CHEMMAT 204 - Module 3

Part 1: Defect in solids, Part 2: Solid state diffusion

What are the three types of point defects?

1. Vacancy (absence of atoms from normal lattice sites)

2. Self - interstitial atom

3. Frenkel defect ie lattice atom moves to adjacent interstitial site which leaves behind a vacancy.

How is a vacancy formed?

• A vacancy is a tiny defect in a crystal where an atom is missing from its normal position.

• An atom gains enough thermal energy to leave its site, creating an empty spot; this requires energy known as the formation energy.

• Atoms moving into these vacancies must overcome an energy barrier called the Activation Energy (Ea) through Thermal Activation.

• Therefore, vacancy formation and movement increase with temperature.

Why is the presence of vacancies energetically favourable?

• Increases entropy (disorder).

• This change in free energy is a combination of the energy consumed to produce a vacancy and the increase in entropy associated with the newly formed vacancy.

What is free energy?

G = H - TS

• The internal energy (H) of a crystal is the sum of Ek and Ep of all the atoms within a structure.

• Ek - vibration of atoms, proportional to temperature

• Ep - interatomic bonds (chemical energy) and bond energy (strain energy)

• Hidden energy - relates to the entropy (S) or degree of order/disorder that reflects the required energy associated with structure changes.

Therefore, Gibbs free energy increases with internal energy or decrease in entropy.

What is this graph?

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What are experimental measurement of vacancy concentrations in a crystal lattce?

1. Differential dilatometry - measuring vacancy concentration by

2. Changes in resistivity - electrical resistivity is sensitive to imperfections due to reduced lattice periodicity, which interfere with electron passage. At low defect concentration, resistivity and defect concentration are proportional.

3. Transmission electron microscopy

What is solid state diffusion?

Matter is transported both through itself and into another solid via atomic movement.

What does the diagram of solid state diffusion look like?

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Mechanisms of diffuion

What is the influence of temperature on diffusivity (D)?

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Where does diffusion take place?

Through movement of vacancies or interstitial atoms

Why does BCC have a faster or slower diffusion than FCC?