Bonding and Structures in Ceramics Flashcards
What is the macro structural level and its features
> 10^-3
Bulk features, what you see
what is the micro structural level and its features
10^-6
grains, grain boundaries, pores, phases, electric and magnetic domains
what is the nano stuctural level and its features
10^-9
atomic/ionic clusters, grain boundaries
what is the crystal structural level and its features
10^-10
unit cells
what is the atomic structural level and its features
10^-10
atomic defects
what is the subatomic structural level and its features
<10^-10
arrangements of electrons giving rise to electronic conductivity, magnetic moments
What are the three types of bonding that are important in ceramic materials?
- ionic
- covalent
- van der waals
do ionic and covalent bonds have small or large bond energy? what does that indicate?
both have large bond energy
large Tm, large E, small a
what is the ratio of the strength of van der waals to ionic/covalent
about 1/10 the strength of ionic/covalent
how does the presence of water affect van der waal forces
promotes particle slipping and facilitates/helps forming
What is the relationship between silica gels and water?
silica gels are porous
the O- on silica attracts water, which is absorbed into the pores
large quantities of water can be absorbed because surface area is high bc of the pores
What are two examples of van der waals bonding in ceramics?
- forming/shaping traditional ceramics
- Porous Desiccants (silica gels)
**both have to do with interactions w water
What do ionic crystal structures contain?
cations and anions
what must each lattice point of a ionic crystal have and why
a cation and anion for electrical neutrality
what kind of structures do large ions form
close packed (HCP or FCC) or SC structures
are large ions usually cations or anions
anions
where do smaller ions usually reside in an ionic crystal structure and cations or anions
interstitial spaces and cations
what kind of structures does covalent bonding result in
open structures due to difficulty in forming close packed structures
what determines the extent of the difference to which a ceramic will be ionic or covalent
the difference in electronegativity
bigger electroneg = more ionic
what does the % ionicity of a ceramic effect
processing characteristics and properties of the ceramic
How do ceramics with a small degree of iconicity behave
small ionicity = large covalency
stronger, more brittle, harder, and smaller diffusion coefficients
If there is a ceramic with a large difference in electronegativity, what does that mean?
large electroneg dif = more ionic
Are ceramics with a higher degree of covalency or ionicity stronger, more brittle, harder, and small diffusion coeff?
high covalency
What is dislocation motion like in metals? covalent ceramics? ionic ceramics? Why?
metals - dislocation easiest
- non-directional bonding, close-packed directions for slip
covalent ceramics - motion difficult
- directional (angular) bonding
ionic ceramics - motion difficult
- need to avoid nearest neighbors of like sign (- and +)
What kinds of structures occur for ionic crystals? why?
many dif kinds can occur because of radial symmetry of the ionic bond (no preferred direction)
in ionic crystals, what determines the sort of coordination environment that would best fit a small ion?
the ratio of the size of the cation to the anion
What ratios of cation to anion radius have which coordination numbers and disposition of central ion?
0.155-0.225 = 3 = triangle
0.225-0.414 = 4 = tetrahedron
0.414-0.732 = 6 = octahedron
0.732-1 = 8 = cube
Describe the structure of NaCl
Ionic structure* / Rock salt structure
r Na / r Cl (anion/cation) = 0.56
coordination number = 6
Cl- FCC, Na+ octahedral interstitial
Describe the structure of CsCl
ionic structure
ratio = 0.94
CN = 9
Cl in SC, Cs in body centered
Describe covalent crystal bonding structures?
they have directional bonding so they have open structures because they can’t form close packed
Give two examples of covalent crystals
- Diamond
- GaAs
Give an important example of ceramic bonding applications
nuclear fission
ceramic UO2 is used instead of metallic U because
1. oxide is stable to high temps
2. the structure can accommodate fission products
What are four examples of polymorphs in ceramics?
- zirconia
- titania
- silicon carbide
- alumina
What are other types of structure that affect properties of ceramics?
- macrostructure
- microstructure
- nanostructure
- atomic structure
What is relevant about polymorphs?
certain polymorphs exhibit different properties and are used in dif. applications
