Here are some tips and techniques for mounting samples into optical cryostats gathered from the researchers who entered our prize draw - ‘How to mount your sample’.
Different types of sample
The range of different samples used in optical cryostats is huge and included liquids and powders, but the most common form of sample are solids.
Solid samples are often mounted directly to a sample holder. Mechanically clamping the sample to ensure good thermal contact is very popular but can only be used for robust and larger samples. This method is not practical for some experiments, for example where minimising strain is a requirement.
If clamping is not possible, a number of alternative techniques are often used. A small amount of grease is a popular method. Apiezon type N-grease was often suggested, particularly when it is important that the sample can be easily removed and perhaps re-used in other experiments. Others suggestions are GE varnish and silver conducting paint or paste. To permanently attach the sample to a holder, various types of adhesives can be used.
Dr Erik van Heumen of Van der Waals - Zeeman Institute, University of Amsterdam is doing IR and UV reflection and transmission experiments on a range of different samples, including high temperature superconductors, topological insulators and correlated oxides. He first cuts his samples to a regular shape making them as large as possible. He suggested that a good technique to improve thermal contact is to cut the sides under a small angle, so that you get a pyramid like shape. He glues the sample with the tip of the pyramid downwards, filling up the remaining space with some glue for additional contact. Dr Heumen’s preferred adhesive is a two part epoxy such as EpoTek E4110.
Chip carriers are often used when electrical connections to the sample are an important requirement.
Dr Michael T. Pettes of Connecticut University is doing micro-thermometry experiments on semiconducting nanostructures in his optical cryostats. He wire bonds his sample to a 16 or 20 pin DIP chip carrier and a micro-device with two four-probe resistors and one nanowire / graphene sheet, etc. in four-probe configuration, with 12 wires in total. Current is passed through one resistor to heat the device and the other resistors are used to measure thermo-electric voltages and temperatures. This allows him to measure electrical resistivity, contact resistance, Seebeck coefficient and thermal conductivity.
Powders are much more difficult to mount and make a good thermal contact. Some suggestion include pressing the powder into pellets and clamping these to the sample holder itself.
Dr Andrei Diaconu of Stefan Cel Mare, University of Suceava is conducting diffuse reflectivity experiments on spin-crossover powder materials in his optical cryostat. The powder is sprinkled on to a patch of silver paste and pressed to obtain a thin layer of powder on top, which fully covers the silver paste. He then uses a heat gun for approximately 5 minutes to dry the paste, before mounting this layered construction into his cryostat.
Liquid samples are typically held in a container of some sort. EPR tubes, thick walled clamped cells and NMR tubes are popular.