Calibration based on a primary pressure scale in a multi-anvil device

Abstract

A key question to all high-pressure research arises from the reliability of pressure standards. There is some indication and discussion of an uncertainty of 10 – 20% for higher pressures in all standards. Independent and simultaneous investigation of the dynamical (ultrasonic interferometry of elastic wave velocities) and static (XRD-measurement of the pressure-induced volume decline) compressibility on a sample reveal the possibility of a standard-free pressure calibration and, consequently an absolute pressure measurement, because all required parameter are collected directly; no additional data, e.g. the volume dependence of the Grüneisen parameter etc. are needed. Ultrasonic interferometry is used to measure velocities of elastic compressional and shear waves in the multi-anvil high-pressure device MAX80 at HASYLAB Hamburg enables XRD, X-radiography, and ultrasonic experiments. Two of the six anvils were equipped with lithium niobate transducers of 33.3 MHz natural frequency. NaCl was used as pressure calibrant, using the equation of state (EoS) of [J. Appl. Phys. 42 (1971) 3239], and sample for ultrasonic interferometry at the same time. From the ultrasonic wave velocity data, v p and v s we calculated the compressibility of NaCl as a function of pressure independent from NaCl-pressure calibrant. To derive the ultrasonic wave velocities from the interferometric frequencies of constructive and destructive interference requires precise in situ sample length measurements. For a NaCl-sample this is of particular importance, because the sample is the most ductile part of the whole set-up. We measured the sample length by XRD-scanning and by X-radiography. The compressibility results, derived from the ultrasonic data, were compared with data of static compression experiments up to 5 GPa [Phys. Rev. 57 (1940) 237] and up to 30 GPa [J. Geophys. Res. 91 (1986) 4949] using experimental data from [J. Phys. Chem. Solids 41 (1980) 517] and [Accurate Characterization of the High Pressure Environment]. At 1.2 and 5.3 GPa our velocity-derived compressibility data agree with the results of static compression. In the range between 2 and 4 GPa our dynamical data have 1.5–3% higher values. In general, the pressure revealed according to [J. Appl. Phys. 42 (1971) 3239] is in accordance to our standard-free pressure calibration. Consequently, up to 8 GPa the NaCl pressure standard has a reliability of at least 1%. However, there is some evidence that at higher pressures the inaccuracy of the NaCl standard seems to exceed 1%. Extrapolation of the compressibility data to higher pressures would also result in an increasing deviation, for EoS-fit and numerical fit of the density more than for the deformation fit .