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Item type:Publication, Access status: with files , A combined compression and indentation study of mechanical metamaterials based on inverse opal coatings(Elsevier, 2020-05-12); ; ; A relatively new class of tailored photonic metamaterials based on the inverse opal (IO) structure shows multifunctionality with exceptional mechanical response due to its periodic porous arch-like structure. Exploiting the smaller is stronger paradigm through varying its pore size and the addition of atomic layer deposition (ALD) films, allow tailorable strength and elastic modulus. Quantification of such properties are achieved through flat punch nanoindentation testing. Results are validated by comparison to microcompression tests, a widely used technique to circumvent the complex stress state normally imposed by indentation, but in the case of high porosity is greatly simplified, approximating uniaxial stress; results from both mechanical loading approaches show strong similarities. All results showed a dependence of strength and elastic modulus on the ratio of the deformation size, i.e., micropillar or flat punch diameter, to the pore size, a trend which is well described by the influence of the boundary conditions of the test method rather than an intrinsic size effect. At larger ratios the values approach a constant value. Furthermore, the mechanical response can be tailored through the deposition of a thin film on the shell structure of the silica IO structures; 34 nm of TiO2 was shown to produce a 10-fold increase in strength and 5-fold increase in elastic modulus.Publicationtype: Journal ArticleScopus© Citations 9 249 289 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Advancing the fabrication of YSZ-inverse photonic glasses for broadband omnidirectional reflector filmsA single-step and all-colloidal deposition method to fabricate yttrium-stabilized zirconia (YSZ)-inverse photonic glasses with 3 μm pores was developed. The process is based on electrostatic attraction and repulsion in suspension, controlled by surface charge of polystyrene (PS)microspheres and YSZ nanoparticles, used as pore templates and matrix material, respectively. The pH was used as a tool to change surface charges and particle-particle interactions. Photonic glass films with 3 μm pores yielded broadband omnidirectional reflection over the wavelengths of 1–5 μm, relevant for thermal radiation at temperatures around 1200 °C. These highly porous materials maintained their structural stability and reflectance after being annealed at 1200 °C for 120 h.Publicationtype: Journal ArticleScopus© Citations 9 819 474 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On the photonic and mechanical properties of ordered and disordered arrays of poresPublicationtype: Doctoral Thesis52 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Determination of the packing fraction in photonic glass using synchrotron radiation nanotomography(Wiley-Blackwell, 2016-11-01); ; ; ; Photonic glass is a material class that can be used as photonic broadband reflectors, for example in the infrared regime as thermal barrier coating films. Photonic properties such as the reflectivity depend on the ordering and material packing fraction over the complete film thickness of up to 100 μm. Nanotomography allows acquiring these key parameters throughout the sample volume at the required resolution in a non-destructive way. By performing a nanotomography measurement at the PETRA III beamline P05 on a photonic glass film, the packing fraction throughout the complete sample thickness was analyzed. The results showed a packing fraction significantly smaller than the expected random close packing giving important information for improving the fabrication and processing methods of photonic glass material in the future.Publicationtype: Journal ArticleScopus© Citations 8 214 328 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Bottom-up Fabrication of Multilayer Stacks of 3D Photonic Crystals from Titanium Dioxide(2016-04-27); ; ; ; A strategy for stacking multiple ceramic 3D photonic crystals is developed. Periodically structured porous films are produced by vertical convective self-assembly of polystyrene (PS) microspheres. After infiltration of the opaline templates by atomic layer deposition (ALD) of titania and thermal decomposition of the polystyrene matrix, a ceramic 3D photonic crystal is formed. Further layers with different sizes of pores are deposited subsequently by repetition of the process. The influence of process parameters on morphology and photonic properties of double and triple stacks is systematically studied. Prolonged contact of amorphous titania films with warm water during self-assembly of the successive templates is found to result in exaggerated roughness of the surfaces re-exposed to ALD. Random scattering on rough internal surfaces disrupts ballistic transport of incident photons into deeper layers of the multistacks. Substantially smoother interfaces are obtained by calcination of the structure after each infiltration, which converts amorphous titania into the crystalline anatase before resuming the ALD infiltration. High quality triple stacks consisting of anatase inverse opals with different pore sizes are demonstrated for the first time. The elaborated fabrication method shows promise for various applications demanding broadband dielectric reflectors or titania photonic crystals with a long mean free path of photons.Publicationtype: Journal Article90 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Refractory absorber/emitter using monolayer of ceramic microparticles(SPIE, 2016-04-21); ; ; ; 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.Publicationtype: Conference Paper105 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Layout and first results of the nanotomography endstation at the P05 beamline at PETRA III(American Institute of Physics (AIP), 2016-01-28); ; ; ; 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 100nm half period with an effective detector pixel size of 15nm. A first tomograph of a photonic glass sample was measured in early 2014.Publicationtype: Conference Paper167 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Yttria-stabilized zirconia microspheres: Novel building blocks for high-temperature photonics(Royal Society of Chemistry (RSC), 2015-11-02); ; ; ; 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.Publicationtype: Journal ArticleScopus© Citations 32 291 361 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Tungsten band edge absorber/emitter based on a monolayer of ceramic microspheres(Soc., 2015-08-27); ; ; ; 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 ZrO2 microparticles on a tungsten layer with a HfO2 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.Publicationtype: Journal Article107 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , High temperature behavior of monodisperse porosity in alumina films(Elsevier, 2015-07-10); ; ; ; Understanding the sintering behavior of tailored porosity in thin-film ceramics is essential for applications in photonics, thermal coatings, catalysis, and electrodes. Alumina films exhibiting a random distribution of templated pores with monodisperse size were produced by spin coating from suspensions of alumina powder and polystyrene microspheres. The microstructural evolution, grain size, pore size, and their respective distributions were determined as a function of the initial volume fraction of templated pores, sintering temperature, and time. The behavior between grains and pores at the surface and in the bulk was evaluated and compared to other disordered and ordered porous alumina systems. Above a pore fraction threshold of ~9 vol%, increasing pore fractions retarded grain growth, with smaller grains observed between close pores. While the average bulk pore size did not change with sintering temperature, time, or pore fraction, it did produce a broader pore size distribution with different pore geometries.Publicationtype: Journal Article74