4. Structure of Glass 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 (Na₂O) 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 Na₂O· 3CaO·6SiO₂ primary phase field (shown as 1:3:6) around the 15Na₂O· 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.

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.

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 (B₃) 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.

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.

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 PO₄ 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.

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 IR-transmitting 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.