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Pressure calibration of a 6-8 type multianvil apparatus is now underway following the recent installment of a 5000-tonne uniaxial press. The press frame is 5 meters in height, over 60 tonnes in weight and houses a 960 mm diameter ram with a travel distance of 200 mm. Six outer anvils are contained within a split cylinder geometry to create a working volume for eight 54 mm cubic inner anvils made of tungsten carbide. Phase transitions in silicates, metals and semiconductors were used to calibrate 18-mm (18M) and 25-mm (25M) edge-length MgO octahedra using 11-mm and 17-mm truncation edge lengths (TEL) on the tungsten carbide anvils, respectively. Calibration of the 18M assembly (Fig. 3.8-1) using 5 x 3 mm pyrophyllite gaskets was carried out at room temperature up to the GaP semiconductor to metal transition at 22 GPa and required 2660 tonnes. Pressure generation from the Bi I-II (2.55 GPa) to the Bi III-V (7.7 GPa) is very efficient with applied force, but efficiency decreases at higher pressures (ZnS at 15.6 GPa, GaAs at 18.3 GPa and GaP at 22 GPa). High-temperature (1000-1200°C) calibrations have been carried out for the same assembly up to the wadsleyite-ringwoodite transition at 18 GPa, which required about 2000 tonnes. For the high-temperature calibration, pressure efficiency was better than at room temperature, beginning with the CaGeO₃ (Gt-Pv) transition at 6 GPa, and continuing with coesite-stishovite at 9 GPa and forsterite-wadsleyite at 14 GPa, but curvature was evident in the pressure vs. load plot approaching the wadsleyite-ringwoodite transition at 18 GPa. Optimization of gasket composition and dimensions may improve efficiency at these higher applied loads. These results provide encouragement that it will be possible to increase sample volumes to the sizes necessary for in situ measurements of physical properties such as thermal conductivity and ultrasonic-wave velocity, on high-pressure phases such as ringwoodite and perovskite.
 

Fig. 3.8-1: Room temperature and high-temperature calibration of 18 mm MgO octahedron (18M) with 11 mm truncation edge length WC cubes in the 5000 tonne multianvil apparatus.