SCHOTT Technical Glasses

58
10. Glass-Ceramics for Industrial Applications
and Home Appliances
10.1 Introduction to glass-ceramics
Glass-ceramics are distinguished from glass and ceramics by
their characteristic manufacturing process (see Figure 45)
and by their properties. Not classifiable as glass or as ceramic,
they represent a completely new class of materials.
They are manufactured in two principal production steps.
In the first step, a batch of exactly defined composition is
melted (as for normal glass). The composition is determined
by the desired properties of the end-product as well as by
the necessary working properties of the glass. After melting,
shapes are produced by pressing, blowing, rolling or casting
and then annealed. In this state, the material still exhibits
all the typical characteristics of glass.
In the second step, the “glassy” articles are subjected to a
specific temperature-time treatment between 800 – 1200 °C
(defined for each composition), by which they are ceramized,
i.e. they are transformed into a mainly polycrystalline
material. Apart from the crystalline phase with crystals of
0.05 – 5 µm in size, this material contains a residual glass
phase of 5 – 50 %.
In the temperature range between 600 – 700 °C, small
amounts of nucleating agents (e.g. TiO 2 , ZrO 2 or F) induce
precipitation of crystal nuclei. As the temperature increases,
crystals grow on these nuclei. Their type and properties
as well as their number and size are predetermined by the
glass composition and the annealing program. By appropriate
program selection, either transparent, slightly opaque, or
highly opaque, non-transparent glass-ceramics can be produced.
Unlike conventional ceramics, these glass-ceramics
are absolutely dense and pore-free. To achieve controlled
ΔI/I in 10 –4 ––>
20
15
10
5
0
Soda-lime glass
DURAN ® /BOROFLOAT ® 33/
SUPREMAX ®
borosilicate glass
“α-0”-glass-ceramics
–5
–200 0 200 400 600 800
Temperature in °C ––>
Fig. 46. Thermal expansion of “α-0”-glass-ceramics compared to
borosilicate glass 3.3 and soda-lime glass
1800
1600
a
C max
Temperature in °C ––>
1400
1200
1000
800
600
400
200
0
b
Fig. 45. Temperature-time schedule for glass-ceramic production
a: melting, b: working, c: nucleation, d: crystallization,
e: cooling to room temperature
c
d
e
Time ––>
Nucleation rate (N) and
Crystal growth rate (C) resp. ––>
N
N max
C
Temperature ––>
Fig. 47. Nucleation rate (N) and crystal growth rate (C) of glasses,
related to temperature