Lateral Heat Spreading Research Articles

Effects of Local Anesthetics on Tissue Penetration Using Diode Lasers in Intraoral Tissues.

Objective: The purpose of this study was to demonstrate heat transfer within oral soft tissues using different lasers under the effect of local anesthetics (LA). Methods: Bovine tongue slices were placed in between two glass slides and at a distance from a thermographic camera. In total, 2-cm-long 240 incisions were made along the surface of the tissue parallel to glass slides and the camera capture field. Incisions were performed using 445-nm and infrared (IR) lasers (970 nm and 980 nm on a continuous wave at 2 W) with 320 µm-initiated (concentrated energy at the tip provided by a blue articulated paper and laser irradiation) and noninitiated (defused energy) fiber (30-sec irradiation period). LA was injected into the specimens before irradiation. The temperature changes in °C (ΔT) and vertical and lateral heat transfer (in mm) were recorded at 10-sec intervals for 30 sec, using thermographic images. The amount of lateral and vertical heat transfer was measured. A repeated analysis of variance statistical comparison test was used to analyze differences between the lateral (width) and the vertical (height) heat transfer for initiated and noninitiated lasers and different lasers. Results: The maximum ΔT in °C utilizing initiated tips of 970, 980, or 445 nm were 11.82 ± 3.46, 7.66 ± 3.24, and 18.94 ± 7.01 and using noninitiated tips were 8.27 ± 1.69, 8.87 ± 2.40, and 12.31 ± 8.65, respectively. Heat transfers (height/width) for initiated were 40.65 ± 10.40/90.65 ± 10.77 mm, 41.50 ± 11.83/83.95 ± 11.20 mm, and 33.70 ± 9.10/95.10 ± 11.17 mm and for noninitiated lasers were 52.95 ± 6.89/96.10 ± 11.17 mm, 47.75 ± 7.41/93.75 ± 14.96 mm, and 31.35 ± 11.40/75.20 ± 19.68 mm, respectively. A statistically significant difference was found between all lasers (p < 0.05) for initiated and noninitiated lasers (except for 970/980 nm for noninitiated lasers). Lower penetration depth (p < 0.05) at 445-nm diode and greater lateral heat spreading (p < 0.05) were identified under LA especially utilizing noninitiated tips without significant difference in IR lasers. Conclusions: LA might negatively influence soft tissues creating scattering when noninitiated tips are used and IR diode laser technology.

Reduced thermal resistance of amorphous Al2O3 thin films on β-Ga2O3 and amorphous SiO2 substrates via rapid thermal annealing

The impact of rapid thermal annealing (1000 °C for 1 min) on the thermal transport properties of amorphous alumina (a-Al2O3) thin films grown by atomic layer deposition on β−Ga2O3 and amorphous silica (a-SiO2) substrates is determined using frequency-domain thermoreflectance measurements. The annealing more than doubles the a-Al2O3 thermal conductivity for both substrates (1.54 ± 0.13 to 3.14 ± 0.27 W m−1 K−1 for β−Ga2O3 and 1.60 ± 0.14 to 3.87 ± 0.33 W m−1 K−1 for a-SiO2) while keeping the film amorphous. The thermal conductivity increase is attributed to partial recrystallization and off-gassing of embedded impurities. Annealing halves the thermal boundary resistance of the a-Al2O3/a-SiO2 interface (10.5 ± 1.0 to 4.47 ± 0.42 m2 K GW−1), which is attributed to compositional mixing and structural reorganization that are enabled by the elastic matching of these two materials. The thermal boundary resistance of the a-Al2O3/β−Ga2O3 interface is not affected by annealing due to the elastic mismatch. Reducing the thermal resistance of a-Al2O3 dielectric films and adjacent interfaces by annealing will promote lateral heat spreading adjacent to hot spots and improve device longevity.

Photothermal superhydrophobic copper nanowire assemblies: fabrication and deicing/defrosting applications

Ice and frost buildup continuously pose significant challenges to multiple fields. As a promising de-icing/defrosting alternative, designing photothermal coatings that leverage on the abundant sunlight source on the earth to facilitate ice/frost melting has attracted tremendous attention recently. However, previous designs suffered from either localized surface heating owing to the limited thermal conductivity or unsatisfied meltwater removal rate due to strong water/substrate interaction. Herein, we developed a facile approach to fabricate surfaces that combine photothermal, heat-conducting, and superhydrophobic properties into one to achieve efficient de-icing and defrosting. Featuring copper nanowire assemblies, such surfaces were fabricated via the simple template-assisted electrodeposition method, allowing us to tune the nanowire assembly geometry by adjusting the template dimensions and electrodeposition time. The highly ordered copper nanowire assemblies facilitated efficient sunlight absorption and lateral heat spreading, resulting in a fast overall temperature rise to enable the thawing of ice and frost. Further promoted by the excellent water repellency of the surface, the thawed ice and frost could be spontaneously and promptly removed. In this way, the all-in-one design enabled highly enhanced de-icing and defrosting performance compared to other nanostructured surfaces merely with superhydrophobicity, photothermal effect, or the combination of both. In particular, the defrosting efficiency could approach ∼100%, which was the highest compared to previous studies. Overall, our approach demonstrates a promising path toward designing highly effective artificial deicing/defrosting surfaces.

The Effect of Cryogenic Temperatures on the Lateral Heat Spreading in InGaAs/InP HEMTs

The Effect of Cryogenic Temperatures on the Lateral Heat Spreading in InGaAs/InP HEMTs

Strain relaxation and self-heating effects of fin AlGaN/GaN HEMTs

In this paper, we present a methodology of 3D electro-thermo-mechanical simulation to analyze the strain relaxation and self-heating effects of fin AlGaN/GaN high electron mobility transistors (HEMTs). The free boundaries of narrow fins cause strain relaxation of the AlGaN barrier and a non-uniform strain distribution near the AlGaN/GaN interface. The strain relaxation not only reduces the surface piezoelectric polarization charges (PPCs), but also introduces space PPCs in AlGaN/GaN, leading to a reduction of two-dimensional electron gas density and a positive shift of threshold voltage (V th). The simulated V th shift with fin width agrees well with experimental results from literature. In addition, the inter-fin trenches facilitate more efficient lateral heat spreading and suppress the self-heating effect compared with the planar HEMTs with the same effective gate width.

Thermal Performance Analysis of Mempool RISC-V Multicore SoC

The presence of multiple cores in modern multicore architectures makes thermal management and temperature estimation a really challenging task for enhancing reliability and lifespan. Due to the presence of many cores, the core/tile spacing needs to be optimized in order to enhance the thermal coupling between interconnect routing blocks and active tiles. In addition, the tiles activity patterns under partial workload conditions significantly affect the maximum on-chip temperature which results in nonuniform temperature distribution. This is due to poor thermal coupling between neighboring tiles owing to the decrease in spacing between cores. In this article, we investigate the thermal performance analysis of a 256-core (i.e., 64 tiles) Mempool reduced instruction set computer (RISC) V-based architecture considering the impact of inter tiles spacing. Simulation results reveal that lateral heat spreading predominantly affects the thermal performance in multicore architectures under partial workload conditions. We also optimize the thermal performance with different tiles activity pattern. Simulation results reveal that both the maximum on-chip temperature and lateral heat spreading are improved for specific tiles activity patterns. Also the thermal performance analysis considering the “tile-insite effect” reveals that there is little impact on on-chip maximum temperature ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\text {max}}$ </tex-math></inline-formula> ), but the on-chip thermal gradient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta T$ </tex-math></inline-formula> ) and the thermal profile pattern are predominantly affected. Finally, the effect of the secondary heat path toward printed circuit board (PCB) is studied in this work.

Device-Level Impact of Highly Anisotropic Thermal Conductivity of AlN/GaN Digital Alloys

AlN/GaN digital alloys (i.e., short period superlattices) have the advantage of being an ultra-wide bandgap due to the quantum confinement effect, preservation of high mobility (no strong alloy scattering), and higher thermal conductivity than AlGaN random alloys.Herein, we investigate the influence of AlN/GaN thickness combinations on digital alloy (DA) thermal conductivity. Using time-domain thermoreflectance and steady-state thermoreflectance, we measure the anisotropic thermal conductivity of AlN/GaN DA’s, with thickness combinations: (2 nm/1 nm), (4 nm/1 nm), (6 nm/1 nm), (11 nm/1 nm). DA's with thick AlN layers exhibit highly anisotropic thermal conductivity (i.e., 4x difference between cross-plane and in-plane).Using finite-element modeling, we thermally simulate AlGaN and AlN/GaN DA devices. Improved lateral heat spreading in DA devices results in lower hot-spot temperatures compared to AlGaN devices. As the first thermal measurements of AlN/GaN DA’s, this study provides guidance for future device design.

DC and RF performance of lateral AlGaN/GaN FinFET with ultrathin gate dielectric

In this study, an enhancement-mode (E-mode) GaN high electron mobility transistor (HEMT) with lateral tri-gate structure field effect transistor (FinFET) is proposed. To passivate the fin width, while keeping the normally-off performance of the FinFET intact, an ultrathin aluminium-oxide/sapphire (Al2O3) gate dielectric is proposed (in a basic single-finger 0.125 mm device). Later, the DC and radio frequency (RF) performances of the proposed FinFET designs (with optimized fin width and Al2O3 thickness) are compared with that of conventional planar HEMT. DC and RF measurements are performed using power transistors in ten-fingers configuration, with a total gate periphery of 2.5 mm. The effect of Fin structure and Al2O3 thickness on the electrical performance of HEMTs, including threshold voltage (V th) shift, transconductance (g m) linearity, small-signal gain, cut off frequency (f t), output power (P out), and power-added efficiency (PAE) are investigated. Based on our findings, FinFET configuration imposes normally-off functionality with a V, while the planar architecture has a V. Originating from passivation property of the alumina layer, the FinFET design exhibits two orders of magnitude smaller drain and gate leakage currents compared to the planar case. Moreover, large signal RF measurements reveals an improved P out density by over 50% compared to planar device, attributed to reduced thermal resistance in FinFETs stemming from additional lateral heat spreading of sidewall gates. Owing to its superior DC and RF performance, the proposed FinFET design with ultrathin gate dielectric could bear the potential of reliable operating for microwave power applications, by further scaling of the gate length.

Experimental Demonstration of Pressure-Driven Flash Boiling for Transient Two-Phase Cooling

This work demonstrates transient cooling for high-heat-flux electronics using pressure-controlled flash boiling. Many advanced electronics packages demand rapid, high-heat flux removal under stringent spatial and temporal constraints such as limited lateral heat spreading area, sharp power delivery transients, and heterogeneous power dissipation among dies. Flash cooling achieved by rapid depressurization of liquid coolant is inherently transient and provides high initial peak cooling that can be used for adaptive cooling demanded by these advanced packages. A coolant loop thermal testbed has been constructed to perform rapid transient thermo-fluid experiments of flash cooling with methanol. This work provides insights into depressurization-based flash incipience, thermodynamic analysis of flash cooling, and the ability to provide transient cooling in two application settings. First, a vapor chamber able to adapt to high initial transients applicable for short-burst applications. Second, a co-designed convective chamber that supports simultaneous power delivery and heat extraction for high-performance wafer-scale systems. A prototype vapor chamber was tested to obtain results for overall cooling and flash incipience experiments. The results demonstrate the efficacy of flash cooling for transient thermal management and highlight directions for further research and necessary data to exploit this technique for future electronics applications.

Experimental Investigation of the Effect of Heat Spreading on Boiling of a Dielectric Liquid for Immersion Cooling of Electronics

Abstract This paper investigated the effect of heat spreading on the boiling of the Novec 649™ for two-phase immersion cooling of electronics. Reference pool boiling tests were performed by attaching a 25.4 mm by 25.4 mm square copper plate to a same-sized heater, thus minimizing lateral heat spreading. Experimental measurements showed that the critical heat flux (CHF) happened at a heat flux of 17.4±0.8 W/cm2. Then, lateral heat spreading through the heat spreader was studied by attaching larger (47 mm by 47 mm) spreaders with four different thicknesses to the copper plate. With an increase in the integrated heat spreader (IHS) thickness from 1 mm to 6 mm, the CHF increased by more than 60% at the saturation condition. One plate was a 1 mm-thick IHS removed from a commercial microprocessor. In this case, the CHF happens at 8.6 W/cm2 (50% lower compared to the reference case) in the saturation condition. At CHF, the boiling can be observed on the whole surface, with columns and slugs regime at the center and the fully developed nucleate boiling regime at the edges. This nonuniform boiling was more pronounced in subcooled conditions, in which the CHF occurred at the center while there were regions at the edges that had no boiling. Finally, the performance of a microporous-coated IHS (with 3.15 mm thickness) was compared to the 6 mm thick IHS. The thermal resistance was almost equal for powers above 200 W. This indicates that lateral heat spreading is a critical parameter for the thermal design of immersion cooling along with microporous coating.

In-situ thermal transport measurement of flowing fluid using modulated photothermal radiometry

In situ thermal transport measurement of flowing fluid could be useful for the characterization and diagnosis of practical thermal systems such as fluid heat exchangers and thermal energy storage systems. Despite abundant reports on the ex-situ thermal conductivity measurement of stagnant fluids, a suitable technique for the thermal conductivity measurement of flowing fluid has been rarely reported. This paper presents the thermal conductivity measurement of flowing fluid within a pipe using a non-contact modulated photothermal radiometry (MPR) technique, where the surface of the pipe is heated by an intensity-modulated laser and the heat diffuses into the fluid with suitable modulation frequency. We design a tube section with small wall thickness suitable for the MPR measurements to maximize the sensitivity of the thermal response to the fluid properties while minimizing the lateral heat spreading effect. Intrinsic thermal conductivity of different fluids was obtained within a proper range of frequency and flow velocity where the forced convection effect is negligible. The forced convection effect became prominent at high flowing velocity and at low modulation frequency, leading to higher effective thermal conductivity of the fluid. It is found that the intrinsic thermal conductivity could be obtained when the flow velocity is less than 100 mm/sec and ReD1/2Pr1/3 < 100 for DI water and Xceltherm oil under the specified experimental conditions, where ReD is the Reynolds number and Pr is the Prandtl number.

Optical engineering of polymer materials and composites for simultaneous color and thermal management

Sustainable architecture requires development of new materials with tailored optical, mechanical, and thermal properties to provide both aesthetic appeal and energy-saving functionalities. Polymers and polymer-based composites emerge as promising lightweight and conformable materials whose optical spectra can be engineered to achieve both goals. Here, we report on the development of new types of organic-inorganic films composed of ultrahigh molecular weight polyethylene with a variety of organic and inorganic nano- and micro-scale inclusions. The films simultaneously provide ultra-light weight, conformability, either visual coloring or transparency on demand, and passive thermal management via both conduction and radiation. The lightweight semi-crystalline polymer matrix yields thermal conductivity exceeding that of many metals, allowing for the lateral heat spreading and hot spots mitigation in the cases of partial illumination of films by sunlight. It also yields excellent broadband transparency, allowing for the opportunities to shape the spectral response of composite materials via targeted addition of inclusions with tailored optical spectra. We demonstrate a variety of dark- and bright-colored composite samples that exhibit reduced temperatures under direct illumination by sunlight, and outline strategies for materials design to further improve material performance.

High anisotropy metamaterial heat spreader

The ability to manipulate heat flow has numerous promising benefits in thermal energy applications such as in heat spreader design. This paper introduces a novel thermal functional metamaterials heat spreader with high anisotropy. The metamaterial heat spreader consists of alternating layers of copper/VHTC and PDMS thin films. The effects of the number of bilayers, bilayers’ thermal conductivity ratio, and thermal spreader thickness on the metamaterial lateral heat spreading have been investigated numerically. In comparison to a simple copper heat spreader, results show significant enhancements of the lateral heat spreading with both the copper and VHTC based metamaterial heat spreaders.

(Invited) Diverse Accessible Heterogeneous Integration (DAHI) Foundry at Northrop Grumman Aerospace Systems (NGAS)

The requirements for future systems are increasingly demanding not only form the point of view of performance, but also from size, weight, power and cost (SWAP-C) considerations. This translates into the need for advanced integration of additional complexity and performance of electronic functions into smaller areas and volumes. In addition, to enable higher performance of the microelectronic technology in which these functions are implemented, it is not enough to use one single semiconductor technology but to combine or integrate several high performance technologies in an efficient and cost effective way. In most cases this involves the integration of semiconductor technologies based on different substrates or different technology nodes based on the same substrate. The combination of two or more dissimilar microelectronic technologies, or heterogeneous integration, leads to significant higher design flexibility and capability. The more adaptable and more intimate is this heterogeneous integration between two or more technologies, the more flexibility is given to the designer for the selection of the technology for a specific function, or even better, for an optimum combination of different transistor technologies in the same function or cell in the design. Northrop Grumman Aerospace Systems (NGAS) under the Diverse Accessible Heterojunction Integration (DAHI) DARPA program is developing heterogeneous integration processes, design kits and thermal simulation tools to integrate submicron CMOS, InP HBT, GaN HEMT and high-Q passive technologies for advanced DoD and other government systems. We have demonstrated integration of NGAS’ TF4 and TF5 InP HBT and GaN20 GaN on SiC HEMT technologies on IBM’s 65nm CMOS and GlobalFoundries’ (GFUS) 45nm CMOS (12SOI). The NGAS DAHI integration process has several advantages. It is scalable to 200 mm and 300 mm CMOS wafers and fully compatible with all Si technologies. All the technologies to be integrated are independently fabricated in parallel for optimum cycle time and decoupled line yield. The NGAS DAHI integration approach also offers advantages from the thermal management point of view. The low thermal resistance heterogeneous interconnects (HICs) enable flexible electrical and thermal routing. The GaN HEMT is integrated with its backside facing the CMOS front side to allow better intra-chip heat spreading and large inter-chip HIC heat sink arrays. The thickness of the Silicon Carbide (SiC) substrate in the GaN HEMT chiplet is designed to be thick enough to allow optimum lateral heat spreading before reaching the thermal HIC interface. The TF InP HBT technology is integrated with its front side facing the CMOS front side for maximum density of interconnects. An additional important task in the development of the DAHI technology is the creation of an integrated DAHI process design kit (PDK). The objective of the DAHI PDK development is to have a flexible design environment that can accommodate schematics, layouts and simulations of integrated DAHI circuits with device-level integration designs of a wide variety of technologies. Integration circuit results will be presented at the conference. Acknowledgements This work is supported by DARPA DAHI Program under AFRL contract No. FA8650-13-C-7312, Daniel Green DARPA program manager and Robert Fitch AFRL COTR. The views expressed are those of the authors and do not reflect the official policy or position of the Department of Defense or the U.S. Government.

(Invited) Diverse Accessible Heterogeneous Integration (DAHI) Foundry at Northrop Grumman Aerospace Systems (NGAS)

The requirements for future systems are increasingly demanding not only form the point of view of performance, but also from size, weight, power and cost (SWAP-C) considerations. This translates into the need for advanced integration of additional complexity and performance of electronic functions into smaller areas and volumes. In addition, to enable higher performance of the microelectronic technology in which these functions are implemented, it is not enough to use one single semiconductor technology but to combine or integrate several high performance technologies in an efficient and cost effective way. In most cases this involves the integration of semiconductor technologies based on different substrates. The combination of two or more dissimilar microelectronic technologies, or heterogeneous integration, leads to significant higher design flexibility and capability.The more adaptable and more intimate is this heterogeneous integration between two or more technologies, the more flexibility is given to the designer for the selection of the technology for a specific function, or even better, for an optimum combination of different transistor technologies in the same function or cell in the design. Northrop Grumman Aerospace Systems (NGAS) under the Diverse Accessible Heterojunction Integration (DAHI) DARPA program is developing heterogeneous integration processes, design kits and thermal simulation tools to integrate submicron CMOS, InP HBT, GaN HEMT and high-Q passive technologies for advanced DoD and other government systems. We have demonstrated integration of NGAS’ TF4 and TF5 InP HBT and GaN20 GaN on SiC HEMT technologies on IBM’s 65nm CMOS and GlobalFoundries’ 45nm SOI. The NGAS DAHI integration process has several advantages. It is scalable to 200 mm and 300 mm CMOS wafers and fully compatible with all Si technologies. All the technologies to be integrated are independently fabricated in parallel for optimum cycle time and decoupled line yield. The NGAS DAHI integration approach also offers advantages from the thermal management point of view. The low thermal resistance heterogeneous interconnects (HICs) enable flexible electrical and thermal routing. The GaN HEMT is integrated with its backside facing the CMOS front side to allow better intra-chip heat spreading and large inter-chip HIC heat sink arrays. The thickness of the Silicon Carbide (SiC) substrate in the GaN HEMT chiplet is designed to be thick enough to allow optimum lateral heat spreading before reaching the thermal HIC interface. The TF InP HBT technology is integrated with its front side facing the CMOS front side for maximum density of interconnects. An additional very important task in the development of the DAHI technology is the creation of an integrated DAHI process design kit (PDK). The objective of the DAHI PDK development is to have a flexible design environment that can accommodate schematics, layouts and simulations of integrated DAHI circuits with device-level integration designs of a wide variety of technologies. Integration circuit results will be presented at the conference.

Quantitative scanning thermal microscopy of graphene devices on flexible polyimide substrates

A triple-scan scanning thermal microscopy (SThM) method and a zero-heat flux laser-heated SThM technique are investigated for quantitative thermal imaging of flexible graphene devices. A similar local tip-sample thermal resistance is observed on both the graphene and metal areas of the sample, and is attributed to the presence of a polymer residue layer on the sample surface and a liquid meniscus at the tip-sample junction. In addition, it is found that the tip-sample thermal resistance is insensitive to the temperature until it begins to increase as the temperature increases to 80 °C and exhibits an abrupt increase at 110 °C because of evaporation of the liquid meniscus at the tip-sample junction. Moreover, the variation in the tip-sample thermal resistance due to surface roughness is within the experimental tolerance except at areas with roughness height exceeding tens of nanometers. Because of the low thermal conductivity of the flexible polyimide substrate, the SThM measurements have found that the temperature rise in flexible graphene devices is more than one order of magnitude higher than those reported for graphene devices fabricated on a silicon substrate with comparable dimensions and power density. Unlike a graphene device on a silicon substrate where the majority of the electrical heating in the graphene device is conducted vertically through the thin silicon dioxide dielectric layer to the high-thermal conductivity silicon substrate, lateral heat spreading is important in the flexible graphene devices, as shown by the observed decrease in the average temperature rise normalized by the power density with decreasing graphene channel length from about 30 μm to 10 μm. However, it is shown by numerical heat transfer analysis that this trend is mainly caused by the size scaling of the thermal spreading resistance of the polymer substrate instead of lateral heat spreading by the graphene. In addition, thermoelectric effects are found to be negligible compared to Joule heating in the flexible graphene devices measured in this work.

Patterning the condenser-side wick in ultra-thin vapor chamber heat spreaders to improve skin temperature uniformity of mobile devices

Vapor chamber technologies offer an attractive approach for passive heat spreading in mobile electronic devices, in which meeting the demand for increased functionality and performance is hampered by a reliance on conventional conductive heat spreaders. However, market trends in device thickness mandate that vapor chambers be designed to operate effectively at ultra-thin (sub-millimeter) thicknesses. At these form factors, the lateral thermal resistance of vapor chambers is governed by the saturation temperature/pressure gradient in the confined vapor core. In addition, thermal management requirements of mobile electronic devices are increasingly governed by user comfort; heat spreading technologies must be designed specifically to mitigate hot spots on the device skin. The current work considers these unique transport limitations and thermal requirements encountered in mobile applications, and develops a methodology for the design of vapor chambers to yield improved condenser-side temperature uniformity at ultra-thin form factors. Unlike previous approaches that have focused on designing evaporator-side wicks for reduced thermal resistance and delayed dryout at higher operating powers, the current work focuses on manipulating the condenser-side wick to improve lateral heat spreading. The proposed condenser-side wick designs are evaluated using a 3D numerical vapor chamber transport model that accurately captures conjugate heat transport, phase change at the liquid–vapor interface, and pressurization of the vapor core due to evaporation. A biporous condenser-side wick design is proposed that facilitates a thicker vapor core, and thereby reduces the condenser surface peak-to-mean temperature difference by 37% relative to a monolithic wick structure.

Two-dimensional hexagonal boron nitride as lateral heat spreader in electrically insulating packaging

The need for electrically insulating materials with a high in-plane thermal conductivity for lateral heat spreading applications in electronic devices has intensified studies of layered hexagonal boron nitride (h-BN) films. Due to its physicochemical properties, h-BN can be utilised in power dissipating devices such as an electrically insulating heat spreader material for laterally redistributing the heat from hotspots caused by locally excessive heat flux densities. In this study, two types of boron nitride based heat spreader test structures have been assembled and evaluated for heat dissipation. The test structures separately utilised a few-layer h-BN film with and without graphene enhancement drop coated onto the hotspot test structure. The influence of the h-BN heat spreader films on the temperature distribution across the surface of the hotspot test structure was studied at a range of heat flux densities through the hotspot. It was found that the graphene-enhanced h-BN film reduced the hotspot temperature by about 8–10 °C at a 1000 W cm−2 heat flux density, a temperature decrease significantly larger than for h-BN film without graphene enhancement. Finite element simulations of the h-BN film predict that further improvements in heat spreading ability are possible if the thermal contact resistance between the film and test chip are minimised.

Improved lateral heat spreading performance for polyvinylidene fluoride composite film comprising silver nanowire in light-emitting diode

Silver nanowire (AgNW) attracts great attention as a heat dissipation material due to its excellent thermal transfer property which exceeds most traditional heat sink materials.

A lock-in thermal imaging setup for dermatological applications.

Lock-in thermal imaging is a thermographic method that is widely used in the nondestructive testing of materials. The technique allows detecting under the sample surface, small variations of the thermophysical properties in a noninvasive and noncontact manner. Surprisingly, this method has, to our knowledge, never been used in dermatology although it is particularly suited. We present in this article the first lock-in thermal imaging setup dedicated to dermatological applications. The apparatus uses a temperature-modulated airflow to periodically stimulate the skin surface. The infrared images recorded by a high sensitive camera are demodulated according to the digital lock-in principle to compute a phase and amplitude image. First results obtained on benign skin lesions are presented. The images allow to detect small variations of the tissue thermophysical properties like for example, perfusion variations. Lock-in thermal imaging has the ability to reject disturbing thermal signals coming from subcutaneous tissues. The localization of the lesions is more accurate due the suppression of the lateral heat spreading. Lock-in thermal imaging is a promising method for the detection of lesions exhibiting modified thermophysical properties compared to the surrounding healthy skin.