Walter Mayr

Thermoluminescence spectroscopy of Eu2+ and Mn2+ doped BaMgAl10O17

Abstract Thermoluminescence (TL) studies of Eu2+ and Mn2+ doped BaMgAl10O17 (BAM) are reported and discussed. The TL spectra that are measured after irradiation with ultraviolet (120– 350 nm ) show a series of TL peaks between 100 and 300 K . The TL spectra are similar for BAM with the two dopants, which suggest that the shallow traps are typical for the BAM host lattice. Using the Hoogstraaten analysis trap depths between 0.1 and 0.2 eV are determined. A model is proposed based on thermally activated recombination in local TL centres (not via the conduction band). Further support for this model is obtained from the observation that the TL signal is strongest for excitation around the band edge of BAM (∼180 nm ) . Upon heating the samples in air all low temperature TL peaks decrease in intensity. In addition a new peak appears in the TL spectrum, which is connected with a deeper trap (0.3 eV ) and also a partial oxidation of Eu2+ to Eu3+ is observed. The luminescence efficiency is lower and the UV induced degradation is faster after annealing in air. These results indicate that the shallow traps are related to oxygen vacancies. The shallow traps do not have a negative influence on performance (efficiency and degradation) of BAM as a lighting phosphor. The luminescence efficiency and stability are strongly influenced by the formation of Eu3+ and a deeper trap during annealing in air. Subsequent annealing in a reducing atmosphere restores the original properties.

Blue emitting BaMgAl10O17:Eu with a blue body color

Abstract The optical properties of Co 2+ -doped BaMgAl 10 O 17 (BAM) and BaMgAl 10 O 17 :Eu (BAM:Eu) are elucidated with emphasis on reflection, (V)UV excitation and luminescence spectra. Doping of BAM with Co 2+ yields a powder with a blue body color equal to that of CoAl 2 O 4 . Co 2+ is incorporated into the spinel blocks of BAM, which are spaced by Ba–O layers hosting the activator Eu 2+ . By means of reflection spectra it is shown that the local environment of Co 2+ is equal to that in CoAl 2 O 4 , which is responsible for the identical body color. The intrinsic pigmentation of BAM:Eu by Co 2+ yields a phosphor with an improved color point due to the self-absorption of the low-energy tail of the 4f–5d emission band of Eu 2+ . Moreover, the presence of Co 2+ in the host lattice of BAM results in resonant afterglow behavior. Only under photoexcitation at the band gap of BAM:Eu afterglow is observed.

Phosphor screens for flat cathode ray tubes

Abstract In the Zeus display panel, as in other flat-panel cathode ray tubes, the anode voltage is limited to a few kV. The effects of the anode voltage on phosphor screen performance are discussed which include energy conversion loss processes at the phosphor surface and variation of the number of luminescent centres available. Phosphor efficiency saturation has been measured in the range of 1 to 10 kV, using a pulse-width-modulation scheme. The luminance performance at different excitation conditions is predicted and confirmed experimentally. Coulomb degradation of phosphor screens is shown to increase with decreasing anode voltage. Novel ultra-thin and dense anti-degradation coatings have been developed for zinc sulphide phosphors which effectively prevent fast degradation due to chemical surface reactions. In this way a satisfactory solution has been found to obtain in the Zeus display a luminance level about equal to the best conventional tubes.