Restrictions to obtain NASICON by a ceramic route


Highly reactive monoclinic ZrO2 powders were mixed with Na3PO4.12H(2)O and amorphous SiO2 in the stoichiometric quantities to obtain Na3Zr2Si2PO12. The mixtures were calcined it different temperatures (T-calc) and for variable periods of time (t(calc)). Their morphology was analysed by SEM and the composition by powder XRD. NASICON reflections are not detected for T-calc=900degreesC and for relatively short t(calc) values of 2 h. At 1100degreesC, the onset of low intensity NASICON peaks indicate the early stages of the formation reaction which is significantly enhanced when T-calc increases up to 1200 and 1300degreesC. For higher temperatures, an apparent decrease of the reaction rate is suggested. On the other hand, the intensities of the ZrO2 peaks suffer a drastic decrease when T-calc varies from 900 to 1300degreesC, followed by a slight increase for T-calc=1400degreesC. The studies were extended for mixtures kept in isothermal conditions at 1100, 1200 and 1300degreesC during 0.5, 1, 2, 4, 8 and 16 h. In the three temperatures, the increase of the t(calc) up to ca. 4 h leads to a significant increase in the intensity of the NASICON reflections while that of mZrO(2) decreases. A plateau is apparent for higher t(calc) values suggesting that equilibrium may be attained. The use of a highly reactive tetragonal zirconia powder (also thermodynamically unstable at low temperature) clearly improved the overall reaction extension and kinetics at moderate temperatures, yielding a single phase product.




Materials Science


Fuentes, RO; Lamas, DG; Fernandez De Rapp, ME; Figueredo, FM; Frade, JR; Marques, FMB; Franco, JI

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