877 research outputs found
Preparation and some properties of cholesterol oxidase from Rhodococcus sp. R14-2
Rhodococcus sp. R14-2, isolated from Chinese Jin-hua ham, produces a novel extracellular cholesterol oxidase (COX). The enzyme was extracted from fermentation broth and purified 53.1-fold based on specific activity. The purified enzyme shows a single polypeptide band on SDS-PAGE with an estimated molecular weight of about 60 kDa, and has a pI of 8.5. The first 10 amino acid residues of the NH2-terminal sequence of the enzyme are A-P-P-V-A-S-C-R-Y-C, which differs from other known COXs. The enzyme is stable over a rather wide pH range of 4.0¿10.0. The optimum pH and temperature of the COX are pH 7.0 and 50°C, respectively. The COX rapidly oxidizes 3ß-hydroxysteroids such as cholesterol and phytosterols, but is inert toward 3¿-hydroxysteroids. Thus, the presence of a 3ß-hydroxyl group appears to be essential for substrate activity. The Michaelis constant (Km) for cholesterol is estimated at 55 ¿M; the COX activity was markedly inhibited by metal ions such as Hg2+ and Fe3+ and inhibitors such as p-chloromercuric benzoate, mercaptoethanol and fenpropimorph. Inhibition caused by p-chloromercuric benzoate, mercuric chloride, or silver nitrate was almost completely prevented by the addition of glutathione. These suggests that -SH groups may be involved in the catalytic activity of the present CO
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Experimental Study of the Cooling Characteristics of Polymer Filaments in FDM and Impact on the Mesostructures and Properties of Prototypes
The bonding quality among polymer filaments in the fused deposition modeling (FDM)
process determines the integrity and mechanical properties of the resultant prototypes. This
research investigates the bond formation among extruded acrylonitrile butadiene styrene (ABS)
filaments in the FDM process. Experimental measurements of the temperature profiles were
carried out for different specimens and their effects on mesostructures and mechanical properties
were observed. Models describing the formation of bonds among polymer filaments during the
FDM process are discussed. Predictions of the degree of bonding achieved during the filament
deposition process were made based on thermal analysis of extruded polymer filaments. The
bond quality was assessed based on the growth of the neck formed between adjacent filaments
and their failure under flexural loading. Further experimental work is underway to assess the
validity of the proposed models.The financial support for this work was provided by the Natural Sciences and
Engineering Research Council of Canada (NSERC) through Research Grants awarded to Drs.
Bellehumeur and Gu.Mechanical Engineerin
Fabrication and characterization of locally resonant acoustic metamaterials made with resonators generated from core-shell drops
Acoustic metamaterials promise the remarkable ability to control, direct, and manipulate sound waves. Within this infant field, a promising approach to fabricate locally resonant acoustic metamaterials is the use of resonators composed of a heavy core surrounded by a rubber shell dispersed in an epoxy matrix. At the resonant frequency, the resonators vibrate 180° out-of-phase with the matrix and a band gap in transmission is observed making these materials excellent sound absorbers. The resonant frequency of the resonators scales with the core mass; therefore, it can be tailored by increasing the core diameter or the density of the core material. A significant challenge in the study and adoption of these materials is the lack of techniques to easily fabricate resonators with a wide range of sizes, and properties. Here, we present a robust yet simple technique to fabricate resonators with diameters ranging from 50 µm to 5 mm from core-shell drops generated in microfluidic and millifluidic devices. We started by fabricating resonators with core diameters ranging from 50 µm to 1 mm at rates ranging from 2000 to 200 drops/second respectively, from double emulsion drops composed of a concentrated ceramic suspension in the core (inner drop) surrounded by a UV-crosslinkable rubber shell (outer drop) using microcapillary microfluidic devices. The double emulsion drops were collected and exposed to UV to crosslink the shell material forming resonators with resonant frequencies ranging from 100 kHz to 25 kHz depending on their size. Lower resonant frequencies down to 6 kHz were obtained by fabricating resonators with core diameters ranging from 1.2 mm to 2 mm from core-shell drops extruded in air from a coaxial nozzle at rates up to 6 drops/minute. The effects of core density were studied by utilizing suspensions composed of ceramic particles of increasing density including silica, alumina, and lead zirconate titanate (PZT). Resonators were harvested, dried and mixed with epoxy to fabricate acoustic metamaterials. The transmission properties of the acoustic metamaterials made with resonators with different core diameters, core materials, and ordering within the matrix, were measured using a shaker/accelerometer setup in the frequency range from 1 kHz to 12 kHz. For example, acoustic metamaterials composed of randomly dispersed 1.8 mm alumina-core resonators at a 30 vol% concentration showed a well defined band-gap at 8.5 kHz. A finite element model was also developed to capture the acoustic transmission physics of these materials. This technique offers a robust path for the fabrication of acoustic resonators and locally resonant acoustic metamaterials
A three-dimensional multidimensional gas-kinetic scheme for the Navier-Stokes equations under gravitational fields
This paper extends the gas-kinetic scheme for one-dimensional inviscid
shallow water equations (J. Comput. Phys. 178 (2002), pp. 533-562) to
multidimensional gas dynamic equations under gravitational fields. Four
important issues in the construction of a well-balanced scheme for gas dynamic
equations are addressed. First, the inclusion of the gravitational source term
into the flux function is necessary. Second, to achieve second-order accuracy
of a well-balanced scheme, the Chapman-Enskog expansion of the Boltzmann
equation with the inclusion of the external force term is used. Third, to avoid
artificial heating in an isolated system under a gravitational field, the
source term treatment inside each cell has to be evaluated consistently with
the flux evaluation at the cell interface. Fourth, the multidimensional
approach with the inclusion of tangential gradients in two-dimensional and
three-dimensional cases becomes important in order to maintain the accuracy of
the scheme. Many numerical examples are used to validate the above issues,
which include the comparison between the solutions from the current scheme and
the Strang splitting method. The methodology developed in this paper can also
be applied to other systems, such as semi-conductor device simulations under
electric fields.Comment: The name of first author was misspelled as C.T.Tian in the published
paper. 35 pages,9 figure
Experimental study of coherent synchrotron radiation in the emittance exchange line at the A0-photoinjector
Next generation accelerators will require a high current, low emittance beam
with a low energy spread. Such accelerators will employ advanced beam
conditioning systems such as emittance exchangers to manipulate high brightness
beams. One of the goals of the Fermilab A0 photoinjector is to investigate the
transverse to longitudinal emittance exchange principle. Coherent synchrotron
radiation could limit high current operation of the emittance exchanger. In
this paper, we report on the preliminary experimental and simulation study of
the coherent synchroton radiation (CSR) in the emittance exchange line at the
A0 photoinjector.Comment: 4 pp. 14th Advanced Accelerator Concepts Workshop, 13-19 Jun 2010:
Annapolis, Marylan
Suppression of the structural phase transition and lattice softening in slightly underdoped Ba(1-x)K(x)Fe2As2 with electronic phase separation
We present x-ray powder diffraction (XRPD) and neutron diffraction
measurements on the slightly underdoped iron pnictide superconductor
Ba(1-x)K(x)Fe2As2, Tc = 32K. Below the magnetic transition temperature Tm =
70K, both techniques show an additional broadening of the nuclear Bragg peaks,
suggesting a weak structural phase transition. However, macroscopically the
system does not break its tetragonal symmetry down to 15 K. Instead, XRPD
patterns at low temperature reveal an increase of the anisotropic microstrain
proportionally in all directions. We associate this effect with the electronic
phase separation, previously observed in the same material, and with the effect
of lattice softening below the magnetic phase transition. We employ density
functional theory to evaluate the distribution of atomic positions in the
presence of dopant atoms both in the normal and magnetic states, and to
quantify the lattice softening, showing that it can account for a major part of
the observed increase of the microstrain.Comment: 7 pages, 4 figure
Donor-specific transfusion in the nude rat prolongs survival of subsequently transplanted hamster cardiac xenografts
Oxygen impurities in NiAl: Relaxation effects
We have used a full-potential linear muffin-tin orbital method to calculate
the effects of oxygen impurities on the electronic structure of NiAl. Using the
supercell method with a 16-atom supercell we have investigated the cases where
an oxygen atom is substitutionally placed at either a nickel or an aluminum
site. Full relaxation of the atoms within the supercell was allowed. We found
that oxygen prefers to occupy a nickel site over an aluminum site with a site
selection energy of 138 mRy (21,370 K). An oxygen atom placed at an aluminum
site is found to cause a substantial relaxation of its nickel neighbors away
from it. In contrast, this steric repulsion is hardly present when the oxygen
atom occupies the nickel site and is surrounded by aluminum neighbors. We
comment on the possible relation of this effect to the pesting degradation
phenomenon (essentially spontaneous disintegration in air) in nickel
aluminides.Comment: To appear in Phys. Rev. B (Aug. 15, 2001
Microwave Surface Impedance of YBCO:123 crystals: Experiment and comparison to a d-wave model
We present measurements of the microwave surface resistance Rs and the
penetration depth lambda of YBCO:123 crystals. At low T obeys lambda(T) a
polynomial behavior, while Rs displays a characteristic non-monotonic
T-dependence.
A detailed comparison of the experimental data is made to a model of d-wave
superconductivity which includes both elastic and inelastic scattering. While
the model reproduces the general features of the experimental data, three
aspects of the parameters needed are worth noting. The elastic scattering rate
required to fit the data is much smaller than measured from the normal state,
the scattering phase shifts have to be close to pi/2 and a strong coupling
value of the gap parameter 2\Delta(0)/kTc = 6 is needed. On the experimental
side the uncertainties regarding the material parameters lambda(0) and
Rs,res(0) further complicate a quantitative comparison.
For one sample does Rs,res(0) agree with the intrinsic value which results
from the d-wave model.Comment: uuencoded tar.Z, 11 pages with 5 figures, used style files: elsart
and graphicx, PS-file available at http://sagar.cas.neu.edu/preprints.htm
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