The electronics market is driven by the desire for higher performance and small size, and therefore constantly increasing power density. While shifting to lower operating voltages and more efficient circuit designs have helped minimize heat loads, greater performance demands will inevitably lead to higher heat fluxes. This high thermal design heat flux is necessary to maintain lower operating temperatures, which ensure reliability and result in reduced gate delay and higher processor speed. Typically, 85 °C is considered the thermal design temperature limit for high performance memory and logic chips, while higher temperature limits may be appropriate for other devices.

One way to manage this thermal load is with a thermosyphon. A two-phase closed thermosyphon consists of an evaporator, a condenser, and an adiabatic section that allows a working fluid to travel between the other two components. Vapor generated at the evaporator rises due to buoyancy forces, and then condenses at the top of the chamber at the condenser, releasing its latent heat. Gravity then returns the condensate back to the evaporator, and the process repeats. Heat generated by a microprocessor could be transferred to the evaporator of a thermosyphon that is bonded with a thin thermally conductive interface to the backside of the chip. At the evaporator, heat would vaporize a working fluid such as FC-72 or FC-87, and ultimately, heat would be dissipated at the condenser.

This research concerns the evaluation of graphitized carbon foam as the evaporator for a thermosyphon. The graphite foam used in this study is a mesophase-pitch-derived carbon foam developed by James Klett at Oak Ridge National Laboratory and produced by Poco Graphite. It is an ideal thermal management material because of its low density, high thermal diffusivity, and a coefficient of thermal expansion that is close to that of silicon.

 

The open-celled structure (photo at right) is easily wetted by FC-72 and FC-87, and the graphitized ligaments provide numerous potential nucleation sites. Although the porosity of the foam can be 80% or higher, the thermal conductivity of the graphite ligaments is about four times that of copper, resulting in a foam thermal conductivity similar to that of solid aluminum. When bonded to a heated surface such as an electronic chip, the heat is dissipated over a wide area within the foam. Nucleate boiling is used to efficiently remove this heat at low wall temperatures.

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Parameters that were optimized in the research were foam size, foam density, liquid level, attachment method, pressure, and fluid. We have been able to obtain heat transfer levels of 140 W from a 1 cm2 surface at a temperature of 85 °C. Boiling data using the foam is documented in the papers below:

1). Coursey, J.S., Roh, H., Kim, J., and Boudreaux, P.J., “Graphite Foam Thermosyphon Evaporator Perfomance: Parametric Investigation of the Effects of Working Fluid, Liquid Level, and Chamber Pressure”, Proceedings of the 2002 ASME IMECE, New Orleans, LA, paper No. 2002-33733.

2). Coursey, J. S., Kim, J., and Boudreaux, P.J., “Performance of graphite foam evaporator for use in thermal management”, Journal of Electronic Packaging, Vol. 127, No. 2, pp. 127-134, 2005.

 

This work would not have been possible without the support of Mr. Paul Boudreaux at the Laboratory for Physical Sciences. Graphite foam samples were provided by Poco Graphite. The support of James Klett at ORNL is gratefully acknowledged.