718 research outputs found
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Computer Aided Design of Advanced Turbine Airfoil Alloys for Industrial Gas Turbines in Coal Fired Environments
Recent initiatives for fuel flexibility, increased efficiency and decreased emissions in power generating industrial gas turbines (IGT's), have highlighted the need for the development of techniques to produce large single crystal or columnar grained, directionally solidified Ni-base superalloy turbine blades and vanes. In order to address the technical difficulties of producing large single crystal components, a program has been initiated to, using computational materials science, better understand how alloy composition in potential IGT alloys and solidification conditions during processing, effect castability, defect formation and environmental resistance. This program will help to identify potential routes for the development of high strength, corrosion resistant airfoil/vane alloys, which would be a benefit to all IGT's, including small IGT's and even aerospace gas turbines. During the first year, collaboration with Siemens Power Corporation (SPC), Rolls-Royce, Howmet and Solar Turbines has identified and evaluated about 50 alloy compositions that are of interest for this potential application. In addition, alloy modifications to an existing alloy (CMSX-4) were also evaluated. Collaborating with SPC and using computational software at SPC to evaluate about 50 alloy compositions identified 5 candidate alloys for experimental evaluation. The results obtained from the experimentally determined phase transformation temperatures did not compare well to the calculated values in many cases. The effects of small additions of boundary strengtheners (i.e., C, B and N) to CMSX-4 were also examined. The calculated phase transformation temperatures were somewhat closer to the experimentally determined values than for the 5 candidate alloys, discussed above. The calculated partitioning coefficients were similar for all of the CMSX-4 alloys, similar to the experimentally determined segregation behavior. In general, it appears that computational materials science has become a useful tool to help reduce the number of iterations necessary to perform laboratory experiments or alloy development. However, we clearly are not able to rely solely on computational techniques in the development of high temperature materials for IGT applications. A significant amount of experimentation will continue to be required
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Thermo-mechanical processing (TMP) of Ti-48Al-2Nb-2Cr based alloys
The effects of heat treatment and deformation processing on the microstructures and properties of {gamma}-TiAl based alloys produced by ingot metallurgy (I/M) and powder metallurgy (P/M) techniques were examined. The alloy selected for this work is the second generation {gamma}-TiAl based alloy -- Ti-48Al-2Nb-2Cr (at %). Homogenization of I/M samples was performed at a variety of temperatures, followed by hot working by isothermal forging. P/M samples were prepared from gas atomized powders, consolidated by both HIP and extrusion and some of the HIPed material was then hot worked by isothermal forging. The effects of processing, heat treatment and hot working on the microstructures and properties will be discussed
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The hot workability and superplasticity of Ti-48Al-2Nb-2Cr alloys
The hot compression behavior and microstructure evolution of ingot metallurgy (I/M) and powder metallurgy (P/M) processed samples of the near-{gamma} Ti-aluminide alloy, Ti-48Al-2Nb-2Cr (at%), were determined. Three I/M conditions and two P/M conditions were examined in this study. Hot compression tests were performed in the temperature range 1,100--1,300 C at strain rates ranging from 1.67 {times} 10{sup {minus}1}/sec to 1.67 {times} 10{sup {minus}4}/sec. P/M materials consolidated by either hot isostatic pressing (HIP`ing) or extrusion exhibited the best hot workability in most cases. The P/M materials possessed finer, more homogeneous microstructures than the I/M materials. It was also noted that improved workability, and in some cases superplastic behavior, was observed in the materials with equiaxed microstructures without any lamellar constituents
Wie kann der Einfluss biologischer Lebensmittel auf den Menschen bewertet werden? Erfahrungen aus einer Klosterstudie.
Sofern der Wert biologischer Lebensmittel nicht ausschließlich durch ethische und ökologische Standards definiert wird, muss er in Bezug auf den Menschen erfasst werden. Die Vergleichsuntersuchung einzelner Nährstoffe ist wenig erfolgreich, da biologische Lebensmittel im Bereich wertgebender Inhaltsstoffe nicht systematisch besser sind als konventionelle Produkte (Strube & Stolz, 2005). Der Hauptgrund besteht jedoch darin, dass die quantitative Bewertung einzelner Substanzen nicht dem ganzheitlichen Ansatz der biologischen Produktion entspricht. Einige Bio-Forscher sind der Meinung, dass der Einfluss von Lebensmitteln auf die menschliche Gesundheit aus analytischer Sicht generell nicht verständlich ist (Busscher et al., 2005), sondern eher mittels kontextabhängiger Konzepte wie z.B. dem Konzept der „Inneren Qualität“ (van der Burgt et al., 2005).
In der Ernährungsforschung können einzelne Nährstoffe oder Produkte sowie Gesamtdiäten aus biologischen Lebensmitteln untersucht werden. Weiters muss die Erforschung des Menschen auf verschiedenen Ebenen, die von biologischen Lebensmitteln beeinflusst werden könnten, erfolgen. Das heißt, wir müssen zwischen physischen, psychischen und geistigen Auswirkungen unterscheiden, wenn wir ein umfassendes Verständnis der Komplexität erreichen wollen.
Eine zukunftsweisende Ernährungsstudie zur Erfassung des Einflusses einer biologisch-dynamischen Gesamtdiät auf das menschliche Wohlbefinden wurde durchgeführt, um erste Erfahrungen in diesem komplizierten und kaum ausgearbeiteten Forschungsbereich zu sammeln. Die Testpersonen waren 32 Nonnen in einem katholischen Kloster. In 3 aufeinander folgenden Testserien von jeweils 2 Wochen erhielten sie konventionelle Fertiggerichte, frisch zubereitete konventionelle und ebenfalls frisch zubereitete biologisch-dynamische Speisen und anschließend dasselbe in umgekehrter Reihenfolge. Am Ende jeder Testserie wurde eine breite Palette von Blutparametern analysiert sowie ein umfassender Fragebogen zum Wohlbefinden beantwortet. Der Fragebogen entsprach den Standardverfahren für die Erfassung psychologischen Wohlbefindens und körperlicher Beschwerden und enthielt Skalierungen für die spezifischen Untersuchungsthemen. Mittels des Fragebogens wurden folgende Ergebnisse gesammelt: demographische Daten, Erwartungen und Motivationen bezüglich der Ernährungsumstellung, Auswirkungen der Lebensmittel, Aktivitäten und Ruhezeiten sowie psychisches und physisches Wohlbefinden.
Trotz einer signifikanten Abnahme des systolischen Blutdruckes zu Beginn der biologisch-dynamischen Diät, konnte keine weitere Veränderung der physiologischen Parameter beobachtet werden; eine deutliche Zunahme der immunologischen Aktivität wurde bei der Umstellung von Fertiggerichten auf frisch zubereitete Speisen nachgewiesen.
Aus den Fragebögen ging aber hervor, dass die biologisch-dynamischen Testperioden signifikante Verbesserungen des subjektiven geistigen, psychischen und physischen Wohlbefindens bewirkten.
Es gibt keine Ergebnisse, die diese subjektiven Erfahrungen im Wohlbefinden auf physiologische Daten zurückführen könnten. Der Wert solcher Ergebnisse muss daher diskutiert werden. Einerseits fehlen objektive und von emotionalen Einflüssen unabhängige Daten, andererseits könnten gerade diese emotionalen und geistigen Veränderungen die wichtigsten Ergebnisse für eine ganzheitliche Betrachtung von Organismen und auch des Menschen sein. Diese Gegebenheit müssen im Zusammenhang mit den Methoden und Ergebnissen der Klosterstudie diskutiert werden
Hydrodynamic modes of a 1D trapped Bose gas
We consider two regimes where a trapped Bose gas behaves as a one-dimensional
system. In the first one the Bose gas is microscopically described by 3D mean
field theory, but the trap is so elongated that it behaves as a 1D gas with
respect to low frequency collective modes. In the second regime we assume that
the 1D gas is truly 1D and that it is properly described by the Lieb-Liniger
model. In both regimes we find the frequency of the lowest compressional mode
by solving the hydrodynamic equations. This is done by making use of a method
which allows to find analytical or quasi-analytical solutions of these
equations for a large class of models approaching very closely the actual
equation of state of the Bose gas. We find an excellent agreement with the
recent results of Menotti and Stringari obtained from a sum rule approach.Comment: 15 pages, revtex, 1 figure
A universal Hamiltonian for the motion and the merging of Dirac cones in a two-dimensional crystal
We propose a simple Hamiltonian to describe the motion and the merging of
Dirac points in the electronic spectrum of two-dimensional electrons. This
merging is a topological transition which separates a semi-metallic phase with
two Dirac cones from an insulating phase with a gap. We calculate the density
of states and the specific heat. The spectrum in a magnetic field B is related
to the resolution of a Schrodinger equation in a double well potential. They
obey the general scaling law e_n \propto B^{2/3} f_n(Delta /B^{2/3}. They
evolve continuously from a sqrt{n B} to a linear (n+1/2)B dependence, with a
[(n+1/2)B]^{2/3} dependence at the transition. The spectrum in the vicinity of
the topological transition is very well described by a semiclassical
quantization rule. This model describes continuously the coupling between
valleys associated with the two Dirac points, when approaching the transition.
It is applied to the tight-binding model of graphene and its generalization
when one hopping parameter is varied. It remarkably reproduces the low field
part of the Rammal-Hofstadter spectrum for the honeycomb lattice.Comment: 18 pages, 15 figure
Dilepton production in heavy ion collisions at intermediate energies
We present a unified description of the vector meson and dilepton production
in elementary and in heavy ion reactions. The production of vector mesons
() is described via the excitation of nuclear resonances ().
The theoretical framework is an extended vector meson dominance model (eVMD).
The treatment of the resonance decays with arbitrary spin is
covariant and kinematically complete. The eVMD includes thereby excited vector
meson states in the transition form factors. This ensures correct asymptotics
and provides a unified description of photonic and mesonic decays. The
resonance model is successfully applied to the production in
reactions. The same model is applied to the dilepton production in elementary
reactions (). Corresponding data are well reproduced. However, when
the model is applied to heavy ion reactions in the BEVALAC/SIS energy range the
experimental dilepton spectra measured by the DLS Collaboration are
significantly underestimated at small invariant masses. As a possible solution
of this problem the destruction of quantum interference in a dense medium is
discussed. A decoherent emission through vector mesons decays enhances the
corresponding dilepton yield in heavy ion reactions. In the vicinity of the
-peak the reproduction of the data requires further a substantial
collisional broadening of the and in particular of the meson.Comment: 32 pages revtex, 19 figures, to appear in PR
Multispecies virial expansions
We study the virial expansion of mixtures of countably many different types of particles. The main tool is the Lagrange–Good inversion formula, which has other applications such as counting coloured trees or studying probability generating functions in multi-type branching processes. We prove that the virial expansion converges absolutely in a domain of small densities. In addition, we establish that the virial coefficients can be expressed in terms of two-connected graphs
Point-contact spectroscopy of the nickel borocarbide superconductor YNi2B2C in the normal and superconducting state
Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in
the normal and superconducting (SC) state (T_c=15.4K) for the main
crystallographic directions are reported. The PC study reveals the
electron-phonon interaction (EPI) spectral function with dominant phonon
maximum around 12 meV and further weak structures (hump or kink) at higher
energy at about 50 meV. No "soft" modes below 12 meV are resolved in the normal
state. The PC EPI spectra are qualitatively similar for the different
directions. Contrary, directional study of the SC gap results in
\Delta_[100]=1.5 meV for the a direction and \Delta_[001]=2.3 meV along the c
axis; however the critical temperature T_c in PC in all cases is near to that
in the bulk sample. The value 2\Delta_[001]/kT_c=3.6 is close to the BCS value
of 3.52, and the temperature dependence \Delta_[001](T) is BCS-like, while the
for small gap \Delta_[100](T) is below BCS behavior at T>T_c/2 similarly as in
the two-gap superconductor MgB2. It is supposed that the directional variation
\Delta can be attributed to a multiband nature of the SC state in YNi2B2C.Comment: 9 pages, 10 figures, to be published in a special issue of J. Low
Temp. Phys. in honour of Prof. H. von Loehneyse
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