Thermal Ground Plane Research Articles

Electric Power Generation Using Buoyancy-Induced Vortices

Thermally-stratified air layers over solar-heated ground are exploited for scalable, low-cost power generation by the deliberate formation of intense buoyancy-induced vertical columnar vortices. Such vortices entrain the ground-heated air layer in regions with high surface solar heating rates, and convert the (gravitational) potential energy into “solar-induced wind” with significant kinetic energy. Unlike naturally occurring “dust devil” vortices, these columnar vortices are deliberately triggered and anchored. Each vortex is sustained by continuous entrainment of ground-heated air through an azimuthal array of stationary ground-mounted vertical vanes. Electric power is generated by using the rotational and vertical air motions within the columnar vortex to drive a vertical-axis turbine. Meter-scale laboratory experiments have demonstrated the nucleation, anchoring, and sustainment of strong, buoyancy-driven vortices centered within an azimuthal array of stationary vertical vanes above a controlled thermal ground plane. Outdoor tests of a meter-scale prototype have demonstrated autonomous start-up, formation, anchoring, and sustainment of a buoyancy-induced vortex generated solely by absorbed solar energy.

Strategies for thermal performance improvements of polymeric thermal ground planes (TGP)

This paper presents a study on the thermal performance and enhancement techniques of polymeric thermal ground planes (TGPs). The literature features a range of polymeric TGPs that exhibit an effective thermal conductivity up to twice that of copper. However, the TGP effective thermal conductivity does not provide insight into the performance and potential improvements that could be made to a specific TGP design and configuration. Relying only on this metric can be limiting to compare different TGPs performances, since effective conductivity is affected by the device size, layout of heated and cooled zones, and the fabrication materials. In this paper, we adapted a model by connecting the TGP pumping capacity to its thermal resistance to predict the polymeric TGP thermal performances as a function of their geometry, test configuration and material used. This approach enables the development of strategies to optimize the polymeric TGPs performances and identify their limitations rather than relying on effective thermal conductivity as a benchmark for comparison. The present model considers vapor flow through a mechanical structure for the first time in TGPs. Indeed, the polymeric TGP in contrast to the metallic one requires a denser mechanical structure to support the vapor core thickness during vacuum due to the flexibility of the polymeric shell. The generated vapor pressure drop along the TGP has a dual impact on its performance as it affects both the capillary limit and vapor resistance resulting in a tradeoff between the TGP pumping capacity and thermal resistance, for a given TGP thickness. To back up the proposed model, experimental characterization was carried out on a one-step assembled polymeric TGP, to extract simultaneously the total and vapor thermal resistances. The model highlighted a trade-off in designing the vapor core and wicking structure, balancing the polymeric TGP pumping capacity and thermal resistance, for a target TGP thickness. Hence, the current study provides guidelines for defining improvement strategies for TGPs thermal performances based on the geometry and the type of material used during the fabrication process. The recommended approach also permits the identification of polymeric TGPs capabilities and limits rather than depending on the thermal metrics, which may be misleading while comparing performances.

A novel battery thermal management system utilizing ultrathin thermal ground planes for prismatic Lithium-ion batteries

Temperature significantly affects the energy efficiency, safety, life and performance of a lithium-ion battery pack in electric vehicles (EVs). High overall temperature and large temperature difference of the battery in fast charging (FC) can cause degradation in performance and even catastrophic failure like a thermal runaway. Besides, the increasing energy density of battery packs limits the space for the thermal management system. Therefore, we developed an ultrathin thermal ground plane-based battery thermal management system (UTTGP-BTMS), which utilizes 0.4 mm thick novel UTTGP with air cooling to dissipate the heat from the gap between batteries. Double-layer high pores per inch (PPI) mesh and wettability modification was adopted to enhance the thermal performance of the UTTGP. Before the evaluation test of the BTMS, the heat generation rates of the batteries in 2.2C to 4C fasting charging conditions are estimated by Bernardi’s model. Then, the thermal performance of the novel battery thermal management system (BTMS) is experimentally investigated under 2.2C to 4C FC regimes at ambient temperatures from 10 °C to 50 °C. The BTMS is able to maintain a battery surface temperature of 55Ah Lithium iron phosphate (LiFeO4, LFP) batteries below 42.7 °C under a 4C charge rate, 57.3 °C at 50°C ambient temperature, respectively, and still achieve good surface temperature uniformity in all cases studied. Compared to a BTMS with copper heat spreaders, the temperature rise temperature non-uniformity, and thermal resistance are reduced by up to 23.3%, 28.4%, and 62.6%, respectively. Besides this, the effects of the pores density of the mesh in the UTTGP-BTMS are also studied by changing the sieve number of mesh, showing that a higher pore density gives better performance under large C-rates. The proposed UTTGP-BTMS showed significant performance with only submillimeter thickness in the thermoregulation of a battery pack, with the potential of being a viable solution for high-power battery thermal management in EVs.

Experimental investigation of heat spreading in a wickless and ultrathin thermal ground plane

A thermal ground plane (TGP) device spreads concentrated heat effectively into a relatively large area. The TGP selected for this study utilizes bubble pumping, instead of wicking, to effectively circulate fluid inside its chamber and keep the hot area wetted. It is thin, light, durable, and its performance is orientation-independent. With a 0.80 mm thick chamber, this TGP can achieve a thermal resistance below 0.2 K/W. Reducing the chamber's thickness by 20% and 50% is shown experimentally to yield a better thermal resistance, as low as 0.15 K/W. The corresponding maximum effective thermal conductivity of the TGP is 1339 W/m-K, 1008 W/m-K, and 821 W/m-K for a chamber thickness of 0.40 mm, 0.64 mm, 0.80 mm, respectively. However, a reduced dryout heat flux is detected with a thinner TGP. This work aims to investigate the effects of the chamber's thickness on the thermal performance of this ultrathin TGP. The performance of the TGP is orientation-independent regardless of thickness and water charging amount in its chamber.

Numerical case study and modeling for spreading thermal resistance and effective thermal conductivity for flat heat pipe

Flat heat pipe, vapor chamber, thermal ground plane, and heat spreader have been emerging topics for various thermal applications such as electric battery packs and high-power computer chips. In this study, computational fluid dynamics is used to model the effective thermal conductivity of a vapor chamber and thermal ground plane. From various literature, simulation conditions are obtained for different flat heat pipe geometries. With the commercial code Fluent (Ansys Inc.), 4,800 cases have been used to examine the spreading thermal resistance tendencies for the different heat sink areas, heat source area, thickness, and heat transfer coefficient. In conclusion, the effective thermal conductivity increases when the thickness decreases or the heat sink area becomes greater with the same spreading thermal resistance value. In particular, the modified model of spreading thermal resistance is proposed for a better accuracy targeting various effective thermal conductivity, based on the original analytical model. A good agreement with a better accuracy is found between the model and the numerical results. This study will be able to provide the thermal system guideline for flat heat pipes.

Effect of integrated copper pad on the performance of boiling-driven wickless thermal ground plane

• A high-capacity thermal ground plane (TGP) with an integrated copper pad is developed. • Critical heat flux in high temperature conductive microporous coating is increased. • Dryout heat flux amplified in the flat and wickless TGP with the integrated copper pad. • Performance of the TGP with the copper pad remains orientation-independent. A solid copper pad is integrated into the heating side of a wickless copper thermal ground plane (TGP) with deionized water as the working fluid. Experimental TGP performance tests, along with pool boiling experiments and conduction thermal numerical simulations, are used to quantify the effect of the copper pad thickness on the overall thermal performance of the TGP. Even though adding the copper pad weakens the effective thermal conductivity of the TGP slightly, it improves the dryout within the TGP and allows operation at up to 1.6 times higher heat flux value. Boiling inside the TGP yields a reduced boiling heat transfer coefficient in the narrow gap, coupled with a higher dryout point due to the bubble-induced flow inside the TGP. The effective thermal conductivity remains 1.5 to 1.8 times higher than that of plain copper, and the weight of the TGP is approximately 60% of the weight of a solid copper block of the same size. The TGP is orientation-independent regardless of the pad thickness and water amounts.

Boiling-driven, wickless, and orientation-independent thermal ground plane

Dissipating heat from a concentrated heat source presents thermal challenges that often impose limits on the compact packaging of electronic devices. While vapor chambers, heat pipes, pulsating heat pipes, and other devices are available, a new concept of a wickless and flat thermal ground plane (TGP) is presented. Since the new concept does not rely on a wick, and its performance is based on boiling instead of evaporation heat transfer, the limits of the new TGP are related to boiling behaviors. Boiling bubbles, in areas close to the heat source, cause pumping of the fluid in the wickless TGP and promote convection two-phase heat transfer within the entire TGP, especially as the heat flux is increased. Following this new concept, a wickless thermal ground plane (80 mm × 80 mm × 1.6 mm-thick) is fabricated from flat copper sheets and its cavity is filled with degassed liquid water. The performance of the device is tested in dissipating concentrated heat from a 10 mm × 10 mm heat source. Depending on the operating temperature of 40 °C–80 °C, this TGP is experimentally proven to achieve a low thermal resistance of 0.2 K/W or better for a heat flux reaching 1500–2200 kW/m2. This performance is due to a copper high-temperature conductive microporous coating (Cu-HTCMC) on the inside of the heating surface. The TGP performs equally well in any orientation, offering an effective thermal conductivity close to twice that of copper. However, its weight is approximately 63% of the weight of a copper block of the same size. The concept and test results, based on the developed TGP, are discussed.

Development of ultrathin thermal ground plane with multiscale micro/nanostructured wicks

In this paper, TGPs with thickness of around 0.53–0.6 mm are developed with copper plate as the casing material, nanostructured copper foam as the wick and electroplated copper pillar as the support of vapor core. The effects of charge amount of water, nanostructured wick, the number of copper foam layers and vapor core thickness on the thermal performance of the developed TGP are experimentally studied. It is found that increasing the number of copper foam layers or vapor core thickness can greatly lower the TGP thermal resistance. Specifically, a TGP with a double layers copper foam wick has a much better thermal performance than that with single layer even though the total wick thickness and porosity remain unchanged. Experimental results also show that the multiscale micro/nanostructured wick is beneficial for improving the TGP thermal performance. Small change of vapor core thickness can lead to great change of TGP thermal performance. The ultrathin TGP can reach an effective thermal conductivity of 2207 W/(m·K) at an input power of 22.66 W over a heating area of 8 mm × 8 mm.

Bubble Behavior in Pool Boiling Heat Transfer between Two Plates with a Narrow Gap

Abstract The pool boiling phenomena, which may occur at high heat flux in the planar heat pipe such as vapor chamber and thermal ground plane (TGP), were visually investigated in the vertical orientation by capturing the bubble behavior within a narrow gap between two plates using distilled water at 1 atm. The size of two copper plates is 80×80 mm2 and the 10×10 mm2 ceramic heater, which simulates the high power chip, was bonded to the backside of the bottom plate. The gap between two plates was adjusted from 3 mm to 0.5 mm using spacers. It was observed that the generated bubble spreads laterally throughout the channel between the plates as decreasing the gap. Thin liquid film forms beneath the flattened bubble and the thin film evaporation improves heat transfer at the narrow gaps of 3 and 1 mm. However, further reduction of the gap down to 0.5 mm brings about an increase of the dried region, resulting in the deterioration of boiling heat transfer. [This work was supported by the Civil-Military Technology Cooperation Program of the Institute of Civil-Military Technology Cooperation (ICMTC), with a grant funded by the Defense Acquisition Program Administration and the Ministry of Trade, Industry and Energy (Grant No. 18CM5017).]

Design, Fabrication, and Performance Evaluation of a Hybrid Wick Vapor Chamber

The growing electrification of transportation systems is dramatically increasing the waste heat that must be dissipated from high-density power electronics. Transformative embedded heat spreading technologies must be developed in next-generation systems to enable air cooling of power semiconductors with heat fluxes exceeding 500 W/cm2 over large hotspot areas up to 1 cm2. In this study, vapor chamber heat spreaders, or thermal ground planes (TGPs), with customized wick structures are investigated as one possibility. A 10 cm × 10 cm TGP with hybrid wick, which is a blend of a biporous wick with a standard monoporous wick, was designed. The TGP was tested in combination with a straight pin fin heat sink under air jet impingement and a 1 cm2 size heat source. The experimental performance of the hybrid wick TGP was compared under the same air-cooled conditions with an off-the-shelf TGP of the same size from a commercial vendor and a TGP with a biporous wick only. The customized hybrid wick TGP exhibits ∼28% lower thermal resistance compared with a traditional commercial TGP, and the capillary limit heat flux is measured as 450 W/cm2. Technical challenges in extending this capillary limit heat flux value and TGP integration into packaged electronics are described.

Design of Thermal Ground Planes for Cooling of Foldable Smartphones

Foldable smartphones are expected to be widely commercialized in the near future. Thermal ground plane (TGP), known as vapor chamber or two-dimensional flat heat pipe, is a promising solution for the thermal management of foldable smartphones. There are two approaches to designing a TGP for foldable smartphones. One approach uses two TGPs connected by a graphite bridge and the other approach uses a single, large, and foldable TGP. In this study, different thermal management solutions are simulated for a representative foldable smartphone with screen dimensions of 144 × 138.3 mm2 (twice the screen of iPhone 6 s with a 10 mm gap). In addition, the simulation includes two heat sources representing a main processor with dimensions of 14.45 × 14.41 mm2 and power of 3.3 W (A9 processor in iPhone 6S) and a broadband processor with dimensions of 8.26 × 9.02 mm2 and power of 2.5 W (Qualcomm broadband processor). For the simulation, a finite element method (FEM) model is calibrated and verified by steady-state experiments of two different TGPs. The calibrated model is then used to study three different cases: a graphite heat spreader, two TGPs with a graphite hinge, and a single, large, and foldable TGP. In the fully unfolded configuration, using a graphite heat spreader, the temperature difference across the spreader's surface is about 17 °C. For the design using two TGPs connected by a graphite bridge, the temperature difference is about 7.2 °C. Finally, for the design using a single large TGP with a joint region, the temperature difference is only 1–2 °C. These results suggest that a single foldable TGP or a configuration with two TGPs outperform the graphite sheet solution for the thermal management of foldable smartphones.

Demonstration of a Pixel-Scale Superconducting Heat Switch for an Ideal Integrating Bolometer

The sensitivity of a bolometer is in principle limited by the phonon noise, which is a function of the conductance between its absorber and the thermal sink. However, practical issues limit the degree to which conductance can be minimized. For example, in a high-radiation environment, extremely low conductance, or, equivalently, long thermal recovery time, can lead to high data loss. To circumvent such limitations, we are developing a technique of modulating the conductance through the use of pixel-scale superconducting heat switches. We have fabricated bolometer-like test devices consisting of isolated regions connected to a thermal ground plane through narrow aluminum strips. Niobium coils are patterned around the aluminum strips. Flowing current through a coil allows the aluminum to be driven into the normal or high thermal conductance state. We have measured the conductance of these devices as a function of temperature and drive coil current.

Flexible Thermal Ground Planes Fabricated With Printed Circuit Board Technology

Thermal ground planes (TGPs) are passive thermal management devices that utilize the phase-change of a working fluid to achieve high thermal conductivity and low thermal resistance. TGPs are flat, two-dimensional heat pipes—similar to vapor chambers—in which liquid is held within a capillary wick, and vapor is held in a sealed vapor layer. Heat is absorbed at an evaporator region, causing the liquid to evaporate. The heated vapor in the vapor core is carried via convection to a condenser region where it condenses as the heat is expelled from the TGP to an external heat sink. The condensed liquid is then pulled back to the evaporator via capillary forces in the wick. In numerous applications, mechanical flexibility of the TGP is required, as is low-cost manufacturing and viable integration routes with electronics. This work describes a flexible TGP (FTGP) fabricated using printed circuit board (PCB) technology, in which commercially available copper-cladded polyimide sheets are used as the casing material. The wick is composed of three layers of fine copper mesh electroplated or sintered together and coated with atomic layer deposited TiO2. A coarse nylon or polyether ether ketone (PEEK) mesh defines the vapor transport layer, and water is used as the working fluid. The perimeter of the device is heat-sealed with flouroethylene propylene (FEP), which has been found to provide a near-hermetic seal for several months and is suitable for flexible applications. This architecture allows the TGP to function with minimal reduction in heat transfer performance while bent by 90 deg, and full functionality is returned when the device is returned to its flat configuration. The FTGP's measured thermal resistance is about half that of an equivalent copper reference for input heat fluxes of 3–6 W/cm2. More than 30 copper-cladded polyimide FTGPs were fabricated and characterized using both simple qualitative and more involved quantitative test setups. The results show that the fabrication and assembly processes developed in this work are repeatable and the devices are durable.

Development of Ultra-Thin Thermal Ground Planes by Using Stainless-Steel Mesh as Wicking Structure

Ultra-thin thermal ground planes (TGPs), which utilize phase-change heat transfer to achieve high performance heat spreading, are of great importance for thermal management of thickness-constrained microsystems. However, both fabrication challenges and performance limitation are encountered when the TGP thickness reduces to 0.5 mm. In this letter, we introduce an ultra-thin TGP by bonding single-layer stainless steel mesh with copper micropillars via electroplating to form a hybrid wicking structure, which enables both high capillary pressure and low flow resistance. Copper-cladded kapton is used as the casing material, and electroplated copper pillars are used to support the casing over a vapor space. The assembled TGP has an active area of 10 cm × 5 cm with a total thickness of 0.28 mm. This TGP can operate with a heat load up to 7.9 W over a heating area of 8 mm × 8 mm, rendering an effective thermal conductivity of 1398 W/m-K.

Probabilistic distributions of pinhole defects in atomic layer deposited films on polymeric substrates

Pinhole defects in atomic layer deposition (ALD) coatings were measured in an area of 30 cm2 in an ALD reactor, and these defects were represented by a probabilistic cluster model instead of a single defect density value with number of defects over area. With the probabilistic cluster model, the pinhole defects were simulated over a manufacturing scale surface area of ∼1 m2. Large-area pinhole defect simulations were used to develop an improved and enhanced design method for ALD-based devices. A flexible thermal ground plane (FTGP) device requiring ALD hermetic coatings was used as an example. Using a single defect density value, it was determined that for an application with operation temperatures higher than 60 °C, the FTGP device would not be possible. The new probabilistic cluster model shows that up to 40.3% of the FTGP would be acceptable. With this new approach the manufacturing yield of ALD-enabled or other thin film based devices with different design configurations can be determined. It is important to guide process optimization and control and design for manufacturability.

Thin Flexible Thermal Ground Planes: Fabrication and Scaling Characterization

Thermal ground planes (TGPs) are passive thermal management devices that utilize the latent heat associated with phase change to achieve high effective thermal conductance, similar to heat pipes. In this paper, we develop flexible TGPs with an ultra-thin thickness of 0.5 mm using copper-cladded polyimide as the encasing material, woven copper mesh as a wick, and electroplated copper pillars to support a vapor core. The lowest thermal resistance of one TGP is characterized to be only 1/3 that of an equivalently sized copper heat spreader. The effects of size scaling of evaporator and condenser, and overall TGP sizes on the thermal resistances of TGPs are experimentally characterized. A simple series thermal resistance model, which accounts for vapor core thermal resistance, is developed to predict the measured results. This experimentally validated model can be used for the design of TGPs with varying sizes of evaporator and condenser, and overall size. [2015-0032]

Microfabricated ultra-thin all-polymer thermal ground planes

Thermal ground planes, or planar heat pipes, can provide highly effective heat transfer by utilizing phase change of an encapsulated fluid. In this article, a flexible thermal ground plane (FTGP) was fabricated using polymer materials. Kapton was employed as a casing material while micropatterned SU-8 was used to provide both a liquid wicking structure and pillars to support the casing over a vapor core. An ultra-thin TiO2 film was deposited over the SU-8 and Kapton via atomic layer deposition, which acted as both a moisture barrier and a hydrophilic coating on polymer surfaces. The assembled FTGP has a thickness of 0.30mm, an active area of 20mm × 60mm, heater area of 20mm × 10mm, and can operate with a heat load up to 9.54W, with an effective thermal conductivity up to 541W/(m K).

Two-Phase Thermal Ground Planes: Technology Development and Parametric Results

Defense Advanced Research Project Agency's (DARPA's) thermal ground plane (TGP) effort was aimed at combining the advantages of vapor chambers or two-dimensional (2D) heat pipes and solid conductors by building thin, high effective thermal conductivity, flat heat pipes out of materials with thermal expansion coefficients that match current electronic devices. In addition to the temperature uniformity and minimal load-driven temperature variations associated with such two phase systems, in their defined parametric space, flat heat pipes are particularly attractive for Department of Defense and commercial systems because they offer a passive, reliable, light-weight, and low-cost path for transferring heat away from high power dissipative components. However, the difference in thermal expansion coefficients between silicon or ceramic microelectronic components and metallic vapor chambers, as well as the need for a planar, chip-size attachment surface for these devices, has limited the use of commercial of the shelf flat heat pipes in this role. The primary TGP goal was to achieve extreme lateral thermal conductivity, in the range of 10 kW/mK–20 kW/mK or approximately 25–50 times higher than copper and 10 times higher than synthetic diamond, with a thickness of 1 mm or less.

Thermal Modeling and Experimental Validation for High Thermal Conductivity Heat Pipe Thermal Ground Planes

Abstract Thermal ground planes (TGPs) are flat, thin (external thickness of 2 mm) heat pipes which utilize two-phase cooling. The goal is to utilize TGPs as thermal spreaders in a variety of microelectronic cooling applications. In addition to TGPs and flat heat pipes, some investigators refer to similar devices as vapor chambers. TGPs are novel high-performance, integrated systems able to operate at a high power density with a reduced weight and temperature gradient. In addition to being able to dissipate large amounts of heat, they have very high effective axial thermal conductivities and (because of nanoporous wicks) can operate in high adverse gravitational fields. A three-dimensional (3D) finite element model is used to predict the thermal performance of the TGP. The 3D thermal model predicts the temperature field in the TGP, the effective axial thermal conductivity, and the evaporation and the condensation rates. A key feature of this model is that it relies on empirical interfacial heat transfer coefficient data to very accurately model the interfacial energy balance at the vapor–liquid saturated wick interface. Wick samples for a TGP are tested in an experimental setup to measure the interfacial heat transfer coefficient. Then the experimental heat transfer coefficient data are used for the interfacial energy balance. Another key feature of this model is that it demonstrates that for the Jakob numbers of interest, the thermal and flow fields can be decoupled except at the vapor–liquid saturated wick interface. This model can be used to predict the performance of a TGP for different geometries and implementation structures. This paper will describe the model and how it incorporates empirical interfacial heat transfer coefficient data. It will then show theoretical predictions for the thermal performance of TGP's, and compare with experimental results.

Thermal-Fluid Modeling For High Thermal Conductivity Heat Pipe Thermal Ground Planes

The thermal ground plane is an advanced planar heat pipe designed for cooling microelectronics in high gravitational fields. A thermal resistance model is developed to predict the thermal performance of the thermal ground plane, including the effects of the presence of noncondensable gases. Viscous laminar flow pressure losses are predicted to determine the maximum heat load when the capillary limit is reached. This paper shows that the axial effective thermal conductivity of the thermal ground plane decreases when the substrate and/or wick are thicker and/or with the presence of noncondensable gases. Moreover, it was demonstrated that the thermal-fluid model may be used to optimize the performance of the thermal ground plane by estimating the limits of wick thickness and vapor space thickness for a recognized internal volume of the thermal ground plane. The wick porosity plays a significant role in maximum heat transport capability. A large adverse gravitational field strongly decreases the maximum heat transport capability of the thermal ground plane. Axial effective thermal conductivity is mostly unaffected by the gravitational field. The maximum length of the thermal ground plane prior to reaching the capillary limit is inversely proportional to input power.