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The realization of multi-level storage of chalcogenide phase change materials (C-PCMs) has been proved to be an effective way to achieve high-density data storage. However, Ge2Sb2Te5 (GST), as one of the most representative C-PCMs, has poor performance in multi-level storage. In this paper, a bi-layer composite structure GST–nAg was proposed and prepared by magnetron sputtering accompanied by thermal annealing. Microstructure and multi-level optical properties of GST–nAg were investigated using x-ray diffraction, scanning electron microscopy, variable angle spectroscopic ellipsometry, Raman spectrometer, micro-area reflectivity testing system, etc. The results showed that the formation of the Ag nanoparticles layer changed the heat distribution and heat transmission in the GST layer when irradiated by a nanosecond pulsed laser. By changing the laser fluence, the microstructure of the GST layer could be adjusted and ten crystal states with different optical reflectivity were obtained. In addition, the introduction of Ag nanoparticles reduced the compressive stress among the Ge/Sb–Te tetrahedral structure and grain size, which benefits to the formation and stable existence of more crystal states. Finally, an optical irradiation device based on GST–nAg was built and the storage density of this optical irradiation device was increased about three times. This paper provides a way to realize multi-level optical storage. [ABSTRACT FROM AUTHOR]

Phase change materials (PCMs) present opportunities for efficient thermal management due to their high latent heat of melting. However, a fundamental challenge for PCM cooling is the presence of a growing liquid layer of relatively low thermal conductivity melted PCM that limits heat transfer. Dynamic phase change material (dynPCM) uses an applied pressure to pump away the melt layer and achieve a thin liquid layer, ensuring high heat transfer for extended periods. This paper investigates heat transfer during dynPCM cooling when the heated surface has extended features made from high thermal conductivity copper (Cu). Using experiments and finite element simulations, we investigate the heat transfer performance of dynPCM paraffin wax on finned Cu surfaces. A total of 102 transient temperature measurements characterize the performance of dynPCM with extended surfaces and compare the performance with other cooling methods including hybrid PCM and air cooling. The study examines the effects of fin geometry, applied power (20–65 W), and pressure (0.97–12.5 kPa). For dynPCM on a finned surface and a heating power of 65 W, the thermal conductance is 0.45 W/cm2-K, compared to 0.22 W/cm2-K for dynPCM on a flat surface and 0.10 W/cm2-K for hybrid PCM. The heat transfer is highest at the fin tips where the melt layer is thinnest, providing valuable design guidelines for future high performance dynPCM cooling technologies. [ABSTRACT FROM AUTHOR]

There is still a gap in understanding phonon scattering by geometrical defects at the nanoscale, and it remains a significant challenge for heat transfer management in nanoscale devices and systems. In this study, we aim to explore the characteristics of phonon scattering by a single pore to gain insights into thermal transport in nanostructures. The paper outlines a methodology for assessing the spatial distribution of the magnitude of the radial, azimuthal, and polar components of the velocity of scattered phonons by a spherical pore. We demonstrated that the size parameter, commonly employed in electromagnetic wave scattering theory, is vital in determining the scattering regime. Specifically, we show that the calculated scattering efficiency has the same pattern as that commonly obtained in classical wave scattering theory. However, we found that crystallographic directions are pivotal in shaping the scattering patterns, especially in the regions where scattering patterns are defined by the Mie resonances. This observation holds significance in understanding the influence of phonon coherence on thermal transport in nanostructured materials. [ABSTRACT FROM AUTHOR]

At present, the thermal switch for localized heating is becoming a hot research topic in MEMS packaging technology. A method of using a liquid-based thermal switch is proposed in this paper. A plate local heating device based on electric field distribution control is constructed, where a thermal liquid column can be formed under the action of the electric field. The electric field controls the formation and dissipation of the liquid column to realize the on-off and off-on function of the thermal switch. Through the control of the different conductive substrates in the plate, the localized heat transfer can be further realized, and the heat transfer position is precisely regulated. Furthermore, due to the fluidity of the liquid, the prepared thermal switch can also realize movable heating. The experimental results show that the heating time and speed can be controlled precisely by adjusting the driven voltage and the conductive area of the upper substrate. [ABSTRACT FROM AUTHOR]

The search for materials with high thermal resistance has promising applications in thermoelectric devices and boiling crisis retardation. In this paper, we study the interfacial heat transfer between water and gold, nanostructuring the gold surface and coating it with graphene. By trapping air (or vacuum in our simulations) between graphene and the nanopatterned surface, we observe a considerable increase in the interfacial resistance compared to the planar gold situation, which is shown to scale with the effective graphene–gold contact surface for both monolayer and multilayer graphene. With the massive thermal resistances we predict (up to 200 nm in terms of Kapitza length), the system proposed here represents a robust alternative to superhydrophobic Cassie materials. Moreover, since the low thermal conductance is achieved primarily due to geometry (vacuum trapping), it is straightforward to extend our results to any material with a structure equivalent to that of the nanopatterned gold wall considered here. [ABSTRACT FROM AUTHOR]

Van der Waals heterostructures have exhibited interesting physical properties. In this paper, heat transfer in hybrid coplanar bilayer/monolayer (BL-ML) graphene, as a model layered van der Waals heterostructure, was studied using non-equilibrium molecular dynamics (MD) simulations. The temperature profile and inter- and intra-layer heat fluxes of the BL-ML graphene indicated that, there is no fully developed thermal equilibrium between layers and the drop in the average temperature profile at the step-like BL-ML interface is not attributable to the effect of Kapitza resistance. By increasing the length of the system up to 1 μm in the studied MD simulations, the thermally non-equilibrium region was reduced to a small area near the step-like interface. All MD results were compared to a continuum model and a good match was observed between the two approaches. Our results provide a useful understanding of heat transfer in nano- and micro-scale layered materials and van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

We present here a controlled measurement of heat flux across a closing gap that is initially less than 10nm wide between two solid surfaces at different temperatures. The measured heat transfer is compared with our published theoretical analyses of this phenomenon that show thermal radiation dominates the heat transfer for gaps wider than about 1-2 nm, but phonon conduction dominates between 1 and 2nm and contact. The experiments employ a thermal actuator mounted on a rocking base block for coarse positioning that supplies Joule heating to an embedded element to cause thermal expansion of a localized region for less than 10 nm spacing control, together with an embedded near-surface resistive temperature sensor to measure its temperature change due to the heat flux across the gap. The measured results are in general agreement with the theoretical predictions, and they also agree with common sense expectations. This paper not only shows nano-scale heat transfer measurement across a closing gap, it also lends additional strong support to the validity of the referenced theoretical developments. The proposed experimental approach can provide support to design of future devices for nano-scale heat transfer measurement. [ABSTRACT FROM AUTHOR]

Regeneration processes are commonly used in cooling devices to improve the device performance. However, irreversible heat loss within the regenerators in many earlier designs of magnetocaloric and electrocaloric (EC) based cooling devices reduces the device performance. In this paper, an electrocaloric based refrigerator without external regenerators is proposed and studied. The regeneration process in this device is realized by direct heat exchange between contacting EC elements which are moving in opposite directions with different applied fields. Simulation results show that a 37 W/cm³ cooling power density is obtained for a Tspan of 20 K while the refrigerator still maintains 57% of Carnot efficiency for a cooling device made of an EC polymer. [ABSTRACT FROM AUTHOR]

Drop impact on superhydrophobic surfaces has gained great attention because of its physics and application in water repellency, drag reduction, and anti-icing. Spreading lengths and the contact time are the crucial parameters determining the extend of drop–surface interaction and effective heat transfer between the two and are, hence, trivial to many engineering applications. Post-collisional dynamics over cylindrical geometries are quite different from that of the flat surfaces due to the asymmetry in spreading and retraction dynamics. The dynamics are mainly governed by the impact Weber number and curvature ratio of impacting surface to drop. The spreading dynamics in axial direction is found to be fairly predicted by the governing laws coined for flat surfaces. However, the spreading dynamics in the azimuthal direction is quite complex. Herein, we propose a simple scaling analysis for the spreading dynamics in the azimuthal direction as well as for the contact time of the impacting drop with the surface. A modified capillary length is proposed accounting the curvature effect of the substrate by incorporating a centrifugal component of acceleration for the expanding lamella over the curved surface. With the proposed modified capillary length, a universal scaling relationship for azimuthal spreading length and contact time is developed. The proposed scaling laws are found to be in good agreement with the experimental results from the present study as well as with the existing literature for a wide range of Weber numbers and surface curvature. [ABSTRACT FROM AUTHOR]

Heat transfer in a planar capacitor containing a dielectric liquid subject to an increasing high-frequency voltage and a fixed temperature gradient under microgravity is investigated using direct numerical simulations. When the intensity of the applied voltage exceeds a critical value, the dielectrophoretic force induces thermoelectric convection in the form of stationary vortices. The increase in the voltage leads to different types of convective patterns and to the increase in the heat transfer coefficient between the electrodes of the capacitor. [ABSTRACT FROM AUTHOR]

We propose and test an exchange gas technique for improving the cooldown times of cryocooled gravitational-wave interferometers. The technique works by utilizing low-pressure dry nitrogen gas to create a path for heat conduction to test masses while protecting the rest of the in-vacuum equipment from unwanted heat leakage. We show that the technique is capable of shortening the total wait time to reach the operating temperature by a factor of 3.5. Additionally, our tests show that the improvement in the heat transfer rate can be predicted to be within 10% error by using the Sherman-Lees interpolation equation. The technique is compatible with vibration isolation requirements of the cryogenic shielding of 124 K silicon interferometers and has the potential to improve the iteration time for research and development. The scalability of the prototype, the ability to predict the heat conduction, and the simplicity of the engineering make the strategy a good candidate to be included in the cryogenic design of future cryocooled gravitational-wave interferometers. The findings mark a first step in the investigation for a strategy to mitigate ice formation on the interferometer optics during initial cooldown. [ABSTRACT FROM AUTHOR]

This paper presents an original modeling approach that enables the calculation of the temperature field within multilayer materials submitted to the flash method. The model takes into account the time-resolved coupled conducto-radiative heat transfer and the temperature of experiments. The compound can be subdivided into as many layers as desired, and their thicknesses and relevant physical properties can be chosen arbitrarily. Unconventional experimental thermograms can be reproduced faithfully by the calculations. This model, thus, makes it possible to correctly estimate the effective thermal diffusivity of semitransparent materials, thereby providing a deeper insight into the analysis of the physical phenomena involved. [ABSTRACT FROM AUTHOR]

Heat exchange between closely positioned bodies has become an important issue for such areas of modern technology as integrated circuits, atomic force microscopy, and high-density magnetic recording, which deal with bodies separated by gaps as narrow as a few nanometers. This paper shows that if the gap's width is below a certain value, estimated as about 10 nm for silicon at room temperature, then, in addition to electromagnetic radiation, significant heat is also carried by acoustic waves. Moreover, as the width of the gap decreases below about 5 nm, acoustic waves rapidly become the dominant heat carrier. [ABSTRACT FROM AUTHOR]

In this paper, we report that the thermal conductivity (TC) of heat transfer nanofluids containing Ni coated single wall carbon nanotube can be enhanced by applied magnetic field. A reasonable explanation for these interesting results is that Ni coated nanotubes form aligned chains under applied magnetic field, which improves thermal conductivity via increased contacts. On longer holding in magnetic field, the nanotubes gradually move and form large clumps of nanotubes, which eventually decreases the TC. When we reduce the magnetic field strength and maintain a smaller field right after TC reaches the maximum, the TC value can be kept longer compared to without magnetic field. We attribute gradual magnetic clumping to the gradual cause of the TC decrease in the magnetic field. We also found that the time to reach the maximum peak value of TC is increased as the applied magnetic field is reduced. Scanning electron microscopy images show that the Ni coated nantubes are aligned well under the influence of a magnetic field. Transmission electron microscopy images indicate that nickel remains attached onto the nanotubes after the magnetic field exposure. [ABSTRACT FROM AUTHOR]

Waste heat is a free and abundant energy source, with 15% of global total energy use existing as waste heat above 600 K. For 600–900 K temperature range, near-field thermophotovoltaics (NFTPVs) are theorized to be the most effective technology to recycle waste heat into electrical power. However, to date, experimental efficiencies have not exceeded 1.5%. In this work, we optimize the efficiency of three modeled InAs/InAsSbP-based room-temperature NFTPV devices positioned 0.1 μm from a 750 K p-doped Si radiator. We couple a one-dimensional fluctuational electrodynamics model for the near field optics to a two-dimensional drift-diffusion model, which we validated by reproducing measured dark current–voltage curves of two previously published InAs and InAsSbP devices. The optimized devices show four to six times higher above-bandgap energy transfer compared to the blackbody radiative limit, yielding enhanced power density, while simultaneously lowering parasitic sub-bandgap energy transfer by factors of 0.68–0.85. Substituting InAs front- and back-surface field layers with InAsSbP show 1.5- and 1.4-times higher efficiency and power output, respectively, from lowered parasitic diffusion currents. Of our three optimized designs, the best performing device has a double heterostructure with an n–i–p doping order from front to back. For radiator-thermophotovoltaic gaps of 0.01–10 μm and radiators within 600–900 K, this device has a maximum efficiency of 14.2% and a maximum power output of 1.55 W/cm2, both at 900 K. Within 600–900 K, the efficiency is always higher with near- vs far-field illumination; we calculate up to 3.7- and 107-times higher efficiency and power output, respectively, using near-field heat transfer. [ABSTRACT FROM AUTHOR]

We build a model to determine the dependency of near-field heat flux on bias voltage using the metal–oxide–semiconductor structures considering gradient distribution of dielectric function. Quantitative dependency of near-field heat flux exchanged by two biased metal–oxide–semiconductor structures on bias voltage is established. The distribution of carrier density and the resultant dielectric function in the semiconductor layer caused by the bias are determined. The corresponding near-field heat flux is calculated using an effective multilayer model. Significant tuning performance is demonstrated, which is due to the increase or decrease in high-frequency surface polariton states induced by the injection or extraction of major carriers. This work deepens the understanding of electrical control of near-field heat transfer with metal–oxide–semiconductor structures, promising for nanoscale thermal management devices and thermal circuits. [ABSTRACT FROM AUTHOR]

Droplet jumping from condensing surfaces induced by droplet coalescence during dropwise condensation of mixed steam on a superhydrophobic surface can significantly enhance condensation heat transfer of mixed steam with non-condensable gas. This phenomenon was visually observed and theoretically analyzed in the present paper. The dynamic evolution of droplet and the velocity distribution inside the droplet during coalescence were simulated using multiphase lattice Boltzmann method. The energy distribution released by droplet coalescence was calculated statistically, and the jumping height induced by droplet coalescence on a superhydrophobic surface was predicted based on the energy conservation method. The theoretical predictions obtained by the modified model proposed in this paper agree well with the experimental observations. [ABSTRACT FROM AUTHOR]

We report on one- to three-dimensional characterization of the pyroelectric properties of aluminum scandium nitride. By means of the laser intensity modulation method, we reconstructed the in-depth distribution of the spontaneous polarization with sub-micrometer resolution. The reconstructed profiles of the spontaneous polarization indicate that the thermal diffusivity and its temperature-dependence differ significantly from what is reported for pure aluminum nitride, which we attribute to the dominant role of phonon-alloy scattering for the heat transfer. [ABSTRACT FROM AUTHOR]

In a recent letter [Appl. Phys. Lett. 92, 011901 (2008)], Tang reported a simulation of structural dynamics in metal films induced by ultrafast laser heating using the two-temperature model [P. B. Allen, Phys. Rev. Lett. 59, 1460 (1987) and R. W. Schoenlein et al., Phys. Rev. Lett. 58, 1680 (1987)] and one-dimensional anharmonic chain model [E. Fermi J. Pasta S. Ulam, No. LA-1940 (1955)]. In this comment, we would like to point out several issues in the physical concepts and formulations in the simulation which we strongly disagree with the author. Consequently, we believe that the main conclusion of Tang's paper that the interpretation of ultrafast diffraction data requires both nonlocal collective atomic motion and the conventional linear thermal expansion lacks physical justification and is questionable. [ABSTRACT FROM AUTHOR]

A correction to the article "Heat transfer at nanoscale contacts investigated with scanning thermal microscopy" that was published in the September 8, 2015 issue is presented.

Generating power at night has recently stimulated interest in using the radiative cooling mechanism with thermoelectric generators (TEGs). These low temperature and passive devices have been shown to generate electricity at night with no active input of heat needed. Here, we optimize both the geometry and operating conditions of radiative cooling driven thermoelectric (RC-TE) generators. We determine the optimal operating conditions, including the maximum power point and maximum efficiency point, by developing a combined thermal and electrical model. Our results show that the optimal operating condition results in larger power output than was previously expected. Moreover, we show that maximum power density occurs when the area ratio between the cooler and P or N element reaches an optimal value and can be improved to nearly 2.2 times larger than what has been achieved with commercial TEGs. Finally, we perform a parametric study that takes account of environmental and structural parameters to improve the performance of the RC-TE device, including enhancing heat transfer between the hot surface and ambient air, suppressing the cooling loss of the radiative cooler, and optimizing the geometry of individual thermocouples. In summary, our work identifies how to maximize the output of RC-TE devices, providing universal guidance for this passive power generation method. [ABSTRACT FROM AUTHOR]

Nanomechanical silicon nitride (SiN) drum resonators are currently employed in various fields of applications that arise from their unprecedented frequency response to physical quantities. In the present study, we investigate the thermal transport in nanomechanical SiN drum resonators by analytical modeling, computational simulations, and experiments for a better understanding of the underlying heat transfer mechanism causing the thermal frequency response. Our analysis shows that radiative heat loss is a non-negligible heat transfer mechanism in nanomechanical SiN resonators, limiting their thermal responsivity and response time. This finding is important for optimal resonator designs for thermal sensing applications as well as cavity optomechanics. [ABSTRACT FROM AUTHOR]

During pool boiling, a significantly high heat flux leads to the transition from nucleate boiling to film boiling, where a vapor film forms over the boiling surface, drastically increasing thermal resistance. This transition at the critical heat flux (CHF) results in an abrupt increase in surface temperature and can lead to catastrophic failure of the boiler. However, reported CHF values vary greatly, even for smooth surfaces of the same material; for example, the CHF values on flat silicon and silicon dioxide surfaces vary across studies by up to 49% and 84%, respectively. Here, we address this discrepancy by accounting for hydrocarbon adsorption on boiling surface. Hydrocarbon adsorption on smooth boiling surfaces decreases surface wettability, hindering the ability to maintain liquid contact with the surface and, thus, lowering the pool boiling CHF. To investigate hydrocarbon adsorption kinetics under ambient conditions and the subsequent effect on CHF, we cleaned flat silicon dioxide samples with argon plasma to remove hydrocarbon contaminants and then exposed them to laboratory air for different periods of time before conducting pool boiling experiments. Pool boiling results along with x-ray photoelectron spectroscopy data showed that the amount of adsorbed hydrocarbon increased with exposure time in air, which resulted in a decrease in wettability and, accordingly, a decrease in CHF. This work has important implications for understanding the spread in CHF values reported in the literature and may serve as a guideline for the preparation of boiling surfaces to achieve consistent experimental results. [ABSTRACT FROM AUTHOR]

We investigate the heat transfer across a closing nanoscale gap between an operational microelectronic device and a static substrate in ambient conditions. The device contains an embedded microheater and a nanoscale metal wire that works as a thermometer. The heater causes a microscale protrusion by thermal expansion such that its surface approaches the substrate until contact occurs. Meanwhile, the metal wire located near the center of the protrusion surface measures the temperature of the protrusion, which is dependent on the size of the gap, the substrate material, and the ambient conditions. We study the nanoscale heat transfer using three different substrates and find that their thermal conductivity plays an essential role. Finally, the experiments are conducted under different relative humidity (RH) conditions. The results show that the ambient humidity can also affect the nanoscale heat transfer when RH > 75%. [ABSTRACT FROM AUTHOR]

Surfaces integrated with controllable wetting behaviors are playing an increasingly important role in a diverse range of applications. But their application in heat transfer is seldom studied. In this work, the excellent performance of the smart-wettability-control surface in boiling heat transfer was investigated experimentally. The experimental results demonstrated that the smart-wettability-control surface integrated perfectly the advantages of hydrophobic (better heat transfer coefficient) and hydrophilic surfaces (higher critical heat flux). We attribute this enhancement to the smart control of nucleation sites and three-phase contact line movement, which could be supported by the visualization image. This enhancement strategy can be used to improve the capacity of heat transfer devices. [ABSTRACT FROM AUTHOR]

Frost formation and ice accretion are major problems for a plethora of industries. Common defrosting and deicing techniques utilize energy-intensive mechanical actuation to dislodge ice/frost or heating methods to melt ice/frost from surfaces. Here, we develop an ultraefficient method to defrost/deice surfaces by spatially and temporally localizing thermal energy at the substrate-ice/frost interface. To remove ice/frost efficiently, it is beneficial only to melt the interfacial layer adhering the ice/frost to the solid surface by using a localized "pulse" of heat, allowing gravity or gas shear in conjunction with the ultrathin lubricating melt water layer to remove the ice/frost. To probe the physics of pulse defrosting, we first developed a transient numerical heat transfer model. Experimental validation of the model was achieved via pulse (≈ 100 ms) joule heating of indium tin oxide on glass samples. Utilizing transient heat fluxes ranging from 10 to 100 W/cm2, spontaneous melting of the interfacial ice/frost layer was achieved, leading to rapid ice removal. We employed our validated model to outline design guidelines for pulse defrosting applications, showing <1% of the energy and <0.01% of the defrosting time needed when compared to conventional thermal-based defrosting methods. [ABSTRACT FROM AUTHOR]

Despite multiple attempts to achieve optical refrigeration in very high (99.5%) external quantum efficiency (EQE) GaAs, no cooling has been observed to date. In this study, we investigate optical refrigeration in GaAs by numerically solving the transient drift-diffusion equation coupled to Poisson's equation. The charge carrier distributions we obtain, together with the heat diffusion equation, allow us to observe the spatial and temporal evolution of cooling/heating within GaAs. Our results indicate that maximum cooling occurs at a laser intensity different from that which maximizes EQE. An 11-fold difference in intensity exists with a corresponding 6-fold difference in cooling power. We ultimately find that samples suspended in vacuum using a 250 μm SiO2 fiber cool to 88 K, starting from room temperature. These results emphasize the critical importance of choosing an appropriate laser excitation intensity to achieve optical refrigeration along with minimizing the conductive heat load on the refrigerator. Furthermore, results of this study are applicable towards analyzing the optical response of other optoelectronic systems where accurate charge and/or heat diffusion modeling is critical. [ABSTRACT FROM AUTHOR]

Convective heat transfer plays an important role in both the fundamental research and the development of high-performance heat exchangers. Inspired by blades of grass vibrating in the wind, we developed a self-agitator for convective heat transfer enhancement. Because of fluid-structure interactions, the agitator, with self-sustained vibration, can generate strong vortices to significantly break the thermal boundary layer and improve fluid mixing for enhanced convective heat transfer. In particular, we establish a methodology to link the vorticity field at a preferred frequency to the optimal improvement in the convective heat transfer. To identify the self-agitator preferred frequency, mode analysis is performed with simulation results using dynamic mode decomposition. Experimental results are also obtained to further validate the proposed approach. These results show that the proposed self-agitator design can improve the convective heat transfer by 120% in a conventional heat exchanger without additional pumping power requirements and can achieve a Nusselt number of up to 30 within the laminar flow region, which is improved by 200% with the same Reynolds number compared to the clean channel. [ABSTRACT FROM AUTHOR]

Elastocaloric cooling serves as a promising environmental friendly candidate with substantial energy saving potential as the next generation cooling technology for air-conditioning, refrigeration, and electronic cooling applications. The temperature change (ΔT) of elastocaloric materials is a direct measure of their elastocaloric effect, which scales proportionally with the device cooling performance based on this phenomenon. Here, the underlying physics between the measured ΔT and the adiabatic temperature span ΔTad is revealed by theoretical investigation of the simplified energy equation describing the coupled simultaneous heat transfer and phase transformation processes. The revealed relation of ΔT depends on a simple and symmetric non-linear function, which requires the introduction of an important dimensionless number Φ, defined as the ratio between convective heat transfer energy and variation of internal energy of the material. The theory was supported by more than 100 data points from the open literature for four different material compositions. Based on the theory, a data sampling and reduction technique was proposed to assist future material characterization studies. Instead of approaching ΔTad by applying an ultrafast strain rate in the old way, the proposed prediction of ΔTad is based on the non-linear least squares fitting method with the measured ΔT dataset at different strain rates within the moderate range. Numerical case studies indicated that the uncertainty associated with the proposed method is within±1K if the sampled data satisfied two conditions. In addition, the heat transfer coefficient can be estimated as a by-product of the least squares fitting method proposed in this study. [ABSTRACT FROM AUTHOR]

Heat flow control plays a significant role in thermal management and energy conversion processes. Recently, two dimensional (2D) materials with unique anisotropic thermal properties are attracting a lot of attention, as promising building blocks for molding the heat flow. Originated from its crystal structure, in most if not all the 2D materials, the thermal conductivity along the Z direction (kz) is much lower than x-y plane thermal conductivity (kxy). In this work, we demonstrate that 2D nanoplates of vertically grown molybdenum disulfide (VG MoS2) can have anomalous thermal anisotropy, in which kxy (about 0.83W/mK at 300 K) is ~1 order of magnitude lower than kz (about 9.2W/mK at 300 K). Lattice dynamics analysis reveals that this anomalous thermal anisotropy can be attributed to the anisotropic phonon dispersion relations and the anisotropic phonon group velocities along different directions. The low kxy can be attributed to the weak phonon coupling near the x-y plane interfaces. It is expected that this 2D nanoplates of VG MoS2 with anomalous thermal anisotropy and low kxy can serve as a complementary building block for device designs and advanced heat flow control. [ABSTRACT FROM AUTHOR]

A near field transducer is employed in the heat assisted magnetic recording technology in order to focus the light energy into a nanoscale spot on the disk. This is necessary to heat the high coercivity magnetic media to their Curie temperature, so the write transducer can record the data. However, the heat transfer mechanism across the head disk interface (HDI) is still not well understood. The current perpendicular media recording systems have a thermal fly-height control means in the air bearing slider near the read/write transducers for placing the transducers within 1 to 2 nm of the rotating disk. In order to monitor this near contact spacing, this system also uses an embedded contact sensor (ECS). Here, we investigate how this ECS can be used to study the heat transfer across the nanoscale gap between the read/write transducer and the disk. This study shows that the self heating effect of the ECS is strong when its current bias is too high. But this self heating effect can be isolated from other heat sources, which allows us to use the ECS for the desired heat transfer measurements. The experiments show that the heat transfer across the HDI is a strong function of the head-disk spacing. [ABSTRACT FROM AUTHOR]

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We study thermal transport in porous Si nanowires (SiNWs) by means of approach-to-equilibrium molecular dynamics simulations. We show that the presence of pores greatly reduces the thermal conductivity, κ, of the SiNWs as long mean free path phonons are suppressed. We address explicitly the dependence of κ on different features of the pore topology-such as the porosity and the pore diameter-and on the nanowire (NW) geometry-diameter and length. We use the results of the molecular dynamics calculations to tune an effective model, which is capable of capturing the dependence of κ on porosity and NW diameter. The model illustrates the failure of Matthiessen's rule to describe the coupling between boundary and pore scattering, which we account for by the inclusion of an additional empirical term. [ABSTRACT FROM AUTHOR]

Laser heating has been introduced in heat-assisted magnetic recording in order to reduce the magnetic coercivity and enable data writing. However, the heat flow inside a couple of nanometers head-disk gap is still not well understood. An experimental stage was built for studying heat transfer in the head-disk interface (HDI) and the heat-induced instability of the HDI. A laser heating system is included to produce a heated spot on the disk at the position of the slider. A floating air bearing slider is implemented in the stage for sensing the temperature change of the slider due to the heat transfer from the disk by the use of an embedded contact sensor, and the gap between the two surfaces is controlled by the use of a thermal fly-height control actuator. By using this system, we explore the dependency of the heat transfer on the gap spacing as well as the disk temperature. [ABSTRACT FROM AUTHOR]

The case presented here deals with plasma flow control applied to a cross-flow configuration, more specifically to a film cooling system. The ability of a plasma dielectric barrier discharge actuator for film cooling effectiveness enhancement is investigated through an experimental set-up, including a film injection from an elongated slot into a thermally uniform cross-flow. Two-dimensional particle image velocimetry and infrared-thermography measurements are performed for three different blowing ratios of M=0.4, 0.5, and 1. Results show that the effectiveness can be increased when the discharge is switched on, as predicted by the numerical results available in literature. Whatever the blowing ratio, the actuator induces a deflection of the jet flow towards the wall, increases its momentum, and delays its diffusion in the cross-flow. [ABSTRACT FROM AUTHOR]

We demonstrate capillary fed porous copper structures capable of dissipating over 1200 W cm-2 in boiling with water as the working fluid. Demonstrated superheats for this structure are dramatically lower than those previously reported at these high heat fluxes and are extremely insensitive to heat input. We show superheats of less than 10 K at maximum dissipation and varying less than 5 K over input heat flux ranges of 1000 W cm-2. Fabrication of the porous copper layers using electrodeposition around a sacrificial template allows fine control of both microstructure and bulk geometry, producing structures less than 40 µm thick with active region lateral dimensions of 2 mm × 0.3 mm. The active region is volumetrically Joule heated by passing an electric current through the porous copper bulk material. We analyze the heat transfer performance of the structures and suggest a strong influence of pore size on superheat. We compare performance of the current structure to existing wick structures. [ABSTRACT FROM AUTHOR]

For nearly two decades, the microchannel flow boiling heat transfer process has been the subject of numerous studies. A plethora of experimental studies have been conducted to decipher the underlying physics of the process, and different hypotheses have been presented to describe its microscopic details. Despite these efforts, the underlying assumptions of the existing hypothesis have remained largely unexamined. Here, using data at the microscopic level provided by a unique measurement approach, we deconstruct the boiling heat transfer process into a set of basic mechanisms and explain their role in the overall surface heat transfer. We then show how this knowledge allows to relate the bubble growth and flow dynamics to the surface heat flux. [ABSTRACT FROM AUTHOR]

The heat dissipation due to the resonant precessional motion of the magnetization in a ferromagnetic metal has been investigated. We demonstrated that the temperature during the ferromagnetic resonance can be simply detected by the electrical resistance measurement of the Cu strip line in contact with the ferromagnetic metal. The temperature change of the Cu strip due to the ferromagnetic resonance was found to exceed 10 K, which significantly affects the spin-current transport. The influence of the thermal conductivity of the substrate on the heating was also investigated. [ABSTRACT FROM AUTHOR]

We consider a near-field thermophotovoltaic device with metal as the emitter and semiconductor as the photovoltaic cell. We show that when the cell is a III-V semiconductor, such as GaSb, parasitic phonon-polariton heat transfer reduces efficiency in the near-field regime, especially when the temperature of the emitter is not high enough. We further propose ways to avoid the phonon-polariton heat transfer by replacing the III-V semiconductor with a non-polar semiconductor such as Ge. Our work provides practical guidance on the design of near-field thermophotovoltaic systems for efficient harvesting of low-quality waste heat. [ABSTRACT FROM AUTHOR]

Realization of the potential of electrocaloric heat pumps includes consideration of not only material properties but also device characteristics and cycle operation. We present detailed models and analysis that elucidate the key parameters for performance optimization. We show that the temperature lift, cooling power, and efficiency of a system driven by heat switches depend on system operating conditions and the combined thermal properties of both the heat switches and the electrocaloric capacitor. We show experimental results that validate the models and draw conclusions about building high-performance systems. [ABSTRACT FROM AUTHOR]

A recently developed enhanced Fourier law is applied to the problem of extracting thermal properties of materials from frequency-domain thermoreflectance (FDTR) experiments. The heat transfer model comprises contributions from two phonon channels: one a high-heat-capacity diffuse channel consisting of phonons of mean free path (MFP) less than a threshold value, and the other a low-heat-capacity channel consisting of phonons with MFP higher than this value that travel quasiballistically over length scales of interest. The diffuse channel is treated using the Fourier law, while the quasi-ballistic channel is analyzed using a second-order spherical harmonic expansion of the phonon distribution function. A recent analysis of FDTR experimental data suggested the use of FDTR in deriving large portions of the MFP accumulation function; however, it is shown here that the data can adequately be explained using our minimum-parameter model, thus highlighting an important limitation of FDTR experiments in exploring the accumulation function of bulk matter. [ABSTRACT FROM AUTHOR]

We investigated nucleate boiling heat transfer with precisely controlled wetting patterns and micro-posts, to gain insights into the impact of surface heterogeneity. To create heterogeneous wetting patterns, self-assembled monolayers (SAMs) were spatially patterned. Even at a contact angle <90, bubble nucleation and bubble frequency were accelerated on SAM patterns, since this contact angle is larger than that found on plain surfaces. Micro-posts were also fabricated on the surface, which interrupted the expansion of generated bubbles. This surface structuring induced smaller bubbles and higher bubble frequency than the plain surface. The resistance provided by surface structures to bubble expansion broke the interface between the vapor mushroom and the heating surface, and water could therefore be continuously supplied through these spaces at high heat flux. To induce synergistic effects with wetting patterns and surface structures on boiling, we fabricated SAM patterns onto the heads of micro-posts. On this combined surface, bubble nucleation was induced from the head of the micro-posts, and bubble growth was influenced by both the SAM pattern and the micro-post structures. In particular, separation of the vapor path on the SAM patterns and the liquid path between micro-post structures resulted in high heat transfer performance without critical heat flux deterioration. [ABSTRACT FROM AUTHOR]

Pyroelectric crystals are used to produce high energy electron beams. We have derived a method to model electric potential generation on LiTaO3 crystal during heating cycle. In this method, effect of heat transfer on the potential generation is investigated by some experiments. In addition, electron emission from the crystal surface is modeled by measurements and analysis. These spectral data are used to present a dynamic equation of electric potential with respect to thickness of the crystal and variation of its temperature. The dynamic equation's results for different thicknesses are compared with measured data. As a result, to attain more energetic electrons, best thickness of the crystals could be extracted from the equation. This allows for better understanding of pyroelectric crystals and help to study about current and energy of accelerated electrons. [ABSTRACT FROM AUTHOR]

Near-to-far-field radiative heat transfer between two macroscopic SiO2 plates--with and without microcavities--was observed using a highly precise and accurate optical gap-measurement method. The experiments, conducted near 300 K, measured heat transfer as a function of gap separation from 1.0 μm to 50 μm and also as a function of temperature differences between 4.1 and 19.5 K. The gap-dependent heat flux was in excellent agreement with theoretical predictions. Furthermore, the effects of microcavities on the plate surfaces were clearly observed and significant enhancement of near-field radiative heat transfer was confirmed between gold-coated microcavities with narrow vacuum separation. [ABSTRACT FROM AUTHOR]

We explore ways to actively control near-field radiative heat transfer between two surfaces that relies on electrical tuning of phonon modes of ferroelectric materials. Ferroelectrics are widely used for tunable electrical devices, such as capacitors and memory devices; however, their tunable properties have not yet been examined for heat transfer applications. We show via simulations that radiative heat transfer between two ferroelectric materials can be enhanced by over two orders of magnitude over the blackbody limit in the near field, and can be tuned as much as 16.5% by modulating the coupling between surface phonon polariton modes at the two surfaces via varying external electric fields. We then discuss how to maximize the modulation contrast for tunable thermal devices using the studied mechanism. [ABSTRACT FROM AUTHOR]

The low thermal conductivity of nano-crystalline materials is commonly explained via diffusive scattering of phonons by internal boundaries. In this study, we have quantitatively studied phononcrystalline boundaries scattering and its effect on the overall lattice thermal conductivity of crystalline bodies. Various types of crystalline boundaries such as stacking faults, twins, and grain boundaries have been considered in FCC crystalline structures. Accordingly, the specularity coefficient has been determined for different boundaries as the probability of the specular scattering across boundaries. Our results show that in the presence of internal boundaries, the lattice thermal conductivity can be characterized by two parameters: (1) boundary spacing and (2) boundary excess free volume. We show that the inverse of the lattice thermal conductivity depends linearly on a non-dimensional quantity which is the ratio of boundary excess free volume over boundary spacing. This shows that phonon scattering across crystalline boundaries is mainly a geometrically favorable process rather than an energetic one. Using the kinetic theory of phonon transport, we present a simple analytical model which can be used to evaluate the lattice thermal conductivity of nano-crystalline materials where the ratio can be considered as an average density of excess free volume. While this study is focused on FCC crystalline materials, where inter-atomic potentials and corresponding defect structures have been well studied in the past, the results would be quantitatively applicable for semiconductors in which heat transport is mainly due to phonon transport. [ABSTRACT FROM AUTHOR]

We investigate the thermal transport originating from the propagation of surface plasmon polaritons (SPPs) in a thin gold stripe. The SPPs are excited by a grating coupler on the Au stripe which was patterned onto a silicon membrane. The transmissivity changes of the Si membrane due to temperature-induced changes of the interference conditions enable measuring the temperature distribution with temporal and spatial resolution better than 1 is and 1 im. With this setup, we demonstrate that SPP excitation, propagation, and decay are accompanied by considerable heating and heat transport. [ABSTRACT FROM AUTHOR]

The impact of the moderate pressure (about 100GPa) on the melting of crystalline (c-) phase change material (PCM) germanium telluride (GeTe) is analyzed, by combining the heat transfer equation in the PCM device scale (10¹-10²nm and beyond), and the ab initio molecular dynamics and the nudged elastic band simulations in the atomistic scale (10-1-100 nm). The multi-scale analysis unravels that a pressure P=1.0 GPa can increase the melting temperature of c-GeTe and the PCM device "reset" operation energy consumption by 6%-7%. It is shown that the melting temperature increase originates from the pressure-induced raise of the energy barrier of the umbrella-flip transition of the Ge atom from the octahedral symmetry site to the tetrahedral symmetry site. It is revealed that when P>1.0 GPa, which is normal in PCM devices, the "reset" energy will be increased even by more. Based on the analysis, suggestions to alleviate pressure-induced raise of melting temperature and "reset" energy are provided. [ABSTRACT FROM AUTHOR]

We report an experimental study on the dynamic thermomagnetic (TM) reversal mechanisms at around Curie temperature (Tc) for isolated 60 nm pitch single-domain [Co/Pd] islands heated by a 1.5 μ m spot size laser pulse under an applied magnetic reversal field (Hr). Magnetic force microscopy (MFM) observations with high resolution MFM tips clearly showed randomly trapped nonswitched islands within the laser irradiated spot after dynamic TM reversal process with insufficient Hr strength. This observation provides direct experimental evidence by MFM of a large magnetization switching variation due to increased thermal fluctuation/agitation over magnetization energy at the elevated temperature of around Tc. The average percentage of non-switched islands/magnetization was further found to be inversely proportional to the applied reversal field Hr for incomplete magnetization reversal when Hr is less than 13% of the island coercivity (Hc), showing an increased switching field distribution (SFD) at elevated temperature of around Tc (where main contributions to SFD broadening are from Tc distribution and stronger thermal fluctuations). Our experimental study and results provide better understanding and insight on practical heat assisted magnetic recording (HAMR) process and recording performance, including HAMR writing magnetization dynamics induced SFD as well as associated DC saturation noise that limits areal density, as were previously observed and investigated by theoretical simulations. [ABSTRACT FROM AUTHOR]

Thermal transport through profiled and abrupt contacts between a nanowire and a reservoir has been investigated by thermal conductance measurements. It is demonstrated that above 1K the transmission coefficients are identical between abrupt and profiled junctions. This shows that the thermal transport is principally governed by the nanowire itself rather than by the resistance of the thermal contact. These results are perfectly compatible with the previous theoretical models. The thermal conductance measured at sub-Kelvin temperatures is discussed in relation to the universal value of the quantum of thermal conductance. [ABSTRACT FROM AUTHOR]