ICT2014 Abstracts

Recently, nanostructured materials have demonstrated to be able to reduce the lattice thermal conductivity, and then exhibited a significant improvement in thermoelectric performance compared with state-of-the-art bulk alloys. This improvement in ZT mainly attributed to the lower lattice thermal conductivity caused by phonon scattering at the interfaces of nano-sized crystal grains. In addition, nanocrystalline is highly strained, which may play a significant role in thermal transport. Strained materials are widely investigated in thermoelectrics, and the strains are known to influence the thermal transport properties. Moreover, the nanostructures of strained thermoelectric materials have recently been investigated, and their thermal conductivity was calculated using equilibrium molecular dynamics simulations.

In this study, to clarify the role of strain in lattice thermal conductivity of bismuth antimony telluride thin films with nano-sized crystal grains, we performed both experimental studies and modeling. The fabricated thin films had preferred crystal orientation along the c-axis, average grain sizes of 30 < d < 100 nm, and strain of −0.8% < e < −1.4% in the c-axis direction, whereas the strain in the a-b-axis direction was constant at 1.7%. The thermal conductivities were measured to be 0.32 < k < 0.52 W/m/K using a 3w method at room temperature. To gain insight into the thermal transport in the strained nanocrystalline thin films, we estimated the lattice thermal conductivity based on the phonon transport model of full distribution of MFPs affected by the grain size effect, the sound velocity and the specific heat influenced by the strained effect. The measured lattice thermal conductivities were in good agreement between the experimental results and modelling, and we found that the decrease of the lattice thermal conductivity of nanocrystalline thin films can be mainly attributed to the nano-size effect rather than the strain effect.