High-performance Thermal Management Research Articles

Correlations for the prediction of natural convection heat transfer of hollow hybrid fin arrays

A hollow hybrid fin (HHF) array is a staggered array of hollow pin fins concatenated with radially-extruded plate fins. Preceding research for a decade shows that natural convection HHF arrays could yield up to 30 % superior mass-based thermal performance and less orientation dependence compared to natural convection pin fin arrays. Hence, the HHF array could be a potential alternative to conventional pin fin arrays for lightweight high-performance thermal management of microelectronics in natural convection. To demonstrate potential and limitation of the natural convection HHF array, optimization is inevitable, and thus correlations for the prediction of natural convection heat transfer around the HHF arrays should be developed to provide design guidelines for the optimization. Nu correlations are developed for HHFs, HHF arrays, and oriented HHF arrays. Due to broad ranges of physical conditions, 168 cases for the HHF, 48 cases for the vertical HHF array, and 24 cases for the oriented HHF arrays are calculated and validated by the measurement and the literature data. Utilizing CFD thermal data, Nu correlations are formulated. Discrepancies between correlation-predicted and CFD-evaluated Nu values are examined. The results show that average discrepancies are 2.9 % for the vertical HHFs, 4.8 % for the vertical HHF arrays, and 5.5 % for oriented HHF arrays. These reasonable discrepancy values demonstrate the correlation validity. Validated Nu correlations are used to analyze parametric effects on convection heat transfer coefficients, h. It is seen that h increases asymptotically with the increase of fin spacing, S, mainly due to the porosity increase. The parametric study finds that h declines till a certain internal diameter, Di. They are 2 mm for external diameter, Do, of 4 mm, 4 mm for Do of 7 mm, and 6 mm for Do of 10 mm. The parametric study shows that after such Di, h consistently increases until the widest Di. The parametric study for the orientation shows that for the sideward orientation, h increases with the increase of porosity. The result also shows that h of the downward case is greater than that of the sideward case, and the difference is greater at lower Ra values.

Reversible Solar Heating and Radiative Cooling Devices via Mechanically Guided Assembly of 3D Macro/Microstructures.

Solar heating and radiative cooling are promising solutions for decreasing global energy consumption because these strategies use the Sun (≈5800K) as a heating source and outer space (≈3K) as a cooling source. Although high-performance thermal management can be achieved using these eco-friendly methods, they are limited by daily temperature fluctuations and seasonal changes because of single-mode actuation. Herein, reversible solar heating and radiative cooling devices formed via the mechanically guided assembly of 3D architectures are demonstrated. The fabricated devices exhibit the following properties: i) The devices reversibly change between solar heating and radiative cooling under uniaxial strain, called dual-mode actuation. ii) The 3D platforms in the devices can use rigid/soft materials for functional layers owing to the optimized designs. iii) The devices can be used for dual-mode thermal management on a macro/microscale. The devices use black paint-coated polyimide (PI) films as solar absorbers with multilayered films comprising thin layers of polydimethylsiloxane/silver/PI, achieving heating and cooling temperatures of 59.5 and -11.9°C, respectively. Moreover, mode changes according to the angle of the 3D structures are demonstrated and the heating/cooling performance with skin, glass, steel, aluminum, copper, and PI substrates is investigated.

Flexible yet impermeable composites with wrinkle structured BNNSs assembling for high-performance thermal management

Efficient thermal management has become one of the most critical issues of electronics because of the high heat flux generated from highly integrated, miniaturized, and increased power. Here we report highly flexible composites with aligned and overlapping interconnected boron nitride nanosheets (BNNSs) assembled in wrinkle structures. Besides high in-plane thermal conductivity of more than 26.58 W m−1 K−1, such structure rendered enhanced through-plane conduction along with increasing pre-stain. As thermal interface materials (TIMs) of both rigid and flexible devices, the composites revealed an outstanding thermal cooling capability outperforming some commercial TIMs. During a record-long bending process of more than 3000 cycles, the maximum temperature fluctuation of the flexible device with 100%-prestrained composite was only within 0.9 °C, less than one-third of that with commercial thermal pad. Moreover, the composite revealed a superior impermeability for flexible seals. Our results illustrate that the composites could be an ideal candidate for the thermal management of emerging flexible electronics.

An approach for topology optimization of heat sinks for two-phase flow boiling: Part 1 – Model formulation and numerical implementation

Two-phase flow boiling in heat sinks and cold plates is attractive for various thermal management applications due to high effective heat capacity rates and high heat transfer coefficients compared to single-phase operation. In addition, multi-physics topology optimization is sought for generation of high performance thermal management components. Yet, the design of heat sinks using topology optimization has been restricted to single-phase flow physics, due to incompatibility between available two-phase flow models and traditional single-phase topology optimization approaches. Interface-capturing two-phase flow models are too computationally expensive for optimization; mixture and two-fluid models that treat the liquid and vapor phases as an interpenetrating continuum with interfacial transport phenomena captured through empirical correlations are computationally tractable for heat-sink-scale optimization. However, the penalization approaches commonly used to define the structures formed during topology optimization results in arbitrary fin and channel geometries for which a universal two-phase flow correlation is not available. To enable coupling with two-phase mixture models, this work leverages a homogenization approach to topology optimization wherein heat sink designs are represented using spatially varying microstructures with optimized dimensions. A predefined microstructure geometry is chosen for which two-phase flow correlations exist and therefore topology optimization can be performed. This work is the first to develop a topology optimization algorithm using a mixture model for simulating two-phase flow boiling to design heat sinks and cold plates. Part 1 of this study develops and implements the algorithm to investigate topology optimized heat sinks at various heat inputs, topology optimization grid sizes, and maximum vapor quality constraints. Topology optimized heat sinks designed for single-phase versus two-phase flow are compared. There are significant differences in hydraulic and thermal responses of the single-phase and two-phase designs due to high effective heat capacity rates and high heat transfer coefficients of flow boiling. A companion paper (Part 2) focuses on manufacturing, model calibration, and experimental validation of the topology optimized heat sinks under two-phase flow boiling. The algorithm demonstrated in this work extends the capabilities of topology optimization to two-phase flow physics, and thereby enables the design of various two-phase flow components such as evaporators, condensers, heat sinks, and cold plates.

Hydrophobic Nanofiber Cellulose-Graphene Films for High-Performance Thermal Management Applications

Hydrophobic Nanofiber Cellulose-Graphene Films for High-Performance Thermal Management Applications

Rigorous Reliability Testing of an Encapsulated Thermal Pyrolytic Graphite (TPG) Heat Spreader for Passive Thermal Management Applications

Continued microelectronic miniaturization has resulted in higher power density, increasing the need for high performance thermal management. One possible solution for board-level thermal management is to fully encapsulate a high thermal conductivity material, such as Thermal Pyrolytic Graphite (TPG; ~1700 W/mK) in a metal (such as 6061 aluminum), producing an efficient passive heat spreader for thermal dissipation. The metal shell provides sufficient mechanical strength at a small detriment to the overall thermal conductivity. Reliability and consistent performance over time are critical for thermal management systems, especially in Defense and Aerospace applications. MIL-STD-883H provides test parameters for reliability involving exposure to mechanical shock (Method 2002.5), mechanical vibration (Method 2005.2), and thermal shock (Method 1011.9), with traditional evaluation criteria involving microelectronic device performance and/or a visual examination. For this work, the reliability of aluminum encapsulated TPG test coupons was evaluated with more demanding criteria: a visual examination, X-ray inspection, acoustic microscopy, and thermal conductivity performance testing, both before and after the previously discussed MIL-STD-883H exposures. The encapsulated coupons showed excellent robustness; the thermal conductivity properties remained constant with an average value around 965 W/mK, and the non-destructive imaging revealed excellent bond quality for the metal encapsulation, with no evidence of delamination. This more stringent evaluation criteria after MIL-STD-883H testing provides a better way to evaluate the reliability of passive thermal management systems. The study results also demonstrate the metal encapsulated TPG heat spreader as a superior thermal management solution with high performance and high reliability for demanding applications.

Preparation and performance evaluation of graphene-based waterborne thermal conductive coatings

Thermal management of electronic products and communication equipment is a key bottleneck restricting the integration of products, and high-performance thermal management materials are an effective means to solve this problem. In this study, graphene prepared by liquid phase exfoliation was used as a functional additive material to successfully prepare a graphene high-thermal-conductivity coating material on stainless steel substrates for enhanced cooling performance. The research found that the thermal conductivity of different coating materials with different graphene contents significantly varies, and when the graphene content is 15 %, the thermal conductivity can reach as high as 9.499 W/(m·K), which can rapidly achieve heat dissipation. Considering the actual working conditions of the thermal coating, the comprehensive performance of the coating was tested, and it has excellent corrosion resistance and excellent temperature cycling resistance. Under experimental conditions, after adding the thermal coating to the stainless steel substrate, its cooling effect significantly improved. Compared with uncoated blank samples, the maximum temperature difference during heat transfer under room temperature can reach 11 °C, and the maximum temperature difference under −80 °C can reach 19 °C. This study's coating has potential application prospects in the field of thermal management.

Ultrafast sintering of boron nitride nanosheet assembled microspheres with strong processability for high-performance thermal management materials

A novel strategy produces BNNS microspheres for isotropic thermal conductivity, utilizing high-temperature ultrafast sintering and surface engineering to enhance processability for producing thermal management materials.

Enhance thermal conductivity via constructing a low heat resistance interface with quaternized graphene

Interfacial thermal resistance of fillers in thermal conductive polymer composites is a barrier restricting heat dissipation function. In-situ graft of quaternary ammonium group on the surface of graphene can form cation-π interaction, providing a medium for heat transfer between graphene layers. Accordingly, we fabricate polyvinyl alcohol (PVA)-based heat conductive composite containing quaternized graphene stacking structure. With the assistance of non-covalent bonding, the in-plane thermal conductivity of the composite formed by self-precipitation method reaches 9.97 W m−1 K−1 at a filling content of 30 vol%. Molecular dynamics (MD) simulation further explores the promoting effect of the interfacial structure constructed between the charged group and graphene on the interfacial heat transfer. The optimized interfacial structure not only alleviates the bottleneck of interface thermal resistance but also enhances the mechanical properties of composite. This strategy provides an ideal solution to preparation of high-performance thermal management materials for electronic equipment.

A review of microfabrication approaches for the development of thin, flattened heat pipes and vapor chambers for passive electronic cooling applications

With the rapid development of microelectronics and the telecommunication industry, a variety of high performance, portable and slim electronic devices have become available. Miniaturization of devices and increased packing density of electronics can generate “hot spots” i.e. a high heat flux on a small area. Thus, in such devices the heat management requirements go beyond the limits of typical approaches and the development of miniaturized, high-performance thermal management concepts to cool high-performance, compact electronic devices is urgently required. To this direction, micro and nanofabrication methods can provide solutions in both miniaturizing existing concepts of passive cooling as well as in improving their performance. In this review, we start by introducing the most commonly used metrics used to evaluate the performance of passive cooling devices (i.e. vapor chambers and flattened heat pipes) together with the most prominent performance limitations. Then, in the main part, we present state of the art examples of microfabricated, thin vapor chambers and flattened heat pipes on rigid substrates (i.e. using metals and silicon), but also vapor chambers on thin and flexible polymeric or composite materials. Finally, the main conclusions and the steps which should be followed to further enhance the performance of such devices are summarized in the conclusions and future perspectives section.

Biomineralization-Inspired Copper Sulfide Decorated Aramid Textiles via In Situ Anchoring toward Versatile Wearable Thermal Management.

Designing smart textiles for personal thermal management (PTM) is an effective strategy for thermoregulation and energy saving. However, the manufacture of versatile high-performance thermal management textiles for complex real-world environments remains a challenge due to the limitations of functional integration, material properties, and preparation procedures. In this study, an aramid fabric based on in situ anchored copper sulfide nanostructure is developed. The textile with excellent solar and Joule heating properties can effectively keep the body warm even at low energy inputs. Meanwhile, the reduced infrared emissivity of the textile decreases the thermal radiation losses and helps to maintain a constant body temperature. Impressively, the textile integrates superb electromagnetic shielding, near-complete UV protection properties, and ideal resistance to fire and bacteria. This work provides a simple strategy for fabricating multi-functional integrated wearable devices with flexibility and breathability, which is highly promising in versatile PTM applications.

Sustainable epoxy composites with high thermal conductivity using poly(lipoic acid) modified liquid metal as crosslinkers

With the increasing demand for electronics integration and the increasing accumulation of e-waste, thermal management materials combing high conductivity and sustainability is highly desired. Herein, bio-based recyclable epoxy composites with significantly improved thermal conductivity were prepared by utilizing poly(lipoic acid) modified liquid metal (LM) as the crosslinkers. The strong interactions between poly(lipoic acid) and LM enable the formation of a poly(lipoic acid) shell layer coating on the LM droplet core, which facilitates the dispersion of LM droplets as well as interfacial thermal transport in the composites. As a result, the composite with only 25 vol% LM exhibits a thermal conductivity of 0.82 W/(m·K), which is higher than those of the previous studies with randomly filled LM at the same loading. In addition, attributed to the existence of dual dynamic bonds (disulfide bonds and β-hydroxyl ester bonds), the composites are reprocessable and their thermal conductivities are maintained after reprocessing. Moreover, the composites can be degraded under mild reducing conditions. This work presents a novel approach for the preparation of high-performance thermal management materials featuring recyclability and degradability.

Study on heat transfer and pressure drop characteristics for nanofluids in microchannel heat exchangers

The microchannel heat exchanger (MCHE) is considered the next-generation heat exchanger owing to its high-performance thermal management systems and fabrication processes. The MCHE is used in many cooling applications, such as x ray medical devices, high-power microelectronics, and high-heat flux devices. Comprehensive exploration of different nanofluids, their concentrations, and impacts on pressure drop and heat transfer within the context of MCHEs was the main focus of the current study. The experiments were conducted at Reynolds numbers ranging from 100 to 500 for laminar flow. Additionally, nanoparticles (np) such as Al2O3, CuO, and carbon nanotubes (CNT) were added to de-ionized water at weight percentages of 0.01, 0.03, and 0.05. The results indicated that heat transfer significantly increased at a particle concentration 0.05 by wt. % for all nanofluids (nf). Furthermore, at a particle of 0.05 by wt. %, the heat transfer of the CNT-based nf increased by 37%, whereas those of the Al2O3- and CuO-based nf increased by 24.01% and 6.23%, respectively. The nanofluid pressure drop (PD) increases with an increase in the Reynolds number owing to the increase in the viscosity of the liquid compared to de-ionized water (base fluid) and requires more pumping power.

Hot-pressing induced alignment of AlN whiskers in polymer matrix leading to enhanced in-plane thermal conductivity

With the increased integration of electronic devices, the demands for polymer-based composites with higher thermal conductivity as thermal management materials are gradually growing. Compared with regular spherical morphology, 1D particles with high aspect ratio are regarded as ideal fillers for polymer matrix due to their favorable construction of thermally conductive channels. In this work, high-aspect-ratio AlN whiskers were successfully synthesized by a modified direct-nitriding method, which were further combined with ultra high molecular weight polyethylene (UHMWPE) to prepare the thermally conductive composites by a facile hot-pressing process. The introduction and alignment of AlN whiskers along horizontal direction resulted in a high in-plane thermal conductivity of 7.12 W/(m·K) at the 62.5 wt% filling fraction, which was 16 times than that of UHMWPE. In addition, due to the cross-linking of 1D AlN whiskers, the composites also exhibited high thermal stability and mechanical strength, making them very promising for application as high-performance thermal management materials.

Microscale Imaging of Thermal Conductivity Suppression at Grain Boundaries.

Grain-boundary engineering is an effective strategy to tune the thermal conductivity of materials, leading to improved performance in thermoelectric, thermal-barrier coatings and thermal management applications. Despite the central importance to thermal transport, a clear understanding of how grain boundaries modulate the microscale heat flow is missing, owing to the scarcity of local investigations. Here, thermal imaging of individual grain boundaries is demonstrated in thermoelectric SnTe via spatially resolved frequency-domain thermoreflectance. Measurements with microscale resolution reveal local suppressions in thermal conductivity at grain boundaries. Also, the grain-boundary thermal resistance - extracted employing a Gibbs excess approach - is found correlating with the grain-boundary misorientation angle. Extracting thermal properties, including thermal boundary resistances, from microscale imaging can provide comprehensive understanding of how microstructure affects heat transport, crucially impacting the materials design of high-performance thermal-management and energy-conversion devices. This article is protected by copyright. All rights reserved.

Adjustable boron nitride segregated framework in epoxy resin for high performance thermal management and flame retardant applications

Excess heat production in miniaturized and integrated of electronic devices has created a strong demand for polymeric thermal management materials with superior thermal conductivity and flame retardancy. For the reason, a phytate-assisted ball-milling method was used to exfoliate and functionalize the boron nitride nanosheets (f-BNNS) with flame retardancy, which was further modified using KH560. Then, EP composites with f-BNNS@KH560 frames were prepared by emulsion polymerization, adhesive self-assembly and hot-pressing strategies. The mass ratio of EP to water in the emulsion polymerization process is closely related to the structure of f-BNNS@KH560 framework. The smaller the ratio of EP to water, the more complex the structure of the framework and the more paths for heat transfer of the obtained EP composites. Ultimately, f-BNNS@KH560/EP-3 exhibits a high K value of 2.4 W m−1 K−1 at a loading of 30 wt% due to more abundant heat transfer paths and good compatibility. In the cooling performance test of LED lamp, f-BNNS@KH560/EP-3 works at a temperature 16.5 °C lower than that of conventional f-BNNS@KH560/EP composites. Besides, the superior flame retardancy of f-BNNS@KH560/EP is mainly attributed to the strong barrier effect of adjustable f-BNNS@KH560 framework. Undoubtedly, f-BNNS@KH560/EP-3 provides ideas for industrial applications of high power devices.

Enhanced capillary and heat transfer performance of asymmetric micropost wicks

We developed asymmetric capillary wicks composed of slanted microposts using inclined photolithography. Then we investigated the effects of inclination angle and wicking direction on the capillary and the heat transfer performances. The working fluid accelerates when it flows in the slope direction of the structure (forward direction, FD) and decelerates when it flows in the opposite slope direction (rear direction, RD). We applied the scaling law to the capillary rise experiment data to verify that the inclination angle and the wicking direction affect the capillary performance. The capillary performance parameter was improved by up to ∼39% with FD case and decreased by 21.3% with RD. The heat transfer performance test showed that the wick-CHF (the enhanced critical heat flux due to the formation of the wick) of the asymmetric FD case was increased by 43.3% compared to symmetric ones while maintaining the heat transfer coefficient. This work shows that asymmetric evaporator wicks can enhance the critical heat flux without sacrificing the heat transfer coefficient, which can help develop high-performance thermal management solutions.

High-performance thermal management system for high-power LEDs based on double-nozzle spray cooling

Thermal management is one of the main challenges for high-power LEDs. As one of the state-of-the-art thermal management approaches, spray cooling can meet the requirements of fast cooling speed, large heat transfer coefficient, and less energy consumption when it provides thermal management for electronics with high heat flux. In this study, a high-performance spray cooling system for high-power LEDs is designed and established. The influence of nozzle configuration, flow rate, and nozzle-to-surface distance on the heat transfer characteristics are experimentally and numerically investigated. The system shows excellent heat dissipation capacity. Two different nozzle configurations are compared and the results show that the double-nozzle configuration possesses the highest single-phase heat transfer coefficient, reaching up to 20.7 kW/m2·K with the flow rate of 150 mL/min. Interestingly, double-nozzle, which owns better heat transfer performance compared to single-nozzle, shows worse temperature uniformity inside the testing module. Simultaneously, the junction temperature of double-nozzle at the center chip can also be controlled to 85 °C corresponding to a 10% reduction for single-nozzle. The proposed cooling system may enable the further application of spray cooling in electronics like high-power LEDs, potentially reducing the energy consumption in cooling electronics and promoting the utilization of low-grade thermal energy.

A Review of Select Patented Technologies for Cooling of High Heat Flux Power Semiconductor Devices

The development of wide band-gap power electronics over the last two decades has stimulated a tremendous amount of research into high performance liquid cooling solutions for high heat flux (200-1000 W/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> ) power semiconductor devices. A patent literature search for electronics cooling technologies was conducted in the late-2000 time frame and promising technologies for high performance thermal management were identified, at that time, for research and development. These promising technologies were separated into four categories classified as single-phase (i.e., liquid) jet impingement cooling, microchannel cooling, phase change or two-phase cooling, and near-junction including direct chip cooling. In this Letter, a perspective is provided on select patents from our group that stem from research into these technology areas from 2010 to 2022. Additionally, the thermal-fluid performance capability of each technology is briefly summarized. As a whole, the Letter provides a demonstration of the historical significance of power electronics cooling technology on the mobility industry and briefly outlines future directions for research and development.

Solar-Radiation-Dependent Anisotropic Thermal Management Device with Net Zero Energy from 4D Printing Shape Memory Polymer-Based Composites.

Reports have pointed out that nearly 50% of the global total energy demand for buildings is used for daily heating and cooling. Therefore, it is very important to develop various high-performance thermal management techniques with low energy consumption. In this work, we present an intelligent shape memory polymers (SMPs)-based device with programmable anisotropic thermal conductivity fabricated by a 4D printing technique to assist in thermal management with net zero energy. Highly thermal conductive BN nanosheets were textured in a poly (lactic acid) (PLA) matrix by 3D printing, and the printed composites lamina exhibited significant anisotropic thermal conductivity. The direction of heat flow in devices could be switched programmably, accompanying the light-activated deformation controlled by grayscale of composite, which was demonstrated by the "windows" arrays composed of in-plate thermal conductivity facets and SMPs-based hinge joints, achieving the programmable movement of opening and closing under different light conditions. Based on solar radiation-dependent SMPs coupled with the adjustment of heat flow along anisotropic thermal conductivity, the 4D printed device has been proved in concept for potential applications in thermal management in a building envelop for dynamic climate adaptation, taking place automatically based on the environment.