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 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 Stress and microstructure of sputter deposited thin copper and Nnobium films(2003) Okolo, Brando Chidi; Mittemeijer, Eric Jan (Prof. Dr. Ir.)Thin film technology has wide applications in science and engineering industry. Cu and Nb have active relevance in micro-electromechanical systems (MEMS). For Cu thin films, as a possible replacement of Al in the metallisation of substrates while in the case of Nb thin films, its relevance comes to bear in corrosion resistance engineering needs and its superconductivity. The optimal use of these thin film materials lies in tailoring their properties to meet demands for their usage. Grain morphology, texture and stress constitute significant aspects of thin film properties. In the present work priority is to investigate the impact of thin film sputter deposition conditions, the nature of substrate and aging on these properties. Cu and Nb films of various thickness ranges (5 – 1000 nm) were produced by magnetron sputter deposition on two commercially available Si based amorphous substrates, SiO2 and Si3N4, under ultra high vacuum conditions. The as-deposited thin films were investigated by X-ray diffraction (XRD), X-ray reflectometry, auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), atomic force microscopy (AFM) and focussed ion beam (FIB) imaging. Sputter cleaning of the substrates at various ion acceleration voltages prior to film deposition was observed to markedly impact on microstructure, texture and stress states owing to the physical (surface roughness) and chemical changes (reduction in contaminant content) on the cleaned substrates. With increase of the ion acceleration voltage the morphology of the ultrathin (5 nm thick) Cu films, which at this thickness were still at the coalescence stage of film growth, changed from larger sized islands to smaller sized islands. For the thicker films a grain morphology change from irregular geometries to columnar structures occurred with the application of substrate sputter cleaning prior to film deposition. This change was accompanied by a reduction in the average grain size from about 1000 nm (only for films on uncleaned substrates) to 300 nm (for films on sputter cleaned substrates). Of novel significance though is that microstructural changes in thin films are achievable at a constant deposition temperature and pressure. A structure zone model developed by J. A. Thornton in 1975 which describes the physical manifestations of thin film microstructure during sputter deposition ascribes morphology changes to temperature and pressure factors. Clearly this is not the case for the present Cu films where the substrate has also been found to significantly influence the film morphology, though it is noted that the model was developed using film thicknesses between 20 to 250 micrometer. Furthermore, in both Cu films and Nb films, a thickness dependency exists whereby the lateral dimensions of grains increases with the thickening of thin films during the growth process. Texture characterisation involved the use of Theta/2Theta scans and pole figure measurements with the powder diffraction (Theta/2Theta) scans more accurately accounting for the presence of a random component while the pole figure measurements fully account for the sharpness of a particular texture component. For the Cu films the {111} fibre texture was the primary component while for the Nb films the {110} fibre was dominant. The films deposited on Si3N4 substrates showed sharper (i.e. a decrease in the value of the HWHM) textures than those deposited on SiO2 substrates. Sputter cleaning of the substrates prior to film deposition markedly sharpened the texture of the films except for the Cu films deposited on SiO2 substrates where no effect of significance was observed. A mechanism is proposed that accounts for the glaring difference in the texture of films deposited on amorphous SiO2 and amorphous Si3N4 substrates. From the residual stress values determined by the XRD sin-sq-psi method, the intrinsic stresses in Cu and Nb films were calculated by subtracting the thermal stress and found to be of compressive nature, though for the ultrathin Cu films a tensile stress regime originating from the coalescence processes during film growth prevailed. A key-driving factor in the development of stress in the Cu films deposited is the evolution of film morphology. In the Nb films, oxide formation at the grain boundaries close to the surface causes a high compressive stress parallel to the film. For ultrathin Nb films a highly compressive stress is generated (in the order of a few GPa). As the film thickness is increased, at a constant penetration depth of oxygen to form oxides at the film GBs, the measured stress, representing an average over the volume, is less in magnitude.Item Open Access Fe-C and Fe-N compound layers : growth kinetics and microstructure(2007) Greßmann, Thomas; Mittemeijer, Eric (Prof. Dr. Ir.)Improvement of surface’s properties of iron and steel work pieces by gas nitriding/nitrocarburising plays an important role in metallurgy. In particular the fatigue, tribological and corrosion properties are enhanced by these processes without changing the properties of the bulk. Gas nitriding is mainly performed in NH3/H2 gas mixtures, whereas by nitrocarburising additionally a carbon delivering species (mostly CO) is present in the gas atmosphere. Typical treatment temperatures range from 773 K to 863 K. If sufficient nitrogen (and carbon) is supplied by the gas phase to the workpiece an iron-(carbo-) nitride compound layer will develop at the surface and the matrix becomes enriched with nitrogen (and carbon). Such compound layers consist in general of a hcp (with respect to Fe) epsilon-Fe3(N,C)1+x layer adjacent to the surface and a fcc-type (with respect to Fe) gamma’-Fe4N layer at the layer/substrate interface. This work addresses (i) growth kinetics studies of Fe-C compound layers produced on pure iron sheets by nitrocarburising as well as (ii) microstructural investigations on iron-nitride compound layers obtained by nitriding of iron. Nitrocarburising of iron usually leads to the formation of epsilon/gamma’ compound layers, where the presence of carbon promotes the epsilon phase which can dissolve considerable amounts of carbon what is not the case for the gamma’ phase. It has been observed previously that additionally to the epsilon and gamma’ phases also some cementite (Fe3C) can form within the compound layer leading to complex microstructures. However, it was found for the first time in this work that it is possible to grow massive Fe3C layers using a certain composition of the gas mixture consisting of CO, H2, NH3 and N2. With this new developed treatment procedure one can also prevent the often observed sooting/graphite formation at the surface, leading in some cases to disintegration of the metastable Fe3C in alpha-Fe and graphite, which is associated with “metal dusting”. The growth kinetics of such Fe3C surface layer is evaluated and discussed. During nitriding a N concentration gradient due to the inwards diffusion of N from the surface to the bulk builds up within the compound layer. Since, especially, the epsilon phase has a wide homogeneity range for N, this concentration gradient leads to a considerable variation of the lattice parameters with depth. Furthermore, macrostresses may build up within the compound layer during growth due to the concentration gradient and after growth during cooling due to different coefficients of thermal expansion of the layer phases and the substrate. High-resolution X-ray diffraction measurements at different sample tilting angles using synchrotron radiation revealed a pronounced anisotropic diffraction-line broadening of the epsilon reflections. The obtained diffraction patterns are successfully described by a newly developed model with which it is possible to fit the evolution of the (strain-free) lattice parameters with depth as well as a stress-depth profile simultaneously. Analysis of gamma’ layers by X-ray stress measurements using several reflections simultaneously revealed a for fcc-type metals unusual elastic anisotropy of gamma’-Fe4N with <100> as stiffest and <111> as most compliant direction. These results are compared with single-crystal elastic constants obtained by ab-initio calculations and are related to the crystal structure of gamma’-Fe4N in order to get a better understanding of the behaviour of the elastic properties of gamma’. The stresses determined on both layers, epsilon and gamma’, can be understood as thermally induced, whereas the stresses in the gamma’ layer (compressive stresses) are much larger than those present in the epsilon layer, which change from tensile at the surface to compressive at the epsilon/gamma’ interface.Item Open Access Initial oxidation of zirconium: oxide-film growth kinetics and mechanisms(2011) Bakradze, Georgijs; Mittemeijer, Eric Jan (Prof. Dr. Ir.)The present thesis addresses the growth kinetics, chemical constitution, morphology and atomic transport mechanisms of zirconium-oxide films, as grown by the dry, thermal oxidation of single-crystalline Zr surfaces at low oxidation temperatures. To this end, bare (i.e. without a native oxide) well-defined single-crystalline Zr(001) and Zr(100) surfaces were exposed to oxygen gas in the temperature range of T = 300-450 K. The current study, for the first time, presents a direct comparison of the initial oxidation of single-crystalline Zr surfaces with basal and prism orientations. The relationships between the oxidation kinetics, the developing microstructure and the crystallographic orientation of the parent metal substrate have been established by application of various (surface-)analytical techniques. On the basis of the thus-obtained results, a Zr oxidation mechanism in the low-temperature regime (< 500 K) has been proposed. As evidenced by ellipsometry, after a short initial stage of fast oxide-film growth, a near-limiting thickness of the oxide film is attained at T < 375 K on both surfaces. Distinct differences in the oxidation kinetics for the two Zr substrate orientations become apparent at T > 375 K: the Zr(100) plane oxidizes more readily than the more densely-packed Zr(001) plane under the same experimental conditions. At T > 375 K, the oxidation rate of Zr becomes governed by thermally-activated dissolution and diffusion of oxygen into the Zr substrate. The changes in the local chemical states of Zr and O in the thin zirconium-oxide films have been investigated with increasing oxidation temperature. To this end, the oxide-film valence-bands (VB) and the Auger-parameters of Zr and O in the grown oxide films were resolved from measured XPS spectra of the oxidized Zr surfaces in the oxidation-temperature range of T = 300-450 K. The changes in the shape of the oxide-film VB spectra and the local chemical states of Zr and O in the oxide films evidence that the oxide films grown at T ≤ 400 K are predominantly amorphous, whereas a tetragonal ZrO2-like phase develops at T > 400 K. The formation of the tetragonal zirconia modification in the oxide films developing at 450 K is supported by HR-TEM analysis. The exposure of the bare Zr surfaces to pure oxygen gas at low leads to the initial, very fast formation of a dense arrangement of small oxide nuclei clusters, which completely cover the bare Zr surface after 300 s of exposure. With increasing temperature the mobility of adsorbed O species and/or Zr species on the oxidizing surface and in the developing oxide becomes promoted, thereby promoting the restructuring/reorientation of the oxide clusters into bigger agglomerates, e.g. with increasing oxidation time at constant temperature, as driven by the Gibbs-Thomson effect. In this thesis for the first time, two-stage tracer oxidation experiments have been applied to study the atomic transport mechanisms in thin (< 10 nm) oxide films, as formed during the initial stages of the low-temperature oxidation of the bare, single-crystalline Zr(001) and Zr(100) surfaces at 450 K. The observed differences in shape of the measured tracer profiles for different stages of oxidation indicate a change in the oxide-film growth mechanism during oxidation: i.e. a change of the predominant transport mechanism from inward oxygen transport by a combination of lattice and short-circuit mechanisms during the initial oxidation stage to inward oxygen transport by only the lattice mechanism during later oxidation stages. A low-temperature oxidation mechanism for zirconium at T = 450 K was proposed. Oxygen transport through the developing oxide film requires coupled fluxes of inwardly migrating O anions and outwardly migrating O vacancies, as supplied by the slow continuous O dissolution into Zr. The resulting Zr(O) solid-solution phases formed at the metal/oxide interface are evidenced by an interfacial suboxide layer in the in-situ AR-XPS and ellipsometry analysis. O vacancies, as generated in the interfacial suboixde layer by the slow, but continuous, dissolution of O into the Zr substrate, diffuse outwardly through the O sublattice of the crystalline zirconia overlayer towards the oxide/gas interface to be filled by chemisorbed O surface species at the gas/oxide interface. The thus-established outward flux of O vacancies is accompanied by a net inward lattice flux of oxygen anions. At the early oxidation stage, oxygen is transported inwardly via both the lattice and short-circuit transport mechanism. At later oxidation stages, the contribution of O short-circuit transport becomes negligible, as attributed to a reduction of grain-boundary (GB) density in the oxide-film in association with an equilibration of the GB structure, in possible combination with the accumulation of space charge in the vicinity of the GB in the oxide-film. The overall oxide-film growth rate at 450 K is limited by the O dissolution rate (i.e. supply of oxygen vacancies into the growing oxide film) at the suboxide/metal interface.