Ceramics Chapter 12: Structures & Properties

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Questions and Answers

What is the coordination number predicted for FeO based on the ionic radii provided?

6 (correct)

What crystal structure is predicted for FeO?

Face-centered cubic

Cubic close-packed

NaCl (correct)

Body-centered cubic

Which type of sites do cations prefer in the case of MgO?

FCC sites

HCP sites

OH sites (correct)

TD sites

What is the key characteristic of bond hybridization in SiC?

Significant covalent bonding Signup and view all the answers

What hybridization do both Si and C prefer in SiC?

sp3 Signup and view all the answers

What types of compounds mainly comprise ceramics?

Oxides, nitrides, and carbides Signup and view all the answers

Which property distinguishes ceramic materials from metals?

Greater resistance to high temperatures Signup and view all the answers

What is a key characteristic of the bonding in most ceramics?

Predominantly ionic with low covalent contribution Signup and view all the answers

How does the ionic character of ceramic bonds change with electronegativity differences?

Increases with increasing electronegativity difference Signup and view all the answers

Which crystal structure do oxide ceramics commonly exhibit?

Face-centered cubic lattice Signup and view all the answers

Which of the following correctly lists types of ceramics based on their applications?

Traditional and advanced ceramics Signup and view all the answers

What effect do impurities have in the ceramic lattice structure?

They can affect the mechanical properties Signup and view all the answers

What special consideration is often made when testing ceramic materials?

Thermal shock resistance testing Signup and view all the answers

What is the primary factor that stable ionic structures maximize?

Number of nearest oppositely charged neighbors Signup and view all the answers

In the general form A m Xp, what does 'm' represent?

The ratio of cations to anions for charge neutrality Signup and view all the answers

What does an increase in coordination number typically indicate about cation and anion sizes?

Both cation and anion sizes are decreasing Signup and view all the answers

For the structure of CaF2, what is the coordination number of Ca2+ ions?

8 Signup and view all the answers

What is the relationship between ionic radii and the maximum number of anions around a cation?

Smaller cations can accommodate more anions than larger cations Signup and view all the answers

How many F- ions surround each Ca2+ ion in the CaF2 structure?

6 Signup and view all the answers

What type of structure is formed in the zincblende (ZnS) lattice?

Diamond cubic Signup and view all the answers

What describes the site occupancy in the CsCl structure based on the CaF2 ratio?

Only half the cation sites are occupied Signup and view all the answers

What are the three polymorphic forms of crystalline SiO2?

Quartz, crystobalite, and tridymite Signup and view all the answers

At what temperature does silica melt?

1710ºC Signup and view all the answers

Which of the following ions contributes to the structural stability of silicates?

Mg2+ Signup and view all the answers

What is the basic unit of glass structure?

SiO4 tetrahedron Signup and view all the answers

What term describes the structure of common glasses that include impurities?

Amorphous Signup and view all the answers

How are the SiO4 tetrahedra in layered silicates configured?

In two-dimensional layers Signup and view all the answers

What is the charge of the silicate ion SiO4?

2- Signup and view all the answers

What type of clay mineral alternates layers of (Si2O5)2- with Al2(OH)42+?

Kaolinite Signup and view all the answers

What is true about interstitial defects in ceramics?

Cations are the only ions that typically form interstitials. Signup and view all the answers

Which defect is characterized by a paired set of vacancies?

Shottky Defect Signup and view all the answers

How does the tensile strength of CNTs compare to 1040 steel?

It is roughly 250 times higher than 1040 steel. Signup and view all the answers

Which statement is true regarding vacancies in ceramics?

Vacancies can exist for both cations and anions. Signup and view all the answers

What distinguishes a Frenkel defect from a Shottky defect?

Frenkel defects occur when a cation is misplaced. Signup and view all the answers

What must impurities satisfy in a ceramic material to ensure stability?

Charge balance Signup and view all the answers

Which feature of ceramics contributes to their brittle nature compared to metals?

Difficult slippage along slip planes Signup and view all the answers

In a 3-point bend test for measuring the elastic modulus of ceramics, what is typically observed at room temperature?

Elastic behavior Signup and view all the answers

What type of impurity is represented by the presence of Ca2+ in place of Na+ in the NaCl lattice?

Substitutional cation impurity Signup and view all the answers

In terms of defect types, which one does NOT correspond to the example provided for ionic solids?

Interstitial cation Signup and view all the answers

What is a common testing method used to evaluate the properties of brittle materials like ceramics?

3-Point Bend Testing Signup and view all the answers

How do ionic solids generally respond to applied stress due to their structure?

They require excessive energy for slippage. Signup and view all the answers

What is the role of impurities in maintaining the structural integrity of ceramics?

To satisfy charge balance Signup and view all the answers

Study Notes

Chapter 12: Structures & Properties of Ceramics

Ceramics are compounds of metallic and non-metallic elements, primarily oxides, nitrides, and carbides.
Examples include alumina (Al₂O₃), silicon carbide (SiC), silicon nitride (Si₃N₄), and zirconia (ZrO₂).
Ceramics are insulating to electricity and heat.
Ceramics are more resistant to high temperature and harsh environments than metals and polymers.
Ceramics are hard and brittle.
Desirable properties of ceramics are achieved through a high-temperature heat treatment called firing.
Traditional ceramics include china clay, porcelain, bricks, tiles, glasses, and cement.
Advanced ceramics are used in electronic, computer, communication, aerospace, and other industries.

Issues to Address

Structures of ceramic materials: How are they different from metals?
Point defects: How are they different from those in metals?
Impurities: How are they accommodated in the lattice and how do they affect properties?
Mechanical properties: What special provisions/tests are made for ceramic materials?

Ceramic Bonding

Mostly ionic, some covalent bonding.
Percentage of ionic character increases with the difference in electronegativity.
Large vs small ionic bond character: CaF₂ (large) and SiC (small)
The electronegativity values for different elements are provided in a table.

Ceramic Crystal Structures

Oxide structures: Oxygen anions are much larger than metal cations.
Oxygen anions are close-packed in a lattice (usually FCC).
Metal cations occupy the holes in the oxygen lattice.
Size of sites: The cation must fit in the site.
Stoichiometry: All of one type of site must be full, and the remaining must go into other types of sites.
Bond hybridization: Significant covalent bonding can have an impact if significant covalent character is present.

Ionic Bonding & Structure

Stable Structures: Maximize the number of nearest oppositely charged neighbors
Charge neutrality: Net charge in the structure should be zero.
General form: AmXp; m and p are determined by charge neutrality.

Coordination # and Ionic Radii

Coordination number increases with the cation/anion ratio.
The ionic radii for different elements are provided.

AX Structures

Consider CaF₂: r cation/r anion = 0.100/0.133 ≈ 0.8
Based on this ratio, coordination number is 8 and the structure is CsCl.
Only half the cation sites are occupied in the CsCl structure.

Cation Site Size

Determine minimum r cation/r anion for OH site (C.N. = 6).
r cation = (√2 - 1)r anion = 0.414r anion

Site Selection II: Stoichiometry

If all of one type of site in a unit cell is full, the remainder must go into other types of sites.
Example: FCC unit cell has 4 O₁ sites and 8 T₁d sites.

Site Selection III: Bond Hybridization

Significant covalent bonding – hybrid orbitals have an impact.
Example: SiC
% ionic character ≈ 11.5% for SiC.
89% covalent bonding in SiC.

Example: Predicting Structure of FeO

Based on ionic radii, the predicted crystal structure for FeO is NaCl.
The ionic radii for different cations and anions are given in a table.

MgO and FeO

Both MgO and FeO have the NaCl structure.
The ratio of rMg/ro = 0.514.

AX Crystal Structures

AX-type Crystal Structures include NaCl, CsCl and zinc blende.
Cesium chloride structure: r Cs+/r Cl- = 0.939.
Cubic sites are preferred in the CsCl structure.
Each Cs⁺ has 8 neighboring Cl⁻.

Zinc Blende Structure

Size arguments predict Zn²⁺ in O㬫 sites, not observed.
Zn²⁺ occupies T₁d sites in the observed structure.
Bond hybridization of Zn favors T₁d sites.
Each Zn²⁺ has 4 neighboring O²⁻.

AX₂ Crystal Structures: Fluorite Structure

Calcium Fluoride (CaF₂) - cations in cubic sites.
UO₂, ThO₂, ZrO₂, CeO₂ display anti-fluorite structure (anion and cation positions reversed).

ABX₃ Crystal Structures: Perovskite Structure

Perovskite structure example: BaTiO₃
Cations (Ba²⁺ and Ti⁴⁺) and anions (O²⁻) are arranged in a cubic structure.

Ceramic Density Computation

Density (ρ) calculation formula is provided: ρ = n(ΣAc + ΣAa)/Vc.NA
n is the number of formula units per unit cell
ΣAc is the sum of atomic weights of cations
ΣAa is the sum of the atomic weights of anions
Vc is the volume of the unit cell.

Silicate Ceramics

Crystalline SiO₂ (silica) has three polymorphic forms: quartz, crystobalite, and tridymite.
The strong Si–O bond leads to a high melting point (1710°C).

Silicates

Bonding of adjacent SiO₄⁴⁻ accomplished by sharing common corners, edges, or faces.
Presence of cations (e.g., Ca²⁺, Mg²⁺, Al³⁺) maintains charge neutrality and ionically bonds SiO₄⁴⁻ to one another.

Glass Structure

Glass is noncrystalline (amorphous).
Fused silica is SiO₂ with no impurities.
Common glasses contain impurity ions (e.g., Na⁺, Ca²⁺, Al³⁺, B³⁺).
Quartz is crystalline SiO₂.

Layered Silicates

Layered silicates (clay silicates): SiO₄ tetrahedra connect to form a two-dimensional plane.
(Si₂O₅)²⁻ is an example of a layered silicate.
Cations are needed to balance the charge.
Kaolinite clay alternates (Si₂O₅)²⁻ layers with Al₂(OH)₄²⁺ layers.

Mechanical Properties

Ceramics are brittle because slippage along slip planes is difficult due to high energy needed to move one anion past another.
Three-point bend testing is often used to determine elastic modulus and flexural strength.

Measuring Elastic Modulus

Room-temperature behavior is usually elastic, with brittle failure.
Three-point bend tests are often used because tensile tests are difficult for brittle materials.
Elastic modulus (E) is calculated using the formula: E = FL³/4bd³.
Elastic modulus (E) is also calculated using the formula: E = FL³/12R⁴.

Measuring Strength

Three-point bend test used to measure room temperature strength.
Flexural strength (σfs): σfs = 1.5FL/bd² for rectangular cross-sections, and σfs = 1.5FL/πR³ for circular cross-sections.

Measuring Elevated Temperature Response

Elevated temperature tensile tests (T > 0.4Tm) are used to measure creep properties.
Creep test: Steady-state creep rate (ε̇ss) is calculated as the slope of the creep curve.
Generally, steady-state creep rate of ceramics is lower than that of metals and polymers.

Point Defects in Ceramics

Vacancies exist for both cations and anions.
Interstitial defects for cations are common but rare for anions (because anions are large relative to interstitial sites).

Defects in Ceramic Structures

Frenkel Defect: cation is out of place
Shottky Defect: a paired set of cation and anion vacancies
Equilibrium concentration of defects

Imperfections in Ceramics

Impurities must satisfy charge balance (electroneutrality).
Substitutional cation or anion impurities alter the initial crystal geometry.

Ceramic Phase Diagrams

Example: MgO-Al₂O₃ diagram showing different phases and their compositions at various temperatures
Diagram shows how the phases evolve depending on the composition of MgO and Al2O3 and temperature.

Carbon Forms

Carbon black – amorphous; surface area ≈ 1000 m²/g
Diamond – tetrahedral carbon; hard, brittle, used for cutting tools; large diamonds are jewelry; small diamonds are manufactured for cutting tools and polishing diamond films.
Graphite – layer structure; aromatic layers; weak van der Waal's forces between layers, planes slide easily, good lubricant.
Fullerenes/carbon nanotubes – sheets of graphite curved into balls or tubes; examples: Buckminsterfullerene (C₆₀) and other carbon structures.
Nanotubes may be wrapped sheets of graphite that have many uses in applications.

Carbon Nanotube (CNT)

Recently discovered polymorph of carbon.
Molecular structure: single sheet of graphite rolled into a tube and capped with C60 hemispheres; diameters are nano-level (less than 100 nm).
High tensile strength (50-200 GPa), elastic modulus (TPa), and fracture strain (5-20%).

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Description

This quiz explores the fundamental aspects of ceramics as discussed in Chapter 12. It covers the structures and properties that differentiate ceramics from metals, including their resistance to heat, electrical insulation, and brittleness. Additionally, the quiz will delve into point defects and the accommodation of impurities in ceramic materials.