2,201 research outputs found
Modeling of convection phenomena in Bridgman-Stockbarger crystal growth
Thermal convection phenomena in a vertically oriented Bridgman-Stockbarger apparatus were modeled by computer simulations for different gravity conditions, ranging from earth conditions to extremely low gravity, approximate space conditions. The modeling results were obtained by the application of a state-of-the art, transient, multi-dimensional, completely densimetrically coupled, discrete-element computational model which was specifically developed for the simulation of flow, temperature, and species concentration conditions in two-phase (solid-liquid) systems. The computational model was applied to the simulation of the flow and the thermal conditions associated with the convection phenomena in a modified Germanium-Silicon charge enclosed in a stationary fused-silica ampoule. The results clearly indicated that the gravitational field strength influences the characteristics of the coherent vortical flow patterns, interface shape and position, maximum melt velocity, and interfacial normal temperature gradient
The quantization of the symplectic groupoid of the standard Podles sphere
We give an explicit form of the symplectic groupoid that integrates the
semiclassical standard Podles sphere. We show that Sheu's groupoid, whose
convolution C*-algebra quantizes the sphere, appears as the groupoid of the
Bohr-Sommerfeld leaves of a (singular) real polarization of the symplectic
groupoid. By using a complex polarization we recover the convolution algebra on
the space of polarized sections. We stress the role of the modular class in the
definition of the scalar product in order to get the correct quantum space.Comment: 33 pages; minor correction
Redox reforming based, integrated solar-natural gas plants: Reforming and thermodynamic cycle efficiency
As demand for energy continues to rise, the concern over the increase in emissions grows, prompting much interest in using renewable energy resources such as solar energy. However, there are numerous issues with using solar energy including intermittency and the need for storage. A potential solution is the concept of hybrid solar-fossil fuel power generation. Previous work has shown that utilizing solar reforming in conventional power cycles has higher performance compared to other integration methods. Most previous studies have focused on steam or dry reforming and on specific component analysis rather than a systems level analysis. In this article, a system analysis of a hybrid cycle utilizing redox reforming is presented. Important cycle design and operation parameters such as the oxidation temperature and reformer operating pressure are identified and their effect on both the reformer and cycle performance is discussed. Simulation results show that increasing oxidation temperature can improve reformer and cycle efficiency. Also shown is that increasing the amount of reforming water leads to a higher reformer efficiency, but can be detrimental to cycle efficiency depending on how the reforming water is utilized.Center for Clean Water and Clean Energy at MIT and KFUPM (Project Number R12-CE-10)King Abdullah University of Science and Technology (KAUST
Nanoengineered Curie Temperature in Laterally-Patterned Ferromagnetic Semiconductor Heterostructures
We demonstrate the manipulation of the Curie temperature of buried layers of
the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the
effect of annealing. Patterning the GaAs-capped ferromagnetic layers into
nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As
layers and thus allows the removal of Mn interstitials using annealing. This
leads to an enhanced Curie temperature and reduced resistivity compared to
unpatterned samples. For a fixed annealing time, the enhancement of the Curie
temperature is larger for narrower nanowires.Comment: Submitted to Applied Physics Letters (minor corrections
Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2
The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides
because the Co sits on a triangular lattice and its valence can be tuned over a
wide range by varying the Na concentration x. Up to now detailed modeling of
the rich phenomenology (which ranges from unconventional superconductivity to
enhanced thermopower) has been hampered by the difficulty of controlling pure
phases. We discovered that certain Na concentrations are specially stable and
are associated with superlattice ordering of the Na clusters. This leads
naturally to a picture of co-existence of localized spins and itinerant charge
carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our
picture brings coherence to a variety of measurements ranging from NMR to
optical to thermal transport. Our results also allow us to take the first step
towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We
suggest the local moments may form a quantum spin liquid state and we propose
experimental test of our hypothesis.Comment: 16 pages, 5 figure
Prehypertensive blood pressures and regional cerebral blood flow independently relate to cognitive performance in midlife
Background
High blood pressure is thought to contribute to dementia in late life, but our understanding of the relationship between individual differences in blood pressure (
BP
) and cognitive functioning is incomplete. In this study, cognitive performance in nonhypertensive midlife adults was examined as a function of resting
BP
and regional cerebral blood flow (
rCBF
) responses during cognitive testing. We hypothesized that
BP
would be negatively related to cognitive performance and that cognitive performance would also be related to
rCBF
responses within areas related to
BP
control. We explored whether deficits related to systolic
BP
might be explained by
rCBF
responses to mental challenge.
Methods and Results
Healthy midlife participants (n=227) received neuropsychological testing and performed cognitive tasks in a magnetic resonance imaging scanner. A pseudocontinuous arterial spin labeling sequence assessed
rCBF
in brain areas related to
BP
in prior studies. Systolic
BP
was negatively related to 4 of 5 neuropsychological factors (standardized β>0.13): memory, working memory, executive function, and mental efficiency. The
rCBF
in 2 brain regions of interest was similarly related to memory, executive function, and working memory (standardized β>0.17); however,
rCBF
responses did not explain the relationship between resting systolic
BP
and cognitive performance.
Conclusions
Relationships at midlife between prehypertensive levels of systolic
BP
and both cognitive and brain function were modest but suggested the possible value of midlife intervention.
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Fundamentals of microcrack nucleation mechanics
A foundation for ultrasonic evaluation of microcrack nucleation mechanics is identified in order to establish a basis for correlations between plane strain fracture toughness and ultrasonic factors through the interaction of elastic waves with material microstructures. Since microcracking is the origin of (brittle) fracture, it is appropriate to consider the role of stress waves in the dynamics of microcracking. Therefore, the following topics are discussed: (1) microstress distributions with typical microstructural defects located in the stress field; (2) elastic wave scattering from various idealized defects; and (3) dynamic effective-properties of media with randomly distributed inhomogeneities
Dry redox reforming hybrid power cycle: Performance analysis and comparison to steam redox reforming
There has been much interest in the use of renewable resources for power generation as the world's energy demand and the concern over the rise in emissions increases. In the near term, however, renewable sources such as solar energy are expected to provide a small fraction of the world's energy demand due to intermittancy and storage problems. A potential solution is the use of hybrid solar-fossil fuel power generation. Previous work has shown the potential of steam redox reforming for hybridization. However, this type of reforming requires some water consumption (which may be infeasible in certain locations) as not all the water can be recovered through recycling. An alternative is to utilize dry (or CO2) redox reforming. In this paper, a system analysis for a CO2 redox reforming hybrid cycle and comparison of cycle and reformer performance between a CO2 redox reformer and steam redox reformer hybrid cycle are presented. The effect of important operating parameters such as pressure, amount of reforming CO2, and the oxidation temperature on the reformer and cycle performance are discussed. Simulation results show that increasing the oxidation temperature or the amount of reforming CO2 leads to higher reformer and cycle efficiencies. In addition, the comparison between the CO2 and steam redox reformer hybrid cycles shows that the CO2 cycle has the potential to have better overall performance.King Fahd University of Petroleum and Minerals (KFUPM (Project No. R12-CE-10)
Quantum planes and quantum cylinders from Poisson homogeneous spaces
Quantum planes and a new quantum cylinder are obtained as quantization of
Poisson homogeneous spaces of two different Poisson structures on classical
Euclidean group E(2).Comment: 13 pages, plain Tex, no figure
A review of solar methane reforming systems
Because of the increasing demand for energy and the associated rise in greenhouse gas emissions, there is much interest in the use of renewable sources such as solar energy in electricity and fuels generation. One problem with solar energy, however, is that it is difficult to economically convert the radiation into usable energy at the desired locations and times, both daily and seasonally. One method to overcome this space-time intermittency is through the production of chemical fuels. In particular, solar reforming is a promising method for producing chemical fuels by reforming and/or water/carbon dioxide splitting. In this paper, a review of solar reforming systems is presented, as well as a comparison between these systems and a discussion on areas for potential innovation including chemical looping and membrane reactors. Moreover, a brief overview of catalysis in the context of reforming is presented
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