Nacre-like surface nanolaminates enhance fatigue resistance of pure titanium.

Fatigue failure is invariably the most crucial failure mode for metallic structural components. Most microstructural strategies for enhancing fatigue resistance are effective in suppressing either crack initiation or propagation, but often do not work for both synergistically. Here, we demonstrate that this challenge can be overcome by architecting a gradient structure featuring a surface layer of nacre-like nanolaminates followed by multi-variant twinned structure in pure titanium. The polarized accommodation of highly regulated grain boundaries in the nanolaminated layer to cyclic loading enhances the structural stability against lamellar thickening and microstructure softening, thereby delaying surface roughening and thus crack nucleation. The decohesion of the nanolaminated grains along horizonal high-angle grain boundaries gives rise to an extraordinarily high frequency (≈1.7 × 103 times per mm) of fatigue crack deflection, effectively reducing fatigue crack propagation rate (by 2 orders of magnitude lower than the homogeneous coarse-grained counterpart). These intriguing features of the surface nanolaminates, along with the various toughening mechanisms activated in the subsurface twinned structure, result in a fatigue resistance that significantly exceeds those of the homogeneous and gradient structures with equiaxed grains. Our work on architecting the surface nanolaminates in gradient structure provides a scalable and sustainable strategy for designing more fatigue-resistant alloys.

Rapid fabrication of physically robust hydrogels.

Hydrogel materials show promise for diverse applications, particular as biocompatible materials due to their high water content. Despite advances in hydrogel technology in recent years, their application is often severely limited by inadequate mechanical properties and time-consuming fabrication processes. Here we report a rapid hydrogel preparation strategy that achieves the simultaneous photo-crosslinking and establishment of biomimetic soft-hard material interface microstructures. These biomimetic interfacial-bonding nanocomposite hydrogels are prepared within seconds and feature clearly separated phases but have a strongly bonded interface. Due to effective interphase load transfer, biomimetic interfacial-bonding nanocomposite gels achieve an ultrahigh toughness (138 MJ m-3) and exceptional tensile strength (15.31 MPa) while maintaining a structural stability that rivals or surpasses that of commonly used elastomer (non-hydrated) materials. Biomimetic interfacial-bonding nanocomposite gels can be fabricated into arbitrarily complex structures via three-dimensional printing with micrometre-level precision. Overall, this work presents a generalizable preparation strategy for hydrogel materials and acrylic elastomers that will foster potential advances in soft materials.

Ultra-high performance humidity sensor enabled by a self-assembled CuO/Ti3C2T X MXene.

An ultra-high performance humidity sensor based on a CuO/Ti3C2T X MXene has been investigated in this work. The moisture-sensitive material was fabricated by a self-assembly method. The morphology and nanostructure of the fabricated CuO/Ti3C2T X composites were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectra. The humidity sensing abilities of the CuO/Ti3C2T X sensor in the relative humidity (RH) range from 0% to 97% were studied. The results showed that the humidity sensor had a high sensitivity of 451 kΩ/% RH, short response time (0.5 s) and recovery time (1 s), a low hysteresis value, and good repeatability. The CuO/Ti3C2T X sensor exhibited remarkable properties in human respiration rate monitoring, finger non-contact sensing, and environmental detection. The moisture-sensitive mechanism of CuO/Ti3C2T X was discussed. The fabricated CuO/Ti3C2T X showed great potential in the application of moisture-sensitive materials for ultra-high-performance humidity sensors.

Collaborative Enhancement of Humidity Sensing Performance by KCl-Doped CuO/SnO2 p-n Heterostructures for Monitoring Human Activities.

In this study, a high-performance humidity sensor based on KCl-doped CuO/SnO2 p-n heterostructures was fabricated by a ball milling-roasting method. The morphology and nanostructure of the fabricated KCl-CuO/SnO2 composite were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and nitrogen sorption analysis. The results showed that the humidity sensor had a high sensitivity of 194 kΩ/%RH, short response and recovery times of 1.0 and 1.5 s, a low hysteresis value, and good repeatability. The energy band structure and complex impedance spectrum of the KCl-CuO/SnO2 composite indicated that the excellent humidity sensing performance originated from the ionic conductivity of KCl, the formation of heterojunctions, the change in the Schottky barrier height, and the depletion of electronic depletion layers. The KCl-CuO/SnO2 sensor has great potential in respiratory monitoring, noncontact sensing of finger moisture, and environmental monitoring.

Creep-Fatigue Crack Initiation Simulation of a Modified 12% Cr Steel Based on Grain Boundary Cavitation and Plastic Slip Accumulation.

High-temperature components in power plants may fail due to creep and fatigue. Creep damage is usually accompanied by the nucleation, growth, and coalescence of grain boundary cavities, while fatigue damage is caused by excessive accumulated plastic deformation due to the local stress concentration. This paper proposes a multiscale numerical framework combining the crystal plastic frame with the meso-damage mechanisms. Not only can it better describe the deformation mechanism dominated by creep from a microscopic viewpoint, but also reflects the local damage of materials caused by irreversible microstructure changes in the process of creep-fatigue deformation to some extent. In this paper, the creep-fatigue crack initiation analysis of a modified 12%Cr steel (X12CrMoWvNBN10-1-1) is carried out for a given notch specimen. It is found that creep cracks usually initiate at the triple grain boundary junctions or at the grain boundaries approximately perpendicular to the loading direction, while fatigue cracks always initiate from the notch surface where stress is concentrated. In addition to this, the crack initiation life can be quantitatively described, which is affected by the average grain size, initial notch size, stress range and holding time.

Effect of ultrasonic surface deep rolling combined with oxygen boost diffusion treatment on fatigue properties of pure titanium.

Ultrasonic surface deep rolling (USDR), oxygen boost diffusion (OBD), and their combination (USDR-OBD) were all used to improve the surface hardening of pure titanium. The microstructure, microhardness, and fatigue life of pure titanium treated by USDR, OBD, and USDR-OBD methods were analyzed. USDR treatment induced a severe deformation area, while OBD treatment produced a brittle oxygen diffusion zone. The USDR-OBD treated samples approached the highest hardness in comparison with other treated samples. The fatigue lives of USDR treated samples were improved, which was due to the high compressive residual stress and refined grains. However, the fatigue lives of both OBD treated samples and USDR-OBD treated samples were decreased due to premature crack initiation and rapid propagation in the oxygen diffusion zone. Finally, the fatigue fracture mechanisms of different samples were proposed.

Brazing Coupling Performance of Piezoelectric Waveguide Transducers for the Monitoring of High Temperature Components.

Piezoelectric waveguide transducers possess great potential for the online monitoring of high temperature critical components, in order to improve their operational safety. Due to the use of a waveguide bar, the sensory device is not susceptible to high temperature environments, which enables the long-term service of the piezoelectric transducers. However, the coupling between the waveguide bar and the high-temperature component has been proven to be the most important part of the monitoring system. In order to effectively transmit waves through the junction of the waveguide bar and the monitoring target, it is necessary to research a reliable coupling method to connect the waveguide transducers with the host structure. In the present research, the feasibility of brazing coupling for wave propagation through the junction was investigated through experiments. Piezoelectric waveguide transducers were welded using various kinds of brazing filler metals. The experimental results indicate that the coupling effects of the brazing welding depend on the filler metals. At the same time, some filler metals for the effective coupling of the transducer and the target monitoring component were identified. The brazing coupling method was verified that it can non-dispersively and effectively propagate waves into the host structure with much better reliability than the conventional dry coupling approach. Moreover, the high-temperature experimental results show that the brazing-coupled waveguide bar system can work reliably and stably in high temperatures at 300 °C for a long time. This work strives to pave a solid foundation for the application of piezoelectric waveguide transducers for the structural health monitoring of high temperature critical components.

Characteristics of high-temperature equipment monitoring using dry-coupled ultrasonic waveguide transducers.

Dry-coupled ultrasonic waveguide transducers enable the long-term monitoring of the critical mechanical components working in high temperature environment. In these systems, waveguide units are applied to isolate the vulnerable piezoelectric elements from the harsh measurement zones. However, the disturbance cannot be overcome, which may arise from inevitably temperature fluctuation and the indirect measurement in long term monitoring. In the present research, a high temperature experimental system using dry-coupled ultrasonic waveguide transducers is investigated. The disturbances caused by this measurement system are studied in the heating period, holding period and cooling period, respectively. Moreover, the correlations between the group velocities, dispersion curves, signal amplitudes, waveforms under temperature variations are investigated. At last, the dispersion characteristics of the shear horizontal wave in high temperature stainless steel plate are analyzed. This work strives to pave a solid foundation for the application of SH waves to online monitoring of high-temperature equipment using dry-coupled ultrasonic waveguide transducers.

Optimal Design Methodology of Tapered Waveguide Transducers for Thickness Monitoring.

For the purpose of providing transducers for long-term monitoring of wall thinning of critical pressure equipment in corrosion or high temperature environments, the optimal design methodology for tapered waveguide units was proposed in the present study. Firstly, the feasibility of the quasi-fundamental shear horizontal (SH0*) wave propagating in the tapered waveguide units was analyzed via numerical simulations, and the transmitting limitations of the non-dispersive SH0* wave were researched. Secondly, several tapered waveguide transducers with varying cross-sections to transmit pure SH0* wave were designed according to the numerical results. Experimental investigations were carried out, and the results were compared with waveguide transducers with a prismatic cross-section. It was found that the tapered waveguide units can transmit non-dispersive shear horizontal waves and suppress the wave attenuation at the same time. The experimental results agreed very well with the numerical simulations. Finally, high-temperature experiments were carried out, and the reliability of thickness measuring by the tapered waveguide transducers was validated. The errors between the measured and the true thicknesses were small. This work paves a solid foundation for the optimal design of tapered waveguide transducers for thickness monitoring of equipment in harsh environments.

Lipids extraction from wet Chlorella pyrenoidosa sludge using recycled [BMIM]Cl.

In this study, experiments on pretreating one species of microalgae (Chlorella pyrenoidosa) using one kind of ionic liquid (IL) of [BMIM]Cl were conducted. The aim of this work is to evaluate the recycling efficacy of expensive IL solvent for effective cell disruption. It was indicated that the molecular structure of IL was stable during the recycling test. Five times antisolvent precipitation of microalgae debris after lipid extraction using methanol recovered 99.8% IL with the energy consumption of 4.46 MJ per kg dry Chlorella pyrenoidosa. The chromatography was used to separate IL and hydrolysates, resulting in the IL loss below 1.97 g per kg dry Chlorella pyrenoidosa.

Critical Excitation of the Fundamental Quasi-Shear Mode Wave in Waveguide Bars for Elevated Temperature Applications.

The safety of critical pressure equipment in elevated temperature is increasingly important. Moreover, the on-line monitoring method is potentially useful to improve their safety. A waveguide bar system can enable monitoring of critical equipment working in elevated temperature using reliable ultrasonic technology. Among the waveguide bar system, the matching mechanism of the transducer and the waveguide bar is crucial to propagate the pure fundamental quasi-shear mode (shorten for SH0*) wave. In the present research, the loading line sources that can excite pure SH0* wave are investigated and the anti-plane shear loading source is selected. The critical values about the geometric dimensions of the junctions between the piezoelectric transducer and the waveguide bar are explored by simulation and experiments. On the condition that the excitation sources satisfy the critical values, the loading can be approximated to an anti-plane shear one to excite the pure SH0* wave. Some waveguide bar systems are designed based on the simulated critical values and some experiments at high temperature are carried out. The experimental results verify that the designed waveguide bar systems can excite the pure SH0* wave at elevated temperatures, which verify the reliability of the simulated critical results.

Design of Waveguide Bars for Transmitting a Pure Shear Horizontal Wave to Monitor High Temperature Components.

Guided wave technique could be a possible method for monitoring components working in high temperature above 350 °C. However, this would require the design of an appropriate waveguide bar to transmit the wave, so that its sensing part is not influenced by the high temperature. In the present study, the shape of waveguide bars is designed based on the analysis of wave source characteristics. The critical frequency-width and frequency-thickness products of waveguide bars are analyzed theoretically and numerically to transmit the zeroth shear horizontal wave SH0* in bars. The results show that waveguide bars can cut off all the other wave modes when their frequency-thickness products are smaller than the critical value fd*, and frequency-width products are not smaller than the critical value fw*. Six waveguide bars are designed and fabricated based on the design criteria, and an experiment system is set up to check their work performance. The testing results indicate that the wave signals of the SH0* mode propagate clearly in waveguide bars, and cut off all the other modes when the frequency-thickness products and frequency-width products of the bars meet the design criteria. It is also demonstrated that the dependency of the experimental group velocity of each waveguide bar on frequency is in good agreement with the numerical result. High-temperature experiments also validate the reliability of the designed waveguide bars. Therefore, the critical frequency-thickness product and frequency-width product can be the basis for the design of the waveguide bars.

An Improved Metal-Packaged Strain Sensor Based on A Regenerated Fiber Bragg Grating in Hydrogen-Loaded Boron-Germanium Co-Doped Photosensitive Fiber for High-Temperature Applications.

Local strain measurements are considered as an effective method for structural health monitoring of high-temperature components, which require accurate, reliable and durable sensors. To develop strain sensors that can be used in higher temperature environments, an improved metal-packaged strain sensor based on a regenerated fiber Bragg grating (RFBG) fabricated in hydrogen (H2)-loaded boron-germanium (B-Ge) co-doped photosensitive fiber is developed using the process of combining magnetron sputtering and electroplating, addressing the limitation of mechanical strength degradation of silica optical fibers after annealing at a high temperature for regeneration. The regeneration characteristics of the RFBGs and the strain characteristics of the sensor are evaluated. Numerical simulation of the sensor is conducted using a three-dimensional finite element model. Anomalous decay behavior of two regeneration regimes is observed for the FBGs written in H2-loaded B-Ge co-doped fiber. The strain sensor exhibits good linearity, stability and repeatability when exposed to constant high temperatures of up to 540 °C. A satisfactory agreement is obtained between the experimental and numerical results in strain sensitivity. The results demonstrate that the improved metal-packaged strain sensors based on RFBGs in H2-loaded B-Ge co-doped fiber provide great potential for high-temperature applications by addressing the issues of mechanical integrity and packaging.

A novel double loop control model design for chemical unstable processes.

In this manuscript, based on Smith predictor control scheme for unstable process in industry, an improved double loop control model is proposed for chemical unstable processes. Inner loop is to stabilize integrating the unstable process and transform the original process to first-order plus pure dead-time dynamic stable process. Outer loop is to enhance the performance of set point response. Disturbance controller is designed to enhance the performance of disturbance response. The improved control system is simple with exact physical meaning. The characteristic equation is easy to realize stabilization. Three controllers are separately design in the improved scheme. It is easy to design each controller and good control performance for the respective closed-loop transfer function separately. The robust stability of the proposed control scheme is analyzed. Finally, case studies illustrate that the improved method can give better system performance than existing design methods.

Development of on-line monitoring systems for high temperature components in power plants.

To accurately detect deformation and extend the component life beyond the original design limits, structural safety monitoring techniques have attracted considerable attention in the power and process industries for decades. In this paper an on-line monitoring system for high temperature pipes in a power plant is developed. The extension-based sensing devices are amounted on straight pipes, T-Joints and elbows of a main steam pipeline. During on-site monitoring for more than two years, most of the sensors worked reliably and steadily. However, the direct strain gauge could not work for long periods because of the high temperature environment. Moreover, it is found that the installation and connection of the extensometers can have a significant influence on the measurement results. The on-line monitoring system has a good alarming function which is demonstrated by detecting a steam leakage of the header.

A modified analysis for thermal-mechanical properties of staggered structure in biomimetic materials.

The thermal-mechanical stress distributions and equivalent coefficient of thermal expansion (CTE) of the staggered arrangement of mineral platelets wrapped by soft matrix are analyzed, which exist in numerous natural biological and biomimetic materials. Two analytical models, 'Stress model' and 'Displacement model', were established from the ways of stress and displacement solution based on the modification of classical shear-lag model. Complementary finite element analysis (FEA) was used to verify the analytical models. Results reveal that, compared to 'Displacement model', 'Stress model' gives a better prediction of the stress distributions within the staggered structure referring to FEA. The equivalent CTE predicted by both models reach constant as the aspect ratio and volume fraction of platelets exceeding the critical values. Nevertheless, the relative error between the results from different models increases with the increase of the ratio of overlap to length of platelets. These provide a benchmark to the optimum design of micro/nano-structure in bio-inspired materials suffering to temperature fluctuation and applied loading.

A simple route for synthesis of PbSe nanocrystals: shape control by ligand and reaction time.

The shape controlled synthesis of high quality colloidal lead selenide (PbSe) nanocrystals (NCs) was achieved through a simple solvothermal process. By using oleic acid (OA) as a ligand and activating agent for the Pb precursor, the evolution of the NCs from nanospheres to nanoflowers and finally to nanocubes was achieved by increasing the reaction time. Further, the shape variation from nanospheres to polyhedrons was readily realized through the increase of OA concentration in the stock solution. More interestingly, the change of the anion ligand was proven to be a facile method to control the structure and size of the nanoflowers. The X-ray diffraction and TEM analysis demonstrated the cubic rock salt structure of the synthesized PbSe NCs. Accompanied by comprehensive analytics, the discussion on the possible mechanisms for the shape evolution was provided.

One-step synthesis of cubic FeS2 and flower-like FeSe2 particles by a solvothermal reduction process.

In this paper, for the first time a simple batch process was utilized for the facile synthesis of cubic FeS(2) and flower-like FeSe(2). By adjusting the amount of solvents and surfactants added, pure pyrite FeS(2) with a defined crystalline structure was obtained. It was found that the reaction temperatures and iron sources had significant influence on the purities and morphologies of FeS(2) and FeSe(2) particles. Raman spectra of the FeS(2) and FeSe(2) samples presented characteristic peaks of S-S and Se-Se active modes at 337, 372 cm(-1), and 180, 217, 254 cm(-1), respectively. The absorption properties of the FeS(2) and FeSe(2) samples were also investigated and the results demonstrated that these samples had broad optical absorption in NIR. Moreover, the synthetic approach demonstrated here may be of great potential for the controlled synthesis of other metal chalcogenides.

Continuous synthesis of CdSe(x)Te(1-x) nanocrystals: chemical composition gradient and single-step capping.

A capillary microreactor was firstly utilized to continuously synthesize near-infrared emitting CdSe(x)Te(1-)(x) nanocrystals (NCs). By using trioctylphosphine oxide and trioctylphosphine as the solvents for anion precursor as well as oleic acid and oleylamine as the solvents for cation precursor, high quantum yield zinc-blend CdSe(x)Te(1-)(x) NCs with a chemical composition gradient internal structure and tunable emission from 634 to 783 nm were synthesized. Thus, the nonlinear relationship between the composition and the emission energies were verified. Moreover, a facile single-step capping approach was developed by using the dissolution of cadmium oxide and free element sulfur in oleic acid, and a very thin CdS shell was successfully epitaxial grown around the as-prepared CdSe(x)Te(1-)(x) NCs to enhance the photostability. After the capping process, the core/shell structured CdSe(x)Te(1-)(x)/CdS NCs remained 15-40% of their initial PL intensity after 3h of illumination.

Biodiesel production from waste cooking oil catalyzed by TiO2-MgO mixed oxides.

Mixed oxides of TiO(2)-MgO obtained by the sol-gel method were used to convert waste cooking oil into biodiesel. Titanium improved the stability of the catalyst because of the defects induced by the substitution of Ti ions for Mg ions in the magnesia lattice. The best catalyst was determined to be MT-1-923, which is comprised of an Mg/Ti molar ratio of 1 and calcined at 923 K, based on an assessment of the activity and stability of the catalyst. The main reaction parameters, including methanol/oil molar ratio, catalyst amount, and temperature, were investigated. The catalytic activity of MT-1-923 decreased slowly in the reuse process. After regeneration, the activity of MT-1-923 slightly increased compared with that of the fresh catalyst due to an increase in the specific surface area and average pore diameter. The mixed oxides catalyst, TiO(2)-MgO, showed good potential in large-scale biodiesel production from waste cooking oil.