Empirical Modelling of Acoustic Emission Impulses

Acoustic emission nondestructive testing method is very widespread diagnostic method based on phenomena of radiation of acoustic waves during the materials destruction. The main advantages of the method are sensitivity to the crack and possibility of remote testing when sensor installed far from the defect. The main drawback of the method is complexity of data processing. Acoustic emission signals are characterized by the variability of the shape and spectrum associated with the dispersive nature of the propagation of the signal along the waveguide. Uncertainty of the signal waveform and spectrum complicates the development of the data processing methods. The article proposes an empirical model of the acoustic emission impulse constructed using generalization of experimental data. The use of this model makes it possible to increase the efficiency of noise filtering by comparing the shape and spectrum of acoustic emission impulses and noise at various distances between the defect and the sensor

Collecting Quality Infrared Spectra from Microscopic Samples of Suspicious Powders in a Sealed Cell

The infrared (IR) microspectroscopical analysis of samples within a sealed-cell containing barium fluoride is a critical need when identifying toxic agents or suspicious powders of unidentified composition. The dispersive nature of barium fluoride is well understood and experimental conditions can be easily adjusted during reflection–absorption measurements to account for differences in focus between the visible and IR regions of the spectrum. In most instances, the ability to collect a viable spectrum is possible when using the sealed cell regardless of whether visible or IR focus is optimized. However, when IR focus is optimized, it is possible to collect useful data from even smaller samples. This is important when a minimal sample is available for analysis or the desire to minimize risk of sample exposure is important. While the use of barium fluoride introduces dispersion effects that are unavoidable, it is possible to adjust instrument settings when collecting IR spectra in the reflection–absorption mode to compensate for dispersion and minimize impact on the quality of the sample spectrum.

Kinetics of the persistent photocurrent in a-Si:H

Although the persistent photocurrent (PPC) in amorphous hydrogenated amorphous silicon (a-Si:H) is known to be dominated by dispersive recombination kinetics which produce the power low decay, [Formula: see text], where [Formula: see text] is the dispersion parameter, the reason for the occurrence of the dispersive reaction is not clear. We discuss the origin of the dispersive nature in the PPC in a-Si:H. It is shown that band edge modulation due to microscopic inhomogeneity may play an important role in the PPC dynamics.

Fano-like spectral profile in TE wave scattering by nanowire of dissipative and dispersive materials

We report the result of extending the study of Fano phenomena in the scattering of TE electromagnetic wave by nanowire having complex and dispersive permittivities, [Formula: see text], on the basis of exact Mie formulation. It is shown that the spectral profiles of scattering coefficients are sensitively affected by imaginary part of the permittivity and its overall frequency dependent properties. The cascaded resonance profile reported previously for real and positive permittivity is suppressed by a large [Formula: see text] while the spectral profile is broadened by the dispersive nature of [Formula: see text] as to increase the asymmetry of the profile. The negative sign of the real part of the permittivity, except for [Formula: see text], is generally found to eliminate the resonace profile. Further, the size (radius) of the nanowire is also shown to affect the resonance profile. Specific scattering coefficients calculated for silicon (Si) and silver (Ag) nanowire using experimental data of corresponding [Formula: see text] are demonstrated to exhibit the spectral profile for certain wire sizes with nicely fitted Fano profile.

Preparation, Structural, Electrical, and Ferroelectric Properties of Lead Niobate–Lead Zirconate–Lead Titanate Ternary System

A ternary system of lead niobate–lead zirconate–lead titanate with composition xPN–yPZ–(x-y)PT where x=0.5 and y=0.15, 0.25, and 0.35 known as PNZT has been prepared by conventional mixed oxide route at a temperature of 1100°C. The formation of the perovskite phase was established by X-ray diffraction analysis. The surface morphology studied by scanning electron microscopy shows the formation of fairly dense grains and elemental composition was confirmed by energy dispersive X-ray analysis. Dielectric properties like dielectric constant and dielectric loss (ε′ and tan⁡δ) indicate poly-dispersive nature of the material. The temperature dependent dielectric constant (ε′) curve indicates relaxor behaviour with two dielectric anomalies. The poly-dispersive nature of the material was analysed by Cole-Cole plots. The activation energy follows the Arrhenius law and is found to decrease with increasing frequency for each composition. The frequency dependence of ac conductivity follows the universal power law. The ac conductivity analysis suggests that hopping of charge carriers among the localized sites is responsible for electrical conduction. The ferroelectric studies reveal that these ternary systems are soft ferroelectric.

4,4′-{Diazenediylbis[(1,4-phenylene)bis(carbonyloxy)]}bis(2,2,6,6-tetramethylpiperidinyloxidanyl): the first crystal structure determination from powder data of a nitroxide radical

The title compound, C32H42N4O6, is a novel nitroxide radical used for pulsed electron-electron double resonance (PELDOR) spectroscopy. Its crystal structure was determined from laboratory X-ray powder diffraction data. The attractive forces between the molecules in the crystal structure are mainly of dispersive nature. A special interaction of the nitroxide radicals was not observed.

Thermodynamics of Dislocation Pattern Formation at the Mesoscale

We introduce a mesoscopic framework that is capable of simulating the evolution of dislocation networks and, at the same time, spatial variations of the stress, strain and displacement fields throughout the body. Within this model, dislocations are viewed as sources of incompatibility of strains. The free energy of a deformed solid is represented by the elastic strain energy that can be augmented by gradient terms to reproduce dispersive nature of acoustic phonons and thus set the length scale of the problem. The elastic strain field that is due to a known dislocation network is obtained by minimizing the strain energy subject to the corresponding field of incompatibility constraints. These stresses impose Peach-Koehler forces on all dislocations and thus drive the evolution of the dislocation network.