Thermal cracks in LTB:Cu luminescent ceramics are connected with the presence of air voids in the material, which decreases the heat conductance. Powder phosphor would be more stable against heat. The effects of sintering time, grinding, and subsequent annealing on the luminescence intensity are studied. Rough grinding decreases the efficiency by 15-20%, but this should be checked more thoroughly. Subsequent annealing does not affect the luminescence properties if the annealing temperature does not exceed 400 °C, but the luminescence yield decreases if the temperature exceeds 500 °C. Due to a large number of luminescence centers and a relatively small amount of electron traps, LTB:Cu should be tolerant of high radiation doses caused by bombardment with electrons in cathodoluminescent UV-radiation sources.
ZnO:W luminescent material was prepared and characterized. We compare nano-sized and bulk material composition and luminescence. Nanoparticles were prepared by a novel acoustoplasma technique (plasma discharge in a liquid at an intensive ultrasonic field above the cavitation threshold). Bulk ZnO:W was prepared by classical annealing way from the mixture of ZnO powder and H2WO4. Under excitation with a pulsed electron beam, the ZnO:W nanoparticles demonstrated very low luminescence efficiency while the bulk sample produced intensive green cathodoluminescence. The most intensive green luminescence was observed after annealing of the sample in air, which excludes a well-known mechanism involving anion vacancies. Probably, tungsten impurity tunes excitonic luminescence towards longer wavelengths, but this point requires further investigations for verification.
Mercury-free UV-radiation sources are described. An electron beam similar to cathode-ray tubes (CRT) excites a luminescent material in a vacuum bulb. A high density of excitation requires the cathode and the luminescent material to be resistant for that and provide the extended lifetime of the UV-radiation source. Carbon fibre and nano-carbon based field-emission cathodes produce long lasting stable emission with a high current density (up to 0.3-0.5 A/cm2 ). Li2B4O7:Cu and Li2B4O7:Ag luminescent ceramics survive under high radiation doses and provide UV luminescence bands peaked at 360-370 nm and 270 nm, respectively. The luminescence band at 360-370 nm has a good overlap with the fundamental absorption edge of TiO2, which is known as a photo-catalyst in air and water cleaning systems. The luminescence band at 270 nm overlaps with DNA absorption and provides a direct disinfection effect. We suggest the structure of complex luminescence centres and energy transfer mechanisms. The electron structure of lithium tetraborate and the contribution of impurities are also discussed in paper.