Diffusion
| | Diffusion | |
4. Experimental Techniques for |
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| General Remarks | ||
 | Let's start
by finding out exactly what it is we want to measure experimentally. The headline says it
all: Diffusion Parameters! OK, but what exactly? When we look at diffusion we have two basic
cases:
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In essence we need to determine the diffusion coefficient and that involves getting
numbers for the migration energy EM and the pre-exponential factor
D0 of the basic equation
for the diffusion coefficient: | |
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True enough. But then we know (after looking up this module) that the migration energy EM for all diffusion via vacancies contains the vacancy formation energy EF. For the case of self-diffusion EM(SD) = EF(V) + EM(V) obtains, with EM(V) = migration energy of the vacancy. We should know about this, too. | |
| In total we want to measure four basic quantities.
They cover essentially everything concerning the diffusion of A in B:
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| Getting
a handle on the first two is not too difficult. The trouble starts with the vacancy properties,
which are sometimes hard to get. I will outline only a few basic techniques here. There are far more ways to measure some property of a material that relates to diffusion then I can cover here. | |
| Self-Diffusion and Impurity Diffusion | |||||||||||||||||||||||||||||||
| The basic experiment for getting diffusion data is simple in principle. It involves the following steps | ||||||||||||||||||||||||||||||
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| Obviously, this is a lot of tricky work. Far
more tricky than you can imagine. It's a feast for graduate students (professors don't have time for things like that!). Let's look at some of these steps in more detail to get an idea of what experiments like that involve |
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| First step: So we want to measure self-diffusion in iron. We put some iron on the surface of an iron crystal and get to work. But how the hell do you measure the concentration of the iron atoms that diffused into the iron? All iron atoms are exactly equal, there is no way to tell one from the other. Or is there? | ||||||||||||||||||||||||||||||
| Yes there is.
All iron atoms are not equal, after all. Iron atoms, as
all atoms, come in variants called isotopes. We need to take some iron isotope that is not part of natural iron. Somebody in possession of a nuclear reactor must make it, and it must and will be radioactive. That means it will not come cheap and that I need to do my experiments in a secure environment (not cheap either). Moreover, often I must do my experiments rather quickly before most of my isotope has decayed. Of course, for elements where there is no suitable or affordable isotope, I will have major problems in getting self-diffusion data. |
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| Second step: This involves all kinds of tricks since you cannot simply put some A down on B with a trowel. You might put your B specimen into a vacuum chamber, shoot of the oxide or whatever with an energetic argon ion beam, and then deposit some A by evaporation or one of the many thin film deposition techniques developed for micro electronics. Mostly this is not so simple and never really cheap. | ||||||||||||||||||||||||||||||
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Third step: This is easy. Regular furnaces are standard equipment and rather cheap. Except if material B is, for example, tungsten (W) or something else with a very high melting point. Than you need a furnace that goes up to very high temperatures (not cheap). Of course, if A should melt long before B, you have a problem, too. | ||||||||||||||||||||||||||||||
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Fourth step: If you consider that it takes quite some time at high temperatures to drive some carbon just half a millimeter into you iron, your slices must be very thin. In many cases they actually need to be ultra-thin, far thinner than a hair. Best is to just have a few atomic layers. So forget about cutting off those slices with your trusty old hack saw. Your best bet is to take of one atomic layer after the other, e.g. by some ion beams in vacuum. I won't say more, except that whatever technique you use will most likely be neither easy nor cheap. | ||||||||||||||||||||||||||||||
| Fifth
step: Now you have your ultra-thin slices (most likely on some substrate) - but how do
you measure the concentration of A in there? Well, that's rather easy if you worked
with radioactive isotopes for whatever A you used, not just for self-diffusion. You
just measure how many decays you have, correct for the decays that happened before you got
around to do the measurement, and here you are. If that doesn't work, you may have to resort to using a mass spectrometer (not cheap) or other sophisticated gear (not cheap either). Forget about chemical analysis, it is never sensitive enough. | ||||||||||||||||||||||||||||||
| Sixth - ninth step: Trivial. | ||||||||||||||||||||||||||||||
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The message is clear: Send us some money! At least pay your taxes, they are needed for my salary (and for running our labs and paying the people who do the work). | ||||||||||||||||||||||||||||||
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| All in all, getting self-diffusion data by
experiments is possible but not all that easy. There are other techniques than the one described
above but they are just as involved or even more so. Getting "regular" diffusion data about some A diffusing in B with A being different from B is typically easier than getting "A in A" data, but of course it depends very much on which one of the about 80 · 80 combinations you are after. | ||||||||||||||||||||||||||||||
| For iron by the way, we are in luck. There are three usable isotopes and that's
why we have good self-diffusion data:
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| Now that we have some idea about how to get data for diffusion coefficients, let's turn to the difficult part: getting data for the vacancies in B. | ||||||||||||||||||||||||||||||
| Measuring Vacancy Data | ||||||||||
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We know that the activation energy in the exponent for self-diffusion is the sum of the vacancy formation and migration energy; | |||||||||
| The c0
term above is new; I haven't bothered you with that so far. Well, c0
is roughly about 1 so we need not be overly concerned about it. If one likes to wallow
in unimportant details one might get rewarded sometimes because c0
does hide tricky and interesting stuff about the inner structure of the vacancy. Don't believe
for a second that missing things have no structure. Ask you wife about that vacancy in your
account that appeared after her recreational shopping, and you will learn that there is a
sophisticated structure to it. Anyway, when we measure vacancy properties, we absolutely want to have a good number for EF and if we get c0 too, we are happy. If we don't get it, we cannot calculate precise concentrations, unfortunately. | |||||||||
| How do you measure a vacancy
concentration? The first thing to realize is that we can forget about all kinds
of microscopy. Yes, some microscopes
actually can "see" a vacancy on the surface of
a sample - here is an example
- but they can never give you reliable numbers for a volume concentration. In essence, we have two major methods and a bunch of special ones. Here I will only look at the two major methods. | |||||||||
| "Delta l minus delta a" Method | ||||||||||
| The first method is conceptually simple. Imagine that you have a crystal full of vacancies. Now ask yourself a question: where are all the atoms that now are missing? They must still be part of the sample because they could not just disappear into nothingness. What happens is that the atoms that made room for a vacancy are now at some internal or external surface, this link gives an idea of how that works. But details don't matter, what happens is that our sample gets bigger if there are vacancies, see the figure below. | |||||||||
| All we have to do is to measure how the length l(T) of a sample changes with increasing temperature T. The problem, of course, is that the length also changes because of thermal expansion, simply because the average distance between the atoms gets larger as schematically shown below | |||||||||
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| Can we measure just the effect of thermal expansion somehow, and then subtract it from the total length change? The difference then must be due to vacancies. | |||||||||
| Yes, we can! Thermal expansion changes the distance between all atoms in the lattice, and thus the lattice constant a. Lattice constants are easy to measure with high precision if you have some good X-ray equipment. I won't go into how it is done but in essence you use interference effects due to the periodic arrangement of atoms. | |||||||||
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What one does is to measure the relative change in length, (l (T) – l0)/l0 = Dl/l 0 with a "ruler" (I won't go into how it is really done) and then subtract the relative change in lattice constant. The vacancy concentration obtained by this "Delta l minus delta a" method (more fancily we call it "differential dilatometry" method) is simply given by | |||||||||
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| Nice, straightforward, and and rather simple. But not without problems, which become apparent by looking at some "classical" examples | |||||||||
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| The basic problem of differential dilatometry is that you need to measure rather small effects with high precision. That limits the method to "large" vacancy concentrations as shown above; it should not be much below 10-5 (10 ppm or 0.001 %). Since the largest vacancy concentrations close to the melting point are in the 10-4 range, the method is restricted to high temperatures. Even then it does not work for crystals with a generally low vacancy concentration like silicon (Si). | |||||||||
| That's why we invented a truly far-out technique for measuring vacancy concentrations called: | |||||||||
| Determining Vacancy Concentrations via Positron Lifetime Measurements | ||||||||||
| If you ever read some science fiction or watched some Star Trek episode, you are familiar with antimatter. All elementary particles like protons, neutrons, electrons, and so on, always come with a kind of mirror image particle called "anti particle". | |||||||||
| We have anti-protons, anti-neutrons and anti-electrons - but for historical reasons
we call the latter positrons. If a particle happens
to meet its anti-particle, they engage in violent activities
that invariably destroy both of them, producing some (typically two) high-energy photons or
light "particles" that we call g-rays. When the universe made itself, there were about equal amounts of particles and antiparticles. All couples have long since disappeared, the matter that is left are just the few survivors resulting from a small imbalance in the original numbers. That's the present wisdom. |
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| So if you need some positrons for an experiment that is supposed to measure the vacancy concentration, you must make your positrons right where you need them. This is typically right at the surface of your specimen, from where they are "shot" into the bulk of the sample. Inside the sample they do a random walk until they find a partner for self-destruction, and off they go. | ||||||||||
| Fortunately, making positrons is not a difficult thing to do. Buy yourself a suitable sodium isotope, typically Na 22. This isotope is radioactive and decays with a life time of 2.6 years into something else by emitting a positron and, at exactly the same time, a g photon with a high energy of 1,28 MeV. | |||||||||
| The great thing
about those positrons is that they announce their birth and
their death with a flash of (g) light that we can easily
measure. Since there are plenty of electrons inside normal matter, a positron let loose in
there doesn't need very long to find an innocent electron who has no choice but to become
a partner for self-destruction. Typically, it's all over after 100 ps (10–10
s). Fortunately once more, the stop watches we use in modern science have no problem in measuring
elapsed time with picosecond precision. To start an experiment we shoot positrons into the
crystal at room temperature, sort of one-by-one, and measure their life time with high precision
in the picosecond region. If that works we start to heat up our sample in order to create an appreciable concentration of vacancies. Inside a vacancy there are no electrons. If a positron on its arandom walk happens to come close to a vacancy, it likes it there and hangs around inside the vacancy for a little while (it is "trapped" in science lingo). Since there are no electrons, it cannot commit suicide quite that easily, and the net effect is that if lives a little longer. In other words, the average positron lifetime increases a bit if a lot of vacancies are around. That we can measure. We also can (with some little difficulty) calculate the vacancy concentration from the positron lifetime data. Here are some results: | |||||||||
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| The major strength of the method is that it can detect vacancies at lower concentrations than differential dilatometry; see the figure above. Sadly, however, it is not sensitive enough to detect anything in silicon, germanium or other important semiconductors as you can see. | |||||||||
| Positron lifetime measurements (and some other
things you can do with positrons) have advantages and problems, but I won't go into that. I will also not go into what one can learn about diffusion and vacancies by using other exotic elementary particles like muons. I'll stop right here about methods. The message is clear: | |||||||||
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| That is the reason that we still know far less than we ought to about the details of diffusion in semiconductors. In particular, the data about vacancies (and self-interstitials, which happen to be important in these materials) are not known very well and still cause a lot of controversies in Materials Scientist circles. | |||||||||
| In the last part of this module I will give you some exemplary data about diffusion and vacancies, which provide a kind of background for the iron data in the main module. | |||||||||
| Some Diffusion and Vacancy Data | ||||||||||||||||||||||||||||||||||||||
| First a few vacancy data. Below is a table from the "vacancy" module, augmented by the maximum vacancy concentration as far as it is known. | |||||||||||||||||||||||||||||||||||||
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| Now let's look at self-diffusion coefficients D(Tm) close to the melting point where the maximum value will be found. | |||||||||||||||||||||||||||||||||||||
| The lower scale gives D(Tm)
in the usual units of m2s–1. The upper scale uses the important relation L = (Dt)½ for the diffusion length L and give this average penetration depth for a diffusion time of 1 second. Multiply by 10 for 100 seconds, 100 for 10 000 seconds or close to 3 hours, and so on. | |||||||||||||||||||||||||||||||||||||
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| Now let's look at some numbers for the exponents
ED of the exponential expression
for the self-diffusion coefficient. We called that diffusion energy or "activation energy
for self-diffusion". In addition we also know that for self-diffusion running by a vacancy
mechanism, we have The graph here calls the diffusion energy by its proper name "activation enthalpy" but never mind. In some rare cases like silicon (Si), self-diffusion is believed to be carried mostly be self-interstitials, so the equation above might not apply to all elements in the figure below. | |||||||||||||||||||||||||||||||||||||
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| As one could expect, the self-diffusion energy scales with the melting point. That allows to guess that number for materials where measurements have not yet been made. You might be off quite a bit, however, as chromium (Cr) and silicon (Si) nicely demonstrate | |||||||||||||||||||||||||||||||||||||
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| Now I only need to supply millions of data for A diffusing in B. Or A diffusing in any compound or alloy B can make with all other elements of the periodic table. For example we might look at the diffusion of carbon in iron, all iron oxides, iron silicides, ..., you get the point. | |||||||||||||||||||||||||||||||||||||
| One example must suffice. In order to get the maximum amount of data in as little
space as possible, it is best to use an Arrhenius plot. Here are the data for some elements diffusing in aluminum (Al). A similar diagram for all kinds of elements diffusing in silicon (Si) can be found in the link. | |||||||||||||||||||||||||||||||||||||
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| Some elements diffuse faster than aluminum (red line), some diffuse slower. Some like cobalt (Co) diffuse slower at lower temperature but faster at higher temperature. There are no obvious rules. If you want to know the diffusion coefficient, you must measure it. | |||||||||||||||||||||||||||||||||||||
| What remains to look at are the migration energies EM for vacancies but I gave you some numbers for those already before. | |||||||||||||||||||||||||||||||||||||
| What also remains to consider, for example, is
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| You get the point once more: | |||||||||||||||||||||||||||||||||||||
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 | You also appreciate the importance of the first law of applied science. A lot of time and effort could be saved if there were a good theory that allows to calculate all those numbers we are after with sufficient precision. | |||||||||||||||||||||||||||||||||||||
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This, however, is a notoriously difficult enterprise.
But there is hope. To quote B. Grabowski et al. who published
the presently last word on this topic in "physica statis solidi B, 248, 2011": "The situation (of not being able to do precise calculations) is likely to change in the near future." | |||||||||||||||||||||||||||||||||||||
| More power to our theoreticians (and the guys who make more powerful computers)! | |||||||||||||||||||||||||||||||||||||
| Back to | |
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Diffusion | |
| 1. Atomic Mechanisms of Diffusion | |
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2. Random Walk | |
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| 3. Phenomenological Modelling of Diffusion | |
| On to | |
| 5. Diffusion in Iron | |
Spring Model and Properties of Crystals
Transmission Electron Microscopes
Phenomenological Modelling of Diffusion
The Story of Self-Interstitials in Silicon
Atomic Mechanisms of Diffusion
© H. Föll (Iron, Steel and Swords script)
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