Nanoindentation of Pressure Quenched Fullerenes and Zirconium Metal from a Diamond Anvil Cell
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Nanoindentation of Pressure Quenched Fullerenes and Zirconium Metal from a Diamond Anvil Cell Shane A. Catledge, Philemon T. Spencer, Jeremy R. Patterson, and Yogesh K. Vohra Department of Physics, University of Alabama at Birmingham (UAB) Birmingham, AL 35294-1170 ABSTRACT The sample size employed in high pressure diamond anvil cells is limited to a diameter of typically 25 to 150 microns. While this size is often sufficient for diagnostics using synchrotron x-ray diffraction and Raman scattering, ex-situ measurements of mechanical properties using conventional microhardness indentation techniques is not feasible. For some materials, the high pressure phase(s) can be quenched to ambient pressure allowing further characterization by other techniques. We make use of the very small probe volume allowed by nanoindentation to investigate the pressure-quenched structures of both C70 fullerene and zirconium. For the case of C70, we show that the amorphous phase established above 35 GPa can be quenched to ambient, and that it shows a largely elastic indentation loading behavior with a hardness of 30 GPa. We establish that this hard carbon phase contains a mixture of sp2- and sp3-bonded carbon and that it can be produced from C70 fullerene by application of pressure at room temperature. With regard to zirconium metal, we confirm the irreversible transformation from the ambient hexagonalclose-packed phase to the simple hexagonal ω-phase (AlB2 structure) and document an 80% increase in hardness that may be attributed to the presence of covalent bonding based on sd2hybridized orbitals forming graphite-like nets in the (0001) plane of the AlB2 structure. INTRODUCTION Due to the advent of the diamond anvil cell (DAC), the physical properties of materials can be investigated under static pressures of millions of atmospheres and temperatures of several thousand Kelvin. Investigation of materials over such a large pressure and temperature range has resulted in the identification of new structures and has provided a fundamental understanding of the physical transformations that occur under extreme conditions [1]. Characterization of materials using DACs is typically performed in situ (under high pressure and/or temperature) via the use of x-ray diffraction, Raman scattering, or electrical transport measurements. In some cases, the high pressure and/or high temperature phase of interest can be quenched to ambient conditions, allowing further ex situ characterization. Of particular interest is investigation of mechanical properties, such as hardness, of the quenched phase [2]. However, this type of measurement has been impractical due to the very small sample volumes required in DAC devices in which the sample chamber is typically only 25-150 µm in diameter, depending on the maximum pressure needed. We have overcome this difficulty by taking advantage of the small probe volume available from a depth-sensing nanoindenter. In this way, we have obtained hardness measurements from pressure quenched samples of C70 fullerene and zirconium m
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