12,792 research outputs found
Space shuttle nonmetallic materials age life prediction
The chemiluminescence from samples of polybutadiene, Viton, Teflon, Silicone, PL 731 Adhesive, and SP 296 Boron-Epoxy composite was measured at temperatures from 25 to 150 C. Excellent correlations were obtained between chemiluminescence and temperature. These correlations serve to validate accelerated aging tests (at elevated temperatures) designed to predict service life at lower temperatures. In most cases, smooth or linear correlations were obtained between chemiluminescence and physical properties of purified polymer gums, including the tensile strength, viscosity, and loss tangent. The latter is a complex function of certain polymer properties. Data were obtained with far greater ease by the chemiluminescence technique than by the conventional methods of study. The chemiluminescence from the Teflon (Halon) samples was discovered to arise from trace amounts of impurities, which were undetectable by conventional, destructive analysis of the sample
Chemical studies of the passivation of GaAs surface recombination using sulfides and thiols
Steady-state photoluminescence, time-resolved photoluminescence, and x-ray photoelectron spectroscopy have been used to study the electrical and chemical properties of GaAs surfaces exposed to inorganic and organic sulfur donors. Despite a wide variation in S2–(aq) concentration, variation of the pH of aqueous HS–solutions had a small effect on the steady-state n-type GaAs photoluminescence intensity, with surfaces exposed to pH=8, 0.1-M HS–(aq) solutions displaying comparable luminescence intensity relative to those treated with pH=14, 1.0-M Na2S·9H2O(aq). Organic thiols (R-SH, where R=–CH2CH2SH or –C6H4Cl) dissolved in nonaqueous solvents were found to effect increases in steady-state luminescence yields and in time-resolved luminescence decay lifetimes of (100)-oriented GaAs. X-ray photoelectron spectroscopy showed that exposure of GaAs surfaces to these organic systems yielded thiols bound to the GaAs surface, but such exposure did not remove excess elemental As and did not form a detectable As2S3 overlayer on the GaAs. These results imply that complete removal of As0 or formation of monolayers of As2S3 is not necessary to effect a reduction in the recombination rate at etched GaAs surfaces. Other compounds that do not contain sulfur but that are strong Lewis bases, such as methoxide ion, also improved the GaAs steady-state photoluminescence intensity. These results demonstrate that a general class of electron-donating reagents can be used to reduce nonradiative recombination at GaAs surfaces, and also imply that prior models focusing on the formation of monolayer coverages of As2S3 and Ga2S3 are not adequate to describe the passivating behavior of this class of reagents. The time-resolved, high level injection experiments clearly demonstrate that a shift in the equilibrium surface Fermi-level energy is not sufficient to explain the luminescence intensity changes, and confirm that HS– and thiol-based reagents induce substantial reductions in the surface recombination velocity through a change in the GaAs surface state recombination rate
Paradoxical popups: Why are they hard to catch?
Even professional baseball players occasionally find it difficult to
gracefully approach seemingly routine pop-ups. This paper describes a set of
towering pop-ups with trajectories that exhibit cusps and loops near the apex.
For a normal fly ball, the horizontal velocity is continuously decreasing due
to drag caused by air resistance. But for pop-ups, the Magnus force (the force
due to the ball spinning in a moving airflow) is larger than the drag force. In
these cases the horizontal velocity decreases in the beginning, like a normal
fly ball, but after the apex, the Magnus force accelerates the horizontal
motion. We refer to this class of pop-ups as paradoxical because they appear to
misinform the typically robust optical control strategies used by fielders and
lead to systematic vacillation in running paths, especially when a trajectory
terminates near the fielder. In short, some of the dancing around when
infielders pursue pop-ups can be well explained as a combination of bizarre
trajectories and misguidance by the normally reliable optical control strategy,
rather than apparent fielder error. Former major league infielders confirm that
our model agrees with their experiences.Comment: 28 pages, 10 figures, sumitted to American Journal of Physic
A Solvable Model for Decoupling of Interacting Clusters
We consider M clusters of interacting particles, whose in-group interactions
are arbitrary, and inter-group interactions are approximated by oscillator
potentials. We show that there are masses and frequencies that decouple the
in-group and inter-group degrees of freedom, which reduces the initial problem
to M independent problems that describe each of the relative in-group systems.
The dynamics of the M center-of-mass coordinates is described by the
analytically solvable problem of M coupled harmonic oscillators. This paper
derives and discusses these decoupling conditions. Furthermore, to illustrate
our findings, we consider a charged impurity interacting with a ring of ions.
We argue that the impurity can be used to probe the center-of-mass dynamics of
the ions.Comment: Version accepted for publication in EP
N=(1,1) super Yang--Mills theory in 1+1 dimensions at finite temperature
We present a formulation of N=(1,1) super Yang-Mills theory in 1+1 dimensions
at finite temperature. The partition function is constructed by finding a
numerical approximation to the entire spectrum. We solve numerically for the
spectrum using Supersymmetric Discrete Light-Cone Quantization (SDLCQ) in the
large-N_c approximation and calculate the density of states. We find that the
density of states grows exponentially and the theory has a Hagedorn
temperature, which we extract. We find that the Hagedorn temperature at
infinite resolution is slightly less than one in units of (g^(2) N_c/pi)^(1/2).
We use the density of states to also calculate a standard set of thermodynamic
functions below the Hagedorn temperature. In this temperature range, we find
that the thermodynamics is dominated by the massless states of the theory.Comment: 16 pages, 8 eps figures, LaTe
Radio Polarization of the Young High-Magnetic-Field Pulsar PSR J1119-6127
We have investigated the radio polarization properties of PSR J1119-6127, a
recently discovered young radio pulsar with a large magnetic field. Using
pulsar-gated radio imaging data taken at a center frequency of 2496 MHz with
the Australia Telescope Compact Array, we have determined a rotation measure
for the pulsar of +842 +/- 23 rad m^-2. These data, combined with archival
polarimetry data taken at a center frequency of 1366 MHz with the Parkes
telescope, were used to determine the polarization characteristics of PSR
J1119-6127 at both frequencies. The pulsar has a fractional linear polarization
of ~75% and ~55% at 1366 and 2496 MHz, respectively, and the profile consists
of a single, wide component. This pulse morphology and high degree of linear
polarization are in agreement with previously noticed trends for young pulsars
(e.g., PSR J1513-5908). A rotating-vector (RV) model fit of the position angle
(PA) of linear polarization over pulse phase using the Parkes data suggests
that the radio emission comes from the leading edge of a conal beam. We discuss
PSR J1119-6127 in the context of a recent theoretical model of pulsar spin-down
which can in principle be tested with polarization and timing data from this
pulsar. Geometric constraints from the RV fit are currently insufficient to
test this model with statistical significance, but additional data may allow
such a test in the future.Comment: 9 pages, including 6 figures and 1 table. Accepted for publication in
Ap
Massive galaxies with very young AGN
Gigahertz Peaked Spectrum (GPS) radio galaxies are generally thought to be
the young counterparts of classical extended radio sources and live in massive
ellipticals. GPS sources are vital for studying the early evolution of
radio-loud AGN, the trigger of their nuclear activity, and the importance of
feedback in galaxy evolution. We study the Parkes half-Jansky sample of GPS
radio galaxies of which now all host galaxies have been identified and 80% has
their redshifts determined (0.122 < z < 1.539). Analysis of the absolute
magnitudes of the GPS host galaxies show that at z > 1 they are on average a
magnitude fainter than classical 3C radio galaxies. This suggests that the AGN
in young radio galaxies have not yet much influenced the overall properties of
the host galaxy. However their restframe UV luminosities indicate that there is
a low level of excess as compared to passive evolution models.Comment: To appear in the proceedings of "Formation and Evolution of Galaxy
Bulges", IAUS 245; M. Bureau, E. Athanassoula & B. Barbuy, ed
Management of intractable bladder neck strictures following radical prostatectomy using the Memokath®045 stent
The incidence of vesicourethral anastomotic stenosis (VUAS) post radical prostatectomy varies from 1 to 26%. Current treatment can be challenging and includes a variety of different procedures. These range from endoscopic dilations to bladder neck reconstruction to urinary diversion. We investigated a 2-stage endoscopic treatment, using the thermo-expandable Memokath®045 bladder neck stent to manage patients with VUAS post radical prostatectomy. We retrospectively reviewed 30 patients, between 2013 and 2017, who underwent a Memokath®045 stent insertion following failed primary treatment (dilation and clean intermittent catheterisation) for VUAS. The mean interval time between prostatectomy and Memokath®045 stent insertion was 13 months. The mean follow-up time was 3.6 years with all patients having a minimum of 12-month follow-up. All patients had two previous attempts at endoscopic dilatation with or without incision and a trial of clean intermittent catheterisation. During stage 1, the anastomotic stricture is dilated/incised to diameter of 30 Fr, the stricture length is measured, and a catheter is left in situ. One to 2 weeks later, post haemostasis and healing, an appropriately sized Memokath®045 stent is inserted. The stent is then removed 1-year post-op. Our series of patients had a median age of 62 (54–72). Most patients (26) had a robot-assisted radical prostatectomy (RARP) or salvage procedure. Results showed improvement in IPSS scores, IPSS quality of life scores, Qmax and PVR after the Memokath®045 stent was removed compared to pre-operation. With a minimum of 12 months post stent removal, 93% of patients were fully continent, whilst 7% of patients were socially continent. 2 (7%) patients had their stents removed and not replaced due to re-stricturing and stone formation. However, no urinary tract infections, stricture recurrence or urinary retention was observed in the rest of the cohort (93%). Overall, the Memokath®045 stent was successful in treating 93% of our patients with VUAS. Our series had minimal complications that were managed with conservative measures and in three patients’ re-operation was needed. In conclusion, the Memokath®045 stent is a minimally invasive technique with faster recovery time compared to other techniques such as bladder neck reconstruction or urinary diversion. Additionally, it provides superior patency results compared to other techniques such as bladder neck incision and injection of Mitomycin C. Therefore, this management option should be considered in the management of VUAS
- …