**20. Co-precipitation and sintering**

The author has developed a simplified co-precipitation technique for the synthesis of CaSO4:Dy TLD phosphor, which circumvent the cumbersome procedure, used so far, namely, evaporation of (highly corrosive) concentrated H2SO4 by recrysttalization. High TL sensitivity, uniform microcrystalline morphology, lower grain size (see SEM pictures shown below left) suitable for manufacturing dosimeters in solid form, better glow curve structure, lesser glow peak shift and better linearity and simplified preparation technique make the new phosphor a better alternative **(Figs.23 & 24).** The new recipe of CaSO4:Dy based on coprecipitation technique is not only economical but also compatible for large scale production. Sintering of the co-precipitated phosphor at 7000C in air increased its TL sensitivity by more than a factor of 2 (see TL glow curves shown below right) due to improved crystallinity and diffusion of Dy3+ ions from the surface to the whole volume of the grains as stated earlier.

In this experiment, initially, CaSO4.2H2O and Dy2O3 salts were dissolved in hot concentrated H2SO4 acid and during slow dilution – water was added drop wise into the above hot solution – CaSO4:Dy was found to precipitate. As per conventional solution chemistry, only CaSO4 should precipitate since CaSO4 does not dissolve in dilute H2SO4 acid. The Dy3+ ions should remain dissolved in the acid-water solution due to the high solubility of Dy2(SO4)3 in water and in H2SO4 acid. The formation CaSO4:Dy, as confirmed by TL and PL studies, however indicated that a considerable amount of Dy ions present in the solution actually diffuse into the CaSO4 host lattice during precipitation. This is considered as an amazing result which confirms that there is a competition between the solubility of Dy ions in CaSO4 lattice and their solubility in acid solution. At low Dy concentrations (~ 0.1 mol%) normally used in TL phosphors, nearly 90% of Dy gets into the CaSO4 lattice. Only at high Dy concentrations, the percentage of Dy getting into CaSO4 lattice goes down.
