Chapter 21 - Calibration based on a primary pressure scale in a multi-anvil device

Abstract

Independent and simultaneous investigation of the dynamical and static compressibility on a sample reveal the possibility of a standard-free pressure calibration and, consequently, an absolute pressure measurement because all required parameters are collected directly; no additional data, e.g. the volume dependence of the Griineisen parameter etc. are needed. Multi-anvil devices are a very successful tool for experimental simulation of mantle conditions with relatively large samples. The formation and the development of gaskets between the anvils causes a deviation between load per anvil surface and pressure inside the set-up because of friction, material variation of the pressure transmitting medium, minor fit variation in the set-up, minor adjustment variation of the set-up and the anvils to each other, and different compressibility of the samples. The chapter details the working of MAX80 that is a single-stage multi-anvil apparatus equipped for ultrasonic interferometry and permanently located at HAmburger SYnchrotron LABor (HASYLAB), Hamburg for having access to synchrotron radiation for in situ X-Ray Diffraction (XRD) measurements. Ultrasonic interferometry is used to measure velocities of elastic compressional and shear waves in the multi-anvil high-pressure device MAX80. Simultaneous XRD- and high-pressure ultrasonic interferometry measurements of compressional and shear wave velocities of polycrystalline NaCl to determine a standard-free pressure scale and to test the existing equation of state (EoS) is presented. Two of the six anvils are equipped with lithium niobate transducers of 33.3 MHz natural frequency. NaCl is used as pressure calibrant, using the equation of state (EoS) of and sample for ultrasonic interferometry at the same time. From the ultrasonic wave velocity data, Vp and Vs, the compressibility of NaCl as a function of pressure is calculated independently from NaCl-pressure calibrant. To derive the ultrasonic wave velocities from the interferometric frequencies of constructive and destructive interference, precise in situ sample length measurements are required. For a NaCl-sample this is of particular importance because the sample is the most ductile part of the whole set-up.