Search

High-resolution ultrasonic imaging, which is highly demanded in nondestructive evaluation, is inherently limited by the detection wavelength. Acoustic metamaterial is an emerging technique to achieve subwavelength-resolution ultrasonic imaging beyond the diffraction limit due to its unprecedented acoustic properties. However, existing reports focus on metalenses for manipulating acoustic waves propagating in fluids like air and water, typically at a low-frequency range below 10 kHz. In this paper, a 0.5 MHz periodic column-structured metalens is designed and fabricated to realize deep-subwavelength ultrasonic imaging for quantitive visualization of subsurface defects in solid structures. The silicon-based metalens is designed based on Fabry–Pérot resonance theory. It consists of silicon columns arranged periodically with a lattice constant of 0.2 mm. The Fabry–Pérot resonance frequency is analyzed theoretically and the wave fields of the metalens at resonance mode are verified numerically. The subwavelength ultrasonic imaging performance of the proposed metalens is numerically proved and experimentally demonstrated. As a result, super-resolution ultrasonic imaging (λ/30, with λ being the wavelength) with a high resolving contrast is realized to identify two separated subsurface defects in a stainless-steel structure experimentally with the designed column-structured metalens. This work demonstrates a valuable deep-subwavelength imaging method that beyond traditional diffraction limits and paves the way for enhanced applications in nondestructive evaluation and biomedical diagnosis. [ABSTRACT FROM AUTHOR]

The conventional machine learning algorithm for analyzing ultrasonic signals to detect structural defects necessarily identifies and extracts either time- or frequency-domain features manually, which has problems in reliability and effectiveness. This work proposes a novel approach by combining convolutional neural networks (CNNs) and wavelet transform to analyze the laser-generated ultrasonic signals for detecting the width of subsurface defects accurately. The novelty of this work is to convert the laser ultrasonic signals into the scalograms (images) via wavelet transform, which are subsequently utilized as the image input for the pretrained CNN to extract the defect features automatically to quantify the width of defects, avoiding the necessity and inaccuracy induced by artificial feature selection. The experimentally validated numerical model that simulates the interaction of laser-generated ultrasonic waves with subsurface defects is first established, which is further utilized to generate adequate laser ultrasonic signals for training the CNN model. A total number of 3104 data are obtained from simulation and experiments, with 2480 simulated signals for training the CNN model and the remaining 620 simulated data together with 4 experimental signals for verifying the performance of the proposed algorithm. This approach achieves the prediction accuracy of 98.5% on validation set, particularly with the prediction accuracy of 100% for the four experimental data. This work proves the feasibility and reliability of the proposed method for quantifying the width of subsurface defects and can be further expanded as a universal approach to various other defects detection, such as defect locations and shapes. [ABSTRACT FROM AUTHOR]

Direct-write piezoelectric transducers are dedicatedly designed and fabricated to generate and detect both fundamental and second harmonic Rayleigh ultrasonic waves for evaluating overload-induced plastic deformation in aluminum (Al) alloy. The Rayleigh ultrasonic signals, generated by concentric focused direct-write transducers and propagating along the Al-alloy specimens, are measured either with other direct-write transducers or laser scanning vibrometer (LSV) to obtain the fundamental and second harmonic ultrasonic signals for analyzing the acoustic nonlinearity. The results show that the acoustic nonlinearity increases over 50% when the plastic strain reaches 7.8%. In comparison with LSV, the direct-write piezoelectric transducers exhibit substantially improved consistency and repeatability in the acoustic nonlinearity measurements. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are conducted to correlate micro-structural changes with the acoustic nonlinearity in the Al alloy. The results and analyses indicate that the direct-write piezoelectric transducers with the appropriate design have significant technical advantages for acoustic nonlinearity testing and yielding determination. [ABSTRACT FROM AUTHOR]

The detection of plastic deformation of metallic alloy materials with second-harmonic Rayleigh ultrasonic wave is first investigated using direct-write piezoelectric ultrasonic transducers, in which piezoelectric poly(vinylidenefluoride/ trifluoroethylene) [P(VDF/TrFE)] polymer coatings and electrodes are directly deposited, processed, and patterned on the alloy to be evaluated. Rayleigh ultrasonic signals, generated by the direct-write transducers on titanium alloy specimens, are characterized by a laser scanning vibrometer. The results show that acoustic nonlinearity increases with plastic strain, and an increase of ~40% in the acoustic nonlinearity corresponding to a plastic strain of 5.1%. The measurement data and technical features with the use of the direct-write transducers are compared with the conventional discrete angle beam piezoelectric transducer. The results and analyses show that compared with the conventional discrete angle beam piezoelectric transducers, implementation of the direct-write piezoelectric transducers has significant technical advantages and is promising for applications in determining nonlinear ultrasonic waves and plastic strain of structural materials. [ABSTRACT FROM PUBLISHER]

Methods for determining yielding of titanium (Ti) alloy material with second harmonic Rayleigh ultrasonic wave are investigated. Both piezoelectric angle beam transducers and high frequency laser scanning vibrometer (LSV) are used to detect ultrasonic signals in the Ti alloy specimens with different plastic strain levels. Technical features and outcomes with use of piezoelectric transducers and LSV are compared. The method using piezoelectric transducers, with much higher signal-to-noise ratio than LSV, has been further improved by deploying two transducers with central frequencies corresponding to the fundamental and second order harmonic signals respectively to improve the testing reliability and accuracy. Both the techniques using piezoelectric transducer and LSV demonstrate consistently that the acoustic nonlinearity increases with plastic strain, and the second harmonic Rayleigh ultrasonic wave can be utilized for effective determination of yielding in Ti alloy. Our experiments further show that the acoustic nonlinearity increases gradually with plastic strain at small plastic strain level, and there is a more significant increase of acoustic nonlinearity when the plastic strain reaches a higher level. Microscopic investigations using scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) are conducted for clarifying the relationship between the observed acoustic nonlinearity and micro-structural changes. [ABSTRACT FROM AUTHOR]

This paper develops an automatic and reliable nondestructive evaluation (NDE) technique that enables quantification of the width and depth of subsurface defects of metallic components simultaneously by using non-contact laser ultrasonic technique and identified machine learning (ML) algorithm. Twenty-two specimens with various subsurface defect dimensions are designed and fabricated for laser ultrasonic experiments, and a total of 220 labeled laser ultrasonic signals are obtained for training and verifying ML models. Twelve features, including four time-domain features (maximum, minimum, peak-to-peak, and |Neg|/Pos value of the laser generated Rayleigh ultrasonic waves) and eight wavelet energy features, are identified and extracted as sensitive feature vectors for establishing the dataset. The principal component analysis (PCA) is implemented as dimensionality reduction method of feature vectors to optimize the recognition algorithm and improve the detection accuracy. Three widely used ML models in NDE, adaptive boosting (Adaboost), extreme gradient boosting (XGBboost), and support vector machine (SVM), combined with the PCA are proposed and compared for detecting both the width and depth of subsurface defects. The PCA-XGBoost achieves the highest recognition rate of 98.48%, and is therefore identified as the most effective approach for analyzing laser-ultrasonic signals. Unlike published reports, the proposed model is trained and evaluated with experimental data covered various classification labels, which is more adaptive and reliable in practical application than the models established using simulated data or limited experimental data. In other applications, as long as sufficient laser ultrasonic data with regards to various defect properties (dimensions, orientations, locations, shapes, etc.) can be acquired, the developed approach can realize accurate detection of corresponding defects. [ABSTRACT FROM AUTHOR]

Lamb wave-based signals from sparse-distributed sensors are complicated and difficult to process for structural health monitoring (SHM), not only due to their dispersive and multi-mode nature, but also due to the increasing complexity of materials and structures. Deep learning (DL) has attracted huge attention to help solve physical problems with a high level of automation and accuracy. However, its reliability and robustness are still questioned when performing the case-by-case model trained by inadequate datasets for practical scenarios, where many variables exist. In this study, a hierarchical deep convolutional regression framework is proposed to solve the impact source localization problem by acoustic emission signals. One-dimensional (1D) network is used due to its capability to process fast with raw time-series data. The window length of input data and the target of output results are discussed to improve the over-fitting issue. The sensor network fail-safe mechanism is designed via generalizing the model to handle abnormal situations with random faulty channels. Data augmentation and transfer learning techniques are utilized to train the fail-safe model without the need for additional experimental data. Pristine case and multiple random-faulty-channel cases are used to test and validate the adaptation performance of the fail-safe model. The whole framework combines both pristine and fail-safe models to achieve high accuracy of impact localization results of both a simple homogeneous plate and a complex inhomogeneous plate with geometric features. The proposed DL framework of greatly improved reliability and robustness, also short processing time, is well suitable for real-time and in-situ SHM applications. [ABSTRACT FROM AUTHOR]

Abstract: In our earlier studies, we have demonstrated that low and high intensity ultrasound can prevent barnacle cyprid settlement. In this study, we found that ultrasound treatment reduced the adhesion of newly metamorphosed barnacles up to 2 days’ old. This was observed in the reduction of adhesion strength of the newly settled barnacles from ultrasound treated cyprids on silicone substrate compared to the adhesion strength of barnacles metamorphosed from cyprids not exposed to ultrasound. Atomic force microscopy (AFM) was used to analyze the effect of ultrasound on barnacle cyprid footprints (FPs), which are protein adhesives secreted when the larvae explore surfaces. The ultrasound treated cyprids were found to secrete less FPs, which appeared to spread a larger area than those generated by untreated cyprids. The evidence from this study suggests that ultrasound treatment results in a reduced cyprid settlement and footprint secretion, and may affect the subsequent recruitment of barnacles onto fouling release surfaces by reducing the ability of early settlement stage of barnacles (up to 2 days’ old) from firmly adhering to the substrates. Ultrasound therefore can be used in combination with fouling release coatings to offer a more efficient antifouling strategy. [Copyright &y& Elsevier]

The determination of elastic constants of a transversely isotropic carbon fiber reinforced plastic (CFRP) composite is the prerequisite of mechanical strength calculation, material degradation evaluation, ultrasonic non-destructive testing for main load-bearing CFRP structures both after manufacturing and in service. Conventional ultrasonic methods request extra sample preparation, water immersion condition or special designed goniometric devices to rotate the test structures, making the elastic constants measurement expensive, inconvenient and not in situ. In this paper, a novel ultrasonic method that enables in situ determination of elastic constants via backwall reflection method (BRM) using a linear array probe is proposed. The BRM can achieve single sided measurement of ultrasonic travel times in various directions including the fiber direction using different pairs of transmitter and receiver array elements. Both quasi longitudinal and quasi shear waves are captured via the mode conversions from the multiple surface reflections. All elastic constants are determined through the particle swarm optimization by minimizing the sum of the squared deviations between the BRM measured and theoretically calculated multi-mode bulk wave travel times in the fiber orthogonal and parallel planes. This method is experimentally verified on a 4.45 mm thick unidirectional T700 carbon fiber/epoxy CFRP composite. The BRM measured Young's modulus in the fiber direction agrees well with that measured by tensile test, with a small deviation of −4.58%. This work proves that the proposed method is single sided, easy to operate, without the necessity of sample preparation, water immersion and extra rotation device, and can determine all elastic constants with high precision, which is therefore promising for in situ applications. [ABSTRACT FROM AUTHOR]

Sequential layer-by-layer (LbL) depositionof polyelectrolytesfollowed by chemical cross-linking was investigated as a method tofabricate functional amphiphilic surfaces for marine biofouling preventionapplications. A novel polyanion, grafted with amphiphilic perfluoroalkylpolyethylene glycol (fPEG) side chains, was synthesized and subsequentlyused to introduce amphiphilic character to the LbL film. The structureof the polyanion was confirmed by FTIR and NMR. Amphiphilicity ofthe film assembly was demonstrated by both water and hexadecane staticcontact angles. XPS studies of the cross-linked and annealed amphiphilicLbL films revealed the increased concentration of fPEG content atthe film interface. In antifouling assays, the amphiphilic LbL filmseffectively prevented the adhesion of the marine bacterium Pseudomonas(NCIMB 2021). [ABSTRACT FROM AUTHOR]

Highlights: [•] Ultrasonic cavitation damaged or removed marine biofouling organism of barnacle. [•] The mechanism was explored using spark generated bubbles and high speed photography. [•] The efficiency of cavitation bubble on barnacle removal was dependent on the dimensionless distance H′. [•] Barnacle was more easily and completely removed at younger ages. [Copyright &y& Elsevier]

Abstract: The inhibitory effect of ultrasound on barnacle (Amphibalanus amphitrite) cyprid was investigated under three excitation frequencies (23, 63, and 102kHz). The linear regression models were built to study the effect of ultrasound pressure and exposure time on the cyprid settlement. The negative slopes of the linear regression lines indicate reduced settlement behavior with increased exposure time and acoustic pressure. The excitation frequency of 23kHz was found to be the most effective on settlement inhibition, with 63 and 102kHz exhibiting similar but weaker response. Separately, ultrasonic cavitation was investigated and confirmed in the filtered seawater (FSW) and partially degassed filtered seawater (PDFSW) via the acoustic spectrum analysis. The cavitation energy was found to be double in FSW than in PDFSW at 23kHz with the acoustic pressure of 20kPa. The much higher settlement reduction and stronger cavitation in FSW at 23kHz suggest that cavitation is a possible if not most likely mechanism for the cyprid inhibition. The cavitation induced force may lead to physical damage to the cyprids which subsequently will result in much higher mortality. The cavitation effect was proven to be stronger at 23kHz than the other two frequencies with the same acoustic energy, which might explain the enhanced efficiency on settlement reduction at this frequency. [Copyright &y& Elsevier]

Simple Summary: The life cycle of marine benthic invertebrates includes a planktonic larval stage and a benthic adult stage, with larval settlement being the critical transition from a planktonic to a benthic lifestyle. Since adult mobility is limited or completely lost in the benthic environment, larval settlement directly affects population distribution, growth, and reproduction. Although larval settlement induced by natural chemical cues is widespread among marine invertebrates, the molecular mechanisms underlying this process remain unclear. Mytilopsis sallei, a typical marine fouling mussel, releases three purine compounds—adenosine, inosine, and hypoxanthine—that synergistically induce larval settlement. In this study, transcriptomic sequencing was employed to identify differentially expressed genes and signaling pathways associated with purine-compound-induced larval settlement. This research not only provides new insights into the molecular mechanisms of larval settlement in marine benthic invertebrates, but also offers a scientific basis and theoretical foundation for the ecological control of marine fouling organisms. The settlement of marine benthic invertebrate larvae is a crucial stage in their life cycle, directly influencing their population distribution, growth, and reproduction. It is generally believed that multiple chemical inducers in the environment synergistically affect larval settlement, though the underlying molecular mechanisms remain unclear. Our previous study has established that three purine compounds (adenosine, inosine, and hypoxanthine) released by adults of the mussel Mytilopsis sallei synergistically induced larval settlement. In this study, we performed transcriptomic sequencing analysis on larvae exposed to individual purine compounds and those exposed to a combination of three purine compounds. A total of 21,850 differentially expressed genes (DEGs) were identified. KEGG enrichment analysis of these DEGs revealed that the AMPK signaling pathway, FoxO signaling pathway, extracellular matrix (ECM)-receptor interaction, focal adhesion, and the thyroid hormone signaling pathway play key roles in the settlement of M. sallei larvae. Moreover, in larvae exposed to the combined action of three purines, the expression of relevant signaling pathways and metabolic processes was more significant. This research provides important information for understanding the molecular mechanisms underlying the induction of larval settlement in M. sallei by purine compounds in natural environments. [ABSTRACT FROM AUTHOR]

A reliable nondestructive evaluation technique that can effectively detect and quantify the internal defects of corner parts in complex-shaped carbon fiber reinforced plastic (CFRP) structures is highly demanded for assuring safety and reliability of safety-critical structures. The conventional methods generally utilize normal incidence ultrasound and suffer from poor image quality due to the unfocused beam. To produce a fully focused image of the CFRP corner part, the delay-and-sum total focusing method (TFM) is preferred. The prerequisite for TFM imaging is to calculate ultrasonic ray paths, which is challenged by the curved shape, elastic anisotropy and multilayered microstructure. In this paper, the complex ultrasonic ray paths transmitting through the corner parts of CFRP structures from the concave side are calculated for ultrasonic array imaging. A ray theory-based homogenization method is firstly proposed to describe the wave velocity distribution of corner parts, which is critically important to the subsequent ray tracing calculation. A ray tracing method based on Dijkstra's algorithm is further developed for rapid calculation of multiple ray paths between two specified points when the ultrasound is incident from the corner's concave surface. To demonstrate the validity of ray tracing method, the TFM images of an CFRP corner part with side drilled holes (SDH) at different depths obtained using three different types of ray paths, are compared and analyzed. Results show that TFM images using ray paths with the minimum or maximum ultrasonic travel time present unsatisfactory image quality due to the obvious structural noises from ply reflections. In contrast, the signal to noise ratio of the image is significantly improved when TFM uses the ray paths with the weakest refraction, and all SDHs are detected and accurately positioned. This paper proves that the proposed ray tracing method is an effective method for imaging the corner parts of CFRP structures. • Wave velocity distribution of CFRP corner part is described using a homogenization method. • Dijkstra's algorithm is used to calculate multiple ray paths through the corner's concave surface. • TFM images reconstruction using different ray paths are compared for CFRP corner part. [ABSTRACT FROM AUTHOR]

A multi-frequency ultrasonic method was proposed to detect and characterize delamination and rich resin in thick composites with waviness. Addressing the challenges for ultrasonic inspection caused by waviness, multi-layer structure, and multiple defect types, ultrasound propagation in thick wavy composite was investigated through a dedicated numerical model built in OnScale®. Key features of ultrasonic testing and composite sample, including water immersion environment, fiber waviness, uneven inter-ply resin distribution, and side-drilled hole (SDH)-simulated delamination, were embraced in the model. Via this model, waviness and thick resin layers were found to cause disturbance and wave vector deviation to inter-ply reflection signal and thus introduce difficulties for delamination detection based just on the signal to noise ratio. In addition, reflections from inter-ply resin and SDH were revealed with different reliance on probe frequencies. Using a 5 MHz probe with time corrected gain, ultrasonic testing in wavy composite was performed experimentally. Based on the reliance of defect characterization on frequency, SDHs and rich resin were differentiated experimentally in the B-scan images with various filtering frequencies, demonstrating the effectiveness of the proposed method for detection and characterization of delamination and rich resin in thick wavy composite structures. • A novel multi-frequency ultrasonic method to characterize delamination and rich resin in thick wavy composite. • A novel ultrasound model embracing multi-layer heterogeneous wavy composite with details. • A frequency-domain filtering method to allow for one-time test to detect delamination and rich resin in wavy composite. [ABSTRACT FROM AUTHOR]

We propose an innovative design for interdigital transducers (IDTs), enabling phase modulation of surface acoustic waves (SAWs) with a dislocated electrode structure. By designing the size and arrangement of these dislocated IDTs, a novel type of Airy SAWs can be generated, exhibiting self-accelerating, self-bending, and self-healing characteristics. The acceleration of the generated Airy SAW is 0.081 cm−1. Furthermore, particles and bubbles can be precisely manipulated using the generated Airy SAW. The proposed dislocated IDTs could be used for generation of many other types of SAWs, hence holding great promise for applications including SAW shaping, particle manipulation/sorting, and acoustic sensing/detection. [ABSTRACT FROM AUTHOR]

Novel direct-write ultrasonic transducers comprising piezoelectric poly(vinylidenefluoride/trifluoroethylene) [P(VDF/TrFE)] polymer coatings which were in-situ deposited, crystallized and patterned on pipe for structural health monitoring purpose. Lamb ultrasonic wave signals, generated and measured by the direct-write transducers and propagating along the pipe structure, were used to monitor the integrity of the pipe structure. The experimental measurements of the axial Lamb wave on pipe structure showed the substantial reduction in the ultrasonic signal by the defects. In addition, pipe thickness was accurately determined with the direct-write transducers to generate and detect the ultrasonic wave in the pipe thickness direction using pulse-echo mode. Our result and analyses suggest that implementation of the unique direct-write ultrasonic transducer technology is promising for realizing structural health monitoring for pipeline structures with improved consistency and reliability. [ABSTRACT FROM AUTHOR]

Tactile sensing with spiking neural networks (SNNs) has attracted increasing attention in the past decades. In this article, a novel SNN framework is proposed for the tactile surface roughness categorization task. In contrast to supervised SNN methods such as ReSuMe and Tempotron that require prespecifying target spike trains, the presented method performs the classification through directly comparing the distance between multineuron spike trains. Unlike simple spike train fusion methods using average pairwise spike train distance or pooled spike train distance, the proposed method merges spike trains from different neurons with the multineuron spike train distance, which can capture the complex correlation of multiple spike trains. Specifically, the spike trains are generated via the Izhikevich neurons from tactile signals. The similarity of the multineuron spike trains is computed using the multineuron Victor–Purpura spike train distance, which can be efficiently implemented in an inductive manner. The classification can be performed by incorporating $k$ -nearest neighbors and the multineuron spike train distance as a similarity metric. The proposed framework is quite general, i.e., other multineuron spike train distances and spike train kernel-based methods can be readily incorporated. The effectiveness of the proposed method has been demonstrated on a tactile data set by comparing it with various feature- and spike-based methods. Note to Practitioners—In the soft neuromorphic implementation of biomimetic tactile sensing and the development of the tactile sensing capability in neurobotic systems, the processing and analysis of spike-like tactile signals are quite common. Inspired by human tactile perception, this article proposes a novel supervised spiking neural network method for tactile sensing tasks. The traditional methods have to prespecify target spike trains, which is still an open question. In addition, the current ways to fuse spike trains from multiple neurons are far from mature. This article tackles these two problems using spike train similarity comparison with multineuron spike train distance. The direct spike train similarity comparison avoids the need to prespecify target spike trains. The multineuron spike train distance can inherently fuse spike trains from different neurons. It is demonstrated that the proposed method is able to effectively perform classification in a tactile roughness discrimination task. [ABSTRACT FROM AUTHOR]

Post-growth rapid thermal annealing of α-MoO3 layered crystals in Pb-species that were in situ evaporated from PbO coatings has been studied using various techniques. They reveal the occurrence of solid–gas reactions between MoO3 and PbO, which is accompanied by MoO3 sublimations, leading to remarkable morphology and structure evolutions when the annealing temperature (Tann) is increased from 400 to 800 °C. Onset of nucleation and crystal growth of PbMoO4, which initiated at the structural defects of α-MoO3, occurred at 600 °C. The growth of PbMoO4 and the consumption of MoO3 at their boundary areas give rise to materials melting over there at 700 °C, which, in turn, connected the localized grown PbMoO4 via capillary actions. After cooling down to room temperature, the PbMoO4 solidified in crystals, while the MoO3 consisted of both amorphous and crystalline precipitates with regular in-plane orientations. In comparison, spherical domes that appeared at Tann = 800 °C resulted from enhanced localized reactions between the molten PbMoO4/MoO3 and the Al2O3 substrate. These results shed new lights on thermal behavior of α-MoO3 and the temperature-dependent reactions between MoO3 and PbO. [ABSTRACT FROM AUTHOR]

Polyoxazolines have received increasing attention as low-fouling materials with good stability and ease of functional group incorporation. We investigated layer-by-layer (LbL) assembly of poly(2-ethyl-2-oxazoline) (PEOX) with poly(acrylic acid) (PAA) to incorporate PEOX into thin conformal coatings with controllable thicknesses ranging from the nano- to micron range. Partial hydrolysis of PEOX (to form PEOX-I) was used to introduce secondary amine groups that enable post-assembly multilayer stabilization by heat-induced crosslinking. While as-assembled multilayers dissolve in aqueous solutions at pH 5 and above, thermally crosslinked multilayers were stable against film loss and instead exhibit pH responsive swelling. The anti-fouling properties of crosslinked coatings were assessed by evaluating the resistance of PEOX-I containing multilayers to fouling by proteins, cells and bacteria. Our study of multilayers with thicknesses ranging from ∼12 nm to ∼1.5 μm revealed thickness dependence of surface fouling resistance to BSA. Crosslinked multilayers of ∼220 nm were found to be highly effective in suppressing surface adsorption of bovine serum albumin (BSA), while thinner or thicker layers were increasingly susceptible to BSA adsorption. We further found that coatings of ∼220 nm and above were all highly effective at preventing surface attachment of fibroblasts, gram-positive ( S. aureus ) and gram-negative ( E. coli ) bacteria. [ABSTRACT FROM AUTHOR]

ZnO/Al 2 O 3 superlattices, i.e., nanolaminate structures, have been grown by atomic layer deposition (ALD) on Si, sapphire, and quartz substrates at 150 °C. Morphological studies show that an increase in the cycle mixing ratios of Al 2 O 3 to ZnO tends to increase the surface smoothness via suppression of nucleation and growth of ZnO crystallites. X-ray diffraction studies reveal that the orientation of the ZnO crystallites is also affected by changing the cycle ratios of Al 2 O 3 to ZnO under certain ALD conditions. Evolutions in the morphological, structural, and optical properties of the ZnO/Al 2 O 3 nanolaminates upon post-growth annealing provide evidence that atomic interdiffusions occurred at elevated temperatures and the activation/diffusion of Zn atoms in the amorphous Al 2 O 3 layers is much easier/faster than that of Al atoms in the crystallite ZnO layers. By comparing the depth profiles of Al-O, Zn, and Al-H ions measured by secondary-ion mass spectroscopy, we find that hydrogen remains in the Al 2 O 3 /ZnO nanolaminates although extensive purge durations were employed in the ALD process. The elemental hydrogen concentrations are higher at the interfaces where the ALD process switches from ZnO to Al 2 O 3 rather than the reverses, which is strongly correlated with the etching behavior observed in an earlier publication [Chem. Mater. 15 , 1020 (2003)]. Our findings shed light on further developments and ALD process of ZnO/Al 2 O 3 superlattice structures for practical devices applications. [ABSTRACT FROM AUTHOR]

Three sodium polyacrylate copolymers PD0x (Poly acrylic acid-co-sodium 4-vinylbenzenesulfonate or PD01; Poly acrylic acid-co-sodium 4-vinylbenzenesulfonate-co-hydroxyethyl methacrylate or PD02 and Poly methyl methacrylate-co-acrylic acid-co-sodium 4-vinylbenzenesulfonate-co-hydroxyethyl methacrylate or PD03) were synthesized as water-based dispersants for grinding red–brown pigment ZnFe1.2Cr0.8O4 particles prepared by the solid phase method (S-ZnF). The particle size distribution, viscosity of suspensions, and adsorption capacity of dispersants were explored by laser particle size analysis, viscometer, and thermogravimetry (TG), respectively. The application of 2 wt.% dispersant PD02 in the S-ZnF suspension ground for 90 min can deliver a finer product with the narrower particle size distribution. The added dispersant PD02 in the grinding process of the S-ZnF particles exhibits a suitable viscosity of the suspension and generates more hydrogen bonds on the S-ZnF particle surface. The sulfonic acid groups (SO3−) and carboxylic acid groups (-COO−) in the dispersant PD02 can also provide a strong charge density, which is favorable for the dispersion and grinding of the S-ZnF particles in the suspensions. Furthermore, the adsorption behavior of polymeric dispersant PD02 adsorbed on the S-ZnF particles surface was simulated and analyzed by adsorption thermodynamic models and adsorption kinetic models. It is indicated that the adsorption thermodynamic behavior of dispersant PD02 adsorbed on the S-ZnF particles surface follows the Langmuir model, and the adsorption process is endothermic and a random process with increased confusion during the grinding process. In addition, the adsorption kinetics of dispersant PD02 adsorbed on the S-ZnF particles surface are more in line with the pseudo-first-order kinetic models. Therefore, the adsorption process of dispersant PD02 on the S-ZnF particles surface can be considered as a single-surface adsorption process. [ABSTRACT FROM AUTHOR]

This paper describes the preparation of ferrocene-containing polymer brushes with different side chain lengths by surface-initiated atom transfer radical polymerization (SI-ATRP). The polymer brushes are characterized by cyclic voltammetry to determine the surface coverage of the ferrocene (Fc) units, their thickness in air by atomic force microscopy (AFM) and ellipsometry, and their mechanical properties by AFM-based nanoindentation. The results are compared against an ideal packing model and we conclude that brushes with short linkers between the Fc units and the polymer back bone are stiff and stand up in air up to 40 nm tall. In contrast, polymers with long linkers collapse and do not stand up in air. Our results indicate that the stiffness and packing structure of Fc-containing polymer brushes are affected by the length of the linkers between the Fc and polymer back bone. [ABSTRACT FROM AUTHOR]

To achieve sensitive biomolecular detection, a gold nanohole array sensing chip was designed by plasmonic simulation and fabricated by high fidelity nanoimprinting technology. The gold nanohole array on a glass substrate serves as an optical antenna array, to produce localized surface plasmon resonance (LSPR) of 10–100 times stronger than the illumination light for quantum dot (QD) excitation. In this work, a robust sandwich bioassay of primary aptamer/thrombin/secondary aptamer with QD655 was established on the chip, to detect thrombin at a limit of detection (LOD) of 1 ng/ml (27 pM), with low non-specific binding even at the presence of 1000 times more concentrated bovine serum albumin (BSA). The conformation and QD enhancement mechanism of the sandwich assay were characterized by atom force microscopy (AFM) and dual polarization interferometry (DPI). This is the first time that DNA is surface functionalized on gold nanostructures for protein detection by plasmon enhanced QD emission. Considering the low-cost, easy regeneration and long shelf-life of the capturing aptamer, low non-specific binding and high sensitivity of the bioassay, as well as the inexpensive mass fabrication of the high quality chips, this novel aptasensor platform is particularly useful for a point-of-care system on medical diagnostics. [ABSTRACT FROM AUTHOR]

In this work, Nb‐doped 0.75SrTiO3‐0.25BiFeO3 (ST‐BF) lead‐free ceramics are designed and synthesized using a conventional solid‐state reaction method. The influence of Nb doping on the microstructure, dielectric, and electrical properties are systematically investigated. With the increase of Nb concentration, the crystal structure of ST‐BF remains pseudo‐cubic as exhibited in the X‐ray diffraction patterns. The grain size is found to increase from 0.33 to 6.23 μm, and then decrease to 1.88 μm by Nb doping, along with a clear heterogeneous core–shell microstructure. A relatively low dielectric loss (∼0.1, at 1 kHz) and a stable dielectric constant (∼700, at 1 kHz) are obtained for the 0.03 Nb‐doped ST‐BF composition at room temperature. Impedance spectroscopy analysis shows that Nb doping in ST‐BF increases the total resistivity, forming an electrically conductive core and a nonconductive shell, with enhanced activation energy. The results may provide a feasible approach to develop novel ST‐based lead‐free dielectric ceramics for capacitor applications. [ABSTRACT FROM AUTHOR]

Sensor drift is one of the severe issues that gas sensors suffer from. To alleviate the sensor drift problem, a gas sensor drift compensation approach is proposed based on local discriminant subspace projection (LDSP). The proposed approach aims to find a subspace to reduce the distribution difference between two domains, i.e., the source and target domain. Similar to domain regularized component analysis (DRCA) which is a recently proposed sensor drift correction method, the mean distribution discrepancy is minimized in the common subspace in our approach. LDSP extends DRCA in two aspects, i.e., it not only takes the label information of the source data into consideration to reduce the possibility of the case that samples in the subspace with different class labels stay close to each other, but also borrows the idea of locality-preserving projection to deal with multimodal data. Specifically, inspired by local Fisher discriminant analysis (LFDA), the label information is utilized to maximize the local between-class variance of source data in the latent common subspace and simultaneously minimize the local within-class variance. The formulation of LDSP is a generalized eigenvalue problem that can be readily solved. The experimental results have shown the proposed method outperforms other gas sensor drift compensation methods in terms of classification accuracy on two public gas sensor drift datasets. [ABSTRACT FROM AUTHOR]

• An improved variational mode decomposition strategy based on particle swarm optimization is proposed to enhance the signal-to-noise ratio. A fitness function based on permutation entropy and mutual information is developed to determine the optimized parameters in variational mode decomposition. • For the different excitation locations, the amplitude of the Rayleigh wave generated by laser irradiation on the defect is larger than on the small area around the defect, and the defect can be effectively detected using B-scans at elevated temperatures according to the abnormal evolution of Rayleigh waves. • For the same excitation locations, the amplitude of R waves gradually decreases at 100–300 °C and is consistent with the simulation, while it is opposite at 400–500 °C. Online monitoring is a critical issue for additive manufacturing components layer by layer to ensure the integrity of the structure. However, the rough surface and high temperature make it difficult to use conventional non-destructive testing methods to carry out. In this work, the laser ultrasonic technique, as a non-contact nondestructive testing method, is applied to detect defects in a 316L stainless steel specimen manufactured using selective laser melting at elevated temperatures. The unreported phenomenon of phase evolution of Rayleigh waves for different excitation locations and temperatures is observed and systematically explored using the finite element method and experiments. Furthermore, an improved variational mode decomposition strategy based on particle swarm optimization is proposed to enhance the signal-to-noise ratio. A fitness function based on permutation entropy and mutual information is developed to determine the optimized parameters in variational mode decomposition. The experimental results indicate that, for the different excitation locations, the amplitude of the Rayleigh wave generated by laser irradiation on the defect is larger than on the small area around the defect, and the defect can be effectively detected using B-scans at elevated temperatures according to the abnormal evolution of Rayleigh waves; for the fixed excitation locations, the amplitude and velocity of Rayleigh waves gradually decreases at 100–500 °C, which will provide a scientific basis for online monitoring and evaluation of additive manufacturing components. [ABSTRACT FROM AUTHOR]

• Overview of the commonly used sensing systems in environmental perception technology. • Give clearer selection guidance for several perception methods. • New challenges from technology, external environment and application. Environmental perception technology is the guarantee of the safety of driverless vehicles. At present, there are a lot of researches and reviews on environmental perception, aiming to realize unmanned driving while ensuring the safety of human life. However, the technology is facing new challenges in the new era. This review paper attempts to systematically summarize environment perception technology and discuss the new challenges currently faced. To this end, we first summarized the advantages, disadvantages and applicable occasions of several commonly used sensing methods to provide a clear selection guide. The new challenges faced by environmental perception technology are discussed from three aspects: technology, external environment and applications. Finally, the article also points out the future development trends and efforts of environmental perception technology. [ABSTRACT FROM AUTHOR]

• α-MoO 3 single crystal belts have been sulfurized at elevated temperatures. • Conversion of α-MoO 3 to MoS 2 /MoO 2 /α-MoO 3 heterostructures occurred at 600–700 °C. • MoS 2 /MoO 2 heterostructures formed, in various configurations, at 800–900 °C. • Absorption edge of MoO 2 flakes exhibits huge blue shifts with thickness decrements. • Possible blue shift mechanism and its application have been discussed and proposed. We have studied thermal vapor sulfurization (TVS) of α-MoO 3 single crystal belts that were grown by chemical vapor deposition and transferred onto sapphire substrates. Micro-Raman scattering spectroscopy reveals an onset of material conversion from α-MoO 3 to MoS 2 /MoO 2 /α-MoO 3 heterostructures at T TVS = 600 °C; the conversion increases with the increase in T TVS , accompanied by monotonically mode hardening of MoO 2 phonons up to T TVS = 900 °C; and the α-MoO 3 base materials completely disappeared when T TVS is increased to higher than 800 °C, giving rise to thermodynamic stable MoS 2 /MoO 2 heterostructures at T TVS = 900 °C. Layered structures consisted of a network of linear structures with regular in-plane orientation at the bottom and densely packed crystallites on top have also been observed, which is attributable to the effect of the epiready c-plane sapphire substrate. Absorption spectroscopy measurements reveal huge blue shifts in the absorption edge of the MoS 2 /MoO 2 heterostructures when the thickness of the MoO 2 layer is reduced, which is attributed to optical interferences at the crystal surfaces. [ABSTRACT FROM AUTHOR]

An effective nondestructive evaluation technique that enables the detection and quantification of subsurface defects is highly demanded for assuring safety and reliability of safety-critical structures. In this work, an improved genetic algorithm-back propagation neural network (GA-BPNN) model and non-contact laser ultrasonic technique are combined to quantify the width of subsurface defects. An experimentally validated numerical model that simulates the interaction of laser-generated Rayleigh ultrasonic waves with subsurface defects is firstly established, which is further utilized to generate a large number of labeled laser ultrasonic signals for training the GA-BPNN model. A total number of 189 data are obtained from simulation and experiments, with 173 simulated signals for training the GA-BPNN model and the remaining 13 simulated signals together with 3 experimental signals for verifying the performance of the trained GA-BPNN model. Five features including three time-domain features (maximum, minimum and peak-to-peak value of the Rayleigh ultrasonic waves) and two frequency-domain features (F c , BW -6dB), which are identified sensitive to the width of subsurface defects by both experiments and simulation, are extracted as inputs to train the machine learning algorithm. The result demonstrates that the GA-BPNN model trained with the combination of time and frequency features has the average error of 2.15%, which is substantially smaller than the errors obtained from the model trained with only time-domain features and frequency-domain features, with the average errors of 4.43% and 21.81%, respectively. This work proves the feasibility and reliability to quantify the width of subsurface defects in metallic structures using laser ultrasonic technique and the improved GA-BPNN algorithm. [ABSTRACT FROM AUTHOR]

• Phase evolution of Rayleigh ultrasonic waves is reported to detect subsurface defects. • The mechanisms of phase evolution with defect width and depth are intensively studied. • The relationships between phase evolution and defect width and depth are established. • A sensitive and robust parameter is proposed to detect and quantify subsurface defect. An unreported phenomenon of phase evolution of Rayleigh ultrasonic waves with subsurface defects is observed and systematically explored for detection of subsurface defects using non-contact line laser ultrasonic technique and numerical simulation. The mechanism of phase evolution of Rayleigh wave signals is explained by the interference of the reflected and direct Rayleigh wave, explored by finite element analysis. Both experiments and simulation show distinct peak evolution of the Rayleigh wave signals with the width and depth of subsurface defect. A dimensionless parameter (|Neg|/Pos), defined by the ratio of absolute negative peak to positive peak of Rayleigh wave, is proposed to evaluate the phase evolution of Rayleigh wave with defect width and depth, which is further used to quantify the subsurface defects. The phase evolution of Rayleigh waves can act as a robust and sensitive feature to detect subsurface defects using laser-generated ultrasound, which has promising applications in life prediction and health monitoring of various engineering structures. [ABSTRACT FROM AUTHOR]

Realistic structures with complex features have been intricate challenges for structural health monitoring (SHM) with permanently installed transducers. Poor conformability of conventional rigid and brittle piezoceramic wafers is a typical issue of applications on surfaces with complex geometry. Moreover, the accompanied high-stress concentration requires high-sensitivity defect detection which is difficult to achieve with large-area monitoring methods like lamb waves at tens to hundreds kHz. Previously, in-situ conformally fabricated piezopolymer coating-based array transducers (PCATs) have been developed to build large-area, lightweight, flexible, and tunable Lamb wave networks. In this study, two novel local monitoring methods were investigated with PCATs, namely ultrasonic bulk wave array inspection and local ultrasonic resonance spectroscopy. For thick structures, ultrasonic bulk waves were generated and detected by PCATs with broadband operating frequencies (1–10 MHz) and flexible array parameters. Simulation tools and imaging algorithms of array inspection in non-destructive testing (NDT) were well implemented based on the analogous directivity pattern and normal-pressure coupling mechanism. As proof of concept, PCATs were applied on example structures with flat, concave, and convex surfaces for internal defect imaging. For thin-walled and/or multilayer structures, PCATs were used to measure local ultrasonic resonances, comparable to conventional non-contact methods. With negligible influence on local mechanical properties and broadband frequency response, multiple resonance peaks from 0 to 25 MHz were identified as zero group velocity (ZGV) Lamb modes and thickness vibration modes of host structures, which can be used as damage indices for local monitoring of corrosion, delamination, stiffness degradation, etc. Through embracing advanced NDT techniques with PCATs, high-sensitivity and quantitative local monitoring could be achieved with conformal networks, offering the possibility to integrate with large-area monitoring as multi-scale SHM for complex structures. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nanocomposite photoacoustic transducer utilized to modulate ultrasonic surface wave for acoustic nonlinearity evaluation. • A structural optimization method of photoacoustic transducer for minimizing unwanted components of surface wave. • High-intensity narrowband surface wave with pure fundamental component acquired with optimized design non-ablatively. • Fully non-destructive microscale fatigue crack localization is achieved by nonlinear wave mixing using optimized designs. Laser ultrasonics is a promising non-contact inspection technique but faces challenges of low signal-to-noise and low amplitude under non-destructive thermoelastic regime. In this paper, a laser ultrasonic surface acoustic wave (SAW) modulation method and photoacoustic transducer (PAT) are proposed and combined with nonlinear wave mixing technique to inspect microscale fatigue crack. PATs comprised of candle soot nanoparticles and polydimethylsiloxane are systematically optimized and combined with a line-arrayed laser source to generate desired high-amplitude and pure fundamental SAWs. Two modulated SAWs with frequencies of 2.1 and 2.9 MHz are excited on a fatigued aluminum plate to generate nonlinear mixed components, and the ultrasonic responses over the fatigue crack regions are acquired with a scanning laser Doppler vibrometer. The localization of fatigue crack with microscale is eventually achieved by mapping the nonlinear parameter of the mixed components, which proves it a reliable and non-destructive technique to inspect the fatigue crack. [ABSTRACT FROM AUTHOR]

Silicone-based coatings are vulnerable to marine fouling organisms in static environments, which seriously hampers its practical application. Here, an amphiphilic acrylate polymer is designed by hydrophilic polyethylene glycol and antibacterial borneol, which is introduced into polydimethylsiloxane (PDMS) by covalent linking to prepare antifouling coatings. XPS and CA tests prove that amphiphilic acrylate polymer can migrate to coating surface in the marine environment to maximize antifouling properties. Mytilus edulis foot protein adhesion test find it can weaken interface force between Mefp-3 and surface. Laboratory antifouling performance evaluations confirm that the coatings are effective against the adhesion of bacteria (86.72 % reduction for E. coli), diatoms (87.76 % reduction for Nitzschia closterium). The good antifouling properties of coatings are ascribed to the synergistic effect among unique bicyclic monoterpene structure of natural non-toxic borneol and amphiphilic acrylate polymer on surface of coatings. Moreover, the coating is less harmful to the marine ecological balance than traditional coatings containing toxic biocides due to the introduction of natural non-toxic borneol. This work contributes new insights in the exploration of environmentally friendly coating with excellent static antifouling performance. • Dynamic migration of amphiphilic substances provide static antifouling capability. • The preparation process of the coating is simple. • Borneol presents environmentally friendly. [ABSTRACT FROM AUTHOR]

[Display omitted] • The transparent and amphiphilic silicone-based coatings are prepared conveniently. • Significantly lower protein adhesion force and reduced protein attachment are obtained. • Excellent fouling release performance is reached. • Significant differences in the expression of adhesion proteins in mussel adhesive plaque are verified. • The main reason is that the introduction of amphiphilic materials significantly reduces the adhesion strength at the coating-fouling interface. The amphiphilic antifouling coatings are effective for inhibiting fouling adhesion. Herein, the amphiphilic coatings were prepared by introducing polyether-modified polydimethylsiloxane (PE-PDMS). Uniform morphology, high transmittance, low surface roughness and amphiphilic property were obtained. PE-PDMS appearing as interspersed chains endowed the coating with amphiphilic characteristic. The artificial seawater promoted the migration of polyether to the surface. The protein adhesion force and quality were significantly reduced according to AFM-based colloidal probe technique and Quartz Crystal Microbalance with Dissipation, respectively. The coating exhibited antibacterial activity against Pseudoalteromonas xiamenensis (84.0%), Escherichia coli (69.2%) and Staphylococcus aureus (63.0%), respectively. 63.1% Halamphora. sp and 75.8% Nitzschia closterium f. had been inhibited, while the coatings possessed outstanding fouling release property with above 76.1% diatom detached after water exposure, proving that the amphiphilic parts weakened the adhesion of fouling. The mussel settlement assay confirmed that less adhesive plaques existed over the amphiphilic coatings. In addition, the expression for mussel adhesion proteins (mfp-5 and mfp-6) were significantly increased, demonstrating that amphiphilic polymers interfered with the adhesion response of the mussels. The effect on the interfacial behavior between fouling and coating is explored, providing a meaningful issue for the development of environmentally friendly antifouling coatings. [ABSTRACT FROM AUTHOR]

Abstract: For the first time, potassium sodium niobate (KNN)‐based lead‐free piezoelectric ceramic coating with strong piezoelectric response was fabricated on stainless steel substrates by thermal spray process, after introducing NiCrAlY and yttria‐stabilized zirconia (YSZ) intermediate layers. A large effective piezoelectric coefficient (d33) of 125 pm/V was obtained with the thermal‐sprayed KNN‐based ceramic coating on the steel substrates. The mechanisms of improving the structure and enhancing the properties of the KNN‐based piezoelectric ceramic coatings by introducing the intermediate layers were analyzed. Ultrasonic transducers were designed and fabricated from the KNN‐based coatings directly formed on a steel plate structure, and the feasibility for generation and detection of ultrasonic waves for structural health monitoring using the thermal‐sprayed lead‐free piezoelectric ceramic coating was demonstrated. [ABSTRACT FROM AUTHOR]

Abstract The detection of fatigue cracks around fastener holes in riveted structures is important to ensure the safety of the structures and to prevent major structural failure. However, due to the complex structure around the fastener holes, the existing non-destructive methods are not effective to address this challenge. Here, we demonstrate a directed acoustic shearography system using a ring-shaped phased array transducer for crack imaging around fastener holes in thick aluminum plates. Supported by numerical simulations, a 16-element ring-shaped phased array transducer was designed, fabricated and tested. The 16-element phased array transducer was excited by an in-house developed phased array drive circuit to control the directions of acoustic waves. Directed acoustic shearography testing with the ring-shaped phased array transducer showed that by controlling the direction of acoustic waves, the imaging contrast of the defects could be significantly improved. Directed acoustic shearography was able to detect subsurface crack which was not detectable by conventional acoustic shearography excited at the same voltage and frequency. This work demonstrates a promising practical NDT tool for fast and full-field detection of surface and subsurface defects with significantly enhanced sensitivity. [ABSTRACT FROM AUTHOR]

Revealing the “structure–property” relationship is nontrivial for the application of materials. The magnetic configurations of FeGe are strongly dependent on the internal structures and positions of Fe and Ge atoms, e.g., B20-type cubic phase configuring into magnetic skyrmions. Although endotaxial FexGe islands on (001)Ge substrate can be obtained with a dynamically controlled growth process, it is not possible to obtain the crystal structural information from conventional approaches, such as X-ray diffraction or 2D electron diffractions due to the embedded growth structures at the nanometer scale, which are too small to be probed. Through a 3D rotational electron diffraction method combined with high resolution transmission electron microscopy, we have successfully determined the specific phase of each FexGe island on the Ge substrate. Three major phases (two hexagonal and one monoclinic) were found existing in the FexGe islands with nearly equal probability and independent of the deposition temperatures and time. All the islands with different phases were inclined to {111}Ge planes, which was due to a comparatively higher energy barrier for Fe atom diffusion through {111}Ge than other Ge planes. The epitaxial growth relationship at the interface was as follows: FeGemono(4−2−1)//Ge(111), FeGemono[15−6]//Ge[1−10], Fe3Ge2(201)//Ge(111) and Fe3Ge2[−112]//Ge[1−10], Fe13Ge8(40−1)//Ge(111) and Fe13Ge8[124]//Ge[1−10]. Ferromagnetism for the as-deposited samples was observed via SQUID, which most probably is attributed to the hexagonal FeGe phases. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel Mo 2 C/Bi 2 WO 6 / indole derivative modified fluorinated acrylate polymer composite film is prepared. • The co-catalysts of Mo 2 C and fluoropolymer facilitate the charge separation. • It has excellent photothermal effect to enhance photocatalytic performance. • Multiple synergies contribute to superior antifouling properties. Micro-dynamic photocatalytic antifouling film based on the construction of heterojunction structure and built-in electric field to solve the insufficient antifouling efficiency of photocatalysis and polymers. Herein, a novel Z-scheme heterojunction hollow photocatalyst Mo 2 C/Bi 2 WO 6 (MB) was prepared and combined with indole derivative modified fluorinated acrylate to construct Mo 2 C/Bi 2 WO 6 /AFIP (MB/AFIP) micro-dynamic composite film. The MB/AFIP film exhibited excellent antifouling performance, and the inhibition ratios against Escherichia coli and Staphylococcus aureus reached 95.20% and 98.52%, respectively, and the anti-settling ratio against Halamphora sp. was 98.58%. The multi-enhanced antifouling performance is attributed to the built-in electric field assisted charge transfer between the Mo 2 C and Bi 2 WO 6 , and the increasing carrier separation via the fluorine component in the polymer. In addition, indole derivative grafted onto fluorinated acrylate polymer further enhanced the antifouling performance. The integration of the environmentally friendly antifouling resin AFIP, the internal hollow photothermal material Mo 2 C, and the typical photocatalyst Bi 2 WO 6 realized multiple synergistic mechanisms, which has potential application prospects in the field of marine antifouling. [ABSTRACT FROM AUTHOR]

A reliable non-destructive evaluation technique that accurately assesses the wall thickness and slag inclusions of ductile iron pipes prior to annealing is crucial to reducing costs in subsequent operations. The rough outer layer of the produced ductile iron pipes leads to considerable background noise, limiting the probability of detection of laser ultrasonic testing equipment. In this study, an improved variational mode decomposition approach based on particle swarm optimization is suggested initially to increase the signal-to-noise ratio. This technique develops a fitness function suitable for processing ultrasound signals. A scanning laser ultrasound technique is designed for simultaneous measuring wall thickness and finding slag inclusions in ductile iron pipes based on excitation locations and flight time. A finite element analysis simulation model is established to assess the feasibility of this scanning method. This experiment is conducted on a 4.2 mm thick sample with slag inclusion defects and a 6.2 mm thick sample without slag inclusion flaws. Experimental results demonstrate that the developed denoising technique may enhance the average signal-to-noise ratio from 34.83 dB to 43.15 dB; the accuracies of thicknesses measurement for two ductile iron pipes are 96.90% and 99.52%, respectively, and the slag inclusion defects can be recognized by ultrasonic amplitude or energy distribution in different B-scan images. It has potential implications for in-service inspection of ductile iron pipe structures. [ABSTRACT FROM AUTHOR]

[Display omitted] • A fully noncontact LU-based method is developed for fast and high-resolution visualization of defects. • A strategy adopted for fast defect localization using Rayleigh waves with circular scans. • High-resolution 3D imaging performed focusing around the located defect areas to quantify defects. • Various defect locations, depths, and sizes in AM parts are analyzed to verify the feasibility of the method. Additive manufacturing (AM) is an emerging technique for efficient fabrication of individually tailored and complex geometry parts. The fabrication process is prone to induce various defects that can have detrimental effects on the AM components. Therefore, a reliable technique that enables monitoring the integrity of AM components and in return helping to optimize the fabrication parameters in mission-critical structures is highly demanded. This work presents a fast and high-resolution damage visualization method using laser-ultrasonic (LU) imaging technique for accurately detecting and quantifying the subsurface defects in printed AM components. Specifically, a fully noncontact LU scanning system is implemented to generate and detect high signal-to-noise ratio laser ultrasonic waves using a pulsed laser and laser Doppler vibrometer, respectively. A strategy for fast defect localization using Rayleigh waves with circular scans is firstly proposed. The high-resolution 3D synthetic aperture focusing technique (SAFT) imaging with raster scans is subsequently performed focusing around the located damage areas to stereoscopically visualize and quantify the subsurface defects. The reconstructed images are further processed and improved using Gaussian filter algorithm to obtain accurate defect shapes, sizes, and positions. The feasibility of the proposed method is eventually verified on AlSi10Mg and stainless steel (316L) components containing subsurface defects with various types and dimensions. The measured sizes are well consistent with the designed values, suggesting that it is a reliable inspection method for AM parts to ensure quality control and feedback. [ABSTRACT FROM AUTHOR]

ABSTRACT Molecular design, fabrication, and properties of thin-film coatings based on poly(2-methyl-2-oxazoline) (PMOX) and its copolymers were investigated to tackle problem of marine and bacterial fouling prevention. The ultraviolet crosslinkable macromonomer poly(2-methyl-2-oxazoline) dimethylacrylate was synthesized by cationic ring-opening polymerization in a microwave reactor initiated by 1,4-dibromobutane. In order to study the charge effect of the PMOX coatings on the adhesion of fouling organisms, PMOX surfaces with negative, neutral, and positive ζ-potential values were prepared by copolymerization with the positively charged monomer [2-(methacryloyloxy)-ethyl]trimethylammonium chloride. The coatings were stable in sea water for at least 1 month without significant reduction in the film thickness. The marine antifouling activity was evaluated against barnacle cyprids Amphibalanus amphitrite and algae Amphora coffeaeformis. Results showed that PMOX coatings provide effective reduction of the settlement regardless of the molar mass and surface charge of the polymer. Bacterial adhesion test showed that PMOX coatings effectively reduce Staphylococcus aureus and Escherichia coli adhesion. Owing to its good stability and antifouling activity PMOX has a great potential as antifouling coating for marine antifouling applications. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 275-283 [ABSTRACT FROM AUTHOR]

A new conjugated polyelectrolyte material, namely, poly [9,9-bis((6′- N , N , N -trimethylamino)hexyl)-fluorene- alt - co -benzoxadiazole dibromide] (PFBD) is reported as electron transport layer (ETL) in polymer solar cells. We observed a light-soaking effect and described how a pre-UV light treatment on PFBD ETL is essential for attaining higher efficiencies (>7%) and negate the light-soaking problem. The pre-UV light treatment on PFBD layer is found to directly influence its molecular structure and result in reduction of the work function and increased electron mobility in PFBD which corroborates well with the observed lower series resistances obtained from dark current analysis and impedance spectra, and therefore enhancement in open-circuit voltage and fill factor. Moreover, after the pre-UV light treatment, the maximal efficiency of the solar cells retains at a nearly similar level for at least 26 days. [ABSTRACT FROM AUTHOR]

Poly(sulfobetaine methacrylate) (PSBMA) films known for their resistance to nonspecific protein adsorption, cell/bacterial adhesion and biofilm formation were produced by surface initiated polymerization on a silicon surface via a batch reaction system in CO 2 expanded liquid (CO 2 -EL) medium. Atom transfer radical polymerization (ATRP) was carried out using 2,2′-bipyridyl as ligand and CuBr as a catalyst in water/methanol mixture with trichloro[4-(chloromethyl)phenyl]silane (CMPS) used as the initiating species. The films were grown in the CO 2 -EL environment at a range of conditions and thickness up to 10 nm. In contrast to films produced by conventional solvent systems at atmospheric pressure, the polymer films grown by the CO 2 -EL process showed uniform thickness and pin-hole free topography. Most importantly, the CO 2 -EL processed PSBMA films showed no trace of copper (used as the catalyst), thus obviating the need for post-deposition processing and avoiding adverse effects of the metal leaching during service. Finally, PSBMA films from both the conventional and CO 2 -EL processes were exposed to Human mesenchymal stem cells (hMSCs) and the results showed that, while in both the cases the cell proliferation rate was inhibited by the charged polymeric brush surface, the CO 2 -EL-processed brush exhibited inhibition to a larger extent due to the reduced occurrence of pinholes. The process can be easily exploited effectively when carrying out surface initiated polymerization on non-flat topographies, such as in trenches and nanostructured features with high aspect ratios. [ABSTRACT FROM AUTHOR]

In-situ fabricated piezopolymer coating-based transducers have been developed to build large-area, lightweight and flexible networks for wave-based structural health monitoring (SHM). Meanwhile, their tunability can be realized by array electrodes for mode control with proper tuning methods. However, conventional standard tuning by phase matching seems not always effective. In this study, the excitation and acquisition mechanism of Piezopolymer Coating-based Array Transducers (PCATs) were first studied for Lamb waves. Distinctly different coupling mechanisms of PCAT actuators and sensors were discussed by analytical models and experimental verification, respectively. Then comprehensive parameter studies were performed to understand the filtering effect with finite temporal pulse duration of PCAT actuators, and finite spatial electrode span of PCAT actuators and sensors. Corresponding bias tuning methods were proposed with analytical solutions to improve mode control in Lamb-wave excitation and acquisition. This new guideline of designing array electrodes for PCAT actuators and sensors has been proven effective by successfully tuning the poor mode-controlled wavefield originated by the standard tuning method. Such tunability has great potential to be applied for detecting various damages in Lamb wave-based SHM, where sensitivity, accuracy, and signal interpretation can be improved with good control of particular frequency-mode selections. [ABSTRACT FROM AUTHOR]

Laser ultrasonic technique has been increasingly implemented for nondestructive testing and structural health monitoring. However, the poor signal-to-noise ratio (SNR) and low amplitude of laser generated ultrasonic signals under thermoelastic regime severely restrict its applications. In this paper, we propose a method for fabricating flexible, high-intensity and readily transplantable photoacoustic transducer (PAT) comprised of candle soot nanoparticles (CSNPs) and polydimethylsiloxane (PDMS), and utilize it to generate high SNR and amplitude ultrasonic signals for inspecting thick structures. A robotic-arm based layer-by-layer automatic scanning strategy is developed to realize candle soot nanoparticles (CSNPs) deposition with excellent homogeneity and thickness controllability, and the optimal thickness of PDMS/CSNPs nanocomposite layer to achieve high SNR and amplitude ultrasonic signal is obtained. In addition, a novel method for optimizing the PAT's structure to generate distinguishable and pure longitudinal ultrasonic signals is proposed, with amplitude over 100 times, and with center frequency (13.5 MHz) and −6 dB bandwidth (20.1 MHz) 29.8% and 35.8% higher than those generated without PAT. The optimized PAT is eventually combined with laser ultrasonic technique to successfully inspect and visualize the internal defects with various sizes (4, 2, 0.8 mm in diameter) of an aluminum component with thickness of 50 mm. With the merits of flexible and high-intensity nanocomposite PAT, the laser ultrasonics can be promisingly implemented for inspecting structure with large thickness and complex geometry under thermo-elastic mechanism. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights: [•] Zwitterionic polymeric brushes based on different monomers have been synthesized. [•] Optimized ATRP protocols for brush fabrication and polymer synthesis in solution are described. [•] Comparative study shows similar good antifouling performance for all monomers investigated. [ABSTRACT FROM AUTHOR]

A reliable non-destructive testing technique that enables effective detection and quantification of defects within safety-critical curved structures is highly demanded both in manufacture and service. Conventional Ultrasonic Testing (UT) methods usually adopt planar ultrasonic probes and it is preferable to adjust the probe axis direction perpendicular to the curved surface, because both the transmitting and receiving sensitivities in the probe axis direction are the highest. This paper proposes a new UT method using a hemispherical-omnidirectional ultrasonic probe. This probe has an omnidirectional directivity pattern and can cover curved surfaces with normal incident ultrasounds at different scan positions without the necessity of adjusting the probe attitude and position to follow the curved surface profiles. A surface reconstruction algorithm is firstly proposed to estimate curved surface profiles, which is based on a fast-imaging algorithm for ultra-wideband radar called the Shape Estimation Algorithm based on the Boundary scattering transform and Extraction of Directly scattered waves (SEABED). This method only uses surface reflection echoes, and thus can inspect curved structures without knowing the surface profiles a prior. Further, the Synthetic Aperture Focusing Technique (SAFT) is implemented to generate a focused image of internal defects by performing delay-and-sum beamforming on defect echoes based on the reconstructed surface profile. The proposed UT method is eventually experimentally verified on a curved plexiglass specimen with 29.4 mm surface curvature radius containing three Ø 3.5 mm Side Drilled Holes (SDHs) at different depths. Results show that compared with the planar ultrasonic probe, the reconstructed surface profile via SEABED is much wider and accurate with a relative error ≤ 0.47%. All SDHs are not only clearly positioned with relative errors ≤ 7.43%, but also can be accurately quantified from the curvature radius of SDH indications in the SAFT images with relative errors ≤ 8.60%. This work proves that the UT method based on the hemispherical-omnidirectional ultrasonic probe can emerge as a promising technique to inspect curved structures with high efficiency and accuracy, which has great potential in practical applications. • A hemispherical-omnidirectional ultrasonic probe is presented for NDT purposes. • Curved structure is inspected by probe linear scanning with fixed attitude. • Surface profile is reconstructed from surface echo travel times with SEABED. • SAFT is used to generate a focused image with high accuracy. [ABSTRACT FROM AUTHOR]

• Significantly reduced conductivity in ST-based ceramics was achieved by excess Bi. • Heterogeneous core–shell microstructure was revealed by SEM and Impedance. • Excellent die/ferroelectric properties were obtained, attractive for capacitors. Lead-free dielectric ceramics 0.75SrTiO 3 -0.25BiFeO 3 (ST-BF) with an extra amount of bismuth (Bi) are synthesized by the conventional solid-state reaction method. The crystal structures of ST-BF retain pseudo-cubic phase as Bi content increases. The grain size is found to significantly increase from 1.8 to 4.9 µm and then fall to 3.3 µm in the presence of excess Bi, exhibiting a distinct heterogeneous core–shell microstructure. Impedance spectroscopy data indicates that with increasing excess Bi content to 2 mol%, the shell resistivity increases significantly to ∼ 9 MΩ cm with the largest volume fraction of the shell/core area, and both the core and shell activation energies are increased, resulting in reduced dielectric loss of ∼ 0.08 at 1 kHz and high breakdown strength of ∼ 140 kV cm−1. This work provides a potential strategy for further improvement of ST-BF for capacitor applications. [ABSTRACT FROM AUTHOR]

Low-temperature magnetic pyrolysis (LMP) of municipal solid waste (MSW) was conducted in a pilot scale continuous reactor to investigate the distribution and transformation of heavy metals (HMs) in biochar. Environmental safety was evaluated by the risk assessment code (RAC) and the modified potential ecological risk index (MRI). Statistical analyses of HMs revealed that the total concentrations of HMs in biochar was higher than that in MSW and the exchangeable fraction of Cd in biochar under 200 °C and 250 °C were at high risk levels. Temperature increment indicates an increase in regular steps not only migrated more HMs into biochar, but also broke the immobilization of HMs, so resulted in higher environmental risks. The lowest direct toxicity to the environment was obtained by LMP at 200 °C. In light of the residual fraction and the high concentration of HMs in biochar produced in this work, it should be mixed with other uncontaminated plant waste for further application in agriculture. The results of economic assessment reveal that the value of net present value (NPV) and the internal rate of return (IRR) can be positive if high quality bio-products are produced with low operating costs. Optimized design of operation, feedstock and the investment are the key factors to improve the economic feasibility of LMP. [Display omitted] • MSW can be converted to biochars by low-temperature magnetic pyrolysis. • LMP enriched the heavy metals in biochar at high reaction temperature. • The immobilization of HMs was broken with the reaction temperature increase. • The RAC and MRI of heavy metals shown relative low and considerable environmental risk at 200 °C. [ABSTRACT FROM AUTHOR]