2,770 research outputs found
Hitting spheres on hyperbolic spaces
For a hyperbolic Brownian motion on the Poincar\'e half-plane ,
starting from a point of hyperbolic coordinates inside a
hyperbolic disc of radius , we obtain the probability of
hitting the boundary at the point . For
we derive the asymptotic Cauchy hitting distribution on
and for small values of and we
obtain the classical Euclidean Poisson kernel. The exit probabilities
from a hyperbolic annulus in
of radii and are derived and the transient
behaviour of hyperbolic Brownian motion is considered. Similar probabilities
are calculated also for a Brownian motion on the surface of the three
dimensional sphere.
For the hyperbolic half-space we obtain the Poisson kernel of
a ball in terms of a series involving Gegenbauer polynomials and hypergeometric
functions. For small domains in we obtain the -dimensional
Euclidean Poisson kernel. The exit probabilities from an annulus are derived
also in the -dimensional case
Mid-infrared photodetectors operating over an extended wavelength range up to 90 K
We report a wavelength threshold extension, from the designed value of 3.1 to 8.9 μm, in a -type heterostructure photodetector. This is associated with the use of a graded barrier and barrier offset, and arises from hole–hole interactions in the detector absorber. Experiments show that using long-pass filters to tune the energies of incident photons gives rise to changes in the intensity of the response. This demonstrates an alternative approach to achieving tuning of the photodetector response without the need to adjust the characteristic energy that is determined by the band structure
Parsec-scale jet properties of the gamma-ray quasar 3C 286
The quasar 3C~286 is one of two compact steep spectrum sources detected by
the {\it Fermi}/LAT. Here, we investigate the radio properties of the
parsec(pc)-scale jet and its (possible) association with the -ray
emission in 3C~286. The Very Long Baseline Interferometry (VLBI) images at
various frequencies reveal a one-sided core--jet structure extending to the
southwest at a projected distance of 1 kpc. The component at the jet base
showing an inverted spectrum is identified as the core, with a mean brightness
temperature of ~K. The jet bends at about 600 pc (in
projection) away from the core, from a position angle of to
. Based on the available VLBI data, we inferred the proper motion
speed of the inner jet as mas yr (), corresponding to a jet speed of about at an inclination
angle of between the jet and the line of sight of the observer. The
brightness temperature, jet speed and Lorentz factor are much lower than those
of -ray-emitting blazars, implying that the pc-scale jet in 3C~286 is
mildly relativistic. Unlike blazars in which -ray emission is in
general thought to originate from the beamed innermost jet, the location and
mechanism of -ray emission in 3C~286 may be different as indicated by
the current radio data. Multi-band spectrum fitting may offer a complementary
diagnostic clue of the -ray production mechanism in this source.Comment: 9 pages, 4 figures, accept for publication in MNRA
Performance and design analyses of various configurations of dew point evaporative cooling-based desiccant air-conditioning (DAC) systems for hot and humid conditions
Thermally driven desiccant- and evaporative cooling-based technologies are promising greener and cheaper alternatives to compressor-based systems due to the separate handling of latent and sensible loads. Desiccant air-conditioning (DAC) systems comprise a desiccant dehumidifier, a sensible cooling unit, a heat source for regeneration, and a heat recovery unit. These components of a DAC system can be arranged in various ways to give different configurations with varying advantages and disadvantages. In this study, five configurations of thermally driven desiccant dehumidifier- and dew point evaporative cooling (DPEC)-based DAC systems were investigated. Seven evaluation criteria namely regeneration temperature, desiccant moisture removal capacity, COPt, DPEC L/H, heat exchanger UA, system size, and fan power requirement were employed. Results show that the standard cycle in ventilation mode offers the highest COPt despite having the highest regeneration temperature. Recirculation of the return room air can operate at a significantly lower regeneration temperature at the expense of larger equipment size and much lower COPt. DAC with an internally cooled dehumidification can operate at low regeneration temperature at the expense of higher fan power and slightly lower COPt. Dividing the dehumidification process into two stages can offer operation at moderately lower regeneration temperature without severely affecting the other criteria. This study can serve as a guide for the selection of an appropriate DAC configuration for space cooling depending on the objective criteria and the resources available
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Intumescent Flame Retardant Polyamide 11 Nanocomposites
Current polyamide 11 and 12 are lacking in fire retardancy and high strength/high heat
resistance characteristics for a plethora of fabricated parts that are desired and required
for performance driven applications. The introduction of selected nanoparticles such as
surface modified montmorillonite (MMT) clay or carbon nanofibers (CNFs), combined
with a conventional intumescent flame retardant (FR) additive into the polyamide
11/polyamide 12 (PA11/PA12) by melt processing conditions has resulted in the
preparation of a family of intumescent polyamide nanocomposites. These intumescent
polyamide 11 and 12 nanocomposites exhibit enhanced polymer performance
characteristics, i.e., fire retardancy, high strength and high heat resistance and are
expected to expand the market opportunities for polyamide 11 and polyamide 12 polymer
manufacturers.
The objective of this research is to develop improved polyamide 11 and 12 polymers with
enhanced flame retardancy, thermal, and mechanical properties for selective laser
sintering (SLS) rapid manufacturing (RM). In the present study, a nanophase was
introduced into the polyamide 11 and combining it with a conventional intumescent FR
additive via twin screw extrusion. Arkema RILSAN® polyamide 11 molding polymer
pellets were examined with two types of nanoparticles: chemically modified
montmorillonite (MMT) organoclays, and carbon nanofibers (CNFs); and Clairant’s
Exolit® OP 1230 intumescent FR additive were used to create a family of FR
intumescent polyamide 11 nanocomposites.
Transmission electron microscopy (TEM) was used to determine the degree of
nanoparticles dispersion. Injection molded specimens were fabricated for physical,
thermal, and flammability measurements. Thermal stability of these intumescent
polyamide 11 nanocomposites was examined by TGA. Flammability properties were
obtained using the Cone Calorimeter at an external heat flux of 35 kW/m
2
and UL 94
Test Method. Heat deflection temperatures (HDT) were also measured. TEM
micrographs, physical, thermal, and flammability properties are presented. FR
intumescent polyamide 11 nanocomposites properties are compared with polyamide 11
baseline polymer. Based on flammability and mechanical material performance, selective
polymers including polyamide 11 nanocomposites and control polyamide 11 will be
cryogenically ground into fine powders for SLS RM processing. SLS specimens will be
fabricated for thermal, flammability, and mechanical properties characterization.Mechanical Engineerin
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Flame Retardant Intumescent Polyamide 11 Nanocomposites – Further Study
The objective of this research is to develop improved polyamide 11 and 12 polymers with
enhanced flame retardancy, thermal, and mechanical properties for selective laser sintering
(SLS) rapid manufacturing (RM). In the present study, a nanophase was introduced into the
polyamide 11 and combine with a conventional intumescent flame retardant (FR) additive via
twin screw extrusion. Arkema Rilsan® polyamide 11 molding polymer pellets were used with
two types of nanoparticles such as: chemically modified montmorillonite (MMT) organoclays
and carbon nanofibers (CNFs). Two types of Clariant’s Exolit® OP 1311 and 1312 intumescent
FR additives were used to generate a family of FR intumescent polyamide 11 nanocomposites
with anticipated synergism.Mechanical Engineerin
Systematic Review and Meta-Analysis of Statin Use and Mortality, Intensive Care Unit Admission and Requirement for Mechanical Ventilation in COVID-19 Patients
There is mounting evidence that statin use is beneficial for COVID-19 outcomes. We performed a systematic review and meta-analysis to evaluate the association between statin use and mortality, intensive care unit (ICU) admission and mechanical ventilation in COVID-19 patients, on studies which provided covariate adjusted effect estimates, or performed propensity score matching. We searched PubMed, Embase, Web of Science and Scopus for studies and extracted odds or hazard ratios for specified outcome measures. Data synthesis was performed using a random-effects inverse variance method. Risk of bias, heterogeneity and publication bias were analyzed using standard methods. Our results show that statin use was associated with significant reductions in mortality (OR = 0.72, 95% CI: 0.67–0.77; HR = 0.74, 95% CI: 0.69, 0.79), ICU admission (OR = 0.94, 95% CI: 0.89–0.99; HR = 0.76, 95% CI: 0.60–0.96) and mechanical ventilation (OR = 0.84, 95% CI: 0.78–0.92; HR = 0.67, 95% CI: 0.47–0.97). Nevertheless, current retrospective studies are based on the antecedent use of statins prior to infection and/or continued use of statin after hospital admission. The results may not apply to the de novo commencement of statin treatment after developing COVID-19 infection. Prospective studies are lacking and necessary
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Polyamide 11-Carbon Nanotubes Nanocomposites: Preliminary Investigation
The objective of this research is to develop an improved polyamide 11 (PA11) polymer with
enhanced flame retardancy, thermal, and mechanical properties for selective laser sintering
(SLS) rapid manufacturing. In the present study, a nanophase was introduced into polyamide 11
via twin screw extrusion. Arkema Rilsan® polyamide 11 molding polymer pellets were used
with 1, 3, 5, and 7 wt% loadings of Arkema’s GraphistrengthTM multi-wall carbon nanotubes
(MWNTs) to create a family of PA11-MWNT nanocomposites.
Transmission electron microscopy and scanning electron microscopy were used to determine
the degree and uniformity of dispersion. Injection molded test specimens were fabricated for
physical, thermal, mechanical properties, and flammability measurements. Thermal stability of
these polyamide 11-MWNT nanocomposites was examined by TGA. Mechanical properties such
as ultimate tensile strength, rupture tensile strength, and elongation at rupture were measured.
Flammability properties were also obtained using the UL 94 test method. All these different
methods and subsequent polymer characteristics are discussed in this paper.Mechanical Engineerin
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