Unsure of Analysis Standard

[mjpavel]

Hello all,

We recently purchased a standard to perform quantitative EDS with specifically for SiC and we went with a vendor who had previously made SiC standards.

We received it and while going through the spec sheet we noticed that their analysis was also only done by EDS and all the weight %'s were normalized. In my mind this throws up some red flags? I already know my quant EDS on SiC is going to raise eyebrows and now my standard I'm backing it up to was also EDS and I cant even tell if they did the analysis right because it was normalized? I would have expected something like LECO or wet chemistry or even microprobe...am I way off on this before I reach out to them and ask questions?

The measured weight fractions are correct-ish - they weren't supposed to coat with carbon (The standards are set in woods metal and we were going to do our own thin coating of something other than C) but they apparently did before running this sample and as such their reported C weight percent is high...I'm presuming due to the carbon coat but I cant be sure how much of it was the coating.

Thanks,

M

[crystalgrower]

Go ahead and ask how the company "qualifies" raw materials.  It is entirely possible that they used WDX (most likely XRF-WDS) to check for impurities.  They would only do this once for any received batch.  If there were no impurities above the detection limit for EPMA then EDS is OK.

It is very important that a company verify each raw material for actual composition And SEPARATELY for impurities.  Ideally their website has some description of how they do this.

Combustion analysis would only give the C content and no impurities.  Wet chemistry and AA or ICP or ICP-MS would not be an option.

Finally do not be concerned that the material may appear brown.  Industrial SiC is brown. 

[Probeman]

The measured weight fractions are correct-ish - they weren't supposed to coat with carbon (The standards are set in woods metal and we were going to do our own thin coating of something other than C) but they apparently did before running this sample and as such their reported C weight percent is high...I'm presuming due to the carbon coat but I cant be sure how much of it was the coating.

I would also ask them what standards they used for Si and C. If they performed standardless analysis, I wouldn't attach any meaning to their reported "results".

Is the composition they reported different from assumed stoichiometry?  That would be 70.045 wt% Si.

[mjpavel]

Go ahead and ask how the company "qualifies" raw materials.  It is entirely possible that they used WDX (most likely XRF-WDS) to check for impurities.  They would only do this once for any received batch.  If there were no impurities above the detection limit for EPMA then EDS is OK.

It is very important that a company verify each raw material for actual composition And SEPARATELY for impurities.  Ideally their website has some description of how they do this.

Combustion analysis would only give the C content and no impurities.  Wet chemistry and AA or ICP or ICP-MS would not be an option.

Finally do not be concerned that the material may appear brown.  Industrial SiC is brown.

Thank you very much for the info - it seems that the impurity analysis was done separately from what I can tell with the results of each impurity less than 0.005%
 

[mjpavel]

The measured weight fractions are correct-ish - they weren't supposed to coat with carbon (The standards are set in woods metal and we were going to do our own thin coating of something other than C) but they apparently did before running this sample and as such their reported C weight percent is high...I'm presuming due to the carbon coat but I cant be sure how much of it was the coating.

I would also ask them what standards they used for Si and C. If they performed standard-less analysis, I wouldn't attach any meaning to their reported "results".

Is the composition they reported different from assumed stoichiometry?  That would be 70.045 wt% Si.

I'm waiting to hear back what they have to say about their quant settings however I did learn that the sample was not carbon coated, they sent the wrong report. On the new report they are still off stoichiometry ~32%C, 68%Si which is better than it was before but still not quite right. I might believe that's the difference due to their quant method, however I know some SiC synthesis methods can leave excess carbon...

At this point I'm hoping the original material provider gives me a chemistry report that I can base off of. I am feeling a bit put off by the experience although it may be my own naivete. I sort of assumed the cost of standards was tied to having a bit of material that was independently verified for chemistry that I could put faith into it's composition and this really almost leaves me no better than having bought a small sputtering target on my own to section and polish.

[mjpavel]

As it turns out the weight %'s they reported are using oxford's factory quant standards inside Aztec...I'm at a bit of a loss what to even do with this block of material now

[crystalgrower]

Send it back for a full refund?

[mjpavel]

That's what were trying to do next...What a headache it's been

[Probeman]

As it turns out the weight %'s they reported are using oxford's factory quant standards inside Aztec...I'm at a bit of a loss what to even do with this block of material now

I talked with our single crystal diffraction expert, Lev Zakharov, and asked him if he could determine the SiC stoichiometry if the materials was single crystal and he responded:

To get the full structure based on X-ray diffraction single crystal method. I can do. Stoichiometric will went out automatically based on the single crystal structure. Powder X-ray diffraction can be used but you need to have powder diffractions from the pure Si and C standards.

Then I asked about the accuracy of such methods and he responded:

I have never been evaluated and never seen such evaluations. From a general consideration I think i t seems to be around 3%-5% if quality of a crystal used for data collection is good.  Typically, 3%-5% is a difference between intensities of symmetrical X-ray diffraction reflections which should be the same based on symmetry.

[mjpavel]

I appreciate the further insight! We have both powder and single crystal XRD so it may be something worth looking into for us. I think we're just going to go shop around another provider and see if someone has a product we can put some trust in to.

SiC is quite hard to work on even by XRD given so many polymorphs. It is supposed to be a line compound and precipitate out the C or Si depending on what side of the phase diagram you are on, but subtle changes in the C/Si ratio push you into other polymorphs and there is some questions about the stoichiometry of those other polymorphs and is there vacancy ordering etc etc.....

I'm interested enough in the problem that I may sink some time into taking a TEM / atom probe lift out and looking into it further (although each of those have issues too). I think our other possibility is to verify it ourselves using APF factors and pure material standards that we have...Honestly all comes out to the same point - I didn't get what I thought we got

[crystalgrower]

You could try simply using the stoichiometric composition of 29.95% C and 70.05% Si.  Use this to see what you get for SiO2 and CaCO3 in-house. 

But as you can see (attached) using EDX would give false C results regardless of how many phases are or are not present.  ONLY SDD can give an accurate EDX analysis for C. 

With respect to this business of phases, it is not worth your time.  It would be more cost effective to go to a jeweller and buy moissanite SiC which is definitely the octahedral phase.