Ani Stoilova

In this study, we determine the refractive index (n), extinction coefficient (k), absorption coefficient (α) and optical band gap (E_g) of thin films based on the commercially available azo polymer (poly [1-[4-(3-carboxy-4-hydroxyphenylazo) benzene-sulfonamido]-1,2-ethanediyl, sodium salt]), usually denoted as PAZO, doped with InP/ZnS quantum dots (QDs) at concentrations of 1 wt.%, 2 wt.%, and 10 wt.%. It was found that the absorption of the composite materials studied is driven by direct allowed electronic transitions, with the band gap energies tunable from 2.92 eV to 3.00 eV depending on the dopant concentration. Furthermore, the results suggest that resonance energy transfer (RET) between the InP/ZnS quantum dots and the azo polymer may occur, actively modulating the optical properties of the composite materials in the 385–750 nm spectral region, where the PAZO absorption spectrum and the emission spectrum of the QDs overlap. The theoretical calculation carried out demonstrate that the energy transfer is more efficient at 600 nm, than at 420 nm, where the two observed peaks in the emission spectrum of the dopant are positioned. We believe that the result obtained may be of interest to researchers working on improving the photoresponse of azo polymers through incorporation of quantum dots, particularly given that most studies in this field focus on cadmium-containing nanocrystals, whereas InP/ZnS quantum dots represent a more environmentally friendly alternative.

Environmental and health concerns drive the search for environmentally benign anodizing electrolytes. This study presents a comparative assessment of anodic films formed on AA2024-T3 alloy in 0.5 M solutions of seven biocompatible organic acids. Anodization was performed at 15 mA cm-2 for 30 min at 20 °C following identical preliminary treatments. The influence of electrolyte composition on film barrier properties, surface morphology, wettability, and chemical composition was evaluated using Electrochemical Impedance Spectroscopy, Potentiodynamic Polarization, Optical Metallographic Microscopy, Scanning Electron Microscopy, sessile drop tests, and X-ray Photoelectron Spectroscopy. The obtained barrier abilities followed the order: malic > malonic > tartaric > citric > oxalic > maleic acid, while glycine exhibited a detrimental effect on film properties. Additional experiments were conducted on films produced in malic acid after thermal treatment in boiling distilled water for 30 min. Although the treatment slightly reduced barrier performance, it significantly improved repeatability by converting the surfaces from completely hydrophobic to fully hydrophilic. The relationships between electrolyte composition, surface characteristics, and corrosion-protective properties are systematically examined and discussed.

The recent endeavor to establish a sustainable society, with respect to environmental protection and occupational health prevention, imposes the need for the development of environmentally friendly anodization electrolytes. In addition, these electrolytes should be composed of biocompatible organic acids derived from renewable sources. In response to these challenges, a need arises to seek environmentally conscious alternatives to the widely used sulfuric acid anodization electrolytes. Accordingly, a comparative study was performed on the anodic polarization of AA2024-T3 aircraft alloy samples for 30 min at 0 or 20 °C. The respective electrolytes were composed of 0.5 M solutions of oxalic, citric, tartaric acids, or glycine. The comparative analysis included optical metallographic microscopy (OMM), scanning electron microscopy (SEM), determination of color characteristics and wettability, chemical composition analysis by X-ray photoelectron spectroscopy (XPS), and assessment of the corrosion protective properties of the obtained layers. The latter were defined by the application of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization scanning (PDS) after 24 h of exposure to a 0.5% NaCl solution. Among the most important conclusions is that the barrier properties of the layers obtained in citric and tartaric acid electrolytes remarkably exceed those of the film obtained in oxalic acid. The use of glycine does not result in film formation at all. The process temperature did not have as strong an effect as the electrolyte composition.

Bulk chalcogenides from the system (GeTe₄)_1−xIn_x, where x = 0; 5 and 10 mol%, were synthesized by a two-step melt quenching technique. New layered composite materials based on them and the azo polymer [1-4-(3-carboxy-4-hydrophenylazo) benzensulfonamido]-1,2-ethanediyl, sodium salt] has been prepared through spin coating, electrospray deposition and via vacuum-thermal evaporation of the chalcogenide and spin coating of the azo polymer onto it. Using the latter technology, a material consisting of one chalcogenide and one azo polymer film and three chalcogenide and three azo polymer films has been fabricated. The carried-out SEM analysis shows that in the materials, initially prepared as a bilayer and multilayer structure, diffusion at the chalcogenide/polymer interface occurs leading to the formation of a homogenous composite environment. Birefringence was induced at 444 nm in all the fabricated thin film materials. The highest value of the maximal induced birefringence has been measured for the material fabricated as a stack, Δn_max = 0.118. For the material prepared as a bilayer structure and the composite material obtained through electrospray deposition, the maximal induced birefringence takes values of Δn_max = 0.101 and Δn_max = 0.095, respectively. The sample prepared via spin coating of the chalcogenide/PAZO dispersion has the lowest value of the maximal induced birefringence (Δn_max = 0.066) in comparison to the pure PAZO polymer film (Δn_max = 0.083).

New bulk chalcogenides from the system (GeTe₆)_1−xCu_x, where x = 5, 10, 15 and 20 mol%, have been synthesized. The structure and composition of the materials were studied using X-ray powder diffraction (XRD) and energy-dispersive spectroscopy (EDS). Scanning electron microscopy (SEM) was applied to analyze the surface morphology of the samples. Some thermal characteristics such as the glass transition, crystallization and melting temperature and some physico-chemical properties such as the density, compactness and molar and free volumes were also determined. The XRD patterns show sharp diffraction peaks, indicating that the synthesized new bulk materials are crystalline. The following four crystal phases were determined: Te, Cu, CuTe and Cu₂GeTe₃. The results from the EDS confirmed the presence of Ge, Te and Cu in the bulk samples in concentrations in good correspondence with those theoretically determined. A layered thin-film material based on Ge₁₄Te₈₁Cu₅, which exhibits lower network compactness compared to the other synthesized new chalcogenides, and the azo polymer PAZO was fabricated, and the kinetics of the photoinduced birefringence at 444 nm was measured. The results indicated an increase in the maximal induced birefringence for the layered structure in comparison to the non-doped azo polymer film.

The study presents calcination of barite recovered from the gangue stored in the tailings dam at a metallurgical plant in Bulgaria with the aim to product BaSO4 with improved whiteness. The calcination was carried out in a rotary kiln at 1150⁰C under air. Samples from three batches of barite with different particle size, namely 40 µm, 20 µm and 6 µm, were processed and the color and brightness value of the product was measured. The residence time of the material in the kiln, respectively in the heated zone, was also determined. The results show that the brightness of the barite recovered from the gangue could be improved by 4 – 20 units through calcination.

New bulk chalcogenides from the system (GeTe6)1-xCux, where x=5, 10, 15 и 20 mol %, has been synthesized. The structure and composition of the materials was studied using X-ray powder diffraction (XRD) and energy dispersive spectroscopy (EDS). Scanning electron microscopy (SEM) has been applied to analyze the surface morphology of the samples. Some thermal characteristics as glass transition, crystallization and melting temperature and some physicochemical properties as density, compactness, molar and free volume were also determined. The XRD pattern shows sharp diffraction peaks indicating that the synthesized new bulk materials are crystalline. The following four crystal phases were determined: Te, Cu, CuTe и Cu2GeTe3. The results from the EDS confirm the presence of Ge, Te and Cu in the bulk samples in a concentration in good correspondence with those theoretically determined. A layered thin film material based on the synthesized new Ge14Te81Cu5 chalcogenide, which has the most flexible structure, and the azo polymer PAZO has been fabricated, and the kinetic of the photoinduced birefringence at 444 nm has been measured. The results indicate an increase in the maximal induced birefringence for the layered structure in comparison to the non-doped azo polymer film.

The article describes the preparation of new thin film composite materials based on the azopolymer (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with three different concentration of InP/ZnS quantum dots with size of 7.5 nm. Clusters of aggregated InP/ZnS were microscopically observed in the fabricated composite samples. Birefringence (Δn) was induced in the films at two different wavelengths of the pump laser (355 nm and 444 nm) and the obtained higher values of Δn_max for the samples doped with quantum dots up to 2 wt% in comparison to the non-doped film and the sample with the highest nanocrystals concentration are discussed, based on the measured fluorescence spectra, in terms of possible local energy transfer between the azobenzene chromophores and the InP/ZnS quantum dots.