University:
York University
Course:
CHEM 1100 | Chemistry and Materials Science for Engineers
Academic year:

2021
Views:

399

Pages:

2
Author:

Nick T.

Definition of Diffusion: Diffusion is the movement of particles from a region of high concentration to a region of low concentration. Concentration Profile: A graph showing how concentration changes with distance. Fick's First Law: The flux (J) is proportional to the concentration gradient (dC/dx). J = -D(dC/dx) Where: J is the flux (kg/m²s) D is the diffusion coefficient (m²/s) dC/dx is the concentration gradient (kg/m⁴) Steady-State Diffusion: Diffusion that occurs when the concentration profile remains constant over time. Diffusion Coefficient: A measure of how quickly a species diffuses through a material. Depends on the material and the diffusing species. Example: Carburization of Iron Plate: A plate of iron is exposed to a carbon-rich atmosphere. Calculate the flux of carbon through the plate at steady state. Steady-State Diffusion: Flux (J) is calculated using Fick's first law: J = -D(dC/dx) Given values: D = 3x10⁻¹¹ m²/s CA = 1.2 kg/m³ CB = 0.8 kg/m³ xA = 5x10⁻³ m xB = 10x10⁻³ m Calculation: J = -D(CA - CB) / (xA - xB) J = -3x10⁻¹¹ (1.2 - 0.8) / (5x10⁻³ - 10x10⁻³) J = 2.4x10⁻⁹ kg/m²s Non-Steady-State Diffusion: Described by Fick's second law: ∂C/∂t = ∂/∂x(D∂C/∂x) Diffusion (flux) changes with time. Concentration profile changes with time and position. D is position-independent. ∂C/∂t = D(∂²C/∂x²)