THERMAL MANAGEMENT OF spacecraft and space station environments is an important issue in both the manned and unmanned exploration of space. Transporting heat away from spacecraft components and bringing heat to other systems often rely on large, liquid-based heat exchange systems. Such active systems add extra weight to the spacecraft and comprise additional mechanical components which can malfunction, thus affecting maximum payload and mission lifetime. A possible alternative is a passive cooling system in which thin coatings or foils would collect or remove heat by radiative absorption or emission.

A technology for the successful fabrication of Micro Column Arrays (MCAs) on thin metal foils has recently been developed in conjunction with Integrated Micro Sensors, Inc. (IMS) of Houston, TX. MCAs consist of densely packed micro cones separated by cone-shaped micro cavities. They exhibit low reflectance (< 0.171) and high absorptance (>0.978) over a wide spectral range in a very close approximation of blackbody behavior. The goal of this project is to explore the use of MCA structures on metal foils for heat acquisition and/or heat rejection through their near-blackbody nature.

In-depth simulation of their heat transport properties will be undertaken using a newly developed Transmission Line Matrix (TLM) methodology. In this approach, a novel TLM link line is introduced to account for the enthalpy heat transport in a fluid or gas. Incorporation of an electrical diode in the new enthalpy link has proven to be an excellent way of accounting for the heat convection without altering the classical TLM algorithm arrangement. Full extension of this model to radiative heat dissipation and collection will be undertaken.

Technical Approach

MCAs are produced by pulsed laser ablation combined with mechanical translation of the substrate material to create cone-shaped micro tips interdigitated with cone-shaped micro cavities^1,2 (Fig. 1). The tips are on the order of 10-20 μm in base diameter and 20-30 μm tall. MCA surfaces feature large (more than 10X) specific areas, low-threshold electron field emission, and unique optical properties.³ To date, MCA fabrication has been realized on a variety of metal foils including stainless steels and refractory metals (Fig. 2).

Figure 1. SEM Image of Micro Column Arrays Generated on Stainless Steel Foil

Figure 2. MCA Samples Fabricated Using Hastelloy, Alloy 321, Ta, Ti, and Mo Foils

Measurements on various MCA samples have been performed at NASA-JSC. Measurements of reflectance from 250 nm to 2.8 μm to calculate an integrated absorbance α over that range and single average reflectance ρ over the spectral range of 2.5-30 μm were undertaken. In both cases, the front (MCA processed) and back (unprocessed) of each metal foil was measured. The results clearly demonstrate the drastic reduction in reflectance with corresponding increase in absorbtance on the MCA-processed side. For the MCA metal foils studied, the average α over the range 250 nm-2.8 μm varied between 0.97 and 0.985 while the average reflectance ρ over the long-wavelength range 2.5-30 μm varied between 0.12 and 0.155.

Previous research has demonstrated that MCAs act as micro cavities to efficiently trap and absorb light similarly to blackbody emitters.⁴ Metal strip samples were resistively headed in vacuum to temperatures up to 1360ºC (for the tantalum). The resulting optical emission was recorded. These spectra were compared to that of a large, cavity-type blackbody simulator. Results indicate that the emission from the MCA structures closely follows that from a blackbody source. In cases where heat acquisition is desired, the high absorbtance of the MCAs over a wide wavelength range could provide efficient heating through the conversion of incident solar energy.

Likewise, the high emissivity of the MCA structures means that they could be used as efficient radiative emission sources. For example, MCAs can be used as passive cooling elements for mechanical or electronic systems by radiating away the excess heat in the IR wavelengths. Blackbody temperatures between 50-100ºC have corresponding peak emission wavelengths from 9.25-7.77 μm, respectively, which match up well with the absorbtance of MCAs (0.88 as averaged over the entire 2.5-30 μm range). The 10X increased surface area from the MCA structures would also provide improved convective cooling in an atmospheric environment in comparison with smooth, unprocessed materials. For temperatures in the 1000-1600°C range, applicable to the leading edges of vehicles upon atmospheric re-entry, the corresponding peak emission wavelengths range from 2.28-1.55 μm, respectively, which also match up well to the spectral regions of high absorbtance of MCA structures.

Heat transfer from one medium to another depends critically on both the thermal properties of the media and the interfacial region area and geometry. While both parameters can be tailored to satisfy a particular application, the media (i.e., the substrate material to heat or cool and the heat dissipater/source are sometimes dictated by other considerations and properties (optical, mechanical, etc.). Their modification/substitution is thus either impractical or expensive. On the other hand, engineering of the existing interface to enhance the heat management characteristics of a system might be realized without significantly perturbing the traditional set up; thus, such an approach is highly desirable.

The use of MCA materials as passive heating or cooling elements could potentially reduce the size, complexity, and weight of thermal management solutions currently used in space. The fact that MCA structuring can be accomplished on most metals means that application-specific choices of materials can be made to balance the issues of weight, thermal stability, and/or thermal conductivity.

Project Work Plan

The project will be divided into the following five tasks:

1. Determine material/thermal requirements for NASA applications. Task leaders: A. Bensaoula and B. Mayeaux.
2. MCA fabrication processing and optical properties measurements. Task leaders: C. Boney and D. Starikov.
3. TLM Simulation. Task leaders: A. Bensaoula and Post-Doctoral Fellow (to be named).
4. Thermal/environmental stability testing. Task leaders: Post-Doctoral Fellow and B. Mayeaux.
5. Prototype fabrication and testing. Task leaders: A. Bensaoula and Post-Doctoral Fellow.

NASA-JSC Resources

Facilities for the measurement of optical emission from MCA samples in the UV-Vis-near IR (up to ~3.0 μm) are available in the UH laboratory. UH investigators would require access to the NASA-JSC spectroreflectrometers mentioned earlier in this proposal in order to measure the absorbtance and reflectance of the MCA materials produced during this project. Additional facilities for the measurement of longer IR wavelength emission would be helpful, if available at NASA-JSC. Potential collaboration with Marshall Flight Center has been discussed with the JCS Project Manager.

References

¹F. Sánchez, J. L. Morenza, R. Aguiar, J. C. Delgado, and M. Varela. “Whiskerlike Structure Growth on Silicon Exposed to ArF Excimer Laser Irradiation,” Appl. Phys. Lett. 69 (1996): 620-22.
²S. I. Dolgaev, S. V. Lavrishev, A. A. Lyalin, A. V. Simakin, V. V. Voronov, and G. A. Shafeev. “Formation of Conical Microstructures Upon Laser Evaporation of Solids,” Appl. Phys. Lett. A 73.2 (2001): 177-81.
³C. Wu, C. H. Crouch, L. Zhao, J. E. Carey, R. Younkin, J. A. Levinson, E. Mazur, R. M. Farrell, P. Gothoskar, and A. Karger. “Near-Unity Below-Band-Gap Absorption by Microstructured Silicon,” Appl. Phys. Lett. 78 (2001): 1850-52.
⁴D. Starikov, C. Boney, R. Pillai, A. Bensaoula, G. A. Shafeev, and A. V. Simakin. “Spectral and Surface Analysis of Heated Micro-Column Arrays Fabricated by Laser-Assisted Surface Modification,” J. Infrared Physics and Technology. (In press.)

PRINCIPAL INVESTIGATORS
Abdelhak Bensaoula, UH / Chris Boney, UH /
Brian M. Mayeaux, NASA-JSC / David Starikov, IMS

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Institute for Space Systems Operations - Y2003 Annual Report
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