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In physical systems, interactions between phenomena of different nature, generally coupled to each other, are often involved. Their comprehensive study requires the use of various physical models sharing a unique set of physical quantities. In an effort to correctly model these systems, numerical methods are frequently used. However, computational tools dedicated to such a specific scope of use are barely available. Furthermore, unsuited numerical discretization and high memory costs are two major drawbacks limiting the use of coupled numerical models. We present in this paper a global method which enables the independent use of existing computational tools, the numerical adaption to each physical model and the reduction of the memory use. This method has resorted to a unique discretization and topology for each physical model. The link between these independent models is ensured by the projection of quantities common to them. Thus computational tools, originally not intended to operate together, can be used again. After a theoretical description of the projection method, we will present successive application to discretizations of different nature. Thus, numerical efficiency of the projections in themselves will be tested. Because of the large range of combination of physical models, additional tests will be carried out in order to determine the most accurate coupling flowchart. A highly coupled problem, involving three different physical models, will be presented using the projection method. Results show a significant gain in flexibility and cuts in memory costs. Present test-cases reveal that accuracy is of same order as the one obtained using dedicated tools.
We propose in this work, contact Schottky Nickel/porous silicon (PSi) system, coupled to nanocrystallites size variation of material for a possible technique to reduce dark current. The device consists of metal- semiconductor-metal photodiode (MSM-PD). Higher barrier ΦΒ enhances the performance of MSM-PD through reduction in dark current (Is), and benefits to resolve noise from signal detection of the devices. In order to reduce much more Is, we proposed different anodization times (5–7–10 min) as method to tune the size of nanocrystallites. As result Is value was reduced to almost two orders of magnitude for 10 min etching time, and the value of Is ≈ 10–10 A. ΦΒ reached the value of 0.882 eV. Among the hypothesis suggested in the reduction of Is was the quantum confinement effects. According to Rhoderick model, the Schottky barrier height is explicitly linked to the band gap energy due to the presence of interface states. The existence of narrow nanocrystallites increased energy band gap of PSi and the Schottky barrier height, which in turn reduces Is. The photoluminescence measurements confirmed our hypothesis. Photosensitivity of the device was established by adopting the MSM configuration, and strong absorption was detected in visible range.
Thin layers of nanoporous silicon PS were synthesized by anodic etching, in order to develop photovoltaic cells. We proposed a diluted concentration of hydrofluoric acid with different etching current densities (1, 3, 5 mA/cm2) on a fairly short time anodization. Observations by scanning electron microscope, electrical measurements and optical measurements revealed that the structural properties of PS layers depended on strong conditions of prints. The reverse and forward component of the I-V characteristics showed an appropriate method to explore and extract the parameters of the diode ideality factor n. The optimum conditions of formation of PS were: HF concentration of 1% and an etching current density of mA/cm2. Unlike silicon, which has a low absorption of short visible wavelengths, it was shown that the PS had wide energy gap of ≈ 2 eV, and a marked improvement in the absorption between 400 and 600 nm. This property has been used to optimize the response of the solar cell Ni/PS/c-Si. Efficiency performance close to 4.2% was obtained with a Voc of 400 mV, and fill factor of 46%. The solar cell exhibited better response than the reference cell Ni/c-Si. These results show that PS/c-Si heterojunction has a potential for photovoltaic applications.
The aim of this work was to investigate a simple and effective method to produce porous silicon from a powder of silicon. The preparation of porous silicon (PS) was realized by exposing silicon powders to acid vapor attack issued from acid solutions containing a 48% of HF and 65% of HNO3. The bond configuration of powder silicon before and after attack with acid vapor was monitored by Fourier transmission infrared spectroscopy (FTIR) and it was found that the PS was produced due to the newly formed Si-H bond during acid vapor attack. From the photoluminescence spectroscopy, it was shown that powder silicon attacked with acid vapor can lead to an increase of photoluminescence (PL) intensity when they are excited by light compared to untreated powder silicon and can provide blue shifts in the PL spectrum by increasing exposing time. This behavior may be attributed to the reduction in the size of the silicon, indicating consequently the formation of PS powder. The experimental results suggest a possibility that the chemical attack with acid vapor of the powder silicon provides a relatively easy way to produce porous silicon.
Organic complementary inverters were fabricated with low-voltage pentacene/SrTiO3 and C60/SrTiO3 field-effect transistors (FETs), without formation of self-assembled monolayer on surface of the dielectric SrTiO3. The inverters showed the highest gain of 18.4 at an operating voltage as low as 3 V, where the pentacene/SrTiO3 and C60/SrTiO3 FETs on the inverters showed hole and electron mobilities of 0.85 and 0.07 cm2/V s with threshold voltages (VT) of −1.1 and −0.2 V, respectively.
Thin films of furfurylidenemalononitrile (FMN) were deposited on different substrates at room temperature by thermal evaporation technique under a high vacuum. The structure of the powder was confirmed by Fourier transformation infrared (FTIR) technique. The unit cell dimensions were determined from X-ray diffraction (XRD) studies. The optical properties were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence of light in the wavelength range from 200 to 2500 nm. The refractive index (n), the absorption index (k) and the absorption coefficient (α) were calculated. The analysis of the spectral behavior of the absorption coefficient in the absorption region revealed an indirect allowed transition. The refractive index dispersion was analyzed using the single oscillator model. Some dispersion parameters were estimated. Complex dielectric function and optical conductivity were determined. The influence of the irradiation with high-energy X-rays (6 MeV) on the studied properties was also investigated.
In order to investigate the effects and optical properties in the mid-infrared region, thin films of PEDOT:PSS were spin-coated on ITO glasses and ITO/Ag nanoparticles substrates. Scanning Electron Microscopy, SEM, has been applied to investigate the surface morphology and measure the size of nanoparticles. Dielectric and optical coefficients such as refractive index, n, and extinction coefficient, k, were calculated by applying Kramers-Kronig dispersion relations to the experimentally recorded near normal reflection FTIR spectra. Different analyses confirm that Ag nanoparticles are present in the structure of the synthesized thin films. The recorded Raman spectra have shown that the intensity enhancement of Raman peaks of PEDOT:PSS increases due to the plasmon resonance of Ag colloidal silver nanoparticles layer and optical properties investigation has confirmed the electrical conductivity increase in the mid-infrared region.
Using molecular dynamics simulations, we exploit a charge-driven flip-flop that is composed several water molecules confined in a single-walled carbon nanotube. The flip-flop has two stable states and can be used to store state information. It can toggle between the two states within 2.5–3.5 ps (286 GHz–400 GHz). We reveal that the underlying mechanism is dominated by the interaction between the water molecules and nonuniform electric field generated by point charges. Namely, each water molecule tends to maintain its lowest electric energy by moving toward the location with the highest field strength. This flip-flop may be of value for molecular computing.
In this study, broadside-coupled triangular split-ring-resonators are designed and simulated as THz sensors. Their double-side sensing capability provides sensitivity enhancement compared with singlesided sensors. The material to be detected is modeled as an over-layer on the structure. The ensuing change of the transmission resonances is investigated as a function in the thickness and permittivity of the substrate and of the one- or two-sided over-layer. The propagation direction of the incident wave also allows to select the excited mode (via the polarization of the radiation) which provides additional flexibility to the proposed sensor device by the possibility to operate it over a larger frequency range. Overall, the polarization dependency of the structure, the double-side sensing and the high sensitivity make such structures attractive as THz sensors.
The transmission of electromagnetic wave through a planar chiral structure, loaded with the gyrotropic medium being under an action of the longitudinal magnetic field, is studied. The frequency dependence of the metamaterial resonance and the angle of rotation of the polarization plane are obtained. We demonstrate both theoretically and experimentally a resonant enhancement of the Faraday rotation. The ranges of frequency and magnetic field strength are defined, where the angle of polarization plane rotation for the metamaterial is substantially higher than that one for a single ferrite slab.
The axial shift and the spin Hamiltonian parameters (zero-field splitting D, g factors and hyperfine structure constants) for Cr+ in BeO are theoretically studied in this work. The calculations are carried out by using the perturbation formulas of these parameters for a 3d5 ion under trigonally distorted tetrahedra based on the cluster approach containing both the crystal-field and charge transfer contributions. It is found that the impurity Cr+ may not occupy exactly the host Be2+ site but experience a small outward shift 0.01 Å away from the ligand triangle along the C3 axis. The above impurity axial shift leads to much smaller trigonal distortion than the host Be2+ site in BeO. The theoretical spin Hamiltonian parameters based on the above impurity axial shift are in good agreement with the observed values.
Carbon steel (C75) is exposed to highly reactive species such as hydroxyl radicals OH created by a gliding arc discharge (GAD) in humid air at atmospheric pressure. The protective properties of carbon steel treated by GAD are studied versus different treatment times (t) and for an immersion in corroding 0.5 M sodium chloride solution during 24 h. Evolutions of corrosion rate are studied using weight loss measurements and electrochemical methods, e.g., electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The results obtained by GAD treatment show that the corrosion rate of steel decreases with the ennoblement of the corrosion potential and the decrease of the corrosion current density. This indicates that the plasma treatment acts as an anodic type inhibitor and suggests the formation of a protective layer. EIS measurements confirm the presence of this film: the charge transfer resistance (Rct) increases with GAD treatment time, leading to a corrosion inhibition efficiency around 73% for a treatment time equal to 60 min. This confirms the importance of the plasma effect. The gliding arc discharge is a clean and efficient technology for the surface treatment of carbon steel; it improves the anticorrosion properties of steel in aggressive environments, forming a resistant and insulating barrier.
A time and spatially resolved technique was used for the investigation of emission signal enhancement in collinear double-pulse LIBS. Two Q-switched Nd:YAG lasers at 1064 nm wavelength have been employed to produce laser-induced plasmas on aluminum-based alloys by single- (SP) and doublepulse (DP) LIBS. Time and spatial evolution of the plasma temperature and electron number density was investigated in these two experimental schemes. The enhancement of the emission line intensities was investigated, and a relation between the increases in intensity and excitation energy level was established. The results shows that the production of the second plasma by second laser pulse was important in DP experiment in collinear geometry.
This paper aims to analyze the behavior of DC bipolar corona discharge in a two wires-to-plane configuration under variable humid air conditions. A circular biased probe was adapted to the plane and used to measure the ground-plane current density and electric field during the bipolar corona. The values of the electric field and the current density at the plane surface were the maximum beneath the two corona wires which decreased when moving away from them. The current-voltage characteristics followed the quadratic Townsend’s law. The experimental results show that the bipolar corona discharge is strongly affected by the air humidity. The current density and the electric field decrease linearly with the humidity for all the tested wire diameters.
Physics of Energy Transfer, Conversion and Storage
This paper presents a stability analysis of a free piston Stirling engine. The model and the detailed calculation of pressures losses are exposed. Stability of the machine is studied by the observation of the eigenvalues of the model matrix. Model validation based on the comparison with NASA experimental results is described. The influence of operational and construction parameters on performance and stability issues is exposed. The results show that most parameters that are beneficial for machine power seem to induce irregular mechanical characteristics with load, suggesting that self-sustained oscillations could be difficult to maintain and control.