Geoff Wehmeyer

High Resolution Image Download MS PowerPoint Slide Films with large anisotropy ratios ( r ) between the in-plane and cross-plane thermal conductivity (κ) can be used for directional heat spreading in electronics thermal management. Here, we show that commercially available solution-spun carbon nanotube (CNT) films with 20 μm thickness and centimeter-scale lateral dimensions exhibit orthotropic thermal conductivity with the highest reported r to date, reaching r = 1400 ± 160 at room temperature ( T ). We find r using laser flash thermal diffusivity (α) measurements over a T range from 198 to 573 K. Dedoping of acid residuals via annealing increases the in-plane-aligned α x of dedoped samples by a factor of 2 compared to the doped samples. These dedoped CNT films also display a strong α x ∝ T –1.1 scaling, indicating that phonon–phonon scattering impacts heat transport along the direction of alignment. Our work motivates further exploration of ultrahigh r in macroscopic CNT materials and applications of CNT films for directional heat spreading.

Films with large anisotropy ratios (<i>r</i>) between the in-plane and cross-plane thermal conductivity (κ) can be used for directional heat spreading in electronics thermal management. Here, we show that commercially available solution-spun carbon nanotube (CNT) films with 20 μm thickness and centimeter-scale lateral dimensions exhibit orthotropic thermal conductivity with the highest reported <i>r</i> to date, reaching <i>r</i> = 1400 ± 160 at room temperature (<i>T</i>). We find <i>r</i> using laser flash thermal diffusivity (α) measurements over a <i>T</i> range from 198 to 573 K. Dedoping of acid residuals via annealing increases the in-plane-aligned α_<i>x</i> of dedoped samples by a factor of 2 compared to the doped samples. These dedoped CNT films also display a strong α_<i>x</i> ∝ <i>T</i>^–1.1 scaling, indicating that phonon–phonon scattering impacts heat transport along the direction of alignment. Our work motivates further exploration of ultrahigh <i>r</i> in macroscopic CNT materials and applications of CNT films for directional heat spreading.

Electrifying industrial heating to reduce Scope 1 emissions will require advanced Joule heating materials and high-power loading devices that enable effective immersion heating of flowing gases. This work shows that solution-spun carbon nanotube fibers (CNTFs) represent promising alternatives to legacy heating materials such as metal alloys for these electrification applications. Annealed CNTFs have similar electrical resistivity to commonly used nichrome alloys while also offering higher specific strength, higher thermal conductivity, higher operating temperatures in non-oxidizing gases, and the ability to be processed with established textile manufacturing techniques. Joule heating experiments are supported by thermal modeling to quantify the power loading of substrate-free devices made entirely of CNTF monofilaments or CNTF fabrics. Single-filament heating experiments in quiescent fluids show that CNTF wires can achieve specific power loadings that are 32x (inert gas) or 3.5x (air) larger than those of nichrome wires with similar diameters, and experiments on self-heated CNTF textiles show 2.4x specific power enhancement in flowing air as compared to nichrome meshes. Thus, this work shows that annealed CNTF wires, arrays, and textiles are a promising material platform to assist in the electrification of industrial gas heating applications.

Quantum interference effects such as weak localization (WL) and universal conductance fluctuations (UCF) normally yield consistent electronic phase-coherence lengths in homogeneous conductors. Here we show that in individual carbon nanotube bundles exfoliated from highly conductive solution-spun fibers, different probes, including the field scales and magnitudes of WL and UCF and nonlocal magnetoconductance, lead to strikingly disparate estimates of coherence lengths. WL magnetoconductance measured in a perpendicular magnetic field yields a phase-coherence length of approximately 50 nm. In contrast, UCF amplitudes are comparable to e squared over h even for an 8 micrometer long segment, and nonlocal magnetoconductance persists across a 4 micrometer separation of electrodes, revealing phase-coherent transport over micrometer length scales within a single bundle. The coexistence of short- and long-range coherence implies that locally diffusive electrons remain partially phase-correlated among nanotubes within the same bundle. These findings challenge the conventional single-scale picture of mesoscopic coherence and establish carbon nanotube bundles as a model platform for emergent, network-level quantum transport.

Quantum interference effects such as weak localization (WL) and universal conductance fluctuations (UCF) normally yield consistent electronic phase-coherence lengths in homogeneous conductors. Here we show that in individual carbon nanotube bundles exfoliated from highly conductive solution-spun fibers, different probes, including the field scales and magnitudes of WL and UCF and nonlocal magnetoconductance, lead to strikingly disparate estimates of coherence lengths. WL magnetoconductance measured in a perpendicular magnetic field yields a phase-coherence length of approximately 50 nm. In contrast, UCF amplitudes are comparable to e squared over h even for an 8 micrometer long segment, and nonlocal magnetoconductance persists across a 4 micrometer separation of electrodes, revealing phase-coherent transport over micrometer length scales within a single bundle. The coexistence of short- and long-range coherence implies that locally diffusive electrons remain partially phase-correlated among nanotubes within the same bundle. These findings challenge the conventional single-scale picture of mesoscopic coherence and establish carbon nanotube bundles as a model platform for emergent, network-level quantum transport.

Abstract Thermomagnetic devices can exhibit time-periodic oscillations between stationary substrates due to temperature-dependent magnetic forces. These oscillations shuttle thermal energy across the device and can be leveraged for thermal rectification and thermomagnetic energy harvesting applications. If the shuttle is thin and made from a high thermal conductivity material, simple thermally lumped modeling can be used to find the time-averaged heat flow. However, there are no existing analytical solutions that describe the full spatiotemporal temperature and heat flow profiles during time-periodic oscillations of multilayered devices with layers of arbitrary thickness and thermal conductivity. Here, we present experimental measurements of such thermomagnetic devices along with analytical solutions for an arbitrary number of stationary hot-side, stationary cold-side, and oscillating layers. We show that the exact solution for the dc component of the heat flow is in good agreement with a simple closed-form approximate expression that spans the lumped and nonlumped regions. We use the analytical solution to interpret experimental measurements of heat flows in multilayer thermal diode devices made with aluminum, steel, or acrylic materials. The aluminum and steel shuttle devices are well-described by the simple lumped thermal model and have thermal rectification ratios near 3 in air, while the acrylic shuttle displays a nonlumped thermal behavior and has thermal rectification ratios of only 1.6 in air. The measurements and analytical solutions developed here provide insight into the thermal performance of thermomagnetic devices for energy scavenging and thermal rectification applications.

Superhydrophobic surfaces, which rely on a combination of surface texture and chemistry, often lose their repellent behavior when contacted by hot water (≳40 °C) because the impinging hot water replaces the requisite air layer within the surface texture via evaporation and recondensation. In contrast to previous approaches targeting this condensation-induced failure mode that rely on intricately tailored surface structures or complex chemical treatments, we present a scalable approach based on thermal design: the multilayered insulated superhydrophobic (MISH) coating mitigates condensation-induced failure by preventing heat transfer. Superhydrophobicity is retained at impinging water temperatures up to 90 °C, with durability demonstrated via long-term (>1 million impacts) droplet impingement experiments. We explain the mechanism for this approach with a detailed thermal model; the model reveals that the underlying physical behavior is self-similar across coating parameters and impinging fluid temperatures. The MISH coating accommodates curved geometries and large surfaces, and it is over 4 orders of magnitude less expensive than cleanroom-nanofabricated alternatives, indicating promise for practical use in the energy sector, chemical processing, and the food and medical industries.

We investigate quantum transport in aligned carbon nanotube (CNT) fibers fabricated via solution spinning, focusing on the roles of structural dimensionality and quantum interference effects. The fibers exhibit metallic behavior at high temperatures, with conductivity increasing monotonically as the temperature decreases from room temperature to $\ensuremath{\sim}36\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Below this temperature, the conductivity gradually decreases with further cooling, signaling the onset of quantum conductance corrections associated with localization effects. Magnetoconductance measurements in both parallel and perpendicular magnetic fields exhibit pronounced positive corrections at low temperatures, consistent with weak localization (WL). To determine the effective dimensionality of electron transport, we analyzed the data using WL models in 1D, 2D, and 3D geometries. We found that while the 2D model can reproduce the field dependence, it lacks physical meaning in the context of our fiber architecture and requires an unphysical scaling factor to match the experimental magnitude. By contrast, we developed a hybrid $3\mathrm{D}+1\mathrm{D}$ WL framework that quantitatively captures both the field and temperature dependences using realistic coherence lengths and a temperature-dependent crossover parameter. Although this combined model also employs a scaling factor for magnitude correction, it yields a satisfactory fit, reflecting the hierarchical structure of CNT fibers in which transport occurs through quasi-1D bundles embedded in a 3D network. Our results establish a physically grounded model of phase-coherent transport in macroscopic CNT assemblies, providing insights into enhancing conductivity for flexible, lightweight power transmission applications.

The ZT figure-of-merit is an index for thermoelectric conversion efficiency that incorporates the Seebeck coefficient, electrical conductivity, and thermal conductivity. In conventional three-dimensional materials, ZT exhibits a local maximum at only one carrier density because these three thermoelectric parameters each have counteracting trends with carrier concentration. Therefore, optimizing the doping level to the single peak in the ZT value is a general strategy to improve the thermoelectric conversion efficiency. Here we show that such a conventional single-peak ZT result is not observed for materials with strong quantum confinement effects. Specifically, we present here a double-peak structure of ZT values observed in films of semiconducting single-walled carbon nanotubes (SWCNTs). The ZT value can be enhanced when the Fermi level is near the van Hove singularities (vHs) in the density of states of the SWCNTs. Using electrolyte gating to shift the Fermi level systematically, we observe two local maxima in ZT corresponding to the two vHs. Such multiple-peak characteristics of the ZT value in quantum materials will alter the concept for improving the thermoelectric conversion efficiency.

Boron nitride (BN) is well-known for its excellent thermal conductivity, high chemical stability, and low dielectric constant, making it widely used as a lubricant, thermal management material, and electrical insulator. For different applications, the nanostructure of BN plays a prominent role. In particular, boron nitride nanotubes (BNNTs) are preferred for enhancing the properties in specific directions. Traditional BNNT synthetic methods often require valuable precursors and catalysts and prolonged reaction time for structure engineering, limiting their practical applications. Here, we present a dimension engineering strategy to controllably synthesize one-dimensional BNNTs and two-dimensional nanosheets (BNNSs) by flash Joule heating (FJH) within 1 min. The scalable production of ∼5 g is achieved per batch. During BN synthesis, sulfur is identified as a crucial additive that accelerates precursor dehydration and facilitates nanotube formation. When applied as additives in composites, BNNTs exhibit enhanced mechanical strength and thermal conductivity compared to BNNSs, highlighting the necessity of BN dimension engineering for diverse applications. This work offers a feasible strategy for tailoring BN nanostructures and optimizing their properties, with potential applicability in the synthesis of other nanomaterials beyond BN.

Neat, densely packed, and highly aligned carbon nanotube fibers (CNTFs) have appealing room-temperature axial thermal conductivity (k) and thermal diffusivity (α) for applications in lightweight heat spreading, flexible thermal connections, and thermoelectric active cooling. Although CNTFs are regularly produced from different input carbon nanotubes (CNTs), prior work has not quantified how the CNT molecular aspect ratio r (i.e., molecular length-to-diameter ratio) influences k and α in well-aligned, packed CNTFs. Here, we perform self-heated steady-state and three-omega thermal measurements at room temperature on CNTF suspended in vacuum. Our results show that k increases from 150 to 380W/mK for viscosity-averaged molecular aspect ratios increasing from r=960 to 5600 and nanotube diameters of ∼2 nm, which we attribute to the effects of thermal resistances between CNT bundles. CNTFs made with varying volume fraction ϕ of constituent high-r and low-r CNT have properties that fall within or below the typical macroscopic rule-of-mixtures bounds. The thermal diffusivity α scales with k, leading to a sample-averaged volumetric heat capacity of 1.5±0.3MJ/m3K. This work's findings that fibers made from longer CNT have larger k and α at room temperature motivate further investigation into thermal transport in solution-spun CNTF.

Patterned solid surfaces with wettability contrast can enhance liquid transport for applications such as electronics thermal management, self-cleaning, and anti-icing. However, prior work has not explored easy and scalable blade-cut masking to impart topography patterned wettability contrast on aluminum (Al), even though Al surfaces are widely used for thermal applications. Here, we demonstrate mask-enabled topography contrast patterning and quantify the resulting accuracy of the topographic pattern resolution, spatial variations in surface roughness, wettability, drop size distribution during dropwise condensation, and thermal emissivity of patterned Al surfaces. The method uses blade-cut vinyl mask templates and a commercially available lacquer resin that serves as a polymer resist against etching. Programmable mask templates enable complex patterning of wettability and emissivity contrast with feature sizes down to ∼1.5 mm. As-fabricated patterned samples show a water contact angle (θ) contrast from <5° to 80° between etched and smooth zones, while patterned samples that are further coated with a hydrophobic promoter show θ contrast between 150° and 120° on etched and smooth zones, respectively. In addition to measuring this wettability contrast via contact angle goniometry, we use condensation visualization experiments to study the spatially controlled condensate morphologies and drop size distributions. These condensation studies demonstrate enhanced droplet shedding on the superhydrophobic regions of striped patterned surfaces compared to homogeneous superhydrophobic surfaces. Motivated by the role of thermal radiation in many phase change processes, we use infrared thermography to map topography-mediated thermal emissivity (ε) contrast between etched (ε ≈ 0.65) and smooth (ε ≈ 0.26) regions. Thus, our study provides a route for researchers to readily create complex and scalable topography-patterned Al surfaces for potential applications in vapor chamber thermal rectification, radiative cooling condensation heat transfer, and high-temperature Leidenfrost or film boiling processes.

Flash Joule heating is an ultrafast, energy-efficient, and scalable technique used in the production of a variety of organic and inorganic compounds, including flash graphene. This technique has also been used in the production of doped graphene by flash Joule heating amorphous carbon in the presence of heteroatom-donating compounds. Herein, we report a modified flash Joule heating technique by which graphene is formed with up to 21 at% of the graphene lattice containing substituted heteroatoms. This is achieved by re-flashing graphene in the presence of heteroatom-donating compounds, allowing this substitution to occur at lower temperatures than previously reported for flash Joule heating-synthesized doped graphene and thereby permitting much higher amounts of heteroatom insertion into the graphene lattice. We demonstrate nitrogen, sulfur, phosphorus, and fluorine atom atomic substitution into or upon the graphene lattice, as well as multi-heteroatom substitution. Finally, the implementation of the nitrogen-substituted re-flashed graphene into battery anodes exhibits improved performance and stability relative to unsubstituted re-flashed graphene battery anodes.

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The applications of carbon nanotube fibers (CNTF) are broad because of their flexibility, high specific strength, and outstanding thermal and electrical properties. Although CNTFs have a hierarchical structure, their macroscopic properties, are usually discussed and investigated at the scale of the whole fiber, with a lack of understanding of the local properties, such as the Seebeck coefficient and the Fermi energy. Here, we study the variation of the Seebeck coefficient along the fibers by using the photothermoelectric (PTE) effect. The photovoltage is measured as a function of position, and the laser-induced temperature profile is obtained by a robust steady-state thermal model. The Seebeck coefficient as a function of position along the fiber can be obtained from the measured, spatially mapped photovoltage and temperature profile. We observe a correlation between the variation of the Seebeck coefficient and the shift of Raman modes, both related to the doping level and the Fermi energy. We find the Seebeck coefficient fluctuation in the pristine fiber is due to the nonuniformity of the doping level and the Fermi energy. With an established model to correlate the thermoelectric response and the Fermi energy, our PTE-based method can probe the Fermi energy fluctuation along the fiber with a resolution better than 1 meV, which is far beyond the capability of commercial Raman spectroscopy. This study shows a nondestructive method to quantify the uniformity of CNTF at the micrometer scale, key for fabricating more uniform and higher quality CNTF and generalizable to other conducting fiber systems.

Abstract 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 &gt;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.

Thermoelectric active cooling uses nontraditional thermoelectric materials with high thermal conductivity, high thermoelectric power factor, and relatively low figure of merit (ZT) to transfer large heat flows from a hot object to a cold heat sink. However, prior studies have not considered the influence of external thermal resistances associated with the heat sinks or contacts, making it difficult to design active cooling thermal systems or compare the use of low-ZT and high-ZT materials. Here, we perform a non-dimensionalized analysis of thermoelectric active cooling under forced heat flow boundary conditions, including arbitrary external thermal resistances. We identify the optimal electrical currents to minimize the heat source temperature and find the crossover heat flows at which low-ZT active cooling leads to lower source temperatures than high-ZT and even ZT→+∞ thermoelectric refrigeration. These optimal parameters are insensitive to the thermal resistance between the heat source and thermoelectric materials, but depend strongly on the heat sink thermal resistance. Finally, we map the boundaries where active cooling yields lower source temperatures than thermoelectric refrigeration. For currently considered active cooling materials, active cooling with ZT &amp;lt; 0.1 is advantageous compared to ZT→+∞ refrigeration for dimensionless heat sink thermal conductances larger than 15 and dimensionless source powers between 1 and 100. Thus, our results motivate further investigation of system-level thermoelectric active cooling for applications in electronics thermal management.

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.

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.