Hi Rory,
At the moment we are in the same position i.e. attempting to calibrate the
temperature and systematic offsets in an HTK10 furnace with Pt strip heater.
One approach is to take materials which have a well defined solid-solid phase
transition or melting point and observe this as a function of set temperature.
Unfortunately, most materials only provide one or two calibration points and
hence you will need to heat several samples in order to cover a wide
temperature range.
At the moment we (and by "we" I mean my PhD student Mark Styles is doing all
the work and I stick my head in occasionally and say "how's it going?") have
taken an 'external standard' approach (we cannot add a standard to our sample
and risk reaction). In this we heat a material with known thermal coefficient
of expansion (TCE), measure the unit cell edge a function of temperature and
then determine a calibration curve. However, the flaw in this approach will be
the impact of other systematic errors (e.g. sample displacement) on the
observed peak positions and hence cell edge.
John Evans (Durham Uni) uses a modified TCE approach in which he adds two
standards with different TCE's (I haven't got the references to hand, but this
is described in one of his papers) and normalises observed cell edge (or cell
volume) to that at the starting temperature (To). Instead of using the absolute
cell edge (volume) for each phase, he then uses the difference between the two
compounds as a way of eliminating the systematic errors which should affect
both equally.
If you pardon the pun, this is a hot topic for us at the moment - we are coming
up to some synchrotron time where T calibration will be important - we have not
completely sorted out the fine details but intend to:-
(1) Use a mixture of NIST silicon and corundum
(2) Heat to various temperatures over the range of interest
(3) Use the 'surface analysis' approach (Stinton and Evans) which relies on
use of the Topas Rietveld software. In this, the cell edge is not refined, but
defined as a function of applied temperature, the temperature offset (a
refinable parameter) and the published TCE equations for each phase.
What we hope to end up with is an analytical method which is quick and easy to
apply to produce a T calibration for every new heater - i.e. it should not be
an onerous task so that it can be carried out frequently.
I hope this helps - we still have much fine detail to sort out.
Cheers
o----------------------oo0oo---------------------------o
Ian Madsen
Team Leader - Diffraction Science
CSIRO Minerals
Box 312
Clayton South 3169
Victoria
AUSTRALIA
Phone +61 3 9545 8785 direct
+61 3 9545 8500 switch
+61 (0) 417 554 935 mobile
FAX +61 3 9562 8919
Email ian.mad...@csiro.au
o----------------------oo0oo---------------------------o
-----Original Message-----
From: r.m.wil...@qmul.ac.uk [mailto:r.m.wil...@qmul.ac.uk]
Sent: Thursday, 8 October 2009 3:14 AM
To: rietveld_l@ill.fr
Subject: HT XRD calibration
Dear All
We have an Anton Paar HTK16 furnace with a Pt strip. I am
hoping to calibrate it both thermally and mechanically (height shift)
using either alpha alumina or ceria so as to better identify HT
phases. Have any of you experience of this, or better suggestions for
calibrants? My only serious worry is whether there is any potential
for ceria to react with, the Pt strip?
(Perhaps under vacuum?) I'd welcome any advice or suggestions.
Yours
Rory
************************************
Dr R.M. Wilson,
Experimental Officer & RPS
School of Eng. and Materials,
G27 & G30 X-ray Laboratories,
Francis Bancroft Building
Queen Mary College
University of London
Mile End Road
LONDON E1 4NS
UK
Tel: +44 (0)20 7882 7938
Fax: +44 (0)20 7882 7931
************************************
