Techniques and Facilities

This page gives a brief overview of some of the techniques and facilities that we in the Ultrasonics Group use to conduct our research. For a more detailed discussion of the synchrotron-based techniques routinely used in our work, please see Li et al., 2004, and for a fuller explanation of the finite-strain method we use for determining the elastic moduli and the pressures in our experiments, see Liu and Li, 2007.

Sample SynthesisMost of the samples that we conduct our Ultrasonic experiments on we synthesize ourselves in the High Pressure Laboratory here at Stony Brook University. For more details on the experimental techniques and cell assemblies used in these sample synthesis experiments, see the High Pressure Lab Homepage. Once these samples have been synthesized, they are characterized using several different methods including X-Ray Diffraction in both the Stony Brook Crystallography Lab and at Beamline X17B2 at the NSLS, and Electron Microprobe. Once the samples are determined to be suitable, they are then prepared for ultrasonics experiments.

High Pressure LabIn addition to simple synthesis experiments, we can also use the Kennedy Press in the High Pressure Lab for conducting in situ ultrasonic interferometry experiments on large-volume samples that have been previously synthesized in either the Kennedy or Sumitomo presses. Shown at right is a schematic cross-section of an octahedral semi-sintered MgO cell assembly used for acoustic measurements in the Kennedy press. This figure was taken from Kung et al., 2004, and for a larger view, simply click the figure at right. Details about this experimental assembly can be found both in the previously linked paper and on the High Pressure Lab Homepage. This type of experiment has the advantage of being relatively quick and easy to assemble, and does not require beamtime at a national facility. However, we cannot use X-rays to study the sample's behavior in situ under the conditions of the experiment, which is a major drawback.

Synchrotron Experiments The development of high-pressure techniques at synchrotron beamlines have allowed for the studying of materials in situ under extreme conditions. Adaptation of the ultrasonic interferometry technique for use with these high-pressure synchrotron beamlines have enabled us to develop and carry out ultrasonic experiments with extreme accuracy and precision. Currently, the Ultrasonics Group makes use of beamline X17B2 at the National Synchrotron Light Source, and the GSECARS beamline at the Advanced Photon Source. What follows is a basic description of the techniques we use in these synchrotron experiments, and schematics of how everything works. Since the majority of our work is done at the NSLS, the following diagrams are tailored to show that experimental setup.

a) Cell Assembly – Schematic diagram of a typical experimental cell assembly used in studies using the DDIA press at X17B2. The NaCl + BN is a powdered mixture in 10:1 proportions to prevent significant grain growth during the experiment. A 1 micron-thick disc of Au foil is placed above and below the sample and at the bottom of the buffer rod to smooth all contact surfaces. For a larger version of the sample cell assembly, click here.

b) Energy Dispersive X-Ray Diffraction – X-Ray diffraction patterns in Energy Dispersive Mode are collected by a 4-element solid-state detector. For these experiments, 2-theta is calibrated using an internal standard. The sample X-Ray pattern shown here is of epsilon-FeSi collected at Room T at 15 tons of Oil Pressure during cold compression. JCPDS peaks for epsilon-FeSi are shown for reference, as are Pb fluorescence peaks.

c) CCD Camera for X-Ray Imaging – X-Ray images of the sample can be collected at each set of conditions during the experiment to monitor the sample length and any changes that occur during the study. The sample image shown here is again of epsilon-FeSi collected at 15 tons of oil pressure during initial cold compression. The black areas to left and right are the anvils. Dark gray area in the center is the FeSi sample. Light area above the sample is the NaCl, and the light area below is the buffer rod. Dark lines at top and bottom of sample are Au foil discs. For a larger view of the sample image, click here.

d) Ultrasonic Interferometer – Ultrasonic measurements were conducted using a dual-mode transducer capable of generating frequencies from 20 to 70 MHz. This allows for the simultaneous collection of both P and S Wave data, which are then convolved offline using the Transfer Function Method, as described in Li et al., 2002. The ultrasonic signal shown here illustrates the P-wave signal of epsilon-FeSi collected at Room T at 15 tons of oil pressure during initial cold compression. First pulse is anvil/buffer rod interface, second is buffer rod/sample interface, third is sample/salt interface. Two-way travel time in sample is determined by overlapping the pulse echoes of the second and third pulses. More information on this experimental setup can be found in Li et al., 2004.

By determining the length of the sample image in pixels at the end of the experiment when the press is opened, and then measuring the absolute length of the sample after the experiment, we can calibrate the pixel to length ratio, and thereby determine the absolute length of the sample at all P-T conditions. We use the pulse echo overlap (PEO) technique to determine the two-way travel times of P and S waves going through the sample. From the sample lengths and two-way travel times, we can then directly obtain the P and S wave velocities for the solid FeSi sample. We use the X-Ray data to determine the cell volume of the sample, which gives us the density of the material under the given conditions. All the velocity and density data are then fitted simultaneously to the third-order finite-strain equations shown at right (Eqs. 1-7), from which we obtain the adiabatic bulk and shear moduli, as well as their first pressure derivatives.

While all of these diagrams have focused on the experimental setup used at beamline X17B2 of the NSLS, the Large Volume Press Facility at GSECARS also can be used for ultrasonic experiments. They have a DDIA insert, which uses a cell assembly similar to the cubic one shown above, and they also have a split-cylinder Walker-type insert that makes use of an octahedral cell assembly similar to that shown for the Kennedy press above.

Again, for more information regarding these techniques, see Li et al., 2004, and for a fuller explanation of the finite-strain method we use for determining the elastic moduli and the pressures in our experiments, see Liu and Li, 2007. Also, for a discussion on how to obtained, process, and utilize temperature-dependent data, see Liu and Li, 2006.