AMIE Exam Lectures- Material Science & Engineering | Diffusion in Solids| its Different States | 5.2

Engineering Subjects: Introduction to Material Science and Engineering:

Material Science and Engineering | Diffusion in Solids| Steady State & Non-Steady State Diffusion | 5.2

Review:
In the previous video, we started with the topic Diffusion in Solids and had seen:
• What diffusion is?
• How diffusion takes place even in solid
• Different types of Diffusion
• Different Diffusion Mechanism
• The fundamentals of Diffusion

In this lecture, we shall discuss further about diffusion and see what steady state and non-steady state diffusions are:
We shall see:
• What they are?
• When each type occurs?
• What are the concerning factors affecting the steady state and non-steady state diffusions?

Steady State Diffusion:-

• Diffusion is a time dependent process
• Rate of diffusion is an important practical parameter
• In different heat treatment the important phenomenon taking place is diffusion and we must know how much time a particular material needs to be heat treated for diffusion to occur.
• Diffusion Flux denoted by J, is used to define the rate of diffusion
• Diffusion Flux states that the amount of matter which is transported per unit are, per unit time.
• It is a very important criterion
• It is the practical parameter based upon which many decisions for heat treatment process is taken
• If the Diffusion Flux remains constant with time, it is said to be steady state diffusion.
• Diffusion occurs (driving force) due to Concentration Gradient
What is Concentration Gradient?
• Concentration Gradient is the rate of change of concentration per unit change of position.
• Higher the Concentration Gradient, faster will be the diffusion as there will be more driving force and more change will be taking place.
• Diffusion Flux (J) can also be expressed in terms of gradient.
This is called Fick’s First Law.
• Fick’s First Law states that the Diffusional Flux (J) is equal to the negative Concentration Gradient times Diffusion Coefficient (D).
• Negative sign is because diffusion is taking place from a higher concentration to a lower concentration.
• Diffusion coefficient depends on several factors including
What is the specimen that is diffusing?
What is the temperature at which the diffusion is taking place and so on
• For higher temperature D will be large and for lower temperature D will be small
• If the material is small D will be large and so on.
• Fick’s First Law is possible in the steady state diffusion because Concentration Gradient does not depend on time.

Non-Steady State Diffusion:-
• When Concentration Gradient is a function of time or when it depends on time then it is called the Non-Steady State Diffusion.
• For most practical application the Concentration Gradient keep changing with time.
• Diffusion Flux therefore, itself is a function of time.
Here Fick’s Second Law holds.
• It relates the change of concentration with time to the change of concentration with position.
• These are the real scenarios that happen during different processes carburising, nitrocarburising, carbonitriding, etc.
• An example of semi-infinite solid with graph is shown.

With these fundamental of diffusion that steady state diffusion means that the Concentration Gradient remains constant and Non-Steady state diffusion means that the Concentration Gradient changing, you have the idea of diffusion which will be handy to understand the different heat treatment further down the line.

In the next lecture, we shall see some of the dominant factor that effect the rate of diffusion
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