deltaH
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Eutectic determination
As a rule of thumb, really low melting eutectics are often multicomponent beasts of three or sometimes even four components. It is a nightmare to
experimentally hunt for a eutectic composition with so many components as the usual approach is to map the liquidus temperatures of multicomponent
compositions incrementally over the range of compositions. So taking just 10% increments in composition for a three component system results in 1000
experiments already 
I was wondering if there isn't a smarter way to do this, so I was thinking, if one just took an equimolar mixture of all the salts (say for example a
mixture of nitrates of various metals), heated it so everything melts, then let it cool slowly while stirring. At some point you will hit the solidus
temperature where solids begin to form, but you continue stirring and cooling slowly until the mixture consists mostly of solids and a very small
amount of liquid melt as a thick slush. At that point, say when most has frozen, you sample the remaining liquid only with a hot glass syringe, maybe
with a small ceramic frit on the tip to filter the crystals. Now you analyse this sample by ICP at your local university to determine the relative
amounts of the metals and presto... you have your eutectic composition???
If necessary, you can take that new composition and repeat the experiment, but starting from there and keep repeating a few times to approach the
global minimum.
If this could work, then you could determine the eutectic composition for any number of components, easy-peasy, so obviously it doesn't, but why not?
[Edited on 8-2-2015 by deltaH]
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subsecret
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Tell me if I'm wrong, but I don't think this would work. You're not dealing with the metals as elements, but rather as ions.
Fear is what you get when caution wasn't enough.
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j_sum1
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As you rightly point out, ternary and quaternary systems are a pain, but the situation need not be quite as complex as that. Simple methods can be
used to find the eutectic in two component systems -- tracing the liquidus as you suggest. And once you know its composition approximately, you can
run some experiments to plot the curves more finely and determine it more accurately.
A third or fourth component is often added to a system to modify the structure of one of the phases and is generally in relatively low quantity. As a
first approximation you generally assume that it doesn't affect the eutectic temperature or composition too much.
Eutectics generally have a very easily observed microstructure -- As the melt solidifies, one phase will crystallise first. Then the local
concentration is tipped to the other side of the eutectic composition and the other phase solidifies. This pushes the local concentration back in the
other direction and the first phase solidifies again. And so forth. Pearlite in steel is a classic, being alternate layers of cementite and ferrite.
(Other structures than layering are possible.) So, if you want the exact composition of a eutectic then you can simply analyse the material
produced. This of course does not tell you the exact composition and temperature of a ternary eutectic, but it does indicate the direction that the
third component will introduce. Once you know that you can increase the amount of the third component and fine-tune the ratios of the first two until
you find the point you need.
I would have to question why you would need to find the eutectic of a ternary system so accurately though. I guess it is useful (eg in solders) to
have a well-defined melting point and it is useful to avoid local concentrations and associated brittle compounds and intermetallics when
crystallising. However, I think that in most practical cases, close enough is good enough. It's an engineering problem and provided there are no
deleterious effects you simply proceed with your project. In any case, metallurgy is your friend. Taking a quick look at the microstructure tells
you a lot about what you have obtained.
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blogfast25
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Quote: Originally posted by deltaH  | So taking just 10% increments in composition for a three component system results in 1000 experiments already
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With 'as little' as 27 compositions tested you'd be able to approx. locate the eutectic with curve fitting.
See Factorial Experiment Designs.
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j_sum1
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by deltaH | So taking just 10% increments in composition for a three component system results in 1000 experiments already
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With 'as little' as 27 compositions tested you'd be able to approx. locate the eutectic with curve fitting.
See Factorial Experiment Designs. |
Good point. However, that presupposes a simple ternary system.
If there are a bunch of intermetallics involved then there will be more than just three liquidus surfaces that need to be modelled. That is why I
would begin with a known eutectic on a two species system and incrementally add the third component (adjusting the ratio of the first two as needed)
to trace out the "valley" of the two liquidus surfaces until the ternary eutectic is reached.
It needn't be too difficult. Suppose you are investigating a system of alpha, beta and gamma.
Find the eutectic composition of alpha and beta. If you are to the left of the required concentration, then microscopic
observations will show a "eutectic phase" (ie, probably layered) and alpha phases. If you are to the right you will observe "eutectic phase" and
beta. In either case you can analyse the eutectic phase and determine what its concentration should be.
Add a small portion of gamma. Again a microscopic analysis will determine for you how the eutectic composition has shifted. If you have alpha
phase present then you are to the left of the line you want. If you have beta then you are to the right. Again, you could analyse the eutectic
structure observed.
Incrementally increase the amount of gamma. As soon as you observe the gamma phase in your microscopy then you know you have overshot the
ternary eutectic point.
you can then do a curve-fit on the data you have -- which may only be three or four points and you will have a good idea where the ternary
eutectic is.
[Spoken with all the authority of someone who has never had to do this himself. There are multiple things that could go wrong here and good lab
practice would be paramount.]
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deltaH
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Thank you all for your points.
I wasn't thinking of metallic alloys though, rather eutectics from salts, for example nitrates and so-called deep eutectic solvents, but some of your
points are universal, nevertheless.
Is there something inherently wrong with the approach I propose? Would it tend to the eutectic?
[Edited on 9-2-2015 by deltaH]
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j_sum1
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I can't think of any particular reason why your approach wouldn't work if it was a simple system. I have no experience of salt systems of the type
you describe however and I don't know what crazy things may be thrown up. Double salts perhaps? Water of crystallisation misbehaving? Unexpected
reactions? Who knows?
I would think your whole process would be a lot easier if you could get close to the eutectic before you start. Otherwise you are going to have
problems with collecting a purely liquid sample. And you really want your sample that you collect to be the final bit of liquid if possible. A kind
of iterative approach may help. Begin with kg quantities. Let most of it solidify and collect the last 20 mL of liquid. Then re-melt this, allow it
to solidify and collect your 1g sample.
And of course, depending on what your salts are you might not need to send the sample away for analysis. If you already know what the components are
then you might be able to get away with a simple titration or something similar.
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