Yttrium-aluminoborate glasses containing Tb2O3, Ce2O3 and Sb2O3 for visualization of UV and X-ray radiation.
Rare-earth (RE) doped glasses and fibers are attractive materials for applications in lighting and optical displays however they suffer from concentration quenching of luminescence resulting in low light yield. The concentration quenching occurs due to solubility limit of RE ions in oxide glass which cause segregation (clustering). Segregation takes place even at low concentrations of RE ions. Clustering facilitates energy migration between RE ions resulting in luminescence quenching due to the interaction between emission activator ions. Large distance between RE ions allows some control of the RE ions clustering. In order to suppress concentration quenching process among the doped ions and initiate intense luminescence in the glass host, searching of matrices with a large RE-RE distances is quite actual.
Among different oxide hosts with large RE-RE distance yttrium-aluminoborate crystals
RExY1-xAl3(BO3)4 with huntite-like structure and the distance between RE about 0,59 nm are indeed promising candidates . Prevailing content of boron oxide in huntite crystals assumes high glass-forming ability of melts and possibility of production of transparent glasses with the composition identical to that in the crystals.
It has been recently shown that the structure of the huntite cation lattice is preserved by passing from SmxY1-xAl3(BO3)4 polycrystals to Sm-containing huntite-like glasses with RE-RE minimum distance of about 0.67 nm. It has been shown that quantum yield of luminescence turns out to be 80% in glass in contrast to that of 55% in polycrystals due to a decrease of [BO3] group content . The presence of [BO3] vibrational groups allows to use RE ions as activator with energy gap, DЕ, between a metastable state and a lower state above 8000 cm-1. The energy gap between a metastable state and a lower state of Tb3+ ions is about 14500 cm-1 and of Ce3+ ions is about 25000 cm-1. Moreover, Ce3+ ion shows an efficient broad band electric dipole allowed 5d-4f transition and acts as a effective sensitizer, transferring a part of its energy to various activator ions, such as Tb3+. That’s why yttrium-aluminoborate glasses containing Tb3+ and Ce3+ ions were selected and synthesize to study their luminescence properties.
Yttrium-aluminoborate glasses with nominal compositions (mol. %) 10(CexTbyY1-x-y)2O3-30Al2O3-60B2O3 (x= 0¸9, y=0¸10) were prepared by melt-quenching method. All chemicals used in the synthesis were of “chemically pure” grade. Batches were calculated to yield 20 g of glass and were mixed homogeneously in a porcelain mortar and subsequently melted in an platinum crucible at 1480 °C for about 1 h in air. The melt was pressed between two stainless steel plates and finally annealed at 660°C for 3 h followed by cooling naturally to room temperature.
Absorption spectra were recorded on UV-3600 (Shimadzu) and Varian Cary-500 (Agilent) spectrophotometers. Excitation, emission spectra and luminescence decay curves were obtained on SDL-2 spectrofluorimeter (LOMO, Russia) and by means of a pulsed dye laser (YAG:Nd3+, Dtimp » 10 nc) a photomultiplier and a digital oscilloscope. XEL spectra were examined using an X-ray tube BSV-21 (30kV, 7,5mA) and detector (FEU-28).
Under melting conditions mentioned above the greater part of cerium turned out to be present in the glass in non-luminescent form as Ce4+. Addition of Sb2O3 leads to stabilization of cerium ions in triply charged state that gives rise to green luminescence under UV excitation.
It was found out that Sb5+ ions also take part in the Tb3+ ions luminescence sensitization. An attenuation of Tb luminescence occurs when the concentration of Sb5+ ions exceeds 1 mol. %. The explanation of this fact may be in the screening of the Tb absorption bands by the Sb5+ ions. A study of Sb luminescence revealed the possibility of using these glasses as quasi white phosphors.
The Judd-Ofelt intensity parameters for f-f transition of Tb3+ ions were determined for yttrium-aluminoborate glasses containing Tb3+ and Ce3+ (TCG) ions and containing only Tb3+ (TG). The following parameters were obtained: Ω2 = 5.45´10-20, Ω4 = 2.82´10-20, Ω6 = 3.76´10-20 cm2 for TCG and Ω2 = 4.99´10-20, Ω4 = 2.18´10-20, Ω6 = 3.51´10-20 cm2 for TG. Thus doping Ce3+ into glass as a co-activator doesn’t affect the local environment of Tb3+ ions.
As a result glasses containing Tb2O3, Ce2O3 and Sb2O3 showed luminescence quantum yield up to ~80% and full absorption of UV radiation at λ ≤ 320 nm in a layer ≤ 100 µm thick. The study of influence of different RE oxides on the spectrum luminescence characteristics allowed to establish that the composition 60B2O3–30Al2O3–1Ce2O3–6Tb2O3+1Sb2O3 is close to optimal and that huntite-like glasses are promising for the fabrication of heavily doped active media and can be used as visualizers of UV radiation for the producing of converters of UV radiation into the yellow-green region of the spectrum.
Yttrium-aluminoborate glasses are found to show green luminescence under X-rays, and the addition of heavy elements such as Pb causes the increase of luminescence, and a high scintillating efficiency of about 30% of that of the CdWO4 crystal was achieved.
This work is financially supported by the Ministry of Education and Science of the Russian Federation (grant 14.Z50.31.0009) and Russian Foundation for Basic Research (grant 16-53-00157).