Interaction between plasticity and damage in the behaviour of [+φ, −φ]n fibre reinforced composite pipes in biaxial loading (internal pressure and tension)

In this paper, we propose a new model which describes the behaviour of [+φ, −φ]_n composite laminates. Tests were performed on glass-epoxy pipes subjected to biaxial tensile and internal pressure loading. Experiments showed that [+55, −55]_n pipes exhibit varying types of damaged elastoplastic behaviour depending on the stress ratio σ_zz/σ_θθ (axial stress/hoop stress). A plastic model is based on the definition of a yield criterion and an associated flow rule. Damaging occurs when transverse microcracks appear in the layer. A micromechanical model defines the anisotropy of the damage. Interaction between plasticity and damage was of major importance in the definition of damage kinetics. This effect was observed on proportional loadings as well as on sequential tests: a preliminary loading in pure internal pressure (σ_zz=0) induced large plastic phenomena which blocked crack propagation in additional internal pressure with closed ends effect (IPCEF) tests (R=σ_zz/σ_θθ=1/2), even though IPCEF caused considerable damage on an unloaded specimen.     

Homogenization kinetics of a cast Ti48A12W0.5Si alloy

The homogenization kinetics of a cast Ti48A12W0.5Si alloy with a duplex microstructure was studied in terms of γ-phase dissolution and -grain growth. It was found that the measured volume fraction of remnant γ grains can be well simulated by a model of interface-controlled dissolution in a dislocation mechanism, instead of a diffusion-controlled one. The activation energy for the /γ interface reaction was found to be Q_int = 476 kJ mol⁻¹, which is much higher than the interdiffusion activation energy in TiAl alloy. The grain growth of phase during homogenization can be categorized into three stages. During the first stage, where the volume fraction of remnant γ grains is higher than about 10%, the growth of grains follows the parabolic law D = k₁t^0.2, and the activation energy for grain growth was calculated to be Q₁ = 442 kJ mol⁻¹, very close to the Q_int for /γ interface reaction. In the second stage, where few fine γ grains (1–10 vol.%) remained, a dramatic grain growth occurs. During the final stage, as the single phase is obtained, the coarsening of grains again satisfies the grain growth law D = k₃t^0.4, with the grain growth activation energy Q₃ of 147 kJ mol⁻¹, lower than the reported interdiffusion energy of γ phase.     

Ultrasonic studies of the binary mixtures of ethyl acetate and cresols—application of Kosower and Dimroth treatments

The ultrasonic velocity (ν) studies were carried out at a frequency of 2 MHz (transducer of x-cut quartz crystal) using ultrasonic pulse echo system (model UX4400-M) on cresols in ethyl acetate at constant temperature of 311 K. The values of internal pressure ( π_i) and molar free volume (V_f) were calculated from measured values of ultrasonic velocity (ν), viscosity (η) and density (ρ). An attempt is made to rationalize the ultrasonic velocity (ν), internal pressure ( π_i) and free volume (V_f) of binary mixtures using Kosower solvent parameter (Z), Dimroth solvent parameter (E_T) and Dielectric constant (). It is found that there is linear correlation between ultrasonic velocity and acidity constant pk⁻¹a, indicating the dependence of acidity. Correlation of Ksower and Dimroth parameters with ultrasonic velocity confirms that solvent polarity is an important factor in the variation of ultrasonic velocity in the present investigation.     

Analysis of Au-, Al-, and Sn/Si semiconductor devices with thin oxide layer by photocurrent measurements

We present herein a photocurrent study on the forward biased Au-, Al-, and Sn/n-Si junctions with the native oxide layer using an intense white light for illumination. The photocurrent transient effect in Metal/n-Si junctions showed a wide variation upon changing the metal. The photocurrent cycles, which consist of fast transient, photocurrent decay, enhanced photocurrent and persistent photocurrent, are identified for each M/Si junction at temperatures 120 K and 300 K. The cycle seems to be strongly dependent on the oxide thickness, and subsequently the potential barrier height φ_B of the junction. The largest degradation in φ_B with the illumination time can be observed for the Sn/Si junction (which has the lowest φ_B), while the smallest change can be seen in the case of Au/Si junction (which has the highest φ_B). Upon illumination, the reactivity of the metal with the SiO₂/Si interface is one of the most important factors affecting the oxide layer, and φ_B. At room temperature, the lower is the potential barrier height φ_B of the diode, the faster the photocurrent cycle is produced. On the other hand, at low temperature the larger is the recombination current of the illuminated junction, the faster the photocurrent cycle is produced.     

高Q值(1-x)(Sr0.2Nd0.208Ca0.488)TiO3-xNd(Ti0.5Mg0.5)O3微波陶瓷的微结构及介电性能研究

采用固相法制备了(1-x)(Sr_0.2Nd_0.208Ca_0.488)TiO₃-xNd(Ti_0.5Mg_0.5)O₃(0.3≤x≤0.4, SNCT-NTMx)系微波介质陶瓷材料, 并研究了该体系的相组成、显微结构、烧结性能和微波介电性能之间的关系。结果表明: 在x = 0.3~0.35范围内, SNCT-NTMx陶瓷形成了正交钙钛矿固溶体, 并伴随有少量未知第二相; 当x增至0.4时, 第二相含量有所增加。介电性能研究结果显示: 随着x的增加, 体系介电常数(ε_r)减小, 但品质因子(Q×f)得到改善; 此外, 体系谐振频率温度系数(τ_f)随NTM含量的增加逐渐向负值方向移动。当x = 0.35, 陶瓷样品在1520℃烧结4 h 得到的微波介电性能较优: ε_r=50.1, Q×f =44910 GHz, τ_f= -1.7×10^-6/℃。     

Magnetic and electrical characterization of heavily boron-doped diamond

For a heavily boron-doped diamond (BDD) film, temperature variations of the electrical conductivity σ and magnetic susceptibility χ are reported. The room temperature σ 143 (Ω-cm)⁻¹ corresponds to a carrier concentration 10³ ppm, and its temperature variation yields an activation energy E_a 28 meV from 140 to 300 K and E_a0.88 meV from 40 to 80 K. It is argued that larger boron doping leads to lower magnitudes of E_a. The χ vs. T data (1.8–350 K) fits the Curie–Weiss law, with the concentration of paramagnetic species 120 ppm and a diamagnetic susceptibility −0.4×10⁻⁶ emu/g Oe. The results obtained from the measurements of σ and χ are discussed and compared.     

Electrical characterization and carrier transport mechanisms of GaAs p/i/n devices for photovoltaic applications

GaAs p/i/n diodes made by Metal-Organic Vapour Phase Epitaxy were examined by electrical measurements for evaluating the optimum i-region for use as solar cells. Four series of samples were prepared and studied each one with a different i-region width. The performance of the devices was examined by means of Admittance spectroscopy as well as classical current–voltage and capacitance–voltage characterization, allowing the calculation of the minority carriers lifetime (τ_eff) and the diodes ideality factors. The values of the τ_eff were found to lie between 8.7 ps and 0.14 ns for i-region widths between 0 and 0.8 μm. These results were used to model the multilayer structure with the two-diode representation and explain the conductance mechanisms inside the diodes. This modeling showed that the recombination/generation currents were dominating in forward biased diodes and the ohmic loss current in reverse bias.     

MgO/SrO/La2O3多元添加对(Zr0.8Sn0.2)TiO4微波陶瓷结构性能影响

基于液相促进固相反应烧结机制, 设计MgO/SrO/La₂O₃多元复合添加(Zr_0.8Sn_0.2)TiO₄(ZST)体系, 探究复合添加剂对ZST陶瓷的物相组成、微观结构、烧结特性以及高频介电性能等参数的影响。实验结果表明: 陶瓷的主晶相均为ZST相; 适量添加MgO/SrO/La₂O₃可以有效地降低ZST陶瓷的烧结温度, 获得较优的微波介电性能; 但MgO添加量的增多对材料的综合性能有小幅度的影响; SrO的添加量过大会造成晶粒的不完全生长、瓷体不致密和气孔的增多, 从而导致材料的密度、介电常数和Q×f值的下降; 此外, 添加剂对陶瓷的频率温度系数(τ_f)影响不大。在复合添加0.2wt%MgO、0.6wt%SrO、1.0wt%La₂O₃时, 1300℃保温5 h的ZST陶瓷综合性能优异: ρ=5.14 g/cm³, ε_r=40.11, Q×f=51000 GHz (f=5.61 GHz), τ_f=-2.85×10^-6℃^-1。     

ZnO掺杂对Ca0.25(Li0.43Sm0.57)0.75TiO3陶瓷烧结性能和微波介电性能的影响

采用溶胶-凝胶法制备Ca_0.25(Li_0.43Sm_0.57)_0.75TiO₃(CLST)微波介质陶瓷纳米粉体, 研究了ZnO掺杂量和烧结温度对CLST+ xmol% ZnO陶瓷烧结性能和微波介电性能的影响。XRD分析结果表明: 随着ZnO掺杂量x的增加, 陶瓷的晶体结构从正交相变为伪立方相, 并在x≥1.5的样品中出现了杂相。CLST+ xmol% ZnO陶瓷的致密化烧结温度随x的增加而降低, x=1.0的样品的致密化烧结温度比x=0的降低了200 ℃。介电常数ε_r和频率品质因数Qf 随x增加和烧结温度的升高具有最优值, 频率温度系数则单调降低。x=1.0的样品在1100 ℃烧结时具有优异的综合性能: ρ = 4.85 g/cm³, ε_r =102.8, Qf = 5424 GHz, τ_f = -8.2×10^-6/℃。表明ZnO掺杂的CLST陶瓷是一种很有发展潜力的微波介质陶瓷。     

Dielectric properties, ac conductivity and thermal behaviour of flux grown cadmium titanate crystals

The dependence of loss tangent (tan δ) and both real and imaginary parts of the dielectric constant (′ and ″) on temperature in the range 298–923 K and frequency in the range 10³–10⁶ Hz for flux grown CdTiO₃ single crystals is reported. The ln σ_ac versus T plots suggest the conduction mechanism to be ionic hopping conduction. From ln σ_ac versus frequency curves, it can be seen that the slope decreases with the rise in temperature, suggesting that the ionic hopping conduction diminishes with the rise in temperature. The activation energy at various fixed frequencies is calculated from the slope of the graph between ln σ_ac versus 1/T (×10³ K⁻¹). Thermal behaviour of flux grown CdTiO₃ crystals using thermoanalytical techniques including TG, DTA and DTG is discussed. Thermal analysis suggests decomposition of CdTiO₃ in the temperature interval of 1386–1693 K leading to the formation of TiO₂ as the final product. Results obtained on application of TG based models viz. Horowitz–Metzger, Coats–Redfern and Piloyan–Novikova are reported. The results of kinetics of thermal decomposition suggest contracting cylinder model as the one that is relevant to the decomposition of CdTiO₃. The kinetic parameters viz. the order of reaction, activation energy, frequency factor, and entropy of activation using the above mentioned models are computed.     

Dynamics of a thin viscoelastic film on an inclined plane

A theory for two-dimensional long and stationary waves of finite-amplitude on a thin viscoelastic fluid (weakly elastic) layer flowing down an inclined plane is investigated. A set of exact averaged equations for the viscoelastic film flow system is described and linearised stability analysis of the uniform flow is performed using normal-mode formulation and the critical condition for linear instability is obtained. The linearised instability for the permanent wave equation, consistent to the second order in (, – unperturbed film thickness, L – characteristic length) is examined and the eigenvalue properties of the fixed points are classified in various parametric regimes. The possible domains of heteroclinic orbits and the regions of possible nonlinear bifurcations are analysed for different values of viscoelastic parameter Γ. Numerical integration of the permanent wave equation as a third order dynamical system is carried out. While wave transitions in real life involve complex spatio-temporal dynamics and many of these transitions lead to chaotic waves that are not stationary traveling waves, bifurcation of stationary traveling waves has been examined as a preliminary study of the more complex transitions. Different bifurcation scenarios leading to multiple hump solitary waves or leading to chaos are exhibited in the parametric space. The results are compared and contrasted with the Newtonian results. A summary of the bifurcation scenarios in the We versus cot θ/Re plane is obtained for different values of viscoelastic parameter Γ, when Re ≈ 13.33 and Re = 100.     

Application of Merchant theory in machining particulate metal matrix composites

Particulate metal matrix composites (PMMC’s) are a category of engineering materials with growing applications in modern industry. Conventional machining processes are used to fabricate engineering PMMC’s components. This article presents a preliminary experimental study based on Merchant theory for PMMC’s (aluminium alloy reinforced with 20% of particulate silicon carbide-SiC). The experiments were carried out on PMMC’s special workpieces using cemented carbides K20 cutting tools in radial turning. The objective of this study is to evaluate the chip compression ratio (R_c), shear angle (Φ), shear strain (ε), shear strain rate (dε/dt), normal stress (σ) and shear stress (τ), under prefixed cutting parameters (cutting velocity and feed rate).     

CdGeAs2晶体的热膨胀行为研究

采用WinTA 100热膨胀仪研究了四方黄铜矿CdGeAs₂晶体在320~620 K温度范围内的热膨胀行为, 探索了CdGeAs₂晶体热膨胀各向异性的物理机制。测定晶体a轴和c轴方向的热膨胀系数α_a和α_c发现, α_a>>α_c>0, 表现出强烈的各向异性热膨胀特性。利用最小二乘法, 拟合出CdGeAs₂晶体的晶格常数(a, c)与温度(T)的函数关系式, 与文献报道值吻合。分别计算出不同温度下的四方畸变因子δ=2-c/a, Cd-As 键长(l_Cd-As)和 Ge-As 键长(l_Ge-As)以及相应的热膨胀系数α_Cd-As和α_Ge-As。结果表明, a、c、δ、l_Cd-As、l_Ge-As和α_Cd-As均随着温度的升高而增大, c/a和α_Ge-As则随着温度的升高而减小。当T=360 K时,α_Cd-As是α_Ge-As的6.36倍, 是造成CdGeAs₂晶体强烈热膨胀各向异性的主要原因。     

On secondary dendrite arm coarsening in peritectic solidification

A model for isothermal coarsening of secondary dendrite arms in peritectic reaction and transformation (liquid + primary-phase → peritectic-phase) is proposed to evaluate the secondary dendrite arm spacing (λ₂) of the primary phase in directional solidification of peritectic alloys. The model defines three stages for thin-arm dissolution (or thick-arm coarsening), i.e. the initial, intermediate and final stages: the initial thin-arm dissolution through the primary phase is sustained solely by the Gibbs–Thomson effect; the intermediate thin-arm dissolution through the peritectic phase is driven by Gibbs–Thomson effect but retarded by the peritectic reaction and transformation; the final dissolution through the primary and peritectic phases is enhanced by the Gibbs–Thomson effect and the phase transformation. The kinetics of peritectic reaction and transformation were found to be crucial to determine the thin-arm dissolution, which were characterized by the reaction constant (f) and the diffusion coefficient of solute in solid peritectic-phase (D_S), respectively. The present model shows that λ₂V^m is constant for a given Pb–Bi peritectic alloy, where V is growth velocity, and the factor, m, ranges from 1/3 to 1/2, rather than that normally observed (e.g. 1/3) for single-phase solidification. It is also notable that the calculated λ₂ for a Zn–7.37 wt.% Cu peritectic alloy was reasonably consistent with our earlier experiments for various growth velocities.     

高居里温度BaTiO3-(Bi0.5Na0.5)TiO3无铅正温度系数电阻陶瓷的制备

用传统的固相反应烧结法制备了(1-xmol%)BaTiO₃-xmol%(Bi_0.5Na_0.5)TiO₃(BBNTx)高温无铅正温度系数电阻( positive temperature coefficient of resistivity, PTCR)陶瓷。X射线衍射表明所有的BBNTx陶瓷形成了单一的四方钙钛矿结构。SEM分析结果显示随着BNT含量的增加, 陶瓷晶粒尺寸减小。空气中烧结的0.2mol% Nb掺杂的BBNT1陶瓷, 室温电阻率为~10² Ω·cm, 电阻突跳为~4.5个数量级, 居里温度为~150℃。氮气中烧结的0.3mol% Nb掺杂的BBNTx(10≤x≤60)陶瓷, 同样具有明显的PTCR效应, 居里温度在180~235℃之间。随着BNT含量的增加, 材料的室温电阻率增大, 同时陶瓷的电阻突跳比下降。     

Ge掺杂对TiO2-Nb2O5-CaCO3压敏陶瓷结构及性能的影响

TiO₂ 压敏电阻是一种典型的非线性电流-电压电子器件, 本文研究了Ge掺杂对TiO₂-Nb₂O₅-CaCO₃压敏陶瓷的非线性系数α和压敏电压E_B的影响。采用传统的球磨-成型-烧结方法成功制备Ge掺杂TiO₂-Nb₂O₅-CaCO₃压敏陶瓷, 用压敏直流参数仪测试样品的非线性系数α、压敏电压E_B和漏电流J_L等电学性质, 并根据相关公式计算样品平均势垒高度。XRD、XPS、SEM和STEM分析表明, Ge掺杂显著改变TiO₂-Nb₂O₅ -CaCO₃压敏陶瓷微结构, 提高非线性系数α和减小压敏电压E_B。当施主Nb₂O₅和受主CaCO₃掺杂浓度分别为0.5mol%时, 掺杂1.0mol% Ge的压敏陶瓷获得了最高的非线性系数和较低的压敏电压(α=10.6, E_B=8.7 V/mm), 明显优于不掺杂Ge的TiO₂-Nb₂O₅-CaCO₃压敏陶瓷。此外, Ge熔点较低, 作为烧结助剂可以降低陶瓷的烧结温度, TiO₂-Nb₂O₅-CaCO₃-Ge压敏陶瓷最佳烧结温度是1300℃。     

Optical properties of Nd:NaLa (WO4)2 single crystal

Nd³⁺-doped NaLa(WO₄)₂ single crystal with a dimension of 20 mm × 40 mm and a good optical quality was grown by Czochralski method. The polarized absorption spectra and emission spectra were measured at room temperature. The absorption cross-section and emission cross-section were presented. The Judd–Ofelt theory, extended to anisotropic system, has been applied to evaluate the intensity parameters Ω_t (t = 2, 4, 6), radiative transition rates A, radiative lifetimes τ_R and fluorescent branching ratios β. The calculated radiative lifetime was compared with the experiment data for the ⁴F_3/2 emitting level. All spectral features are strongly affected by an inhomogeneous broadening connected with the ‘disordered crystal’ character of the title compound.     

T Violation in the weak scalar and tensor interaction: neutron and nuclei

The motivation and status of a search for time-reversal violation in nuclear and neutron beta decay are discussed. A new experiment for free neutron decay is proposed. A hitherto unmeasured amplitude, R, of the directional correlation J·( p×σ), between the neutron spin J, the electron momentum p and the electron spin σ, will be determined. An accuracy well below 0.01 can be achieved using an intense cold neutron beam and an electron tracking detector, where the spin sensitivity is provided by large angle Mott scattering. This study provides an unique access to the exotic scalar S and tensor T interaction. Finite values, or tight constraints on the time-reversal violating scalar components, can be deduced in a combined analysis of the proposed experiment and a precise determination of the tensor couplings from a recent study of ⁸Li decay. The great interest in weak scalar interaction is stimulated by a multitude of scalar bosons which are introduced in most extensions of the Standard Model.     

Powder X-ray diffraction, infrared and conductivity studies of AgSbMP3O12 (M = Al, Ga, Fe and Cr)

New Nasicon type of compounds of composition AgSbMP₃O₁₂ (M = Al, Ga, Fe and Cr) are synthesized by solid-state method. All the compounds crystallize in the hexagonal lattice with space group . The infrared spectra of these compounds show characteristic bands due to PO₄ group. The frequency independent conductivity of these compounds shows Arrhenius type behavior and the activation energy for conduction is in the range 0.40–0.55 eV. Frequency independent conductivity (σ_dc) studies and frequency dependent (σ_ac) impedance measurements correlate well. The Cole–Cole plots do not show any spikes on the lower frequency side indicating negligible electrode effects. The activation energies obtained from the plots of log σ_dcT versus 1/T, log σ_ac(0) versus 1/T and log τ versus 1/T are approximately the same. The peak width at half height for electric modulus (M″) plot is 1.24 decades for all samples, which is close to 1.14 decades observed for Debye solid. The height of electric modulus (M″) obtained from the experimental plots are close to that of M″ (max) = C₀/2C indicating the Debye nature of the samples.     

Determining crystallization kinetic parameters of Li2O–Al2O3–SiO2 glass from derivative differential thermal analysis curves

The kinetic parameters such as crystallization activation energy, E, and the frequency factor, ν, of Li₂O–Al₂O₃–SiO₂ glass were determined by a new non-isothermal method. The method is described by the equation

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, where β is the heating rate and T_f is the inflection-point temperature of differential thermal analysis (DTA). The value of T_f is determined as the maximum peak temperature on derivative differential thermal analysis (DDTA) curves. Values of E and ν of Li₂O–Al₂O₃–SiO₂ glass were also determined by two existing non-isothermal methods, namely the Kissinger plot and the Ozawa plot, and compared with those determined by isothermal method. Values of E and ν determined by the proposed equation were 332 kJ/mol and 1.4×10¹³ s⁻¹, respectively. They are excellent agreement with the isothermal analysis results, 336 kJ/mol and 1.8×10¹³ s⁻¹, respectively. In contrast, both the Kissinger equation and the Ozawa equation give much higher values of E and ν.