Amy S. Fleischer

Thermal Challenges in Next-Generation Electronic Systems

Thermal challenges in next-generation electronic systems, as identified through panel presentations and ensuing discussions at the workshop, Thermal Challenges in Next Generation Electronic Systems, held in Santa Fe, NM, January 7-10, 2007, are summarized in this paper. Diverse topics are covered, including electrothermal and multiphysics codesign of electronics, new and nanostructured materials, high heat flux thermal management, site-specific thermal management, thermal design of next-generation data centers, thermal challenges for military, automotive, and harsh environment electronic systems, progress and challenges in software tools, and advances in measurement and characterization. Barriers to further progress in each area that require the attention of the research community are identified.

The Experimental Exploration of Embedding Phase Change Materials With Graphite Nanofibers for the Thermal Management of Electronics

Phase change materials PCMs are materials that undergo aphase transformation, typically the solid-liquid phase transforma-tion, at a temperature within the operating range of the thermalapplication. The latent heat absorption inherent in the phasechange process results in the maintenance of a constant operatingtemperature during the melt process. In transient applications,PCMs can thus be used to absorb heat and maintain operation at aspecified temperature. PCMs have been shown to be effective intransient thermal abatement by slowing the rate of temperatureincrease during transient operation 1 .While basic PCM systems have proven to be effective in lowvolume applications 2–12 , in larger volumes, the low thermalconductivity of the PCM for example, 0.2 W/m K for tricosaneimpedes the thermal performance. The low thermal conductivitycreates a high conductive thermal resistance and leads to the iso-lation of the melt process near the heat source. Pal and Joshi 13numerically analyzed the melting of PCM using a uniformly dis-sipating flush mounted heat source in a rectangular enclosure andestablished that for low thermal conductivity PCMs, melting islocalized near the heat source, whereas for higher conductivity,heat is more effectively distributed throughout the mass. Krishnanet al. 14 studied a hybrid heat sink/paraffin combination for usein electronics cooling applications, finding that paraffin alone isunsuitable for transient heating applications due to its low thermaldiffusivity. Therefore, for high power applications the design mustbe adapted to facilitate more effective heat flow into the PCM.The PCM is typically contained within a sealed container mod-ule located adjacent to the heat source. The PCM can melt as itabsorbs heat and then resolidify at the end of a power cycle withinthis container module. In some cases, embedded finned heat sinks 15–19 or metallic foams 20–22 have been used to facilitate theheat penetration from the module walls into the contained PCMby providing a heat flow path to the module center and thus en-suring effective heat absorption through an even melt process.However, the use of embedded heat sinks and metallic foams hasseveral significant disadvantages, including added weight, dis-placed PCM, and the difficulty of manufacturing foams in thickenough layers for larger modules. This project investigates the useof graphite nanofibers suspended within the PCM to increase ther-mal performance without significantly increasing module weightor size.One of the most commonly studied PCMs is paraffin wax. Par-affin waxes in general are inexpensive, thermally and chemicallystable, and have a low vapor pressure in the melt 23 . In thisproject, graphite nanofibers are mixed uniformly into a paraffinwax blend with a melt temperature of 56°C and the thermal per-formance of the system is quantified.Graphite nanofibers GNFs generally have diameters of2–100 nm and lengths of up to 100 m 24 . The advantage ofusing GNF as the conductivity enhancer is that they exhibit highsurface area 25 and possess thermal properties, which are of thesame order of magnitude of carbon nanotubes 24 , but with asignificantly easier and less expensive production process 25 .The suspension of graphite nanofibers in the PCM is expected toimprove the thermal diffusivity and thus the thermal performanceby reducing the bottlenecking of heat flux at the source. The em-bedding of graphite nanofibers will accomplish this through in-creased conductivity of the composite material and possiblythrough an additional nanofluid-type enhancement effect throughBrownian motion of the particles when suspended in the liquidphase. This will be accomplished with low fiber loading levels,thus preserving a maximum volume for PCM and maximizing thepossible heat absorption and duration of melt process.The GNFs used in this study are grown through the catalyticdeposition of hydrocarbons and/or carbon monoxide over metalcatalysts in a reducing atmosphere using a process previously de-scribed 25 , which will be thus only covered in summary here.The carbon precipitates as graphite, which initially encapsulatesthe metal particle. The catalyst particle is “squeezed” through,leaving a perfectly formed graphite plane. As each graphite planeis formed, the fiber grows longer along an axis extending out-wards from the metal catalyst particle. Through precise manage-ment of the deposition process, the resulting orientation of these

Dynamics of the vortex structure of a jet impinging on a convex surface

Abstract Smoke–wire flow visualization is used to investigate the behavior of a round jet issuing from a straight tube and impinging on a convex surface. Video analysis of the impinging jet shows the initiation and growth of ring vortices in the jet shear layer and their interaction with the cylindrical surfaces. Effects of relative curvature, nozzle-to-surface distance, and Reynolds number on vortex initiation, vortex separation from the surface and vortex breakup are described. Examples of vortex merging are discussed.

Effect of graphene layer thickness and mechanical compliance on interfacial heat flow and thermal conduction in solid-liquid phase change materials.

Solid-liquid phase change materials (PCMs) are attractive candidates for thermal energy storage and electronics cooling applications but have limited applicability in state-of-the-art technologies due to their low intrinsic thermal conductivities. Recent efforts to incorporate graphene and multilayer graphene into PCMs have led to the development of thermal energy storage materials with remarkable values of bulk thermal conductivity. However, the full potential of graphene as a filler material for the thermal enhancement of PCMs remains unrealized, largely due to an incomplete understanding of the physical mechanisms that govern thermal transport within graphene-based nanocomposites. In this work, we show that the number of graphene layers (n) within an individual graphene nanoparticle has a significant effect on the bulk thermal conductivity of an organic PCM. Results indicate that the bulk thermal conductivity of PCMs can be tuned by over an order of magnitude simply by adjusting the number of graphene layers (n) from n = 3 to 44. Using scanning electron microscopy in tandem with nanoscale analytical techniques, the physical mechanisms that govern heat flow within a graphene nanocomposite PCM are found to be nearly independent of the intrinsic thermal conductivity of the graphene nanoparticle itself and are instead found to be dependent on the mechanical compliance of the graphene nanoparticles. These findings are critical for the design and development of PCMs that are capable of cooling next-generation electronics and storing heat effectively in medium-to-large-scale energy systems, including solar-thermal power plants and building heating and cooling systems.

Transient thermal management using phase change materials with embedded graphite nanofibers for systems with high power requirements

Phase change materials (PCMs) exhibit excellent thermal storage capacity due to their high latent heat of transformation and have been successfully utilized in small volumes for transient thermal management of electronics. However, their low thermal conductivity makes it difficult to utilize large volumes of PCMs for transient thermal management of larger systems. To improve the thermal performance, high thermal conductivity graphite nanofibers are embedded into a paraffin PCM. The thermal effects of fiber loading levels, measured in weight percent (0 to 10%) are examined for a system with power loads between 100 and 700 W. The use of the graphite nanofiber enhancement is found to double the useful performance time of the PCM and lower the system operating temperature.

Development of Methods to Fully Saturate Carbon Foam With Paraffin Wax Phase Change Material for Energy Storage

In this work, the effect of infiltration method on the saturation rate of paraffin phase change material within graphite foams is experimentally investigated. Graphite foams infiltrated with paraffin have been found to be effective for solar energy storage, but it has been found that it is difficult to completely saturate the foam with paraffin. The effectiveness of the fill will have a significant effect on the performance of the system, but the data on fill ratio are difficult to separate from confounding effects such as type of graphite or phase change material (PCM) used. This will be the first detailed quantitative study that directly isolates the effect of infiltration method on fill ratio of PCM in graphite foams. In this work, the two most commonly reported methods of infiltration are studied under controlled conditions. In fact, the effect of the infiltration method on the paraffin saturation rate is found to be highly significant. It was found that the more commonly used simple submersion technique is ineffective at filling the voids within the graphite foam. Repeated tests showed that at least 25% of the reported open space within the foam was left unfilled. In contrast, it was found that the use of a vacuum oven lead to a complete fill of the foam. These high saturation rates were achieved with significantly shorter dwell times than in previously reported studies and can be of significant use to others working in this area.

Experimental Investigation of the Thermal Performance of Graphite Foam for Evaporator Enhancement in Both Pool Boiling and an FC-72 Thermosyphon

High-conductivity graphite foam is investigated for use as a surface enhancement for improved thermal performance in both pool boiling and an FC-72 thermosyphon. The influences of heat load and fluid level on the overall system thermal performance including surface superheat, effective heat transfer coefficient, and thermal resistance are examined. The thermal resistance of the foam heat sink is found to be extremely low at a minimum of 0.024 K/W, well below that of many other methods. The featured low thermal resistance is the primary benefit of this system. The thermal resistance is found to rise with increasing heat flux, but still remains advantageously low and exhibits excellent potential for high heat flux dissipation with low surface superheat, making it suitable for thermal management of advanced electronics.

Evaluation of methods to fully saturate carbon foam with paraffin wax phase change material for energy storage

In this work, the effect of infiltration method on the saturation rate of paraffin phase change material within graphite foams is experimentally investigated. Graphite foams infiltrated with paraffin have been found to be effective for energy storage, but it is difficult to completely saturate the foam with paraffin. In this work, two methods are used to infiltrate the foam: a simple submersion technique and a vacuum infiltration technique. The effect of the infiltration method on the paraffin saturation rate is found to be significant with the vacuum system yielding better results.

Forced convective cooling of electro-optical components maintained at different temperatures on a vertically oriented printed circuit board

Forced convective heat rejection from electro-optical components maintained at different maximum operating temperatures, 60/spl deg/C and 100/spl deg/C above ambient (25/spl deg/C), on the same vertically orientated single circuit board (either FR4 or copper clad FR4) was experimentally studied. Reynolds numbers ranged from 0-20 000 in which forced ambient air was passed in the horizontal direction parallel to the plane of the board in a wind tunnel. The effect of component proximity and orientation on maximum power dissipation was explored. Observed thermal behavior patterns included an increase in power dissipation with Reynolds number, an increase in power dissipation with component spacing, and in increase in power dissipation with circuit board thermal conductivity. A significant influence of component arrangement (on the same horizontal plane versus on the same vertical plane) and relative location of the hotter component on the power dissipated was also observed and was influenced by board conduction, thermal wake interactions and/or wake shedding. Results provide placement criteria needed for designers to optimally place optical and electrical components in close proximity to each other while still achieving maximum power dissipation within given thermal management constraints.

An investigation into the solidification of nano-enhanced phase change material for transient thermal management of electronics

Cyclically utilized electronics provide an interesting challenge for thermal management. Phase Change Materials (PCM) are ideal for cyclic operations due to their high capacity to store heat, however, many phase change materials do not exhibit sufficient conductivity to be effective in larger sizes. Conductivity enhancement can be done in a number of ways including the use of foams or nanomaterials. This experimental study examines the thermal behavior of PCMS with carbon nanofibers conductivity enhancement during solidification. The enhanced PCM is found to exhibit lengthened melt times and shortened cool-down times.