**18. Scanning electron microscopy**

It is known that luminescent properties of a phosphor depend on its particles shape and size. On this account, the morphology of Tb3+ activated YAG samples has been studied by scanning electron microscopy (SEM). SEM micrographs of YAG:Tb3+(5%) powder recorded at 800× magnification are shown in Fig.22. From the first picture related to the as-prepared xerogel **(Fig.22a),** it can be seen irregular size blocks. Voids and pores are also observed. The micrograph of heat-treated sample **(Fig.22b)** exhibits a denser network with fewer voids and narrow size distribution. For the two samples, the largest particles can reach several tens of micrometers.

Fig. 22. SEM images recorded at 800× magnification from Y3Al5O12:Tb3+(5%) unheated (a) and annealed at 1100◦C for 4 h (b) **[20].**

The Role of Sintering in the Synthesis of Luminescence Phosphors 347

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

A number of display phosphors has been recently synthesized in the author's laboratory using combustion and pyrolysis technique. In the pyrolysis technique, the constituent chemicals decompose and fuse by the action of heat at relatively low external temperatures (≤ 10000C) in air with adequate ventilation so that the gaseous products released during decomposition reactions escape and the fusion process is complete, Limitations of conventional solid state method are inhomogeneity of the product, formation of large particles with low surface area and hence mechanical particle size reduction is required, which introduces impurity and defects and presence of defects, which are harmful to luminescence. The problem of inhomogeneity could be mitigated by the use of nonconventional methods (wet-chemical) which include solution combustion. Combustion is an exothermic reaction and occurs with the evolution of heat and light. This method was accidentally discovered in 1988 in Prof. Patil's laboratory in India. The first synthesis of Eu3+ doped LnBO3 (Ln=La, Gd and Y) borate phosphors by combustion method was made by his group. The emission spectrum of LaBO3:Eu3+ consisted of two bands at 615 and 595 nm and these bands were attributed to 5D0→7F2 and 5D0→7F1 transition of 9-coordinated Eu3+ ions, respectively. But, there were three bands at 625, 610 and 595 nm observed for GdBO3:Eu3+ and YBO3:Eu3+ phosphors. The band at 595 nm was attributed to magnetic dipole 5D0→7F1 transition of Eu3+, whereas the bands at 625 nm and 610 nm were attributed to electric dipole 5D0→7F2 transition for 12 and 8 coordinated Eu3+ ions, respectively. Since electric dipole transition, 5D0→7F2 depends upon the structure, two bands were observed for differently

(Y,Gd)BO3:Eu3+ phosphor used in PDP displays was prepared earlier by combustion method using amino acetic acid as the combustion agent and then sintered at 1000 °C for 30 min. However, details of the recipe used and the PL sensitivity comparisons with commercial phosphor were not reported. In author's lab, raw materials used for the synthesis of (Y,Gd)BO3:Eu3+ and YBO3:Eu3+ phosphors using combustion technique were Y2O3, Gd2O3, H3BO3 and Eu2O3. NH4NO3 and urea (CH4N2O) were used as oxidizer (O) and fuel (F) respectively. The O/F ratio was kept at unity. After wet mixing in a porcelain crucible, combustion was carried out at 600 °C for 15 min in a muffle furnace with adequate ventilation so that the gaseous products released during combustion escape and the combustion process is complete. Combustion of the borate materials with urea fuel resulted in smoldering without flame. In contrast, with Oxalyl dihydrazide (ODH — C2H6N4O2) fuel, the combustion was reported to be flaming and the flame temperature measured using optical pyrometer was about 1400±100 °C. ODH process showed very sharp powder XRD pattern whereas urea process exhibited very broad powder XRD peaks. Therefore, by changing the fuel one could control particulate properties. However, ODH is nearly 50 times more expensive than urea

lattice goes down.

**21. Combustion and pyrolysis** 

coordinated Eu3+ in GdBO3:Eu3+ and YBO3:Eu3+.
