Test 3. Chapter 12 Flashcards
ceramic bonding?
- mostly ionic, some covalent
- % ionic character increase with difference in electronegativity
ceramic bonding depends on what?
bonding
large vs small ionic bond character
define coordination number
maximize the number of nearest oppositely charged neighbors
what is charge neutrality
net charge in the structure should be zero
coordination number increases with ???
rcation/ranion
AX-type crystal structures include, _____________-
NaCl, Cs,Cl, and zinc blende
AX2 crystal structure includes
UO2, ThO2, ZrO2, CeO2, fluorite
why are ceramics more brittle than metals?
slippage along planes
- in ionic solids this slippage is very difficult
- too much energy needed to move one anion past another anion
How can you measure elastic modulus
Room T behavior is usually elastic, with brittle failure.
why is 3 point bend testing often used?
tensile tests are difficult for brittle materials
most common elements on earth are _____________?
Si, O
silicate ceramics
crystobalite
They are so small that the they have unusually unit structure. They share oxygen atoms
Crystalline SiO2 (silica) structures are quartz, crystobalite, & tridymite
• The strong Si-O bond leads to a strong, high melting
material (1710ºC)
armorphous silica
done have organized patter for the way we assemble the basic structure together
not enough atoms to cancel out the charge of the oxygen atoms so now you have HYDROGEN atoms added
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
define silica glass
a dense form of amorphous silica
- has Na+ ions to correct charge inbalance. “counter cations”
– Borosilicate glass is the pyrex glass used
in labs
• better temperature stability & less brittle than sodium
glass
silicates
Combine SiO4
4- tetrahedra by having them share
corners, edges, or faces
Cations such as Ca2+, Mg2+, & Al3+ act to
neutralize & provide ionic bonding
layered silicates
– SiO4 tetrahedra connected
together to form 2-D plane
- (Si2O5)2-
- So need cations to balance charge
carbon forms
amorphous/olycrystalline-surface area ca 1000m^2/g
what are different forms for diamond? and describe?
– tetrahedral carbon
• hard – no good slip planes
• brittle – can cut it
– large diamonds – jewelry
– small diamonds • often man made - used for cutting tools and polishing – diamond films (artificial) • hard surface coat
– tools,
medical devices, etc.
carbon forms - graphite
- layer structure-aromatic layers
- weak van der waals forcer between layers
- planes slip easily, good lubricant
carbon forms- fullerenes and nanotubes
- fullerenes or carbon nanotubes
- wrap the graphite sheet by curving into ball or tube
- buckminister fullerness
what are some defects in ceramic structures?
frenkel defect- a cation is out of place ( a pair of cation vacancy +cation interstitial_
shottky defect- a pair set of cations and anion vacancy
impurities must also satisfy __________= electroneutrailty
charge valance
ceramic materials have _________ and __________ bonding
covalent and ionic bonding
structures based on :
CHARGE NEUTRALITY
maximizing # of nearest oppositely charged neighbors
structures may be predicted based on:
ratio of the cation and anion radi
defects ?????//
must have preserved charge neutrality
have a concentration that varies exponentially w/T
room T mechanical response is elastic, but fracture is brittle negligible deformation
-elevated T creep properties are generally superior to those of metals (and polymers)
general
