Thermal Transistor and Thermal Regulator Devices Utilizing Temperature-Dependent Magnetic Forces

Switchable and nonlinear thermal devices are enabling technologies for applications ranging from waste heat scavenging to solid-state cooling and battery thermal management. Existing high-performance passive switching devices operating near room temperature rely primarily on thermal expansion to open or close gaps between surfaces; however, many thermal expansion switches have substantial thermal hysteresis and/or require large device thicknesses.<br/>Here, we demonstrate passive thermal switching with several devices that use temperature-dependent magnetic forces to make and break mechanical contact between thermally conductive elements. Our devices rely on the reversible paramagnetic/ferromagnetic phase transition of the magnetically soft material gadolinium, which has a convenient Curie temperature near 20^oC for room temperature switching. We use this magnetically-actuated mechanism to construct a thermal regulator,¹ which is a two-terminal device in which the steady-state thermal conductance can be passively tuned between an ON or OFF value by the temperature of the control thermal terminal. We also demonstrate the first reported thermal transistor,² which is a three-terminal device in which the temperature at the gate terminal controls the source-drain heat flow in a manner that leads to heat flow switching and amplification.<br/>The thermal regulator displays an average thermal switch ratio of 34 (uncertainty +30,-13) in vacuum with switching temperatures near 20^oC and a 5^oC thermal hysteresis. The magnetic actuation mechanism leads to relatively low thermal hysteresis and smaller thicknesses (&lt;2 cm) as compared to the prior thermal expansion demonstrations. Similarly, the thermal transistor displays an average source-drain thermal switch ratio of 109 (uncertainty +62, -38) in vacuum with switching gate temperatures near 25^oC and a 7^oC thermal hysteresis. The thermal transistor displays measured heat flow amplifications near 30, meaning that a small gate heat flow can drive a larger source-drain heat flow, and has characteristic thermal time constants ranging from 4 to 17 minutes. We quantify the durability and performance of both devices over &gt;1000 cycles and find that the regulator switch ratio decreases by a factor of four after cycling and long-duration (0.5 year) storage. We construct proof-of-concept thermal circuits to demonstrate that these nonlinear thermal devices enable thermal logic operations, passive heat flow routing, and temperature regulation capabilities. Thus, our demonstrations of passive heat switching using a magnetic thermal regulator and thermal transistor will motivate further research implementing nonlinear thermal elements for advanced thermal management.<br/><br/>This work was supported by an Early Career Faculty grant from NASA’s Space Technology Research Grants Program (Grant #80NSSC20K0066), by a NASA Space Technology Graduate Research Opportunity (Grant # 80NSSC20K1220), and by a grant from the Welch Foundation (Grant No. C-2023).<br/><br/>References:<br/>1. L. Castelli, A. Garg, Q. Zhu, P. Sashital, T. J. Shimokusu, G. Wehmeyer, A thermal regulator using passive all-magnetic actuation, <i>Cell Reports Physical Science</i>, <b>4</b>, 101556, 2023<br/>2. L. Castelli, Q. Zhu, T. J. Shimokusu, G. Wehmeyer, A three-terminal magnetic thermal transistor, <i>Nature Communications</i>, <b>14</b>, 393, 2023