泛函理论,本文采用了第一性原理计算方法,系统地研究了层状热电材料BiCuOCh (Ch=S, Se, Te)和层状过渡金属二硫化物Td-WTe2的晶格热导率以及相关的声子振动频率分布、Griineisen参数、德拜温度、热容、热膨胀系数等热输运性质。主要成果如下：(1)层状热电材料BiCuOCh (Ch=S, Se, Te)热输运性质的研究。运用第一性原理计算方法,并结合Klemens热导率模型,我们发现BiCuOCh (Ch=S, Se, Te)本征的晶格热导率都非常低,并且表现出各向异性。其中,在300K下,晶格热导率的最高值出现在沿着BiCuOSe的(001)面,值为1.16 Wm-1K-1；最低值沿着BiCuOTe的[001]方向,只有0.14 Wm-1K-1。三种材料中,BiCuOTe具有最低的晶格热导率,是由于它拥有最低的德拜温度决定的。我们也研究了它们热导率的各向异性,BiCuOS、BiCuOSe和BiCuOTe三种材料面内热导率和面间热导率之比分别为2.68,2.21,和3.14。同时,我们发现三种材料热导率的各向异性和它们声子群速度各向异性是一致的。最后我们讨论了它们晶格热导率对材料尺寸的依赖关系,发现引入纳米结构可以进一步有效地降低它们的晶格热导率,从而继续提升其热电效率。(2) Td-WTe2晶格热导率的研究。在这个工作中,我们基于第一性原理方法,研究了块体WTe2的晶格结构性质和本征热导率。我们发现,范德华(van der Waals)作用主导着WTe2层间的相互作用,要得到WTe2的精确结构必须要考虑范德华作用。我们的计算表明,WTe2的晶格热导率是各向异性的,300 K下的最高热导率(11.06 Wm-1K-1)沿着b轴,而最低热导率(1.04 Wm-1k-1)则沿着c轴方向。它的平均热导率是2.06 Wm-1K-1。声子群速度的各向异性是决定热导率各向异性的重要原因。而且,WTe2晶体在垂直于层的方向上拥有超低的热导率,甚至比WSe2在相同方向上的热导率更低。我们同样也研究了WTe2晶格热导率对尺寸的依赖关系,结果表明引入纳米尺寸可以进一步降低材料的晶格热导率,并可能大大提高它的热电效率。(3)Td-WTe2晶体热膨胀的研究。本文利用Gruneisen公式,从第一性原理出发,研究了Td-WTe2的各向异性的热膨胀以及相关热性质。Td-WTea晶体属于对称性较低的正交晶系,我们利用Gruneisen公式,对晶体仅使用了6个形变来获得广义Gruneisen参数。相比于基于简谐近似的直接最小化晶体自由能的方法,我们避免了大量的计算需求。我们研究了WTe2晶体的弹性常数,广义Gruneisen参数,宏观Gruneisen函数,线性热膨胀系数,体积热膨胀系数以及定压热容。我们发现,WTe2晶体的线性热膨胀系数是各向异性的。低温下在b方向有轻微的负热膨胀现象。但是体积热膨胀在整个研究的温度范围内都是正的热膨胀。在远高于德拜温度时,各个方向的线性热膨胀系数和体积热膨胀系数都分别达到了饱和值。a,b,和c三个晶格方向上的线性热膨胀系数分别为10.06,7.54,和4.45x10-6K-1。体积热膨胀系数是22.05×10-6K-1。
[Abstract]:In recent years, the experiment found many layered materials. Because of their unique and rich light, electric, magnetic, thermal and catalytic properties of these materials, superconducting in power, and has broad application prospects in many fields such as ferroelectric, so the researchers are focused on. The lattice thermal conductivity and other related heat transport properties for the practical application of various materials is a very important property. Based on the density functional theory, the calculation method of the first principle, systematic study of the layered thermoelectric materials BiCuOCh (Ch=S, Se, Te) and two layered transition metal sulfide Td-WTe2 lattice thermal conductivity and the phonon frequency the distribution of the Griineisen parameter, Debye temperature, heat capacity, thermal expansion coefficient and thermal transport properties. The main results are as follows: (1) layered thermoelectric materials BiCuOCh (Ch=S, Se, Te) to study the transport properties of heat transport by the first. The calculation method of principle, and combined with the Klemens thermal conductivity model, we found that BiCuOCh (Ch=S, Se, Te) the lattice thermal conductivity is very low, and exhibit anisotropy. Among them, in 300K, the highest value of the lattice thermal conductivity occurs along the BiCuOSe (001), a value of 1.16 Wm-1K-1; the lowest value along the [001] direction of BiCuOTe Wm-1K-1., only 0.14 of the three kinds of materials, BiCuOTe has the lowest lattice thermal conductivity, because it has the lowest Debye temperature. We also studied the BiCuOS anisotropy, their thermal conductivity, BiCuOSe and BiCuOTe materials of three kinds of heat conductivity and thermal conductivity of the surface the ratios were 2.68,2.21 and 3.14., at the same time, we found that three kinds of thermal conductivity anisotropy and their phonon group velocity anisotropy is consistent. Finally we discuss their lattice thermal conductivity dependence on the size of the material, found that the introduction of Nano structure can further effectively reduce their rate of lattice thermal conductivity, and continue to enhance the thermoelectric efficiency. (2) Td-WTe2 on the lattice thermal conductivity. In this work, we study the first principle method based on lattice structure property of block WTe2 and the intrinsic thermal conductivity. We found that Fan Dehua (van der Waals) leading role interaction between WTe2 layers, the precise structure to get WTe2 must consider the Fan Dehua effect. Our calculations show that the lattice thermal conductivity of WTe2 is anisotropic, the highest thermal conductivity rate under 300 K (11.06 Wm-1K-1) along the b axis, and the low thermal conductivity (1.04 Wm-1k-1) is along the C axis direction. The average rate of its thermal conductivity anisotropy is 2.06 Wm-1K-1. phonon group velocity is an important factor of determining anisotropic thermal conductivity. Moreover, WTe2 crystal has low thermal conductivity rate in the direction perpendicular to the layers, even more than WSe2 The thermal conductivity in the same direction of the lower rate. We also study the lattice thermal conductivity of WTe2 depends on the size. The results show that the introduction of nano size can further decrease the lattice thermal conductivity, and can greatly improve the thermoelectric efficiency of it. (3) Td-WTe2 on the thermal expansion of the crystal. In this paper, by using the Gruneisen formula, starting from the first principle, the Td-WTe2 anisotropy of thermal expansion and thermal properties of.Td-WTea crystal belongs to orthorhombic symmetry is low, we use the Gruneisen formula, the crystal using only 6 deformation to obtain generalized Gruneisen parameters. Compared to the direct method to minimize the crystal free energy based on the harmonic approximation, we avoided the large computational demands. We studied the elastic constants of WTe2 crystal, the generalized Gruneisen parameters, Gruneisen macro function, linear thermal expansion coefficient, volume thermal expansion coefficient And the heat capacity at constant pressure. We found that the linear thermal expansion coefficient of WTe2 crystal is anisotropic. Under low temperature in B direction is slightly negative thermal expansion phenomenon. But the volume thermal expansion in the temperature range of the research are positive. In the thermal expansion is much higher than the Debye temperature, linear thermal expansion in all directions the coefficient of thermal expansion coefficient and volume respectively reach the saturation value of.A, B, and C three linear thermal expansion coefficient of the lattice direction were 10.06,7.54 and 4.45x10-6K-1., the volume thermal expansion coefficient is 22.05 * 10-6K-1.

【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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