Flash-Sintering of MnCo2O4 and (La, Sr)(Co, Fe)O3 Ceramics for Potential Application in SOFC

The innovative flash-sintering technique has been considered to investigate the effect of DC electric field on sintering and electrical conductivity of two oxide ceramics MnCo2O4 and La0.6Sr0.4Co0.2 Fe0.8O3 (LSCF). These oxides are renowned for their high electrical conductivity and oxygen reduction properties, and therefore find application in solid oxide fuel cell as interconnect coating and cathode respectively. Flash-sintering of LSCF composite with 10 mol% Gd doped ceria (GDC), which is considered more promising than pure LSCF for cathodic application, is also reported. In flash-sintering-effect, when an adequately high DC electric field is applied on a green specimen, which is subjected to heating at a constant rate, the field drives rapid increase in the conductivity at a suitable furnace temperature. This event drives sintering, and the rate of shrinkage is so rapid that the material, generally, sinters in a couple of seconds without requiring elevated temperature. The aim of the present work is to better understand such electric field effect on sintering as it may affect the material properties (microstructure, conductivity etc.). Till now, the technique has been mostly reported for weakly conducting or insulating materials. The present work deals with the conducting-edge materials to point out the versatility of flash-sintering technique. The present work demonstrates the flash-sintering of conductive oxides, MnCo2O4 and LSCF. These oxides are sintered under electric field ranging from 7.5 to 12.5 V/cm at furnace temperatures 100-200°C, which are 1000°C lower than traditional heat treatments. LSCF, being highly conductive, surprisingly sinters at 25°C under 12.5 V/cm. On the other hand, the composite phases (LSCF/GDC: 60/40, 50/50 and 40/60 weight ratios) flash sinters at higher temperatures and electric fields which is systematic with GDC additions. The role of electric field and temperature in sintering is realized from specimen temperature which helps to understand the observed outstanding event. The flash-sintering occurs at conventional sintering temperatures and therefore, enhanced sintering at a particular temperature at this stage is expected to result from increased defect concentrations. Rapid increase in the conductivity possibly provides diffusion-able sites at rapid rate and the high sintering rate can be explained to occur using these sites. At drastic conductivity increase, the thermal effect is so dominant that ions diffuse for sintering more with the support of local temperature and less by the electric field. The extent of sintering is confirmed through scanning electron microscopy. The microstructural analysis carried on flash-sintered samples suggests that the surface morphology and grain growth homogeneity resemble that of traditionally-sintered samples. With the proper choice of processing parameters (electric field and current density), dense and pore-free microstructure for MnCo2O4 coating and porous microstructure of LSCF for cathodic application are obtained in very short sintering time. From the case of composites, current is clearly pointed to be a determining parameter in controlling the density. An enhanced electric field effect is recorded on the microstructure of MnCo2O4 starting at 1100°C. No such temperature discrimination is observed for LSCF, the sintering effects being regular with the temperature. From XRD analysis, MnCo2O4 undergoes to a significant phase reduction for the specimen temperature in excess of 1100°C. The LSCF and its composite phase’ are stable and compatible with GDC10 against flash-sintering. These observations are in agreement with conventional sintering. The flash-effect is investigated by analyzing electrical conductivity property of dense specimen. Consistent changes in current-voltage characteristics with temperature suggest that the electric field controls the conductivity in the same way as temperature does. At sufficient temperature, the electric field enhances the conductivity by increasing thermal excitation of charge carrier. At low temperature, the field enhances the conductivity by direct energy transfer; these temperature and field has a fundamental role in flash-sintering. On the basis of conductivity analysis, flash-sintering is proposed to be accelerated by utilizing defect complexes formed during the “polaron-hopping” which is the usual conduction mechanism of these materials, MnCo2O4 and LSCF. On the basis of correlation between the microstructure and phase stability, a constraint about the utilization of defect complex is realized. Sintering is enhanced when there is stabilization of reduction reaction in hopping process. This condition assures the availability of defect complexes for sintering; under equal probability of hopping transitions, the reduction is more under the tendency of oxidation. It was realized in the sintering of MnCo2O4 is enhanced over conventional for temperatures higher than 1080°C which corresponds to the reduction temperature. Such implication was verified on LSCF where sintering is found to be enhanced for all the considered temperatures as it undergoes reduction as sintering starts. Therefore, sintering is proposed to occur by the movement of reduced transition metal cations during usual hopping mechanism. Sintering at unusually low temperatures, in very short times and involving quite usual ionic transitions shows its potentials for ceramics manufacturing especially in multilayer production and temperature sensitive application.