Structural and thermal characterization of SiO2-P2O5 sol-gel powders upon annealing at high temperatures


This study deals with SiO2-P2O5 powders obtained by sol-gel process, starting from tetraethoxysilane (TEOS) as precursor for SiO2 and either triethylphosphate (TEP) or phosphoric acid (H3PO4) as precursors for P2O5, In the case of samples prepared with H3PO4, 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 H3PO4 showed specific peaks for silicophosphate compounds such as Si-3(PO4)(4), Si2P2O9, and SiP2O7. 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 SiO2 in the sample might be expected. Thus, we have searched for any SiO2 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. (C) 2009 Elsevier B.V. All rights reserved.




Materials Science


Elisa, M; Sava, BA; Volceanov, A; Monteiro, RCC; Alves, E; Franco, N; Oliveira, FAC; Fernandes, H; Ferro, MC


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