摘要:

AbstractVarious organic molecules, in particular polymers, are extremely good glass formers and allow the study of supercooled melts near the glass transition in metastable equilibrium. Theories of the glass transition imply such an equilibrium (e.g. mode‐coupling theory, or Gibbs‐di Marzio theory) and can hence be tested in these systems.Simplified lattice models for polymer melts (e.g. the bond fluctuation model) have been developed that can very efficiently be studied by Monte‐Carlo simulation, and although they fail to accurately describe the local structure, they describe many features of the experiments very well. In this model, the mechanism of the glass transition is a competition between the (energetically favorable) local stretching of effective bonds, and the (entropically favorable) dense packing of effective monomers (“geometric frustration”). It is shown that the model exhibits a dramatic slowing down upon cooling, and the properties of the frozen‐in glass distinctly depend on the cooling rate. While the Gibbs‐di Marzio theory predicts correctly the temperature region where the configurational entropy of the supercooled melt strongly decreases, the vanishing of the entropy is traced back to a severe underestimation of the entropy of the reference state (high temperature melt). The idealized version of the mode‐coupling theory (involving critical singularities above the freezing transition) is found to be not very useful either, as the singularities are strongly rounded. But the extended version of the mode‐coupling theory which describes this rounding, is in nice agreement wit

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000902

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractA method is presented that allows the detailed study of regions of multi‐minima energy landscapes close to some deep‐lying minimum. Two versions are described the “lid algorithm” and the “threshold algorithm” for discrete and continuous landscapes, respectively. Applications of these algorithms to the travelling salesman problem (TSP), a two‐dimensional layer of Ne atoms, and the system Mg2F

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000903

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractWe combine the results of dielectric relaxation spectroscopy and solvation dynamics measurements for the glass‐forming liquid 2‐methyltetrahydrofuran spatially confined to the pores of sol‐gel glasses with pore diameters ϕ = 2.5, 5.0, and 7.5 nm. Although both techniques probe the orientational polarization of the polar liquid, a dielectric experiment is bound to sense macroscopic effects, whereas the solvation method probes the molecular dynamics on microscopic scales. When the temperature approaches the glass‐transition atTgupon cooling, an interfacial liquid layer with strongly frustrated dynamics grows in thickness and is thus able to block the connectivity among the pores. As a result the structural α‐process is subject to an enhanced spatial restriction nearTgand a transition from constant pressure to constant volume conditions might occur as the pore connectivity becomes obstructed by immo

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000904

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe transformation from crystallineα‐quartz to silica glass under shock pressure is followed, in the intermediate phase where both crystalline and amorphous SiO2are detected, by Raman microspectrometry. Experiments are carried out on samples recovered from increasing peak pressure. The evolution of the crystalline phase is compared to the evolution ofα‐quartz with a thermal annealing. No observation is made for aβ‐phase transformation before full amorphization of the

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000905

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractWe use molecular dynamics computer simulations to study the dependence of the properties of amorphous silica on the cooling rate with which the glass has been produced. In particular we show that the density, the glass transition temperature, the radial distribution function and the distribution of the size of the rings depend on the cooling rate.

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000906

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractWe review results from Raman scattering experiments on different types of glass formers which have been analysed in order to describe the fast relaxations occurring aboveTg. Both fragile and strong glass formers exhibit similar dynamics which can be qualitatively reproduced by mode coupling theory (MCT). In particular, a crossover temperatureTccan be extracted whereTc>Tg. Quantitatively, deviations from the MCT predictions are larger, the stronger the glass former is. One reason for the deviations stems from low‐frequency vibrations which also contribute to the dynamic structure factor and which are small in fragile but large in strong liquids. A change of dynamics atT>Tgis also indicated in simple liquids by the emergence of a slow secondary relaxation process and by the break‐down of rotational‐translational coupling. Furthermore, a viscosity analysis is performed by introducing a crossover temperatureTxwhich marks the boundary between two dynamical regimes. For all glass formers a master curve is obtained atT

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000907

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractA rate equation is developed by which the dissolution rates of multicomponent silicate glasses in diluted aqueous solutions can be calculated as a function of pH, CO2partial pressure, and salinity. Gibbs free energies of hydration are derived by treating the aqueous solution as a diluted system of aqueous species of the various cations in the glass, and by treating the glass as a mixture of the constitutional phases taken from its basic phase diagram. Beside the free energies, the amounts of H+and OH−absorbed at the glass surface are taken into account as rate determining factors. Calculated dissolution rates of silica, A type, E type, rock, and slag glass fibres agree with experimental values within ±30% on a linear sca

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000908

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractUsing low‐melting enamel glasses it could be managed to develop a protecting glassy embedding for luminescent pigments. Such a lasting protection from greying ‐ especially against UV light in the presence of humidity ‐ offers new operational areas to luminescent pigments like the outdoor area. In the present work the luminescence spectra and decay functions of phosphorescent glasses are examined. For a ZnS pigment and a newly developed SrO‐aluminate pigment of high phosphorescence efficiency it could be shown that the emission spectra of the phosphorescent glasses do not essentially differ from those of the pure pigments. The phosphorescence efficiency of ZnS phosphorescent glass could be increased by more than 10 per cent by reducing the TiO2content regarded as optimal up to now. First experiments show that an SrO‐aluminate pigment can also be embedded in a glassy matrix. Indeed, intensity losses of 70 per cent appeared. Here a high potential of improvement is to be assumed. Further investigations are necessary. In addition we have found that the phosphorescence decay functions of the samples under investigation depend on the excitation time. The phosphorescent glass can be universally applied as granules or as formed bodies and offers manifold app

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000909

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractA survey on the kinetics of nucleation and the mechanisms of crystal growth is given. Theoretical and experimental developments verifying the more general, non steady state formulation of the theory of nucleation are discussed. It is argued that the experimentally observed transiency in the kinetics of nucleation in glass forming systems may be due to both non steady state time lag as well as to growth effects. In order to distinguish between both cases, an analysis of the temperature dependence of the induction times,t *0, and a comparison with the temperature dependence of the non steady state time lag and the rates of nucleation and growth are required. Experimental results on the mechanisms of crystal growth are provide

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000910

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractTo prepare α‐cordierite glass ceramics, we demonstrate two types of phase diagrams of a composition 2MgO·2Al2O3·5SiO2with time courses. One is the continuous‐cooling‐transformation (CCT) diagram; the other is the temperature‐time‐transformation (TTT) diagram. At ∼ 1273 K, μ‐cordierite is crystallized as the primary phase and then it transforms to α‐cordierite with a lath‐like martensite transformation. The volume expansion derived from this transformation results in a lot of cracks in crystallized glasses. Since the addition of MgF2for the substitution of MgO (ca. 5%) suppresses the μ‐to α‐cordierite transformation but promotes the direct crystallization of α‐cordierite from glass, we can prepare α‐cordierite glass ceramics without cracks. In this paper, we point out the needs and usefulness of CCT and TTT diag

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000911

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY