Impurities in Solids Quiz

Impurities in Solids

• Impurities are atoms different from the host atoms
• May be intentional or unintentional
• All real solids are impure
• Very pure metals are ~99.9999% pure
• Typically one impurity per 10⁶ atoms

Examples of Impurities

• Carbon added to iron creates steel, which is stronger than pure iron
• Boron or phosphorus added to silicon changes its electrical properties (doping)

Point Defects

• Vacancies - a lattice position that is vacant because the atom is missing
• Interstitials - an atom that occupies a place outside the normal lattice position
  • May be the same type of atom as the others (self-interstitial) or an impurity atom (interstitial atom)

Types of Point Defects

• Vacancy
• Self-interstitial
• Interstitial impurity
• Substitutional impurities

Properties of Self-Interstitials

• Produce large distortions in the surrounding lattice
• The energy of self-interstitial formation is about 3 times larger than the energy of vacancy formation
• Equilibrium concentration of self-interstitials is very low (less than one self-interstitial per cm³ at room temperature)

0D Point Defects

• Atoms missing or in irregular places in the lattice
• Include vacancies, interstitials, and impurities

1D Linear Defects/Dislocations

• Groups of atoms in irregular positions
• Include edge dislocations and screw dislocations

2D Planar Defects

• Interfaces between the homogeneous regions of the material
• Include external surfaces, grain boundaries (tilt and twist)

3D Bulk/Volume Defects

• Extended defects (pores, cracks)
• Atomic vibrations

Solid Solutions

• Made of a host (solvent or matrix) that dissolves a minor component (solute)
• Solubility is the ability to dissolve
• Solvent: in an alloy, the element or compound present in greater amount
• Solute: element or compound present in lesser amount
• Solid solution: homogeneous, crystal structure is maintained, contains randomly dispersed impurities (substitutional or interstitial)
• Second phase: as solute atoms are added, new compounds/structures are formed, or solute forms local precipitates

Alloys

• Deliberate mixtures of metals
• Example: Sterling silver is a 92.5% silver, 7.5% copper alloy. It is stronger than pure silver
• Other examples include gold alloys (yellow, red, rose, white, etc., each with a different composition of metals)

Factors for High Substitutional Solid Solubility

• Atomic size factor: the solute and solvent atomic radii should be within ~15% of each other
• Crystal structures of solute and solvent should be the same
• Electronegativities of solute and solvent should be comparable
• Generally, solubility increases if solute has a higher valence than solvent

Factors for High Interstitial Solid Solubility

• For FCC, BCC, HCP structures, the voids (interstices) between the host atoms are relatively small
• The atomic radius of the solute should be significantly less than the solvent
• Usually the max solute concentration is ≤10% (Example: 2% for C-Fe)

Composition/Concentration

• Composition can be expressed as weight percent (wt%) or atom percent (at%)
• Weight percent is useful for making solutions
• Atom percent is useful for understanding the material at the atomic level

Composition/Concentration Examples

• Calculating weight percent involves comparing the weight of the particular element to the total weight of the alloy.
• Calculation of atomic percent involves comparing the number of moles (or atoms) of a particular element to the total number of moles (or atoms) in the alloy.

Linear Defects

• The interatomic bonds are significantly distorted near the dislocation line (dislocation core)
• Dislocations create small elastic deformations in the lattice

Burgers Vector

• Describes the size and direction of the main lattice distortion caused by a dislocation

Edge Dislocations

• Burgers vector perpendicular to the dislocation line

Screw Dislocations

• Burgers vector parallel to the dislocation line

Mixed/Partial Dislocations

• Dislocations are often split into partial dislocations, with cores spread over a larger area, adding to the complexity of real defect structures

Planar/Interfacial Defects

• External Surfaces: surface atoms have unsatisfied atomic bonds, higher energy than bulk. Areas tend to minimize (e.g., liquid drop). Solids can reconstruct to satisfy atomic bonds.
• Grain Boundaries: Polycrystalline materials are composed of many small crystals (grains) with different crystallographic orientations. Mismatch exists in the regions where grains meet, called grain boundaries.

Grain Boundaries - Tilt and Twist

• Tilt boundary: an array of aligned edge dislocations
• Twist boundary: the boundary region consisting of arrays of screw dislocations

Grain Boundaries - Twin Boundaries

• Low-energy twin boundaries with mirrored atomic positions across the boundary
• Produced by deformation of materials. Allows shape memory metals to recover their original shape when heated to high temperatures.

Bulk/Volume Defects

• Pores: greatly affect optical, thermal, and mechanical properties
• Cracks: greatly affect mechanical properties
• Foreign inclusions: greatly affect electrical, mechanical, and optical properties

Atomic Vibrations

• Heat causes atoms to vibrate
• Vibration amplitude increases with temperature
• Melting occurs when vibrations are sufficient to rupture bonds