1,061 research outputs found
Nanoscale thermal properties of carbon nanotubes/epoxy composites by atomistic simulations
Carbon nanotubes/epoxy composites are increasingly employed in several industrial fields, because of the enhanced material properties provided by the nanofillers. In particular, the thermal conductivity of these nanocomposites is determined by heat transfer mechanisms occurring over multiple scales, thus causing a complex relation between effective response and microscopic characteristics of the material. Here, the thermal properties of epoxy composites reinforced by carbon nanotubes are investigated using atomistic simulations. For a better understanding of how the effective thermal conductivity arises from the characteristics of the composite at the nanoscale, the thermal properties of its constituents are studied separately according to different geometrical, physical and chemical characteristics. The thermal conductivity of carbon nanotubes and epoxy resin alone is first investigated by molecular dynamics; then, the Kapitza resistance at the nanotube–nanotube and nanotube–epoxy interfaces is studied as well. The effective thermal conductivity of the carbon nanotubes/epoxy composite is finally computed and the observed behavior interpreted on the basis of the properties of the nanofillers, matrix and interfaces alone. Results – verified against effective medium theory predictions – show that, for the considered configurations, the effective thermal conductivity of the nanocomposite increases with the nanotube length and volume fraction, with the curing degree of the epoxy and system temperature. In perspective, the presented approach could be employed to investigate other constitutive materials or properties of nanocomposites
The ortho-to-para ratio of ammonia in the L1157 outflow
We have measured the ortho-to-para ratio of ammonia in the blueshifted gas of
the L1157 outflow by observing the six metastable inversion lines from (J, K) =
(1, 1) to (6, 6). The highly excited (5, 5) and (6, 6) lines were first
detected in the low-mass star forming regions. The rotational temperature
derived from the ratio of four transition lines from (3, 3) to (6, 6) is
130-140 K, suggesting that the blueshifted gas is heated by a factor of ~10 as
compared to the quiescent gas. The ortho-to-para ratio of the NH3 molecules in
the blueshifted gas is estimated to be 1.3--1.7, which is higher than the
statistical equilibrium value. This ratio provides us with evidence that the
NH3 molecules have been evaporated from dust grains with the formation
temperature between 18 and 25 K. It is most likely that the NH3 molecules on
dust grains have been released into the gas phase through the passage of strong
shock waves produced by the outflow. Such a scenario is supported by the fact
that the ammonia abundance in the blueshifted gas is enhanced by a factor of ~5
with respect to the dense quiescent gas.Comment: 16 pages, including 3 PS figures. To appear in the ApJ (Letters).
aastex macro
The Effects of Excess Copper on Antioxidative Enzymes, Lipid Peroxidation, Proline, Chlorophyll, and Concentration of Mn, Fe, and Cu in Astragalus neo-mobayenii
To probe the physiological and biochemical tolerance mechanisms in Astragalus neo-mobayenii Maassoumi, an endemic plant around the Cu-rich areas from the North West of Iran, the effect of different copper concentrations at toxic levels on this plant was investigated. Copper was applied in the form of copper sulfate (CuSO4·5H2O) in four levels (0, 50, 100, and 150 μM). We observed no visible symptoms of Cu toxicity in this plant species. During the exposure of plants to excess copper, the antioxidant defense system helped the plant to protect itself from the damage. With increasing copper concentration, superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities increased in leaves and roots () compared with that of the control group. The chlorophyll amount gradually declined with increasing Cu concentrations. However, reduction in the 50 μM level showed insignificant changes. Enhanced accumulation of proline content in the leaves was determined, as well as an increase of MDA content (oxidative damage biomarker) (). The results indicated that Cu contents in leaves and roots enhanced with increasing levels of Cu application. The Fe and Mn contents in both shoots and roots significantly decreased with increasing Cu concentration. Finally, the mechanisms of copper toxicity and copper tolerance in this plant were briefly discussed
Toward harmonization of clinical molecular diagnostic reports: findings of an international survey
Toward harmonization of clinical molecular diagnostic reports: findings of an international survey
Systematic Molecular Differentiation in Starless Cores
(Abridged) We present evidence that low-mass starless cores, the simplest
units of star formation, are systematically differentiated in their chemical
composition. Molecules including CO and CS almost vanish near the core centers,
where the abundance decreases by one or two orders of magnitude. At the same
time, N2H+ has a constant abundance, and the fraction of NH3 increases toward
the core center. Our conclusions are based on a study of 5 mostly-round
starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have
mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and
(2,2), and the 1.2 mm continuum. For each core we have built a model that fits
simultaneously the radial profile of all observed emission and the central
spectrum for the molecular lines. The observed abundance drops of CO and CS are
naturally explained by the depletion of these molecules onto dust grains at
densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to
densities of several 10^5, while the NH3 abundance may be enhanced by reactions
triggered by the disappearance of CO from the gas phase. With the help of our
models, we show that chemical differentiation automatically explains the
discrepancy between the sizes of CS and NH3 maps, a problem which has remained
unexplained for more than a decade. Our models, in addition, show that a
combination of radiative transfer effects can give rise to the previously
observed discrepancy in the linewidth of these two tracers. Although this
discrepancy has been traditionally interpreted as resulting from a systematic
increase of the turbulent linewidth with radius, our models show that it can
arise in conditions of constant gas turbulence.Comment: 25 pages, 9 figures, accepted by Ap
Role of L- and T-Type Calcium Channels in Regulation of Absence Seizures in WAG/Rij Rats
In chronic experiments on five groups of WAG/Rij rats (a genetic model of absence epilepsy;
six animals in each group), we recorded EEG activity from the S1po cortical area through
implanted electrodes and subjected the cortex to the action of four agents affecting L- and
T-type calcium channels (injections through an implanted cannula). A blocker of L-type channels, verapamil hydrochloride, an agonist of these channels, Bay K8644, an antagonist of
T-type calcium channels, L-ascorbate, and an agonist of the latter channels, PMA, were used.
The parameters of 7- to 10-Hz spike-wave discharges, SWDs, spontaneously generated in the
cortex of this rat strain (frequency of SWDs, mean duration of the latter, and their number)
were measured within the baseline interval (before injections) and within three subsequent
20-min-long post-injection intervals. Normal saline was injected in the control group. There
were no significant differences in the mean peak frequency in SWDs between all examined
groups (P > 0.05 in all cases). Verapamil significantly (by more than 40%; P < 0.05) decreased the mean SWD duration throughout the entire period of post-injection observation.
The dynamics of the Bay K8644 effects were rather similar, but the intensity of SWD duration changes was somewhat smaller. Both the above agents in the doses used dramatically
decreased the number (frequency of appearance) of SWDs within the observation period.
L-ascorbate also suppressed SWD generation. The duration of these phenomena decreased
mildly, while their number dropped dramatically. In the PMA group, the number of SWDs
increased significantly (by 50%, P < 0.05) within the first 20-min-long interval, but this was
not observed within subsequent intervals. These findings confirm that blocking or activating
of L- and T-type Ca2+ channels in the S1po area (cortical focus area) can significantly control
generation of SWDs during absence seizures. Possible mechanisms underlying actions of the
tested agents are discussed.У хронічних експериментах на п’яти групах щурів лінії
WAG/Rij (генетична модель абсанс-епілепсії; шість тварин у кожній групі) ми відводили ЕЕГ-активність від кортикальної зони S1po через імплантовані електроди і піддавали кору дії чотирьох агентів, що впливають на кальцієві
канали L- та T-типів (ін’єкції через імплантовану канюлю).
Використовували блокатор кальцієвих каналів L-типу верапамілу гідрохлорид, агоніст цих каналів Bay K8644, антагоніст кальцієвих каналів T-типу L-аскорбат та агоніст останніх каналів PMA. Вимірювали параметри розрядів пік-хвиля
(РПХ, частота 7–10 Гц), котрі спонтанно генеруються у корі
головного мозку щурів указаної лінії (частоту в межах РПХ,
середню тривалість останніх та їх кількість) протягом інтервалу порівняння (перед ін’єкціями) та трьох послідовних
20-хвилинних інтервалів після ін’єкцій. Щурам контрольної групи ін’єкували фізіологічний розчин. Істотних різниць
між величинами частоти в межах РПХ у всіх досліджених
групах не спостерігалося (P > 0.05 у всіх випадках). Верапаміл істотно (більш ніж на 40 %; P < 0.05) зменшував
середню тривалість РПХ протягом усього періоду спостереження після ін’єкцій. Динаміка ефектів Bay K8644 була
вельми подібною, але інтенсивність скорочення тривалості
РПХ – дещо меншою. Обидва вказані агенти у використаних
дозах зумовлювали драматичне зменшення кількості (тобто
частоти виникнення) РПХ протягом усього періоду спостереження. L-аскорбат також істотно пригнічував генерацію
РПХ. Тривалість цих феноменів зменшувалася помірно, а
їх кількість скорочувалася дуже сильно. У групі РМА кількість РПХ протягом першого 20-хвилинного інтервалу спостереження істотно зростала (на 50 % , P < 0.05), але цього
не спостерігалося в межах наступних інтервалів. Наші результати підтверджують, що блокування або активація кальцієвих каналів L- та T-типів у фокальній кортикальній зоні
S1po здатні істотно контролювати генерацію РПХ під час
прояву абсансних судом. Обговорюються можливі механізми дії тестованих агентів
Superradiance Transition in Photosynthetic Light-Harvesting Complexes
We investigate the role of long-lasting quantum coherence in the efficiency
of energy transport at room temperature in Fenna-Matthews-Olson photosynthetic
complexes. The excitation energy transfer due to the coupling of the light
harvesting complex to the reaction center ("sink") is analyzed using an
effective non-Hermitian Hamiltonian. We show that, as the coupling to the
reaction center is varied, maximal efficiency in energy transport is achieved
in the vicinity of the superradiance transition, characterized by a segregation
of the imaginary parts of the eigenvalues of the effective non-Hermitian
Hamiltonian. Our results demonstrate that the presence of the sink (which
provides a quasi--continuum in the energy spectrum) is the dominant effect in
the energy transfer which takes place even in absence of a thermal bath. This
approach allows one to study the effects of finite temperature and the effects
of any coupling scheme to the reaction center. Moreover, taking into account a
realistic electric dipole interaction, we show that the optimal distance from
the reaction center to the Fenna-Matthews-Olson system occurs at the
superradiance transition, and we show that this is consistent with available
experimental data.Comment: 9 page
Herschel/HIFI deepens the circumstellar NH3 enigma
Circumstellar envelopes (CSEs) of a variety of evolved stars have been found
to contain ammonia (NH3) in amounts that exceed predictions from conventional
chemical models by many orders of magnitude. The observations reported here
were performed in order to better constrain the NH3 abundance in the CSEs of
four, quite diverse, oxygen-rich stars using the NH3 ortho J_K = 1_0 - 0_0
ground-state line. We used the Heterodyne Instrument for the Far Infrared
aboard Herschel to observe the NH3 J_K = 1_0 - 0_0 transition near 572.5 GHz,
simultaneously with the ortho-H2O J_Ka,Kc = 1_1,0 -1_0,1 transition, toward VY
CMa, OH 26.5+0.6, IRC+10420, and IK Tau. We conducted non-LTE radiative
transfer modeling with the goal to derive the NH3 abundance in these objects'
CSEs. For the latter two stars, Very Large Array imaging of NH3
radio-wavelength inversion lines were used to provide further constraints,
particularly on the spatial extent of the NH3-emitting regions. Results. We
find remarkably strong NH3 emission in all of our objects with the NH3 line
intensities rivaling those obtained for the ground state H2O line. The NH3
abundances relative to H2 are very high and range from 2 x 10-7 to 3 x 10-6 for
the objects we have studied. Our observations confirm and even deepen the
circumstellar NH3 enigma. While our radiative transfer modeling does not yield
satisfactory fits to the observed line profiles, it leads to abundance
estimates that confirm the very high values found in earlier studies. New ways
to tackle this mystery will include further Herschel observations of more NH3
lines and imaging with the Expanded Very Large Array.Comment: 4+2 page
Symbolic powers of monomial ideals and Cohen-Macaulay vertex-weighted digraphs
In this paper we study irreducible representations and symbolic Rees algebras
of monomial ideals. Then we examine edge ideals associated to vertex-weighted
oriented graphs. These are digraphs having no oriented cycles of length two
with weights on the vertices. For a monomial ideal with no embedded primes we
classify the normality of its symbolic Rees algebra in terms of its primary
components. If the primary components of a monomial ideal are normal, we
present a simple procedure to compute its symbolic Rees algebra using Hilbert
bases, and give necessary and sufficient conditions for the equality between
its ordinary and symbolic powers. We give an effective characterization of the
Cohen--Macaulay vertex-weighted oriented forests. For edge ideals of transitive
weighted oriented graphs we show that Alexander duality holds. It is shown that
edge ideals of weighted acyclic tournaments are Cohen--Macaulay and satisfy
Alexander dualityComment: Special volume dedicated to Professor Antonio Campillo, Springer, to
appea
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