Competitive oxidation behavior of Ni-based superalloy GH4738 at extreme temperature

A high thrust-to-weight ratio poses challenges to the high-temperature performance of Ni-based superalloys. The oxidation behavior of GH4738 at extreme temperatures has been investigated by isothermal and non-isothermal experiments. As a result of the competitive diffusion of alloying elements, the oxide scale included an outermost porous oxide layer (OOL), an inner relatively dense oxide layer (IOL), and an internal oxide zone (IOZ), depending on the temperature and time. A high temperature led to the formation of large voids at the IOL/IOZ interface. At 1200℃, the continuity of the Cr-rich oxide layer in the IOL was destroyed, and thus, spallation occurred. Extension of oxidation time contributed to the size of Al-rich oxide particles with the increase in the IOZ. Based on this finding,the oxidation kinetics of GH4738 was discussed, and the corresponding oxidation behavior at 900-1100℃ was predicted.

无谐振腔结构的四级级联拉曼光纤激光器

利用20 W连续波掺Yb光纤激光器泵浦一段520 m长的高非线性光纤,在无谐振腔的简单结构中实现了四级级联拉曼激光输出,各阶斯托克斯波长分别为1121 nm、1178 nm、1247 nm和1319 nm,总的频移量为53 THz,最大输出功率达到3.2 W。实验中描述了斯托克斯光的转换过程,并进行了相应分析。无谐振腔结构中多级拉曼激光的产生被认为是基于瑞利散射效应的随机分布反馈,并且证实通过高非线性光纤作为拉曼增益介质可以缩短无谐振腔拉曼光纤激光器所需的光纤长度。

Simulation of Hydrodynamic Performance of Drag and Double Reverse Propeller Podded Propulsors

The unsteady performance of drag and double reverse propeller podded propulsors in open water was numerically simulated using a computational fluid dynamics （CFD） method. A moving mesh method was used to more realistically simulate propulsor working conditions, and the thrust, torque, and lateral force coefficients of both propulsors were compared and analyzed. Forces acting on different parts of the propulsors along with the flow field distribution of steady and unsteady results at different advance coefficients were compared. Moreover, the change of the lateral force and the difference between the abovementioned two methods were mainly analyzed. It was shown that the thrust and torque results of both methods were similar, with the lateral force results having the highest deviation

Experimental Study on Hydrodynamics of L-type Podded Propulsor in Straight-ahead Motion and Off-Design Conditions

Experimental tests were conducted to evaluate the hydrodynamic performance of an L-type podded propulsor in straight-ahead motion and off-design conditions using an open-water measuring instrument developed by the authors for podded propulsors, a ship model towing tank, and under water particle image velocimetry （PIV） measurement systems. Under the three types of conditions, the main parameters of an L-type podded propulsor were measured, including the propeller thrust and torque, as well as the thrust, side force, and moment of the whole pod unit.In addition, the flow field on the section between the propeller and the strut was analyzed. Experimental results demonstrate that the dynamic azimuthing rate and direction and the turning direction affect the forces on the propeller and the whole pod unit. Forces are asymmetrically distributed between the left and right azimuthing directions because of the effect of propeller rotation. The findings of this study provide a foundation for further research on L-type podded propulsors.

Ag nanoparticles anchored organic/inorganic Z‐scheme 3DOMM‐TiO_(2‒x)‐based heterojunction for efficient photocatalytic and photoelectrochemical water splitting

Narrow spectral response,low charge separation efficiency and slow water oxidation kinetics of TiO_(2)limit its application in photoelectrochemical and photocatalytic water splitting.Herein,a promising organic/inorganic composite catalyst Ag/PANI/3DOMM‐TiO_(2–x)with a three‐dimensional ordered macro‐and meso‐porous(3DO MM)structure,oxygen vacancy and Ti^(3+)defects,heterojunction formation and noble metal Ag was designed based on the Z‐scheme mechanism and successfully prepared.The Ag/PANI/3DOMM‐TiO_(2–x)ternary catalyst exhibited enhanced hydrogen production activity in both photocatalytic and photoelectrochemical water splitting.The photocatalytic hydrogen production rate is 420.90μmol g^(–1)h^(–1),which are 19.80 times and 2.06 times higher than the commercial P25 and 3DOMM‐TiO_(2),respectively.In the photoelectrochemical tests,the Ag/PANI/3DOMM‐TiO_(2–x)photoelectrode shows enhanced separation and transfer of carriers with a high current density of 1.55 mA cm^(–2)at equilibrium potential of 1.23 V under simulated AM 1.5 G illumination,which is approximately 5 times greater than the 3DOMM‐TiO_(2).The present work has demonstrated the promising potential of organic/inorganic Z‐scheme photocatalyst in driving water splitting for hydrogen production.

Study on Unsteady Hydrodynamic Performance of Propeller in Waves

The speed of a ship sailing in waves always slows down due to the decrease in efficiency of the propeller. So it is necessary and essential to analyze the unsteady hydrodynamic performance of propeller in waves. This paper is based on the numerical simulation and experimental research of hydrodynamics performance when the propeller is under wave conditions. Open-water propeller performance in calm water is calculated by commercial codes and the results are compared to experimental values to evaluate the accuracy of the numerical simulation method. The first-order Volume of Fluid(VOF) wave method in STAR CCM+ is utilized to simulate the three-dimensional numerical wave. According to the above prerequisite, the numerical calculation of hydrodynamic performance of the propeller under wave conditions is conducted, and the results reveal that both thrust and torque of the propeller under wave conditions reveal intense unsteady behavior. With the periodic variation of waves, ventilation, and even an effluent phenomenon appears on the propeller. Calculation results indicate, when ventilation or effluent appears, the numerical calculation model can capture the dynamic characteristics of the propeller accurately, thus providing a significant theory foundation forfurther studying the hydrodynamic performance of a propeller in waves.

A Non-geometrically Similar Model for Predicting the Wake Field of Full-scale Ships

The scale effect leads to large discrepancies between the wake fields of model-scale and actual ships, and causes differences in cavitation performance and exciting forces tests in predicting the performance of actual ships. Therefore, when test data from ship models are directly applied to predict the performance of actual ships, test results must be subjected to empirical corrections. This study proposes a method for the reverse design of the hull model. Compared to a geometrically similar hull model, the wake field generated by the modified model is closer to that of an actual ship. A non-geometrically similar model of a Korean Research Institute of Ship and Ocean Engineering （KRISO）’s container ship （KCS） was designed. Numerical simulations were performed using this model, and its results were compared with full-scale calculation results. The deformation method of getting the wake field of full-scale ships by the non-geometrically similar model is applied to the KCS successfully.

Numerical and Experimental Analysis of the Hydrodynamic Performance of a Three-Dimensional Finite-Length Rotating Cylinder

The hydrodynamic performance of a three-dimensional finite-length rotating cylinder is studied by means of a physical tank and numerical simulation.First,according to the identified influencing factors,a hydrodynamic performance test of the rotating cylinder was carried out in a circulating water tank.In order to explore the changing law of hydrodynamic performance with these factors,a particle image velocimetry device was used to monitor the flow field.Subsequently,a computational field dynamics numerical simulation method was used to simulate the flow field,followed by an analysis of the effects of speed ratio,Reynolds number,and aspect ratio on the flow field.The results show that the lift coefficient and drag coefficient of the cylinder increase first and then decrease with the increase of the rotational speed ratio.The trend of numerical simulation and experimental results is similar.

Expression changes of Akt and GSK-3β during vascular inflammatory response and oxidative stress induced by high-fat diet in rats

Aim: To observe the expression changes of Akt and GSK-3β during vascular inflammatory response and oxidative stress induced by high-fat diet in rats. Methods: 20 male Sprague-Dawley rats were separately fed for 18 weeks with two types of diets;a normal diet (control group, CON) or high-fat diet hyperlipidmia group, HLP). Then the body weight, lipid parameter, plasma hepatocyte growth factor (HGF), serum superoxide dismutase (SOD) , malondialdehyde (MDA), Tumor necrosis factor-α (TNF-α), a Soluble intercellular adhesion molecule-1 (sICAM-1), Lectin-like oxidized cellulose low density lipoprotein receptor-1 (LOX-1), as well as aortic endothelial p-GSK-3β, GSK-3β, p-Akt, Akt expressions were determined. Results: In comparison with the control group, the model group showed a significant increase in the levels of serum total cholesterol (TC), triglyceride (TG), low density lipoprotein cholesterol ( LDL-C) and significant decrease in the level of serum high density lipoprotein cholesterol (HDL-C) after high-fat diet for 18 weeks (p < 0.05 or p < 0.01). Meanwhile, a more obvious increase of plasma HGF, LOX-1 and serum MDA, TNF-α, and sICAM-1 levels were observed relative to the control group (p < 0.05 or p < 0.01). Moreover, high-fat diet significantly increased the phosphorylation of Akt and GSK-3β in rat aorta. Conclusion: Short-term high-fat diet could induce vascular endothelium injury by increasing inflammation and oxidative stress. And PI3K/Akt pathway could play an important role in hyperlipidemia-induced vascular endothelium injury.

PVDF/6H-SiC composite fiber films with enhanced piezoelectric performance by interfacial engineering for diversified applications

Piezoelectric silicon carbide(SiC)has been quite attractive due to its superior chemical and physical properties as well as wide potential applications.However,the inherent brittleness and unsatisfactory piezoelectric response of piezoelectric semiconductors remain the major obstacles to their diversified applications.Here,flexible multifunctional PVDF/6H-SiC composite fiber films are fabricated and utilized to assemble both piezoelectric nanogenerators(PENGs)and stress/temperature/light sensors.The open cir-cuit voltage(V_(oc))and the density of short circuit current(I_(sc))of the PENG based on the PVDF/5 wt%6H-SiC composite fiber films reach 28.94 V and 0.24μA cm^(-2),showing a significant improvement of 240%and 300%compared with that based on the pure PVDF films.The effect of 6H-SiC nanoparticles(NPs)on inducing interfacial polarization and stress concentration in composite fiber films is proved by first-principles calculation and finite element analysis.The stress/temperature/light sensors based on the composite fiber film also show high sensitivity to the corresponding stimuli.This study shows that the PVDF/6H-SiC composite fiber film is a promising candidate for assembling high-performance energy harvesters and diverse sensors.

High-repetition-rate and high-power efficient picosecond thin-disk regenerative amplifier

We present an effective approach to realize a highly efficient,high-power and chirped pulse amplification-free ultrafast ytterbium-doped yttrium aluminum garnet thin-disk regenerative amplifier pumped by a zero-phonon line 969 nm laser diode.The amplifier delivers an output power exceeding 154 W at a pulse repetition rate of 1 MHz with custom-designed 48 pump passes.The exceptional thermal management on the thin disk through high-quality bonding,efficient heat dissipation and a fully locked spectrum collectively contributes to achieving a remarkable optical-to-optical efficiency of 61%and a near-diffraction-limit beam quality with an M2 factor of 1.06.To the best of our knowledge,this represents the highest conversion efficiency reported in ultrafast thin-disk regenerative amplifiers.Furthermore,the amplifier operates at room temperature and exhibits exceptional stability,with root mean square stability of less than 0.33%.This study significantly represents advances in the field of laser amplification systems,particularly in terms of efficiency and average power.This advantageous combination of high efficiency and diffraction limitation positions the thin-disk regenerative amplifier as a promising solution for a wide range of scientific and industrial applications.

Mode-locked fiber laser of 3.5 μm using a single-walled carbon nanotube saturable absorber mirror

We report on a mid-infrared fiber laser that uses a single-walled carbon nanotube saturable absorber mirror to realize the mode-locking operation.The laser generates 3.5 μm ultra-short pulses from an erbium-doped fluoride fiber by utilizing a dual-wavelength pumping scheme.Stable mode-locking is achieved at the 3.5 μm band with a repetition rate of 25.2 MHz.The maximum average power acquired from the laser in the mode-locking regime is 25 mW.The experimental results indicate that the carbon nanotube is an effective saturable absorber for mode-locking in the mid-infrared spectral region.

Maximizing the mechanical performance of Ti_(3)AlC_(2)-based MAX phases with aid of machine learning

Mechanical properties consisting of the bulk modulus,shear modulus,Young’s modulus,Poisson’s ratio,etc.,are key factors in determining the practical applications of MAX phases.These mechanical properties are mainly dependent on the strength of M–X and M–A bonds.In this study,a novel strategy based on the crystal graph convolution neural network(CGCNN)model has been successfully employed to tune these mechanical properties of Ti_(3)AlC_(2)-based MAX phases via the A-site substitution(Ti_(3)(Al1-xAx)C_(2)).The structure–property correlation between the A-site substitution and mechanical properties of Ti_(3)(Al1-xAx)C_(2)is established.The results show that the thermodynamic stability of Ti_(3)(Al1-xAx)C_(2)is enhanced with substitutions A=Ga,Si,Sn,Ge,Te,As,or Sb.The stiffness of Ti_(3)AlC_(2)increases with the substitution concentration of Si or As increasing,and the higher thermal shock resistance is closely associated with the substitution of Sn or Te.In addition,the plasticity of Ti_(3)AlC_(2)can be greatly improved when As,Sn,or Ge is used as a substitution.The findings and understandings demonstrated herein can provide universal guidance for the individual synthesis of high-performance MAX phases for various applications.

High-power, ultra-broadband supercontinuum source based upon 1/1.5 μm dual-band pumping

We experimentally demonstrate an all-fiber supercontinuum source that covers the spectral region ranging from visible to mid-infrared. The ultra-broadband supercontinuum is realized by pumping a cascaded photonic crystal fiber and a highly nonlinear fiber with a 1/1.5 μm dual-band pump source. A maximum output power of 9.01 W is achieved using the system,which is the highest power ever achieved from a supercontinuum source spanning from the visible to mid-infrared.

Generation of single solitons tunable from 3 to3.8μm in cascaded Er^(3+)-doped and Dy^(3+)-doped fluoride fiber amplifiers

High-power tunable femtosecond mid-infrared(MIR)pulses are of great interest for many scientific and industrial applications.Here we demonstrate a compact fluoride-fiber-based system that generates single solitons tunable from 3 to 3.8μm.The system is composed of an Er:ZBLAN fiber oscillator and amplifier followed by a fusion-spliced Dy:ZBLAN fiber amplifier.The Er:ZBLAN fiber amplifier acts as a power booster as well as a frequency shifter to generate Raman solitons up to 3μm.The Dy:ZBLAN fiber amplifier transfers the energy from the residual 2.8μm radiation into the Raman solitons using an in-band pumping scheme,and further extends the wavelength up to 3.8μm.Common residual pump radiation and secondary solitons accompanying the soliton self-frequency shift(SSFS)are recycled to amplify Raman solitons,consequently displaying a higher output power and pulse energy,a wider shifting range,and an excellent spectral purity.Stable 252 fs pulses at3.8μm with a record average power of 1.6 W and a pulse energy of 23 n J are generated.This work provides an effective way to develop high-power widely tunable ultrafast single-soliton MIR laser sources,and this method can facilitate the design of other SSFS-based laser systems for single-soliton generation.

Effect of Sn doping concentration on the oxidation of Al-containing MAX phase(Ti_(3)AlC_(2))combining simulation with experiment

Sn doping is usually adopted to prepare Ti_(3)AlC_(2)in mass production because it can reduce the synthesis temperature while increasing the phase purity.However,excessive Sn doping usually deteriorates the oxidation resistance of Ti_(3)AlC_(2).Therefore,an appropriate Sn doping concentration is a vital issue.In this work,the effect of Sn doping concentration on the oxidation behavior of Ti_(3)AlC_(2)was systematically investigated by combining theoretical calculations and experimental methods.Density function theory calculations suggest that the oxygen adsorption mechanisms for the(001)surface of Ti_(3)AlC_(2)with and without Sn doping are similar,and Ti-O bonds are always preferentially formed.The molecular dynamics simulation further indicates that Al atoms have a faster diffusion rate during the oxidation process.Therefore,a continuous Al_(2)O_(3)layer can form rapidly at high temperature.Nevertheless,when the Sn doping concentration exceeds 10 mol%,the continuity of the Al_(2)O_(3)layer is destroyed,thereby impairing the oxidation resistance of Ti_(3)AlC_(2).Furthermore,oxidation experiments verify the above results.The oxidation mechanisms of Ti3AlC2 with different Sn doping concentrations are also proposed.

Mild fabrication of SiC/C nanosheets with prolonged cycling stability as supercapacitor

A facile and mild route to synthesize C-coated SiC nanosheets(SiC/C NSs)via wet-chemical etching in hydrofluoric acid(HF)at 60°C for 48 h using carbon aluminum silicate(Al_(4)SiC_(4))as raw materials is reported for the first time.HF molecule leads to the breaking of C-Al bonds in Al_(4)SiC_(4),which eventually results in the formation of two-dimensional SiC nanosheets.A carbon layer with a thickness of approximately 1.5 nm is formed on the surface of SiC nanosheets due to the excess carbon.The prepared SiC/C NSs possess a smooth and rectangular sheet with a mean 150 nm in width,500 nm in length and10 nm in thickness,respectively.The crystallographic characterization indicates that 3C-SiC and 2H-SiC coexist and the parallel plane relationship of 3C/2H-SiC heterojunction is(111)_(3C-SiC)//(001)_(2H-SiC).Due to the formed 3C-SiC/2H-SiC heterojunction and graphitic carbon,the fabricated electrode based on SiC/C NSs exhibits prolonged cycling stability and high specific areal capacitance as a promising supercapacitor candidate.It remains 91.2%retention even after 20,000 cycles and 734μF/cm^(2)at a scan rate of 10 m V/s.

High-power mid-infrared femtosecond master oscillator power amplifier Er:ZBLAN fiber laser system

High-power femtosecond mid-infrared(MIR)lasers are of vast importance to both fundamental research and applications.We report a high-power femtosecond master oscillator power amplifier laser system consisting of a singlemode Er:ZBLAN fiber mode-locked oscillator and pre-amplifier followed by a large-mode-area Er:ZBLAN fiber main amplifier.The main amplifier is actively cooled and bidirectionally pumped at 976 nm,generating a slope efficiency of 26.9%.Pulses of 8.12 W,148 fs at 2.8μm with a repetition rate of 69.65 MHz are achieved.To the best of our knowledge,this is the highest average power ever achieved from a femtosecond MIR laser source.Such a compact ultrafast laser system is promising for a wide range of applications,such as medical surgery and material processing.

New design concept for stableα-silicon nitride based on the initial oxidation evolution at the atomic and molecular levels

As the dominated composition of Si_(3)N_(4)ceramics,α-silicon nitride(α-Si_(3)N_(4))can satisfy the strength and fracture toughness demand in the applications.However,α-Si_(3)N_(4)is oxygen-sensitive at high temperatures,which limits its high-temperature performance.To improve the oxidation resistance ofα-Si_(3)N_(4)ceramics,it is necessary to shed light on the oxidation mechanism.Herein,the initial oxidation ofα-Si_(3)N_(4)was systematically studied at the atomic and molecular levels.The density functional theory(DFT)calculation denotes that the(001)surface ofα-Si_(3)N_(4)has the best stability at both room temperature and high temperature.Besides,the oxidation process of theα-Si_(3)N_(4)(001)surface consists of O adsorption and N desorption,and the consequent formation of nitrogen-vacancy(VN)is the key step for further oxidation.Moreover,the molecular dynamics(MD)simulation indicates that the oxidation rate ofα-Si_(3)N_(4)(100)surface is slower than that ofα-Si_(3)N_(4)(001)surface due to the lower N concentration at the outermost layer.Therefore,the oxidation resistance ofα-Si_(3)N_(4)can be improved by regulating the(100)surface as the dominant exposure surface.In addition,reducing the concentration of N on the final exposed surface ofα-Si_(3)N_(4)by mean of constructing the homojunction of the Si-terminal(100)surface and other N-containing surfaces(such as(001)surface)should be also a feasible approach.

High-power all-fiber 1.0/1.5μm dual-band pulsed MOPA source

The simultaneous dual-band pulsed amplification is demonstrated from an Er/Yb co-doped fiber（EYDF）, and consequently a high-power all-fiber single-mode 1.0/1.5 μm dual-band pulsed master oscillator power amplifier（MOPA） laser source is realized for the first time, to the best of our knowledge, based on one singlegain fiber. The simultaneous outputs at 1061 and 1548 nm of the laser source have the maximum powers of 10.7 and 25.8 W with the pulse widths of 9.5 ps and 2 ns and the pulse repetition rates of 178 and 25 MHz, respectively. This EYDF MOPA laser source is seeded by two separate preamplifier chains operating at 1.0 and 1.5 μm wavebands. The dependence of the laser output powers on the length of the large-mode area EYDF, the ratio of the powers of the two signals launched into the booster amplifier, and the wavelength of the 1 μm seed signal are also investigated experimentally.