• Semmar, Nadjib
• Colombier, Jean-Philippe
• Thomann, Anne-Lise
• Champeaux, Corinne
• Karim, Wael
• Tabbal, Malek
• Briois, Pascal
• Vulliet, Julien

Abstract

In ceramic solid oxide electrochemical cells (SOEC), a thin film of gadolinium-doped cerium oxide (CGO) is deposited (by plasma sputtering) between the electrolyte (yttrium-stabilized zirconia, YSZ) and the oxygen electrode (lanthanum-strontium-cobalt ferrite, LSCF) to reduce the formation of insulating phases at the electrode/electrolyte interface. In order to improve the adhesion between these layers as well as the ion exchange surfaces, we have considered a micro/nanoscale morphological structuring process using laser beams. The challenge is to successfully fabricate well-organized nanostructures on complex oxide thin films such as CGO, and thus assess the feasibility of forming laser-induced periodic surface structures (LIPSS). One of the objectives of this thesis is therefore to increase the specific surface area of the material through the formation of LIPSS. Our study specifically focuses on the generating LIPSS patterns on a 700 nm CGO film under both static and scanning irradiation conditions. A picosecond Nd: YAG laser (355 nm, 40 ps, 10Hz) with a relatively large laser beam spot (~500 μm) was used. In static mode, parallel and perpendicular LSFL (low spatial frequency LIPSS) were formed simultaneously in the irradiated area. The generation of parallel LSFL// is mainly attributed to a thermochemical process produced in the center of the irradiated zone corresponding to the highest local effluence, while the perpendicular LSFL that are detected at the edges of the laser spot are due to a soft ablation process. A transition zone between these two types of LSFL manifested by the appearance of 'nano-squares' is due to the superposition of LSFL// and LSFL. The soft ablation process of the CGO layer also induces the formation of organized patterns in form of square shaped cracks on YSZ sublayer. The double irradiation of the laser beam in the same area with two different conditions leads to the formation of 2D structuring that were also obtained in scanning mode. Different surface characterization techniques (SEM, AFM, etc.), as well as COMSOL Mutliphysics simulations, have been used to interpret the obtained results. LSFL were also identified on the YSZ surface, as well as HSFL (high spatial frequency LIPSS) formed at a very large number of laser pulses (~1000). Using a geometric model, we were able to estimate the theoretical increase in developed area of 57% and 78% for 1D (regular LIPSS) and 2D periodic structures respectively. In this work, the largest experimental value reached is 42% for combined 2D structures of the 'nano-squares' type.

Topics

• impedance spectroscopy
• surface
• phase
• scanning electron microscopy
• thin film
• simulation
• Oxygen
• atomic force microscopy
• crack
• Strontium
• Lanthanum
• forming
• cobalt
• Yttrium
• ceramic
• Gadolinium
• Cerium