Devitrificated matrix

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

It is interesting that this cement has been known for over 100 years and yet certain features of its chemistry remain obscure. The exact nature of the matrix is still a matter for conjecture. It is known that the principal phase is amorphous, as a result of the presence of aluminium in the liquid. It is also known that after a lapse of time, crystallites sometimes form on the surface of the cement. A cement gel may be likened to a glass and this process of crystallization could be likened to the devitrification of a glass. Therefore, it is reasonable to suppose that the gel matrix is a zinc aluminophosphate and that entry of aluminium into the zinc phosphate matrix causes disorder and prevents crystallization. It is not so easy to accept the alternative explanation that there are two amorphous phases, one of aluminium phosphate and the other of zinc phosphate. This is because it is difficult to see how aluminium could act in this case to prevent zinc phosphate from crystallizing. 

Ion exchange (production of OH in the reaction layer) and matrix hydrolysis (consumption of OH from the reaction layer and the surrounding environment) are intimately coupled, devitrification being autocatalysed by ion exchange and both processes being enhanced by water diffusion (Grambow 1992). This step is then followed by reaction (3). 

At still higher temperatures (above 160 K), close to that of matrix devitrification, the decay of etr appears to proceed along the diffusion channel. This conclusion can be made on the basis of the results obtained from the kinetic studies of the reactions of et with additives, electron acceptors in the same water-alkaline matrices (see Sect.4). 

Tunneling reactions of e near the temperature of matrix devitrification. Effect of translational mobility of reagents on the kinetics... 

The model of thermal diffusion, however, suffers from the following shortcomings. First, it does not agree with the results of direct measurements of the rate of diffusion-controlled electron transfer reactions near the temperatures of solid matrix devitrification (cf. Chap. 6, Sect. 4). Extrapolation of the values obtained in these experiments to the region of lower temperatures has shown that at these temperatures the rate of diffusion must be many orders of magnitude less than the observed rates of electron transfer reactions. 

Bol shakov A.I., Barkalov I.M. Chain transfer effect in low-temperatures polymerizations of vinyl monomers at devitrification ethyl alcohol matrixes. Polymer Science, 23,1981, pp. 1086-1089. 

The dependence of heating on pressure itself may be due to a decrease, with increasing pressure, of the characteristic size of dispersion grains. There is also an alternative explanation, connected with increasing devitrification temperature of a compressed matrix this factor causes a displacement of the temperature region of the intense recombination of active centers and creates the conditions for a higher degree of conversion at the front. 

Because of its effect on matrix viscosity, devitrification of the intergranular matrix has a pronounced effect on the creep behavior of the composite. In... 

Solution-precipitation theory cannot be used to justify creep asymmetry or high tensile stress exponents for ceramic matrix composites. The theory suggests that creep is symmetric in stress and that the stress exponent is equal to 1. Justification of creep asymmetry by solution-precipitation would require other parameters in Eqn. (4) to depend on the sign of the applied stress. A nonlinear dependence on stress would be required. Diffusion and devitrification may play a role in this regard however, the data needed to support this possibility have yet to be obtained. 

Flame Vitrification and Spheridization of Silicate Particles. The alumino-silicate particles vitrify and take a spherical shape in the flame and are partially recrystallized on cooling. Micro-needles of mullite up to 10 pm long and crystalloids of quartz are the principal devitrification products enveloped in a glassy material matrix. 

Unique nanocrystalline microstructures can be produced by controlled crystallization of the fully amorphous product, including nanocrystalline precipitates homogeneously distributed in an amorphous alloy matrix. In some systems, both the strength and ductility increase in this partially crystalline state [24], Other alloys produce nanocrystalline intermetallic or quasicrystalline precipitates, providing a credible path for increasing the specific stiffness. Thus, a significant effort is required to study the kinetics of crystallization, the devitrification pathways and the microstructures and properties that may be produced upon devitrification. The potential for exploration of novel compositions and microstructures in this class of materials is clearly promising. 

The effect of °Ar loss during devitrification of the glassy matrix of Kirkpatrick Basalt flows was investigated by Fleck et al. (1977) by means of the step-wise heating technique of the Ar/ Ar method. In preparation of this study, Fleck et al. (1977, p. 19) adopted the following definition of the plateau date ... 

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