Materials Engineering Lecture 11: Ceramics. Glass

What is ceramics? Oxides: aMexOy(∙bMe’mOn)∙cNMeuOw∙dH2O Nitrides: aMexNy(∙bMe’mNn) Carbides: aMexCy Silicides: aMexSiy Generally, very hard, very tough, very brittle, refractory.

Ceramic materials

Chemical Bonds in ceramic materials Si-O, Si-C, Si-N: purely covalent Me-O, Me-C, Me-N, Me-Si: partly covalent SiC ZnS Si3N4 SiO2 Al2O3 NaCl MgO CaF2 Material 12 18 30 51 63 67 73 89 % ionic bond Even when a typical metal is bound to a typical non-metal, some orbital overlapping takes place => each ionic bond has some fraction of covalent. Ceramics is built by covalent bonds and the ceramic properties are the result of covalent bonding

Crystal structure of ceramics Metal structures are formed by non-directional metal bonds => they are dense (close packing) Ceramic structures are formed by directional covalent bonds => these are loose and brittle

Ceramics and glass Ceramic materials are always crystalline Glass materials are amorphous

Silicate ceramics Clay, sand, quartz, talс and others are silicates: MekMe’lSipOq = aMexOy(∙bMe’mOn)∙cNMeuOw The main structural element of all silicates is SiO44- tetrahedron

Silica Silica: SiO2: 2.2 gr/cm3, m.p. 1575 - 1725ºC Polymorhism: Crystobalite Metastable (stable above 1470ºC) Quartz: 2.65 g/cm3, HM=7, UV-transparent Sand: dispersed quartz

Silicates Simple: forsterite (Mg2SiO4) and others; Layered: talc, kaolinite and others. Clay is a mixture of layered silicates

Carbon allotropy Carbon is not ceramic. However it is technically close to ceramics. Diamond Graphite Fullerene Carbon nanotubes: the strongest known material in the Universe, very light, ductile, electric anisotropy (metal conductor along the tube, semiconductor across)

Defects in ceramics Due to defects ceramics can be: Cation conductive Anion conductive Electron conductive Insulator Two types of point defects are especially important: Schottky and Frenkel defects

Mechanical properties of ceramics Brittle fracture Testing: compression and bending Elasticity (almost like of metals)  fracture (no plasticity) Porosity is very important (influences on elasticity and strength) Hardness (Knoop hardness, HK) Creep at elevated temperatures Viscose flow at elevated temperatures (and for glasses)

Glasses characterisitcs other B2O3 Al2O3 CaO Na2O SiO2 glasses High m.p., low thermal expansion => glassware for high temperatures; UV-transparent > 99.5 Fused silica (quartz) Thermal shock and chemically resistant => chemical glassware 4 96 Vycor Thermal shock and chemically resistant => ovenware 13 2.5 3.5 81 Borosilicate (pyrex) Easily worked, durable => regular glass 4MgO 1 5 16 74 Soda-lime (container) Easily drawn into fibers => glass-resin composites 10 15 55 Fiberglass High density, high refraction index => optics 37PbO, 8K2O 54 Optical flint Easily fabricated, strong, resists thermal shock => ovenware 6.5TiO2, 0.5As2O3 5.5 30 14 43.5 Glass-ceramic (pyroceram)

Refractory ceramics Service temperature Porosity, % TiO2 CaO Fe2O3 Cr2O3 MgO SiO2 Al2O3 < 1587ºC 10-25 1-2 0-1 70-50 25-45 fireclay < 1890ºC 18-25 1-4 10-45 90-50 Alumina fireclay < 1650ºC 25 2.2 0.6 96.3 0.2 Silica 22 2.5 3.0 0.3 90.0 1.0 Periclase 21 2.0 8.2 73.0 5.0 9.0 Periclase – chrome ore Other refractories: alumina, zirconia, carbides, SiC, graphite Firing: raw materials contain large and fine particles (of different compositions). The large particles form a bonding phase (glassy or crystalline) and this phase is responsible for the strength of the brick.

Cements, Abrasives, Special ceramics Portland cement (the most popular) Cement + water  (setting) concrete Preparation: clay + lime are fired at ~ 1440ºC  clinker Clinker is ground + gypsum  portland cement Portland cement is hydraulic (set by water) Special cements are set by other materials Abrasive ceramics (HK>1000): diamond, SiC, WC, corundum (Al2O3), silica sand Special ceramics: Piezoelectric (strain ~ electricity): BaTiO3, PZT (lead zirconate-titanate), PbTiO3, NaKNbO3 MEMS (microelectromechanical systems) Optical fibers (high purity silica) Ceramic bearings (Si3N4)

Glass production Press-and-blow technique Drawing Thermal processing: annealing Thermal tempering

Ceramic production Hydroplastic forming Slip casting Drying Firing Powder pressing Extrusion Sintering

Musts of this lecture Ceramics: composition and properties; classification Ceramic structure and its influence on properties Glass Silicates Defects in ceramics Mechanical properties of ceramics Refractories, cements, abrasives, special ceramics Glass production Ceramic production