**10. Effect of atmosphere during sintering**

Phosphors tend to get oxidised during sintering in air. For example CaSO4 gets oxidized to CaO at temperatures above 8000C in air. As a result the luminescence efficiency of CaSO4:Dy gets reduced at high sintering temperatures. In addition there are certain activators such as Mn, Cu etc which tend to get oxidised during sintering in air. Certain other activators such Eu and Ce tend to exhibit dual valence state. Quite often during sintering, reducing atmosphere is essential to prevent Eu2+ and Ce3+ from getting oxidized to Eu3+ and Ce4+, respectively. While phosphors containing Eu2+ activator (eg., BAM:Eu2+) gives intense blue emission, those containing Eu3+ activator (eg., Y2O3:Eu3+) gives intense red emission. So depending on the phosphor and activator, sintering should be carried out either in a reducing atmosphere or in air. Reducing atmospheres are usually obtained with H2/N2 mixture. Alternately, the phosphors to be sintered are covered with carbon powder in closed crucibles so as to create reducing CO atmosphere when burnt with limited oxygen.

CaS:Eu2+ red-emitting phosphors particles, were prepared by the precipitation method with calcium acetate and Na2S as starting materials, followed by sintering in the atmosphere over the mixture of sulfur powder, Na2CO3, and carbon-containing compounds such as tartaric acid, citric avid, glucose, and cane sugar. CaS:Eu2+ particles without additive show inhomogeneous, rough and aggregation with the size of 75–125 nm, but the spherical particles with mean size of about 110 nm were obtained by adding carbon-containing compounds **(Fig.13).** Compared with phosphor without additive, the addition of carbon-containing materials induced a remarkable increase of PL, in the order of cane sugar, glucose, citric acid, and tartaric acid. This enhancement is due to the improvement of crystallinity, particle morphology and size distribution of the samples by adding carbon-containing additive.

Fig. 13. Transmission electron (TEM) micrographs of CaS:Eu2+ obtained by the precipitation method without additive (a) and with cane sugar additive (b) **[9].**

The Role of Sintering in the Synthesis of Luminescence Phosphors 337

grain shape and size were observed with an increase in the sintering time. Optimal crystallization was realized in the case of the ZnWO4 phosphor synthesized using 50 mol% WO3 at 1,100°C for 3 h. The maximum emission intensity was achieved when the phosphor

Fig. 14. The relative emission intensity versus the calcined temperature of YInGe2O7:5 mole% Eu3+ under an excitation of 393 nm. The signals were detected at 611 nm **[10].**

Luminescence properties (both PL and TL) of Tricalcium phosphate (TCP) are very sensitive to its crystal phase (α/β). TCP crystals, in both α and β phases, were synthesized through two different routes, viz. wet precipitation and high temperature solid state reaction **[12].** The doping was done during the synthesis using suitable compounds of Dy and Eu. In the wet precipitation method used, the wet reaction is carried out using calcium nitrate and diammonium hydrogen phosphate in an ammoniated solution. The precipitation of tricalcium phosphate occurs through the chemical reaction 3Ca(NO3)2 + 2(NH4)2HPO4 + 2NH4OH → Ca3(PO4)2+ 6NH4NO3 + 2H2O. the supernatant liquid was decanted to collect the precipitate. It was then centrifuged thrice using distilled water and finely filtered. The filtrate was dried at 100°C in a hot air oven overnight and then calcined at 300°C in a muffle furnace for 3 h to remove any traces of other compounds. The calcined material was ground to form fine powder and graded using standard sieves. It was then sintered at high temperatures for 2 h in a programmable furnace to obtain the required phase (900°C for β-TCP and 1300°C for α-TCP). Various samples were prepared using dysprosium and europium as dopant. The doping was done by adding oxides of the dopant elements (dysprosium and europium) dissolved in minimum quantity of dilute nitric acid. The solid state synthesis of tricalcium phosphate was done through a high temperature firing of the

exhibited optimal crystallization.

**12. Phase change during sintering** 

**Yttria-stabilized zirconia** (YSZ) is a zirconium-oxide based ceramic, in which the particular crystal structure of zirconium oxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" (ZrO2) and "yttria" (Y2O3), hence the name. The addition of yttria to pure zirconia replaces some of the Zr4+ ions in the zirconia lattice with Y3+ ions. This produces oxygen vacancies, as three O2- ions replace four O2- ions. It also permits yttrium stabilized zirconia to conduct O2- ions, provided there is sufficient vacancy site mobility, a property that increases with temperature. This ability to conduct O2- ions makes yttria-stabilized zirconia well suited to use in solid oxide fuel cells, although it requires that they operate at high enough temperatures.
