Multiferroic properties of Co and Nd co-substituted Bi5Ti3FeO15 thin films

Co and Nd co-substituted Bi₅Ti₃FeO₁₅ thin films were prepared on Pt/Ti/SiO₂/Si substrates via metal organic decomposition method. The structural and multiferroic properties of the films were investigated. It was found that Co ions enter into the lattice and occupy the Fe site. The Bi_4.15Nd_0.85Ti₃Fe_0.5Co_0.5O₁₅ films simultaneously exhibit ferroelectric and ferromagnetic properties at room temperature, and its 2P_r and 2M_r are 38 μC/cm² and 3 kA/m, respectively. Moreover, substitutions create local ferromagnetic order and antiferromagnetic order depending on whether the local bonding is FeOCo or FeOFe/CoOCo, respectively. The competing interaction of the ferromagnetic and antiferromagnetic phases results in an interesting magnetic behavior of the films.     

Effect of Gd substitution on ferroelectric and magnetic properties of Bi4Ti3O12

Polycrystalline samples Bi_4 − xGd_xTi₃O₁₂ (x = 1, 1.5, 2) were investigated by X-ray diffraction, piezoresponse force microscopy and SQUID-magnetometry techniques. Increasing the gadolinium content was shown to suppress the spontaneous polarization in Bi_4 − xGd_xTi₃O₁₂, resulting in a polar-to-nonpolar phase transition near x = 1.5. In contrast to previous expectations, all these samples were found to be paramagnets. It was thus proven that introducing magnetically-active Gd ions into the lattice of the ferroelectric Aurivillius-type compound should not be considered as an effective way to achieve multiferroic behavior.     

Automatic differentiation of micromechanics incremental schemes for coupled fields composite materials: Effective properties and their sensitivities

A modeling approach based on automatic differentiation and micromechanics incremental schemes for coupled fields composite materials is presented in this work. In the multi-sites framework, the micromechanics incremental schemes presented herein are able to account for the anisotropic behavior of the constituents, the morphological and the topological textures and the strong contrast between the properties of the individual phases of these composite materials. By applying automatic differentiation to these micromechanics incremental schemes, the first order and high order sensitivities of the effective material properties can be easily computed in the same analysis. An application on three-phase magneto-electro-elastic composite material is presented in the framework of mono-site micromechanics to show the effectiveness of this composite materials modeling approach. The details on the implementation of this modeling approach in the multi-sites framework will be discussed in a future work. The composite materials modeling methodology reported here may be used for material microstructure sensitive design in material by design strategies.     

Effect of citric acid content on the structural and magnetic properties of SrFe12O19 thin films

Strontium hexaferrite SrFe₁₂O₁₉ thin films have been synthesized on the Si (100) substrate using spin coating sol-gel process. The influence of different citric acid to metal ions (CA/MN) molar ratios of 0.5, 1 and 1.5 on the formation of SrFe₁₂O₁₉ was investigated. Fourier transform infrared analysis and theoretical calculations were conducted to determine and help control the concentration of metal citrates in solution precursors. X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer techniques were also applied to evaluate the composition, microstructure, crystallite size and magnetic properties of the SrFe₁₂O₁₉ thin films. The results showed that single phase strontium hexaferrite thin films with optimum physical properties can be obtained from the solution with the CA/MN molar ratio of 1 after calcination at 800 °C. The SrFe₁₂O₁₉ thin films synthesized with the CA/MN molar ratio of 1 exhibited crystallite size of 42 nm and isotropically magnetic properties of M_s = 267.5 kA/m (at 795.8 kA/m), M_r = 134 kA/m, and H_c = 341.4 kA/m.     

Magnetic coupling properties of multiple metal wear particles for high-precision electromagnetic debris detection applications

The electromagnetic wear particle detector is one of effective method to monitor wear debris in lubricating oil for assessing the wear condition of mechanical equipment. However, the motion of wear particles, especially the aggregation behavior in both fluid field and magnetic field, may make the particle detector generate false wear signals. Therefore, to estimate the impact on the detection accuracy of wear debris by the particle aggregation effect, the magnetic coupling model of multiple wear particles is established for studying the magnetic coupling effect between adjacent metal particles. The research results show that the effect changes the total magnetic energy variation induced by the particles and then affects the amplitude of particle signal. Meanwhile, the variation degree is closely associated with the frequency of magnetic field of particle detectors. Overall, the aggregation of wear particles with the same magnetic properties (both ferromagnetic or non-ferromagnetic) leads to false alarms of particle detectors; and the aggregation of wear particles with different magnetic properties causes missing alarms of particle detectors. Meanwhile, increasing the field frequency may increases the probability of missing alarm failure of particle detectors.     

Apparatus for calorimetric measurements of losses in MgB2 superconductors exposed to alternating currents and external magnetic fields

Inexpensive superconducting wires with low AC losses would open up for a large superconductor market in AC electrical power applications. One candidate for this market is the MgB₂ conductor. In the development of an AC optimized superconductor, high-quality measurements of the AC losses under application-like conditions must be available. This article describes an apparatus built for this purpose. The measurement method is calorimetric. The temperature increase of the superconductor sample is measured and compared to the temperature increase due to a heater with known heat input. The system is designed for measurements of losses due to magnetic fields combined with transport currents. Results from tests verifying the capabilities of the system are given, as well as from initial AC loss measurements on a tape-shaped MgB₂ superconductor.     

Core-shell composite nanoparticles with magnetic and temperature dual stimuli-responsive properties for removing emulsified oil

Core-shell magnetic and thermosensitive composite nanoparticles, named as M-DMEA, were prepared by grafting polyoxyalkylated N,N-dimethylethanolamine onto magnetite (Fe₃O₄) nanoparticles. A series of characterizations were performed to figure out the properties of the as-prepared M-DMEA. The results reveal that the M-DMEA is superparamagnetic and has irregular shape with a core-shell structure. The flocculation performance of M-DMEA in oily wastewater produced from flooding (OWPF) treatment was investigated for the first time. It was found that the M-DMEA could separate emulsified oil droplets from OWPF rapidly under an external magnetic field with high oil removal (92.3% at the dosage of 2.5 g/L) at 65 °C. Moreover, due to its excellent magnetic response, the M-DMEA can be collected and recycled for another 3 cycles. Factors affecting the flocculation performance were discussed in detail. It is anticipated that as-prepared M-DMEA might find its potential application in practical oily wastewater treatment.     

Formation mechanism and magnetic performance of Fe-Si soft magnetic composites coated with MnO-SiO2 composite coatings

Fe-Si soft magnetic composites (SMCs) coated with MnO-SiO₂ composite coatings were fabricated via sintering the ball milled Fe-Si/MnO₂ composite powders. The transformation mechanism from MnO₂ coating to MnO-SiO₂ composite coatings was studied. The effect of MnO₂ content on the microstructure and magnetic properties of Fe-Si soft magnetic composites has also been investigated systematically. The analysis results indicated that MnO₂ would be reduced to MnO by Si in the Fe-Si particles at 780.1 ℃. When the temperature reached 985.8 ℃, the generated MnO and SiO₂ would further combine to form 2MnO·SiO₂, finally forming the MnO-SiO₂ composite coatings. With the increase of the MnO₂ content from 0.0 wt% to 10.0 wt%, the MnO-SiO₂ composite coatings became thicker, thus the saturation magnetization and permeability decreased gradually. However, exorbitant MnO₂ content above 5.0 wt% would result in excessive in-situ Fe existed in the composite coatings and obvious reduction of Si content in the Fe-Si particles due to the above solid-state reactions. Therefore, the increasing trend of the resistivity was weakened. And the 5.0 wt% MnO₂ coated Fe-Si SMCs exhibited superior comprehensive performance such as high saturation magnetization (186.5 A·m²/kg), good permeability (71), low total core loss (362.6 mW/cm³ at 50 kHz).     

Development of smart composite structures with small-diameter fiber Bragg grating sensors for damage detection: Quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing

The authors and Hitachi Cable, Ltd. have recently developed small-diameter optical fiber and its fiber Bragg grating (FBG) sensor for embedment inside a lamina of composite laminates without strength reduction. The outside diameters of the cladding and polyimide coating are 40 and 52 μm, respectively. First, a brief summary is presented for applications of small-diameter FBG sensors to damage monitoring in composite structures. Then, we propose a new damage detection system for quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing. In this system, a piezo-ceramic actuator generates Lamb waves in a CFRP laminate. After the waves propagate in the laminate, transmitted waves are received by an FBG sensor attached on or embedded in the laminate using a newly developed high-speed optical wavelength interrogation system. This system was applied to detect interlaminar delamination in CFRP cross-ply laminates. When the Lamb waves passed through the delamination, the amplitude decreased and a new wave mode appeared. These phenomena could be well simulated using a finite element analysis. From the changes in the amplitude ratio and the arrival time of the new mode depending on the delamination length, it was found that this system could evaluate the delamination length quantitatively. Furthermore, small-diameter FBG sensors were embedded in a double-lap type coupon specimen, and the debonding progress could be evaluated using the wavelet transform.     

Nonintrusive coupling of 3D and 2D laminated composite models based on finite element 3D recovery

In order to simulate the mechanical behavior of large structures assembled from thin composite panels, we propose a coupling technique, which substitutes local 3D models for the global plate model in the critical zones where plate modeling is inadequate. The transition from 3D to 2D is based on stress and displacement distributions associated with Saint‐Venant problems, which are precalculated automatically for a simple 3D cell. The hybrid plate/3D model is obtained after convergence of a series of iterations between a global plate model of the structure and localized 3D models of the critical zones. This technique is nonintrusive because the global calculations can be carried out using commercial software. Evaluation tests show that convergence is fast and that the resulting hybrid model is very close to a full 3D model. Copyright © 2014 John Wiley & Sons, Ltd.     

Fe/C复合纳米炭粉的制备及其磁学性能研究

以高温煤焦油沥青为原料,以体积比7∶3的浓硫酸和浓硝酸混合酸为氧化剂,制备水性中间相沥青;采用溶胶-凝胶法先形成碳基溶胶,加入FeCl3后进一步形成复合Fe/C凝胶;凝胶经醇水交换、常温干燥和900℃炭化制备出Fe/C复合磁性纳米炭粉。利用FT-IR、XRD、TG和TEM等对水性中间相沥青、磁性纳米炭原粉以及磁性纳米炭粉进行表征。结果表明:采用溶胶-凝胶和常温干燥的方法可以制备出粒度均匀、形状近似于椭圆形的Fe/C复合磁性纳米炭;其磁性纳米炭粉的平均粒径约5 nm,以聚集成粒度为20 nm～30 nm的团聚体形式存在。磁性纳米炭粉中的碳以无定型结构的形式存在,Fe元素以α-Fe、Fe2O3和Fe3C的形式存在,Fe/C复合磁性纳米炭粉具有软磁性和较高的磁响应性。     

Impedimetric biosensor based on magnetic nanoparticles for electrochemical detection of dopamine

One of the difficulties which limit the use of electrochemical sensors for detection of dopamine is the interference from ascorbic acid. We have sought to address this problem through the synthesis and characterization of a suitable electrode material based on magnetic nanoparticles. The interference from the ascorbic acid was overcome by fabricating a negatively charged electrode surface using PEGylated arginine functionalized magnetic nanoparticles (PA-MNPs). The nanoparticles were characterized by various techniques viz., X-ray diffraction, FT-Infrared spectroscopy, transmission electron microscopy and vibrating sample magnetometer. The electrochemical behavior of the proposed sensor was investigated by cyclic voltammetry and the sensor showed high sensitivity and selectivity for dopamine. The response mechanism of the modified electrode is based on the interaction between the negatively charged electrode and the positively charged dopamine. Under optimized conditions, linear calibration plots were obtained for amperometric detection of dopamine (DA) over the concentration range of 1–9 mM dopamine, with a linear correlation coefficient of 0.9836, sensitivity of 121 μA/mM and a detection limit of 7.25 μM. Electrochemical impedance spectroscopy (EIS) has been used to study the interface properties of modified electrodes. The value of the polarization resistance (R_p) increases linearly with dopamine concentration in the range of 10 μM to 1 mM and the limit of detection (LOD) was calculated to be 14.1 μM. High sensitivity and selectivity, micromolar detection limit, high reproducibility, along with ease of preparation of the electrode surface make this system suitable for the determination of DA in pharmaceutical and clinical preparations.     

Determination of dynamic strain profile and delamination detection of composite structures using embedded multiplexed fibre-optic sensors

This paper studies the dynamic strain measurement and delamination detection of clamped–clamped composite structures using embedded multiplexed Fibre-optic Bragg grating (FBG) sensors through experimental and theoretical approaches. A dynamic strain calibration of the FBG sensors and surface mounted strain gauges to find out the correlation between the strain and photovoltage is reported. The embedded FBG sensors were used to measure the natural frequency and dynamic strain of intact and delaminated composite structures. The strain profile of these structures subjected to external excitations was evaluated by using experimental measured data and a modified vibration theory. The results revealed that the use of the embedded FBG sensors is able to actually measure the dynamic strain and identify the existence of delamination of the structures. This allows the continuous estimation of fatigue life and minimises the need of in-site inspection of the structures.     

具有多层结构的聚合物复合薄膜的介电和磁性能

采用旋涂法制备了 Fe3 O4/聚偏氟乙烯(PVDF)复合薄膜(A)、多壁碳纳米管(MWCNT)/PVDF复合薄膜(B)以及纯PVDF薄膜(P)。利用热压法制备具有3层结构的AAA、ABA及APA 复合薄膜。为了探究层状结构对复合薄膜介电和磁性能的影响,制备了单层膜A作为对比(厚度与AAA复合薄膜相同)。分别研究了薄膜的介电和磁性能。结果表明：由于界面效应,同等厚度的AAA复合薄膜较A膜而言具有较高的介电常数;以B和P薄膜替代AAA结构中间层薄膜后,其中ABA复合薄膜的介电常数高于AAA及APA复合薄膜,同时保持较低的介电损耗。对于磁性能,层状结构对复合薄膜的饱和磁化强度及矫顽力均无明显的影响,而ABA复合薄膜的饱和磁化强度高于AAA及APA复合薄膜,且ABA和APA复合薄膜的矫顽力增加。层状结构设计不仅能够调节复合材料的介电性能和磁性能,而且有利于不同纳米填料的分散,为制备多功能聚合物复合材料提供了一定的借鉴作用。     

Calculation of electro and thermo static fields in matrix composite materials of regular or random microstructures

In the work, a numerical method for calculation of electro and thermo static fields in matrix composite materials is considered. Such materials consist of a regular or random set of isolated inclusions embedded in a homogeneous background medium (matrix). The proposed method is based on fast calculation of fields in a homogeneous medium containing a finite number of isolated inclusions. By the solution of this problem, the volume integral equations for the fields in heterogeneous media are used. Discretization of these equations is carried out by Gaussian approximating functions that allow calculating the elements of the matrix of the discretized problem in explicit analytical forms. If the grid of approximating nodes is regular, the matrix of the discretized problem proves to have the Toeplitz structure, and the matrix-vector product with such matrices can be calculated by the Fast Fourier Transform technique. The latter strongly accelerates the process of iterative solution of the discretized problem. In the case of an infinite medium containing a homogeneous in space random set of inclusions, our approach combines a self-consistent effective field method with the numerical solution of the conductivity problem for a typical cell. The method allows constructing detailed static (electric or temperature) fields in the composites with inclusions of arbitrary shapes and calculating effective conductivity coefficients of the composites. Results are given for 2D and 3D-composites and compared with the existing exact and numerical solutions.     

Highly selective and sensitive magnetic silica nanoparticles based fluorescent sensor for detection of Zn ions

In this work, quinoline group modified multifunctional silica nanoparticles having high magnetization and excellent Zn²⁺ selectivity have been successfully prepared. These multifunctional nanoparticles were characterized by high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The characterization data indicated that the organic ligand was successfully grafted on the surface of the magnetic silica nanoparticles. The fluorescent properties of the nanosensor were characterized and employed to detect Zn²⁺ with excellent selectivity and sensitivity (0.1 μM) toward Zn²⁺ over other cations even in trace amount.     

Interfacial effects on the magnetic profiles and interlayer exchange coupling of Co/CoSi multilayers

Density functional calculations are conducted to investigate the interlayer exchange coupling (IEC) between ferromagnetic Co slabs mediated by a CoSi spacer in Co/CoSi(001) multilayers with CsCl crystalline structure. For both sharp and mixed Co-Si interfaces we calculated the magnetic moment distribution and the energy stability for ferromagnetic (F) IEC and antiferromagnetic (AF) IEC between the Co slabs as function of the spacer thickness. We show that mixing near to the interface noticeably modifies the IEC to the extent that this can change from an oscillatory IEC as function of the spacer thickness to an exponentially decaying AF behavior.     

The study of the electric and magnetic properties of PbZr0.2Ti0.8O3-BiFeO3 multilayers

The results of the electric and magnetic measurements performed on PbZr_0.2Ti_0.8O₃-BiFeO₃ symmetric structures, deposited on Pt/Si wafers, were compared for different number of layers in order to analyse the effect of interfaces over the macroscopic properties. It was found that the shape and magnitude of the capacitance-voltage characteristic, as well as the shape and parameters of the ferroelectric and magnetic hysteresis, depend on the number of interfaces in the intended multilayer structure. A temperature induced gradual transition from a magnetically disordered spin glass like phase of low temperature to an uncompensated antiferromagnetic phase at room temperature takes place in the BiFeO₃ films, under low applied magnetic fields. A partial ferromagnetic like order can be obtained at low temperatures by increasing the field. The observed changes in the electric and magnetic behaviour of the systems were related to an increased degree of disorder for electric dipoles and magnetic moments, due to the increased number of layers and crystallization treatments.     

A novel predictive model for mechanical behavior of single-lap GFRP composite bolted joint under static and dynamic loading

Mechanical connection of composite is critical due to its complicated meso-structure and failure mode, which has become a bottleneck on reliability of composite material and structure. Although many researches on composite bolted joints have been carried out, the theory and experiment on mechanical behavior of such a joint structure under dynamic loading were rarely reported. Here, we propose a novel predictive model for quasi-static and dynamic stiffness of composite bolted joint by introducing the strain rate dependent elastic modulus into the mass spring model. Combined with the composite laminate theory and Tsai-Hill theory, the present model was capable of predicting the strain rate dependent stiffness and strength of the composite bolted joint. Quasi-static and impact loading experiments were carried out by Zwick universal hydraulic testing machine and split Hopkinson tension bar, respectively. The stiffness and strength predicted by our model showed good accordance with the experiment data with errors below 12% under quasi-static loading and below 30% under impact loading. The results indicated that under impact loading, stiffness and strength of the composite bolted joint were significantly higher than their quasi-static counterparts, while the failure mode of the joint structure trended towards localization which was mainly bearing failure. Among various lay-up ratios studied, the optimal lay-up ratio for quasi-static and dynamic stiffness was 0:±45:90 = 3:1:1.     

Influence of shape and size on the alignment of multi-wall carbon nanotubes under magnetic fields

The influence of the shape and size of carbon nanotubes (CNTs) on the alignment of multi-wall CNTs was investigated. A CNT suspension with polyethylenimine (PEI) added as a dispersant showed stable dispersion. Stable CNT dispersion had a relatively high zeta potential value compared with poor dispersion. In addition, a strong magnetic field of 12 T was applied to the CNT suspension to investigate the alignment behavior of the CNTs. Good alignment of the CNTs according to the direction of the magnetic field was obtained. The degree of alignment depended on the shape and size of the CNTs, with the thick, straight CNTs showing better alignment than the thin, curved CNTs.