Browsing by Author "Wehmeyer, Geoff"
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Item A thermal regulator using passive all-magnetic actuation(Elsevier, 2023) Castelli, Lorenzo; Garg, Ajay; Zhu, Qing; Sashital, Pooja; Shimokusu, Trevor J.; Wehmeyer, GeoffThermal 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.Item A three-terminal magnetic thermal transistor(Springer Nature, 2023) Castelli, Lorenzo; Zhu, Qing; Shimokusu, Trevor J.; Wehmeyer, GeoffThree-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.Item Macroscopic weavable fibers of carbon nanotubes with giant thermoelectric power factor(Springer Nature, 2021) Komatsu, Natsumi; Ichinose, Yota; Dewey, Oliver S.; Taylor, Lauren W.; Trafford, Mitchell A.; Yomogida, Yohei; Wehmeyer, Geoff; Pasquali, Matteo; Yanagi, Kazuhiro; Kono, Junichiro; Carbon HubLow-dimensional materials have recently attracted much interest as thermoelectric materials because of their charge carrier confinement leading to thermoelectric performance enhancement. Carbon nanotubes are promising candidates because of their one-dimensionality in addition to their unique advantages such as flexibility and light weight. However, preserving the large power factor of individual carbon nanotubes in macroscopic assemblies has been challenging, primarily due to poor sample morphology and a lack of proper Fermi energy tuning. Here, we report an ultrahigh value of power factor (14 ± 5 mW m−1 K−2) for macroscopic weavable fibers of aligned carbon nanotubes with ultrahigh electrical and thermal conductivity. The observed giant power factor originates from the ultrahigh electrical conductivity achieved through excellent sample morphology, combined with an enhanced Seebeck coefficient through Fermi energy tuning. We fabricate a textile thermoelectric generator based on these carbon nanotube fibers, which demonstrates high thermoelectric performance, weavability, and scalability. The giant power factor we observe make these fibers strong candidates for the emerging field of thermoelectric active cooling, which requires a large thermoelectric power factor and a large thermal conductivity at the same time.Item Measurements and modeling of passive nonlinear thermal devices: diodes and regulators(2024-08-05) Shimokusu, Trevor J.; Wehmeyer, GeoffThis 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.Item Mechanical Reshaping of Inorganic Nanostructures with Weak Nanoscale Forces(American Chemical Society, 2021) Rehn, Sarah M.; Gerrard-Anderson, Theodor M.; Qiao, Liang; Zhu, Qing; Wehmeyer, Geoff; Jones, Matthew R.Inorganic nanomaterials are often depicted as rigid structures whose shape is permanent. However, forces that are ordinarily considered weak can exert sufficient stress at the nanoscale to drive mechanical deformation. Here, we leverage van der Waals (VdW) interactions to mechanically reshape inorganic nanostructures from planar to curvilinear. Modified plate deformation theory shows that high-aspect-ratio two-dimensional particles can be plastically deformed via VdW forces. Informed by this finding, silver nanoplates were deformed over spherical iron oxide template particles, resulting in distinctive bend contour patterns in bright-field (BF) transmission electron microscopy (TEM) images. High-resolution TEM images of deformed areas reveal the presence of highly strained bonds in the material. Finally, we show that the distance between two nearby template particles allows for the engineering of several distinct curvilinear morphologies. This work challenges the traditional view of nanoparticles as static objects and introduces methods for postsynthetic mechanical shape control.Item Probing phonon heat conduction in multi-scaled systems via ray tracing simulation and three omega measurements(2024-06-27) Song, Yingru; Wehmeyer, GeoffNanostructured materials display unique thermal and electrical properties, but it can be challenging to scale up the nanoscale phenomena for micro/macroscale applications. In addition to experimental challenges in assembling and controlling multiple nanostructures, it is difficult to model multiscale phonon heat transfer and capture nanoscale size effects for microscale experimental geometries with complex features. In this first half of this thesis, I have developed a Landauer-based phonon Monte Carlo ray tracing simulation methods to determine the phonon mean free path and local temperature/heat flux profiles in realistic micromaterials. I will discuss applications of the ray tracing method to experimentally relevant microstructures that display interesting phonon transport phenomena including locally inverted temperature gradients and ballistic phonon focusing. In the second half of the thesis, I will discuss experimental measurements of high-conductivity carbon nanotube fiber (CNTFs), which consist of long individual carbon nanotubes (CNTs) that are highly aligned along the fiber’s axis. My electrothermal three-omega measurements show that the thermal conductivity increases with increasing CNT molecular aspect ratio (i.e., length to diameter ratio) up to values as large as 380 W/m.K, showing that the interfaces between CNT bundles still impede thermal transport even at high degrees of alignment and with CNT lengths up to 10 μm. Thus, my simulations and experiments focus on understanding and optimizing the relationship between nanoscale phonon physics and macroscale properties in complex geometries.Item 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, GeoffJumping 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.Item Ultrahigh strength, modulus, and conductivity of graphitic fibers by macromolecular coalescence(AAAS, 2022) Lee, Dongju; Kim, Seo Gyun; Hong, Seungki; Madrona, Cristina; Oh, Yuna; Park, Min; Komatsu, Natsumi; Taylor, Lauren W.; Chung, Bongjin; Kim, Jungwon; Hwang, Jun Yeon; Yu, Jaesang; Lee, Dong Su; Jeong, Hyeon Su; You, Nam Ho; Kim, Nam Dong; Kim, Dae-Yoon; Lee, Heon Sang; Lee, Kun-Hong; Kono, Junichiro; Wehmeyer, Geoff; Pasquali, Matteo; Vilatela, Juan J.; Ryu, Seongwoo; Ku, Bon-Cheol; The Carbon HubTheoretical considerations suggest that the strength of carbon nanotube (CNT) fibers be exceptional; however, their mechanical performance values are much lower than the theoretical values. To achieve macroscopic fibers with ultrahigh performance, we developed a method to form multidimensional nanostructures by coalescence of individual nanotubes. The highly aligned wet-spun fibers of single- or double-walled nanotube bundles were graphitized to induce nanotube collapse and multi-inner walled structures. These advanced nanostructures formed a network of interconnected, close-packed graphitic domains. Their near-perfect alignment and high longitudinal crystallinity that increased the shear strength between CNTs while retaining notable flexibility. The resulting fibers have an exceptional combination of high tensile strength (6.57 GPa), modulus (629 GPa), thermal conductivity (482 W/m·K), and electrical conductivity (2.2 MS/m), thereby overcoming the limits associated with conventional synthetic fibers.Item Understanding the structural properties in two-dimensional halide perovskites under external stimuli(2021-08-23) Li, Wenbin; Mohite, Aditya D.; Kono, Junichiro; Wehmeyer, GeoffUnderstanding the structural and electrical behaviors of organic-inorganic (hybrid) halide perovskites under practical environments is critical for building an efficient and stable optoelectronic device. In this thesis, we report a light-activated interlayer contraction in 2D hybrid halide perovskite. This effect is reversible and is strongly dependent on the structural phase and interlayer distance. X-ray photoelectron spectroscopy and density function theory simulation suggest that photogenerated hole carriers are accumulating at the inter-slab iodide atoms which results in the enhancement of interlayer I---I interactions across the organic barrier. In-situ structural (device) and transport (space charge limited current-SCLC) measurements directly correlate the light-induced interlayer contraction to the onset of a three-fold increase in carrier mobility and conductivity. Furthermore, light intensity dependent SCLC measurement reveals a percolation based mechanism for the enhancement of the charge transport. The increase in transport properties boost the photovoltaic efficiency of Dion-Jacobson 2D n=4 perovskite solar cells from 15.6% to 18.3%.