Transition-Metal-Doped Alkali Borosilicate Glass as a Phosphor Material

Abstract

Alkali-borosilicate glasses doped with transition metals were studied to test their suitability as phosphor materials. The goal of this work is to employ a low-processing temperature glass system doped with photoluminescent activators as a direct coating on long-wavelength UV and blue GaN and InGaN LEDs. The blue or UV LED emission would act as the excitation source, so that the combination of emission colors yields white light via additive mixing. The LED materials must be processed at moderate temperatures (approximately ≤ 500° C), so a thermally suitable glass host is of importance. The transition metal ions that were studied here are Cr3+ and Mn2+, as well as a reduced niobium-containing glass that contains a mix of niobium oxidation states—primarily Nb4+ and Nb5+. The green-colored Cr3+-containing glass was found to emit weak red/near IR light upon excitation with 450 nm light. Both Mn2+- and Nb-doped glasses were transparent, and found to emit intense orange light with a peak emission wavelength of around 600 nm upon excitation with 366 nm UV radiation. Niobium in the reduced state [Nb4+] was detected via electron spin resonance [ESR] measurements. Glasses near the base glass composition of 11.7 Na_{2}O•36.7 B_{2}O_{3}•51.6 SiO_{2} are known to be spinodally phase-separated on the order of tens of Angstroms, but this was not found to be detrimental to the fluorescence properties. The host glass exhibited onset of glass transition temperature [Tg ≈ 460° C] and a coefficient of linear thermal expansion [CTE] of around 4-5 ppm*K-1, which matches the typical LED materials well. The reduced state of manganese was readily obtained via traditional batching and melting techniques, and the Mn2+-doped alkali borosilicate would make a suitable orange-emitting fluorescent material. The luminescence of the niobium-containing glass may also be useful for the orange emission if an appropriate model for the fluorescence were determined. The 600 nm emissions would require the addition of other colors to achieve white light, so a mix of luminescent activators such as rareearths would be necessary in the proposed phosphor system.