Controlling the thermal conductivity of multilayer graphene by strain

Straintronics is a new concept to enhance electronic device performances by strain for next-generation information sensors and energy-saving technologies. The lattice deformation in graphene can modulate the thermal conductivity because phonons are the main heat carriers. However, the device fabrication process affects graphene’s heat transport properties due to its high stretchability. This study experimentally investigates the change in the thermal conductivity when biaxial tensile strain is applied to graphene. To eliminate non-strain factors, two mechanisms are considered: pressure-induced and electrostatic attraction–induced strain. Raman spectroscopy and atomic force microscopy precisely estimate the strain. The thermal conductivity of graphene decreases by approximately 70% with a strain of only 0.1%. Such thermal conductivity controllability paves the way for applying graphene as high-efficiency thermal switches and diodes in future thermal management devices.

Wide range continuously tunable and fast thermal switching based on compressible graphene composite foams

Thermal switches have gained intense interest recently for enabling dynamic thermal management of electronic devices and batteries that need to function at dramatically varied ambient or operating conditions. However, current approaches have limitations such as the lack of continuous tunability, low switching ratio, low speed, and not being scalable. Here, a continuously tunable, wide-range, and fast thermal switching approach is proposed and demonstrated using compressible graphene composite foams. Large (~8x) continuous tuning of the thermal resistance is achieved from the uncompressed to the fully compressed state. Environmental chamber experiments show that our variable thermal resistor can precisely stabilize the operating temperature of a heat generating device while the ambient temperature varies continuously by ~10 °C or the heat generation rate varies by a factor of 2.7. This thermal device is promising for dynamic control of operating temperatures in battery thermal management, space conditioning, vehicle thermal comfort, and thermal energy storage.

A theory of glacier dynamics and instabilities Part 1: Topographically confined glaciers

We present a theoretical framework that integrates the dynamics of glaciers with and without the topographic confinement. This Part 1 paper concerns the former, which may exhibit surge cycles when subjected to thermal switches associated with the bed condition. With the topographic trough setting the glacier width and curbing the lateral drainage of the meltwater, the problem falls under the purview of the undrained plastic bed (UPB) formalism. Employing the UPB, we shall examine the external controls of the glacial behavior and test them against observations. Through our non-dimensionalization scheme, we construct a 2-D regime diagram, which allows a ready prognosis of the glacial properties over the full range of the external conditions, both climate- and size-related. We first discern the boundaries separating the glacial regimes of steady-creep, cyclic-surging and steady-sliding. We then apply the regime diagram to observed glaciers for quantitative comparisons. These include the Svalbard glaciers of both normal and surge types, Northeast Greenland Ice Stream characterized by steady-sliding, and Hudson Strait Ice Stream exhibiting cyclic surges. The quantitative validation of our model containing no free parameters suggests that the thermal switch may unify the dynamics of these diverse glaciers.

Thermal switches between delayed fluorescence and persistent phosphorescence based on a keto-BODIPY electron acceptor

A new type of twisted donor–acceptor molecular material 3a and 3b containing carbazole as an electron donor and keto-BODIPY bearing keto-isoindolinyl and pyridyl subunits as an acceptor has been prepared and characterized.

Testing System for the On-Site Checking of Magneto-Thermal Switches with Arc Fault Detection

Arcing is a harmful condition that can lead to electrical fires. The U.S. National Electrical Code requires the installation of electric arc breakers in all residential areas. However, such devices can fail, and therefore, may not work when needed. At present, there are no tests to confirm the correct operation of this type of device. This study aimed to propose an experimental test method to verify the proper functioning of magneto-thermal switches with an electric arc protection function for the occurrence of arc faults. The results show that the proposed method adequately detected the correct operations and tripping time by using different surge suppressors. The proposed system lays the foundations for a portable test system for the periodic checking of electrical systems in which arc fault detection devices (AFDDs) are installed.

Regulative Characteristic of Methanol–Copper Heat Pipes for Asteroid Lander “MASCOT”

Variable conductance heat pipes (VCHPs) are the main part of the MASCOT (mobile asteroid surface SCOuT) lander thermal control system (TCS). They provide variable conductivity by utilizing the heat transfer limitations. This allows the heat pipes to act as thermal switches without additional constructive elements, thus leveraging the simple and compact design of conventional heat pipes. Two cylindrical methanol–copper heat pipes with shell length of 0.482 m and 0.438 m and external diameter of 0.006 m, having copper discrete metal fiber wick and copper shell were constructed and verified in the temperature range between −75 and +60 °C. The purpose is to apply this design into the MASCOT TCS and to investigate the heat pipes' regulative characteristics and heat transfer limitations. VCHPs show a change of thermal resistivity from 70 K/W at a heat sink temperature of −60 °C, to 0.8 K/W at a heat sink temperature of +60 °C; with an obtained maximal heat transfer rate of 5 W and 16 W, respectively. It is found that the switching effect of the heat pipes is governed by the sonic velocity limitation, the saturation vapor pressure of the working fluid, and the maximal capillary pressure of the wick. The operation of the heat pipes as the part of the TCS has confirmed their variable thermal properties.

Development of Thermal Switches Based on Shape Memory Alloy Actuators

In machine tool engineering, the impact of thermal issues on machine precision and efficiency has been outlined in numerous studies. One of the major challenges is the energy-efficient distribution of heat within the machine structure. In order to control occurring heat fluxes without additional energy input into the machine tool, smart materials can be used for load-dependent adjustment of heat transfer characteristics. The present study illustrates the development and examination of heat transfer switch mechanisms using shape memory alloys. Experimental and numerical results demonstrate how different types of actuators can be used to enable an energy self-sufficient thermal switch function between heat source and heat sink. Different scenarios are considered and the combination of thermal switches with highly conductive heat-transfer devices and latent heat storages is evaluated.

Influence of thermal conditions on the electrocaloric effect in a multilayer capacitor based on doped BaTiO3

We present the results of studies of thermal expansion, heat capacity, permittivity, polarization and the influence of different thermal conditions on the intensive electrocaloric effect (ECE), [Formula: see text], in a commercial multilayer capacitor based on doped BaTiO3. Investigations in a wide temperature range revealed one anomaly in the behavior of the physical properties at about 290[Formula: see text]K characteristic for the relaxors. Direct measurements showed a high reversibility of ECE under equilibrium thermal conditions. Good agreement was found between the values of [Formula: see text] determined by direct and indirect measurements at the electric field up to 15.4[Formula: see text]kV/cm. Quasi-isothermal conditions lead, firstly, to decrease of large [Formula: see text]0.94[Formula: see text]K obtained in adiabatic conditions under [Formula: see text][Formula: see text]kV/cm to 0.87[Formula: see text]K, and, secondly, the appearance of a difference between the values of [Formula: see text] and [Formula: see text] determined when the electric field is applied and removed. Using this phenomenon and changing the frequency of the periodic electric field, [Formula: see text][Formula: see text]kV/cm, the effect of cooling was obtained equal to [Formula: see text][Formula: see text]K. The results obtained are useful for further development of the electrocaloric refrigeration technique without thermal switches.