Janis Grabis

The role of Nb in intensity increase of Er ion upconversion luminescence in zirconia

It is found that Nb co-doping increases the luminescence and upconversion luminescence intensity in rare earth doped zirconia. Er and Yb-doped nanocrystalline samples with or without Nb co-doping were prepared by sol-gel method and thermally annealed to check for the impact of phase transition on luminescence properties. Phase composition and grain sizes were examined by X-ray diffraction; the morphology was checked by scanning- and high-resolution transmission electron microscopes. Both steady-state and time-resolved luminescence were studied. Comparison of samples with different oxygen vacancy concentrations and different Nb concentrations confirmed the known assumption that oxygen vacancies are the main agents for tetragonal or cubic phase stabilization. The oxygen vacancies quench the upconversion luminescence; however, they also prevent agglomeration of rare-earth ions and/or displacement of rare-earth ions to grain surfaces. It is found that co-doping with Nb ions significantly (>20 times) increases upconversion luminescence intensity. Hence, ZrO2:Er:Yb:Nb nanocrystals may show promise for upconversion applications.

Luminescence Properties of ZnO Nanocrystals and Ceramics

The luminescence excitation spectra, luminescence spectra and the nanosecond-scale decay kinetics were studied. The ZnO and ZnO:Al nanopowders were prepared by vaporization-condensation in a solar furnace using different raw powders: commercial, hydrothermal and those obtained by plasma synthesis. Exciton-phonon as well as exciton-exciton interaction processes in nanopowders, a bulk crystal and ZnO ceramics were studied and compared. The fast decay and low afterglow intensity of ZnO nanopowders and ceramics support these materials for scintillators.

A goal model-driven supply chain design

Modern decision-making problems are often characterised by a high degree of complexity and supply chain design is a good example of such a complex problem. In order to solve it, different problem-solving techniques, including descriptive modelling, quantitative modelling, computerised implementation and the presentation of results, should be considered. To achieve that, the goal model-driven supply chain design approach is proposed in this paper. This approach uses the principles of model- driven development to combine various problem-solving techniques. The paper describes the use of goal modelling to formally represent the supply chain design problem, defines transformations to obtain the multi-objective optimisation model on the basis of the goal model and uses business intelligence methods to represent modelling results. An example demonstrating the goal model-driven supply chain design is provided.

Nanosized Powders of Refractory Compounds for Obtaining of Fine-Grained Ceramic Materials

Most of ceramic materials are made of powders and therefore their properties depend to a great extent on the quality of the starting powders. The powder determines the processing, sintering behavior and the subsequent formation of the microstructure, which strongly affects many of the dense materials properties. One of the ways to produce ceramic materials with fine-grained structure is the application of nanopowders. Different methods are used for the production of nanopowders. One of them is the method of plasmachemical synthesis. Different nanopowders of refractory materials can be obtained by this method. Preparation of nanosized powders of nitrides and oxides and their composites by the method of plasmachemical synthesis, as well as the characteristics and possible advantages of nanopowders were investigated.

Spectral characterization of bulk and nanostructured aluminum nitride

Spectral characteristics including photoluminescence (PL) spectra and its excitation spectra for different AlN materials (AlN ceramics, macro size powder and nanostructured forms such as nanopowder, nanorods and nanotips) were investigated at room temperature. Besides the well known UV-blue (around 400 nm) and red (600 nm) luminescence, the 480 nm band was also observed as an asymmetric long-wavelength shoulder of the UV-blue PL band. This band can be related to the luminescence of some kind of surface defects, probably also including the oxygen-related defects. The mechanisms of recombination luminescence and excitation of the UV-blue luminescence caused by the oxygen-related defects were investigated. It was found that the same PL band is characteristic for different AlN materials mentioned above; however, in the nanostructured materials (nanorods, nanotips and nanopowder) the intensity of UV-blue PL is remarkable lower than in the bulk material (ceramics). In the case of nanostructured AlN materials, excitation of the oxygen-related defect is mainly realized through the energy transfer from the host material (electron/hole or exciton processes) to the defects and this mechanism prevails over the mechanism of direct defect excitation.

Configurable supply chain: framework, methodology and application

Dynamic nature of modern supply chains leads to frequent changes in the supply chain structure. Appropriate decision-modelling mechanisms are required to support decision-making in such an environment. This paper discusses general requirements for a methodology governing modelling of reconfigurable supply chain and develops supply chain configuration methodology according to these requirements. Two main features of the proposed methodology are information modelling and hybrid modelling. Information modelling enables systematic formalisation of modelling problems and processes. Hybrid modelling combining optimisation and simulation allows for comprehensive evaluation of alternative configurations subject to dynamic and stochastic factors. The methodology is applied to deal with a multicriteria supplier selection problem. Specific optimisation and simulation models are developed for this particular problem. These models are integrated using meta-modelling.

Doped zirconia phase and luminescence dependence on the nature of charge compensation

Zirconia is a relatively new material with many promising practical applications in medical imaging, biolabeling, sensors, and other fields. In this study we have investigated lanthanide and niobium doped zirconia by luminescence and XRD methods. It was proven that charge compensation in different zirconia phases determines the incorporation of intrinsic defects and activators. Thus, the structure of zirconia does not affect the Er luminescence directly; however, it strongly affects the defect distribution around lanthanide ions and the way in which activator ions are incorporated in the lattice. Our results demonstrate the correlation between the crystalline phase of zirconia and charge compensation, as well as the contribution of different nanocrystal grain sizes. In addition, our experimental results verify the theoretical studies of metastable (tetragonal, cubic) phase stabilization determined using only oxygen vacancies. Moreover, it was found that adding niobium drastically increases activator luminescence intensity, which makes Ln3+ doped zirconia even more attractive for various practical applications. Although this study was based on the luminescence of the Er ion, the phase stabilization, charge compensation, and luminescence properties described in our results are expected to be similar for other lanthanide elements. Our results suggest that the luminescence intensity of other oxide matrices where lanthanides incorporate in place of tetravalent cations could be increased by addition of Nb ions.

Plasma-Chemical Synthesis of Nanosized Powders – Nitrides, Carbides, Oxides, Carbon Nanotubes and Fullerenes

In this article the plasma-chemical synthesis of nanosized powders (nitrides, carbides, oxides, carbon nanotubes and fullerenes) is reviewed. Nanosized powders - nitrides, carbides, oxides, carbon nanotubes and fullerenes have been successfully produced using different techniques, technological apparatuses and conditions for their plasma-chemical synthesis.

Blue luminescence in ZnO single crystals, nanopowders, ceramic

The luminescence spectra and luminescence decay processes were studied in a ZnO single crystal, nanopowders and ceramic at liquid helium and room temperature under VUV synchrotron radiation as well as under pulsed laser excitation. The exciton-exciton and exciton-multiphonon processes were compared in different ZnO nanopowders (commercial powder, powders obtained by vaporization-condensation technique) and ceramic. The possibility of luminescence decay time modification by Al3+ doping was shown.

Nanosized lithium titanates produced by plasma technique

The synthesis of nanosized lithium titanates is studied by evaporation of coarse grained commercially available titanium and lithium carbonate particles in radio-frequency plasma flow with subsequent controlling formation and growth conditions of product particles. In accordance with the XRD analysis the phase composition of the obtained powders is determined by feeding rate of precursors and strongly by ratio of lithium and titanium. The Li2TiO3 and Li4Ti5O12 particles containing small amounts of extra phases were obtained at ratio of Li/Ti = 2 and Li/Ti = 0.8 respectively, feeding rate of precursors being in the range of 0.6-0.9 kg/h. Specific surface area of powders is in the range of 20-40 m2/g depending on concentration of vapours in gas flow and cooling rate of the products. Additional calcination of nanosize particles at 800-900 °C improves phase composition of lithium titanates.