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Item type:Publication, Access status: with files , Influence of alumina addition on the optical properties and the thermal stability of titania thin films and inverse opals produced by atomic layer deposition(Multidisciplinary Digital Publishing Institute (MDPI), 2021-04); ; ; ; TiO2 thin films deposited by atomic layer deposition (ALD) at low temperatures (lower than 100 °C) are, in general, amorphous and exhibit a smaller refractive index in comparison to their crystalline counterparts. Nonetheless, low-temperature ALD is needed when the substrates or templates are based on polymeric materials, as the deposition has to be performed below their glass transition or melting temperatures. This is the case for photonic crystals generated via ALD infiltration of self-assembled polystyrene templates. When heated up, crystal phase transformations take place in the thin films or photonic structures, and the accompanying volume reduction as well as the burn-out of residual impurities can lead to mechanical instability. The introduction of cation doping (e.g., Al or Nb) in bulk TiO2 parts is known to alter phase transitions and to stabilize crystalline phases. In this work, we have developed low-temperature ALD super-cycles to introduce Al2O3 into TiO2 thin films and photonic crystals. The aluminum oxide content was adjusted by varying the TiO2:Al2O3 internal loop ratio within the ALD super-cycle. Both thin films and inverse opal photonic crystal structures were subjected to thermal treatments ranging from 200 to 1200 °C and were characterized by in- and ex-situ X-ray diffraction, spectroscopic ellipsometry, and spectroscopic reflectance measurements. The results show that the introduction of alumina affects the crystallization and phase transition temperatures of titania as well as the optical properties of the inverse opal photonic crystals (iPhC). The thermal stability of the titania iPhCs was increased by the alumina introduction, maintaining their photonic bandgap even after heat treatment at 900 °C and outperforming the pure titania, with the best results being achieved with the super-cycles corresponding to an estimated alumina content of 26 wt.%.Publicationtype: Journal ArticleScopus© Citations 11 188 176 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Conductive and radiative heat transfer inhibition in YSZ photonic glass(2020-08-01); ; ; ; A high temperature stable ceramic photonic structure is demonstrated with low thermal conductivity and suppressed external radiative heat transfer. The structure is based on a disordered arrangement of yttria-stabilized zirconia (YSZ) microparticles, called photonic glass (PhG). The prepared YSZ-PhG film exhibits low thermal conductivity of 0.03 Wm−1K−1 comparable to that of the air. The small point contacts of the adjacent YSZ particles are the main cause of such low thermal conductivity. After annealing at 1400 °C for 5 h, the solid thermal conductivity increased to 0.3 Wm−1K−1 at room temperature due to the thermally induced neck formation, associated with an increased contact area between adjacent particles. This thermal conductivity is still much lower than that of conventional YSZ thermal barrier coatings (TBCs) with approximately 1 Wm−1K−1. At the same time, the PhG structure is an efficient scatterer for thermal radiation in the wavelength range between 1 and 6 μm. In an only 100 μm thick structure an average reflection of 84% was obtained. At 1400 °C, the effective thermal conductivity is 0.2 Wm−1K−1. The presented structure is applicable to other oxides with even lower bulk thermal conductivity and can be considered for future TBCs.Publicationtype: Journal Article262 - 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: with files , Controlling thermal emission with refractory epsilon-near-zero metamaterials via topological transitions(Nature Publishing Group, 2016-06-06); ; ; ; Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO2 metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the OTT in a thermally stable metamaterial at high temperatures of 1,000 °C.Publicationtype: Journal ArticleScopus© Citations 245 584 459 - 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: 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 , Self-assembled ultra high strength, ultra stiff mechanical metamaterials based on inverse opals(Wiley-VCH, 2015-03-30); ; ; ; Researchers show that mechanical metamaterials based on ceramic inverse opaline structures with densities in the range of 330-910 kgm-3 are suitable as photonic crystals and have better stiffness- and compressive strength-to-weight ratios as compared to microfabricated optimized strut-based structures. The stiffness- and compressive strength-to-weight ratios are also lower than carbon nanoframes fabricated by interference lithography. Pure silica inverse opal structures and silica inverse opals whose pores have been internally coated by a thin TiO2 layer have been fabricated and their structural and mechanical behavior has been investigated.Publicationtype: Journal Article101 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Perfect narrow-band absorber based on a monolayer of metallodielectric microspheres(American Institute of Physics (AIP), 2013-11-19); ; We have studied how two-dimensional arrays of metallodielectric core-shell microspheres on a metal substrate can efficiently absorb infrared electromagnetic radiation in a narrow wavelength range under normal incidence. Our simulations indicate that perfect absorption efficiencies can be achieved for resonance wavelengths. The influence of core-shell microspheres geometry and lattice geometry is studied on absorption properties. For wavelength from 1.2 μm to 2.6 μm, an optimal combination of sphere and core radius was obtained to provide perfect absorption which can be wavelength adjusted. © 2013 AIP Publishing LLC.Publicationtype: Journal Article65