Specialized in phase-change heat transfer, thermal management of power electronics, development of ultra-thin vapor chamber and battery modeling and control.
Controlling the solid-liquid-vapor tri-phase interface is of fundamental importance for a broad r... more Controlling the solid-liquid-vapor tri-phase interface is of fundamental importance for a broad range of industrial applications ranging from biomedical engineering, energy production and utilization, environmental control, water production, and thermal management. Despite that a lot of progress has been made over the past few decades on surface manipulation for promoting droplet removal, it is challenging to accelerate both droplet growth and surface refreshing for enhancing vapor-to-liquid condensation. Here we present a superhydrophobic hierarchical mesh-covered (hi-mesh) surface to
Stable and efficient jumping droplet condensation is demonstrated on 3D superhydrophobic copper n... more Stable and efficient jumping droplet condensation is demonstrated on 3D superhydrophobic copper nanowire networks. Due to the interconnections among nanowires, the micro-defects in straight nanowire arrays are eliminated while the spacing between neighboring nanowires is reduced. By spatially controlling nucleation for highly mobile droplets, 100% higher heat flux is achieved compared with that on the state-of-the-art hydrophobic surface. The remarkable water repellency of 3D nanowire networks can be applied to a broad range of water-harvesting and phase-change heat transfer applications.
Thermal ground planes, or planar heat pipes, can provide highly effective heat transfer by utiliz... more 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 TiO 2 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 0.30 mm, an active area of 20 mm 9 60 mm, heater area of 20 mm 9 10 mm, and can operate with a heat load up to 9.54 W, with an effective thermal conductivity up to 541 W/(m K).
— Thermal ground planes (TGPs) are passive thermal management devices that utilize the latent hea... more — 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] Index Terms— Vapor chamber, thermal ground plane, flexible heat pipe, thermal management.
www.advmat.de www.MaterialsViews.com Figure 4. Specifi c charge capacities and Coulombic effi cie... more www.advmat.dewww.MaterialsViews.com Figure 4. Specifi c charge capacities and Coulombic effi ciencies of SiNW-cPAN/L33 full-cells assembled with PYR 13 FSI (1.2 M LiFSI) and conventional EC/DEC (1 M LiPF 6 ) electrolytes. 193 wileyonlinelibrary.com
Thermal management of high power electronic systems such as advanced lasers, light emitting diode... more Thermal management of high power electronic systems such as advanced lasers, light emitting diodes, radars, microprocessors, electrical machines, and power inverters, is becoming critical in defense, space, and commercial applications. As a liquid-vapor phase-change process, capillary evaporation on wicking structures has received increased interest owing to its capability dissipating high heat flux by increasing the effective evaporation area and sustaining the liquid supply. However, the approaches based on capillary evaporation suffer from intrinsic tradeoffs between the low thermal resistance for heat flow and the high wicking flow rate for liquid supply, which make it challenging to optimize both heat transfer coefficient (HTC) and critical heat flux (CHF) on the same wicking structure. Here, we present a cost-effective hybrid wicking structure, that can be scalably manufactured using commercial copper micromeshes along with simple etching processes, to enable a novel capillary-driven liquid film boiling heat transfer by simultaneously improving liquid supply and increasing nucleation sites. The interconnected microchannels between woven mesh layers and the substrate serve as low flow resistance passages for high volumetric flow rate of the liquid. Through creating nanograsses and micro-cavities on the multilayer micromeshes, the liquid flow is enhanced for higher CHF due to the increased surface wettability. Moreover, a plural of microcavities on the mesh surface decrease the superheat for early onset of nucleate boiling (ONB), resulting in higher HTC. Enhanced CHF (198.6 W/cm 2) and HTC (138.7 kW/m 2 K) are observed on the hybrid mesh wicking structure with a 10 mm × 10 mm heated area due to more than twice the flow rate for liquid supply and more than one third reduced superheat at the ONB, compared to that on the plain mesh wicking structure. The insights gained from this study can be used to guide the design and manufacturing of highly efficient wicking structures in heat pipes, vapor chambers, and other high flux thermal systems.
Stable and efficient jumping droplet condensation is demonstrated on 3D superhydrophobic copper n... more Stable and efficient jumping droplet condensation is demonstrated on 3D superhydrophobic copper nanowire networks. Due to the interconnections among nanowires, the micro-defects in straight nanowire arrays are eliminated while the spacing between neighboring nanowires is reduced. By spatially controlling nucleation for highly mobile droplets, 100% higher heat flux is achieved compared with that on the state-of-the-art hydrophobic surface. The remarkable water repellency of 3D nanowire networks can be applied to a broad range of water-harvesting and phase-change heat transfer applications.
— Ultra-thin thermal ground planes (TGPs), which utilize phase-change heat transfer to achieve hi... more — 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. [2016-0097]
Condensation is a ubiquitous phase-change phenomenon and has been widely used in energy-intensive... more Condensation is a ubiquitous phase-change phenomenon and has been widely used in energy-intensive industrial applications. By promoting self-propelled droplet jumping, superhydrophobic surfaces offer an avenue to improve condensation heat transfer performance. However, the tendency of condensed droplets to form as pinned states greatly limits its applicability in condensation heat transfer, especially under large surface subcooling. Here, we report the development of a micromesh-covered superhydrophobic surface via a simple fabrication method that allows for high-efficiency jumping condensation at small surface subcooling and the continuous droplet suction-enhanced condensation at large surface subcooling. We show that the cooperation of droplet jumping and suction effect can lead to enhanced heat transfer performance throughout the large range of surface subcooling: an 80% higher heat flux at ΔT <; 4 K and a 35% higher heat flux at 4 K <; ΔT <; 30 K compared to dropwise condensation on the state-to-the-art plain hydrophobic surface. The new insights about the cost-effective surface fabrication and the novel condensation modes induced by the integration of droplet jumping and suction can guide the development of new technology for enhancing phase-change heat transfer.
Self-propelled droplet jumping on nanostruc-tured superhydrophobic surfaces is of interest for a ... more Self-propelled droplet jumping on nanostruc-tured superhydrophobic surfaces is of interest for a variety of industrial applications including self-cleaning, water harvesting, power generation, and thermal management systems. However , the uncontrolled nucleation-induced Wenzel state of condensed droplets at large surface subcooling (high heat flux) leads to the formation of unwanted large pinned droplets, which results in the flooding phenomenon and greatly degrades the heat transfer performance. In this work, we present a novel strategy to manipulate droplet behaviors during the process from the droplet nucleation to growth and departure through a combination of spatially controlling initial nucleation for mobile droplets by closely spaced nanowires and promoting the spontaneous outward movement of droplets for rapid removal using micropatterned nanowire arrays. Through the optical visualization experiments and heat transfer tests, we demonstrate greatly improved condensation heat transfer characteristics on the hierarchical superhydrophobic surface including the higher density of microdroplets, smaller droplet departure radius, 133% wider range of surface subcooling for droplet jumping, and 37% enhancement in critical heat flux for jumping droplet condensation, compared to the-state-of-art jumping droplet condensation on nanostructured superhydrophobic surfaces. The excellent water repellency of such hierarchical superhydrophobic surfaces can be promising for many potential applications, such as anti-icing, antifogging, water desalination, and phase-change heat transfer.
—SiC and GaN power electronic devices enable smaller die sizes and increase power densities with ... more —SiC and GaN power electronic devices enable smaller die sizes and increase power densities with high heat fluxes ranging from 100 to 1,000 W/cm 2 , which calls for efficient thermal management solutions with matched ultra-thin form factors. Thermal ground plane is a vapor chamber fabricated using printed circuit board technologies. It is recognized as one of the most effective heat spreaders to dissipate high heat fluxes. Similar to heat pipes, thermal ground planes rely on self-sustaining capillary evaporation and condensation cycle. Nanostructures are being explored for improving such phase-change heat transfer devices; however, these nanoscale solutions are expensive and susceptible to reliability concerns. Here, we develop a low-cost and reliable thermal ground plane enabled by commercial copper micromesh wicking structures with enhanced capillary evaporation heat transfer, which can remove high heat fluxes, up to 425 W/cm 2 .
Controlling the solid-liquid-vapor tri-phase interface is of fundamental importance for a broad r... more Controlling the solid-liquid-vapor tri-phase interface is of fundamental importance for a broad range of industrial applications ranging from biomedical engineering, energy production and utilization, environmental control, water production, and thermal management. Despite that a lot of progress has been made over the past few decades on surface manipulation for promoting droplet removal, it is challenging to accelerate both droplet growth and surface refreshing for enhancing vapor-to-liquid condensation. Here we present a superhydrophobic hierarchical mesh-covered (hi-mesh) surface to
Stable and efficient jumping droplet condensation is demonstrated on 3D superhydrophobic copper n... more Stable and efficient jumping droplet condensation is demonstrated on 3D superhydrophobic copper nanowire networks. Due to the interconnections among nanowires, the micro-defects in straight nanowire arrays are eliminated while the spacing between neighboring nanowires is reduced. By spatially controlling nucleation for highly mobile droplets, 100% higher heat flux is achieved compared with that on the state-of-the-art hydrophobic surface. The remarkable water repellency of 3D nanowire networks can be applied to a broad range of water-harvesting and phase-change heat transfer applications.
Thermal ground planes, or planar heat pipes, can provide highly effective heat transfer by utiliz... more 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 TiO 2 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 0.30 mm, an active area of 20 mm 9 60 mm, heater area of 20 mm 9 10 mm, and can operate with a heat load up to 9.54 W, with an effective thermal conductivity up to 541 W/(m K).
— Thermal ground planes (TGPs) are passive thermal management devices that utilize the latent hea... more — 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] Index Terms— Vapor chamber, thermal ground plane, flexible heat pipe, thermal management.
www.advmat.de www.MaterialsViews.com Figure 4. Specifi c charge capacities and Coulombic effi cie... more www.advmat.dewww.MaterialsViews.com Figure 4. Specifi c charge capacities and Coulombic effi ciencies of SiNW-cPAN/L33 full-cells assembled with PYR 13 FSI (1.2 M LiFSI) and conventional EC/DEC (1 M LiPF 6 ) electrolytes. 193 wileyonlinelibrary.com
Thermal management of high power electronic systems such as advanced lasers, light emitting diode... more Thermal management of high power electronic systems such as advanced lasers, light emitting diodes, radars, microprocessors, electrical machines, and power inverters, is becoming critical in defense, space, and commercial applications. As a liquid-vapor phase-change process, capillary evaporation on wicking structures has received increased interest owing to its capability dissipating high heat flux by increasing the effective evaporation area and sustaining the liquid supply. However, the approaches based on capillary evaporation suffer from intrinsic tradeoffs between the low thermal resistance for heat flow and the high wicking flow rate for liquid supply, which make it challenging to optimize both heat transfer coefficient (HTC) and critical heat flux (CHF) on the same wicking structure. Here, we present a cost-effective hybrid wicking structure, that can be scalably manufactured using commercial copper micromeshes along with simple etching processes, to enable a novel capillary-driven liquid film boiling heat transfer by simultaneously improving liquid supply and increasing nucleation sites. The interconnected microchannels between woven mesh layers and the substrate serve as low flow resistance passages for high volumetric flow rate of the liquid. Through creating nanograsses and micro-cavities on the multilayer micromeshes, the liquid flow is enhanced for higher CHF due to the increased surface wettability. Moreover, a plural of microcavities on the mesh surface decrease the superheat for early onset of nucleate boiling (ONB), resulting in higher HTC. Enhanced CHF (198.6 W/cm 2) and HTC (138.7 kW/m 2 K) are observed on the hybrid mesh wicking structure with a 10 mm × 10 mm heated area due to more than twice the flow rate for liquid supply and more than one third reduced superheat at the ONB, compared to that on the plain mesh wicking structure. The insights gained from this study can be used to guide the design and manufacturing of highly efficient wicking structures in heat pipes, vapor chambers, and other high flux thermal systems.
Stable and efficient jumping droplet condensation is demonstrated on 3D superhydrophobic copper n... more Stable and efficient jumping droplet condensation is demonstrated on 3D superhydrophobic copper nanowire networks. Due to the interconnections among nanowires, the micro-defects in straight nanowire arrays are eliminated while the spacing between neighboring nanowires is reduced. By spatially controlling nucleation for highly mobile droplets, 100% higher heat flux is achieved compared with that on the state-of-the-art hydrophobic surface. The remarkable water repellency of 3D nanowire networks can be applied to a broad range of water-harvesting and phase-change heat transfer applications.
— Ultra-thin thermal ground planes (TGPs), which utilize phase-change heat transfer to achieve hi... more — 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. [2016-0097]
Condensation is a ubiquitous phase-change phenomenon and has been widely used in energy-intensive... more Condensation is a ubiquitous phase-change phenomenon and has been widely used in energy-intensive industrial applications. By promoting self-propelled droplet jumping, superhydrophobic surfaces offer an avenue to improve condensation heat transfer performance. However, the tendency of condensed droplets to form as pinned states greatly limits its applicability in condensation heat transfer, especially under large surface subcooling. Here, we report the development of a micromesh-covered superhydrophobic surface via a simple fabrication method that allows for high-efficiency jumping condensation at small surface subcooling and the continuous droplet suction-enhanced condensation at large surface subcooling. We show that the cooperation of droplet jumping and suction effect can lead to enhanced heat transfer performance throughout the large range of surface subcooling: an 80% higher heat flux at ΔT <; 4 K and a 35% higher heat flux at 4 K <; ΔT <; 30 K compared to dropwise condensation on the state-to-the-art plain hydrophobic surface. The new insights about the cost-effective surface fabrication and the novel condensation modes induced by the integration of droplet jumping and suction can guide the development of new technology for enhancing phase-change heat transfer.
Self-propelled droplet jumping on nanostruc-tured superhydrophobic surfaces is of interest for a ... more Self-propelled droplet jumping on nanostruc-tured superhydrophobic surfaces is of interest for a variety of industrial applications including self-cleaning, water harvesting, power generation, and thermal management systems. However , the uncontrolled nucleation-induced Wenzel state of condensed droplets at large surface subcooling (high heat flux) leads to the formation of unwanted large pinned droplets, which results in the flooding phenomenon and greatly degrades the heat transfer performance. In this work, we present a novel strategy to manipulate droplet behaviors during the process from the droplet nucleation to growth and departure through a combination of spatially controlling initial nucleation for mobile droplets by closely spaced nanowires and promoting the spontaneous outward movement of droplets for rapid removal using micropatterned nanowire arrays. Through the optical visualization experiments and heat transfer tests, we demonstrate greatly improved condensation heat transfer characteristics on the hierarchical superhydrophobic surface including the higher density of microdroplets, smaller droplet departure radius, 133% wider range of surface subcooling for droplet jumping, and 37% enhancement in critical heat flux for jumping droplet condensation, compared to the-state-of-art jumping droplet condensation on nanostructured superhydrophobic surfaces. The excellent water repellency of such hierarchical superhydrophobic surfaces can be promising for many potential applications, such as anti-icing, antifogging, water desalination, and phase-change heat transfer.
—SiC and GaN power electronic devices enable smaller die sizes and increase power densities with ... more —SiC and GaN power electronic devices enable smaller die sizes and increase power densities with high heat fluxes ranging from 100 to 1,000 W/cm 2 , which calls for efficient thermal management solutions with matched ultra-thin form factors. Thermal ground plane is a vapor chamber fabricated using printed circuit board technologies. It is recognized as one of the most effective heat spreaders to dissipate high heat fluxes. Similar to heat pipes, thermal ground planes rely on self-sustaining capillary evaporation and condensation cycle. Nanostructures are being explored for improving such phase-change heat transfer devices; however, these nanoscale solutions are expensive and susceptible to reliability concerns. Here, we develop a low-cost and reliable thermal ground plane enabled by commercial copper micromesh wicking structures with enhanced capillary evaporation heat transfer, which can remove high heat fluxes, up to 425 W/cm 2 .
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Papers by Shanshan Xu