Ultra low thermal conductivity in layered disordered crystalline materials

This dissertation presents an alternative route to achieve ultralow thermal conductivity in a dense solid. Thin films of disordered layered crystalline materials were deposited using Modulated Elemental Reactants (MER) method. Cross-plane thermal conductivity was measured using Time-Domain Thermo Reflectance (TDTR) method; elastic properties were investigated using picosecond acoustics. The results are applied to reducing the thermal conductivity in misfit layer materials and multilayers containing disordered layered crystalline materials. The cross-plane thermal conductivity of thin films of WSe2 is as small as 0.05 W m-1 K-1 at room temperature, 30 times smaller than the c-axis thermal conductivity of single-crystal WSe2 and a factor of 6 smaller than the predicted minimum thermal conductivity for this material. The ultralow thermal conductivity is attributed to the anisotropic bonding of the layered WSe2 and orientational disorder in the stacking of well-crystallized WSe2 sheets along the direction perpendicular to the surface. Disordering of the layered structure by ion bombardment increases the thermal conductivity. I measured the room-temperature, cross-plane thermal conductivities and longitudinal speeds of sound of misfit-layer dichalcogenide films [(PbSe)m (TSe2)n]i (T = W or Mo, m = 1-5, n = 1-5) synthesized by the MER. The thermal conductivities of these nanoscale layered materials are 5-6 times lower than the predicted minimum thermal conductivity ??min of PbSe. Thermal conductivity decreases with increasing content of the main source of anisotropy in the sample, the layered chalcogenide, and it is largely unaffected by variations in superlattice period. I investigated the lower limit to the lattice thermal conductivity of Bi2Te3 and related materials using thin films synthesized by MER. The thermal conductivities of single layer films of Bi2Te3 , Bi2Te3 and Sb-doped Bi2Te3 and multilayer films of (Bi2Te3)m(TiTe2)n and [(BixSb1-x)2Te3]m(TiTe2)n are measured by TDTR; the thermal conductivity data are compared to a Debye-Callaway model of heat transport by acoustic phonons. The homogeneous nanocrystalline films have average grains sizes 30 < d < 100 nm as measured by the width of the (003) x-ray diffraction peak. Multilayer films incorporating turbostratic TiTe2 enable studies of the effective thermal conductivity of Bi2Te3 layers as thin as 2 nm. In the limit of small grain size or layer thickness, the thermal conductivity of Bi2Te3 approaches the predicted minimum thermal conductivity of 0.31 W m-1 K-1. The dependence of the thermal conductivity on grain size is in good agreement with the Debye-Callaway model. The use of alloy (Bi,Sb)2Te3 layers further reduces the thermal conductivity of the nanoscale layers to as low as 0.20 W m-1 K-1