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Ultrasonic non-destructive evaluation in pulse-echo mode is used for the inspection of single-lap aluminum adhesive joints, which contain interface defects in bonding area. The aim of the research is to increase the probability of defect detection in addition to ensuring that the defect sizes are accurately estimated. To achieve this, this study explores additional ultrasonic features (not only amplitude) that could provide more accurate information about the quality of the structure and the presence of interface defects. In this work, two types of interface defects, namely inclusions and delaminations, were studied based on the extracted ultrasonic features in order to evaluate the expected feasibility of defect detection and the evaluation of its performance. In addition, an analysis of multiple interface reflections, which have been proved to improve detection in our previous works, was applied along with the extraction of various ultrasonic features, since it can increase the probability of defect detection. The ultrasonic features with the best performance for each defect type were identified and a comparative analysis was carried out, showing that it is more challenging to size inclusion-type defects compared to delaminations. The best performance is observed for the features such as peak-to-peak amplitude, ratio coefficients, absolute energy, absolute time of flight, mean value of the amplitude, standard deviation value, and variation coefficient for both types of defects. The maximum relative error of the defect size compared to the real one for these features is 16.9% for inclusions and 3.6% for delaminations, with minimum errors of 11.4% and 2.2%, respectively. In addition, it was determined that analysis of the data from repetitive reflections from the sample interface, namely, the aluminum-adhesive second and third reflections, that these contribute to an increase in the probability of defect detection.
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We have recently reported the self-pulsation phenomenon under strong optical feedback in terahertz (THz) quantum cascade lasers (QCLs). One important issue, however, we left open: the effect of multiple round trips in the external cavity on the laser response to feedback. Our current analysis also casts additional light on the phenomenon of self-pulsations. Using only one external cavity round trip (ECRT) in the model has been the common approach following the seminal paper by Lang-Kobayashi in 1980. However, the conditions under which the Lang-Kobayashi model, in its original single-ECRT formulation, is applicable has been rarely explored. In this work, we investigate the self-pulsation phenomenon under multiple ECRTs. We found that the self-pulsation waveform changes when considering more than one ECRT. This we attribute to the combined effect of the extended external cavity length and the frequency modulation of the pulsation frequency by the optical feedback. Our findings add to the understanding of the optical feedback dynamics under multiple ECRTs and provide a pathway for selecting the appropriate numerical model to study the optical feedback dynamics in THz QCLs and semiconductor lasers in general.
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We study the dynamics of classical particles confined in a time-dependent potential well. The dynamics of each particle is described by a two-dimensional nonlinear discrete mapping for the variables energy en and phase Ïn of the periodic moving well. We obtain the phase space and show that it contains periodic islands, chaotic sea, and invariant spanning curves. We find the elliptic and hyperbolic fixed points and discuss a numerical method to obtain them. We study the dispersion of the initial conditions after a single iteration. This study allows finding regions where multiple reflections occur. Multiple reflections happen when a particle does not have enough energy to exit the potential well and is trapped inside it, suffering several reflections until it has enough energy to exit. We also show deformations in regions with multiple reflection, but the area remains constant when we change the control parameter NC. Finally, we show some structures that appear in the e0e1 plane by using density plots.
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The interaction of guided waves with wall thinning can be complex, depending on the thinning geometry and the frequency. At a high frequency-thickness, when a shear-horizontal (SH) guided wave mode impinges upon a tapered wall thinning region, there is mode conversion to other propagating SH modes, either in reflection or transmission, which heavily depends on the shape of the taper. In this paper, we have combined the reciprocity theorem of elastodynamics and the theory of multiple reflections, in order to analytically calculate the scattered SH wavefield in plates, due to the interaction with an arbitrary tapered thinning. The taper is discretized into several sections and the formulation is addressed in matrix notation, in order to tackle several modes which arise due to mode interconversion distributed within the taper. The method was validated with experimental and numerical data at linear tapered thinning, in the high-frequency-thickness regime. It was also applied to provide understanding of the reflection behaviour within smoother taper profiles, namely, raised-cosine and Blackman window tapers, and to visualize the propagating field of each mode. It is shown that for a linear taper profile, the reflection within the taper is virtually constant, which produces an interference pattern in the overall reflection from the whole taper. Such a mechanism is broken with smoother tapers, since they impose lower reflection close to the taper ends. The method proves itself useful for analytically investigating the scattering from arbitrary wall thinning when mode-conversion arises.
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Two-dimensional graphene is of great interest for electromagnetic interference (EMI) shielding owing to its inherent electrical conductivity, lightweight, and excellent mechanical flexibility even at minor thicknesses. However, the complex synthesis and quality-control difficulties limit its application. In this study, we demonstrate that electrochemically exfoliated graphene (EEG) with post-reduction treatment is a promising candidate for lightweight EMI shielding materials. A facile electrochemical exfoliation approach produces a high-quality multilayer graphene with a high electrical conductivity of â¼600 S cm-1, owing to its low degree of oxidation. The reduction of EEG by three different methods, including chemical, thermal, and microwave treatments, causes the removal of surface functional groups as well as significant changes in the microstructure of the final films. The reduced graphene films by microwaves, which are driven by the improved electrical conductivity and large volume expansion, exhibit an EMI shielding effectiveness of 108 dB at a thickness of 125 µm, one of the largest EMI shielding values ever reported for graphene at comparable thicknesses.
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With the rapid development of electronic communications, coated fabrics with EMI shielding capability have attracted increasing attention due to their broad applications in military, civilian, and commercial fields. The coating structure plays a vital role in EMI shielding performance and the fundamental understanding of how the coating structure affects the EMI shielding performance of the coated fabric is urgently needed. In this work, the coating structure has been systematically modulated to study its effects on the shielding performance of the corresponding coated fabric for the rational design of the high-performance EMI shielding materials. Owing to the multiple reflections of the electromagnetic waves triggered by the graphene oxide (GO)/ polypyrrole (PPy) interfaces, the shielding effectiveness (SE) of the coated fabric reaches 39.1 dB by increasing the amount of interface in the coating. Furthermore, more GO/PPy interfaces in the coating results in stronger EMI shielding enhancement once the conductive network is built. This work provides a guideline for the judicious design of shields to achieve excellent EMI shielding performances and offers opportunities for new-generation portable and wearable EMI shielding products.
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Lightweight and high-efficiency microwave absorbers are determined by structure and composition of materials. In this research, a novel core-shell ZnFe2O4@MoS2 composite with a flower-like heterostructure was synthesized successfully by a facile hydrothermal process. The unique 3D heterostructure (porous ZnFe2O4 and MoS2 nanosheets as core and outer shells, respectively) endows the synthesized sample with high-efficiency electromagnetic wave absorption performance. The exploration of microwave absorption properties reveals that the maximum reflection loss displayed by the ZnFe2O4@MoS2 composite is up to -61.8 dB at 9.5 GHz with a filler content of 20 wt%, and the corresponding effective bandwidth (RL exceeding -10 dB) achieves 5.8 GHz (from 7.2 to 13 GHz). The enhanced microwave absorption performance is benefitted by the porous core-shell structure, intense interfacial polarization, multiple reflections, matched impedance and favorable synergistic effect between ZnFe2O4 core and MoS2 shell. Consequently, this strategy provides inspiration for the design of novel microwave absorber with high-performance.
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We present thickness measurements with millimeter and terahertz waves using frequency-modulated continuous-wave (FMCW) sensors. In contrast to terahertz time-domain spectroscopy (TDS), our FMCW systems provide a higher penetration depth and measurement rates of several kilohertz at frequency modulation bandwidths of up to 175 GHz. In order to resolve thicknesses below the Rayleigh resolution limit given by the modulation bandwidth, we employed a model-based signal processing technique. Within this contribution, we analyzed the influence of multiple reflections adapting a modified transfer matrix method. Based on a brute force optimization, we processed the models and compared them with the measured signal in parallel on a graphics processing unit, which allows fast calculations in less than 1 s. TDS measurements were used for the validation of our results on industrial samples. Finally, we present results obtained with reduced frequency modulation bandwidths, opening the window to future miniaturization based on monolithic microwave integrated circuit (MMIC) radar units.
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In this paper, we report a theoretical framework on the effect of multiple resonances inside the dielectric cavity of insulator-insulator-metal-insulator (IIMI)-based surface plasmon sensors. It has been very well established that the structure can support both long-range surface plasmon polaritons (LRSPP) and short-range surface plasmon polaritons (SRSPP). We found that the dielectric resonant cavity under certain conditions can be employed as a resonator to enhance the LRSPP properties. These conditions are: (1) the refractive index of the resonant cavity was greater than the refractive index of the sample layer and (2) when light propagated in the resonant cavity and was evanescent in the sample layer. We showed through the analytical calculation using Fresnel equations and rigorous coupled wave theory that the proposed structure with the mentioned conditions can extend the dynamic range of LRSPP excitation and enhance at least five times more plasmon intensity on the surface of the metal compared to the surface plasmon excited by the conventional Kretschmann configuration. It can enhance the dip sensitivity and the dynamic range in refractive index sensing without losing the sharpness of the LRSPP dip. We also showed that the interferometric modes in the cavity can be insensitive to the surface plasmon modes. This allowed a self-referenced surface plasmon resonance structure, in which the interferometric mode measured changes in the sensor structure and the enhanced LRSPP measured changes in the sample channel.
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Hollow structures with an efficient light harvesting and tunable interior component offer great advantages for constructing a Z-scheme system. Controlled design of hollow cobalt sulfide (Co9 S8 ) cubes embedded with cadmium sulfide quantum dots (QDs) is described, using hollow Co(OH)2 as the template and a one-pot hydrothermal strategy. The hollow CdS/Co9 S8 cubes utilize multiple reflections of light in the cubic structure to achieve enhanced photocatalytic activity. Importantly, the photoexcited charge carriers can be effectively separated by the construction of a redox-mediator-free Z-scheme system. The hydrogen evolution rate over hollow CdS/Co9 S8 is 134 and 9.1 times higher than that of pure hollow Co9 S8 and CdS QDs under simulated solar light irradiation, respectively. Moreover, this is the first report describing construction of a hollow Co9 S8 based Z-scheme system for photocatalytic water splitting, which gives full play to the advantages of light-harvesting and charges separation.
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In nature, many living organisms exhibit low-angle-dependent or even angle-independent structural colors for wide viewing angle, such as the blue skin of mandrill, feather of blue bird and shell of longhorn beetle, etc. To mimic these structural colors, based on periodic semicircular micro-grooved (PSMG) substrate, silica colloidal photonic crystals provide anisotropic angle-independent structural colors. The PSMG photonic crystals were fabricated by self-assembling monodispersed silica nanoparticles on a micro-grooved template, which can be easily prepared in large scale by a hot embossing method, and no additional materials are needed in the whole preparation process. The PSMG photonic crystals exhibit identical structural colors around the groove axis, whereas it is angle-dependent along the groove axis. In addition, this PSMG surface of photonic crystal also leads to color separation effect and monocolor polarization conversion. This system provides a facile and scalable means to prepare anisotropically angle-independent colloidal photonic crystal, which is considered important in applications in the field of anti-counterfeiting or the manufacture of monochromatic light reflector with wide viewing angles and other novel optical devices.
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Complex oxide YVO4 multi-shell hollow spheres with uniform morphologies and controllable shell numbers are successfully prepared by using a newly developed and general composite yttrium-carbonaceous sphere templated approach. The prominent upconversion luminous intensity of the YVO4 :Yb3+ /Er3+ hollow spheres might be attributed to the enhanced near-infrared excitation light harvesting efficiency originated from the multiple reflections.
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In this study, yolk-shell Ni@SnO2 composites with a designable interspace were successfully prepared by the simple acid etching hydrothermal method. The Ni@void@SnO2 composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results indicate that interspaces exist between the Ni cores and SnO2 shells. Moreover, the void can be adjusted by controlling the hydrothermal reaction time. The unique yolk-shell Ni@void@SnO2 composites show outstanding electromagnetic wave absorption properties. A minimum reflection loss (RLmin) of -50.2 dB was obtained at 17.4 GHz with absorber thickness of 1.5 mm. In addition, considering the absorber thickness, minimal reflection loss, and effective bandwidth, a novel method to judge the effective microwave absorption properties is proposed. On the basis of this method, the best microwave absorption properties were obtained with a 1.7 mm thick absorber layer (RLmin= -29.7 dB, bandwidth of 4.8 GHz). The outstanding electromagnetic wave absorption properties stem from the unique yolk-shell structure. These yolk-shell structures can tune the dielectric properties of the Ni@air@SnO2 composite to achieve good impedance matching. Moreover, the designable interspace can induce interfacial polarization, multiple reflections, and microwave plasma.
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Ethylenediammonium sulfate (EDS) crystals were grown from aqueous solution and cleaved into thin (100-500 micron) plates. The 422 point group of EDS was confirmed by X-ray diffraction. The constitutive relations of EDS crystals were determined through generalized ellipsometry with an instrument that uses four photoelastic modulators (4PEM). The optical rotation at 500 nm, for example, was + 22.9°/mm along the optic axis and - 12.1°/mm perpendicular to the optic axis for the P41 21 2 crystals. Enantiomorphous twins frequently form across the (001) plane. Mirrored halves must be separated by cleavage in advance of optical measurements. Chirality 28:460-465, 2016. © 2016 Wiley Periodicals, Inc.
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The fabrication of low-density and compressible polymer/graphene composite (PGC) foams for adjustable electromagnetic interference (EMI) shielding remains a daunting challenge. Herein, ultralightweight and compressible PGC foams have been developed by simple solution dip-coating of graphene on commercial polyurethane (PU) sponges with highly porous network structure. The resultant PU/graphene (PUG) foams had a density as low as â¼0.027-0.030 g/cm(3) and possessed good comprehensive EMI shielding performance together with an absorption-dominant mechanism, possibly due to both conductive dissipation and multiple reflections and scattering of EM waves by the inside 3D conductive graphene network. Moreover, by taking advantage of their remarkable compressibility, the shielding performance of the PUG foams could be simply adjusted through a simple mechanical compression, showing promise for adjustable EMI shielding. We believe that the strategy for fabricating PGC foams through a simple dip-coating method could potentially promote the large-scale production of lightweight foam materials for EMI shielding.
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Based on simple principles, spectrophotometry nevertheless demands a lot of precautions to avoid errors. The following properties of spectrophotometers will be discussed together with methods to test them: Spectral properties-wavelength accuracy, bandwidth, stray light; photometric linearity; interactions between sample and instrument-multiple reflections, polarization, divergence, sample wedge, sample tilt, optical path length (refractive index), interferences. Calibration of master instruments is feasible only by complicated procedures. With such a master instrument standards may be calibrated which greatly simplify performance checks of instruments used for practical work. For testing high quality spectrophotometers the use of emission lines and nearly neutral absorbing solid filters as standards seems to be superior, for some kinds of routine instruments the use of absorption bands and liquid filters may be necessary.