Polarization Caloritronics: a pathway to electrically-controlled heat switches

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Most human energy consumption goes to create heat, for example, to generate electricity in power plants or to power of cars and planes with fuel combustion. Heat switches are important devices that control electricity flow based on temperature. They enable important applications such as magnetic and electric greenhouse-gas-free cooling technologies, solar-thermal power generators, and automotive exhaust waste heat recovery systems. Most heat switches are either mechanical switches that wear out and cannot support millions of cycles, or they are based on phase transitions, which work only at one fixed temperature. Ferroelectric materials have potential to overcome these limitations. This project will develop the fundamental knowledge needed to design heat switches from bulk ferroelectric materials.Heat is carried by vibrations of the atoms, called “phonons” in electrically insulating solid. Magnets, such as iron, are solids in which each atom carries its own little magnet, called a “magnetic moment”, and when all the atomic magnetic moments align, the solid gains a net magnetization. In ferroelectric solids, atoms have local atomic dipole moments where one side of the atom is more positively charged and the opposite side is more negative. These atomic dipole moments align in ferroelectric materials just like the atomic magnetic moments align in magnets, giving a net polarization. In magnets, heat can be carried by thermal fluctuations (somewhat like Brownian motion) of the atomic magnetic moments, called “magnons”. The proposal investigates the nature of the thermal fluctuations of the dipole moments in ferroelectrics, the analogs of magnons. These are tentatively labeled “ferrons” and have not been studied explicitly before. Preliminary results suggest that ferrons are phonons that involve the vibrations of the atoms that carry dipole moments and, like most phonons, they carry heat. Theoretically, an applied voltage should influence their heat-carrying capability; preliminary experiments confirm this. The detailed theory of the influence of an external electric field on the thermal conductivity of ferrons will be developed and tested experimentally in this project. The goal is to provide design rules to develop new materials from the ground up in which the thermal conductivity is affected greatly by applied voltage, making them suitable for heat switches.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.