4. Structure of Glass
4.1. Structure of Silica Glass
The atomic arrangements in glass are similar to those in a solid except
that there is no long-range order. Some similarity to a crystalline type of
order, perhaps out to about 10 to 20 Å, is actually expected in most
multicomponent glasses. Figure 4.1 shows the arrangement of atoms in
fused silica glass in a “pancaked” version. The building block (Fig 4.1a)
is an SiO4 tetrahedron comprising a silicon ion bonded to four oxygens.
One negative charge of each of the four oxygens satisfies the four
positive charges on the silicon, which leaves each oxygen to form a
corner shared with an adjacent tetrahedron, thus acting as a bridging
oxygen (BO). Corner sharing allows three angles, shown in Fig 4.1b,the
bond angle -Si-O-Si-, the angle α1, and the twist angle α2, to assume
random values over a large range. Variations in these angles create a
random, three-dimensional, structure lacking long-range periodicity, as
shown in Fig 4.1c. Randomness is described in terms of the ring statistic,
which is the number of silicon atoms contacted when a spider takes the
shortest path to come back to its origin atom. Much of the randomness is
believed to arise from variations in the three angles shown in Fig. 4.1b.
FIGURE 4.1 Silica glass structures: (a) the SiO4 building block––line through the oxygen implies a bridging end;
(b) intermediate range structures in silica glass; (c) A two-dimensional representation of silica glass structure.
The tetrahedron does not itself have to be deformed significantly.
The Si-O-Si bond angles are continuously distributed randomly between
120° and 180° and have a maximum probability at 144°. Physical models
made with balls and sticks and computer simulations using molecular
dynamics generally support the structure of silica glass as presented in
Figs. 4.1c. Disagreements tend to be limited to the description of ring
statistics.
4.1.1. Structure of Alkali Silicate Glass
The addition of one molecule of sodium oxide (Na 2O) to silica glass
breaks up a bridge and creates two nonbridging oxygens (NBO) as
follows:
[O]3Si-O-Si[O]3 + Na2O = [O]3Si-O−Na+ + Na+ −O-Si[O]3
Each added sodium ion is attached more or less ionically to one
nonbridging oxygen. The overall electrical neutrality of the structure is
thus maintained.
If the oxygen bridges are broken up, the structure begins to lose
connectivity and, as a result, becomes more fluid relative to the fully
connected silica glass at comparable temperatures.
FIGURE 4.2 Phase equilibria and glass formation region in the soda-lime-silica system. (After “Phase
equilibria in the system Na2O-CaO-SiO2".
4.1.2. Structure of Alkali-Alkaline Earth-Silicate Glass
When an alkaline earth ion containing oxide, such as CaO, is added to
silica, the bivalent alkaline earth ion is attached to two NBOs. The bridge
via the Ca++ ion is not as strong as the direct -O- bridge, but is not as
weak as the broken bridge with alkalis. The increased stability of a soda
lime silica glass relative to the sodium silicate glass may be explained in
such qualitative terms. Excellent glasses are formed in the Na 2O·
3CaO·6SiO2 primary phase field (shown as 1:3:6) around the 15Na 2O·
10CaO·75SiO2 composition in the soda lime- silica phase equilibrium
diagram (Fig. 4.2). These forms the basis of most of the commercial
container, flat, and household lamp (both incandescent and fluorescent)
glass industry.
4.2.Structure of Boric Oxide, Borate, and Borosilicate Glasses
Pure boric oxide is perhaps the best glass former (even better than silica).
Crystals of B2O3 are hard to obtain from a melt at the slowest cooling
rates. (All of the commercially sold anhydrous boric oxide is actually
boric oxide glass.) The structure is composed of fully connected BO3
triangles with the boron atoms slightly out of the plane formed by the
oxygens.
FIGURE 4.3 The boroxol building block in B2O3 glass.
According to current beliefs, a significant fraction of the triangles make
up a boroxol unit shown in Fig. 4.3. Variation in the three bridging
oxygen bond angles at the ends provides most of the randomness. The
amount of these units decreases with increases in temperature.
Addition of an alkali ion, M+, to boric oxide brings about two
possibilities, as shown in Fig. 4.4:
1. Each sodium ion converts one bridging oxygen to one nonbridging
oxygen (NBO) and attaches itself to the NBO as in silicate glasses.
2. One triangularly coordinated boron (B 3) is converted to a tetrahedrally
coordinated boron (B4). Since boron is a trivalent ion, the presence of
four oxygens leaves the BO4 group with one net negative charge, which is
satisfied by the univalent alkali ion bonded loosely to the group.
Conversion of B3 to B4 yields a greater level of network connectivity
(without the creation of any NBO), resulting in increasing glass transition
temperature and decreasing thermal expansion coefficient.
It has been suggested that the addition of alkali to boric oxide proceeds
through option 2 initially up to about 33 mol% added alkali oxide,
corresponding to about 50 percent B4 conversion. Continuing additions of
alkali subsequently create NBO through option 1. Thus, it is also agreed
that the appearance of extrema in physical properties such as the thermal
expansion coefficient at 13 to 17 mol% alkalis in alkali borate glasses.
FIGURE 4.4 Structural changes in B2O3 on alkali addition: (a) formation of nonbridging oxygen;
(b) conversion of triangularly coordinated boron to tetrahedrally coordinated boron.
In alkali borosilicate glasses, the alkali is believed to prefer its association
with boron; thus, no NBOs are present in the structure for mol
Na2O/B2O3 < 0.5. Thereafter, the alkalis are distributed between borons
and silicons, depending on the composition and temperature.
4.3. Structure of Alkali Aluminosilicate Glasses
In alkali aluminosilicate system, it is believed that the Al3+ ion initially
goes into the network as a former creating AlO4 tetrahedra. Like the
boron ion, the trivalency then forces the alkali ion M+ to be loosely
connected to the AlO4 group. Such is the case for mol Al2O3/M2O < 1. On
further addition, the Al3+ ion enters the network as a network modifier
with an octahedral coordination of oxygens at Al2O3/M2O > 1. An oxygen
atom may be shared between three tetrahedra of SiO4 and AlO4, forming
a tricluster arrangement.
Possibilities also exist that all the three tetrahedra could be either AlO4 or
SiO4, leading to a phase-separated structure.
4.4. Structure of Phosphate Glasses
In phosphate glasses, the network is composed of PO 4 tetrahedra. Since P
is a pentavalent ion, only three of the four oxygens are corner-shared with
adjacent tetrahedra. The fourth oxygen is attached to the P with a double
bond to satisfy charge neutrality and acts as a terminator.
FIGURE 4.5 Structures in P2O5 glasses: (a) PO4 tetrahedron; (b) P4O10 molecule.
Pure P2O5 glass may contain isolated P4O10 molecules (Fig. 4.5) with van
der Waals forces between the molecules. With additions of network
modifiers, such as Na2O and CaO, the molecules begin to break up to
form linear chains and sheet-like structures.
Glass structures are, therefore, not as rigid and stable as those of the
silicates, and may readily crystallize or be attacked by chemical media.
4.5. Structure of Lead and Zinc Silicate Glasses
Although classified as an (Network Modifier), PbO can go into the
network as both a former and a modifier. Pure PbO does not make a
glass; however, glasses with as high as 90 wt% PbO can be made. It is
possible that two SiO4 tetrahedra are joined via a Pb atom. Another
suggestion has been the formation of twisted PbO4 pyramids with a Pb
atom sitting at the apex. Similar considerations hold for ZnO, though not
as strongly.
5. Non-Oxide Glasses
Other commercial glass systems include fluoride-based glasses;
chalcogenide and chalcohalide glasses; the amorphous semiconductors,
silicon and germanium; and glassy metals.
In fluoride glasses, fluorine rather than oxygen is the primary glass
network anion. BeF2 (beryllium fluoride), alone and in combination with
alkali fluorides, has sometimes been considered a low-temperature model
for silica. Those glasses transmit even farther into the ultraviolet region,
and have lower refractive index and lower dispersion than silica, but
because of health hazards associated with handling beryllium compounds
and the difficulty of producing high-purity melts, these glasses have seen
little commercialization.
Heavy metal fluoride (HMF) glasses also transmit farther into the infrared
region than silica and can be produced with sufficiently high purity that
they have found optical applications, including optical fiber (mostly for
short hauls and sensors).
The chalcogen elements, sulfur (S), selenium (Se), and tellurium (Te), are
elements from group 16 (previously called VI-A) of the chemical periodic
table.
Sulfur and selenium themselves form glasses. All three, in combination
with certain group 14 (IV) and group 15 (V) elements, such as arsenic
(As) and antimony (Sb), form glasses over considerably broad
composition ranges. When modified by adding halogens, the materials
are known as chalcohalides.
The major interest in these glasses is for their semiconducting,
photoconducting, and IR-transmitting properties. The photoconductivity
of amorphous selenium was the historical basis for the xerographic
approach to photocopying.
Although these materials are generally opaque to visible light, their IRtransmitting capabilities, in some cases extending to 18μm wavelength, or
more, are unique among glasses, and have made them candidates for
optic-fiber transmission of high-intensity CO2 laser light (10.6μm
wavelength) for laser-assisted surgery “Traditional Fiber Optics”.
Amorphous silicon, in thin-film form, is a widely used electronic
semiconductor. Its excellent photovoltaic properties, grain-boundary-free
structure, and relative ease of fabrication have made it widely used for
solar energy conversion (solar cells) and as the thin-film transistor (TFT)
switching elements in active-matrix liquid crystal display (AMLCD)
television screens and computer monitors, particularly portable units.
Chalcogenides and amorphous semiconductors have been considered for
other electronic and electrooptic applications such as computer memories.
Glassy metals, which are essentially metals, metal alloys, or metals in
combination with metalloid elements, having a glass-like atomic
structural arrangement, are generally prepared by extremely rapid
quenching (105 to 108°C/s) from the molten state. While they are of
commercial value for their significantly enhanced electrical, magnetic,
and structural strength aspects.
Oxyhalide, oxynitride, and oxycarbide glasses have also been made and
studied. The high anionic electrical conductivity observed for some
oxyhalides has raised interest in them as possible solid electrolytes.