DTA/TGA and XRD investigation of Lithium tetragermanate synthesized via sol-gel

Lithium tetragermanate gel was synthesized via sol-gel method [1]. A flow-chart indicating the preparation procedure and the compositions employed is given in Fig. 1. Water-free ethanol was obtained by distillation with metallic sodium of commercial anhydrous ethanol, since the Ge(C2H5O)4 (TEOG) is a very water-sensitive reagent and to permit the control of TEOG/H2O. Bi-distillated water was used for hydrolysis reaction. The alcoholic solutions were prepared in a drybox at room temperature. The alcoholic solution of TEOG was mixed at 0°C with a water-alcoholic solution of lithium hydroxide. Under these conditions complete gelation occurred at room temperature in two days. The obtained gel appeared homogeneous. The gelled system was held for one day more at room temperature before drying. The gel was fully dried in air at 50°C in an electric oven for one day. After these treatments, an amorphous powder was obtained. The gel behaviour was examined by differential thermal analysis (DTA), simultaneous thermogravimetric (TGA) and X-ray diffraction (XRD). Fig. 2 shows the TGA and DTA curves of the dried gel. A large endothermic peak, from room temperature to about 250°C, appears on the DTA curve, with a maximum at about 150°C, and a simultaneous weight loss occurs in the TGA curve. The weight loss was 14.9%. These effects were due to evaporation from open pores of the water and alcohol physically trapped in the gel. No appreciable effects were observed on TGA and DTA curves in the range of 250-500°C, due to the absence of organic substances which can be produced in sol-gel processing [1,2]. The DTA curve of the gel exhibits a slope change that may be attributed to the glass transition. The inflection point of the DTA curve was taken as the glass transition temperature (Tg = 532°C). A high and sharp exothermic peak appears, just above the Tg, on the DTA curve at the temperature of 565°C. At a higher temperature, 604°C, the DTA curve exhibits a second exo-peak, smaller than the first one. The presence of two exothermic effects on the DTA curve of the studied gel suggests a crystallization process in two steps. The XRD diffraction patterns (data not shown) of the dried gel, the dried gel heated in the DTA furnace up to the temperature of the first exo-peak and the dried gel after a DTA run carried out from room temperature to 700°C, revealed that the powder has broad humps characteristic of the amorphous state of the dried gel. The broad reflections of the dried gel heated in the DTA furnace up to the temperature of the first exo-peak were attributed to Li2Ge4O9 and GeO2 microcrystaIlites, while the reflections of the dried gel after a DTA run corresponded to Li2Ge4O9 and Li2Ge7O15 crystals. The values of activation energy for the two stages are found to be 557 and 405 kJmol-1 [3]. Activation energies are comparable with those reported with conventional melt glasses from oxides. Figure 2. DTA/TGA curves of the dried gel, recorded in N2 at 10°C/min. Acknowledgements The work was financially supported by V:ALERE 2019 grant support from Università degli studi della Campania “L. Vanvitelli” of CHIMERA. Bibliography [1] Brinker C.J. , Scherer G.W. Sol-Gel Science, Academic Press, San Diego, CA, 1990. [2] Osaka A. , Yuasa M. , Miura Y. , Takahashi K. Sodium borosilicateglasses prepared by the sol-gel process, Non-Cryst. Solids, 1988, 100(1-3), 409-412 [3] Ozawa T. Kinetics of non-isothermal crystallization, Polymer, 1971, 12(