4.14 Fick's Second Law - Modifications to the Thin Film Solution

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From the course by Georgia Institute of Technology

Material Behavior

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Georgia Institute of Technology

Material Behavior

326 ratings

Have you ever wondered why ceramics are hard and brittle while metals tend to be ductile? Why some materials conduct heat or electricity while others are insulators? Why adding just a small amount of carbon to iron results in an alloy that is so much stronger than the base metal? In this course, you will learn how a material’s properties are determined by the microstructure of the material, which is in turn determined by composition and the processing that the material has undergone. This is the first of three Coursera courses that mirror the Introduction to Materials Science class that is taken by most engineering undergrads at Georgia Tech. The aim of the course is to help students better understand the engineering materials that are used in the world around them. This first section covers the fundamentals of materials science including atomic structure and bonding, crystal structure, atomic and microscopic defects, and noncrystalline materials such as glasses, rubbers, and polymers.

From the lesson

Point Defects and Diffusion [Level of Difficulty: Medium || Student Effort: 2hrs 30mins]

In the previous module, we learned how the lattice structure of a crystalline material in part determines the properties of that material. In this module, we will begin to learn how defects - deviations from the expected microstructure - also have a large effect on properties. This module covers one-dimensional, or point, defects which can be missing atoms (vacancies) or excess atoms (interstitial solution) or the wrong type of atom at a lattice point (substitutional solution). Building on these concepts, part of this module will cover diffusion - the movement of atoms through the crystal structure.

4.13 Fick's Second Law - The Thin Film Solution8:56

4.14 Fick's Second Law - Modifications to the Thin Film Solution7:07

4.15 Case Hardening a Gear7:29

4.16 Case Hardening a Gear - Example Problem9:32

4.17 Development of a Useful Approximation4:49

4.18 Appllication to Engineering Materials9:43

Meet the Instructors

Thomas H. Sanders, Jr.
Regents' Professor
School of Materials Science and Engineering

In this lesson, we're going to describe how we might modify

Fick's Second Law to account for the diffusion in a slightly different

manner than what we did when we looked at the thin film.

We're going to attempt to use the solution that we came up for

the thin film to describe the behavior of rather than

two pieces of a surrounded by the thin film b.

This time, what we're going to do is to see if we can, and under what conditions,

we can actually model the diffusion process where we have pure a on

the left and pure b on the right, and look at the time dependent

change in the composition that occurs as a result of this problem.

And we're going to modify the solution of the thin

film that we developed in the previous lesson.

So what we'll do here is we're going to look at a thin film.

And what we've done is we're making b,

a whole series of individual slabs that we're

going to allow to diffuse independently of the compositions that lie around it.

So here is our thin film.

And now, we're going to allow that thin film to diffuse.

So that's the composition profile after diffusion has occurred

in that particular film.

Now, what we're going to do is look at a whole series of these

as we section through the material.

So here is the thin film of a slab that's right to the front surface.

We put in another one, we put in another one, we add one more, and we add another.

And so what we're looking at is a multitude of thin film

solutions that will describe the process of diffusion.

And so what we have to make sure that we understand here,

because we are modifying the thin film problem.

What we have to understand is that there are going to be certain

assumptions that we have to make.

And the major assumption that we're going to make is that the diffusion

of b is going to be independent of its surrounding compositions.

So, when we do that and we wind up summing all those

individual thin films, what we will see is a master plot,

which tells us that the composition is now going to

be a function in any particular interval delta alpha.

It's going to be a function of the contribution that that particular thin

film makes, along with the contributions that are made by the other thin films that

surround it and are in close proximity to that particular thin film region.

So now what we're going to do is when we look at the solution of composition

as a function of position in time, now what we have to do is, because we're

adding up all those contributions to those thin films, we now have a summation.

If we take that summation and re-replace the summation with an interval,

then what we find is a function that has been termed the error function.

And what we will now have is an equation that describes for

us the time dependent and position dependent, change in composition

during diffusion, and that particular solution has this form.

So we have the composition that's given here that we begin with.

And what we have is, in the error function, we have x, which is the position

where we are in space, d is the diffusivity and t represents the time.

Now, we need to go back and consider what is meant by the error function.

And the error function is like any of these functions that you deal with

in engineering like an exponential function or a sign function,

we can use the arrow function precisely the same way.

So what we have plotted here is what the value of z is on the x axis and

the corresponding value of the arrow function of that number, and

the properties of this particular arrow function is that it

increases with increasing z.

And what we also find is the range of values which

seem to be reasonable with respect to z and the error function of z.

Now what is in z?

Well, what is in z is x divided by two onto the square root of Dt.

So our experimental information goes in here, that is, what is the temperature,

and hence the temperature will tell us what the diffusivity is.

And how long is this material sitting in the particular furnace and

x is where we want to look at the particular value

of position to determine the composition at that position.

So we'll look at the values of z that we have on the x axis and

look at the corresponding value of the error function of z.

Now, one of the things that I want you to pay close attention to is the values that

we're looking at, that is x divided by the square root of Dt times two.

Those numbers generally will range between zero and one.

If you get numbers that are outside of that range,

you've done something wrong with your calculations.

The reason that I bring this up is because of the magnitudes

of the numbers like the diffusivity.

They can be on the order of 10 to the -11.

And until you get used to working with these numbers, you don't necessarily have

a feel for the values that are reasonable with respect to the error function.

So generally, the problems that we're going to be working with and

trying to use this error function, we're going to be looking at numbers that lie

within the range of say, .2 up to about .6.

So as long as your z comes into that range,

then you know reasonably that your calculations have

been correct with respect to position diffusivity and time.

We now have a solution that we can apply

to a variety of important problems that we're going to be

using in material science and engineering for processing materials.

Thank you.

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