Ceramics Chapter 12: Structures & Properties

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%).

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.