Chapter 12 Flashcards
What types of elements are ceramics usually compounds of?
metallic and nonmetallic elements
What interatomic bonds do ceramics have?
they’re either totally ionic or predominately ionic but with some covalent character
How are the desirable properties of ceramics normally achieved?
Firing, a high-temperature heat treatment process
What is the primary material of “traditional ceramics”?
clay
How many elements are in ceramics?
At least 2
What does the degree of ionic character of a ceramic depend on?
Electronegativities of the atoms
How should you think of the crystal structures for ceramic materials whose atomic bonding is predominately ionic?
Think of them as composed of electrically charged ions rather than atoms
metallic ions/cations: positive
nonmetallic ions/anions: negative
What two characteristics of the component ions in crystalline ceramic materials influence the crystal structure?
- The magnitude of the electrical charge on each of the component
- The relative sizes of the cations and anions
What is the charge of a ceramic crystal?
Electrically netural
Are cations or anions ordinarily smaller once ionized?
cations because metallic elements give up electrons
What is the criteria for ceramic ionic radii size of cations and anions?
- radius cation < radius anion
- cations want to have the most nearest-neighbor anions as possible and vice versa
In terms of cations and anions, how do you determine a stable ceramic crystal structure? What minimum value does this create?
The anions surrounding a cation are all in contact with the cation
This creates a minimum value of rC/rA
What does the coordination number stand for in ceramics?
Number of anion nearest neighbors for a cation
If rC/rA < 0.155 what is the cation-anion orientation? Coordination number?
a very small cation is bonded to two anions in a linear manner
coordination number = 2
If 0.155 < rC/rA < 0.255, what is the cation-anion orientation? Coordination number?
Each cation (small) is surrounded by three anions in the form of a planar equilateral triangle
Coordination number = 3
If 0.225 < rC/rA < 0.414, what is the cation-anion orientation? Coordination number?
The cation is located at the center of a tetrahedron, with anions at each of the corners
Coordination number = 4
If 0.414 < rC/rA < 0.732, what is the cation-anion orientation? Coordination number?
The cation is at the center an octahedron surrounded by six anions, one at each corner
Coordination number = 6
If 0.732 < rC/rA < 1.0, what is the cation-anion orientation? Coordination number?
Anions at all corners of a cube and a cation positioned at the center
Coordination number = 8
What is the coordination number for a radius ratio greater than unity (1) ?
12
What are the most common coordination numbers for ceramic materials?
4,6,8
What does the size of an ion depend on?
1) Coordination number: the ionic radius increases as the number of opposite charge nearest-neighbor ions increases
2) The charge of an ion: cations are smaller than anions
What are AX compounds?
Common ceramic materials with equal numbers of cations and anions (A-cations, X-anions)
What is the most common AX crystal structure?
Rock Salt/ sodium chloride (NaCl) type
What is the coordination number for cations and anions in rock salt structure? What is the cation-anion radius ration between?
6
0.414-0.732
What is the crystal structure for rock salt type?
FCC
one cation at the center and all the cube edges
What are five ceramic materials that form rock salt structure?
NaCl, MgO, MnS, LiF, FeO
What are the AX compounds?
1) Rock Salt Structure
2) Cesium Chloride
3) Zinc Blende
What is the coordination number for cesium chloride (CsCl)?
8 for both ion types
What does the structure of a cesium chloride cube look like?
Single cation at the center with anions located on the corners of the cube
NOT bcc structure
If you interchange cations and anions it’s still the same crystal structure
What is the coordination number of the Zinc Blende Structure? What are the two atoms it’s made from?
4- they’re tetrahedrally coordinated
ZnS
What’s another name for zinc blende structure (ZnS)?
sphalerite
What does the structure look like for Zinc Blende?
All corner and face positions are occupied by S atoms, and Zn atoms fill the interior tetrahedral position
It can be reversed
What is the primary type of bonding for Zinc Blende structure?
highly covalent
What are three examples of Zinc Blende structure?
ZnS, ZnTe, SiC
If the charges on cations and anions are not the same, what is the chemical formula of the ceramic?
AmXp
m and/or p can’t be 1
ex: CaF2: 2x F - ions as Ca 2+
What does the crystal structure of a AmXp ceramic look like?
One unit cell consists of 8 different cubes, each with an cation at the center and anion at the corners
What are compounds with AmXp crystal structure?
ZrO2, UO2, PuO2, ThO2
What kind of ceramic compound chemical formula has more than one type of cation?
AmBnXp
A- cation
B- cation
X- anion
What is an example of a compound with a AmBnXp type crystal structure? What is this called?
BaTiO3
Ba 2+
Ti 4+
O 2-
Perovskite crystal structure
What does the perovskite crystal structure look like?
Ba 2+ ions at the eight corners, Ti 4+ at the cube center, and O 2- ions at the center of each of the six faces
What is the summary of rock salt crystal structure? Structure type? Anion packing? cation? anion? examples?
Type: AX
Anion Packing: FCC
Cation: 6
Anion: 6
Examples: NaCl, MgO, FeO
What is the summary of cesium chloride crystal structure? Structure type? Anion packing? cation? anion? examples?
Type: AX
Anion Packing: Simple Cubic
Cation: 8
Anion: 8
Examples: CsCl
What is the summary of Zinc Blende (sphalerite) crystal structure? Structure type? Anion packing? cation? anion? examples?
Type: AX
Anion Packing: FCC
Cation: 4
Anion: 4
Examples: ZnS, SiC
What is the summary of Fluorite crustal structure? Structure type? Anion packing? cation? anion? examples?
Type: AX2
Anion Packing: Simple cubic
Cation: 8
Anion: 4
Examples: CaF2, UO2, ThO2
What is the summary of Perovskite crystal structure? Structure type? Anion packing? cation? anion? examples?
Type: ABX3
Anion Packing: FCC
Cation: 12 (A) 6 (B)
Anion: 6
Examples: BaTiO3, SrZrO3, SrSnO3
What is the summary of Spinel crystal structure?
Type: AB2X4
Anion Packing: FCC
Cation: 4(A) 6(B)
Anion: 4
Examples: MgAl2O4, FeAl2O4
What are the two types of interstitial positions in ceramics?
Tetrahedral position and octahedral position
What is the coordination number for tetrahedral positioning? Octahedral?
4
6
What formula do you use to compute the theoretical density for ceramic materials?
n’ = number of formula units within the unit cell
Ac= the sum of the atomic weights of all cations in the formula unit
Aa = the sum of the atomic weights of all anions in the formula unit
Vc= the unit cell volume
Na= Avogadro’s number, 6.022x10^23
What is a silicate ceramic?
A material composed of primarily silicon and oxygen
What type of ceramic are most soils, rocks, clays, and sand?
Silicate
What does the arrangement of a silicate look like?
SiO4 (4-)
Each atom of Si 4+ is bonded to four oxygen atoms, which are at the corners of the tetrahedron
Are silicates usually considered positively or negatively charged entity’s?
Negatively (4-)
What is silica?
Also known as silicon dioxide (SiO2)
It’s the most simple silicate material
What are the three structures of silica?
quartz, cristobalite, and tridymite
The picture is cristobalite
What are four properties of silica?
1) electrically neutral = stable electric structures
2) Si to O atoms 1:2 ratio
3) not closely packed: low density
4) high melting temperature
What are silica glasses?
Silica existing as a noncrystalline solid
Called fused silica or vitreous silica
What is the basic unit of silica glasses?
SiO4 (4-) tetrahedron, but there’s a high degree of randomness
The image is sodium-silicate glass
What are some oxides that can form glassy structures? What are they called?
B2O3, FeO2, SiO2, SiO4
Network formers
What are network modifiers? Examples?
Cations that are incorporated into the SiO4 network to modify the glass
CaO, Na2O
What is a silicate? What are some typical chemical formulas?
one, two, or three of the corner oxygen atoms of the SiO4 tetrahedra are shared by other tetrahedra
SiO4, SiO7, SiO9
What are some common cations that are added to silicates?
Ca 2+, Mg 2+, Al 3+
What is a simple silicate?
A structurally simple silicate that has an isolated tetrahedra (ex: Mg2SiO4)
What is a layered silicate?
A 2D sheet/layered structure produced by the sharing of 3 oxygen atoms in each of the tetrahedra
What is the repeating unit formula in layered silicates?
(Si2O5)^2-
net charge is negative– associated with the unbonded oxygen atoms
How is net neutrality established in layered silicates?
A second planar sheet structure that has an excess of cations bonds to the unbonded atoms in the Si2O5 sheet
What is one of the most common clay minerals with a layered silicate structure?
Kaolinite clay: has a 2 layer silicate sheet structure
What is Kaolinite’s formula? What are the layers?
Si2O5 is a layer and Al2(OH)4 is a layer
What is the midplane of Kaolinite made from? What are the different types of bonding?
Midplane is made from O from the Si2O5 layer and OH from the Al2(OH)4 layer
within the midplane is strong ionic-covalent bonds
adjacent sheets are loosely bonded by van der Waals forces
What are carbon’s two allotropic forms?
Diamond and graphite
What is diamond?
A metastable carbon polymorph at room temperature and atmospheric pressure
What is diamond’s crystal structure? What kind of hybridization does each carbon atom have? What are the bonds?
Diamond Cubic crystal structure
It’s structure is a variant of the zinc blende structure where carbon occupies all positions
sp3 hybridization (each carbon bonds tetrahedrally to four other carbons)
extremely strong covalent bonds
What is the structure of graphite? What are the types of orbitals? Bonding type?
Carbon atoms are located at the corners of interlocking regular hexagons that lie in parallel planes
sp2 orbitals
strong covalent bonds with a fourth delocalized electron
How do vacancy/interstitial defects exist in ceramics?
Since ceramics contain ions of at least two kinds, defects for each ion type may occur.
Ex: in NaCl, there are Na interstitial and vacancy and Cl interstitial and vacancy
Is it more common to have a cation or anion interstitial in ceramics?
Cations because they are smaller and anions are large and to fit in a small interstitial position requires more strains
What is electroneutrality?
A state that has equal numbers of positive and negative charges for ions
What is the Frenkel defect?
a cation-vacancy and cation-interstitial pair
think of the cation moving from it’s normal position to an interstitial one
No change in net charge
What must be maintained in a defect structure?
electroneutrality
What is a Schottky defect?
A cation-vacancy anion-vacancy pair
What must be maintained when impurities are present in ceramics?
Electroneutrality (charge balance)
How do you determine which host ion an impurity substance will substitute for?
It substitutes for the one with the most similar electrically
basically, cations for cations and anions for anions
How is electroneutrality maintained for impurity ions having a charge different from that of the host ion for which it substitutes?
The formation of lattice defects– vacancies or interstitials of both types (cations or anions)
What happens when Ca 2+ wants to be put into a NaCl lattice?
It’s a substitutional cation impurity
The Ca2+ substitutes for a Na + and creates a cation vacancy to balance the lattice
What happens when O 2- wants to be put into NaCl?
It’s a substitutional anion impurity
The O 2- substitutes for a Cl- and then the lattice creates an anion vacancy to the main electroneutrality in the lattice
Why are ceramics more brittle than metal?
Ceramics are highly ionic so dislocation motion is difficult
They have fewer slip systems
The resistance of dislocation motion of ions makes ceramics more brittle
Is it harder to dislocate metals or ceramics?
Ceramics because they are often composed of highly ionic bonds.
Strong and ridged structure, making the movement of dislocations more difficult compared to metals
What is the substitute for tensile testing in ceramics?
the flexure test
What is the ratio of tensile to compressive strengths of ceramics?
Tensile strengths are about 1/10 of their compressive strengths
What is flexural strength?
The stress at fracture using the flexure tests
What temperature is used with flexural tests?
Room temperature
Usually elastic with brittle failure
How do you measure the elastic modulus of ceramics?
Flexural tests- 3-Point Bend Testing often used
What do you measure in ceramics when behavior is linear-elastic? Brittle fracture?
Linear-elastic: elastic modulus
Brittle-fracture: flexural modulus
What is the formula for determining the elastic modulus of a ceramic?
gamma= midpoint deflection
bd (area of rectangle)
R- radius of cicule
What is the formula for determining the flexural strength of a ceramic?
bd- area of rectangle
R- radius of circle
What are 3 properties of oxide structures?
1) oxygen anions are larger than metal cations
2) close-packed oxygen in a lattice (usually FCC)
3) cations fit into interstitial sites among oxygen ions
What are the two factors that determine crystal structure?
1) relative sizes of ions: to form stable structures you want to maximize the # of oppositely charged neighbors
2) Maintenance of charge neutrality: net charge should be 0
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