ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 1 Bruce Mayer, PE Engineering-45: Materials of Engineering Bruce Mayer, PE Licensed Electrical & Mechanical Engineer Engineering 45 Ceramic Structure/Props

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 2 Bruce Mayer, PE Engineering-45: Materials of Engineering 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

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 3 Bruce Mayer, PE Engineering-45: Materials of Engineering 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

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 4 Bruce Mayer, PE Engineering-45: Materials of Engineering Ceramic Atomic Bonding  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 Metal CATion NonMetal ANion

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 5 Bruce Mayer, PE Engineering-45: Materials of Engineering Ceramic Atomic Bonding  Bonding Form Primarily IONIC; Secondarily COVALENT  Ionic Dominance INCREASES with INCREASING ELECTRONEGATIVITY Ionic CoValent

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 6 Bruce Mayer, PE Engineering-45: Materials of Engineering Ceramic Crystal Structure  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? 1.Size of sites → Does the cation fit in the site? 2.Stoichiometry  if all of one type of site is full the remainder have to go into other types of sites. 3.Bond Hybridization

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 7 Bruce Mayer, PE Engineering-45: Materials of Engineering Ionic Bonding & Structure  Charge Neutrality Requirement The NET Electronic Charge on a Macroscopic Piece, or “Chunk”, of a Material MUST be ZERO  The General Ceramic Chemical Formula: CaF 2 : Ca 2+ cation F - F - anions + Where –A  Metal Atom –X  Nonmetal Atom –m & p  atom ratio required to satisfy CHARGE NEUTRALITY

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 8 Bruce Mayer, PE Engineering-45: Materials of Engineering 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 stable UNstable

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 9 Bruce Mayer, PE Engineering-45: Materials of Engineering CoOrdination No. & Atomic Radii  The CoOrdination Number (CN) Increases With Increasing ratio of  Trends for CN vs r C /r A

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 10 Bruce Mayer, PE Engineering-45: Materials of Engineering Cation Site Size  Determine minimum r cation /r anion for O H site (C.N. = 6) a  2r anion

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 11 Bruce Mayer, PE Engineering-45: Materials of Engineering Site Selection II 2.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 O H and 8 T D sites If for a specific ceramic each unit cell has 6 cations and the cations prefer O H sites –4 in O H → These Sites Fill First –2 in T D → These Sites Take the remainder

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 12 Bruce Mayer, PE Engineering-45: Materials of Engineering Site Selection II 3.Bond Hybridization – significant covalent bondingy the hybrid orbitals can have impact if significant covalent bond character present For example in SiC: X Si = 1.8 & X C = 2.5 –ca. 89% covalent bonding – both Si and C prefer sp3 hybridization –Therefore in SiC get T D sites

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 13 Bruce Mayer, PE Engineering-45: Materials of Engineering Exmpl: Predict Fe 2 O 3 Structure  Using Ionic Radii Data Predict CoOrd No. and Crystal Structure for Iron Oxide Ionic radius (nm) Cation Al 3+ Fe Ca 2+ Anion O 2- Cl - F -  Check Radii Ratio  From r C :r A vs CN Table (Previous Sld) Predict: CN = 6 Xtal Type = NaCl

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 14 Bruce Mayer, PE Engineering-45: Materials of Engineering Ceramic Structure: NaCl  a.k.a., “Rock Salt” Structure  CN = 6 r C /r A =  Two Interpenetrating FCC Lattices Cations Anions  NOT a Simple Cubic Structure  Examples MgO, MnS, LiF, FeO

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 15 Bruce Mayer, PE Engineering-45: Materials of Engineering Ceramic Structure: CsCl  CN = 8  Structure of Interlaced Simple Cubic Structures  NOT a BCC Structure

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 16 Bruce Mayer, PE Engineering-45: Materials of Engineering Ceramic Structure: ZincBlende  CN = 4 (covalent Bonding)  Basically the DIAMOND Structure with Alternating Ions DiamondZinc Sulfide  Examples SiC, GaAs

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 17 Bruce Mayer, PE Engineering-45: Materials of Engineering Ceramic Structure: Fluorite  Formula = AX 2  r C /r A  0.8  CN Cation → 8 Anion → 4  Simple Cubic Structure with “A” (e.g. Ca) at Corners “X” (e.g. F) at Centers  Examples UO 2, PuO 2, ThO 2

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 18 Bruce Mayer, PE Engineering-45: Materials of Engineering Ceramic Structure: Perovskite CaTiO 3 – Two Types of Cations Ca 2+ CoOrdination Ti 4+ CoOrdination  Chemical Formula = A m B n X p  CN Cation-A → 12 Cation-B → 6 Anion → 4

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 19 Bruce Mayer, PE Engineering-45: Materials of Engineering Silicate Ceramics  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) crystobalite

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 20 Bruce Mayer, PE Engineering-45: Materials of Engineering Amorphous Silica  Silica gels  amorphous SiO 2 Si 4+ and O 2- not in well-ordered lattice Charge balanced by H + (to form OH - ) at “dangling” bonds –very high surface area > 200 m 2 /g SiO 2 is quite stable, therefore unreactive –makes good catalyst support

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 21 Bruce Mayer, PE Engineering-45: Materials of Engineering Silica Glasses (NonCrystalline)  NonXtal, or Amorphous Structure Called Fused-Silica or Vitreous-Silica  SiO 2 Forms a Tangled Network (Network Former)  Other Materials Added to Change Properties (e.g., Lower Melting Temp) Network Modifiers Na as SiO 2 Network Modifier

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 22 Bruce Mayer, PE Engineering-45: Materials of Engineering Composition of Common Glasses  All Compositions in WEIGHT%

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 23 Bruce Mayer, PE Engineering-45: Materials of Engineering 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) Shottky Defect: Frenkel Defect Defects Must Maintain Charge Neutrality Defect Density is Arrhenius

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 24 Bruce Mayer, PE Engineering-45: Materials of Engineering 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

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 25 Bruce Mayer, PE Engineering-45: Materials of Engineering Impurities in Ceramics cont.1  Impurities Must also Satisfy Charge Neutrality  Example: NaCl →  O 2- ANion Substitution-Defect in Rock Salt Structure To maintain Charge Neutrality ONE O 2- ion Subs for TWO Cl - ions

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 26 Bruce Mayer, PE Engineering-45: Materials of Engineering Ceramic Phase Diagrams  Same form and Use as Metallic Phase Diagrams Except Horizontal Axis is based on Ceramic COMPOUNDS –e.g. ; Al 2 O 3 → SiO 2 Typically Much Higher Phase-Change Temperatures  Can use Lever Rule on Compounds

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 27 Bruce Mayer, PE Engineering-45: Materials of Engineering 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  Then E F x linear-elastic behavior  F  slope = E  F  L 3 4bd 3  F  L 3 12  R 4 rect. X-Section circ. X-Section

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 28 Bruce Mayer, PE Engineering-45: Materials of Engineering Measuring Strength  Use the Same 3-Pt Bend Test to Measure Room Temperature Ultimate Strength  Then the FLEXURAL Strength at Fracture x F F max   rect.  fs  m fail  1.5F max L bd 2  F max L  R 3 circ.

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 29 Bruce Mayer, PE Engineering-45: Materials of Engineering Creep Performance  As with Metals Characterize Creep Performance as the Steady-State Creep Rate during Secondary-Phase Creep time   x slope =  ss = steady-state creep rate. T test >0.4T melt  Generally The Creep-Resistance Ceramics > Metal >> Plastics

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 30 Bruce Mayer, PE Engineering-45: Materials of Engineering 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

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 31 Bruce Mayer, PE Engineering-45: Materials of Engineering 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

ENGR-45_Lec-26_Ceramic_Structure-Props.ppt 32 Bruce Mayer, PE Engineering-45: Materials of Engineering WhiteBoard Work  Problem Fe 3 O 4 Ceramic (LodeStone or Magnetite) –Unit Cell for Edge Length, a = nm –Density, ρ = 5240 kg/m 3 –Fe 3 O 4 = FeOFe 2 O 3 Find Atomic Packing Factor, APF