Thermal Management Of High-power Electronics Research Articles

Analysis of Confined Jet Impingement in Converging Annular Microchannel Heat Sinks

Jet impingement cooling is deemed an excellent choice for the thermal management of high-power electronics. However, high-pressure drop penalties and low local heat transfer coefficients in regions far from the jet zone are its drawbacks. Although it is reported that recirculation areas appear because of the entrainment, the effects of recirculation size on thermal behavior are not understood well enough. Here, jet impingement heat sinks with converging annular channels are employed in a numerical investigation to minimize the adverse cooling effects associated with an impinging jet in a microchannel. The realizable k−ε turbulent model is used for modeling thermal and turbulent flow fields (Re=5,000 to 25,000). It was found that the different flow recirculation zones in small scales are responsible for the enhanced heat transfer rate. While the thermal performance of a converging wall jet impingement heat sink is higher than its flat wall counterpart at low Re numbers, the thermal performance results are in favor of the flat wall jet impingement heat sink at high Re numbers. The flow recirculation area shrinks in converging channels at high Re numbers, thereby deteriorating the thermal performance of the converging channel compared with a flat wall jet heat sink. Also, it was found that employing steeper converging channels shrinks the flow recirculation region, resulting in up to 59% lower pressure drops at Re=25,000. The present study examines the role of flow recirculation at different Re numbers on the thermohydraulic performance of jet impingement converging annular heat sinks.

A heat transfer model for liquid film boiling on micro-structured surfaces.

High heat transfer coefficient (HTC) and critical heat flux (CHF) are achieved in liquid film boiling by coupling vibrant vapor bubbles with a capillary liquid film, which has thus received increased interest for thermal management of high-power electronics. Although some experimental progress has been made, a high-fidelity heat transfer model for liquid film boiling is lacking. This work develops a thermal-hydrodynamic model by considering both evaporation atop the wick and nucleate boiling inside the wick to simultaneously predict the HTC and CHF. Nucleate boiling is modeled with microlayer evaporation theory, where a unified scaling factor is defined to characterize the change of microlayer area with heat flux. The scaling factor η is found to be independent of wicking structure and can be determined from a few measurements. This makes our model universal to predict the liquid film boiling heat transfer for various micro-structured surfaces including micropillar, micropowder, and micromesh. This work not only sheds light on understanding fundamental mechanisms of phase-change heat transfer, but also provides a tool for designing micro-structured surfaces in thermal management.

Enhanced boiling heat transfer performance on wettability-patterned surface

As an efficient heat transfer method, boiling heat transfer (BHT) plays an important role in strengthening the thermal management of high-power electronics, etc. In this work, the wettability patterns are prepared on copper substrate by mask-assisted chemical processing method, which enhances the BHT performance. We firstly treat the copper surface into superhydrophilic by chemical immersion, and then it is modified by fluorosilane into superhydrophobic, and wettability patterns are formed with the assistance of mask. Then, the BHT performances and the corresponding bubble dynamics of the wettability-patterned surfaces are experimentally investigated. Finally, the optimal BHT performances of the wettability-patterned surface prepared by the proposed method are compared with other modified surfaces. The results show that the BHT is improved dramatically on the wettability-patterned surface, the critical heat flux (CHF) and the heat transfer coefficient (HTC) improve by 23% and 56%, respectively. Under high heat flux, the decrease of hydrophobic pattern size can significantly improve the BHT performances, while under low heat flux, reducing the hydrophobic pattern pitch can improve the BHT performances to a certain extent. The reduction of the hydrophobic area has a significant effect on the CHF and HTC.

Efficient thermal management of high-power electronics via jet-enhanced HU-type manifold microchannel

To efficiently remove the increasingly higher heat fluxes from electronic chips, this numerical study presents an embedded single-phase water-cooled heat sink which consists of a microchannel layer and a jet-enhanced HU-type manifold layer. The manifold features two liquid inlet plenums symmetrically positioned on two opposite sides and some outlets arranged in the middle. In order to reduce the average thermal resistance, the fluid impinging velocity is boosted by narrowing the manifold channel inlet. However, narrower inlet also leads to flow maldistribution which increases the maximum thermal resistance. To address this problem, we demonstrate two approaches: either adding a flat plate at the manifold outlet or adjusting the manifold channel walls to form a tapered pathway. Further, the impact of the plenum and manifold channel inlet profiles on fluid flow and heat transfer is investigated. Negligible difference is found between rectangular and trapezoidal plenums. As to the manifold channel inlet, although similar heat transfer performance is obtained with different shapes, the total pressure drop can be substantially different. To reduce the large pressure drop at the inlet, a semicircular or triangular inlet profile is preferred, instead of a rectangular one. Quantitatively, we show that for a 3 × 3 mm2 heated area, our heat sink can dissipate fluxes up to 2000 W cm−2 with a maximum temperature rise of 65 K and a pressure drop below 40 kPa, which represents one of the highest performances and energy efficiencies reported to date.

A hybrid slot jet impingement/microchannel heat sink for cooling high-power electronic devices: A combined experimental and numerical study

Jet impingement cooling and microchannel cooling have been extensively adopted as the efficient cooling technologies in the thermal management of high-power electronics. In this research, an optimal hybrid slot jet impingement/microchannel heat sink, integrating the merits of two cooling methods, obtained from multi-objective optimization is machined and tested experimentally. By varying test conditions, the impact of inlet mass flow rate, inlet temperature and heat flux variations on the flow resistance and heat removal capacity of the optimal heat sink are investigated. Moreover, by conducting overall 3D numerical simulation, the internal flow and heat transfer behaviours of the heat sink are also investigated. By analyzing the flow field inside the heat sink, the experimental phenomena are interpreted. Comparing the relative errors between the numerical simulation and experimental measurement values under the same working conditions, it could be observed that the maximum relative error of the pressure drop is about 9.7 %, and the maximum relative error of the average temperature of the heated surface is only 0.9 %. According to the experimental results, as the heat flux is 200 W/cm2, the inlet temperature is 293 K and the inlet mass flow rate is 28 g/s, the average temperature of the heated surface does not surpass 348 K, while the pressure drop is only 2.46 kPa. Additionally, compared with the literature results, the hybrid heat sink obtained after multi-objective optimization in this study realizes a significant enhancement in cooling capacity, while the pumping power to heating power ratio is not significantly increased.

Vertically Aligned Boron Nitride Nanosheets Films for Superior Electronic Cooling.

Thermally conductive and electrically insulating thermal interface materials (TIMs) are highly desired for electronic cooling. To improve heat transfer efficiency, thermally conductive fillers with a high loading content have been incorporated into the polymer-based TIMs. However, this is usually at the expense of the interfacial thermal resistance reduction and reliability. In this study, vertically aligned boron nitride nanosheet films (VBNFs) have been prepared by a scalable microfluidic spinning process and template-assisted chemical vapor deposition conversion method. A further high-temperature annealing was applied to achieve high crystallinity. VBNFs have been applied as fillers to fabricate TIMs and achieve a superior through-plane thermal conductivity of 6.4 W m-1 K-1 and low modulus of 2.2 MPa at low BN loading of 9.85 vol %, benefitting from the well-aligned vertical sheet structure and high crystallinity. In addition, the fabricated TIMs present high-volume resistivity and breakdown strength, satisfying the electrical insulation demands. The high thermal conductivity and low modulus contribute an outstanding cooling performance to the TIMs in the heat dissipation application for high-power LEDs. This template-assisted conversion technology for the fabrication of orientated BN nanosheets structure and the prepared high-performance TIMs pave the way for efficient thermal management of high-power electronics.

Microfluidic production of silica nanofluids for highly efficient two-phase cooling with micro pin-fins structure

Flow boiling heat transfer in microchannels is fundamentally important for the thermal management of high-power electronics. The combination of silica nanofluids and micro pin-fins structure is expected to solve the limitations of the internal trade-off between the heat transfer coefficient (HTC) and critical heat flux (CHF). However, the high cost, preparation complexity, poor stability, and particle agglomeration limit the further development of silica nanofluids. Herein, we present a simple and straightforward microfluidic strategy for continuous, ultrafast, and high-throughput synthesis of functional silica nanofluids with excellent stability. Numerical simulation and experimental validation are conducted to investigate the excellent mixing performance of spiral microreactor. The morphology and size of SiO2 can be precisely controlled by simple flow rate. The excellent stability of time (∼30 d) and temperature (∼80 ℃) significantly improved the problem of easy aggregation of nanofluid particles previously used for heat transfer. The flow boiling heat transfer in microchannels on micro pin-fins silicon chip demonstrates the simultaneous enhancement of CHF (124%) and HTC (153%) at extremely low power consumption (pressure drop less than 1.5 kPa) compared to the base fluid. These findings provide important guidelines for the embedded cooling and shed new light on significant energy savings on electronics.

Enhancement of heat transfer on micro- and macro- structural surfaces in close-loop R410A flashing spray cooling system for heat dissipation of high-power electronics

High effective spray cooling system is urgently required for the thermal management of high-power electronics in small areas. These electronic devices need to be cooled at functional operating temperature, and it is particularly lacking in current spray cooling studies. For this purpose, a hybrid structural surface that integrates micro- roughness and macro- structure is investigated in close-loop R410A flash spray cooling system. The heat transfer performance is tremendously enhanced as the increase of roughness on flat surface until it reaches the value of 3.5 μm. Meanwhile, a novel tetrahedral fin surface is proposed and compared with modified surfaces including flat, wavy, square, and pyramid fins. It performs high heat flux of 411.2 W cm−2, a 101.5 % enhancement over the flat surface. What’s more, the hybrid structure with 3.5 μm roughness can further improve the maximum critical heat flux and heat transfer coefficient by up to 424 W cm−2 and 814 kW K−1 m−2, respectively, while keeping the surface temperature below 50 °C. Besides, correlations and energy efficiency are studied to explore the performance of structural surfaces. These findings in a wider range of roughness and macrostructure will provide insights to guide the surface modification in flash spray cooling.

Architected lattices embedded with phase change materials for thermal management of high-power electronics: A numerical study

• TPMS-PCM based heat sinks under high heat flux were numerically investigated. • The effect of TPMS structure type, PCM type, Applied Heat Flux and Heat Sink Material were studied. • For gallium-TPMS heat sinks, the effect of TPMS structure was not pronounced. For docosane-TPMS heat sinks, the effect of TPMS was considerably significant. • Docosane-TPMS heat sink severely underperformed than gallium-TPMS heat sink in controlling the heat sink base temperature. • An increase in applied heat flux caused faster PCM melting time and higher maximum heat sink temperatures. • Copper based TPMS heat sinks outperformed AlSi10Mg based heat sink owing to superior thermo-physical properties of copper. The boom of additive manufacturing has opened the doors to manufacture complex architectures with ease. The ever-increasing demands of high computational power has garnered a lot of research interest in more advanced and efficient cooling systems. Recently, additively manufactured and mathematically modeled Triply Periodic Minimal Surface (TPMS) based lattices have found widespread attention as thermal conductivity enhancers for phase change materials (PCMs). In this numerical study, architected lattices based on TPMS structures and impregnated with PCM have been studied as heat sinks for potential application in high power electronics cooling application. To authors’ best knowledge, heat sinks based on TPMS structures impregnated with PCM have never been studied for high heat flux/electronics cooling applications. Two TPMS structures i.e., IWP and Primitive have been selected as candidates based on reported work on TPMS-PCM composites performance in thermal energy storage applications. Two materials for architected lattices were considered i.e., Aluminum powder (AlSi10Mg) and Copper. Furthermore, two PCMs are taken into account, one an organic PCM (Docosane) and the other being a metallic PCM (Gallium metal). Besides, three values of applied heat flux replicating to-be-cooled electronic chips were considered i.e., 50 kW/m 2 , 100 kW/m 2 and 150 kW/m 2 . The results indicated that TPMS structures can help in temperature mitigation under high heat flux conditions. In the case of metallic PCM, the performance of both Primitive and IWP structure came out to be nearly identical. Hence, there was no architecture effect noticed in heat transfer performance of the lattices at all the three heat flux values. However, in the case of paraffinic PCM, Primitive structure showed better performance than IWP due to superior natural convection of liquid PCM in Primitive structure. However, paraffinic PCM could not aid in temperature mitigation to a realistic value despite being embedded inside metallic TPMS lattice owing to its inferior thermo-physical characteristics even at the smallest value of the heat flux. Gallium based heat sink outperformed paraffinic PCM as expected for both IWP and Primitive cases. Moreover, Copper based TPMS structures outperformed their AlSi10Mg based counterparts in mitigating the heat sink temperature owing to its superior thermo-physical properties. Therefore, this study offers a perspective of possible utilization and advancement of heat sinks for electronics cooling application.

IONIC WIND FLOW CHARACTERISTICS OF A MAGNETIC-FIELD-ENHANCED SOLID-STATE FAN AND ITS PERFORMANCE IN LIGHT-EMITTING DIODE COOLING APPLICATIONS

Solid-state fans (SSFs) have distinct advantages over traditional cooling fans in the thermal management of high-power electronics. In this work, a magnetic-field-enhanced SSF is proposed, and the physical model of negative corona discharge superimposed by an electromagnetic field is established. A computational model is used to calculate and analyze the effect of the magnetic field on the ionic wind distribution. The magnetic flux density and permanent magnet position distribution in the SSF are optimized experimentally, and the optimized SSF is applied to LED chip cooling. The findings show that adding a magnetic field encourages electrons to collide with neutral gas molecules and enhances the driving force of charged particles. The ionic wind velocity and mean driving force at the SSF's outlet will grow as magnetic flux density rises, and the ionic wind flow distribution will show apparent divergence. When the permanent magnet spacing is 15 mm, the highest ionic wind velocity is 2.82 m/s, and the mean driving force of SSF increases by 30.2%. The magnetic-field-enhanced SSF has a better LED-chip cooling effect, the maximum junction temperature drop is 11.6°C, and the cooling efficiency is higher. This research introduces a novel way of improving the cooling of electronics.

Recent Advances in Thermal Interface Materials for Thermal Management of High-Power Electronics

With the increased level of integration and miniaturization of modern electronics, high-power density electronics require efficient heat dissipation per unit area. To improve the heat dissipation capability of high-power electronic systems, advanced thermal interface materials (TIMs) with high thermal conductivity and low interfacial thermal resistance are urgently needed in the structural design of advanced electronics. Metal-, carbon- and polymer-based TIMs can reach high thermal conductivity and are promising for heat dissipation in high-power electronics. This review article introduces the heat dissipation models, classification, performances and fabrication methods of advanced TIMs, and provides a summary of the recent research status and developing trends of micro- and nanoscale TIMs used for heat dissipation in high-power electronics.

Thermal management of power electronics using nanofluids and nucleate boiling heat transfer technique

Nucleate boiling is an efficient solution for thermal management of power electronics where high-intensity heat flux needs to be transferred in a compact space. Both micro surfaces and nanofluids are used in literature to enhance the performance of nucleate pool boiling heat transfer systems; however, the effect of nanofluids over the microporous surface still needs to explore in several phase change heat transfer applications. The study’s objective is to examine the performance of nanofluid over microporous surfaces in Nucleate Boiling Heat transfer (NBHT) and performs a comparative analysis with plain and micro-porous surfaces. The study analyzed three ceramic (Alumina) nanofluids for nucleate pool boiling heat transfer (NBHT) over plain and microporous surfaces. The literature reported enhanced performance of nanofluids over the plane surface is verified in the study before proceeding to the detailed analysis over the microporous surface. Three different concentrations of 0.005%, 0.0005%, and 0.00005% were tested for the nanofluid. For plane surface, the critical heat flux (CHF) has been increased to 1035 [kW/m2] for Alumina from 783 [kW/m2] for deionized water, with the average increase in heat transfer coefficient (HTC) being 46.5%. The maximum CHF has been reported for micro-porous surfaces with a value of 1110 kW/m2. Out of the tested surfaces and fluids combination, the micro-porous surfaces with deionized water have been concluded as the most efficient surface with the highest heat transfer capacity for thermal management of high-power electronics and other high heat flux applications.

High-performance thermal energy storage and thermal management via starch-derived porous ceramics-based phase change devices

Low thermal conductivity and leakage of phase change materials (PCMs) have severely limited their applications in thermal energy storage and thermal management of electronic devices. Here, we propose starch-derived porous SiC ceramics to achieve high thermal conductivity and prevent leakage of PCMs simultaneously. Porous SiC ceramics with high thermal conductivity of 30 W/m-K at high porosity of 80% are obtained, benefiting from directional pore structures and dense grains enabled via facile directional freeze-drying of starch combined with liquid silicon infiltration technology. Thermal conductivity and thermal energy storage density of SiC/paraffin composite PCMs (CPCMs) attenuated only slightly by 2.75% and 2.80% after 500 repeated heating-cooling cycles, respectively, confirming their longevity and good stability. The phase change enthalpy achieves 331.56 J/g with high thermal conductivity of 24.27 W/m-K maintained by replacing paraffin with LiOH-LiF eutectics. When applying into transient cooling of high-power chips, the chip temperature is 10 ℃ lower if replacing traditional copper by porous SiC/paraffin CPCMs as a cooling medium. Our work demonstrates a promising route to realize efficient thermal energy storage and thermal management of high-power electronics via starch-derived porous ceramics-based phase change devices.

Numerical analysis of evaporation from nanopores using the direct simulation Monte Carlo method

Liquid evaporation from nanopores has been more recent with progress in nanotechnology. The relevant application ranges from solar-driven water harvesting using nanoporous membranes to thermal management of high-power electronics via coolant evaporation from nanoscale pores. In this work, the direct simulation Monte Carlo (DSMC) method was used to simulate evaporation from nanopores into the half-space, with an axisymmetric computational domain which conforms to the real nanopore geometry. According to the position of liquid–vapor interface, the evaporation regime was divided into: pinning and receding. In the pinning regime, the interface was pinned at the top of the nanopores. A range of Knudsen numbers and Mach numbers were compared, and the structure of the Knudsen layer was also identified in detail. The Knudsen layer thickness was less than 10λ at low Mach number and grew to nearly 40λ at Ma∞ = 0.785. We also examined the effects of porosity on evaporation mass flux. The effects of the receding length were researched in the receding regime. The downstream density decreased from 2.4 × 10−3 to 4.29 × 10−4 kg/m3 with the receding ratio from 1 to 20. Furthermore, the evaporation coefficient was established to determine its impact on gas flow. The present results highlighted the non-equilibrium and non-continuum features of nanoporous evaporation, providing insights into the rarefied gas dynamics within nanopores coupled to the liquid–vapor phase change.

Flat plate two-phase heat spreader on the thermal management of high-power electronics: a review

Flat plate two-phase heat spreader on the thermal management of high-power electronics: a review

Thermal effect of epilayer on phonon transport of semiconducting heterostructure interfaces

Heat dissipation through nanoscale heterostructures is of technological importance and fundamental research interest. Diamond is a desirable heat spreader, for integrating with semiconductors to dissipate quickly large amount of heat generated in electronics. The rapid development of GaN-on-diamond devices holds much promise for thermal management of high-power electronics and devices. Hitherto, the interfacial effect of an inherent epilayer between GaN and diamond to thermal dissipation is less investigated. One aim of this study is to understand and analyze the interfacial thermal boundary resistance (TBR) across a diamond/GaN heterostructure with the presence of an epilayer of either AlN, or Si3N4, or Si. It is found that with a 5 nm epilayer the Si3N4 gives the lowest TBR (4.58 m2K/GW) among the three types of diamond/GaN interface. The thickness effect of epilayer is also revealed; and it is found from theoretical calculations that the lowest TBR exists for the AlN and Si3N4 epilayers around 50 and 70 nm, respectively. Besides, c-BN is drawing increasing attention because it not only owns diamond-like thermal properties but also is a promising material for fabricating optoelectronic devices. The interfacial TBR reduced when the diamond cap or the GaN substrate was replaced by the c-BN. Further, c-BN can be directly grown on diamond without use of epilayer, resulting in an unprecedented low TBR of 0.4 m2K/GW in the diamond/c-BN interface.

Thermal conductance of nanostructured interfaces from Monte Carlo simulations with ab initio-based phonon properties

Interfaces are ubiquitous in electronics, photonics, and advanced materials. Interface engineering has become an essential strategy for developing functional materials with low thermal conductivities such as thermoelectric materials and thermal barrier coatings. On the other hand, interfaces are becoming a bottleneck for thermal management in electronic devices. Recent experiments have shown that a fin-like nanostructured interface with a size of 30–100 nm could enhance thermal transport across interfaces. Since phonon mean free paths span from several nanometers to dozens of micrometers, depending on the material, the size of the interface features may significantly affect the phonon transport regime and interface conductance. Here, the Monte Carlo simulation, with ab initio-based phonon properties as input parameters, was developed to study thermal conductance of a fin-like nanostructured interface. Simulated results indicate that the nanofin size (i.e., width, spacing, and height) significantly affects interface thermal conductance. Interface conductance is found to first increase and then decrease with increasing width of the nanofin when its height is 100 nm. This phenomenon is attributed to competition between the enlarged interface area and increased backscattering of transmitted phonons. This study demonstrates the existence of an optimal nanofin size for maximizing interface conductance, which could be important for thermal management of high-power electronics using nanostructured interfaces.

Gallium-based liquid metal alloy incorporating oxide-free copper nanoparticle clusters for high-performance thermal interface materials

Thermal interface materials (TIMs) with high thermal conductivity, fluidic characteristics, and surface wettability are crucial for the effective thermal management of high-power electronics. Gallium-based liquid metal (LM) can be a candidate for obtaining the purposes due to their thermo-physical properties. Several studies attempted to further increase the thermal conductivity of composites by adding conductive fillers. However, these efforts have been limited due to the oxidation and solidification issues at high vol% of fillers. In this work, we apply three strategies to obtain the enhanced thermal conductivity while maintaining the fluidity: 1) suppressing Ga-induced oxidation, 2) reducing the matrix-fillers interfacial resistance, and 3) forming additional heat-pathways within the LM matrix. An oxide-free ultrasonication-assisted particle internalization method has been developed, in which the copper nanoparticles (Cu NPs) are incorporated into the gallium-indium-tin (GaInSn) LM matrix. The fabricated composite shows ~180% enhancement of thermal conductivity (~64.8 Wm−1K−1) with only 4 vol% of nano-fillers compared to the untreated GaInSn. In the droplet impacting test, the synthesized GaInSn/Cu NPs composite presents almost identical spreading diameters with the untreated one, which confirms the maintained fluidity. The predicted thermal conductivity using the theoretical model, calculating the measured cluster fraction, is well-matched to the experimental data, which clarifies that the nanoparticle clusters created effective heat-pathways. A high-quality interface with a silicon substrate is confirmed by the X-ray photoelectron spectroscopy, demonstrating the presence of a thin-film Ga2O3 adhesion layer. In the cooling performance test, the developed TIM provides over 20% lower hot-spot temperature than that of the grease-type TIM at high heat flux regime (>150 W/cm2), showing excellent thermal stability over 230 cycles of acceleration test. This work will help develop high-performance TIMs, especially for high power density semiconductor applications.

High performance heat sink with counter flow diverging microchannels

Effective thermal management of high-power electronics is urgently demanded for the further development of electric vehicles. Two-phase flow boiling in microchannels is well recognized as a prominent approach for high density heat dissipation. This study develops a novel high performance heat sink with counter flow diverging microchannels to further enhance the flow boiling performance. A unique and significant heat exchange between the neighboring channels is demonstrated to dramatically alter the flow pattern transition in the counter-flow microchannel manifold. In particular, as the flowing bubbles in the microchannels can shrink or even disappear before exiting, no back flow or partial dry-out is observed throughout all the tested conditions. Therefore, this additional heat transfer mechanism in the present design enables significant enhancement of temperature uniformity, up to a heat flux of 2677 kW·m−2 with 45.1% increment of heat transfer coefficient, 73.8% reduction of pressure drop and 123.1% increment of coefficient of performance when compared with those of traditional co-current flow design. Overall, with the excellent flow boiling performance, the present study offers a highly promising microchannel design for a variety of applications requiring high heat flux dissipation.

Flow Boiling of Low-Pressure Water in Microchannels of Large Aspect Ratio

Flow boiling heat transfer in microchannels can provide a high cooling rate, while maintaining a uniform wall temperature, which has been extensively studied as an attractive solution for the thermal management of high-power electronics. The depth-to-width ratio of the microchannel is an important parameter, which not only determines the heat transfer area but also has dominant effect on the heat transfer mechanisms. In the present study, numerical simulations based on the volume of fraction models are performed on the flow boiling in very deep microchannels. The effects of the depth-to-width ratio on the heat transfer coefficient and pressure drop are discussed. The bubble behavior and heat transfer characteristics are analyzed to explain the mechanism of heat transfer enhancement. The results show the very deep microchannels can effectively enhance the heat transfer, lower the temperature rise and show promising applications in the thermal management of high-power electronics.