Manufacturing & Prototyping

Integrated PEMFC Flow Field Design for Gravity-Independent Passive Water Removal

The design solves safety as well as reliability issues. A gravity-independent PEM (proton exchange membrane) fuel cell stack has been developed that will operate at high-pressure H2 and O2 conditions with the requirement for relatively modest H2 and O2 gas circulation. Until now, in order to get higher efficiency, excess reactant gas flow was required to prevent water slug formation in gas channels, thus reducing fuel cell performance. In addition, this excess gas flow is typically supported by mechanical pumps and/or a high-pressure ejector system. All of these in a closed space environment contributed to potential safety as well as reliability issues due to the potential failure of mechanical pumps and ejectors.

Posted in: Briefs, Manufacturing & Prototyping

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Metal-Assisted Fabrication of Biodegradable Porous Silicon Nanostructures

Silicon nanostructures are fabricated from single-crystal silicon by an electroless chemical etch process. Porous silicon nanowires are fabricated by two-step, metal-assisted electroless chemical etching of p-type or n-type silicon wafers. This method, in combination with nanolithography or nanopatterning, can be applied to fabricate porous silicon nanostructures of different shapes and sizes, such as nanorods, nanobelts, nanostrips, and nanochains. The specific resistivity of the silicon substrate, and composition of the etching solution, determine the porosity and pore size or lack thereof of the resulting nanostructures. Silicon doping, type of metal catalyst, concentrations of H2O2, and solvent all affect the formation of porous nanostructures at various resistivity ranges of silicon. A phase diagram summarizing the relation of porosification and doping, metal, concentrations of H2O2, and solvent can be generated. In this innovation, high-aspect-ratio porous silicon nanostructures, such as those previously mentioned, were fabricated from single-crystal silicon by an electroless chemical etch process. A metal film, metal nanofeatures, or metal nanoparticles were coated on the silicon substrate first, and a solution of HF and hydrogen peroxide was then used to anisotropically etch the silicon to form the porous silicon nanostructures. Up to hundreds of micron-long high-aspect-ratio porous silicon nanostructures can be fabricated, and the patterns of the cross-section of porous silicon structures can be controlled by photolithography, nanolithography, or nanoparticle-assisted patterning. The porosity is related to the resistivity range of the silicon substrate, the metal catalysts, the chemical concentration, and the additive solvent. The fabricated porous silicon nanostructure is biodegradable, and the degradation time can be controlled by surface treatments. Porous silicon nanowires can be fabricated with a two-step process. A nanostructured metal layer can be deposited on a silicon substrate by an electroless chemical deposition or electrochemical deposition. This step determines the shape of the final nanowires. Alternatively, metal nanoparticles can be spun on the silicon surface to form a metal layer, or a metal layer can be physically or chemically deposited on the silicon through a nanopatterned mask. The metal-coated silicon can be etched in a solution of HF, water, and H2O2 to produce porous silicon nanowires. Solvent can be added to the solution to modulate the features of the porous silicon nanowires. This work was done by Mauro Ferrari, Xuewu Liu, and Ciro Chappini of the University of Texas Health Science Center at Houston for Johnson Space Center. For further information, contact the JSC Innovation Partnerships Office at (281) 483-3809. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to: The University of Texas Health and Science Center at Houston Office of Technology Management 7000 Fannin Street, Suite 720 Houston, TX 77030 MSC-24690-1

Posted in: Briefs, Manufacturing & Prototyping, Semiconductors & ICs

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Post-Growth, In Situ Adhesion of Carbon Nanotubes to a Substrate for Robust CNT Cathodes

This technology can be used down-hole in oil wells, and in high-temperature, high-pressure, corrosive environments in the automotive industry. The field emission electron sources using carbon nanotubes (CNTs) are being targeted for low-power vacuum microelectronic applications for harshenvironment operation (high temperature, pressure, and corrosive atmosphere). While CNTs have demonstrated excellent properties in terms of low threshold field, low-power operation, and high current densities, one problem with vacuum electronic applications is poor adhesion of CNTs to the substrate on which they are synthesized. The chemical vapor deposition (CVD) process used to grow CNTs on silicon or other metallic substrates using an iron catalyst with a thin oxide diffusion barrier layer has consistently provided reproducible growth. The CNTs are only surface- adhering in these cases, and are easily removed from the surface with the application of minor forces — typically pressures of 20 to 60 kPa. This causes catastrophic failures of CNT field emitters since the applied field could exceed the adhesion strength of CNTs to the substrate.

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Thermal Mechanical Preparation of Glass Spheres

The forming process allows a very wide variety of material to be processed into spheres. Samples of lunar regolith have included small glass spheres. Most literature has suggested the small spheres were formed by meteorite impacts. The resulting transformation of kinetic energy to thermal energy caused the lunar surface to melt. The process yielded glass spheres. Recreating a meteorite impact that yields glass spheres is very challenging. Furthermore, the melting temperature of certain minerals on the Moon precludes the use of standard thermal techniques.

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Development of a Precision Thermal Doubler for Deep Space

A copper thermal doubler is used to spread the thermal loads. Thermal requirements and a need for a very flat mechanical interface led to the development of a copper doubler for the titanium vault on the Juno Spacecraft. The vault is designed to contain the science instruments on the spacecraft, protecting them from damage due to the extreme radiation environment of Jupiter. The titanium used in the vault creates unwanted thermal effects due to the poor thermal conductivity of titanium. To remove heat from the telecommunication equipment mounted to the interior of the vault, a copper thermal doubler was used to spread the thermal loads over the entire area of the radiator (located on the outside of the vault), which decreased the effective thermal resistance through the vault wall. A method of bonding a copper doubler to the titanium preserves the mounting interface flatness to less than 0.005 in. (0.13 mm) while providing a superior thermal path to the radiators, which are fitted with thermal control louvers. The precisely controlled titanium surface, and that of the milled copper doubler with integral spacing features, provides the mechanical interface flatness, structural integrity, and thermal performance required by the telecommunications subsystem.

Posted in: Briefs, TSP, Manufacturing & Prototyping

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Improving Friction Stir Welds Using Laser Peening

This technique can be used in any application of friction stir welding, including automotive, railroad, and maritime industries. Friction stir welding (FSW) has emerged as a promising solid-state process with encouraging results, particularly when used on high-strength aerospace aluminum alloys that are generally difficult to weld. Laser peening has been applied to the mechanical and fatigue properties of welded joints. Laser peening introduces a compressive residual stress at the surface that can extend several millimeters or deeper into the material. These residual stresses resulting from laser peening can be significantly higher and deeper than for conventional shot peening, resulting in superior mechanical and fatigue properties in FSW.

Posted in: Briefs, TSP, Manufacturing & Prototyping

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Methodology of Evaluating Margins of Safety in Critical Brazed Joints

This methodology provides a guide consisting of design, testing, and structural analysis steps developed to assure positive strength margins of safety (MS) in critical brazed joints used for assembly of flight and non-flight structures.

Posted in: Briefs, TSP, Manufacturing & Prototyping

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