Avram Bar-Cohen

Bio: Avram Bar-Cohen is an academic researcher from University of Maryland, College Park. The author has contributed to research in topics: Heat transfer & Heat sink. The author has an hindex of 50, co-authored 329 publications receiving 8329 citations. Previous affiliations of Avram Bar-Cohen include Auburn University & DARPA.

Impact of integrated superlattice μtec structures on hot spot remediation

Abstract: Driven by shrinking feature sizes, microprocessor "hot-spots" - with their associated high heat flux and sharp temperature gradients - have emerged as the primary "driver" for on-chip thermal management of today's advanced IC technology. Proposed uses of solid state thermoelectric microcoolers for hot spot remediation have included the formation of a superlattice layer on the back of the microprocessor chip, but there have been few studies on the cooling performance of such devices. The present study provides the results of three-dimensional, electro-thermal, finite-element modeling of a superlattice microcooler, focusing on the achieved hot spot temperature and superlattice surface temperature reductions, respectively. Simulated temperature distributions and heat flow patterns in the silicon, associated with variations in microcooler geometry, chip thickness, hot spot size, hot spot heat flux, and superlattice thickness are provided. Comparison is made to hot spot cooling achieved by the Peltier effect in the silicon microprocessor chip itself. The numerical results suggest that, for a variety of operating conditions and geometries, while increasing the superlattice thickness serves to decrease the exposed superlattice surface temperature, it is ineffective in reducing the hot spot temperature below that due to the silicon Peltier effect.

Review of Most Recent Progress on Development of Polymer Heat Exchangers for Thermal Management Applications

Abstract: Polymeric materials have several favorable properties for heat transfer systems, including low weight, low manufacturing cost, antifouling, and anticorrosion. Additionally, polymers are typically electrical insulators, making them favorable for applications in which electrical conductivity is a concern. Examples of utilizing these favorable properties are discussed. The drawbacks to raw polymer materials include low thermal conductivity, low structural strength, and poor stability at elevated temperatures. Methods of mitigating these unfavorable properties, including loading the polymer with other materials and developing new polymers, are discussed. Enhanced geometric designs enabled by additive manufacturing can also improve thermal performance of polymer heat exchangers. Results of a research study utilizing additive manufacturing toward developing high-performance and cost-effective polymer heat exchangers for an air-to-liquid application are reviewed and discussed. Finally, needs for further research on enhancing polymer thermal performance are discussed.Copyright © 2015 by ASME

Combined pressure and subcooling effects on pool boiling from a PPGA chip package

Abstract: This study presents a detailed experimental investigation of the combined effects of pressure and subcooling on nucleate pool boiling and critical heat flux (CHF) for degassed fluorocarbon FC-72 boiling on a plastic-pin-grid-array (PPGA) chip package. In these experiments, pressure was varied between 101.3 and 303.9 kPa, and the subcooling ranged from 0 to 65/spl deg/C. As expected, lower wall superheats resulted from increases in pressure, while subcooling had a minimal effect on fully-developed pool boiling. However, the superheat reductions and CHF enhancements were found to be smaller than those predicted by existing models. The saturation CHF increased by nearly 17% for an increase in pressure from 101.3 to 202.7 kPa. In experiments with both FC-72 and FC-87, further increases in pressure did not produce any significant increase in CHF. At a pressure of 101.3 kPa, a subcooling of 30/spl deg/C increased CHF on horizontal, upward-facing chips by approximately 50% as compared to 70% on vertically oriented packages. The enhancement in CHF due to subcooling decreased rapidly with increasing pressure and the data showed that the influence of pressure and subcooling on CHF is not additive. A correlation to predict pool boiling CHF under the combined effects of pressure and subcooling is proposed.

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

Light-emitting diodes

Abstract: Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

A review of Ga2O3 materials, processing, and devices

Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

Low loss dielectric materials for LTCC applications: a review

Abstract: Small, light weight and multifunctional electronic components are attracting much attention because of the rapid growth of the wireless communication systems and microwave products in the consumer electronic market. The component manufacturers are thus forced to search for new advanced integration, packaging and interconnection technologies. One solution is the low temperature cofired ceramic (LTCC) technology enabling fabrication of three-dimensional ceramic modules with low dielectric loss and embedded silver electrodes. During the past 15 years, a large number of new dielectric LTCCs for high frequency applications have been developed. About 1000 papers were published and ∼500 patents were filed in the area of LTCC and related technologies. However, the data of these several very useful materials are scattered. The main purpose of this review is to bring the data and science of these materials together, which will be of immense help to researchers and technologists all over the world. The comme...