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Chapter Outline: Ceramics - PowerPoint PPT Presentation

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Chapter Outline: Ceramics

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  1. Chapter Outline: Ceramics • Chapter 12: Structure and Properties of Ceramics • Crystal Structures • Silicate Ceramics • Imperfections in Ceramics • Carbon Skip: 12.9 – 12.11

  2. Ceramics • keramikos - burnt stuff in Greek -properties achieved through high-temperature heat treatment (firing). • Usually metallic + non-metallic elements • Always composed of more than one element (e.g., Al2O3, NaCl, SiC, SiO2) • Bonds are partially or totally ionic • Hard and brittle • Electrical and thermal insulators • Optically opaque, semi-transparent, or transparent • Traditionally based on clay (china, bricks, tiles, porcelain) and glasses • “New ceramics” for electronic, computer, aerospace industries.

  3. Bonding in Ceramics (Chapter 2) Electronegativity – ability of atoms to accept electrons (subshells with one electron - low electronegativity; subshells with one missing electron -high electronegativity). Electronegativity increases from left to right. Bonding is mixed: ionic + covalent Degree of ionic depends on difference in electronegativities Cations(+); Anions(-)

  4. Crystal Structures: Predominantly Ionic • Crystal structure is defined by • Magnitude of electrical charge on each ion Charge balance dictates chemical formula (Ca2+ and F- form CaF2). • Relative sizes of cations and anions Cations want maximum possible number of anion nearest neighbors and vice-versa. Ceramic crystal structures: anions surrounding a cation are all in contact with it. For a specific coordination number there is a critical or minimum cation-anion radius ratio rC/rAfor which this contact can be maintained

  5. C.N. rC/rA Geometry • 2 <0.155 • 3 0.155-0225 • 4 0.225-0.414 • 6 0.414-0.732 • 8 0.732-1.0 The critical ratio determined by geometrical analysis 30° Cos 30= 0.866 = R/(r+R)  r/R = 0.155

  6. Crystal Structures Rock Salt Structure NaCl rC = rNa = 0.102 nm, rA = rCl = 0.181 nm  rC/rA = 0.56 From table for stable geometries: C.N. = 6 Two interpenetrating FCC lattices NaCl, MgO, LiF, FeO have this crystal structure

  7. Other crystal structures in ceramics (will not be included in the test) Cesium Chloride Structure: rC = rCs = 0.170 nm, rA = rCl = 0.181 nm  rC/rA = 0.94 From table for stable geometries: C.N. = 8

  8. Other crystal structures in ceramics (will not be included in the test) Zinc Blende Structure: typical for compounds where covalent bonding dominates. C.N. = 4 ZnS, ZnTe, SiC have this crystal structure

  9. Other crystal structures in ceramics (will not be included in the test) Fluorite (CaF2): rC = rCa = 0.100 nm, rA = rF = 0.133 nm  rC/rA = 0.75 From table for stable geometries: C.N. = 8 FCC structure with 3 atoms per lattice point

  10. Density computation (similar to Chapter 3.5 for metals)  = n’(AC + AA) / (VcNA) n’: number of formula units in unit cell (all ions included in chemical formula of compound = formula unit) AC: sum of atomic weights of cations AA: sum of atomic weights of anions Vc: volume of the unit cell NA: Avogadro’s number, 6.0231023 (formula units)/mol Example: NaCl n’ = 4 in FCC lattice AC= ANa = 22.99 g/mol AA= ACl = 35.45 g/mol Vc = a3 = (2rNa+2rCl)3 = (20.10210-7 + 20.18110-7)3 cm3

  11. Silicate Ceramics • Mainly of silicon and oxygen, the two most abundant elements in earth’s crust (rocks, soils, clays, sand) • Basic building block: SiO44- tetrahedron • Si-O bonding is largely covalent, but overall SiO4 block has charge of –4 • Various silicate structures – different ways to arrange SiO4-4 blocks

  12. Silica = silicon dioxide = SiO2 • Every oxygen shared by adjacent tetrahedra • Silica is crystalline (quartz) or amorphous, as in glass (fused or vitreous silica) 3D network of SiO4 tetrahedra in cristobalite High melting temperature of 1710 C

  13. Window glasses Common window glass is produced by adding oxides (e.g. CaO, Na2O) whose cations are incorporated within SiO4 network. The cations break the tetrahedral network. Glasses melt at lower temperature than pure amorphous SiO2. Lower melting T makes it easier to form objects (e.g, bottles). Some other oxides (TiO2, Al2O3) substitute for silicon and become part of the network

  14. Imperfections in Ceramics (I) Point defects in ionic crystals are charged. Coulomb forces are large. Any charge imbalance has a strong tendency to balance itself. To maintain charge neutrality several point defects can be occur: Frenkel defect: a pair of cation (positive ion) vacancies and a cation interstitial. Also be an anion (negative ion) vacancy and anion interstitial. Anions are larger than cations so not easy for an anion interstitial to form Schottky defect is a pair of anion and cation vacancies Schottky defect Frenkel defect

  15. Imperfections in Ceramics (II) • Frenkel and Schottky defects do not change ratio of cations to anions  compound is stoichiometric • Non-stoichiometry (composition deviates from the one predicted by chemical formula) occurs when one ion type can exist in two valence states, e.g. Fe2+, Fe3+ • In FeO, Fe valence state is 2+. • Two Fe ions in 3+ state  an Fe vacancy is required to maintain charge neutrality •  fewer Fe ions  non-stoichiometry

  16. Impurities in Ceramics • Impurity atoms can be substitutional or interstitials • Substitutional: substitute for ions of like type • Interstitials: small compared to host structure – formation of anion interstitials is unlikely • Solubilities higher if ion radii and charges match • Incorporation of ion with different charge state requires compensation by point defects Interstitial impurity atom Substitutional impurity ions

  17. Mechanical Properties of Ceramics • Brittle Fracture stress concentrators are very important. (Chap. 8: measured fracture strengths are much smaller than theoretical due to stress risers) Fracture strength greatly enhanced by creating compressive stresses in the surface region (similar to shot peening, case hardening in metals, Chap. 8) • Compressive strength is typically ten times the tensile strength Therefore ceramics are good structural materials under compression (e.g., bricks in houses, stone blocks in the pyramids).

  18. Plastic Deformation in Ceramics • Crystalline ceramics: Slip (dislocation motion) is difficult because ions of like charge have to be brought close together  large barrier for dislocation motion In ceramics with covalent bonding slip is not easy (covalent bonds are strong)  ceramics are brittle. • Non-crystalline ceramic: no regular crystalline structure  no dislocations or slip. Materials deform byviscous flow (breaking and reforming bonds, allowing ions/atoms to slide past each other (like in a liquid) • Viscosity is a measure of glassy material’s resistance to deformation.

  19. Viscosity Viscosity: measure of non-crystalline (glass or liquid) resistance to deformation. High-viscosity fluids resist flow; low-viscosity fluids flow easily. How readily a moving layer of molecules drags adjacent layers of molecules along determines its viscosity. Units are Pa-s, or Poises (P) 1 P = 0.1 Pa-s Viscosity of water at room temp is ~ 10-3 P Viscosity of typical glass at room temp >> 1016 P

  20. Carbon Carbon not a ceramic Exists in various polymorphic forms: sp3 diamond and amorphous carbon, sp2 graphite and fullerenes/nanotubes, one dimensional sp carbon

  21. Carbon: Diamond • Diamond-cubic structure • One of the strongest/hardest materials • High thermal conductivity (unlike ceramics) • Transparent in visible and infrared, high index of refraction, looks nice, costs $$$ • Semiconductor (can be doped to make electronic devices) • Metastable (transforms to carbon when heated) Hydrogenated diamond {111} surface with the dangling bonds or radicals terminated by hydrogen atoms Diamond turning into graphite at elevated temperature Figures from http://www.people.virginia.edu/~lz2n/Diamond.html

  22. Carbon: Graphite • Layered structure: Strong bonding within planar layers. Weak, van der Waals bonding between layers • Easy interplanar cleavage, applications as a lubricant and for writing (pencils) • Good electrical conductor • Chemically stable even at high temperatures • Applications: furnaces, rocket nozzles, welding electrodes

  23. Carbon: buckyballs and nanotubes Buckminsterfullerenes (buckyballs) + carbon nanotubes expected to be important in future nanotechnology applications (nanoscale materials, sensors, machines, computers) Carbon nanotube T-junction Nanotubes as reinforcing fibers in nanocomposites Nano-gear Nanotube holepunching/etching Figures from http://www.nas.nasa.gov/Groups/SciTech/nano/

  24. Summary Make sure you understand language and concepts: • Anion • Cation • Defect structure • Frenkel defect • Electroneutrality • Schottky defect • Stoichiometry • Viscosity