Elisabeth W. Leib

Synthesis and thermal stability of zirconia and yttria-stabilized zirconia microspheres

HYPOTHESIS Zirconia microparticles produced by sol-gel synthesis have great potential for photonic applications. To this end, identifying synthetic methods that yield reproducible control over size uniformity is important. Phase transformations during thermal cycling can disintegrate the particles. Therefore, understanding the parameters driving these transformations is essential for enabling high-temperature applications. Particle morphology is expected to influence particle processability and stability. Yttria-doping should improve the thermal stability of the particles, as it does in bulk zirconia. EXPERIMENTS Zirconia and YSZ particles were synthesized by improved sol-gel approaches using fatty acid stabilizers. The particles were heated to 1500 °C, and structural and morphological changes were monitored by SEM, ex situ XRD and high-energy in situ XRD. FINDINGS Zirconia particles (0.4-4.3 μm in diameter, 5-10% standard deviation) synthesized according to the modified sol-gel approaches yielded significantly improved monodispersities. As-synthesized amorphous particles transformed to the tetragonal phase at ∼450 °C with a volume decrease of up to ∼75% and then to monoclinic after heating from ∼650 to 850 °C. Submicron particles disintegrated at ∼850 °C and microparticles at ∼1200 °C due to grain growth. In situ XRD revealed that the transition from the amorphous to tetragonal phase was accompanied by relief in microstrain and the transition from tetragonal to monoclinic was correlated with the tetragonal grain size. Early crystallization and smaller initial grain sizes, which depend on the precursors used for particle synthesis, coincided with higher stability. Yttria-doping reduced grain growth, stabilized the tetragonal phase, and significantly improved the thermal stability of the particles.

Tungsten band edge absorber/emitter based on a monolayer of ceramic microspheres.

We report on a band edge absorber/emitter design for high-temperature applications based on an unstructured tungsten substrate and a monolayer of ceramic microspheres. The absorber was fabricated as a monolayer of ZrO(2) microparticles on a tungsten layer with a HfO(2) nanocoating. The band edge of the absorption is based on critically coupled microsphere resonances. It can be tuned from visible to near-infrared range by varying the diameter of the microparticles. The absorption properties were found to be stable up to 1000°C.

Yttria-stabilized zirconia microspheres: novel building blocks for high-temperature photonics

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.

Layout and first results of the nanotomography endstation at the P05 beamline at PETRA III

The Helmholtz-Zentrum Geesthacht operates the P05 Imaging Beamline at the DESY storage ring PETRA III. This beamline is dedicated to micro- and nanotomography with two endstations. This paper will present the nanotomography endstation layout and first results obtained from commissioning and test experiments. First tests have been performed with CRLs as X-ray objectives and newly developed rolled X-ray prism lenses as condenser optics. This setup allows a resolution of 100 nm half period with an effective detector pixel size of 15nm. A first tomograph of a photonic glass sample was measured in early 2014.

Refractory absorber/emitter using monolayer of ceramic microparticles

We present a self-assembled refractory absorber/emitter without the necessity to structure the metallic surface itself, still retaining the feature of tailored optical properties for visible light emission and thermophotovoltaic (TPV) applications. We have demonstrated theoretically and experimentally that monolayers of zirconium dioxide (ZrO2) microparticles on a tungsten layer can be used as large area, efficient and thermally stable selective absorbers/emitters. The band edge of the absorption is based on critically coupled microsphere resonances. It can be tuned from visible to near-infrared range by varying the diameter of the microparticles. We demonstrated the optical functionality of the structure after annealing up to temperatures of 1000°C under vacuum conditions. In particular it opens up the route towards high efficiency TPV systems with emission matched to the photovoltaic cell.

Determination of the packing fraction in photonic glass using synchrotron radiation nanotomography

Synchrotron radiation nanotomography has been used to quantify the packing fraction in a photonic glass sample.