In this paper, molecular dynamics simulations are used to study the effect of molecular water and composition (Si/Al ratio) on the structure and mechanical properties of fully polymerized amorphous sodium aluminosilicate geopolymer binders. The X-ray pair distribution function for the simulated geopolymer binder phase showed good agreement with the experimentally determined structure in terms of bond lengths of the various atomic pairs. The elastic constants and ultimate tensile strength of the geopolymer binders were calculated as a function of water content and Si/Al ratio; while increasing the Si/Al ratio from one to three led to an increase in the respective values of the elastic stiffness and tensile strength, for a given Si/Al ratio, increasing the water content decreased the stiffness and strength of the binder phase. An atomic-scale analysis showed a direct correlation between water content and diffusion of alkali ions, resulting in the weakening of the AlO tetrahedral structure due to the migration of charge balancing alkali ions away from the tetrahedra, ultimately leading to failure. In the presence of water molecules, the diffusion behavior of alkali cations was found to be particularly anomalous, showing dynamic heterogeneity. This paper, for the first time, proves the efficacy of atomistic simulations for understanding the effect of water in geopolymer binders and can thus serve as a useful design tool for optimizing composition of geopolymers with improved mechanical properties.

No abstract

No abstract

We report on a phononic crystal ͑PC͒ consisting of a square array of cylindrical polyvinylchloride inclusions in air that can be used to control the relative phase of two incident acoustic waves with different incident angles. The phase shift between waves propagating through the crystal depends on the angle of incidence of the incoming waves and the PC length. The behavior of the PC is analyzed using the finite-difference-time-domain method. The band structure and equifrequency contours calculated via the plane wave expansion method show that the distinctive phase controlling properties are attributed to noncollinear wave and group velocity vectors in the PC as well as the degree of refraction.Phononic crystals ͑PCs͒ are composite materials which derive their spectral ͑-space͒ and wave vector ͑k-space͒ properties from the scattering of elastic waves by periodic arrays of elastic inclusions embedded in an elastic matrix. Bulk or defected PCs have been shown to exhibit numerous useful spectral capabilities including transmission band gaps, local modes for guiding, filtering, and multiplexing. 1-15 k-space properties result from features in the band structure that impact refraction. 16-28 These properties parallel many of those found in photonic crystals. 29,30 The -space and k-space properties are directly related to the size, geometry, scale, and composition of the constitutive materials of the PC.In the present letter, we demonstrate that the band structure of a two-dimensional PC constituted of a square array of cylindrical polyvinylchloride ͑PVC͒ inclusions in an air matrix can be used to control the relative phase of elastic waves. Phase control is due to the propagation of elastic waves in the PC with wave vectors that are not collinear with their group velocity vectors. This condition implies that excited Bloch waves travel at different phase velocities in the direction of their group velocity. Additionally, this crystal shows near zero-angle refraction permitting wave collimation as well as enabling the superposition of beams with different wave vectors in the same volume of crystal. Phase manipulation of these superposed waves can result in constructive or destructive interferences between noncollinear incident beams. Finally, there are operating frequencies for which the circular equifrequency contour ͑EFC͒ in air is larger than the first Brillouin zone of the PC, allowing several Bloch modes to exit the crystal, leading to the phenomenon of beam splitting. The work presented in this letter constitutes a significant move toward broadening the range of properties and applications of PCs beyond their more common spectral and wave number properties.The PVC-air system parameters are: PVC = 1364 kg/ m 3 , c t,PVC = 1000 m / s, c l,PVC = 2230 m / s, AIR = 1.3 kg/ m 3 , c t,AIR =0 m/ s, and c l,AIR = 340 m / s, where is density, c t is transverse speed of sound, and c l is longitudinal speed of sound. The inclusion radius is 12.9 mm and the lattice parameter is 27 mm. The plane wave expansion ͑PWE͒ method was e...

A phononic crystal (PC) consisting of a square array of cylindrical polyvinylchloride inclusions in air is used to construct a variety of acoustic logic gates. In a certain range of operating frequencies, the PC band structure shows square-like equi-frequency contours centered off the gamma point. This attribute allows for the realization of non-collinear wave and group velocity vectors in the PC wave vector space. This feature can be utilized to control with great precision, the relative phase between propagating acoustic waves in the PC. By altering the incidence angle of the impinging acoustic beams or varying the PC thickness, interferences occur between acoustic wave pairs. It is recognized that information can be encoded with this mechanism (e.g., wave amplitudes/interference patterns) and accordingly to construct a series of logic gates emulating Boolean functions. The NAND, XOR, and NOT gates are demonstrated with finite-difference time-domain simulations of acoustic waves impinging upon the PC.