Volumen 55 (2018): Heft 6 (December 2018)

We have studied luminescence of LiGaO₂, Al₂O₃-Ga and Al₂O₃-Li crystals in order to reveal the nature of luminescence centres and mechanisms in these crystals. In Al₂O₃-Ga presence of Ga impurities determines occurrence of the 280 nm emission band, which demonstrates intra-centre character in photoluminescence and recombination character under X-ray irradiation. In Al₂O₃-Li crystal lithium induced luminescence is presented with the 326 nm band, which has a recombination character. Basing on spectral similarity of the main luminescence bands in pure LiGaO₂ crystal with the dopant-induced emission bands in Al₂O₃, and on peculiarities of the X-ray induced thermoluminescence, the adjustment of the previous luminescence interpretation is done. It is proposed that the donor-acceptor pairs with random separation distribution responsible for the 280 nm emission are represented with gallium Ga (plus an electron) and O (plus a hole) pairs, while the donor-acceptor pairs, producing the 330 nm emission band contain a lithium ion, presumably in the interstitial position Li_i⁰, and a neighbouring oxygen ion with a caught hole.

X-ray absorption spectroscopy at the Cu K-edge is used to study X-ray induced photoreduction of copper oxide to metallic copper. Although no photoreduction has been observed in microcrystalline copper oxide, we have found that the photoreduction kinetics of nanocrystalline CuO depends on the crystallite size, temperature and pressure. The rate of photoreduction increases for smaller nanoparticles but decreases at low temperature and higher pressure.

The projector augmented-wave method within the density functional theory is applied to investigate the oxygen diffusion in the intermetallic Ti-Al alloys. It is shown that the highest oxygen absorption energies in Ti-Al alloys correspond to the octahedral Ti-rich sites but the presence of aluminium in the nearest neighbours leads to a substantial decrease in the oxygen absorption energy in the alloys. The migration barriers for the oxygen diffusion between various interstices in the crystal lattice of the Ti-Al alloys are estimated. The preferred migration paths along a and c axes and limiting barriers of the oxygen diffusion in the alloys are determined. The dependence of the oxygen diffusion coefficient on Ti-Al alloy composition is discussed.

Substitution of fossil-based chemical processes by the combination of electrochemical reactions driven by sources of renewable energy and parallel use of H₂O and CO₂ to produce carbon and hydrogen, respectively, can serve as direct synthesis of bulk chemicals and fuels. We plan to design and develop a prototype of electrochemical reactor combining cathodic CO₂-reduction to ethylene and anodic H₂O oxidation to hydrogen peroxide. We perform ab initio calculations on the atomistic 2D graphene-based models with attached Cu atoms foreseen for dissociation of CO₂ and H₂O containing complexes, electronic properties of which are described taking into account elemental electrocatalytical reaction steps. The applicability of the model nanostructures for computer simulation on electrical conductivity of charged Cu_n/graphene (0001) surface is also reported.

Transparent ZnO ceramics are of interest for use as material for high-efficiency fast scintillators. Doping ZnO ceramics in order to improve complex of their properties is a promising direction. In the present research, the role of indium in the ZnO nanopowders surface interactions and in the change of microstructures and photoluminescence (PL) characteristics of sintered cera-mics is considered. Undoped and 0.13 wt% In doped ZnO ceramics are obtained by hot pressing sintering. It has been found that indium leads to the transition of initially faceted ZnO particles to rounded, contributing to good sintering with formation of diffusion active grain boundaries (GBs). Unlike ZnO ceramics, ZnO:In ceramics microstructure is characterised by the trans-crystalline mode of fracture, faceted GBs with places of zig-zag forms and predominant distribution of In at the GBs. Such indium induced modifications of GBs promote removal of point defects and reduce PL parameter α = I_def/I_exc in comparison with the undoped ceramics. Results characterise ZnO:In cera-mics with improved GBs properties as a prospective material for scintillators.

The MgGa₂O₄ ceramics co-doped with Mn²⁺ and Eu³⁺ ions were synthesized via a high-temperature solid-state reaction technique. The samples with various Eu³⁺ concentrations were characterised using high-resolution photoluminescence (PL) spectroscopy. The PL spectra show weak matrix emission in a blue spectral region with dominant excitation band around 380 nm. Manganese ions are highly excited deeply in UV region and exhibit emission band peaked at 502 nm. The Eu³⁺ ions show characteristic f-f excitation and emission lines. The energy transfer between host defects and activator ions was observed. Luminescence decay curves of Mn²⁺ and Eu³⁺ emission showed complex kinetics with both Eu³⁺-ion concentration and excitation wavelength changes.

The article presents an overview of the current situation of awareness of the Latvian citizens in the field of state-of-the-art energy-saving technologies. The authors present a wide range of data obtained as a result of a survey on the attitude of residents to new technologies and readiness to follow the development trends of a smart city.

The article contains the analysis and recommendations for improving the efficiency of introducing new energy-saving and energy-efficient technologies into each household in order to create the most favourable conditions for the implementation of long-term plans for the development of smart cities in Latvia.