Droplet glass phase

With phase separation, material transport delivers the molecular building blocks for the nucleus to the nucleation site (droplet glass phase). The term AGq in Eq. 1-10 appears to have been reduced to allow nucleation to progress directly and rapidly to the desired primary nucleus in the droplet... 

Other Aluminosilicates, Transparent mullite glass-ceramics can be produced from modified binary Al C —Si02 glasses (21). In these materials, the bulk glass phase separates into tiny alumina-rich droplets in a siliceous matrix. Further heat treatment causes these droplets to crystallize to mullite spherulites less than 0.1 Jim in size. When doped with ions such as Cr3+, transparent mullite glass-ceramics can be made to absorb broadly in the visible while fluorescing in the near-ii (22,23), thereby making them potentially useful for luminescent solar collectors. 

No large molecules can be evaporated thermally without decomposition. If one tries to place flexible macromolecules into the gas phase by evaporation of the solvent molecules from a dispersion of droplets of a solution with only one macro-molecule per droplet, the macromolecules become solid microphase particles and collect at the bottom of the container. Typical examples of single polymer glass phases and crystals are shown in Chap. 5. 

Water in its several forms is the substance most essential to life on earth. Some of its crystalline forms are stable in certain temperature-pressure ranges and others are metastable. Although the stable form of water at sufficiently low temperatures is crystalline, inside this stable phase, water can also exist in liquid form. When this occurs, water is said to be supercooled. Supercooled water occurs naturally in the form of small droplets in clouds. If liquid water is cooled quickly enough, the crystalline phase can be bypassed and a noncrystalline (amorphous) solid, that is, a glass, is the result. This amorphous glass phase of water is polymorphic, that is, it can exists in two different forms. Glassy water is undoubtedly the most common form of water in the universe. Scientists puzzle over the anomalous properties of glassy water when it is cooled it becomes more compressible, when compressed it is less viscous, and when cooled sufficiently, it expands. 

The next part of the crystallization process at temperatures above approximately 750 C was characterized by the development of the humite mineral norbergite, Mg2SiO MgF2. These crystals were produced by means of a solid-state reaction between chondrodite and the droplet-shaped glass phase, in the process of which both were entirely dissolved. The microstructure containing norbergite crystals is shown in Fig. 2-19c. 

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.

The characteristics of the rubber additive are of critical importance in determining the toughness of the final product. Several important factors include the size of the rubber droplets, the phase structure within the rubber particles (see Figures 4.14 and 11.2), grafting of the rubber to the plastic, and the glass transition of the rubber. 

Figure 5.10 In these glasses, the main phase is depleted in calcium and fluoride, which reduces Its reactivity. Acid attack occurs selectively at the phase-separated droplets which are rich in calcium and fluoride (Hill Wilson, 1988a).

Wood Hill (1991b) induced phase-separation in the clear glasses by heating them at temperatures above their transition temperatures. They found evidence for amorphous phase-separation (APS) prior to the formation of crystallites. Below the first exotherm, APS appeared to take place by spinodal decomposition so that the glass had an intercoimected structure (Cahn, 1961). At higher temperatures the microstructure consisted of distinct droplets in a matrix phase. 

Figure 5.14 The microstructure of the set cement is clearly revealed by Nomarski reflectance optical microscopy. Glass particles are distinguished from the matrix by the presence of etched circular areas at the site of the phase-separated droplets (Barry, Clinton Wilson, 1979).

Figure 6.13 Electron micrograph of a single-stage replica of a dental silicate cement glass, showing phase-separated droplets rich in calcium and fluoride large droplets 400 nm in diameter and small droplets 20 to 30 nm in diameter (Wilson et at., 1972).

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