Surface Modification of Ba0.5Sr0.5Co0.8Fe0.2O3-d By Using Atomic Layer Deposition
Authors: Hayamizu Yoshiaki , Oikawa Itaru , Takamura Hitoshi 
Published: February 2020

For the application in the combustion process, oxygen permeable membrane has been gathering much attention. It requires both of bulk ionic and electronic transport to maintain the charge balance. Many perovskite-type oxides have been investigated since Teraoka et al. reported high oxygen permeability for La-Sr-Co-Fe series oxides [1]. Among many mixed conductors, Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) shows outstanding performance of oxygen permeability [2]. However, it is widely accepted that the surface exchange reaction limits the permeation rate at reducing temperature and thickness [3]. Consequently, the improved bulk property may be wasted in such regime. Thus, it is significant to understand the effects of the surface state on the oxygen transport property. Recent studies indicated the surface state takes the dominant role in the surface exchange reaction because it decides the electronic structure of the surface and availability of active sites for the reaction [4]. In this study, Atomic Layer Deposition (ALD) was employed to apply various oxide coating such as TiO2 and SrO on BSCF. Introducing gaseous precursor and oxidant alternately, ALD can grow thin film in nm order not only on a flat surface, but also on a complex shape surface such as porous structure. Applying single nm-thick oxide coating, different function can be given to the BSCF surface such as protective layer for higher chemical stability and purposely degraded terminal. BSCF substrates were synthesized by the solid state reaction method. Subsequent XRD and SEM-EDS analysis indicated that the substrates were in single phase of cubic pereovskite-type structure and in nominal cation composition. ALD was conducted on the mirror polished BSCF substrates and on quartz substrates at the table temperature of 200°C. Film thickness was determined by the spectroscopic ellipsometry for the quartz substrate. Oxygen permeation measurements were performed from 850°C to 700°C, using He for sweep gas. The thickness of TiO2 coating on BSCF was estimated to be 7.6 nm by the spectroscopic ellipsometry. In order to investigate the effect of TiO2 coating on the chemical stability of BSCF, high temperature XRD (HT-XRD) measurements were performed in N2-CO2 atmosphere at 700°C. It is reported that perovskite-type oxides containing alkaline-earth metal are critically unstable under CO2 atmosphere due to strong adsorption of CO2 molecule and / or carbonates segregation [5][6]. After applying TiO2 coating, the peak from SrCO3 was suppressed from 0 to 10%-CO2 while the sample without coating showed the same peak at the lowest CO2 content, 2.1%-CO2. This result suggests that higher tolerance of BSCF to CO2 can be given by the TiO2 coating. Moreover, it is also worth noting that very thin TiO2 coating in single nm can work as a protective layer while previous study applied much thicker coating of 250 nm [7]. Oxygen permeation measurements showed that TiO2 coating did not degrade high permeation rate of original BSCF above 775°C. Lowering the temperature, larger slope from coated BSCF resulted in lower oxygen permeation rate, and it became about 60% of that of uncoated BSCF at 700°C. The activation energy of oxygen permeation was increased by the coating, from 60.8 kJ·mol-1 to 75.9 kJ·mol-1 and from 108 kJ·mol-1 to 152 kJ·mol-1 in the temperature range of 850°C to 800°C and 775°C to 700°C, respectively. Judging from the higher activation energy for coated sample, it is suggested the TiO2 coating affects both of the diffusion and surface exchange kinetics. Much larger activation energy may imply limited area for the surface exchange reaction such as cracks in the coating, which might be introduced by the lattice mismatch between TiO2 layer and BSCF. However, permeation rate can be improved by the porous layer coating according to our previous study [8]. More details of the effects of TiO2 coating and also effects of other oxides coating will be presented along with possible mechanisms of them.
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