Sample Assemblies & Expendables
One of the most common sample configurations employed in piston-cylinder work is shown in the schematic cross-section. A halite or BaCO3 cell, pyrex sleeve, graphite furnace and sample container are arranged in a nested cylindrical arrangement. Crushable alumina or MgO is packed into the space surrounding the sample capsule and the assembly is capped by a steel base plug and pyrophyllite sleeve. The plug makes electrical contact to the graphite furnace and top plate of the stack, which is electrically isolated. The remaining contact to the furnace element is made through the press frame via the piston, pressure vessel and lower stack components. Sample temperature is measured by means of a thermocouple which is placed in a hole through the base plug and ceramic filler
The sample crucible is usually a cup cut from hollow alumina or magnesia tubing. Sealed gold or platinum capsules are used if potential reactions with ceramic crucibles are a concern. Several hundred milligrams of material can be reacted in the apparatus using a 3/4" diameter assembly and sample cup of the scale shown above. A deeper sample cup can be used, but care must be exercised to ensure that the length of the specimen is not subjected to a substantial temperature gradient. Special tapered graphite heaters which minimize the axial gradient are available from Rockland Research, in addition to the standard variety, which are supplied with our sample assemblies.
The materials chosen to fabricate sample assemblies and crucibles for experiments depend on the demands of the investigation, and there is considerable room for individual innovation. If required, an oxygen atmosphere can be created by mixing an oxidizer such as KClO3 with the reactants, or it can be separated from the reactants with a ceramic or noble metal partition. Reducing environments can be achieved by using the proper metal/metal oxide buffer in the reaction mixture.