It has been reported that tablet computer surface temperatures can rise from room temperature up to 47°C. Holding a warm or hot computer surface might cause user’s thermal discomfort and possibly skin burns. The use of a tablet often requires holding the device for prolonged time with multiple fingers and palm areas in contact with the tablet lower surface. Previous research has not tested whole finger/palm thermal sensation at a specific surface temperature in a moderate environmental heat range. The current research investigates user’s thermal sensations on the palm and fingers, in response to warm/heat stimuli in a tablet size device with a longer contact duration than used in previous studies, to provide ergonomic design guidelines for electronic device designers and manufacturers. A tablet-size heating surface was developed comprising of nine rectangular aluminum heating pads connected with computer-controlled heaters and thermal sensors. Participants were asked to report their finger/palm thermal sensation and comfort every 45 seconds when they held the prototype for 90 seconds. Results showed a positive linear relationship between surface temperature and user’s thermal sensation and thermal discomfort. Duration of holding the prototype had no significant effect on user’s thermal comfort, but it did significantly affect thermal sensation ratings.

Graphene has great potential for ultra-sensitive strain sensors applications due to its high mechanical strength and good compatibility with the traditional semiconductor process. In the current study, we investigated the effect of tensile and bending deformations on the electronic states of graphene nanoribbons (GNRs) using density functional theory (DFT) to clarify the underlying mechanism of the piezoresistive properties of graphene. It is found that the electronic structure of armchair graphene nanoribbons (AGNRs) is very sensitive to the tensile deformation. When a uniaxial tensile stress is applied to AGNRs with width N a = 10, the band structure is modified, leading to the change in band gap approximately from 0 eV to 1.0 eV. The band gap values of bent AGNRs decrease significantly when the maximum local dihedral angle exceeds a critical value due to the orbital hybridization. Based on these knowledge, we fabricated a strain sensor using the graphene film grown by thermal chemical vapor deposition (CVD) method on Cu foil. The strain sensor is fabricated directly on the graphene-coated Cu foils by using the standard photolithography process and reactive ion etching (RIE) and then transferred onto a stretchable and flexible polydimethysiloxane (PDMS) substrate. The one-dimensional tensile test and three-dimensional bending test are performed to investigate the piezoresistive properties. A gauge factor 3.4 was achieved under the tensile deformation. The fabricated strain sensor also exhibits good performance to detect bending deformation.

The development of gallium nitride (GaN) on silicon (Si) substrates is a critical technology for potential low cost power electronics. These devices can accommodate faster switching speeds, hotter temperatures, and high voltages needed for power electronics applications. However, the lattice mismatch and difference in crystal structure between 111 Si and c-axis hexagonal GaN requires the use of buffer layers in order to grow device quality epitaxial layers. For lateral high electron mobility transistors, these interfacial layers act as a potential source of increased thermal boundary resistance (TBR) which impedes heat flow out of the GaN on Si devices. In addition, these interfacial layers impact the growth and residual stress in the GaN epitaxial layer which can play a role in device reliability. In this work we use optical methods to experimentally measure a relatively low TBR for GaN on Si with an intermediate buffer layer to be 3.8 ± 0.4 m 2 K/GW. The effective TBR of a material stack that encompasses GaN on Si with a superlattice (SL) buffer is also measured, and is found to be 107 ± 1 m 2 K/GW. In addition the residual state of strain in the GaN layer is measured for both samples, and is found to vary significantly between them. Thermal conductivity of a 0.8μm GaN layer on AlN buffer is determined to be 126 ± 25 W/m-K, while a 0.84 μm GaN layer with C-doping on a SL structure is determined to be 112 ± 29 W/m-K.

In 2012, 12% of US electricity consumption was due to residential and commercial lighting. By 2030, LED lightbulbs will be more plentiful, projected to drive a 40% reduction in energy [1]. Yet today’s LED lightbulbs have bulky heat sinks that do not leverage modern thermal management techniques. Reducing the size of the heat sink offers lower cost and increased energy efficiency. To leverage modern techniques, it is valuable to understand how thermal performance degrades light quality as people utilize lights over weeks and months. However, using current data acquisition systems to obtain such massive data sets is expensive, time-consuming, and cable-ridden. We have developed a robust ground-to-cloud system to acquire temperature and luminosity data for building lighting systems. The data acquisition units are small and may be positioned in existing lighting installations. During acquisition, data is backed up automatically. Usability studies demonstrate that novice users can configure an experiment in less than four minutes.

Thermal management for electric machines (motors/generators) is important as the automotive industry continues to transition to more electrically dominant vehicle propulsion systems. Cooling of the electric machine(s) in some electric vehicle traction drive applications is accomplished by impinging automatic transmission fluid (ATF) jets onto the machine’s copper windings. In this study, we provide the results of experiments characterizing the thermal performance of ATF jets on surfaces representative of windings, using Ford’s Mercon LV ATF. Experiments were carried out at various ATF temperatures and jet velocities to quantify the influence of these parameters on heat transfer coefficients. Fluid temperatures were varied from 50°C to 90°C to encompass potential operating temperatures within an automotive transaxle environment. The jet nozzle velocities were varied from 0.5 to 10 m/s. The experimental ATF heat transfer coefficient results provided in this report are a useful resource for understanding factors that influence the performance of ATF-based cooling systems for electric machines.

The spectral components of the phonon transport in the locally thermally excited graphene samples were studied by molecular dynamics (MD) method. In order to be able to select and analyze separate phonon modes in the time of propagation, the transient Green-Kubo approach to the definitions of density of states (DOS) and thermal conductivity was tested in quasi-equilibrium regimes for limited region of the graphene sample studied. Propagation of single modes at the background of diffusional phonon distribution and energy decay of such modes are studied by calculation of the DOS and dispersion relations, their dependence on the heating condition and temperature is studied. Similar conditions can be generated at localized heating of small areas of graphene structures in electronic devices. In transient regime, many issues of thermal transport evaluation still remain not sufficiently tested, especially phonon dynamics. Thermal conductivity of graphene samples related to transport of separate phonon modes is still not completely investigated, however, recent result give indication on the difference in the contribution of phonon modes. In the study, we consider mostly high temperature transport modes that are generated at the heated spot in order to be able to define their velocities and lifetimes in the limit of transient MD sampling. The single-layer graphene nanoribbon of 150 nm to 40 nm was relaxed and prepared in equilibrium in zigzag and armchair orientations. REBO potential for graphene was utilized. Our calculation has shown that at the heating to high temperatures of 1000K and higher, the G mode of graphene remains stationary and has a minimal contribution into thermal transport by coherent modes. The coherent phonon mode or modes that contribute the most into thermal transport were confined in the vicinity of 30 THz and can possibly be attributed to the D modes of graphene.

This paper describes the workflow and improved accuracy of a combined characterization and simulation of an LED luminaire. The achieved measurement results of the thermal and radiometric characterization, the process of implementing these results into a thermal simulation, and the benefit for the luminaire designer are presented.

Thermal boundary conductance (TBC) at Ti-graphene-Ti and Au-graphene-Au interfaces was measured using time-domain thermoreflectance (TDTR). Graphene grown on Cu foil by chemical vapor deposition was transferred to metallized Si substrates and TDTR was performed following the deposition of a top metal contact to form the metal-graphene-metal sandwiched structure. The results show that TBC was reduced at Au-graphene-Au interface in comparison to Au-Au interface and the value was similar to TBC measured at Au-HOPG interface. TBC at Ti-graphene-Ti interface was measured to be in the same range as Ti-Ti interface. This result is believed to be due to formation of oxide on surface of Ti, but further investigation is required. To the best of our knowledge, this is the first experimental study of TBC in metal-graphene-metal architecture.

This paper presents a detailed approach to provide improved cooling and heat spreading in electric machine rotors using centrifugally-pumped revolving thermosiphons. Design concepts are discussed that offer the following advantages: (1) high thermal performance across a wide range of operating points; (2) low-impedance heat paths; (3) excellent opportunities for integration with electric machine design for improved electromagnetic performance and structural design, as well as practical, cost-effective manufacturing. It takes advantage of centrifugal force to provide effective inertial pumping over a wide range of operating conditions. In addition, the new thermosiphon design is compatible with existing standard electric machine manufacturing techniques and cooling needs. A condenser section fin and ramp structure provides consistently high condensation performance. Surface texture design to promote effective nucleate boiling at high speeds is discussed, and fluid fill factor is analyzed. Applications include induction and PM synchronous machines. Benefits of these thermosiphons include increased steady-state power and torque density, increased and more consistent efficiency, and reduced permanent magnet volume and cost in PM synchronous machines. Other applications may include centrifugal gas compression, chemical processes, and machine tools.

Vapor chamber technologies offer an attractive approach for passive cooling in portable electronic devices. Due to the market trends in device power consumption and thickness, vapor chamber effectiveness must be compared with alternative heat spreading materials at ultra-thin form factors and low heat dissipation rates. A test facility is developed to experimentally characterize performance and analyze the behavior of ultra-thin vapor chambers that must reject heat to the ambient via natural convection. The evaporator-side and ambient temperatures are measured directly; the condenser-side surface temperature distribution, which has critical ergonomics implications, is measured using an infrared camera calibrated pixel-by-pixel over the field of view and operating temperature range. The high thermal resistance imposed by natural convection in the vapor chamber heat dissipation pathway requires accurate prediction of the parasitic heat losses from the test facility using a combined experimental and numerical calibration procedure. Solid Metal heat spreaders of known thermal conductivity are first tested, and the temperature distribution is reproduced using a numerical model for conduction in the heat spreader and thermal insulation by iteratively adjusting the external boundary conditions. A regression expression for the heat loss is developed as a function of measured operating conditions using the numerical model. A sample vapor chamber is tested for heat inputs below 2.5 W. Performance metrics are developed to characterize heat spreader performance in terms of the effective thermal resistance and the condenser-side temperature uniformity. The study offers a rigorous approach for testing and analysis of new vapor chamber designs, with accurate characterization of their performance relative to other heat spreaders.

Forests comprised of nominally vertically aligned carbon nanotubes (CNTs) are excellent candidates for thermal interface materials (TIMs) due to their theoretically predicted outstanding thermal and mechanical properties. Unfortunately, due to challenges in the synthesis and characterization of these materials reports of the thermal conductivity and thermal contact resistance of CNT forests have varied widely and typically fallen far short of theoretical predictions. In particular, the micro- and nano-length scales characteristic of the heat transfer in CNT forests pose significant challenges and may lead to misreported results. Here we examine the ability of a popular and well-established thermal metrology technique, time-domain thermoreflectance (TDTR), to resolve the properties of CNT forest TIMs. The characteristic heating frequencies of TDTR (1–10 MHz) are used to probe heat transfer at length scales spanning ∼0.1–1 μm, applicable for measuring the contact resistance between the CNT forest free tips and an opposing substrate. We identify the range of CNT forest-opposing substrate interface resistances that can be resolved with TDTR, and simultaneously demonstrate the effectiveness of several processes developed to reduce the resistance of these interfaces. The limitations of characterizing CNT forests with TDTR are discussed in terms of uncertainty and sensitivity to parameters of interest.

In this paper, we describe the system-level packaging of a 30 kW continuous, 55 kW peak, traction inverter to showcase the electro-thermal-mechanical performance enhancements of silicon carbide (SiC), a wide bandgap (WBG) semiconductor, over silicon. Higher efficiency, larger gravimetric and volumetric power densities, and smaller thermal management system requirements may be achieved through higher operating junction temperatures afforded by SiC versus silicon power devices. By applying advanced system-level packaging techniques, high-temperature control circuitry, utilizing 105°C-rated capacitors, and reducing the number of system interconnects and attaches to enable higher system reliability, a substantial cost reduction from the die level to the system level can be demonstrated by completely eliminating an electric vehicle’s secondary low-temperature cooling loop. The endgame is to reduce the traction inverter size (≥ 13.4 kW [peak]/L), weight (≥ 14.1 kW [peak]/kg), and cost (≤ $182/100,000) relative to output power while maintaining 15-year reliability metrics [1].

GaN on Diamond has been demonstrated to enable notable increases in RF power density without impacting High Electron Mobility Transistor (HEMT) peak junction temperature. However, Monolithic Microwave Integrated Circuits (MMICs) fabricated using GaN on Diamond substrates are subject to the same packaging thermal limitations as their GaN on SiC counterparts. Therefore, efforts to exploit GaN on Diamond to achieve substantial increases in MMIC power are stymied by external packaging thermal resistances that characterize the current “remote cooling” paradigm. This paper explores an intra-chip cooling alternative to the “remote cooling” paradigm, eliminating various heat spreader, heat sink and thermal interface layers in favor of integral microfluidic cooling in close proximity to the device junction. We describe an intra-chip cooling structure comprised of GaN on Diamond with integral micro-channels fed using a Si fluid distribution manifold. This structure exploits GaN on Diamond substrate technology to support increased HEMT areal power density while employing diamond microfluidics to affect scalable, low thermal resistance die-level heat removal. Thermal-electrical-mechanical co-design of integrated circuit (IC) features is performed to optimize conjugate heat transfer performance and manage the electrical and mechanical impacts associated with the presence of fluidic cooling near the electrically active region of the device. Through this, MMICs with significantly greater RF output than typical of the current state-of-the-art (SoA), dissipating die and HEMT heat fluxes in excess of 1 kW/cm 2 and 30 kW/cm 2 , respectively, can be operated with junction temperatures that support reliable operation. The modeling, simulation and micro-fabrication results presented here demonstrate the potential of diamond microfluidics-based intra-chip cooling as a means to alleviate thermal impediments to exploitation of the full electromagnetic potential of GaN.

Use of Thermal Interface Materials (TIM) is a common thermal solution approach in handheld devices to reduce junction temperature and control device skin temperature. This work summarizes the thermal benefits of using a TIM for enhancing the user experience and increasing System on Chip (SoC) performance. On the other hand, TIM induces a load on the package which in turn can impose stress on the package solder joints. This paper explains the impact of a variety of parameters such as TIM material and thickness and system boundary conditions on thermal performance of the SoC/system and the load distribution on solder joints. The complexity of mechanical load distribution is discussed through extensive data collection and simulation in phone and tablet form factors. Design guidelines for selection of appropriate TIM are proposed to improve the thermal performance without compromising the reliability of the SoC package.

Under the DARPA-sponsored ICECool Applications program, a microchannel cooling system using a 50-50 ethylene glycol-water mixture was optimized for cooling a high-power GaN-on-Diamond Monolithic Microwave Integrated Circuit (MMIC). Automated multi-objective optimization of the microchannel passages yielded an optimized design with a predicted thermal resistance of 22.4 K·cm 2 /kW at a pressure drop of only 121.4 kPa for an inlet temperature of 40°C. These values were corroborated by a coupled thermofluid analysis that included a detailed treatment of both the gate region and microchannel cooling geometry. Several versions of prototype coolers were fabricated, with one set consisting of pairs of coolers joined at their heated faces. These cooler pairs were used in heat exchange tests to characterize the average thermal resistance and the flow performance of the coolers. The performance testing results were consistent with the analytic predictions. Based on the analytical and experimental results, the system may be operated at inlet temperatures as high as 65°C without exceeding the transistor junction temperatures of 240 °C required for 10 6 hour mean-time-to -failure. The higher inlet temperature ameliorates system penalties associated with rejection of waste heat to ambient heat sinks.

In this paper, we present a non-intrusive case temperature measurement method of the direct-water-cooled power module. It uses the structure function, which in this case comprises the cumulative thermal capacitance and the cumulative thermal resistance. Since the effective heat transfer rate of the pinfin heatsink varies with the water flow rate, in this study we assumed the inflection point of the structure function corresponding to the change in the flow rate was junction-case thermal resistance. We compared numerical simulation results with experimental results and present our findings. Finally, we show that the design area in which the heat spreading angle of 45 degrees, the well-known rule of thumb, is suitable.

Even as the use of flexible graphite heat spreaders becomes ubiquitous in mobile electronics, numerically quantifying the heat dissipation remains a challenge. The rapid pace of development of mobile devices has deterred the industry from establishing standards, and rules of thumb are few, as are closed-form solutions. Users have requested numerical methods and tools to simplify the selection of flexible graphite heat spreaders from among the standard thicknesses and grades, as well as to quantify the effect of changing heat transfer area and configuration. In the presence of adjacent layers — adhesives, dielectrics, or still air gaps — the thin nature of the materials and the high, orthogonal thermal conductivity ratios of the graphite combine to create a complex conjugate heat transfer problem. Although the thinnest of these sheets constitute but a tiny fraction of the thickness of a cell phone or tablet, their dominant role in the heat transfer requires that they not be neglected in the calculations. Some CFD software guidelines advise using multiple meshing layers to capture fully the heat transfer in these spreaders, while others (primarily FEA based) provide a plate element that negates the need for discretization. In the former, a fully meshed spreader confounds the goal of a quick calculation, but the flexibility of 3D solution also demands meticulous attention to the details, provides “an answer” that is easy to misinterpret, and in the hands of an unskilled user, invites error. The goal of this project is to establish the guidelines for computing heat spreading in graphite, including cell dimension ratio, mesh density, spreading radius, and transport capacity and to marry the orthogonal properties of the material with the row-column format of a spreadsheet or matrix software. It also reviews methods for addressing the non-orthotropic situations such as angled plates, and the curved surfaces seen in the case of graphite wraps and flexible hinges. There are cases in which a simple contact resistance value adequately represent a graphite thermal interface material, but others that require an accounting for the lateral conductivity that increases the efficacy of the TIM. Finally, the error of the calculation is assessed for a simple representative geometry.

The present work is generally related to the design of a manifold microchannel heat sink with high modularity and performance for electronics cooling, utilizing two well established (i.e., jet impingement and channel flow) cooling technologies. The present cold plate design provides flexibility to assemble manifold sections in five different configurations to reach different flow structures, and thus different cooling performance, without redesign. The details of the modular manifold and possible configurations of a cold plate comprising three manifold sections are shown herein. A conjugate flow and heat transfer 3-D model is developed for each configuration of the cold plate to demonstrate the merits of each modular design. Parallel flow configurations are used to satisfy a uniform cooling requirement from each module, but a “U-shape” parallel flow “base” configuration cools the modules more uniformly than a “Z-shape” flow pattern due to intrinsic pressure distribution characteristics. A serial fluid flow configuration requires the minimum coolant flow rate with a gradually increasing device temperature along the flow direction. Two mixed (i.e., parallel + serial flow) configurations achieve either cooling performance similar to the “U-shape” configuration with slightly more than half of the coolant flow rate, or cooling of a specific module to a much lower temperature level. Generally speaking, the current cold plate design significantly extends its application to different situations with different cooling requirements.

In this paper an attempt has been made to demonstrate various package design considerations to accommodate series connection of high voltage Si-IGBT (6500V/25A die) and SiC-Diode (6500V/25A die). The effects of connecting the cathode of the series diode to the collector of the IGBT versus connecting the emitter of the IGBT to the anode of the series diode has been analyzed in regards to gate terminal operation and the parasitic line inductance of the structure. ANSYS Q3D/MAXWELL software have been used to analyze and extract parasitic inductance and capacitances in the package along with electromagnetic fields, electric potentials, and current density distributions throughout the package for variable parameters. SIMPLIS-SIMETRIX is used to simulate typical switch behavior for different parasitic parameters under hard switched conditions. Various simulation results have then been used to redesign and justify the optimized package structure for the final current switch design. The thermal behavior of such a package is also conducted in COMSOL in order to ensure that the thermal ratings of the power devices is not exceeded, and to understand where potentially harmful hotspots could arise and estimate the maximum attainable frequency of operation. The main motivation of this work is to enumerate detailed design considerations for packing a high voltage current switch package.

As demands on performance for mobile electronics continue to increase, traditional packaging technology is facing its limit in number of input/outputs (I/Os) and thermal challenges. Glass interposers offer many advantages over previous packaging technology for mobile electronics, including ultra-high electrical resistivity, low loss, and lower cost at processed interposer levels. However, it has two fundamental limitations; brittleness and relatively low thermal conductivity (∼1 W/mK), compared to Si (∼150 W/mK). This paper presents a study on thermal performance enhancement of glass interposer based on thermal modeling, and compares it with silicon interposer. The model captures in-plane and out-of-plane thermal performance enhancement with copper structures incorporated in the interposer. To further study the effect of advanced cooling schemes on interposer technology, an integrated vapor chamber design is evaluated through computational modeling.