9-14 GLASS-CERAMICS

9-14 GLASS-CERAMICS

Glass-ceramics cover the crystallized or devitrified glass produced by a controlled crystallization of a solid glass body. The term glass-ceramics should not be confused with ceramics made by bonding glass or other powder, even though crystallization may occur during the bonding process. Crystallization in glass can be induced in certain circumstances, although it is avoided in making transparent glass. Glasses that will crystallize reasonably easily usually have a relatively high proportion of modifying oxides. This weakens the three-dimensional glass network by introducing non-bridging oxygen ions, making possible the atomic rearrangements necessary for crystallization. It appears that the smaller cations of greater polarizing power enhance crystallization more than the larger cations. The following compositions are typical of glasses in which nucleation and crystallization have been commercially produced:

Li 2 O − A1 2 O 3 − SiO 2

(9-16)

(9-17) Li 2 O − MgO − SiO 2

MgO − A1 2 O 3 − SiO 2

(9-18) Li 2 O − ZnO − SiO 2

A process of controlled crystallization involves the addition of nucleating agents such as TiO to the molten glass. These nucleating agents can be TiO 2 , ZrO 2 , CaF 2 , or metallic colloidal particles such as Pt, Au, Ag, and Cu. Then the melt is shaped by the usual glass-forming techniques to clear glass articles, which are subjected to a special heat treatment to convert the glass to a microcrystalline ceramic. The heat treatment consists of two steps (Fig. 9-9). First, the object is heated to a nucleation temperature T, which

10 corresponds to glass viscosities in the range 10 11 to 10 Pa.s, and is soaked at this temperature. Second, after the nucleation period the temperature of the glass is raised at a

rate of about 5°C/mm (9°F/mm) to a temperature of optimum crystal growth, T cr . This is usually about 100°C (212°F) below the liquidus temperature. The resultant microstructure consists of very fine crystals ranging from 0.01 to 1 µ m which ideally

18 21 should be uniformly dispersed in a concentration from 10 3 to 10 nuclei/m . On prolonged heating the number of crystals decreases and their size increases. This

crystallization process is accompanied by optical changes from a transparent glass to an opaque polycrystalline material. The opacity is due to light scattering at interfaces between the crystal and the residual glass matrix, which have different refractive indexes. When crystals are very small, the glass-ceramic may be translucent and even transparent. Polycrystalline glasses are used in industrial and domestic applications under the trade name Pyroceram.

FIGURE 9-9 Heat treatment of glass ceramics. T n is the nucleation temperature T crist is the crystallization temperature.

Photosensitive Glasses. Photosensitive glasses have been developed using a lithium— alumina—silicate composition and inducing crystallization by metals such as Cu, Ag, and Au, which are photosensitive constituents. When such glasses are irradiated with ultraviolet light through a mask or a negative, a latent image forms in the glass because of the production of atoms of the photosensitive metals. On subsequent heating to a temperature just below the annealing point, the submicroscopic crystals of copper, silver, or gold are first formed by the aggregation of the irradiated metal atoms. These crystals serve as nucleation centers for lithium metasilicate crystals to form and grow. Since this crystallized region of glass is more soluble in hydrofluoric acid than the original glass matrix, intricate patterns can be etched. After proper machining, the glass may be exposed again to high temperatures for enhanced crystallization to produce a strong Photosensitive Glasses. Photosensitive glasses have been developed using a lithium— alumina—silicate composition and inducing crystallization by metals such as Cu, Ag, and Au, which are photosensitive constituents. When such glasses are irradiated with ultraviolet light through a mask or a negative, a latent image forms in the glass because of the production of atoms of the photosensitive metals. On subsequent heating to a temperature just below the annealing point, the submicroscopic crystals of copper, silver, or gold are first formed by the aggregation of the irradiated metal atoms. These crystals serve as nucleation centers for lithium metasilicate crystals to form and grow. Since this crystallized region of glass is more soluble in hydrofluoric acid than the original glass matrix, intricate patterns can be etched. After proper machining, the glass may be exposed again to high temperatures for enhanced crystallization to produce a strong