M. Elisa, B. A. Sava, A. Volceanov, R. C. C. Monteiro, E. Alves, N. Franco, F. A. Costa Oliveira, H. Fernandes, M. C. Ferro, Structural and thermal characterization of SiO2–P2O5 sol–gel powders upon annealing at high temperatures, J. Non-Cryst. Solids, 356 (2010) 495 - 501.

Title: Structural and thermal characterization of SiO2–P2O5 sol–gel powders upon annealing at high temperatures

Abstract: This study deals with SiO(2)-P(2)O(5) powders obtained by sol-gel process, starting from tetraethoxysilane (TEOS) as precursor for SiO(2) and either triethylphosphate (TEP) or phosphoric acid (H(3)PO(4)) as precursors for P(2)O(5), In the case of samples prepared with H(3)PO(4), TG-DTA data showed an accentuated weight loss associated to an endothermic effect up to about 140 degrees C, specific for the evaporation of water and ethylic alcohol from structural pores, and also due to alkyl-amines evaporation. Sol-gel samples prepared with TEP exhibited different thermal effects, depending on the type of atmosphere used in the experiments, i.e. argon or air. XRD analysis revealed that annealed sol-gel samples prepared with H(3)PO(4) showed specific peaks for silicophosphate compounds such as Si(3)(PO(4))(4), Si(2)P(2)O(9), and SiP(2)O(7). XRD results for annealed sol-gel samples prepared with TEP indicated mainly the presence of a vitreous (amorphous) phase, which could be correlated with SEM images. The presence of SiO(2) in the sample might be expected. Thus, we have searched for any SiO(2) polymorph possible to crystallize. Only potential peaks of cristobalite were identified but some of them are overlapping with peaks of other crystalline phosphates. SEM analysis indicated a decrease of the amount of crystalline phases with the increase in the annealing temperature.

Key words: Scanning electron microscopy; Phosphates; Silicates; Sol-gels (xerogels); Glass transition; X-ray diffraction