THE THERMAL-PROPERTIES OF NANOCRYSTALLINE PD FROM 16-K TO 300-K

Quantitative X-ray diffraction measurements made over a temperature range 16-300 K on a Pd sample of 8-3 nm median grain size were compared with data acquired from a coarse-grained Pd reference sample. The larger Debye-Waller parameter of nanocrystalline Pd was found to be due to increased static displacements of atoms from their equilibrium sites compared with the static displacements in coarse-grained material. Consistent with this behaviour, the strain distribution determined from the width of intensity peaks was significantly broader in the nanocrystalline sample than in the coarse-grained sample. No grain-size-correlated differences in either thermal vibrational amplitude or lattice parameter were observed. In contradiction to previous reports of greatly increased thermal expansion coefficients in nanocrystalline metals, the change in lattice parameter of Pd from 16 to 300 K was observed to be independent of grain size. The results indicate that some regions of the nanocrystalline sample are in a state of compressive stress, while other regions are under a tensile stress. Comparisons with results for unconsolidated nanometre-sized powders indicate that the origin of the increased static displacements in nanocrystalline Pd may be due to the ultrafine powder production or subsequent consolidation processes, rather than being an inherent feature associated with a particular grain size.