60 research outputs found
Les amers du choix pertinent des signes à la manipulation de l'espace
El mar no ofereix referències, l'única alternativa per descriure l'espai és la terra. Aixà neixen els senyals de pescadors i mariners. Un invent que va ser el resultat d'una cerca a l'atzar portat a terme a cop d'intuïció, d'assaig i error i coronat finalment pel descobriment de la geografia del fons marÃ. L'article se centra en els origens i la prà ctica dels senyals relacionats amb la navegació i la pesca a la costa del departament francès dels Pirineus Orientals
Correlation between floppy to rigid transitions and non-Arrhenius conductivity in glasses
Non-Arrhenius behaviour and fast increase of the ionic conductivity is
observed for a number of potassium silicate glasses with
potassium oxide concentration larger than a certain value .
Recovering of Arrhenius behaviour is provided by the annealing that enhances
densification. Conductivity furthermore obeys a percolation law with the same
critical concentration . These various results are the manifestation of
the floppy or rigid nature of the network and can be analyzed with constraint
theory. They underscore the key role played by network rigidity for the
understanding of conduction and saturation effects in glassy electrolytes.Comment: 4 pages, 4 EPS figure
Onset of rigidty in glasses: from random to self-organized networks
We review in this paper the signatures of a new elastic phase that is found
in glasses with selected compositions. It is shown that in contrast with random
networks, where rigidity percolates at a single threshold, networks that are
able to self-organize to avoid stress will remain in an almost stress- free
state during a compositional interval, an intermediate phase, that is bounded
by a flexible phase and a stressed rigid phase. We report the experimental
signatures and describe the theoretical efforts that have been accomplished to
characterize the intermediate phase. We illustrate one of the methods used in
more detail with the example of Group III chalcogenides and finally suggest
further possible experimental signatures of self-organization.Comment: 27 pages, 6 figures, Proceedings of the Conference on Non-Crystalline
Materials 10, to appear in Journal of Non-Crystalline Solid
Percolative conductivity in alkaline earth silicate melts and glasses
Ion conducting glasses and melts show a threshold
behaviour in dc conductivity near , with conductivities increasing
linearly at . We show that the behaviour can be traced to a rigid
() elastic phase transition near . In the
floppy phase, conductivity enhancement is traced to increased mobility or
diffusion of carriers as the modified network elastically softens.Comment: 15 pages, 5 figures. Europhysics Letters (2003), in pres
Abrupt boundaries of intermediate phases and space filling in oxide glasses
Modulated DSC measurements on bulk (Na2O)x(GeO2)1-x glasses show a sharp
reversibility window in the 14% < x < 19% soda range, which correlates well
with a broad global minimum in molar volumes. Raman and IR reflectance TO and
LO mode frequencies exhibit anomalies between xc(1) = 14% (stress transition)
and xc(2) = 19% (rigidity transition), with optical elasticity power-laws
confirming the nature of the transitions . Birefringence measurements dramatize
the macroscopically stress-free nature of the Intermediate Phase in the
reversibility window
Intermediate Phases, structural variance and network demixing in chalcogenides: the unusual case of group V sulfides
We review Intermediate Phases (IPs) in chalcogenide glasses and provide a
structural interpretation of these phases. In binary group IV selenides, IPs
reside in the 2.40 < r < 2.54 range, and in binary group V selenides they shift
to a lower r, in the 2.29< r < 2.40 range. Here r represents the mean
coordination number of glasses. In ternary alloys containing equal proportions
of group IV and V selenides, IPs are wider and encompass ranges of respective
binary glasses. These data suggest that the local structural variance
contributing to IP widths largely derives from four isostatic local structures
of varying connectivity r; two include group V based quasi-tetrahedral (r =
2.29) and pyramidal (r = 2.40) units, and the other two are group IV based
corner-sharing (r = 2.40) and edge-sharing (r = 2.67) tetrahedral units.
Remarkably, binary group V (P, As) sulfides exhibit IPs that are shifted to
even a lower r than their selenide counterparts; a result that we trace to
excess Sn chains either partially (As-S) or completely (P-S) demixing from
network backbone, in contrast to excess Sen chains forming part of the backbone
in corresponding selenide glasses. In ternary chalcogenides of Ge with the
group V elements (As, P), IPs of the sulfides are similar to their selenide
counterparts, suggesting that presence of Ge serves to reign in the excess Sn
chain fragments back in the backbone as in their selenide counterparts
Microscopic Aspects of Stretched Exponential Relaxation (SER) in Homogeneous Molecular and Network Glasses and Polymers
Because the theory of SER is still a work in progress, the phenomenon itself
can be said to be the oldest unsolved problem in science, as it started with
Kohlrausch in 1847. Many electrical and optical phenomena exhibit SER with
probe relaxation I(t) ~ exp[-(t/{\tau}){\beta}], with 0 < {\beta} < 1. Here
{\tau} is a material-sensitive parameter, useful for discussing chemical
trends. The "shape" parameter {\beta} is dimensionless and plays the role of a
non-equilibrium scaling exponent; its value, especially in glasses, is both
practically useful and theoretically significant. The mathematical complexity
of SER is such that rigorous derivations of this peculiar function were not
achieved until the 1970's. The focus of much of the 1970's pioneering work was
spatial relaxation of electronic charge, but SER is a universal phenomenon, and
today atomic and molecular relaxation of glasses and deeply supercooled liquids
provide the most reliable data. As the data base grew, the need for a
quantitative theory increased; this need was finally met by the
diffusion-to-traps topological model, which yields a remarkably simple
expression for the shape parameter {\beta}, given by d*/(d* + 2). At first
sight this expression appears to be identical to d/(d + 2), where d is the
actual spatial dimensionality, as originally derived. The original model,
however, failed to explain much of the data base. Here the theme of earlier
reviews, based on the observation that in the presence of short-range forces
only d* = d = 3 is the actual spatial dimensionality, while for mixed short-
and long-range forces, d* = fd = d/2, is applied to four new spectacular
examples, where it turns out that SER is useful not only for purposes of
quality control, but also for defining what is meant by a glass in novel
contexts. (Please see full abstract in main text
Rings and rigidity transitions in network glasses
Three elastic phases of covalent networks, (I) floppy, (II) isostatically
rigid and (III) stressed-rigid have now been identified in glasses at specific
degrees of cross-linking (or chemical composition) both in theory and
experiments. Here we use size-increasing cluster combinatorics and constraint
counting algorithms to study analytically possible consequences of
self-organization. In the presence of small rings that can be locally I, II or
III, we obtain two transitions instead of the previously reported single
percolative transition at the mean coordination number , one from a
floppy to an isostatic rigid phase, and a second one from an isostatic to a
stressed rigid phase. The width of the intermediate phase and the
order of the phase transitions depend on the nature of medium range order
(relative ring fractions). We compare the results to the Group IV
chalcogenides, such as Ge-Se and Si-Se, for which evidence of an intermediate
phase has been obtained, and for which estimates of ring fractions can be made
from structures of high T crystalline phases.Comment: 29 pages, revtex, 7 eps figure
Contribution a l'etude des transitions de phase structurales des perovskites CsCaCl3 et CsPbCl3 par resonance paramagnetique electronique de l'ion Gd3+
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