GOALI: Experimental Tests of Nonequilibrium Thermodynamics Beyond the Onsager Relation: Nonlinear and Far-From-Equilibrium Thermoelectrics

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Nontechnical Abstract:Management of heat flow to control temperature plays a critical role in the proper operation of a wide variety of modern processes, ranging from running computer chips to heating/cooling a house to cryptocurrency mining. Interestingly, the physics of heat flow is well understood only for what is known as “near-equilibrium” situations, where temperature differences and heat transfers between hot and cold regions are small. However, many large-scale processes, such as generating electrical power or running a modern data center, involve sizeable heat flows between objects at very different temperatures. It is unclear whether existing understanding of near-equilibrium processes can be used to model accurately such “far-from-equilibrium” conditions. The main goal of this project is to conduct experiments on heat flow that violate the main assumptions of the near-equilibrium theory and test whether its conclusions continue to hold or need to be amended in far-from-equilibrium situations. The results will provide a foundation on which to build advanced models to manage far-from-equilibrium heat flows produced in the industrial production of almost all goods and services. Being an NSF GOALI partnership between industry and academia, this project will expose students to an industrial perspective and approach to research, which will be particularly valuable training to both students and society when they enter the workforce.Technical Abstract:Nonequilibrium thermodynamics is concerned with evolution of heat and entropy when energy, mass, and charge flows result from various driving forces. A central question is how macroscopic energy dissipation and entropy production develop when the microscopic particle dynamics are time reversible and dissipationless. The principle known as the Onsager reciprocal relation (ORR) is considered a cornerstone of current understanding in nonequilibrium thermodynamics but was derived assuming the simplest of conditions: microscopic time-reversal symmetry, linear force-flow dynamics, and a macroscopic system in near-equilibrium. More complicated cases have been examined theoretically but with no agreement on whether the ORR can be applied. No reliable empirical guidance exists to steer the direction of theoretical work. This project aims to fill in this gap by experimentally exploring to what extent the ORR, as applied to thermoelectric (TE) physics, continues to remain valid. In TE systems the ORR predicts the ratio of Peltier coefficient to thermopower equals the absolute temperature. This project will test the ORR by measuring this ratio under conditions that violate the ORR’s basic assumptions. These conditions include going to large enough temperature differences to qualify as far-from-equilibrium and testing the ORR prediction on semiconductor diodes that have an intrinsic nonlinear force-flow relation. The main questions to be answered are whether the ORR continues to be valid even outside its foundational assumptions and, if not, whether the ORR can be revised to generalize it to a wider range of conditions. Results of this research will establish quantitative guidance on any such revisions and thus broaden the understanding of nonequilibrium thermodynamics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.