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What sealing material can be used?

In NorECs we use only gold as sealing material. The gold rings which we use are made of pure gold. We use this material because
a) The seal can easily be detached by melting the gold, allowing the support tube and possibly also sample to be reused.
b) The seal is softer during use, allowing sample and support to have bigger differences in thermal expansions. For instance we can use perovskite membranes such as SrCeO3 with large thermal expansions on alumina supports.
c) We avoid contamination with silicon in the sample, on surfaces, and in the support tube.

Of course, the ProboStat can also be used with glass seals; in this case the sample can be cut off the support using a diamond saw, and sometimes the support used again. Glass or glass ceramics give unfortunately more challenges in terms of thermal expansion mismatch. Other alternatives are either silver or brazing alloy. They require special procedures and testing for gaining experience by the user. Pt washers as sealing material is not recommended as Pt is harder than Au and would need higher temperature to seal. Possibly, one could use Pt paint instead, but we have not tested that.

One can say that making a good gas-tight seal with gold gaskets is never easy in any case; it takes practice and patience. We make many gas-tight seals between alumina support and many test materials, and are successful in around 70 % of the cases. If the samples are thin and fragile however, this rate may go down, and glass sealing may be a better option.

In order to use Au sealing rings, you will need to go up in temperature to 1055-1060 °C.

If you need to seal at temperatures as high as 1400°C, Pt or ceramic felt are possible candidates, but none of these materials will give perfect sealing. The user will have to accept small leakages. For the SOFC field, the small leakage is not a big problem, but the user needs to be aware of this when applying gases in the two compartments, in order to run the measurements safely. For SOFC, a possible sealing material is Thermiculite® 866 (based on Vermiculite®). It is designed as a compression seal. Vermiculite is closely related to mica, a mineral noted for high temperature capability, high chemical resistance and an electrical insulation. We can supply plates with thickness 0.5mm which can be cut relatively easily in any gasket shape using a scalpel or similar. It is however recommended to keep at least 3-4mm between the outer and inner cut in order to avoid any cracks in the material. Thermiculite is stable up to 1000°C, is soft and compressible, and with no organic content. From our experience, Thermiculite gives better sealing in the higher temperature region (800°C) when tested in the ProboStat. At 400°C, a complete gas-tight seal was difficult to achieve. Stiff springs is recommended.

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