Study Of Growth And Electrical Properties Of Gd Doped Ceria Electrolyte Thin Film

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 was prepared by reactive radio-frequency magnetron sputtering method on amorphous quartz substrates,using Gd/Ce metallic target at different temperatures. The phases,grain size,texture,growth morphologies and electrical property of the films are analyzed by X-Ray Diffraction,Atomic Force Microscopy and Alternating Current impedance analysis.The results show that:Components of GDC Films are almost independent of deposition temperature,and the mol ratio of Gd is about 14.3%.Growth orientation of GDC film varied with the substrate temperatures.At the temperatures of 300 and 400℃,GDC films show strong(111) textures, while at higher temperatures of 500℃and 600℃,the films show random growth.Grain sizes of the GDC films varies with deposition temperature,it decreases gradually from 300℃to 500℃and becomes comparable at 500℃and above.Surface morphologies of GDC films also transform from deposition temperature:At the temperature of 300℃and 400℃,the films are covered with sphenoid faceted growth islands in an ordered orientation,and the film roughness R_rms＞12nm.At 500℃and 600℃,however,the morphologies changed into small, dense,round growth islands,and the film roughness R_rms≈2nm.The grain shape of the film deposited at 300℃is triangular,and the grain size is bigger,while at 400-600℃,that is small size and random shape grains.The volume fraction of grain boundary is bigger at 600℃.All the properties of the GDC film suggest that 500℃is the temperature point of the film growth nature change.The Nynist plot has two constituents,one approximate arc in high frequency and another small arc in low frequency,and the equivalent circuit is a parallel RC.Capacitance of a single GDC Film is almost independent of deposition temperature,and resistances increase exponentially with the test temperatures decrease.Conductivities increase as the test temperatures increase,the max value is 0.0151s.cm^-1,and to different GDC films it increase at 300-500℃deposition temperature but decrease at 600℃in the range of test temperature.The activation energy(～1.3eV) of GDC films is almost independent of deposition temperature, and it close to the reported value for the grain boundary.It indicates that grain boundary resistance dominates the electrical properties of GDC films.Conductivity varies with grain size of GDC films due to the grain boundary space charge effect,the smaller the grain size, the higher the conductivity and vice versa.The coincidence of the impedance Z' and Z" values at higher frequencies at all temperature indicates a possible release of space charge.