Question

Diffusion in solids is faster when

• A. pressure decreases
• B. grain size increases
• C. temperature increases
• D. density increases

Hint:

Solid-state diffusion is a thermally activated process. As temperature increases, the concentration of vacancies increases, providing more pathways for atomic movement.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks which physical parameter changes in a way that accelerates the rate of solid-state atomic diffusion.

Step 2: Key Formula or Approach:
The temperature dependence of the diffusion coefficient (\( D \)) is mathematically expressed by the Arrhenius equation:
\[ D = D_0 \exp\left(-\frac{Q}{RT}\right) \]
where:
\( D_0 \) is the pre-exponential factor,
\( Q \) is the activation energy for diffusion,
\( R \) is the universal gas constant, and
\( T \) is the absolute temperature.

Step 3: Detailed Explanation:

• Temperature Effect: An increase in temperature exponentially increases the value of the diffusion coefficient \( D \).
Higher thermal energy increases atomic vibration amplitudes, providing more atoms with the kinetic energy necessary to overcome the activation barrier \( Q \) and jump to adjacent vacancy or interstitial sites.

• Analysis of Other Options:
-

Grain size increases (Option B) actually reduces the total boundary area. Since diffusion along grain boundaries is faster than bulk lattice diffusion, larger grains lead to slower overall diffusion.
-

Density increases (Option D) generally means tighter atomic packing, which increases the activation energy barrier for atomic jumps, slowing down diffusion.
-

Pressure decreases (Option A) has a minor effect on solid-state diffusion compared to temperature.


Step 4: Final Answer:
Thus, diffusion in solids is faster when temperature increases, aligning with Option (C).