Structure and characteristics

10.4.1 Structure and characteristics

Some materials can exist in both the crystalline and non-crystalline form, e.g. SiO 2 ,B 2 O 3 , as shown in Figure 10.11 for boric oxide. In the non-crystalline form the material is termed glassy, vitreous or amorphous and is characterized by the absence of any long-range order. In the figure, each triangular group (CN

= 3) represents three oxygen anions around a small B 3 + cation and, overall, forms a random network in three dimensions. For silica, it is SiO 4 4 − tetrahedra that form a 3-D network.

Oxides other than SiO 2 and B 2 O 3 have the ability to form networks, e.g. P 2 O 5 ,V 2 O 5 , while others such as Na 2 O, K 2 O, CaO have no tendency to form networks but are important network modifiers. Apart from fused silica, the two glass formers are added together, often with network modifiers, to produce commercial glasses (Vycor 96% SiO 2 , 4% B 2 O 3 ; Borosilicate 81% SiO 2 , 13% B 2 O 3 , 4% Na 2 O, 2% Al 2 O 3 ). In general, the glass network becomes unstable and tends to crystallize if the additives increase the numerical ratio of oxygen to silicon ions above 2.5. The rate of cooling from the molten or fused state is also important in controlling the glass formation in oxides. Rapid cooling restricts the time for ordering and favors glass formation. 2 Glassy materials do, however, cool in a different way to crystalline materials. The glass becomes more and more viscous with decreasing temperature in a continuous manner and there is no definite temperature at which the liquid transforms to a solid, as shown in Figure 10.12. The specific volume (volume per

2 Cooling rates of 10 6 Ks −1 by melt spinning or splat cooling have been used to produce metallic glasses with transition metal (Fe, Ni, Co, Pd, Mo) and metalloids (B, C, P, Si).

Non-metallics I – Ceramics, glass, glass-ceramics 529

Liquid

Supercooled liquid

Glass Specific volume

Crystal

T f m.p. Temperature

Figure 10.12 Comparison of the formation of glass and crystals from a melt.

unit mass, m 3 kg −1 ) versus temperature curve shows a decrease in slope at a ‘fictive’ temperature T f , which may be considered a glass transition temperature, but it does depend on cooling rate. Viscous flow is given by the Newtonian equation

dγ/dt = τ/η, where dγ/dt is the shear strain rate, τ is the applied shear stress and η the coefficient of viscosity. In

its melting range, a typical SiO 2 –Na 2 O–CaO glass has a viscosity 3 of 5–50 N s m −2 , which may be compared with the viscosity of liquid metals of roughly 1 mN s m −2 . As glass is hot-drawn, its cross- sectional area decreases at a rate which depends solely upon the drawing force and viscosity, not upon area. For this reason, the glass extends uniformly and does not ‘neck’. An increase in temperature and/or addition of network-modifying cations will reduce the viscosity. The temperature dependence of viscosity is given by an Arrhenius relation

η = Aexp(Q/kT ), from which the activation energy Q for viscous flow may be obtained from a plot of log η against 1/T .

Addition of a nucleating agent, usually TiO 2 , can reduce the activation energy and induce the formation of crystalline regions within the glassy matrix. This process of devitrification can result in the product having some desirable properties, i.e. high thermal and mechanical shock resistance. Controlled devitrification is now an accepted processing method for producing such glass-ceramic materials (see

Section 10.4.3). Figure 10.13 shows a plot of log viscosity versus temperature for a typical SiO 2 – Na 2 O–CaO glass. Two practical operating temperatures are known as (1) the softening point and

3 Absolute viscosity is the force required to move 1 m 2 of plane surface at a velocity of 1 m s −1 to a second plane surface which is parallel to the first and separated 1 m from it by a layer of the fluid phase. Kinematic viscosity = absolute

viscosity/density. Unit of viscosity =1Nsm −2 = 10 P (poise). Fluidity is the reciprocal of viscosity.

530 Physical Metallurgy and Advanced Materials

6.6 Working range

5 h ⫽ 10 3 –10 2 Annealing

point

Log viscosity

Temperature (⬚C)

Figure 10.13 Viscosity versus temperature curve for a typical SiO 2 –Na 2 O–CaO glass.

Log viscosity (N

Boric oxide

Sheet glass

Temperature (⬚C)

Figure 10.14 Viscosity curves for typical glasses.

(2) the annealing point. The softening point gives the maximum temperature at which the glass can

be handled and for ordinary silica glass it is about 1000 K. At the annealing temperature, the ions are sufficiently mobile to allow residual stresses to be relieved in about 15 min. The point in the curve at which the slope is a maximum corresponds to the fictive or glass transition temperature. The working

range for commercial silica glass corresponds to a viscosity range of 10 3 –10 7 Nsm −2 . The curve for this glass is quite steep, indicating that close temperature control is necessary during working (i.e. drawing, blowing, rolling, etc.). Figure 10.14 provides a comparison of the viscosity curves for different types of glass. Even at a temperature of 1300 ◦

C silica has a viscosity of about 10 12 Nsm −2 , which is still too high for working. Glasses with special chemical and physical properties often have a steep viscosity curve and tend to devitrify, presenting difficulties during traditional working temperatures. One solution is to hot extrude through a graphite die. High pressure (up to 10 GN m −2 ) can overcome a high viscosity and reduce the extrusion temperature to prevent devitrification.