960 research outputs found
Thermal conductivity and diffusion-mediated localization in Fe_{1-x}Cr_{x} Alloys
We apply a new Kubo-Greenwood type formula combined with a generalized
Feynman diagram- matic technique to report a first principles calculation of
the thermal transport properties of disordered Fe_{1-x}Cr_{x} alloys. The
diagrammatic approach simplifies the inclusion of disorder-induced scattering
effects on the two particle correlation functions and hence renormalizes the
heat current operator to calculate configuration averaged lattice thermal
conductivity and diffusivity. The thermal conductivity K(T) in the present case
shows an approximate quadratic T-dependence in the low temperature regime (T <
20 K), which subsequently rises smoothly to a T-independent saturated value at
high T . A numerical estimate of mobility edge from the thermal diffusivity
data yields the fraction of localized states. It is concluded that the complex
disorder scattering processes, in force-constant dominated disorder alloys such
as Fe-Cr, tend to localize the vibrational modes quite significantly.Comment: 5 pages, 5 figure
Anomalous dynamic back-action in interferometers
We analyze the dynamic optomechanical back-action in signal-recycled
Michelson and Michelson-Sagnac interferometers that are operated off dark port.
We show that in this case --- and in contrast to the well-studied canonical
form of dynamic back-action on dark port --- optical damping in a
Michelson-Sagnac interferometer acquires a non-zero value on cavity resonance,
and additional stability/instability regions on either side of the resonance,
revealing new regimes of cooling/heating of micromechanical oscillators. In a
free-mass Michelson interferometer for a certain region of parameters we
predict a stable single-carrier optical spring (positive spring and positive
damping), which can be utilized for the reduction of quantum noise in
future-generation gravitational-wave detectors.Comment: 9 pages, 5 figures. Paper reorganize
Tracer diffusion inside fibrinogen layers
We investigate the obstructed motion of tracer (test) particles in crowded
environments by carrying simulations of two-dimensional Gaussian random walk in
model fibrinogen monolayers of different orientational ordering. The fibrinogen
molecules are significantly anisotropic and therefore they can form structures
where orientational ordering, similar to the one observed in nematic liquid
crystals, appears. The work focuses on the dependence between level of the
orientational order (degree of environmental crowding) of fibrinogen molecules
inside a layer and non-Fickian character of the diffusion process of spherical
tracer particles moving within the domain. It is shown that in general
particles motion is subdiffusive and strongly anisotropic, and its
characteristic features significantly change with the orientational order
parameter, concentration of fibrinogens and radius of a diffusing probe.Comment: 8 pages, 12 figure
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Numerical Simulation of Bi-Materials Laser Densification 386
The dominant procedure currently used for permanent fixed prosthodontics is porcelainfused-to-metal (PFM) restoration that is a time consuming and labor intensive process. To address these shortcomings, this project will develop a solid freeform fabrication (SFF) technique for dental restoration. Thus, a dental restoration can be built from a computer model without part-specific tooling and human intervention. The SFF technique to be developed is called multi-materials laser densification (MMLD) and capable of dealing with multiple dental materials such as dental alloys and porcelains. In order to provide guidelines for laser
densification of multiple materials, numerical simulation has been carried out using the ANSYS code with 3-dimensional elements to model the temperature and stress fields during the MMLD process. Effects of laser scanning patterns and scanning rates have been investigated. Implications of these results on laser densification of multiple materials are discussed.The authors gratefully acknowledge financial support provided bythe National Science Foundation under Grant No: DMI-9908249.Mechanical Engineerin
Thermoelectric properties of Co, Ir, and Os-Doped FeSi Alloys: Evidence for Strong Electron-Phonon Coupling
The effects of various transition metal dopants on the electrical and thermal
transport properties of Fe1-xMxSi alloys (M= Co, Ir, Os) are reported. The
maximum thermoelectric figure of merit ZTmax is improved from 0.007 at 60 K for
pure FeSi to ZT = 0.08 at 100 K for 4% Ir doping. A comparison of the thermal
conductivity data among Os, Ir and Co doped alloys indicates strong
electron-phonon coupling in this compound. Because of this interaction, the
common approximation of dividing the total thermal conductivity into
independent electronic and lattice components ({\kappa}Total =
{\kappa}electronic + {\kappa}lattice) fails for these alloys. The effects of
grain size on thermoelectric properties of Fe0.96Ir0.04Si alloys are also
reported. The thermal conductivity can be lowered by about 50% with little or
no effect on the electrical resistivity or Seebeck coefficient. This results in
ZTmax = 0.125 at 100 K, still about a factor of five too low for solid-state
refrigeration applications
Generation and detection of very high frequency acoustic waves in solids Final report
Techniques for generation and detection of very high frequency acoustic waves in solid
Observation of generalized optomechanical coupling and cooling on cavity resonance
Optomechanical coupling between a light field and the motion of a cavity
mirror via radiation pressure plays an important role for the exploration of
macroscopic quantum physics and for the detection of gravitational waves (GWs).
It has been used to cool mechanical oscillators into their quantum ground
states and has been considered to boost the sensitivity of GW detectors, e.g.
via the optical spring effect. Here, we present the experimental
characterization of generalized, that is, dispersive and dissipative
optomechanical coupling, with a macroscopic (1.5mm)^2-sized silicon nitride
(SiN) membrane in a cavity-enhanced Michelson-type interferometer. We report
for the first time strong optomechanical cooling based on dissipative coupling,
even on cavity resonance, in excellent agreement with theory. Our result will
allow for new experimental regimes in macroscopic quantum physics and GW
detection
Effects of nano-void density, size, and spatial population on thermal conductivity: a case study of GaN crystal
The thermal conductivity of a crystal is sensitive to the presence of
surfaces and nanoscale defects. While this opens tremendous opportunities to
tailor thermal conductivity, a true "phonon engineering" of nanocrystals for a
specific electronic or thermoelectric application can only be achieved when the
dependence of thermal conductivity on the defect density, size, and spatial
population is understood and quantified. Unfortunately, experimental studies of
effects of nanoscale defects are quite challenging. While molecular dynamics
simulations are effective in calculating thermal conductivity, the defect
density range that can be explored with feasible computing resources is
unrealistically high. As a result, previous work has not generated a fully
detailed understanding of the dependence of thermal conductivity on nanoscale
defects. Using GaN as an example, we have combined physically-motivated
analytical model and highly-converged large scale molecular dynamics
simulations to study effects of defects on thermal conductivity. An analytical
expression for thermal conductivity as a function of void density, size, and
population has been derived and corroborated with the model, simulations, and
experiments
Towards the grain boundary phonon scattering problem: an evidence for a low-temperature crossover
The problem of phonon scattering by grain boundaries is studied within the
wedge disclination dipole (WDD) model. It is shown that a specific q-dependence
of the phonon mean free path for biaxial WDD results in a low-temperature
crossover of the thermal conductivity, . The obtained results allow to
explain the experimentally observed deviation of from a
dependence below in and .Comment: 4 pages, 2 figures, submitted to J.Phys.:Condens.Matte
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