Improved Test Cell Material for the Tetrahedral Anvil Apparatus

Abstract

TEST cells for ultra-high-pressure apparatus of the ‘belt’ or ‘anvil’ types¹ are typically made from pyrophyllite, a naturally occurring aluminium silicate. It is unlikely, however, that this material possesses an ideal combination of mechanical properties for this purpose, a view which is supported by the shape of the load/pressure calibration curve. Fig. 1 (curve A) shows this experimental relation for a 0.75-in. edge National Bureau of Standards type tetrahedral anvil apparatus using pyrophyllite tetrahedra. The usual resistance transitions in bismuth at 25.4 kbars (ref. 2), thallium at 36.7 kbars (ref. 2), and barium at 59 kbars (ref. 3) were used, with the specimen geometry shown. It will be noted that although at 25.4 kbars loads close to the nominal pressure (that is, load/anvil area) line B are obtained, at higher pressure there is a more or less linear fall-off towards high loads, the efficiency of pressure generation falling away rapidly. No explanation of this has as yet been published, but two likely hypotheses may be put forward: either (a) it is due to the load on the compressible gaskets, or (b) it is due to stress difference in the pyrophyllite (which can be high at high pressures)¹, since the maximum principal stress would be expected to be normal to the anvil face in the pyrophyllite just below the surface. Experiments have been made with specially profiled and lubricated anvils⁴: these indicate that gasket loads are probably not significant throughout the pressure range, so that (b) is the more likely hypothesis. Since this involves a fundamental mechanical property, an alternative test cell material was sought with a different relation between shear strength and compressibility.