Understanding how diffusion takes place within nanocrystals is of great importance for their stability and for controlling their synthesis. In this study, we used the strain sensitivity of Bragg coherent diffraction imaging (BCDI) to study the diffusion of iron into individual gold nanocrystals in situ at elevated temperatures. The BCDI experiments were performed at the I-07 beamline at Diamond Light Source, UK. The diffraction pattern of individual gold nanocrystals was measured around the (11-1) Bragg peak of gold before and after iron deposition as a function of temperature and time. Phase retrieval algorithms were used to obtain real space reconstructions of the nanocrystals from their measured diffraction patterns. Alloying of iron with gold at sample temperatures of 300°C-500°C and dealloying of iron from gold at 600°C were observed. The volume of the alloyed region in the nanocrystals was found to increase with the dose of iron. However, no significant time dependence was observed for the structure following each iron deposition, suggesting that the samples reached equilibrium relatively quickly. The resulting phase distribution within the gold nanocrystals after the iron depositions suggests a contraction due to diffusion of iron. Our results show that BCDI is a useful technique for studying diffusion in three dimensions and alloying behaviour in individual crystalline grains.
This paper presents an experimental methodology for using multistage, drained triaxial tests on compacted soils under unsaturated conditions to estimate soil-specific relationships between mean effective stress and matric suction. Tests were performed by applying a matric suction to a soil specimen in a triaxial cell using the axis translation technique with back-pressure, then shearing the specimen under drained conditions until reaching stress-path tangency. The specimen was then unloaded, a new suction was applied, and the shearing process was repeated. The points of maximum principal stress difference for the unsaturated specimen were plotted versus mean effective stress, defined using the degree of saturation as the effective stress parameter, and they were found to correspond well with the critical state line defined from triaxial tests on saturated specimens. The suction stress for the compacted soil tested in this study was observed to increase nonlinearly with suction, tending toward a constant value with increasing suction. Although there are potential changes in soil structure in the specimen during loading, unloading, and reloading, the results indicate that the multistage testing method may be useful for estimating soil-specific effective stress parameters for compacted soils in unsaturated conditions. Furthermore, the fact that differences in the soil-water retention curve of soil specimens subjected to different net confining pressures were observed for the soil tested in this study emphasizes the importance of using soil-specific tests to validate predictive relationships between suction stress and matric suction.
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