381 research outputs found
Catalytic Membrane Reactor: Multilayer membranes elaboration
International audienceMethane conversion to syngas is very attractive for hydrogen or clean fuel production and provides an alternative to oil products. An efficient architecture for the membrane reactor is constituted of a porous support, a thin dense membrane and a catalyst layer. This work is focused on the elaboration process of such asymmetric membranes by co-sintering of at least the porous support and the dense membrane and specially the choice of well adapted materials. La0.8Sr0.2Fe0.7Ga0.3O3-ÎŽ perovskite material has been chosen as the dense membrane because it exhibits a good compromise between oxygen flux and stability. The choice of the material for the porous support is mainly oriented by the sintering behaviour of the membrane, the thermal expansion behaviour of both layers to avoid cracks formation under working conditions and the chemical inertness of both materials. Several formulations fulfilling these three requirements were synthesized by liquid phase reaction and tape-cast. A pore forming agent was added in the support tapecasting slurry in order to create a controlled porosity. Then, the porous support has been characterized in term of gas permeability and thermal expansion under working conditions. Keywords: Ceramic membrane, co-sintering, perovskite, syngas, mixed conducting materials
Oxygen permeation and dimensional stability under pO2 gradient of (La,Sr)(Fe, Ga)O3-delta perovskite membranes
International audienceNatural gas conversion into syngas, is very attractive for hydrogen or cleanfuel production and provides a new alternative to oil products ......
Pressure-induced amorphization, crystal-crystal transformations and the memory glass effect in interacting particles in two dimensions
We study a model of interacting particles in two dimensions to address the
relation between crystal-crystal transformations and pressure-induced
amorphization. On increasing pressure at very low temperature, our model
undergoes a martensitic crystal-crystal transformation. The characteristics of
the resulting polycrystalline structure depend on defect density, compression
rate, and nucleation and growth barriers. We find two different limiting cases.
In one of them the martensite crystals, once nucleated, grow easily
perpendicularly to the invariant interface, and the final structure contains
large crystals of the different martensite variants. Upon decompression almost
every atom returns to its original position, and the original crystal is fully
recovered. In the second limiting case, after nucleation the growth of
martensite crystals is inhibited by energetic barriers. The final morphology in
this case is that of a polycrystal with a very small crystal size. This may be
taken to be amorphous if we have only access (as experimentally may be the
case) to the angularly averaged structure factor. However, this `X-ray
amorphous' material is anisotropic, and this shows up upon decompression, when
it recovers the original crystalline structure with an orientation correlated
with the one it had prior to compression. The memory effect of this X-ray
amorphous material is a natural consequence of the memory effect associated to
the underlying martensitic transformation. We suggest that this kind of
mechanism is present in many of the experimental observations of the memory
glass effect, in which a crystal with the original orientation is recovered
from an apparently amorphous sample when pressure is released.Comment: 13 pages, 13 figures, to be published in Phys. Rev.
Melting and Pressure-Induced Amorphization of Quartz
It has recently been shown that amorphization and melting of ice were
intimately linked. In this letter, we infer from molecular dynamics simulations
on the SiO2 system that the extension of the quartz melting line in the
metastable pressure-temperature domain is the pressure-induced amorphization
line. It seems therefore likely that melting is the physical phenomenon
responsible for pressure induced amorphization. Moreover, we show that the
structure of a "pressure glass" is similar to that of a very rapidly (1e+13 to
1e+14 kelvins per second) quenched thermal glass.Comment: 9 pages, 4 figures, LaTeX2
Oxygen permeation, thermal and chemical expansion of (La, Sr)(Fe, Ga)O3âÎŽ perovskite membranes
International audienceDense ceramic membranes made from mixed conductors are interesting because of their potential applications formethane conversion into syngas (H2 and CO mixture). Such membranes need to present a low differential dimensional variation between the opposite faces submitted to a large gradient of oxygen partial pressure, in order to minimize mechanical stresses generated through the membrane thickness. Besides, high oxygen permeability is required for high methane reforming rate. La(1âx)SrxFe(1ây)GayO3âÎŽ materials fulfil these two main requirements and were retained as membranes in catalytic membrane reactors (CMR). The variations of expansion and oxygen permeation of La(1âx)SrxFe(1ây)GayO3âÎŽ perovskite materials with the partial substitution of lanthanum and iron cations, temperature and oxygen partial pressure, were studied. For low temperatures (800 âŠC), TEC, then dimensional stability of the membrane, and oxygen permeation of La(1âx)SrxFe(1ây)GayO3âÎŽ materials, are significantly affected by Sr content and oxygen partial pressure. Ga has a stabilisation effect on the TEC and has no influence on oxygen permeation flux. A good compromise between dimensional stability and oxygen permeation of materials was found to be La0.7Sr0.3Fe0.7Ga0.3O3âÎŽ compositio
Temperature Evolution of Sodium Nitrite Structure in a Restricted Geometry
The NaNO nanocomposite ferroelectric material in porous glass was
studied by neutron diffraction. For the first time the details of the crystal
structure including positions and anisotropic thermal parameters were
determined for the solid material, embedded in a porous matrix, in ferro- and
paraelectric phases. It is demonstrated that in the ferroelectric phase the
structure is consistent with bulk data but above transition temperature the
giant growth of amplitudes of thermal vibrations is observed, resulting in the
formation of a "premelted state". Such a conclusion is in a good agreement with
the results of dielectric measurements published earlier.Comment: 4 pages, 4 figure
Mechanical versus thermodynamical melting in pressure-induced amorphization: the role of defects
We study numerically an atomistic model which is shown to exhibit a one--step
crystal--to--amorphous transition upon decompression. The amorphous phase
cannot be distinguished from the one obtained by quenching from the melt. For a
perfectly crystalline starting sample, the transition occurs at a pressure at
which a shear phonon mode destabilizes, and triggers a cascade process leading
to the amorphous state. When defects are present, the nucleation barrier is
greatly reduced and the transformation occurs very close to the extrapolation
of the melting line to low temperatures. In this last case, the transition is
not anticipated by the softening of any phonon mode. Our observations reconcile
different claims in the literature about the underlying mechanism of pressure
amorphization.Comment: 7 pages, 7 figure
Search for Fingerprints of Tetrahedral Symmetry in
Theoretical predictions suggest the presence of tetrahedral symmetry as an
explanation for the vanishing intra-band E2-transitions at the bottom of the
odd-spin negative parity band in . The present study reports on
experiment performed to address this phenomenon. It allowed to determine the
intra-band E2 transitions and branching ratios B(E2)/B(E1) of two of the
negative-parity bands in .Comment: presented by Q.T. Doan at XLII Zakopane School of Physics: Breaking
Frontiers: Submicron Structures in Physics and Biology, May 2008. 5 pages,
minor corrections. To be published in the proceeding
Frequency dependent specific heat of viscous silica
We apply the Mori-Zwanzig projection operator formalism to obtain an
expression for the frequency dependent specific heat c(z) of a liquid. By using
an exact transformation formula due to Lebowitz et al., we derive a relation
between c(z) and K(t), the autocorrelation function of temperature fluctuations
in the microcanonical ensemble. This connection thus allows to determine c(z)
from computer simulations in equilibrium, i.e. without an external
perturbation. By considering the generalization of K(t) to finite wave-vectors,
we derive an expression to determine the thermal conductivity \lambda from such
simulations. We present the results of extensive computer simulations in which
we use the derived relations to determine c(z) over eight decades in frequency,
as well as \lambda. The system investigated is a simple but realistic model for
amorphous silica. We find that at high frequencies the real part of c(z) has
the value of an ideal gas. c'(\omega) increases quickly at those frequencies
which correspond to the vibrational excitations of the system. At low
temperatures c'(\omega) shows a second step. The frequency at which this step
is observed is comparable to the one at which the \alpha-relaxation peak is
observed in the intermediate scattering function. Also the temperature
dependence of the location of this second step is the same as the one of the
peak, thus showing that these quantities are intimately connected to
each other. From c'(\omega) we estimate the temperature dependence of the
vibrational and configurational part of the specific heat. We find that the
static value of c(z) as well as \lambda are in good agreement with experimental
data.Comment: 27 pages of Latex, 8 figure
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