ENR116 Module 4: Ceramic Structures

• Module 4 of ENR116 Engineering Materials covers Non-Metals and Corrosion.
• The presentation covers an introduction to Ceramic Structures.

Intended Learning Outcomes

• Identify how crystal structures of ceramics differ from metals.
• Describe a range of crystal structure formulations and arrangements.
• Understand why knowing a materials crystal structure is important for designing materials.

Ceramic Bonding

• Atomic bonding ranges from purely ionic to a mixture of ionic and covalent.
• % ionic character increases with increasing difference in electronegativity of atoms.
• Ceramics are inorganic, nonmetallic compounds between metallic and nonmetallic elements.
• Interatomic bonds are either totally ionic or predominantly ionic with some covalent character.
• The percentage of ionic character depends on the electronegativity difference between bonded elements.
• Calcium fluoride (CaF2) has a large percentage of ionic character (89%).
• Silicon carbide (SiC) has a small percentage of ionic character (12%).

Ionic Bonding & Structure

• Stable ceramic structures maximize the number of nearest, oppositely-charged neighbors.
• The Net charge in a ceramic structure should be zero.
• Ceramic compounds with high ionic bonding consist of a cation and an anion.
• The cation is a metallic ion with a positive charge, having given up valence electrons.
• The anion is a nonmetallic ion with a negative charge, having accepted valence electrons from the cation.
• Cations are smaller than anions due to giving up valence electrons.
• Cation and anion characteristics define the resulting crystal structure of the ceramic compound.
• The relative size and charge of the ions are key factors.
• Stable structures maximize the number of nearest neighbor oppositely-charged atoms.
• Atom charges define the ratio of cation to anion atoms needed to balance the total charge.
• The general formula for ceramic compounds is AmXp, where m and p are determined by charge balance.

Coordination Number and Ionic Radii

• The Coordination Number is the number of nearest neighbors an atom has.
• The Coordination Number increases with an increase in the relative size of the cation to anion.
• Cation and anion sizes are defined by ionic radii.
• The Coordination Number can be determined geometrically by packing anion spheres around a central cation.
• The ratio of ionic radii predicts the Coordination Number and unit cell structure of the ceramic compound.
• ZnS unit cells are tetrahedral.
• NaCl unit cells are octahedral.
• CsCl unit cells are cubic.

Predicting Structure

• The ratio of ionic radii can predict the Coordination Number, as illustrated by the CaO example.
• Ca2+ ionic radius is 0.100 nm, O2- ionic radius is 0.140 nm, ratio is 0.714.
• Based on this ratio, the Coordination Number is predicted to be 6, indicating a rock salt type structure.

Types of Crystal Structures

• Ceramic compounds are labelled using the AmXp convention.
• Variable m and p capture the relative charge of the atoms.
• Structure depends on the relative size of the cation and anion.
• The first category is where m equals p, indicating the cation and anion have the same charge magnitude.
• Examples are sodium chloride, cesium chloride, and zinc sulfide.
• The second category is where m doesn't equal p, indicating different charge magnitudes.
• Examples are calcium fluoride and zirconium oxide (zirconia).
• The third category is AmBnXp, indicating the ceramic compound is made from mixing three different elements.
• An example from this category is barium titanate.

AX Structures

• In the first category (m=p), AX ceramic compounds exists
• These compounds have several observed structures for different cation/anion combinations of the same charge magnitude.
• The most common AX structure is the sodium chloride type, aka rock salt.
• Both the cation and anion have a coordination number of 6.
• The structure is an interpenetrating network of face-centered cubic lattices of cations and anions.
• The ionic radius ratio of cation to anion lies between 0.414 and 0.732 e.g. NaCl it is 0.564.
• Magnesium oxide and iron oxide also have a sodium chloride or rock salt structure
• Cesium Chloride Structure
• It may appear to be a body-centered cubic structure, but the central atom is a cesium atom.
• The ionic radius ratio of the cation to anion is 0.939, thus structure is cubic.
• Each cesium atom has a coordination number of 8.
• If the positions of cations and anions swapped within the crystal structure, the cubic structure is still maintained.
• Zinc Blende (ZnS) Structure
• The final AX ceramic compound structure is zinc sulfide.
• The predicted structure is octahedral.
• The zinc ions are found in a tetrahedral structure.
• The interatomic bonds is low in ionic character, and are highly covalent.
• To satisfy the electronic structure of zinc, four lone pairs of electrons are donated by the anions.
• Each zinc atom has four neighboring sulfur atoms, hence the tetrahedral structure.
• This is similarly observed for other ceramics where the charge is +2 on the cation and -2 on the anion.
• Examples include zinc oxide and silicon carbide.

AmXp structures

• Calcium fluorite is in this category, with the charge on calcium being 2+ and each fluorine -1
• In that case m = 1 and p = 2 to obtain charge neutrality
• For those atoms, the ratio of the ionic radius of the cation to anion is 0.8
• This leads to a coordination number for the cation of 8
• The crystal structure is the same observed for the cesium chloride ceramic in the AX category
• Zirconium oxide (cubic zirconia) is another example of an oxide in this category.

AmBnXp structures

• Involves the mixing of three different elements, where A and B are typically cations.
• Perovskite is the general name given to the observed crystal structure within this category of ceramic compounds.
• Example: A ceramic compound having the Perovskite crystal structure is barium titanate
• Perovskite structure illustrated is cubic.
• Interesting and useful electromechanical properties that can be exploited in certain applications are found in compounds having this structire.

Crystal Structures from the close packing of anions

• Crystal structure can be defined by close packing of the anions.
• Anions are close packed within a layer, and then subsequent layers arranged to build up the three dimensional structure.
• Layers can be close-packed in face-centered cubic or hexagonally close-packed arrangements (ABCABC or ABABAB layer sequences).
• Cations reside in interstitial positions between the anion layers.
• Cations can reside in tetrahedral or octahedral positions.
• To describe ceramic compounds crystal structure using this method, you can examine the rock salt crystal structure ie. sodium chloride
• Chlorine anions pack in a face centred cubic structure
• To achieve a coordination number of six, the sodium cation must reside in the octahedral position.

Ceramic Density Computation

• Density is an important property in defining material performance.
• Density of a ceramic can be theoretically determined similarly.
• n' =number of formula units within the unit cell
• where the formula unit is the total number of atoms (not type of atoms) that make up the chemical formula
• For instance, in the typical unit cell of rock salt there are four sodium atoms and four chlorine atoms.
• Since the chemical formula has one sodium and one chlorine, there are four formula units per unit cell.
• Vc is the unit cell volume.
• and Na is Avogadro's number, 6.022 times 10 to the power 23 formula units per mole of material.

Silicate ceramics

• Silicates are the most abundant elements in the Earth's crust.
• Silicates consists of soils, rocks, clays, and sand fall.
• They are described by the various arrangements of an SiO4 tetrahedron rather than the geometry of the unit cell,.
• The ratio of the ionic radius of the cation to anion is approximately 0.286,
• The interatomic bond between Si and O is highly covalent.
• The most simple of the silicates is silicon dioxide, or silica.
• Silica examples are quartz, crystobalite and tridymite.
• Atoms are not closely packed together.
• Quartz at room temperature has a density of 2.65 grams per cubic centimeter
• Si-O bond is quite strong, which is reflected in a relatively high melting temperature of 1710°C
• Silica Glasses
• Silicates also form silica glasses.
• These noncrystalline compounds consist of silicon and oxygen and are also named amorphous silica.
• The high degree of randomness in the atomic arrangement is more characteristic of a liquid than a solid
• Hydrogen atoms counter the “dangling” bonds.
• Materials that form such glassy structures are termed network formers.
• Network modifiers are oxide additives.
• Borosilicate glass commercially known as Pyrex is a composition where boron is used as a cation to balance the SiO4 charge.
• Combinations of SiO4 tetrahedra, which share one, two or three of the oxygen atoms give rise to other silicates
• Cations achieve charge neutrality and to provide ionic bonding between the different SiO4 tetrahedra.
• Clay silicates involves layered silicates (clay silicates)
• The interatomic bonding within a given sheet is strong and has reasonable ionic character.
• Bonding between sheets are defined by hydrogen and oxygen interation
• Kaolinite is an example of a common clay mineral

Carbon forms

• Carbon Black
• Carbon is used in a variety of forms.
• Carbon black is the amorphous form of carbon.
• It's readily used in inks and carbon toners as black for printing.
• Can also exist as diamond (zinc blende type structure).
• The change in material properties between the amorphous and well-ordered form results in a significant difference in the cost.
• the atoms within a layer are arranged in a hexagonal pattern.
• Graphite has strong covalent bonding between neighboring carbon atoms.
• Graphite has a weak van der Waal's forces.
• Graphite is a good lubricant.
• Fullerenes and Nanotubes
• It exists when a sheet of the graphite is wrapped upon itself to form a ball or tube

Summary

• Ceramic crystal structures are based on maintaining charge neutrality, the ratio of ionic radii, and the ionic character of bonding.
• Silicates and carbon display a range of different structures.
• Compare and contrast these different structures within either the silicates or carbon demonstrates why knowing a material's structure is very useful and important when selecting or designing materials for a given application.