Project Description

Thermal management of spacecraft and space station environments is an important issue in both manned and unmanned exploration of space. Transporting heat away from spacecraft components as well as bringing heat to other systems often rely on large, liquid-based heat exchange systems. Such active systems add extra weight to the spacecraft and have 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 and exhibit low reflectance (<0.171) and high absorbtance (>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 though 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 been revealed to be an excellent way to account 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 (Figure 1). The tips are on the order of 10 - 20 mm in base diameter and 20 - 30 mm tall. MCA surfaces feature large (more than 10 X) 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 (Figure 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, an Mo foils.

Measurements on various MCA samples have been performed at NASA JSC. Measurements of reflectance from 250 nm to 2.8 mm to calculate an integrated absorbance a over that range and single average reflectance r over the spectral range of 2.5 - 30 mm 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 a over the range 250 nm - 2.8 mm varied between 0.97 and 0.985 while the average reflectance r over the long-wavelength range 2.5 - 30 mm 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) and 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 mm, respectively, which matches up well with the absorbtance of MCAs (0.88 as averaged over the entire 2.5 - 30 mm range). The 10X increased surface area from the MCA structures would also provide improved convective cooling in an atmospheric environment when compared to the smooth, unprocessed materials. For temperatures in the 1000 - 1600ºC range, applicable to the leading edges of vehicles upon atmospheric re-entry, the corresponding peak emission wavelengths range from 2.28 - 1.55 mm, 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 and 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 in the following five tasks:

Necessary JSC Resources for the Project

Facilities for the measurement of optical emission from MCA samples in the UV-Vis-near IR (up to ~3.0 mm) are available in our laboratory. We would require access to the 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 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, 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," Journal of Infrared Physics and Technology (in press).

Position Description

The ideal candidate should have a recent Ph.D. in Materials Science, Physics, Electrical Engineering or related fields. A dissertation topic or proven expertise in thin film optical devices fabrication, characterization and simulation is a requirement. Familiarity with high vacuum technology and thin film growth is highly desirable.  The candidate should also have an excellent command of the English language and good communication skills.

The post-doctoral fellow will concentrate primarily on issues related to the application of MCA technology to thermal management in NASA environments. His contributions will be critical in (a) understanding the specific needs of NASA in the area of thermal management; and (b) tailoring MCA technology to NASA requirements - weight, thermal conductivity, operating temperature range, stability, reliability, etc. Most importantly, the post doctoral fellow will hopefully serve as a catalyst to foster a long term collaboration between UH and JSC researchers in this important area of thermal management technologies for space environments.

Professional Biographies of the Principal Investigators  

Abdelhak Bensaoula, Full Research Professor
TcSAM
University of Houston
4800 Calhoun Rd, SR-1, Rm 724
 Houston, TX 77204-5004
(713-743-3621); FAX: (713)- 747-7724
bens@svec.uh.edu

Professional Experience
2000- Present  Research Professor of Physics, The University of Houston
1992- 2000  Research Associate Professor, The University of Houston
1990-1992  Research Assistant Professor, The University of Houston
1988-1999  Senior Research scientist, University of Houston, Space Vacuum Epitaxy Center
1986-1988  Research Scientist, University of Houston, Space Vacuum Epitaxy Center

Patents
Tandem Solar Cell with Improved Tunnel Junction (patent # 5,407,491)
Strained quantum well photovoltaic energy conventional (patent # 5,851,310)
Tandem solar cell with indium phosphide tunnel junction (patent # 5,800,630)
A microelectromechanical machined array valve (5,927,325)
Real Time Etch Rate Determination and Enhanced Etch End Point Detection Using Isotopically Engineered Materials (6,054,333).
Group III Nitride Field Emitters (6,218,771)
Capacitor and method of storing energy (6,570,753)
One-chip micro-integrated optoelectronic sensor (6,608,360)

Research Activities
(1995- 2004) Developed a Multi-faceted Nitride Materials and Devices Program. The focus is on in-situ monitoring of thin film growth and development of high temperature environmental microsensors.
(1992 -1995) Initiated a program for high efficiency Photovoltaic device research using chemical beam epitaxy. This program is continuing under Professor Alex Freundlich.
(1992-2000) Developed various thin film monitoring instrumentation in collaboration with a local industry. Some of the tools are currently commercialized (www@Ionwerks.com).
(1990-2001) Developed a Graduate/undergraduate training program between various Universities in Algeria, France and Mexico and the University of Houston (12 BS, 9 MS and 5 Ph.D. students were trained to date).

Publications
Over 150 in peer reviewed journals

Chris Boney, Ph.D., Research Scientist
TcSAM
University of Houston
4800 Calhoun Rd, SR-1, Rm 724
Houston, TX 77204-5004
(713) 743-3621; FAX: (713) 747-7724
cboney@svec.uh.edu

Dr. Boney is a research scientist at the Texas Center for Superconductivity and Advanced Materials. He has expertise in the areas of II-VI and III-N growth by MBE and MOCVD, thin film characterization techniques, and device fabrication and testing. His experience in optical characterization of semiconductor films has been recently applied to optical measurements of MCA structures. Dr. Boney's current scientific interests include the growth and fabrication of III-nitride optoelectronic sensors, study of the physical and optical properties of Micro Column Arrays, and investigation of ferromagnetic semiconductor-based materials.

Selected Publications
Carreno, L. A., C. Boney, and A. Bensaoula. In-situ Determination of Surface Composition, Polarity, Crystallographic Relationship and Periodicity of GaN Films by Mass Spectroscopy of Recoiled Ions and Direct Recoil Spectroscopy," J. Appl. Phys. 94 (2003): 7883.
Mouffak, Z., N. Medelci-Djezzar, C. Boney, A. Bensaoula, and L. Trombetta. "Effect of Photo-Assisted RIE Damage on GaN," MRS Internet J. Nitride Semicond. Res. 8 (2003): 7.
Starikov, D., 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," Journal of Infrared Physics and Technology (in press).
Starikov, D., C. Boney, J-W. Um, N. Medelci, and A. Bensaoula. "Experimental simulation of integrated optoelectronic sensors based on III Nitrides," J. Vac. Sci. Tech. B 20.5 (2002): 1815-20.
Starikov, D., C. Boney, I. Berishev, I. C. Hernandez, and A. Bensaoula, "Radio-frequency molecular beam epitaxy growth of III nitrides for microsensor applications," J. Vac. Sci. Tech. B 19.4 (2001): 1404-08.
Brown, J. D., J. Boney, J. Matthews, P. Srinivasan, J. F. Schetzina, T. Nohava, W. Yang, and S. Krishnankutty. "UV-Specific (320-365 nm) Digital Camera Based On a 128x128 Focal Plane Array of GaN/AlGaN p-i-n Photodiodes," MRS Internet J. Nitride Semicond. Res. 5.6 (2000). <http://nsr.mij.mrs.org/5/6/>

Brian M. Mayeaux, Ph.D., Materials Research Engineer
Materials and Processes Branch
Mail Code ES4
(281) 244-5802
brian.mayeaux1@jsc.nasa.gov

Dr. Mayeaux received his Ph.D. in Materials Science from Rice University while working at the NASA Johnson Space Center in the Materials and Processes Branch.  He has worked for NASA for over 12 years and has experience in a variety of projects and programs including Space Shuttle mission simulations, spacesuit engineering and flight control, and failure analysis.  Prior to the Columbia accident in 2003, he served as the lead for Failure Analysis Integration at Johnson Space Center in support of the Space Shuttle and Space Station Programs.  Additionally, he conducts research on thermal properties of nanotube composites and space applications, and serves as a Science Advisor in the Clear Creek School District. He is currently serving as the Systems Engineering Lead for Materials Development in support of Space Shuttle Return To Flight activities.

David Starikov, Ph.D., Director of Research
Integrated Micro Sensors, Inc. (IMS)
10814 Atwell Drive
Houston, TX 77096
(713) 748-7926; FAX: (713) 747-7724
dstarikov@imsensors.com

Dr. David Starikov serves as a Director of Research at Integrated Micro Sensors Inc. His background is in the development and fabrication of optoelectronic components based on wide bandgap materials. He has extensive expertise in employment of laser ablation for wide bandgap material growth and processing. As a Principal Investigator Dr. Starikov has completed several Phase I and Phase II SBIR projects dedicated to development of advanced optoelectronic systems, and employment of the MCA technology in several military and industrial applications. Dr. Starikov holds 5 Russian and 2 US patents on the development of optoelectronic devices for advanced miniature multifunctional optoelectronic biochemical sensors for super-ambient environments.

Selected publications
Ageev, V., S. Klimentov, M. Ugarov, E. Loubnin, A. Bensaoula, N. Badi, A.Tempez, and D. Starikov; "Enhanced free carrier generation in boron nitride films by pulsed laser radiation," Applied Surface Science 138-139 (1999): 364-69.
Bensaoula, A., C. Boney, R. Pillai, G.A. Shafeev, A.V. Simakin, and D. Starikov Arrays of 3D micro-columns generated by laser ablation of Ta and steel: modeling of a black body emitter," European Journal of Appl. Phys. A 00 (2004): 1-3.
Starikov, D., N. Badi, I. Berishev, N. Medelci, O. Kameli, M. Sayhi, V. Zomorrodian, and A. Bensaoula. "Metal-insulator-semiconductor Schottky barrier structures fabricated using interfacial BN layers grown on GaN and SiC for optoelectronic device applications," J. Vac. Sci. Technol. A 17.4 (1999): 1235-38.
Starikov, D., I. Berishev, J.-W. Um, N. Badi, N. Medelci, A. Tempez, and A. Bensaoula. Diode Structures Based on p-GaN for Optoelectronic Applications in the Near-Ultraviolet Range of the Spectrum," J. Vac. Sci. Tech. B 18.6 (2000): 2620-23.
Starikov, D., C. Boney, I. Berishev, I.C. Hernandez, and A. Bensaoula. Radio-frequency molecular beam epitaxy growth of III nitrides for microsensor applications," J. Vac. Sci. Tech. B 19.4 (2001): 1404-08.
Starikov, D., C. Boney, N. Medelci, J-W. Um, A. Bensaoula , M. Larios Sanz and G. E. Fox, "Experimental simulation of integrated optoelectronic sensors based on III Nitrides," J. Vac. Sci. Tech. B 20.5 (2002): 1815-20.
Starikov, D., 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," Journal of Infrared Physics and Technology 45.3 (2004): 159-67.