20,399 research outputs found
A neural network based spatial light scattering instrument for hazardous airborne fiber detection
This paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited. Copyright OSA (www.osa.org/pubs/osajournals.org)A laser light scattering instrument has been designed to facilitate the real-time detection of potentially hazardous respirable fibers, such as asbestos, within an ambient environment. The instrument captures data relating to the spatial distribution of light scattered by individual particles in flow using a dedicated multi-element photodiode detector array. These data are subsequently processed using an artificial neural network which has previously been trained to recognise those features or patterns within the light scattering distribution which may be characteristic of the specific particle types being sought, such as for example, crocidolite or chrysotile asbestos fibers. Each particle is thus classified into one of a limited set of classes based upon its light scattering properties, and from the accumulated data a particle concentration figure for each class may be produced and updated at regular intervals. Particle analysis rates in excess of 103 per second within a sample volume flow-rate of 1 litre per minute are achievable, offering the possibility of detecting fiber concentrations at the recommended maximum exposure limit of 0.1 fibers/ml within a sampling period of a few seconds.Peer reviewe
Sudden collapse of a colloidal gel
Metastable gels formed by weakly attractive colloidal particles display a
distinctive two-stage time-dependent settling behavior under their own weight.
Initially a space-spanning network is formed that for a characteristic time,
which we define as the lag time \taud, resists compaction. This solid-like
behavior persists only for a limited time. Gels whose age \tw is greater than
\taud yield and suddenly collapse. We use a combination of confocal
microscopy, rheology and time-lapse video imaging to investigate both the
process of sudden collapse and its microscopic origin in an refractive-index
matched emulsion-polymer system. We show that the height of the gel in the
early stages of collapse is well described by the surprisingly simple
expression, h(\ts) = \h0 - A \ts^{3/2}, with \h0 the initial height and
\ts = \tw-\taud the time counted from the instant where the gel first yields.
We propose that this unexpected result arises because the colloidal network
progressively builds up internal stress as a consequence of localized
rearrangement events which leads ultimately to collapse as thermal equilibrium
is re-established.Comment: 14 pages, 11 figures, final versio
Reliability Modeling in Spatially Distributed Logistics System
This article proposes methods for modeling service reliability in a supply chain. The logistics system in a supply chain typically consists of thousands of retail stores along with multiple distribution centers (DC). Products are transported between DC & stores through multiple routes. The service reliability depends on DC location layouts, distances from DC to stores, time requirements for product replenishing at stores, DC\u27s capability for supporting store demands, and the connectivity of transportation routes. Contingent events such as labor disputes, bad weather, road conditions, traffic situations, and even terrorist threats can have great impacts on a system\u27s reliability. Given the large number of store locations & multiple combinations of routing schemes, this article applies an approximation technique for developing first-cut reliability analysis models. The approximation relies on multi-level spatial models to characterize patterns of store locations & demands. These models support several types of reliability evaluation of the logistics system under different probability scenarios & contingency situations. Examples with data taken from a large-scale logistics system of an automobile company illustrate the importance of studying supply-chain system reliability
Two-color photoassociation spectroscopy of ytterbium atoms and the precise determinations of s-wave scattering lengths
By performing high-resolution two-color photoassociation spectroscopy, we
have successfully determined the binding energies of several of the last bound
states of the homonuclear dimers of six different isotopes of ytterbium. These
spectroscopic data are in excellent agreement with theoretical calculations
based on a simple model potential, which very precisely predicts the s-wave
scattering lengths of all 28 pairs of the seven stable isotopes. The s-wave
scattering lengths for collision of two atoms of the same isotopic species are
13.33(18) nm for ^{168}Yb, 3.38(11) nm for ^{170}Yb, -0.15(19) nm for ^{171}Yb,
-31.7(3.4) nm for ^{172}Yb, 10.55(11) nm for ^{173}Yb, 5.55(8) nm for ^{174}Yb,
and -1.28(23) nm for ^{176}Yb. The coefficient of the lead term of the
long-range van der Waals potential of the Yb_2 molecule is C_6=1932(30) atomic
units J nm^6).Comment: 9 pages, 7 figure
Transport critical current density in Fe-sheathed nano-SiC doped MgB2 wires
The nano-SiC doped MgB2/Fe wires were fabricated using a powder-in-tube
method and an in-situ reaction process. The depression of Tc with increasing
SiC doping level remained rather small due to the counterbalanced effect of Si
and C co-doping. The high level SiC co-doping allowed creation of the
intra-grain defects and nano-inclusions, which act as effective pinning
centers, resulting in a substantial enhancement in the Jc(H) performance. The
transport Jc for all the wires is comparable to the magnetic Jc at higher
fields despite the low density of the samples and percolative nature of
current. The transport Ic for the 10wt% SiC doped MgB2/Fe reached 660A at 5K
and 4.5T (Jc = 133,000A/cm2) and 540A at 20K and 2T (Jc = 108,000A/cm2). The
transport Jc for the 10wt% SiC doped MgB2 wire is more than an order of
magnitude higher than for the state-the-art Fe-sheathed MgB2 wire reported to
date at 5K and 10T and 20K and 5T respectively. There is a plenty of room for
further improvement in Jc as the density of the current samples is only 50%.Comment: 4 pages, 7 figures, presented at ASC 2002, Housto
Valence band offset of InN/AlN heterojunctions measured by X-ray photoelectron spectroscopy
The valence band offset of wurtzite-InN/AlN (0001) heterojunctions is determined by x-ray photoelectron spectroscopy to be 1.52±0.17 eV. Together with the resulting conduction band offset of 4.0±0.2 eV, a type-I heterojunction forms between InN and AlN in the straddling arrangement
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