Non-Metals
5.7. GLASSES AND GLASS CERAMICS 1. Introduction
5.7.3. Non-Tetrahedral Glasses
Apart from polymers, low temperature data for non-tetrahedrally bonded glasses are largely restricted to B203 [Ste73, WhiS4a], AS2S3 [BarSO, MorSla], and AS2Se3 [Hor78, Cla78], as summarized in Table 5.7. Chieffeatures are as follows:
1. the maxima in Cp/T3 (or the equivalent minima in 0
c)
are less pronounced than for the tetrahedral glasses or crystals (see lower part of Fig. 5.1S);2. the elastic moduli do not soften under pressure;
3. there is no indication of negative expansion, at least above 1 K. Ackerman et al. [AckS4] have measured one of these glasses, AS2S3, below 1 K; their extrapolation suggests that a tunnelling (T -) term has an associated 'Y value of about -2.
Non-Metals 185 Table 5.7. Experimental data for some non-silicate glasses from
references in the text including [Bar80, p. 669] and [Ste73, Ack84].
Vitreous SiOl is included for comparison. 'YlO is from thermal data at 10 K and
'Yl
is from the respective T -terms (tunnelling) below 1The tenn 'glass ceramics' describes those materials which can be fonned in the glassy state and then heat treated to partially recrystallize with the help of a nucleating agent. The resulting mix of small crystallites ('" ILm or less) in a glass matrix is achieved with very little change in volume or shape, and has zero porosity and high mechanical strength.
The history of the development and technical importance of low thermal ex-pansion glass ceramics is discussed in a book by Hans Bach of Schott Glaswerke [Bac95]. The process of photonucleation was discovered at Corning (New York) about 1940. There followed research on the lithium-alumina-silicates such as f3-eucryptite (LhO·Alz03.2SiOz) which have negative values of volume expansion at ambient temperatures, and also the discovery of nucleation by addition of TiOz.
Another convenient nucleating agent was found to be zrOz. Development work at Corning, Owens-Illinois and Schott centered on the LiAISiO family with f3(hi)-quartz structure. This structure can be stabilized below the a-f3 quartz transition (573°C) by additions of MgO, ZnO, Alz03, etc. Generally the addition of LiAlOz leads to strong negative values of a, ZnAlz04 gives smaller negative values, AIP04 has little effect, and MgAlz04 gives a strong positive contribution to the expansion.
These developments led to the production of glass ceramics for kitchen ware which were resistant to thermal shock. Later came their potential use for large zero-expansion blanks for telescope mirrors. This imposed additional requirements of homogeneity, suitable polishing characteristics and adhesion of Al films. One such material from Schott was called Zerodur and typically contained (in wt%) about 57%
SiOz, 25% Ah03, 6.5% PzOs, 3.4% LiOz, 1 % each of MgO and ZnO, and 2% each of TiOz and ZrOz. Later Schott developed Zerodur M with less MgO to improve
186 ChapterS
stability on thermal cycling (see also [Hea94, Ch. 14]).
Partly because of their use in space telescopes, there are thermal data extending to liquid helium temperatures, particularly for thermal expansion. At temperatures below about 100 K, the expansion coefficient is negative with values comparable to those for vitreous silica.
Measurements of heat capacity have been largely confined to below 30 K, due largely to a fundamental interest in the magnitude of T -and T3 -terms in C p. At temperatures between about 50 K and room temperature the observed values of Cp for these aluminosilicates are not very different from values for silica or many silicate glasses.
J1-Eucryptite. LiAISi04 or (LhO·AI203.2Si02) is a hexagonal Li-stuffed derivative of {3-quartz, and plays an important rOle in determining the expansion coefficient of ceramic glasses. There are no high resolution expansivity data, but there are lattice spacing measurements at 20, 100, 200, 300 K, and also at higher temperatures, which reveal markedly anisotropic expansion. Along the c-axis. all is negative above 20 K, with a value of ~ - 20 x 1 0-6K -I at 300 K; while a 1-is positive (except possibly below 50 K) with a magnitude roughly half that of all. The volume coefficient is near zero over a wide range. The relative importance of low-frequency TA modes, rigid unit modes (which may include some low frequency TA modes) and the Li+ ion is yet unresolved; see [Pil73, Lic98] and [Hea94, Ch. 3].
Glass Ceramics. The heat capacity of most glass ceramics at room temperature is about 0.8 J.g-1·K- 1 or 48 J·mol-I·K- 1, compared with 44 J·mol-I·K-1 (±1%) for a-quartz, cristobalite and vitreous silica at 293 K. For all these latter three forms of Si02. Cp values agree within 1 or 2 per cent from 300 K down to 80 K [Wes63].
They diverge increasingly below this temperature: at 25 K. Cp
=
1.32 J·mol-I·K- 1 for quartz, 2.33 for cristobalite. 2.26 for silica and 1.7 J·mol-I·K- 1 for Cer-Vit.Some of the measurements of Cp on samples of Cer-Vit (Owens-Illinois) and Zerodur (Schott) are illustrated in Fig. 5.22 [CoI85b]. The Cer-Vit samples had varying degrees of crystallinity and were stated [Lea77] to be ' ... essentially a lithium aluminium silicate containing several percent of Ti02 and Zr02 as nucleating agents ... '. Below 4 K. data could be represented by Cp ~ (5T
+
0.8T3) pJ.g-l·K- I• the linear term being of similar magnitude for differing proportions of glass-to-crystallite.
The Zerodur samples had average crystal sizes 50 to 135 nm and had 70 to 80%
crystallinity [CoI85b]. Below 5 K, measurements fitted Cp
=
AT+
BT3 with values of A from 5.4 to 7.8 pJ.g-I·K-2 andB ~ 0.7 pJ.g-I.K-4; these are rather similar to the Cer-Vit data. Note that the linear (tunnelling) term is much larger per gram than the 1.2p.J.g-I·K-2 observed for vitreous silica (Fig. 5.15).The thermal expansion coefficient for these aluminosilicates is much more sen-sitive than Cp to composition, particularly at intermediate temperatures. say 50 to
Non-Metals
-•
4
crl.tob.llte
'lIII: 3
';at
.,
.!
l::
Go 2CJ
1
o
OL..L-L~.L...J.-L-'-8.I.--L-1...-'-1"':-2-'-~-:1:':8-'-~--' T (K)Fig. 5.22. Cp IT3 against T for some silica-based materials [CoI85b].
187
350 K. Berthold and Jacobs [Ber76a] (see also [Bar80, p. 673]) measured the ex-pansion of more than 40 samples of Cer-Vit between 150 K and 540 K, finding a at 150 K varying from -0.3 to +0.8 x 1O-6K-1• Figure 5.23 shows some selected data on Zerodur samples and MGC (or MACOR, a machineable glass ceramic from Corning); and for comparison vitreous silica (aged at 1000°C) and Corning ULE [CoI91, Whi76a]. The maximum in a near 100 K for many samples of Cer-Vit and Zerodur raises interesting questions. Obviously there is a balance between negative and positive contributions to a, but it is difficult to reconcile the large differences near 100 K with the rather similar values below 10 or 15 K. At the lowest temperatures transverse modes of vibration (giving negative expansion as in vitreous silica) must be dominant, and near 100 K other modes become important. Near room temper-ature there may be a strong negative contribution from a crystalline phase such as l3-eucryptite.
The machineable glass ceramic, MGC or MACOR, contains small (5-10 ILm) blocks of a ftuorophlogopite mica phase crystallized in a boraluminosilicate matrix.
Its machinability, shock-resistance, and non-magnetic qualities make it useful for cryogenic equipment. The heat capacity per gram measured below 20 K by Lawless [Law75] is similar to that of silica. The density is 2.52 g·cm-3, and a293 = 8.8 x 10-6 K-1. Values of a down to 2 K are shown in Fig. 5.23 [Whi76a].
188 Chapter 5
Fig. 5.23. a(T) of Zerodur samples. Z-1 and Z-M. vitreous Si02. ULE 7971 and MGC (Macor) [Whi76a.
CoI91].
S.S. IDGHLY ANISOTROPIC CRYSTALS