Thermal expansion phase separated glasses

Phase separation has almost no effect on the thermal expansion coefficient of glasses. The measured thermal expansion coefficient is a volume average of the thermal expansion coefficients of the two phases present, in much the same way as the density. 

Figure 7.11 Effect of phase separation on the thermal expansion curves of glasses...

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.

Addition of modifier ions to silica fills the interstices, preventing bond bending, and hence increases the thermal expansion coefficient. The thermal expansion coefficients of binary alkali silicate glasses increase in the order Li < Na < K. The thermal expansion coefficient is virtually independent of the existence of phase separation, increasing linearly with increasing alkali oxide content for all three oxides over the... 

The thermal expansion curve of a phase separated sample containing two glassy phases may show two glass transitions, as is shown in Figure 7.11. This type of curve will be observed if the more viscous phase is continuous, and if the immiscibility temperature for the glass lies above the Tg of the more viscous phase. Curves of this type have been observed for lead borate and barium silicate glasses, where the immiscibility temperature is greater than the Tg of either phase. 

In borosilicate glasses, BjOj acts as a flux for silica, and when combined with alkali oxides, the thermal expansion can be controlled to match various materials. Alkali borosil-icates tend to phase-separate. Alumina helps to reduce this tendency. A typical composition (alkali borosilicate) is SlSiOj ISBjOj 4Na20 - 2AI2O3 (wt%). This is close to that of Corning code 7740, used for Pyrex brand products. Its approximate properties are listed in Table 6.2. 

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