ch12

• Sketch/describe unit cells for sodium chloride, cesium chloride, zinc blende, diamond cubic, fluorite, and perovskite crystal structures

  1. Sketch unit cell

  2. Describe unit cell
• Sketch/describe the atomic structures of graphite and a silica glass

  1. Sketch atomic structure

  2. Describe atomic structure
• Given the chemical formula for a ceramic compound and the ionic radii of its component ions

  Predict the crystal structure
• Eight different ionic point defects that are found in ceramic compounds

  • List defect 1
  • List defect 2
  • List defect 3
  • List defect 4
  • List defect 5
  • List defect 6
  • List defect 7
  • List defect 8
• There is normally significant scatter in the fracture strength for identical specimens of the same ceramic material
• Compute the flexural strength of ceramic rod specimens that have been bent to fracture in three-point loading

  Compute flexural strength
• Slip considerations


  Explain why crystalline ceramic materials are normally brittle
• Ceramics are inorganic and nonmetallic materials
• Most ceramics are compounds between metallic and nonmetallic elements for which the interatomic bonds are either totally ionic, or predominantly ionic but having some covalent character
• Ceramic


  Comes from the Greek word keramikos, which means "burnt stuff"
• Desirable properties of ceramic materials are normally achieved through a high-temperature heat treatment process called firing
• Traditional ceramics are those for which the primary raw material is clay; products considered to be traditional ceramics are china, porcelain, bricks, tiles, and, in addition, glasses and high-temperature ceramics
• A new generation of ceramic materials has evolved, and the term ceramic has taken on a much broader meaning
• These new ceramic materials have a rather dramatic effect on our lives; electronic, computer, communication, aerospace, and a host of other industries rely on their use
• Ceramic structures are generally more complex than those for metals
• The atomic bonding in ceramic materials ranges from purely ionic to totally covalent; many ceramics exhibit a combination of these two bonding types, the degree of ionic character being dependent on the electronegativities of the atoms
• Cations


  Positively charged metallic ions
• Anions


  Negatively charged nonmetallic ions
• The crystal must be electrically neutral; that is, all the cation positive charges must be balanced by an equal number of anion negative charges
• Cations are ordinarily smaller than anions, and, consequently, the ratio rC/rA is less than unity
• Stable ceramic crystal structures form when those anions surrounding a cation are all in contact with that cation
• Coordination number


  Number of anion nearest neighbors for a cation
• Coordination numbers and nearest-neighbor geometries for various rC/rA ratios


  • Coordination number 2, rC/rA < 0.155
  • Coordination number 3, 0.155 <= rC/rA <= 0.225
  • Coordination number 4, 0.225 < rC/rA <= 0.414
  • Coordination number 6, 0.414 < rC/rA <= 0.732
  • Coordination number 8, 0.732 < rC/rA <= 1.0
  • Coordination number 12, rC/rA > 1.0
• The most common coordination numbers for ceramic materials are 4, 6, and 8
• Ionic radius


  Depends on coordination number and charge on the ion
• AX compounds


  Ceramic materials in which there are equal numbers of cations and anions
• Rock salt structure


  Common AX crystal structure, coordination number 6 for both cations and anions
• Cesium chloride structure


  AX crystal structure, coordination number 8 for both ion types
• Zinc blende structure


  AX structure, coordination number 4, highly covalent bonding
• Fluorite structure


  Common AX2 crystal structure, coordination number 8
• AmBnXp-type crystal structures


  Ceramic compounds with more than one type of cation
• AmXp-Type Crystal Structures


  Compounds where the charges on the cations and anions are not the same, with the chemical formula AmXp where m and/or p ≠ 1
• Fluorite (CaF2) crystal structure

  • Ionic radii ratio rC/rA is about 0.8, giving a coordination number of 8
  • Calcium ions are positioned at the centers of cubes, with fluorine ions at the corners
  • There are only half as many Ca2+ ions as F- ions, so the structure is similar to CsCl but with only half the center cube positions occupied by Ca2+ ions
• Other compounds with fluorite crystal structure

  • ZrO2 (cubic)
  • UO2
  • PuO2
  • ThO2
• AmBnXp-Type Crystal Structures


  Ceramic compounds with more than one type of cation, with the chemical formula AmBnXp
• Perovskite crystal structure of BaTiO3


  • At temperatures above 120°C, the crystal structure is cubic
  • Ba2+ ions are situated at all eight corners of the cube
  • A single Ti4+ is at the cube center
  • O2- ions are located at the center of each of the six faces
• Crystal structures summarised


  • Rock salt
  • Cesium chloride
  • Zinc blende
  • Fluorite
  • Perovskite
• Ceramic crystal structures from close packing of anions

  • Close-packed planes of anions create interstitial tetrahedral and octahedral sites for cations
  • Coordination numbers for cations in tetrahedral and octahedral sites are 4 and 6 respectively
  • Each anion sphere has one octahedral and two tetrahedral positions
• Rock salt crystal structure

  • Unit cell has cubic symmetry
  • Each cation (Na+ ion) has six Cl- ion nearest neighbors
  • Can be considered as an FCC array of close-packed planes of anions, all of {111} type
  • Cations reside in octahedral positions
• Other ceramic crystal structures can be treated similarly to the rock salt structure, including zinc blende and perovskite