FDTD simulation of the nonlinear gain dynamics in active optical waveguides and semiconductor microcavities

We use the finite-difference time-domain (FDTD) solution of the full-wave vectorial Maxwell-Bloch equations for a two-level quantum system developed earlier , to investigate the nonlinear gain spatio-temporal dynamics of active optical waveguides and semiconductor microcavities. The numerical model has been successfully validated against density matrix theory of gain saturation in homogeneously broadened two-level quantum systems for optical waveguides containing resonant gain nonlinearities. The semiclassical equations have been extended employing the Langevin formalism to account for the quantum noise and the spontaneous emission. We have numerically demonstrated the time evolution of the coherent oscillations build up at the output laser facet identifying the lasing threshold and the fast relaxation oscillations until the settlement of a steady-state emission. Our simulation predictions of the lasing wavelength in a number of vertical-cavity surface-emitting laser geometries, when the spontaneous emission is the only source of radiation, agree very well with standard results and, thus, allow us to infer and subsequently optimize important emission characteristics, such as the spontaneous emission rate, the laser line shape, and the relaxation oscillation frequencies and decay rates.     

Spectral and time domain characteristics of single muscle fibre action potentials during continuous activity extracted from model considerations

Galanin (GAL) has been proposed to be an inhibitory modulator of cholinergic memory pathways because it acts within the hippocampus to inhibit the release and antagonize the postsynaptic actions of acetylcholine. Here we have used: 1) slice binding and quantitative autoradiography to assess the density and occupancy of GAL receptors; and 2) in situ hybridization histochemistry to assess expression of the GALR1 receptor subtype in the ventral hippocampus of 3-month-old and 21-month-old Fischer 344 male rats. We detected a small but significant (p < or = 0.0003) age-related reduction in 125I-GAL binding-site density in the ventral hippocampus and entorhinal cortex under standard binding conditions. Post-hoc analysis indicated that this reduction with age persisted in the CA1 radiatum and entorhinal cortex following GTP-induced desaturation to unmask pre-existent GAL receptors occupied by endogenous ligand. It was not associated with a significant change in peak GALR1 gene expression in the hippocampus. Because a portion of GAL receptors in this region have been postulated to function as presynaptic auto-receptors on cholinergic fiber terminals, the reduction in GAL binding sites with age may be a consequence of age-related alterations in GAL receptor expression by basal forebrain cholinergic neurons which project to the ventral hippocampus.     

Effect of the molecular structure of some quinones on their corrosion inhibiting action

The molecular characteristics obtained by quantum chemical self‐consistent field (SCF) calculations of a series of quinones are correlated with the experimentally determined inhibitor efficiency for mild steel corrosion in neutral aqueous medium. It was established that the decrease of the ionization potential and the increase of the dipole moment of the quinone molecules favor the higher protective effect. The electron density and the geometric molecular structure have also been computed and are discussed in view of the corrosion inhibition.     

ECN‐measurements at copper in artificial tap water – Investigation of anion‐effects

Electrochemical noise analysis was succesfully applied to study anion effects in the first 20 h starting period of copper corrosion in tap water. The influence of chloride, sulfate and bicarbonate as single anions and the interactions between them in three anion solutions were characterized in terms of standard deviation and PSD(I) data as well as interaction coefficients obtained from the evaluation of a set of experiments designed according to a statistical 2³‐factorial plan. The results let conclude that sulfate has an activating and chloride a passivating effect. Bicarbonate seems to act as a buffering agent. It is obviously the combination of the presence of all three anions in a narrow range of concentration ratios which finally leads to pitting corrosion. The relevance of these results from short term exposure for the long term performance of copper in tap water and the likelihood of pitting corrosion should now be investigated in long term exposure tests.     

Preparation and characterization of Co-Ru/TiO2/MWCNTs electrocatalysts in PEM hydrogen electrolyzer

The subject of this study is preparation and characterization of hypo-hyper d-electrocatalysts with reduced amount of precious metals aimed for water electrolysis. The studied electrocatalysts contain 10% mixed metallic phase (Co:Ru = 1:1 wt., Co:Ru = 4:1 wt. and Co:Ru:Pt = 4:0.5:0.5 wt.), 18% TiO₂ as a crystalline anatase deposited on multiwalled carbon nanotubes (MWCNTs). Previously, MWCNTs were activated in 28% nitric acid. As a reference electrocatalyst for hydrogen evolution reaction, corresponding electrocatalysts with pure Pt metallic phase and mixed CoPt (Co:Pt = 1:1 wt.) metallic phase were prepared. Also, as a reference electrocatalyst for oxygen evolution reaction, electrocatalyst with pure Ru metallic phase was prepared.The prepared electrocatalysts were structurally characterized by means of XPS, XRD, TEM, SEM and FTIR analysis.Electrochemical characterization was performed by means of cyclic voltammetry and potentiodynamic method in the PEM hydrogen electrolyzer. The range of the catalytic activity for hydrogen evolution of studied electrocatalysts was the following: CoRuPt (4:0.5:0.5) > CoPt (1:1) > Pt > CoRu (1:1) > CoRu (4:1). The order of the catalytic activity for oxygen evolution was the following: CoRu (1:1) > Ru > CoRu (4:1) > Pt > CoRuPt (4:0.5:0.5) > CoPt (1:1).     

New nano-structured and interactive supported composite electrocatalysts for hydrogen evolution with partially replaced platinum loading

This work is concerned with preparation and characterization of nano-structured composite electrocatalytic material for hydrogen evolution based on CoPt hyper d-metallic phase and anatase (TiO₂) hypo d-phase, both deposited on multiwalled carbon nanotubes (MWCNTs) as a carbon substrate. The main goal is partially or completely to replace Pt as the electrocatalytic material. Four electrocatalytic systems were prepared with common composition 10% Me + 18% TiO₂ + MWCNTs, where Me = Co, CoPt (4:1, wt. ratio), CoPt (1:1, wt. ratio) and Pt. The structural changes and their influence on electrocatalytic activity were studied by means of XRD, TEM, SEM and FTIR. The electrocatalytic activity was assessed in aqueous alkaline and polymer acidic electrolytes by means of steady-state galvanostatic method. It was found that Co strongly affects the platinum particle size. The addition of Co reduces platinum particle's size from 11 nm (in pure Pt metallic system) to 4 nm (in both systems 4:1 and 1:1), i.e. almost by 3 times. The corresponding increase of the surface area and the number of the active catalytic centres improves the efficiency, despite the fact that the amount of used platinum was decreased up to 5 times. The catalyst based on CoPt (1:1) performed the best, while the activity of the pure platinum and CoPt (4:1) systems were very close. Generally, the studied electrocatalysts have shown good and stable performances for hydrogen evolution in PEM electrochemical cell. The influence of the hydrogen electrodes under investigation on the water electrolysis efficiency at current density of 0.3 A cm⁻² was assessed, using previous data oxygen evolution on IrO_x electrode. Related to the performances of commercial Pt (ELAT) electrode, when hydrogen electrodes with the prepared mixed electrocatalysts were used, the water electrolysis efficiency was only 5% lower for CoPt (1:1), nearly 10% lower for CoPt (4:1) and 13% lower in the case of pure Co-based electrocatalyst.     

Investigation of montmorillonite as carrier for OER

The aim of this work is to investigate the natural mineral Montmorillonite (MMT) as catalytic support and to assess the efficiency of the composite MMT-supported Ir toward OER in acidic electrochemical water splitting. MMT is a phyllosilicate layered clay with 2:1 type sheet structure with high cation exchange capacity, high surface area and low cost. Three different catalyst with iridium loadings of 10, 20, and 30 wt% Ir supported on MMT are synthesized. Their phase identification, crystallite size, elemental analysis, and thermal stability are studied by means of XRD, HRTEM, EDX, and TGA, respectively. The catalytic performance is examined in 0.5 M H₂SO₄ and in electrolysis cell with proton conductive polymer membrane (PEMEC). The results obtained prove that montmorillonite is a promising alternative of the conventional carbon supports with the advantage of being both easily available and cost favourable. Ir/MMT loaded with 30 wt% Ir is the best performed catalyst. In PEMEC operated at 80 °C the catalyst loading of 0.5 mg_Ir cm^?2 ensures intensive and sustainable oxygen evolution with current density reaching 200 mA cm^?2 already at 1.6 V.     

Oxygen evolution on Ebonex-supported Pt-based binary compounds in PEM water electrolysis

Ebonex-supported Pt-based binary electrocatalysts (Pt–Fe, Pt–Co) in different metal ratios were prepared by wet sol–gel method using acetilacetonate precursors (M[(C₅H₇O₂)_n]_m, M = Pt, Fe, Co) and deposited on nonstoichiometric titanium oxide support. The composition of the synthesized composites were studied by X-ray diffraction (XRD) and X-ray-photoelectron spectroscopy (XPS) analysis. Their electrocatalytic activity toward oxygen evolution in PEM water electrolysis was investigated using the common electrochemical techniques of cyclic voltammetry and steady state polarisation. The XRD spectra registered a formation of solid solution between the metallic components accompanied by decrease in the lattice parameter and the crystal size. The effects observed resulted in enhanced efficiency toward oxygen evolution reaction of the synthesized Pt–Fe/Ebonex and Pt–Co/Ebonex catalysts compared to pure Pt.     

Simulations and study of electrochemical hydrogen energy conversion in EasyTest Cell

An EasyTest Cell concept is applied to study the performance characteristics of the electrochemical processor for polymer electrolyte membrane electrochemical hydrogen energy converters (PEM EHEC), broadly known as a membrane electrode assembly (MEA). A series of MEAs consisting of Nafion 117 polymer electrolyte and magnetron sputtered Pt, IrO_x, and composite IrO_x/Pt/IrO_x catalysts with varying catalytic loadings were investigated. The partial electrode reactions proceeding in the real PEM EHEC, namely hydrogen oxidation (HOR), hydrogen evolution (HER), oxygen reduction (ORR), and oxygen evolution (OER), are simulated and studied in a recently developed test cell with a unitized gas compartment. The EasyTest Cell design gives possibilities for strict control of the experimental conditions by avoiding the usage of any auxilliary gas conditioning equipment. By varying the thickness of the sputtered Pt film, the catalyst loading is remarkably reduced (from 0.5 to 0.06 mg cm⁻² or about 8 times) for both HOR and HER without any sacrifice of the electrode performance. The electrode with 0.2 mg cm⁻² sputtered IrO_x shows the best OER performance. The composite IrO_x/Pt/IrO_x electrode demonstrated a bi-functional catalytic activity toward both OER and ORR, as well as improved gas diffusion properties toward ORR compared to the single Pt layer with the same catalytic loading.A phenomenological criterion for evaluating the gas diffusion properties of the electrodes is proposed. The applied testing approach is validated via comparison of the results obtained in the EasyTestCell and the common laboratory PEM electrolytic cell.     

HF-contact elements for testing and multi chip module applications

In this paper a platform based on the composite IC concept (Semiconductor Industry Association in The NTRS, pp 63, 1994) will be presented that is optimized for high frequency applications. Key elements of the platform are coplanar waveguides and micro springs that are processed using MEMS technologies. Rotation symmetric micro springs with diameters between 500 and 125 μm with different widths and thicknesses were designed, simulated and fabricated. The best mechanical properties were obtained from springs with a diameter of 250 μm. They were made by electroplating of a nickel–tungsten-alloy. FEM simulation of the high frequency performance shows a good performance up to frequencies of 10 GHz.