Glasses phase separation processes

Direct evidence of nucleation during the induction period will also solve a recent argument within the field of polymer science as to whether the mechanism of the induction of polymers is related to the nucleation process or to the phase separation process (including spinodal decomposition). The latter was proposed by Imai et al. based on SAXS observation of so-called cold crystallization from the quenched glass (amorphous state) of polyethylene terephthalate) (PET) [19]. They supposed that the latter mechanism could be expanded to the usual melt crystallization, but there is no experimental support for the supposition. Our results will confirm that the nucleation mechanism is correct, in the case of melt crystallization. 

Yycor , a high-temperature glass that often can be substituted for quartz glass, is also made by a phase separation process (see page 16). 

Recently in the field of physics of semiconductors and materials science a great attention has been paid to formation and optical properties of semiconductor nanocrystals (quantum dots, QDs) dispersed in inorganic matrixes. An interest to glassy materials with QDs is associated with their unique physical properties and possibility to create elements of optoelectronic devices. Phase separation processes followed by crystallization are the basic in production of such materials. They result in formation of semiconductor nanocrystals stabilized within a glass matrix. The materials are advanced for various applications because of optical and thermal stability and possibility to control optical features through the technology of glass preparation and post-synthesis thermal treatment. 

The majority of todays membranes used in microfiitration, dialysis or ultrafiltration and reverse osmosis cire prepared from a homogeneous polymer solution by a technique referred to as phase inversion. Phase inversion can be achieved by solvent evaporation, non-solvent precipitation and thermcd gelation. Phase separation processes can not only be applied to a large number of polymers but also to glasses and metal alloys and the proper selection of the various process parameters leads to different membranes with defined structures and mass transport properties. In this paper the fundamentals of membrane preparation by phase inversion processes and the effect of different preparation parameters on membrane structures and transport properties are discussed, and problems utilizing phase inversion techniques for a large scale production of membranes are specified. 

In order to crystallize silicate glasses it is not necessary to remove the modifier cations since they can interact with more than one oxygen tetrahedron due to their ionic bonding character. Thus, these cations act as connecting links between the non-bonding oxygen atoms. First of aU one deals with a kinetic problem, and often a previous phase separation process is needed to get crystallizable components. 

Nucleation may be initiated by phase-separation processes. In a special glass-ceramic heated at 1000°C for sealing applications and additionally... 

The phase separation process of slag sital is characterized by the fact that the base glasses demonstrate substantial liquid-liquid phase separation as a result of the high P2O5 content. This type of phase separation in the base glasses, which was determined by Pavluskin (1986) with scanning electron microscopy, is more strongly influenced by the phosphate additives than by the sulfides. Hence, the phospates are of particular importance for the entire nucleation procedure and for the formation of crystals. 

Most apatite glass-ceramics have been developed as biomaterials. This section, however, addresses biocompatible glass-ceramics without bioactivity. Volume nucleation and crystallization are used to produce these glass-ceramics. In addition, phase separation processes in the base glass are important for nucleation. 

Figures 3-4, 3-5, and 3-6 show the individual phases and the interface magnified 20,000, 30,000, and 50,000 times. The glass phase (Fig. 3-4) exhibits phase-separation processes in the form of droplet phases less than 200 nm in size. This phase separation creates the opal effect of the glass-ceramic. Although the crystals of the leucite type (Fig. 3-5) in the coastal areas (marked 2 in Fig. 3-3) measure only approximately 1 pm, they produce a highly translucent effect in the glass-ceramic. The crystals provide the material with a very high coefficient of thermal expansion. The crystal-glass interface is shown in Fig. 3-6. Clearly, crystal growth was interrupted at a specific st e of growth once a crystal front of some micrometer thickness had formed.

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