Crystal structure and defects Flashcards
What are the primary interatomic bonds? (3)
1) Ionic
2) Covalent
3) Metallic Bonds
What are the secondary interatomic bonds? (2)
1) Van der Waals
2) Hydrogen bonding
What is ionic bonding?
Transfer of electrons creates electrically charged ions which are attracted together.
What is covalent bonding?
Sharing of valence electrons between adjacent atoms.
What are metallic bonds?
Valence electrons not bound to any atom in the solid, firing a sea of electrons or an electron cloud.
What are Van der Waal bonds?
Considerably weak compared to primary ones; arise from atomic or molecular dipoles.
What is hydrogen bonding?
A special type of bonding where hydrogen is involved as one of the constituents.
What are atoms and ions separated by?
An equilibrium distance (r0) where the interatomic bonding energy is minimum (E0).
What is the net force at equilibrium distance?
Zero (attractive=repulsive).
Define bonding energy.
The energy required to separate two atoms to infinite separation.
What do materials which high bonding energies have?
High melting temperature and therefore high strengths.
What dictates the shape of the force or energy vs interatomic separation curve?
1) Modulus of elasticity or mechanical stiffness.
2) Linear coefficient of thermal expansion
Define Linear coefficient of thermal expansion.
How much materials expand upon heating and contract upon cooling.
How do atoms form a solid structure?
1) Through Crystalline materials- periodic arrangement of atoms over long distances, therefore, long-range order.
2) Through non-crystalline or amorphous materials.
What type of materials form crystalline structures? (3)
1) All metals
2) Many ceramics
3) Some polymers
What is a non-crystalline or amorphous material?
Short-range order, sd long-range order do not exist. They are complex structures that rapidly cool.
What does a crystal structure describe?
Describe the manner of how atoms, ions and molecules are spatially arranged.
What is lattice?
A 3D array of points coinciding with atom positions.
What are unit cells?
Smallest repetitive entities (building units).
What builds a crystal?
Repeated translation of unit cells.
What is the hard-sphere model?
Atoms or ions are thought of as being solid spheres ave well-defined diameters.
How many crystal systems are there?
7
What happens when you combine one of the crystal systems with and entering?
Creates 14 bravos lattices such as BCC (Body-centered-cubic).
What are the lattice parameters?
a,b & c in x,y & z.
What are the interaxial angles?
Angles between the two edges: angles between a and b is gamma, b and c is alpha and a and c is beta.
How are crystalline structures formed?
From solidification from a liquid.
What forms first when a crystalline structure forms?
The nucleus, consisting of only a few unit cells. The nucleus grows by the addition of unit cells forming a larger crystal. (crystallite).
When multiple nuclei form at the same time what is the orientations of these?
All different.
How are grains formed?
When different crystallites impinge on each other, they stop growing forming a structure consisting of multiple crystallites called grains.
What are the unit cells of grains?
Misaligned at their interfaces calling them “grain boundaries”
Define a polycrystalline structure?
Structure made of multiple crystalline grains.
What are vacancies?
Missing atoms leaving holes in the lattice reducing mechanical strength of the material.
What are the 3 defects in crystal lattices?
1) Point defects
2) Line defects
3) Surface defects
What is a point defect also known as?
zero-dimensional defect concerning the position of one or, pre isolated atoms in a crystal lattice.
What do point defects do?
Tune the properties of a material.
What is an interstitial atom defect?
An extra atoms/ion inserted at an interstitial site, small void space not usually occupied.
What are the properties of interstitial atoms?
Smaller than the ones located in the lattice sites.
What is the substitutional defect?
A lattice atom or ion is replaced by a different atom/ion.
What are the properties of the substitutional atoms?
Can be larger or smaller than the host atoms in the structure.
What do substitutional atoms do?
Increase the strength of metallic materials.
What is the Frankel defect?
A vacancy interstitial pair, formed when a lattice atom jumps to an interstitial site, leaving behind a vacancy.
What is the Schottky defect?
Removal of a stoichiometric number of anions and cations to preserve the electrical neutrality.
What are line defects also called?
One dimensional defects or dislocations.
What are the two types of line defects?
1) Edge dislocation
1) Screw dislocation
What is the typical movement of an edge dislocation?
The sliding of dislocation on straight planes called lattice planes.
What do dislocations contribute to?
Strain hardening a macroscopic phenomenon.
What is the material surface?
A crystal abruptly ends, the atomic bonding is disrupted and each atom at the surface has no longer got a proper coordination number.
Define Grain boundaries.
Narrow zones where the atoms are not properly spaced, separating the individual grains.
What happens if you reduce the grain size?
Increase the grain boundaries leads to an increase in materials strength.
What is the Hall-Petch equation?
σy = σ0 + kd^(-1/2) where: σy = yield strength σ0 and k = material constant d = average grain diameter
What are stacking faults?
An error in the stacking sequence of close-packed planes.
What are twin boundaries?
A plane across which there is a mirror image disorientation of the crystal structure.
Define strain hardening?
Process of strengthening a material by deforming it increases dislocation density.
How does grain size strengthening occur?
Increasing the number of grains or reducing grain size.
What are important in semi-conductors to achieving the desired performance?
Dopants
How can we produce dramatic colour changes in the original material?
Adding small amounts of foreign atoms. e.g. coloured lass.
What effect do point defects have?
Dramatic effects on optical, magnetic and electrical properties of materials.
