Photonic band structure and omnidirectional band gap in anisotropic superlattice

We investigate theoretically the photonic band structure of one-dimensional superlattices (SL) composed of alternating anisotropic layers with their principal axes oriented at arbitrary directions. The dispersion relation of second order is calculated analytically by using the 4×4 matrix method which is based on the boundary conditions of the electric and magnetic fields at each interface. It is shown that such structures can exhibit coupled electromagnetic modes between transverse magnetic TM and transverse electric TE modes, and dispersion curves that do not exist in superlattices composed only of isotropic layers. For a given value of the wave vector k (parallel to the layers), the dispersion curves (frequency ω) versus k_B (where k_B is the Bloch wave vector of the periodic system along the axis of the superlattice) are illustrated. Specific applications of these results are given for the case of the biaxial superlattice. We show that with an appropriate choice of the superlattice parameters an absolute (or omnidirectional) band gap for these coupled electromagnetic waves can be obtained. The band gap width depends on the anisotropic parameters of the media forming the SL.     

Numerical simulation of laser-induced transient temperature field in film-substrate system by finite element method

The transient temperature fields generated by a pulsed laser in film-substrate system are obtained by using the finite element method. Time integrations of the semi-discrete finite element equations are achieved by using approximate one order derivative of temperature. The temperature dependences of material properties are taken into account, which has a great influence on the temperature fields indicated by the numerical results. The pulsed laser-induced transient temperature fields in aluminum/methyl-methacrylate system and aluminum/copper system are obtained, which will be useful in the research on thermoelastic excitation of laser ultrasonic waves in film-substrate system.     

How hydrophobic side chain design affects water cluster connectivity in model polymer electrolyte membranes: Linear versus Y-shaped side chains

Dissipative Particle Dynamics is used to model water clustering in ion exchange membranes. We consider polymers with hydrophobic backbone alternatingly grafted with short hydrophilic ([C]) and hydrophobic linear (A₆) or Y-shaped side chains (ie. Ap [As][As]; p(s) = 6(0), 4(1), 2(2), 0(3)). At 16% water volume fraction water cluster connectivity (D_MC(W), water diffusivity, and inter water domain distance increase with length p and average number of bonds (<N_bond^A−C>) separating A and nearest C beads. Exchange of the pendent A and C beads for (A [C]A [A₆])₁₀ to form (A [A]A [A₅C])₁₀ decreases water cluster connectivity. This is because the lower number of bonds between nearest C beads for (A [C]A [A₆])₁₀ forces them to join the same water domain. This cooperative mechanism also explains deviations from the increase of D_MC(W) with <N_bond^A−C> for similar ion exchange capacity (IEC). At high IEC (A [A_x+1C])₁₀ architectures favour good connected pores which is surpassed by the H-phobic side chain (A [C]A [Ax])₁₀ architectures at low IEC.     

Thermoelastic wave propagation in a transversely isotropic thick plate under Green–Naghdi theory due to gravitational field

The aim of this article is to study the gravitational response in an infinite, homogeneous, transversely isotropic thick plate. The upper surface of the plate is free from traction, and the surface temperature distribution is imposed on it. The lower surface of the plate is kept on a rigid base and is thermally insulated. The heat conduction equations in the presence of heat source are of hyperbolic types among them (i) without energy dissipation (GN-II model) and (ii) with energy dissipation (GN-III model). The Laplace–Fourier double-transform technique has been developed to solve the nondimensional coupled field equations. The physical quantities are obtained in the space–time domain using a numerical inversion method. Numerical calculations are performed for a thick plate made of magnesium (Mg) metal and have been depicted graphically to estimate the effect of gravity. The results for the isotropic material (Cu) and that of the transversely isotropic material (Mg) have been compared for both the models.     

Novel sonochemical synthesis of Zn2V2O7 nanostructures for electrochemical hydrogen storage

Although utilization of diverse classes of metal oxides as hydrogen storage materials has been reported, but there is still a major need to introduce efficient materials. Herein, mesoporous Zn₂V₂O₇ nanostructures were produced by a new sonochemical method using hydrazine, zinc nitrate, and ammonium vanadate as the starting reagents and then annealed at 700 °C. Prior to annealing, Zn₃V₃O₈ was produced in the presence of ultrasonic waves, whereas in the absence of ultrasonic waves, Zn₂(VO₄)₂ was the major product. In fact, ultrasonic waves interfered with the reaction mechanism and reduced V⁵⁺ to V⁴⁺ and V³⁺. Because of the proper composition and structure of these nanostructures, they were used for electrochemical storage of hydrogen. Storage of over 2899 mAh/g after 20 cycles by flower-like nanostructures revealed their high capability. The results also showed that morphology affects efficiency such that three-dimensional spherical nanostructures had a storage capacity of 2247 mAh/g after 20 cycles.     

Forced convection heat transfer inside an anisotropic porous channel with oblique principal axes: Effect of viscous dissipation

An analytical study on laminar and fully developed forced convection heat transfer in a parallel-plate horizontal channel filled with an anisotropic permeability porous medium is performed. The principal axis of the anisotropic porous medium is oriented from 0 to 90 degrees. A constant heat flux is applied on the outer wall of the channel. Both clear (Newtonian) fluid and Darcy viscous dissipations are considered in the energy equation. Directional permeability ratio parameter A¹ is defined to combine both the effect of the dimensionless permeability ratio parameter K¹=(K₁/K₂) and orientation angle φ into one parameter. The effects of the parameter A¹, the Darcy number Da and the modified Brinkman number Br¹ on the heat transfer and fluid flow characteristics in the channels are investigated and presented in graphs. The obtained results show that the parameters A¹, Da and Br¹ have strong effects on the dimensionless normalized velocity and temperature profiles as well as on the Nusselt number. It is found that for a particular value of A¹, called as critical value A_cr¹, the external heat applied to the surface of the channel is balanced by the internal heat generation due to viscous dissipation and the bulk mean temperature approaches the wall temperature. Hence, the Nusselt number approaches infinity for the critical values A_cr¹.     

The gasification of wet biomass using Ca(OH)2 as CO2 absorbent: The microstructure of char and absorbent

Aimed at the hydrogen production by gasification of wet biomass coupled CO₂ absorption, the microstructures of chars produced from gasification of wet/pre-dried biomass and several possible stages of absorbent were studied with N₂ adsorption, XRD and SEM. The results show that all the chars examined have essentially microporous structures of pore size less than 2 nm, and the char obtained from gasification of wet biomass at 923 K exhibits larger surface area (A_BET) of 430.87 m² g⁻¹ and total pore volume (V_pore) of 0.188 m³ g⁻¹ than those (368.15 m² g⁻¹ and 0.157 m³ g⁻¹) from pre-dried biomass at the same temperature. The great change of microstructures in the absorbents was observed in the gasification process. The CaO generated from the dehydration of Ca(OH)₂ has larger A_BET and V_pore, than its former Ca(OH)₂, and the increase of pore mainly belongs to mesopores ranges (pore size between 2 nm and 50 nm). However, the generated CaO suffers from a jam of mesopores with the formation of CaCO₃ which will cause incomplete utilization. Although the burning of SR sample can realize CaO generation, the regenerated CaO with a totally deteriorated pore networks suggests poor reactivity. With the increase of temperature the CO₂ absorption by CaO is weakened gradually except in low temperature range of less than 973 K, meaning that the maximum temperature of the process should not exceed the decomposition temperature of CaCO₃.     

Optimal control of discrete and differential inclusions with distributed parameters in the gradient form

This paper is devoted to optimization of so-called first-order differential (P _C ) inclusions in the gradient form on a square domain. As a supplementary problem, discrete-approximation problem (P _A ) is considered. In the Euler–Lagrange form, necessary and sufficient conditions are derived for the problems (P _A ) and partial differential inclusions (P _C ), respectively. The results obtained are based on a new concept of locally adjoint mappings. The duality theorems are proved and duality relation is established.     

Ab initio calculation of NbH phases with low H compositions

Ab initio calculation shows that the BCC NbH_x (0 ≤ x ≤ 0.5) structure with H at tetrahedral(T) site is the most thermodynamically stable one among all the BCC, FCC, and HCP phases, and its negative heat of formation decreases linearly with the increase of H composition. Calculation also reveals that the elastic moduli of BCC(T) NbH_x phases all increase with the increase of H composition, and the BCC(T) NbH_x phases remain ductile within the studied composition range (0 ≤ x ≤ 0.5). Moreover, it is found that the percentage anisotropy in shear (A_G) and the universal anisotropic parameter (A^U) are all appropriate to describe the elastic anisotropy of NbH phases, and that H location should play an important role in elastic anisotropy. The fundamental mechanism of various properties is deeply understood by means of electronic structures, and the present results agree well with experimental investigations in the literature.     

Detection of axial crack in the bend region of a pipe by high frequency electromagnetic waves

High frequency electromagnetic guided waves are used to detect axial narrow slit or crack in bend-section of the U-shape pipe. In the previous studies performed by the authors, electromagnetic waves (EM-waves) of TM₀₁- and TE₁₁-modes have used to detect circumferential and axial crack in straight section of a pipe, appropriately. In this paper to show potential of this technique for detection of any shape of piping system, especially bend-section of the steam generator (SG) and feeder pipe of pressurized water reactor (PWR), axial crack in bend-section is investigated. Due to axial orientation of the crack, TE₁₁-mode, a suitable mode for detection of axial crack is implied for this purpose. To excite TE₁₁-mode in the test pipe, microwave signals generated by the network analyzer, are sent to the inspected pipe by a coaxial line and a mode converter. Presence of the crack causes change in the reflection coefficient of the reflected signals from the crack. The behavior of the crack characteristic signals as a function of time is studied to estimate time of flight of the reflected wave and consequently crack position. Suitable frequency range is chosen in order to generate only TE₁₁-mode in the test pipe. To show effect of frequency range of the electromagnetic waves to the crack, crack positions are also evaluated for several smaller frequency ranges. Two crack positions were examined and comparisons of measurement results with theoretical calculations indicate that the microwave guided waves technique has high capability to detect defect in bend-section of a piping systems.     

The electro-optical properties of amorphous indium tin oxide films prepared at room temperature by pulsed laser deposition

The electrical and optical properties of pulsed laser deposited amorphous indium tin oxide films at room temperature are discussed. The films were grown from indium oxide (In₂O₃) targets of different tin (Sn) doping content (0, 5 and 10 wt%) at different oxygen pressures (P_O2) ranging from 1×10⁻³ to 5×10⁻² Torr. The electrical and optical properties of the films were examined by Hall measurements and optical spectrophotometry. It was found that high conductivity amorphous films could be prepared at room temperature irrespective of the Sn doping content. The properties of these films deposited from 0, 5, 10 wt% Sn-doped In₂O₃ targets show a similar response to changes in P_O2. The maximal conductivity of (4.0, 2.1 and 1.8)×10³ S/cm and optical transmittance (visible) higher than 90% were obtained at P_O2 region of (1–1.5)×10⁻² Torr. An undoped In₂O₃ film produced the highest conductivity of 4×10³ S/cm in these studies.     

Stress-Focusing Effect Following Dynamically Transforming Anisotropic Strains in a Spherical Zirconia Inclusion

When an infinite elastic medium with a spherical inclusion of zirconia is suddenly subjected to an instantaneous transversely anisotropic phase transformation caused by impact cooling, stress waves occur at the surface of spherical inclusion the moment instantaneous transformation strains are applied. The stress waves in an inclusion proceeds radially inward to the center of the inclusion and show the stress-focusing effects. This paper analyzes the stress-focusing effects caused by the transversely isotropic phase transformation in the spherical inclusion of zirconia embedded in the infinite domain. By using the ray theory, the numerical results give a clear indication of the mechanism of stress-focusing effects caused by the phase transformation.     

Pulsed laser deposition of W–V–O composite films: Preparation, characterization and gasochromic studies

Modified tungsten oxide films by vanadium oxide provide neutrally coloring electrochromic electrodes for smart windows technology. In this study W–V–O mixed oxide films were fabricated by Nd:YAG pulsed laser deposition (PLD), λ=1064 nm, from mixed pressed powders of (WO₃)_1−x(V₂O₅)_x, x=0, 0.09, 0.17, 0.23, 0.29 and 0.33, at 13.3 Pa oxygen partial pressure and 200 °C temperature on glass substrates. X-ray photoelectron spectroscopy (XPS) revealed V⁵⁺, V⁴⁺, W⁶⁺ and W⁵⁺ surface oxide states, where the ratio of W⁵⁺/W⁶⁺ enhances by the amount of vanadium in the films. Surface morphology was studied by scanning electron microscope (SEM) and optical properties by transmission-reflection spectra. Results showed that films with a low amount of vanadium oxide have better porosity and higher optical band gaps. The gasochromic response to hydrogen gas exposure was found better for x=0.09 in the sense of both deeper and faster coloring. Weak responses of samples with more vanadium oxide were attributed to higher amounts of W⁵⁺ in the films and also to lower porosity.     

An experimental study of heat transfer in oscillating flow through a channel filled with an aluminum foam

An experimental study has been conducted on the heat transfer of oscillating flow through a channel filled with aluminum foam subjected to a constant wall heat flux. The surface temperature distribution on the wall, velocity of flow through porous channel and pressure drop across the test section were measured. The characteristics of pressure drop, the effects of the dimensionless amplitude of displacement and dimensionless frequency of oscillating flow on heat transfer in porous channel were analyzed. The results revealed that the heat transfer in oscillating flow is significantly enhanced by employing porous media in a plate channel. The cycle-averaged local Nusselt number increases with both the kinetic Reynolds number Re_ω and the dimensionless amplitude of flow displacement A₀. The length-averaged Nusselt number is effectively increased by increasing the kinetic Reynolds number from 178 to 874 for A₀ = 3.1-4.1. Based on the experimental data, a correlation equation of the length-averaged Nusselt number with the dimensionless parameters of Re_ω and A₀ is obtained for a porous channel with L/D_h = 3.     

Generalized thermo-elastodynamics for semiconductor material subject to ultrafast laser heating. Part II: Near-field response and damage evaluation

Thermal waves and near-field responses including the axial and radial displacements and stresses are investigated in a time window as wide as 10 ns using the generalized thermo-elastodynamic model of axisymmetric geometry presented in Part I. Ultrashort laser-induced thermal waves are found to be fast-attenuating; while transverse thermal stress waves are dispersive and characteristically of broadband and extremely high frequency. In addition, near-field responses and wave dispersion described by the presented model formulation are characteristically different from those modeled using parabolic transport equations for the same laser input parameters. The methodology of accumulated damage evaluation (Oh et al., 2008 [11]) making use of high cycle fatigue and time-frequency analysis is utilized to conclude that thermal stress waves induced by 500 fs ultrafast laser pulses of 0.775 μm in wavelength, 0.005 J/cm² in fluence and 10 μm in spot size are insufficient to initiate fatigue cracking in the silicon thin section considered in the study.     

Al-doped vanadium dioxide thin films deposited by PLD

Thin films of vanadium dioxide (VO₂) and Al³⁺-doped VO₂ were deposited on silicon and glass substrates using pulsed laser deposition (PLD). Optimized processing conditions were determined for depositing pure VO₂ with monoclinic phase by laser ablation of a V₂O₅ target. Al³⁺-doping levels in the VO₂ films were varied by altering the relative laser ablation time on the Al₂O₃ and V₂O₅ targets. The change in electrical conductivity with temperature in the semiconductor to metallic phase transition was measured for pure VO₂ and Al³⁺-doped VO₂ films. Doping the VO₂ films with Al³⁺ lowered the transition temperature directly on increasing the Al³⁺ content from 67 °C for the pure VO₂ films to 40 °C at 10% Al³⁺. The magnitude of the resistance change from semiconductor to metallic states also decreased with increase in Al³⁺ doping. The results imply that Al³⁺-doped VO₂ films could be a good candidate for energy-efficient “smart window” coatings used for architecture applications.     

ON THE PROPAGATION OF THERMOELASTIC WAVES IN MEDIA WITH LONG-RANGE COHESIVE FORCES

Abstract

After establishing the fundamental equations of nonlocal coupled thermoelasticity in Fourier space, nonlocal longitudinal thermoelastic waves in an infinite space are analyzed. Identification of the elastic dispersion equation obtained with its counterpart derived in lattice dynamics leads to values of the nonlocal elastic moduli. Similar identification of the velocity of thermal waves with its counterpart in second sound theory yields the values of the nonlocal thermal moduli. A numerical example involving solid helium is given.     

Kinetics of photocurrent generation and quantum efficiency of a dye sensitized platinized solid state photovoltaic cell made from p-type CuCNS and n-SnO2 transparent film on glass

The mechanism of photocurrent enhancement of a platinized dye sensitized solid state photovoltaic cell based on p-CuCNS and SnO₂ coated transparent glass is discussed. The variation of photocurrent quantum efficiency (φ) with dye concentration (D₀) of the device is presented for both platinized and unplatinized systems. According to the experimental results the parameters A and B in the model equation φ=AD₀ − BD₀² are calculated. It is found that parameter B is decreased and parameter A is increased for platinized systems compared with unplatinized systems. Photocurrent enhancement due to platinization is further discussed with the photophysical processes and charge transfer processes associated with the device.     

Sonochemical synthesis of CeVO4 nanoparticles for electrochemical hydrogen storage

In this study, the synthesis of cerium vanadate (CeVO₄) nanoparticles using ammonium metavanadate, cerium (III) nitrate hexahydrate as the primary reactant and hydrazine as the source of OH^? was presented in the absence and presence of ultrasonic waves. Reaction control was performed using OH^? and ethylenediamine sources. Other parameters such as solvent, surfactant, power, and time were also examined. Nanostructures were analyzed by XRD, FESEM, FTIR, DLS, BET, and EDS. FESEM results showed that using ultrasonic irradiation, relatively fine spherical nanoparticles were formed in one step while uniform spherical nanostructures were formed in a two-step path. The obtained product was used for electrochemical storage of hydrogen. The discharge capacity of spherical nanoparticles of CeVO₄ with high uniformity was recorded at about 4299 mAh/g.     

Heat transfer augmentation by swirl generators inserted into a tube with constant heat flux

A novel conical injector type swirl generator (CITSG) is devised in this study. Performances of heat transfer and pressure drop in a pipe with the CITSG are experimentally examined for the CITSGs' angle (α) of 30°, 45° and 60° in Reynolds number (Re) range of 10,000–35,000. Moreover, circular holes with different numbers (N) and cross-section areas (A_h) are drilled on the CITSG. In this way, total areas (A_t = N · A_h) of the holes on the CITSG are equaled each other. Besides, flow directors having three different angles (β = 30°, 60° and 90°) to radial direction are attached to every one of the holes. This study is a typical example for decaying flow. All experiments were conduced with air accordingly; Prandtl number was approximately fixed at 0.71. The local Nusselt number (Nu_x), heat transfer enhancement ratio (Nu_ER) and heat transfer performance ratio (Nu_PR) are calculated and discussed in this paper. It is found that the Nu_ER decreases with increase in Reynolds number, the director angle (β), the director diameter (d), and with decrease in the CITSG angle (α). Likewise, variation of Nu_PR and Nu_ER is also essentially similar for the same independent parameters.