Papers by Author: Heiko Paul

Abstract: Employing x-ray diffractometry and electron microscopy, we have investigated thermally induced microstructural evolution in ball-milled nanocrystalline Fe. At low annealing temperatures, the early-stage growth of the area-weighted and volume-weighted average grain sizes deviates strongly from the parabolic behavior expected for normal grain growth. Analysis of the ratio of these two averages indicates that the width of the grain-size distribution changes with time. This result is more consistent with the occurrence of a transient stage of abnormal grain growth than with a grain-size-dependent change in the rate-limiting mechanism for grain-boundary migration.

Abstract: Due to its high quantum efficiency (QE) for luminescence, conventional coarse-grained YAG:Ce (Y3Al5O12:Ce) finds widespread use in light conversion and scintillator applications. Nanocrystalline YAG:Ce may possess modified optical properties which are advantageous for technological applications, but this will depend on highly efficient energy conversion. In this work, the effect of the particle size and Ce concentration on the quantum efficiency and the optical lifetime of the YAG:Ce emission will be characterized and discussed. Nanocrystalline YAG:Ce with an average particle size of 20 to 50 nm was synthesized by the chemical vapour reaction (CVR) method and subsequently analyzed using various techniques. When comparing the nanocrystalline samples to a coarse-grained reference sample, the particle size and doping concentration was found to have a significant influence on quantum efficiency. It was established that the nanocrystalline samples investigated exhibit a lower QE at ambient temperature than the coarse-grained reference. The results of the optical lifetime measurements are discussed in relation to this reduction in QE.

Abstract: Nanocrystalline (Y1-xEux)2O3 powder was synthesized via a chemical vapour reaction. Xray diffraction revealed the structure of cubic yttria with crystallite sizes of about 5 nm. The Eudopand concentrations x for the samples in the range from 0.003 up to 0.165 were determined by EDX-spectra. The luminescence of the nanopowders was investigated by continuous and timeresolved UV-fluorescence spectroscopy and compared to a microcrystalline Y2O3:Eu phosphor as a reference. The emission spectra show an increasing intensity for higher doping concentrations. However, compared to the microcrystalline material the yield was significantly lower. The lifetime of the 5D0 – 7FJ transition in the nanocrystalline Y2O3:Eu was found to be significantly longer than for the microcrystalline reference sample. For increasing Eu-content the lifetime in the nanocrystalline samples decreased continuously from 3.71 ms to a value of 1.20 ms for the highest doping concentration. The concentration dependent lifetime behaviour was interpreted by energy transfer between Eu ions and from Eu ions to impurities as a competing process to the radiative 5D0 – 7F2 transition.

Abstract: Nano-crystalline YAG:Ce (Y3Al5O12:Ce) with an average particle size of 10-50 nm was synthesized by the vapour phase reaction (CVR) in a hot-wall reactor. This nano-crystalline luminescent material has the advantage of a high light-conversion efficiency associated with superior light-scattering properties. Phases and compositions were analyzed by XRD and EDX, respectively. Photoluminescence was used to investigate the optical properties of the YAG host lattice doped with Cerium. As a first step the conditions for the in-situ production of the YAG phase were established. On the basis of previous DSC investigations of YAG phase formation, the DSC results were analyzed according to the Kissinger theory for thermally activated transformations. From the results it is predicted that, with sufficient heat transfer during the residence time of the particles in the hot zone of the reactor, YAG can be obtained in-situ during the CVR process. By modifying the parameters and the CVR setup it was possible to achieve conditions, which allow the production of in-situ YAG. Samples with Ce doping concentrations ranging from 0 to 2.65 at.% Ce were prepared to investigate the influence of the doping concentration on the luminescence of YAG:Ce.