Microscopic and Macroscopic Fragmentation Characteristics under Hypervelocity Impact Based on MD and SPH Method

This work investigates the difference in the fragmentation characteristics between the microscopic and macroscopic scales under hypervelocity impact, with the simulations of Molecular Dynamics (MD) and Smoothed Particle Hydrodynamics (SPH) method. Under low shock intensity, the model at microscopic scale exhibits good penetration resistance due to the constraint of strength and surface tension. The bullet is finally embedded into the target, rather than forming a typical debris cloud at macroscopic scale. Under high shock intensity, the occurrence of unloading melting of the sample reduces the strength of the material. The material at the microscopic scale has also been completely penetrated. However, the width of the ejecta veil and external bubble of the debris cloud are narrower. In addition, the residual velocity of bullet, crater diameter and expansion angle of the debris cloud at microscopic scale are all smaller than those at macroscopic scale, especially for low-velocity conditions. The difference can be as much as two times. These characteristics indicate that the degree of conversion of kinetic energy to internal energy at the microscopic scale is much higher than that of the macroscopic results. Furthermore, the MD simulation method can further provide details of the physical characteristics at the micro-scale. As the shock intensity increases, the local melting phenomenon becomes more pronounced, accompanied by a decrease in dislocation atoms and a corresponding increase in disordered atoms. In addition, the fraction of disordered atoms is found to increase exponentially with the increasing incident kinetic energy.

Structure Refinement and Thermal Stability Studies of the Uranyl Carbonate Mineral Andersonite, Na2Ca[(UO2)(CO3)3]·(5+x)H2O

A sample of uranyl carbonate mineral andersonite, Na2Ca[(UO2)(CO3)3]·5−6H2O, originating from the Cane Springs Canyon, San Juan Co., UT, USA was studied using single-crystal and powder X-ray diffraction at various temperatures. Andersonite is trigonal, R−3m, a = 17.8448(4), c = 23.6688(6) Å, V = 6527.3(3) Å3, Z = 18, R1 = 0.018. Low-temperature SCXRD determined the positions of H atoms and disordered H2O molecules, arranged within the zeolite-like channels. The results of high-temperature PXRD experiments revealed that the structure of andersonite is stable up to 100 °C; afterwards, it loses crystallinity due to release of H2O molecules. Taking into account the well-defined presence of H2O molecules forming channels’ walls that to the total of five molecules p.f.u., we suggest that the formula of andersonite is Na2Ca[(UO2)(CO3)3]·(5+x)H2O, where x ≤ 1. The thermal behavior of andersonite is essentially anisotropic with the lowest values of the main thermal expansion coefficients in the direction perpendicular to the channels (plane (001)), while the maximal expansion is observed along the c axis—in the direction of channels. The thermal expansion around 80 °C within the (001) plane becomes negative due to the total release of “zeolitic” H2O molecules. The information-based structural complexity parameters of andersonite were calculated after the removal of all the disordered atoms, leaving only the predominantly occupied sites, and show that the crystal structure of the mineral should be described as complex, possessing 4.535 bits/atom and 961.477 bits/cell, which is comparative to the values for another very common natural uranyl carbonate, liebigite.

Contact and Friction at Nanoscale

Molecular Dynamics (MD) simulations of indentation and scratch over crystal nickel (100) were carried out to investigate the microstructure evolution at nanoscale. The dislocation nucleation and propagation during process were observed preferably between close-packed planes. Dislocation loops are formed under both indentation and scratch process, and indentation and friction energy were transferred to the substrate in the form of phonon of disordered atoms, then part of the energy dissipated and rest is remain in the form of permanent plastic deformation.

A crystal structure of ultra-dispersed form of polytetrafluoroethylene based on X-ray powder diffraction data

An X-Ray powder diffraction study of ultra-dispersed polytetrafluoroethylene was carried out. As well as a regular polytetrafluoroethylene the ultra-dispersed form contents a high proportion of the crystalline phase. The X-ray diffraction pattern could be described with two-dimensional hexagonal unit cell [a=5.685(1) Å, symmetry group p6mm]. Structural modeling with a continuous electron density approach as well as with a discrete disordered atoms distribution was accomplished. The model was refined using the Rietveld method. The structure is characterized by a spiral arrangement of polymers (CF2-)n along the z-axis with complete mutual disordering by rotational displacement around z, as well as a partial molecular translation along the z-axis. Molecular disordering results in a systematic absence of reflections with 1≠0 and as a sequence in two-dimensional unit cell effect. The presence of complete rotational disordering distinguishes the ultra-dispersed form of polytetrafluoroethylene from the standard one (fluoroplast-4), where only partial disordering is observed.

Residual short-range order in the heavy fermion compound CeInCu2

By using the X-ray diffraction technique, residual short-range order was detected in CeInCu2, cerium dicopper indium, which is known to be a heavy fermion compound. In spite of the long-range order of this substance, diffuse scattering exhibiting short-range order was observed at room temperature. The correlation parameters obtained showed that an incorrectly occupied lattice site has a tendency to gather atoms of different species at the neighboring sites along the 〈111〉 directions. Thus, the disordered region would form a cluster composed of several disordered atoms. Furthermore, a chain-type correlation which has a period of 20–23 Å along the same directions was indicated. The superstructure-like feature of the disordered atoms hardly increases the residual resistivity. It is consistent with the behavior of the residual resistivity under high pressure.