Ceramics and Composite Materials

Biography

Biography: Maik Finsel

Abstract

In recent years, zirconia (ZrO₂) micro- and sub-microparticles have attracted considerable attention due to their outstanding properties, including chemical inertness, thermal stability and high refractive index. They are suited for a broad variety of applications ranging from fuel cells, catalysis, electro- and bio ceramics to building blocks in photonic structures. The latter are promising materials for high-temperature applications, including thermal barrier coatings (TBC) and structural colors (SC). The thermal stability, surface smoothness and optical properties of zirconia particles can be improved or modified by encapsulation with a suitable shell material. By using different types of shells, e.g. Al₂O₃, SiO₂, TiO₂ or polymers, desired properties of the resulting core-shell composites for various applications are achievable. For example, silica shells ensure a strong refractive index contrast for application as structural colors. Previously, it was shown that silica shells could be grown on ZrO₂ cores after surface modification with organic additives (polymer, citric acid). However, such additives most likely preclude high-temperature applications. For the preparation of ZrO₂@SiO₂ core-shell sub-microparticles, we developed a straightforward approach without the need of additional organic capping agents by using SiO₂ seeds as self-adhesive layer. The zirconia core particles were synthesized according to the sol-gel method by Widoniak et al. modified to achieve smaller diameters. In the pre- encapsulation step, silica seeds are formed on the core surface. The silica shell can then grow smoothly on its self-adhesive layer and its thickness is controlled by successive addition of silica precursor. The obtained core-shell particles withstand temperatures up to 1000°C whereas size-comparable zirconia particles disintegrate when heated to 800°C. Also, core-shell particles synthesized using polyvinylpyrrolidone (PVP) as interfacial coupling agent disintegrate when heated to 800°C, most likely due to decomposition of PVP. Using XRD, SEM and cross-sectional TEM characterization, we show how grain growth and phase transitions are influenced by the SiO₂ encapsulation of ZrO₂ submicron particles. Additionally, the thermal stability can be improved by doping zirconia cores with yttrium or yttrium/lanthanum, as recently shown for zirconia microparticles

Figure 1: Synthesis of ZrO₂@SiO₂ core-shell particles with PVP as coupling agent (red arrows, according to previous work) and with SiO₂ seeds as self-adhesive layer (blue arrows, novel method). After temperature treatment at 1000°C, the shell of the PVP containing particles partially disintegrated whereas most silica shells without interfacial PVP layer stayed intact. Heated zirconia particles (without shell) after 1000°C are shown for comparison (black arrow).

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