Intrinsic and extrinsic effects near orthorhombic-tetragonal phase transition in barium titanate ceramics doped with small amounts of zirconium

Pure BT and BT doped with 1 and 2 mol% Zr, with orthorhombic-tetragonal (O-T) phase transitions near room temperature were prepared, with different grain sizes, by conventional ceramic method, at various sintering temperatures. Intrinsic and extrinsic effects of Zr addition on structural, dielectric, piezoelectric and ferroelectric properties of BT ceramics, near O-T phase transition were studied. In coarse grained ceramics, the intrinsic effects manifested in low field measurements (dielectric and piezoelectric constants), were reduced by Zr addition, while the extrinsic effects which control the high field response (remanent polarization and coercive field) were significantly enhanced. Instead, in fine grains ceramics, of either pure or doped BT, the extrinsic effects related to increased domain wall and grain boundaries density were dominant in both low and high field measurements, overlapping Zr effects. The results were explained in terms of crystal anisotropy correlated with grain size effects.     

Fatigue performance of friction stir welded marine grade steel

An extensive study on the fatigue performance of friction stir welded DH36 steel was carried out. The main focus of this experimental testing programme was fatigue testing accompanied by tensile tests, geometry measurements, hardness and residual stress measurements, and fracture surface examination. The S–N curve for friction stir butt welded joints was generated and compared with the International Institute of Welding recommendations for conventional fusion butt welds. Friction stir welds of marine grade steel exceeded the relevant rules for fusion welding. This newly developed S–N curve is being proposed for use in the relevant fatigue assessment guidelines for friction stir welding of low alloy steel. Fracture surfaces were examined to investigate the fatigue failure mechanism, which was found to be affected by the processing features generated by the friction stir welding tool.     

Design of controlled-morphology NiCo2O4 with tunable and excellent microwave absorption performance

In recent years, the high-performance microwave absorbers with strong loss, broad frequency bandwidth, thin thickness and light weight have been intensively investigated to address the problem of electromagnetic pollution and improve stealth technology. Considering the fact that microwave absorption performance is quite sensitive to morphology, studying NiCo₂O₄ with different morphologies is a valuable step towards developing a high-performance microwave absorber. The different morphologies are prepared by adjusting the addition of the structure-directing agent NH₄F. When the amount of added NH₄F is 1 mmol, a flower-like NiCo₂O₄ morphology (NC–F1) is obtained with a large specific surface area of 158.97 m²/g and pore volume of 0.3525 cm³g^-1, which easily generates conductive loss, polarization loss, and multiple scattering, thereby enhancing its microwave absorption performance. The maximum reflection loss reaches −50.3 dB at 3 mm, and the effective bandwidth is 4 GHz with the matching thickness of 2 mm when the fill ratio is only 30 wt% in the epoxy resin. As the thicknesses range from 1.5 mm to 5 mm, the effective bandwidth is 14.2 GHz (3.8 GHz–18 GHz) and covers the entire C, X, and Ku bands. Therefore, the defined-morphology NiCo₂O₄ is expected to be a novel wide-band and strong-loss microwave absorber.     

Interfacial reaction mechanism of SiC joints joined by pure nickel foil

To investigate the reaction mechanism of Ni/SiC system, 0.1 μm-thick pure nickel foil is used to join SiC ceramic at 1245 °C for different times. Interfacial melting is calculated to occur at 964 °C along the interface due to the low eutectic point of θ-Ni₂Si and NiSi, a periodical layered structure, consisting alternating layers of silicides/silicides+graphite, is formed along the interface due to periodical detachment of graphite from SiC reaction interface during the reactions between Ni-Si liquid phase and SiC. The reaction products of Ni/SiC are successfully predicted by CALPHAD method, based on a home-made Ni-Si-C ternary database. The reaction processes between Ni and SiC and the morphology changes of the joined seam are discussed in detail in this paper. The average shear strengths of SiC joints held for 15 min, 30 min and 60 min are tested to be 16.53 MPa, 19.32 MPa and 29.43 MPa, respectively.     

Vibration modal analysis of three-dimensional and four directional braided composites

The composite has found extensive applications because of its excellent mechanical properties, but most conditions for the composite serving are subjected to dynamical loads. The parameters relevant to the performance under dynamical loads are system dynamical parameters, for example natural frequency, mode shape. For this aim, the three-dimensional (3D) and four directional braided composite was exemplified to explore the effect rule of the microstructure properties on the dynamical parameters. These macroscopic parameters are employed in discrete beam models and finite element models of continuous body to determine the natural frequencies and shapes. The simulation results were compared with experimental vibration modal analysis. Our study shows that the natural frequencies and shapes computed from both the discrete beam models and finite element models fit the experimental results well. This validates both our research approach and our theory from the microstructure properties to systematical dynamical properties.     

Preparation of highly dispersive solid microspherical α-Al2O3 powder with a hydrophobic surface for stereolithography-based 3D printing technology

The present research demonstrates the feasibility of the preparation of a highly dispersive, solid microspherical α-Al₂O₃ powder with a hydrophobic surface using a simple, rapid, and low-cost method for stereolithography-based 3D printing technology, including the synthesis of a precursor based on the in situ hydrothermal reaction of aluminum sulfate and urea, heat treatment, and surface modification using sodium oleate. The dependence of the morphologies of the precursors and α-Al₂O₃ powders on the type of aluminum salt, reaction time, and temperature was studied. The formation conditions of the solid spherical structure were explored. The results indicate that the formation of a solid spherical structure strongly depends on the type of aluminum salt, reaction time, and temperature. A spherical structure is only obtained by mixing aluminum sulfate with urea. Very high reaction times and temperatures facilitate the transformation of the microstructure from a solid sphere to a hollow sphere. The optimum hydrothermal reaction conditions are as follows: aluminum sulfate concentration of 0.05 mol/L, urea concentration of 0.2 mol/L, reaction temperature of 120 °C, and reaction time of 2 h. The optimum surface modification is obtained by using 3 wt% of sodium oleate, which can be used for the preparation of a 50 vol% α-Al₂O₃ ceramic UV-curable suspension with a viscosity of 1.66 Pa s at 30 s^-1. An alumina part with a sintering density of 95.5% was fabricated using stereolithography-based 3D printing technology. The analysis of the formation mechanism of the solid spherical structure indicates that the high decomposition temperature of urea, homogeneous dissolution of urea in aluminium sulfate solution, slow OH^- release by urea in the hydrothermal system, and strong coordination ability of SO₄²⁻ effectively restrain the hydrolysis of Al³⁺ and facilitate the formation of the solid spherical precursor Al₄(OH)₁₀SO₄. The solid microspherical α-Al₂O₃ powder is prepared by heat treatment of the precursor.     

Mechanism of MnS-mediated pit initiation and propagation in carbon steel in an anaerobic sulfidogenic media

In a saline anaerobic sulfidogenic environment, pitting on 1018 carbon steel was initiated within a 20–30 nm zone at the MnS inclusion boundary. Nanoscale analysis was performed using scanning electron microscopy and a scanning Auger nanoprobe. The pitting was more pronounced in the presence of a biofilm of sulfate-reducing bacteria than in abiotic sulfide medium. It is proposed that initiation of an anodic reaction leading to dissolution of Fe matrix and subsequent pitting of steel in MnS inclusion boundary regions is due to disorder and strain exerted on the Fe matrix by MnS contamination of the interface from metallurgical processes.     

Investigating permeation and transport of H isotopes in tungsten by first-principles

We have investigated permeation and transport of hydrogen (H) isotopes in tungsten (W) single crystal employing first-principles calculations in junction with Fick’ law. Permeability was approximately evaluated according to the solubility and diffusion coefficient of H. The solubility for H in bulk W from present calculation is consistent with the experimental results measured by Frauenfelder. The permeation fluxes of H isotopes are examined at the different thickness of W wall. The permeation fluxes of deuterium with the W thickness of 21 μm at the temperature of 770 K and with the W thickness of 50 μm at the temperature of 893 K were 0.68 × 10¹³ atom/m²s and 0.34 × 10¹⁴ atom/m²s, respectively. The dissociation coefficients of H isotopes are also evaluated. We believe that the present first-principles combined with Fick’ law method can be also generalized to investigate permeation and transport of H isotopes in most metals since such H isotopes behaviors in most metals are similar to those of H isotopes in W.     

变焦系统凸轮的优化设计及有限元分析

介绍了凸轮的运动基本形式、光学连续变焦系统的工作原理以及光路计算过程。利用UG三维设计软件,对凸轮的轮廓进行了三维设计,并对设计凸轮的结构进行了有限元分析,准确还原了连续变焦系统中补偿镜组和变倍镜组透镜的运动规律,提高了变焦系统凸轮结构的设计精度。     

Effect of stress peening on surface layer characteristics of (TiB + TiC)/Ti–6Al–4V composite

Residual stresses and microstructure on surface layer of (TiB + TiC)/Ti–6Al–4V are investigated after stress peening. The values of domain sizes and microstrain of surface deformation layers are calculated from the integral breadth of diffractive peaks via Voigt method. The results show that the compressive residual stresses and microhardness are improved significantly after stress peening, and the variations of residual stresses are affected by both the prestresses and the directions of measurement. Microstructure investigations reveal that, the deformation amount increase after stress peening, and smaller domain grain sizes and higher density dislocations are introduced. The changes of microstructures are mainly influenced by the values of prestresses. According to these investigations, it is can be found that the stress peening is superior to the conventional shot peening treatments and it is an effective method to improve the surface properties of (TiB + TiC)/Ti–6Al–4V composite.