Diffusion in Solids

• Diffusion is the movement of particles from an area of high concentration to an area of low concentration

Diffusion in Solids

• Diffusion can occur in gases, liquids, or solids
• In solids, diffusion results from the thermally activated random motion of atoms
• Atoms vibrate and move within the material unless it is at absolute zero temperature (zero Kelvin)

Examples of Diffusion in Solids

• Chalk particles diffuse into a blackboard making it difficult to clean after a prolonged time
• Atoms from one metal can diffuse into the lattice of another forming a solid solution or alloy when two metals are heated
• Zinc atoms diffusing into a copper lattice is an example of brass formation

Importance of Diffusion in Solids

• Diffusion is crucial because it allows atoms to move within a solid material
• Diffusion enables compound formation, finding lattice positions, and the movement of charges and defects
• Diffusion significantly impacts material properties
• Plays a crucial role in metallurgy with recrystallization, precipitation, and mechanical strength
• Diffusion ultimately affects the performance of manufactured materials across various engineering applications

Diffusion Mechanism

• Diffusion mechanisms describe how an atom moves from one position in the lattice to another, generally through an activated process that sees the ion move over an energy barrier

Two Main Diffusion Mechanisms

• Vacancy Mechanism
• Interstitial Mechanism

Vacancy Mechanism

• A dominant process for diffusion in FCC (Face-Centered Cubic), BCC (Body-Centered Cubic), and HCP (Hexagonal Close-Packed) metals and solid solution alloys
• Activation energy for the process includes the energy to create a vacancy and move an atom into it
• Vacancy diffusion occurs when atoms move into adjacent vacant sites
• Diffusion causes concentration changes over time
• Vacancies are continually created and destroyed at surfaces, grain boundaries, and interior positions like dislocations
• The rate of diffusion increases rapidly with increasing temperature

Interstitial Mechanism

• Interstitial mechanism, where an atom changes positions using an interstitial site does not usually occur in metals for self-diffusion but is favored when interstitial impurities are present because of the low activation energy
• Interstitial solutions occur when a solid consists of two or more elements with significantly different atomic radii
• Larger atoms occupy lattice sites, while smaller atoms fit into the voids (interstices) created by the larger atoms
• Activation energy is associated with interstitial diffusion
• To reach a vacant site, an atom must squeeze past neighboring atoms, with energy supplied by the vibrational energy of the moving atoms

Law of Diffusion

• Fick’s First Law (Steady-State Diffusion): describes diffusion as a mass flow process where atoms or molecules change positions relative to neighbors in a given phase due to thermal energy and a gradient
• Fick’s First Law: J = -D (dc/dx)
• Fick’s Second Law (Nonsteady-State Diffusion): Most practical diffusion situations are nonsteady-state
• Diffusion flux and gradient concentrations vary with time

Applications of Diffusion

• Heat Treatment of Metals: Carburizing uses diffusion, where carbon atoms diffuse into the surface of steel to improve its hardness and wear resistance
• Semiconductor Manufacturing: Dopants such as phosphorus or boron diffuse into silicon to control electrical properties like conductivity in semiconductors
• Battery Technology: Ion Diffusion plays a key role in the movement of ions within lithium-ion batteries, affecting their charge and discharge cycles
• Corrosion: Diffusion contributes to the transport of oxygen and moisture in corrosion, leading to the degradation of metals, like the formation of rust on iron
• Surface Coatings: protective elements like chromium or aluminum are diffused onto the surface of a material to enhance it, which improves wear resistance or corrosion resistance
• Alloying: Diffusion allows for the uniform distribution of elements within the metal matrix in metal alloys, improving mechanical and thermal properties