Jiao-Ping Yang

Facile surfactant-free synthesis of monodisperse Ni particles via a simple solvothermal method and their superior catalytic effect on thermal decomposition of ammonium perchlorate

Successful synthesis of monodisperse nickel particles with narrow particle size distributions is reported for the first time via a simple solvothermal method with ultrasonic treatment using no surfactants. The Ni products are characterized by X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy. The as-synthesized monodisperse Ni particles show a spherical morphology with the mean particle size of 46.2, 60.1 and 113.7 nm for the reaction time of 0.5, 1 and 3 h, respectively at a low temperature of 100 degrees C. The catalytic effect is investigated for the Ni particles on the thermal decomposition of ammonium perchlorate (AP) using differential thermal analysis and thermogravimetry. For the purpose of comparison, as-prepared and commercial non-monodisperse Ni particles are used for examining the catalytic effect. The monodisperse Ni particles show a superior catalytic effect over the non-monodisperse Ni counterparts with a similar mean particle size. The surface areas of Ni particles are measured by Brunauer-Emmett-Teller (BET) technique. A theoretical consideration is also presented for analysis of the surface areas of monodisperse and non-monodisperse particles. Both the BET and theoretical results show that the surface area of monodisperse particles is higher than that of non-monodisperse counterparts. Consequently, the catalytic effect of Ni particles on thermal decomposition of AP is explained in terms of the surface areas.

Observation of Dislocation Microstructures and Simulation of Stress Field during Fatigue Crack Initiation in a Copper Single Crystal

How a crack initiates from the smooth surface of single crystals subjected to uniaxial cyclic loading is unclear. Experiments were conducted to observe in detail the dislocation microstructures during the saturation stage of cyclic deformation in a copper single crystal using scanning electron microscopy and the electron channeling contrast (SEM–ECC) technique. Some dark zones were found in the dislocation microstructures, which were located either at the edge region of the specimen or within the persistent slip bands (PSBs) at the matrix/PSB interfaces. Hence, fatigue cracks will initiate at these sites with high stress concentrations, i.e., the dark zones. Also, dislocation dynamics (DD) simulation was adopted to calculate internal stress distributions induced by dislocations, and finite element analysis (FEA) used to obtain stress distribution at the matrix/PSB interfaces and neighboring micro-regions caused by an externally applied load. Simulation results show that the external shear stresses distribute uniformly in all specimens; while near the free-surface regions, the maximum value of internal stresses not only occurs at interfaces between PSBs and dislocation matrix, but also at locations where these interfaces cross the freesurface. Consequently, the interfaces are most probable sites for nucleated cracks. Finally, the simulation results agree well with experimental observations.