LIQUID-PHASE SINTERING, ELECTRICAL-CONDUCTIVITY, AND CHEMICAL-STABILITY OF LANTHANUM CHROMITE DOPED WITH CALCIUM AND NICKEL

The substitution of 10 mol% nickel for chromium in calcium doped lanthanum chromite has been shown to promote rapid densification of the compound at low temperatures in air by the formation of a transient liquid phase. Liquids were generated via the decomposition of CaCrO4 second phase material present in calcined powders. During electrical conductivity measurements at 1000-degrees-C, severe microstructural degradation occurred in atmospheres of H-2 and CO2. The processes leading to chemical degradation are attributed to the precipitation of Ni from the (La,Ca)(Cr,Ni)O3 solid solution and to the decomposition of small amounts of residual CaCrO4 which remains at grain boundaries and triple points after sintering. DTA studies on CaCrO4 in CO2 and H-2 atmospheres suggested that the degradation due to CaCrO4 decomposition was primarily a result of the reaction of CaO with CO2 to form CaCO3. The presence of H-2 gas is thought to catalyse this reaction via the formation of large amount of CaO during the decomposition of CaCrO4. Degradation is more severe in atmospheres containing a mixture of CO2 and H-2 gases than in either gas on its own.