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3.8 b. Pressure calibration using ultrasonic measurements at high pressures and room temperature in the multianvil press (J. Kung)

Calibration of multianvil presses requires knowledge of the relationship between oil pressure (or ram force) and the resulting pressure on the sample. This is typically performed by monitoring electrical resistance changes caused by phase transitions in standard materials at room temperature. However, the shapes of the calibration curves between these widely spaced transition points are unknown. To address this problem, we have measured the elastic wave speeds of standard materials while simultaneously identifying fixed-point phase transitions in Bi and ZnTe. At any given pressure the sound wave speeds are related to the density and elastic moduli of the material. By using materials with well-determined elastic properties this technique can be employed to calculate the pressure and, therefore, provide a continuous pressure calibration between the "fixed points."

The ultrasonic measurements were performed using techniques similar to those described in the Annual Report of 1997, in both the Hymag (1000 tonne) and Sumitomo (1200 tonne) presses. We used a commercial grade of high quality polycrystalline alumina (LucaloxTM, from General Electric Corporation) as the standard material because of its high acoustic quality and well-known elasticity data at high pressure. In addition, Bi and ZnTe were inserted into the sample assemblies in order to confirm the pressure calibration at "fixed-points".

Using the known equation of state of Al2O3, the measured travel times of LucaloxTM can be used to determine the pressures present in the cell assembly. Figure 3.8-2 shows the calculated cell pressures vs. oil pressure in four separate experiments in the Hymag press. In runs H916 (P wave) and H939 (P wave), Bi pressure calibrations were also performed. For H963 (S wave), Bi and ZnTe were loaded together for pressure calibration. The transitions of Bi occurred at average oil pressures of 31 to 33 bar (I-II, 2.55 GPa) and 128 to 134 bar (III-V, 7.70 GPa) and the transitions of ZnTe (I, 9.6 GPa) at 180 bar. The fixed-point pressure calibrations and ultrasonic data sets display a similar scattering trend. The scattering may be due to a small degree of non-reproducibility in the sample cell assembly and/or gasket extrusion.

Fig. 3.8-2: Pressure calibrations determined from Bi/ZnTe and Al2O3 (Lucalox) on the Hymag press Solid symbols are the pressures determined using ultrasonic measurements on Lucalox. Open symbols are the fixed-point pressure calibrations.

In order to compare the effective pressure environments in different presses, travel time measurements with an identical cell assembly and sample were carried out in the Sumitomo press. Figure 3.8-3 shows that the phase transitions of Bi (S2035) in the Sumitomo press were observed at higher ram force than those (H963) in the Hymag press, but the phase transitions of ZnTe (I) were observed at the same ram force in both presses. The pressure calibrations (S2035P and H963P) determined from ultrasonic measurements in the two presses, however, are almost identical.

This work has improved our understanding of ultrasonic measurements at high pressures and highlights the importance of cell assembly reproducibility in achieving consistent pressure calibrations. These results also provide a basis for extending the ultrasonic measurements to simultaneous high pressure, high temperature conditions.

Fig. 3.8-3: Comparison of the pressure calibration on the Hymag and Sumitomo presses. Open and closed symbols are the same as in Fig. 3.8-2.

Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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