Yttria-stabilized zirconia microspheres: novel building blocks for high-temperature photonicsElectronic supplementary information (ESI) available: Synthetic details, heating rate profiles, SEM and XRD data for the particles as well as SEM, XRD and IR-spectroscopic data for the photonic glass films. See DOI: 10.1039/c5tc03260a

Zirconia-based ceramics cover a huge variety of applications, including refractories, electro- and bioceramics, fuel cells, catalysts, and many more. For various photonic applications considered for energy systems and heat management, zirconia microspheres are interesting building blocks due to their high refractive index, as well as their chemical and mechanical robustness. However, instabilities caused by thermally-induced phase transitions and grain growth at temperatures above ∼1000 °C preclude high-temperature applications of pure zirconia particles. Here, we present a synthetic route for yttria-stabilized zirconia microparticles with significantly improved thermal stability. With these particles we conducted the first study on their thermal stability as a function of the yttrium content and at temperatures up to 1500 °C. Using X-ray diffraction and scanning electron microscopy, the optimum Y content was determined to be 8-10%, which was marked by stabilization of the tetragonal or cubic phase and significantly attenuated grain growth. Furthermore, with diameters ranging from 2 to 5 μm, the particles covered a size range perfectly suited for photonic applications in the IR spectral range. To demonstrate this, photonic glass coatings were prepared with these particles and their IR reflectivity and microstructural stability was studied after subjecting them to various heating cycles. While heating beyond 1200 °C led to failure and delamination of undoped particle films, films doped with 6 and 10% Y displayed quite stable broadband IR reflection of up to 80% in the wavelength range from 1-5 μm, even after prolonged heating at 1400 °C. A detailed analysis of the X-ray diffraction patterns revealed that prolonged heating at 1400 °C resulted in phase decomposition due to Y segregation into Y-lean and Y-rich domains, confirming the presence of the solute-drag effect. Yttria-stabilized zirconia microspheres show significantly enhanced thermal stability and are interesting building blocks for IR-reflective photonic glasses operated above 1200 °C.