Synchrotron X-ray Diffraction Studies of Phase Transitions and Mechanical Properties of Nanocrystalline Materials at High Pressure PDF Download

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The behavior of nanocrystals under extreme pressure was investigated using synchrotron x-ray diffraction. A major part of this investigation was the testing of a prototype synchrotron endstation on a bend magnet beamline at the Advanced Light Source for high pressure work using a diamond anvil cell. The experiments conducted and documented here helped to determine issues of efficiency and accuracy that had to be resolved before the construction of a dedicated ''super-bend'' beamline and endstation. The major conclusions were the need for a cryo-cooled monochromator and a fully remote-controllable pressurization system which would decrease the time to change pressure and greatly reduce the error created by the re-placement of the diamond anvil cell after each pressure change. Two very different types of nanocrystal systems were studied, colloidal iron oxide (Fe[sub 2]O[sub 3]) and thin film TiN/BN. Iron oxide nanocrystals were found to have a transition from the[gamma] to the[alpha] structure at a pressure strongly dependent on the size of the nanocrystals, ranging from 26 GPa for 7.2 nm nanocrystals to 37 GPa for 3.6 nm nanocrystals. All nanocrystals were found to remain in the[alpha] structure even after release of pressure. The transition pressure was also found, for a constant size (5.7 nm) to be strongly dependent on the degree of aggregation of the nanocrystals, increasing from 30 GPa for completely dissolved nanocrystals to 45 GPa for strongly aggregated nanocrystals. Furthermore, the x-ray diffraction pattern of the pressure induced[alpha] phase demonstrated a decrease in intensity for certain select peaks. Together, these observations were used to make a complete picture of the phase transition in nanocrystalline systems. The size dependence of the transition was interpreted as resulting from the extremely high surface energy of the[alpha] phase which would increase the thermodynamic offset and thereby increase the kinetic barrier to transition that must be overridden with pressure. The anomalous intensities in the x-ray diffraction patterns were interpreted as being the result of stacking faults, indicating that the mechanism of transition proceeds by the sliding of[gamma](111) planes to form[alpha](001) planes. The increasing transition pressure for more aggregated samples may be due to a positive activation volume, retarding the transition for nanocrystals with less excess (organic) volume available to them. The lack of a reverse transition upon decompression makes this interpretation more difficult because of the lack of an observable hysteresis, and it is therefore difficult to ascertain kinetic effects for certain. In the case TiN/BN nanocomposite systems, it was found that the bulk modulus (B[sub 0]) of the TiN nanoparticles was not correlated to the observed hardness or Young's modulus of the macroscopic thin film. This indicates that the origin of the observed super-hard nature of these materials is not due to any change in the Ti-N interatomic potential. Rather, the enhanced hardness must be due to nano-structural effects. It was also found that during pressurization the TiN nanoparticles developed a great deal of strain. This strain can be related to defects induced in individual nanoparticles which generates strain in adjacent particles due to the highly coupled nature of the system.