Diffusion Mechanisms in Solids

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Questions and Answers

What is the primary driving force behind diffusion in solids?

Concentration of the solute

Random movements of atoms (correct)

Presence of external forces

Pressure variation

Which mechanism of diffusion involves an atom moving to a nearby vacancy in the crystal lattice?

Surface diffusion

Vacancy migration (correct)

Random walk

Interstitial diffusion

What is the term for the net number of atoms crossing a unit area per unit time?

Flux (correct)

Atom migration

Diffusive force

Concentration gradient

Which of the following factors does NOT affect diffusion in solids?

Size of the solid object Signup and view all the answers

Which process is usually faster in solid diffusion?

Random walk Signup and view all the answers

Fick's first law states that diffusion occurs from areas of?

High concentration to low concentration Signup and view all the answers

What distinguishes steady-state diffusion from non-steady-state diffusion?

Behavior of the flux parameter Signup and view all the answers

Which term refers to the majority atom type in a diffusion process?

Solvent Signup and view all the answers

What does the diffusion coefficient represent?

The amount of substance diffusing across a unit area per unit time Signup and view all the answers

Which of the following correctly explains the negative sign in Fick's 1st Law?

It signifies particle flow occurs from higher to lower concentration. Signup and view all the answers

In steady state diffusion, what is true about the concentration profile?

It remains constant with time. Signup and view all the answers

What is the main implication of Fick's First Law concerning concentration profiles?

The steeper the concentration profile, the greater the flux. Signup and view all the answers

What is the expression for diffusion flux (Jx) based on Fick's First Law?

$Jx = -D \cdot (dC/dx)$ Signup and view all the answers

During case hardening, what role do carbon atoms play?

They make the gear harder by locking dislocation planes. Signup and view all the answers

What unit can the diffusion flux (J) be expressed in?

atoms/cm2s Signup and view all the answers

What does a steady state in diffusion signify?

Flux values are equal on both sides of a system. Signup and view all the answers

Study Notes

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.

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Description

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.