Development of synchrotron X-ray micro-tomography under extreme conditions of pressure and temperature.

X-ray tomography is a non-destructive three-dimensional imaging/microanalysis technique selective to a wide range of properties such as density, chemical composition, chemical states and crystallographic structure with extremely high sensitivity and spatial resolution. Here the development of in situ high-pressure high-temperature micro-tomography using a rotating module for the Paris-Edinburgh cell combined with synchrotron radiation is described. By rotating the sample chamber by 360°, the limited angular aperture of ordinary high-pressure cells is surmounted. Such a non-destructive high-resolution probe provides three-dimensional insight on the morphological and structural evolution of crystalline as well as amorphous phases during high pressure and temperature treatment. To demonstrate the potentials of this new experimental technique the compression behavior of a basalt glass is investigated by X-ray absorption tomography, and diffraction/scattering tomography imaging of the structural changes during the polymerization of C60 molecules under pressure is performed. Small size and weight of the loading frame and rotating module means that this apparatus is portable, and can be readily installed on most synchrotron facilities to take advantage of the diversity of three-dimensional imaging techniques available at beamlines. This experimental breakthrough should open new ways for in situ imaging of materials under extreme pressure-temperature-stress conditions, impacting diverse areas in physics, chemistry, geology or materials sciences.

"Compressed graphite" formed during C60 to diamond transformation as revealed by scattering computed tomography.

The collapsing of C60 into polycrystalline diamond has been studied after nonhydrostatic pressurization at ambient temperature using x-ray scattering computed tomography. Using this selective structural probe we provide evidence of concentric coexistence of "compressed graphite" (d(00l)∼3.09-3.11 Å), sp2-graphitelike phase (d(00l)∼3.35-3.42 Å), and sp3-like amorphous carbon surrounding polycrystalline diamond (a∼3.56-3.59 Å). The so-called "compressed graphite" exhibits a collapsed c axis and is textured with disordered layers. This latter phase is better described as a short interlayered carbon phase with buckled sp2-sp3 layers with possible interlayer bonding. Additionally, our 3D maps of phase distribution and of the residual stress retained in the polycrystalline diamond phase support the importance of stressed synthesis conditions for diamond formation.

High efficiency multichannel collimator for structural studies of liquids and low-Z materials at high pressures and temperatures.

A high efficiency multichannel collimator (MCC) device has been developed at the high pressure beamline ID27 of the European Synchrotron Radiation Facility to drastically reduce the x-ray background from the sample environment in the Paris-Edinburgh press. The main technical difficulty, which resides in the minimum slits size achievable using the classical mono-bloc design, has been resolved using an original concept based on a set of independent slits. Then, a very small slit size of 50 µm was manufactured resulting in a great improvement of the signal to background ratio. In addition, the transfer function of the MCC has been measured using the x-ray diffusion signal of a metal doped glass and efficiently applied to correct the raw data. The potential of this new device is illustrated in two challenging examples: iron-sulfur liquid structures and C(60) polymerization process at high pressure and high temperature.