Phase transition vitreous

As opposed to the liquid-crystal transformation, the liquid-glass transformation is not a phase transition and therefore it can not be characterized by a certain transition temperature. Nevertheless, the term "the vitrification temperature , Tv, is widely used. It has the following physical meaning. As opposed to crystallization, vitrification occurs when the temperature changes continuously, i.e. over some temperature interval, rather than jump-wise. Inside this interval, the sample behaves as a liquid relative to some of the processes occurring in it, and as a solid relative to other processes occurring in it. The character of this behaviour is determined by the ratio between the characteristic time of the process, t, and the characteristic relaxation time of the matrix, x = t//G, where tj is the macroscopic viscosity and G is the matrix elasticity module. If t x, then the matrix should be considered as a solid relative to the process, and if t > x it should be considered as a liquid. The relation tjx = 1 can be considered as the condition of the matrix transition from the liquid to the solid (vitreous) state, and the temperature Tv at which this condition is realized as the temperature of vitrification. Evidently, Tv determined by such means will be somewhat different for the processes with different characteristic times t. However, due to the rapid (exponential) dependence of the viscosity rj on T, the dependence of Tw on t (i.e. on the kind of process) will be comparatively weak (logarith-... 

Phase transition of crystalline a-Te2Mo07to the vitreous p-form, surface composition, and activity in the vapor-phase selective oxidation of ethyl lactate to pyruvate over TeOa-MoOa catalysts... 

Phase transition of crystalline a-Tc2Mo07 to less active vitreous p-form and the regeneration of a-form by recalcination with evidence of powder X-ray diffraction(XRD), metal-oxygen distances analyzed by extended X-ray absorption fine structure (EXAFS) for Tc2Mo07 and the component oxides, and enriched Te-content at surface by X-ray photoelectron spectroscopy (XPS), are given in the present paper. 

Phase transition of crystalline a-TeaMoO to the vitreous P-Form... 

Binary oxide, Te02-Mo03, converted ethyl lactate selectively to pyruvate in a vapor-phase fixed-bed flow system. A synergy in activity suggested a-Te2Mo07 as the active species. Phase transition of crystalline active phase, a-Te2Mo07, to the vitreous p-form are demonst-... 

This means, for example, the study of the three dimensional order on very small single crystals, phase transitions or transformations under physical or chemical factors of crystalline powders but also vitreous transition of glasses, large angle and small angle X-ray scattering (LAXS—SAXS) of amorphous phases, liquids or coordination complexes in solution. 

Highly densified vitreous silica may be an example of an amorphous polymorph. Recently the amorphous to amorphous (pressure induced high-density) reversible phase transition has received much attention in connection with the development of bulk metal glasses such as La68Al2oCu2oC 02 [Liu and Hong, 2007[. Amorphous fluid phases are also possible. 

The spectroscopy of the SD0 level is also a valuable tool to investigate the point symmetry of Eu3+ ions in materials. It is well-known that the 5D0 - 7F2 transition is electric-dipole in nature, while 5D0- -7Fi shows a magnetic-dipole character. 5D0-> 7F2 is totally forbidden in presence of an inversion center and it is allowed in the opposite case. Usually, in vitreous phases, where the symmetry is low, electric dipolar transitions exhibit the strongest intensity. The fluorescence spectra displayed in Fig. 8 show that both transitions have about the same intensity indicating that Eu3+ ions are in high-symmetry sites in fluorozirconate glasses, relatively to oxide glasses. 

Nevertheless, the reliability of the assessment of the drop in titer by accelerated aging is disputable. The whole calculation principle relies on the fact that the kinetics follow laws of the Arrhenius type throughout the whole studied temperature domain. It so happens that this particular point is not that obvious if the dehydrated product contains an amorphous phase, which is quite frequently the case. As a matter of fact, a freeze-dried vaccine whose substrate contains sugars and proteins often has a vitreous transition temperature (Tg Lyo) greater than zero [12,25]. 

In vitreous systems, it is to be noted that the collapse temperature, T., can exceed the glass transition temperature by several degrees or more [2]. The retention of pellet structure below arises from the fact that when the frozen solution passes through the glass transition temperature, it returns from a glass to a highly viscous amorphous material. It is only when the viscosity of this material has decreased significantly that the fluidity of the interstitial phase is sufficient to cause collapse. 

Anionic Polymerizations (15). These experiments were performed with acrylonitrile. To favor the anionic polymerization, N,N-dimethyl-formamide (DMF), triethylamine (TEA), and isopropylamine (IPA) were selected as solvents. Acrylonitrile, N,N-dimethylformamide, and isopropylamine easily crystallize on cooling. Triethylamine, on the other hand, forms a glass below —170 °C. Some of the binary mixtures investigated were partially crystalline at —196 °C., but optically clear glasses were found over limited ranges of concentrations. Glass transition temperatures (Tg) were determined by differential thermal analysis (DTA). Some of the data are listed in Table I. No phase separation could be detected in any of the vitreous mixtures. 

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