Publications

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We are developing a low-emissivity thermal management coating system to minimize radiative heat losses under a high-vacuum environment. Good adhesion, low outgassing, and good thermal stability of the coating material are essential elements for a long-life, reliable thermal management device. The system of electroplated Au coating on the adhesion-enhancing Wood's Ni strike and 304L substrate was selected due to its low emissivity and low surface chemical reactivity. The physical and chemical properties, interface bonding, thermal aging, and compatibility of the above Au/Ni/304L system were examined extensively. The study shows that the as-plated electroplated Au and Ni samples contain submicron columnar grains, stringers of nanopores, and/or H{sub 2} gas bubbles, as expected. The grain structure of Au and Ni are thermally stable up to 250 C for 63 days. The interface bonding is strong, which can be attributed to good mechanical locking among the Au, the 304L, and the porous Ni strike. However, thermal instability of the nanopore structure (i.e., pore coalescence and coarsening due to vacancy and/or entrapped gaseous phase diffusion) and Ni diffusion were observed. In addition, the study also found that prebaking 304L in the furnace at {ge} 1 x 10{sup -4} Torr promotes surface Cr-oxides on the 304L surface, which reduces the effectiveness of the intended H-removal. The extent of the pore coalescence and coarsening and their effect on the long-term system integrity and outgassing are yet to be understood. Mitigating system outgassing and improving Au adhesion require a further understanding of the process-structure-system performance relationships within the electroplated Au/Ni/304L system.

There has been significant debate in the literature about the role of strain-induced martensite in hydrogen-Assisted fracture of metastable austenitic stainless steels. It is clear that α'-martensite is not necessary for hydrogen-Assisted fracture since hydrogen affects the tensile ductility and fracture properties of stable austenitic stainless steels. Martensite, however, is believed to facilitate hydrogen transport in austenitic stainless steel and numerous studies propose that martensite contributes to fracture. Yet conclusive evidence that strain-induced α'-martensite plays an important mechanistic role on fracture processes in the presence of hydrogen has not been clearly articulated in the literature. In this study, we report microstructural evidence suggesting that α'-martensite does not play a primary role in hydrogen-Assisted fracture during tensile testing of metastable austenitic stainless steel. This microstructural evidence also suggests that thermal twin boundaries are susceptible sites for hydrogen-Assisted fracture.

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This study investigates a pathway to nanoporous structures created by hydrogen and helium implantation in aluminum. Previous experiments for fusion applications have indicated that hydrogen and helium ion implantations are capable of producing bicontinuous nanoporous structures in a variety of metals. This study focuses specifically on implantations of hydrogen and helium ions at 25 keV in aluminum. The hydrogen and helium systems result in remarkably different nanostructures of aluminum at the surface. Scanning electron microscopy, focused ion beam, and transmission electron microscopy show that both implantations result in porosity that persists approximately 200 nm deep. However, hydrogen implantations tend to produce larger and more irregular voids that preferentially reside at defects. Implantations of helium at a fluence of 10{sup 18} cm{sup -2} produce much smaller porosity on the order of 10 nm that is regular and creates a bicontinuous structure in the porous region. The primary difference driving the formation of the contrasting structures is likely the relatively high mobility of hydrogen and the ability of hydrogen to form alanes that are capable of desorbing and etching Al (111) faces.

This study investigates a pathway to nanoporous structures created by hydrogen implantation in aluminum. Previous experiments for fusion applications have indicated that hydrogen and helium ion implantations are capable of producing bicontinuous nanoporous structures in a variety of metals. This study focuses specifically on hydrogen and helium implantations of aluminum, including complementary experimental results and computational modeling of this system. Experimental results show the evolution of the surface morphology as the hydrogen ion fluence increases from 10{sup 17} cm{sup -2} to 10{sup 18} cm{sup -2}. Implantations of helium at a fluence of 10{sup 18} cm{sup -2} produce porosity on the order of 10 nm. Computational modeling demonstrates the formation of alanes, their desorption, and the resulting etching of aluminum surfaces that likely drives the nanostructures that form in the presence of hydrogen.

Bi-Te-based thermoelectric (TE) alloys are excellent candidates for power generation modules. We are interested in reliable TE modules for long-term use at or below 200 C. It is known that the metallurgical characteristics of TE materials and of interconnect components affect the performance of TE modules. Thus, we have conducted an extensive scientific investigation of several commercial TE modules to determine whether they meet our technical requirements. Our main focus is on the metallurgy and thermal stability of (Bi,Sb){sup 2}(Te,Se){sup 3} TE compounds and of other materials used in TE modules in the temperature range between 25 C and 200 C. Our study confirms the material suite used in the construction of TE modules. The module consists of three major components: AlN cover plates; electrical interconnects; and the TE legs, P-doped (Bi{sub 8}Sb{sub 32})(Te{sub 60}) and N-doped (Bi{sub 37}Sb{sub 3})(Te{sub 56}Se{sub 4}). The interconnect assembly contains Sn (Sb {approx} 1wt%) solder, sandwiched between Cu conductor with Ni diffusion barriers on the outside. Potential failure modes of the TE modules in this temperature range were discovered and analyzed. The results show that the metallurgical characteristics of the alloys used in the P and N legs are stable up to 200 C. However, whole TE modules are thermally unstable at temperatures above 160 C, lower than the nominal melting point of the solder suggested by the manufacture. Two failure modes were observed when they were heated above 160 C: solder melting and flowing out of the interconnect assembly; and solder reacting with the TE leg, causing dimensional swelling of the TE legs. The reaction of the solder with the TE leg occurs as the lack of a nickel diffusion barrier on the side of the TE leg where the displaced solder and/or the preexisting solder beads is directly contact the TE material. This study concludes that the present TE modules are not suitable for long-term use at temperatures above 160 C due to the reactivity between the Sn-solder and the (Bi,Sb){sup 2}(Te,Se){sup 3} TE alloys. In order to deploy a reliable TE power generator for use at or below 200 C, alternate interconnect materials must be used and/or a modified module fabrication technique must be developed.

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The mechanical properties of some materials (Cu, Ni, Ag, etc.) have been shown to develop strong dependence on the geometric dimensions, resulting in a size effect. Several theories have been proposed to model size effects, but have been based on very few experiments conducted at appropriate scales. Some experimental results implied that size effects are caused by increasing strain gradients and have been used to confirm many strain gradient theories. On the other hand, some recent experiments show that a size effect exists in the absence of strain gradients. This report describes a brief analytical and experimental study trying to clarify the material and experimental issues surrounding the most influential size-effect experiments by Fleck et al (1994). This effort is to understand size effects intended to further develop predictive models.

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Electrical resistance anomalies noted in EPDM gaskets have been attributed to zinc-enriched surface sublayers, about 10-{micro}m thick, in the sulfur cured rubber material. Gasket over-compression provided the necessary connector pin contact and was also found to cause surprising morphological changes on the gasket surfaces. These included distributions of zinc oxide whiskers in high pressure gasket areas and cone-shaped features rich in zinc, oxygen, and sulfur primarily in low pressure protruding gasket areas. Such whiskers and cones were only found on the pin side of the gaskets in contact with a molded plastic surface and not on the back side in contact with an aluminum surface. The mechanisms by which such features are formed have not yet been defined.

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