**17. Infrared spectroscopy**

344 Sintering of Ceramics – New Emerging Techniques

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].** 

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