Measuring a phase with fine inclusions...advice needed

[Jeff C.]

Hi all,

We are trying to measure the composition of some synthetic glasses with our JEOL8200 EPMA. They were made by melting synthetic chemical mixtures at high temperature and then quenched into water.

These glasses contain essentially FeO, SiO2 and small amount of Cu2O. Some of these glass samples looked fine and homogeneous under SEM, but some show fine precipitations (sub-micron) of Cu metal which formed during cooling quenching process. (See photo(left) attached)

We initially used 0 probe diametre (roughly 1um) and did a line scan across the sample as shown in red circles in the photo and we found the concentration of Cu/Cu2O scatters depending on the position where measurement was made, that's understandable. Later we tried to increase the probe size to 5um and measurement results of Cu/Cu2O show much less scattering.

What we are really interested is the average composition of this glass phase(including Cu metal precipitates).

Here is the question, to what extent by increasing the probe size for the measurement in this case would give results that are closer to the average composition?

[Probeman]

Hi all,

We are trying to measure the composition of some synthetic glasses with our JEOL8200 EPMA. They were made by melting synthetic chemical mixtures at high temperature and then quenched into water.

These glasses contain essentially FeO, SiO2 and small amount of Cu2O. Some of these glass samples looked fine and homogeneous under SEM, but some show fine precipitations (sub-micron) of Cu metal which formed during cooling quenching process. (See photo(left) attached)

We initially used 0 probe diametre (roughly 1um) and did a line scan across the sample as shown in red circles in the photo and we found the concentration of Cu/Cu2O scatters depending on the position where measurement was made, that's understandable. Later we tried to increase the probe size to 5um and measurement results of Cu/Cu2O show much less scattering.

What we are really interested is the average composition of this glass phase(including Cu metal precipitates).

Here is the question, to what extent by increasing the probe size for the measurement in this case would give results that are closer to the average composition?

Hi Jeff,
This is an excellent question and difficult to answer. Well, I can give you an answer: do not defocus the beam on heterogeneous materials.  I think it was Chuck Fiori who said: "if the interaction volume is heterogeneous, all bets are off!".

Here is a post related to your question that you might find helpful:

http://probesoftware.com/smf/index.php?topic=198.msg896#msg896

also this analysis by Julie Chouinard comparing averaging intensities vs averaging concentrations.  The latter is what we really want to do!

http://probesoftware.com/smf/index.php?topic=44.msg145#msg145

and then this on pixel boundary effects:

http://probesoftware.com/smf/index.php?topic=49.msg159#msg159

Ideally you'd want to analyze both the inclusions and the matrix and sample enough of your material to obtain a representative ratio between the matrix and the inclusions and then calculate the average concentrations.  But because your inclusions are so small this will be difficult. What exactly is the size range of these inclusions?

I'd probably run at a very low overvoltage, with a highly focused beam and acquire a quant map over a "representative" area that includes enough matrix and inclusions and then calculate the average composition by averaging the concentrations of all the pixels.

But as to your specific question on how much of an error one would get from defocussing the beam... I'd probably need to run a Monte Carlo model with that geometry.

You'd only have two materials to model, but the geometry model would be complex. I'd take a look at this post here:

http://probesoftware.com/smf/index.php?topic=59.msg1340#msg1340

and run several simulations each with a different beam "aperture" in Penepma to see the effect of spreading the beam out on the relative intensities.

[Les Moore]

Hi Jeff,

The heterogeneous micro nature is the issue and the big concern is when the substrate affects the yield of the elements from the inclusion.  In your system, the Fe in the substrate is unlikely to do much but if the phases were reversed, and you had iron silicate inclusions in the Cu matrix you would have a differential effect on these two elements.

A simple exercise would be to model it with PENEPMA or CASINO and look at the predicted K ratios for a simple physical model of a hemisphere embedded in the glass and then compare it to a homogeneous calculated composition single phase.  The only difference should be that the absorption of the Cu X-Rays will be different.

Then, if this doesn't worry you, try changing the Fe in the substrate for something that will fluoresce the Cu.

If you look at this post you will find a discussion on how to....
http://probesoftware.com/smf/index.php?topic=59.msg221#msg221

[Jeff C.]

Many thanks for the suggestions.

The Cu precipitates are spherical with size ranging from 0.1 to 0.4um.

I think I'll model the situation to evaluate the effect of this heterogeneity first.

[jon_wade]

Jeff

did you model this, and if so, what did you get?

if you didn't model it, mind if I do it?  I am running a similar simulation (which may complete a week before the heat death of the universe ) which takes puts random spots over a simulated quenched metal so this is kind of similar (and its easy!)

[Jeff C.]

No I have not done it, please do it and let me know the results.
However, I managed to run some measurement on samples of same composition but with various extents of inhomogeneity. Would be interesting to compare the predictions and actual results.

[jon_wade]

Its underway Jeff

Ash Norris and I have commandeered the dept cluster and are busy raising the UK's power consumption.
Will let you know in a week or two.