Showing papers by "Avram Bar-Cohen published in 2008"

Heat driven cooling of portable electronics using thermoelectric technology

Abstract: A novel ldquoshunt attachrdquo configuration for chip-scale thermoelectric (TE) generation of electric power from microprocessor waste heat is described, modeled, and parametrically analyzed. The generated electricity is used to drive a cooling fan that convectively cools the chip. A prototype using heat-driven cooling through off-the-shelf TE modules and a low-voltage fan was built and successfully applied to the thermal management of a high-power mobile processor in a portable equipment form factor.

Integrated Measurement Technique for Curing Process-Dependent Mechanical Properties of Polymeric Materials Using Fiber Bragg Grating

Abstract: We propose an integrated technique to measure critical mechanical properties of polymeric materials. The method is based on a fiber Bragg grating (FBG) sensor. A polymer of interest is cured around a glass FBG and the Bragg wavelength (BW) shift is measured and documented while polymerization progresses at the curing temperature. After complete polymerization, the BW shift is monitored continuously as the temperature of the cured polymer changes. The desired material properties are then found inversely from the relationship between the Bragg wavelength shift and the deformation of the polymer caused by the changes in the material properties.

Thermofluid characteristics of two-phase flow in micro-gap channels

Abstract: Microgap coolers provide direct contact between chemically inert, dielectric fluids and the back surface of an active electronic component, thus eliminating the significant interface thermal resistance associated with thermal interface materials and/or solid-solid contact between the component and a microchannel cold plate. This study focuses on the two-phase thermo-fluid characteristics of a dielectric liquid, FC- 72, flowing in an asymmetrically-heated chip-scale micro-gap channel, 10 mm wide by 37 mm long, with channel heights varying from 110 mum to 500 mum and channel wall heat fluxes of 200 kW/m2 (20 W/cm2). The two-phase, area-averaged heat transfer coefficients of FC-72 reached 15.5 kW/m2-K, significantly higher than the single phase FC-72 values, thus providing cooling capability in the range associated with water under forced convection. Data obtained for single phase water yielded very good agreement with predictions for the convective heat transfer coefficients and served to validate the accuracy of the experimental apparatus and measurement technique.

Hotspot remediation with anisotropic thermal interface materials

Abstract: Shrinking chip feature sizes and increasing performance demands are resulting in non-uniform on-chip power dissipation. Sub-millimeter regions of high heat flux are developing with heat fluxes exceeding the average chip flux by a factor of six to ten and peak heat fluxes approaching 1000 W/cm2. These "flux-spots" can yield locally high temperatures ("hotspots") and extreme thermal gradients which can degrade chip performance and compromise reliability. This paper will explore the potential for spreaders and thermal interface materials (TIMs) of anisotropic thermal conductivity to mitigate on-chip hotspots. When used together with an existing global cooling solution, such anisotropic materials, bonded directly to the back of the silicon chip, can conduct heat laterally away from the flux-spot and towards cooler areas of the chip that are subjected to lower heat flux. An available analytical solution for the temperature distribution in a perfectly attached bi-layer slab, subjected to a central rectangular heat flux region, is used to study hotspot remediation with such an orthotropic TIM/spreader. The parametric sensitivity of the hotspot temperature to the in- plane conductivity, TIM/spreader thickness, chip thickness, flux-spot size, and heat transfer coefficient are studied, along with the detrimental effects of an interfacial contact resistance.

Forced convection and flow boiling of a dielectric liquid in a foam-filled channel

Abstract: Porous metal foams, inserted into the channels of a liquid cold plate, can be used to enhance forced convection and flow boiling heat transfer and may be especially useful for direct, dielectric liquid cooling of electronic and photonic components. This study explores the thermofiuid characteristics of three porous copper foam configurations: 95% porosity and 10 PPI, 95% porosity and 20 PPI, and 92% porosity and 20 PPI, which were soldered to the heated wall of a 10 mm wide times37 mm long and 7 mm high channel. The results for water are in good agreement with the available sparse porous matrix correlations, using the recommended dispersion conductivity coefficient of 0.06. Despite this relatively low dispersion coefficient, the porous foam is found to more strongly enhance the convective heat transfer coefficients for FC-72 than water. For the two-phase heat transfer rate, the high porosity and large pore size foam, i.e. the 95%, 10 PPI copper foam, was found to provide the best result, achieving a 10 kW/m2-K heat transfer coefficient.

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.

Enhanced Hot Spot Cooling Using Bonded Superlattice Microcoolers With a Trench Structure

Abstract: In this paper, we describe how to use Si/SiGe superlattice microcoolers to cool the target hot spots and how a trench structure could enhance its cooling performance. The microcooler chip is gold fusion bonded with a 65 mum-thick silicon chip, where heaters are fabricated on the opposite of fusion bonding layer to simulate the hot spots. Our 3-D electrothermal simulations showed that with a trench structure, the maximum cooling and cooling power density could be doubled at hot spot region. Our experimental prototype also demonstrated a maximum cooling of ~ 2degC reduction at hot spot or a maximum cooling power density of 110 W/cm with trench structure as compared with the 0.8degC cooling without trench structure. This two-chip bonded configuration will allow the integration of spot coolers and ICs without impact on microelectronics processing process. It could be a potential on-chip hot spot cooling solution.

Simultaneous measurements of effective chemical shrinkage and modulus evolution during polymerization

Abstract: An integrated measurement technique is proposed to measure the effective chemical shrinkage and the modulus of polymeric materials simultaneously, as a function of time, during polymerization (evolution history). The method is based on a fiber Bragg grating (FBG) sensor. A polymer is cured around a glass FBG and the Bragg wavelength (BW) shift is continuously documented while polymerization progresses at the curing temperature. Based on the theoretical relationship between the BW shift and the stress field in the FBG, the measured BW shift is used to determine the evolving effective chemical shrinkage and the modulus. The results can be employed to predict curing-induced residual stresses in geometrically-complex packaged assemblies. The proposed method is implemented on a high temperature curing epoxy.

Optimization of Pool Boiling Heat Sinks Including the Effects of Confinement in the Interfin Spaces

Abstract: A finite element analysis approach is developed and used to efficiently evaluate and optimize the boiling performance of longitudinal rectangular plate fin heat sinks, including the explicit dependence of fin spacing on boiling heat transfer coefficients and on the critical heat flux (CHF). Polished silicon heat sinks are shown to dissipate at nearly five times the CHF limit of the unfinned base area and outperform comparable aluminum heat sinks by a factor of 2. For optimum heat sink geometries, over the parameter ranges explored, the fin thickness is found to be approximately equal to the fin spacing, and the relationship between the optimum thickness and spacing is demonstrated to be relatively insensitive to the fin thermal conductivity. Results suggest that even greater performance enhancements may be gained with appropriately-selected advanced materials.

Energy Efficient Polymers for Gas-Liquid Heat Exchangers

Abstract: The present study explores the thermofluid characteristics of a seawater-methane heat exchanger that could be used in the liquefaction of natural gas on offshore platforms. The compression process generates large amounts of heat, usually dissipated via plate heat exchangers using seawater as a convenient cooling fluid. Such an application mandates the use of a corrosion resistant material. Metals such as titanium, expensive in terms of both energy and currency, are a common choice. The “total coefficient of performance,” or COPT , which incorporates the energy required to manufacture a heat exchanger along with the pumping power expended over the lifetime of the heat exchanger, is used to compare conventional metallic materials to thermally conductive polymers. The results reveal that heat exchangers fabricated of low energy, low thermal conductivity polymers can perform as well as, or better than, those fabricated of conventional materials, over the full lifecycle of the heat exchanger. Analysis of a prototypical seawater-methane heat exchanger, built from a thermally conductive polymer, suggests that a COPT nearly double that of aluminum, and more than ten times that of titanium, could be achieved.Copyright © 2008 by ASME

Numerical and experimental investigations of boiling enhancement in buoyancy-driven microchannels

Abstract: In this study, confinement-driven boiling enhancement trends and experimental data from narrow parallel plate channels are presented and analyzed via comparison with numerical simulations of buoyancy-driven boiling and two phase flow using the commercially-available Fluent CFD software package. An Euler-Euler multiphase approach, known as the volume of fluid (VOF) method, is employed, as bubbles sizes are on the order of the channel dimensions. Numerical results suggest that enhanced natural convection already accounts for a large portion of the unconfined pool boiling heat flux. While the increased buoyancy from large vapor fractions in narrow channels may lead to an order of magnitude increase in channel mass flux, confinement-driven convective enhancement is found to increase the unconfined boiling heat flux by less than 10%. Further, simulated convective enhancement is found to be a maximum for intermediate size channels, in direct contrast to experimental data which show maximum enhancement (500%) for the smallest channels investigated. Experimental results for different channel wall materials suggest an enhancement mechanism highly dependent on boiling surface characteristics.

Application of Thermally-Enhanced Thermoplastics to Seawater-Cooled Heat Exchangers

Abstract: In this study, the potential benefits of using thermally-enhanced polymers in high-performance seawater heat exchangers are assessed. The thermal and mechanical properties of commercially available, thermally-conductive resins are reviewed and compared with those of polymer heat exchangers described in the literature, as well as those of metals commonly used in compact heat exchangers, including in seawater-based cooling systems (i.e., Cu-Ni alloys, stainless steel, and titanium). This survey reveals that engineered thermoplastics have sufficiently high thermal conductivities to compete with their metal counterparts. The thermo-fluid performance of a conceptual, doubly-finned plate liquid-liquid heat exchanger module, is analytically evaluated using the -NTU method. The heat transfer rate and coefficient of performance (COP) of this heat exchanger are parametrically assessed for various fin spacings, wall thicknesses, and a near 3 orders-of magnitude range of wall thermal conductivities. This analysis shows that the thermal conductivities achievable with enhanced thermoplastics, 20 W/mK, can provide approximately half the heat transfer rate of an aluminum heat exchanger operating under the same conditions, and 80% of the heat transfer rate provided by a corrosion-resistant, metallic heat exchanger. This study indicates that thermally conductive thermoplastics offer a promising alternative to the use of conventional and/or corrosion-resistant metals in compact, high performance heat exchangers in seawatercooled applications. While conventional metal heat exchangers are generally incapable of providing reliable long-term service with seawater (and other corrosive fluids), to date the cost, complexity, and the restricted availability of exotic corrosion-resistant materials have limited the use of seawater as the ultimate heat sink for energy conversion processes. Currently available polymer heat exchangers, fabricated with thermally un-enhanced thermoplastics, are an alternative option, but are limited to relatively low heat transfer rates. In this study, the potential benefits of using thermally-conductive polymers in high-performance seawater heat exchangers are assessed. These advanced materials could provide reduced weight, greater resistance to corrosion and fouling, and reduced energy of formation and fabrication, as well as greater geometric flexibility and ease of manufacturing, relative to the conventional material technologies in use today. On the premise that such materials have the required thermal and mechanical performance characteristics, they could facilitate the development of seawater heat exchangers for the power industry, naval applications, and coastal petroleum refineries. In this paper, commercial, prototype, and research polymer heat exchangers described in the literature are firstly discussed, with an emphasis on their performance limits and physical characteristics. A review of commercially available, thermally-conductive resins is then presented, with a comparison of their thermal and mechanical properties to those of metals commonly used in compact heat exchangers, including in seawater-based cooling systems (i.e. Cu-Ni alloys, stainless steel, and titanium). Based on this review, the thermo-fluid performance of a notional, doubly-finned plate liquid-liquid heat exchanger module is analytically evaluated using the e-NTU method. The heat transfer rate and coefficient of performance (COP) 1 of this heat exchanger are assessed for various fin spacings, wall thicknesses, and a near 3 orders-of magnitude range of wall thermal conductivities.