| Abstract [eng] |
Barium cerate and barium zirconate-based materials are well known for their ionic conductivity properties at temperatures between 200 and 1000 ºC. The properties of these materials vary from random-oriented crystal to highly oriented heteroepitaxial films. The orientation forms surfaces with different textures. Textured structure is a result of the residual stress in the film that affects the surface. The residual stress as well affects the entire film that results on the hetero-epitaxial ultra-thin films, offering an advantage to obtain a detailed study of the surface and interfaces: they exhibit a bending or formation of cracks in the entire film effect due to the mismatch or poor mechanical properties. Textured thin films and the stress in the formed structures influence their chemical stability and catalytic, mechanical and electronic properties. There are two main issues for the production of microsystems on metal supports, i.e., processing cost and scalability [3]. Moreover, the use of metal-supported fuel cell causes another problem, which is the degradation of the metal and the migration of atoms, such as Ni, which decomposes the crystal at high sintering temperatures [4]. High sintering temperatures are necessary to obtain a high percentage of crystallinity, high-density and thin films with large grain size. Physical vapor deposition (PVD) techniques have the advantage of forming dense, crystalline films at low forming temperatures. A variation in the deposition parameters controls the microstructure and properties of the formed thin film. In this work, there were investigated the formation of barium cerate (BaCeO3-δ, BCO) and their doped composition with variation in the atomic concentration as barium cerate doped yttrium 10% (BaCe0.9Y0.1O3-δ, BCY10), barium cerate doped yttrium 20% (BaCe0.8Y0.2O3-δ, BCY20), barium zirconate (BaZrO3-δ, BZO) and their doped composition with variation in the atomic concentration as barium zirconate doped yttrium 20% (BaZr0.8Y0.2O3-δ, BZY) and barium zirconate 10% yttrium with 10% Ceria (BaZr0.8Ce0.1Y0.1O3-δ, BZCY) thin films, formed using the e-beam evaporation method. The influence of the microstructure of the formed thin films on ionic conductivity, chemical and mechanical stability was investigated. The e-beam evaporation process of barium cerates and barium zirconates demonstrated the formation of textured thin films. The microstructure of the formed thin films is influenced by the kinetic energy of the arriving particles gained from the evaporation process [5, 6], substrate temperature and the relaxation processes. The dopant concentration as well influences the microstructure and residual stress of the formed thin films. The Thornton diagram clearly describes the dependence of technological parameters on the microstructure [7]. The microstructure is classified into different zones (Zone 1, Zone T, Zone 2 and Zone 3). Zone 1 is characterized by a porous and fibrous amorphous phase structure in the cross-section due to the diffusion limited movements of adatoms. The increase of the substrate temperature enhances the diffusion of adatoms filling the voids and forming the Zone T, but the amorphous phases are still present in the thin films. By further increasing the temperature, in Zone 2, the diffusion of the adatom is further enhanced resulting in the formation of broad highly crystalline phases columns. At the higher temperatures, in Zone 3, the bulk and boundary diffusions participate by forming near-equiaxed and highly crystalline structures [8, 9]. Barium cerate thin films with columnar growth (Zone T) and highly dense structure with the smallest grain sizes were obtained when the support temperature was below 400 ºC . Although the support temperature is higher than 500 ºC , thin films exhibit columnar growth (Zone 2) with improved grain sizes compared to the Zone T. Ionic conductivity varies due to the change in microstructure. The barium cerate (BCO) thin film formed on Al2O3 (0001), YSZ (001) and MgO (001) supports corresponds to Zone 2. It demonstrates the influence of residual stress on the ionic conductivity. The highest ionic conductivity in BCO thin films (~0.1 S/cm, 400 ºC) was obtained when the thin films were formed on YSZ support at 600 ºC formation temperature. YSZ support induces oxygen vacancy formation in BaCeO3 and then enhances the ionic conductivity. Furthermore, barium zirconates thin films were highly unstable when the formation temperatures were below 400 ºC. Then, they exhibit stability when support temperature is higher than 500 ºC. Summarising, it was shown how the microstructure and ion conductivity of the formed barium cerates and barium zirconates thin films are influenced by the deposition parameters (deposition rate and substrate temperature) and lattice mismatch between the thin films and supports, which have different thermal and ionic conductivities. |
