Systematic coarse-graining of epoxy resins with machine learning-informed energy renormalization

A persistent challenge in molecular modeling of thermoset polymers is capturing the effects of chemical composition and degree of crosslinking (DC) on dynamical and mechanical properties with high computational efficiency. We established a coarse-graining (CG) approach combining the energy renormalization method with Gaussian process surrogate models of molecular dynamics simulations. This allows a machine-learning informed functional calibration of DC-dependent CG force field parameters. Taking versatile epoxy resins consisting of Bisphenol A diglycidyl ether combined with curing agent of either 4,4-Diaminodicyclohexylmethane or polyoxypropylene diamines, we demonstrated excellent agreement between all-atom and CG predictions for density, Debye-Waller factor, Young’s modulus, and yield stress at any DC. We further introduced a surrogate model-enabled simplification of the functional forms of 14 non-bonded calibration parameters by quantifying the uncertainty of a candidate set of calibration functions. The framework established provides an efficient methodology for chemistry-specific, large-scale investigations of the dynamics and mechanics of epoxy resins.

Effect of the non-ideal axial ratio c/a on anharmonic EXAFS oscillation of h.c.p. crystals

The temperature and wavenumber dependence of the extended X-ray absorption fine-structure (EXAFS) oscillation of hexagonal close-packed (h.c.p.) crystals have been calculated and analyzed under the effect of the non-ideal axial ratio c/a. The anharmonic EXAFS oscillation is presented in terms of the Debye–Waller factor using the cumulant expansion approach up to the fourth order. An effective calculation model is expanded and developed from the many-body perturbation approach and correlated Debye model using the anharmonic effective potential. This potential, depending on the non-ideal axial ratio c/a, is obtained from the first-shell near-neighbor contribution approach. A suitable analysis procedure is performed by evaluating the influence of EXAFS cumulants on the phase shift and amplitude reduction of the anharmonic EXAFS oscillation. The numerical results for crystalline zinc are found to be in good agreement with those obtained from experiments and other theoretical methods at various temperatures. The obtained results show that the present theoretical model is essential and effective in improving the accuracy for analyzing the experimental data of anharmonic EXAFS signals of h.c.p. crystals with a non-ideal axial ratio c/a.

The Features of X-Ray Phase Analysis of Rocks with Complex Mineral Composition

The article presents the results of identification and quantitative analysis of the phase composition, fine structure parameters of minerals in carbonate and terrigenous rocks by the use of modern X-ray diffraction (XRD) analysis. To make the XRD analysis, we optimized the modes of x-ray pattern shooting by changing the radius of the goniometer, the system of primary and secondary slits, Soller slits, and the system of detecting the low-content minerals. In processing the obtained x-ray patterns, we considered the size and defects of the crystal grains, the crystallographic mode of arrangements, atomic population of the crystal lattice, the Debye-Waller factor and the instrumental line broadening by the use of the Caliotti function for LaB6. So we determined the type and content of minerals, estimated the period of the crystal lattice, the size of the coherent scattering domains and micro-distortion crystal lattice of the mineral. We compared the obtained data on the presence and quantitative content of minerals with the data of X-ray fluorescence (XRF) analysis and scanning electron microscopy (SEM). Based on the obtained data, reference intensity ratio (RIR) coefficients were selected for a number of minerals typically contained in core materials for quantitative phase analysis by the use of the corundum number method.

The electron–phonon coupling constant for single-layer graphene on metal substrates determined from He atom scattering

A theory, previously formulated for conducting surfaces, is extended to extract the electron-phonon coupling strength λ for graphene supported on metal substrates from the thermal attenuation (Debye–Waller factor) of helium scattering reflectivity.

Pressure effects on the EXAFS Debye–Waller factor of iron

The pressure effects on atomic mean-square relative displacement characterizing the extended X-ray absorption fine structure (EXAFS) Debye–Waller factor of iron metal have been investigated based on the Debye model. The analytical expressions of the Debye frequency and EXAFS Debye–Waller factor have been derived as functions of crystal volume compressibility. Based on the well established equation-of-state including the contributions of the anharmonic and electronic thermal pressures, numerical calculations have been performed for iron up to a pressure of 220 GPa and compared with experimental data when possible. These results show that the Debye frequency increases rapidly with compression, and beyond 150 GPa it behaves as a linear function of pressure. Meanwhile the mean-square relative displacement curve drops robustly with pressure, especially at pressures smaller than 100 GPa. This phenomenon causes the enhancement of EXAFS signals at high pressure. Reversely, the increasing of temperature will reduce the amplitude of EXAFS spectra.