Julian Plewa

Synthesis and optical properties of Li3Ba2La3(MoO4)8:Eu3+ powders and ceramics for pcLEDs

A series of polycrystalline Li3Ba2La3−xEux(MoO4)8 samples were prepared by the conventional solid-state reaction. The phase formation of the samples was investigated by X-ray diffraction measurements. The luminescence spectra and decay curves were studied as a function of Eu3+ concentration and temperature. It turned out that the optical band gap of the undoped molybdates is at 3.65 eV. The quantum efficiency (QE) of the Eu3+ doped luminescent materials increases with increasing Eu3+ concentration and almost 100% QE was obtained for those samples doped with 70, 80, or 90% Eu3+. A sample containing 100% Eu3+ showed solely a slight decrease in quantum efficiency. The luminous efficacy (LE) was 330 and 312 lm Wopt−1 for the 10 and 100% Eu3+ doped samples, respectively. The decrease of LE values is caused by a slight shift of the colour point to the red spectral range with increasing Eu3+ content. Temperature dependent measurements revealed that Li3Ba2Eu3(MoO4)8 loses only 15% of efficiency up to 400 K, which demonstrates that the investigated phosphors are attractive for application in pcLEDs.

The influence of Ag on univariant reactions and the oxygen content of the eutectic melt in the Y–Ba–Cu–(Ag)–O system

Abstract The influence of Ag admixtures on the phase equilibria of YBa2Cu3O7−δ near the liquidus surface was investigated at 0.21×105 Pa oxygen partial pressure using thermoanalytic methods (DTA/TG). The temperatures at which the liquid phase occurs are decreased by about 30 K in comparison with pure Y–Ba–Cu–O compositions. The peritectic decomposition of YBa2Cu3O7−δ is replaced by two sequential univariant reactions in the presence of silver: a eutectic reaction with a silver rich liquid at 944°C and a monotectic-like reaction at 980°C resulting in the Y–Ba–Cu–O rich melt besides Y2BaCuO5. Depending on the content of Ag, an enhanced oxygen concentration was detected in the melts which is available as an internal oxygen source during the crystallization of the superconducting phase by melt processing. The solubility of Ag in the melt was found to be 2.5 mol.% in the monotectic-like equilibrium at 980°C.

Thick films of ceramic, superconducting, and electro-ceramic materials

The use of thick films becomes more and more important in particular for electronic and microelectronic applications. The term “thick film ” does not relate so much to the thickness of the film but more to the kind of deposition. Thick films are made by low-priced processes such as doctor (dr) blading, screen-printing, or spraying methods, etc. The preparation of thick films of ceramic material by these methods generally implies a processing sequence of the following steps: preparation of the oxide powders; preparation of pastes and slurries; painting/printing, etc.of the pastes onto a suitable substrate; drying at low temperature; and sintering at high temperature to get a consolidated layer. These technologies and the fabricated thick films of thermoresistive and superconducting materials will be discussed.

Synthesis and dielectric properties of Am-1Bi2BmO3m+3 ceramic ferroelectrics with m=1.5

Abstract Bismuth containing ceramic materials of molecular formula Bi 5 TiWNbO 15 , Bi 5 Ti 1/2 W 1/2 Nb 2 O 15 , Bi 5 Nb 3 O 15 have been synthesised and fabricated. The materials are known as the Aurivillius phases. The Aurivillius phases are a family of oxide ferroelectrics generally formulated as A m −1 Bi 2 B m O 3 m +3 . The process of preparation was investigated by X-ray diffraction method as well as the simultaneous thermal analysis, in which both thermal (DTA) and mass change effects (TG) were measured on the same sample. The synthesis of the Aurivillius phases was carried out according to the solid state reaction from the conventional mixture of oxides (Nb 2 O 5 , WO 3 , Bi 2 O 3 , and TiO 2 ). DTA and TG traces showing the synthesis effects have been recorded. Investigations of the crystalline structure, microstructure and basic dielectric properties were performed.

Kinetic Analysis Crystallization of α-Al2O3 by Dynamic DTA Technique

AbstractThe phase transformation of seeded (5 mass% Fe2O3 as a Fe(NO3)3 solution) boehmite derived alumina gel to α-Al2O3 was studied with DTA technique and compared with unseeded and α-Al2O3 seeded boehmite gels. Data for kinetic analysis of α-Al2O3 crystallization were obtained from quantitative DTA curves. The kinetic parameters were analysed by traditional Kissinger analysis and Friedman and Ozawa-Flynn-Wall methods using the Netzsch Thermokinetics program. Results of the comparison of values of activation energies for all three gels and methods are the process of α-Al2O3 transformation for originally γ-AlOOH/Fe(NO3)3 gels goes like that of unseeded boehmite gels,only under lower temperatures (lower about 200°C).

Superconducting materials for electronic applications

Abstract Shielding elements were made of bulk material or thick films. The bulk route is determined by large-scale processing. After an axial or isostatic pressing of the fine-grained powder composed of BSCCO, Y-123 and Nd-123, the material was sintered/textured and annealed in special programs. Then the characterization of the materials and a measurement of magnetic fields was done with the help of a specially constructed measuring apparatus with Hall- and SQUID-sensors. The objective of the thick film route was to get large, coated superconducting work pieces of YBCO and BSCCO (plates or cylinders) with current density of >1000 kA/cm 2 (and/or a shielding effect of >1 mT). Thick films were made by plasma spraying and paste technology. Substrates used were ZrO 2 , MgO, SrTiO 3 , BaZrO 3 and others. In some cases the work pieces were treated with laser radiation. After laser and thermal treatment the surface was more dense, the cracks were cured and the current density was increased by more than two times.

Improvement of HTSC-materials by treatment with laser

Abstract The influence of different kinds (Nd, ruby and CO2) of laser radiation on HTSC (Y-123, Bi-2212, Bi-2223) ceramics are shown. Irradiation of HTSC leads to modification of the near-surface layers, by processes such as ceramic densification, granular melting and the appearance of continuous melting with increasing laser irradiation energy. On the whole, the density and smoothness of the surface were improved and content of carbon was minimised with the help of radiation and subsequent thermal treatment. The current density has higher values (>500 A/cm2) than those which were achieved before the laser treatment (20–340 A/cm2).

Thermische Untersuchungen der Kristallisation von Korund

With thermal analysis (DTA/TG) the crystallisation behaviour of corundum with influence of the fluor compounds were investigated. The formation of aluminium fluoridhydroxide in the temperature range 400-600°C are observed. Through the influence of AIF3 or HF the strong exothermic effect increased and a rapid loss of mass were noticed. The particle of corundum grow more, when the exothermic effects are large.We supposed, that also these exothermic effect will be due to the decomposition of may be adsorbed AIOF gas molecules.

Peritektische Zersetzung von YBa2Cu3O7−x

During the heating of YBCO a peritectic reaction takes place at 1020‡C, which can be described by: 2YBa2Cu3O7−x⇄Y2BaCuO5+L+(1-2x)/2O2 (1) whereL = 3BaCuO2 +2CuO is a fluid with limited amount of yttrium.It has been reported, that many parameters can influence the reaction. From one side not only the starting size of grains but also the heating rate have an influence on the resulting Y2BaCuO5-phase. From the other side, there is a change of the peritectical temperature caused by changing of the partial pressure of O2 and the presence of parasitic phase.From general kinetic consideration one can draw the conclusion, that different mechanisms (nucleation, phase-boundary reaction and diffusion) can control the reaction.Using DTA/TG measurements, the peritectic reaction has been examined. Classical kinetic methods (Kissinger and Friedman) has been used. The Friedman method has given the dependence of the activation energy from reaction degree. This suggests many steps reactions. The dependence of the DTA-peaks from the heat rate suggest a parallel steps of reaction. This assumption can be motivated by evaluation of free O2, one solid and liquid phase formation. Amount of this phases depends on the heating rate. Additionally X-ray and microscopic methods has been used. In this way was shown, that the perovskit structure is stable up to peritectical temperature and than is dramatically destroyed. From microscopic observations has been got information about shape and size of solid phase and its creation as a function of temperature, time and starting grain size.

The crystallization kinetics of Er/Yb co-doped oxyfluoride glasses

The intention of the authors is to show a possibility of controlled crystallization of Er/Yb co-doped oxyfluoride glass fibers provided for glass-ceramics core fiber lasers or optical amplifiers at 1550 nm. Selected glasses (the batch composition 48SiO211Al2O3-7Na2CO3-10CaO-10PbO-12PbF2-1.5/0.6YbF3-0.5/0.2ErF3) were examined in the form of powders of diameter 45-100 μm (diameter comparable to that of standard multimode fiber core 62 μm). Powders were annealed at various temperatures and time periods in order to obtain glass-ceramics with different crystalline fraction. DTA measurements and calculations (isothermal and non-isothermal annealing) enabled to determine JMAK (JohnsonMehl-Avrami-Kissinger) equation parameters and to estimate crystal growth rate and certain features of nucleation stage. XRD/TEM/SAED/EDS/HRTEM/SEM/EDS techniques (X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy) enabled to determine crystal structure of growing crystals (erbium and ytterbium enriched hexagonal PbF2, erbium and ytterbium enriched cubic PbF2 isomorphic to the fluorite structure). Luminescence intensity at 1550 nm under 488/980 nm excitation has shown comparable values for glassceramics powder and for bulk glass what suggests a considerably higher emission yield for glass-ceramics fiber. The lifetime of erbium excited state for glass-ceramics powder (~4 ms) is also comparable to that of bulk glass and is promising from the point of view of a stimulated emission. Computed parameters of JMAK equation enable to establish heat treatment conditions for glass fibers and hence to control the level (fraction) of glass crystallization.