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.

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.

High-performance computing systems, especially 3D ICs, are yet facing thermal exacerbation. Inter-tier liquid cooling microchannel layers have been introduced into 3D ICs as an integrated cooling mechanism to tackle thermal degradation. Many research works optimize microchannel designs based on runtime-expensive numerical simulations or inaccurate thermo-fluid models. In this work, we propose accurate closed-form models on tapered microchannel to capture the relationship between channel geometry and heat transfer performance. To improve the accuracy, our correlation is based on developing flow model and derived from numerical simulation using a subset of multiple channel parameters. Our models reduce error by 57 % in Nusselt number and 45 % in pressure drop for channels with inlet width 100-400 mu m compared to commonly used fully developed flow based models in optimization. Obtained correlations show potential as solid foundation to achieve close to optimal design through runtime-efficient microchannel design optimization.

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.

Thermoelectric devices have attracted a great attention for renewable energy harvesters and solid-state coolers. For practical applications, the mechanical properties of thermoelectric materials become critical for the device reliability, a persistent performance with a long time and high operation cycles. Bi-Te based single-crystals, mostly used in commercial thermoelectric devices, are intrinsically brittle with weak van der Waals bonding, often leading to device failures such as crack and debonding during fabrication and operation. Thus, it is highly desirable to enhance the mechanical property of Bi-Te based alloys as well as the thermoelectric property. Here, we investigate the effect of B4C nanoparticles (less than 0.5 wt%) dispersed in p-type Bi0.4Sb1.6Te3 matrix on the mechanical properties. X-ray diffraction (XRD) result confirms that B4C-dispersed Bi0.4Sb1.6Te3 has a single phase. We observe that the grain size of Bi0.4Sb1.6Te3 becomes decreased with the B4C nanoparticle concentration by electron backscatter diffraction (EBSD) technique. Hardness, Young's modulus, and flexural strength of B4C-dispersed Bi0.4Sb1.6Te3 are enhanced, compared to the B4C-free Bi0.4Sb1.6Te3 polycrystals. On the other hand, the thermoelectric figure-of-merit of B4C-dispersed Bi0.4Sb1.6Te3 is almost identical to that of the pure Bi0.4Sb1.6Te3. Such enhancements of the mechanical properties of the B4C-dispersed Bi0.4Sb1.6Te3 are attributed to the grain boundary hardening and second-phase hardening. Beyond thermoelectric materials, our result implies that the grain refinement by nanoparticle dispersion is a simple and promising way to strengthen the mechanical properties of other brittle materials with layered structure. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The effect of Sn doping on the thermoelectric properties of p-type Bi0.4Sb1.6Te3 (BST) thin films was studied. Sn-doped BST films were grown on 4A degrees tilted GaAs (001) substrates by metal-organic chemical vapor deposition. To control the Sn ion concentration in the films, we systematically controlled the dose of the Sn precursor by varying the H-2 flow rate from 0 sccm to 100 sccm. The hole carrier concentration increased as the H-2 flow rate was increased. Interestingly, the Seebeck coefficient of the films simultaneously increased with the carrier concentration when the H-2 flow rate was increased up to 60 sccm. This might be attributed to the formation of virtual bound states in the valence band by Sn doping. Consequently, the Sn ion doping contributed to the thermopower enhancement of the BST films.

We measure and model the dynamic temperature response of electrocaloric (EC) multilayer capacitors (MLCs) which have been recently highlighted as novel solid-state refrigerators. The MLC temperature responses depend on the operation voltage waveform, thus we consider three types of voltage waveforms, which include square, triangular, and trapezoidal. Further, to implement an effective refrigeration cycle, the waveform frequency and duty cycle should be carefully chosen. First, our model is fitted to the measurements to evaluate an effective EC power and thermal properties, and calculates an effective cooling power for an EC MLC. The prediction shows that for a MLC with a thermal relaxation time for cooling, trc, a square voltage waveform with a duty cycle of 0 < d les 0.3 and a period of trc < P les 1.4 trc provides the maximum cooling power. This work will help to improve the implementing methods for EC refrigeration cycles.

We measure and model the dynamic temperature response of electrocaloric (EC) multilayer capacitors (MLCs) which have been recently highlighted as novel solid-state refrigerators. The MLC temperature responses depend on the operation voltage waveform, thus we consider three types of voltage waveforms, which include square, triangular, and trapezoidal. Further, to implement an effective refrigeration cycle, the waveform frequency and duty cycle should be carefully chosen. First, our model is fitted to the measurements to evaluate an effective EC power and thermal properties, and calculates an effective cooling power for an EC MLC. The prediction shows that for a MLC with a thermal relaxation time for cooling, t(rc), a square voltage waveform with a duty cycle of 0< d <= 0.3 and a period of t(rc)< P <= 1.4t(rc) provides the maximum cooling power. This work will help to improve the implementing methods for EC refrigeration cycles. (C) 2014 AIP Publishing LLC.

The performance of infrared (IR) sensing bimaterial cantilevers depends upon the thermal, mechanical and optical properties of the cantilever materials. This paper presents bimaterial cantilevers that have a layer of black silicon nanocone arrays, which has larger optical absorbance and mechanical compliance than single crystal silicon. The black silicon consists of nanometer-scale silicon cones of height 104-336 nm, fabricated using a three-step O-2-CHF3-Ar+Cl-2 plasma process. The average cantilever absorbance was 0.16 over the 3-10 mu m wavelength region, measured using a Fourier transform infrared (FTIR) microspectrometer. The measured cantilever responsivity to incident IR light compares well to a model of cantilever behavior that relate the spectral absorbance, heat transfer, and thermal expansion. The model also provides further insights into the influence of the nanocone height on the absorbance and responsivity of the cantilever. Compared to a cantilever with smooth single crystal silicon, the cantilever with black silicon has about 2x increased responsivity. The nanocone array fabrication technique for silicon bimaterial cantilevers presented here could be applied to other IR sensors. (C) 2013 Elsevier B.V. All rights reserved.

Highlights • We develop a differential method to measure thermoelectric module performance. • Differential method compensates for thermal losses and reduces the experiment time. • We develop a heat flow meter for the thermoelectric module measurement. • We compare the direct measurement and Harman method. Abstract An accurate and rapid characterization of a thermoelectric module (TEM) is critical to understand the problems in module design and fabrication. We describe an apparatus and a method that directly measure the cooling performance of a TEM such as current for maximum cooling (Imax), maximum cooling power (Qc,max), and maximum temperature difference (ΔTmax). The apparatus is designed based on a finite element model to ensure a simple heat flow measurement. We evaluate the module performance metrics based on differential measurement between the cooling powers with temperature difference across the module under transient conditions. The use of transient data reduces measurement time, and the use of a differential technique enables compensation of the thermal losses. The measured data fit well with conventional theoretical relations for the TEM performance metrics. We test a commercial TEM and validate the results using the Harman method.

This article reports the design, fabrication, and demonstration of additively manufactured air jet impingement coolers for the thermal management of high-power gallium nitride (GaN) transistors. The polymer jet coolers impinge high-speed airflow with a velocity of 42 & x2013;195 m/s (Reynolds number between onto working GaN devices mounted on a printed circuit board (PCB). The air jet provides cooling heat fluxes of up to 58.4 W/cm(2), cooling rates of up to 6.6 & x00B0;C/s, and convective heat transfer coefficient ranging from 5.2 to 17.0 kW/). The cooling performance is comparable to that of jet coolers made from other materials and manufacturing technologies. A key benefit of additive manufacturing (AM) is design freedom and geometric complexity, which we highlight by demonstrating three different packaging configurations, each enabled by a different jet cooler design that is customized for different types of packaging configurations: Cooler 1 directs two parallel impinging jets onto the top side of two devices; cooler 2 directs two air jets onto the front side and two air jets onto the back side of two devices; and cooler 3 directs air jets onto the front side of four devices mounted on parallel adjacent circuit boards. The second benefit of AM is the ability to consolidate multiple components into a single part, which we highlight by combining a nozzle, a fluidic delivery system, and a flow distributor within a volume of 80 mm mm. This work demonstrates the potential of AM to create complex, lightweight, fluidic delivery systems to achieve thermally and hydrodynamically optimized air jet cooling for high-power-density electronic devices.

Sn-doped Bi2Te3 films were grown on vicinal GaAs (0 0 1) substrates by metal-organic chemical vapor deposition at 360 degrees C. Trimethylbismuth and diisopropyltellurium, which are alkyl-based, were used as the Bi and Te sources, respectively. Tetrakis(dimethylamino)tin (TDMASn) and tetramethyltin (TMSn) were used as the Sn precursors. Both Sn precursors successfully converted the carrier type of the Bi2Te3 films from n- to p-type and achieved a high Seebeck coefficient. In the case of the Sn-doped Bi2Te3 films with TDMASn, however, the Sn concentration could not be monotonically controlled by the amount of the precursor, and even the hole concentration was almost invariant despite the drastic increase in the amount of the precursor. In the case of the Sn-doped Bi2Te3 films grown with TMSn, on the other hand, the Sn and hole concentrations could be easily controlled by the variation in the flow rate of the H-2 carrier gas. In particular, the hole concentration varied over a range of 1-5 10(19)/cm(3) in which a thermoelectric power factor can be maximized despite a very high vapor pressure of TMSn. The growth of high-quality Sn-doped Bi2Te3 films was possible using all alkyl-based precursors. (C) 2015 Elsevier B.V. All rights reserved.

A critical challenge in using thermoelectric generators (TEGs) for charging the portable or wearable electronics has been their limited outputs, as available temperature differential on human body (Delta T-ext) is typically less than 10 K. Furthermore, the thermal resistance (R-th) at the TEG-air interface often overwhelms R-th of TEG itself, which makes the temperature differential within the TEG merely a small fraction of Delta T-ext. Here, the designs of TEG systems for wearable applications based both on theory and systematic experiments are studied. First, this study fabricates the TEGs having different fill factors (equivalently, varied internal R-th of the TEGs) and finds an optimum fill factor that is determined by both thermal matching condition and the electrical contact resistance. Next, to investigate the effects of heat sink and external air flow, this study combines plate fin heat sinks with the TEGs and evaluates their performance under three different convection conditions: natural convection, and convection with either parallel or impinging flow. Lastly the effect of R-th at the skin-TEG interface is studied. Although the TEG system produces an output power of 126 mu W cm(-2) (Delta T-ext =3D 7 K) on a smooth heat source (Cu heater), it generates reduced power of 20 mu W cm(-2) (Delta T-ext =3D 6 K) on wrist (uneven heat source).

This paper investigates heat transfer enhancement by means of additively manufactured static mixers during liquid water cooling of a horizontal, heated flat plate. The static mixers disrupt the thermal boundary layer and induce mixing, resulting in an increased heat transfer rate of about 2X larger than flows without mixers. Simulations of the flows provided insights into the flows near the mixers, and guided selection of specific mixer geometries. The mixers were fabricated directly into the flow channels using additive manufacturing and then assembled onto the heated plate. Two types of mixing structures were analyzed: twisted tape structures that are similar to conventional static mixers; and novel chevron shaped offset wing structures. Heat transfer performance was measured for liquid water (510 <=3D Re <=3D 1366) cooling the heated section with convective heat flux ranging between 0.1 and 0.8 W/cm(2). This work demonstrates the potential of additive manufacturing to enable novel flow geometries that can enhance convection heat transfer whilst minimizing pressure drop penalties and volumes. (C) 2019 Elsevier Ltd. All rights reserved.