SCHOTT Technical Glasses

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crystallization in the glass, the temperature difference between
the nuclei formation region and the crystal growth
region must be sufficiently large (Figure 47). In this way,
spontaneous crystallization during hot forming and unwanted
crystal growth during nucleation can be avoided.
Like the glass composition, the composition of glass-ceramics
is highly variable. Well-known compositions lie within the
following systems:
Li 2 O–Al 2 O 3 –SiO 2 ; MgO–Al 2 O 3 –SiO 2 ; CaO–P 2 O 5 –Al 2 O 3 –SiO 2 .
Glass-ceramics of the Li 2 O–Al 2 O 3 –SiO 2 system that contain
small amounts of alkali and alkaline earth oxides as well as
TiO 2 and ZrO 2 as nucleating agents have achieved great
commercial significance. Based on this system, glass-ceramics
with coefficients of linear thermal expansion of close to zero
can be produced (Figure 46 and Table 18). This exceptional
property results from the combination of crystalline constituents
(such as solid solutions of h-quartz, h-eucryptite or
h-spodumene), with negative coefficients of thermal expansion,
and the residual glass phase of the system, with a positive
coefficient of thermal expansion.
Such “α-0”-glass-ceramics can be subjected to virtually
any thermal shock or temperature variation below 700 °C,
whereby wall thickness, wall thickness differences, and
complicated shapes are of no significance.
Another technical advantage is the exceptionally high dimensional
and shape stability of objects made from these materials,
even when subjected to considerable temperature
variations.
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An application for CERAN ® glass-ceramic cooktop panels:
e.g. SCHOTT CERAN ® for induction