Ceramic introduction

What is "ceramic"? – from Greek meaning: "burnt earth" – non-metal, inorganic • Examples – sand, stone – porcelain (china), pottery – brick, concrete – glass – cutting tools – turbine engines • Unique properties – hard and strong in compression, brittle – refractory (resistance to heat) – electrical insulator (?) – chemically stable – special functions, e.g. optical

Types of ceramics • Rocks and minerals • Clays – composed of Al2O3,SiO2 & chemically bound water – attributes: easy to form with water addition – applications: bricks, tiles, porcelain, pottery, etc • Glasses – SiO2 based – attributes: transparent to light; easy to form – applications: windows, containers, lenses, etc • Cements – consisting of CaO, SiO2 & Al2O3 – attributes: form paste withwater and then harden – applications: construction,road, concrete • Engineering ceramics (advanced ceramics, high-performance ceramics, technical ceramics) – e.g. Al2O3, SiC, Si3N4, ZrO2 – attributes: excellent physical, chemical or mechanical properties – applications: heat engines, cutting tools, die material, superconductors

Engineering ceramics • Aluminium oxide (Al2O3) – cutting tool - increased cutting speed – die inserts - better wear resistance – bearings under low load - e.g. tiny sapphire spheres for watches (e.g. 19 jewel watch) – abrasives - e.g. grinding wheel, grinding powder – package material for Si chips – laser generator – crucible for metal casting – refractory furnace lining – reinforcing phase in MMC • Aluminium nitride (AlN) – good thermal conductivity (> 200 W/mK) – applications in electronic engineering when high thermal conductivity is needed • Barium titanate (BaTiO3) – high dielectric constant (up to 9500) – used in capacitor (high charge-storage capability desirable) • Beryllium oxide (BeO) – high thermal conductivity (> 250 W/mK) – applications similar to AlN

• Graphite (C) – solid lubricants – seals – crucibles for molten metals – heating element in an inert atmosphere: >2000°C • Silicon carbide (SiC) – heating element - up to 1500°C – abrasives - e.g. grinding wheel, grinding powder – dies for hot pressing – reinforcing phase in MMC – in heat engines – under development • Boron carbide (B4C) – low density (~2.5 g/cm3) and hard – armor for helicopter and personnel • Boron nitride (BN) – hexagonal BN: good solid lubricant – cubic BN: hard - e.g. as abrasives or for cutting tools • Diamond (C) – cutting tools – abrasives – jewelry – coating films

• Silicon nitride (Si3N4) – bearings under high load – reinforcing phase in MMCs – metal cutting tools – in heat engines – under development • Zirconium oxide (ZrO2) – very low thermal conductivity (~1.8W/mK) – thermal barrier coating - e.g. coating on the surface of superalloy gas turbine engine component (resulting in a temperature drop of ~300°C across the layer) – heating element at >1800°C • Glass ceramics – ease of forming – no porosity – unique properties: e. g. transparency with tiny crystals; low thermal expansion – example: LiAlSi2O6 • low thermal expansion • low thermal conductivity • good radiation ability • good impact strength • high hardness • e.g. for cooker hob (flat separation between heating coil and cooking utensil)

Characteristics of ceramics • Elastic modulus – Al2O3 = ~390 GPa – SiC = ~420 GPa – diamond = >1000 GPa – Comparison with metals steel = ~200 GPa Al = ~70 GPa) • Strength – tensile - Al2O3 = ~200-300 MPa - Si3N4 = up to 550-600 MPa - Comparison with metals steels = >1000 MPa Al alloys = 200-700 MPa – flexural • Al2O3 = 300-500 MPa • SiC = 600-800 MPa • Si3N4 = 700-900 MPa • Fracture toughness – ceramics = < 10 MPa m1/2 – metals = > 25-30 MPa m1/2 • Ductility – ceramics = nil in most cases – metals = > 10-20% • Density – ceramics: 2-6 g/cm3 – metals 1.7 - 4.5 g/cm3 for light alloys > 7 g/cm3 for many others

• Thermal conductivity – diamond = ~900 W/mK – graphite = ~100 W/mK – SiC = > 100 W/mK – Al2O3 = ~30-40 W/mK – Si3N4 = ~15 W/mK – ZrO2 = ~1.5 W/mK – Comparison with metals • Cu = ~380-390 W/mK • Al = ~220-230 W/mK • carbon steel = 50-60 W/mK • Thermal expansion coefficient – SiC = 4.3 x 10-6 /K (20 -1000°C) – Al2O3 = 8.6 x 10-6 /K (0 -1000°C) – Si3N4 = 3.2 x 10-6 /K (25 -1225°C) – Comparison with metals • Cu = ~18 x 10-6 /K (20 -300°C) • Al = ~26 x 10-6 /K (20 -300°C) • carbon steel = ~13 x 10-6/K (20 - 400°C)

The ionic radii for K+ and O2- as 0. 138 and 0. 140 nm, respectively The ionic radii for K+ and O2- as 0.138 and 0.140 nm, respectively. What would be the coordination number for each O2- ion? Briefly describe the resulting crystal structure for K2O. Explain why this is called the antifluorite structure.