Bonding and Structures in Ceramics

Question 1

What is the macro structural level and its features

Answer 1

> 10^-3

Bulk features, what you see

Question 2

what is the micro structural level and its features

Answer 2

10^-6

grains, grain boundaries, pores, phases, electric and magnetic domains

Question 3

what is the nano stuctural level and its features

Answer 3

10^-9

atomic/ionic clusters, grain boundaries

Question 4

what is the crystal structural level and its features

Answer 4

10^-10

unit cells

Question 5

what is the atomic structural level and its features

Answer 5

10^-10

atomic defects

Question 6

what is the subatomic structural level and its features

Answer 6

<10^-10

arrangements of electrons giving rise to electronic conductivity, magnetic moments

Question 7

What are the three types of bonding that are important in ceramic materials?

Answer 7

1. ionic
2. covalent
3. van der waals

Question 8

do ionic and covalent bonds have small or large bond energy? what does that indicate?

Answer 8

both have large bond energy

large Tm, large E, small a

Question 9

what is the ratio of the strength of van der waals to ionic/covalent

Answer 9

about 1/10 the strength of ionic/covalent

Question 10

how does the presence of water affect van der waal forces

Answer 10

promotes particle slipping and facilitates/helps forming

Question 11

What is the relationship between silica gels and water?

Answer 11

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

Question 12

What are two examples of van der waals bonding in ceramics?

Answer 12

1. forming/shaping traditional ceramics
2. Porous Desiccants (silica gels)

**both have to do with interactions w water

Question 13

What do ionic crystal structures contain?

Answer 13

cations and anions

Question 14

what must each lattice point of a ionic crystal have and why

Answer 14

a cation and anion for electrical neutrality

Question 15

what kind of structures do large ions form

Answer 15

close packed (HCP or FCC) or SC structures

Question 16

are large ions usually cations or anions

Answer 16

anions

Question 17

where do smaller ions usually reside in an ionic crystal structure and cations or anions

Answer 17

interstitial spaces and cations

Question 18

what kind of structures does covalent bonding result in

Answer 18

open structures due to difficulty in forming close packed structures

Question 19

what determines the extent of the difference to which a ceramic will be ionic or covalent

Answer 19

the difference in electronegativity

bigger electroneg = more ionic

Question 20

what does the % ionicity of a ceramic effect

Answer 20

processing characteristics and properties of the ceramic

Question 21

How do ceramics with a small degree of iconicity behave

Answer 21

small ionicity = large covalency

stronger, more brittle, harder, and smaller diffusion coefficients

Question 22

If there is a ceramic with a large difference in electronegativity, what does that mean?

Answer 22

large electroneg dif = more ionic

Question 23

Are ceramics with a higher degree of covalency or ionicity stronger, more brittle, harder, and small diffusion coeff?

Answer 23

high covalency

Question 24

What is dislocation motion like in metals? covalent ceramics? ionic ceramics? Why?

Answer 24

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 +)

Question 25

What kinds of structures occur for ionic crystals? why?

Answer 25

many dif kinds can occur because of radial symmetry of the ionic bond (no preferred direction)

Question 26

in ionic crystals, what determines the sort of coordination environment that would best fit a small ion?

Answer 26

the ratio of the size of the cation to the anion

Question 27

What ratios of cation to anion radius have which coordination numbers and disposition of central ion?

Answer 27

0.155-0.225 = 3 = triangle

0.225-0.414 = 4 = tetrahedron

0.414-0.732 = 6 = octahedron

0.732-1 = 8 = cube

Question 28

Describe the structure of NaCl

Answer 28

Ionic structure* / Rock salt structure

r Na / r Cl (anion/cation) = 0.56

coordination number = 6

Cl- FCC, Na+ octahedral interstitial

Question 29

Describe the structure of CsCl

Answer 29

ionic structure

ratio = 0.94

CN = 9

Cl in SC, Cs in body centered

Question 30

Describe covalent crystal bonding structures?

Answer 30

they have directional bonding so they have open structures because they can’t form close packed

Question 31

Give two examples of covalent crystals

Answer 31

1. Diamond
2. GaAs

Question 32

Give an important example of ceramic bonding applications

Answer 32

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

Question 33

What are four examples of polymorphs in ceramics?

Answer 33

1. zirconia
2. titania
3. silicon carbide
4. alumina

Question 34

What are other types of structure that affect properties of ceramics?

Answer 34

1. macrostructure
2. microstructure
3. nanostructure
4. atomic structure

Question 35

What is relevant about polymorphs?

Answer 35

certain polymorphs exhibit different properties and are used in dif. applications