Damping characteristics of a spray-deposited shape memory alloy beam

The damping characteristics of an Ni–Ti shape memory alloy (SMA) beam are theoretically and experimentally studied with interest in identifying an appropriate damping model for the material. The SMA beam is manufactured by a spray deposition method followed by heat treatment and found to have nanocrystalline structure in which damping capacity is high. The beam is then tested to obtain an impulse response and the frequency response function (FRF). By using the Hilbert transform technique it is shown that damping of the beam is almost amplitude independent in the tested range of displacement. It is also shown from the FRF that the damping of the spray-deposited shape memory alloy beam is well represented by a model including both linear viscous and hysteretic dampings.     

Inelastic response of beams under sinusoidal and random loads

A new analytical model has been suggested for the hysteretic behaviour of beams. The model can be directly used in a response analysis without bothering to locate the precise point where the unloading commences. The model can efficiently simulate several types of realistic softening hysteretic loops. This is demonstrated by computing the response of cantilever beams under sinusoidal and random loadings. Results are presented in the form of graphs for maximum deflection, bending moment and shear.     

Effect of enhancement of interface performance on mechanical properties of shape memory alloy hybrid composites

Effect of enhancement of interface performance on mechanical properties of shape memory alloy hybrid composites (SMAHCs) was investigated in this work. Composite laminates without Ni-Ti shape memory alloy (SMA) fibers, with Ni-Ti SMA fibers polished, corroded and modified by silane coupling agent KH550 were taken into comparison to investigate the effect of surface treatments. Surface morphology of Ni-Ti fibers under different treatments were observed with scanning electron microscope. The mechanical performance of specimens without Ni-Ti fibers and with Ni-Ti fibers under different treatments were investigated through tensile, three-point bending and low-velocity experiments. The morphology and micromorphology were observed to study the effect of different surface treatment methods. The conclusion shows that embedded Ni-Ti SMA fibers can enhance the mechanical performance of composite laminates. However, the performance of Ni-Ti SMA fibers was restrained by the poor interface performance. After surface treatments, SMAHCs illustrate better mechanical performance owing to the enhanced interface performance. Among all the surface treatment methods, modification with silane coupling agent KH550 shows the best effect.     

Thermo-magnetic history effects in the giant magnetostriction across the first-order transition and minor hysteresis loops modeling in Fe0.955Ni0.045Rh alloy

Results of temperature- and magnetic field-dependent strain measurements across the first-order antiferromagnetic to ferromagnetic phase transition in Fe(0.955)Ni(0.045)Rh are presented. Distinct thermal and magnetic field hystereses are observed in the measured strain across the phase transition. The minor hysteresis loops inside the hysteretic regime across the temperature-driven transition are modeled using the Preisach model of hysteresis. The applicability of the Preisach model to explain the general features of minor hysteresis loops is discussed for a disorder influenced first-order transition. The minor hysteresis loops show the property of retaining the memory of the starting or end point of the temperature cycle followed within the hysteretic region. A larger temperature excursion within the hysteretic region wipes out the memory of a smaller temperature cycle which contains one of the extrema of the larger cycle. The end-point memory and the wiping-out property of the minor hysteresis loops can be described quite well within the Preisach model, irrespective of the temperature history followed to reach a particular starting point. Thermo-magnetic history effects across the magnetic field-induced transition are explained, which will enable the choice of the starting point of an experimental cycle in the field-temperature phase space so as to achieve the desired functionality. Our results highlight the necessity to understand the influence of disorder on a first-order phase transition so as to achieve a repeatable performance of materials whose functionalities are based on such a transition.     

基于OTDR和SMA原理的分布温度阈值执行器的设计

温度阈值执行器的设计是准分布光纤温度报警系统对温度报警空间定位的关键.本文分析了基于OTDR和SMA设计温度阈值执行器的原理,提出了执行器的物理模型.通过实验测试,从SMA弹簧样品中选出了满足执行器技术要求的记忆弹簧,完成了执行器的机械设计.系统运行表明:执行器能有效运作,能够实现准分布温度报警,并且系统具有接近1.5 m的空间分辨率和低于16 s的响应时间等优良的性能参数.     

Interfaces of correlated electron systems: proposed mechanism for colossal electroresistance

Mott's metal-insulator transition at an interface due to band bending is studied by the density matrix renormalization group approach. We show that the result can be recovered by a simple modification of the conventional Poisson's equation approach used in semiconductor heterojunctions. A novel mechanism of colossal electroresistance is proposed, which incorporates the hysteretic behavior of the transition in higher dimensions.     

Analysis of synchronized regimes for injection-locked spin-transfer nano-oscillators

The large-angle magnetization dynamics of an injection-locked spin-transfer nano-oscillator (STNO) is studied. The magnetic system is subject to the action of time-varying spin-polarized currents and external magnetic fields. The uniform mode theory is developed and describes the hysteretic synchronization mechanism in terms of bifurcations of equilibria and limit cycles of appropriate dynamical systems. Analytical predictions of control parameters for the synchronization between the magnetization self-oscillation and the external microwave excitations (current, field) are provided. The effect of temperature on the locking band and the hysteretic character of the oscillation response is analyzed. An analytical approach is developed to determine the thermally induced sidebands in the power spectral density of phase-locked oscillations as a function of control parameters. The analytical predictions are in good agreement with the results of numerical simulations.     

Current-driven magnetic excitations in permalloy-based multilayer nanopillars

We study current-driven magnetization switching in nanofabricated Ni(84)Fe(16)/Cu/Ni(84)Fe16 trilayers at 295 and 4.2 K. The shape of the hysteretic switching diagram at low magnetic field changes with temperature. The reversible behavior at higher fields involves two phenomena, a threshold current for magnetic excitations closely correlated with the switching current, and a peak in differential resistance characterized by telegraph noise, with an average period that decreases exponentially with current and shifts with temperature. We interpret both static and dynamic results at 295 and 4.2 K in terms of thermal activation over a potential barrier, with a current-dependent effective magnetic temperature.     

Room-temperature magneto-optics of ferromagnetic transition-metal-doped ZnO thin films

Magneto-optic studies of ZnO doped with transition metals Co, Mn, V, and Ti indicate a significant magnetic circular dichroism (MCD) at the ZnO band edge at room temperature, together with an associated dispersive Faraday rotation. Similar spectra occur for each dopant, which implies that the ferromagnetism is an intrinsic property of the bulk ZnO lattice. At 10 K, additional paramagnetic contributions to the MCD are observed, but above about 150 K, the magnitude of the MCD signal is dominated by the ferromagnetism and is almost temperature independent. The MCD at the ZnO band edge shows room temperature hysteretic behavior.     

Surface melting of the vortex lattice

We discuss the effect of an (ab) surface on the melting transition of the pancake-vortex lattice in a layered superconductor within a density functional theory approach. Both discontinuous and continuous surface melting are predicted for this system, although the latter scenario occupies the major part of the low-field phase diagram. The formation of a quasiliquid layer below the bulk melting temperature inhibits the appearance of a superheated solid phase, yielding an asymmetric hysteretic behavior which has been seen in experiments.     

Large eddy simulation of bluff-body stabilized swirling non-premixed flames

Large eddy simulations (LES) using a subgrid mixing and combustion model are carried out to study two bluff-body stabilized swirling non-premixed flames (SM1 and SMA2). The similarities and differences between the two flames are highlighted and discussed. Flow features, such as, the recirculation zone (RZ) size and the flame structure are captured accurately in both cases. The SM1 flame shows a toroidal RZ just behind the bluff body and a vortex breakdown bubble (VBB) downstream. In addition, a highly rotational non-recirculating region in-between the RZ and VBB is observed as well. On the other hand, the SMA2 shows a single elongated recirculation zone downstream the bluff body. Flame necking is observed downstream the bluff body for the SM1 flame but not for the SMA2 flame. The time-averaged velocity and temperature comparison also shows reasonable agreement. The study shows that the sensitivity of the flame structure to inflow conditions can be captured in the present LES without requiring any model changes.     

Low-temperature biomimetic formation of apatite/TiO2 composite coatings on Ti and NiTi shape memory alloy and their characterization

Through a low temperature process, a bilayer composite coating was formed on Ti and NiTi shape memory alloy (SMA). The composite coating consisted of a layer of titania, which was formed using a H₂O₂-oxidation and hot water aging technique, and a layer of apatite, which was formed through an accelerated biomimetic process by immersing as-oxidized metals in a high-strength simulated body fluid (5SBF). Various techniques including X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the surfaces of samples at different stages of coating formation and the coatings formed. Bioactive apatite/TiO₂ coatings could be formed on NiTi SMA and firmly bonded to the metal substrate. But there were differences for the formation of the composite coating on Ti and NiTi SMA substrates. The composite coatings formed will render both metals bioactive and hence bone-bonding.     

软体微手鲁棒非线性控制设计

软体驱动器是构建智能软体机器人的基石。然而,由于软体驱动器具有非线性、耦合和不确定性等复杂的特性,如何对其进行有效建模与控制是目前极需解决的难题。以一种由三支单腔双向弯曲软驱动器构成的软体微手为研究对象,对其进行了鲁棒非线性控制设计研究。首先,进行了鲁棒非线性控制系统的总体结构设计分析。其次,对如何设计算子控制器、跟踪控制器、算子观测器实现其对软体微手的弯曲角度和力进行控制进行了分析和讨论。接着,分析和研究了鲁棒稳定和跟踪条件。最后,通过基于实验数据的仿真验证了所提方案的可行性和有效性。     

Consideration of piezoceramic actuator nonlinearity in the active isolation of deterministic vibration

There is an increasing need to effectively control micro-vibration in such fields as metrology, optics and micro-electronics. This paper describes the design of an adaptive feedforward strategy for vibration isolation of harmonic disturbance using a piezoelectric actuator with hysteretic behavior. A nonlinear analytical model of the piezoelectric actuator including a ferroelectric-like behavior is built using a Preisach model of hysteresis. Pre-multiplication of a single-frequency reference signal by the nonlinear model of the stack is investigated in order to effectively compensate the actuator nonlinearity. It is observed that a simple linear model of the stack is sufficient in the adaptation of a filtered-X LMS feedforward controller to effectively compensate the actuator nonlinearity, provided the reference signal has frequency components at the disturbance frequency and its higher harmonics.     

Distribution of supercurrent switching in graphene under the proximity effect

We study the stochastic nature of switching current in hysteretic current-voltage characteristics of superconductor-graphene-superconductor junctions. We find that the dispersion of the switching current distribution scales with temperature as σ(I) proportional to T(α(G)) with α(G) as low as 1/3. This observation is in sharp contrast to the known Josephson junction behavior where σ(I) proportional to T(α(J)) with α(J)=2/3. We propose an explanation using a generalized version of Kurkij?rvi's theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.     

PASSIVE DAMPING AUGMENTATION OF A VIBRATING BEAM USING PSEUDOELASTIC SHAPE MEMORY ALLOY WIRES

This paper examines the effectiveness of pseudoelastic shape memory alloy (SMA) wires for passive damping of flexural vibrations of a clamped-free beam with a tip mass. A finite-element model of the system is developed and validated with experimental results. The SMA behavior is modelled using amplitude-dependent complex modulus. Numerical simulations indicate that the damping introduced by the SMA wires will increase for higher excitation-force amplitudes that produce higher strain levels in the SMA wires. Increasing the wire cross-section area provides more damping at low force-excitation amplitudes, but reduced damping at higher amplitudes. The angle between the beam and the SMA wires is an influential parameter, and a value in the 10-20° range was found to introduce maximum damping. The underlying physical mechanisms are examined in detail. System damping depends only mildly on the SMA wire length, and is unaffected by the tip mass.     

Magnetic properties of jet-printer inks containing dispersed magnetite nanoparticles

Two ferrofluid inks for jet-printing, containing magnetite NPs of slightly different average radius (sample A: 6 nm; sample B: 8 nm) were prepared by adding a dispersion of magnetite nanopowders in n-hexane to an insulating ink. Isothermal magnetization loops of inks were measured by means of a vibrating sample magnetometer in the temperature interval 5–300 K up to 70 kOe. The inks were then ejected at room temperature on standard paper by means of either a thermal ink jet head (TIJ; sample A) or a piezoelectric ink jet head (PIJ; sample B). Magnetic properties of prints on paper (FC/ZFC curves, isothermal magnetic loops and related hysteretic properties) were measured between 10 and 300 K using an alternating gradient force magnetometer up to 20 kOe. The inks display a different magnetic behavior with respect to both prints. In particular, the dispersed NPs are characterized by an effective radius (and ensuing magnetic interaction) larger than expected on the basis of the properties of the starting powders. Instead, the NP radii in both prints are closer to the starting values. The printed magnetic films show an almost perfect superparamagnetic (SP) response around room temperature; however, at temperatures lower than 100 K the SP scaling is not observed and both samples behave as interacting superparamagnetic (ISP) materials. The evolution from the SP to the ISP regime is marked by a steady increase in the hysteretic properties of both samples. Particular attention will be paid to the study of magnetic interactions occurring among NPs. The effect of the ejection process on the degree of aggregation of magnetite NPs will be here studied.     

Multi-Wavelength-Switchable Double Clad Yb3+-Doped Fiber Laser Based on Reflectivity Control of Fiber Bragg Gratings by Induced Bend Loss

We propose and demonstrate an all-fiber multi-wavelength switchable double-clad Yb³⁺-doped fiber laser based on the reflectivity cavity control by induced bend loss. The wavelength switching is realized by a variable reflecting mirror that employs a cascaded array of three high reflection (>99%) fiber Bragg gratings at 1064, 1080 and 1096 nm, with a bending controller inserted between each grating. The order of the Bragg gratings is decided according to the gain profile of the Yb³⁺-doped fiber and the induced bending loss allows us to select the Bragg-wavelength laser operation. The laser is capable of switching continuously from one wavelength to another, and slope efficiencies over 50% are obtained at each wavelength.     

Magnetic resonance, phase transition, and phase coexistence in a highly doped La0.8Sr0.2MnO3 manganite near the Curie point

Magnetic resonance and microwave impedance are studied in a highly doped La_0.8Sr_0.2MnO₃ manganite near the Curie temperature. A splitting of the resonance line is observed in the temperature region 6° below the temperature corresponding to the maximum impedance at zero magnetic field. The microwave impedance exhibits a hysteretic behavior in increasing and decreasing temperature runs with jumps at the ends of a 2°-wide temperature interval. The splitting of magnetic resonance occurs immediately above the temperature corresponding to the impedance jump in zero magnetic field. The jumps and the hysteretic behavior of the impedance are interpreted as a manifestation of the magnetic structure phase transition of the first order.     

Effective temperature, thermodynamical functions, and equilibrium properties of excited domain structure

The theory of the equilibrium properties of stripe domain structure (DS) of nonhomogeneous magnets, induced by a variable magnetic field, is developed. It is shown that an induced DS is a thermodynamic system characterized by an effective temperature which may be many orders of magnitude higher than the room temperature. For a DS of this kind, the thermodynamic functions are derived and the equilibrium conditions are determined. It is found that the entropy term in the free energy function is responsible for DS fragmentation under the variable field action, for the hysteretic dependence of the DS period on the frequency and the amplitude of the variable field, and for phase transitions attended with a jump in the number of domains.