4,206 research outputs found
Development and evaluation of die and container materials
X = 0.75 Beta prime Sialon (a silicon aluminum oxynitride) and Sibeon (silicon beryllium oxynitride) are promising die materials. In sessile drop tests in contact with molten silicon, beryllium contamination was less than ppm and aluminum contamination 50 ppm. A shaping die of the Sialon material was successfully fabricated. Dry milling studies for the preparation of Si3N4-Al2O3-ALN mixtures were performed with butanol, acetic anhydride, oleic acid, and triethanolamine milling aids. Optimum mixing was achieved with 0.15 percent triethanolamine using a milling time of 8 hours. Preliminary evaluation of Sibeon materials indicates that they are more resistent to molten silicon attack than Sialon. Silicon contamination from the beryllium was less than aluminum contamination even though the aluminum impurity level in the Sibeon was only 450 to 1300 ppm. Work designed to produce an aluminum-free Sibeon is described
Rotating black hole hair
A Kerr black hole sporting cosmic string hair is studied in the context of the abelian Higgs model vortex. It is shown that such a system displays much richer phenomenology than its static Schwarzschild or Reissner-Nordstrom cousins, for example, the rotation generates a near horizon ‘electric’ field. In the case of an extremal rotating black hole, two phases of the Higgs hair are possible: large black holes exhibit standard hair, with the vortex piercing the event horizon. Small black holes on the other hand, exhibit a flux-expelled solution, with the gauge and scalar field remaining identically in their false vacuum state on the event horizon. This solution however is extremely sensitive to confirm numerically, and we conjecture that it is unstable due to a supperradiant mechanism similar to the Kerr-adS instability. Finally, we compute the gravitational back reaction of the vortex, which turns out to be far more nuanced than a simple conical deficit. While the string produces a conical effect, it is conical with respect to a local co-rotating frame, not with respect to the static frame at infinity
van der Waals phase transition in protein solutions
The van der Waals equation of state for imperfect gases is applied to solutions of macromolecules, especially to explain the fluid-fluid phase transition in protein solutions, a phenomenon of much interest in relation to protein crystallization. The van der Waals b parameter corresponds to the total excluded volume per pair of molecules and can be calculated from independently known molecular properties. It is comprised of terms resulting from hard-sphere and net charge-charge interactions. The experimentally determined second virial coefficient B can then be used to obtain the equilibrium constant for dimerization K, a phenomenologically accessible measure of the van der Waals a parameter. Sedimentation equilibrium is recommended as the technique for measuring B most accurately. More general results are used to make a minor quantitative correction to the van der Waals prediction concerning the criterion for the fluid-fluid phase transition. Calculations of the effect of inert co-solutes on the phase transition may prove useful in choosing crystallization conditions
Predictive learning, prediction errors, and attention: evidence from event-related potentials and eye tracking
Prediction error (‘‘surprise’’) affects the rate of learning: We learn more rapidly about cues for which we initially make incorrect predictions than cues for which our initial predictions are correct. The current studies employ electrophysiological measures to reveal early attentional differentiation of events that differ in their previous involvement in errors of predictive judgment.
Error-related events attract more attention, as evidenced by features of event-related scalp potentials previously implicated in selective visual attention (selection negativity, augmented anterior N1). The earliest differences detected occurred around 120 msec after stimulus onset, and distributed source localization (LORETA)
indicated that the inferior temporal regions were one source of the earliest differences. In addition, stimuli associated with the production of prediction errors show higher dwell times in an eyetracking procedure. Our data support the view that early attentional processes play a role in human associative learning
Second Epoch Global VLBI Observations of Compact Radio Sources in the M82 Starburst Galaxy
We have presented the results of a second epoch of global Very Long Baseline
Interferometry observations, taken on 23 February 2001 at a wavelength of 18
cm, of the central kiloparsec of the nearby starburst galaxy Messier 82. These
observations were aimed at studying the structural and flux evolution of some
of the compact radio sources in the central region that have been identified as
supernova remnants. The objects 41.95+575 and 43.31+592 have been studied,
expansion velocities of 2500 +/- 1200 km/s and 7350 +/- 2100 km/s respectively
have been derived. Flux densities of 31.1 +/- 0.3 mJy and 17.4 +/- 0.3 mJy have
been measured for the two objects. These results are consistent with
measurements and predictions from previous epochs.Comment: 5 pages, 3 figures. To be published on the accompanying CD of the
Proceedings of IAU Colloquium 192: Supernova
Toward Perfection: Kapellasite, Cu3Zn(OH)6Cl2, a New Model S = 1/2 Kagome Antiferromagnet
The search for the resonating valence bond (RVB) state continues to underpin
many areas of condensed matter research. The RVB is made from the dimerisation
of spins on different sites into fluctuating singlets, and was proposed by
Anderson to be the reference state from which the transition to BCS
superconductivity occurs. Little is known about the state experimentally, due
to the scarcity of model materials. Theoretical work has put forward the S =
1/2 kagome antiferromagnet (KAFM) as a good candidate for the realization of
the RVB state. In this paper we introduce a new model system, the S = 1/2 KAFM
Kapellasite, Cu3Zn(OH)6Cl2. We show that its crystal structure is a good
approximation to a 2-dimensional kagome antiferromagnet and that susceptibility
data indicate a collapse of the magnetic moment below T = 25 K that is
compatible with the spins condensing into the non-magnetic RVB state.Comment: Communication, 3 pages, 3 figure
Prototype Backscatter Moessbauer Spectrometer for Measurement of Martian Surface Mineralogy
We have designed and successfully tested a prototype of a backscatter Moessbauer spectrometer (BaMS) targeted for use on the Martian surface to (1) determine oxidation states of iron, and (2) identify and determine relative abundances of iron-bearing mineralogies. No sample preparation is required to perform measurements; it is only necessary to bring sample and instrument into physical contact. The prototype meets our projected specification for a flight instrument in terms of mass, power, and volume. A Moessbauer spectrometer on the Martian surface would provide wide variety of information about the current state of the Martian surface, and this information is described
High-velocity-resolution observations of OH main line lasers in the M82 starburst
Using the VLA, a series of high velocity resolution observations have been
made of the M82 starburst at 1.6 GHz. These observations follow up on previous
studies of the main line OH maser emission in the central kiloparsec of this
starburst region, but with far greater velocity resolution, showing significant
velocity structure in some of the maser spots for the first time. A total of
thirteen masers were detected, including all but one of the previously known
sources. While some of these masers are still unresolved in velocity, these new
results clearly show velocity structure in spectra from several of the maser
regions. Position-velocity plots show good agreement with the distribution of
H{\sc i} including interesting velocity structure on the blueward feature in
the west of the starburst which traces the velocity distribution seen in the
ionised gas.Comment: MNRAS in press. 15 pages, 9 figure
Deceleration and Dispersion of Large-scale Coronal Bright Fronts
One of the most dramatic manifestations of solar activity are large-scale
coronal bright fronts (CBFs) observed in extreme ultraviolet (EUV) images of
the solar atmosphere. To date, the energetics and kinematics of CBFs remain
poorly understood, due to the low image cadence and sensitivity of previous EUV
imagers and the limited methods used to extract the features. In this paper,
the trajectory and morphology of CBFs was determined in order to investigate
the varying properties of a sample of CBFs, including their kinematics and
pulse shape, dispersion, and dissipation. We have developed a semi-automatic
intensity profiling technique to extract the morphology and accurate positions
of CBFs in 2.5-10 min cadence images from STEREO/EUVI. The technique was
applied to sequences of 171A and 195A images from STEREO/EUVI in order to
measure the wave properties of four separate CBF events. Following launch at
velocities of ~240-450kms^{-1} each of the four events studied showed
significant negative acceleration ranging from ~ -290 to -60ms^{-2}. The CBF
spatial and temporal widths were found to increase from ~50 Mm to ~200 Mm and
~100 s to ~1500 s respectively, suggesting that they are dispersive in nature.
The variation in position-angle averaged pulse-integrated intensity with
propagation shows no clear trend across the four events studied. These results
are most consistent with CBFs being dispersive magnetoacoustic waves.Comment: 15 pages, 18 figure
15 years of VLBI observations of two compact radio sources in Messier 82
We present the results of a second epoch of 18cm global Very Long-Baseline
Interferometry (VLBI) observations, taken on 23 February 2001, of the central
kiloparsec of the nearby starburst galaxy Messier 82. These observations
further investigate the structural and flux evolution of the most compact radio
sources in the central region of M82. The two most compact radio objects in M82
have been investigated (41.95+575 and 43.31+592). Using this recent epoch of
data in comparison with our previous global VLBI observations and two earlier
epochs of European VLBI Network observations we measure expansion velocities in
the range of 1500-2000km/s for 41.95+575, and 9000-11000km/s for 43.31+592
using various independent methods. In each case the measured remnant expansion
velocities are significantly larger than the canonical expansion velocity
(500km/s) of supernova remnants within M82 predicted from theoretical models.
In this paper we discuss the implications of these measured expansion
velocities with respect to the high density environment that the SNR are
expected to reside in within the centre of the M82 starburst.Comment: Accepted for publication in MNRAS, 9 pages, 8 figure
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