The effective absorption cross-section of thermal neutrons in a medium containing strongly or weakly absorbing centres

The structure of a heterogeneous system influences diffusion of thermal neutrons. The thermal-neutron absorption in grained media is considered in the paper. A simple theory is presented for a two-component medium treated as grains embedded in the matrix or as a system built of two types of grains (of strongly differing absorption cross-sections). A grain parameter is defined as the ratio of the effective macroscopic absorption cross-section of the heterogeneous medium to the absorption cross-section of the corresponding homogeneous medium (consisting of the same components in the same proportions). The grain parameter depends on the ratio of the absorption cross-sections and contributions of the components and on the size of grains. The theoretical approach has been verified in experiments on prepared dedicated models which have kept required geometrical and physical conditions (silver grains distributed regularly in Plexiglas). The effective absorption cross-sections have been measured and compared with the results of calculations. A very good agreement has been observed. In certain cases the differences between the absorption in the heterogeneous and homogeneous media are very significant. A validity of an extension of the theoretical model on natural, two-component, heterogeneous mixtures has been tested experimentally. Aqueous solutions of boric acid have been used as the strongly absorbing component. Fine- and coarse-grained pure silicon has been used as the second component with well-defined thermal-neutron parameters. Small and large grains of diabase have been used as the second natural component. The theoretical predictions have been confirmed in these experiments.     

Parametric amplification of ultrasound in ferromagnetic piezoelectric composites at the vicinity of the orientational phase transition

Direct parametrical amplification of ultrasound was theoretically considered for the ferromagnetic/piezoelectric thin-layered structure composites, for which within the long-wavelength approach some, material specific, yet homogeneous bulk properties could be assumed. Having chosen appropriate homogeneous components, it is possible to construct a desirable composite with such properties that are absent in the starting-up “building blocks” (components).Sound frequencies are assumed to be such that only the dispersion due to magnetic subsystem is thought relevant; the piezoelectric components are set to have a nonlinear dielectric response. Presence of orientational phase transition for the ferromagnetic subsystem allows to vary frequency of a ferromagnetic resonance, and consequently to alter the dispersion at the chosen frequency of sound.While solving coupled equations describing dynamics of mechanical, magnetic and piezoelectric subsystems, an effective sound amplification for realistic values of physical parameters is shown possible.     

Perturbation Expansion Method and Effective Medium Approximation for Effective Conductivity of Nonlinear Composite Media

In this paper, we establish an effective medium approximation (EMA) for effective conductivity of nonlinear composite media. As an example, we consider a two-dimensional composite medium with a cylindrical inclusion embedded in a homogeneous host, both the host and the inclusion having nonlinear current-voltage constitutive relations, and apply the perturbation expansion method to derive its analytic series solution. Using the nonlinear EMA we derive the formulae of the first, the third and the fifth-order effective conductivities, which are valid for nonlinear composite media with middle concentration of inclusion.     

Comprehensive T-matrix reference database: A 2007-2009 update

The T-matrix method is among the most versatile, efficient, and widely used theoretical techniques for the numerically exact computation of electromagnetic scattering by homogeneous and composite particles, clusters of particles, discrete random media, and particles in the vicinity of an interface separating two half-spaces with different refractive indices. This paper presents an update to the comprehensive database of T-matrix publications compiled by us previously and includes the publications that appeared since 2007. It also lists several earlier publications not included in the original database.     

Temperature profiles in a two-component heterogeneous system heated with a cw laser

A formalism has been developed to calculate the steady-state temperature profiles induced by a cw laser in a composite system comprising a thick medium covered with a film of another medium. The analysis is carried out assuming that thermal and optical properties of the media are temperature-independent. A concept of the effective thermal conductivity for the system is introduced in terms of the maximum temperature rise. Temperature-distributions are numerically computed in typical cases for a circular Gaussian beam to illustrate dependence on film thickness, thermal conductivities and absorption coefficients of the media and size of the laser beam.     

Handedness of magnetic-dipolar modes in ferrite disks

For magnetic-dipolar modes in a ferrite, components of the magnetic flux density in a helical coordinate system are dependent on both an orientation of a gyration vector and a sign of a pitch. It gives four types of helical harmonics for magnetostatic-potential wave functions in a ferrite disk. Because of the reflection symmetry breaking, coupling between certain types of helical harmonics takes place in the reflection points. The reflection feature leads to exhibition of two types of resonances: the “right” and “left” resonances. These resonances become coupled for a ferrite disk placed in a homogeneous tangential RF magnetic field. One also observes such resonance coupling for a ferrite disk with a symmetrically oriented linear surface electrode, when this ferrite particle is placed in a homogeneous tangential RF electric field. In a cylindrical coordinate system handedness of magnetic-dipolar modes in a ferrite disk is described by spinor wave functions.     

Energy characteristics of radiation of oscillating dipoles moving in a nondispersive medium and a cold plasma

We consider radiation of moving oscillating electric and magnetic dipoles, whose moments are oriented along their velocity. We have derived general expressions for field components and radiation power, which are valid for isotropic homogeneous nonabsorbing media. Special cases of a nondispersive medium and a cold plasma are considered. In these cases, the dependences of energy spectral distributions and radiation powers are analyzed as a function of the velocity of the sources and the parameters of media.     

Effect of thermal stresses on the phase transition temperature in a ferroelectric-dielectric nanocomposite

The mechanism of an increase in the phase transition temperature of a ferroelectric-dielectric nanocomposite above the corresponding temperature of an unbounded homogeneous ferroelectric has been revealed. The significant thermal stresses induced in the composite material due to the difference in the thermal and elastic characteristics of its components affect the polarized state of the ferroelectric component via the electrostrictive coupling and can lead to an increase in the phase transition temperature of the composite.     

Functional multi-walled carbon nanotube/polyaniline composite films as supports of platinum for formic acid electrooxidation

Embedding of carbon nanotubes in conducting polymeric matrices for various nanocomposites material is now a popular area. In this article, a concise chemical method has been described for the preparation of homogeneous nanocomposite of multi-walled carbon nanotube (MWNT)/polyaniline (PANI) by electrochemical codeposition. For this we functionalized the MWNTs via the diazotization reaction. This helped to disperse the nanotubes in aniline. The composite films were dispersed Pt by electrodeposition technique. The presence of MWNTs and platinum in the composite films was confirmed by XRD analysis and transmission electron microscopy (TEM). Four-point probe investigations revealed that the MWNT/PANI composite films exhibited a good conductivity. Cyclic voltammograms (CV) showed that Pt-modified MWNT/PANI composite films perform higher electrocatalytic activity and better long-term stability than Pt-modified pure PANI film toward formic acid oxidation. The results imply that the MWNT/PANI composite films as a promising support material improves the electrocatalytic activity for formic acid oxidation greatly.     

Effective thermal conduction in composite materials

The problem of determining the bounds and/or estimating the effective thermal conductivity (λ _eff) of a composite (multiphase) system given the volume fractions and the conductivities of the components has been investigated. A comparison between the measured data and the results predicted by theoretical models has been made for seven heterogeneous samples. The tested models include those of the effective medium theory (EMT), Hashin and Shtrikman (HS) bounds, and Wiener bounds. These models can be used to characterize macroscopic homogeneous and isotropic multiphase composite materials either by determining the bounds for the effective thermal conductivity and/or by estimating the overall conductivity of the random mixture. It turns out that the most suitable one of these models to estimate λ _eff is the EMT model. This model is a mathematical model based on the homogeneity condition which satisfies the existence of a statistically homogeneous medium that encloses inclusions of different phases. Numerical values of thermal conductivity for the samples that satisfy the homogeneity condition imposed by the effective medium theory are in best agreement with the experimentally measured ones.     

The Effective AC Response of Nonlinear Composites

A perturbative approach is used to study the AC response of nonlinear compostie media,which obey a current-field relation of the form J=σE χ|E|^2E with comp[onents having nonlinear response at finite frequencies.For a sinusoidal applied field,we extend the local potential in terms of sinusoidal components at fundamental frequency and high-order harmonic frequencies to treat the nonlinear comosites.For nonlinear composite media with a low concentrations of spherical inclusions,we give the formulae of the nonlinear effective AC susceptibility χ3ω at the third harmonic frequency.     

Binary metal and semiconductor quantum dot metamaterials with negative optical dielectric constant and compensated loss for small volume waveguides,modulators and switches

We study numerically and analytically a binary mixture of quantum dots exhibiting gain and loss. For a mixture of gain quantum dots and silver nanoparticles, we find conditions when the composite shows negative dielectric constant operation and lossless operation. The composites of this kind may be used for dense integration of photonic components as well as modulation and switching in optical interconnect systems L. Thylen is also at Dept of Microelectronics and Applied Physics, Royal Institute of Technology (KTH), 164 40 Kista, Sweden.     

Modeling and Computing Example for Effective Electromagnetic Parameters of Multiphase Composite Media

A method using strong fluctuation theory （SFT） to compute the effective electromagnetic parameters of multiphase composite media, and common materials used to design radar-absorbing materials, is demonstrated. The effective electromagnetic parameters of ultrafine carbonyl-iron （DT-50） and fiber fabric, which are both multiphase composite media and represent coated and structured radar absorbing materials, respectively, are investigated, and the corresponding equations of electromagnetic parameters by using the SFT are attained. Moreover, we design a program to simplify the solutions, and the results are discussed.     

Time-resolved laser optoacoustic tomography of inhomogeneous media

The methods of time-resolved laser optoacoustic tomography of inhomogeneous media and related problems are reviewed. Time-resolved laser optoacoustic tomography allows one to measure the distribution of light absorption in turbid media with depth resolution up to several microns in real time. The theory of laser excitation of acoustic waves by absorbing of light in particles, dispersed in transparent, light-absorbing or scattering media, is developed. The distribution of light absorption can be obtained from the temporal course of acoustic pressure. Two schemes of acoustic wave detection — in the medium under testing (direct detection) and in transparent medium, coupled to the investigated one (indirect detection) — are discussed. In both cases the reconstruction of light absorption can be made by simple calculations. Test experiments with homogeneous and layered media confirm the proposed theoretical models and the possibility of using the proposed experimental schemes. Light absorption in homogeneous, inhomogeneous media and in absorbing particles dispersed in turbid media was investigated. The experimental setup allows one to measure the absorption coefficients over the range 1-500 cm^–1 with the depth resolution 10–15 m over the depth 1–1.5 mm.     

Minimum entropy deconvolution of pulse-echo signals acquired from attenuative layered media.

In this article deconvolution of ultrasonic pulse-echo data acquired from attenuative layered media is considered. The problem is divided in two subproblems: treating the sparse reflection sequence caused by the layered structure of the media and treating the frequency-dependent attenuation. The first subproblem is solved by means of joint maximum a posteriori estimation of the assumed zero mean, white, nonstationary reflection sequence and its corresponding sequence of unknown standard deviations. This approach leads to an algorithm that seeks minimum entropy solutions for the reflection sequence and therefore the algorithm serves as a novel link between the classical Wiener filter and methods for sparse or minimum entropy deconvolution. The second subproblem is solved by introducing a new signal processing-oriented, linear discrete-time model for frequency-dependent attenuation in isotropic and homogeneous media. The deconvolution algorithm is tested using simulated data and its performance for real normal incidence pulse-echo data from a composite material is also demonstrated. The results show that the algorithm, in combination with the attenuation model, yields estimates that reveal the internal structure of the composite and, thus, simplify the interpretation of the ultrasonic data.     

Ferroelectric and phase transition studies in cesium nitrate: poly(vinyl?alcohol)?composite films

The composite films of cesium nitrate (CsNO₃) and poly(vinyl alcohol) (PVA) with varying composition were prepared using the solvent cast method. The hysteresis loop characteristics show optimum remnant polarization (P _r ) of 2.75 μC/cm² at 50 wt.% composition. The field emission scanning electron microscope images show a nearly homogeneous distribution of CsNO₃ grains in the 50 wt.% composite film. The temperature dependence of the remnant polarization shows a diffused transition temperature range from the ferroelectric to the paraelectric phase and this has been attributed to the reduced enthalpy. The butterfly features of the dielectric constant–voltage (ε–V) characteristics have been attributed to polarization switching.     

Thermal Conductivity of Periodic Composite Media with Spherical Inclusions

The thermal conductivity of periodic composite media with spherical inclusions embedded in a homogeneous matrix is discussed. Using Green's function, we show that the Rayleigh identity can be generalized to deal with the thermal properties of these systems. A technique for calculating effective thermal conductivities is proposed. Systems with cubic symmetries (including simple cubic, body centered cubic and face centered cubic symmetry) are investigpted in detail, and useful formulae for evaluating effective thermal conductivities are derived.     

Carbon nanotubes as reinforcement of styrene-butadiene rubber

This study reports an easy technique to produce cured styrene-butadiene rubber (SBR)/multi-walled carbon nanotubes (MWCNT) composites with a sulphur/accelerator system at 150 °C. Significant improvement in Young's modulus and tensile strength were achieved by incorporating 0.66 wt% of filler without sacrificing SBR elastomer high elongation at break. A comparison with carbon black filled SBR was also made. Field emission scanning electron microscopy was used to investigate dispersion and fracture surfaces. Results indicated that the homogeneous dispersion of MWCNT throughout SBR matrix and strong interfacial adhesion between oxidized MWCNT and the matrix are responsible for the considerable enhancement of mechanical properties of the composite.     

非连续反馈控制激发介质中的螺旋波

采用非连续反馈方法来控制Fitz-Hugh-Nagumo方程描述的激发介质中的螺旋波. 在控制过程中,对于系统各个格点快变量的幅值进行观测并和设定的阈值进行比较,当采样格点的快变量的值大于这个阈值时,则对系统进行直接小幅度的负反馈. 研究发现：在对系统所有格点快变量幅值观测时选择比较小的阈值则更容易将系统的螺旋波消除掉并使系统达到稳定均匀态. 在比较大的阈值下,系统的螺旋波则变得稀疏,也可以导致螺旋波的破裂. 在任意选择单个格点的快变量观测下,比较小的反馈强度仍然可以消除螺旋波,系统也达到稳定均匀态. 当 关键词： 螺旋波 Fitz-Hugh-Nagumo方程 反馈控制