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to rule out deformation of ware during heat treatment; crystallization
must yield phases which impart the desired properties (Table 22). Phase
diagrams may be utilized for evaluating the phase composition; it should
be borne in mind, however, that non-equilibrium phases are often formed
in particular at lower temperatures.
Despite the above limitations, large number of various glass-ceramics
with a wide assortment of compositions have been described in the
The several types of glass-ceramics considered below are already being
manufactured and/or have possibilities of further development (Table 23).
The system Li20 - Al203 - Si02. The low-expansion-coefficient
materials of the system Li20-A1203 - Si02 are the most widely
manufactured types of technical glass-ceramics. According to the patents
of Corning Glass Works, the range of composition in which a < 15 x 10-7
is defined within the following limits: 53 to 75 wt. % Si02, 12 to 36%
AI203, 2 to 15% Li20, 3 to 7 % ÒÞ2 as nucleating agent. The U20 : : A1203
ratio should amount to 0.1-0.6. Melting can be promoted by introducing
further components in amounts up to 5%. P205 can also be used as a
The low-expansion glass-ceramics manufactured on an industrial scale
have a composition close to that of lithium feldspar; in most cases they
contain 65 to 70 wt. % Si02, 20 to 22% AI203, 3 to 5% Li20 and about 4%
ÒÞ2 as nucleating agent. This type requires a comparatively high melting
temperature. Melting is facilitated by replacing some Si02 with B203, or
other components. Crystallization at about 830 °C yields /Ç-eucryptite
(Li20. AI203 . 2 Si02) and spodumene (Li20 . . A1203 . 4 Si02) at about
1150 °C. The thermal expansion coefficients for the above compounds in
the 20 to 1000 °C range are a = -64 x 10-7 and a = 9 x 10"7 respectively.
A thermal expansion coefficient approaching zero can be attained by
suitably adjusting the proportion of the two phases. The materials
obtained do not change their dimensions with temperature and are
insensitive to thermal shock. Their bending strength of over 100 MPa
exceeds that of the original glass.
A very low thermal expansion is also exhibited by high-temperature
quartz* which forms solid solutions with eucryptite by substitution Si4+
Li+ + Al3 + . The (Si04) tetrahedra are replaced by (ÀÞ4); the Li+ ion
located in the lattice cavities compensates for the lower charge of Al3 +
. A similar substitution is also possible on the basis of combinations 2
Si4+ Mg2+ + 2 Al3 (Zn2+ + 2 Al3 + , Al3+ + Ps + , etc.) The
transformation temperature of a# /?-quartz (573 °C with pure quartz)
decreases with increasing content of substituting species and the high-
temperature low--expansion form is stabilized. The low thermal expansion
glass-ceramics may therefore also be based on solid solutions of high-
temperature quartz stabilized by suitable amounts of additions.**
Materials of this type are formed by crystallization at low temperatures
(up to 900 °C) and when the sizes of the crystals are smaller than the
* In the literature on glass-ceramics, the substance is designated
as the /?-modification which is a mineralogical-geological term, whereas
in the literature dealing with high-temperature equilibria, the high-
temperature form is designated as y..
** So-called stuffed /?-quartz.
wavelength of visible light (roughly 400 nm), the product is transparent.
The solid solution of quartz is the cause of very low (even zero)
expansion. At higher temperatures (above 900 °C), the solid solution will
decompose producing stable spodmncne and eucryptite, thus rendering the
transparent glass-ceramic non-transparent.
FIG. 147. The system Li20 - A1203 - Si02 (Eppler, 1963): P -
petalite Li20 . A1203 . 8 Si02, R - lithium feldspar
Li20 . A1203 . 6 Si02, E - eucryptite Li20 . Al203 . 2 Si02.