Author Topic: Preparation of non-conductive powders  (Read 415 times)

Tom

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Preparation of non-conductive powders
« on: April 13, 2018, 12:18:17 pm »
Hello all,
I've searched the forum for help in this area but no luck, unless I missed it.
Occasionally, I analyze by EDS mineral coated and/or filled papers that have been ashed at 525degrees C, which removes all organics but does not destroy clays and calcium carbonate. The objective is to quantify the mineral content or, at least, obtain mineral ratios. Presently I grind the ash with a mortar and pestle with alcohol or water to reduce particle size and then dropper the suspension onto a polished carbon mount. Two things I want to try to accomplish. 1) Prepare a homogeneous, flat preparation/pellet or such, and 2) create some reference standards. I have relatively pure stock clay (kaolin slurry), Titanium dioxide slurry and precipitated calcium carbonate, the three most common mineral fillers and the ones I encounter most of the time. Any advice on how to generate suitable reference standards? Also, how can I reduce the particle size? I'm considering using a Wig-L-Bug mixer-grinder with a stainless steel vial. I tried the agate vial but the cover fractured after about 60 seconds of use. Besides, the cover does not seal. I realize there are several obstacles to overcome.

Your advice would be greatly appreciated.
Tom

jeffchen

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Re: Preparation of non-conductive powders
« Reply #1 on: April 13, 2018, 01:33:46 pm »
Hi Tom,

I'm not sure if I understand your question correctly, but if you are after proportion of the phases (except for amorphous) in your sample, then it is a job for XRD. EPMA/WDS/EDS measures local chemical composition of the phases of interest and does not directly provide information on how much each phase is in your sample. Other technique like MLA or QEMSCAN may provide both information, but they are generally time consuming and expensive.

Probeman

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Re: Preparation of non-conductive powders
« Reply #2 on: April 14, 2018, 11:57:51 am »
Hi Tom,
If what you are really after is the ratio of phases in a sample, then Jeff is correct. XRD is usually the way to go.

As for elemental composition, even other bulk methods such as XRF, which has a much larger interaction volume than electron beam methods, one usually grinds the sample into a fine power or dissolves it in a flux to get the average composition.

The problem with electron beam methods for fine grained materials is that when the grain size is somewhat smaller than the beam interaction volume, then as Chuck Fiori used to say: "all bets are off". This is because the x-rays from the emission volume will not have interacted completely or even at all with the other phase compositions.   Note that I changed the term to emission volume rather than interaction volume, because what really matters is the volume from which the x-rays are emitted.  We really don't care what happens below the emission depth (except for cases of substrate fluorescence, like trying to measure phosphorus in a film on a Si substrate!). 

But also note that high energy x-rays tend to come from deeper in the sample, while lower energy x-rays tend to come from closer to the surface. So we have a sampling issue as well, if we don't ask where our x-rays coming from.  Of course the overvoltage matters too because at low overvoltages, even high energy x-rays will tend to come from only the surface as well (hence the rationale for Cameca's "Shallow Probe" instrument).  So it's a mix of beam energy, emission energy and overvoltage that needs to be considered.

Think of it like this: in a homogeneous interaction volume we have "bulk" matrix physics and we can apply normal matrix corrections to our measured x-ray intensities.  At the other extreme, if our grain sizes are very small compared to our interaction/emission volume (say approaching atomic scale), then every x-ray tends to interact with every phase composition, and we again approach the situation of bulk matrix physics.  And in between, well, it's in between physics!   :D  The worst case of course is an interaction volume with two phases.

This is why we utilize particle or thin film geometry corrections when analyzing particles or thin films.   Applying bulk matrix corrections to heterogeneous volumes will produce significant errors. Basically we will get a different answer depending on the beam energies involved.

Because of these geometry effects, if one does want an average composition of different phases, then one needs to quant at the finest scale possible, then quant each pixel/point, and only *then*, average the results to get an average composition:

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

Let's consider the situation of an interaction/emission volume at a phase boundary:



Here we have a boundary between pure Cu and pure Al with the electron beam sitting at the boundary. Note that depending on the detector orientation relative to the sample we could have the emitted Cu Ka x-rays absorbed only by Cu, but in other orientations we will have Cu Ka x-rays absorbed by both Cu and Al.  Conversely for Al ka, in some detector orientations we will have Al Ka x-rays absorbed by only Al, but in other detector orientations we will have Al Ka x-rays absorbed by both Al and Cu.  And everything in between for in between orientations!

It gets worse.  What does our detector see in any detector orientation?  We see lots of Cu Ka x-rays, and also lots of Al ka x-rays.  So our detector thinks it's seeing Cu-Al *alloy*!   But again, some emitted/detected x-rays are only absorbed by one of the phases.  And remember, the matrix correction for pure Cu Ka in Cu is 1.0 (relative to a pure Cu standard), and for Al Ka in pure Al it's also 1.0 (again relative to a pure Al standard).

And while the matrix correction for Cu Ka in a Cu-Al alloy is close to 1.0 (it's about a 10% correction in a 50:50 composition), the matrix correction for Ak Ka in a Cu-Al alloy is around 50% (actually 60% in a 50:50 Cu-Al alloy).  So our bulk matrix correction happily churns out a composition that totals 150% or so.  Of course if one isn't using standards (shudder), then one sees a 100% total and it all looks just fine!

This by the way is often the reason we often see high totals in EPMA as our point traverses cross phases boundaries.  Because in a heterogeneous volume at a phase boundary, the application of bulk physics often over estimates our matrix corrections. Of course it could go either way depending on the physics details, but because absorption usually dominates and each phase by itself might be "simpler" physics than a "mix" of the two phases, it's often a high total that we see.

Here's a question to consider: in what detector orientation (relative to a phase boundary) will the emitted Cu and Al x-rays be absorbed *only* in their own respective phases?

If you download the CalcZAf utility:

http://probesoftware.com/smf/index.php?topic=81.0

You can get a better idea of the issues involved, especially see the Run | Calculate Electron and Xray Ranges dialog.

This is probably more information than you needed, but let us know what exactly you are trying to accomplish and we can fine tune our responses.
john
« Last Edit: April 14, 2018, 06:17:00 pm by Probeman »
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Tom

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Re: Preparation of non-conductive powders
« Reply #3 on: April 17, 2018, 09:40:06 am »
Thank you Jeff, I agree that XRD is the preferred method because, yes, I am looking at mineral content (crystalline phases). However, I do not  have access to one, nor do I have XRF.

John, thank you for the mini-tutorial. It is very instructive. Most often, I simply need to know qualitatively which minerals are present and how they are distributed, which is best done from a cross section of the intact paper. At this point, I am looking into the possibility of estimating the relative amounts of each phase. Kaolin clay and calcium carbonate, with or without titanium dioxide are the most common. Again, I realize that XRD of the ash would be the best approach. I am *confined*, shall I say, to using a SEM (JSM-840) with an Oxford INCA EDS system. I wish to identify what my errors are and how best to minimize them. The crystallite size of the mineral residue is very small already for the purposes of paper-making but they are aggregated in the paper-making process. So, disaggregating and mixing is one challenge. I also have the stock raw materials (calcium carbonate, kaolin clay and TiO2 ) that can be mixed in various ratios. I can't grind the material (ash or reference minerals) finer than their present crystallite size. If that will result in large errors for electron probe analysis, then I may have to take a different approach. It seems that mixing the ash or pigment mixtures with a flux and fusing is an option; I have a muffle furnace with a top temp of 1,200C.

Again, I appreciate your feedback.

BenjaminWade

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Re: Preparation of non-conductive powders
« Reply #4 on: April 17, 2018, 04:40:25 pm »
Hi Tom
Not sure if I quite understand what you are after, but can you simply do some "manual" MLA? Take a bunch of representative BSE images carefully thresholded to pick for maximum dynamic range between the different phases, then do some simple image analysis after to get the area% of each mineral, max width etc, which you could then ratio if needed?

Cheers