Поиск оптимальных условий монолитизации реакторного порошка сверхвысокомолекулярного полиэтилена

In order to clarify the possibility of sintering reactor powders of ultra-high molecular weight polyethylene under pressure at a temperature higher than its equilibrium melting point at atmospheric pressure (T0m) without catastrophic changes in the internal structure of particles, a comparative WAXS analysis was carried out of the samples sintered below (T <T0m) and above (T> T0m) this temperature and cooled under different conditions. A multifunctional structural analysis of the X-ray scattering curves recorded in the Bragg-Brentano mode on a 2D Phaser Bruker diffractometer from UHMWPE reactor powders sintered under different conditions was carried out, and the crystallite sizes were calculated. It was found that an increase in the sintering temperature above T0m does not significantly change the crystal structure of the polymer, and the precursors produced can be used for further orientational hardening.

Constrained minimal-interface structures in polycrystalline copper with extremely fine grains

Metals usually exist in the form of polycrystalline solids, which are thermodynamically unstable because of the presence of disordered grain boundaries. Grain boundaries tend to be eliminated through coarsening when heated or by transforming into metastable amorphous states when the grains are small enough. Through experiments and molecular dynamics simulations, we discovered a different type of metastable state for extremely fine-grained polycrystalline pure copper. After we reduced grain sizes to a few nanometers with straining, the grain boundaries in the polycrystals evolved into three-dimensional minimal-interface structures constrained by twin boundary networks. This polycrystalline structure that underlies what we call a Schwarz crystal is stable against grain coarsening, even when close to the equilibrium melting point. The polycrystalline samples also exhibit a strength in the vicinity of the theoretical value.

Addressing the discrepancy of finding the equilibrium melting point of silicon using molecular dynamics simulations

We performed molecular dynamics simulations to study the equilibrium melting point of silicon using (i) the solid–liquid coexistence method and (ii) the Gibbs free energy technique, and compared our novel results with the previously published results obtained from the Monte Carlo (MC) void-nucleated melting method based on the Tersoff-ARK interatomic potential (Agrawal et al. Phys. Rev. B 72 , 125206. ( doi:10.1103/PhysRevB.72.125206 )). Considerable discrepancy was observed (approx. 20%) between the former two methods and the MC void-nucleated melting result, leading us to question the applicability of the empirical MC void-nucleated melting method to study a wide range of atomic and molecular systems. A wider impact of the study is that it highlights the bottleneck of the Tersoff-ARK potential in correctly estimating the melting point of silicon.

Molecular Dynamics Simulations on Melting of Aluminum

Molecular dynamics simulations were performed to calculate the melting points of perfect crystalline aluminum to high pressures. Under ambientpressure, there exhibits about 20% superheating before melting compared to the experimental melting point. Under high pressures, thecalculated melting temperature increases with the pressure but at a decreasing rate, which agrees well with the Simon's melting equation. Porosity effect was also studied for aluminum crystals with various initial porosity at ambient pressure, which shows that the equilibrium melting point decreases with the initial porosity as experiments expect.