Hi Alex,
May be we worked with the same furnace: Inel? I had also the same problem and
solved it by making a new sample holder with a smaller thermocouple placed on
the side of the sample holder.
Patrick
Patrick Weisbecker
LCTS - Pessac
- Message d'origine
De : Yokochi, Alexandre alexandre.yoko...@oregonstate.edu
À : Rietveld list rietveld_l@ill.fr
Envoyé le : Jeu 15 Octobre 2009, 21 h 33 min 31 s
Objet : RE: Re : HT XRD calibration
Hi all,
I've never used one of these Anton Paar furnaces, but with some of my
instruments (externally heated, not Pt resistive foil heated) I could actually
use the thermocouple as the sample mount. I also had an instrument that had a
thermocouple in contact with the powder sample, which due to thermal expansion
mismatch would upon heating expand and split the sample surface giving rise to
ugly split peaks so be careful... (you can imagine that implementation lasted
as many samples as it took me to figure out what the peak splitting problem
was).
Alex Y
Dr. Alexandre (Alex) F. T. Yokochi
Assistant Professor
Department of Chemical Engineering
Oregon State University
Corvallis, OR 97331-2702
Voice: (541) 737-9357
Fax: (541) 737-4600
http://oregonstate.edu/~yokochia
-Original Message-
From: Payzant, E. Andrew [mailto:payza...@ornl.gov]
Sent: Thursday, October 15, 2009 6:27 AM
To: Rietveld list
Subject: Re: Re : HT XRD calibration
Patrick,
That is a good idea. Note that to actually get rid of the uncertainty you
need to be certain that the thermocouple is either reading the temperature
of the actual sample volume contributing to the diffraction pattern, or
that it is reading the temperature of a location where the temperature is
exactly the same as the sample volume.
Andrew
On 10/14/09 11:25 AM, Patrick Weisbecker weis...@yahoo.fr wrote:
Hi,
One way to get rid of the temperature uncertainty is to introduce a second
thermocouple inside the furnace and to fix it as close as possible from
the sample. If the sample is a bulk sample:Drill a hole inside the sample
and put the thermocouple inside the hole and if the the sample is made of
powder put it inside the powder.
The are some miniature thermocouple (0.25-0.5mm diameter), mineral
insulated which are unlikely to react with the samples.
Patrick
- Message d'origine
De : Payzant, E. Andrew payza...@ornl.gov
À : Mikko Heikkilä mikko.j.heikk...@helsinki.fi
Cc : Rietveld list rietveld_l@ill.fr
Envoyé le : Mer 14 Octobre 2009, 16 h 39 min 58 s
Objet : Re: HT XRD calibration
All,
The two calibration standard method was described by Andy Drews (Ford
Research Lab) in Advances in X-ray Analysis (the proceedings of the Denver
X-ray Conference), vol. 44, page 44 (2001).
Scott Misture (Alfred Univ), Stephen Skinner (Imperial College), Christian
Resch (Anton Paar), and I ran a workshop on non-ambient diffraction at the
Denver X-ray Conference in 2008, where we discussed many of these issues.
There was a lively discussion with the attendees regarding various
experiences with HTXRD, and the difficulty in abtaining the right
answer. Too often we are so focused on the precision and accuracy of our
refinements that we neglect the temperature issue, to our peril.
One issue with the HT16 hot stage is that you will likely need to
recalibrate with every new heater strip - don't rely on them behaving the
same. Even for a particular strip, use of an internal standard is best if
you can use one that does not react with either the strip or the sample.
If you push this furnace to temperatures above 1200degC or so, the
platinum will creep and the tensioning will change, so that the
height/temperature calibration may subsequently be off.
I agree fully with the suggestion to use parallel beam optics to eliminate
the sample height error. You do lose some angular resolution of course, at
least on a laboratory diffractometer, but you take one variable (sample
height) out of the refinement.
The other BIG problem with the HT16 (and many other non-ambient stages) is
knowing the temperature of the SAMPLE (as opposed to the strip). Powders
have hopelessly poor thermal conductivity, and the gradients in these
furnaces can be substantial. We rarely measure the temperature of the
sample directly, and this is another reason why internal standards are
very useful - you can use either a known CTE to estimate the delta T over
the full temperature range, or a known phase transformation to determine
delta T at a single point. I personally prefer the former approach myself.
Note that a solid-solid transformation may give a different result than a
melting point standard, since the liquid will change the thermal
conductivity between the strip and the sample. You can find more
discussion of these issues in Mark Rodriguez's chapter in the book
Industrial Applications of X-ray Diffraction, or mine
