Browsing by Author "Shimokusu, Trevor J."

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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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    Measurements and modeling of passive nonlinear thermal devices: diodes and regulators
    (2024-08-05) Shimokusu, Trevor J.; Wehmeyer, Geoff
    This thesis investigates the thermal science of three nonlinear thermal devices: the jumping droplet thermal diode (JDTD), the heterojunction diode, and the oscillating heat pipe (OHP). I use experiments and modeling to gain insight into physical mechanisms and characteristics that underpin thermal nonlinearities required for thermal rectification and regulation. Furthermore, in contrast to existing research that has primarily focused on understanding and improving the steady-state performance of thermal diodes and regulators, my work also considers the durability and transient performance of passive nonlinear thermal devices. I begin by discussing my work developing superhydrophobic aluminum surfaces with improved robustness against steam degradation. Using these surfaces in a JDTD, I parametrically study steady-state thermal rectification and demonstrate half-wave thermal rectification in response to an alternating current (ac) thermal input. Then, building on insight gained from half-wave thermal rectification experiments, I present our work studying the time-periodic thermal response of a heterojunction diode. Using analytical perturbation methods supported by finite element modeling, I show that a direct current (dc) heat flux emerges in response to an ac thermal input, and that this dc response may differ depending on which side the ac input is applied. The emergence of additional harmonics (e.g., dc heat flow) from a single harmonic input (i.e., sinusoidal temperature input) for nonlinear heat conduction in heterojunction diodes agrees with our half-wave thermal rectification measurements with the JDTD and could be useful for transient applications such as thermal energy storage. Lastly, I discuss my strain gauge and temperature measurements of an oscillating heat pipe (OHP). By correlating the strain frequency response with thermal resistance measurements as function of heat flow, I delineate the transition between operating regimes characterized by intermittent and stable fluid oscillations, which is responsible for strong nonlinearity and thermal regulatory behavior in OHPs. Overall, my research motivates to the need to conduct more nonlinear thermal device- and circuit-level research that complements steady-state demonstrations, such as transient thermal measurements and modeling and durability characterization.
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    Teflon AF–Coated Nanotextured Aluminum Surfaces for Jumping Droplet Thermal Rectification
    (Wiley, 2024) Shimokusu, Trevor J.; Nathani, Alia; Liu, Zhen; Yap, Te Faye; Preston, Daniel J.; Wehmeyer, Geoff
    Jumping droplet thermal diodes (JDTDs) are promising candidates to achieve thermal rectification for next-generation thermal control. However, most prior demonstrations of JDTDs have relied on monolayer-coated copper-based superhydrophobic (SHPB) surfaces, while lower-cost aluminum JDTDs with more durable thin polymeric coatings have not been explored. In this work, a JDTD is constructed that employs SHPB aluminum surfaces coated with protective thin films of Teflon AF (amorphous fluoropolymer) 1601. Measurements for different heating orientations, gap heights (H), and fill ratios (ϕ) show that a maximum thermal rectification ratio of 7 can be achieved for H = 2.4 mm and ϕ = 10%. A thermal circuit is demonstrated that uses the JDTD to rectify time-periodic temperature profiles, achieving thermal circuit effectiveness values up to 30% of the ideal-diode limit. Coupon-level durability tests and device-level cycling show that dip coated Teflon AF enables stable operation of Al JDTDs over >20 cycles, improving on the performance of a monolayer-coated surface that fails after 5 cycles. The findings of this work signify that Teflon AF coated Al SHPB surfaces can be used for thermal rectification and motivate future research into Al JDTDs for advanced thermal management applications.