Diffusion Mechanisms in Solids

Diffusion

• Diffusion is a mass flow process where atoms change position due to thermal and concentration gradients.
• Atoms migrate from higher to lower concentration areas.
• Diffusion in solids occurs, but at a significantly slower rate compared to gases and liquids.
• Point defects are crucial for diffusion in solids.

Diffusion Mechanisms in Solids

• Vacancy Migration: Atoms move from a lattice position to a vacancy.
• Random Walk (Interstitial Diffusion): Atoms move from one interstitial site to another.
• Interstitial diffusion is typically faster than vacancy migration.

Basic Concepts

• Diffusion is a random process, not due to external force.
• Diffusion processes can be either steady-state or non-steady-state.
• Flux is the rate of atoms crossing a unit area perpendicular to a given direction.
• Solvent: The majority atom type.
• Solute: The element with lower concentration.
• Interstitial: Solute atoms located within gaps between host atoms.

Factors Affecting Diffusion

• Temperature: Higher temperature increases atomic mobility and diffusion rate.
• Time: Longer time allows more atoms to move.
• Position in the solid: Diffusion speed varies depending on the location within the material.
• Diffusion mechanisms (vacancy or interstitially, interdiffusion/self-diffusion): Different mechanisms have varying rates.
• Crystal structure: Open structures facilitate diffusion
• Path of diffusion: Surface, grain boundary, volume, and dislocation affect the pathways for atoms.
• Concentration of solute: Usually not the primary factor.

Fick's First Law

• Fick's first law relates diffusive flux to the concentration gradient.
• Flux goes from high to low concentration regions.
• Flux is proportional to the concentration gradient.
• Diffusion coefficient reflects the substance's ability to diffuse through a given area with a concentration gradient.

Modeling Diffusion: FLUX (J)

• Flux (J) is the rate of transfer across a given surface (fluid, particles, or energy).
• Flux is a directional quantity.
• Flux is measurable for vacancies, host atoms and impurities.

Fick's 1st Law (Steady State)

• Steady-state diffusion means the flux doesn't change with time.
• Flux is related to the diffusion coefficient and concentration gradient. The relationship includes a negative sign, indicating flow in the direction of the decreasing concentration.
• Flux can be measured in units of atoms/cm²/s, moles/cm²/s, or equivalents.

Concentration Profiles & Flux

• Steeper concentration profiles result in higher fluxes.
• Fick's first law governs the relationship between flux and concentration gradients.

Steady-State Diffusion

• Steady-state means that the concentration profile doesn't change with time.
• dC/dx (concentration gradient) must be constant in a steady-state system.

Case Hardening

• Case hardening involves diffusing carbon atoms into the surface of iron.
• This process makes the hardened material hard to deform (due to the 'locking' of atomic planes) and hard to crack.
• A common example of case hardening is a hardened gear

PBL-1: Steady-State Diffusion

• A problem illustrating steady-state diffusion calculation, considering a carbon diffusion process in an iron plate.

Diffusion Summary

• Open crystal structures, lower melting temperatures, materials with secondary bonding and smaller atoms diffuse faster.
• Close-packed structures, higher melting temperatures, materials with covalent bonding and larger atoms diffuse slower.
• Cations and lower density materials diffuse faster.
• Anions and higher density materials diffuse slower.

Explore the fundamental concepts and mechanisms of diffusion, focusing on how atoms migrate within solids. This quiz covers critical processes like vacancy migration and interstitial diffusion, emphasizing the differences in rates and conditions. Test your understanding of how concentration gradients influence atomic movement in various states of matter.