CH 12 - Structures & Properties of Ceramics Flashcards
What is the bonding found in ceramics?
- can be ionic and/or covalent
- % ionic character increases with difference in electronegativity of atoms
- degree of ionic character may be large or small
Describe oxide structures
- oxygen anions larger than metal cations
- close packed oxygen in a lattice (usually FCC)
- cations fit into interstitial sites among oxygen ions
What are factors that determine the crystal structure of ceramics?
- relative size of ions; formation of stable structures maximizes the # of oppositely charge ion neighbors
- maintenance of charge neutrality
What is the tetrahedral position? (ceramics)
- type of interstitial site
- four atoms surround it
- three in one plane, single atom in the adjacent plane
- lines drawn from the centers of the surrounding spheres form a four sided tetrahedron
What is the octahedral position? (ceramics)
- type of interstitial site
- six ion spheres
- three in each plane
- octahedron forms when lines are drawn connecting all six atoms
How are silicates formed?
bonding of adjacent SiO4(4-) accomplished by the sharing of common corners, edges or faces
Describe the glass structure
- basic unit: SiO4(4-)
- fused silica to which no impurities have been added
- noncrystalline
What are layered silicates?
- SiO4 tetrahedra connected to form a 2D plane
- net negative charge associated with each (Si2O5)2- unit
- neg charge balanced by adjacent plane rich in cations
Describe kaolinite clay
- alternating layers of (Si2O5)2- with Al2(OH)4(2+)
- adjacent sheets are loosely bound by van der Waal’s forces
What are the properties of diamond?
- tetrahedral bonding of carbon
- hardest material known
- very high thermal conductivity
- large single crystals = gemstones
- small crystals used to grind/cut other materials
- diamond thin films used as surface coatings for cutting tools & medical devices
What are the properties of graphite?
- layered structure
- parallel hexagonal arrays of carbon atoms
- weak van der Waal’s forces between layers
- planes slide easily over one another
- good lubricant
What are fullerenes?
- spherical cluster of 60 carbon atoms
- shaped like soccer ball
What are carbon nanotubes?
- sheet of graphite rolled into a tube
- ends capped with fullerene hemispheres
What are the types of point defects in ceramics?
vacancies & interstitials
What is a Frenkel defect?
cation vacancy, cation interstitial pair
