Preparation, Characterization And Electrical Properties Of Al Doped Lanthanum Silicate Electrolyte

Apatite-type lanthanum silicate(La10-x(SiO4)6O2±y) have been considered as the solid oxide fuel cell electrolyte materials of great potential for application due to high ionic conductivity and lower activation energy. Though substituting Al3+ increases the ionic conductivity of apatite-type lanthanum silicate effectively, it requires high temperatures and dwell times as compared to undoped silicates.In this paper, La(NO3)3-xH2O, sodium silicate and Al(NO3)3·xH2O were used as raw materials and the precursors were prepared by chemical precipitation method using ammonium bicarbonate as precipitant. Apatite-type powder of La9.67Al1Si5O26 and La9.67Al1.5Si4.5O25.75 were obtained after freeze-dried and calcination at a certain temperature. The powder was characterized by TG-DSC and XRD. The sintered pellets were obtained through high temperature sintering and characterized by XRD, SEM, density measurement and AC impedance. The relationship between the electrical properties and the microstructure of sintered pellets were obtained. The results show that:(1) The precursor was prepared by chemical coprecipitation with La(NO3)3·xH2O, sodium silicate and Al(NO3)3·xH2O. The apatite-type powder La9.67Al1Si5O26 could be obtained at 900℃after calcined for 4 hours from the freeze-dried precursor. XRF characterization revealed that the ratio of elements agreed with the experimental design. Apatite-type La9.67Al1.5Si4.5O25.75 could be obtained at 1000℃after calcined for 4 hours by the same method.(2) For the sintered pellets of La9.67Al1Si5O26, SEM results show that particle size was increased from the 0.5μm at 900μto 0.8～1.0μm at 1100℃. The maximum density of the sintered pellets could be obtained at 1000℃, but the powder density was decreased when the temperture increased to 1100℃. For La9.67Al1.5Si4.5O25.75, the particle size of sintered pellet was increased as the powder rising calcination temperature from the 0.5μm at 800℃to 1.0μm at 1100℃, and then was decreased. The maximum density of the sintered pellet could be obtained at 1100℃.(3) For the solid electrolyte La9.67Al1Si5O26, the electrical properties results showed that the total conductivities of the sintered pellets sintered at 1550℃for 2 h were increased with the rising of the powder calcining temperature, and then were decreased. The conductivity was highest when the sintered powder was treated at 1000√, from 1.60×10-3 S/cm at 500℃to 1.69×10-2 S/cm at 800℃For the sintered pellets sintered at 1550℃for 4 h, the conductivity was highest when the sintered powder was treated at 800℃, from 2.1×10-3 S/cm at 500℃to 2.06×10-2S/cm at 800℃. For the sintered pellets sintered for 2 h, the grain conductivity was highest when the sintered powder was treated at 1000℃; however, for the the sintered for 4 h, the grain conductivity was highest when the sintered powder was treated at 900℃. For sintered pellets sintered for 2 h, the grain boundary conductivity was highest when the sintered powder was treated at 1100℃, with 1.60×10-3 S/cm at 450℃; however, for the sintered for 4 h, the grain boundary conductivity was highest when the sintered powder was treated at 800℃, with 2.23×10-3 S/cm at 450℃. The results indicated that prolonging the sintering temperature led to improve the total conductivity of the electrolyte and the grain boundary.(4) For the solid electrolyte La9.67Al1.5Si4.5O25.75, the electrical properties results showed that the total conductivities of the sintered pellets sintered at 1550℃for 2 h were increased with the rising of the powder calcining temperature, and then were decreased. The conductivity was highest when the sintered powder was treated at 1100℃, with 1.48×10-2S/cm at 800℃. The grain conductivity was highest when the sintered powder was treated at 1100℃. The grain boundary conductivity was highest when the sintered powder was treated at 800℃, with 6.14×10-4 S/cm at 450℃.(5) When the calcined temperature and test temperature were identical, the total conductivity, the grain conductivity and the grain conductivity of apatite-type were higher than La9.67Al1.5Si4.5O25.75.