Density Of Active Centers Research Articles

Performance and wavelength tuning of green emitting terbium lasers.

Wavelength tuning experiments in the green spectral range applying Tb3+-doped LiLuF4 and Tb3+/Na+-codoped CaF2 single crystals are presented. While in the former case almost 100 mW could be obtained throughout a total range of 7 nm, the inhomogeneously broadened emission features of Tb3+ in the CaF2-matrix allowed for a continuous tuning range exceeding 10 nm. For initial characterization, free running continuous wave laser experiments using Tb3+-doped LiLuF4, Na+:CaF2, and stoichiometric TbF3 crystals were carried out resulting in efficient continuous wave laser operation in the green spectral region. The density of active centers in TbF3 is among the highest that can be obtained in solid state gain materials. Nevertheless, a slope efficiency as high as 32% was obtained from this crystal. High Tb3+-densities are essential to compensate for the adherent low absorption cross sections at useful pump-wavelengths.

Characterization of Fresh Oil Hydro Treating Catalysts

Catalysts are of great importance in catalytic processes. Key features of oil hydrotreating catalysts are highlighted. These catalysts play a significant role in removing impurities, as sulfur and other hetero-atoms existing in these oil fractions. These catalysts are composed of mainly, molybdenum salts spread in the porous structure of the carrier, mainly γ-Alumina (γ-Al2O3). Other metal salts supporting the action of the catalytic activity of Molybdenum, as Cobalt, Nickel, are added. The first step in the process of catalyst production is the drying of the wet salts mixture to remove moisture. The oxidic precursor is produced by the calcination of the dried mixture. The choice of the appropriate catalyst is based on its ability to maintain a high level of activity for long periods before re-activation. This ability depends on the chemical composition and on the porous structure of the carrier, which ensures the appropriate properties such as the density of active centers, particle strength, and porous structure parameters. Also, the catalyst resistance to chemical poisons destroying active centers in particular, is considered as a limiting factor in selecting the catalyst.

Determination of the strength of the joint in homogeneous metals in electric resistance welding

Investigations were carried out into the mechanism of increasing the density of active centres during the thermomechanical cycle of electric resistance welding of homogeneous metals. The dependences for the calculation of the density of active centres and the relative strength of the joint between the welded and base metals have been determined.

Hopping conduction on conductive inks for wearable electronics

This work describes the preparation and electrical characterization of conducting carbon black on polyurethane and polyvinyl commercially available screen printing inks. Resistivity dependence with temperature for samples containing 3.0, 4.4, 5.0, 8.0 and 10.7 % filler mass concentration were investigated from 5 to 310 K. By using the differential activation energy method, it will be demonstrated that Mott type variable range hopping 3D conduction model is the predominant transport mechanism on the studied composites at low temperatures. Besides, the variable range hopping parameter and its subproducts were obtained: the mean hopping distance, the density of active centres and the energy associated with it. Variable range hopping 3D conduction is associated with an increase of the interchain connectivity. In the present work, this role is played by the carbon black. The carbon black/polyurethane and carbon black/polyvinyl composites are promising conducting coatings for using in wearable electronics as suggested by experimental results.

Hopping conduction on carbon black/styrene–butadiene–styrene composites

This study describes the preparation and electrical characterisation of conducting carbon black-filled/styrene–butadiene–styrene tri-block copolymer. Composites containing 5 and 10 vol% of carbon black were studied. The carbon black distribution, conductive mechanism and electrical properties of the composites were investigated from 8 to 304 K. For the first time, an analysis of the differential activation energy is carried out for carbon black composites, allowing the classification of the variable range hopping as one-dimensional for composite containing 5% of carbon black and as three-dimensional for the composite containing 10% of carbon black. Such results are found to be determined by two factors: the concentration of carbon black in the filler-rich phase and by the structural continuity of this phase. Increasing the carbon black content induces structural transition that results in variable range hopping conduction changing from one-dimensional to three-dimensional. From the hopping parameters, the mean hopping distance, the density of active centres and the energy associated with it were obtained.

DC electrical conductivity of carbon black polymer composites at low temperatures

The understanding of the charge transport mechanism of the conducting polymers and their composites is important for the successful fabrication of semiconductor devices. In this work, the DC electrical conductivity of carbon black particles in a polystyrene matrix, in the temperature range of 80 to 250 K is examined, for concentrations below the critical percolation concentration. We show that three-dimensional variable range hopping is observed, that is, the temperature behavior of the DC conductivity can be expressed by σ = σ 0 exp − T 0 T 1 4 . From the hopping parameter we obtained the mean hopping distance, the density of active centers and the energy associated with it.

Hopping conduction on PAni/PSS blends

Conducting blends of polyaniline (PAni) and sulfonated polystyrene (PSS) were obtained by in situ polymerization. We prepared PAni/PSS blends with molar ratio of 1:2 and PSS sulfonated at the degrees of 13, 21 and 30 mol%. The blends were investigated by d.c. conductivity and scanning electron microscopy (SEM). Increasing sulfonation degree increases the conductivity and the miscibility of the blends was observed by SEM. The electrical conductivity of blends was evaluated from 100 to 300 K. We show that three-dimensional variable range hopping is observed. From the hopping parameter we obtained the mean hopping distance, the density of active centers and the energy associated with it. The hopping distance was compared to the mean distance of sulfonic groups of the PSS, indicating that interchain 3D variable range hopping is dominant at low temperatures.

Diffusional-relaxational model of the formation of polymer boundary layers in adhesional compounds

The variation in density of active centers is described by a diffusion equation of hyperbolic type, which is solved by numerical methods. The results of the calculation are compared with known experimental data.

Effect of titanium, copper and iron on silicon solar cells

The effect of Ti, Cu and Fe on silicon solar cells has been investigated. Ti severely degrades cell performance above a concentration of 10 11 cm −3. The presence of 2 × 10 14 cm −3 Ti results in a 63% loss in cell performance and more than an order of magnitude reduction in carrier lifetime. Ti gives rise to two deep levels in Si at Ev + 0.30 eV and Ec − 0.27 eV. Copper, at concentrations below 10 16 cm −3, has negligible effect on cell performance and carrier lifetime. Above 10 16 cm −3 copper occasionally produces a 10–15% loss in cell performance with a noticeable increase in junction excess current. No recombination centers were found due to Cu, instead considerable precipitation in the starting material was observed. Fe begins to hurt the cell performance above a concentration of 2 × 10 14 cm −3. Iron at 1.7 × 10 15 cm −3 results in 46% loss in cell efficiency and about an order of magnitude reduction in lifetime. Fe induces a deep level in silicon at Ev + 0.4 eV. The active center density, for both Ti and Fe, is only a very small fraction of the total impurity content in the starting silicon wafer.

Threshold density of active centers with a three-step heterogeneous reaction

A three-step reaction (A+Z=AZ, B2+2Z=2BZ, AZ+BZ=AB+2Z) on a decaying catalyst is considered. Irreversibility of the first step yields a threshold effect as to the density (zo) of active centers Z the reaction collapses for zo smaller than a critical value. The dependence of this effect on the rate constants of the three steps has been investigated.