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Terrace Park, OH, United States

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.
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.,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.

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.

Mhi Inc. | Date: 1975-01-21


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