Chemical and structural changes in Ln2NiO4+δ (Ln=La, Pr or Nd) lanthanide nickelates as a function of oxygen partial pressure at high temperature
Résumé
The chemical stability of lanthanide nickelates Ln2NiO4+δ (Ln=La, Pr or Nd) has been studied in the temperature range 25–1300 °C, either in air or at low pO2 (down to 10−4 atm). Thermal gravimetry analysis (TGA) measurements coupled with X-ray diffraction (XRD) characterization have shown that all compounds retain their K2NiF4-type structure in these conditions, while remaining over-stoichiometric in oxygen up to 1000 °C. Only Nd2NiO4+δ starts to decompose into Nd2O3 and NiO above 1000 °C, at pO2=10−4 atm. In addition, a careful analysis of the lanthanide nickelates structural features has been performed by in situ XRD, as a function of temperature and pO2. For all compounds, a structural transition has been always observed in the temperature range 200–400 °C, in air or at pO2=10−4 atm. In addition, their cell volume did not vary upon the variation of the oxygen partial pressure. Therefore, these materials do not exhibit a chemical expansion in these conditions, which is beneficial for a fuel cell application as cathode layers. Additional dilatometry measurements have revealed that a temperature as high as 950 °C for Pr2NiO4+δ or 1100 °C for La2NiO4+δ and Nd2NiO4+δ has to be reached in order to begin the sintering of the material particles, which is of primary importance to obtain an efficient electronic/ionic conduction in the corresponding designed cathode layers. Besides, excellent matching was found between the thermal expansion coefficients of lanthanide nickelates and SOFC electrolytes such as 8wt% yttria stabilized zirconia (8YSZ) or Ce0.8Gd0.2O2−δ (GDC), at least from 400 °C up to 1400 °C in air or up to 1200 °C at pO2=10−4 atm.
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