Hyun-Suk Kim
Lecture 6:
Formation, Structure, and Properties of Glass 51570-00, Spring 2018
Ceramics Materials: Science & Engineering
(세라믹스 특론)
3. Glass Structure
▪ In principles, if the requisite data were available, the TTT diagram for any material could be generated, and the CCR that would be required to keep it from crystallizing could be calculated. In other words, if cooled rapidly enough, any liquid will form a glass, and indeed glasses have been formed from ionic, organic, and metallic melts.
What is of interest here, however, is the so-called inorganic glasses formed from covalently bonded, and for the most part silicate-based, oxide melts.
These glass-forming oxides are characterized by having a continuous three- dimensional network of linked polyhedra and are known as network
formers.
They include silica (SiO2), boron oxide (B2O3), phosphorous pentoxide (P2O5), and germania (GeO2).
Commercially, silicate-based glasses are by far the most important and the most studied and consequently are the only ones discussed here.
Since glasses possess only short-range order, they cannot be as elegantly and succinctly described as crystalline solids e.g., there are no unit cells.
The best way to describe a glass is to describe the building block that
possess the short-range order (i.e., the coordination number of each atom) and then how these blocks are put together.
The simplest of the silicates is vitreous silica (SiO2), and understanding its structure is fundamental to understanding the structure of other silicates.
Vitreous silica SiO2
▪ The basic building block for all crystalline silicates is the SiO2 tetrahedron.
In the case of quartz, every silica tetrahedron is attached to four other tetrahedra, and a three-dimensional periodic network results (see top of Table 3.4).
Increase in the stability of the structure
▪ Silica
SiO2 (∵ O/Si ratio = 2)
each oxygen is linked to two Si ( no NBOs) and each Si is linked to four O, resulting in a three-dimensional network.
all allotropes of silicas which, depending on the exact arrangement of the tetrahedra, include quartz, tridymite, and cristobalite.
if long range order is lacking, the resulting solid is labeled amorphous silica or fused quartz.
The structure of vitreous silica is very similar to that of quartz, except that the network lacks symmetry or long-range periodicity.
This so-called random network model, first proposed by Zachariasen, is generally accepted as the best description of the structure of vitreous or fused silica and is shown schematically in two-dimensions in Fig. 1.1b.
Quantitatively it has been shown that the Si-O-Si bond angle in vitreous
silica while centered on 144°, which is the angle for quartz, has a distribution of roughly 10 percent. In other words, most of the Si-O-Si bond angles fall between 130° and 160°, which implies the structure of fused silica is quite uniform at a short range order, but that the order does not persist beyond several layers of tetrahedra.
In Sec. 3.6, the formation of nonbridging oxygens upon the addition of alkali or alkaline earth oxides to silicates melts was discussed in some detail.
Because, as discussed shortly, these oxides usually strongly modify the properties of a glass, they are referred to as network modifiers. The resulting structure is not unlike that of pure silica, except that now the
continuous three-dimensional network is broken up due to the presence of nonbridging oxygens, as shown in Fig. 9.7.
Table 9.2 lists typical compositions of some of the more common commercial glasses and their softening points. Most of these glasses are predominantly composed of oxygen and silicon.
Multicomponent silicates
4. Glass Properties
▪ The noncrystalline nature of glasses endows them with certain characteristics unique to them as compared to their crystalline counterparts.
Once formed, the changes that occur in a glass upon further cooling are quite subtle and different from those that occur during other phase
transitions such as solidification or crystallization.
The change is not from disorder to order, but rather from disorder to disorder with less empty space.
In this section, the implication of this statement on glass properties is discussed.
▪ The Glass Transition Temperature
▪ The temperature dependences of several properties of crystalline solids and glasses are compared schematically in Fig. 9.8.
Typical crystalline solids will normally crystallize at their melting point, with an abrupt and significant decrease in the specific volume and configuration entropy (Fig. 9.8a and b).
The changes in these properties for glasses, however, are more gradual, and there are no abrupt changes at the melting point, but rather the
properties follow the liquid line up to a temperature where the slope of the specific volume or entropy versus temperature curve is markedly decreased.
The point at which the break in slope occurs is known as the glass transition temperature and denotes the temperature at which a glass- forming liquid transforms from a rubbery, soft plastic state to a rigid, brittle, glassy state.
In other words, the temperature at which a supercooled liquid becomes a glass, i.e., a rigid, amorphous body, is known as the glass transition
temperature or 𝑇𝑔.
In the range between the melting and glass transition temperatures, the material is usually referred to as a supercooled liquid.
Given that (see Fig. 9.8) at the glass transition temperature, the specific volume 𝑉𝑠 and entropy 𝑆 are continuous, whereas the thermal expansivity 𝛼 and heat capacity 𝑐𝑝 are discontinuous, at first glance it is not unreasonable to characterize the transformation occurring at 𝑇𝑔 as a second-order phase transformation.
After all, recall that, by definition, second-order phase transitions require that the properties that depend on the first derivative of the free energy 𝐺 such as
𝑉 and 𝑆
be continuous at the transition temperature, but that the ones that depend on the second derivative of 𝐺, such as
𝛼 and 𝑐 𝑇
be discontinuous.
Thermodynamic considerations
In a very real sense, 𝑇𝑔 is a measure of the rigidity of the glass network;
In general, the addition of network modifiers tends to reduce 𝑇𝑔, while the addition of network formers increase it.
This observation is so universal that experimentally one of the techniques of determining whether an oxide goes into network of forms nonbridging
oxygen is follow the effect of its addition on 𝑇𝑔. Effect of composition on 𝑻𝒈
What is occurring at 𝑇𝑔, however, is more complex, because it is
experimentally well established that 𝑇𝑔 is a function of the cooling rate, as shown in Fig. 9.8a; the transition temperature 𝑇𝑔 shifts to lower temperatures with decreasing cooling rates.
This implies that with more time for the atoms to rearrange, a denser glass will result and strongly suggests that 𝑇𝑔 is not a thermodynamic quantity, but rather a kinetic one.
