Experimental investigation of microchannel coolers for the high heat flux thermal management of GaN-on-SiC semiconductor devices

Experiments on removing high heat fluxes from GaN-on-SiC semiconductor dies using microchannel coolers are described. The dies contain an AlGaN/GaN heterostructure operated as a direct current resistor, providing a localized heat source. The active dimensions of the heat source are sized to represent the spatially-averaged heat flux that would appear in microwave power amplifiers. A wide variety of microchannel materials and configurations are investigated, allowing a comparison of performance and the resulting GaN temperatures. Silicon and AlN microchannel coolers exhibit good performance at lower power densities (1000–1200 W/cm² over 3 × 5 mm² to 2 × 5 mm² active areas). Polycrystalline chemical vapor deposited (CVD) SiC microchannel coolers are found to be extremely promising for higher power densities (3000–4000 W/cm² over 1.2 × 5 mm² active areas with 120 °C GaN temperature). A hybrid microchannel cooler consisting of low-cost CVD diamond on polycrystalline CVD SiC exhibits moderately better performance (20–30%) than polycrystalline CVD SiC alone.     

Control of self-organization of drop-casted Nafion film for improving proton conduction in a polymer-electrolyte-membrane fuel cell to raise its output power density

A simple drop-cast method to directly deposit Nafion polymer electrolyte membrane (PEM) on nanostructured thin-film catalyst layer composed of stacked Pt nanoparticles prepared by pulsed laser deposition (PLD) was demonstrated. Through optimization of solvent composition and drying temperature of Nafion solution to control self-organization of Nafion, a uniform PEM with better bulk and interface microstructures could be produced, leading to a significant improvement in the output current density of a PEM fuel cell over that using reference commercial PEMs. The formation of facile proton conduction pathways in the bulk Nafion membrane resulted in a 35% reduction in ohmic resistance compared to that with the commercial membrane. Moreover, the infiltration of Nafion in the catalyst layer formed suitable proton transport network to render more catalyst nanoparticles effective and thus lower charge-transfer resistance. With the optimized PLD, drop-cast, and hot-pressing conditions, the current density of PEMFCs using drop-casted PEM reached 1902 mA cm⁻² at 0.6 V at 2 atm H₂ and O₂ pressures with a cathode Pt loading of 100 μg cm⁻², corresponding to a power density of 1.14 W cm⁻² and a cathode mass-specific power density of 11.4 kW g⁻¹.     

Evaluation of efficiency factors and internal resistance of thermoelectric materials

It is well known that the figure of merit (ZT) is unreliable in calculating the efficiency (?) of micro thermoelectric generators system level and unrealistic when comparing the performance of thermoelectric (TE) materials in the same metric units. To solve this problem, we have used COMSOL multiphysics to design a single leg of micro thermoelectric generators model for computing efficiency factors (? ) and internal resistance using TE materials' constants, such as electrical conductivity (σ ), TE conductivity (K ), and Seebeck coefficient (α ). The TE materials were placed between two copper electrodes, and the first data analyzed were the voltages per meter and electric currents per meter. The internal resistances were calculated by taking the ration of voltages to electric currents, and at the same time, the electric powers were calculated from the products of electric currents and voltages yielding power per unit area in μW cm^?2. The ? were calculated using changes in power (ΔP ), temperature gradient (ΔT ), and the surface area (A ). The obtained results showed that the TE materials with highest ? when the temperatures are between 375 and 550 K are n‐type SiGe and p‐type SiGe. When the temperatures are between 550 and 780 K, the TE materials with the highest ? are PbTe‐Pbl₂, PbTe‐CdTe, and PbTe‐SrTe‐Na. We noted that the ? obtained from eight TE materials in this work are within the range as those reported in the literature between 0.001 and 0.091 μW cm^?2 K^?2. The TE materials with high internal resistances such as PbS, PbTe, and PbSe have ? that is <0.0001 μW cm^?2 K^?2, and those with low internal resistances have ? in the range between 0.002 and 0.0091 μW cm^?2 K^?2. This work has shown that COMSOL multiphysics is a powerful computational tool that can be used to analyze internal resistances and ? of TE materials in the same temperature ranges. Copyright © 2016 John Wiley & Sons, Ltd.     

A Honeycomb Microchannel Cooling System for Microelectronics Cooling

A honeycomb porous microchannel cooling system for electronics cooling was proposed in this article. The design, fabrication, and test system configuration of the microchannel heat sink were summarized. Preliminary experimental investigation was conducted to understand the characteristics of heat transfer and cooling performance under steady single-phase flow. In the experiments, a brass microchannel heat sink was attached to a test heater with 8 cm² area. The experimental results show that the cooling system is able to remove 18.2 W/cm² of heat flux under 2.4 W pumping power, while the junction wall temperature is 48.3°C at the room temperature of 26°C. Extensive experiments in various operation conditions and parameters for the present cooling system were also conducted. The experimental results show that the present cooling system is able to perform heat dissipation well.     

Multi-Objective Optimization of Micro Pin-Fin Arrays for Cooling of High Heat Flux Electronics with a Hot Spot

ABSTRACT

This article presents fully three-dimensional conjugate heat transfer analysis and a multi-objective, constrained optimization to find sizes of pin-fins, inlet water pressure, and average speed for arrays of micro pin-fins used in the forced convection cooling of an integrated circuit with a uniformly heated 4 × 3 mm footprint and a centrally located 0.5 × 0.5 mm hot spot. Sizes of micro pin-fins having cross sections shaped as circles, symmetric airfoils, and symmetric convex lenses are optimized to completely remove heat due to a steady, uniform heat flux of 500 W cm^?2 imposed over the entire footprint (background heat flux) and a steady, uniform heat flux of 2000 W cm^?2 imposed on the hot spot area only (hot spot heat flux). The two simultaneous objectives are to minimize maximum substrate temperature and minimize pumping power, while keeping the maximum temperature constrained below 85°C and removing all of input thermal energy by convection. The design variables are the inlet average velocity and size of the pin-fins. A response surface is generated for each of the objectives and coupled with a genetic algorithm to arrive at a Pareto frontier of the best trade-off solutions. Numerical results show that, for a specified maximum temperature, optimized arrays with pin-fins having symmetric convex lens shapes create the lowest pressure drop, followed by the symmetric airfoil and circular cross-section pin-fins. An a posteriori three-dimensional stress–deformation analysis incorporating hydrodynamic and thermal loads shows that Von-Mises stress for each pin-fin array is significantly below the yield strength of silicon, thus, confirming structural integrity of such arrays of micro pin-fins.     

Experimental investigation on controlled cooling by coupling of thermoelectric and an air impinging jet for CPU

In this study, experimental tests have been carried out on the coupling thermoelectric cooling module with minichannel heatsink subjected to impinging airflow for cooling desktop central processing unit (CPU). A controlled thermoelectric-forced test system was designed for this purpose. This was designed using electronic Arduino card. The proposed hybrid cooling system was compared with the conventional forced air-cooling technique. Three power of heat source (CPU) were adopted, investigated, and compared, namely 60, 87, and 95 W. Performance of controlled thermoelectric cooling with three preset temperature were experimentally examined. The effects of air velocity and thermoelectric input current on the case temperature (T_case), thermal resistance, and heat transfer coefficient were analyzed. Results showed that the T_case increases with the increase of its input power. In addition, increasing air jet velocity and thermoelectric input current improve CPU cooling significantly. For a CPU power of 95 W, the recorded T_case temperature was 57°C with the conventional system. While it was maintained below 50°C in the hybrid system. The thermoelectric cooler has had a major effect on CPU cooling, having 15% improvement over conventional forced air-cooling. However, this was accompanied by an increase in energy consumption in the range of 45 W.     

Heat transfer characteristics of spray cooling in a closed loop

A closed loop spray cooling test setup is established for the cooling of high heat flux heat sources. Eight miniature nozzles in a multi-nozzle plate are used to generate a spray array targeting at a 1 × 2 cm² cooling surface. FC-87, FC-72, methanol and water are used as the working fluids. Thermal performance data for the multi-nozzle spray cooling in the confined and closed system are obtained at various operating temperatures, nozzle pressure drops (from 0.69 to 3.10 bar) and heat fluxes. It is exhibited that the spray cooler can reach the critical heat fluxes up to 90 W/cm² with fluorocarbon fluids and 490 W/cm² with methanol. For water, the critical heat flux is higher than 500 W/cm². Air purposely introduced in the spray cooling system with FC-72 fluid has a significant influence on heat transfer characteristics of the spray over the cooling surface.     

Properties of pyroxene glass ceramics,heat treated in the Big Solar Furnace

Pyroxene glass ceramic materials were obtained on the basis of the basaltic rock of the Koitashskii ore field. It is shown that the glass ceramic crystallization process under heat treatment in the electric furnace is fully complete at 1180°C. In the case of heat treatment in the Big Solar Furnace at low powers of 40–60 W/cm², the crystallization process is initiated on the exposed surface of the samples, and bulk crystallization is observed at a power of 80 W/cm².     

Enhancement of optical absorption for below 5 μm thin-film poly-Si solar cell on glass substrate

Optical confinement effect of thin-film polycrystalline-Si (poly-Si) solar cell on glass substrate fabricated at low-temperature has been investigated as a function of cell thickness of less than 5 μm. We found that it is possible to fabricate the textured Si thin film in situ on a glass substrate and that the reflectance at long-wavelength light is reduced by surface texturing. Thin-film poly-Si solar cell and a-Si:H/(0.45 μm)/poly-Si (5 μm) tandem solar cell exhibit the efficiency of 8.6% and 12.8%, respectively. The numerical study in terms of the light trapping explains the excellent high short-circuit current density (_sc above 27 mA/cm² at the 4.7 μm thin-film poly-Si solar cell.     

A closed-loop electronics cooling by implementing single phase impinging jet and mini channels heat exchanger

This paper reports our works in the design and testing of a closed-loop electronics cooling system that adopts bi-technologies: single phase impinging jet and mini channels heat exchanger. The system has the cooling capacity of 200 W over a single chip with a hydraulic diameter of 12 mm. The equivalent heat flux is 177 W/cm². The cooling system maintains the chip’s surface temperature below 95 °C maximum when the ambient temperature is 30 °C. De-ionized water is the working fluid of the system. For the impinging jet, two different nozzles are designed and tested. The hydraulic diameters (d_N) are 0.5 mm and 0.8 mm. The corresponding volume flow rates are 280 mL/min and 348 mL/min. Mini channels heat exchanger has 6 (six) copper tubes with the inner diameter of 1.27 mm and the total length of about 1 m. The cooling system has a mini diaphragm pump and a DC electric fan with the maximum power consumptions of 8.4 W and 0.96 W respectively. The coefficient of performance of the system is 21.4.     

Heat transfer enhancement of spray cooling with ammonia by microcavity surfaces

In the present work, spray cooling heat transfer performances with ammonia as coolant were experimentally investigated on three self-manufactured microcavity surfaces and the enhancement of heat transfer over that of flat surface was also examined. The experimental results showed that almost the same heat transfer performance was obtained at low surface superheats for different heat transfer surfaces due to the fact that the single phase convection dominated the heat transfer process. The microcavity surfaces exhibited uniform temperature distribution and higher heat transfer coefficient than that on the flat surface at high surface superheats once the heat transfer was dominated by the nucleate boiling. This was because that the capillary effect induced by the microcavity structure results in dramatic reduction in heat transfer resistance and then enhancement of the nucleate boiling. It was also found that the microcavity surface with the lowest Bo number of 0.1004 yielded the maximum heat transfer coefficient of 148,245 W/m²·K at the heat flux of 451 W/cm² as a result of the strongest capillary effect. In the meantime, low surface temperature of below 0 °C and uniform temperature distribution with deviation below ±1.5 °C at the heat flux of 420 W/cm² was simultaneously achieved.     

Laser-printed thick-film electrodes for solid-state rechargeable Li-ion microbatteries

Laser-printed thick-film electrodes (LiCoO₂ cathode and carbon anode) are deposited onto metallic current collectors for fabricating Li-ion microbatteries. These microbatteries demonstrate a significantly higher discharge capacity, power and energy densities than those made by sputter-deposited thin-film techniques. This increased performance is attributed to the porous structure of the laser-printed electrodes, which allows improved ionic and electronic transport through the thick electrodes (∼100 μm) without a significant increase in internal resistance. These laser-printed electrodes are separated by a laser-cut porous membrane impregnated with a gel polymer electrolyte (GPE) in order to build mm-size scale solid-state rechargeable Li-ion microbatteries (LiCoO₂/GPE/carbon). The resulting packaged microbatteries exhibit a power density of ∼38 mW cm⁻² with a discharge capacity of ∼102 μAh cm⁻² at a high discharge rate of 10 mA cm⁻². The laser-printed microbatteries also exhibit discharge capacities in excess of 2500 μAh cm⁻² at a current density of 100 μA cm⁻². This is over an order of magnitude higher than that observed for sputter-deposited thin-film microbatteries (∼160 μAh cm⁻²).     

Primary Li/SOCl2 cells. X. Optimization of D cells with respect to energy density,storability and safety

Hermetically-constructed Li/SOCl₂ D cells were used for the studies reported in this publication. Optimization with respect to energy density resulted in a capacity recovery of 18 – 19 A h at 3.5 volt at 25 °C, at 0.01 A corresponding to 20 W h/in.³ (1.24 W h/cm³) and 300 W h/lb. (661 W h/kg). The optimization with respect to storability resulted in cells having no voltage-delays after three months of storage at 72 °C and test at ?30 °C at 3.0 A. The optimization with respect to safety resulted in cells which are resistant to abuses such as shorting and force-discharge. Approaches have also been developed to stabilize the partially-discharged cells and thus prevent spontaneous explosions on storage.     

Interfacial electric properties of beta″-alumina and electrode by AC impedance

In order to enhance cell power density and to study the interfacial electric property between beta″-alumina and an electrode, test cells of Na(l)/beta″-alumina/M, where M=TiN or TiB₂ or Na–Sn or Na–Pb molten alloys as electrode materials, were set up and run within the temperature range of 400°–800°C. The performance of the test cells and the interfacial electric properties were investigated by measuring current–voltage characteristics and AC impedance. The maximum power density of 0.18 W cm⁻² for TiN and 0.24 W cm⁻² for TiB₂ could be achieved with a large electrode-area of 30 cm² at 800°C. A simplified model and equivalent circuit were given, based on the impedance data. The effect of microstructure of the porous electrode and roughness of the beta″-tube on the cell electric performance and impedance has been studied and discussed. The electron-transport through the porous electrode to the interface of the electrode and the beta″-tube surface is the control step for the electrode reaction, Na⁺+e→Na, rather than the mass-transport step, for a cell of Na(l)/beta″-alumina/porous thin film electrode. The AC impedance data demonstrated that wetting of the beta″-alumina electrolyte plays an important roll in reducing the cell resistance for the molten Na–Sn or Na–Pb electrode, and the molten alloy electrodes have a smaller cell-resistance, 0.3–0.35 Ω cm², at 700°C after 10–20 h. The comparison with sputtered thin, porous film electrodes, showed that the microstructure and thickness of electrode, and the interfacial resistance between electrode and the surface of the beta″-alumina is crucial to enhance cell power density.     

Evaluation of Jet Impingement,Spray and Microchannel Chip Cooling Options for High Heat Flux Removal

Thermal management for high heat flux removal from microelectronic chips is gaining critical importance in many earth-based and space-based systems. Heat fluxes greater than 1 MW/m² (100 W/cm²) have already been realized in high-end server applications, while cooling needs in next generation chips and advanced systems such as high-power electronics and electrical systems, pulsed power weapons systems, solid-state sensors, and phased-array radars are expected to reach 5–10 MW/m² (500–1000 W/cm²). After evaluating the contributions from different thermal resistances in the chip-to-ambient thermal path, this paper presents a critical review and research recommendations for three prominent contending technologies: jet impingement, spray cooling, and microchannel heat sinks.     

State of the Art of High Heat Flux Cooling Technologies

The purpose of this literature review is to compare different cooling technologies currently in development in research laboratories that are competing to solve the challenge of cooling the next generation of high heat flux computer chips. Today, most development efforts are focused on three technologies: liquid cooling in copper or silicon micro-geometry heat dissipation elements, impingement of liquid jets directly on the silicon surface of the chip, and two-phase flow boiling in copper heat dissipation elements or plates with numerous microchannels. The principal challenge is to dissipate the high heat fluxes (current objective is 300 W/cm²) while maintaining the chip temperature below the targeted temperature of 85°C, while of second importance is how to predict the heat transfer coefficients and pressure drops of the cooling process. In this study, the state of the art of these three technologies from recent experimental articles (since 2003) is analyzed and a comparison of the respective merits and drawbacks of each technology is presented. The conclusion is that two-phase flow boiling in microchannels is the most promising approach; impingement cooling also has good prospects but single-phase liquid cooling is probably only a short-term solution. As an example of the state of the first technology, the Heat and Mass Transfer Laboratory at Ecole Polytechnique Fédérale de Lausanne has already achieved 200 W/cm² of cooling in a first prototype, with a low pumping power, good temperature uniformity, and at the required maximal operating temperature.     

Foil mounted thin-film solar cells for space and terrestrial applications

The developing photovoltaic market for civilian communications satellites has caused a reassessment of the traditional single-crystal cells used in space. Thin-film cells mounted on metal foil substrates offer considerable weight, manufacturing, and radiation resistance advantages. These same cells can solve the manufacturing problems of glass breakage and temperature uniformity in cells intended for terrestrial applications. This paper describes work on a backwall CdS/CdTe cell on molybdenum foils. Large grained adherent films of CdTe are formed. A thin interlayer of Cu and Te between the CdTe and the molybdenum formed a low resistance (less than 1 Ω cm²) ohmic contact. The cell is completed with a sputtered indium tin oxide top contact layer and a V_oc of 580 mV was obtained.     

Status of the development of a direct methanol fuel cell

In the past, most papers on direct methanol fuel cells (DMFC) reported about systems using pure oxygen instead of air supplied to the cathode. The status of the work on DMFC at Siemens was characterized by more than 200 mW/cm² at a cell voltage of 0.5 V under oxygen operation (4–5 bar abs.) at high temperatures (140°C). High oxygen pressure operation at high temperatures is only useful in special market niches. Low air pressure up to 1.5 bar abs. and therefore low operation temperatures in the range of 80–110°C are necessary technical features and economic requirements for widespread application of the DMFC. Today, our system produces 50 mW/cm² under air operation at low over pressure and at 80°C, while the cell voltage again amounts to 0.5 V. These measurements were carried out in single cells between 3 and 60 cm². First results for a cell design with an electrode area of 550 cm², which is appropriate for assembling a DMFC-stack, are shown. In the new cell it was possible to achieve the same power densities as in the experimental cells at low air over pressure. Also a three-celled stack based on this design revealed nearly the same performance. At 80°C a power output of 77 W at a stack voltage of 1.4 V can be obtained in the air mode. The low pressure air operation results in a lower performance which must be compensated by future improvements of the activity of the anode catalyst and by an adequate membrane with a low methanol and water permeation, which would be a great progress for the DMFC.     

Thermal management of a proton exchange membrane fuel cell stack with pyrolytic graphite sheets and fans combined

This work characterizes the thermal management of a proton exchange membrane fuel cell (PEMFC) stack with combined passive and active cooling. A 10-cell PEMFC stack with an active area of 100 cm² for each cell is constructed. Six thermally conductive 0.1-mm-thick Pyrolytic Graphite Sheets (PGSs) are cut into the shape of flow channels and bound to the six central cathode gas channel plates. These PGSs, which are lightweight and have high thermal conductivity, function as heat spreaders and fins and provide passive cooling in the fuel cell stack, along with two small fans for forced convection. Three other cooling configurations with differently sized fans are also tested for comparisons (without PGSs). Although the maximum power generated by the stack with the configuration combining PGSs and fans was 183 W, not the highest among all configurations, it significantly reduced the volume, weight, and cooling power of the thermal management system. Net power, specific power, volumetric power density, and back work ratio of this novel thermal management method are 179 W, 18.54 W kg⁻¹, 38.9 kW m⁻³, and 2.1%, respectively, which are superior to those of the other three cooling configurations with fans.