Universität Stuttgart
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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 Reinforcement of precursor-derived Si-(B-)C-N ceramics with carbon nanotubes(2005) Katsuda, Yuji; Aldinger, Fritz (Prof. Dr.)Incorporation of carbon nanotubes (CNTs) into the precursor-derived Si-(B-)C-N ceramics has been investigated for the reinforcement of the materials. Different types of CNTs consisting of multi-wall (MW) and single-wall (SW) were examined as the reinforcement of the Si-(B-)C-N ceramics to make a comparison of the effect. Mechanical properties demonstrated in the Si-(B-)C-N/CNT nanocomposites have been discussed in connection with their microstructural features characterized by scanning electron (SEM) and transmission electron microscope (TEM). Other material properties of the nanocomposites as revealed on the thermal stability and the crystallization behavior have been also considered in relation to the microstructural characteristics of the nanocomposites. Dense Si-C-N/CNT nanocomposites containing different types of MWCNTs were successfully prepared by casting of a mixture of MWCNTs and a liquid precursor polymer followed by cross-linking and thermolysis. In these processes, the sonication for deagglomeration and dispersion of CNTs in the precursor polymer as well as the thermolysis condition for ceramization of the cross-linked precursor/CNT nanocomposites was examined to obtain homogeneously CNT distributed Si-C-N ceramics. Fracture toughness behavior of the Si-C-N/CNT nanocomposites has been evaluated by a thermal loading technique on the disc shaped materials. The results reveal a dependence of the fracture toughness on the type of the MWCNTs. The MWCNTs showing high integrity in the tube structure exhibit a remarkable increase in the fracture toughness at the CNT content of 1 – 2 mass %, whereas the other ones possessing amorphous nature exhibits no effect. The microstructural analyses at the fracture surfaces have demonstrated different features of CNTs between both nanocomposites, where pulling out and breaking of CNTs are considered to be reasons for the observed fracture toughness increase. No significant influences observed on the material properties of the Si-C-N/CNT nanocomposites besides the toughening indicates that CNTs can simply work as the reinforcement for the Si-C-N ceramics. SWCNTs incorporation into the Si-C-N materials has revealed toughening effect with similar microstructural features to the MWCNT reinforced Si-C-N nanocomposites. In this system, it was found that the deagglomeration and debundle of the SWCNTs are major issues to make the best use of SWCNTs as the reinforcements. Concerning the Si-B-C-N/CNT nanocomposites, preparation processes via a casting and a warm pressing from different types of boron-containing precursors have been investigated to produce rigid MWCNT nanocomposites. The observed pulling out and breaking CNTs structure at the fracture surfaces of the Si-B-C-N/CNT nanocomposites indicate the toughening effect of CNTs similar to Si-C-N/CNT ceramics. Moreover, the interaction between CNTs and the matrix has appeared to be changed with increasing thermolysis temperature. However, the crystallization of the Si-B-C-N matrix and the deterioration of thermal stability have been disclosed in the Si-B-C-N/CNT nanocomposites. It is revealed that embedded CNTs have an effect to accelerate or to generate nucleation sites for the crystallization of Si-B-C-N matrix.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 Low temperature sintering additives for silicon nitride(2003) Matovic, Branko; Aldinger, Fritz (Prof. Dr.)Pressureless sintering of Si3N4 with two new additives based on the Li2O-Y2O3 system (LiYO2) and on the Li2O-Al2O3-SiO2 system (LiAlSiO4) were investigated in this study. Experiments were conducted in the areas of powder processing, sintering optimization, phase transformation characterization and microstructural development. Sintered materials were characterized by fracture toughness and thermal diffusivity measurements. The experimental results are summarized in the following: Using three different mechanical mixing processes (attrition milling, ball milling and vibratory milling) for the introduction of additives (LiYO2) into Si3N4 powders, the best results are obtained for attrition milling. This method yields a good dispersion of the additive powder in fine unagglomerated Si3N4 without contamination. It also yields good sintering characteristics. For the LiYO2 system, the densification depends largely on the content of sintering additive. A larger amount of additive means a higher volume of liquid phase, which is favorable for efficient particle rearrangement resulting in higher values of density rate. In case of the LiAlSiO4 additive, it is found that the densification is less dependent on the additive content. The overall sintering kinetics at the low temperatures is less retarded when using the LiYO2 additive in comparison to the LiAlSiO4 additive, resulting in higher densities obtained at lower temperatures and shorter annealing times. With prolonged heating time, the differences in the degree of densification become smaller. The kinetics of phase transformation in the both systems are found to be of first order. In the LiYO2 system, the transformation rate constant increases with increasing additive content. While the opposite behavior is noticed in case of the LiAlSiO4 additive, i.e. the rate constant decreases with higher additive content. The phase transformation is always completed at a later stage than the densification. The lag between the two phenomena in the sintering process is more pronounced with the Li2O-Al2O3-SiO2 additive system. The alpha to beta Si3N4 transformation is accompanied by grain growth. Upon prolongation of the annealing time the grain size and the morphology of the growing beta-Si3N4 particles are significantly changed from equiaxed to elongated. The grain growth becomes anisotropic, leading to rod-like betaƒ{Si3N4 crystals. The growth rate is higher in the LiYO2 system than in the LiAlSiO4 system. At 1600„aC, the microstructure of Si3N4 ceramics sintered with both the additives is characterized by a homogeneous distribution of elongated beta Si3N4 grains and glassy phase located in thin layers at grain boundaries and at triple points. The maximum values obtained for fracture toughness are 6.8 and 6.2 for the materials sintered with LiYO2 and LiAlSiO4 additives, respectively, at 1600„aC for 8 h. The higher value of fracture toughness in the LiYO2 system is attributed to its microstructure with a higher aspect ratio of the elongated beta-Si3N4 grains. Thermal conductivity of the material sintered with the LiYO2 additive is higher in comparison to that sintered with LiAlSiO4 additive. In the LiAlSiO4 system, partial dissolution of Al3+ in the beta-Si3N4 grains results in increasing phonon scattering and hence decreases the thermal conductivity.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 Segregation and phase transformations at interfaces(2004) López, Gabriel Alejandro; Mittemeijer, Eric Jan (Prof. Dr. Ir.)Important properties of metallic materials are strongly influenced by the behavior of interfaces, e.g. grain boundaries and free surfaces. Therefore the understanding of processes, which can change this behavior, have become of great importance from both the theoretical and the practical point of view. To these processes belong the grain boundary and surface segregation, the discontinuous precipitation as well as the grain boundary wetting. A particular goal of this work was to investigate the grain boundary and surface segregation in the Cu-Bi system under exactly the same conditions. The diffusivities of moving grain boundaries was determined in the Cu-In system applying, for the first time, a local analysis of growth kinetics of the discontinuous precipitation reaction. The Al-MG and Al-Zn systems were studied systematically regarding grain boundary wetting, in order to find an explanation for the enormous superplastic behavior of alloys based on these systems. The grain boundary and surface segregation were studied in the Cu-Bi system under identical conditions by using a special sample preparation procedure. After annealing of the samples at temperatures between 1073 and 1223 K segregation of Bi at grain boundaries and internal free surfaces in Cu bicrystals was accomplished. For the first time, Bi segregation at free surfaces was determined under equilibrium conditions. The segregation of Bi at the free surfaces was clearly stronger than at the grain boundaries. The morphology and kinetics of the discontinuous precipitation reaction were comprehensively studied in a Cu-4.5 At,% in alloy. Special attention was given to the determination of the concentration gradients remaining in the solute-depleted matrix. The grain boundary diffusion coefficient was determined applying a local interpretation of growth kinetics and thus the discrepancy between two differently models was eliminated. Finally the wetting behavior in Al-Mg and Al-Zn alloys was studied by a metallographic investigation. With this purpose cross-sections of samples with different compositions, which had been annelaed at different temperatures, were prepared and examined afterwards. The wetting behavior plays a substantial role regarding the mechanical characteristics of these alloys. For the first time, the formation of a second solid layer at the grain boundaries was disccussed in terms of wetting by a solid phase. During the accomplished investigations the possibilities of the analytic transmission electron microscopy were mainly used. Concentration profiles within nm range could be determined thanks to the high resolution of this technique. Furthermore Auger electron spectroscopy, light and scanning electron microscopy, as well as microanalysis and X-ray diffraction analyses were used, in order to achieve the above-mentioned goals of this work.Item Open Access Precipitation of nitrides in iron-based binary and ternary alloys; influence of defects and transformation-misfit stresses(2015) Akhlaghi, Maryam; Mittemeijer, Eric Jan (Prof. Dr. Ir.)The initial microstructure of the unnitrided specimen has a significant influence on the nitriding response of binary Fe-Me (Me: Mo or Al) alloys specimens. This effect was not investigated until now for the case of nitrided ternary Fe-Me1-Me2 alloys, the role of the initial microstructure was studied upon nitriding Fe-4.1 at.% Cr-7.9 at.% Al specimens. To this end, the recrystallized and cold-rolled specimens were nitrided at low nitriding temperature of 400 °C. Upon precipitation of misfitting coherent nitrides during nitriding of thin-foils of binary Fe-Me (Me: Cr and V) alloys, a hydrostatic tensile lattice-stain component results from the elastic accommodation of volume misfit of nitrides and ferrite matrix. The change of the ferrite-matrix lattice parameter can be traced upon precipitation of the nitrides by X-ray diffraction measurements. The theory originally developed for the case of imperfections (by Eshelby) in solids can be applied for quantitatively describing the lattice-parameter changes of the matrix, the nitrides and the aggregate (matrix+ nitrides) as function of volume fraction and type of nitrides.Item Open Access Formation of lath martensite(2015) Löwy, Sarah; Mittemeijer, Eric Jan (Prof. Dr. Ir.)In this thesis the formation of different lath martensites was investigated upon cooling, particularly with regard to the mechanisms contributing to the transformation process. Upon very slow cooling of different Fe-Ni alloys and a maraging steel, all forming lath martensite, a discontinuous transformation behaviour was observed. This modulation of the transformation rate is ascribed to the interplay of chemical driving force, developing strain energy and its relaxation upon slow cooling. It is proposed that the modulation is caused by simultaneous formation of blocks in different martensite packages. Additionally, the influence of the Ni content on the transformation behaviour is presented as well as the influence of an externally applied force.Item Open Access Hochtemperatur-Thermochemie im System Al-Cr-Ni-O(2003) Saltikov, Pavlo; Aldinger, Fritz (Prof. Dr.)Das System Al-Cr-Ni ist von großer Bedeutung in der Energieerzeugungs- und der Luftfahrttechnik als Basis für die Superlegierungen. Zur Steigerung des Wirkungsgrads einer Gasturbine müssen die neuen Herstellungstechniken (z.B. gerichtete Erstarrung) bzw. Konstruktionskonzepte (keramische Wärmedämmschichten) eingeführt werden, die den Betrieb bei höheren Verbrennungstemperaturen ermöglichen. Obwohl die Ni-Basis-Superlegierungen in den letzten Jahren sehr ausgiebig untersucht worden waren, bleiben immer noch viele offene Fragen bezüglich der Herstellung, des Betriebs und des Schutzes dieser Legierungen. In dieser Arbeit wurde ein Beitrag zum Verständnis dieser unterschiedlicher Aspekte geleistet. Da die auftretende Probleme sehr vielschichtig sind, wurden hier unterschiedliche Methoden der Thermochemie eingesetzt. Die für die Simulation der gerichteten Erstarrung notwendigen thermodynamischen Daten flüssiger Al-Cr-Ni-Legierungen wurden in dieser Arbeit bestimmt. Die partiellen Mischungsenthalpien flüssiger Al-Cr-Ni-Legierungen wurden mittels der Hochtemperatur-Mischungskalorimetrie gemessen. Die experimentelle Ergebnisse wurden ausgewertet und zur Ermittlung der Werte der integralen Mischungsenthalpie im ganzen Konzentrationsbereich verwendet. Zum Vergleich der Mischungsfunktionen wurden diese auch thermodynamisch berechnet. Weiterhin wurden die experimentellen Daten nach dem Assoziatmodell behandelt. Auf diese Weise wurden die Mischungsfunktionen sowie die thermodynamische Aktivitäten berechnet. Außerdem wurden die Beziehungen zwischen den thermodynamischen Funktionen und der chemischen Nahordnung in der Schmelze mit dem Assoziatmodell beschrieben. Aus diesen Untersuchungen hat sich ergeben, dass nur die schwache Wechselwirkungen zwischen den Spezies in binären Al-Cr- und Cr-Ni-Schmelzen vorliegen. Die stärkste chemische Nahordnung im System Al-Cr-Ni bei 1727 K wurde nahe der Zusammensetzung Al2Cr1Ni1 beobachtet. Es wurde festgestellt, dass die flüssigen Al-Ni-Legierungen eine größere Tendenz zur Bildung von chemischer Nahordnung als flüssige Cr-Ni- und Al-Cr-Legierungen aufweisen, was ein Grund für einen starken Einfluss des Al-Ni-Systems auf die Mischungsfunktionen in einem weiten Konzentrationsbereich darstellt. Um die Versagensursachen eines Wärmedämmschichtsystems besser zu verstehen, wurde die Oxidation einer Ni-Cr-Al-Haftvermittlerschicht bei 1373 K thermodynamisch modelliert. Die Berechnungen von lokalen Phasengleichgewichten haben Aufschlüsse darüber gegeben, in welcher Reihenfolge sich die Phasen in der Oxidschicht bilden. Dazu wurde in dieser Arbeit eine neue Art von Phasenmengendiagrammen vorgeschlagen. Sie wurden mit Hilfe von computergestützten thermodynamischen Berechnungen konstruiert und zur Erörterung der Grenzflächenreaktionen verwendet. Es wurde gezeigt, dass die Entstehung des (Al,Cr)2O3-Mischoxids und der Ni(Al,Cr)2O4-Spinell-Mischphase in den unteren bzw. mittleren Zonen der Oxidschicht sowie des Nickeloxids mit der Spinell-Mischphase in der Deckschicht des thermisch gewachsenen Oxids möglich ist. Es wurde auch gezeigt, dass das Al2O3 sich nur auf der Oberfläche der beta-reichen Körner bilden kann. Nach einer ausreichend langen Oxidationsdauer kommt es zur Al-Verarmung sowie zur Abschnürung der beta-Phase und zur Erhöhung der Anteile der gamma- und gamma'-Phasen im Oberflächenbereich der Haftvermittlerschicht.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).