Pressure-induced stiffness of Au nanoparticles to 71 GPa under quasi-hydrostatic loading

Abstract

The compressibility of nanocrystalline gold (n-Au, 20 nm) has been studied by x-ray total scattering using high-energy monochromatic x-rays in the diamond anvil cell under quasi-hydrostatic conditions up to 71 GPa. The bulk modulus, K0, of the n-Au obtained from fitting to a Vinet equation of state is ~196(3) GPa, which is about 17% higher than for the corresponding bulk materials (K0: 167 GPa). At low pressures (<7 GPa), the compression behavior of n-Au shows little difference from that of bulk Au. With increasing pressure, the compressive behavior of n-Au gradually deviates from the equation of state (EOS) of bulk gold. Analysis of the pair distribution function, peak broadening and Rietveld refinement reveals that the microstructure of n-Au is nearly a single-grain/domain at ambient conditions, but undergoes substantial pressure-induced reduction in grain size until 10 GPa. The results indicate that the nature of the internal microstructure in n-Au is associated with the observed EOS difference from bulk Au at high pressure. Full-pattern analysis confirms that significant changes in grain size, stacking faults, grain orientation and texture occur in n-Au at high pressure. We have observed direct experimental evidence of a transition in compressional mechanism for n-Au at ~20 GPa, i.e. from a deformation dominated by nucleation and motion of lattice dislocations (dislocation-mediated) to a prominent grain boundary mediated response to external pressure. The internal microstructure inside the nanoparticle (nanocrystallinity) plays a critical role for the macro-mechanical properties of nano-Au.

High-energy X-ray focusing and applications to pair distribution function investigation of Pt and Au nanoparticles at high pressures

“…At 71 GPa, the volume of n-Au is 55.5 Å 3 , which is larger than the bulk one (54.2 Å 3 ) calculated from ref. 31 , but smaller than the EOS value of Gu et al 28 (56.5 Å 3 ), suggesting a smaller sized-related stiffness as reported recently 13 . The blue line ( Fig.…”

“…Because almost all the EOS parameters have been based on Rietveld refinements so far, we then add the offset of 7.57 × 10 −4 Å to the PDF data (solid circles, Figure 4 ), leading to a good agreement with the data of Rietveld refinements. To get rid of the influence of lattice stiffness caused either by size 27 28 or microstructure (nanocrystallinity) 13 , the EOS curve at 12.35–12.70 GPa is normalized to the average value of Rietveld refinement (right axis, Fig. 4 ) for a direct comparison (right axis, Fig.…”

“…5 ) shows the calculated PDF based on the initial size of n-Au reduced by the effects of compression as determine from the EOS. It is clear that there is obvious pressure-induced extra damping in the G(r) function, which reflects a fundamental change of nanocrystallinity at high pressure 13 .…”

“…This reversible behavior may reflect a pressure dependence of the large screw dislocations found recently in Pt nanopartiles 39 . Detailed analysis of the n-Pt microstructure and stiffness will be published in a separated paper while that of n-Au has been published recently 13 .…”

“…High-energy synchrotron radiation can be especially valuable for providing structural information covering short-to-medium range atomic order in crystals but this requires obtaining data covering a large Q range 10 11 , where Q = (4πsinθ)/λ is the scattering wave vector. The atomic pair distribution function (PDF) method using high-energy X-ray or neutron diffraction is a powerful tool for studying crystalline, disordered, and nanocrytalline materials 10 11 12 13 . Experimentally, the atomic pair distribution function, G(r) , can be obtained by Fourier transform, where S(Q) is the total scattering structure function.…”