Thermoelectric effects in polar liquids and ionic conductors

Transport theory describes non-equilibrium conditions. The Onsager equations set a linear relationship between thermodynamic forces and macroscopic currents. When a temperature gradient is applied, it can generate heat and ions diffusion. Similarly, an electric field can induce electric currents and heat transport. The off-diagonal elements of the Onsager matrix address coupled, e.g. thermoelectric, effects. In the 1950s, Green and Kubo developed a sound equilibrium framework for transport phenomena based on linear response theory. Despite the theoretical depth, computational inefficiencies hindered its broad application to equilibrium molecular dynamics simulations. In the last decade, the introduction of gauge and convective invariance principles addressed common misconceptions in the Green Kubo theory, concerning thermoelectric and heat transport. The aim of this Thesis is threefold. I investigate thermoelectric effects in classical fluids. In insulating liquids, thermopolarization is a quasi-equilibrium phenomenon. After being perturbed by a temperature inhomogeneity, the dielectric degrees of freedom relax on a microscopic time scale. In conducting fluids, a temperature gradient induces electric currents. The Seebeck effect refers to the electric field that must be applied to counterbalance the temperature gradient and then achieve zero net charge flow. We study thermoelectric transport in several ionic conductors. We develop a Bayesian strategy to compute both diagonal and off-diagonal Onsager coefficients from equilibrium molecular dynamics simulations. The Bayesian approach leverages the statistical properties of Green-Kubo estimators. A single statistical model can be designed to estimate the entire Onsager matrix. Finally, I participated in a collaboration studying heat transport in glasses. In disordered solids, the acoustic contribution to thermal conductivity is challenging to estimate. The investigation of nearly 140000-atoms harmonic models of glasses, reveals the presence of Rayleigh scattering in the low-frequency regime. In harmonic disordered systems, our findings predict a divergent bulk thermal conductivity at all temperatures. The divergence is cured by proper accounting for anharmonic effects.