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 Synthesis
Most of the samples that we
conduct our Ultrasonic experiments on are synthesized by ourselves in the High
Pressure Laboratory here at
Ultrasonic Measurements at High Pressure
In High Pressure Lab, we can use the Kennedy Press for conducting in situ ultrasonic interferometry experiments at room temperature to as high as 20 GPa. Shown at right is a schematic cross-section of an octahedral semi-sintered MgO cell assembly used for such measurements. Details about this experimental assembly can be found in the links on the High Pressure Lab Homepage (similar cell also used in Kung et al., 2004).
Combined Ultrasonic and Synchrotron X-radiation 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 on a much wider range of materials. the techniques developed on beamline X17B2 at the NSLS/BNL now have been adapted to many other synchreotron facilities, including the GSECARS at the Advanced Photon Source, APring8/Japan, HASHYLAB/Germany, and soon to ESRF/France. What follows is a basic description of the techniques we use in these synchrotron experiments, and schematics of how everything works.
Experimental Set-up at X17B2, NSLS, Brookhaven national
Lab
a) Cell Assembly . For a larger version of the sample cell
assembly, click here.
b) Energy Dispersive X-Ray Diffraction: For monitoring structural change, unit
cell parameters and unit cell volume
determination
c) CCD Camera for X-Ray Imaging –Imaging the entire sample in 2D which yields
the direct determination of sample length change at any P T conditions. For a
larger view of the sample image, click
here.
d) Ultrasonic Interferometry –A dual-mode transducer capable of generating
frequencies from 20 to 70 MHz for both P and S Waves. A newly developed
Transfer Function Method, as described in Li et al., 2002, is used to obtain two-way travel time in the
sample. More information on this experimental setup can be found in Li et al., 2004.
From the sample lengths and two-way travel times, we can then directly obtain
the P and S wave velocities of the sample. With in-situ X-ray data, we 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 to the
third-order finite-strain equations, from which we obtain not only the adiabatic
bulk and shear moduli and their pressure and temperature derivatives, but also
the absolute pressure exerted on the sample directly, an accurate method for
pressure scale determination.
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.
Compression Studies Using Diamond Anvil Cell (DAC)
In High Pressure Lab, we are also equipped with diamond anvil cells, which are primarily used for the study of phase transformation and equation of state of materials at high pressure. It has been shown that with DAC, we can study materials at the Earth’s core pressures, but the sample size is only a few tens micrometers. The transparency of diamonds allows for many spectroscopic approaches for the in-situ investigation of the sample at these extreme pressure, such as X-ray diffraction, absoption, Brillouin scattering, nuclear resonance scatteing, and many more. More details can be found in a recent review Bass, Sinogeikin and Li (2008).
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