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The technology of glass and ceramics - Hlavac J.

Hlavac J. The technology of glass and ceramics - Oxford, 1983. - 429 p.
Download (direct link): tehnologyofglass1983.djvu
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(b) Metastable separation brings about the formation of phases some
of which exhibit a greater crystallization ability. For instance, the
boric phase separated in the system Na20 -B203 - Si02 crystallizes at
relatively low temperatures. Acceleration of nucleation by phase
separation is also established in the system Li20 -Si02. In the system
MgO - A1203 - Si02 containing Ti02 as nucleating agent, MgO . Ti02
crystallizes in the separated phase and serves as nuclei for the
crystallization of cordierite and cris^balite (Maurer, 1962). This
mechanism is frequently considered in connection with oxidic nucleating
agents; however, in some systems, substances promoting nucleation have no
effect on metastable separation and do not precipitate primarily. For
example, in the system Li20-A1203 - Si02, eucryptite can crystallize
before the Ti02-containing crystalline phase appears.

1
232
() Nucleating agents reduce interfacial energy between the crystal
and the glass, thus facilitating homogeneous and heterogeneous
nucleation.
With the exception of metallic nucleating agents, which have been
ranked into group (a)^ classification of the other currently employed
substances according to their functional mechanism is somewhat ambiguous.
Of the oxidic nucleating agents employed, the most efficient and
universal is Ti02, followed by Zr02, P2O5, 2, and others. Further
highly efficient nucleating agents includc fluorides known as opacifiers,
and sulphides (FeS, CuS, PbS, CdS). In all these instances, the required
amount of nucleating agent is considerably higher than in the case of
metals, being in the order of a per cent. Various combinations are also
frequently used, for example TiOa + Zr02.
2. MAIN TYPES OF TECHNICAL GLASS-CERAMICS
The choice of compositions for glass-ceramics is limited by a number of
conditions.
Apart from suitable melting and forming properties, the nuclcating-agent-
containing glass has to exhibit a suitable rate of crystal growth at a
satisfactorily high viscosity
TABLE 22. The properties of crystalline phases in glass-ceramics (after
Pavlushkin, 1970)

Coefficient
Crystal Density Meltin of
thermal
Mineral Summary formula system (gem'3) point
expansion
C C)
ax IOVc-1)
Anorthite CaO . AI20* . 2 SiG2 mono 2.75 1550 45
(100-200)
Celsiane BaO . Al203 . 2 Si02 mono 3.3-3.4 1640 27 (20-
100)
Cordierite 2 MgO . 2 A1203 . rh. (pseudo- 2.53 3470 26 (25-
700)
. 5 Si02 hexa)
Diopside CaO . MgO . 2 Si02 mono 3.27 1391 50
--- Li20 . 2 Si02 rh. 2.45 3032 110
(20-600)
Eucryptite Li20 . 12 . 2 Si02 hexa 2.67 1388 --- 64
(20---1000)
Gehlenite 2 CaO . Al203 . Si02 tetra 3.04 1590 ---
Haematite a-Fe203 hexa 5.2 1350 -
Hedenbergite FeO . CaO . 2 Si02 mono 3.54 1391
Magnetite FeO . Fe203 cubic 5.17 1590 70
Mullite 3 A1203 . 2 Si02 rh. 3 1810 53
Quartz (high-
-temperature) Si02 hexa 2.53 1470 -5
Rutile Ti02 tetra 4.23 1825 78
Sphen CaO . Ti02 . Si02 mono 3.4 --- 3.6 1382 -
Spinel MgO . A1,03 cubic 3.55 2135 88
Spodumene Li20 . Al203 . 4 Si02 tetra 2.35 1380 9 (20-
1000)
Wollastonite CaO . Si02 tricl. 2.92 1540 94
(100-200)
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