CERAMICS

• Ceramics


  Inorganic and nonmetallic materials, typically compounds between metallic and nonmetallic elements, with predominantly ionic or partially covalent bonds
• Origin of the term "ceramic"

  Derived from the Greek word "keramikos" meaning "burnt stuff", reflecting the high-temperature firing process used in ceramic production
• Primary raw material for traditional ceramics

  Clay
• Ceramics


  • Robust atomic bonding, resulting in high hardness and chemical inertness
  • Lack free electrons, making them poor conductors of electricity and heat
  • Some ceramics with additives can rival metals in toughness
  • Industrial ceramics, primarily inorganic metal oxides, offer mechanical strength and low thermal conductivity
• Ceramics are composed of at least two elements and exhibit complex crystal structures, with atomic bonding ranging from purely ionic to entirely covalent</b>
• Rock salt crystal structure


  Composed of interpenetrating face-centered cubic lattices of cations and anions
• Common ceramics with rock salt structure

  • NaCl
  • MgO
  • MnS
  • LiF
  • FeO
• AX compounds in ceramics

  • Equal presence of cations and anions
  • Prominent example: Sodium chloride (NaCl)
  • Coordination number: Both cations and anions have a coordination number of 6
  • Cation-Anion Radius Ratio: Ranges from approximately 0.414 to 0.732
  • Unit Cell: Originates from a face-centered cubic (FCC) arrangement of anions
  • Conceptualization: Comprising two interpenetrating FCC lattices—one for cations, the other for anions
• Cesium chloride structure


  • Coordination is 8 for both ion types
  • Not a BCC crystal structure because ions of two different kinds are involved
  • Interchanging anions with cations maintains the same crystal structure
• Zinc blende structure


  • Coordination number of 4
  • All corner and face positions of the cubic cell are occupied by S atoms, while the Zn atoms fill interior tetrahedral positions
  • Named after the mineralogical term for zinc sulfide (ZnS)
  • Exhibits highly covalent atomic bonding
• AmXp-type crystal structures


  • If cations and anions do not have the same charge, a compound can exist with the chemical formula AmXp, where m and/or p ≠ 1
  • In the crystal structure, calcium ions are positioned at the centers of cubes, with fluorine ions at the corners
  • The ionic radii ratio for CaF2 is about .8, giving a coordination number of 8

  
• AmBnXp-type crystal structures


  • Ceramic compounds can contain more than one type of cation
  • At temperatures above 120C (248F), the crystal structure is cubic
  • Example: Barium titanate, which exhibits a perovskite crystal structure and unique electromechanical properties
• Tetrahedral position


  Defined by four atoms surrounding one type in a specific arrangement, forming a four-sided tetrahedron
• Octahedral position


  Produced by joining six sphere centers, with a coordination number of 6 for cations
• Silicates


  • Materials primarily composed of silicon and oxygen, abundant in the Earth's crust
  • The fundamental building block is the tetrahedron, where each silicon atom is bonded to four oxygen atoms
  • Silicates exhibit covalent character due to the strong and directional Si–O bonds
  • Silicate structures can be one-dimensional, two-dimensional, or three-dimensional, depending on how the tetrahedral units combine

  
• Silica is chemically known as silicon dioxide (SiO₂)
• Silica


  • Structurally forms a three-dimensional network where each corner oxygen atom in a tetrahedron is shared by adjacent tetrahedra
  • Electrically neutral, with all its atoms having stable electronic structures
  • The ratio of Si to O atoms is 1:2
  • When tetrahedra are arranged in a regular and ordered manner, a crystalline structure is formed
  • There are three primary polymorphic crystalline forms: quartz, cristobalite, and tridymite
  • These crystalline silicas have relatively low densities due to their comparatively open structures
  • The strong Si-O interatomic bonds contribute to a high melting temperature of 1710°C

  
• Silica glasses


  • Silica can exist as a noncrystalline solid or glass, known as fused silica or vitreous silica
  • Similar to crystalline silica, the tetrahedron is the basic unit, but there is considerable disorder beyond this structure
  • Silica glasses are used for various purposes, including containers and windows
  • Network modifiers (such as CaO and Na₂O) are added to silica glasses to lower the melting point and viscosity, making it easier to form at lower temperatures
• Silicate minerals


  • Consist of tetrahedral structures where corner oxygen atoms are shared by other tetrahedra
  • Positively charged cations play two roles: compensate negative charges from the units to achieve charge neutrality, and ionically bond the tetrahedra together
• Simple silicates


  • Olivine: Contains isolated tetrahedra
  • Akermanite (Ca₂MgSi₂O₇): Has two ions and one ion bonded to each unit
• Layered or sheet silicates


  • A two-dimensional sheet or layered structure can be formed by sharing three oxygen ions in each tetrahedron
  • The repeating unit formula for this structure can be represented as (Si₂O₅)
  • The net negative charge arises from the unbonded oxygen atoms projecting out of the plane of the sheet
  • Electroneutrality is achieved by a second planar sheet structure containing an excess of cations that bond to these unbonded oxygen atoms from the Si₂O₅ sheet

  
• Kaolinite


  One of the most common clay minerals, with a relatively simple two-layer silicate sheet structure consisting of a silica tetrahedral layer and an adjacent layer that makes the silica tetrahedral layer electrically neutral
• Kaolinite crystal structure


  • A crystal of kaolinite consists of a series of these double layers or sheets stacked parallel to each other, forming small flat plates, typically less than in diameter and nearly hexagonal

  
• Silicate sheet structures are not limited to clays; other minerals in this group include talc and the micas (e.g., muscovite)
• Carbon


  Exists in various polymorphic forms, defying traditional classification schemes—neither purely metal, ceramic, nor polymer
• Diamond


  • A remarkable metastable carbon polymorph, with a diamond cubic crystal structure where each carbon atom bonds to four others, creating strong covalent bonds
  • Renowned for its hardness, low electrical conductivity, high thermal conductivity, optical transparency, and high index of refraction
• Graphite


  • Consists of layers of hexagonally arranged carbon atoms, with strong covalent bonds within the layers and weak van der Waals bonds between the layers
  • Exhibits facile interplanar cleavage, high electrical conductivity in directions parallel to the hexagonal sheets, high strength and thermal conductivity, low coefficient of thermal expansion, and effective gas adsorption

  
• Fullerenes


  • Discrete molecular clusters, with a typical fullerene (C60) consisting of sixty carbon atoms arranged in a hollow spherical cluster
  • Within each C60 molecule, carbon atoms form both hexagonal (six-carbon atom) and pentagonal (five-carbon atom) geometrical configurations
  • The arrangement of 20 hexagons and 12 pentagons resembles a soccer ball, with no two pentagons sharing a common side
• Buckminsterfullerene


  The material composed of C60 molecules, named after R. Buckminster Fuller, the inventor of the geodesic dome
• While diamond and graphite are network solids, where carbon atoms form primary bonds throughout the entire solid, buckminsterfullerene is different, as carbon atoms bond together to form spherical molecules
• Carbon nanotubes


  • Consist of a single sheet of graphite, rolled into a tube, with both ends capped with C60 fullerene hemispheres
  • Diameters are on the order of a nanometer (approximately 100 nm or less)
  • Each nanotube is a single molecule composed of millions of atoms, with its length significantly greater than its diameter
  • Multiple-walled carbon nanotubes (consisting of concentric cylinders) also exist
• Carbon nanotubes as the "ultimate fiber"


  Due to their exceptional properties, carbon nanotubes are often called the "ultimate fiber" and hold immense promise as reinforcements in composite materials
• Ultimate Fiber


  Due to their exceptional properties, carbon nanotubes are often called the "ultimate fiber"
• Carbon nanotubes hold immense promise as reinforcements in composite materials
• Carbon nanotubes represent a fascinating intersection of material science, nanotechnology, and electronic engineering
• When considering defect structures, electroneutrality must be maintained. This is the state where there are equal numbers of positive and negative charges from the ions.
• Frenkel defect


  Involves a cation-vacancy and a cation-interstitial pair
• Schottky defect


  A cation vacancy-anion vacancy pair
• The formation of either a Frenkel or a Schottky defect does not alter the ratio of cations to anions. If no other defects are present, the material is said to be stoichiometric.

  
• Nonstoichiometric


  A ceramic compound is nonstoichiometric if there is any deviation from the exact ratio of cations to anions as predicted by the chemical formula