Ultra low thermal conductivity in layered disordered crystalline materials

Low Thermal Conductivity;Misfit layer;turbostratic;disordered;layered;thermal transport

Chiritescu, Catalin

关键词: Low Thermal Conductivity;    Misfit layer;    turbostratic;    disordered;    layered;    thermal transport;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/16101/Chiritescu_Catalin.pdf?sequence=2&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This dissertation presents an alternative route to achieve ultralow thermalconductivity in a dense solid. Thin films of disordered layered crystalline materials weredeposited using Modulated Elemental Reactants (MER) method. Cross-plane thermalconductivity was measured using Time-Domain Thermo Reflectance (TDTR) method;elastic properties were investigated using picosecond acoustics. The results are appliedto reducing the thermal conductivity in misfit layer materials and multilayers containingdisordered layered crystalline materials.The cross-plane thermal conductivity of thin films of WSe2 is as small as 0.05 Wm-1 K-1 at room temperature, 30 times smaller than the c-axis thermal conductivity ofsingle-crystal WSe2 and a factor of 6 smaller than the predicted minimum thermalconductivity for this material. The ultralow thermal conductivity is attributed to theanisotropic bonding of the layered WSe2 and orientational disorder in the stacking ofwell-crystallized WSe2 sheets along the direction perpendicular to the surface.Disordering of the layered structure by ion bombardment increases the thermalconductivity.I measured the room-temperature, cross-plane thermal conductivities andlongitudinal 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 ofthese nanoscale layered materials are 5-6 times lower than the predicted minimumthermal conductivity Λmin of PbSe. Thermal conductivity decreases with increasingcontent of the main source of anisotropy in the sample, the layered chalcogenide, and it islargely unaffected by variations in superlattice period.I investigated the lower limit to the lattice thermal conductivity of Bi2Te3 andrelated materials using thin films synthesized by MER. The thermal conductivities ofsingle 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 thermalconductivity data are compared to a Debye-Callaway model of heat transport by acousticphonons. The homogeneous nanocrystalline films have average grains sizes 30 < d < 100nm as measured by the width of the (003) x-ray diffraction peak. Multilayer filmsincorporating turbostratic TiTe2 enable studies of the effective thermal conductivity ofBi2Te3 layers as thin as 2 nm. In the limit of small grain size or layer thickness, thethermal conductivity of Bi2Te3 approaches the predicted minimum thermal conductivityof 0.31 W m-1 K-1. The dependence of the thermal conductivity on grain size is in goodagreement with the Debye-Callaway model. The use of alloy (Bi,Sb)2Te3 layers furtherreduces the thermal conductivity of the nanoscale layers to as low as 0.20 W m-1 K-1.