509 research outputs found
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
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 ......
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
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
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