Luminescence in CaSO₄: Dy phosphor - dependence on grain agglomeration, sintering temperature, sieving and washing

Abstract

In the recently developed high-sensitive CaSO4 : Dy phosphor, sieving before the high-temperature sintering treatment has successfully eliminated particle agglomeration during subsequent sintering, and has further enhanced its thermostimulated luminescence (TSL) sensitivity to γ-rays. The reduction in TSL sensitivity of higher sized grains observed earlier following the procedure of sieving after sintering has also more or less vanished. Maximum TSL sensitivity is seen after sintering around 700°C, whereas maximum photoluminescent (PL) sensitivity is seen after sintering around 325°C. While the observed increase in TSL sensitivity (by 30%) with increasing sintering temperature in the range 325-700°C is explained on the basis of diffusion of Dy3+ ions from the surface to the whole volume of the grains (0-75 µm), the drastic decrease (by a factor of 3) in PL sensitivity with increasing sintering temperature is explained on the basis of change in the Dy3+ environment on the grain surface perhaps due to oxygen incorporation. Washing with water and acetone, which affect mainly the surface traps, enhances the PL sensitivity of CaSO4 : Dy slightly; however, it does not influence TSL sensitivity very significantly. Grinding reduces PL in general, but no such trend was noticed in TSL which supports the conclusion that PL originates mainly from surface traps since grinding affects mainly the grain surface. However, the sharp reduction in TSL and PL sensitivities observed at 400°C indicates that an unusual process takes place near that sintering temperature.

The Role of Sintering in the Synthesis of Luminescence Phosphors

“…Significantly, on UV illumination, a white-light phosphor could be achieved if the bluishgreen ZnWO 4 and red Y 2 O 3 :Eu 3+, Li + phosphors were blended.…”

“…However, the sharp reduction in TL and PL sensitivities observed at 400°C indicates that an unusual process takes place near that sintering temperature. TG-DTA (thermogravimetric and differential thermal analysis) data indicate that dysprosium sulphate dopant in CaSO 4 which are hydrated at RT become anhydrous at 400 0 C and the water molecules released possibly damage the crystal lattice which get restored at higher sintering temperatures. As a result, CaSO 4 :Dy annealed at 400 0 C show a slightly reduced TL while those annealed at 300 0 C or at 700 0 C do not show any such reduction.…”

“…After the complete dissolution of these oxides, NH 4 OH solution was added drop wise until the pH of the solution reached 9. White precipitates were instantaneously formed and so-obtained precipitates were washed several times with distilled water by centrifugation.…”

“…The amount of H 3 BO 3 in the solution was adjusted so as to have an appropriatemole ratio with respect to Gd and Eu to give GdBO 3 :Eu 3+ . NH 4 OH was used to adjust pH of the precursor solution to 7-10. After vigorous stirring, the precursor solution was placed in a Teflon-lined stainless steel autoclave with a volume of 100 ml.…”

Effect of NaF flux on microstructure and thermoluminescence properties of Sm3+ doped CdSiO3 nanophosphor

“…Even though considerable amount of work has been done on TL of bulk materials, the TL properties of their nanocrystalline counterparts have often not been reported. Therefore, the effort towards improving the TL characteristics are still being continued for the same materials, either by synthesizing them at nano level by different methods or by varying the dopant impurities [10,11] or developing other new TL sensitive nanomaterials [12,13].…”