First-principle Study Of Thermal Transport Properties Of Layered Compounds

There are many kinds of layered compounds. Many of them have attracted significant interest and have promising applications due to their unique optic, electrical, thermal, magnetic and catalytic properties.Thermal conductivity, thermal expansion, and related thermal transport properties are the keys for practical applications of materials. In this work, through the first principles calculations based on the density functional theory (DFT), we systemically study the thermal conductivities of layered BiCuOCh (Ch= S, Se, Te) and Td-WTe2. And their related thermal transport properties such as phonon spectra, heat capacity, Debye temperature, Gruneisen parameters, and thermal expansion are also investigated. The main conclusions are listed as follows:1. Thermal properties of layered oxychalcogenides BiCuOCh (Ch= S, Se, and Te). Using first-principles calculations and Klemens model, we have investigated the lattice thermal conductivity and related thermal properties of BiCuOCh(Ch= S, Se, and Te). It is found that the intrinsic lattice thermal conductivities of the materials are quite low and anisotropic. The highest lattice thermal conductivity is 1.16 Wm-1K-1(at 300 K) in the (001) plane of BiCuOSe, while the lowest is only 0.14 Wm-1K-1 (at 300 K) along the [001] direction of BiCuOTe, which is attributed to its lowest Debye temperature. We also have investigated the anisotropy of their thermal conductivities, and found the ratios of the thermal conductivities between the in-plane and out-of-plane are 2.68,2.21, and 3.14 for BiCuOS, BiCuOSe, and BiCuOTe, respectively. We find the anisotropy of phonon velocities is in accordance with the lattice thermal conductivities. At last we investigate the size dependence of thermal conductivity, and find that introducing nanostructure can further decrease the lattice thermal conductivities of the three materials, which may increase their figure of merit ZT.2. First-principles study of lattice thermal conductivity of Td-WTe2. We have investigated the structural properties and the intrinsic lattice thermal conductivity of bulk Td-WTe2 from first principles. It is found that van der Waals force plays an important role in the inter-layer interaction of WTe2. Our calculations indicate that the thermal conductivity of WTe2 is anisotropic, and the highest thermal conductivity (11.06 Wm-1K-1 at 300 K) is along b-axis, while the lowest one is along c-axis (1.04 Wm-1K-1 at 300 K). The averaged thermal conductivity is 2.60 Wm-1K-1. The anisotropic group velocities of phonon are responsible for the anisotropy of thermal conductivity. In addition, the WTe2 has the ultralow low cross-plane thermal conductivity which is even lower than that of WSe2. We also studied the size dependent thermal conductivity of WTe2. Our results indicate that introducing nanostructure can possibly further decrease the lattice thermal conductivity in WTe2.3. Anisotropic thermal expansion of Td-WTe2. We have extended the Griineisen formalism to treat Td-WTe2 which processes a low-symmetry orthorhombic structure. We just apply six deformations to calculate the Griineisen parameters of Td-WTe2. We obtain the generalized Gruneisen parameters, macroscopic Griineisen parameters, linear and bulk thermal expansion coefficient, and specific heat at constant pressure Cp. We find the thermal expansion of WTe2 is anisotropic. At low temperature the linear thermal expansion coefficient along b-axis is slightly negative, but the bulk thermal expansion coefficient is always positive in the whole temperature range. When T>?D, the linear thermal expansion coefficients along three directions and bulk thermal expansion coefficient reach saturation values. The saturation values are 10.06,7.54, and 4.45×10-6K-1 for the linear thermal expansion coefficients along a, b, and c directions, respectively. And the saturation value of bulk thermal expansion coefficient is 22.05 ×10-6K-1.