Tailoring Glass Properties: Why Chemical Composition and Thermal Treatments Matter

Architectural use of glass dates back from the beginning of our era when it wasused to make windows. Its range of chemical composition was close to that ofcurrent flat or hollow glass, illustrating early optimization of both productionprocess and material properties. In modern buildings glass is ubiquitous, highlyvisible as in facades or hidden as fibers for thermal insulation or for high-speedtelecommunication. This short review describes the main factors that have madethis variety of uses possible. The fundamental point is the amorphous nature ofglass, which allows pieces of any shape and size to be produced and the propertiesof the material to be tailored through thermal treatments and incorporation of a hostof chemical elements in widely different proportions

Amorphization induced by pressure: results for zeolites and general implications

We report an {\sl ab initio} study of pressure-induced amorphization (PIA) in zeolites, which are model systems for this phenomenon. We confirm the occurrence of low-density amorphous phases like the one reported by Greaves {\sl et al.} [Science {\bf 308}, 1299 (2005)], which preserves the crystalline topology and might constitute a new type of glass. The role of the zeolite composition regarding PIA is explained. Our results support the correctness of existing models for the basic PIA mechanim, but suggest that energetic, rather than kinetic, factors determine the irreversibility of the transition.Comment: 4 pages with 3 figures embedded. More information at http://www.icmab.es/dmmis/leem/jorg

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.

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-δ 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

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 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 ......

High-Temperature Thermodynamic Properties Of Forsterite

The high-temperature thermodynamic properties of forsterite were reviewed in the light of a new determination of the isobaric heat capacity (C(p)), up to 1850 K, and Raman spectroscopic measurements, up to 1150 K and 10 GPa. The C(p) measurements and available data on thermal expansion (alpha) and bulk modulus (K) show that the isochoric specific heat (C-nu) exceeds the harmonic limit of Dulong and Petit above 1300 K. This intrinsic anharmonic behavior of C-nu) can be modeled by introducing anharmonic parameters a(i) = (partial-lnv(i)/partial-T)V which are calculated from the measured pressure and temperature shifts of the vibrational frequencies. These parameters are all negative, with absolute values lower for the stretching modes of the SiO4 tetrahedra (a(i) almost-equal-to - 1 x 10(-5) K-1) than for the lattice modes (a(i) almost-equal-to 2 x 10(-5) K-1). Through the relation C(p) = C-nu) + alpha-2K(T)VT, the calculated anharmonic C-nu) and the measured C(p) are then used to determine the temperature dependences of the thermal expansion and bulk modulus of forsterite, up to 2000 K, in agreement with recent experimental results. Finally, all these data point to an inconsistency for the Gruneisen parameter of forsterite, whereby the macroscopic parameter gamma = alpha-VK(T)/C-nu) cannot be evaluated simply at high temperature by summation of the individual isothermal mode Gruneisen parameters gamma-iT = K(T) (partial-lnv(i)/partial-P)

Oxygen permeation, thermal and chemical expansion of (La, Sr)(Fe, Ga)O3−δ 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(1−x)SrxFe(1−y)GayO3−δ 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(1−x)SrxFe(1−y)GayO3−δ perovskite materials with the partial substitution of lanthanum and iron cations, temperature and oxygen partial pressure, were studied. For low temperatures (800 ◦C), TEC, then dimensional stability of the membrane, and oxygen permeation of La(1−x)SrxFe(1−y)GayO3−δ 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 La0.7Sr0.3Fe0.7Ga0.3O3−δ compositio