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Sekhar J.A.,Institute of Thermodynamics and Design | Sekhar J.A.,University of Cincinnati | Sekhar J.A.,MHI Inc. | Mantri A.S.,University of Cincinnati | And 4 more authors.
JOM | Year: 2015

This article presents, for the first time, evidence of nanocrystalline structure, through direct transmission electron microscopy (TEM) observations, in a Cu-32 wt.% Sn alloy that has been made by an age-old, uniquely crafted casting process. This alloy has been used as a metal mirror for centuries. The TEM images also reveal five-sided projections of nano-particles. The convergent beam nano-diffraction patterns obtained from the nano-particles point to the nano-phase being quasicrystalline, a feature that has never before been reported for a copper alloy, although there have been reports of the presence of icosahedral ‘clusters’ within large unit cell intermetallic phases. This observation has been substantiated by x-ray diffraction, wherein the observed peaks could be indexed to an icosahedral quasi-crystalline phase. The mirror alloy casting has been valued for its high hardness and high reflectance properties, both of which result from its unique internal microstructure that include nano-grains as well as quasi-crystallinity. We further postulate that this microstructure is a consequence of the raw materials used and the manufacturing process, including the choice of mold material. While the alloy consists primarily of copper and tin, impurity elements such as zinc, iron, sulfur, aluminum and nickel are also present, in individual amounts not exceeding one wt.%. It is believed that these trace impurities could have influenced the microstructure and, consequently, the properties of the metal mirror alloy. © 2015, The Minerals, Metals & Materials Society.

Connelly M.C.,MHI Inc.
Current Opinion in Chemical Engineering | Year: 2014

The need for clean and renewable energy sources due to environmental concerns and resource depletion has continued the trend for innovation in green energy technologies. Recent literature has displayed a strong interest and a wide variety of research, development and innovation in many areas of renewable energy including solar, wind and biofuel disciplines. Publically available data concerning U.S. energy production and issued patents both show growth trends that correlate with each other and are supported by the interest in green energy innovation shown in the current literature. © 2013 Elsevier Ltd.

Sekhar J.A.,MHI Inc. | Sekhar J.A.,University of Cincinnati
Journal of Materials Science | Year: 2011

The maximum entropy production rate (MEPR) in the solid-liquid zone is developed and tested as a possible postulate for predicting the stable morphology for the special case of steady state directional solidification (DS). The principle of MEPR states that, if there are sufficient degrees of freedom within a system, it will adopt a stable state at which the entropy generation (production) rate is maximized. Where feasible, the system will also try and adopt a steady state. The MEPR postulate determines the most probable state and therefore allows pathway selections to occur in an open thermodynamic system. In the context of steady state solidification, pathway selections are reflected in the corresponding morphological selections made by the system in the solid-liquid (mushy) zone in order to cope with the required entropy production. Steady state solidification is feasible at both close to, and far from equilibrium conditions. Based on MEPR, a model is proposed for examining the stability of various morphologies that have been experimentally observed during steady state directional solidification. This model employs a control volume approach for entropy balance, including the entropy generation term (S gen), which depends on the diffuse zone and average temperature of the solid-liquid region within the control volume. In this manner, the model takes a different approach from the successful kinetic models that have been able to predict key features of stable morphological patterns. Unstable planar interfaces, faceted cellular arrays, cell-dendrite transitions, half cells both faceted and smooth, and other transitions such as the absolute stability transition at high solid/liquid velocities are examined with the model. Uncommon solidification morphological features such as non-crystallographic dendrites and discontinuous cell-tip splitting are also examined with the model. The preferred morphological changedirection for the emergence of the stable morphological feature is inferred with the MEPR postulate in a manner analogous to the free energy minimization principle(s) when used for predicting phase stability and metastable phase formation. Aspects of mixed-mode order transformation characteristics are also discussed for nonequilibrium solidification containing a diffuse interface, in contrast to classifying solidification as purely a first order transformation. The MEPR model predictions are shown to follow the experimental transitions observed to date in several historical studies. © Springer Science+Business Media, LLC 2011.

Connelly M.C.,MHI Inc. | Reddy G.S.,MHI Inc. | Nadagouda M.N.,Wright State University | Sekhar J.A.,MHI Inc. | Sekhar J.A.,University of Cincinnati
Clean Technologies and Environmental Policy | Year: 2016

The relationship between microbe populations that are active on engineered-product surfaces and their relationship to surface corrosion or human health is increasingly being recognized by the materials engineering community as a critically important study-direction. Microbial contamination from biofilms and germ colonies leads to costs that are reported to be extremely high every year in infection control, epidemics, corrosion loss and energy/infrastructure materials loss throughout the world. Nanostructured surfaces, particularly those that are hard-surface nanoporous (pore radii between 2 and 1000 nm), are an emerging class of surfaces that have recently been recognized as important for the prevention of microbial colony growth and biofilm formation. Such nanostructured/nanoporous surfaces, whether made with deposited nanoparticles (welded nanoparticles), or formed by ion-assisted growth on a surface have been found to display biocidal activity with varying efficacy that depends on both the microbe and the nanosurface features. The rate of mortality from common pathogens that are resident in ubiquitous bio-films when attached to common engineering surfaces made of steels, titanium and zirconium appears to be increasing. In this short review, we look at methods of manufacture of durable (i.e., highly scratch resistant) nanostructuring on commonly used engineering surfaces. The microstructures, energy dispersive X-ray analysis, X-ray photo-electron spectroscopy and other types of characterization of a few such surfaces are presented. Simple tests are required by the surface engineering community for understanding the efficacy of a surface for antimicrobial action. These are reviewed. The surface drying rate and the dynamics of the droplet spread have been proposed in the literature as quick methods that correlate well with the residual antimicrobial activity efficacy even after some surface abrasion of the nanostructured surface. A categorization of a surface against short-term antimicrobial action and long-term action is proposed in this review article. Test periods that span time-frames greater than 5 years have demonstrated a high efficacy of the nanoporous nanostructures for preventing bio-film formation. New comparative results for diamond- and graphite-containing surfaces are presented. A brief discussion on a recently developed plasma application technique for creating durable nanoporous surfaces is presented. Although considerable information is now available regarding tunable surface nanofeatures for antimicroabial efficacy, there is a need for more research activity, particularly directed toward the low cost manufacture and rapid characterization of durable (wear and chemical resistant) surfaces that display permanent antimicrobial behavior. © 2016 Springer-Verlag Berlin Heidelberg

Li H.P.,Jinwen University of Science and Technology | Sekhar J.A.,University of Cincinnati | Sekhar J.A.,MHI Inc.
Key Engineering Materials | Year: 2012

The principle of maximum entropy generation rate principle is reviewed for its applicability in Materials Science. The principle of MEPR states that, if there are sufficient degrees of freedom within a system, it will adopt a stable state at which the entropy generation (production) rate is maximized. Where feasible, the system will also try and adopt a steady state. MEPR determines the most probable state. MEPR thus allows for pathway selections that can occur in an open thermodynamic system. Recent work also shows that isolated systems and closed thermodynamic systems also display this principle. The Belousov-Zhabitonsky reaction is also described in the Sgen context. Both solidification morphologies and micropyretic process generated morphologies are studied as examples of the Sgen and MEPR. © (2012) Trans Tech Publications, Switzerland.

Reddy G.S.,MHI Inc. | Nadagouda M.N.,U.S. Environmental Protection Agency | Sekhar J.A.,MHI Inc. | Sekhar J.A.,University of Cincinnati
Key Engineering Materials | Year: 2012

Provided in this article are the quantitative and qualitative morphological results describing the action of several nanostructured surfaces for bactericidal and bacteriostatic action. Results are also provided to illustrate microbial corrosion and its impact. Biofilm formation is correlated to colony formation. Nanostructured surfaces, i.e. surfaces with welded nanoparticles are noted to display biocidal activity with varying efficacies. Porous nanostructures, on stainless steel and copper substrates, made of high purity Ag, Ti, Al, Cu, MoSi2, and carbon nanotubes, are tested for their efficacy against bacterial colony formation for both gram-negative, and gram-positive bacteria. Silver and Molybdenum disilicide (MoSi2) nanostructures are found to be the most effective bactericidal agents with MoSi2 being particularly effective in both low and high humidity conditions. Bacteriostatic activity is also noted. The nanostructured surfaces are tested by controlled exposures to several microbial species including (Gram+ve) bacteria such as Bacillus Cereus and (Gram-ve) bacteria such as Enterobacter Aerogenes. The resistance to simultaneous exposure from diverse bacterial species including Arthrobacter Globiformis, Bacillus Megaterium, and Cupriavidus Necator is also studied. The nanostructured surfaces were found to eliminates or delay bacterial colony formation, even with short exposure times, and even after simulated surface abrasion. The virgin 316 stainless steel and copper substrates, i.e. without the nanostructure, always displayed rapid bacterial colony evolution indicating the lack of antimicrobial action. The efficacy of the nanostructured surface against colony formation (bacterial recovery) for E-Coli (two strains) and virus Phi 6 Bacteriophage with a host Pseudomonas Syringae was also studied. Preliminary results are presented that also show possible anti-fungal properties by the nanostructured MoSi2. When comparing antimicrobial efficacy of flat polished surfaces (no curvature or nanostructure) with nanostructure containing surfaces (high curvature) of the same chemistry, shows that bacterial action results from both the nanostructure size and chemistry. Single point dynamic surface adherence tests were also performed to gauge the quality of adherence of the nanostructures with stainless steel substrates before and after simulated abrasion. The surface drying rate and the dynamics of the droplet spread rate are proposed as quick methods that correlate well with the nanostructure retention efficacy and residual anti-microbial activity after surface abrasion. Evidence from tests that span a 7-year period is presented to demonstrate the viable efficacy of the nanostructures for preventing biofilm formation. Early stage conclusions are offered for the chemical and topological influence of the nanostructures on bioactivity, including on the long-term, airborne biofilm formation behavior. This is the first article that comprehensively connects airborne biofilm elimination tendencies to bacterial colony elimination tendencies for strongly adherent nanostructured surfaces. The growth, rotation, and clustering of typical airborne biofilms during their early stages of formation on virgin surfaces is presented. The nanostructuring method is presented in this article as a possible method for the elimination of biofilms. © (2012) Trans Tech Publications, Switzerland.

Savov S.S.,University of Cambridge | Atkins N.R.,University of Cambridge | Uchida S.,MHI Ltd
Proceedings of the ASME Turbo Expo | Year: 2016

The effect of purge flow, engine-like blade pressure field and mainstream flow coefficient are studied experimentally for a single and double lip rim seal. Compared to the single lip, the double lip seal requires less purge flow for similar levels of cavity seal effectiveness. The double lip seal has both a weaker vane pressure field in the rim seal cavity and a smaller difference in seal effectiveness across the lower lip. The smaller gradient across the lower lip of the double lip seal suggests that it is less sensitive to mainstream-cavity interactions across all length scales. Unlike the double lip seal, the single lip seal is sensitive to overall Reynolds number, the addition of a simulated blade pressure field and large-scale non-uniform ingestion. In both seals, the addition of blades is seen to suppress unsteady activity attributed to shear between the rim seal and mainstream flows. The data suggests that in the case of the single lip seal, the blade pressure field has a more dominant effect in promoting ingress than the unsteadiness it suppresses at an engine-matched flow coefficient. At higher flow coefficients, increased shear between the rim seal cavity flow and the mainstream drives more mixing, reducing the seal effectiveness for both configurations. Copyright © 2016 by ASME.

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