Polarization Caloritronics: a pathway to electrically-controlled heat switches

人类的大部分能源消耗都用于产生热量，例如，在发电厂发电或通过燃料燃烧为汽车和飞机提供动力。热开关是根据温度控制电流的重要装置。它们使磁性和电无温室气体冷却技术、太阳能热发电机和汽车废气废热回收系统等重要应用成为可能。大多数热开关要么是磨损的机械开关，不能支持数百万次循环，要么是基于相变，只能在一个固定的温度下工作。铁电材料有潜力克服这些限制。该项目将开发设计大块铁电材料热交换器所需的基础知识。热是由在电绝缘固体中称为“声子”的原子的振动携带的。磁铁，如铁，是固体，其中每个原子都有自己的小磁铁，称为“磁矩”，当所有原子磁矩对齐时，固体获得净磁化。在铁电固体中，原子具有局部原子偶极矩，其中原子的一侧带更多正电而另一侧带更多负电。这些原子偶极矩在铁电材料中排列，就像原子磁矩在磁体中排列一样，产生净极化。在磁体中，热量可以通过称为“磁振子”的原子磁矩的热波动（有点像布朗运动）来传递。该提案研究了磁振子类似的铁电体中偶极矩的热波动的性质。它们被暂时标记为“铁子”，以前还没有被明确地研究过。初步结果表明，铁子是一种声子，它涉及携带偶极矩的原子的振动，并且像大多数声子一样，它们携带热量。理论上，施加的电压应该影响它们的热承载能力；初步实验证实了这一点。外电场对铁子热导率影响的详细理论将在本项目中发展和实验测试。其目标是为开发新材料提供设计规则，这些新材料的导热性受外加电压的影响很大，使其适合于热开关。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.