Grain Boundary Regulation Of Laminated YSZ Thin Films And Its Effect On Conductivity Behavior

As an excellent solid oxygen-ion conducting electrolyte, the YSZ has been widely applied in the solid oxide fuel cell (SOFC), oxygen pump and oxygen sensor. It is very important to improve the ionic conductivity of YSZ thin films for both theoretical research and realistic application.In this paper, the polycrystalline YSZ thin films have been prepared on the sapphire substrates by magnetron sputtering technology. The XRD patterns of both the as-deposited and annealing at 800℃ are consistent with the diffraction data for single polycrystalline YSZ phase with cubic fluorite structure. SEM analysis the film with characteristic columnar structure and 2μm thickness shows a good adhesion to the substrate. TEM analysis demonstrated that the diameters of the columnar grains were about 50-100nm. The Fe-, Co-and Si-containing sols were chosen respectively to coat these YSZ coatings by dip-coating process, followed by the grain-boundary diffusions at appropriate temperatures. The EDS results demonstrated the strong segregation of Fe as well as Si in the triple grain junctions. The results of electrical conductivity testing showed that the electrical conductivity of Fe-doped YSZ thin film which not depends on PO2 was 8 times higher than that of the as-deposited one while the activation energy did not show any variations, demonstrating that the electrical conductivity of the Fe-doped YSZ thin film is oxygen-ionic,In order to figure out the scavenging effect of doped transition element Fe on the intergranular siliceous phase in the YSZ thin films, the as-deposited thin films were firstly doped with Si-containing sol, and then discussed the change of the electrical conductivity with the later Fe doping. The results showed that compared with the as-deposited YSZ thin film, the conductivity of thin film only doped with Si is decreased by 3 times, however, the conductivity of thin film doped with Si first then secondly dopped with Fe is increased by 3 times while the three films have the same activation energy for ionic conduction illustrating the doped transition element Fe has a scavenging effect on intergranular siliceous phase, so that it can improve-the ionic conductivity of YSZ thin films with columnar grain structure.In order to figure out the thickness effect on the electrical conductivities of YSZ films, different thicknesses of YSZ films were prepared by magnetron sputtering through controlling the deposition time. All the films exhibit a typical columnar grain structure with thicknesses of 0.67-2.52 μm based on the fractured cross-sectional SEM observations. The diameters of the columnar grains increase from 40 μm to 100 nm with the increment in film thickness from 0.67 μm to 2.52 μm according to TEM analysis. The thinnest YSZ film, i.e., the 0.67μm-thick film, shows the highest apparent electrical conductivity in the four films in 400-800 ℃ due to the contribution from the highly conductive film/substrate interface region. After eliminating the effects of the film/substrate interface, the real electrical conductivities of YSZ films increase with the increment in film thickness because of the decreasing volumetric fraction of the resistive grain boundary region with increasing thickness based on the space charge theory analysis.