03 Fakultät Chemie
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Item Open Access Designing covalent organic framework‐based light‐driven microswimmers toward therapeutic applications(2023) Sridhar, Varun; Yildiz, Erdost; Rodríguez‐Camargo, Andrés; Lyu, Xianglong; Yao, Liang; Wrede, Paul; Aghakhani, Amirreza; Akolpoglu, Birgul M.; Podjaski, Filip; Lotsch, Bettina V.; Sitti, MetinWhile micromachines with tailored functionalities enable therapeutic applications in biological environments, their controlled motion and targeted drug delivery in biological media require sophisticated designs for practical applications. Covalent organic frameworks (COFs), a new generation of crystalline and nanoporous polymers, offer new perspectives for light‐driven microswimmers in heterogeneous biological environments including intraocular fluids, thus setting the stage for biomedical applications such as retinal drug delivery. Two different types of COFs, uniformly spherical TABP‐PDA‐COF sub‐micrometer particles and texturally nanoporous, micrometer‐sized TpAzo‐COF particles are described and compared as light‐driven microrobots. They can be used as highly efficient visible‐light‐driven drug carriers in aqueous ionic and cellular media. Their absorption ranging down to red light enables phototaxis even in deeper and viscous biological media, while the organic nature of COFs ensures their biocompatibility. Their inherently porous structures with ≈2.6 and ≈3.4 nm pores, and large surface areas allow for targeted and efficient drug loading even for insoluble drugs, which can be released on demand. Additionally, indocyanine green (ICG) dye loading in the pores enables photoacoustic imaging, optical coherence tomography, and hyperthermia in operando conditions. This real‐time visualization of the drug‐loaded COF microswimmers enables unique insights into the action of photoactive porous drug carriers for therapeutic applications.Item Open Access Experimental and computational phase studies of the ZrO2-based systems for thermal barrier coatings(2006) Wang,Chong; Aldinger, Fritz (Prof.)The ZrO2-based materials are practically important as the thermal barrier coatings (TBC) for high temperature gas turbines, due to their low thermal conductivity, high temperature thermal stability and excellent interfacial compatibility. Studies of the phase equilibira, phase transformation, and thermodynamics of the ZrO2-based systems can provide the necessary basic knowledge to develop the next generation TBC materials. In the thesis, the systems ZrO2 - HfO2, ZrO2 - LaO1.5, ZrO2 - NdO1.5, ZrO2 - SmO1.5, ZrO2 - GdO1.5, ZrO2 - DyO1.5, ZrO2 - YbO1.5 and ZrO2 - GdO1.5 - YO1.5 were experimentally studied. The samples were prepared by the chemical co-precipitation method, with aqueous solutions Zr(CH3COO)4, HfO(NO3)2, and RE(NO3)3×xH2O (RE=La, Nd, Sm, Gd, Dy, Yb) as starting materials. Various experimental techniques, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), differential thermal analysis (DTA), and high temperature calorimetry were employed to study the phase transformation, phase equilibria between 1400 and 1700°C, heat content and heat capacity of the materials. A lot of contradictions in the literature were resolved and the phase diagrams were reconstructed.Item Open Access Mechanische Spektroskopie an dünnen Kupferschichten(2001) Hagen, Joachim von der; Arzt, Eduard (Prof. Dr. phil.)In dieser Arbeit wurden erstmalig dünne Kupferschichten, die für die Mikroelektronik von zunehmendem technologischen Interesse sind, systematisch mit Hilfe der mechanischen Spektroskopie untersucht. Dabei handelt es sich um eine empfindliche und zerstörungsfreie Messmethode, mit der man Informationen über Defektstrukturen in der Schicht und in der Substrat/Schicht-Grenzfläche erhalten kann. Darüber hinaus wurden die spektroskopischen Ergebnisse ebenfalls erstmalig vor dem Hintergrund der thermomechanischen Eigenschaften dünner Schichten diskutiert. Die Voraussetzung hierfür wurde durch eine apparative Neuentwicklung geschaffen. Bei den untersuchten Systemen handelte es sich um Kupferschichten auf den Trägermaterialien Silizium und Saphir. Die Messungen beruhen auf der Dämpfung von Eigenschwingungen zwischen 20 bis 530°C. Daneben wird die Eigenfrequenz gemessen, aus der man prinzipiell Rückschlüsse auf den E-Modul von Schicht und Substrat, bzw. auf die Haftung ziehen kann. Es wurden vor allem passivierte und unpassivierte Kupferschichten zwischen 1 und 4 µm auf Siliziumsubstraten untersucht. Kupferschichten auf Silizium-Substraten zeigen ein breites, bei Temperaturzyklen stabiles, Dämpfungsmaximum zwischen 280 und 380°C. Mit zunehmender Schichtdicke wächst dessen Intensität, während sich seine Position zu höheren Temperaturen verschiebt. Auf Grund seiner Aktivierungsenthalpie kann dieses Maximum auf Versetzungsbewegungen zurückgeführt werden. Man nimmt an, dass die Versetzungen thermisch aktivierte, lokale Bewegungen um ihre Gleichgewichtslage ausführen, während sie an ihren Enden fest verankert sind. Als Verankerungspunkte sind vor allem die Grenz-, bzw. die Oberfläche, sowie weitere Versetzungen anzusehen. Die Relaxationsparameter der Dämpfungsmaxima zeigen, dass Einengungseffekte die Mobilität der beweglichen Versetzungssegmente maßgeblich bestimmen, wie es im Zusammenhang mit den hohen inneren Spannungen in dünnen Schichten diskutiert wird.Item Open Access The initial oxidation of Al-Mg alloys(2009) Panda, Emila; Mittemeijer Eric J. (Prof. Dr.)The oxide film present on an alloy surface influences many of its physical and chemical properties, such as corrosion resistance, adhesion, electrical and thermal conductivity, friction and wear resistance. The technological demand to control and optimize these properties by tailoring both the alloy-substrate and oxide-film microstructure has led to a large interest for the thermal oxidation behavior of metallic alloys in the last decades. However, up to date, investigations on the oxidation of metallic alloys have been performed mainly at relatively high temperatures (i.e. T > 800 K) and high pressures (i.e. 0.1 < p < 105 Pa). At these high temperatures, relatively thick (i.e. in the μm-range) oxide scales, composed of multiple, crystalline oxide phases, develop on the alloy surface by sequential, preferential oxidation of the alloy constituents. The oxidation of metallic alloys at low temperatures (i.e. T < 600 K), on the other hand, has been investigated only very scarcely up to date. At these low temperatures, thermally activated diffusion of reactants through the developing oxide-film is negligibly small and, consequently, ultra-thin (< 3 nm) oxide films of near-limiting thicknesses are formed, which are generally constituted of a metastable (often amorphous), multi-metal oxide phase. However, the detailed microstructures (i.e. thickness, morphology, crystallographic structure, chemical composition and constitution) of these initial oxide films as a function of the alloy microstructure (e.g. alloying content, surface orientation), the surface pretreatment (e.g. with or without a native oxide) and the growth conditions (e.g. temperature, time and partial oxygen pressure) are often unknown. This PhD thesis addresses the initial stages of dry, thermal oxidation of bare (i.e. without a native oxide) Al-based Al-Mg alloy surfaces as a function of the oxidation conditions (here: oxidation temperature, time and partial oxygen pressure) and for different pre-treatments of the bare alloy surface prior to oxidation. To this end, first, a thermodynamic model, which accounts for the crucial role of surface and interface energetics in such ultra-thin oxide-film systems, was developed to predict the initial, amorphous oxide overgrowth (i.e. am-Al2O3, am-MgO and/or am-MgAl2O4) developing on a bare AlMg alloy substrate as a function of the growth temperature, the Mg alloying content at the alloy/oxide interface and the oxide-film thickness (≤ 5 nm). To this end, experimental or empirically-estimated values for the surface energies of the competing amorphous oxide phases of am-Al2O3, am-MgO or am-MgAl2O4 (further denoted as , and ) as a function of the growth temperature were employed. Required values for the interface energy between the alloy substrate and the competing amorphous oxide phases as a function of the temperature and the Mg alloying content at the alloy/oxide interface were estimated from corresponding expressions, as derived on the basis of the macroscopic atom approach. Further, comprehensive experimental investigation of the interrelationships between the oxide growth kinetics, the microstructural evolutions in the oxide overgrowth and the alloy subsurface and the oxidation conditions was conducted. To this end, polycrystalline AlMg alloy specimens with nominal Mg alloying contents of 0.8 and 1.1 at. % were thermally oxidized in a dedicated UHV system (base pressure < 3×10-8 Pa) for specimen processing (i.e. cleaning, annealing and oxidation) and in-situ analysis. After introduction of the polished alloy sample surface in the UHV system, first the native oxide on the alloy surface was removed by sputter cleaning with a focussed 1 keV Ar+ ion beam rastering the entire alloy surface (of 7×7 mm2). The thus obtained sputter-cleaned (as verified by in-situ AR-XPS) alloy surfaces will be further designated as SC-substrate. These SC-substrates were subsequently in-situ exposed to pure O2 gas in the partial oxygen pressure range of pO2 = 10-4 - 10-2 Pa for durations varying from 15 s up to 6 hrs and for various temperatures in the range of T = 300 – 610 K. As an additional surface pretreatment, some SC-substrates were in-vacuo annealed for 1200 s at T = 460 K prior to the oxidation. The thus obtained sputter-cleaned, annealed alloy surfaces will be further designated as SC/Ann-substrate. Real-time in-situ spectroscopic ellipsometry (RISE) was applied to establish the oxide-film growth kinetics. The thicknesses, compositions and chemical constitutions of the grown films were determined by in-situ angle-resolved X-ray Photoelectron Spectroscopy (AR-XPS). Furthermore, the microstructures of some of the grown oxide films were analysed on an atomic scale by cross-sectional high resolution-Transmission Electron Microscopy (HR-TEM).Item Open Access Einfluss der Herstellungsparameter auf die mechanischen Eigenschaften von Si-(B-)C-N-Precursorkeramiken(2002) Bauer, Arndt; Aldinger, Fritz (Prof. Dr.)Ziel dieser Arbeit ist es, den Herstellungsprozess für Precursorkeramiken zu optimieren und den Einfluss einzelner Herstellungsparameter auf die Eigenschaften und insbesondere auf die Hochtemperaturkriechverformung von Si-(B)-C-N-Precursorkeramiken zu charakterisieren. Am Beispiel eines kommerziell erhältlichen Polysilazans Ceraset werden als wichtigste Parameter die Vernetzungstemperatur der Polymere, die Polymerpartikelgröße nach dem Mahlen und Sieben sowie die Press- und Pyrolysebedingungen betrachtet. Dabei zeigt sich, dass die geringste Porosität einer Keramik, die über das Pressen von Polymerpulver erhalten werden kann, bei etwa 11 % liegt. Es stellt sich heraus, dass vor allem die Porosität des gepressten Grünlings für die Enddichte der Keramik ausschlaggebend ist und im optimalen Fall etwa 7 % beträgt. Von spezieller Bedeutung ist dabei die offene Porosität. Die nach außen offene Porenkanäle werden benötigt, um während der Pyrolyse ein Entweichen der Gase aus dem Grünling zu ermöglichen. Die Porosität des Grünlings hängt von der Viskosität des Polymers während des Pressvorgangs ab und kann deshalb über die Parameter Vernetzungsgrad und Warmpresstemperatur gesteuert werden. Zusätzlich zur Dichte und Hochtemperaturstabilität werden weitere Eigenschaften wie die Hochtemperaturverformung, die Bruchzähigkeit, der Elastizitätsmodul und die Biegefestigkeit von Precursorkeramiken in Abhängigkeit von den Herstellungsbedingungen ermittelt. Eine simultane Maximierung aller untersuchten Eigenschaften ist nicht möglich, so dass für bestimmte Anforderungsprofile Kompromisse gesucht werden müssen. Als Parameter mit der größten Wirkung auf die Eigenschaften hat sich die Partikelgröße erwiesen. Große Partikel haben einen positiven Einfluss auf die Bruchzähigkeit und die Hochtemperaturstabilität, wohingegen kleine Partikel sich vorteilhaft auf die Hochtemperaturkriechverformung und die Biegefestigkeit auswirken. Die Hochtemperaturverformung wurde durch Experimente bei konstanten Spannungen und Temperaturen zwischen 1350 und 1500 °C untersucht. Es wurde eine detaillierte Charakterisierung der Zeit-, Spannungs- und Temperaturabhängigkeit sowohl unter Druck- (bis 300 MPa) als auch unter Biegebeanspruchung (bis 50 MPa) durchgeführt. Dabei wird bei beiden Beanspruchungsarten selbst bei einer Versuchsdauer von bis zu zwei Wochen kein stationäres Kriechen beobachtet. Die Dehnraten sinken vielmehr auf Werte unterhalb der Nachweisgrenze ab. Es wird versucht, das Freie-Volumen-Modell, das ursprünglich zur Beschreibung der Relaxation in metallischen Gläsern entwickelt und inzwischen vor kurzem erfolgreich auf Precursorkeramiken übertragen wurde, auch bezüglich der Gültigkeit für die hier untersuchten Materialien zu überprüfen.Item Open Access Mikrostruktur und Kriechverhalten von Magnesium-Druckgusslegierungen im System Mg-Zn-Al-Ca(2002) Vogel, Michael; Arzt, Eduard (Prof. Dr.)Ziel dieser Arbeit war, ein besseres Verständnis über die grundlegenden Mechanismen der Kriechverformung von Mg-Druckgusslegierungen zu gewinnen. Dazu wurden systematische Untersuchungen zur Mirostruktur und zum Kriechverhalten einer nicht kommerziell erhältlichen Mg-Legierung mit 8 Gew.% Zink und 5 Gew.% Aluminium (ZA85) durchgeführt. Zusätzlich wurden zwei mit 0.3 und 0.9 Gew.% Calcium (ZACa8503 und ZACa8509) modifizierte Varianten dieser Legierung hergestellt und untersucht. Licht- und verschiedene elektronenmikroskopische Verfahren zeigen, dass die Legierungen ein zweiphasiges Gefüge besitzen. Neben den dendritischen Mg-Körnern, die in der Nähe der Korngrenzen stark übersättigte Seigerungszonen aufweisen, tritt auf den Korngrenzen eine intermetallische Phase mit quasikristalliner Kristallstruktur auf. In Folge einer thermischen Beanspruchung kommt es speziell in den Seigerungszonen zur Bildung von Ausscheidungen, die in Folge von Ostwald-Reifung kontinuierlich vergröbern. Neben Gefügeuntersuchungen standen Kriechversuche die an unterschiedlichen Gefügezuständen durchgeführt wurden im Vordergrund dieser Arbeit. Die Kriechbeständigkeit der ZA85 ist dabei größer als die konventioneller, aluminiumreicher AZ-Legierungen. Weiterhin konnte als Folge der Ca-Zugabe eine signifikante Steigerung der Kriechfestigkeit beobachtet werden. Die Korrelation der mikrostrukturellen Untersuchungen mit den Ergebnissen der Kriechversuche deutet darauf hin, dass die Hochtemperaturverformung durch Versetzungskriechen dominiert wird, welches wiederum von Ausscheidungs- und Alterungsvorgängen beeinflusst wird. Das Verhalten der ZA85 Basislegierung, sowie der Einfluss des Calciums lassen sich mittels eines Schwellspannungskonzeptes, das auf der Versetzungs-Teilchen-Wechselwirkung beruht, phänomenologische beschreiben. Die in dieser Arbeit gewonnen Erkenntnisse werfen einerseits ein neues Licht auf das Kriechverhalten von Mg-Legierungen und geben zum anderen Hinweise für zukünftige Legierungsentwicklungen.Item Open Access Interaction of carbon and nitrogen in iron(Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2016) Göhring, Holger; Mittemeijer, Eric Jan (Prof. Dr. Ir.)Item Open Access Mechanisms of intrinsic stress formation in thin film systems(2013) Flötotto, David; Mittemeijer, Eric Jan (Prof. Dr.)Functionalization of thin-film systems on the basis of their functional properties requires precise control of the intrinsic film stresses that develop during the growth process. Although the intrinsic stress evolution during thin film growth has been extensively studied for a huge diversity of different materials, deposition techniques and deposition conditions, the technological potential to optimize and control the properties of thin film systems is still limited due to the lack of fundamental and comprehensive understanding of the stress-inducing mechanisms and their complex correlation with atomic scale processes during thin film growth. The effect of the adatom surface diffusivity on the evolution of the microstructure and the intrinsic stress of thin metal films was investigated for the case of growth of polycrystalline Ag films on amorphous SiO2 and amorphous Ge substrates, with high and low Ag adatom surface diffusivity, respectively. As evidenced by AR-XPS, Ge continuously segregates at the surface of the growing film and thus suppresses the surface diffusivity of the deposited Ag adatoms on the a-Ge substrate also after coalescence of Ag islands and subsequent thickening of the laterally closed Ag film. The abatement of the Ag adatoms surface diffusivity by (segregated) Ge leads to the development of a fine, equiaxed, texture less microstructure for Ag film growth on a-Ge substrates, which is in striking contrast to the development of a columnar, surface energy minimizing {111} fiber textured microstructure for Ag film growth on a-SiO2 substrates. Nevertheless, the real-time in-situ stress measurements revealed a compressive-tensile-compressive stress evolution for the developing Ag films on both types of substrates, however on different time scales and with stress-component values of largely different magnitudes. On the basis of experimental results an assessment could be made of the role of adatom surface diffusivity on the microstructural development and the intrinsic stress evolution during film growth: The microstructural development of polycrystalline metallic thin films is predominated by the surface diffusivity of the adatoms, and the intrinsic stress evolution is largely controlled by the developing microstructure and the grain-boundary diffusivity of atoms. It is revealed that the in-plane film stress oscillates with increasing film thickness at the initial stage of epitaxial single-crystalline Al(111) film growth on a Si(111) substrate, with a periodicity of two times the Fermi wavelength of bulk Al and a stress amplitude as large as 100 MPa. Such macroscopic stress oscillations are shown to be caused by a hitherto unrecognised stress generating mechanism resulting from the quantum confinement of free electrons in the ultrathin metal film: A freestanding film would energetically prefer specific thicknesses and lateral dimensions as a result of the optimal, net effect of quantum confinement of the electrons and the associated elastic deformation (straining). Because the film is attached to the (rigid) substrate, the (oscillating) preferred lateral dimensions cannot be realized and consequently an oscillating component of stress is induced in the plane of the film. The amplitude, period and phase of the observed stress oscillations are consistent with predictions based on the free electron model and continuum elasticity. Furthermore, it is revealed that oxygen exposure of clean Si(111)-7x7, Si(100)-2x1 and a-Si surfaces results in compressive adsorption-induced surface stress changes for all three surfaces due to the incorporation of O atoms into Si backbonds. The measured surface-stress change decreases with decreasing atomic packing density of the clean Si surfaces, in correspondence with the less-densily packed Si surface regions containing more free volume for the accomodation of adsorbed O atoms. It is demonstrated for the first time that pronounced intrinsic stresses can be generated in ultrathin amorphous Al2O3 films formed by thermal oxidation of bare Al surfaces at low temperatures. The magnitude of the growth stress strongly depends on the Al surface orientation: Oxide films formed on Al(100) are stress free, whereas oxide films formed on Al(111) exhibit a thickness averaged in-plane tensile film stress as large as 1.9 GPa. The striking dependence of the stress evolution on the Al surface orientation at the very beginning of oxygen exposure is a direct consequence of the different initial oxygen-adsorbate structures at the Al surfaces inducing distinctly different adsorption induced changes of surface stress. In contrast, the observed striking dependence of the stress evolution on the Al surface orientation during continued oxide-film growth is the result of competing processes of free volume generation and structural relaxation originating from the different microstructural developments for amorphous oxide film growth on Al(111) and Al(100).Item Open Access Dynamic ultrasound projector controlled by light(2022) Ma, Zhichao; Joh, Hyungmok; Fan, Donglei Emma; Fischer, PeerDynamic acoustic wavefront control is essential for many acoustic applications, including biomedical imaging and particle manipulation. Conventional methods are either static or in the case of phased transducer arrays are limited to a few elements and hence limited control. Here, a dynamic acoustic wavefront control method based on light patterns that locally trigger the generation of microbubbles is introduced. As a small gas bubble can effectively stop ultrasound transmission in a liquid, the optical images are used to drive a short electrolysis and form microbubble patterns. The generation of microbubbles is controlled by structured light projection at a low intensity of 65 mW cm-2 and only requires about 100 ms. The bubble pattern is thus able to modify the wavefront of acoustic waves from a single transducer. The method is employed to realize an acoustic projector that can generate various acoustic images and patterns, including multiple foci and acoustic phase gradients. Hydrophone scans show that the acoustic field after the modulation by the microbubble pattern forms according to the prediction. It is believed that combining a versatile optical projector to realize an ultrasound projector is a general scheme, which can benefit a multitude of applications based on dynamic acoustic fields.Item Open Access Deposition of metal oxide thin films from solutions containing organic additives(2007) Lipowsky, Peter; Aldinger, Fritz (Prof. Dr.)In bio-inspired materials synthesis the principles of biomineralization are employed for the fabrication of materials with favourable functional properties at near-ambient temperature and with little expenditure: Organic templates direct the formation of inorganic matter. In aqueous solution, zinc compounds with manifold morphologies are produced by thermal hydrolysis of zinc nitrate in the presence of biomolecules like amino acids and dipeptides. In methanol, ZnO films are deposited by hydrolysis of zinc acetate in the presence of polymers like polyvinylpyrrolidone (PVP) and polyethylene glycol. With PVP, particularly smooth, uniform and stable films are fabricated. Their thickness is determined by the deposition time and the polymer concentration. Various microscopic and spectroscopic measurements prove that the films consist of textured nanocrystalline zinc oxide. Selected properties of the films, such as their photoluminescence, are investigated. Film deposition is possible on substrates with organic coatings bearing certain functional groups. Patterned films can be deposited after local decomposition of the organic coating by UV light. The mechanism of film formation is treated in detail. Like in biomineralization, an amorphous transient state of matter occurs before crystallization. This state succumbs to ZnO nanocrystals, which either aggregate in solution or adsorb to the substrate. It is demonstrated in what way the additive controls the reaction. Sulfonate-modified polystyrene beads are coated with zinc oxide and used as sacrificial templates for the fabrication of zinc oxide hollow spheres. Laminates of alternating layers of zinc oxide and poly(amino acids) are deposited and exhibit an improved mechanical performance compared to the monolithic zinc oxide.