Papers by Author: Harald Kaps

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.