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Title: Mass transport in mixed-conducting LSCrF-ScSZ dual phase composites for oxygen transport membrane applications
Author: Shen, Zonghao
ISNI:       0000 0004 7969 813X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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The (La,Sr)(Cr,Fe)O3-δ (LSCrF)-Scandia-Stabilised Zirconia (ScSZ) based dual-phase composite system has been investigated in this work. As the dense layer in an oxygen transport membrane device, transport properties profoundly affect the membrane performance. Isotopic Exchange Depth Profile (IEDP) combined with Secondary Ion Mass Spectroscopy (SIMS) was performed to investigate the oxygen ionic transport in the composite materials and the electrical conductivity was measured by the 4-Probe DC method. In order to achieve improved performance, optimisation strategies were also applied, including varying the Cr substitution level in the LSCrF single phase materials and changing the phase ratios in the composite systems. For the electrical conductivity behaviour, in the composite system a percolation threshold was observed at around 20-30 vol% for all Cr:Fe ratios (LSCrF37, LSCrF55 and LSCrF73). Among these three systems, the LSCrF73-based composites presented the highest electrical conductivity. The diffusion measurements were firstly carried out at the macroscopic scale and the 'effective' diffusion kinetics were obtained. In a dry oxygen atmosphere, the predominant phase for the surface exchange process was determined to be the LSCrF phase while the ScSZ-based ionic conductor dominated the bulk diffusion. The percolation limit for the diffusion coefficients was observed at a composition of around 30 vol% ionic conductor for the three measured systems (LSCrF37-10Sc1CeSZ, LSCrF55-10Sc1CeSZ, and LSCrF73-10Sc1CeSZ) and the LSCrF73 based dual-phase composites displayed the best bulk diffusivity. Subsequently, in oxygen diffusion studies at the microscopic scale using Focused Ion Beam (FIB)-SIMS, a synergistic effect between the two phases was observed in the composite materials: a decreased surface exchange coefficient (k) was observed for the MIEC phase LSCrF while an enhanced k was obtained for the pure ionic conductor (10Sc1CeSZ) compared to the corresponding isolated single-phase materials. Further mechanism studies were performed on a specialised sample by applying IEDP, SIMS and Low Energy Ion Scattering (LEIS) techniques. The plausible origin of the synergistic effect is suggested to be a combination of the spillover type mechanism and the self-cleaning behaviour of the LSCrF based upon these observations. Prior to the dual-phase composite materials, the starting single-phase materials, LSCrF and ScSZ were firstly characterised. As the Cr substitution increased, the electrical conductivity increased and the maximum value was achieved in the LSCrF73 phase while the bulk diffusivity of the LSCrF decreased. Fast grain boundary diffusion behaviour was observed in the LSCrF73 single phase material. Additionally, the diffusion behaviour of both the single-phase and dual-phase samples in a pure water vapour environment was lastly studied by using labelled H218O to reflect operating atmospheres. The diffusion coefficient (D*) at 800 ̊C of LSCrF was found to increase dramatically by 3 orders of magnitude compared to an isothermal experiment under dry oxygen atmosphere. The surface exchange coefficient of 10Sc1CeSZ has been improved to 1.5×10-6 cm s-1 at 800 ̊C while in dry O2 conditions almost no 18O has been exchanged into the sample. For the LSCrF-ScCeSZ composite, ScCeSZ single phase dominates both the surface exchange process and the bulk diffusion in pure water vapour atmosphere.
Supervisor: Skinner, Stephen ; Kilner, John Sponsor: Praxair Inc., USA
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral