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Síntese e caracterização ferroelétrica de compósitos cerâmicos planares de BaTiO3/BaTi1-xZrxO3
(2015)
Compósitos cerâmicos planares de titanato zirconato de bário, BaTi1-xZrxO3, foram produzidos e os efeitos da quantidade de Zr4+ em suas propriedades funcionais foram estudados. As amostras foram fabricadas pelo método convencional de processamento cerâmico e pela técnica de deposição de fitas cerâmicas a partir de BaTi1-xZrxO3 com x=0, 0,05, 0,1, 0,15 e 0,2 sintetizados pelo método hidrotermal e pelo método dos precursores poliméricos. Foram realizadas caracterizações estrutural (difração de Raios X pelo método do pó e espectroscopia Raman), composicional (espectroscopia por dispersão de energia), microestrutural (microscopia eletrônica de varredura, ensaios de dilatometria) e funcional (permissividade elétrica, coeficiente piezoelétrico d33, coeficiente piroelétrico e histerese ferroelétrica). Além disso, a regra da mistura de fase foi utilizada para prever a permissividade elétrica dos compósitos e as tensões mecânicas internas e seus efeitos na permissividade elétrica e na temperatura de Curie de policristais de BaTiO3 foram simulados. As análises dos dados e as discussões foram realizadas considerando o modelo fenomenológico de Devonshire, a modificação de Forsbergh deste modelo para incluir efeitos de tensões mecânicas bidimensionais e o modelo de policristal tetragonal de BaTiO3 sugerido por Buessem. Os resultados mostram a existência de tensões residuais bidimensionais que surgem após o resfriamento dos compósitos devido às diferenças nos coeficientes de expansão térmica das fases constituintes. Os métodos de obtenção das amostras afetaram o tamanho final dos grãos e as espessuras das interfaces dos compósitos, sendo que, em geral, eles são menores e mais finos nas fitas cerâmicas homogêneas do que nas correspondentes cerâmicas homogêneas. Os compósitos apresentaram deslocamento da Tc para maiores temperaturas e aumento do grau de difusividade da transição. A presença das tensões mecânicas residuais e as características microestruturais, juntamente com os modelos utilizados, explicam qualitativamente a permissividade elétrica obtida. Concluí-se que a quantidade de Zr4+ modifica o comportamento das camadas durante a sinterização e altera o coeficiente de dilatação. Estas mudanças geram tensões mecânicas residuais que afetam a microestrutura e as propriedades funcionais dos compósitos. Portanto, a produção de compósitos cerâmicos ferroelétricos deve considerar a correlação existente entre microestrutura e tensões residuais para que suas propriedades sejam otimizadas.
The cubic Laves-phase aluminides REAl2 with RE = Sc, Y, La, Yb and Lu were prepared from the elements by arc-melting or using refractory metal ampoules and induction heating. They all crystallize in the cubic crystal system with space group Fd[3 with combining macron]m and adopt the MgCu2 type structure. The title compounds were characterized by powder X-ray diffraction and spectroscopically investigated using Raman and 27Al and in the case of ScAl2 by 45Sc solid-state MAS NMR. In both, the Raman and NMR spectra, the aluminides exhibit only one signal due to the crystal structure. DFT calculations were used to calculate Bader charges illustrating the charge transfer in these compounds along with NMR parameters and densities of states. Finally, the bonding situation was assessed by means of ELF calculations rendering these compounds aluminides with positively charged REδ+ cations embedded in an [Al2]δ− polyanion.
Nanotechnology provides useful solutions in fields of science and technology ranging from energy, environment, biomedicine and optics to electronics, among others. This workshop aimed at strengthening the Brazilian‐German cooperation in nanotechnology and its applications. Our intention has been to bring together scientists and technologists from different communities to communicate and to identify prospective lines of future cooperations. This Workshop Proceedings contains articles based on the ten talks given during the workshop on topics such as carbon nanomaterials, optics, electronics, as well as
nanotechnology for sensors and for magnetic and biomedical applications.
Polyrotaxane bestehen aus linearen Polymeren, auf die zyklische Moleküle aufgefädelt sind. Durch Vernetzung von Polyrotaxanen erhält man elastische Gele, sogenannte Slide-Ring Gels (SRGs), die nach mechanischer Beanspruchung selbst wieder ausheilen können. Polyrotaxane sind zwar seit etwa 25 Jahren bekannt, jedoch war ihre Synthese bislang zu aufwändig für eine industrielle Anwendung. Cyclodextrine eignen sich in besonderer Weise zur Synthese von Polyrotaxanen, da sie in wässriger Lösung auf Polymere wie Polyethylenglykol spontan auffädeln können. Polyrotaxane wurden bislang durch Auffädeln von Cyclodextrinen auf Polymere wie Polyethylenglykol oder Polyimino-undecamethylen erzeugt. Ein besonderes Problem bereitet dabei die Anknüpfung der Stopper-Gruppen, ohne die die Ringe wieder abfädeln würden. Die Synthese von Polyrotaxanen wurde in einer Reihe von Publikationen beschrieben. Bislang gibt es nur ein mehrstufiges Verfahren, das die Synthese im kg-Maßstab erlaubt. Diese Synthese erfordert jedoch den Einsatz des giftigen und reproduktionstoxischen Lösungsmittels Dimethylformamid. Die aufgefädelten Ringe im Polyrotaxan sind zwar an die Polymerkette gebunden, besitzen aber dennoch Translations- und Rotationsfreiheitsgrade. Diese einzigartige Topologie erlaubt die Konstruktion von mobilen, dreidimensionalen Netzwerken, den Slide Ring Gels (SRG)s, durch kovalente Verknüpfung der aufgefädelten Ringe. Bei mechanischer Beanspruchung können sich die Ringe auf der Kette bewegen und so einem Stoß ausweichen. Folglich zeigen SRGs hohe elastische Dehnungen und sehr gute Quellfähigkeiten.[...]
Cu(ln,Ga)Se2 (CIGS) ist ein Material für Dünnschicht-Solarzellen, mit dem Wirkungsgrade erreicht werden können, die nur wenig unter denen kristalliner Silizium-Solarzellen liegen. CIGS-Dünnschichtmodule auf Glassubstraten sind seit einigen Jahren erfolgreich auf dem Markt. Die Verwendung von Folienmaterial als Substrat verspricht jedoch gegenüber Glas einige Vorteile, einerseits auf der Seite der Herstellungskosten, die durch kontinuierlich von Rolle zu Rolle arbeitende Verfahren deutlich gesenkt werden könnten, andererseits durch eine Erweiterung des Spektrums möglicher Anwendungen, bedingt durch Flexibilität und geringeres Gewicht.
Bedingt durch die hohen Temperaturen des CIGS-Herstellungsprozess kommen als Substrat eigentlich nur Metallfolien in Betracht. Das einzige Polymermaterial, das sich (mit starken Einschränkungen) eignet, ist recht teures Polyimid. Metallfolien sind aber entweder ebenfalls recht teuer (Kupfer, Titan) oder aber eine Quelle für Ionen, die in den CIGS-Absorber hineindiffundieren und die Zelleneffizienz beeinträchtigen (Stahl- oder Aluminiumfolie).[...]
Continuous roll-to-roll fabrication is essential for transferring the idea of bio-inspired, fibrillar dry adhesives into large-scale, synthetic, high-performance adhesive tapes. Toward this aim, we investigated process parameters that allow us to control the morphology and the resulting adhesion of mushroom-shaped micropatterned surfaces. Flexible silicone templates enabled the replication process of the polyurethane acrylate pre-polymer involving UV-light-induced cross-linking. For this paper, we particularly tailored the polyurethane acrylate pre-polymer by adding chemical components to tune UV curing kinetics and to reduce oxygen inhibition of radicals. We found that higher intensities of the UV light and faster reaction kinetics improved the quality of the microstructures, i.e., a larger cap diameter of the mushroom tips was achieved. The polymer blend U6E4 exhibited the fastest curing kinetics, which resulted in a micromorphology similar to that of the Ni-shim master structures. Best adhesion results were obtained for adhesive tapes made from U6E4 with 116 kPa pull-off stress, 1.4 N cm−1 peel strength and 71 kPa shear strength. In addition, repeated attachment–detachment tests over 100,000 cycles demonstrated strong robustness and reusability.
For the next generation of handling systems, reversible adhesion enabled by micropatterned dry adhesives exhibits high potential. The versatility of polymeric micropatterns in handling objects made from various materials has been demonstrated by several groups. However, specimens reported in most studies have been restricted to the laboratory scale. Upscaling the size and quantity of micropatterned adhesives is the next step to enable successful technology transfer. Towards this aim, we introduce a continuous roll-to-roll replication process for fabrication of high-performance, mushroom-shaped micropatterned dry adhesives. The micropatterns were made from UV-curable polyurethane acrylates. To ensure the integrity of the complex structure during the fabrication process, flexible templates were used. The compression between the template and the wet prepolymer coating was investigated to optimize replication results without structural failures, and hence, to improve adhesion. As a result, we obtained micropatterned adhesive tapes, 10 cm in width and several meters in length, with adhesion strength about 250 kPa to glass, suitable for a wide range of applications
Two-dimensional photonic structures such as nanostructured pillar gratings are useful for various applications including wave coupling, diffractive optics, and security features. Two-photon lithography facilitates the generation of such nanostructured surfaces with high precision and reproducibility. In this work, we report on nanopillar diffraction gratings fabricated by two-photon lithography with various laser powers close to the polymerization threshold of the photoresist. As a result, defect-free arrays of pillars with diameters down to 184 nm were fabricated. The structure sizes were analyzed by scanning electron microscopy and compared to theoretical predictions obtained from Monte Carlo simulations. The optical reflectivities of the nanopillar gratings were analyzed by optical microscopy and verified by rigorous coupled-wave simulations.
Natural functional surfaces often rely on unique nano- and micropatterns. To mimic such surfaces successfully, patterning techniques are required that enable the fabrication of three-dimensional structures at the nanoscale. It has been reported that two-photon polymerization (TPP) is a suitable method for this. However, polymer structures fabricated by TPP often tend to shrink and to collapse during the fabrication process. In particular, delicate structures suffer from their insufficient mechanical stability against capillary forces which mainly arise in the fabrication process during the evaporation of the developer and rinsing liquids. Here, we report a modified development approach, which enables an additional UV-treatment to post cross-link created structures before they are dried. We tested our approach on nanopillar arrays and microscopic pillar structures mimicking the moth-eye and the gecko adhesives, respectively. Our results indicate a significant improvement of the mechanical stability of the polymer structures, resulting in fewer defects and reduced shrinkage of the structures.
Abstract What happens when the extremely adhesive and versatile chemistry of polydopamine (PDA) is in contact with the extremely slippery surfaces known as slippery liquid-infused porous substrates (SLIPS)? Inspired by the pitcher plant, SLIPS possess excellent repellence against a variety of complex liquids and have been proposed as promising antifouling surfaces because of their successful performance even in marine environments. In the counterpart, inspired by the adhesive proteins enabling the strong adhesion of mussels to multiple substrates, PDA has been extensively studied for its ability to adhere on nearly every type of substrate. The interaction between various SLIPS systems and the highly fouling medium from the oxidative polymerization of dopamine is explored here. A PDA coating is observed on all the SLIPS evaluated, modifying their hydrophobicity in most cases. In-depth study of silicone-based SLIPS shows that hydrophobicity of PDA coated SLIPS partially recovers with time due to percolation of the lubricant through the coating. “Strongly” bound PDA species are attributed to the formation of dopamine-polydimethylsiloxane species on the crosslinked matrix, rendering a coating that withstands repeated washing steps in various solvents including water, hexane, and toluene. The results not only satisfy scientific curiosity but also imply a strategy to modify/bond SLIPS.
