Dynamic tuning of thermal transport in solids is scientifically intriguing with wide applications for thermal transport control in electronic devices. In this work, we demonstrate a thermal ...transistor, a device in which heat flow can be regulated using external control, realized in a topological insulator (TI) through the topological surface states. The tuning of thermal transport is achieved by using optical gating of a thin dielectric layer deposited on the TI film. The gate-dependent thermal conductivity is measured using micro-Raman thermometry. The transistor has a large ON/OFF ratio of 2.8 at room temperature and can be continuously and repetitively switched in tens of seconds by optical gating and potentially much faster by electrical gating. Such thermal transistors with a large ON/OFF ratio and fast switching times offer the possibilities of smart thermal devices for active thermal management and control in future electronic systems.
Materials that can be switched between low and high thermal conductivity states would advance the control and conversion of thermal energy. Employing in situ time-domain thermoreflectance (TDTR) and ...in situ synchrotron X-ray scattering, we report a reversible, light-responsive azobenzene polymer that switches between high (0.35 W m−1 K−1) and low thermal conductivity (0.10 W m−1 K−1) states. This threefold change in the thermal conductivity is achieved by modulation of chain alignment resulted from the conformational transition between planar (trans) and nonplanar (cis) azobenzene groups under UV and green light illumination. This conformational transition leads to changes in the π-π stacking geometry and drives the crystal-to-liquid transition, which is fully reversible and occurs on a time scale of tens of seconds at room temperature. This result demonstrates an effective control of the thermophysical properties of polymers by modulating interchain π-π networks by light.
•Autonomous solar adsorption chiller for off-grid application presented.•Performance of thermally activated controls investigated.•Diurnal cycle dependence eliminated by novel control system.•System ...COPs in the range of 0.12–0.24 achieved.
Attempts to develop autonomous solar adsorption chillers without additional controls to regulate system operation have yielded low cooling capacity and poor coefficients of performance (COPs) due to the use of the sun’s diurnal cycle for control of system operation. A thermally driven, solar adsorption chiller with consistent cooling rates is investigated here. A model for a single-bed system is considered with activated-carbon/ammonia as the working pair. The modeled heat input corresponds to 1 m2 of collector area. Autonomous operation is achieved using novel thermally activated controls to regulate the heating and cooling of the adsorbent bed, rather than relying on the solar cycle. Heat from the collector is stored in a hot-side thermal mass during the adsorption phase, and cooling is delivered by an air-cooled heat sink. The system does not require pumping power or any auxiliary systems for operation. The simple design and elimination of expensive complex components make this system ideal for cooling applications in the developing world or in installations not connected to the grid.
The use of magnetic nanoparticles for the remote control of heat transfer in electronic devices can overcome the current limitations of appliance engineering. In this work, we demonstrate that a ...thermal switch based on a low-cost and stable MnFe2O4/Ethylene Glycol:Water (MFO/EG:W) dispersion can increase the span temperatures as high as 60% in the 0.01–0.60 Hz operating frequency range under the same heat supply for different recipient filling ratios (FR). Under the optimum condition of FR = 80%, the efficiency of our new MFO/EG:W colloidal dispersion is twice the obtained for the commercial Fe3O4/paraffin oil fluid. From numerical calculation, we demonstrate that the improved heat exchange efficiency relates to the three-steps effective thermal conductivity variation during operation, expanding the contact time between the heat and cold sources. Thus, the combination of an EG:W refrigerant solution and superparamagnetic MFO nanoparticles with high saturation magnetization allows their use for heat management control of electronic systems for long operation periods.
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•A magnetic thermal switch was developed based on an innovative refrigerant dispersion.•The refrigerant fluid doubles the MATS performance, enabling span temperatures up to 60%.•An ON-OFF switching time of ms allows the control of heat flux.•Numerical calculations confirm the change in thermal conductivity during operation.
With the appearance of energy crisis, greenhouse effect, and heat management problem, the control especially the active and reversible control of heat transport or thermal conductivity is becoming ...urgent. However, phonon transport as controllable as electron transport has not yet been achieved. The difficulty lies in the lack of direct connection between phonons and external stimuli. To realize the goal of controllable phonon transport, a comprehensive and systematic understanding of thermal switching is essential. Consequently, we review recent progress and efforts on thermal switching in five different types of solid materials including ferroelectric materials, ferromagnetic materials, nanomaterials and nanostructures, polymers, and phase change materials, considering their thermal switching ability. Within each type of material, different controlling methods are reviewed and the underlying mechanisms are discussed, aimed at improving their thermal switching performance. Among the five types of solid materials, systematic comparison and analysis are provided, aimed at combining the advantages from different materials. In addition, current challenges and future perspectives are provided to highlight new and emerging research directions in this growing field.
Caloric energy conversion is an emerging field of cooling, heat-pumping and power-generation technologies. The potentially high energy efficiency and use of environmentally friendly and safe ...solid-state working substances in the form of refrigerants has stimulated increased research activity in the past two decades. Most of today's caloric devices apply so-called active regeneration, which involves the oscillation of the working fluid through the matrix of the caloric material – the caloric regenerator. However, the unavoidable, irreversible viscous and heat-transfer losses apply limits to the caloric device's performance as well as its size. The quest for better caloric-device performance has led to the development of thermal control elements, which control the heat flux on different size and time scales. In this paper we describe the working principles of these elements: thermal switches, thermal diodes and thermal regulators. This is followed by the first up-to-date critical review of the research activities and applications of thermal control elements in all types of caloric devices. We show that thermal control elements have the potential to improve the power density of caloric devices. Finally, we propose target features for these elements with respect to future research activities in the field of caloric technologies.
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•The operating principles of thermal control elements are described.•The applications of thermal control elements in caloric technologies are explained.•A generalized figure of merit is proposed for different thermal control elements.•Future research guidelines for thermal control elements in calorics are proposed.
•A novel concept of the rare-earth-free electromagnetic field source is proposed.•The new concept introduces the magnetic energy recovery.•The device was designed, numerically analysed and ...experimentally validated.•A comparative analysis of different types of magnetic-field-sources was conducted.•The results reveal the new concept can strongly outperform existing electromagnets.
This article reports on the novel resistive electromagnetic field source with the magnetic energy recovery, which enables the use of the static magnetocaloric regenerator. Most of the existing prototype magnetocaloric devices that operate near room temperature, use magnetic field sources consisting of permanent magnets. The alternating of the magnetic field that is required for the thermodynamic cycle often comes from the rotation of magnets over the refrigerant, that is, an active magnetocaloric regenerator (AMR). Such systems require moving parts and a motor drive, both of which cause additional costs and reduced energy efficiency. Further restrictions in existing devices result from the speed of the magnetisation/ demagnetisation process, which is, in addition to efficient heat transfer, crucial for the compactness of the device. Another drawback is that the instant change of the magnetic field is not feasible, regardless of the principle of movement. Permanent-magnet assemblies based on neodymium are also constrained by the use of this rare-earth-material. Therefore, a number of global research activities relate to the optimization of permanent-magnet-based magnetic field sources. However, ohmic loss, the active cooling of magnets, and considerable energy consumption are the reasons why another type of magnetic field source, that is, the electromagnet, was generally avoided by the magnetocaloric community. This article presents a novel and unique approach that enables substantially improved energy efficiency and applicable operation of rare-earth-free and static electromagnetic field sources, by implementing for the first time the magnetic energy recovery for magnetic refrigeration and heat pumping. To prove the advantages of such a system, a large number of numerical simulations, as well as an experimental proof, were conducted. A comparative analysis was made for the evaluation of the energy efficiency of the proposed novel system vs an example of the existing rotating-magnet assembly. The results of this study reveal that this new type of electromagnetic field sources provides a number of different and important advantages that can lead to new frontiers in research. However, the energy efficiency is still lower than that of the comparable rotating-magnet assembly.
In the present study, we theoretically demonstrate a vacuum thermal switch based on near-field thermal radiation between phase transition materials, i.e., vanadium dioxide (VO2), whose phase changes ...from insulator to metal at 341K. Strong coupling of surface phonon polaritons between two insulating VO2 plates significantly enhances the near-field heat flux, which on the other hand is greatly reduced when the VO2 emitter becomes metallic, resulting in strong thermal switching effect. Fluctuational electrodynamics incorporated with anisotropic wave propagation predicts more than 80% heat transfer reduction at sub-30-nm vacuum gaps and 50% at vacuum gap of 1μm. Furthermore, the penetration depth inside the uniaxial VO2 insulator is studied at the vacuum gap of 50nm, suggesting the possible impact of reduced VO2 thickness on the near-field thermal radiation with thin-film structures. By replacing the bulk VO2 receiver with a thin film of several tens of nanometers, the switching effect is further improved over a broad range of vacuum gaps from 10nm to 1μm. Finally, the effect of SiO2 substrate for the thin-film emitter or receiver is also considered to provide insights for future experimental demonstrations. By controlling heat flow with near-field radiative transport, the proposed vacuum thermal switch would find practical applications for energy dissipation in microelectronic devices and for the realization of thermal circuits.
•Near-field thermal switching was theoretically demonstrated with phase change VO2.•Radiative heat flux was reduced by 80% at sub-30-nm vacuum gaps or 50% at 1μm.•Strong phonon coupling between insulating VO2 emitter and receiver was elucidated.•Thin-film structures were studied for achieving stronger thermal switching effect.•Effect of SiO2 substrate was investigated for thin-film vacuum thermal switches.
Active insulation systems (AISs) in buildings are envelopes that integrate thermal insulation, thermal energy storage, and controls. Although different designs for AISs have been proposed in the ...literature, a comprehensive analysis of feasible AISs is lacking. This paper discusses the energy performance, peak demand reduction potential, and performance characteristics of an AIS that uses a concrete wall as thermal mass sandwiched between two solid-state thermal switches (STSs). These STSs change their thermal conductivity using an on/off metal switch to create or break a thermal bridge across the STS. This paper first describes the experimental setup, used to determine the ratio of thermal resistance during R-high (low thermal conductivity) and R-low (high thermal conductivity) states of the STSs. This ratio was then used in whole-building energy simulations to evaluate the performance of AIS walls across different climate zones with/without a freeze timer of 60 min. The timer was added to reduce the number of switches of STSs from one state to another, and hence the energy needed for these switches. Analysis of the switching frequency and interval of STSs, thermal conductivity of walls, impact of wall orientation, and heat transfer through the wall from the use of AIS at different climate zones/locations were performed. The simulation results show that the AIS can achieve energy savings ranging from ~980 to 2,290 kWh in a single-family home with a floor area of ~220 m2 compared with an IECC 2018 baseline. We found the energy savings was higher in dry climate zones which represent 17% of residential buildings in the United States, compared to humid or marine climate.
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•Smectic nanostructured electrolytes were prepared for solvent-free li-ion batteries.•High content LiBF4 formed dynamic lamellar conducting channels in smectic electrolytes.•Smectic ...electrolytes showed robust stability and high efficiency for li-ion battery.•Phase transition of electrolytes could thermally switch on/off li-ion batteries.•The work gave new light on developing efficient and smart electrolytes for li-ion battery.
Developing electrolytes with desirable electrochemical performance and robust stability remains a huge challenge to meet the efficiency and durability demands of energy storage and conversion devices. Herein, the potential of ionic liquid crystals as high-performance electrolytes is demonstrated. Smectic ionic liquid crystals only consisting of conventional C14MimBF4 and LiBF4 are designed and prepared as electrolytes for solvent-free lithium-ion batteries. C14MimBF4 has good miscibility with LiBF4 to form smectic liquid crystals. LiBF4 is highly concentrated by C14MimBF4 layers in the smectic electrolytes to form dynamic lamellar conducting channels, which enable considerable ion conductivities of 10–4 ∼ 10–3 S cm−1. The smectic electrolytes possess robust cyclic stability and energy conversion efficiency in Li/LiFePO4 batteries, which show a charge/discharge capacity of 154.7 mA h g−1 at a 1 C current density, as well as stable performance after 100 charge/discharge cycles even at 80 ℃. Additionally, the phase transition behavior of the electrolytes can be applied as a smart switch to thermally tune on/off the batteries. The electrolytes show almost no charge/discharge behavior under crystal phase, but automatically operate efficiently with charge/discharge capacities >150 mA h g−1 once entering the smectic phase. The present work sheds new light on developing high-performance and smart electrolytes for solvent-free Li-ion batteries.
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