Granular Composites Research Articles

Magneto-optical effects calculated for granular composites with magnetized nano-onions dispersed in matrixes

For granular composite having nano-onions (nanosized, multicore-shell structured particles) dispersed in a matrix we derived an effective dielectric permittivity tensor including off-diagonal terms, based on the Maxwell Garnett effective field approximation. The magneto-optical Kerr effect was calculated from the effective permittivity tensor for the composite containing nano-onions with Au (core)/Fe (inner shell)/Au (outer shell) structure. The off-diagonal term, 〈εxy〉=〈εxy′〉+i〈εxy″〉, of the effective permittivity tensor and Kerr effect derived from it exhibit a dispersion relation, 〈εxy′〉 making a peak and 〈εxy″〉 crossing zero, at a wavelength around 630 nm. The dispersion may be ascribed to the plasma resonance of Au, because similar peak was found in the Kerr rotation spectra for compositionally modulated multilayered Au/Fe films due to the plasma resonance of Au.

Enhanced magnetoresistance in granular La2/3Ca1/3MnO3/ polymer composites

Polymer embedded granular composites, La2/3Ca1/3MnO3/polyparaphenylene (LCMO)1−x(PPP)x (x is the weight fraction of PPP), were prepared and their magnetoresistance (MR) properties were investigated. A significant enhancement in MR is observed for the composites especially in the low temperature range. The MR reaches the maximum at x=0.1 when T=5 K and at x=0.2 when T=260 K. We argue that such enhancement in MR is attributed to the enhanced spin-polarized tunneling, which is manipulated by the spin disorder at the LCMO surfaces caused by PPP.

Theoretical investigation on the possibility of preparing left-handed materials in metallic magnetic granular composites

We investigate the possibility of preparing left-handed materials in metallic magnetic granular composites. Based on the effective medium approximation, we show that by incorporating metallic magnetic nanoparticles into an appropriate insulating matrix and controlling the directions of magnetization of metallic magnetic components and their volume fraction, it may be possible to prepare a composite medium of low eddy current loss which is left-handed for electromagnetic waves propagating in some special direction and polarization in a frequency region near the ferromagnetic resonance frequency. This composite may be easier to make on an industrial scale. In addition, its physical properties may be easily tuned by rotating the magnetization locally.

A Note on the Theory of Short-Term Microdamageability of Granular Composites under Thermal Actions

The theory of microdamageability of granular composites is stated with allowance made for the thermal effect. Microdamages in the components are modeled by pores, hollow or filled with particles of the destroyed material that resist compression. The fracture criterion is assumed to have the Schleicher–Nadai form, which takes into account the difference between the tensile and compressive ultimate loads, with the ultimate strength being a random function with a power or Weibull distribution. The stress–strain state and effective properties of the material are determined from the stochastic thermoelastic equations for granular composites with porous components. The equations of deformation and microdamage are closed by the equation of porosity balance corrected for the thermal effect. Nonlinear diagrams are plotted for the concurrent processes of deformation of a granular material and microdamage of the matrix as functions of macrostrains and temperature. The influence of the physical and geometrical parameters on the processes is analyzed.

Giant magnetoresistance in a three-dimensional lattice of dipolar interacting magnetic nanoparticles

The magnetization and the giant magnetoresistance (GMR) of three-dimensional multilayered lattice of dipolar interacting fine anisotropic magnetic particles embedded in a nonmagnetic metallic matrix are numerically investigated. Using a Monte Carlo method the dependence of the magnetization curve and the GMR effect on the magnetic field H, temperature, the number of layers, the magnetic anisotropy, the separation between neighboring particles, and the particle-size distribution are examined systematically. We found that the enhanced dipolar interaction at high particle densities and for wide particle-size distributions reveals itself as a substantial suppression in the change in the negative MR with the applied field H. For out-of-plane magnetic field orientation, the negative MR is shown to increase more rapidly with H as the number of layers increases. This effect is attributed to the demagnetization field arising from the free magnetic poles forming at the sample surface. The relevance of the present results to the understanding of the magnetic and transport properties in granular composites is discussed.

Homogenization principles and their application to continuum damage mechanics

The concept of homogenization is utilized herein to develop simplified methodologies for predicting the response of solids that develop multi-scale damage. This methodology is cast within a computational framework in order to produce efficient algorithms for a variety of media including granular and layered composites. Examples are given to exemplify the usefulness of this approach.

Synthesis and characterization of compatibilized poly(ϵ‐caprolactone)/granular starch composites

To improve the mechanical properties of granular corn starch-filled poly(e-caprolactone) (PCL) compositions, three strategies were investigated including the hydrophobic coating of starch granules by reaction with n-butyl isocyanate, the addition of PCL-grafted dextran (PGD) as an amphiphilic compatibilizer, and the use of PCL-grafted granular starch (PGS). Except for the chemical modification of granular starch by reaction with n-butyl isocyanate, the synthesis of both PGD and PGS relies upon the controlled ring-opening polymerization (ROP) of e-caprolactone (CL) initiated by Al-alkoxides generated onto the polysaccharide, either dextran or starch particles. While the hydrophobic coating of starch only provides higher tensile strength and elongation at break, these properties as well as Young's modulus and strength at yield of the PCL/starch blends were remarkably increased by locating the PCL-grafted dextran at the filler/matrix interface. It is however worth pointing out that a tougher and stiffer material was obtained by melt blending PGS and pure PCL. These property changes were analyzed and clearly related to parameters such as filler dispersion, interfacial tension, interfacial adhesion and reinforcement by PCL crystallites.

Synthesis and characterization of compatibilized poly(ε-caprolactone)/granular starch composites

Synthesis and characterization of compatibilized poly(ε-caprolactone)/granular starch composites

Alkali Hydrothermal Synthesis of Zeolites from Coal Fly Ash and Their Uptake Properties of Cesium Ion

Zeolites were synthesized from coal fly ash by hydrothermal treatment with KOH solutions. K-H zeolite (K2Al2Si4O12-nH2O) was produced under optimum conditions of 160°C, 3 d, 1 M (=mol/dm3) KOH and liquid-solid ratio of 15cm3/g. The uptake behavior of radioactive cesium for the products was investigated by batch and column methods. The uptake equilibrium of Cs+ for the above product was attained within 2h yielding the distribution coefficient of above 104 cm3/g. The uptake of Cs+ was followed by a Langmuir adsorption isotherm and the maximum uptake capacity was estimated to be 3.34 mmol/g. The successive removal of Cs+ was accomplished through the column packed with granular composites of product-alginate gel polymer.

Effective nonlinear optical properties of metal-dielectric composite media with shape distribution.

The effective linear and nonlinear optical properties have been investigated in granular metal-dielectric composites taking the geometric shape of inclusions into account. Recently derived Maxwell-Garnett-type approximations are generalized to study the spectral function for composites in which the metal inclusions have a uniform distribution of geometric shapes. The numerical results show that the spectral density function becomes a prominent peak around small spectral value s within the non-self-consistent Maxwell-Garnett approximation but a broad continuous spectrum around large s besides the prominent peak around small s within the self-consistent Maxwell-Garnett approximation. Based on the spectral representation, we investigate the optical absorption and third-order nonlinear optical susceptibility. The results indicate that the shape distribution can lead to the separation of the nonlinearity enhancement peak from the absorption peak and thereby make the figure of merit more attractive.

Dependence of the giant magnetoresistance on the concentration of magnetic particles in granular composites

We study the dependence of giant magnetoresistance (GMR) on the volume concentration of magnetic particles in a magnetic granular composite via a Monte Carlo method and by modeling the composite as a random resistor network. We assume the nanosized magnetic particles are spherical in shape and are randomly distributed in a square or cubic lattice. The uniaxial anisotropy of the particles and the classical dipolar interaction among the particles are taken into account. By considering the difference in electron scatterings for spin-up and spin-down conduction electrons at the magnetic and nonmagnetic interface, and the scatterings within the magnetic regions and the nonmagnetic host medium in the composite, the value of GMR is found to depend sensitively on the spatial distribution of the particles, the magnetic states of the particles, and the densities of the spin polarized conduction electrons. There is an optimum concentration (about 25% in two-dimensional and 30% in three-dimensional cases) of magnetic particles at which the magnetoresistance shows a maximum. This phenomenon was also observed in experiments.

A phenomenological theory of giant magnetoresistance in magnetic granular alloys

We combine effective medium theory (EMT) with the two-channel conducting model to study the magnetic granular concentration dependence of a giant magnetoresistance (GMR) in magnetic granular composites. The composite is composed of small magnetic granules (such as Co) embedded in an immiscible nonmagnetic metallic matrix (such as Ag). We consider the influence of granule size and concentration, respectively. Firstly, granules have a distribution in size, resulting in superparamagnetic (SPM) granules and ferromagnetic granules which have different contributions to conductance. Secondly, the effective surface scattering is changed with the changes of concentration. The calculated result of GMR is in agreement with the experimental data.

Electrical properties and giant magnetoresistance of the CoFeB–SiO 2 amorphous granular composites

The (Co 41Fe 39B 20) x (SiO 2) 100− x metal–insulator granular films ( x=33, 43, 50, 72 and 100 at.%) with amorphous structure have been obtained by ion-beam sputtering technique. Resistivity and magnetoresistance have been studied in these materials. It has been observed that all the granular composites crystallize within the same temperature region 700–900 K independent of the concentration. Electrical conductance by hopping mechanism with a variable jump length is found to dominate in the (Co 41Fe 39B 20) x (SiO 2) 100− x granular composites containing less than 72 at.% of the metal phase (33, 43 and 50 at.% Co 41Fe 39B 20) at temperatures below 300 K. The negative giant magnetoresistance (GMR) was observed in the same granular composites. The maximum value of the GMR (3.4%) was found in the (Co 41Fe 39B 20) 50(SiO 2) 50 film which is close to the percolation threshold. The GMR is associated with the electron tunneling through the insulator barriers separating the ferromagnetic granules.

Low-field magnetoresistance in La 2/3Sr 1/3MnO 3 granular composites by a sol–gel coating process

The La 2/3Sr 1/3MnO 3 granular composites have been prepared by a sol–gel coating process, in which the transport and magnetic properties have been systematically measured. It has been found that the low-field MR is enhanced owing to tunneling between the large and small La 2/3Sr 1/3MnO 3 grains. Furthermore, the hysteretic MR behavior can be attributed to the different magnetization processes of the large and small ferromagnetic grains.

Enhanced magnetoresistance in La07Sr03MnO3 nanoscaledgranular composites

By soft chemical processing, two kinds of nanoscaled lanthanum manganite granular composites, La07Sr03Mn3 / CoFe2O4 (LSMO/CF) and La07Sr0.3Mn3 / Nd07Sr03MnO3(LSMO/NSMO), were fabricated. Enhancement in magnetoresistance is observed in both of the composites. Different magnetic compounds, CF and NSMO, embedded in LSMO matrix, result in different temperature ranges where the enhancement occurs. It is considered that the phenomenon is associate with electron tunneling between sorts of grains as well as spin-dependent scattering.

Effects of highly conducting interface and particle size distribution on optical nonlinearity in granular composites

The optical nonlinearity has been investigated in granular metal/dielectric composites taking the effects of highly conducting interfaces between the constituent phases as well as the distribution of particle sizes into account. We compute analytically the spectral function for composites with a binary distribution of particle sizes. For a log-normal distribution of width σ, numerical results show that the spectral density m(s) changes from a delta function for zero width to a prominent peak, accompanied by a broad spectrum for a finite width σ. As a result, the locations of the nonlinearity enhancement peak and the absorption spectrum shift to small frequencies with the increase of the interfacial factor I. The strength of the absorption and the optical nonlinearity are always decreased near resonance with the increase of σ, while the absorption peak and optical nonlinearity peak shift to smaller volume fraction f as I is increased. Moreover, the variation of I will further increase the optical absorption (optical nonlinearity) by more than one (three) order of magnitude, depending on the volume fractions.

Response of thermites to dynamic high pressure and shear

The paper begins with a review of the literature on high pressure and shear properties of materials. This is an area in which further interest was recently stimulated by published Russian work on s...

Nonlinear response of partially resonant composites

Nonlinear resonance conditions for nonlinear cylinder or sphere granular composites with nonlinear cores or nonlinear shells are investigated. It is found that the nonlinear partially resonant conditions which are associated the linear part of nonlinear component with linear component cannot be realized. However, under the linear partially resonant conditions, the nonlinear dielectric response of the composite will still be greatly enhanced.

Theory of Short-Term Microdamageability of Granular Composite Materials

The theory of microdamageability of granular composites is outlined through the simulation of microdamages in the components by pores filled with compression-resisting particles of a destroyed material. The damage criterion for a microvolume of a component is taken in the Schleicher–Nadai form, which allows for the difference between the ultimate tensile and compressive loads. The ultimate strength is a random function of Weibull-distributed coordinates. The stress–strain state and the efficient properties of the material are determined from the stochastic equations of elastic theory for a granular composite with porous components. The equations of deformation and microdamage are closed by the equations of porosity balance in the components. Nonlinear diagrams of the concurrent processes of deformation in the granular material and microdamage in the matrix are plotted. The effect of the physical and geometrical parameters on them is studied

Double-Particle Approximation Analysis of the Residual Thermostressed State of Granular Composites

The local thermostressed state near spherical inclusions in SiC–TiB2 composite ceramics is analyzed. The boundary value problem that is rigorously solved corresponds to the double-particle approximation of the mechanics of composites. The calculated results are in qualitative agreement with the well-known experimental dependences of the microfracture zone near a microcrack on the volume concentration and average size of the particles of the dispersed phase