SOFC용 LSGM계 단일상 전해질 박막의 성장 및 특성 연구

Abstract

In SOFC, one of the most important issues is to decrease the operating temperature of SOFC. Low operating temperature gives the advantages for the elongation of SOFC lifetime and cost reduction in fabrication and total system operation. Two possible approaches can be considered: One is to reduce the thickness of the YSZ electrolyte layer, and the other is to search for alternative electrolyte materials with higher oxygen ion conductivities. Namely, a solid electrolyte with high conductivity and in the form of a thin film is essential in order to achieve high power density at low operating temperature. Hereupon, Doped-LaGaO3 electrolyte materials have especially attracted considerable attention in recent years as promising alternative electrolytes for lowering the operating temperature to around 600？700？C, which are currently significantly low to obtain adequate power densities with YSZ based SOFCs. In this study, we investigated growth characteristics and electrical properties for three doped-LaGO3 electrolyte thin films of LSGM8282 (La0.8Sr0.2Ga0.8Mg0.2O3-？), LSGM9182 (La0.9Sr0.1Ga0.8Mg0.2O3-？) and LSGMC (La0.8Sr0.2Ga0.8Mg0.115Co0.085O3-？) deposited by pulsed laser deposition (PLD). Based on the systematic experiments, we found that the optimum deposition conditions including the substrate temperature, the oxygen partial pressure, and the strain between the thin film and the substrate to influence the thin film growth. From the XRD results, it was founded that the crystallization substrate temperature for three doped-LaGO3 electrolyte thin films of LSGMC, LSGM8282, and LSGM9182 is around 700 oC, which was also supported by the AFM observations of the grains consisting of the thin films. The LSGMC thin film on the Pt (111)/TiO2/SiO2/Si substrate showed the preferentially (110)-oriented growth with the low (100) XRD peak because the Pt (111) and Pt (200)/TiO2/SiO2/Si substrates have a low lattice misfit for the doped-LaGO3 electrolyte materials. On the other hands, the LSGMC thin film on the Pt (200)/TiO2/SiO2/Si substrate the preferentially (100)-oriented growth. Differently from two Pt substrates, the LSGMC thin film deposited on the (0001) sapphire substrate did not exhibited a preferential growth and it just showed the (110), (220), (111), (222), and (210) XRD peaks without any secondary phases. The LSGM8282 thin film on the (0001) sapphire substrate exhibited the (110), (100), (111), (200), and (211) XRD peaks and the LSGM9182 thin film deposited on (0001) sapphire substrate (110), (220), (111), (100), (200), and (211) XRD peaks. In particular, the LSGM8282 thin film on the Pt (200) substrate showed the preferentially (110)-oriented growth. From the result of the thin film growth affected by the oxygen partial pressure, it was confirmed that the optimum oxygen partial pressure as a key parameter is put in narrow windows around 150 mTorr for the LSGMC, 300 mTorr for LSGM9182 and, 500 mTorr for LSGM8282 for the formation of the single phase doped-LaGaO3 electrolyte thin films. The scanning electron microscope (SEM) images showed that the single phase LSGMC thin film has a dense structure without cracks and particulates. The LSGMC thin film showed large grains in the top view SEM and the columnar structure in the cross-sectional view of the SEM image, indicating the island growth mode of the LSGMC thin film. The atomic force microscope (AFM) observations of the LSGMC thin films revealed that the surface morphology is also affected by the substrate temperature. Finally, the electrical conductivity for the single phase doped-LaGaO3 thin film and bulk samples was investigated and compared. Both of the LSGMC and LSGM8282 thin films exhibited the similar conductivity curves to those of the bulk materials. For the single phase LSGM9182 thin film, the conductivity was lower than that of the bulk. The results in conductivity obviously demonstrated that there is no significant enhancement of ionic conductivity originated from the formation of nano-crystalline microstructures. The activation energies of the thin films on sapphire substrates were lower than those of bulk samples. The origin for these different activation energies is not clear at present, but it may originate partially from the interface effect between the substrate and the film.