Preparation Of Doped Ceria Solid Electrolyte Thin Film By Magnetron Sputtering

To satisfy the demand of intermediate-temperature (500—800℃)Solid Oxide Fuel Cell (SOFC), several kinds of new electrolyte materials with higher oxygen ion conductivity than the traditional high-temperature (above 1000℃) yttria-stabilized zirconia (YSZ) electrolyte has been investigated in recent years. Among them, ceria doped with 10～20% mol ratio Gd₂O₃ (GDC) has been considered as one of the most promising candidates due to its excellent ionic conductivity at intermediate temperatures.In the thesis, GDC films with the thickness smaller than 2μm was prepared by reactive radio-frequency magnetron sputtering method using Gd/Ce metallic target. Substrate temperature and oxygen partial pressure was varied during the film deposition. The relationship between the experimental parameters and the component, phases, grain size, texture, roughness and growth morphologies of the films was analyzed and surface evolution of the films at different growth conditions was discussed. The key results obtained are listed below:1. Substrate temperature influences the film growth: face-centered cubic (f.c.c) GDC solid solution dominates, while a little amount of body-centered cubic Gd₂O₃ intermediate phase coexists in the films prepared at different temperatures. Orientation of the films varies with the temperature with random structure at 200℃, (220) texture at 500℃ and (111) texture at 700℃. Deposition rate decreases with higher temperature. Growth of films proceeds via Volmer-Weber mode with increased islands and smaller island density at higher temperature. Surface evolution of the films varies with the temperatures. Smoothing at high deposition rate proceeds at temperatures below 200℃, while the films become roughened at temperatures of 300-700℃ mainly due to the oriented grain growth roughening mechanism.2. Oxygen partial pressure (P02) influences the film growth: Film components are almost independent of, while the phases vary with, the oxygen partial pressure. Single phase of face-centered cubic (f.c.c) GDC solid solution with (111) texture is formed at low Po₂ (0.08Pa), and f.c.c GDC phase with (220) texture structure with a little mount of body-centered cubic Gd₂O₃ intermediate phase coexists in the films prepared at higher Po₂. AFM analysis results show that, at low Po₂ (0.08Pa), growth islands coalescence together forming well-oriented island chains, which, we believe, is related to the high (111) texturestructure of GDC film;At higher P02 (0.15-0.21 Pa) individual round islands appear with larger sizes and finally become irregular in shape at the high Po2 of 0.26Pa. It indicates that grain growth can be enhanced by increase of the amount of the active oxygen to a proper extent. The surface evolution mechanism varies with P02: at low P02 (0.08-0.21Pa), the oriented grain growth and the shadowing effect are the main roughening mechanisms;while at high P02 of 0.26Pa, anti-sputtering of ions of O2' on the substrate results in a much more roughened film surface.