Percolative proton transport in hexagonal boron nitride membranes with edge-functionalization.

Two-dimensional layered materials have been used as matrices to study the structure and dynamics of trapped water and ions. Here, we demonstrate unique features of proton transport in layered hexagonal boron nitride membranes with edge-functionalization subject to hydration. The hydration-independent interlayer spacing indicates the absence of water intercalation between the h-BN sheets. An 18-fold increase in water sorption is observed upon amine functionalization of h-BN sheet edges. A 7-orders of magnitude increase in proton conductivity is observed with less than 5% water loading attributable to edge-conduction channels. The extremely low percolation threshold and non-universal critical exponents (2.90 ≤ α ≤ 4.43), are clear signatures of transport along the functionalized edges. Anomalous thickness dependence of conductivity is observed and its plausible origin is discussed.

Dielectric response and proton transport in water confined in graphene oxide.

Graphene oxide (GO) membranes possess a hierarchical microstructure, with well-ordered crystalline lamellae combining to form a macroscopic membrane. Water can intercalate in GO either in the sub-nanometer interlayer spaces or in the gaps between the lamellae known as voids; distinguishing the contribution of these two has been challenging. Addressing this challenge, we systematically study various properties of GO membranes exposed to controlled humidity levels ranging from 0% to 90% RH. Thickness-dependent dynamic vapor sorption is used to quantify the water content under different humidity environments. Complementing the vapor sorption studies, the AC impedance response of the GO membrane is determined at different humidity values. Our findings suggest that (a) most water gets absorbed in interlayer spaces at low humidity (<25% RH), (b) the fraction of water in the void spaces increases with RH%, (c) the lower bound for the dielectric constant of confined water is estimated to be εwater > 17, and (d) the conductivity increases by 5 to 6 orders of magnitude over a narrow range of water content (13 wt% to 31 wt%). The rapid increase in conductivity over a narrow range of water content suggests a percolative process for the protons. The dielectric constant estimates suggest that confined water behaves distinctly differently in a hydrophilic environment than in a hydrophobic one.

Frenkel line crossover of confined supercritical fluids.

We investigate the temperature evolution of dynamics and structure of partially confined Lennard Jones (LJ) fluids in supercritical phase along an isobaric line in the P-T phase diagram using molecular dynamics simulations. We compare the Frenkel line (FL) crossover features of partially confined LJ fluids to that of the bulk LJ fluids in supercritical phase. Five different spacings have been chosen in this study and the FL crossover characteristics have been monitored for each of these spacings for temperatures ranging from 240 K to 1500 K keeping the pressure fixed at 5000 bar. We characterize the FL crossover using density of states (DoS) function and find that partially confined supercritical fluids (SCF) exhibit a progressive shift of FL crossover point to higher temperatures for smaller spacings. While the DoS perpendicular to the walls shows persistent oscillatory modes, the parallel component exhibits a smooth crossover from an oscillatory to non-oscillatory characteristics representative of FL crossover. We find that the vanishing of peaks in DoS parallel to the walls indicates that the SCF no longer supports shear mode excitations and could serve as an identifier of the FL crossover for confined systems just as is done for the bulk. Layer heights of density profiles, self-diffusivity and the peak heights of radial distribution function parallel to the walls also feature the FL crossover consistent with the DoS criteria. Surprisingly, self-diffusivity undergoes an Arrhenius to super-Arrhenius crossover at low temperatures for smaller spacings as a result of enhanced structural order evidenced via pair-excess entropy. This feature, typical of glass-forming liquids and binary supercooled liquids, is found to develop from the glass-like characteristic slowdown and strong caging in confined supercritical fluid, evidenced via mean squared displacement and velocity autocorrelation function respectively, over intermediate timescales.

Non-contact in situ microwave material measurements for high temperature process monitoring.

Non-contact real time microwave measurement and signal analysis techniques to extract high temperature material parameters from the mono-static reflections gathered by a compact air cooled corrugated horn are presented in this work. Non-contact in situ microwave measurements gathered over 20-24 GHz inside a closed furnace were processed to identify the thermodynamic phase change temperature of metal and glass melts. The melting point of aluminum alloy and glass transition of a borosilicate glass matrix extracted from the time gated and processed microwave measurements were in good agreement with differential scanning calorimetry measurements. Thus, the ability to measure high temperature material process parameters using non-contact microwave measurements is demonstrated.

Soft-wall induced structure and dynamics of partially confined supercritical fluids.

The interplay between the structure and dynamics of partially confined Lennard Jones (LJ) fluids, deep into the supercritical phase, is studied over a wide range of densities in the context of the Frenkel line (FL), which separates rigid liquidlike and non-rigid gaslike regimes in the phase diagram of the supercritical fluids. Extensive molecular dynamics simulations carried out at the two ends of the FL (P = 5000 bars, T = 300 K, and T = 1500 K) reveal intriguing features in supercritical fluids as a function of stiffness of the partially confining atomistic walls. The liquidlike regime of a LJ fluid (P = 5000 bars, T = 300 K), mimicking argon, partially confined between walls separated by 10 Å along the z-axis, and otherwise unconstrained, reveals amorphous and liquidlike structural signatures in the radial distribution function parallel to the walls and enhanced self-diffusion as the wall stiffness is decreased. In sharp contrast, in the gas-like regime (P = 5000 bars, T = 1500 K), soft walls lead to increasing structural order hindering self-diffusion. Furthermore, the correlations between the structure and self-diffusion are found to be well captured by excess entropy. The rich behaviour shown by supercritical fluids under partial confinement, even with simple interatomic potentials, is found to be fairly independent of hydrophilicity and hydrophobicity. The study identifies persisting sub-diffusive features over intermediate time scales, emerging from the strong interplay between density and confinement, to dictate the evolution and stabilization of structures. It is anticipated that these results may help gain a better understanding of the behaviour of partially confined complex fluids found in nature.

Structural behavior of supercritical fluids under confinement.

The existence of the Frenkel line in the supercritical regime of a Lennard-Jones (LJ) fluid shown through molecular dynamics (MD) simulations initially and later corroborated by experiments on argon opens up possibilities of understanding the structure and dynamics of supercritical fluids in general and of the Frenkel line in particular. The location of the Frenkel line, which demarcates two distinct physical states, liquidlike and gaslike within the supercritical regime, has been established through MD simulations of the velocity autocorrelation (VACF) and radial distribution function (RDF). We, in this article, explore the changes in the structural features of supercritical LJ fluid under partial confinement using atomistic walls. The study is carried out across the Frenkel line through a series of MD simulations considering a set of thermodynamics states in the supercritical regime (P=5000 bar, 240K≤T≤1500K) of argon well above the critical point. Confinement is partial, with atomistic walls located normal to z and extending to "infinity" along the x and y directions. In the "liquidlike" regime of the supercritical phase, particles are found to be distributed in distinct layers along the z axis with layer spacing less than one atomic diameter and the lateral RDF showing amorphous-like structure for specific spacings (packing frustration) and non-amorphous-like structure for other spacings. Increasing the rigidity of the atomistic walls is found to lead to stronger layering and increased structural order. For confinement with reflective walls, layers are found to form with one atomic diameter spacing and the lateral RDF showing close-packed structure for the smaller confinements. Translational order parameter and excess entropy assessment confirms the ordering taking place for atomistic wall and reflective wall confinements. In the "gaslike" regime of the supercritical phase, particle distribution along the spacing and the lateral RDF exhibit features not significantly different from that due to normal gas regime. The heterogeneity across the Frenkel line, found to be present both in bulk and confined systems, might cause the breakdown of the universal scaling between structure and dynamics of fluids necessitating the determination of a unique relationship between them.

Wave localized finite-difference-time-domain modelling of scattering of elastic waves within a polycrystalline material.

Ultrasonic studies based on the first arrived signals are of utmost importance when dealing with heterogeneous material especially to seismology, biomedical imaging, as well as for nondestructive evaluation and structural health monitoring applications. Numerical modelling of elastic waves through polycrystalline features has been primarily held back by huge computational requirements. This article discusses the development of a robust and efficient numerical scheme based on finite-difference-time-domain (FDTD) by introducing wave-localized approach to simulate elastic waves in polycrystalline media. The numerical scheme adopts a rotated staggered grid in velocity-stress configuration. The numerical efficiency is improved by adopting parallel computing using efficient graphical processors and by introducing wave-localized computations. It is demonstrated that the proposed tool, especially with the introduction of wave-localized approach, is computationally faster and can handle large-scale grains in comparison with the commercial finite element software, especially when dealing with first arrived signals. This article reports an optimal ratio of FDTD grids per grain to minimize the staircasing effects at the polycrystalline boundaries and was found to be valid over a range of grain sizes. The article also addresses the orientation averaging requirements achieving statistically significant first arrived signal and suggests optimal averaging trials for various grain size models. The developed two-dimensional model shows good agreement with the prediction across the Rayleigh and Stochastic scattering regimes for the chosen model material (Inconel 600) having a cubic symmetry.

Improving the signal amplitude of meandering coil EMATs by using ribbon soft magnetic flux concentrators (MFC).

This paper presents a new method of improving the ultrasonic signal amplitude from a meander line EMAT by using soft magnetic alloy ribbon (Fe60Ni10V10B20) as a magnetic flux concentrator (MFC). The flux concentrator is a thin soft amorphous magnetic material (Fe60Ni10V10B20) which is very sensitive to a small flux change. The MFC is used with the EMAT to improve the signal amplitude and it was observed that the peak signal amplitude increases by a factor of two compared to the signal without MFC. Two dimensional numerical models have been developed for the EMAT with MFC to quantify the improvement of the received signal amplitudes. Model calculations and experiments have been carried out for a wide range of ultrasonic frequencies (500 kHz-1 MHz) in different materials.

Interaction of Lamb mode (A(o)) with structural discontinuity and generation of "Turning modes" in a T-joint.

In the present work, the interaction of the fundamental anti-symmetric guided Lamb mode (A(o)) with a structural discontinuity in a composite structure was studied through Finite Element numerical simulations and experiments. The structural component selected for this study was a T-joint section made from glass/epoxy material. This co-cured composite structure is made-up of an upper shell (skin) and a spar as the sub-components. It was observed that when A(o) mode interacts with the junction (structural discontinuity) of these sub-components, a mode-converted S(o) mode is generated. Experiments were conducted using air-coupled ultrasound to validate the numerical simulations. The back-propagating "Turning modes", which propagate from the thin region to the spar web and vice versa, were also numerically simulated and experimentally verified.

Simultaneous measurement of ultrasonic longitudinal wave velocities and thicknesses of a two layered media in the absence of an interface echo.

A measurement technique has been developed to extract the phase information of successive echoes for the simultaneous estimation of thicknesses and ultrasonic velocities of individual layers in a two layered media. The proposed method works in the absence of an interface echo and requires the total thickness of the sample to be known. Experiments have been carried out on two layered samples of white cast iron and gray cast iron with layer thickness variation in the range of 2-8 mm for total thickness variation in the range of 12-13 mm. Comparison with micrographs of a few samples confirmed the model predictions. The model is found to be sensitive to the total sample thickness but fairly insensitive to noise in the data.

Blind inversion method using Lamb waves for the complete elastic property characterization of anisotropic plates.

A novel blind inversion method using Lamb wave S(0) and A(0) mode velocities is proposed for the complete determination of elastic moduli, material symmetries, as well as principal plane orientations of anisotropic plates. The approach takes advantage of genetic algorithm, introduces the notion of "statistically significant" elastic moduli, and utilizes their sensitivities to velocity data to reconstruct the elastic moduli. The unknown material symmetry and the principal planes are then evaluated using the method proposed by Cowin and Mehrabadi [Q. J. Mech. Appl. Math. 40, 451-476 (1987)]. The blind inversion procedure was verified using simulated ultrasonic velocity data sets on materials with transversely isotropic, orthotropic, and monoclinic symmetries. A modified double ring configuration of the single transmitter and multiple receiver compact array was developed to experimentally validate the blind inversion approach on a quasi-isotropic graphite-epoxy composite plate. This technique finds application in the area of material characterization and structural health monitoring of anisotropic platelike structures.

Ultrasonic imaging using a computed point spread function.

An explicit point spread function (PSF) evaluator in the frequency domain is described for an ultrasonic transducer operating in the pulse-echo mode. The PSF evaluator employs the patch element model for transducer field determination and scattered field assessment from a small but finite "point" reflector. The PSF for a planar transducer in a medium has been evaluated in the near and the far field. The computed PSFs were used to deconvolve and restore surface images, obtained experimentally, of a single hole and a five-hole cluster in an Al calibration block. A calibration plot is arrived at for estimating, without the need for deconvolution, the actual diameters of circular reflectors from apparent diameters obtained experimentally for a single-medium imaging configuration. The PSF, when the transducer and the point reflector are in two media separated by a planar interface, was evaluated in the near and far field. The computed PSFs were used to deconvolve and restore subsurface images, obtained experimentally, of flat bottom holes (FBHs) in an Al calibration block. We show that the PSF, in the presence of a planar interface, can be obtained from a single-medium PSF model using an effective single-medium path length concept. The PSFs and modulation transfer functions (MTFs) are evaluated for spherical focused and annular transducers and compared with those for the planar transducer. We identify imaging distances to get better-resolved images when using planar, spherical focused, and annular transducers.