Kwon, Beomjin
Maniscalco, Nicholas I.
Jacobi, Anthony M.
King, William P.
This paper reports two-phase cooling in compact cross-flow microchannel heat exchangers with high power density up to 180 W/cm(3). The performance is enabled by high-speed air flow through microchannels and two-phase condensation of refrigerant R245fa. The heat exchangers were realized in 1 cm(3) blocks of copper alloy, using micro-electrical-discharging machining. Two heat exchanger designs were analyzed, fabricated, and tested. The first device has 150 air-side channels of diameter 520 mu m, and the second device has 300 air-side channels of diameter 355 mu m. In both cases the refrigerant channels are 2.0 x 0.5 mm(2). The heat exchangers were operated with Reynolds number between 7500 and 20,500 for the air flow and with mass flux between 330 and 750 kg/m(2) s for the refrigerant flow. The refrigerant temperature at the channel entrance was 80 degrees C, which is near the maximum operating temperature for some electronic devices. For comparison purposes, the devices were also tested with single-phase refrigerant flows. This work demonstrates the potential of high power density heat exchangers that leverage advanced manufacturing technologies to fabricate miniature channels.
Kwon, Beomjin
Maniscalco, Nicholas I.
Jacobi, Anthony M.
King, William P.
We present single-phase heat transfer in a compact cross-flow microchannel heat exchanger, with air flowing through the heat exchanger to remove heat from a closed-loop flow of refrigerant R245fa. The 1 cm(3) heat exchanger was monolithically fabricated from a block of copper alloy using micro electrical-discharge machining. Air carrying channels of diameter 520 gm were oriented in cross-flow to the refrigerant-carrying channels of size 2.0 x 0.5 mm(2). High-speed air flowed with Reynolds number between 1.2 x 10(4) and 2.05 x 10(4), which corresponded to air speeds between 20 and 100 m/s, while refrigerant flowed at Reynolds number between 1000 and 2300. Using an equivalent fin model and finite element simulations, we predicted the heat exchanger performance and used the simulations to interpret the measured behavior. Temperature, pressure, and flow rates were measured over a variety of operating conditions to determine heat transfer rate, j-factor, and friction factor. We observed a maximum power density of 60 W/cm(3) when the air inlet temperature was 27 degrees C and the refrigerant inlet temperature was 80 degrees C. The high speed of air flow caused large friction on the air side, resulting in goodness factor j/f near 0.5. This work demonstrates that high power density can be achieved in miniature heat exchangers, and that micromachined metal devices can enable this performance. The results could be broadly applied to other types of microchannel devices. (C) 2017 Elsevier Ltd. All rights reserved.
Dong, Liang
King, William P.
Raleigh, Mark
Wadley, Haydn N. G.
A scalable technique for making octet microlattice topology mechanical metamaterials with a cell size of 1 mm or less from thin stainless steel sheets is described. The microfabrication process used a perforation operation to form planar truss sheets by the periodic removal of rectangular shaped material. A simultaneous perforation and corrugation operation was also used to create a single layer of trusses with a pyramidal topology. A 3D lattice was then assembled by alternatively stacking the pyramidal and planar truss sheets, taking care to align their nodes to form the octet lattice truss topology. A vacuum brazing method was used to metallurgically bond the assembly. The compressive properties of the microlattices are shown to be comparable to those of much larger cell-size structures of similar material and topology. An assessment of geometric imperfection sensitivity indicates where further mechanical property improvements could be realized by improving the precision of both the perforation and assembly process. (C) 2018 Elsevier Ltd.
Yang, Tianyu
Kwon, Beomjin
Weisensee, Patricia B.
Kang, Jin Gu
Li, Xuejiao
Braun, Paul
Miljkovic, Nenad
King, William P.
Devices capable of actively controlling heat flow have been desired by the thermal management community for decades. The need for thermal control has become particularly urgent with power densification resulting in devices with localized heat fluxes as high as 1 kW/cm(2). Thermal switches, capable of modulating between high and low thermal conductances, enable the partitioning and active control of heat flow pathways. This paper reports a millimeter-scale thermal switch with a switching ratio >70, at heat fluxes near 10 W/cm(2). The device consists of a silicone channel filled with a reducing liquid or vapor and an immersed liquid metal Galinstan slug. Galinstan has a relatively high thermal conductivity (approximate to 16.5W/mK at room temperature), and its position can be manipulated within the fluid channel, using either hydrostatic pressure or electric fields. When Galinstan bridges the hot and cold reservoirs (the "ON" state), heat flows across the channel. When the hot and cold reservoirs are instead filled with the encapsulating liquid or vapor (the "OFF" state), the cross-channel heat flow significantly reduces due to the lower thermal conductivity of the solution (approximate to 0.03-0.6 W/mK). We demonstrate switching ratios as high as 15.6 for liquid filled channels and 71.3 for vapor filled channels. This work provides a framework for the development of millimeter-scale thermal switches and diodes capable of spatial and temporal control of heat flows. Published by AIP Publishing.
Weisensee, Patricia B.
Wang, Yunbo
Qian Hongliang
Schultz, Daniel
King, William P.
Miljkovic, Nenad
Condensation is a ubiquitous phenomenon in nature and industry. Heat transfer rates during dropwise condensation on non-wetting substrates can be 6-8X higher than heat transfer rates during traditional filmwise condensation on wetting substrates. Dropwise condensation on lubricant-infused surfaces (LIS, or SLIPS) is particularly interesting due to high droplet mobility on these surfaces. To accurately predict heat transfer rates during dropwise condensation, the distribution of droplet sizes must be known. Here we present condensation studies of water on aluminum-based lubricant-infused surfaces with a wide range of lubricant viscosities (12-2717 cSt) to determine droplet size distributions. Through optical imaging and microscopy, we show that the distribution of droplet sizes on LIS is independent of lubricant viscosity, and agrees well with the model developed by Rose for the distribution of droplet sizes on hydrophobic surfaces, especially in the range 10 < r < 100 mu m. Using artificial sweeping experiments and numerical modeling, we investigate the dependence of sweeping rates on the distribution of droplet sizes and on average heat transfer rates. The maximum size to which droplets grow before being swept decreases rapidly with only a modest decrease in sweeping period, from 750 to 62 mu m. Yet, the distribution of droplet sizes and heat transfer rates are nearly unaffected by the change in sweeping period, due to a relative insensitivity of heat transfer to droplets with radii r > 100 mu m due to a high conduction resistance within these droplets. Our work provides an experimental and analytical framework to predict heat transfer and sweeping rates for water condensation on a vertical plate coated with a LIS or SLIPS surface. (C) 2017 Elsevier Ltd. All rights reserved.
High power density microbatteries could enable new capabilities for miniature sensors, radios, and industrial electronics. There is, however, a lack of understanding on how battery architecture and materials limit power performance when battery discharge rates exceed 100 C. This paper describes the development and application of an electrochemical model to predict the performance of microbatteries having interdigitated bicontinuous microporous electrodes, discharged at up to 600 C rates. We compare predicted battery behavior with measurements, and use the model to explore the underlying physics. The model shows that diffusion through the solid electrodes governs microbattery power performance. We develop design rules that could guide the development of improved batteries. (C) The Author(s) 2017. Published by ECS. All rights reserved.
Estrada, David
Li, Zuanyi
Choi, Gyung-Min
Dunham, Simon N.
Serov, Andrey
Lee, Jungchul
Meng, Yifei
Lian, Feifei
Wang, Ning C.
Perez, Alondra
Haasch, Richard T.
Zuo, Jian-Min
King, William P.
Rogers, John A.
Cahill, David G.
Pop, Eric
Estrada, David
Li, Zuanyi
Choi, Gyung-Min
Dunham, Simon N.
Serov, Andrey
Lee, Jungchul
Meng, Yifei
Lian, Feifei
Wang, Ning C.
Perez, Alondra
Haasch, Richard T.
Zuo, Jian-Min
King, William P.
Rogers, John A.
Cahill, David G.
Pop, Eric
New technologies are emerging which allow us to manipulate and assemble 2-dimensional (2D) building blocks, such as graphene, into synthetic van der Waals (vdW) solids. Assembly of such vdW solids has enabled novel electronic devices and could lead to control over anisotropic thermal properties through tuning of inter-layer coupling and phonon scattering. Here we report the systematic control of heat flow in graphene-based vdW solids assembled in a layer-by-layer (LBL) fashion. In-plane thermal measurements (between 100 K and 400 K) reveal substrate and grain boundary scattering limit thermal transport in vdW solids composed of one to four transferred layers of graphene grown by chemical vapor deposition (CVD). Such films have room temperature in-plane thermal conductivity of similar to 400 Wm(-1) K-1. Cross-plane thermal conductance approaches 15 MWm(-2) K-1 for graphene-based vdW solids composed of seven layers of graphene films grown by CVD, likely limited by rotational mismatch between layers and trapped particulates remnant from graphene transfer processes. Our results provide fundamental insight into the in-plane and cross-plane heat carrying properties of substrate-supported synthetic vdW solids, with important implications for emerging devices made from artificially stacked 2D materials.
Pikul, James H.
Ozerinc, Sezer
Liu, Burigede
Zhang, Runyu
Braun, Paul V.
Deshpande, Vikram S.
King, William P.
This paper describes a nickel-based cellular material, which has the strength of titanium and the density of water. The material's strength arises from size-dependent strengthening of load-bearing nickel struts whose diameter is as small as 17 nm and whose 8 GPa yield strength exceeds that of bulk nickel by up to 4X. The mechanical properties of this material can be controlled by varying the nanometer-scale geometry, with strength varying over the range 90-880 MPa, modulus varying over the range 14-116 GPa, and density varying over the range 880-14500 kg/m(3). We refer to this material as a "metallic wood," because it has the high mechanical strength and chemical stability of metal, as well as a density close to that of natural materials such as wood.
Chen, Sihan
Kim, SunPhil
Chen, Weibing
Yuan, Jiangtan
Bashir, Rashid
Lou, Jun
van der Zande, Arend M.
King, William P.
Monolayer MoS2 is a promising material for nanoelectronics; however, the lack of nanofabrication tools and processes has made it very challenging to realize nanometer-scale electronic devices from monolayer MoS2. Here, we demonstrate the fabrication of monolayer MoS2 nanoribbon field-effect transistors as narrow as 30 nm using scanning probe lithography (SPL). The SPL process uses a heated nanometer-scale tip to deposit narrow nanoribbon polymer structures onto monolayer MoS2. The polymer serves as an etch mask during a XeF2 vapor etch, which defines the channel of a field-effect transistor (FET). We fabricated seven devices with a channel width ranging from 30 to 370 nm, and the fabrication process was carefully studied by electronic measurements made at each process step. The nanoribbon devices have a current on/off ratio > 10(4) and an extrinsic field-effect mobility up to 8.53 cm(2)/(V s). By comparing a 30 nm wide device with a 60 nm wide device that was fabricated on the same MoS2 flake, we found the narrower device had a smaller mobility, a lower on/off ratio, and a larger subthreshold swing. To our knowledge, this is the first published work that describes a working transistor device from monolayer MoS2 with a channel width smaller than 100 nm.
A manufacturing apparatus for manufacturing extruded parts having microstructures comprising: a support structure; a hopper carried by the support structure for receiving feedstock; an extrusion chamber operatively associated with the hopper for receiving the feedstock from the hopper and melting the feedstock above a feedstock melting temperature; a die carried by the support structure having die microstructures disposed on an inner surface of the die, the die microstructures having a plurality of microfeatures each having an upper surface and a lower surface, the melted feedstock being forced through the die to produce an extrudate having extrudate microstructures; and, a cooling assembly wherein the extrudate microstructures of the pre-cooled extrudate have larger physical dimensions than that of the extrudate microstructures of the cooled extrudate.
Park, Ji Yong
Gardner, Andrew
King, William P.
Cahill, David G.
We use pump-probe thermal transport measurements and high speed imaging to study the residence time and heat transfer of small (360 mu m diameter) water droplets that bounce from hydrophobic surfaces whose temperature exceeds the boiling point. The structure of the hydrophobic surface is a 10 nm thick fluorocarbon coating on a Si substrate; the Si substrate is also patterned with micron-scale ridges using photolithography to further increase the contact angle. The residence time determined by high-speed imaging is constant at approximate to 1 ms over the temperature range of our study, 110 < T < 210 degrees C. Measurements of the thermal conductance of the interface show that the time of intimate contact between liquid water and the hydrophobic surface is reduced by the rapid formation of a vapor layer and reaches a minimum value of approximate to 0.025 ms at T > 190 degrees C. We tentatively associate this time-scale with a similar to 1 m s(-1) velocity of the liquid/vapor/solid contact line. The amount of heat transferred during the impact, normalized by the droplet volume, ranges from 0.028 J mm(-3) to 0.048 J mm(-3) in the temperature range 110 < T < 210 degrees C. This amount of heat transfer is approximate to 1-2% of the latent heat of evaporation.
Cahill, David G.
Braun, Paul V.
Chen, Gang
Clarke, David R.
Fan, Shanhui
Goodson, Kenneth E.
Keblinski, Pawel
King, William P.
Mahan, Gerald D.
Majumdar, Arun
Maris, Humphrey J.
Phillpot, Simon R.
Pop, Eric
Shi, Li
A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of similar to 1 nm, the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interfaces between materials. Major advances in the physics of phonons include first principles calculation of the phonon lifetimes of simple crystals and application of the predicted scattering rates in parameter-free calculations of the thermal conductivity. Progress in the control of thermal transport at the nanoscale is critical to continued advances in the density of information that can be stored in phase change memory devices and new generations of magnetic storage that will use highly localized heat sources to reduce the coercivity of magnetic media. Ultralow thermal conductivity-thermal conductivity below the conventionally predicted minimum thermal conductivity-has been observed in nanolaminates and disordered crystals with strong anisotropy. Advances in metrology by time-domain thermoreflectance have made measurements of the thermal conductivity of a thin layer with micron-scale spatial resolution relatively routine. Scanning thermal microscopy and thermal analysis using proximal probes has achieved spatial resolution of 10 nm, temperature precision of 50 mK, sensitivity to heat flows of 10 pW, and the capability for thermal analysis of sub-femtogram samples. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
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