WEB Tailoring electrostatic surface potential and adsorption capacity of porous ceramics by silica-assisted sintering
The surface charge of oxide ceramic materials critically determines the material’s performance in application areas such as filtration, biomaterials, biomedical devices and catalyst supports. Chemical functionalization, like silanization, is frequently used to modify surface charge and other characteristics of ceramic materials without affecting their bulk properties. However, such strategies often require poorly scalable protocols, especially if a well-defined, ultrathin surface coating is desired, and they show a limited stability against hydrolysis, especially at extreme pH, or against decomposition at elevated temperatures. As an alternative, inorganic amorphous sintering additives might be used to modify ceramic surfaces without the need for chemical post-functionalization to generate a resistant and well-defined surface coating with tailored surface charge.
To this end, we apply silica-assisted sintering to develop porous yttria stabilized zirconia (YSZ) ceramics with tailored electrostatic surface potential and adsorption capacity as a promising alternative to chemical functionalization. Porous bodies are formed by partial sintering at 1050 °C and are investigated regarding the influence of admixtures of silica particles on sintering behavior, microstructural evolution and the resulting mechanical and surface properties of the material, particularly the surface potential. With increasing silica content, the sintering mechanism gradually changed from solid state to liquid phase sintering, due to the wetting of YSZ by liquid silica and a resulting inhibition of mass transport, particle growth and diffusion-induced densification. Most importantly, due to the silica layer development, the isoelectric point (IEP) of the YSZ/silica material surfaces could be systematically shifted towards the IEP of silica from pH 9.4 to 1.2 resulting in a more pronounced negative surface potential at neutral pH. The relationship between surface IEP and silica concentration was mathematically described using the IEPs of the starting materials, the YSZ particle radius and the glass layer thickness. This estimation allows us to tailor the surface coverage of the YSZ matrix with silica as well as the resulting electrostatic surface potential. We further demonstrate how the applied processing route can be effectively used to develop ceramics with specified adsorption capacities for protein immobilization for use in filtration, bioprocessing or biomaterial applications.