Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege

1. Engineering 45 CeramicStructure/Props Bruce Mayer, PE Licensed Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

2. Learning Goals – Ceramic Props • How the Atomic Structure of Ceramics Differs from that of Metals • How Point Defects Operate in Ceramics • Impurities • How The Crystal Lattice Accommodates them • How They Affect Properties • Mechanical Testing of Ceramics • Must Allow for Their Brittle Nature

3. Ceramics • The term CERAMIC comes from the Greek – KERAMIKOS, which means “burnt stuff”…. • Ceramics are often Processed by a high-temp heat treating process called FIRING

4. Ceramics are Compounds between METAL and NONmetal IONS Metals are the oxidANT; They GIVE UP e- to Become Positively Charged CATions The nonmetals are the oxidIZER; They RECEIVE e- to Become Negatively Charge ANions Ceramic Atomic Bonding MetalCATion NonMetalANion

5. Bonding Form Primarily IONIC; Secondarily COVALENT Ionic Dominance INCREASES with INCREASING ELECTRONEGATIVITY CaF : Large He H 2 - 2.1 Li C Be F N e SiC: Small 1.0 1.5 - 2.5 4.0 Na Si Cl Ar Mg 0.9 3.0 - 1.2 1.8 K Ca B r Ti Cr Fe Ni Zn As K r 0.8 2.8 1.5 1.6 1.8 1.8 1.8 2.0 - 1.0 Rb Sr I Xe 0.8 1.0 2.5 - Cs Ba At Rn 0.7 0.9 2.2 - Fr Ra Table of Electronegativities 0.7 0.9 Ceramic Atomic Bonding Ionic CoValent

6. Oxide Structures oxygen anions are much larger than metal cations close packed oxygen in a lattice (usually FCC) Cations (usually a metal) in the holes of the oxygen lattice Site Selection → Which sites will Cations occupy? Size of sites → Does the cation fit in the site? Stoichiometry if all of one type of site is full the remainder have to go into other types of sites. Bond Hybridization Ceramic Crystal Structure

7. Charge Neutrality Requirement The NET Electronic Charge on a Macroscopic Piece, or “Chunk”, of a Material MUST be ZERO The General Ceramic Chemical Formula: - F 2+ Ca + CaF : 2 anions cation - F Ionic Bonding & Structure • Where • A  Metal Atom • X  Nonmetal Atom • m & p  atom ratio required to satisfy CHARGE NEUTRALITY

8. - - - - - - + + + - - - - - - UNstable stable stable Ceramic MicroStructural Stability • Two Primary Issues • Due to Coulombic Forces (Electrical Attraction) the Solid State Ions Need OPPOSITELY Charged Nearest Neighbors • Anion↔Cation Radial Contact Holds the Smaller Cation in Place

9. r cation Coord # r ZnS anion (zincblende) < .155 2 .155-.225 3 NaCl .225-.414 4 (sodium chloride) .414-.732 6 Cs Cl (cesium .732-1.0 8 chloride) CoOrdination No. & Atomic Radii • The CoOrdination Number (CN) Increases With Increasing ratio of • Trends for CN vs rC/rA

10. Cation Site Size • Determine minimum rcation/ranion for OH site (C.N. = 6) a= 2ranion

11. Site Selection II • Stoichiometry • If all of one type of site is full the remainder have to go into other types of sites. • Ex: FCC unit cell has 4 OH and 8 TD sites • If for a specific ceramic each unit cell has 6 cations and the cations prefer OH sites • 4 in OH → These Sites Fill First • 2 in TD → These Sites Take the remainder

12. Site Selection II • Bond Hybridization – significant covalent bondingy • the hybrid orbitals can have impact if significant covalent bond character present • For example in SiC: XSi = 1.8 & XC = 2.5 • ca. 89% covalent bonding • both Si and C prefer sp3 hybridization • Therefore in SiC get TD sites

13. Using Ionic Radii Data Predict CoOrd No. and Crystal Structure for Iron Oxide Ionic radius (nm) Cation 3+ 0.053 Al 2 + 0.077 Fe 3+ 0.069 Fe 2+ 0.100 Ca Anion 2- 0.140 O - 0.181 Cl - 0.133 F Exmpl: Predict Fe2O3 Structure • Check Radii Ratio • From rC:rA vs CN Table (Previous Sld) Predict: • CN = 6 • Xtal Type = NaCl

14. a.k.a., “Rock Salt” Structure CN = 6 rC/rA = 0.414-0.732 Two Interpenetrating FCC Lattices Cations Anions NOT a Simple Cubic Structure Examples MgO, MnS, LiF, FeO Ceramic Structure: NaCl http://www.webelements.com/webelements/index.html

15. CN = 8 Structure of Interlaced Simple Cubic Structures NOT a BCC Structure Ceramic Structure: CsCl

16. CN = 4 (covalent Bonding) Basically the DIAMOND Structure with Alternating Ions Ceramic Structure: ZincBlende • Examples • SiC, GaAs Diamond ZincSulfide

17. Formula = AX2 rC/rA 0.8 CN Cation → 8 Anion → 4 Simple Cubic Structure with “A” (e.g. Ca) at Corners “X” (e.g. F) at Centers Examples UO2, PuO2, ThO2 Ceramic Structure: Fluorite

18. Ceramic Structure: Perovskite Ca2+CoOrdination Ti4+CoOrdination CaTiO3 – Two Types of Cations • Chemical Formula = AmBnXp • CN • Cation-A → 12 • Cation-B → 6 • Anion → 4

19. Si and O are the MOST abundant Elements in the Earth’s Crust By Valence e- Ratio Si & O Form Compounds With a Si:O ratio of 1:2 Have Low Densities About 33%-50% That of Steel Si-O Bond is Mostly CoValent and Quite Strong Yields a High Melting Temperature (1710 °C) Silicate Ceramics crystobalite

20. Silica gels amorphous SiO2 Si4+ and O2- not in well-ordered lattice Charge balanced by H+(to form OH-) at “dangling” bonds very high surface area > 200 m2/g SiO2 is quite stable, therefore unreactive makes good catalyst support Amorphous Silica

21. NonXtal, or Amorphous Structure Called Fused-Silica or Vitreous-Silica SiO2 Forms a Tangled Network (Network Former) Other Materials Added to Change Properties (e.g., Lower Melting Temp) Network Modifiers Silica Glasses (NonCrystalline) Na as SiO2 Network Modifier

22. Composition of Common Glasses • All Compositions in WEIGHT%

23. Shottky Defect: Frenkel Defect Defects In Ceramic Structures • FRENKEL Defect → Cation (the Smaller one) has Moved to an Interstitial Location • SHOTTKY Defect → Anion-Cation Pair-Vacancy (electrical neutrality conserved) • Defects Must Maintain Charge Neutrality • Defect Density is Arrhenius

24. Impurities in Ceramics • Impurities Must Also Satisfy Charge Neutrality • Example: NaCl → • Ca-CATion Substitution-Defect in Rock Salt Structure • To maintain Charge Neutrality ONE Ca+2 ion Subs for TWO Na+ ions

25. Impurities in Ceramics cont.1 • Impurities Must also Satisfy Charge Neutrality • Example: NaCl → • O2- ANion Substitution-Defect in Rock Salt Structure • To maintain Charge Neutrality ONE O2- ion Subs for TWO Cl- ions

26. Same form and Use as Metallic Phase Diagrams Except Horizontal Axis is based on Ceramic COMPOUNDS e.g. ; Al2O3 → SiO2 Typically Much Higher Phase-Change Temperatures Can use Lever Ruleon Compounds Ceramic Phase Diagrams

27. F cross section L/2 L/2 d R F 3 3 b F L F L d = midpoint x rect. circ. = = E deflection 3 4 F d d p 4 bd 12 R slope = rect. circ. d X-Section X-Section d linear-elastic behavior Measuring Young’s Modulus • Ceramic Room Temp behavior is Linear-Elastic with BRITTLE Fracture • Due to Brittle Failure, 3-Point Bending Tests are used to Assess Elastic Modulus • ThenE

28. F cross section L/2 L/2 1 . 5 F L F L fail max max R d s = s = = fs m 2 3 p bd R b rect. circ. rect. circ. location of max tension F x F max d d max Measuring Strength • Use the Same 3-Pt Bend Test to Measure Room Temperature Ultimate Strength • Then the FLEXURAL Strength at Fracture

29. s x Ttest >0.4Tmelt . e slope = = steady-state creep rate ss s time Creep Performance • As with Metals Characterize Creep Performance as the Steady-State Creep Rate during Secondary-Phase Creep • Generally The Creep-Resistance • Ceramics > Metal >> Plastics

30. Summary • Ceramics Exhibit Primarily IONIC Bonding • Some Ceramics; e.g., Silica. are Covalently Bonded • Crystal Structure is a Function of • Charge Neutrality • Maximizing of NEAREST, OPPOSITELY-Charged, Neighbors • Cation:Anion Size-Ratio

31. Summary cont.1 • Defects • Must Preserve Charge NEUTRALITY • Have a Concentration that Varies EXPONENTIALLY with Temperature • Room Temperature Mechanical Response is ELASTIC, but Fracture is Brittle, with negligible ductility • Elevated Temp Creep Properties are Generally Superior to those of Metals

32. WhiteBoard Work • Problem 12.21 • Fe3O4 Ceramic (LodeStone or Magnetite) • Unit Cell for Edge Length, a = 0.839 nm • Density, ρ = 5240 kg/m3 • Fe3O4 = FeO•Fe2O3 • Find Atomic Packing Factor, APF