The thermal conductivity of crystalline materials is typically one or two orders of magnitude higher than that of their amorphous structures. The phase transition in barium titanate is generally ...considered to exhibit order–disorder character, suggesting the potential for thermal conductivity switching if this order–disorder transition can be controlled. To investigate this possibility computationally, following the method of Fu and Bellaiche, here electric fields are applied to align the polarizations and transform disordered paraelectric structures to ordered ferroelectric structures. Solving the Boltzmann transport equation, the theoretical limit of a perfectly disordered structure is found to have thermal conductivity of a factor of 3.9 lower than the perfectly ordered structure. The thermal conductivity of the ordered structure can be further enhanced by up to another 2.4 times under electric fields due to the reduction in phonon scattering rates, implying a theoretical maximum thermal conductivity switching ratio of 9.4. This study yields two guidelines in searching for high thermal conductivity switch ratio in ferroelectric materials: the structure should be single domain under electric field and the phase transition should be fully order–disorder rather than displacive.
Applying strong electric fields to a ferroelectric material having order–disorder phase transition can transform its structure from disordered to ordered, reminiscent of an amorphous–crystalline transition. First‐principles calculations of this effect in barium titanate show a thermal conductivity switching ratio of up to 9.4 comparing perfectly ordered and disordered states, suggesting a mechanism for electric field controlled solid‐state thermal switch.
Dynamical control of thermal transport at the nanoscale provides a time-domain strategy for optimizing thermal management in nanoelectronics, magnetic devices, and thermoelectric devices. However, ...the rate of change available for thermal switches and regulators is limited to millisecond time scales, calling for a faster modulation speed. Here, time-resolved X-ray diffraction measurements and thermal transport modeling reveal an ultrafast modulation of the interfacial thermal conductance of an FeRh/MgO heterostructure as a result of a structural phase transition driven by optical excitation. Within 90 ps after optical excitation, the interfacial thermal conductance is reduced by a factor of 5 and lasts for a few nanoseconds, in comparison to the value at the equilibrium FeRh/MgO interface. The experimental results combined with thermal transport calculations suggest that the reduced interfacial thermal conductance results from enhanced phonon scattering at the interface where the lattice experiences transient in-plane biaxial stress due to the structural phase transition of FeRh. Our results suggest that optically driven phase transitions can be utilized for ultrafast nanoscale thermal switches for device application.
A metamaterial made of submicron aluminum disks on phase-change vanadium dioxide (VO2) thin film, which is synthesized by furnace oxidation method, has been fabricated by metal deposition, ...photolithography, and lift-off processes. By varying over-exposure time during the photolithography process with a stepper, metamaterials with submicron disk diameters down to 0.55 μm were successfully fabricated. Characterized by a Fourier transform infrared microscope, the metamaterial exhibits an absorption peak close to unity at the wavelength around 7 μm at room temperature as a selective absorber, while it is highly reflective once VO2 becomes metallic after phase transition at higher temperatures. Elucidated by numerical simulation, the spectral absorption peak is attributed to the excitation of magnetic polariton (MP) within the insulating VO2 film. Once the metamaterial is heated above the transition temperature of VO2, MP cannot be excited within the metallic VO2 film, resulting in disappearance or “switch-off” of the absorption peak. The switchable behavior is further explained by an equivalent inductor-capacitor circuit model, which predicted absorption peak wavelengths in excellent agreement with experimentally observed ones of fabricated metamaterials with different disk diameters. This thermally-switchable spectrally-selective metamaterial could facilitate applications in dynamic infrared camouflage and active radiative thermal management.
•Thermally-switchable spectrally-selective infrared absorber/emitter is demonstrated.•Selectively absorptive at room temperature and reflective at elevated temperature.•It was achieved by tuning magnetic polariton with a phase-change VO2 layer.•Tunable absorption peaks were verified through experiment and modelling.
•A new thermal switch method was proposed to improve the output performance of TEG system with fluctuating heat source.•The turning on temperature and turning off temperature of the TEG system with ...thermal switch can be easily set to maximally increase the output power and electricity efficiency.•A one-dimensional dynamic model considering the thermoelectric effects depend on the temperature was put forward to analyze the detailed characteristics of the new system with thermal switch.•An integrated experimental platform has been built to verify the validity of the new TEG system by experimental data.
The stability of output performance plays a very important role in the thermoelectric generation (TEG) system since the fluctuation problem of heat source widely exists in the industry which releases the waste heat, and the fluctuating process will cause the instability and low efficiency of the TEG system. In this article, a novel TEG system with thermal switch was proposed to address this serious problem. In order to obtain the detailed characteristics of the TEG system with thermal switch, experimental and dynamic modeling methods were employed. The experimental and modeling results show that the thermal switch can efficiently reduce the temperature fluctuation and increase the output power and efficiency of the TEG system; in addition, there is an optimal turning on/off temperature to maximally increase the output power and electricity efficiency of the TEG system. The comparison between the numerical data and the experimental results has further demonstrated the proposed model.
•Effective magnetocaloric cycles can be achieved with appropriate thermal switches.•Temperature spans near 2 K can be achieved with only one thermal switch.•The use of two thermal switches is ...beneficial if both show near-ideal behaviours.
Magnetic refrigeration, based on the magnetocaloric effect, is an attractive alternative to the conventional vapor-compression technology. In a solid state magnetic refrigerator the traditional alternated fluid flow is replaced by two solid thermal switches (TSs) that control the heat flux. These TSs are materials, or devices, whose thermal conductivity changes with an external stimulus. Here, we numerically investigate how the performance of a solid state magnetic refrigerator depends on the TS thermal conductivities (k) and corresponding k-variations (Δk) with the magnetic field. Two different scenarios were considered: a single TS and a double TS magnetic refrigerator. We have numerically varied the thermal conductivity of the thermal switches, operating frequency and working temperature. The use of only one thermal switch with near-ideal Δk is enough if the required performance does not overcome half of the adiabatic temperature change (ΔTad) of the magnetocaloric material. To overcome this value the device must use two effective thermal switches. Using an additional thermal switch is only beneficial when ΔTad needs to be overcome, or when the first thermal switch shows a modest Δk. These results simplify the operability control, which will increase the interest for magnetically activated thermal switches operating at acceptable magnetic fields.
Abstract
Describing the dynamic characteristics of glacier surge events is a precursor to being able to understand their driving mechanisms. Here, a comprehensive suite of surface velocities and ...surface elevation changes for the surging South Rimo Glacier (SRG), situated in the East Karakoram region, are obtained by offset-tracking from Sentinal-1A and geodetic method from TerraSAR-X/TanDEM-X and Ice, Cloud, and land Elevation Satellite-2 Advanced Topographic Laser Altimeter System. The surge of SRG initiated in the summer of 2018, and the rapid and dramatic increase in surface velocities reached a peak (∼12 m d
−1
) in August 2019. By the summer of 2020, the surface velocity of SRG had returned to its pre-surge level. We interpret that the evolution of the latest SRG surge was probably triggered by changes in subglacial thermal conditions, and was ultimately accelerated by hydrological processes. Based on historical analysis, a surge return period of ∼25–30 years prevails at SRG. Spatiotemporal analyses of surface velocities and elevation changes such as these can provide useful information about surge mechanisms and their controls.
•The near-field radiative thermal switch made of the n-type doped silicon and graphene-covered silicon dioxide plates is comprehensively investigated.•The modulation is due to the surface mode of the ...graphene-covered silicon dioxide, which is significantly affected by the graphene chemical potential value via applying the external voltage bias.•The D-Si doping concentration and the breakdown voltage of dielectric materials also can significantly affect the performance of the near-field radiative thermal switch.•The thermal switching factor of the near-field radiative thermal switch can be improved to 93.5% across a 10 nm vacuum gap by altering the graphene chemical potential value from 0 to 1 eV.
The key to the micro/nanoscale thermal management of precision instruments is flexibly controlling the heat flux in the near-field following the actual demand. In this paper, a near-field radiative thermal switch (NFRTS) made of the n-type doped silicon (D-Si) and graphene-covered silicon dioxide (SiO2) plates is proposed to achieve the active near-field radiative heat transfer modulation. The radiative heat flux, thermal switching factor, and thermal modulation factor are calculated for different graphene chemical potential values from 0 to 1 eV and D-Si doping concentrations at different vacuum gaps. This is achieved by considering the breakdown voltage of the SiO2, and with the fluctuational electrodynamics and fluctuation-dissipation theorem. The SiO2 is also replaced by the silicon carbide (SiC) in the thermal switch to clarify further the effect of the breakdown voltage on the performance of the NFRTS. In combining the graphene chemical potential value and D-Si doping concentration, it is obtained that the optimal thermal switching factor is 93.5% for a doping concentration of 1018 cm−3 at a 10 nm vacuum gap for SiO2 system when the heat source and heat sink temperatures are 400 K and 300 K, respectively. This is mainly due to the combination of the major angular frequency band of the transmission coefficients for bulk D-Si determined by the doping concentration and the surface mode of the graphene-covered SiO2 modulated by altering the graphene chemical potential value via applying the external voltage bias. Results also reveal that the performance of the NFRTS with SiC substrate is significantly affected and weakened by the limited breakdown voltage of SiC, and almost always worse than that of the NFRTS with SiO2 substrate. This work paves a way for designing the active near-field thermal management devices for simple structures.
Reversible thermal conductivity regulation at the nanoscale is of great interest to a wide range of applications such as thermal management, phononics, sensors, and energy devices. Through a series ...of large-scale molecular dynamics simulations, we demonstrate a thermal conductivity regulation utilizing the phase transition of polyethylene nanofibers, enabling a thermal conductivity tuning factor of as high as 12, exceeding all previously reported values. The thermal conductivity change roots from the segmental rotations along the polymer chains, which introduce along-chain morphology disorder that significantly interrupts phonon transport along the molecular chains. This phase transition, which can be regulated by temperature, strain, or their combinations, is found to be fully reversible in the polyethylene nanofibers and can happen at a narrow temperature window. The phase change temperature can be further tuned by engineering the diameters of the nanofibers, making such a thermal conductivity regulation scheme adaptable to different application needs. The findings can stimulate significant research interest in nanoscale heat transfer control.
Traditional building envelopes have passive insulation systems that cannot respond to dynamic changes in the environment. An Active Insulation System (AIS) consists of Active Insulation Materials ...(AIMs) that dynamically vary the thermal conductivity of the insulation system. Several researchers have evaluated the impact of AIS on building thermal and energy performance by using simulation tools. Up to 70% savings in annual heating and cooling energy and significant reductions in peak demand have been predicted for some climates with wall systems employing AIS. However, materials and assembly development still need a cost-effective product that achieves the required performance. Here, in this study, we present the process of developing an AIS that we will install in a test hut for its performance evaluation. Minimum performance criteria of the AIS system are developed based on R-low/R-high ratio, required time and efficiency to switch states, and cost estimates. The following steps during this study are creating the concept to meet the requirements, predicting the performance via simulations, developing the experimental setup for bench-scale testing, and finally, constructing a full-scale wall assembly and monitoring the performance when exposed to environmental chamber tests. The selected approach uses off-the-shelf products to create an AIS that can switch R-value between 0.98 ft2·°F·h/BTU (0.173 m2·K/W) and 5.81 ft2·°F·h/BTU (1.02 m2·K/W) and have a switching time of less than one minute between R-high and R-low.
We explore that two ferromagnetic insulator slabs host a strong twist-induced near-field radiative heat transfer in the presence of twisted magnetic fields. Using the formalism of fluctuational ...electrodynamics, we find the existence of a large twist-induced thermal switch ratio in large damping conditions and nonmonotonic twist manipulation for heat transfer in small damping conditions, associated with the different twist-induced effects of nonreciprocal elliptic surface magnon-polaritons, hyperbolic surface magnon-polaritons, and twist-nonresonant surface magnon-polaritons. Moreover, the near-field radiative heat transfer can be significantly enhanced by the twist-nonresonant surface magnon-polaritons in the ultrasmall damping condition. Such a twist-induced effect is applicable for other kinds of anisotropic slabs with time-reversal symmetry breaking. Our findings provide a way to twisted and magnetic control in nanoscale thermal management and improve it with twistronic concepts.
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