本文選題：第一性原理　切入點：BiCuOSe　出處：《南京大學》2016年博士論文

【摘要】：近年來,實驗發(fā)現(xiàn)了許多層狀化合物材料。由于它們具有獨特而豐富的光、電、磁、熱、催化等性能,這些材料在熱電、超導、鐵電等很多領(lǐng)域具有廣闊的應用前景,因此得到了研究者廣泛的關(guān)注。晶格熱導率及其它相關(guān)熱輸運性質(zhì),對于各種材料的實際應用是一個非常重要的性質(zhì)�；诿芏确汉碚�,本文采用了第一性原理計算方法,系統(tǒng)地研究了層狀熱電材料BiCuOCh (Ch=S, Se, Te)和層狀過渡金屬二硫化物Td-WTe2的晶格熱導率以及相關(guān)的聲子振動頻率分布、Griineisen參數(shù)、德拜溫度、熱容、熱膨脹系數(shù)等熱輸運性質(zhì)。主要成果如下：(1)層狀熱電材料BiCuOCh (Ch=S, Se, Te)熱輸運性質(zhì)的研究。運用第一性原理計算方法,并結(jié)合Klemens熱導率模型,我們發(fā)現(xiàn)BiCuOCh (Ch=S, Se, Te)本征的晶格熱導率都非常低,并且表現(xiàn)出各向異性。其中,在300K下,晶格熱導率的最高值出現(xiàn)在沿著BiCuOSe的(001)面,值為1.16 Wm-1K-1；最低值沿著BiCuOTe的[001]方向,只有0.14 Wm-1K-1。三種材料中,BiCuOTe具有最低的晶格熱導率,是由于它擁有最低的德拜溫度決定的。我們也研究了它們熱導率的各向異性,BiCuOS、BiCuOSe和BiCuOTe三種材料面內(nèi)熱導率和面間熱導率之比分別為2.68,2.21,和3.14。同時,我們發(fā)現(xiàn)三種材料熱導率的各向異性和它們聲子群速度各向異性是一致的。最后我們討論了它們晶格熱導率對材料尺寸的依賴關(guān)系,發(fā)現(xiàn)引入納米結(jié)構(gòu)可以進一步有效地降低它們的晶格熱導率,從而繼續(xù)提升其熱電效率。(2) Td-WTe2晶格熱導率的研究。在這個工作中,我們基于第一性原理方法,研究了塊體WTe2的晶格結(jié)構(gòu)性質(zhì)和本征熱導率。我們發(fā)現(xiàn),范德華(van der Waals)作用主導著WTe2層間的相互作用,要得到WTe2的精確結(jié)構(gòu)必須要考慮范德華作用。我們的計算表明,WTe2的晶格熱導率是各向異性的,300 K下的最高熱導率(11.06 Wm-1K-1)沿著b軸,而最低熱導率(1.04 Wm-1k-1)則沿著c軸方向。它的平均熱導率是2.06 Wm-1K-1。聲子群速度的各向異性是決定熱導率各向異性的重要原因。而且,WTe2晶體在垂直于層的方向上擁有超低的熱導率,甚至比WSe2在相同方向上的熱導率更低。我們同樣也研究了WTe2晶格熱導率對尺寸的依賴關(guān)系,結(jié)果表明引入納米尺寸可以進一步降低材料的晶格熱導率,并可能大大提高它的熱電效率。(3)Td-WTe2晶體熱膨脹的研究。本文利用Gruneisen公式,從第一性原理出發(fā),研究了Td-WTe2的各向異性的熱膨脹以及相關(guān)熱性質(zhì)。Td-WTea晶體屬于對稱性較低的正交晶系,我們利用Gruneisen公式,對晶體僅使用了6個形變來獲得廣義Gruneisen參數(shù)。相比于基于簡諧近似的直接最小化晶體自由能的方法,我們避免了大量的計算需求。我們研究了WTe2晶體的彈性常數(shù),廣義Gruneisen參數(shù),宏觀Gruneisen函數(shù),線性熱膨脹系數(shù),體積熱膨脹系數(shù)以及定壓熱容。我們發(fā)現(xiàn),WTe2晶體的線性熱膨脹系數(shù)是各向異性的。低溫下在b方向有輕微的負熱膨脹現(xiàn)象。但是體積熱膨脹在整個研究的溫度范圍內(nèi)都是正的熱膨脹。在遠高于德拜溫度時,各個方向的線性熱膨脹系數(shù)和體積熱膨脹系數(shù)都分別達到了飽和值。a,b,和c三個晶格方向上的線性熱膨脹系數(shù)分別為10.06,7.54,和4.45x10-6K-1。體積熱膨脹系數(shù)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.

【學位授予單位】：南京大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：O469

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