Modern Physical Metallurgy and Materials Engineering Part 12

Tham khảo tài liệu 'modern physical metallurgy and materials engineering part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Ceramics and glasses 321 concerned with structure property relations. One can predict that a ceramic structure with a fine grain crystal size and low porosity is likely to offer advantages of mechanical strength and impermeability to contacting fluids. It is therefore scientifically appropriate to classify ceramic materials in microstructural terms in the following manner 1. Single crystals of appreciable size . ruby laser crystal 2. Glass non-crystalline of appreciable size . sheets of float glass 3. Crystalline or glassy filaments . E-glass for glass-reinforced polymers single-crystal whiskers silica glass in Space Shuttle tiles 4. Polycrystalline aggregates bonded by a glassy matrix . porcelain pottery silica refractories hot-pressed silicon nitride 5. Glass-free polycrystalline aggregates . ultra-pure fine-grained zero-porosity forms of alumina magnesia and beryllia 6. Polycrystalline aggregates produced by heattreating glasses of special composition . glassceramics 7. Composites . silicon carbide or carbon filaments in a matrix of glass or glass-ceramic magnesiagraphite refractories concrete . This approach to classifying ceramics places the necessary emphasis upon the crystalline and noncrystalline glassy attributes of the ceramic body the significance of introducing grain boundary surfaces and the scope for deliberately mixing two phases with very different properties. General properties of ceramics The constituent atoms in a ceramic are held together by very strong bonding forces which may be ionic covalent or a mixture of the two. As a direct consequence their melting points are often very high making them eminently suited for use in energy-intensive systems such as industrial furnaces and gas turbines. For instance alumina primarily owes its importance as a furnace refractory material to its melting point of 2050 C. The type of inter-atomic bonding is responsible for the relatively low electrical conductivity of ceramics. For .

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