Near-infrared radiative properties of porous zirconia ceramics

Infrared radiative properties of zirconia ceramics of porosity about 16% are studied by means of the measurements of directional–hemispherical reflectance and transmittance in the wavelength range from 2.5 to 9 μm. The recently suggested modified two-flux approximation is examined as a simplified basis of the identification procedure. A comparison with the exact numerical solution confirms a good accuracy of this approach for identification of the absorption coefficient of ceramics. An analysis of the results for transport scattering coefficient showed that scattering is determined by isotropic pores with characteristic average radius about 1 μm. The corresponding approximate theoretical model of radiative properties of ceramics is suggested. The absorption coefficient of bulk zirconia in the semi-transparency range is obtained from the data for porous zirconia ceramics.     

A study on electromagnetic wave absorber for the collision-avoidance radar

In this paper, we designed and fabricated the EM wave absorber for eliminating false image and system-to-system interference in a collision-avoidance radar using Permalloy and CPE, the mixing ratio of which was Permalloy:CPE = 70:30 wt%. The EM wave absorption ability was simulated according to different thicknesses of the EM wave absorber using measured permittivity and permeability, and then the EM wave absorber was fabricated based on the simulated design. The simulated results agree well with the measured ones. As a result, we suggest Permalloy for use as a new EM wave absorber in W-band, and the fabricated EM wave absorber with the thickness of 1.4 mm has absorption ability more than 20 dB in frequency range of 76–77 GHz.     

Coupling of Dirichlet-to-Neumann boundary condition and finite difference methods in curvilinear coordinates for multiple scattering

The applicability of the Dirichlet-to-Neumann technique coupled with finite difference methods is enhanced by extending it to multiple scattering from obstacles of arbitrary shape. The original boundary value problem (BVP) for the multiple scattering problem is reformulated as an interface BVP. A heterogenous medium with variable physical properties in the vicinity of the obstacles is considered. A rigorous proof of the equivalence between these two problems for smooth interfaces in two and three dimensions for any finite number of obstacles is given. The problem is written in terms of generalized curvilinear coordinates inside the computational region. Then, novel elliptic grids conforming to complex geometrical configurations of several two-dimensional obstacles are constructed and approximations of the scattered field supported by them are obtained. The numerical method developed is validated by comparing the approximate and exact far-field patterns for the scattering from two circular obstacles. In this case, for a second order finite difference scheme, a second order convergence of the numerical solution to the exact solution is easily verified.     

Dielectric relaxation of ZnO nanostructure synthesized by soft chemical method

Thioglycerol capped nanoparticles of ZnO have been prepared in methanol through chemical technique. Nanostructures of the prepared ZnO particles have been confirmed through X-ray diffraction measurement. The Debye–Scherrer formula is used to obtain the particle size. The average size of the prepared ZnO nanoparticles is found to be 50 nm. The frequency-dependent dielectric dispersion of the sample is investigated in the temperature range from 293 to 383 K and in a frequency range from 100 Hz to 1 MHz by impedance spectroscopy. An analysis of the complex permittivity (ε′ and ε′′) and loss tangent (tan δ) with frequency is performed assuming a distribution of relaxation times. The frequency-dependent maxima of the imaginary part of impedance are found to obey Arrhenius law with activation energy ∼1 eV. The scaling behavior of dielectric loss spectra suggests that the relaxation describes the same mechanism at various temperatures. The frequency-dependent electrical data are analyzed in the framework of conductivity and modulus formalisms. The frequency-dependent conductivity spectra obey the power law.     

Fast atom diffraction at metal surface

Fast atoms with energies from 300 eV up to 1.7 keV are scattered under a grazing angle of incidence from a clean and flat Ni(1 1 0) surface. For scattering under ”axial surface channeling” conditions, we observe – as reported recently for insulator and semiconductor surfaces – defined diffraction patterns in the angular intensity distributions for scattered fast ³He and ⁴He atoms. We have investigated the domain of scattering conditions where decoherence phenomena are sufficiently small in order to observe for metal targets quantum scattering of fast atomic projectiles. As a consequence, fast atom diffraction appears to be a general technique with a wide range of applicability in surface science.     

Improved Analytical Approximations to the Scattering Solutions of the Schrdinger Equation with a Hyperbolical Potential

Using an improved approximate formula to the centrifugal term, we present arbitrary l-state scattering solutions of the hyperbolic potential. The approximate analytical formula of scattering phase shifts and normalized wavefunctions are presented. All data calculated by the above approximate analytical formula are compared with those obtained by using the numerical integration method in the scattering state cases. We find that this improved approximate formula is better than previous one since the calculated results are in good agreement with those exact ones.     

Improved Analytical Approximations to the Scattering Solutions of the Schrödinger Equation with a Hyperbolical Potential

Using an improved approximate formula to the centrifugal term, we present arbitrary l-state scattering solutions of the hyperbolic potential. The approximate analytical formula of scattering phase shifts and normalized wavefunctions are presented. All data calculated by the above approximate analytical formula are compared with those obtained by using the numerical integration method in the scattering state cases. We find that this improved approximate formula is better than previous one since the calculated results are in good agreement with those exact ones.     

Improvements to the Johnson–Allard model for rigid-framed fibrous materials

Measurements of the surface impedance and the physical parameters of seven glass wool samples and six polyester fibre samples with flow resistivities between 4100 and 69,900 Pa s m⁻² have been made. Comparisons of measured absorption coefficients and those predicted from the Johnson–Allard formulae using the measured and deduced physical parameters show discrepancies that exceed 20% for some samples and frequencies. By modifying the Johnson–Allard formula for effective density and by introducing a correction factor that is a function of flow resistivity based on data fitting, it has been found possible to improve the predictions. However, it has also been found that a similar modification of the formula for bulk modulus is necessary to reduce the discrepancies with data to below 5% in the frequency range between 800 Hz and 5 kHz.     

浅海波导中目标散射的边界元方法

提出了一种计算三维散射体在声速剖面随深度变化、距离无关浅海波导中散射声场的数值方法波导边界元方法。当散射体不十分靠近波导界面因而边界多次散射可以忽略时,在边界元计算中可以用自由场格林函数近似波导格林函数。应用镜像法和球波函数加法定理推导了理想波导中球体散射声场的解析解,用来验证波导边界元方法的计算精度,证明该数值方法是准确的。对浅海波导中水下潜器散射声场数值模拟的结果表明,浅海波导海面、海底界面反射、声速剖面等对目标散射声场的幅值和方向性都有很大的影响。      

Charged surface states scattering in AlGaN/GaN heterostructures

The electron mobility limited by charged surface states scattering was calculated accurately based on a self-consistent Schrödinger and Poisson equations solver. For a AlGaN thickness less than 5 nm, this new scattering mechanism is the dominated scattering for electron mobility. Moreover, there is a “high electron mobility window” for a certain AlGaN thickness range from 4 to 7 nm, where the electron mobility can be as high as 3000 cm²/Vs at a low temperature, which is consistent with reported experimental data [1] (Cao and Jena, 2007).     

Exact solution of the Dirac equation with the Mie-type potential under the pseudospin and spin symmetry limit

We investigate the exact solution of the Dirac equation for the Mie-type potentials under the conditions of pseudospin and spin symmetry limits. The bound state energy equations and the corresponding two-component spinor wave functions of the Dirac particles for the Mie-type potentials with pseudospin and spin symmetry are obtained. We use the asymptotic iteration method in the calculations. Closed forms of the energy eigenvalues are obtained for any spin-orbit coupling term κ. We also investigate the energy eigenvalues of the Dirac particles for the well-known Kratzer-Fues and modified Kratzer potentials which are Mie-type potentials.     

Error analysis of the absorption coefficient calculated by the materials' transmissivities and refractive indices

In this paper, a practical method for obtaining the exact absorption coefficient of optical media was presented, which can be applied to amend the error attributed to multi-reflections. According to the materials' transmissivities and refractive indices, a formula was derived, which can be used for calculating the absorption coefficient. The feasible region with the relative error lower than 10% was represented by a mathematical expression. The calculated formula was confirmed to be applicable to optical materials with strong-absorbent (>0.1 cm^?1), especially laser or Raman media. This study can provide a useful approach for getting exact absorption coefficients of new optical materials and other derivative parameters.     

Experimental and theoretical study of AC electrical conduction mechanisms of Organic–inorganic hybrid compound Bis (4-acetylanilinium) tetrachlorocadmiate (II)

A new organic–inorganic bis (4-acetylaniline) tetrachlorocadmate [C₈H₁₀NO]₂[CdCl₄] can be obtained by slow evaporation at room temperature and characterized by X-ray powder diffraction. It crystallized in an orthorhombic system (Cmca space group). The material electrical properties were characterized by impedance spectroscopy technique in the frequency range from 209 Hz–5 MHz and temperature 413 to 460 K. Besides, the impedance plots show semicircle arcs at different temperatures and an electrical equivalent circuit has been proposed to interpret the impedance results. The circuits consist of the parallel combination of a resistance (R), capacitance (C) and fractal capacitance (CPE). The variation of the exponent s as a function of temperature suggested that the conduction mechanism in Bis (4-acetylanilinium) tetrachlorocadmiate compound is governed by two processes which can be ascribed to a hopping transport mechanism: correlated barrier hopping (CBH) model below 443 K and the small polaron tunneling (SPT) model above 443 K.     

Strong suppression of Coulomb corrections to the <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> pair production cross section in ultrarelativistic nuclear collisions

The semiclassical correction to Molière’s formula for multiple scattering is derived. The consideration is based on the scattering amplitude obtained with the first semiclassical correction taken into account for an arbitrary localized but not spherically symmetric potential. Unlike the leading term, the correction to Molière’s formula contains the target density n and thickness L not only in the combination nL (areal density). Therefore, this correction can be referred to as the bulk density correction. It turns out that the bulk density correction is small even for high density. This result explains the wide range of applicability of Molière’s formula. The article is published in the original.     

Correction to Molière’s formula for multiple scattering

The semiclassical correction to Molière’s formula for multiple scattering is derived. The consideration is based on the scattering amplitude obtained with the first semiclassical correction taken into account for an arbitrary localized but not spherically symmetric potential. Unlike the leading term, the correction to Molière’s formula contains the target density n and thickness L not only in the combination nL (areal density). Therefore, this correction can be referred to as the bulk density correction. It turns out that the bulk density correction is small even for high density. This result explains the wide range of applicability of Molière’s formula.     

Maximum length sequence and Bessel diffusers using active technologies

Active technologies can enable room acoustic diffusers to operate over a wider bandwidth than passive devices, by extending the bass response. Active impedance control can be used to generate surface impedance distributions which cause wavefront dispersion, as opposed to the more normal absorptive or pressure-cancelling target functions. This paper details the development of two new types of active diffusers which are difficult, if not impossible, to make as passive wide-band structures. The first type is a maximum length sequence diffuser where the well depths are designed to be frequency dependent to avoid the critical frequencies present in the passive device, and so achieve performance over a finite-bandwidth. The second is a Bessel diffuser, which exploits concepts developed for transducer arrays to form a hybrid absorber–diffuser. Details of the designs are given, and measurements of scattering and impedance used to show that the active diffusers are operating correctly over a bandwidth of about 100 Hz to 1.1 kHz. Boundary element method simulation is used to show how more application-realistic arrays of these devices would behave.     

Exact Spin and Pseudospin Symmetry Solutions of the Dirac Equation for Mie-Type Potential Including a Coulomb-like Tensor Potential

We solve the Dirac equation for Mie-type potential including a Coulomb-like tensor potential under spin and pseudospin symmetry limits with arbitrary spin–orbit coupling quantum number κ. The Nikiforov–Uvarov method is used to obtain analytical solutions of the Dirac equation. Since it is only the wave functions which are obtained in a closed exact form; as for the eigenvalues, only the eigenvalue equations have been given and they have been solved numerically. It is also shown that the degeneracy between spin doublets and pseudospin doublets is removed by tensor interaction.     

Multifrequency super-thin cloaks

Bandwidth and thickness have become the most troublesome problems for EM cloaks. In this paper, we propose to solve the two problems using connected patches based on the microwave network model. By covering an obstacle with combined connected patches, cloaking effect can be achieved at multiple frequencies so as to expand the operating band. As an explicit example, a dual-band super-thin cloak using two different connected patch unit cells is demonstrated. Cloaking effect can be achieved at 3.50 GHz and 4.14 GHz simultaneously with an 8 dB transmission enhancement. The cloak design method provides a new route to broadening the bandwidth of thin EM cloaks.