Planar, semiconductor heat arrays have been previously proposed and developed; however, this design makes use of a novel, microscale black silicon wick structure that provides increased capillary pumping pressure of the internal working fluid, resulting in increased effective thermal conductivity of the device, and also enables operation of the device in any orientation with respect to the gravity vector.

SEM photo showing black silicon formed by ICP cryo etching.
In a heat pipe, the efficiency of thermal transfer from the case to the working fluid is directly proportional to the surface area of the wick in contact with the fluid. Also, the primary failure mechanism for heat pipes operating within the temperature range of interest is inadequate capillary pressure for the return of fluid from the condenser to the wick. This is also what makes the operation of heat pipes orientation- sensitive. Thus, the two primary requirements for a good wick design are a large surface area and high capillary pressure. Surface area can be maximized through nanomachined surface roughening. Capillary pressure is largely driven by the working fluid and wick structure.

The proposed nanostructure wick has characteristic dimensions on the order of tens of microns, which promotes menisci of very small radii. This results in the possibility of enormous pumping potential due to the inverse proportionality with radius. Wetting, which also enhances capillary pumping, can be maximized through growth of an oxide layer or material deposition (e.g. TiO2) to create a superhydrophilic surface.

In addition, the wick fabrication technique produces nanostructure forests that are planar, and can take advantage of 2D heat spreading over a surface vs. stateof- the-art 1D heat transport associated with heat pipes. The combined result of these benefits promises to be a two-phase heat transfer device, which is very insensitive to a gravity field. Although liquid pressure drops may be relatively large depending on the nanostructure density, the overall device dimensions of ≈ 7×7 cm are expected to be well within the overall capillary limit. The novel aspects of the currently proposed effort include the use of the phenomenon of superhydrophilicity in a heat pipe, and the wick geometry, control of the nanotip height density, and the method for creating this nanotip texture. A cryo-etch inductively coupled plasma is used to make the nanotips, enabling the cost-effective, mask-free formation of a uniform black silicon surface over a large area, with nanotip heights exceeding 100 microns. Unlike nanotextured surfaces formed by the growth or deposition of materials (e.g. carbon nanotubes), the resulting formations are robust and compatible with liquid processes.

This work was done by Karl Y. Yee, Eric T. Sunada, Gani B. Ganapathi, Harish Manohara, and Andrew Homyk of Caltech, and Mauro Prina of SpaceX for NASA’s Jet Propulsion Laboratory. NPO-47299

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Micro-Textured Black Silicon Wick for Silicon Heat Pipe Array

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This article first appeared in the March, 2013 issue of NASA Tech Briefs Magazine.

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