Module 3: Ceramics

Atomic bonding in ceramics:

What 4 factors influence atomic bonding in ceramics?

1. Electronegativity

2. Ionic/covalent character

3. Ionic radius and relative sizes of cations and anions

4. Coordination numbers

Factor 1: Electronegativity (1 of the factors which influences atomic bonding in ceramics)

What is electronegativity? Do metals have a high or low electronegativity?

Electronegativity is a measure of an atom's ability to attract shared electrons to itself.

Metals have a low electronegativity.

True or false: ceramics must have an overall positive charge

False, ceramics must have an overall neutral charge

Factor 2: Ionic/Covalent character (1 of the factors which influences atomic bonding in ceramics)

What does atomic bonding in ceramics look like?

• Ranges from purely ionic

• To totally covalent

• Combination of the two

What does an ionic crystal structure look like?

• Cations (+ve) give valence electrons to anions (-ve)

Factor 3: Ionic Radii and Cation/Anion ratio (1 of the factors which influences atomic bonding in ceramics)

What influences an ionic crystal structure?

• crystal must be electrically neutral

• ratio of cation/anion must be less than 1

Factor 3: Ionic Radii and Cation/Anion ratio (1 of the factors which influences atomic bonding in ceramics)

Why must rcation/ranion be less than 1?

• ratio maximizes the number of anion neighbors that a cation has → stable structure

Factor 4: Coordination numbers (1 of the factors which influences atomic bonding in ceramics)

What does a higher coordination number mean in ceramics and why do you want a higher coordination number?

Higher coordination number → larger r_c:r_a ratio

larger r_c:r_a ratio → greater stability within ceramics

What are the three common ceramic crystal structures?

1. AX type

2. AmXp type

3. AmBnXp type

Tell me more about an AX type crystal

• Cations and anions?

• Example

• Type of atomic bonding?

• equal numbers of cations and anions

• One type of cation and one type of anion

• Example: NaCl (Na is the cation, Cl is the anion)

• Highly covalent atomic bonding

Tell me more about an AmXp type crystal

• Cations and anions?

• Example

• Type of atomic bonding?

• charges on the cations and anions are not
  the same, not an equal number of cations and anions

• One type of cation and two types of anions

• Example: Fluorite (CaF2), Calcium is the cation and the two fluoride ions are the anions

Tell me more about an AmBnXp type crystal

• Cations and anions?

• Example

• Type of atomic bonding?

• Two types of cations (A and B)

• Example: Barium titanate (BaTiO3), Ba2+ and Ti4+ are the cations, O3(2-) is the anion

• Barium titanate→perovskite crystal structure

What are silicate ceramics?

• SiO4 tetrahedron

• Covalent because of Si-O bond (strong)

True or false: Silicate ceramics are ionic

False, silicate ceramics are covalent

What is the significance of SiO4-?

SiO4- is the basic unit of silicates

• SiO4- has different 1D, 2D, and 3D arrangements

Tell me more about the SiO2 arrangement?

• Polymorphs if crystalline?

• Densities?

3 polymorphs:

1. Silica

2. Quartz

3. Cristobalite

Polymorphs→relatively low densities but high melting points

Why do polymorphs have relatively low densities but high melting points?

The Si-O interatomic bonds are very strong so they require a lot of energy to break

Silica glasses…tell me more about them

• Atomic randomness

• Basic unit?

• Network formers, modifiers, intermediates

• great disorder

• SiO4- is the basic unit

• Types include network formers, modifiers, and intermediates

What are network formers?

• Backbone

• Building blocks of glass (“backbone”)

• Have a direct influence on the structure of glass

Examples: Silicon, Boron, Phosphorus

What are network modifiers?

• modify, not form

• Don’t form the backbone of glass like network formers;

• Network modifiers sit in the network structure and change it to stabilize the network

Examples: CaO and Na2O

What are intermediates?

• substitute teachers

intermediates substitute for silicon to stabilize the network

What does the addition of network modifiers or intermediates do?

Lower the melting point and viscosity of glass

• Make glass easier to form at lower temperatures

What is the difference between simple and layered silicates?

Layered:

Net charge?

Electroneutrality?

Simple: involve isolated tetrahedra

Layered: 2D sheet

Net negative charge

Electroneutrality: second layered sheet with an excess of cations cancels out the first layered sheet with the net negative charge

True or false: Layered silicates form a 3D structure with the tetrahedra

False: they form a 2D layered structure by sharing 3 O’s in the tetrahedra

True or false: Layered silicates have neutral base units

False- (Si₂O₅)^2- has an unshared oxygen atom that is balanced by a cation-rich planar sheet structure

Are clays examples of simple or layered silicates?

• Answer the question

• Example of clay

• How many layers (if any)

• Charge

• Bonding within the sheet and between the two sheets?

• Layered

• Kaolinite

• 2 layer sheet structure

• Neutral

• Bonding within the two-layered sheet: strong (ionic/covalent)

• Bonding between the two sheets: weak Van Der Waals

Which of the following are examples of network formers in silica glasses?

1. Silicon, boron, and phosphorus

2. Sodium and potassium

3. Aluminum and zinc

4. Calcium and magnesium

(Pick one)

Silicon, boron, and phosphorus

True or false: Network formers form glassy structures and polyhedral oxide structures

True

True or false: Network modifiers form glassy structures and polyhedral oxide structures

False: modifiers sit in the network and modify it

True or false: Intermediates form glassy structures and polyhedral oxide structures

False: intermediates like TiO₂ and Al₂O₃ substitute for silicon to stabilize the network

True or false: Silica glasses are crystalline solids

False, silica glasses are noncrystalline solids with high atomic randomness

Carbon

• 2 famous allotropes?

• Metal, ceramic or polymer?

• Other allotropes?

• Diamond and graphite

• None of the above, graphite is sometimes classified as a ceramic

• Nanocarbons

What is the primary structural difference between diamond and graphite?

1. Diamond has a hexagonal layer structure, while graphite has a 3D tetrahedral network

2. Diamond is held by weak Van der Waals forces, while graphite is bonded covalently

3. Diamond and graphite have identical structures but differ in density

4. Diamond has a tetrahedral network, while graphite has hexagonal layers with weak interlayer forces

(Pick one)

Diamond has a tetrahedral network, while graphite has hexagonal layers with weak interlayer forces

True or false: Graphite has basal plates connected to each other with covalent bonds

False- the basal plates are covalently bonded, but they are connected to each other by Van der Waals forces

Defects in ceramics?

Frenkel and Schottky defects

Defects

What is the difference between a Frenkel and a Schottky defect?

Frenkel: cation leaves its place in the crystal structure and moves to a nearby interstitial site, 1 vacancy

Schottky: cation-anion pair leave their place in the crystal structure, 2 vacancies

Does rcation/ranion change with the presence of Schottky or Frenkel defects?

No

Stoichiometry vs Nonstoichiometry?

• Examples for each

• What does nonstoichiometry induce in a crystal structure?

Stoichiometry: exact rcation/ranion ratio based on the chemical formula

• Example: NaCl (ratio is 1:1)

Nonstoichiometry: deviation from rcation/ranion ratio predicted by the chemical formula

• Example: FeO

• Induces a defect to achieve electroneutrality

Impurities

• 2 types of impurities?

Interstitial and substitutional

Interstitial: impurity atoms occupy interstitial spaces of lattice structure

• ionic radius of impurity must be smaller than the interstitial space!

Substitutional: foreign atom replaces host atom in crystal lattice without changing the overall structure of the material

True or false: Interstitial impurities in ceramics should be the same size as the anion

False, they should be relatively small compared to the anion

True or false: Substitutional impurities in ceramics always substitute for cations

False: they substitute for whichever host ion is most similar (electrically)

Solid solubility

What must the ionic radius and charge be compared to the host atom for a substitutional impurity to achieve solid solubility?

What if the ionic radius or charge is different?

What does the crystal want to achieve?

To achieve solid solubility, the charge and ionic size of the impurities must be nearly the same as the host ion

• If the charge or ionic radius of the foreign atom is different from the host, a defect is induced to achieve electroneutrality

• YOU ALWAYS WANT TO TRY TO DO WHATEVER POSSIBLE TO ACHIEVE ELECTRONEUTRALITY!!

Binary phase diagrams

two components are compounds that
share ___

Oxygen

Stress-strain curve

What conclusions can you draw from this stress-strain curve of aluminum oxide and glass?

The curve for aluminum oxide is steeper than the curve for glass. This means that aluminum oxide has a higher Young’s Elastic Modulus (stress/strain) than glass which corresponds with higher stiffness.

Fracture toughness

What is fracture toughness?

measure of a ceramic
material’s ability to resist
fracture when a crack is
present

Porosity

Increased porosity increases E and reduces strength

False: it reduces both E and strength

How does porosity affect the properties of ceramics?

1. Makes ceramics more flexible

2. Improves electrical conductivity

3. Increases strength and toughness

4. Reduces strength but enhances applicability in filtration

(Pick one)

Reduces strength but enhances applicability in filtration

Properties and applications of glass-ceramics, clay products, refractory ceramics, and abrasives

What is a glass-ceramic?

Glass processed by heat treatment to produce a fine-grained polycrystalline structure

Are all glass-ceramics transparent?

Only some glass-ceramics may be made optically transparent

• only very fine uniform grains are transparent

What are the most attractive attributes of glass-ceramics?

• Ease of fabrication

• Strength and excellent resistance to thermal shock

True or false: Glasses only crystallize above the critical cooling rate

False: they only crystallize below the critical cooling rate

True or false: Shifting transformation curves in glass crystallization affects how glasses crystallize with time

True

True or false: Transformation curves in glass crystallization are shifted by changes in viscosity

False: they are shifted by the use of nucleating agents

True or false: Glass-ceramics have high coefficients of thermal expansion

False: they have low coefficients to reduce cracks from cycling stress (and therefore avoid thermal shock)

True or false: Larger grains increase transparency

False: this will only work if it is a single crystal

True or false: Glass-ceramics are electrical and thermal insulators

True

What are the steps in manufacturing clay products?

1. Clay and water are mixed for shaping

2. Shaped piece is dried

3. Fired at a high temperature to improve mechanical strength

2 classifications of clay products?

1. Structural clay products

2. Whitewares

True or false: Structural clay products have very high structural integrity

True

True or false: Refractory ceramics are more reactive than other ceramics

False: they don’t degrade (even in high temps) and they are inert in extreme environments

Refractory ceramics provide thermal insulation and support mechanical loads

True

True or false: Refractory ceramics are used for products that need to withstand high temperatures

True

True or false: Abrasives are used to cut harder materials

False: they can only grind/cut softer materials

True or false: Abrasives need to be hard, wear resistant, and tough

True

What is the Mohs hardness of most abrasive materials?

7 or greater

True or false: Abrasive particles remove large chunks of material

False: they remove very small amounts of material

Phase transformation of glass ceramics

Most inorganic glasses transform from _____ to ______ at high-temperatures

through ____

noncrystalline to crystalline, nucleation

What is the critical cooling rate?

“just misses the nose”

What happens above the critical cooling rate?

• does recrystallization/nucleation happen?

no, nucleation is prevented

What happens below the critical cooling rate?

nucleation takes place, some glass-ceramic material will form

Glass forming

What are some glass-forming examples?

Pressing, blowing, drawing, fiber forming

Particulate-forming examples

casting (slip/tape), powder pressing

Ceramics are produced by these 3 methods:

• glass-forming (bottles, jars, sheet glass)

• particulate-forming (electronic and magnetic ceramics)

• cementation (dental veneers)

What is the glass transition temperature (Tg)?

temperature @ which amorphous polymer transitions from a hard, glassy state→softer, viscous state

What is the difference between
glass and crystalline solids?

At the melting temperature (Tm), crystalline materials experience a sharp drop in specific volume (crystallization) before they become a crystalline solid.

At the melting temperature (Tm), liquid glass becomes a supercooled liquid and at the glass transition temperature, it becomes hardened glass.

Interpret this graph

Both glass and crystalline solids start out as liquids. At the melting temperature (Tm), liquid glass becomes a supercooled liquid while crystalline solids experience a sharp drop in specific volume (crystallization) and become crystalline solids. The glass transition temperature (Tg) is significant for glass as that is when glass transitions from a supercooled rubbery state to a hard, brittle glass.

True or false: Glass is always amorphous

True

What is the difference between amorphous and crystalline materials?

Crystalline materials: repeating atomic structure

Amorphous materials: random arrangement of atoms in structure

If there is no crystallization, how do
you define Tm?

Viscosity

Is glass crystalline or amorphous?

Glass is amorphous. However, glass can be heat-treated to become crystalline (glass-ceramics)

What are the different points in glass forming and describe each?

Melting point: glass is fluid enough to be a liquid

Working point: glass is easily deformed

Softening point: glass can be handled without causing significant dimensional alternations

Annealing point: glass is heated to relieve any internal stresses that arise because of the glass working process

Strain point: Glass solidifies completely and will fracture if any extra loads are added

True or false: The melting point of glasses is determined by when they become liquids

False, the melting point of glasses is determined by viscosity

True or false: The glass transition temperature is below the strain point

False, the glass transition temperature is above the strain point

What are the points in glass forming from low to high viscosity and high to low temperatures?

Melting→working→softening→

annealing→strain

True or false: Glass-forming should be done between the working and softening points

True, this is called the working point of glass

True or false: At the strain point, glass will plastically deform and then fracture

False, at the strain point, glass will fracture and then plastically deform

Does glass have residual stresses when heated?

No, glass has residual stresses when cooled

True or false: Stresses from cooling can be fixed just above the annealing point

True

How does viscosity influence the temperature-dependent behavior of glass, and where does the glass transition temperature (Tg) fall in relation to viscosity levels?

• Viscosity as glass is heated?

• Viscosity as glass is cooled?

• Viscosity before Tg?

• Viscosity above Tg?

• As glass is heated, viscosity decreases (resistance to flow decreases).

• As glass is cooled, viscosity increases (resistance to flow increases).

• Before Tg (material is glassy), viscosity is higher than above Tg.

• Above Tg (material is rubbery), viscosity is lower than below Tg.

Which of the following statements is true about the viscosity that glass reaches at different points?

1. All statements are true

2. Viscosity at the melting point is very high, i.e. glass is fluid enough to behave as a liquid.

3. Viscosity is very high at the strain point, at which the material becomes so rigid that it will fracture before undergoing plastic deformation.

4. Viscosity at the annealing point is lower than at the softening point, such that atomic diffusion occurs rapidly enough to relieve residual stresses within about 15 minutes at the annealing point.

5. Viscosity at the working point is lower than at the melting point, such that the glass can be easily shaped and deformed at the working point.

(Pick one)

Viscosity is very high at the strain point, at which the material becomes so rigid that it will fracture before undergoing plastic deformation.

What are the steps in the press and blow technique?

1. Put gob into mold

2. Press it to make a concave shape

3. Blow in compressed air to fit the mold

True or false: The float process makes hollow containers

False: the float process makes sheets

True or false: Glass tempering decreases strength

False, it increases strength by inducing compressive surface stresses

True or false: Glass tempering occurs above the softening point

False, it occurs below the softening point

How is glass cooled in glass tempering?

With air or oil

True or false: Powder processing compacts powder into a solid

False: it compacts powder, but it needs to be sintered to be a bonded solid

What is the difference between ceramic jet 3D printing and the stereolithographic technique for 3D printing?

Difference in curing:

• SLA uses a laser to cure

• Ceramic jet 3D printing uses ceramic particles to cure