Far-field radiative thermal rectification with bulk materials

Thermal control devices and thermal circuits

“…While research on thermal switches and thermal regulators leans heavily on experimental methods aligned with specific applications, thermal diodes are predominantly evaluated numerically, which limits the understanding of their actual performance. Numerous parametric analyses, such as in [33,34], which evaluate a wide range of materials, aim to identify optimal performance but lack a link to actual experiment is missing. In contrast to thermal switches and thermal regulators, thermal diodes have received less attention in analyses of transient operation [35][36][37], although higher RR is showed in transient behaviour compared to steady-state conditions [35,38].…”

Far-Field Radiative Thermal Rectification Based on Asymmetric Emissivity

“…In solid-state systems, various mechanisms have previously been proposed to achieve thermal rectification. These mechanisms include, but are not limited to, variations in thermal boundary resistances between two materials, − the presence of anharmonic interatomic potentials, dissimilar bulk materials exhibiting distinct temperature-dependent thermal conductivities ( k ), − and the use of asymmetrically structured materials (such as those involving load mass, ballistic scattering, mass gradient ,, and asymmetric thermal radiation − ). While various metrics and definitions for the thermal rectification efficiency (η) are found in the literature, here we define by η as the ratio between the heat fluxes: , η = italicF max − italicF min italicF min where F max ( F min ) is the absolute value of the maximum (minimum) thermal flux.…”

“…However, it should be noted that realizing such small gaps and large thermal biases, especially when nanoparticles are involved, requires considerable challenges to be overcome. When comparing NF RTR to FF RTR, the latter offers the significant advantage of simplicity and ease of fabrication, as it does not require the combination and alignment of objects separated by micrometric or nanometric gaps. ,− FF RTR, also known as radiative cooling, is a passive cooling mechanism in which an object can effectively dissipate heat by emitting thermal radiation to the colder surroundings, typically the sky. − In this case, this phenomenon exploits the difference in temperature between an object and the atmosphere, with heat generally being released (i.e., high material absorption/emissivity) over the atmospheric window in the infrared where there is limited optical absorption of thermal radiation by atmospheric gases. The advent of novel materials and designs has revolutionized radiative cooling, enabling subambient cooling even under direct sunlight. ,, Under a different context, exploiting this effect for achieving thermal rectification represents a promising addition to the toolbox of heat transfer control.…”

“…Within this context, the radiative thermal rectification (RTR) effect has emerged as a promising way for achieving directional heat flow . This concept harnesses the directional nature of radiative heat transfer to create thermal rectifiers capable of regulating the heat flow in a controlled manner.…”

Transient thermal rectification effect of one-dimensional heterostructure

“…导系统多余热量，另一方面可以隔热，因此热整流机理及器件的相关研究在节 能、热防护和热输运上有重要的应用价值 [2,3] ，此外基于热整流机理的热二极管 [4] 、热三极管 [5] 和热逻辑电路 [6] 为热学计算机的实现、热交换器的设计和航天器 热控制系统的开发提供重要的器件支撑。 稳态条件下热整流机理的研究涉及到材料热物性的非线性和各种不同的物 理机制，包括不对称结构 [7] 和热传递方向 [8,9] 引起的热整流效果。对于经典的宏 观异质结构热二极管，基于傅立叶传热定律的研究表明实现热整流的必要条件 是材料热导率为温度或空间的函数 [10] ，其中利用两种材料热导率随温度变化的 不同产生热整流的机制最为常见 [11,12] ，通过温度梯度来调控双段式复合结构的 等效热导率从而在不同方向上影响热传输能力，基于此的热整流机制也在实验 中得到论证 [13] 。热整流器的研究也逐步从实现热整流效果转换为优化热整流效 果，研究发现通过调节材料热导率的温度相关性来优化热整流效果的设计是可 行的 [14][15][16] 。与此同时，结构的不对性也成为实现热整流的一个重要手段，采用 变截面结构结合材料热导率的温度相关性可以实现热整流，比如多孔结构 [17] 、 金字塔形 [18] 、双矩形 [19] 、圆柱状和球形热整流器 [20,21] 。在非对称传热的热整流 机理研究中，界面的接触热阻也是影响热整流效果的一个重要因素。研究表 明，界面热阻的存在可以增大正、反向情况下热流量的差异，从而在经典双段 热整流器的基础上优化热整流系数 [22,23] 。考虑到界面热阻极大地受到界面应 力、界面间隙和界面形貌的影响，应用热膨胀效应定向控制界面热阻也可以优 化热整流效果 [24,25] 。在热弹性系统中由外力引起结构应变的非均匀分布，从而 改善结构的热导率，也可实现结构的热整流效果 [26,27] 。此外，利用复合结构不 同材料热导率和热膨胀系数的差异性，通过热膨胀实现热开关来调控界面热 3 / 25 阻，也可以实现热整流系数的最优化 [28,29] 。在微纳米材料器件中，如纳米梯形 板 [30] 、异质结碳纳米管 [31,32] 、带孔硅纳米薄膜 [33] 也是热整流效应的研究热点。 值得注意的是，此前的研究中绝大多数都假设热整流器处于稳态，而在实 际应用中热环境是动态变化的，比如热控应用中芯片的生热过程，对于热环境 随时间变化时的热整流研究目前还刚刚起步，此时热整流不仅受材料热导率的 影响，还受其比热容的影响。瞬态异质结构的瞬态热整流效果最早是由 Herrera 等人于 2017 年给出的 [34] ，研究结果表明比热容效应可以提高初始瞬时的热整流 系数，合理选择材料热扩散率和热导率可以提高瞬态下的热整流效果。进一步 的理论研究表明固态磁热制冷循环的异质结热二极管的最大热整流系数可以达 到稳态条件下的 295 倍，其平均热整流可能大于稳态值 [35] 。而基于热导率和比 热容时空调节机制的热波二极管也获得了大于 86%的热整流系数 [36] 。如果热二 极管的边界条件随时间变化，此时瞬态热整流机理的研究更具有实际意义。 Zhang 等人基于热二极管针对随时间变化的废热进行了回收利用，通过实验验 证了俘能效率的提高 [37] ，Shimokusu 等人研究了周期性变化的温度边界条件下 的瞬态热整流效果 [...…”