The effect of CNT modification on the mechanical properties of polyimide composites with and without MoS<Subscript>2</Subscript>

The aim of this work was to investigate the effect of surface modification of carbon nanotubes (CNTs) on the mechanical properties of polyimide (PI) composites with and without MoS₂. A three-phase system of CNT/PI/ MoS₂ laminates were fabricated with an extrusion-grade PI to produce CNT-reinforced laminates with MoS₂ volume fractions of 1–5%. The tensile and impact properties of CNT/PI composites and CNT/PI/MoS₂ laminates were also measured and compared. Results showed that the introduction of CNTs as reinforcing additives improved the tensile properties of the composites, but worsened their impact properties. Furthermore, the addition of MoS₂ increased the impact strength of the CNT/PI composites greatly. The optimum contents of CNT and MoS₂ have been found.     

Heating effect of BaTiO3 in microwave field and microstructure of BaTiO3

Microwave equipment at 2 450 MHz was employed to prepare BaTiO₃. The heating effect of the system in the microwave field, which was influenced by several factors including dielectric properties of synthesis system and thermal insulate structures, was discussed in detail. The heating rates of the synthesis system were mainly determined by BaCO₃ and TiO₂ at low temperature and by TiO₂ and BaTiO₃ at high temperature. The results show that the heating effects in microwave field are greatly different from those in conventional furnace. The reaction of BaCO₃ and TiO₂ only lasts for 3 min at 1 100°C, and the fine, narrow-distributed and well-crystallized powders were prepared. Project supported by the National Natural Science Foundation of China.     

On generalized hamiltonian systems

1. PreliminaryIt is well known that{1] a 8ymPlectic form is invariant along the trajectory of a Hamilto-nian system. Based on this fundamental property, certain techniques have been developed.The purpose of this paper is to extend such an approach to a wider class of dynamic systeIns,namely, genera1ized Hamiltonian systems. Our purpose is to investigate a class of dynaInicsystems, which possess a certain "geometric structure".Deflnition 1.1[1'2]. Let M be a tIlallifo1d. w E fl'(M) is call…     

FEM-analysis of a multiferroic nanocomposite – comparison of experiment and numerical simulation

The combination of electric and magnetic materials opens new possibilities in the field of sensor technologies and data storage [1]. These magneto-electric (ME) materials have the property to change a physical ferroic quantity into another, i.e. a magnetic field can change the electric polarization and vice versa. The combination of multiple ferroic characteristics within materials is called multiferroic. Since magneto-electric single-phase materials are rare in nature and typically operate only at very low temperature, they are not favorable in technical applications. However, ME composites, consisting of ferroelectric and ferromagnetic phases, produce a strain-induced magneto-electric product property at room temperature [2]. In these composites, two different effects can be differentiated, the direct and the converse ME effect. The first one describes a polarization which is magnetically caused. In detail, a magnetic field is applied which produces a deformation of the magneto-active phase which is transferred to the electro-active phase and as a consequence this phase exhibits a polarization. Therefore, one can discover a strain-induced polarization. The second effect to observe is a magnetization caused by an electric field. In our contribution, we focus on a (1-3) composite, where cobalt ferrite nanopillars are embedded in a barium titanate matrix, see the experiments described in [3]. In the numerical simulations we compare the changes of the strain-induced inplane polarizations of the ferroelectric matrix with experimental measurements. Furthermore, we analyze the magneto-electric coupling coefficient. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Micromechanics of Electro- and Magneto-active Soft Composites

We study the coupled behavior in soft active microstructured materials undergoing large deformations in the presence of an external electric or magnetic field. We focus on the role of the microstructures on the coupled behavior, and examine the phenomenon in the composites with (a) periodic composites with rectangular and hexagonal periodic unit cells, and (b) in composites with the random distributions of active particles embedded in a soft matrix. We show that for these similar microstructures exhibit very different responses in terms of the actuation, and the coupling phenomenon. Next, we consider the macroscopic and microscopic instabilities in the active composites. We show that the external field has a significant influence of the instability phenomena, and can stabilize or destabilize the composites depending on the direction relative to composite geometry. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal dependence of electric conductivity in thermoplastic composites

Thermal dependence of the electric conductivity of thermoplastic composites based on both amorphous (hiPS) and crystallized (PP) polymers is investigated in this study. Two types of carbon black fillers with different values of BET surface area were used as charge conductors. Composites based on crystallized polymer matrices indicate the sharp growth of electric resistivity just before the melting range. This maximum is followed by substantial decrease of resistance at T > T_melt. With the decrease of carbon black concentration both relative growth of resistance at the T T_melt and further dropping resistance at T > T_melt increase. Composites filled with particles of higher surface area are characterized by less pronounced matrix influence on thermal dependence of electric conductivity than composites filled with particles of lower surface area; this can be caused by more pronounced matrix/filler interaction in the first case. The range of temperatures at which the resistance increase occurs does not depend on the type of carbon filler and its concentration. Composites with amorphous matrices are characterized by distinct increase of resistance above glass transition. Thermal treatment of the sample significantly affects the initial values and intensity of the temperature dependence of the resistance.To be presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga, October, 1995.Published in Mekhanika Kompozitnykh Materialov, Vol. 31, No. 4, pp. 526–532, July–August, 1995.     

Water uptake and mechanical characteristics of wood fiber-polypropylene composites

The influence of mixing process (in a two-roll mill, high-speed mixer, or twin-screw extruder) on the strength properties of polypropylene/wood fiber composites was studied. The best results were obtained for composites compounded in a twin-screw extruder. The water uptake and the influence of moisture on the flexural strength (σ_fl) and modulus (E_fl) were studied by immersion of the composites in water at 20, 50, and 90°C. Most strongly the moisture affected the value of E_fl, but the degree of water uptake and the change in σ_fl and E_fl also depended on temperature and the presence of a modificator—maleated polypropylene (MAH). MAH improved the strength properties of the composites both in dry and wet states and also decreased the extent of water uptake and swelling in cyclic (soaking/drying) tests. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 1, pp. 101–114, January–February, 2006.     

Unidirectional synchronization of Hodgkin–Huxley neurons exposed to ELF electric field

In this paper, a hybrid control strategy, H_∞ variable universe adaptive fuzzy control, is derived and applied to synchronize two Hodgkin–Huxley (HH) neurons exposed to external electric field. Firstly, the modified model of HH neuron exposed to extremely low frequency (ELF) external electric field is established and its periodic and chaotic dynamics in response to sinusoidal electric field stimulation are described. And then the statement of the problem for unidirectional synchronization of two HH neurons is given. Finally H_∞ variable universe adaptive fuzzy control is designed to synchronize the HH systems and the simulation results demonstrate the effectiveness of the proposed control method.     

Numerical analysis of two-phase magneto-electric composites

The focus of this paper is on the simulation of two-phase magneto-electric (ME) composites, consisting of a piezoelectric matrix with piezomagnetic inclusions. In such composites, the coupling between electric and magnetic fields is strain-induced and thus ME coupling arises as a product porperty. In order to compute the effective properties of the composite a computational homogenization scheme based on the Finite Element Method will be applied. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

An exact deformation analysis for the magneto-electro-elastic fiber-reinforced thin plate

A closed form expressions for bending problem of magneto-electro-elastic (MEE) rectangular thin plates are derived, the exact solutions for the deformation behaviors of the fiber-reinforced BaTiO₃/CoFe₂O₄ composites subjected to certain types of surface loads are analytically obtained. Based on Kirchhoff thin-plate theory, structural characteristics such as elastic displacements, electric potential and magnetic induction for magneto-electro-elastic (MEE) rectangular plates are investigated, the governing equation in terms of the transverse displacement is presented in a rather compact form due to the omission of the transverse shear deformation and rotatory inertia. The material coefficients for the MEE plate can be uniquely expressed by the volume-fraction (v.f.) of piezoelectric constituent BaTiO₃ in the fiber-reinforced composite, and are tabulated with 25% offset of the volume-fraction. The deformation variations of the MEE thin plate with closed-circuit electric restriction are evaluated analytically according to their specified boundary conditions, and the effects of the volume-fractions on the deformations variations are discussed. It can be found that all the results obtained by using the proposed model have reached good agreements with the other available research works, whereas, the present study provides a much simpler way in seeking the analytic solutions for the interactively coupled quantities of a multiphase medium.     

Simulation of Atomic Force Microscopy for investigating BaTiO3 and LiMn2O4 nanostructures based on Phase Field Approach

Atomic Force Microscopy (AFM) probes the surface features of specimens using an extremely sharp tip scanning the sample surface while the force is applied. AFM is also widely used for investigating the electrically non-conductive materials by applying an electric potential on the tip. Piezoresponse Force Microscopy (PFM) and Electrochemical Strain Microscopy (ESM) are variants of AFM for different materials. Both PFM and ESM signals are obtained by observing the displacement of the tip when applying electric fields during the scanning process. The PFM technique is based on converse piezoelectric effect of ferroelectrics and the ESM technique is based on electrochemical coupling in solid ionic conductors. In this work, two continuum-mechanical formulations for simulation of PFM and ESM are discussed. In the first model, for PFM simulation, a phase field approach based on the Allen-Cahn equation for non-conserved order parameters is employed for ferroelectrics. Here, the polarization vector is chosen as order parameter. Since ferroelectrics have highly anisotropic properties, this model accounts for transversely isotropic symmetry using an invariant formulation. The polarization switching behavior under the electric field will be discussed with some numerical examples. In the simulation of ESM, we employ a constitutive model based on the work of Bohn et al. [8] for the modeling of lithium manganese dioxide LiMn₂O₄ (LMO). It simulates the deformation of the LMO particle according to an applied voltage and the evolution of lithium concentration after removing a DC pulse. The modeling results are compared to experimental data. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

EXPECTED DISCOUNTED PENALTY FUNCTION AT RUIN FOR RISK PROCESS PERTURBED BY DIFFUSION UNDER INTEREST FORCE

In this article, the risk process perturbed by diffusion under interest force is considered, the continuity and twice continuous differentiability for Фδ（u,w） are discussed,the Feller expression and the integro-differential equation satisfied by Фδ （u ,w） are derived. Finally, the decomposition of Фδ（u,w） is discussed, and some properties of each decomposed part of Фδ（u,w） are obtained. The results can be reduced to some ones in Gerber and Landry＇s,Tsai and Willmot＇s, and Wang＇s works by letting parameter δ and （or） a be zero.     

Influence of nano-SiO<Subscript>2</Subscript> and carbon fibers on the mechanical properties of POM composites

Polyoxymethylene/carbon fiber(POM/CF)composites containing nano-SiO₂ were prepared, and their mechanical properties were investigated. At a content of 1-5 vol.%, the nano-SiO₂ exerted an obvious reinforcing effect on POM, leading to an increase in the elastic modulus and stiffness of the composites.     

Analysis of elastomeric composites based on fiber-reinforced systems. 1. Development of design methods for composite materials

A method for calculating the elastic properties of fiber-reinforced composites is discussed. The method is based on the structural macroscopic theory for reinforced media [1, 2], which can be used for analysis of stiff and soft composites. As a measure of the elastic properties of composites, the parameters of macroscopic deformations of the base system of Cartesian coordinates are used, with the axes oriented in a certain direction relative to the general reinforcement and loading field. The corresponding macrostresses in the loaded composites are found by a solution of the microboundary problem for a composite macroelement with sides parallel to reinforcement planes of the system. The microboundary-value problem is multiply connected and is formulated based on the information about the homogeneous field of macroscopic displacements specified by the parameters of macroscopic deformation. The problem is solved using the local system of coordinates whose axes are directed along some of the reinforcement trajectories.State Metallurgical Academy of Ukraine, Dniepropetrovsk, Ukraine. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 6, pp. 733–745, November–December, 1998.     

Blow-up of Electric Fields between Closely Spaced Spherical Perfect Conductors

The electric field increases toward infinity in the narrow region between closely adjacent perfect conductors as they approach each other. Much attention has been devoted to the blow-up estimate, especially in two dimensions, for the practical relevance to high stress concentration in fiber-reinforced elastic composites. In this paper, we establish optimal estimates for the electric field associated with the distance between two spherical conductors in n-dimensional spaces for n ≥ 2. The novelty of these estimates is that they explicitly describe the dependency of the blow-up rate on the geometric parameters: the radii of the conductors.     

The effect of polar magnetic sub-storms on the equatorial sporadic E

It is known that equatorial sporadic E disappears at night when dynamo field is east to west. During some DP₂ type magnetic sub-storms, which cause a depression of the geomagnetic horizontal field at the equator, theq type of sporadic E is found to disappear at the equatorial stations Huancayo and Kodaikanal. This suggests that one of the mechanisms causing the temporary disappearance of E _s —q during daytime in equatorial ionograms is the replacement of the east to west dynamo electric field by a west to east electric field due to the imposition of an electric field opposing the normal daytime S _q field.     

Artificial Intelligence in numerical modeling of nano sized ceramic particulates reinforced metal matrix composites

Artificial neural network models have the capacity to eliminate the need for expensive experimental investigation in various areas of manufacturing processes, including the casting methods. An understanding of the inter-relationships between input variables is essential for interpreting the sensitivity data and optimizing the design parameters. Aluminum is the best metal for producing metal matrix composites which are known as one of the most useful and high-tech composites in our world. Combining aluminum and nano Al₂O₃ particles will yield a material with high mechanical and tribological properties. In this investigation, the accuracy of various artificial neural network training algorithms in FEM modeling of Al₂O₃ nano particles reinforced A356 matrix composites has been studied.     

Experimental study of scaling properties and roughing mechanisms in C60-Ag thin films

Surface morphologies and microstructures of C₆₀/Ag composite films were studied by atomic force microscope (AFM) and transmission electron microscope (TEM). The surface roughness depended on the substrate temperature, and the transition of surface morphology of rough→smooth→rough was observed when the substrate temperature increased from −50 to 120°C. Although the rms values are similar, the scaling properties of the thermal roughing and the kinetic roughing surfaces are quite different. The relations between the scaling properties, microstructures and roughing mechanisms are discussed based on the AFM and TEM results. Project supported by the National Natural Science Foundation of China (Grant No. 59529204).     

Quantum scattering near the lowest Landau threshold for a Schrödinger operator with a constant magnetic field

For fixed magnetic quantum number m results on spectral properties and scattering theory are given for the three-dimensional Schrödinger operator with a constant magnetic field and an axisymmetrical electric potential V. In various, mostly singular settings, asymptotic expansions for the resolvent of the Hamiltonian H _m+H_om+V are deduced as the spectral parameter tends to the lowest Landau threshold. Furthermore, scattering theory for the pair (H _m, H _om) is established and asymptotic expansions of the scattering matrix are derived as the energy parameter tends to the lowest Landau threshold.     

Epitaxial growth and electrical transport properties of La0.5Sr0.5Co03 thin films prepared by pulsed laser deposition

Epitaxial growth of the La_0.5Sr_{0.5 3}(LO) thin films has been realized on Lin3, SrTiC₃ and MgO substrates by pulsed laser deposition. The epitaxial growth behavior and the electrical transport properties of these films were studied systematically. The temperature dependencies of the resistivity of the film have been determined. Studies indicate that close dependencies exist between the crystal structures and the electrical transport properties of the epitaxial LSCO films, and that the epitaxial thin films are of low resistivity and metallic conductive features. The epitaxial films deposited on the LaA10₃ substrates at about 700 °C possess the optimal properties compared with the others. Discussions of the dependencies and the mechanisms of the epitaxial structures on the electrical transport properties of the LSCO films have been made. Project supported by the National Natural Science Foundation of China (Grant No. 19574003 and No. 19674001).