Développement de couches absorbantes à base de composites Al2O3-NiO pour des applications en énergie solaire thermique

Efficient conversion of solar radiation into useful heat is characterized by the ability of the thermal solar conversion systems to capture and absorb the largest amount of solar radiation while minimizing losses by thermal emission. The solar collector performances depend on several parameters such as the elaboration processes, the optical and thermal properties of materials. Various elaborations methods are employed and the composite coatings based on various materials are widely studied in order to improve yields. This is due to the increased interest in the development and efficient use of renewable energy sources. However, thermal efficiency of solar collectors is low ( 40 %) compared to optical yields ( 80 %). These efficiencies, in particular the thermal one, should be significantly improved. In addition their durability should be taken into account, because the thermal and chemical performance of some solar collector on the market are middling due to the poor quality of the materials which are used. The main objective of this study is to develop layers absorbents based on composites in order to improve the optical and thermal performance and possibly the sustainability of the solar energy conversion technologies into heat and / or electricity. The layers have been developed using electrophoretic deposition technique (EPD) of dispersed nickel oxide (NiO) nanoparticles in alumina (Al2O3) matrix on nickel (Ni), aluminum (Al) or stainless steel (SS) substrates. EPD process is based on migration of charged particles suspended in a fluid medium under the influence of an electric field (electrophoresis process) and the formation of deposit onto the oppositely charged electrode (deposition process). The mechanism studies of charges appearance and the suspension stability are primordial for any electrophoresis experience. Effectively, it is important to determine these aspects because the particles in suspension migrate only if they carry sufficient charge and only a very stable suspension is able to produce uniform deposits with high microstructural homogeneity. Mechanism of creating the charge on particles surface: Oxide particle surfaces in air are usually covered by amphoteric hydroxyl groups. These suffer reaction with the H+/or OH- of a suspension liquid, which ion depending on the suspension pH and depending on pH, the particles surface charge can be positive or negative. x Mechanism of suspension stability: The presence of this surface charge around particles leads to the formation of the ionic atmosphere called the double-layer (Stern and diffuse layer). The measure of the potential at boundary of the Stern layer gives the value of the particles zeta potential is the magnitude characteristic of the state of particles surface charge. The particles zeta potential measurement could predict the stability of the suspension (high zeta potential is the sign of a suspension stability). The first model used to describe EPD process is attributed to Hamaker [1] who proposed that, the deposited mass per unit area is proportional to the parameters related to the suspension stability properties, which are the suspension concentration and the electrophoretic mobility and those related to the physical and electrical fixed conditions of the deposition which the applied electric field strength and the deposition time. Electrophoretic mobility is proportional to the parameters related to the zeta potential and some parameters related to the solvent properties which the permittivity and the viscosity. The solvent has therefore a direct influence on the deposited mass and must be carefully chosen. Usually in EPD, organic (ethanol) and aqueous (water) solvents are used. Water is benefic because it provides a good stability and a fast kinetic but the disadvantages of electrolysis interferes with the quality of the film deposited [2]. Organic solvents (ethanol) have the advantage of high density and appropriate conductivity [3, 4]. However, the electric charge on the particles in these solvents is very low so that it's difficultly to obtain a stable suspension for oxide powders deposition [5]. In the first step of our study and in order to achieve our overall aim, we found it necessary to experiment two solvents whose pure ethanol and mixture ethanol-water. Monolithic Al2O3 and NiO films were deposited separately on Ni substrate by EPD from pure ethanol and mixture ethanol-water suspensions, stabilized with acetic acid in order to determine the optimum parameters of stability and deposits of each powder. However it was found from these studies that ethanol-water was more advantageous than pure ethanol for the electrophoretic deposition of Al2O3 and NiO suspensions. This was due to the higher stability and electrophoretic mobility of the powders suspension in this solvent than in pure ethanol. Denser and more homogeneous deposits are also obtained with the ethanol-water solvent than in ethanol. Furthermore the Al2O3 and NiO particles were positive electrical charges and their electrophoretic motilities are similar in this solvent. Accordingly the ethanol-water solvent will be used in the following works of this thesis to optimize xi the EPD deposition of the Al2O3-NiO composites. Therefore these results are very useful for a proper achievement of the following works of the thesis. In particular the similar electrophoretic mobility of Al2O3 and NiO particles obtained in the ethanol-water solvent made possible these studies. In the second step of our study, Al2O3-NiO composite containing 0 to 100 % NiO have been fabricated from the mixture powder in ethanol-water suspensions by adjusting the previous optimum parameters. All films were deposited on the cathode and theirs thickness could be controlled by adjusting the NiO content and the deposition time. Our experimental results illustrated clearly the advantage of EPD for the synthesis of Al2O3-NiO composites. The microstructures showed that the NiO particles were uniformly dispersed in the matrix of Al2O3. If the NiO content increases the particles distribution becomes more symmetrical, forming an interconnected matrix which inhibit Al2O3 grain growth. These different aspects spark our interest to study the relationship between the microstructures of the Al2O3-NiO composites and their mechanical properties. It was found that increasing the voltage beyond 150 V have generated the porous microstructures and reduced the deposited yield at NiO ≤ 40 %, probably due to the formation of spinel NiAl2O4 revealed by the XRD analysis. In the third step of our study, we investigated the effects of NiO content, the sintering temperature and the applied voltage on the relative density, mechanical and electrical properties of these composites. The results revealed that, in contrast to the dense monolithic Al2O3, the addition of NiO ≥ 50 % nanoparticles has promoted sintering of composite coatings at relatively medium temperatures and a marked increment in the density, electrical conductivity and toughness of deposits. The micro-hardness was found inversely dependent of fracture toughness. In the last step of our work, we have characterized the optical and thermal properties of Al2O3-NiO composites containing 60-85 % NiO on Ni, Al and stainless steel substrates. The comparison of their coefficients of solar absorption and thermal emissivity was determined using reflectance measurements in the solar spectrum and in infrared respectively. The effects of the nature of the substrate, the NiO content, the sintering temperature, the applied voltage of the EPD process and the deposition times on the optical properties of the fabricated composites were studied and xii analyzed. Durability and thermal stability tests were performed. These various aspects studied in this thesis are presented in chapters V, VI, VII and VIII as four articles published or submitted.