Optical modulators. The quantum well intersubband kind

Perfecting thin film growth techniques have made possible today's search for new, ultrafast optoelectronic devices. The techniques include: molecular beam epitaxy (MBE), organometallic chemical vapor deposition (OMCVD) and atomic layer epitaxy (ALE). By using these methods, scientists and engineers can grow multilayered semiconductor structures of different materials, with extremely high purity, sharp interfaces, and narrow doping profiles. The thickness of each layer has as its lower limit only one atomic layer. By superposing successive materials of different bandgaps, we can create quantum well structures in which electrons or holes are confined to potential wells in the conduction or valence bands. This quantization has allowed for the observation of quantum effects impossible to observe in the bulk materials. One effect is the electron or hole subband-to-subband transition within a conduction or a valence band in a quantum well. This is best known as the inter-subband transition. Before to this concept is examined, the author reviews the optical modulator, its pros and cons, and how quantum wells are used to realize a modulator     

Unsteady heat transfer from an oblate/prolate spheroid

Numerical methods are used to investigate the transient heat transfer from an oblate/prolate spheroid to a steady stream of viscous, incompressible fluid. The temperature of the spheroid is considered spatially uniform but not constant in time. The momentum and heat balance equations were solved numerically in oblate/prolate spheroidal coordinates system. The solutions span the parameter ranges 10 ? Re ? 100 (for the oblate spheroid), 10 ? Re ? 200 (for the prolate spheroid), Pr = 1, 10 and axis ratio ε, 0.1 ? ε ? 0.9. The computations were focused on the influence of the axis ratio and volume heat capacity ratio on the heat transfer rate.     

Multi-element including rare earth content of lichens, bark, soils, and waste following industrial closure

The fate of rare earth and other rare elements entering the environment is largely unknown. The lichen Hypogymnia physodes was transplanted over a 40 km long transect centered on a major metallurgical waste dump close to the Zlatna town center two weeks after smelter closure. Lichens, bark, soil, and waste dump materials were analyzed for 56 elements (including REE). Lichen and bark multi-element compositions were alike, reflecting fixation of elements of environmental concern and the ability for tree canopies to concentrate substances leading to enhanced deposition to both lichens and bark. Higher REE enrichment in lichens than in soil confirm efficient fixation in lichens. The negative europium anomaly in lichens and soil, similar to that in upper crust, confirm a strong crustal influence on lichen signatures across the transect area. Multi-element analysis supports the view that epiphytic lichens, unlike trees, are not influenced by lower groundwater, and they are excellent indicators for REE and other rare elements entering the surface environment, difficult to detect by conventional means.     

Interface characterisation of ZnSe/CuGaSe2 heterojunction

The ZnSe/CuGaSe₂ heterojunctions were fabricated by flash evaporation technique of CuGaSe₂ onto the (110) surface of ZnSe crystals. CuGaSe₂ layers had thickness 2–4 μm and showed a hole concentration up to (1.5–18.0)×10¹⁸ cm⁻³ and mobility μ4–24 cm² V⁻¹ s⁻¹ at 300 K. The charge carrier concentration in ZnSe crystals at 300 K was n=5.6×10¹⁶ cm⁻³ and their mobility μ=300 cm² V⁻¹ s⁻¹. The investigated ZnSe/CuGaSe₂ heterojunctions have at the interface an intermediate layer with a thickness of 450–750 Å and a linear graded band gap as well as an i-ZnSe compensated layer with a thickness of 1–2 μm and resistivity ρ10⁸–10⁹ Ω cm. The i-ZnSe layer is highly compensated due to the presence of Cu acceptor impurities. In this layer the Fermi level position E_c−F₀690 meV and a trap level position E_t−F₀17 meV were determined. The total trap concentration in the i-ZnSe layer is N_t5×10¹⁴ cm⁻³. The mean free path of excited charge carriers in the graded band gap region was calculated as λ55 Å. On the basis of experimental data analysis of electrophysical properties of both ZnSe/CuGaSe₂ heterojunctions and constituent materials the energetic band diagram of the investigated heterostructures is proposed. The current transport mechanism through ZnSe/CuGaSe₂ heterojunctions is consequently elucidated.     

Preconditioned Conjugate Gradient and Multigrid Methods for Numerical Solution of Multicomponent Mass Transfer Equations I. Diffusion-Reaction Equations

In a series of two papers, we analyze the numerical performances of the preconditioned conjugate gradient algorithms (BICGSTAB and GMRES) and multigrid method in solving two-dimensional multicomponent mass transfer equations. The present test problems are steady-state, linear and nonlinear diffusion-reaction equations. The nonlinear algorithm used in connection with the preconditioned conjugate gradient methods is the modified Picard iteration. Theoretical results about the preconditioning are presented. The multigrid algorithm used is the Full Approximation Storage algorithm. The finite difference method was used to discretize the mathematical model equations. The numerical results obtained show good numerical performances.     

Synthesis and hydrogen adsorption properties of a new iron based porous metal-organic framework

A new metal-organic framework [Fe₃O(OOC-C₆H₄-COO)₃(H₂O)₃]Cl·(H₂O)_x was synthesized with a specific surface area of 2823 m²/g and a lattice parameter of 88.61 Å. Isostructural with MIL-101, this compound exhibits similar hydrogen adsorption properties, with maximum adsorption capacity of 5.1wt.% H at 77 K. The adsorption enthalpy of hydrogen for MIL-101 and ITIM-1 (MIL-101Fe) at zero coverage was calculated for a wide temperature range of 77 K ÷ 324 K, considering corrections for the variation of hydrogen gas entropy with the temperature. The resulted adsorption enthalpy is 9.4 kJ/mol for MIL-101, in excellent agreement with the value reported in literature from microcalorimetric measurements, and a value of 10.4 kJ/mol at zero coverage was obtained for ITIM-1 (MIL-101Fe).     

Problems of village electrification based on PV systems in Romania: Individual solar home systems for settlements in the Cerna Valley

The autonomous PV systems cover a wide area of the applications that can be developed in the country, in places far from the national grid and with a low demand of electric power. The electrification of homes and farms in rural areas by using PV systems would contribute to improving the inhabitants' social, economic and cultural standard.In Romania, it is estimated that there are about 10,000 homes, farms and schools located in more than 500 settlements which could be electrified by using PV systems. The individual solar home systems would allow each user to adjust his own electricity consumption according to weather variations. A major problem in the development of these systems is the identification of the optimum technical and economic solutions as well as of the main financing sources. The paper aims to present the technical, economic and social problems involved by the implementation of such a project of PV-based electrification in Romania, presenting the particular case of two settlements (Scarisoara and Prisacina) located in the Cerna Valley - the Banat Mountains.     

Chemical transformation of PVC by phase transfer catalysis

Summary Suspension type PVC was reacted with sodium phenoxide in presence of different quaternary ammonium halides in THF or THF/water mixtures. The competition elimination vs. substitution of chlorine atoms depends on the homogeneity of the system, i.e. the more homogeneous the reaction mixture the less substituted the polymer.Dedicated to the 60th anniversary of Prof. Otto Vogl     

Remembrance of Petre T. Frangopol (1933–2020) the promoter of scientometrics in Romania

Scientometrics - The article is an obituary aimed to pay tribute to Petre T. Frangopol, a brilliant Romanian scientist (chemist and biophysicist). He had many fields of interest: Chemistry,...