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In dieser Arbeit wurde eine Synthese zur Herstellung von Ti-Nb-Oxid-Nanopartikeln für unterschiedliche Verhältnisse zwischen Ti und Nb entwickelt. Es entstehen mikrometer-große kugelförmige Agglomerate mit einer hierarchisch aufgebauten Kern-Schale-Struktur, deren Kern Nb-arm im Vergleich zur Schale ist. Die Partikel wurden später für (opto-)elektronische Anwendungen getestet. Als erstes wurde der Einsatz als transparent leitfähiges Oxid (TCO) untersucht. Die deagglomerierten Partikel wurden zu Tinten verarbeitet und daraus Schichten auf Glas hergestellt. Der Zusatz der Vorstufen, die auch für die Synthese verwendeten wurden, wirkte sich im Vergleich zu Acrylaten als Binder positiv auf den Widerstand aus. Im Gegensatz zu anderen TCOs zeigt Nb:TiO2 eine photokatalytisch Aktivität, wel- che bei der Nachbehandlung berücksichtigt werden muss. Weiterhin wurden die elektrischen Eigenschaften von Pellets untersucht, die aus dem Pulver gepresst und nachbehandelt wurden. Der Widerstand der Pellets hängt maßgeblich von den Temperaturen und Gasen während der Nachbehandlung sowie den erhaltenen Strukturen und Phasen ab. Der geringste Widerstand wurde bei Nb-Gehalten um 20 und 66 at% und Temperaturen über 750◦C erzielt. Als weitere Anwendung wurden die Mikrokugeln für Photoanoden von farbstoffsensibilisierten Solarzellen mit zwei unterschiedlichen Farbstoffen verwendet. Der maximale Wirkungsgrad lag bei 4,1 % und 8,0 % für Eosin Y und N719.
2D and 3D photonic crystals active in the visible wave range are highly interesting for applications, such as waveguiding elements, sensors, or counterfeiting features. However, the tuneable production of such crystals with the current processes is challenging. Two-photon lithography (TPL), which is mainly used to manufacture microstructures, offers this versatility but currently suffers from insufficient structure resolution and mechanical stability for sub-micrometre structures. In the course of this work several novel approaches including an improved development and standing wave enhanced two-photon lithography are presented. These approaches improve the structure resolution and quality, and thus, allow stable features sizes down to 120 nm in horizontal and 45 nm in vertical direction. The new capabilities were used to fabricate distinct photonic crystals inspired by the 2D-pillar grating found on the moth eye and the 3D ‘Christmas tree’-like structures covering the wings of the Morpho-butterflies. Resulting structures were analysed in detail regarding their sizes and optical properties, showing highly effective diffraction, promising anti-reflection properties, and outstanding angle independent iridescence. The experimental work is supported by correlated simulations investigating the optical properties, structures sizes, and the influences of different experimental parameter settings relevant for the fabrication.
Numerical analysis of interfacial stress distributions and adhesion behaviour of fibrillar surfaces
(2016)
The climbing abilities of geckos have inspired many researchers to develop reusable, reversible adhesives. The fabrication of such synthetic adhesives has been well investigated. However, a full theoretical description is still lacking. The objective of the thesis is to improve the theoretical understanding of the mechanics of fibrillar adhesion and also to uncover the various factors influencing the adhesion of the compliant fibrils adhered to a rigid surface using finite element analysis. The effect of fibril geometry on the adhesion was examined. Straight punch and mushroom fibrils were examined numerically and it was found that mushroom fibrils show better adhesion compared to straight punch. Mushroom fibrils with higher stalk to cap ratio and smaller flap height show better adhesion when the corner singularity is considered as driving force for delamination. For these fibrils the detachment will begin from centre instead of corner. Some other shapes were also studied by introducing a fillet radius at the corner joining stalk and the cap. We propose a novel composite fibril with a stiff stalk and a softer tip to replicate the benefits shown by mushroom fibrils but with reduced manufacturing complications. The influence of Young’s modulus and tip height were studied along with different interfacial shapes joining the stiff stalk and soft tip. It is found that higher Young’s modulus ratio and smaller soft tip height result in higher adhesion strength. The results support the rational optimization of synthetic micropatterned adhesives.
With steps towards Industry 4.0, it becomes imperative to the development of next-generation industrial assembly lines, to be able to modulate adhesion dynamically for handling complex and diverse substrates. The inspiration for the design and functionality of such adhesive pads comes from gecko’s remarkable ability to traverse rough and smooth topographies with great ease and agility. The emphasis in this thesis was to equip artificial micropatterned adhesives with such functionalities of tunability and devise an on-demand release mechanism. The project evaluates the potential of electric fields in this direction. The first part of this work focusses on integrating electric fields with polymeric micropatterns and studying the synergistic effect of Van der Waals and electrostatic forces. An in-house electroadhesion set up was built to measure the pull-off forces with and without electric fields. As a function of the applied voltage, adhesion forces can be tuned. The second part of the work demonstrates a novel route that exploits the in-plane actuation of the dielectric elastomeric actuators integrated with microstructure to induce peeling in them. Voltage-dependent actuation has been harnessed to generate the requisite peel force to detach the micropatterns. Overall, the findings of this thesis combine disciplines of electroadhesion, electroactuation, and reversible dry adhesives to gain dynamic control over adhesion.
In this work, a new process of generating nanobubbles* in transparent polymers was developed. The azo-chemical initiators AIBN (Azobisisobutyronitrile) and ABVN (2,2’-Azobis(2,4-dimethylvaleronitrile)) were used as chemical blowing agents (CBA) that deliver nitrogen gas to form the bubbles. Specifically, the photo initiator Irgacure 819, which could be activated by a longer wavelength of UV (405 nm) than that of AIBN (345 nm), was added to the solution in order to minimize the decomposition of the azo chemicals during the first stage of sample preparation by UV-pre-curing. As a result, the CBAs remained unreacted, thus they could decompose and be used merely as a blowing agent. In addition, the post-foaming processes were conducted under the different foaming conditions to investigate critical factors affecting the nucleation and growth of bubbles. The results were discussed based on thermodynamics, mainly by conventional nucleation theory (CNT). Finally, stepwise optimization of those factors led to the generation of nanobubbles. This technique could be applicable to UV curable polymer systems in general. This novel approach was used to make prototype optical devices, such as security mark coating and light out-coupling in optical waveguide. This study also gives important information about the nucleation and growth of bubbles in meta-stable state of polymers, which could help to verify the background theories.
Adhesion to substrates with surface roughness is a research field with many unsolved questions. A more thorough understanding of the underlying principles is important to develop new technologies with potential implications for instance in robotics, industrial automatization and wearable interfaces. Nature is a vast source of inspiration as animals have mastered climbing on various surfaces at high speed with several attachment and detachment events in a short time. In this work, new designs for dry adhesives inspired by natural blueprints are presented. Different strategies were explored to understand and tune adhesion on a range of substrates from smooth glass to polymers with skin-like roughness. Both the material properties and the geometry of the dry adhesives were utilized to improve adhesion strength. Three concepts are presented in this work: (i) composite structures with tunable interface, (ii) soft pressure sensitive adhesive layers, and (iii) funnel-shaped microstructures. This thesis aims for better understanding of the adhesion behavior as a function of several important factors including hold time, substrate material and roughness. The new concepts for bioinspired structures investigated in the present thesis will contribute to the development of performant, reversible adhesives for a variety of applications where surface roughness is involved.
Switchable microtopographies based on the two-way shape memory effect in nickel-titanium alloys
(2016)
Nickel-titanium (NiTi) shape memory alloys are functional materials that are capable of undergoing a reversible temperature-induced shape change. Specifically in martensitic NiTi alloys, a reversible two-way shape memory effect can be induced using indentation techniques enabling a temperature-induced change in topography. Combining switchable topographies with nano- or microstructures could expand the properties of functional surfaces, and in addition make the surfaces responsive to their environment. For example, it would be possible to change the adhesive properties of surfaces with switchable dry adhesive microstructures or to control celladhesion on implant materials with specific nano- and micro-switchable structures. In this study, the indentation induced two-way shape memory effect was investigated in different NiTi alloys. In particular, the effects of alloy microstructure, deformation parameters (training) and thermal treatments on switchability were explored. In an austenitic NiTi alloy a specific thermal treatment led to the formation of coherent precipitates, which were shown to be crucial for the two-way shape memory behavior; exceeding the phase transformation temperature considerably decreased the switchability of the topography. At higher temperatures the stabilized martensite, which is required for an oriented phase transformation and consequently for the two-way shape memory behavior, transforms to austenite. An embossing and an electrochemical forming process were developed to prepare switchable topographies on larger areas. Both methods led to surface arrays on NiTi with two-way shape memory topographies. Finally, two approaches were presented, which use the switchable topographies to enable switching of a formerly passive surface function. In combination with bioinspired dry adhesive structures, the switchable NiTi topography led to a reversible, temperature-induced change of the adhesive properties of the surface. Secondly, the two-way shape memory effect was transferred to an alloy system with a phase transformation temperature near body temperature and a small width of hysteresis. By this, a switchable topography was induced, which is controllable within a physiological temperature range. The only issue impeding the use of this switchable surface for experiments on cell-surface interactions is an increased leakage of harmful copper ions. Therefore, surface passivation through oxidation is presented as a method to reduce the ion leakage.
Having emerged from basic science, nanoparticles are now applied in many fields of science and technology. Agglomeration is ubiquitous and occurs, often inadvertently, in the formulation of particle-polymer mixtures, during the chemical functionalisation of nanoparticles and even during storage. This dissertation outlines recent studies on the agglomeration behaviour of different gold nanoparticle types. A modular fluidic system was specifically designed to perform the experiments. It enables the preparation of a steady flow of stable or unstable nanoparticle samples. This allows to observe different steps of a time-dependant process in steady state over practically arbitrary times in flow. These capabilities are used in this research to investigate the agglomeration of destabilised aqueous gold nanoparticles, reassess the solvent induced self-assembly of unpolar gold nanoparticles and examine the shear-assisted orientation of gold nanowires. The performance of the system is demonstrated and new insights on the influence of solvent conditions, temperature and the flow field are presented. The relevant process parameters as well as the microscopic interactions responsible for the structure formation mechanisms are discussed.
The gecko is of high interest for scientists due to its ability to attach and to move on different surfaces with various roughnesses. To date, research groups worldwide aim to study adhesion mechanisms of gecko-like structures and to mimic gecko adhesion. However, most investigations have been performed in controlled environments and under near to ideal conditions, which present a significant constraint for transferring the results to applications. Therefore, two important parameters have been the subject of investigations in the present work, the surface roughness and elevated temperatures. For the first time, the impact of roughness on the adhesion of gecko-like, micropatterned structures was systematically studied. Two adhesive regimes, which are dependent on the pillar geometry and the roughness parameters, were discovered: an adhesive and a non –adhesive regime. The influence of the temperature on adhesion was studied on micropatterned samples fabricated out of three materials, which are interesting candidates for industrial applications. Promising correlations were determined between the temperature dependent mechanical properties and the adhesion values: the glass transition temperature was identified as the temperature of maximum adhesion. These results can support the improvement of bioinspired adhesives for industrial applications.
Colloidal crystals can exhibit novel properties arising from the combination of particle properties and collective phenomena of particle packings. Particular colloidal crystals composed of nanoparticles are interesting, because of the unique properties of nanoscale objects, and because of the formation of three-dimensional structures on scales that can be manufactured using established methods only with great technical effort. The aim of this work was to develop appropriate ways to produce the crystals. Two approaches were chosen. In the first approach, colloid particles were deposited on surfaces in a process similar to dip coating. Large-area crystalline particle films with low defect density were obtained by an optimized deposition geometry. In the second approach attractive interactions between particles were used. Reducing the thermal energy induced agglomeration of the particles. This approach allowed production of a variety of particle structures. Besides the expected result, formation of hexagonal particle packings, unexpected results were obtained. In the first approach a superposition of two crystallization mechanisms ensured a robust formation of hexagonal particle packings. In the second approach crystallization among the particles was suppressed in a pure thermally induced agglomeration.
