Understanding size and interface dependent anisotropic thermal conduction in correlated multilayer structures

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It has recently been established that a very effective method for enhancement of thermoelectric efficiency is through suppression of thermal conductivity by structuring at the nanoscale, such as in multilayer systems. However, the mechanisms of this effect, i.e. the scattering and blocking of phonons at various wavelengths by interfaces are not well understood. The objective of this proposed joint project is to further develop the understanding of anisotropic and size dependent thermal conduction in oxide multilayer systems. We have optimized the 3-omega method in top and bottom electrode geometries for reliable measurements of thermal conductivity κ. The use of neural networks in combination with detailed finite element modelling of the coupled thermal and electrical conditions will allow for high-precision determination of in-plane and cross-plane contributions, κ┴ and κ||. Our continuum modelling of phonon dispersion has led to the selection of promising thermoelectric oxide systems for multilayers, namely doped SrTiO3 and Pr1-xCaxMnO3 and the misfit layered compound Ca3Co4O9, for our further studies. Separating out the influence of acoustic impedance mismatch of layers from various scattering contributions of interfaces, disorder, and dopants will be performed by careful experimental studies combined with atomic level modelling of thermal conductivity using non-equilibrium molecular dynamics and the phonon Boltzmann equation.