4,627 research outputs found
Heat conduction in multifunctional nanotrusses studied using Boltzmann transport equation
Materials that possess low density, low thermal conductivity, and high
stiffness are desirable for engineering applications, but most materials cannot
realize these properties simultaneously due to the coupling between them.
Nanotrusses, which consist of hollow nanoscale beams architected into a
periodic truss structure, can potentially break these couplings due to their
lattice architecture and nanoscale features. In this work, we study heat
conduction in the exact nanotruss geometry by solving the frequency-dependent
Boltzmann transport equation using a variance-reduced Monte Carlo algorithm. We
show that their thermal conductivity can be described with only two parameters,
solid fraction and wall thickness. Our simulations predict that nanotrusses can
realize unique combinations of mechanical and thermal properties that are
challenging to achieve in typical materials
Properties of superconducting MgB_2 wires: "in-situ" versus "ex-situ" reaction technique
We have fabricated a series of iron-sheathed superconducting wires prepared
by the powder-in-tube technique from (MgB_2)_{1-x}:(Mg+2B)_x initial powder
mixtures taken with different proportions, so that x varies from 0 to 1. It
turned out that "ex-situ" prepared wire (x = 0) has considerable disadvantages
compared to all the other wires in which "in-situ" assisted (0 < x < 1) or pure
"in-situ" (x = 1) preparation was used due to weaker inter-grain connectivity.
As a result, higher critical current densities J_c were measured over the
entire range of applied magnetic fields B_a for all the samples with x > 0.
Pinning of vortices in MgB_2 wires is shown to be due to grain boundaries.
J_c(B_a) behavior is governed by an interplay between the transparency of grain
boundaries and the amount of "pinning" grain boundaries. Differences between
thermo-magnetic flux-jump instabilities in the samples and a possible threat to
practical applications are also discussed.Comment: To be published in Supercond. Sci. Technol. (2003), in pres
Template epitaxial growth of thermoelectric Bi/BiSb superlattice nanowires by charge-controlled pulse electrodeposition
© The Electrochemical Society, Inc. 2009. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in The Journal of The Electrochemical Society, 156(9), 2009.Bi/BiSb superlattice nanowires (SLNWs) with a controllable and very small bilayer thickness and a sharp segment interface were grown by adopting a charge-controlled pulse electrodeposition. The deposition parameters were optimized to ensure an epitaxial growth of the SLNWs with a preferential orientation. The segment length and bilayer thickness of the SLNWs can be controlled simply by changing the modulating time, and the consistency of the segment length can be well maintained by our approach. The Bravais law in the electrodeposited nanowires is verified by the SLNW structure. The current–voltage measurement shows that the SLNWs have good electrical conductance, particularly those with a smaller bilayer thickness. The Bi/BiSb SLNWs might have excellent thermoelectric performances.National Natural Science Foundation
of China and the National
Major Project of Fundamental Research for Nanomaterials and
Nanostructures
The Generalized Spectral Kurtosis Estimator
Due to its conceptual simplicity and its proven effectiveness in real-time
detection and removal of radio frequency interference (RFI) from radio
astronomy data, the Spectral Kurtosis (SK) estimator is likely to become a
standard tool of a new generation of radio telescopes. However, the SK
estimator in its original form must be developed from instantaneous power
spectral density (PSD) estimates, and hence cannot be employed as an RFI
excision tool downstream of the data pipeline in existing instruments where any
time averaging is performed. In this letter, we develop a generalized estimator
with wider applicability for both instantaneous and averaged spectral data,
which extends its practical use to a much larger pool of radio instruments.Comment: 5 pages, 2 figures, MNRAS Letters accepte
A complete quasiclassical map for the dynamics of interacting fermions
We present a strategy for mapping the dynamics of a fermionic quantum system
to a set of classical dynamical variables. The approach is based on imposing
the correspondence relation between the commutator and the Poisson bracket,
preserving Heisenberg's equation of motion for one-body operators. In order to
accommodate the effect of two-body terms, we further impose quantization on the
spin-dependent occupation numbers in the classical equations of motion, with a
parameter that is determined self-consistently. Expectation values for
observables are taken with respect to an initial quasiclassical distribution
that respects the original quantization of the occupation numbers. The proposed
classical map becomes complete under the evolution of quadratic Hamiltonians
and is extended for all even order observables. We show that the map provides
an accurate description of the dynamics for an interacting quantum impurity
model in the coulomb blockade regime, at both low and high temperatures. The
numerical results are aided by a novel importance sampling scheme that employs
a reference system to reduce significantly the sampling effort required to
converge the classical calculations
Nature of band-gap states in V-doped TiO2 revealed by resonant photoemission
Band-gap states in V-doped TiO2 have been studied by photoemission spectroscopy over a range of photon energies encompassing the Ti 3p and V 3p core thresholds. The states show resonant enhancement at photon energies significantly higher than found for Ti 3d states introduced into TiO2 by oxygen deficiency or alkalimetal adsorbates. This demonstrates that the gap states relate to electrons trapped on dopant V cations rather than host Ti cations
Drag reduction in a flat-plate turbulent boundary layer flow by polymer additives
This paper presents a theoretical study on the velocity distribution and the friction factor of boundary layer flows with polymer additives starting from the concept of “stress deficit.” A novel method of order of magnitude analysis is developed, which converts the governing equations of boundary layer flow into a solvable ordinary differential equation, thus the total shear stress distribution is obtained, then the formulas for the mean velocity profiles and the friction factor for a boundary layer flow are derived after introducing appropriate expressions for the “effective viscosity” and the thickness of viscous sublayer. The derived velocity equation is able to depict the velocity from a solid wall to the outer edge of boundary layer with or without polymer additives using only one fitted parameter D* that is a function of polymer species, its concentration, and Reynolds number. By integrating the velocity profiles, the friction factor and the thickness of boundary layer development are obtained. Experimental data agree well with the theoretical results
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