Free damped vibrations of a viscoelastic oscillator based on Rabotnov’s model

Free damped vibrations of a linear viscoelastic oscillator based on Rabotnov’s model involving one fractional parameter and several relaxation (retardation) times are investigated. The analytical solution is obtained in the form of two terms, one of which governs the drift of the system’s equilibrium position and is defined by the quasi-static processes of creep occurring in the system, and the other term describes damped vibrations around the equilibrium position and is determined by the systems’s inertia and energy dissipation. The drift is governed by an improper integral taken along two sides of the cut of the complex plane. Damped vibrations are determined by two complex conjugate roots of the characteristic equation, which are located in the left half-plane of the complex plane. The behaviour of the characteristic equation roots as function of the system’s parameters is shown in the complex plane. Dedicated to the bright memory of Academician Yury N. Rabotnov.     

Facile loading of metal ions in the nanopores of polymer thin films and in situ generation of metal sulfide nanoparticle arrays

Two facile strategies for loading metal ions in the nanopores of polystyrene-b-polyisoprene (PS-b-PI) polymeric thin films have been developed. In the first approach, through the controlled epoxidation of the polyisoprene (PI) component of the template, the hydrophilicity of the interior wall of the nanopores was increased, and the penetration of metal salt solutions into the nanopores was dramatically enhanced. However, thin film damage was observed sometimes during the PI epoxidation. Using a 'fully wetted' method, large capillary forces were suppressed, and the metal ions were easily introduced into the nanopores. The validation of the methods was illustrated for the generation of large-area and high-density CdS and ZnS nanoparticle arrays in nanoporous PS-b-PI polymeric thin films, which were characterized using atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), electron diffraction (ED), and UV-vis spectroscopy.     

Electrochemical behaviour of self-assembly multilayer films based on iron-substituted α-Keggin polyoxotungstates

Ultrathin multilayer films containing metal-substituted polyoxometalates, [PW₁₁Fe^III(H₂O)O₃₉]⁴⁻ (PW₁₁Fe) or [SiW₁₁Fe^III(H₂O)O₃₉]⁵⁻ (SiW₁₁Fe), and poly(ethylenimine) (PEI) were prepared by the electrostatic layer-by-layer self-assembly method on a glassy carbon electrode. The multilayer films were characterized by cyclic voltammetry and scanning electron microscopy and UV-Vis absorption spectroscopy on a quartz slide was used to monitor film growth. Cyclic voltammetry indicates that the electrochemical properties of the polyoxometalates are completely maintained in the multilayer films, and the influence of scan rate on the voltammetric features showed that the first tungsten reduction process for immobilized PW₁₁Fe and SiW₁₁Fe is a surface-confined process. Studies with [Fe(CN₆)]^3−/4− as electrochemical probe showed that their permeability depends on the thickness of the multilayer films, if the outermost layer is negatively charged. Additionally, the (PEI/SiW₁₁Fe)_n multilayer films showed electrocatalytic properties towards nitrite reduction.     

Platinum nanoparticles on carbonaceous materials: the effect of support geometry on nanoparticle mobility, morphology, and melting

Molecular dynamics simulations have been used to investigate the morphology and mobility of platinum nanoparticles of various sizes supported by carbon materials. The embedded-atom method was used to model Pt-Pt interactions, and the Lennard-Jones potential was used to model the Pt-C interactions. The C?atoms in the supports were held fixed during the simulations. The supports considered were a single graphite sheet and three bundles of carbon nanotubes. Three sizes of Pt nanoparticles were considered: 130?atoms, 249?atoms, and 498?atoms (Pt(130), Pt(249), and Pt(498) respectively). It was found that for all three sizes, diffusion coefficients were approximately one order of magnitude higher for graphite-supported nanoparticles than for carbon nanotube-supported nanoparticles. In addition, increasing the size of the nanoparticle decreased its diffusion coefficient, with Pt(130) having the highest and Pt(498) the lowest diffusion coefficients. More interestingly, we found that for the Pt nanoparticles of all three sizes the diffusion coefficient increases as temperature increases, reaches a maximum at the melting temperature of the nanoparticle, and then decreases. The melting temperature was found to be strongly dependent on the particle size, but only slightly dependent on the features of the supports. While the size of the nanoparticle was seen to affect the particles' mobility, it did not significantly affect their structure. The nanoparticles supported by graphite have density profiles that indicate a highly ordered, fcc-like structure, while the particles supported by carbon nanotubes have a more disordered structure. An order parameter confirms that the nanoparticles' structure depends on the support morphology.     

A parity-time symmetry single-mode laser based on graphene

The development of advanced optical systems, especially coherent optical systems demands high-performance single-mode lasers. Here, we proposed a parity-time symmetry single-mode laser based on graphene with superior performance over the widely utilized technologies of the index-coupled DFB laser working at telecommunication wavelength. The unique properties of graphene have been used to tune the III-V/Silicon hybrid laser to the PT symmetry broken phase where the lasing mode has a minimum overlap with graphene nanostructures while all other modes have been suppressed by the loss in graphene. Our results suggest a high-performance silicon-based laser source for photonic integrated circuits. Such a compact single-mode laser source can be widely used in some applications, such as on-chip optical interconnects, optical spectrometry, biochemical sensing and imaging.     

Inference based on censored samples from exponentiated populations

Maximum likelihood and Bayes estimators of the parameters, survival and hazard rate functions of an exponentiated population whose cumulative distribution function takes the form H(x)=[G(x)] ^α , where α is an unknown positive parameter and G(x) is a cumulative distribution function, are obtained in two cases: when the parameters of G(x) are all known so that α is the only unknown parameter of H and when the k-dimensional vector of parameters β of G(x) are all unknown so that the (k+1)-dimensional vector of parameters (α,β) of H is unknown. In both cases, estimation is based on a Type II censored sample. In such cases, the predictive density and predictive survival functions of future observables are obtained when based on the two-sample scheme. This enables us to obtain Bayes prediction bounds of future observables. Subjective priors are used which reflect the prior belief of the experimenter. Numerical results are obtained under three exponentiated models and compared with the results obtained by using the MCMC algorithm.     

Detailed analysis of particle–substrate interaction based on a centrifugal method

The interaction between particles and inclined substrates in a centrifuge was investigated theoretically and experimentally. First, the balance of the force acting on a particle adhering to the substrate, with an inclination angle from 0 to 90° to the horizontal, was formulated separately in the normal and tangential directions. The adhesion force was then derived based on the point-mass model as a function of the angular velocity. Next, the balance of the moments of the forces acting on a particle adhering to the substrate was formulated; theoretical equations for the adhesion force and the effective contact radius were then derived from the angular velocities, obtained at any two inclination angles, based on the rigid-body model. Finally, the removal fraction curves of spherical/nonspherical particles with median diameters of less than 10 µm were experimentally obtained by increasing the angular velocity at each inclination angle. The experimentally obtained angular velocities were substituted into the theoretical equations to compare the point-mass and rigid-body models. The effects of the particle shape on the adhesion force and effective contact radius and that of the inclination angle on the removal fraction curves based on the theoretical equation were also investigated.     

Optimal age replacement scheduling for a random work system with random lead time

System maintenance and spare parts are two closely related logistics activities since maintenance generates the demand for spare parts. Most studies on integrated models of preventive replacement and inventory of spare parts have focused on age replacement scheduling, while random replacement policy, which is sensible and necessary in practice, is rarely discussed and applied. The purpose of this paper is to present a generalised age replacement policy for a system which works at random time and considers random lead time for replacement delivery. To model an imperfect maintenance action, we consider that the system undergoes minimal repairs at minor failures and corrective replacements at catastrophic failures. Before catastrophic failures, the system is replaced preventively at age T or at the completion of a working time, whichever occurs first. The main objective is to determine an optimal schedule of age replacement that minimises the mean cost rate function of the system in a finite time horizon. The existence and uniqueness of optimal replacement policy are derived analytically and computed numerically. It can be seen that the proposed model is a generalisation of the previous works in maintenance theory.     

Novel hybrid solar cells based on α-copper phthalocyanine–cadmium sulfide planar heterojunction

Cadmium sulfide (CdS) has been employed as an alternative acceptor for planar heterojunction solar cell based on copper phthalocyanine (CuPc). Spin-coated poly-3,4-ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) on indium tin oxide (ITO)-coated glass substrates was used for the vacuum deposition of CuPc and CdS planar heterojunction. In the present study, we have fabricated two different architectures of CuPc/CdS devices: (1) ITO/PEDOT:PSS/CuPc/CdS/Al and (2) ITO/PEDOT:PSS/CuPc/CdS/LiF/Al. Our results indicate that the CdS could effectively facilitate charge transport in the nanostructured network, and be a good acceptor. The fabricated bare CuPc/CdS device shows 0.13 % conversion efficiency while incorporation of LiF layer between CuPc/CdS and Al contact facilitates low-recombination rate results ~43 % enhancement in efficiency. The ITO/PEDOT:PSS/CuPc/CdS/LiF/Al device shows 0.30 % power conversion efficiency.     

Effect of sonication time on the synthesis of the CdS nanoparticle based multiwall carbon nanotube – maleic anhydride – 1-octene nanocomposites

Effect of sonication time on the synthesis of the CdS nanoparticles within the matrix obtained through the covalent functionalization of multiwall carbon nanotube (MWCNT) with maleic anhydride (MA) – 1-octene copolymer was investigated. Cadmium chloride and thiourea were used as the raw materials. MWCNTs used for the matrix were synthesized by Catalytic Chemical Vapor Deposition using Fe-Co/Al₂O₃ as the catalyst. The obtained nanostructures were characterized by FTIR, XRD, Raman spectroscopy, TEM, SEM, TG and UV-Vis spectroscopy. Electrophysical properties of the polymer nanocomposites obtained using different periods of time for sonication were comparably investigated. The average CdS particle diameter was between 3.9–7.9 nm as confirmed independently by TEM and XRD. UV-Vis spectroscopy revealed that the obtained nanostructures are appropriate base materials for making optical devices.     

Electrochemical Corrosion Behavior of Nanocrystalline Materials—a Review

Nanocrystallization significantly influences the electrochemical corrosion behaviors of metals/alloys in liquid system. In active dissolution, nanocrystallization accelerates the corrosion reactions. If the corrosion products are dissoluble, the corrosion rate is increased by nanocrystallization; if the corrosion products are insoluble, the corrosion rate is decreased on the contrary because the corrosion products act as a block layer to delay the dissolution. In passivation, nanocrystallization changes the composition of the passive film, and results into different morphology and growth process of the passive film, both of which improves the formation of compact film and influences the semiconductor property. It influences the passivation depending on fast element diffusion and special adsorbed ability. The small grain size improves the element diffusion, which leads to the different composition of passive film (passive elements enrichment such as Cr, Ti). The small grain size also changes the surface condition, which influences the ions adsorption. All increase the corrosion resistance of materials. In local corrosion, nanocrystallization increases the unstable points on the surface of the materials, which increases the possibility of local corrosion. However, the excellent ability of element diffusion helps heal the local corrosion points, which inhibits the growth of the local corrosion.     

Characterization of the Response of CaWO4 on Recoiling Nuclei from Surface Alpha Decays

A potentially harmful background for experiments attempting direct dark matter detection like the CRESST (= Cryogenic Rare Event Search with Superconducting Thermometers) experiment is caused by recoiling nuclei from ²¹⁰Po alpha decays on surfaces close to the detector. In order to characterize this kind of background in CRESST, calibration measurements have been performed at the TU München. A for this purpose an optimized version of the CRESST detector has been developed consisting of a 38 g CaWO₄ crystal and a separate cryogenic light detector, both equipped with Ir/Au transition edge sensors (TESs). The simultaneous measurement of the phonon signal and the scintillation light from the CaWO₄ crystal allows to discriminate between electron and nuclear recoils using their different light outputs. The unexpected results of a first measurement with a ²¹⁰Po source can be understood with the help of a Monte Carlo simulation performed for a similar system.      

Analysis of water drop erosion on turbine blades based on a nonlinear liquid–solid impact model

Water drop erosion is regarded as one of the most serious reliability concerns in the wet steam stage of a steam turbine. One of the most challenging aspects of this problem involves the fundamental solution of the transient pressure field in the liquid drop and stress field in the metal substrate, which are coupled with each other. In this paper, we first solve the fundamental problem of high-speed liquid–solid impact, both analytically and numerically, based on a nonlinear wave model. The transient pressure distribution in liquid (include shock wave) and transient stress distribution in solid are obtained for representative water drop-1Cr13 impacts, with impact speed varying from 10 m/s to 500 m/s. The relationship between the most important parameters characterizing impact and incident speed is established. With the statistics of water drop impact on the blade, a simple fatigue model is employed in this paper to obtain the lifetime map on a blade surface under typical working conditions, in terms of both impact times and operation hours. The most dangerous water drop erosion regions and operating conditions of the steam turbine blade are deduced. These results are useful for evaluating the water drop erosion mechanisms based on the fundamental solution of liquid–solid impact.     

The effect of residual stresses in the ZnO buffer layer on the density of a ZnO nanowire array

Density control is a valuable concern in the research of ZnO nanowire arrays. In this study, unannealed and annealed ZnO thin films were used as substrates to fabricate ZnO nanowire arrays. In the unannealed thin film, an inhomogeneous distribution of the nanowire array was found: the density of nanowires decreases with the increase of distance to the edge. In the annealed thin film, the density of nanowire array becomes larger and more homogeneous. Moreover, nanowires are found in high density along microcracks. It is proposed that the residual stresses in the thin film and the density of the nanowire array are in inverse proportion, leading to the results mentioned above. The relationship between residual stresses and the density of nanowires will have potential applications in modifying the density of ZnO nanowire arrays.     

Equal-Channel Multi-Angle Pressing Effect on the Properties of NbTi Alloy

An improvement of NbTi alloy functional properties by equal-channel multi-angle pressing (ECMAP) combined with hydrostatic extrusion, drawing and thermal treatment is revealed. The ECMAP method allows to increase the billet accumulated deformation with preserving its initial dimensions. The formed highly dispersed and homogeneous nanocrystalline structure with a more uniform distribution of α-phase precipitations as a result of ECMAP treatment improves the functional properties of the alloy. In the field of 5 T, the critical current density in wire samples produced with application of the ECMAP method is enhanced approximately by a factor of 2 in comparison with the values obtained for the samples produced without the ECMAP treatment.     

Effect of cannibalism on a predator–prey system with nutritional value: a model based study

Enrichment of a simple predator–prey system results in the destruction of stable steady state and further enrichment leads to the extinction of the species, which is a classical problem in ecology and known as the paradox of enrichment. The Paradox of enrichment is a controversial issue. Most theoretical studies show the destabilization of predator–prey system due to enrichment, but there is a discrepancy between the empirical evidence and theoretical predictions. In spite of the debate and cross-debate, research is still being carried out on the paradox of enrichment (16 papers and nearly 500 citations in 2008), which in general does not include the effect of cannibalism. Here we present a simple predator–prey system in the presence of cannibalism among predators and offer a novel resolution to the paradox of enrichment. The concept of nutritional value is introduced by many authors to resolve the issue. It is observed that even in the face of sufficient enrichment the system remains stable in the vicinity of critical nutritional value. In the case of a lower cannibalism rate we also observe similar kinds of behaviour, but for a higher cannibalism rate, the system remains always stable and does not depend upon the nutritional value of prey, i.e. paradox of enrichment does not hold at all in such a situation. We also observe that cannibalism can have a positive as well as a negative effect on population abundance, depending on the cannibalism rate.     

Influence of minute amount of elements Bi, Ag and In on surface tension and soldering process performance of tin–lead based solders

A new method of measuring surface tension of liquid alloy solders, based on the interference principle between laser wave and liquid surface tension stationary wave, was employed to characterize the surface-active effect of some elements which can remarkably reduce the surface tension of liquid solders. The focus was placed on investigating the influence of minute amounts of elemental bismuth (Bi), silver (Ag) and indium (In) on surface tension and soldering process performance of tin–lead (Sn–Pb) based solders. The dependences of the surface tension of Sn–Pb solders vs. weight percentage of elements Bi, Ag and In in the solders were investigated. The experimental results showed that elements Bi and In have a strong effect on reduction of the surface tension of solders with the minute addition amount of 0.8–1.0 wt %, and the effect of element Ag is relatively weak. However, Ag has the strongest effectiveness on improving the wettability of the solders due to the comprehensive excellent function of reducing both the surface tension of liquid solders and the interface tension between liquid solder and solid base metal.