Browsing by Author "Castelli, Lorenzo"

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    A thermal regulator using passive all-magnetic actuation
    (Elsevier, 2023) Castelli, Lorenzo; Garg, Ajay; Zhu, Qing; Sashital, Pooja; Shimokusu, Trevor J.; Wehmeyer, Geoff
    Thermal regulators are two-terminal devices used for passive temperature control of electronics, batteries, or buildings. Existing thermal expansion regulators suffer from large thicknesses and substantial hysteresis. Here we report an all-magnetic thermal regulator in which the temperature of the control terminal (Tcontrol) leads to passive steady-state surface mating/demating that enables/blocks heat conduction. The mechanism relies on Tcontrol-dependent magnetic forces between gadolinium and neodymium iron boron magnets when Tcontrol is near gadolinium’s Curie temperature of 21oC. Our centimeter-scale prototype has a thermal switch ratio of 34−13+30 in vacuum and 2.1−0.2+0.2 in air, a vacuum OFF state thermal conductance of 3.5 mW/K, an average switching temperature of 20oC, a small thermal deadband of 5oC, and a relatively compact thickness <2 cm. We quantify the regulator performance over >2,000 cycles and construct the regulator using commercially available materials, showing that this thermomagnetic device can be used for effective thermal regulation near room temperature.
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    A three-terminal magnetic thermal transistor
    (Springer Nature, 2023) Castelli, Lorenzo; Zhu, Qing; Shimokusu, Trevor J.; Wehmeyer, Geoff
    Three-terminal thermal analogies to electrical transistors have been proposed for use in thermal amplification, thermal switching, or thermal logic, but have not yet been demonstrated experimentally. Here, we design and fabricate a three-terminal magnetic thermal transistor in which the gate temperature controls the source-drain heat flow by toggling the source-drain thermal conductance from ON to OFF. The centimeter-scale thermal transistor uses gate-temperature dependent magnetic forces to actuate motion of a thermally conducting shuttle, providing thermal contact between source and drain in the ON state while breaking contact in the OFF state. We measure source-drain thermal switch ratios of 109 ± 44 in high vacuum with gate switching temperatures near 25 °C. Thermal measurements show that small heat flows into the gate can be used to drive larger heat flows from source to drain, and that the switching is reversible over >150 cycles. Proof-of-concept thermal circuit demonstrations show that magnetic thermal transistors can enable passive or active heat flow routing or can be combined to create Boolean thermal logic gates. This work will allow thermal researchers to explore the behavior of nonlinear thermal circuits using three-terminal transistors and will motivate further research developing thermal transistors for advanced thermal control.
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    Multi-season passive variable insulation for buildings using magnetic thermal diodes
    (Elsevier, 2024) Castelli, Lorenzo; Kumar, Monisha Vijay; Wehmeyer, Geoff
    Passive variable insulation (PVI) could reduce the energy consumption for heating and cooling of buildings compared to traditional static insulation without requiring active control and sensing. However, existing PVI systems are limited to single-season operation. Here, we demonstrate a PVI thermal circuit consisting of two anti-parallel oscillating thermomagnetic diodes that enables passive heating during a warm winter day and passive cooling during a cold summer night. Centimeter-scale prototype experiments show that the PVI has a narrow 4oC temperature regulation band, functions in all gravitational orientations, is durable over >104 oscillation cycles, and has a circuit-level passive thermal switch ratio of 3 under ambient conditions. Thermal modeling over the US climate zones shows that PVI can reduce undesirable building envelope heat flows by up to 47% for base-case parameters, motivating further study of passive variable thermal insulation strategies for the building envelope.