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3.10 d. Extension of the Pd activity sensor technique to high pressure (L. Chamberlin)

It would be valuable to extend application of the Pd technique (see Sect. 3.7 h) to high pressure so the effect of water content on the thermodynamics of melt phases can be studied. In order to accomplish this, P, T, and fO2 conditions must be found under which measurable amounts of Mg, Al, and Si can be dissolved into Pd in equilibrium with oxide-based materials. One must be able to maintain the experimental charge in the piston-cylinder press at this low fO2 for sufficient time for the alloying reaction to take place. Experiments have been performed to determine whether the assemblages Fe-FeO, NbO2-Nb2O5, Cr-Cr2O3, or V-VO were suitable as solid fO2 buffers for the double-capsule technique. All of these assemblages proved either insufficiently stable to buffer the Pd + pure-oxide samples for a long enough time to ensure equilibration, or were so reducing that they caused melting of the inner capsules containing the samples. However, Pd equilibrated with quartz for 48 hours inside a graphite capsule (C-CO) was shown upon analysis to contain 360 ppm dissolved Si. This is a concentration that is similar to those routinely achieved in the experiments at 1 atmosphere and can be measured to good precision by the electron microprobe. This result indicates that the technique is feasible at high pressure if the solution behaviour of the Pd alloy at a given P can be established.

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