Plasma-Sprayed Coating of an Apatite-Type Lanthanum Silicate Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs)

Apatite-type lanthanum silicate was successfully synthesized via a solid state re- action protocol at 1400~C in a vacuum for 4 hours. The powder was synthesized faster and at a lower reaction temperature than by conventional solid state reaction methods. The resulting powder was used in the fabrication of a coating deposited by atmospheric plasma spray （APS） technology. The microstructure of the coating was analyzed by X-ray diffraction and scanning electron microscopy. Heat treatment was found to fully crystallize the coating, increasing its den-sity. The ionic conductivity of the apatite coating was 0.39 （0.054） mS/cm at 850 （700） ℃, and its activation energy was 0.67 eV.

Electrical properties of iron doped apatite-type lanthanum silicates

The effect of Fe doping on the electrical properties of lanthanum silicates was investigated. The apatite-type lanthanum silicates La10Si6-xFexO27-x/2 （x=0.2, 0.4, 0.6, 0.8, 1.0） were synthesized via sol-gel process. The unit cell volume increased with Fe doping because the ionic radius of Fe3＋ ion is larger than that of Si4＋ ion. The conductivities of La10Si6-xFexO27 x/2 first increased and then decreased with the increasing of Fe content. The increase of the conductivity might be attributed to the distortion of the cell lattice, which assisted the migration of the interstitial oxygen ions. The decrease of the conductivity might be caused by the lower concentration of interstitial oxygen ions. The optimum Fe doping content in lanthanum silicates was 0.6. La10Si5.4Fe0.6O26.7 exhibited the highest ionic conductivity of 2.712× 10^-2 S/cm at 800 ℃. The dependence of conductivity on oxygen partial pressure p（O2） suggested that the conductivity of La10Si6-xFexO27-x/2 was mainly contributed by ionic conductivity.

Pressure-induced structural evolution of apatite-type La_(9.33)Si_6O_(26)

The pressure-induced structural evolution of apatite-type La9.33Si6026 was systematically studied using in situ syn- chrotron x-ray diffraction （XRD）. The XRD spectra indicated that a subtly reversible phase transition from P63/m to P63 symmetry occurred at ~ 13.6 GPa because of the tilting of the SiO4 tetrahedra under compression. Furthermore, the La9.33Si6026 exhibited a higher axial compression ratio for the a-axis than the c-axis, owing to the different axial arrange- ment of the SiO4 tetrahedra. Interestingly, the high-pressure phase showed compressibility unusually higher than that of the initial phase, suggesting that the low P63 symmetry provided more degrees of freedom. Moreover, the La9.33Si6026 exhibited a lower phase transition pressure （PT） and a higher lattice compression than LaloSi6027. Comparisons between La9.33Si6026 and LaloSi6027 provided a deeper understanding of the effect of interstitial oxygen atoms on the structural evolution of apatite-type lanthanum silicates （ATLSs）.

Wet chemical synthesis of La_(9.83–x)Sr_xSi_6O_(26+δ)(0≤x≤0.50) powders, characterization of intermediate and final products

In this paper we reported the preparation and extensive characterization of La9.83–x Sr x Si6O26＋δ（0≤x≤0.50） precursors, intermediate and final products. The sintering reactions, the phase formation, the structure as well as the powders＇ morphology were studied by means of thermogravimetric analysis, X-ray diffraction（XRD）, Fourier-transform infrared spectroscopy（FTIR） and scanning electron microscopy（SEM）. Moreover, the effect of stoichiometry on precursor＇s structure and morphology as well as on intermediate and final products was reported. As was concluded pure La9.83Si6O26＋δ, La9.38Sr0.45Si6O26＋δ and La9.33Sr0.50Si6O26＋δ could be prepared after sintering at 1400 °C for 20 h while La9.68Sr0.15Si6O26＋δ and La9.53Sr0.30Si6O26＋δ compounds contained minor traces（〈3.5%） of La2Si2O7 secondary phase. Concerning the synthesis, there have been no previous reports on the preparation of pure La9.83Si6O26＋δ, La9.38Sr0.45Si6O26＋δ and La9.33Sr0.50Si6O26＋δ compounds. The final powders consisted of spherical particles and an increase of Sr content seemed to inhibit sintering phenomena. The existence of interstitial oxygen at intermediate crystallographic positions of apatite structure had great effect on Si O4 sub-structure distortion. The increase of Sr content led to a major reduction of interstitial oxygen quantity and the refutation of silicon tetrahedron distortion.