Simulations of shock waves in solids

Molecular dynamic shock wave simulations have been carried out for face centered cubic (f.c.c.) and body centered cubic (b.c.c.) solids using Lennard-Jones and Morse potentials for the interatomic interactions. The Hugoniot conservation relations were accurately obeyed in all of these calculations. The shock wave profiles may vary with the interatomic potential and the crystal structure, effects most clearly shown by the temperature profile near the shock front. The Lennard-Jones solids are intensitive to a change in structure but the Morse solids appear sensitive to crystal structure, at least in comparing b.c.c. with f.c.c. It was shown that the average shock wave temperature can be calculated from a combination of the Hugoniot conservation relations and the Mie-Grüneisen equation of state. The temperature calculated this way is in good agreement with the average shock wave temperature obtained in the computer simulations.     

Effects of different composition ratio on the dielectric relaxation and dynamic mechanical properties of poly(ω‐dodecalactam‐co‐ε‐caprolactam‐co‐propylene oxide) copolymers

The molecular dynamics of new poly (ω‐dodecalactam‐co‐ε‐caprolactam‐co‐propylene oxide) copolymers (DL/CL/PAC) has been investigated by using dynamic mechanical thermal analysis (DMTA) and dielectric relaxation spectroscopy (DRS) measurements. The copolymers were synthesized via anionic polymerization of relevant lactams activated with carbamoyl derivatives of telechelic hydroxyl terminated polypropylene oxide with isophorone diisocyanate (PAC). The calorimetric, X‐ray diffraction, and DMTA measurements were performed to recognize the influence of the composition ratio and the type of PAC on the physical, thermal, and mechanical properties of the synthesized copolymers. The DRS was used to study the frequency dependence of the dielectric permittivity of some isotherms from ?110 to 145 °C. Copolymerization of ε‐caprolactam with about 10 wt % ω‐dodecalactam results in a copolymer that has lower water absorption, a melting point close to that of polyamide 6 and has a high enough degree of crystallinity in respect to high storage modulus. Five dielectric relaxations have been observed in the dielectric spectra, three at lower temperature and two at higher temperature. The copolymers have two glass transition temperatures for polyamide segments and polyether blocks, indicating microphase separation in the copolymers. Other studies directed toward molecular dynamics of polyamide DL/CL/PAC copolymers have not been reported. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Self-consistent and quasiharmonic approximations: The equation of state and the high temperature lattice instability

The self-consistent average-phonon (SCAP) treatment, developed in earlier papers for monoatomic solids was extended to the alkali halides for which self-consistent calculations were not available. The quantitative comparison of SCAP with an average quaiharmonic (AQH) treatment for Ar and for NaCl showed that AQH highly overestimated the thermal pressure. Because of this overestimation, we conclude that the lattice instability reported by Boyer for NaCl, which he considered to be related to melting, is a consequence of using a quasiharmonic approximation.     

Experimental method for the measurement of hyperfine transition saturation intensities in a gaseous medium

A laser system capable of generating picosecond pulses of 2 ps duration and peak powers of up to 50 MW, tunable throughout the emission spectrum of the lasing medium is described. The system consists of a cw dye laser which is tuned and is actively mode-locked by means of a novel interferometer. Single-pulse amplification is achieved via a two-stage N₂-laser-pumped amplifier arrangement. One of the most attractive features of the system here described is its inherent versatile broad tunability, which should allow, with different lasing media and pump sources, ps pulse generation from 400 to 700 nm using the same interferometer device.     

A molecular dynamical study of the equation of state of solids at high temperature and pressure

The molecular dynamics method of computer simulation was used to study the pressure and energy as a function of volume and temperature for f.c.c. neon with 6–12 Lennard-Jones interatomic forces and for b.c.c. iron with Morse interatomic forces. A self-consistent cell model was used to derive analytical formulae which describe the results of the computer experiments. A method for correcting the inherently classical results of the molecular dynamics calculations for the effect of quantum statistics is proposed. The results of the calculations are compared with experimental data and with various approximate methods for calculating the Grüneisen constant.     

The self-consistent cell model equation of state for solids

An anharmonic equation of state for solids using the Self-Consistent Cell Model (SCCM) is given in a form useful for calculating the usual thermodynamic properties. Following Cowley and Shukla, using the Jaswal-Girifalco potential for copper, calculations are compared with other models and with experiment. The results from the analytic expressions using the SCCM are as good or better than those obtained from far more complicated theories of anharmonicity. Using the Lindemann criterion, the pressure at the melting point was obtained as a function of the melting temperature. A melting line was also obtained for iron and the longitudinal velocity and isothermal bulk modulus along the melting line were calculated. The Hugoniot pressures were calculated and compared with experiment. For both the copper and iron the agreement between theory and experiment is remarkably good considering the empirical nature of the potentials, the simplifying approximations of the SCCM calculations, and the large range of densities and pressures that are compared with experiment.     

One-dimensional thermomechanical model for lateral melting and ignition of a thin sheared viscous layer

We compute the temperature profile near a laterally growing uniformly sheared viscous layer, a melt of the host material. A similarity variable is identified and an analytical solution is found for the temperature field and phase boundary trajectory for the case of uniform sliding. A numerical method for the iterative solution of this non-linear Stefan problem is implemented. In the case of a chemically reactive material with an Arrhenius dependence of the reaction rate on temperature we compute the ignition times. The dependence of the ignition time on the sliding velocity and latent heat of the material is determined numerically.