Key Properties of Quartz and Other Silicas

Quartz is abundant and hence inexpensive, relatively hard and chemically inert. Similar to other ceramics, high hardness is a useful property of quartz. Knoop hardness data for a number of ceramic materials including quartz are given in Table 5 [32]. The densities of a number of ceramic materials including quartz are given in Table 6 [32].

Extensive reviews have been reported on the mechanical behavior of vitreous silica [33]. The Young’s modulus at 25°C is 73 GPa, the shear modulus is 31 GPa, and Poisson’s ratio is reported as 0.17. Vitreous silica and silicates are notable solids because of their unique set of properties such as its ability to transmit visible light, ultraviolet and infrared radiation, good refractory and dielectric properties, chemical inertness, and low thermal expansion with resulting high thermal shock resistance. In the infrared region, water incorporated in the structure as hydroxyl (OH-) has strong absorption bands at specific wavelengths. The Si-O vibration has two strong absorption bands that affect the transmission of silica. Transmission curves are typically compared

Key Properties of Quartz and Other Silicas

Fig. 8 The structure of vitreous silica is composed of a (a) basic building block, the (SiO4) 4 tetra­hedron (corner spheres denote oxygen and the central sphere silicon), which forms (b) a 3D noncrys­talline network of fully connected tetrahedra [26]

Key Properties of Quartz and Other Silicas

Ring Size

Fig. 9 The distribution of ring sizes in vitreous silica follows a lognormal distribution. The distribu­tion broadens under increasingly high pressures

for several types of vitreous silica. Ultra-pure vitreous silica with elevated high trans­parency is required in telecommunication fiber optics.

Different sources of radiation can affect the physical and optical properties of vitreous silica. For instance, a dose of 1 x 1020 neutrons cm-2 has been reported to increase the density of vitreous silica by about 3% (to 2.26 g cm-3) [27]. Similar increases in density are reported in quartz, tridymite, and cristobalite after comparable irradiation levels. On the other hand, ionizing radiation such as X-rays, y-rays, electrons, or protons carry

Table 5 Knoop hardness for quartz and some common ceramic materials [32]

Material

Knoop hardness (100 g load) (in kg mm-2)

Boron carbide (B4C)

2800

Silicon carbide (SiC)

2,500-2,550

Tungsten carbide (WC)

1,870-1,880

Aluminum oxide (Al2O3)

2,000-2,050

Zirconium oxide (ZrO2)

1,200

Quartz (SiO2)

Parallel to optical axis

710

Perpendicular to optical axis

790

SiO2 glass

500-679

Table 6 Densities for quartz and some common ceramic materials [32]

Material

Density (g cm 3)

Boron carbide (B4C) Silicon carbide (SiC)

2.51

Hex.

3.217

Cub.

3.210

Tungsten carbide (WC)

15.8

Aluminum oxide (Al2O3)

3.97-3.986

Zirconium oxide (ZrO2)

5.56

Quartz (SiO2)

2.65

SiO2 glass

2.201-2.211

enough energy to produce absorption centers, known as color or defect centers, in vitreous silica. A characteristic band at 215 nm is produced by long exposures to X-radiation [27]. This band is also reported in irradiated a-quartz and is often associated with the existence of E’ centers, a type of defect assumed to be a pyramidal SiO3 unit having an unpaired electron in the Si sp3 orbital. Various types of defect centers in silica glass can be classified as either intrinsic (melt-quench) or extrinsic (radiation-induced).