850 research outputs found
The Development of Global Science.
How do we build research capacity throughout the world and capture the great human potential? To us, the answer is rather straightforward: the time-honored tradition of scientific mentoring must be practiced on a wider scale across borders. Herein, we detail the necessity for expanding mentorship to a global scale and provide several important principles to be considered when designing, planning, and implementing programs and centers of research around the world
Adsorption Mechanism and Uptake of Methane in Covalent Organic Frameworks: Theory and Experiment
We determined the methane (CH_4) uptake (at 298 K and 1 to 100 bar pressure) for a variety of covalent organic frameworks (COFs), including both two-dimensional (COF-1, COF-5, COF-6, COF-8, and COF-10) and three-dimensional (COF-102, COF-103, COF-105, and COF-108) systems. For all COFs, the CH_4 uptake was predicted from grand canonical Monte Carlo (GCMC) simulations based on force fields (FF) developed to fit accurate quantum mechanics (QM) [second order MĂžllerâPlesset (MP2) perturbation theory using doubly polarized quadruple-ζ (QZVPP) basis sets]. This FF was validated by comparison with the equation of state for CH_4 and by comparison with the experimental uptake isotherms at 298 K (reported here for COF-5 and COF-8), which agrees well (within 2% for 1â100 bar) with the GCMC simulations. From our simulations we have been able to observe, for the first time, multilayer formation coexisting with a pore filling mechanism. The best COF in terms of total volume of CH_4 per unit volume COF absorbent is COF-1, which can store 195 v/v at 298 K and 30 bar, exceeding the U.S. Department of Energy target for CH_4 storage of 180 v/v at 298 K and 35 bar. The best COFs on a delivery amount basis (volume adsorbed from 5 to 100 bar) are COF-102 and COF-103 with values of 230 and 234 v(STP: 298 K, 1.01 bar)/v, respectively, making these promising materials for practical methane storage
Covalent Organic Frameworks as Exceptional Hydrogen Storage Materials
We report the H_2 uptake properties of six covalent organic frameworks (COFs) from first-principles-based grand canonical Monte-Carlo simulations. The predicted H_2 adsorption isotherm is in excellent agreement with the only available experimental result (3.3 vs 3.4 wt % at 50 bar and 77 K for COF-5), also reported here, validating the predictions. We predict that COF-105 and COF-108 lead to a reversible excess H_2 uptake of 10.0 wt % at 77 K, making them the best known storage materials for molecular hydrogen at 77 K. We predict that the total H_2 uptake for COF-108 is 18.9 wt % at 77 K. COF-102 shows the best volumetric performance, storing 40.4 g/L of H_2 at 77 K. These results indicate that the COF systems are most promising candidates for practical hydrogen storage
Chaotic Phase Synchronization in Bursting-neuron Models Driven by a Weak Periodic Force
We investigate the entrainment of a neuron model exhibiting a chaotic
spiking-bursting behavior in response to a weak periodic force. This model
exhibits two types of oscillations with different characteristic time scales,
namely, long and short time scales. Several types of phase synchronization are
observed, such as 1 : 1 phase locking between a single spike and one period of
the force and 1 : l phase locking between the period of slow oscillation
underlying bursts and l periods of the force. Moreover, spiking-bursting
oscillations with chaotic firing patterns can be synchronized with the periodic
force. Such a type of phase synchronization is detected from the position of a
set of points on a unit circle, which is determined by the phase of the
periodic force at each spiking time. We show that this detection method is
effective for a system with multiple time scales. Owing to the existence of
both the short and the long time scales, two characteristic phenomena are found
around the transition point to chaotic phase synchronization. One phenomenon
shows that the average time interval between successive phase slips exhibits a
power-law scaling against the driving force strength and that the scaling
exponent has an unsmooth dependence on the changes in the driving force
strength. The other phenomenon shows that Kuramoto's order parameter before the
transition exhibits stepwise behavior as a function of the driving force
strength, contrary to the smooth transition in a model with a single time
scale
The Discovery of a Very Narrow-Line Star Forming Obat a Redshift of 5.66ject
We report on the discovery of a very narrow-line star forming object beyond
redshift of 5. Using the prime-focus camera, Suprime-Cam, on the 8.2 m Subaru
telescope together with a narrow-passband filter centered at
= 8150 \AA with passband of = 120 \AA, we have obtained a very
deep image of the field surrounding the quasar SDSSp J104433.04012502.2 at a
redshift of 5.74. Comparing this image with optical broad-band images, we have
found an object with a very strong emission line. Our follow-up optical
spectroscopy has revealed that this source is at a redshift of
, forming stars at a rate
yr. Remarkably, the velocity dispersion of Ly-emitting gas is
only 22 km s. Since a blue half of the Ly emission could be
absorbed by neutral hydrogen gas, perhaps in the system, a modest estimate of
the velocity dispersion may be 44 km s. Together with a linear
size of 7.7 kpc, we estimate a lower limit of the dynamical mass
of this object to be . It is thus suggested that
LAE J10440123 is a star-forming dwarf galaxy (i.e., a subgalactic object or
a building block) beyond redshift 5 although we cannot exclude a possibility
that most Ly emission is absorbed by the red damping wing of neutral
intergalactic matter.Comment: 6 pages, 2 figures. ApJ Letters, in pres
Metal-organic and covalent organic frameworks (MOFs and COFs) as adsorbents for environmentally significant gases (H2, CO2, and CH4)
A series of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) possessing
various functionalities, pore structures, and surface areas were evaluated for sorption and storage
properties of environmentally significant gases (H_2, CO_2, and CH_4). It was concluded that the gas
sorption behavior follows a general trend that materials with high surface area show enhanced gas
uptake performance. For example, MOF-177 (SA = 5200 m^2/g) captures 7.2 wt% of H_2 at 77 K and 19
wt% of CH_4 at 298 K. In addition, MOF-177 exhibits exceptionally high gravimetric CO_2 uptake up to
120 wt% at 298 K. Similarly, the gas storage capacity for COFs seems to follow the same trend and it is
determined by the apparent surface area. The architectural stability of both COFs and MOFs upon high
pressure H_2 and CH_4 gas sorption measurements were manifested by isotherms which reach saturation
without significant hysteresis
Effects of gas density on the structure of liquid jets in still gases
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77334/1/AIAA-11098-570.pd
Evidence for a companion to BM Gem, a silicate carbon star
Balmer and Paschen continuum emission as well as Balmer series lines of P
Cygni-type profile from H_gamma through H_23 are revealed in the violet spectra
of BM Gem, a carbon star associated with an oxygen-rich circumstellar shell
(`silicate carbon star') observed with the high dispersion spectrograph (HDS)
on the Subaru telescope. The blue-shifted absorption in the Balmer lines
indicates the presence of an outflow, the line of sight velocity of which is at
least 400 km s^-1, which is the highest outflow velocity observed to date in a
carbon star. We argue that the observed unusual features in BM Gem are strong
evidence for the presence of a companion, which should form an accretion disk
that gives rise to both an ionized gas region and a high velocity, variable
outflow. The estimated luminosity of ~0.2 (0.03-0.6) L_sun for the ionized gas
can be maintained by a mass accretion rate to a dwarf companion of ~10^-8 M_sun
yr^-1, while ~10^-10 M_sun yr^-1 is sufficient for accretion to a white dwarf
companion. These accretion rates are feasible for some detached binary
configurations on the basis of the Bond-Hoyle type accretion process. We
concluded that the carbon star BM Gem is in a detached binary system with a
companion of low mass and low luminosity. However, we are unable to determine
whether this companion object is a dwarf or a white dwarf. The upper limits for
binary separation are 210 AU and 930 AU for a dwarf and a white dwarf,
respectively. We also note that the observed features of BM Gem mimic those of
Mira (omi Cet), which may suggest actual similarities in their binary
configurations and circumstellar structures.Comment: 11 pages, 2 figures, 1 table, accepted for publication in Ap
Covalent Organic Frameworks as Exceptional Hydrogen Storage Materials
We report the H_2 uptake properties of six covalent organic frameworks (COFs) from first-principles-based grand canonical Monte-Carlo simulations. The predicted H_2 adsorption isotherm is in excellent agreement with the only available experimental result (3.3 vs 3.4 wt % at 50 bar and 77 K for COF-5), also reported here, validating the predictions. We predict that COF-105 and COF-108 lead to a reversible excess H_2 uptake of 10.0 wt % at 77 K, making them the best known storage materials for molecular hydrogen at 77 K. We predict that the total H_2 uptake for COF-108 is 18.9 wt % at 77 K. COF-102 shows the best volumetric performance, storing 40.4 g/L of H_2 at 77 K. These results indicate that the COF systems are most promising candidates for practical hydrogen storage
Pattern formation of reaction-diffusion system having self-determined flow in the amoeboid organism of Physarum plasmodium
The amoeboid organism, the plasmodium of Physarum polycephalum, behaves on
the basis of spatio-temporal pattern formation by local
contraction-oscillators. This biological system can be regarded as a
reaction-diffusion system which has spatial interaction by active flow of
protoplasmic sol in the cell. Paying attention to the physiological evidence
that the flow is determined by contraction pattern in the plasmodium, a
reaction-diffusion system having self-determined flow arises. Such a coupling
of reaction-diffusion-advection is a characteristic of the biological system,
and is expected to relate with control mechanism of amoeboid behaviours. Hence,
we have studied effects of the self-determined flow on pattern formation of
simple reaction-diffusion systems. By weakly nonlinear analysis near a trivial
solution, the envelope dynamics follows the complex Ginzburg-Landau type
equation just after bifurcation occurs at finite wave number. The flow term
affects the nonlinear term of the equation through the critical wave number
squared. Contrary to this, wave number isn't explicitly effective with lack of
flow or constant flow. Thus, spatial size of pattern is especially important
for regulating pattern formation in the plasmodium. On the other hand, the flow
term is negligible in the vicinity of bifurcation at infinitely small wave
number, and therefore the pattern formation by simple reaction-diffusion will
also hold. A physiological role of pattern formation as above is discussed.Comment: REVTeX, one column, 7 pages, no figur
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