Interfacial reaction between nickel oxide and lanthanum gallate during sintering and its effect on conductivity

The results of interdiffusion and reaction between NiO and strontium- and magnesium-doped lanthanum gallate (LSGM) during sintering were investigated. Electron microprobe analysis showed that the NiO content of LSGM sintered 4 h at 1350 degrees C was slightly >2 wt% within 5 mu m of the interface, which is equivalent to 7 mol% nickel on B cation sites. This concentration decreased rapidly to <0.25% at a distance of 15 mu m. The interfacial reaction between NiO and LSGM, however, led to the formation of a LaSrGa(Ni)O4-delta-type phase. A separate experiment involving conductivity measurements of nickel-doped LSGM indicated that the oxygen-ion conductivity did not decrease significantly if the nickel content was <5%-8%, At higher nickel contents, the conductivity increased in air because of hole conduction and markedly decreased under conditions of low oxygen pressure because of the formation of the LaSrGa(Ni)O4-delta-type phase. The nickel ions in the LSGM electrolyte and in the reaction product, LaSrGa(Ni)O4-delta, were stable as Ni2+ at reduced oxygen pressures and, therefore, did not cause electronic shorting.