ABSTRACT

McWhan and collaborators at Grenoble presented in 1974 a piston-cylinder device which became for the following 25 years the working horse for high pressure neutron scattering beyond ∼ 1 GPa [29]. The characteristic element of the McWhan cell (Figures 6.1 and 6.2) is a biconical cylinder made of high density sintered Al2O3 alumina

1 (“Lucalox” from GE or “Widalox R” from Krupp) or ZrO2 zirconia with a bore of ca 5 mm which contains the sample (plus other elements as discussed further below) and two pistons made of tungsten carbide on each side. Lucalox is composed of 99.9% pure Al2O3 and the rest MgO as binder and has a compression strength of approximately 3.8 GPa [32]. The cylinder is on both ends radially supported by a steel ring which is pushed over the two conical parts. In McWhan’s original design [29], the device had two independently driven hydraulic rams: One to drive the upper tungsten carbide piston to compress the sample and another to push the support rings around the bicone. The force on the support rings was carefully adjusted as the pressure in the sample was increased. Using this method, the maximal reported pressure is 4.2 GPa, derived from the observed first-order phase-transition in Cs. The today’s accepted value for this transition is still the same, so it is clear that the McWhan design is capable to generate pressures beyond 4 GPa.