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Crystallization of melts in the system Li20 - A1203 -Si02 brings about
the formation of solid solutions between individual compounds in the
series Si02-petalite--spodumene-eucryptite, LiA102 being the final member
of this series (cf. the phase diagram in Fig. 147). In agreement with the
phase diagram, Li20 . 2 Si02 is the main crystalline phase precipitating
from glasses with a lower A1203 content (aprox. 4%) and higher Li20
content (12 to 13%). The glass-ceramics of this type exhibit a high
mechanical strength (up to 400 MPa) and a low electrical conductivity.
A modified type of glass-ceramic contains up to 15% MgO or ZnO in
addition to LizO, A1203 and Si02; this constitutes a transition between
the spodumene and cordierite materials and shows excellent resistance to
abrasion. Its thermal expansion coefficient depends on the MgO content.
The main crystalline phase is stuffed /?--quartz.
The low-expansion spodumene-eucryptite glass-ceramics are manufactured
under the trade mark Pyroceram (with a numerical code), Cer-Vit, Hercuvit
(USA), Sitall with a numerical code (USSR), Heatron, Neoceram, Miraclon
(Japan), Jena 2000, Zerodur (FRG).
The system Mg0 - Al203 - Si02. Cordierite 2 MgO . 2 A1203 . 5 Si02 is
crystalline substance with a low thermal expansion coefficient (cf. Table
22). It constitutes the basis of cordierite-type glass-ceramics which do
not contain alkali oxides. In agreement with the phase diagram (Fig.
189), mullite and spinel, and possibly rutile (Ti02) and other
crystalline phases may also precipitate, besides cordierite. In the first
stages of crystallization, solid solutions with high-temperature quartz
structure are usually separated. Cordierite also enters into solid
solution with other oxides.
The main components are in the following ranges: 45 to 60 wt. % Si02,
20 to 30% A1203, 2 to 18% MgO. This proportion of components corresponds
approximately to the region of primary crystallization of cordierite in
the phase diagram. The nucleating agent is usually Ti02 (7 to 12%) or
P205 (1 to 6%). Temperatures in excess of 1500 °C are necessary for
melting; the melts have a comparatively low viscosity and can be formed
by casting. After crystallization, they have a high strength (up to 250
MPa in bending), a high electrical resistivity and low dielectric losses,
and therefore find application in electronics and electrical engineering.
Their relatively low coefficient of thermal expansion renders them highly
resistant to thermal shocks.
Other types of technical glass-ceramics. Glasses of the system PbO -
- B203 -Si02 are readily fusible and are used as solder glass in vacuum
electronics. Originally use was made of solder glass solidifying as glass
(for example, in the system PbO -ZnO -B203); however, in some
applications it is desirable to let the solder glass crystallize after
joining the respective parts, so that the joint can be reheated during
later processing without any danger of softening and deformation. The
soldering and subsequent crystallization take place at temperatures of
350 to 450 °C. A considerable number of solder glasses of various
compositions are known besides the system in question, so that the
expansion coefficient can be adjusted according to the type of materials
to be joined over a very wide range.
Ferroelectric glass-ceramics can be made within the systems BaO-AI203
- Ti02-Si02 (the main crystalline phases: BaTi03 and celsian BaO . A1303
. . 2 Si02) or Na20 -Nb2Os - Si02 (crystallization of NaNb03 -f Si02)and
others. From systems also containing PbO, PbTi03 or PbNb2Ob crystallize.
Because of the inert diluting phase, the permittivity is lower than in
the case of ceramic ferroelectrics, usually in the range 400 to 1500.
Advantageous use can be made of the wide spectrum of properties
attainable by adjusting the composition, particle size and content of the
active crystalline phase, and in particular through the possibility of
shaping extremely thin sheets (Herczog, 1967; Layton and Herczog, 1969).
Glass-ceramics which are machinable to precise dimensions can be made
from melts of the system K20 -MgO - A1203-B203 -Si02,by crystallization
of solid solutions of fluorophlogopite KMg3AlSi3O30F2 - a fluorine-
containing mica showing an interlocking microstructLirc of mica flakes
(Beall et al., 1972, 1978).
In addition to the types of glass-ceramics mentioned above, there are
many others, some still at the stage of research and development. More
recently, considerable attention has also been paid to the possibility of
simplifying and shortening the
crystallization process which has so far been time consuming and thus
uneconomical. With some systems, this goal can be attained by suitable
selection of composition and nucleating agents causing the ware 10
crystallize uniformly throughout its volume during continuous cooling,