Lubrication, Gasketing, and Precision in Multianvil Experiments

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American Mineralogist
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Lubrication, gasketing, and pressure medium effects on pressure generation and reproducibility were investigated with 35 experiments on the Bi transitions in a multianvil apparatus. The system of integral gasket fins on octahedral pressure media and sliding, cylindrical anvil-driving wedges within a retaining ring, similar to that of Walker et al. (1990), was used for all experiments. Internal friction in this device of approximately 10% of thrust, the existence of which was previously known from ring-strain hysteresis, has been found to be at the interface between the anvils and their cylindrical driving wedges. This friction is a common property of all MA6/MA8 devices, not just the design employed here. Truncated edge length (TEL) anvil facets of 8 mm on MgO (5% Cr2O3) octahedra with epoxy-Al2O3 gaskets achieved Bi III --> V at 242 +/- 6 tons (equivalent to 2.15 +/- 0.05 MN, 1 ton force = 2000 lb force = 8.90 kN), in six experiments between 233 and 250 tons with standard deviation of the mean, sigma = +/- 2.5%. This gasket system gives excellent pressurization efficiency and precision at room temperature and also affords nearly perfect blowout suppression. A procedure for fabricating the pressure medium and gasket fins from castable ceramics in a single operation was developed for high-temperature experiments. Using an MgO-based castable ceramic at room temperature, Bi III --> V on 8 mm TEL was found at 266 +/- 11 tons (eight experiments, 249-282 tons, sigma +/- 4%). whereas for 6 mm TEL it was found at 159 +/- 6 tons (eight experiments, 149-168 tons, sigma +/- 4%). The advantages of castable ceramics, in addition to ease of fabrication and rigorous dehydration, include their excellent high-temperature characteristics of thermal insulation, pressure stability, and thermocouple survival. Calibrations on the coesite to stishovite transition at 1200-degrees-C confirm the good performance of these ceramics. Experiments of several days' duration within the melting range have been routinely achieved with this single-substance gasket and pressure medium system.


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