The microstructure and magnetic properties of Ca2+ ion doped GdCrO3

In this paper, the crystal structure, vacancy defect, local electron density and magnetic properties of Gd_1-xCa_xCrO₃ (0 ≤ x ≤ 0.3) polycrystalline samples were investigated systematically. The crystal structural analyses show that all the samples are orthorhombic phase and a structural distortion happens around x = 0.3. Due to the formation of Cr⁴⁺ ions, both the lattice constant and the Cr–O bond length decrease. The results of positron annihilation spectrum reveals that the vacancy defect concentration increases and the local electron structure changes with the introduction of Ca²⁺ ions. The field-cooled (FC) and zero-field cooled (ZFC) curves of Gd_1-xCa_xCrO₃ samples measured under H = 100 Oe exhibits negative magnetization characteristics due to the interaction between Gd³⁺ and Cr³⁺ ions, and the magnetism can be affected by the structural distortion.     

The effects of hydrogen-induced lattice distortion and Nb-D formation on its permeation through niobium

Abnormal permeation behavior of hydrogen through niobium has been investigated in this paper, i.e. the permeation flux saturated with long-term decrease after reaching a maximum. The diffusivity and permeability have been deduced from the decay edge of permeation transient. Three kinds of polycrystalline niobium foils with different annealing temperature have been compared, to verify the effect of defects and grain properties on the permeability and diffusivity. In the temperature range of (773–1023) K, the heat treatment along with the permeation cycles could either reduce or increase the permeability and diffusivity depending sensitively on temperature and showing a temperature threshold around 950 K. The permeation flux is proportional to square root of pressure, revealing that the abnormal permeation was still bulk diffusion-limited. The diffusivity gradually decreased with permeation cycles, and became more and more sensitive to pressure. The niobium foil expanded macroscopically along the gradient of hydrogen concentration, which reveals the strong and unrecoverable lattice distortion in this temperature and pressure range. The X-ray diffraction studies showed that splitting of all the Nb peaks and shifting of Nb-D peaks along with hydrogen loadings. The phase transition was expected to eliminate the lattice strain during hydrogen loading and which in turn acted as a diffusion barrier.     

Features of crystal structures and thermo-mechanical properties of weberites RE3NbO7 (RE=La,Nd, Sm,Eu, Gd) ceramics

The features of crystal structures, thermo-mechanical properties and their dominant mechanisms of weberites RE₃NbO₇ were studied as high-temperature oxides. We concentrated on connections between structures and thermo-mechanical properties, the influences of bond lengths, lattice distortion degrees and microstructures on these properties were estimated. The shortening of bond length and increment of bonding strength would lead to the increase of mechanical properties. The Vickers hardness (4.5-7.8 GPa) and toughness (0.5-1.6 MPa·m^1/2) of weberites RE₃NbO₇ are enhanced by grain refinement and increment of bond strength, while crystal structures, bond lengths, and lattice distortion degrees influenced their Young's modulus (100-170 GPa). Nano-indentation was applied to test the influence of microstructures on modulus and hardness. The dominant mechanisms for mechanical properties and thermal conductivity were proposed, which was conducive to properties tailoring and engineering applications of weberites RE₃NbO₇ oxides.     

Phase-change regulation criterion based on size-dependent lattice distortion rate and born theory for VO2 nanomaterials

Vanadium dioxide (VO₂) is a promising thermally induced phase transition material because of the abrupt changes in electrical and optical properties. However, the high phase transition temperature of VO₂ and its unspecified modulation relationship need to be resolved urgently. Herein, we proposed a simple and precise regulation criterion for VO₂ materials based on size-dependent lattice distortion rate and Born theory. The results indicated that the application of a tensile stress changed the elastic properties of the VO₂, which promoted VO₂ phase transition, and regulated the phase transition temperature. Moreover, the specific modulation relationship between the stress and phase transition temperature of VO₂ was confirmed experimentally. These results show that our criterion provides theoretical guidance to regulate VO₂ thermally induced phase transition materials.     

航天用多弧段封头淬火装置优化设计

提出了某航天产品用多弧段封头产品的淬火处理方法;通过有限元分析,得出了多弧段封头在底部约束情况下的热处理畸变趋势;建立了多弧段封头淬火装置的三维模型,介绍了多弧段封头淬火装置的基本结构。通过有限元分析得出在淬火装置约束下,多弧段封头热处理,总畸变量为16.0 mm,Z向15.0 mm,X向8.5 mm。通过对多弧段封头淬火装置的热畸变情况分析,提出了淬火装置的优化设计方案,优化后的多弧段封头的热处理,总畸变量为7.0 mm,Z向10.0 mm,X向3.0 mm。解决了多弧段封头热处理畸变过大问题,并通过生产验证。     

The relationship between enhanced dielectric property and structural distortion in Ca doped Ba2NaNb5O15 tungsten bronze ceramics

The relationship between enhanced dielectric property and structural distortion in tungsten bronze structure ceramics was discussed in this work. The ceramics with the composition of (Ba_1-xCa_x)₂NaNb₅O₁₅ (x = 0, 0.1, 0.2, 0.3, 0.4) were fabricated via conventional solid-state method. All ceramics were pure without secondary phase and the distinct lattice distortion in structure was testified by Rietveld XRD refinement. Compared with the un-doped composition, the maximum polarization and energy storage density were strongly enhanced according to the ferroelectric property measurements, which were contributed to the distortion of NbO₆ octahedron induced by the variation of ionic radius. The actually pulsed charge-discharge property of x = 0.3 ceramic was tested, whist excellent power density (P_D = 35.106 MV/cm³) and discharge energy density (W_d = 0.29 J/cm³) were obtained at 100 °C under 120 kV/cm, revealing the potential for application of Ba₂NaNb₅O₁₅ system-based ceramics in harsh environment.     

Effects of welding parameters and welding sequence on residual stress and distortion in Al6061-T6 aluminum alloy for T-shaped welded joint

The distribution of temperature and then the distribution of residual stress and distortion in the stiffened aluminum alloy Al6061-T6 plates under the metal inert gas (MIG) welding process were investigated by three dimensional thermo-mechanical coupled finite element model using Ansys software. The properties of materials were considered temperature-dependent and the filler metal was added to the workpiece by the element birth and death technique. In three modes of current, two different speeds and two various sequences, the distribution of residual stress and distortion were calculated and analyzed. The results showed that increase in welding speed decreased the vertical deflection in the plate, transverse shrinkage and angular distortion of plate and the lateral deflection of stiffener, but increased the maximum longitudinal tensile stress in the plate and stiffener. Furthermore, increase in current increased the residual stress and deformation in the plate and stiffener, and the change in the welding sequence changed the distribution of the distortion in the plate and the stiffener without significant change in the distribution of the longitudinal residual stress.     

Anomalous thermal expansion behavior in Er1-xCaxMnO3

Perovskite Er_1-xCa_xMnO₃ (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5) was synthesized using a solid-state method. Thermal expansion behavior was tested using a thermal dilatometer and high-temperature X-ray diffraction (XRD). The experimental results indicated the doping contents of Ca (x) in the Er_1-xCa_xMnO₃ have a dramatic effect on their thermal expansion behavior. The samples of Er_1-xCa_xMnO₃ (x = 0.1,0.2 and 0.25) exhibit positive thermal expansion (PTE) characteristics while Er_0.7Ca_0.3MnO₃ (x = 0.3) exhibits a negative thermal expansion (NTE) property with a thermal expansion coefficient of −3.1 × 10⁻⁶ K⁻¹ in room temperature (RT) −750 K. In addition, Er_0.6Ca_0.4MnO₃ (x = 0.4) exhibits NTE properties only at RT–500 K, and Er_0.5Ca_0.5MnO₃ (x = 0.5) exhibits PTE properties at RT–750 K. The thermal shrinkage mechanism is the Jahn–Teller effect of the Mn³⁺ ions and the double exchange of Mn³⁺–O–Mn⁴⁺ in Er_0.7Ca_0.3MnO₃. This phenomenon causes Mn–O octahedral distortion and oxygen vacancy, causing Er_0.7Ca_0.3MnO₃ to become anisotropic. This feature results in the elastic deformation of Er_0.7Ca_0.3MnO₃ during heating, which consumes the void and displays NTE at macro level.     

A Strategy of Signal Detection for Performance Improvement in Clipping Based OFDM System

In this paper, the supervised Deep Neural Network (DNN) based signal detection is analyzed for combating with nonlinear distortions efficiently and improving error performances in clipping based Orthogonal Frequency Division Multiplexing (OFDM) ssystem. One of the main disadvantages for the OFDM is the high Peak to Average Power Ratio (PAPR). The clipping is a simple method for the PAPR reduction. However, an effect of the clipping is nonlinear distortion, and estimations for transmitting symbols are difficult despite a Maximum Likelihood (ML) detection at the receiver. The DNN based online signal detection uses the offline learning model where all weights and biases at fullyconnected layers are set to overcome nonlinear distortions by using training data sets. Thus, this paper introduces the required processes for the online signal detection and offline learning, and compares error performances with the ML detection in the clipping-based OFDM systems. In simulation results, the DNN based signal detection has better error performance than the conventional ML detection in multi-path fading wireless channel. The performance improvement is large as the complexity of system is increased such as huge Multiple Input Multiple Output (MIMO) system and high clipping rate.