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# Materials Science MCQ’s- Diffusion Mechanisms II

1 - Question

In steady state diffusion, which of the following remain unchanged with time?

a) Concentration at source

b) Concentration at sink

c) Concentration profile

d) All of mentioned

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Answer: dExplanation: In steady state diffusion, the concentration of the diffusing species is kept constant at both ends and hence, the concentration at different points remain unaffected as time passes. Purification of hydrogen gas employs steady-state diffusion through a thin palladium sheet.

2 - Question

For steady-state diffusion, diffusion flux is proportional to the concentration gradient. Concentration gradient is:

a) Rate of change of concentration with respect to space

b) Rate of change of concentration with respect to time

c) Difference in concentrations at the source and sink

d) Ratio of concentrations at source and sink

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Answer: aExplanation: Concentration gradient is the major driving force behind diffusion. It is the slope of the concentration profile and measures the difference in concentration of the diffusing species per unit length.

3 - Question

Which of the following is not true for steady-state diffusion?

a) The concentration profile is linear

b) The concentration gradient is constant

c) There is no net transfer of mass

d) The diffusion flux is constant

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Answer: cExplanation: Although, the concentration at each point in the diffusion medium remains constant, yet the diffusing species is being transferred from a source end to sink end.

4 - Question

Concentration of hydrogen gas across a 2mm thick palladium sheet differs by 4 kg/m3. Considering steady-state diffusion with diffusion constant 10-10 m2/s, diffusion flux is ______ kg/m2.s:

a) 2 x 1010

b) 2 x 10-10

c) 5 x 10-11

d) 5 x 109

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Answer: bExplanation: Magnitude of diffusion flux for steady-state diffusion is given by the product of diffusion constant and concentration gradient.

5 - Question

The expression J = M/(A.t), where J, M, A, & t are diffusion flux, mass of diffusing species, cross sectional area, and time respectively is:

a) incorrect

b) valid for steady-state diffusion only

c) valid for non steady state diffusion only

d) valid for both steady and non steady state diffusion

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Answer: dExplanation: It follows directly from the definition of diffusion flux. It is the mass being transferred per unit area per unit time.

6 - Question

If C is the concentration of the diffusing species at distance x from the source and time t, then according to Fick’s second law (assuming diffusion coefficient, D is constant):

a) ∂C/∂x = D.(∂C/∂t)

b) ∂C/∂t = D.(∂2C/∂x2)

c) ∂C/∂x = D.(∂2C/∂t2)

d) ∂C/∂t = D.(∂C/∂x)

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Answer: bExplanation: According to Fick’s second law, the rate of change of concentration of diffusing species with time at a given point in space is proportional to the slope of the concentration gradient curve at that point.

7 - Question

The relation between temperature and diffusion coefficient is:

a) Linear

b) Exponential

c) Sinusoidal

d) Diffusion coefficient is not related to temperature

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Answer: bExplanation: Diffusion coefficient is indicative of rate of diffusion. Its relation with temperature is given by the expression: D = D0.exp( -Qd / R.T) where D0 is diffusion coefficient at initial temperature, Qd is the activation energy, R is gas constant, and T is the change in temperature.

8 - Question

Predeposition and drive-in diffusion are two heat treatments used in the manufacture of integrated circuits. Assertion: Predepostion is carried at a higher temperature than drive-in diffusion.

a) True

b) False

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Answer: bExplanation: Drive-in diffusion is used to push the already added (through predeposition step) impurity atoms farther into the Si lattice to gain the desired impurity distribution and is carried out at approximately 1200℃ (contrary to 900℃ of predeposition).

9 - Question

Following plot shows the concentration profile at two different time, T1 and T2 for the same non-steady diffusion process.

What is the correct relationship between the two?

a) T1 > T2

b) T2 > T1

c) Insufficient data

d) Same process can’t have two concentration profiles

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Answer: bExplanation: As more time passes, the diffusing species reaches farther into the solid and tends to approach a linear shape.

10 - Question

Diffusion through grain boundaries is very fast and is called short-circuit diffusion. Assertion: Their contribution to overall diffusion is negligible.

a) True

b) False

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Answer: aExplanation: Due to very small cross sectional areas of the grain boundaries, the actual amount of mass transferred through them is much smaller than that through the bulk.