Typical applications of glass-ceramics The versatility and potential for development of glass-

10.4.4.3 Typical applications of glass-ceramics The versatility and potential for development of glass-

10.4.4.2 Development of glass-ceramics ceramics quickly led to their adoption in heat engines, Glass-ceramics date from the late 1950s and are an

chemical plant, electronic circuits, seals, cladding for offshoot of research by S. D. Stookey at the Corn-

buildings, aerospace equipment, nuclear engineering, ing Glass Works, USA, on photosensitive glasses. In

etc. They can offer a remarkable combination of prop- the normal procedure for these glasses, after prior

erties. For instance, glass-ceramic hob plates for elec- irradiation with ultraviolet light, their structure can

tric cookers are strong, smooth and easy to clean, stable

be altered by heat-treatment. Metals, such as cop- over long periods of heating, transparent to infrared per, silver or gold, act as nucleating agents for lim-

radiation from the tungsten halogen lamp, relatively ited crystallization. It was fortuitously discovered that

opaque to visible light (reducing glare) and resistant to higher heat-treatment temperatures could induce com-

thermal shock. The last property originates primarily plete crystallization. Subsequent research at Corn-

from the low thermal coefficient of thermal expan- ing established that certain oxides could also act as

sion (˛) of this particular glass-ceramic. In the ver- effective nucleating agents and led to development

satile SiO 2 –Al 2 O 3 –LiO 2 class of glass-ceramics, the of the first glass-ceramics, which were based on the

˛ -value can be ‘tailored’ from zero to 12 ð 10 ° C SiO 2 –Al 2 O 3 –LiO 2 system. Conveniently, these mate-

by controlling structure. Thus, in types containing rials do not require prior irradiation. Oxides which

about 10% alumina, crystals of lithium disilicate promote crystallization include titania, zirconia and

and quartz (or cristobalite) form to give a relatively phosphorus pentoxide.

high expansion coefficient. Increasing the alumina to Practical considerations of production greatly

about 20% favours the formation of two types of influence the development and exploitation of glass-

lithium aluminium silicate crystals, ˇ-spodumene and ceramics. As a general rule, the temperatures of

ˇ -eucryptite. Over the temperature range 20–1000 ° C, melting and refining should not exceed 1400 –1500 ° C. the ˛-values for these two compounds are 0.9 ð

Fortunately, some nucleating agents, such as titania,

10 ° C ° C , respectively. Careful

334 Modern Physical Metallurgy and Materials Engineering balancing of their relative amounts against the residual

have been identified by X-ray diffraction analysis. glass content can reduce the overall expansion coeffi-

For example, many different stacking sequences are cient towards zero. This capability is ideal for the large

possible in hexagonal ˛-SiC (e.g. 4H, 6H). The nomen- mirror blanks used for telescopes where dimensional

clature for such variants (polytypes) indicates the num- stability is essential. In heat engines such as lorry gas

ber of atomic layers in the stacking sequence and the turbines, a low expansion coefficient is required for the

crystal system (e.g. 15R, 3C). Tetrahedral grouping of regenerative heat exchanger which is alternately heated

carbon atoms around a central silicon atom is a com- and cooled by exhaust gases and combustion air. In

mon and basic feature of these various structures. This glass/glass and metal/glass seals, precise matching of

covalent bonding of two tetravalent elements gives expansion characteristics is possible. In the mass pro-

exceptional strength and hardness and a very high duction of colour television tubes, devitrifiable solder

melting point (>2700 ° C).

Despite its carbon content, silicon carbide offers tem have a fairly high ˛ value and have been used to

glass-ceramics based upon the PbO–ZnO–B 2 O 3 sys-

useful resistance to oxidation. At elevated tempera- seal the glass cone to the glass face plate at a relatively

tures, a thin impervious layer of silica (cristobalite) low temperature without risk of distortion. The seal is

forms on the grains of carbide. On cooling, the ˛/ˇ subsequently heated to form the glass-ceramic.

transition occurs in cristobalite and it can crack, allow-

A fine-grained structure of interlocking crystals ing ingress of oxygen. Above a temperature of 1500 ° C, favours mechanical strength; modulus of rupture val-

the silica layer is no longer protective and the carbide ues are comparable to those for dense alumina. In a

degrades, forming SiO and CO. machinable variety of glass-ceramic, interlocking crys-

For service applications where resistance to ther- tals of platey mica deflect or blunt forming cracks

mal shock is important, it is customary to compare and make it possible for complex machinable com-

candidate ceramic materials in terms of a parame- ponents to be designed. Chemical stability, as well

ter which allows for the effect of relevant properties. as wear-resistance, is essential when service involves

Many versions of this parameter have been proposed. contact with fluids (e.g. valves, pumps, vessel linings).

A typical parameter (R) for sudden thermal shock is It is well-known that the fluxing oxides of sodium

and potassium lower the chemical resistance of sil-

(N m

ica glass to aqueous solutions; in glass-ceramics, this

E is the modulus of elasticity (N m ), and ˛ is the susceptibility is countered by stabilizing any residual

linear coefficient of thermal expansion ⊲K glass phase with boric oxide. Attempts are being made ⊳ . In the to extend their use to much higher temperatures (e.g.

case of silicon carbide, the product in the denomi- 1300–1700 °

C) by exploiting refractory systems, such nator tends to be high, giving a low index. Silicon as SiO 2 –Al O

2 3 –BaO, which can provide a liquidus temperature above 1750 °

nitride gives a higher index and it is understandable

that Si 3 N 4 -bonded silicon carbide is preferred when difficult to melt.

service involves thermal cycling. The severity of shock Conversion from glass to crystals causes mobile ions

C but such glasses are very

can affect the rating of different materials. Thus, for to disappear from the structure and become ‘bound’

less rapid shock, a thermal conductivity term (k) is to crystals, consequently the electrical resistivity and

included as a multiplier in the numerator of the above dielectric breakdown strength of glass-ceramics are

parameter.

high, even at temperatures up to 500–700 °

C. The

dielectric loss is low.

10.4.5.2 Production of silicon carbide powder Utilization of plentiful and cheap waste byproducts

and products

from industry for the bulk production of glass-ceramics Silicon carbide is a relatively costly material because has stimulated much interest. Although the chemical

its production is energy-intensive. The Acheson carbo- complexity of materials such as metallurgical slags

thermic process, which is the principal source of com- makes it difficult to control and complete crystalliza-

mercial-quality silicon carbide, requires 6–12 kW h tion, strong and wear-resistant products for architecture

per kg of silicon carbide. Locations served by hydro- and road surfacing have been produced (e.g. Slagce-

electric power, such as Norway and the Niagara Falls ram, Slagsitall ).

in Canada, are therefore favoured for synthesis plant. In this unique process, a charge of pure silica sand

10.4.5 Silicon carbide

(quartz), petroleum coke (or anthracite coal), sodium

10.4.5.1 Structure and properties of silicon chloride and sawdust is packed around a 15 m long carbide

graphite conductor. A heavy electric current is passed through the conductor and develops a temperature in

The two principal structural forms of this synthetic

C. The salt converts impurities into compound are ˛-SiC (non-cubic; hexagonal, rhombo-

excess of 2600 °

volatile chlorides and the sawdust provides connected hedral) and ˇ-SiC (cubic). The cubic ˇ-form begins

porosity within the charge, allowing gases/vapours to to transform to ˛-SiC when the temperature is raised

escape. The essential reaction is: above 2100 °

C. Conditions of manufacture determine the exact crystal structure and a number of variants

SiO 2

Ceramics and glasses 335 The reaction product from around the electrode is

and is an example of reaction-sintering (reaction- ground and graded according to size and purity. The

bonding). A mixture of ˛-SiC, graphite and a plas- colour can range from green (99.8% SiC) to grey (90%

ticizing binder is compacted and shaped by extrusion, SiC).

pressing, etc. This ‘green’ compact can be machined, Numerous chemical conversion and gas-phase syn-

after which the binder is removed by heating in an thesis processes have been investigated. The temper-

oven. The pre-form is then immersed in molten silicon atures involved are generally much lower than those

C. Graphitic developed in the Acheson process; consequently, they

under vacuum at a temperature of 1700 °

carbon and silicon react to form a strong intergran- yield the cubic ˇ-form of silicon carbide. Chemical

ular bond of ˇ-SiC. A substantial amount of ‘excess’ vapour deposition (CVD) has been used to produce

silicon (say, 8–12%) remains in the structure; the max- filaments and ultra-fine powders of ˇ-SiC.

imum operating temperature is thus set by the melting point of silicon (i.e. 1400 Silicon carbide shapes for general refractory appli- °

C. Beyond this temperature cations are produced by firing a mixture of SiC grains

there is a rapid fall in strength. Ideally, neither unre- and clay at a temperature of 1500 °

acted graphite nor unfilled voids should be present in bond forms mullite and a glassy phase, absorbing the

the final structure. The dimensional changes associated thin layer of silica which encases the grains. Other

C. The resultant

with the HP process are small and close tolerances can bonding media include ethyl silicate, silica and sili-

be achieved; the shrinkage of 1–2% is largely due to the bake-out of the binder.

con nitride. The latter is developed in situ by firing a SiC/Si compact in a nitrogen atmosphere. This parti-