Randy D. Weinstein

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

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.

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.

An Experimental Study of Minimum-time Optimal High Pressure Gas Storage System Recharging

This work describes the implementation and experimental application of a nonlinear optimal control strategy for recharging gas storage tank systems from a high pressure source. The objective of the open-loop optimal control strategy presented here is to refill a high pressure gas storage tank in minimum time with a specified quantity of gas subject to pressure and temperature constraints. The nonlinearity in the system model arises from the non-ideal thermodynamic behavior of the gas at the high operating pressures under consideration. The control strategy is illustrated using an experimental carbon dioxide recharging system. We choose this system because there are significant deviations from ideal behavior at relatively low pressure.

Analysis of transient thermal management characteristics of PCM with an embedded carbon fiber heat sink

There has been a strong interest in the use of phase change materials (PCMs) for the transient thermal abatement of small size electronics and several experimental studies have examined various aspects of PCM implementation. However, the adaptation of this technology for larger systems will require a scale-up in both physical size and power density. In larger volumes, the poor thermal conductivity of the PCM itself (around 0.2 W/mmiddotK) becomes a considerable limitation. The thermal resistance into a large volume of PCM is significant and the design must be adapted to facilitate greater heat flow into the PCM container. This study experimentally investigates the performance of PCM with an embedded light weight carbon fiber heat sink as an effective thermal management technique for high power transient applications

A Minimum-Time Optimal Recharging Controller for High Pressure Gas Storage Systems

A minimum-time optimal recharging control strategy for high pressure gas storage tank systems is described in this work. The goal of the nonlinear model-based controller is to refill the tank in minimum time with a two-component gas mixture of specified composition subject to hard constraints on the component flow rates, tank temperature, and tank pressure. The nonlinearity in this system arises from the non-ideal behavior of the gas at high pressure. The singular minimum-time optimal control law can not be reliably implemented in the target application due to a lack of sensors. Minimum-time optimal control is therefore approximated by a nonlinear model-based constraint controller. In order to account for the uncertainty in the unmeasured state of the storage tank, the state sensitivities to the control and process measurements are propagated along with the state to obtain a state variance estimate. When the variance of the state exceeds a maximum threshold, the constraint control algorithm automatically degrades into a fail-safe operation.

Thermal Analysis of Phase Change Materials With Embedded Graphite Nanofibers for Thermal Management of Electronics

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 diffusivity makes it difficult to utilize large volumes of PCMs for transient thermal management of high power density systems. To improve the thermal performance of a paraffin PCM, high thermal conductivity graphite nanofibers are embedded into the paraffin PCM. The thermal effects of graphite fiber loading levels, measured in weight percent, are examined for a 131 cm3 volume cubic system with power loads of 3 and 7 W. It is found that the thermal response of the system improves with increased fiber loading levels.Copyright

Thermal Management of Heat Generating Devices in Close Proximity on Printed Circuit Boards

This study uses experimentally validated computational fluid dynamics models to predict the behavior of the localized thermal interactions between adjacent components on vertically orientated circuit boards in natural convection. Using the developed models, the effects of power density, component proximity, component geometry, circuit board material, board packing density and board separation distance on the maintenance of optimal operating temperatures for all components are investigated. The device separation distance beyond which the components no longer thermally influence each other is identified and the influence of various parameters on this distance is studied. The parametric study is designed using Design of Experiments methodology to best interpret the interaction between parameters and can easily be applied to other packaging situations to allow designers to optimally place components on a circuit board in close proximity to minimize negative thermal interactions.Copyright

Liquid and Supercritical Carbon Dioxide Loading into Chewing Gum Base

The sorption and loading of liquid and supercritical CO2 into chewing gum base spheres with radii of 0.5 cm was experimentally explored over the temperature and pressure ranges of 25−45 °C and 70−276 bar, respectively. Maximum loading amounts were found to be independent of temperature and to increase with increasing CO2 density. The time required to achieve maximum loading decreased with increasing temperature. A loading as high as 0.094 g of CO2 per gram of gum base was achieved. The sorption process was modeled by unsteady-state radial Fickian diffusion assuming a constant diffusion coefficient at a particular temperature and pressure (CO2 density). Volume changes of the polymer blend spheres under the conditions explored were small and ignored in the modeling. Sorption diffusion coefficients were on the order of 10-10 m2/s and increased with increasing temperature and CO2 density.