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Elements > Chapter 12 > Flashcards

Chapter 12 Flashcards

1
Q

What types of elements are ceramics usually compounds of?

A

metallic and nonmetallic elements

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2
Q

What interatomic bonds do ceramics have?

A

they’re either totally ionic or predominately ionic but with some covalent character

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3
Q

How are the desirable properties of ceramics normally achieved?

A

Firing, a high-temperature heat treatment process

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4
Q

What is the primary material of “traditional ceramics”?

A

clay

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5
Q

How many elements are in ceramics?

A

At least 2

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6
Q

What does the degree of ionic character of a ceramic depend on?

A

Electronegativities of the atoms

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7
Q

How should you think of the crystal structures for ceramic materials whose atomic bonding is predominately ionic?

A

Think of them as composed of electrically charged ions rather than atoms

metallic ions/cations: positive
nonmetallic ions/anions: negative

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8
Q

What two characteristics of the component ions in crystalline ceramic materials influence the crystal structure?

A
  1. The magnitude of the electrical charge on each of the component
  2. The relative sizes of the cations and anions
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9
Q

What is the charge of a ceramic crystal?

A

Electrically netural

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10
Q

Are cations or anions ordinarily smaller once ionized?

A

cations because metallic elements give up electrons

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11
Q

What is the criteria for ceramic ionic radii size of cations and anions?

A
  1. radius cation < radius anion
  2. cations want to have the most nearest-neighbor anions as possible and vice versa
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12
Q

In terms of cations and anions, how do you determine a stable ceramic crystal structure? What minimum value does this create?

A

The anions surrounding a cation are all in contact with the cation

This creates a minimum value of rC/rA

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13
Q

What does the coordination number stand for in ceramics?

A

Number of anion nearest neighbors for a cation

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14
Q

If rC/rA < 0.155 what is the cation-anion orientation? Coordination number?

A

a very small cation is bonded to two anions in a linear manner

coordination number = 2

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15
Q

If 0.155 < rC/rA < 0.255, what is the cation-anion orientation? Coordination number?

A

Each cation (small) is surrounded by three anions in the form of a planar equilateral triangle

Coordination number = 3

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16
Q

If 0.225 < rC/rA < 0.414, what is the cation-anion orientation? Coordination number?

A

The cation is located at the center of a tetrahedron, with anions at each of the corners

Coordination number = 4

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17
Q

If 0.414 < rC/rA < 0.732, what is the cation-anion orientation? Coordination number?

A

The cation is at the center an octahedron surrounded by six anions, one at each corner

Coordination number = 6

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18
Q

If 0.732 < rC/rA < 1.0, what is the cation-anion orientation? Coordination number?

A

Anions at all corners of a cube and a cation positioned at the center

Coordination number = 8

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19
Q

What is the coordination number for a radius ratio greater than unity (1) ?

A

12

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20
Q

What are the most common coordination numbers for ceramic materials?

A

4,6,8

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21
Q

What does the size of an ion depend on?

A

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

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22
Q

What are AX compounds?

A

Common ceramic materials with equal numbers of cations and anions (A-cations, X-anions)

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23
Q

What is the most common AX crystal structure?

A

Rock Salt/ sodium chloride (NaCl) type

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24
Q

What is the coordination number for cations and anions in rock salt structure? What is the cation-anion radius ration between?

A

6

0.414-0.732

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25
Q

What is the crystal structure for rock salt type?

A

FCC

one cation at the center and all the cube edges

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26
Q

What are five ceramic materials that form rock salt structure?

A

NaCl, MgO, MnS, LiF, FeO

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27
Q

What are the AX compounds?

A

1) Rock Salt Structure
2) Cesium Chloride
3) Zinc Blende

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28
Q

What is the coordination number for cesium chloride (CsCl)?

A

8 for both ion types

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29
Q

What does the structure of a cesium chloride cube look like?

A

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

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30
Q

What is the coordination number of the Zinc Blende Structure? What are the two atoms it’s made from?

A

4- they’re tetrahedrally coordinated

ZnS

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31
Q

What’s another name for zinc blende structure (ZnS)?

A

sphalerite

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32
Q

What does the structure look like for Zinc Blende?

A

All corner and face positions are occupied by S atoms, and Zn atoms fill the interior tetrahedral position

It can be reversed

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33
Q

What is the primary type of bonding for Zinc Blende structure?

A

highly covalent

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34
Q

What are three examples of Zinc Blende structure?

A

ZnS, ZnTe, SiC

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35
Q

If the charges on cations and anions are not the same, what is the chemical formula of the ceramic?

A

AmXp

m and/or p can’t be 1

ex: CaF2: 2x F - ions as Ca 2+

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36
Q

What does the crystal structure of a AmXp ceramic look like?

A

One unit cell consists of 8 different cubes, each with an cation at the center and anion at the corners

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37
Q

What are compounds with AmXp crystal structure?

A

ZrO2, UO2, PuO2, ThO2

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38
Q

What kind of ceramic compound chemical formula has more than one type of cation?

A

AmBnXp

A- cation
B- cation
X- anion

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39
Q

What is an example of a compound with a AmBnXp type crystal structure? What is this called?

A

BaTiO3

Ba 2+
Ti 4+
O 2-

Perovskite crystal structure

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40
Q

What does the perovskite crystal structure look like?

A

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

41
Q

What is the summary of rock salt crystal structure? Structure type? Anion packing? cation? anion? examples?

A

Type: AX
Anion Packing: FCC
Cation: 6
Anion: 6
Examples: NaCl, MgO, FeO

42
Q

What is the summary of cesium chloride crystal structure? Structure type? Anion packing? cation? anion? examples?

A

Type: AX
Anion Packing: Simple Cubic
Cation: 8
Anion: 8
Examples: CsCl

43
Q

What is the summary of Zinc Blende (sphalerite) crystal structure? Structure type? Anion packing? cation? anion? examples?

A

Type: AX
Anion Packing: FCC
Cation: 4
Anion: 4
Examples: ZnS, SiC

44
Q

What is the summary of Fluorite crustal structure? Structure type? Anion packing? cation? anion? examples?

A

Type: AX2
Anion Packing: Simple cubic
Cation: 8
Anion: 4
Examples: CaF2, UO2, ThO2

45
Q

What is the summary of Perovskite crystal structure? Structure type? Anion packing? cation? anion? examples?

A

Type: ABX3
Anion Packing: FCC
Cation: 12 (A) 6 (B)
Anion: 6
Examples: BaTiO3, SrZrO3, SrSnO3

46
Q

What is the summary of Spinel crystal structure?

A

Type: AB2X4
Anion Packing: FCC
Cation: 4(A) 6(B)
Anion: 4
Examples: MgAl2O4, FeAl2O4

47
Q

What are the two types of interstitial positions in ceramics?

A

Tetrahedral position and octahedral position

48
Q

What is the coordination number for tetrahedral positioning? Octahedral?

A

4

6

49
Q

What formula do you use to compute the theoretical density for ceramic materials?

A

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

50
Q

What is a silicate ceramic?

A

A material composed of primarily silicon and oxygen

51
Q

What type of ceramic are most soils, rocks, clays, and sand?

A

Silicate

52
Q

What does the arrangement of a silicate look like?

A

SiO4 (4-)

Each atom of Si 4+ is bonded to four oxygen atoms, which are at the corners of the tetrahedron

53
Q

Are silicates usually considered positively or negatively charged entity’s?

A

Negatively (4-)

54
Q

What is silica?

A

Also known as silicon dioxide (SiO2)

It’s the most simple silicate material

55
Q

What are the three structures of silica?

A

quartz, cristobalite, and tridymite

The picture is cristobalite

56
Q

What are four properties of silica?

A

1) electrically neutral = stable electric structures
2) Si to O atoms 1:2 ratio
3) not closely packed: low density
4) high melting temperature

57
Q

What are silica glasses?

A

Silica existing as a noncrystalline solid

Called fused silica or vitreous silica

58
Q

What is the basic unit of silica glasses?

A

SiO4 (4-) tetrahedron, but there’s a high degree of randomness

The image is sodium-silicate glass

59
Q

What are some oxides that can form glassy structures? What are they called?

A

B2O3, FeO2, SiO2, SiO4

Network formers

60
Q

What are network modifiers? Examples?

A

Cations that are incorporated into the SiO4 network to modify the glass

CaO, Na2O

61
Q

What is a silicate? What are some typical chemical formulas?

A

one, two, or three of the corner oxygen atoms of the SiO4 tetrahedra are shared by other tetrahedra

SiO4, SiO7, SiO9

62
Q

What are some common cations that are added to silicates?

A

Ca 2+, Mg 2+, Al 3+

63
Q

What is a simple silicate?

A

A structurally simple silicate that has an isolated tetrahedra (ex: Mg2SiO4)

64
Q

What is a layered silicate?

A

A 2D sheet/layered structure produced by the sharing of 3 oxygen atoms in each of the tetrahedra

64
Q

What is the repeating unit formula in layered silicates?

A

(Si2O5)^2-

net charge is negative– associated with the unbonded oxygen atoms

64
Q

How is net neutrality established in layered silicates?

A

A second planar sheet structure that has an excess of cations bonds to the unbonded atoms in the Si2O5 sheet

65
Q

What is one of the most common clay minerals with a layered silicate structure?

A

Kaolinite clay: has a 2 layer silicate sheet structure

66
Q

What is Kaolinite’s formula? What are the layers?

A

Si2O5 is a layer and Al2(OH)4 is a layer

67
Q

What is the midplane of Kaolinite made from? What are the different types of bonding?

A

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

68
Q

What are carbon’s two allotropic forms?

A

Diamond and graphite

69
Q

What is diamond?

A

A metastable carbon polymorph at room temperature and atmospheric pressure

70
Q

What is diamond’s crystal structure? What kind of hybridization does each carbon atom have? What are the bonds?

A

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

71
Q

What is the structure of graphite? What are the types of orbitals? Bonding type?

A

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

72
Q

How do vacancy/interstitial defects exist in ceramics?

A

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

73
Q

Is it more common to have a cation or anion interstitial in ceramics?

A

Cations because they are smaller and anions are large and to fit in a small interstitial position requires more strains

74
Q

What is electroneutrality?

A

A state that has equal numbers of positive and negative charges for ions

75
Q

What is the Frenkel defect?

A

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

76
Q

What must be maintained in a defect structure?

A

electroneutrality

77
Q

What is a Schottky defect?

A

A cation-vacancy anion-vacancy pair

78
Q

What must be maintained when impurities are present in ceramics?

A

Electroneutrality (charge balance)

79
Q

How do you determine which host ion an impurity substance will substitute for?

A

It substitutes for the one with the most similar electrically

basically, cations for cations and anions for anions

80
Q

How is electroneutrality maintained for impurity ions having a charge different from that of the host ion for which it substitutes?

A

The formation of lattice defects– vacancies or interstitials of both types (cations or anions)

81
Q

What happens when Ca 2+ wants to be put into a NaCl lattice?

A

It’s a substitutional cation impurity

The Ca2+ substitutes for a Na + and creates a cation vacancy to balance the lattice

82
Q

What happens when O 2- wants to be put into NaCl?

A

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

83
Q

Why are ceramics more brittle than metal?

A

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

84
Q

Is it harder to dislocate metals or ceramics?

A

Ceramics because they are often composed of highly ionic bonds.

Strong and ridged structure, making the movement of dislocations more difficult compared to metals

85
Q

What is the substitute for tensile testing in ceramics?

A

the flexure test

86
Q

What is the ratio of tensile to compressive strengths of ceramics?

A

Tensile strengths are about 1/10 of their compressive strengths

87
Q

What is flexural strength?

A

The stress at fracture using the flexure tests

88
Q

What temperature is used with flexural tests?

A

Room temperature

Usually elastic with brittle failure

89
Q

How do you measure the elastic modulus of ceramics?

A

Flexural tests- 3-Point Bend Testing often used

90
Q

What do you measure in ceramics when behavior is linear-elastic? Brittle fracture?

A

Linear-elastic: elastic modulus
Brittle-fracture: flexural modulus

91
Q

What is the formula for determining the elastic modulus of a ceramic?

A

gamma= midpoint deflection
bd (area of rectangle)
R- radius of cicule

92
Q

What is the formula for determining the flexural strength of a ceramic?

A

bd- area of rectangle
R- radius of circle

93
Q

What are 3 properties of oxide structures?

A

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

94
Q

What are the two factors that determine crystal structure?

A

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

95
Q
A
96
Q

Bla

A

Bla

Elements (15 decks)

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