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Item type:Publication, Access status: with files , Spin‐ and Stress‐Depending Electrical Transport in Nanoparticle Supercrystals: Sensing Elastic Properties of Organic Tunnel Barriers via Tunneling Magnetoresistance(Wiley-VCH, 2022-05-26); ; ; ; Abstract The spin‐dependent electrical transport in rigid inorganic‐inorganic layered systems is extensively applied for the detection of magnetic fields in data storage. In this work, spin‐dependent electrical transport in flexible organic‐inorganic supercrystals based on superparamagnetic iron oxide nanoparticles is investigated. These nanoparticles are stabilized by oleic acid ligands, which in turn are serving as tunneling barriers between individual magnetic nanoparticles. The resulting tunneling magnetoresistance (TMR) is tunable due to the elastic properties of these organic barriers. Applying external mechanical stress on this composite material will change the average distance between adjacent nanoparticles and will hence determine the resulting TMR‐effect amplitude. Thus, measured stress‐induced changes in the barrier thickness at sub‐nanometer scale allow for determining the mechanical properties of organic barrier molecules in the confined space between the particles. These results provide the foundation for a new type of mechanical sensor.Publicationtype: Journal ArticleScopus© Citations 3 88 163 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Using polystyrene spheres to produce thin, porous interlayers in alumina laminatesPorous alumina layers were produced by colloidal processing of alumina with the addition of 15, 30, and 45 vol% polystyrene spheres (PS) as pore formers. Alumina laminates were designed with dense layers alternated with porous interlayers using 45 vol% of PS spheres and sintered at 1350°C. The layers’ thickness ranged from 2 to 15 μm, with a random distribution of pores. The higher volume fraction of pores tends to decrease the alumina average grain size, but does not influence the final size of bulk and surface pores. The obtained values of hardness and Young's modulus for the porous interlayer are ∼30% of the values obtained for the dense layer. Vickers indentations suggested that crack propagation can be opposed by the porous interlayers. However, values of mechanical strength, fracture toughness (KIC), and work of fracture presented no relevant difference compared to a monolithic reference. R-curves presented a slight increase and KIC a decrease due to crack propagation through the porous interlayers. Although no macrodeviations of the crack path were observed in the fractured surfaces, microdeviations were detected in the interlayer regions.Publicationtype: Journal Article58 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Improved thermal stability of zirconia macroporous structures via homogeneous aluminum oxide doping and nanostructuring using atomic layer deposition(Elsevier, 2021-02-11); ; ; ; Dopants are regularly used in sol-gel and powder metallurgy routes, however, the controlled insertion of such is quite challenging, especially in the case of nanostructures. Here we investigate the use of atomic layer deposition (ALD) as a potential technique to precisely introduce aluminum oxide as dopant or second phase into zirconia 3D macroporous nanostructures. The results show that the introduction of high Al O contents into the zirconia nanostructures successfully inhibited sintering when in comparison to undoped zirconia. Moreover, for the multi-nanolaminated and full-mix structures, the tetragonal phase was stabilized up to 1200 °C. Furthermore, the structures presented a photonic band gap even after heat treatment at 1200 °C for 2 h, enabling its application as inverse opal photonic crystals in high-temperature environments. The enhancement of thermal stability and high-temperature tetragonal phase stabilization is enabled jointly by the nanostructuring and homogeneous distribution of aluminum oxide provided by ALD super-cycles. 2 3Publicationtype: Journal ArticleScopus© Citations 8 191 376 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Microstructure and mechanical behavior of TiO₂-MnO-doped alumina/alumina laminatesTapes of TiO2-MnO-doped alumina (d-Al2O3) and pure alumina (Al2O3) were shaped via tape casting. Laminates with three different layer numbers and respective thicknesses were produced and sintered at 1200°C. The microstructure and mechanical behavior of laminates were investigated and compared to the respective monolithic references (d-Al2O3 and Al2O3). The use of dopants in alumina decreased the initial sintering temperature, leading to higher densification at 1200°C (~98% theoretical density (TD)) when compared to Al2O3 (~73% TD). The higher density was reflected in a higher Young's modulus and hardness for doped alumina. A region of diffusion of dopants in pure alumina layers was observed along the interface with doped layers. The mechanical strength of d-Al2O3 samples sintered at 1200°C was not statistically different from Al2O3 samples sintered at 1350°C. The strength of laminates composed of doped layers with undoped, porous interlayers did not change. Nevertheless, as the thickness of these porous interlayers increases, a loss of strength was observed. Monolithic references showed constant values of fracture toughness (KIC), ~2 MPa·m1/2, and linear crack path. On the other hand, KIC of laminates increases when the crack propagates from weak Al2O3 layers to dense d-Al2O3 layers.Publicationtype: Journal ArticleScopus© Citations 5 195 294 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Influence of pores arrangement on stability of photonic structures during sintering(Elsevier, 2020-05-11); ; ; ; Discrete Element Method (DEM) has been used for numerical investigation of sintering-induced structural deformations occurring in inverse opal photonic structures. The influence of the initial arrangement of template particles on the stability of highly porous inverse opal α-Al2O3 structures has been analyzed. The material transport, densification, as well as formation of defects and cracks have been compared for various case studies. Three different stages of defects formation have been distinguished starting with local defects ending with intrapore cracks. The results show that the packing of the template particles defined during the template self-assembly process play a crucial role in the later structural deformation upon thermal exposure. The simulation results are in very good agreement with experimental data obtained from SEM images and previous studies by ptychographic X-ray tomography.Publicationtype: Journal ArticleScopus© Citations 7 203 243 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Alumina-doped zirconia submicro-particles : synthesis, thermal stability, and microstructural characterization(Multidisciplinary Digital Publishing Institute (MDPI), 2019-09-05); ; ; ; Zirconia nanoceramics are interesting materials for numerous high-temperature applications. Because their beneficial properties are mainly governed by the crystal and microstructure, it is essential to understand and control these features. The use of co-stabilizing agents in the sol-gel synthesis of zirconia submicro-particles should provide an effective tool for adjusting the particles’ size and shape. Furthermore, alumina-doping is expected to enhance the particles’ size and shape persistence at high temperatures, similar to what is observed in corresponding bulk ceramics. Dispersed alumina should inhibit grain growth by forming diffusion barriers, additionally impeding the martensitic phase transformation in zirconia grains. Here, alumina-doped zirconia particles with sphere-like shape and average diameters of 300 nm were synthesized using a modified sol-gel route employing icosanoic acid and hydroxypropyl cellulose as stabilizing agents. The particles were annealed at temperatures between 800 and 1200 degree Celsius and characterized by electron microscopy, elemental analysis, and X-ray diffraction. Complementary elemental analyses confirmed the precise control over the alumina content (0–50 mol%) in the final product. Annealed alumina-doped particles showed more pronounced shape persistence after annealing at 1000 degree Celsius than undoped particles. Quantitative phase analyses revealed an increased stabilization of the tetragonal/cubic zirconia phase and a reduced grain growth with increasing alumina content. Elemental mapping indicated pronounced alumina segregation near the grain boundaries during annealing.Publicationtype: Journal ArticleScopus© Citations 13 731 508 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Transparency induced in opals via nanometer thick conformal coating(Macmillan Publishers Limited, part of Springer Nature, 2019-08-06); ; ; ; Self-assembled periodic structures out of monodisperse spherical particles, so-called opals, are a versatile approach to obtain 3D photonic crystals. We show that a thin conformal coating of only several nanometers can completely alter the reflection properties of such an opal. Specifically, a coating with a refractive index larger than that of the spherical particles can eliminate the first photonic band gap of opals. To explain this non-intuitive effect, where a nm-scaled coating results in a drastic change of optical properties at wavelengths a hundred times bigger, we split the permittivity distribution of the opal into a lattice function convoluted with that of core-shell particles as a motif. In reciprocal space, the Bragg peaks that define the first Brillouin zone can be eliminated if the motif function, which is multiplied, assumes zero at the Bragg peak positions. Therefore, we designed a non-monotonic refractive index distribution from the center of the particle through the shell into the background and adjusted the coating thickness. The theory is supported by simulations and experiments that a nanometer thin TiO2 coating via atomic layer deposition (ALD) on synthetic opals made from polystyrene particles induces nearly full transparency at a wavelength range where the uncoated opal strongly reflects. This effect paves the way for sensing applications such as monitoring the thicknesses growth in ALD in-situ and in real time as well as measuring a refractive index change without spectral interrogation.Publicationtype: Journal ArticleScopus© Citations 6 478 380 - 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 , Highly selective photonic glass filter for saturated blue structural color(2019-04-02); ; ; ; Angle independent non-absorbing spectral filters are required for many applications such as sunscreens, structural colors, photovoltaics, and radiative cooling. One of the promising and simple to manufacture structures is based on the disordered arrangement of monodisperse spherical particles by self-assembly, also called photonic glasses. So far, reported photonic glasses inherently show poor spectral selectivity with a smooth transition in reflection. No significant improvement is usually expected from particles optimization as the Mie resonances are broad for small dielectric particles with a moderate refractive index. Via Fourier space engineering, we show here that it is, nonetheless, possible to obtain sharp spectral transitions from the synergetic effect of a core-shell geometry of the particles with the short range order of the photonic glass. We apply the developed approach to demonstrate a high color saturation of a non-iridescent blue structural color employing a photonic glass with hollow sphere particles, which features a sharp spectral transition in reflection. The experimental results support the theoretical predictions from the first-order approximation. © 2019 Author(s).Publicationtype: Journal ArticleScopus© Citations 18 338 347 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , High-temperature stable inverse opal photonic crystals via mullite-sol-gel infiltration of direct photonic crystalsThree-dimensionally ordered macroporous materials for photonic or refractory applications have been developed by an innovative approach based on mullite sol-gel infiltration of direct photonic crystals followed by burn-out and calcination. Direct photonic crystals were obtained using polystyrene spheres templates either by vertical convective self-assembly or by drop casting. The samples were then infiltrated by spin coating with mullite sol-gels prepared with two different compositions (74 wt.% Al 2 O 3 , 26 wt.% SiO 2 and 80 wt.% Al2O 3 , 20 wt.% SiO 2 ). The inverse opal photonic crystals prepared with both sol-gels presented a highly ordered porosity and the high-alumina composition showed stability up to 1500°C. After inversion of the structure (polymeric template burn-out), the high-alumina composition showed roundness of the PS templated pores closer to an ideal sphere (Ø = 0.967) when compared to the low-alumina composition (Ø = 0.954). Although the inverse opal photonic crystals did not present a photonic bandgap, they showed structural stability at high temperatures, which enable their application as refractory materials.Publicationtype: Journal ArticleScopus© Citations 10 277 389