Combined in situ mechanical testing and scale-bridging 3D analysis of nanoporous gold

In this work we present results on in situ small scale testing of nanoporous gold (npg) in scanning electron microscopy (SEM) and transmission electron microscopy (TEM). By combining nano- and micromechanical testing of pillar structures with advanced tomographic imaging, a 3D characterization of the plastic deformation process in different states of deformation is achieved. For small strut sizes 360° electron tomography (ET) is applied enabling high quality reconstructions of the 3D morphology of npg without missing-wedge artefacts. Combining the geometric information with mechanical data from in situ testing in SEM and TEM the yield strength is precisely determined. Furthermore, the experimentally derived 3D data are used as input for large-scale molecular dynamics (MD) simulations in order to understand the role of strain localization and identify predominant defect processes. Please click Additional Files below to see the full abstract

Charakterisierung von Rußpartikeln aus Dieselmotoren und Zinndioxid-Partikeln gemahlen in Rührwerkskugelmühlen

Transmission electron microscopy, electron energy loss spectroscopy and Raman spectroscopy are used to investigate soot particles taken form the combustion chamber and exhaust of low emission diesel engines. The present work represents an enormous progress in soot particle characterization and the understanding of the complex soot formation mechanism. Nascent particles are found in the early pre-combustion phase and are assumed to be nuclei for the further soot formation. First soot particles are observed at the beginning of the main-combustion phase and the evolution of their internal and electronic structure is shown as a function of crank angle / time after the combustion of the diesel fuel begins, as well as a function of injection pressure. A sample taken at the late phase of the main-combustion shows that the chain-like agglomerates, which consist of several primary soot particles, are already formed in the combustion chamber of a diesel engine. Such chain-like agglomerates were previously only found in the exhaust of diesel engines. Investigations of the soot samples taken from the exhaust of diesel engines show the dependence of the morphology and nanostructure on the operating conditions (for instance injection pressure, exhaust gas recirculation) given in a diesel engine. The present work points out that the transmission electron microscopy techniques are powerful methods to investigate soot particles taken from the exhaust and combustion chamber of diesel engines. Coupling the engine parameters with the results from electron microscopy enabled to understand more in detail the mechanisms which lead to the formation of diesel engine soot. Furthermore, the combination of results achieved from exhaust diesel soot and combustion chamber soot might be very useful for the elimination of diesel soot particles and the lowering of diesel soot emissions. One possibility for the fabrication of nanoparticles is wet grinding in stirred media mills. Motivated by the fact that the fracture mechanisms at the nanoscale are not yet fully understood, in the present work the evolution of the microstructure within tin dioxide particles, grinded in a stirred media mill, is investigated. HRTEM images show particles with sizes below 10 nm, while mean crystallite sizes of ≈ 9 nm were measured from XRD. TEM analysis were conducted to gain detailed insight into the microstructural effects which govern the grinding process. Using TEM the formation of stacking faults, shear bands and mechanical twins on nanoscale are revealed. MD simulations are performed, where the uniaxial compression of particles with a diameter of 30 nm was simulated. The simulated particles shared microstructural details with the real samples, most importantly the shear bands which lead to significant plastic deformation. In situ nanoindentation tests describe the mechanical behaviour of tin dioxide particles to be plastic, but in some cases also brittle in nature. Deformation induced events from load-displacement curves are directly coupled to images recorded during the in situ nanoindentation tests. The internal microstructure produced during multiple particle stressing events in the mill and also observed in the simulations is directly linked to the fracture mechanism and the experimentally observed grinding limit. The studies in this work indicate that TEM and MD simulations are suitable methods for the observation of structural changes that occur in a comminution process. Quantitative comparison between intraparticulate structures produced in a real mill and those observed in molecular dynamics simulations was not possible. One reason is that the stressing events in a stirred media mill are very complex and not yet known in sufficient detail in terms of number, intensity and direction. Another reason is the limited computer capacities in MD simulations, so several simplifications in the simulation model must be made. While in the milling experiment the stressing events represent an unknown combination of compression, shear, impact and friction, the loading case in MD simulations and in situ nanoindentation testing is simplified to single compression modes. In order to understand in detail the microstructural changes occurring during a nanomilling experiment, fluid mechanics, motion of the milling beads, and multiple stressing events of all kinds must be taken into account, which represents a complex process and thus a motivation for further investigations.Transmissionselektronenmikroskopie, Elektronenenergieverlustspektroskopie und Ramanspektroskopie wurden zur Untersuchung von Rußpartikeln aus dem Brennraum und Abgas von Niedrigst-Emissions-Dieselmotoren verwendet. Die vorliegende Arbeit stellt einen enormen Fortschritt in der Charakterisierung von Rußpartikeln und dem Verständnis der komplexen Rußbildung dar. Junge Partikel wurden in der frühen Vorverbrennungsphase gefunden und werden als Nukleationszentren für die weitere Rußbildung angesehen. Erste Primärrußpartikel wurden zu Beginn der Hauptverbrennungsphase gefunden und die Entwicklung ihrer inneren und elektronischen Struktur ist sowohl als Funktion des Kurbelwinkels / Zeit nach Brennbeginn von Dieselkraftstoff als auch als Funktion des Einspritzdrucks gezeigt. Eine Probe, die in der späten Phase der Hauptverbrennung entnommen wurde, zeigt, dass kettenförmige Agglomerate, die aus mehreren Primärpartikeln bestehen, bereits im Brennraum eines Dieselmotors gebildet werden. Solche kettenförmigen Agglomerate wurden vorher nur im Abgas von Dieselmotoren gefunden. Die Untersuchungen von Abgasrußproben aus Dieselmotoren zeigen eine Abhängigkeit der Morphologie und Nanostruktur von den Betriebsbedingungen (z.B. Einspritzdruck, Abgasrückführung), die in einem Dieselmotor gegeben sind. Die vorliegende Arbeit verdeutlicht, dass die Techniken der Transmissionselektronenmikroskopie leistungsfähige Methoden sind, um aus dem Abgas und dem Brennraum entnommene Rußpartikel von Dieselmotoren zu untersuchen. Die Verknüpfung von Motorenparametern mit den Ergebnissen aus der Elektronenmikroskopie ermöglichte es die Mechanismen zu verstehen, die zur Rußbildung von Dieselruß führen. Des Weiteren ist die Kombination der Ergebnisse vom Abgasruß und dem Ruß aus dem Brennraum möglicherweise für die Beseitigung der Dieselrußpartikel und für die Minderung der Dieselrußemissionen nützlich. Eine Möglichkeit für die Herstellung von Nanopartikeln stellt die Naßmahlung in Rührwerkskugelmühlen dar. Motiviert durch die Tatsache, dass die Bruchmechanismen auf der Nanoskala noch nicht vollständig verstanden sind, wurde in der vorliegenden Arbeit die Entwicklung der Mikrostruktur von in Rührwerksmühlen gemahlenen Zinndioxidpartikeln untersucht. HRTEM Aufnahmen zeigen Partikel mit Größen von unterhalb 10 nm, während die mittlere Kristallitgröße von ≈ 9 nm mit Hilfe von XRD gemessen wurde. TEM Analysen wurden durchgeführt, um einen genauen Einblick in die mikrostrukturellen Effekte zu bekommen, die den Zerkleinerungsprozess steuern. Mittels TEM wurde die Entstehung von Stapelfehlern, Scherbändern und mechanischen Zwillingen auf der Nanoskala aufgedeckt. MD Simulationen wurden durchgeführt, wobei eine einaxiale Kompression von Partikeln mit einem Durchmesser von 30 nm simuliert wurde. Die simulierten Partikel zeigten einen Anteil mikrostruktureller Details von realen Proben, hauptsächlich Scherbänder, die zu einer nennenswerten plastischen Verformung führen. In situ Nanoindentierungsuntersuchungen beschreiben das mechanische Verhalten von Zinndioxid als plastisch, aber in manchen Fällen auch als spröde. Die durch Verformung herbeigeführten Vorgänge aus Kraft-Eindringkurven sind direkt mit den Abbildungen gekoppelt, die während eines in situ Nanoindentierungsversuchs aufgenommen werden. Die interne Mikrostruktur, die durch mehrfache Belastungsvorgänge von Partikeln in der Mühle erzeugt und auch mit Simulationen ermittelt wurde, ist direkt mit den Bruchmechanismen und der experimentell ermittelten Mahlgrenze verknüpft. Die Untersuchungen in dieser Arbeit zeigen auf, dass TEM und MD Simulationen adäquate Methoden sind, um strukturelle Änderungen, die in einem Zerkleinerungsprozess auftreten, aufzuklären. Quatitative Vergleiche von intrapartikulären Strukturen, die in einer realen Mühle erzeugt werden und in Molekulardynamik Simulationen beobachtet wurden, waren nicht möglich. Ein Grund dafür ist, dass die Belastungsvorgänge in einer Rührwerkskugelmühle sehr komplex sind und nicht ausreichend Details in Bezug auf ihre Anzahl, Intensität und Richtung bekannt sind. Ein anderer Grund sind die begrenzten Rechnerkapazitäten in MD Simulationen, weshalb einige Vereinfachungen im Simulationsmodell vorgenommen werden mussten. Während im Mahlexperiment die Belastungsvorgänge eine unbekannte Kombination aus Kompression, Scherung, Stoß und Reibung darstellen, ist die Belastung im Falle von MD Simulationen und in situ Nanoindentierungsversuchen auf den einaxialen Kompressionsmodus beschränkt. Um mikrostrukturelle Änderungen im Detail zu verstehen, die sich während eines Nanomahlexperiments abspielen, müssen Aspekte aus der Strömungsmechanik, die Bewegung der Mahlkörper, und mehrfache Belastungsvorgänge aller Art in Betracht gezogen werden, die einen komplexen Prozess und daher eine Motivation für weitere Untersuchungen darstellen

Low-temperature oxidation-induced crack healing in Ti₂Al_0.5Sn_0.5C–Al₂O₃ composites

The oxidation-induced crack healing of an Al2O3 composite loaded with various volume fractions of Ti2Al0.5Sn0.5C repair filler particles was investigated by annealing in air at a relatively low temperature of 700°C. After annealing a composite with 20 vol.% repair fillers (with a particle size of ~5.6 µm) for 48 hours, artificial indentation cracks prepared on the surface, as well as pores near the surface, were completely healed by filling with condensed oxidation products. Additionally, minor fraction of metallic Sn was detected. A complementary study by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy revealed that nano-sized oxidation products (SnO2, TiO2, and α-Al2O3 phase) were formed as major crack-filling species. After healing, the strength recovery of the Al2O3 composites was significantly improved in the composites loaded with more than 10 vol.% repair fillers and achieved 107% at 700 for 48 hours.(OLD) MSE-

ZnO superstructures via oriented aggregation initiated in a block copolymer melt

A fast and simple one pot synthesis of ZnO nano- and microparticles initiated and driven by an amino block copolymer O,O′-bis(2-aminopropyl)polypropylene glycol-block-polyethylene glycol-block-polypropylene glycol (Jeffamine®) is reported. The specific building mechanism of ZnO mesocrystals is investigated in detail using electron microscopy and diffraction methods. Mesocrystals with a complex superstructure are formed as a result of a consecutive and oriented multiple stage aggregation process: first a 0D → 1D aggregation process is observed, then a 1D → 3D aggregation process occurs in which secondary particles form cones and multiple cone symmetries. Dots, rods, cones, and multiple cones have been isolated within a time resolved study which clearly supports the growth model. To control the morphology of the product particles, the influence of relevant synthesis parameters including stirring and sonication of the intermediate were investigated. Extensive surface characterization of the resulting mesocrystals is presented using infrared and photoluminescence spectroscopies as well as thermogravimetric analysis. Even after multiple washing steps, the particles exhibit a Jeffamine® coated surface that allows for easy dispersion in both polar and nonpolar solvents. The obtained mesocrystals efficiently scatter in the whole range of visible light

Inducing a nanotwinned grain structure within the TiO2 nanotubes provides enhanced electron transport and DSSC efficiencies > 10 %

Titania is one of the key materials used in 1, 2 and 3D nanostructures as electron transport media in energy conversion devices. In the present work we show that the electronic properties of TiO2 nanotubes can be drastically improved by inducing a nanotwinned grain structure in the nanotube wall. This structure can be exclusively induced for “single-walled” nanotubes with a high temperature treatment in pure oxygen atmospheres. Nanotubes with a twinned grain structure within the tube wall show a strongly enhanced conductivity and photogenerated charge transport compared to classic nanotubes. We exemplify this remarkable improvement in the electronic properties by using nanotwinned TiO2 nanotubes in dye-sensitized solar cells where a significant increase in efficiency of up to 10.2% is achieved

Mechanical properties and failure of Ag nanowire transparent electrodes studied by means of in situ tensile testing

Organic (opto)electronics have undergone a rapid development in recent years. Applications are for instance organic solar cells (OSCs), flexible displays or “smart clothing”, which means clothing with integrated electronics. For such applications it is decisive that the active layers as well as the electrodes withstand mechanical loading. Indium tin oxide (ITO), which is still a common material as electrode, behaves brittle under deformation and is relatively expensive. Metallic nanowire networks, especially silver nanowires (AgNWs), are highly promising alternatives. They fulfill the requirement of a low sheet resistance combined with high transmittance. On a macroscopic scale bending tests as well as tensile tests revealed the excellent performance of Ag NW films, since the increase of resistance is small compared to ITO films. In order to understand failure mechanisms and prospectively optimize the deformation behavior of AgNW electrodes in situ mechanical testing in the transmission electron microscope (TEM) is conducted. In situ tensile tests of single Ag NWs were performed with a Hysitron PI95 TEM PicoindenterTM. Moreover, tensile tests of AgNW networks on different flexible substrates such as PEDOT:PSS and PET, which are both used for flexible OSCs, were performed with a Gatan TEM straining holder 654. For these straining tests a dogbone-shaped sample of the AgNW polymer composite is cut in a Focused Ion Beam (Fig. 1(a)). The Ag NW network is prepared by doctor blading. On a nanometer scale tensile tests of single 5-fold twinned Ag NWs show a ductile behavior and a size effect of the strength. The pristine NWs are almost defect free except for the twin boundaries. In situ testing reveals nucleation and propagation of partial dislocations during straining. Tensile tests of Ag NW networks in the TEM provide direct insights in the behavior of the network as well as the response of the whole AgNW polymer composite. A map of the local strain was determined via digital image correlation and depicts a homogeneous strain distribution. The fracture behavior of the Ag NWs is ductile, whereas the PEDOT:PSS film fracture appears to be rather brittle (Fig 1. (b,c)). Since the interaction of mechanical and electrical properties is crucial for applications in (opto)electronics, we are currently working on combining in situ mechanical and electrical measurements in the TEM. Please click Additional Files below to see the full abstract

Interaction of light with hematite hierarchical structures: Experiments and simulations

Mesocrystalline particles have been recognized as a class of multifunctional materials with potential applications in different fields. However, the internal organization of nanocomposite mesocrystals and its influence on the final properties have not yet been investigated. In this paper, a novel strategy based on electrodynamic simulations is developed to shed light on how the internal structure of mesocrystals influences their optical properties. In a first instance, a unified design protocol is reported for the fabrication of hematite/PVP particles with different morphologies such as pseudo-cubes, rods-like and apple-like structures and controlled particle size distributions. The optical properties of hematite/PVP mesocrystals are effectively simulated by taking their aggregate and nanocomposite structure into consideration. The superposition T-Matrix approach accounts for the aggregate nature of mesocrystalline particles and validate the effective medium approximation used in the framework of the Mie theory and electromagnetic simulation such as Finite Element Method. The approach described in our paper provides the framework to understand and predict the optical properties of mesocrystals and more general, of hierarchical nanostructured particles.Fil: Distaso, Monica. Universitat Erlangen-Nuremberg; AlemaniaFil: Zhuromskyy, Oleksander. Universitat Erlangen-Nuremberg; AlemaniaFil: Seemann, Benjamin. Universitat Erlangen-Nuremberg; AlemaniaFil: Pflug, Lukas. Universitat Erlangen-Nuremberg; AlemaniaFil: Mačković, Mirza. Universitat Erlangen-Nuremberg; AlemaniaFil: Encina, Ezequiel Roberto. Universitat Erlangen-Nuremberg; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Taylor, Robin Klupp. Universitat Erlangen-Nuremberg; AlemaniaFil: Müller, Rolf. Lanxess Deutschland GmbH Inorganic Pigments; AlemaniaFil: Leugering, Günter. Universitat Erlangen-Nuremberg; AlemaniaFil: Spiecker, Erdmann. Universitat Erlangen-Nuremberg; AlemaniaFil: Peschel, Ulf. Universitat Erlangen-Nuremberg; AlemaniaFil: Peukert, Wolfgang. Universitat Erlangen-Nuremberg; Alemani

Effect of nano-sized bioactive glass particles on the angiogenic properties of collagen based composites

Angiogenesis is essential for tissue regeneration and repair. A growing body of evidence shows that the use of bioactive glasses (BG) in biomaterial-based tissue engineering (TE) strategies may improve angiogenesis and induce increased vascularization in TE constructs. This work investigated the effect of adding nano-sized BG particles (n-BG) on the angiogenic properties of bovine type I collagen/n-BG composites. Nano-sized (20-30 nm) BG particles of nominally 45S5 Bioglass composition were used to prepare composite films, which were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The in vivo angiogenic response was evaluated using the quail chorioallantoic membrane (CAM) as an model of angiogenesis. At 24 h post-implantation, 10 wt% n-BG containing collagen films stimulated angiogenesis by increasing by 41 % the number of blood vessels branch points. In contrast, composite films containing 20 wt% n-BG were found to inhibit angiogenesis. This experimental study provides the first evidence that addition of a limited concentration of n-BG (10 wt%) to collagen films induces an early angiogenic response making selected collagen/n-BG composites attractive matrices for tissue engineering and regenerative medicine.Fil: Vargas, Gabriela Elizabet. Universidad Católica de Salta. Facultad de Ingeniería e Informática; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; ArgentinaFil: Haro Durand, Luis Alberto. Universidad Católica de Salta. Facultad de Ingeniería e Informática; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; ArgentinaFil: Cadena, María Vanesa. Universidad Católica de Salta. Facultad de Ingeniería e Informática; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; ArgentinaFil: Romero, Marcela. Universidad Nacional de Salta. Consejo de Investigación; ArgentinaFil: Vera Mesones, Rosa. Universidad Nacional de Salta. Consejo de Investigación; ArgentinaFil: Mačković, Mirza. Universitat Erlangen Nuremberg; AlemaniaFil: Spallek, Stefanie. Universitat Erlangen Nuremberg; AlemaniaFil: Spiecker, Erdmann. Universitat Erlangen Nuremberg; AlemaniaFil: Boccaccini, Aldo R.. Universitat Erlangen Nuremberg; AlemaniaFil: Gorustovich Alonso, Alejandro Adrian. Universidad Católica de Salta. Facultad de Ingeniería e Informática; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; Argentin

Microscopic Deformation Modes and Impact of Network Anisotropy on the Mechanical and Electrical Performance of Five-fold Twinned Silver Nanowire Electrodes

Silver nanowire (AgNW) networks show excellent optical, electrical, and mechanical properties, which make them ideal candidates for transparent electrodes in flexible and stretchable devices. Various coating strategies and testing setups have been developed to further improve their stretchability and to evaluate their performance. Still, a comprehensive microscopic understanding of the relationship between mechanical and electrical failure is missing. In this work, the fundamental deformation modes of five-fold twinned AgNWs in anisotropic networks are studied by large-scale SEM straining tests that are directly correlated with corresponding changes in the resistance. A pronounced effect of the network anisotropy on the electrical performance is observed, which manifests itself in a one order of magnitude lower increase in resistance for networks strained perpendicular to the preferred wire orientation. Using a scale-bridging microscopy approach spanning from NW networks to single NWs to atomic-scale defects, we were able to identify three fundamental deformation modes of NWs, which together can explain this behavior: (i) correlated tensile fracture of NWs, (ii) kink formation due to compression of NWs in transverse direction, and (iii) NW bending caused by the interaction of NWs in the strained network. A key observation is the extreme deformability of AgNWs in compression. Considering HRTEM and MD simulations, this behavior can be attributed to specific defect processes in the five-fold twinned NW structure leading to the formation of NW kinks with grain boundaries combined with V-shaped surface reconstructions, both counteracting NW fracture. The detailed insights from this microscopic study can further improve fabrication and design strategies for transparent NW network electrodes