2,731 research outputs found
Transient non-isothermal model of a polymer electrolyte fuel cell
In this paper we present a one-dimensional transient model for the membrane electrode assembly of a polymer-electrolyte fuel cell. In earlier work we established a framework to describe the water balance in a steady-state, non-isothermal cathode model that explicitly included an agglomerate catalyst layer component. This paper extends that work in several directions, explicitly incorporating components of the anode, including a micro-porous layer, and accounting for electronic potential variations, gas convection and time dependence. The inclusion of temperature effects, which are vital to the correct description of condensation and evaporation, is new to transient modelling. Several examples of the modelling results are given in the form of potentiostatic sweeps and compared to experimental results. Excellent qualitative agreement is demonstrated, particularly in regard to the phenomenon of hysteresis, a manifestation of the sensitive response of the system to the presence of water. Results pertaining to pore size, contact angle and the presence of a micro-porous layer are presented and future work is discussed
A transient PEMFC model with CO poisoning and mitigation by O2 bleeding and Ru-containing catalyst
In this paper we present a transient, fully two-phase, non-isothermal model of carbon monoxide poisoning and oxygen bleeding in the membraneelectrode assembly of a polymer electrolyte fuel cell. The model includes a detailed description of mass, heat and charge transport, chemisorption,electrochemical oxidation and heterogeneous catalysis (when oxygen is introduced). Example simulation results demonstrate the ability of themodel to qualitatively capture the fundamental features of the poisoning process and the extent of poisoning with respect to channel temperatureand concentration. Further examples show how the multi-step kinetics can interact with other physical phenomena such as liquid-water flooding,particularly in the anode. Carbon monoxide pulsing is simulated to demonstrate that the complicated reaction kinetics of oxygen bleeding canbe captured and even predicted. It is shown that variations in the channel temperature have a convoluted effect on bleeding, and that trends inperformance on relatively short time scales can be the precise opposite of the trends observed at steady state. We incorporate a bi-functionalmechanism for carbon monoxide oxidation on platinum–ruthenium catalysts, demonstrating the marked reduction in the extent of poisoning, theeffect of variations in the platinum–ruthenium ratio and the influence of temperature. Finally, we discuss the implications of the results, extensionsto the model and possible avenues for experimental work
Maximum thickness of a two-dimensional trapped Bose system
The trapped Bose system can be regarded as two-dimensional if the thermal
fluctuation energy is less than the lowest energy in the perpendicular
direction. Under this assumption, we derive an expression for the maximum
thickness of an effective two-dimensional trapped Bose system.Comment: 1 pages, 0 figure
Effects of Boron Purity, Mg Stoichiometry and Carbon Substitution on Properties of Polycrystalline MgB
By synthesizing MgB using boron of different nominal purity we found
values of the residual resistivity ratio () from 4 to
20, which covers almost all values found in literature. To obtain high values
of , high purity reagents are necessary. With the isotopically pure boron
we obtained the highest 20 for the stoichiometric compound. We also
investigated MgB samples with 0.8 1.2. For the range
MgB up to MgB we found average values
of between 14 and 24. For smaller variations in stoichiometry () . All of our data point to the conclusion that high
() and low () are intrinsic
material properties associated with high purity MgB. In addition we have
performed initial work on optimizing the formation of carbon doped MgB
via the use of BC. Nearly single phase material can be formed by reaction
of nominal Mg(BC) for 24 hours at . The
for this composition is between and (depending on
criterion).Comment: accepted to Physica C, special MgB2 issu
Temperature-dependent Raman spectroscopy in BaRuO systems
We investigated the temperature-dependence of the Raman spectra of a
nine-layer BaRuO single crystal and a four-layer BaRuO epitaxial film,
which show pseudogap formations in their metallic states. From the polarized
and depolarized spectra, the observed phonon modes are assigned properly
according to the predictions of group theory analysis. In both compounds, with
decreasing temperature, while modes show a strong hardening, (or
) modes experience a softening or no significant shift. Their different
temperature-dependent behaviors could be related to a direct Ru metal-bonding
through the face-sharing of RuO. It is also observed that another
mode of the oxygen participating in the face-sharing becomes split at low
temperatures in the four layer BaRuO. And, the temperature-dependence of
the Raman continua between 250 600 cm is strongly correlated to
the square of the plasma frequency. Our observations imply that there should be
a structural instability in the face-shared structure, which could be closely
related to the pseudogap formation of BaRuO systems.Comment: 8 pages, 6 figures. to be published in Phys. Rev.
Asymptotics and local constancy of characters of p-adic groups
In this paper we study quantitative aspects of trace characters
of reductive -adic groups when the representation varies. Our approach
is based on the local constancy of characters and we survey some other related
results. We formulate a conjecture on the behavior of relative to
the formal degree of , which we are able to prove in the case where
is a tame supercuspidal. The proof builds on J.-K.~Yu's construction and the
structure of Moy-Prasad subgroups.Comment: Proceedings of Simons symposium on the trace formul
Determination of 3D Trajectories of Knots in Solar Prominences Using MSDP Data
In this paper we present a new method of restoration of the true
thee-dimensional trajectories of the prominence knots based on ground-based
observations taken with a single telescope, which is equipped with a
Multi-Channel Subtractive Double Pass imaging spectrograph. Our method allows
to evaluate true three-dimensional trajectories of the prominence knots without
any assumptions concerning the shape of the trajectories or dynamics of the
motion. The reconstructed trajectories of several knots observed in three
prominences are presented.Comment: 14 pages, 9 figures, accepted for publication in Solar Physic
Prominent bulk pinning effect in the MgB_2 superconductor
We report the magnetic-field dependence of the irreversible magnetization of
the recently discovered binary superconductor MgB. For the temperature
region of , the contribution of the bulk pinning to the
magnetization overwhelms that of the surface pinning. This was evident from the
fact that the magnetization curves, , were well described by the
critical-state model without considering the surface pinning effect. It was
also found that the curves at various temperatures scaled when the field
and the magnetization were normalized by the characteristic scaling factors
and , respectively. This feature suggests that the
pinning mechanism determining the hysteresis in is unique below .Comment: 4pages and 4 figures. Phys. Rev. B (accepted
Field-induced quantum critical point in CeCoIn_5
The resistivity of CeCoIn_5 was measured down to 20 mK in magnetic fields of
up to 16 T. With increasing field, we observe a suppression of the non-Fermi
liquid behavior, rho~T, and the development of a Fermi liquid state, with its
characteristic rho=rho_0+AT^2 dependence. The field dependence of the T^2
coefficient shows critical behavior with an exponent of ~4/3. This is evidence
for a new field-induced quantum critical point, occuring in this case at a
critical field which coincides with the superconducting upper critical field
H_c2.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
Fate of Z(N) walls in hot holographic QCD
We first study Z(N) walls in a deconfined phase of Witten's D4-brane
background of pure SU(N) Yang-Mills theory, motivated by a recent work in the
case of N=4 SYM. Similarly to it, we propose that for a large wall charge k ~
N, it is described by k D2-branes blown up into a NS5-brane wrapping S^3 inside
S^4 via Myers effect, and we calculate the tension by suitable U-duality. We
find a precise Casimir scaling for the tension formula. We then study the fate
of Z(N)-vacua in a presence of fundamental flavors in quenched approximation
via gauge/gravity correspondence. In the case of D3/D7 system where one can
vary the mass m_q of flavors, we show that there is a phase transition at T_c ~
m_q, below which the Z(N)-vacua survive while they are lifted above the
critical temperature. We analytically calculate the energy lift of k'th vacua
in the massless case, both in the D3/D7 system and in the Sakai-Sugimoto model.Comment: 24 pages, v2: references updated, v3: A clarification on the meaning
of Z(N) walls in Euclidean space added, citations update
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