Thermal Switching Research Articles

A novel thermal bus architecture for large cryogenic space telescopes utilizing helium pulsating heat pipes

Future space telescopes, such as those proposed for the Far-infrared Surveyor Mission, are expected to employ actively cooled optical arrays with a similar overall surface area compared to the James Webb Space Telescope. Therefore, there is a need for a cryogenic cooling system with a thermal bus architecture that can distribute cooling to these large optical arrays. Recent experimental research of helium Pulsating Heat Pipes (PHPs) has shown that helium PHPs can transfer heat over long distances (on the order of 2 m) with high efficiency, and also have the ability to act as a passive thermal switch upon the removal of the cooling source. PHPs’ high thermal performance, passive switching capability, low mass, and ease of manufacturing make them an appealing option compared to high-purity metal straps for a thermal bus architecture on large cryogenic space telescopes. A novel architecture for the thermal control of optical arrays is proposed utilizing unique configurations of helium pulsating heat pipes that minimize mass, maximize thermal performance, and reduce the risk of mission failure.

Thermal Conductivity Switching in Pd-Catalyzed Gd and GdH2 Films Upon Gasochromic Hydrogenation and Dehydrogenation

Thermal conductivity switching in Pd-catalyzed Gd hydride films was investigated using in situ analyses of electrical, optical, structural, and thermophysical properties upon gasochromic hydrogenation/dehydrogenation at room temperature. These reactions allow the films to reversibly switch between the metallic (GdH2) phase in the hydrogenated state and the semiconductor (GdH3) phase in the dehydrogenated state. We fabricated three-layered films comprising Pd (5 nm)/Gd or GdH2 (300 nm)/Mo (100 nm) using DC magnetron sputtering using Pd, Gd, and Mo targets, respectively, on unheated synthetic quartz substrates. Ar or mixture of Ar + H2 (H2: 5 % to 50 %) gases were used for the deposition of the Gd or GdH2 films, respectively. The thermal conductivities of the Gd or GdH2 films were analyzed using in situ rear-heating/rear-detection type time-domain picosecond pulsed light heating thermoreflectance measurements by employing a gas mixture of N2/Ar and H2 (H2: 3 %) at 1 atm, which is below the explosive limit for hydrogen. The thermal conductivities of Gd and GdH2 films were measured for one and two hydro-/dehydrogenation cycles, respectively. The thermal switching ratios of the ON state thermal conductivity to the OFF state thermal conductivity were 6.4 and 2.9–5.4 for the Gd film and the GdH2 (H2: 5 %–50 %) films, respectively. The thermal conductivity of the Gd hydrides films changed along with those estimated from the Wiedemann–Franz Law and electrical conductivities.

Switchable Heat Pipes for Eco-Friendly Battery Cooling in Electric Vehicles: A Life Cycle Assessment

Battery thermal management systems (BTMSs) ensure that lithium-ion batteries (LIBs) in electric vehicles (EVs) are operated in an optimal temperature range to achieve high performance and reduce risks. A conventional BTMS operates either as an active system that uses forced air, water or immersion cooling, or as a complete passive system without any temperature control. Passive systems function without any active energy supply and are therefore economically and environmentally advantageous. However, today’s passive BTMSs have limited cooling performance, which additionally cannot be controlled. To overcome this issue, an innovative BTMS approach based on heat pipes with an integrated thermal switch, developed by the Fraunhofer Cluster of Excellence Programmable Materials (CPM), is presented in this paper. The suggested BTMS consists of switchable heat pipes which couple a passive fin-based cold plate with the battery cells. In cold state, the battery is insulated. If the switching temperature is reached, the heat pipes start working and conduct the battery heat to the cold plate where it is dissipated. The environmental benefits of this novel BTMS approach were then analysed with a Life Cycle Assessment (LCA). Here, a comparison is made between the suggested passive and an active BTMS. For the passive system, significantly lower environmental impacts were observed in nearly all impact categories assessed. It was identified as a technically promising and environmentally friendly approach for battery cooling in EVs of the compact class. Furthermore, the results show that passive BTMS in general are superior from an environmental point of view, due their energy self-sufficient nature.

Continuously‐Tunable and Ultrawide‐Range Thermal Regulator Based on Superaligned Carbon Nanotube Aerogels for Dynamic Thermal Management of Batteries and Buildings

AbstractEfficient heat transfer control is highly demanded for dynamic thermal management of equipment and buildings especially when the environmental temperature dramatically changes. Thermal switches, the current approach of heat transfer control, suffer from the issues of low switching ratios below eight and sharp state transition between “on” and “off”. Herein, a continuously‐tuned thermal regulator with an ultrahigh thermal‐conductivity change ratio of 43 based on superaligned carbon nanotube aerogel is reported, which works on the regulation of both thermal interfacial resistance and conduction pathways by compressive deformation‐induced microstructure evolution. This thermal regulator can stabilize the device temperature at 25 °C when the environmental temperature varies by 7 and 11 °C under the natural convection and forced convection conditions, respectively. Toward practical application, the thermal regulator with flexibility is wrapped around a cylindrical lithium‐ion battery to control the operation temperature within the optimal range of 20–40 °C for enhanced discharging performance even at an environmental temperature of −20 °C. Besides, by combining the thermal regulator with radiative cooling film, the house model temperature can be lowered by 2.7 °C during daytime and raised by 1.1 °C during nighttime compared with the bare one. This efficient thermal regulation approach offers an effective solution for practical thermal management.

Four-terminal graphene-superconductor thermal switch controlled by the superconducting phase difference

Four-terminal graphene-superconductor thermal switch controlled by the superconducting phase difference

Revealing the Role of the Tetragonal Distortion in the Metal-Insulator Transition of Co- and Fe-Doped NiS.

Although the NiS exhibits the most widely adjustable metal-to-insulator (MIT) properties among the chalcogenides, the mechanisms, with respect to the regulations in their critical temperatures (TMIT), are yet unclear. Herein, we demonstrate the overlooked role associated with the structurally tetragonal distortion in elevating the TMIT of NiS; this is in distinct contrast to the previously expected hybridization and bandwidth regulations that usually reduces TMIT. Compared to the perspective of structure distortions, the orbital hybridization and band regulation of NiS are ∼19 times more effective adjustment in TMIT. As a result, the respective abruptions in both the electrical and thermal resistive switches across the TMIT of NiS can be better preserved in the low-temperature range (<273 K), shedding light on their optimum usage at cryogenic temperatures.

Strong enhancement of near-field radiative heat transfer between anisotropic magneto-optical gratings by a weak magnetic field

The highly tunable surface magneto-plasmon polaritons supported by magneto-optical materials provide reliable means for active adjusting of near-field radiative heat transfer (NFRHT). In our study, we proposed an anisotropic system consisting of two graphene-covered magneto-optical gratings, and theoretically investigate the NFRHT between them controlled by a weak magnetic field. We find that an external magnetic field open a broad-spectrum photo tunneling channel and modify the energy distribution of surface electromagnetic modes in the wave-vector space. The near-field heat flux between two magneto-optical gratings can achieve a remarkable maximum enhancement of 6 times at the magnetic field H=0.9T compared to that without a magnetic field. Owing to the coupling between highly tunable surface magneto-plasmon polaritons of graphene and surface modes of InSb, the regulation ratio of the heat flux exceeds 10 from 0.5 T to 0.9 T. In addition, the effects of the separation distance, chemical potential of graphene and structural configuration of two gratings are also investigated. We look forward to the proposed system being used as a thermal switch, providing convenience for tuning thermal radiation and improving the efficiency of energy-utilization devices.

Thermal control over phosphorescence or thermally activated delayed fluorescence in a metal-organic framework.

By integrating a tailor-made donor-acceptor (D-A) ligand in a metal-organic framework (MOF), a material with unprecedented features emerges. The ligand combines a pair of cyano groups as acceptors with four sulfanylphenyls as donors, which expose each a carboxylic acid as coordination sites. Upon treatment with zinc nitrate in a solvothermal synthesis, the MOF is obtained. The new material combines temperature-assisted reverse intersystem crossing (RISC) and intersystem crossing (ISC). As these two mechanisms are active in different temperature windows, thermal switching between their characteristic emission wavelengths is observed for this material. The two mechanisms can be activated by both, one-photon absorption (OPA) and two-photon absorption (TPA) resulting in a large excitement window ranging from ultraviolet (UV) over visible light (VL) to near infrared (NIR). Furthermore, the emission features of the material are pH sensitive, such that its application potential is demonstrated in a first ammonia sensor.

Mechanical regulation to interfacial thermal transport in GaN/diamond heterostructures for thermal switch.

Gallium nitride offers an ideal material platform for next-generation high-power electronics devices, which enable a spectrum of applications. The thermal management of the ever-growing power density has become a major bottleneck in the performance, reliability, and lifetime of the devices. GaN/diamond heterostructures are usually adopted to facilitate heat dissipation, given the extraordinary thermal conduction properties of diamonds. However, thermal transport is limited by the interfacial conductance at the material interface between GaN and diamond, which is associated with significant mechanical stress at the atomic level. In this work, we investigate the effect of mechanical strain perpendicular to the GaN/diamond interface on the interfacial thermal conductance of heterostructures using full-atom non-equilibrium molecular dynamics simulations. We found that the heterostructure exhibits severe mechanical stress at the interface in the absence of loading, which is due to lattice mismatch. Upon tensile/compressive loading, the interfacial stress is more pronounced, and the strain is not identical across the interface owing to the contrasting elastic moduli of GaN and diamond. In addition, the interfacial thermal conductance can be notably enhanced and suppressed by tensile and compressive strains, respectively, leading to a 400% variation in thermal conductance. More detailed analyses reveal that the change in interfacial thermal conductance is related to the surface roughness and interfacial bonding strength, as described by a generalized relationship. Moreover, phonon analyses suggest that the unequal mechanical deformation under compressive strain in GaN and diamond induces different frequency shifts in the phonon spectra, leading to an enhancement in phonon overlapping energy, which promotes phonon transport at the interface and elevates the thermal conductance and vice versa for tensile strain. The effect of strain on interface thermal conductance was investigated at various temperatures. Based on the mechanical tunability of thermal conductance, we propose a conceptual design for a mechanical thermal switch that regulates thermal conductance with excellent sensitivity and high responsiveness. This study offers a fundamental understanding of how mechanical strain can adjust interface thermal conductance in GaN/diamond heterostructures with respect to mechanical stress, deformation, and phonon properties. These results and findings lay the theoretical foundation for designing thermal management devices in a strain environment and shed light on developing intelligent thermal devices by leveraging the interplay between mechanics and thermal transport.

Negative to positive axial thermal expansion switching of an organic crystal: contribution to multistep photoactuation.

Materials displaying negative thermal expansion (NTE), in contrast to typical materials with positive thermal expansion (PTE), are attractive for both fundamental research and practical applications, including the development of composites with near-zero thermal expansion. A recent data mining study revealed that approximately 34% of organic crystals may present NTE, indicating that NTE in organic crystals is much more common than generally believed. However, organic crystals that switch from NTE to PTE or vice versa have rarely been reported. Here, we report the crystal of N-3,5-di-tert-butylsalicylide-3-nitroaniline in the enol form (enol-1) as the first organic crystal in which the axial thermal expansion changes from negative to positive at around room temperature. When heated, the crystal shrinks along the a-axis below 30 °C and then it expands above 30 °C. Geometric calculations revealed that below 30 °C, the decrease in the tilt angle of the molecule exceeds the increase in the interplanar distance, causing NTE, whereas above 30 °C, the increase in the interplanar distance outweighs the decrease in the tilt angle, resulting in PTE. By combining photoisomerisation and the NTE-PTE switching induced by the photothermal effect, multistep crystal photoactuation was achieved. Moreover, actuation switching of the same crystal sample by changing atmosphere temperature was realised by utilising the NTE-PTE change. Such NTE-PTE switching without a thermal phase transition provides not only new insight into organic crystals but also a new strategy for designing crystal actuators.

Above room temperature spin crossover in mononuclear iron(II) complexes featuring pyridyl-benzimidazole bidentate ligands adorned with aliphatic chains.

Two bidentate ligands (L1 = 1-pentyl-2-(pyridin-2-yl)-1H-benzimidazole and L2 = 1-heptyl-2-(pyridin-2-yl)-1H-benzimidazole) were employed for the synthesis of five mononuclear Fe(II) coordination compounds 1-5 containing perchlorate, tetrafluoroborate and triflate counterions. Single-crystal X-ray diffraction analysis confirmed the expected molecular structures of all the reported compounds, revealing a moderately distorted octahedral geometry of {FeN6} coordination chromophores. All five compounds exhibit thermal spin crossover with T1/2 temperatures allocated above 400 K. The theoretical calculations supported the experimental magnetic investigation and helped to explain the electronic structures of the reported complexes with respect to the occurrence of thermal spin state switching. In addition, compound 4 was employed for the preparation of Langmuir-Blodgett films and fabrication of molecular films using the method of spontaneous evaporation of the subphase. While the formation of Langmuir-Blodgett films was unsuccessful due to the instability of the compound at the water/air interface, the latter technique allowed the formation of molecular films of 4 with well-defined thickness and homogeneity.

Detour to success: photoswitching via indirect excitation.

Photoswitchable molecules that undergo nanoscopic changes upon photoisomerisation can be harnessed to control macroscopic properties such as colour, solubility, shape, and motion of the systems they are incorporated into. These molecules find applications in various fields of chemistry, physics, biology, and materials science. Until recently, research efforts have focused on the design of efficient photoswitches responsive to low-energy (red or near-infrared) irradiation, which however may compromise other molecular properties such as thermal stability and robustness. Indirect isomerisation methods enable photoisomerisation with low-energy photons without altering the photoswitch core, and also open up new avenues in controlling the thermal switching mechanism. In this perspective, we present the state of the art of five indirect excitation methods: two-photon excitation, triplet sensitisation, photon upconversion, photoinduced electron transfer, and indirect thermal methods. Each impacts our understanding of the fundamental physicochemical properties of photochemical switches, and offers unique application prospects in biomedical technologies and beyond.

Implementing L-DNA analogs as mirrors of PCR reactant hybridization state: theoretical and practical guidelines for PCR cycle control.

In previous reports, we described a PCR cycle control approach in which the hybridization state of optically labeled L-DNA enantiomers of the D-DNA primers and targets determined when the thermal cycle was switched from cooling to heating and heating to cooling. A consequence of this approach is that it also "adapts" the cycling conditions to compensate for factors that affect the hybridization kinetics of primers and targets. It assumes, however, that the hybridization state of the labeled L-DNA analogs accurately reflects the hybridization state of the D-DNA primers and targets. In this report, the Van't Hoff equation is applied to determine the L-DNA concentration and ratio of L-DNA strands required by this assumption. Simultaneous fluorescence and temperature measurements were taken during L-DNA controlled cycling, and the optical and thermal switch points compared as a function of both total L-DNA concentration and ratio of strands. Based on the Van't Hoff relationship and these experimental results, L-DNA best mirrors the hybridization of PCR primers and targets when total L-DNA concentration is set equal to the initial concentration of the D-DNA primer of interest. In terms of strand ratios, L-DNA hybridization behavior most closely matches the behavior of their D-DNA counterparts throughout the reaction when one of the L-DNA strands is far in excess of the other. The L-DNA control algorithm was then applied to the practical case of the SARS-CoV-2 N2 reaction, which has been shown to fail or have a delayed Cq when PCR was performed without nucleic acid extraction. PCR Cq values for simulated "unextracted" PCR samples in a nasopharyngeal background and in an NaCl concentration similar to that of viral transport media were determined using either the L-DNA control algorithm (N = 6) or preset cycling conditions (N = 3) and compared to water background controls run in parallel. For preset cycling conditions, the presence of nasopharyngeal background or a high salt background concentration significantly increased Cq, but the L-DNA control algorithm had no significant delay. This suggests that a carefully designed L-DNA-based control algorithm "adapts" the cycling conditions to compensate for hybridization errors of the PCR D-DNA reactants that produce false negatives.

Low voltage–driven high-performance thermal switching in antiferroelectric PbZrO 3 thin films

Effective control of heat transfer is vital for energy saving and carbon emission reduction. In contrast to achievements in electrical conduction, active control of heat transfer is much more challenging. Ferroelectrics are promising candidates for thermal switching as a result of their tunable domain structures. However, switching ratios in ferroelectrics are low (&lt;1.2). We report that high-quality antiferroelectric PbZrO 3 epitaxial thin films exhibit high-contrast (&gt;2.2), fast-speed (&lt;150 nanoseconds), and long-lifetime (&gt;10 7 ) thermal switching under a small voltage (&lt;10 V). In situ reciprocal space mapping and atomistic modelings reveal that the field-driven antiferroelectric-ferroelectric phase transition induces a substantial change of primitive cell size, which modulates phonon-phonon scattering phase space drastically and results in high switching ratio. These results advance the concept of thermal transport control in ferroic materials.

A flux pump driven non-soldering closed-loop HTS magnet and its electromagnetic-thermal semi-analytical modelling method

We propose a new flux pump driven non-soldering closed-loop (NSCL) high temperature superconducting (HTS) magnet free of any soldering joints throughout the magnet for persistent current mode (PCM) operation. All the superconducting parts of the magnet are wound directly with HTS closed-loop ring split from an 8 mm REBCO tape using our new cutting scheme free of any soldering requirements. The magnet contains two single-pancake coils connected in series forming a closed circuit through two parallel bridge branches. Two thermal switches set on the two bridge branches control the on–off of the two bridges. A copper coil with iron core installed around one of the bridges is employed as the flux pump to drive the HTS magnet. An electromagnetic-thermal semi-analytical modelling method is proposed to analyse the pumping process by which the transport current in the magnet is calculated. The theoretical limit equation of the saturation current is improved as well. The proposed method can predict the current of the NSCL HTS magnet during the pumping process and provide results that are close to experiments. Experiments verify both the feasibility of the proposed flux pump driven NSCL HTS magnet and the modelling method. The results show that the NSCL HTS magnet works well in the PCM, which provides inspiration to the design of PCM operation of high field HTS magnets. The proposed modelling method also helps guide the design of different forms of HTS magnets and the flux pumps driving them.

Near-field thermal emission from metasurfaces constructed of SiC ellipsoidal particles

We model near-field thermal emission from metasurfaces structured as two-dimensional arrays of ellipsoidal SiC particles. The modeling approach is developed from fluctuational electrodynamics and is applicable to systems of ellipsoidal particles within the dipole limit. In all simulations, the radial lengths of particles are restricted to the range of 10–100 nm, and interparticle spacing is constrained to at least three times the particle characteristic length. The orientation and dimensions of constituent ellipsoidal particles are varied to tune localized surface phonon resonances and control the near-field energy density above metasurfaces. Results show that particle orientation can be used to regulate the relative magnitude of resonances in the energy density, and particle dimensions may be changed to adjust the frequency of these resonances within the Reststrahlen band. Metasurfaces constructed from particles with randomized dimensions display comparatively broadband thermal emission rather than the three distinct resonances seen in metasurfaces made with ellipsoidal particles of equivalent dimensions. When the interparticle spacing in a metasurface exceeds about three times the particle characteristic length, the spectral energy density above the metasurface is dominated by individual particle self-interaction and can be approximated as a linear combination of single-particle spectra. When interparticle spacing is at the lower limit of three times the characteristic length, however, multiparticle interaction effects increase and the spectral energy density above a metasurface deviates from that of single particles. This work provides guidance for designing all-dielectric, particle-based metasurfaces with desired near-field thermal emission spectra, such as thermal switches.

Numerical analysis of a novel coaxial annular tube convective heat switch

Heat switches are commonly used in cryogenic instruments to provide thermal contact or isolation between the hot and cold masses over a temperature range from millikelvin to room temperature. This study presents a new type of convective thermal switch that implements an annular tube design replacing the twin-tube structure. To study the effects of geometry and operating parameters on thermal performance, a computational fluid dynamics (CFD) model is developed. The results show that the designed heat switch provides a thermal conductance of 33.79 mW/K when the end temperatures are 4.2 K and 40 K. In addition, it can automatically change to thermal isolation mode with a residual thermal conductance of 0.19 mW/K. The thermal conductance is increased from 33.79 mW/K to 38.90 mW/K by extending the length of the inner tube. This novel heat switch holds promise for precooling the modern cryogen-free dilution refrigerator (DR) and adiabatic demagnetization refrigerator (ADR).

Thermal conductivity switching for a Y–Mg alloy hydride thin film due to hydrogenation/dehydrogenation reactions using dilute H2 gas

Thermal switching requires a significant contrast in thermal conductivity between the on and off states. We focus on thermal conductivity switching performance and mechanism for switchable mirror materials, which changes reversible metallic and semiconductor states due to hydrogenation and dehydrogenation. A thin film of yttrium–magnesium (Y–Mg) alloy hydride covered with a Pd catalyst layer was fabricated on quartz glass substrates by dc magnetron sputtering using a 60 at. % Y and 40 at. % Mg alloy target and a mixture of 50% Ar and 50% H2 gases. The crystal structure, electrical conductivity, and thermal conductivity in each state were measured using in situ x-ray diffraction analysis, Hall effect measurement, and thermoreflectance apparatus, respectively. The Y–Mg alloy hydride film was hydrogenated and dehydrogenated on exposure to a mixture of 3% H2 in N2 gas and air, respectively. The structural change in Y hydrides due to hydrogenation and dehydrogenation was clarified, whereas Mg or Mg hydride in the film showed no apparent crystallization. The thermal conductivity of the on-state was 4.5 times larger than that of the off-state. The thermal conductivity change from hydrogenated to dehydrogenated state was ∼5.4 W m−1 K−1, and ∼2.5 W m−1 K−1 of thermal conductivity change could be attributed to electron contribution based on the estimation using Wiedemann–Franz law. The thermal conductivity changes of Y–Mg alloy hydrides due to hydrogenation/dehydrogenation resulted from both electrons and phonons.

Flexible and elastic thermal regulator for multimode intelligent temperature control

AbstractAs nonlinear thermal devices, thermal regulators can intelligently respond to temperature and control heat flow through changes in heat transfer capacities, which allows them to reduce energy consumption without external intervention. However, current thermal regulators generally based on high‐quality crystalline‐structure transitions are intrinsically rigid, which may cause structural damage and functional failure under mechanical strain; moreover, they are difficult to integrate into emerging soft electronic platforms. In this study, we develop a flexible, elastic thermal regulator based on the reversible thermally induced deformation of a liquid crystal elastomer/liquid metal (LCE/LM) composite foam. By adjusting the crosslinking densities, the LCE foam exhibits a high actuation strain of 121% with flexibility below the nematic–isotropic phase transition temperature (TNI) and hyperelasticity above TNI. The incorporation of LM results in a high thermal resistance switching ratio of 3.8 over a wide working temperature window of 60°C with good cycling stability. This feature originates from the synergistic effect of fragmentation and recombination of the internal LM network and lengthening and shortening of the bond line thickness. Furthermore, we fabricate a “grid window” utilizing photic‐thermal integrated thermal control, achieving a superior heat supply of 13.7°C at a light intensity of 180 mW/cm2 and a thermal protection of 43.4°C at 1200 mW/cm2. The proposed method meets the mechanical softness requirements of thermal regulator materials with multimode intelligent temperature control.

Moisture modulated refractive index difference of engineered pore interface for indoor temperature and humidity regulation

Regulation of indoor environments like temperature and humidity consumes enormous energy and brings environmental crises. Therefore, the development of energy-free approaches is critical as it simultaneously improves living comfort and decreases the carbon footprint. Here we report a moisture-modulated strategy to access switchable thermal regulation by tuning solar radiative scattering through manipulation of the refractive index difference (Δn) of a porous surface. This approach is based on a double-layered film containing an upper porous polymer layer and a lower photothermal layer. Via moisture absorption and evaporation, the film performs a reversible thermal switching between solar heating and radiative cooling, allowing efficient space temperature manipulation in the range from + 12.0 to − 8.0 °C. Besides, the moisture-modulated characteristic also offers an opportunity for solar dehumidification by automatically absorbing ambient moisture while mitigating temperature rise. This work bridges a thermal nexus between moisture with solar radiation, while providing a guideline for energy-free indoor environment manipulation.