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Cooling samples at ultra-low temperatures

Low Temperature Cells

Low Temperature Cells
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Sample mounting methods during an ultra-low temperature experiment can raise many issues; the issues can be dependent on whether the sample is a crystal or a powder, how good or bad a conductor it is and how well it is coupled to its mounting or even the fridge.

The following paragraphs are a guide that should be acted upon well before an experiment commences.

Single Crystals
The first objective to achieve from an experimental point of view is the alignment of the sample, which is of paramount importance; however the joining of the sample to its mount is of equal importance and must be achieved with a conductive cryogenic resin

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Conduction through a single point of contact is far from ideal and there are a number of other techniques that can be employed to get the sample cold.
These include:

A.      Wrapping the crystal with copper wire, foil or coating the mount and sample with a thermally conductive paint.

B.      Enclosing the single crystal in an indium sealed can that has been sealed with Helium exchange gas.

Powders
The difficulties encountered when introducing powders to the ultra-low temperature region, where cooling is through conduction only, can be reduced immediately by avoiding the use of vanadium cans. If this is unavoidable then a copper wire can be joined to the can lid and pushed through the sample, although this is far from ideal.

Where possible indium sealed copper cans should be used, that are closed in a Helium atmosphere so that some exchange gas it residual with the sample; this however will not work for poorly conducting powders, some other options available for cooling powders are:

A.      Mixing the powder with a conductive powder or liquid; for example copper.

B.      Introducing Helium exchange gas into the powder by using gas capillaries.  

If you have any questions about mounting samples for experiments at cryogenic temperatures please contact the sample environment team and your instrument scientist to discuss your requirements in more detail.

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