Showing papers by "Avram Bar-Cohen published in 2011"

Near-Junction Thermal Management for Wide Bandgap Devices

Abstract: Near-junction thermal management is critical to achieving the promise of electronic and photonic devices using wide bandgap materials. In such devices, including GaN HEMTs in PAs, the thermal resistance associated with the "near-junction" region dominates the heat removal path and is often as large as the thermal resistance of all the other elements in the resistance chain. As part of DARPA's portfolio in Thermal Management Technologies (TMT), efforts are underway to develop transformative, paradigm-changing cooling techniques. This paper will briefly review the thermal management needs of WBG devices and DARPA's Thermal Management Technologies portfolio, with emphasis on the goals and status of these efforts relative to the current State-of-the-Art. Attention will then turn to promising options in near-junction cooling and the challenges inherent in realizing their potential for WBG device thermal management.

Considerations in the Use of the Laser Flash Method for Thermal Measurements of Thermal Interface Materials

Abstract: With increasing thermal fluxes, the performance of thermal interface materials (TIMs) that are used to reduce the thermal resistance between contacting surfaces in electronic devices, such as at the die-to-heat sink or heat spreader-to-heat sink interfaces, is becoming critical. However, measuring the thermal resistances of TIMs in a manner representative of actual applications is difficult. The laser flash method is a technique that may be used to determine the thermal resistance of TIMs and their degradation under environmental exposure. This paper examines three issues associated with using the laser flash method that could limit its effectiveness in calculating thermal resistance: sample holder heating, clamping, and error in the Lee algorithm outputs due to coupon-TIM thermal diffusivity differences. As a case study, the thermal performance of polymer TIMs in pad form, as well as an adhesive and a gel, were examined. Finite element simulations indicated that, without proper consideration, sample holder heating can lead to significant error in the calculated TIM thermal conductivity values.

Self-Cooling on Germanium Chip

Abstract: As the scaling of silicon complementary metal-oxide-semiconductor devices becomes more and more challenging, both innovative device structures and new materials with high carrier mobility are needed to continue improving device performances. A metal-oxide-semiconductor field-effect transistor with germanium channel currently receives a resurgence of interest as a possible candidate for next-generation high mobility devices because germanium offers much higher mobility for both electrons and holes in comparison with silicon. While germanium solid-state device provides outstanding electrical benefits, it also offers significant challenges in thermal management and raises a major concern over the effect of on-chip hot spot on the reliability and performance of germanium chips. Current thermal management technologies, with a major focus on chip-level global cooling, offer very few choices for on-chip micro-scale hot spot cooling. The inherent thermoelectric properties of single crystal germanium support development of a novel thermal management strategy for micro-scale hot spot cooling which relies on thermoelectric self-cooling by electric current flowing into the back of the germanium chip. In this paper, the concept of germanium self-cooling for on-chip micro-scale hot spot is proposed and investigated. 3-D thermal-electric coupling simulations are used to evaluate the hot spot cooling performance on a germanium chip with a wide range of system parameters, including applied current, doping concentration, hot spot heat flux, micro-cooler size, and germanium chip thickness. The results suggest that localized thermoelectric self-cooling on the germanium chip can effectively reduce the temperature rise resulting from micro-scale high heat flux hot spots and shows a great promise as a novel on-chip cooling solution.

THE IMPACT OF GaN/SUBSTRATE THERMAL BOUNDARY RESISTANCE ON A HEMT DEVICE

Abstract: The present work uses finite element thermal simulations of Gallium Nitride High Electron Mobility Transistors (GaN HEMTs) to evaluate the impact of device design parameters on the junction temperature. In particular the effects of substrate thickness, substrate thermal conductivity, GaN thickness, and GaN-to-substrate thermal boundary resistance (TBR) on device temperature rise are quantified. In all cases examined, the TBR was a dominant factor in overall device temperature rise. It is shown that a TBR increase can offset any benefits offered through a more conductive substrate and that there exists a substrate thickness independent of TBR which results in a minimum junction temperature. Additionally, the decrease of GaN thickness only provides a thermal benefit at small TBRs. For TBRs on the order of 10 -4 cm 2 K/W or greater, decreasing the GaN thickness can actually increase the temperature as the heat from the highly localized source is not sufficiently spread out before crossing the GaN-substrate boundary. The tradeoff between GaN heat spreading, substrate heat spreading, and temperature rise across the TBR results in a GaN thickness with minimum total temperature rise. For the TBR values of

Pool Boiling Critical Heat Flux in Dielectric Liquids and Nanofluids

Abstract: With the recent advances in consumer and power electronics, efficient thermal management of high heat flux components has taken on new urgency. While its inherent advantages have made air cooling the method of choice for the vast majority of electronic systems, the relatively poor thermophysical properties of air limit its ability to meet today's more demanding thermal requirements. Therefore, researchers have continued to investigate liquid cooling techniques such as microchannel forced convection, jet impingement, and pool boiling heat transfer. Although water has superior thermal properties, concern over its dielectric strength and chemical activity have directed attention to the use of the inert and dielectric perfluorocarbons as alternative cooling fluids, with particular emphasis on harnessing boiling and evaporative heat transfer processes. Passive pool boiling with these dielectric fluids, including the effects of subcooling, pressure, length scale, mixtures, surface enhancements, and nanoadditives, has been investigated by a large number of researchers. The present study focuses on the critical heat flux for pool boiling of these dielectric liquids, representing the upper limit of the nucleate pool boiling regime, and provides a review and summary of the work reported in more than 100 archival papers by research groups from around the globe.

Incorporating Moldability Considerations During the Design of Polymer Heat Exchangers

Abstract: Incorporating moldability considerations during the design of thermally enhanced polymer heat exchangers. Recently, available formulations of thermally enhanced polymer composites are attractive in heat exchanger applications due to their low cost and improved corrosion resistance compared to the conventional metal options. This paper presents a systematic approach to the design of plate-fin heat exchangers made out of thermally enhanced polymer composites. We have formulated the design problem as the life cycle cost minimization problem. The integrated design model introduced here accounts for heat transfer performance, molding cost, and assembly costs. We have adopted well known models to develop individual parametric models that describe how heat transfer performance, molding cost, and assembly cost varies as a function of the geometric parameters of the heat exchanger. Thermally enhanced polymer composites behave differently from the conventional polymers during the molding process. The desired thin walled large structures are expected to pose challenges during the filling phase of the molding process. Hence we have utilized experimentally validated simulations to develop a metamodel to identify difficult and impossible to mold design configurations. This metamodel has been integrated within the overall formulation to address the manufacturability considerations. This paper also presents several case studies that show how the material and labor cost strongly influence the final design.

Modeling and Validation of a Prototype Thermally-Enhanced Polymer Heat Exchanger

Abstract: Polymer heat exchangers (PHXs) have received considerable attention since their invention more than 40 years ago due to their corrosion resistance, low density and low manufacturing cost. New polymer composites with higher strengths, thermal conductivities and thermal stability promise to bridge the performance gap between polymers and corrosion resistant metals. In the present study, PHX components were injection molded using thermally enhanced polyamide 12 resin and assembled into a crossflow finned-plate heat exchanger prototype. The prototype was implemented in an air-to-water experimental test apparatus and the heat transfer results were compared to an analytical model. This comparison confirmed that a polymer composite heat exchanger (PCHX) can offer significantly enhanced heat transfer relative to a pure polymer. A thermomechanical finite element model of the PCHX was developed and validated using experimental results. At fluid pressures near ambient, the heat transfer rate of the PCHX was 28% less than could be attained with an identical titanium heat exchanger. As fluid pressures increased, the through wall conduction resistance had a larger effect on heat transfer rate, reducing the performance of the PCHX relative to the titanium heat exchanger. Stress analysis of the thermally enhanced PCHX revealed that the stresses due to pressure loading were more sensitive to heat exchanger geometry, while the stresses due to thermal loading were more sensitive to material property anisotropy.Copyright © 2011 by ASME

Two-Phase Minichannel Cold Plate for Army Vehicle Power Electronics

Abstract: Army programs have focused on increasing the use of power-dense electronic components to improve system weight, fuel usage, design flexibility, and overall functionality, thus, stressing the thermal management requirements. Recent cooling designs focused on flowing 80–100 °C engine coolant through single-phase microchannel cold plates but concern over pumping power, heat dissipation, cold plate temperature inconsistency, and contaminate clogging have prompted interest in two-phase flow in a minichannel cold plate. In the course of this study, both single- and two-phase experiments were conducted with a 6.8 × 2.7 × 0.9 cm offset fin minichannel cold plate using 25 °C, 80 °C, and 99 °C de-mineralized water, respectively, with flowrates ranging from 0.33 cm3 /s to 45 cm3 /s. Heat dissipation using solder attached chip resistors was incrementally increased from 0 W to more than 1000 W while simultaneously measuring cold plate pressure drop, chip surface temperature, inlet and outlet fluid temperature, and flowrate. Preliminary results indicate that utilizing a minichannel cold plate with two-phase heat transfer offers the ability to significantly reduce clogging potential, flowrate, and associated pumping power, while improving thermal resistivity by more than a factor of 4 and temperature consistency by greater than a factor of 10. Single- and two-phase correlations were used to compare performance with theoretical values.Copyright © 2011 by ASME

Evaluation of Two-Phase Cold Plate for Cooling Electric Vehicle Power Electronics

Abstract: Rapid increases in the power ratings and continued miniaturization of semiconductor devices have pushed the heat flux of power electronics well beyond the range of conventional thermal management techniques, and thus maintaining the IGBT temperature below a specified limit has become a critical issue for thermal management of electric vehicle power electronics. Although two-phase cold plates have been identified as a very promising high flux cooling solution, they have received little attention for cooling of power electronics. In this work, a first-order analytical model and a system-level thermal simulation are used to compare single-phase and two-phase cold plate cooling for Toyota Prius motor inverter, consisting of 12 pairs of IGBT’s and diodes. Our results demonstrate that with the same cold plate geometry, R134a two-phase cooling can substantially reduce the maximum IGBT temperature, operate all the IGBT’s at very uniform temperatures, and lower the pumping power and flow rate in comparison to single-phase cold plate cooling. These results suggest that two-phase cold plate can be developed as a low-cost, small-volume, and high-performance cooling solution to improve system reliability and conversion efficiency for electric vehicle power electronics.© 2011 ASME

Silicon Hot Spot Remediation With a Germanium Self Cooling Layer

Abstract: Driven by shrinking feature sizes, microprocessor hot spots have emerged as the primary driver for on-chip cooling of today’s IC technologies. Current thermal management technologies offer few choices for such on-chip hot spot remediation. A solid state germanium self-cooling layer, fabricated on top of the silicon chip, is proposed and demonstrated to have great promise for reducing the severity of on-chip hot spots. 3D thermo-electrical coupled simulations are used to investigate the effectiveness of a bi-layer device containing a germanium self-cooling layer above an electrically insulated silicon layer. The parametric variables of applied current, cooler size, silicon percentage, and total die thickness are sequentially optimized for the lowest hot spot temperature compared to a non-self-cooled silicon chip. Results suggest that the localized self-cooling of the germanium layer coupled with the higher thermal conductivity of the silicon chip can significantly reduce the temperature rise resulting from a micro-scaled hot spot.© 2011 ASME

IR Based Inverse Calculation of Two-Phase Heat Transfer Coefficients in Microgap Channels

Abstract: IR thermography of the heated wall for the two-phase flow of FC-72 in microgap channels provides explicit evidence of the quality-driven M-shaped variations in the two-phase microgap heat transfer coefficients. Data obtained from a 210μ microgap channel, operated with an FC-72 mass flux of 195 and 780 kg/m2 -s and asymmetric heat fluxes of 28 W/cm2 to 35 W/cm2 are presented and discussed.© 2011 ASME

Polymer Heat Exchangers: An Enabling Technology for Water and Energy Savings

Abstract: Polymer heat exchangers (PHXs), using thermally-enhanced composites, constitute a “disruptive” thermal technology that can lead to significant water and energy savings in the thermoelectric energy sector. This paper reviews current trends in electricity generation, water use, and the inextricable relationship between the two trends in order to identify the possible role of PHXs in seawater cooling applications. The use of once-through seawater cooling as a replacement for freshwater recirculating systems is identified as a viable way to reduce the use of freshwater and to increase power plant efficiency. The widespread use of seawater as a coolant can be made possible by the favorable qualities of thermally-enhanced polymer composites: good corrosion resistance, higher thermal conductivities, higher strengths, low embodied energy and good manufacturability. The authors use several seawater cooling case studies to explore the potential water and energy savings made possible by the use of PHX technology. The results from three case studies suggest that heat exchangers made with thermally enhanced polymer composites require less energy input over their lifetime than corrosion resistant metals, which generally have much higher embodied energy than polymers and polymers composites. Also, the use of seawater can significantly reduce the use of freshwater as a coolant, given the inordinate amounts of water required for even a 1MW heat exchanger.Copyright © 2011 by ASME

An Integrated Approach to Design of Enhanced Polymer Heat Exchangers

Abstract: Recently available formulations of thermally enhanced polymers are attractive in heat exchanger applications due to their low cost and improved corrosion resistance compared to the conventional metal options. This paper presents a systematic approach to the design of plate-fin heat exchangers made out of thermally enhanced polymers. We have formulated the design problem as the life cycle cost minimization problem. The integrated design model introduced here accounts for heat transfer performance, molding cost, and assembly costs. We have adopted well known models to develop individual parametric models that describe how heat transfer performance, molding cost, and assembly cost varies as a function of the geometric parameters of the heat exchanger. Thermally enhanced polymers behave differently from the conventional polymers during the molding process. The desired thin walled large structures are expected to pose challenges during the filling phase of the molding process. Hence we have utilized experimentally validated simulations to develop a metamodel to identify difficult and impossible to mold design configurations. This metamodel has been integrated within the overall formulation to address the manufacturability considerations. This paper also presents several case studies that show how the material and labor cost strongly influence the final design.Copyright © 2011 by ASME

Hierarchical Reliability Model for Life Prediction of Actively Cooled LED-Based Luminaire

Abstract: The interest in light-emitting diodes (LEDs) for illumination applications has been increasing continuously over the last decade due to two key attributes of long lifetime and low energy consumption compared to the conventional incandescent light and compact fluorescent light. Although LEDs are attractive for lighting applications due to the aforementioned advantages, unique technical challenges, such as the extreme sensitivity of luminous output and useful lifetime to LED junction temperature, need to be overcome for their large-scale commercialization.