J. Li, J. Xie, M. Zhang
Florida State University,
Keywords: carbon nanotube, foam, heat spreader
Summary:Semiconductor devices have suffered large on-chip temperature gradients due to localized high heat flues resulting from the substantial non-uniformity in power dissipation. To solve the heat problems, the flat heat pipes (HPs) are excellent candidates for cooling electronics as heat spreader . The wick materials in HPs play a key role on the cooling performance. In this work, we fabricated nanocarbon foams and evaluated their performance as wick material. Nanocarbon foam is porous carbon material based on carbon nanotubes (CNTs). Different from various methods used to produce CNT foams [2-4], we made nanocarbon foams by using polymer spheres as templates and achieved the foams with regular cell shape and controllable pore size. The process involves coating CNTs on PMMA (poly(methyl methacrylate)) microspheres in a solvent with polyacrylonitrile (PAN), obtaining a densely packed CNT/polymer composite, then removing PMMA from the solid by heating the composite to a temperature higher than PMMA’s boiling point, and finally annealing at the temperature of PAN carbonization . The previous positions of the PMMA microspheres are the cells of the foam and the CNT networks form the struts of the foam. Here, PAN is used to reinforce the CNT network by locking the contacted CNTs and generating connections among CNTs because PAN is converted into nano-graphitic structures through carbonization process . Figure 1 shows a typical structure of the nanocarbon foam. The nanocarbon foams with cell size from 6 μm to 100 μm, and the density from 29 to 200 mg/cm3 were prepared. Three factors, liquid delivery, vapor removal, and heat transfer, are used to evaluate wick structures. Nanocarbon foams have very good heat transfer property. Their capability in liquid delivery is tested by measuring the capillary rise of the liquid in the foams. The infrared (IR) thermal imaging method was used to track the moving of the wicking front. Figure 2 shows an IR image of the experiment and the capillary rise height over time. The capillary rise height is extracted from the IR thermal image and the tip of the wicking front is chosen as the location of capillary rise height. We found that the capillary rise of liquid is faster and higher in the foams with bigger pores and relatively higher density. The weight change of the samples before and after the capillary rise shows that ~90% pore space below the wicking front is filled with liquid, which indicates that the foam has very high capacity to store and transfer liquid due to its nano- and micro- wick structures. Nanocarbon foams are lightweight, electrically and thermally conductive, elastic, and stable. As a wick material, it will not only have excellent heat transfer function, but also provide flexibility and reliability.