**16. Sol-gel method**

The sol–gel process refers broadly to the room temperature solution routes for preparing oxide materials. The solutions of precursors are reacted to form the irreversible gels that dry and shrink to rigid oxide glasses and powders. Traditionally, phosphor films are generally deposited by using the sputtering technique and only in recent decades, the sol–gel method has been considered as a low cost alternative approach for the preparation of novel nanostructured materials including luminescent powders and films. The sol-gel route presents a lot of advantages: low-temperature synthesis, possible formation of powders with uniform grain morphology and achievement of homogeneous multicomponent films. An advantage in using the nano-crystalline phosphors is that non-radiative transition could significantly be controlled with a decrease in the crystal grain size. The preparation of the Zn2SiO4 precursor followed a simple route and was completed at room temperature and room humidity: firstly, Zn(Ac)2 was dissolved in a mixture solution of de-ionized water and ethanol; secondly, stoichiometric TEOS was added into the above solution with agitation for

patterns show that the YAG phase can be produced with this method by sintering at 1000 °C for 2 h. This temperature is much lower than that required to synthesize the YAG phase using the conventional solid-state reaction method. No intermediate phases such as yttrium aluminum perovskite (YAP; YAlO3) or yttrium aluminum monoclinic (YAM; Y4Al2O9) were observed as a result of the sintering process. The powders were found to absorb excitation energies in the range of 410–510 nm. Furthermore, the crystalline YAG:Ce powder phosphor produced broad emission peaks in the range of 480–600 nm with the maximum intensity at 528 nm **[18]. Fig. 18** shows the TEM picture of BAM:Eu nano particles prepared by solution

Fig. 18. TEM picture of BAM:Eu nano particles prepared by solution combustion synthesis [15].

The sol–gel process refers broadly to the room temperature solution routes for preparing oxide materials. The solutions of precursors are reacted to form the irreversible gels that dry and shrink to rigid oxide glasses and powders. Traditionally, phosphor films are generally deposited by using the sputtering technique and only in recent decades, the sol–gel method has been considered as a low cost alternative approach for the preparation of novel nanostructured materials including luminescent powders and films. The sol-gel route presents a lot of advantages: low-temperature synthesis, possible formation of powders with uniform grain morphology and achievement of homogeneous multicomponent films. An advantage in using the nano-crystalline phosphors is that non-radiative transition could significantly be controlled with a decrease in the crystal grain size. The preparation of the Zn2SiO4 precursor followed a simple route and was completed at room temperature and room humidity: firstly, Zn(Ac)2 was dissolved in a mixture solution of de-ionized water and ethanol; secondly, stoichiometric TEOS was added into the above solution with agitation for

combustion synthesis [15].

**16. Sol-gel method** 

homogenization; thirdly, Eu(Ac)3 and Tb(Ac)3 were added into the precursor in the amount of 10 mol% (Eu:Zn2SiO4). Finally, a proper amount of 0.1M HCl was applied as catalyst for the hydrolysis of TEOS. The obtained precursor was transparent and clear, and was stable for several months if sealed at room temperature. Transparent gel can be obtained by leaving the precursor in air for about 2 or 3 days. Dry gel was prepared by baking the gel at 1200C for 5 min. Powder phosphors were prepared by sintering the dry gel for 30 min at different temperatures from 650 to 8500C. XRD data revealed that good crystallization of the powders could be obtained at about 8500C, which is about 4500C lower than that of the conventional solid-state reaction method **(Fig.19).** Phase analysis indicates that the obtained product has a willemite structure (rhombohedral). At the same time, a little amount of triclinic Zn2SiO4 was also detected. The TEM observation of the powders indicates that the particles are in the range 40–100 nm in diameter and have a good crystallinity.

Fig. 19. XRD patterns of sol–gel derived Zn2SiO4 powders [19].

**TG and DTA analysis in sol-gel route** - In order to study the thermal decomposition of undoped xerogels, thermal gravimetry analysis (TGA) combined with infrared (IR) spectroscopy were used. TG and DTA (differential thermal analysis) results show cinetics of emitted gases during TG analysis, determined through the intensity evolution of the infrared main characteristic bands of each species. The total weight loss remains between -35 and -40% whatever the Tb3+ concentration is and occurs in three steps as shown in **Fig.20**. The first one (25–200◦C) corresponds to the *departure of adsorbed moisture and chemically bonded alcohol molecules, i*n agreement with **Fig. 21.** This stage leads to a wide *endothermic peak*. The second and main weight loss (−19%) lying from 200 to 600◦C is associated with an *exothermic phenomenon.* This stage can be *assigned to the pyrolysis of the organic parts* of the alkoxide groups and likely to more strongly adsorbed alcohol molecules. It is in good agreement with IR measurements which show that most of the residual organic groups are removed before

The Role of Sintering in the Synthesis of Luminescence Phosphors 345

which showed that YAG sample begins to crystallize between 850 and 900◦C. No significant weight loss appears thereafter, assuming the formation of the final product

In the xerogel FTIR spectrum of of xerogel and undoped YAG sintered for 4 h at 1100◦C, the presence of the characteristic organic bands is noticed. The broad band ranging from 2600 cm−1 to 3800 cm−1 is ascribed to −CH3 and −OH stretching in the isopropanol and alkoxy groups. This latter also involves −OH stretching resulting from water added in excess for hydrolysis and adsorbed from air moisture. A peak of weak intensity at about 2350 cm−1 is attributed to the adsorbed carbon dioxide from the atmosphere. The bands lying from 1200 to 1600 cm−1 are attributed to C-O and -CH3 stretches bonds of organic groups whereas the peaks at about 1638 cm−1 is likely due to water added in excess to hydrolyze the heterometallic isopropoxide sol. Moreover, the several broad bands observed within the 400–850 cm−1 region of the IR spectrum correspond to M-O bonds (M=Y or Al) vibrations in YAG lattice. After sintering, specific bands related to the solvent and alkoxy groups significantly decrease or disappear. The carbon dioxide peak remains. Specific Al-O and Y-O

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

Fig. 22. SEM images recorded at 800× magnification from Y3Al5O12:Tb3+(5%) unheated

vibrations peaks below 800 cm−1 are clearly identified as YAG ones.

Y3Al5O12.

micrometers.

**17. Infrared spectroscopy** 

**18. Scanning electron microscopy** 

(a) and annealed at 1100◦C for 4 h (b) **[20].**

600◦C. Above 600◦C, only a weak weight loss takes place, ascribed to the withdrawal of the last residual alkoxy groups embedded in YAG matrix. Furthermore, *a sharp exothermic peak at about 894*◦*C indicates the onset of YAG crystallization.* This is consistent with the XRD results

Fig. 20. TG and DTA profiles obtained fromYAG precursor gel **[20].**

Fig. 21. Cinetics of gases emitted during TG analysis **[20].** 

which showed that YAG sample begins to crystallize between 850 and 900◦C. No significant weight loss appears thereafter, assuming the formation of the final product Y3Al5O12.
