Understanding the key parameters for the rational design of layered oxide materials by composite sol–gel procedures

Abstract

Previous works have well demonstrated that particle size of the filler used in layered oxide formulation is the first important parameter and must be decreased below 5 μm (Agrafiotis, 1999–2000 [10]). But once the particle size is set what are the next formulation parameters to highlight as critical? How do we improve cohesion and adhesion of the coatings? To highlight the key parameters driving the quality of coating, a model layered oxide material was prepared inside a pan granulator. The model composite sol–gel formulation is based on boehmite nanoparticles (binder) and a monomodal two micrometer grain size gamma alumina (filler) which is applied onto alpha alumina beads substrate. The influences of the wetting method and relative amount of filler and binder were investigated. Extensive characterization and imaging of the layered materials (SEM, Cryo-SEM, EPMA, Washburn test, mechanical tests, Hg-porosimetry) were used in order to follow the microstructure evolution of coating during and at the end of drying. Several crack propagation schemes were observed and explained qualitatively. Overall quality of coating is mainly related to the sol–gel transition of the binder. It defines if prior to shaping, the binder primer will be able to improve the coating adhesion and it defines also the nature and extent of damages that the coating undergoes during drying. The mechanical properties of layered oxide materials obtained using composite sol–gel formulation are definitely correlated with the binder gel shrinkage during drying.