Materials T5 - Ceramics

Ceramic Composition

Mixture of metal and non-metal with mixed bonds (ionic and covalent)

Cation

Positively charged ion with missing e- and smaller atomic radius

Anion

Negatively charged ion with extra e- and larger atomic radius

Ionic Crystal Composition

Determined by charge neutrality and physical stability

Less Stable

Effect of greater size difference in atoms within ionic bond

AX-Type Crystal Structure

1:1 ratio of cations and anions, e.g. NaCl

A_mX_p-Type Crystal Structure

1:2 ratio of cations and ions to maintain charge neutrality, e.g. CaF₂

A_mB_nX_p-Type Crystal Structure

Has more than one type of cation, e.g. BaTiO₃

Rock Salt Structure

AX-type with FCC anion packing (6 cations, 6 anions)

Cesium Chloride

AX-type with simple cubic anion packing (8 cations, 8 anions)

Zinc Blende (sphalerite)

AX-type with FCC anion packing (4 cations, 4 anions)

Fluorite

AX₂ with simple subic anion packing (8 cations, 4 anions)

Perovskite

ABX₃ with FCC anion packing (12 A cations, 6 B cations, 6 anions)

Spinel

AB₂X₄ with FCC anion packing (4 A cations, 6 B cations, 4 anions)

Anion Vacancy

Missing anion

Cation Vacancy

Missing cation

Cation Interstitial

Additional cation in interstitial space

Frenkel Defect

Cation moved into another interstitial (vacancy + cation interstitial)

Shottky Defect

Equal amount of cations and anions missing, charge neutrality maintained

Intertitial Impurity Atom

Additional cation of different element

Substitutional Impurity Atom

Anion substituted with different element

Effect of Porosity

Changes fracture behaviour, detrimental for tension as cracks are pulled apart

Compression

Best type of stress for ceramic

Brittle Fracture

Type of fracture experienced by ceramics before plastic deformation

Reasons for Brittle Fracture

Ceramics have restricted slip, electrostatic repulsion difficult to break, and covalent/ionic bonds are very strong

Slipping of Amorphous Ceramics

Impossible due to not having crystal planes

Flexural Strength

Maximum stress when break occurs (similar to UTS)

Weibull Modulus

Width of stress/fracture frequency graph, wider = ceramic

Concrete

A composite comprised of cement, aggregate and water

Hydration of Cement

Water is added to cement, causing minerals to dissolve

Cement Setting

C-S-H crystals interconnnect and form solid structure

Interfacial Transition Zone

Transition between aggregate/cement

Properties of Interfacial Transition Zone

Has larger crystals + porosity and weaker than cement bulk (cracks follow transition zone during fracture)

Increased Relative Strength

Higher curing temperature

Silica Glass

Amorphous network former composed of SiO₂

Fused Silica

Glass made of pure SiO₂, silicon atom connected to tetrahedrom of oxygem atoms

Si-O Bonds

Strong, high melting temperature (T_m)

Oxide Additive in Glass

Breaks network bonds and becomes network modifier, which lowers melting point and decreases viscosity

Carbon Black

Amorphous carbon

Diamond

Covalently bonded carbon in cubic structure

Graphite

Hexagonal close-packed carbon structure

Graphite Structure

Covalent bonds within sheets, Van der Waal’s forces between sheets

Graphene

Single sheet of graphite

Highly Conductive

Conductivity of graphite due to p-orbitals forming cloud of delocalised e- on surface

Causes of Graphite Strength

Covalent bonding, short bond length, and no grain boundaries/porosity