Unformatted text preview: 13.5 Glass–Ceramics ● 423 Table 13.10 Compositions and Characteristics of Some of the Common Commercial Glasses
Glass Type Fused silica SiO2 99.5 Na2O Composition (wt% ) CaO Al2O3 B2O3 Other Characteristics and Applications High melting temperature, very low coefﬁcient of expansion (shock resistant) Thermally shock and chemically resistant—laboratory ware Thermally shock and chemically resistant—ovenware Low melting temperature, easily worked, also durable Easily drawn into ﬁbers—glass–resin composites High density and high index of refraction—optical lenses Easily fabricated; strong; resists thermal shock—ovenware 96% Silica (Vycor) Borosilicate (Pyrex) Container (soda–lime) Fiberglass Optical ﬂint Glass–ceramic (Pyroceram) 96 81 74 55 54 43.5 1 14 30 3.5 16 5 16 2.5 1 15 4 13 4MgO 10 4MgO 37PbO, 8K2O 6.5TiO2 , 0.5As2O3 5.5 13.4 GLASSES
The glasses are a familiar group of ceramics; containers, windows, lenses, and ﬁberglass represent typical applications. As already mentioned, they are noncrystalline silicates containing other oxides, notably CaO, Na2O, K2O, and Al2O3 , which inﬂuence the glass properties. A typical soda–lime glass consists of approximately 70 wt% SiO2 , the balance being mainly Na2O (soda) and CaO (lime). The compositions of several common glass materials are contained in Table 13.10. Possibly the two prime assets of these materials are their optical transparency and the relative ease with which they may be fabricated. 13.5 GLASS –CERAMICS
Most inorganic glasses can be made to transform from a noncrystalline state to one that is crystalline by the proper high-temperature heat treatment. This process is called devitriﬁcation, and the product is a ﬁne-grained polycrystalline material which is often called a glass–ceramic. A nucleating agent (frequently titanium dioxide) must be added to induce the crystallization or devitriﬁcation process. Desirable characteristics of glass–ceramics include a low coefﬁcient of thermal expansion, such that the glass–ceramic ware will not experience thermal shock; in addition, relatively high mechanical strengths and thermal conductivities are achieved. Some glass–ceramics may be made optically transparent; others are opaque. Possibly the most attractive attribute of this class of materials is the ease with which they may be fabricated; conventional glass-forming techniques may be used conveniently in the mass production of nearly pore-free ware. Glass–ceramics are manufactured commercially under the trade names of Pyroceram, Corningware, Cercor, and Vision. The most common uses for these materials are as ovenware and tableware, primarily because of their strength, excellent resistance to thermal shock, and their high thermal conductivity. They also serve as electrical insulators and as substrates for printed circuit boards, and are utilized ...
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This note was uploaded on 11/04/2010 for the course ACC 411 taught by Professor Kim during the Spring '08 term at Aberystwyth University.
- Spring '08