14 Externe wissenschaftliche Einrichtungen

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/15

Browse

Filters

2004 - 2009122010 - 201610

Settings

Publication Type: search.filters.UBSTyp.DissertationSubject: search.filters.subject.500

Search Results

Now showing 1 - 10 of 22
  • Thumbnail Image
    ItemOpen Access
    Vortex-Kern-Korrelation in gekoppelten Systemen
    (2014) Jüllig, Patrick; Schütz, Gisela (Prof. Dr.)
    In der vorliegenden Arbeit wurden strukturierte ferromagnetische Dreischichtsysteme zum einen auf ihre statische in-plane- sowie out-of-plane-Magnetisierungsverteilung als auch auf deren dynamisches Verhalten hin untersucht. Die sowohl quadratischen als auch kreisförmigen Strukturen bestanden aus zwei ferromagnetischen Lagen mit einer Dicke von jeweils 50nm, welche durch eine nicht magnetische Cu-Zwischenschicht getrennt waren. Die Dicke dieser Zwischenschicht variierte schrittweise von t(Cu)=3nm bis 15nm. Als Magnetmaterialien kamen für die untere Schicht Kobalt (Co) und für die obere Schicht das magnetisch isotrope Permalloy (Ni80Fe20) zum Einsatz. Die lateralen Abmessungen sowie das Aspektverhältnis der beiden Einzelschichten wurden so gewählt, dass der Vortexzustand die stabile Domänenkonfiguration ist. Somit resultierten zwei vertikal übereinander angeordnete Vortexkonfigurationen, sodass deren Wechselwirkung sowohl im statischen als auch im dynamischen Fall untersucht werden konnte. Aufgrund der gewählten Cu-Schichtdicke von mindestens 3nm war gewährleistet, dass die Kopplung der in-plane-Schichtmagnetisierung hauptsächlich durch die elektrostatische Streufeldenergie beeinflusst wurde und somit der Beitrag der Oszillatorischen Zwischenschichtaustauschwechselwirkung vernachlässigt werden konnte. Im Falle zweier vertikal übereinander angeordneter Vortexstrukturen kann man bezüglich der Zirkulation C (beschreibt die Orientierung der in-plane-Magnetisierung) und der Polarisation P (beschreibt die Orientierung der out-of-plane-Komponente des Vortexkerns) unter Berücksichtigung der Symmetrie vier verschiedene Konfigurationen voneinander unterscheiden: Die beiden Fälle, bei denen C und P jeweils bzw. orientiert sind, sowie die beiden Fälle, bei denen lediglich C oder P parallel ausgerichtet ist. Der erste Schritt dieser Arbeit bestand in der Probenpräparation. Als Strukturierungsverfahren kamen zum einen das Ionenstrahlätzen und zum anderen die Elektronenstrahllithographie zum Einsatz. Anhand von Röntgenbeugungsexperimenten konnte herausgefunden werden, dass beide Schichtmaterialien, sowohl das Permalloy als auch das Kobalt, eine polykristalline, fasertexturierte Schichtstruktur mit einer fcc-Gitterstruktur aufwiesen. Diese Erkenntnisse waren vor allem für die korrekte Parameterwahl für die nachfolgend durchgeführten mikromagnetischen Simulationen von großer Bedeutung. Messungen der Oberflächenrauigkeiten mittels des AFM ließen darauf schließen, dass neben dem Beitrag der Streufeldenergie ebenso korrelierte bzw. unkorrelierte Zwischenschichtrauigkeiten zur gegenseitigen Ausrichtung der in-plane-Schichtmagnetisierungen beitrugen. Mit Hilfe von SQUID-Messungen bei T=40K an unstrukturierten Co/Cu/Py-Dreischichtsystemen konnte nachgewiesen werden, dass erst für Proben mit Cu-Schichtdicken ab t(Cu)=2,0nm beide ferromagnetische Materialien chemisch voneinander getrennt vorlagen und keine direkte ferromagnetische Kopplung aufgrund von sogenannten Pinholes auftrat. Somit konnte geschlussfolgert werden, dass erst ab einer Dicke von t(Cu) größer gleich 2,0nm eine vollständig geschlossene Cu-Schicht vorlag. Die ersten statischen in-plane-Messungen am STXM zeigten, dass Proben, welche im as-sputtered Zustand eine undefinierte metastabile Mehrdomänenkonfigurationen aufwiesen, durch einen Entmagnetisierungsprozess in den stabilen Vortexzustand überführt werden konnten. Neben antiparallel gekoppelten Systemen bezüglich der Zirkulation C wurden mit einer ähnlich hohen Wahrscheinlichkeit Proben mit einer parallelen Ausrichtung der in-plane-Magnetisierung gefunden. Dies zeigte, dass die Kopplung der Schichtmagnetisierungen nicht allein durch die Streufelder realisiert wurde, sondern ein weiterer Beitrag hinzukam, dessen Ursache mit hoher Wahrscheinlichkeit in den Zwischenschichtrauigkeiten zu finden war. Statische mikromagnetische Simulationen an quadratischen Co/Spalt/Py-Strukturelementen haben gezeigt, dass die in-plane-Magnetisierungsverteilung der Systeme mit C=parallel eine merklich verzerrte Landaustruktur aufwies. Zudem lag bei Konfigurationen mit P=antiparallel ein lateraler Shift bezüglich der Gleichgewichtspositionen der Kerne vor, was aufgrund der Interaktion der out-of-plane-Streufelder zu erwarten war. Dies spiegelte sich auch in der Energiebetrachtung wieder, wobei die beiden Systeme mit der Konfiguration C=parallel deutlich höhere Gesamtenergien aufwiesen als diejenigen mit C=antiparallel. Allgemein lagen im Falle von parallelen Kernpolarisationen die Energiewerte etwas niedriger als bei antiparallel ausgerichteten Kernen. Die dynamische Anregung der ferromagnetischen Schichtsysteme wurde experimentell mittels eines in-plane-Magnetfeldpulses realisiert, welcher durch die lineare Stripline generiert wurde. Die Pulsdauer betrug je nach Element 0,5 bis 1,6ns, und bezüglich der Pulsamplitude mussten Feldstärken von B(Puls)=3,1mT bis zu 6,0mT angelegt werden, um eine Gyrationsbewegung beobachten zu können.
  • Thumbnail Image
    ItemOpen Access
    Grain growth and texture evolution in copper thin films
    (2010) Sonnweber-Ribic, Petra; Arzt, Eduard (Prof. Dr. phil)
    An improved basic understanding of mechanisms causing grain growth and texture evolution in Cu thin films contains the potential to improve performance and reliability of components and devices. In this work, the influence of film thickness, strain and temperature on grain growth and texture evolution in Cu thin films was investigated. By varying the parameters, information about the underlying mechanisms were revealed. The 0.5 to 10 micrometer thick Cu films were deposited on 125 micrometer thick polyimide substrates (Kapton®, DuPont) using a UHV magnetron sputtering system. For detailed observation of grain growth and texture evolution an EBSD-based in situ testing appliance was constructed. This system allowed the simultaneous observation of grain growth and texture evolution, giving new insight into growth kinetics and details of grain growth. In a first step, Cu thin films of thicknesses in between 0.5 and 10 micrometer were deposited on polymer substrates and annealed at 330°C for 30 min. Their resulting texture and microstructure were investigated by EBSD. A texture transition from (111) to (100) was observed at film thicknesses between 3 and 5 micrometer. The experimental findings were explained by the texture evolution model of Thompson and Carel. A significant observation which cannot be explained by a purely energetic argument is the broad texture transition. In order to get more information about the critical role of strain energy, uniaxial tensile tests were carried out on 3 micrometer thick films. In contrast to theoretical predictions, various tensile tests revealed no influence of strain on grain growth behaviour. Neither at room temperature nor at elevated temperatures, further (100) grain growth was observed. In a next step, the abnormal growth of individual (100) oriented grains was recorded for more than 24 hours at temperatures between 90 and 118°C. Annealing was carried out inside a Leo 1530-VP SEM equipped with a heating facility. Detailed analysis of grain growth and estimates of the possibly acting driving forces indicated that the reduction of dislocation density played an important role for abnormal grain growth. A further hint for the critical importance of defect density was given by the HWHM of the (100) texture fraction. Nevertheless, it was not clear why this driving force favours the growth of (100) oriented grains. A possible answer could be given by the strain energy release maximization (SERM) model developed by Lee. In addition, when analysing the activation energy for grain growth, they were found to possess a higher grain boundary mobility, supporting the preferred growth of (100) oriented grains. A new texture map, considering dislocation density as driving force, was constructed. Assuming dislocation density to play a significant role for grain growth and texture evolution in Cu thin films, the influence of deposition parameters is pointed out.
  • Thumbnail Image
    ItemOpen Access
    Effects of oxide incorporation in proton conducting organic electrolytes
    (2009) Sörgel, Seniz; Maier, Joachim (Prof. Dr.)
    In this work, the effects of incorporation of various types of oxide particles (e.g. ZrO2, TiO2, Al2O3) into proton conducting organic electrolytes is investigated. As a weak liquid model electrolyte, moderately proton conducting imidazole is chosen. As a highly proton conducting strong polymer electrolyte, and simultaneously practically very important electrolyte, Nafion® is selected for the second part of the work. In the first part of this work, for the first time, the applicability of the concept of heterogeneous doping to imidazole is demonstrated. Imidazole exhibits moderate proton conductivity due to low intrinsic charge carrier concentration. Therefore, a perceptible conductivity increase by heterogeneously doping imidazole is expected. Ac-impedance spectroscopy measurements of composites of imidazole with various types of nanometer sized oxide particles, which were performed as a function of temperature and oxide concentration show that the composites exhibit significantly enhanced ionic conductivities compared to the pure imidazole. The highest measured composite ionic conductivity is observed for the composite with heated sZrO2, viz. 1.66x10-2 -1 cm-1 at 90 °C corresponding to an enhancement by a factor of 10 compared to the pure ImiH at the same temperature. The composites prepared with the oxides having the highest activity and density of the acidic sites on the surface show the most pronounced improvement in conductivity. These results were quantitatively analyzed in light of the concept of heterogeneous doping. The proton conductivities calculated according to the heterogeneous doping concept are consistent with the experimentally observed conductivities. The results of zeta potential measurements show that the surface charge of the inorganic oxides becomes strongly more negative on the addition of imidazole. This is consistent with the formation of a space-charge layer on the oxide surface as a consequence of an adsorptive interaction: trapping of imidazolate anions (Imi-) on the oxide surface results in an increased concentration of imidazolium cations (ImiH2+) in the space charge region at the interface of oxide and conductor. The second part of this work focuses on the investigation of the effects of inorganic oxide admixture on proton conductivity, microstructure and mechanical properties of a strong polymer electrolyte, namely Nafion®. Various composite and respective bare membranes were investigated for which performance improvements had been proven in literature before. Thermal and hydrothermal treatments were applied to the membranes in order to get an insight into the properties of the materials at high temperature and low humidity conditions. According to the attenuated total reflection infrared (ATR-IR) spectroscopy results, upon hydrothermal treatments a condensation reaction and consequently an anhydride formation (R-O2S-O-SO2-R) is suggested to occur in the membranes. The thermal treatment above Tg may also lead to the same kind of products. In addition, sulphur formation (aging) is proposed to occur in such conditions which can be derived by X-ray powder diffractometry and energy dispersive microanalysis. These reactions (condensation and sulphur formation) result in an increase of the equivalent weight (EW) and local ordering between polymer crystallites which were detected by acid-base titrimetry and small-angle X-ray scattering (SAXS) measurements, respectively. The conductivity of the membranes is observed to decrease upon thermal and hydrothermal treatments. At high water contents, the decay of conductivity can be explained by the equivalent weight increase. However, at low water contents the mobility of the charge carriers is observed to be slightly suppressed which can explain the conductivity behavior. The lower mobility at low water contents can be due to the less favorable microstructure of the membranes for proton conduction. The proposed condensation reaction and/or sulphur formation (aging) lead to a decrease of hydrophilicity of the side chains. This negatively affects the nanophase separated morphology since hydration of the ionic clusters decreases. Thereby, the water content in the membranes decreases. It is observed by dynamic mechanical analysis (DMA) measurements that the lower amount of water in the membranes is unfavorable for the mechanical properties of the membranes at high temperatures as water acts as a stiffener in such conditions. The above explained effects of thermal and hydrothermal treatments on EW, proton conductivity, activation enthalpy, mobility and microstructure of the membranes without oxide particles are more severe than they are for the composite membranes. A probable condensation reaction and/or aging and therefore changes in microstructure and transport properties of the material are suppressed in the presence of oxide particles. DMA measurement results show that the composite membranes also keep a higher amount of water at elevated conditions and they are thermally and mechanically slightly more stable compared to the respective bare membranes. The incorporation of the oxide particles also increases the glass transition temperature about 10 °C which indicates that the composites have slightly higher thermal stability. In conclusion, in this work it is shown that the oxide incorporation has a positive effect on both weak and strong proton conducting electrolytes: while in the former the proton conductivity is improved by charge carrier concentration increase in the space charge layer, in the latter one it is the structural, thermal and mechanical stability of the material that is beneficially affected at elevated conditions. This study may encourage further developments of electrolyte materials for alternative energy conversion devices.
  • Thumbnail Image
    ItemOpen Access
    Synthesis and characterization of carbon nanotube reinforced copper thin films
    (2006) Otto, Cornelia; Arzt, Eduard (Prof. Dr.)
    Two model composites of copper and carbon nanotubes were fabricated by very different deposition methods. Copper electrodeposition in a plating bath containing nanotubes created a 3D matrix of randomly oriented CNTs within a thick, 20 micron Cu film. In contrast, sandwiching a layer of well-separated nanotubes between two sub-micron sputtered Cu layers produced a 2D-composite with nanotubes lying parallel to the substrate surface. These composites, which were mechanically tested using various techniques, proved to be well suited to explore the nature of the CNT/Cu matrix interface. Columns approximately 600 nm in diameter and 1.4 microns high were cut from the sputter-deposited composite and microcompression tested in a nanoindenter. No influence of the presence of nanotubes on the stress-strain-curves was observed, which was attributed to the low nanotube content. On the other hand, microscopic analysis showed an influence of the nanotube on the copper immediately surrounding it, resulting in funnel-like depressions on the column surface. In addition to deformation by slip, twinning was observed in some columns, which has never before been reported in the literature for micron-sized columns. Macroscopic tensile tests were performed on the electrodeposited films and the samples with the highest carbon content showed an increase in toughness of over 100% with respect to the CNT-free control samples produced by the same method. Finally, short copper electrodepositions into carbon nanotube carpets revealed large regions with conformally coated nanotubes. Until now, it was assumed that copper would not wet the nanotubes and that the interfacial strength between copper and CNTs would be low, since copper does not form a carbide. However, these experiments all revealed clear evidence of adhesion exceeding the copper shear strength. To our knowledge, this is the first time such strong adhesion was demonstrated between a nanotube and a metal matrix. We attribute this unexpected, but highly desirable adhesion and wetting behaviour to the defect structure in the nanotubes used. Most of the experiments were done with nitrogen-doped carbon nanotubes, which are known to be rich in defects. The nanotube carpets used in the last experiment were not doped but had a high defect density due to the synthesis method used. As a good adhesion between fiber and matrix is a prerequisite for the successful use of carbon nanotubes in metal matrix composites, these results are very encouraging. The composites and methods presented here provide a foundation for further studies needed to understand the nanotube-metal interaction in more detail and thus ultimately for successful metal-carbon nanotube composites.
  • Thumbnail Image
    ItemOpen Access
    Microstructural changes and intermetallic compound formation in metallic bilayers
    (Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2016) Rossi, Paul J.; Mittemeijer, Eric Jan (Prof. Dr. Ir.)
    This thesis investigates interdiffusion and intermetallic compound (IMC) formation, as well as their effects on the microstructure, in metallic thin-film bilayers. The investigation focuses on bilayers based on the Ag-Sn and Ag-In binary systems, which are technologically important as basis for lead free solders. Due to the enhanced diffusional mechanisms in these systems, diffusion occurs readily even at room and low temperatures. The proceeding interdiffusion eventually leads to IMC formation in the bilayers, allowing for the investigation of the kinetics of IMC formation and the associated microstructural changes at room and low temperatures. The combination of the properties special to thin films with the diffusional mechanisms in the binary Ag-Sn and Ag-In systems leads to interesting effects, such as the dependence of IMC formation on the stacking sequence in the bilayers. The obtained experimental results for both systems could be explained using thermodynamic and kinetic models. Experimental characterization of the bilayers mainly relied on X-ray diffraction (XRD) and electron microscopy. In order to investigate the effect of the deposition process on IMC formation and the microstructure of the bilayers, different physical vapor deposition (PVD) techniques, especially thermal evaporation and magnetron sputtering, were used for the preparation of the bilayers. During investigation of the Ag-Sn system it was found that ambiguity exists among the published crystal structures of the Ag3Sn IMC. Therefore, the crystal structure of Ag3Sn has been reinvestigated using high-resolution XRD in connection with Rietveld refinements.
  • Thumbnail Image
    ItemOpen Access
    Anion exchange membranes for fuel cells and flow batteries : transport and stability of model systems
    (2015) Marino, Michael G.; Maier, Joachim (Prof. Dr.)
    Polymeric anion exchange materials in membrane form can be key components in emerging energy storage and conversions systems such as the alkaline fuel cell and the RedOx flow battery. For these applications the membrane properties need to include good ionic conductivity and sufficient chemical stability, two aspects, that are not sufficiently understood in terms of materials science. Materials fulfilling both criteria are currently not available. The transport of ions and water in a model anion exchange membrane (AEM) as well as the alkaline stability of their quaternary ammonium functional groups is therefore investigated in this thesis from a basic point of view but with the aim to bring these technologies one step closer to large scale application, as they have several advantages compared to existing energy storage and conversion systems. The hydroxide exchanging alkaline fuel cell (AFC), for example, is in principle more cost-effective than the more common acidic proton exchange fuel cell (PEMFC). Unfortunately AFCs suffer from base induced decomposition of the membrane. Especially the quaternary ammonium (QA) functional groups are easily attacked by the nucleophilic hydroxide. QAs with higher alkaline stability are required but there is considerable disagreement regarding which QAs are suitable, with widely varying and partially contradicting results reported in the literature. In this thesis, the decay of QA salts was investigated under controlled accelerated aging conditions (up to 10 M NaOH and 160 °C). This allowed a stability comparison based solely on the molecular structure of the QAs. A number of different approaches to stabilize the QAs which potentially inhibit degradation reactions such as β-elimination, substitution and rearrangements were compared. These include β-proton removal, charge delocalization, spacer-chains, electron-inducing groups and conformational confinement. Heterocylic 6-membered QAs based on the piperidine structure proved to be by far the most stable cations at the chosen conditions. This was not readily apparent from their structure since they contain β-protons in anti-periplanar positions, which generally cause rapid decomposition in other types of QAs. The geometry of the cyclic structure probably exerts strain on the reaction transition states, kinetically inhibiting the degradation reactions. Other stabilization approaches resulted in markedly less stable compounds. Noticeably the benzylic group, which is the current standard covalent tether between QA and polymer, degrades very fast compared to almost all aliphatic QAs. The results of this stability study suggest that hydroxide exchange membranes for alkaline fuel cells, which are significantly more stable than current materials are achievable. Besides stability, the transport of anions and water in AEMs was investigated in this Hydroxide exchange membranes (HEM) have been reported to exhibit surprisingly low ionic conductivities compared to their proton exchange membrane (PEM) counterparts. This is partially because hydroxide charge carriers are rapidly converted to carbonates when a HEM comes into contact with ambient air. Careful exclusion of CO2 was required to investigate pure hydroxide form membranes. For this purpose a custom glove box was designed and built that allowed preparation and measurements of HEM samples in a humidified CO 2 -free atmosphere. It was found that the conductivity reduction of a carbonate contaminated HEM is not only due to the reduced ionic mobility of carbonate charge carriers compared to hydroxide, but also because of reduced water absorption of the corresponding membrane which decreases conductivity even further. Pure HEMs can in fact achieve conductivities within a factor of two of PEMs at equal ion exchange capacity at sufficient hydration, according to the differences in the ionic mobility of hydroxide and hydronium. At lower water contents though, the hydroxide mobility decreases faster than that of hydronium in comparable PEMs due to reduced dissociation and percolation as well as a break-down of structural diffusion Apart from the HEM, membranes in other ionic forms were investigated. Generally, all investigated AEM properties were found to change if the type of anion was exchanged. This comprises the degree of dissociation, conductivity, membrane morphology and sometimes even water diffusion. Remarkably, at low water contents, the ionic conductivity of the HEM sank below that of the halides, despite the much higher hydroxide mobility in aqueous solution. A gradual break-down of the hydroxide structural diffusion is probably responsible. Another noticeable observation was that the degree of dissociation for at least the bromide and chloride form membranes remains almost constant over a considerable water content range, suggesting the formation of associates consisting of several ions, which probably also exists in other ionic forms.
  • Thumbnail Image
    ItemOpen Access
    Einfluß relativistischer Effekte auf die Chemie von Platin und Thallium
    (2006) Karpov, Andrey; Jansen, Martin (Prof. Dr. Dr. h. c.)
    Im Rahmen der vorliegenden Arbeit wurden zum ersten Mal Verbindungen mit Platinid-Anionen dargestellt und charakterisiert. Die negativen Valenzzustände der Platinatome wurden sowohl theoretisch durch quantenchemische Analyse als auch experimentell mittels der Photoelektronenspektroskopie für die chemische Analyse (ESCA) bestätigt. Das Vorliegen der Platinid-Ionen liefert einen weiteren eindrucksvollen Beweis für die Bedeutung der relativistischen Effekte in der Chemie der schweren Elemente. Bemerkenswerte Parallelen der chemischen Eigenschaften des Edelmetalls Platins zu der Chemie der Hauptgruppenelemente (16. Gruppe) werden auf die relativistische Kontraktion des 6s-Orbitals zurückgeführt. Des Weiteren wurden Versuche zur Darstellung der isolierten, vermutlich diamagnetischen (Tl-)-Anionen durchgeführt, um den Einfluß des zweiten relativistischen Effekts, der Spin-Bahn-Aufspaltung, als ein chemisch relevantes Phänomen nachzuweisen. Mit dieser Zielsetzung wurden ternäre Systeme Alkalimetall-Thallium-Sauerstoff untersucht und dabei Verbindungen mit neuartigen Kristallstrukturen und Bindungsverhältnissen entdeckt.
  • No Thumbnail Available
    ItemMetadata only
    Internal precipitation of nitrides in iron-based alloys
    (Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2016) Steiner, Tobias; Mittemeijer, Eric J. (Prof. Dr. Ir.)
    X‐ray diffraction has become a standard method of microstructural analysis. However, in systems with a complex microstructure, the interpretation of the measured diffractograms may not be very straightforward. The influence of the precipitation of fine alloying element nitrides and the changes in precipitation morphology that occur upon continued nitriding on the shape and position of XRD peaks have been identified. The thus obtained quantitative model has been applied to a variety of precipitation systems and in general good agreement of predicted values and experimental results is found. The precipitation of finely distributed alloying element nitrides is the main strengthening mechanism in the diffusion zone of nitrided parts. Various alloying elements having an affinity for N show considerably different nitriding behavior. Cr shows a strong N‐affinity and therefore readily precipitates in the presence of N. However, Mo has a weak strength of interaction with N and reacts only slowly. In order to better understand the nitriding behavior of nitrided steels containing both Cr and Mo, the nitriding behavior of ternary Fe‐Cr‐Mo model alloys is investigated. The complex precipitation sequence of ternary mixed Cr-Mo-nitrides has been identified and the role of the Cr/Mo-ratio of the alloy is exposed. X‐ray diffraction has become a standard method of microstructural analysis. However, in systems with a complex microstructure, the interpretation of the measured diffractograms may not be very straightforward. The influence of the precipitation of fine alloying element nitrides and the changes in precipitation morphology that occur upon continued nitriding on the shape and position of XRD peaks have been identified. The thus obtained quantitative model has been applied to a variety of precipitation systems and in general good agreement of predicted values and experimental results is found.
  • Thumbnail Image
    ItemOpen Access
    Nitriding of Fe-Mo alloys and maraging steel : structure, morphology and kinetics of nitride precipitation
    (2012) Selg, Holger; Mittemeijer, E.J. (Prof. Dr. Ir.)
    The nitriding behaviour of Fe-Mo alloys was investigated in terms of sequence of nitride precipitation and its morphology and kinetics. Upon nitriding Fe-1at.% Mo alloy cubic Mo2N-type precipitates develop as platelets of several hundreds of nm length and thickness of a few atomic layers. Upon prolonged treatment a discontinuous precipitation reaction initiating at ferrite-matrix grain boundaries takes place replacing the submicroscopical Mo2N-type platelets by a lamellar ferrite/MoN (hexagonal crystal structure) microstructure. The morphology of the formed compound layer consisting of iron-nitride at the surface is strongly dependant of the defect density of the matrix: a compact layer develops in case of cold rolled specimen having a high defect density, whereas a plate-like morphology develops in case of recrystallized specimens having a low defect density. The development of the nitrogen concentration-depth profiles in maraging steel upon nitriding can be successfully modelled with a combined diffusion and precipitation kinetic model, provided that the role of excess nitrogen is recognized. The model employs the following fit parameters: the composition parameter of the nitride (expressing the presence of immobile, i.e. adsorbed excess nitrogen at the nitride/matrix interface), the solubility of nitrogen in the matrix (recognizing the presence of mobile, i.e. additionally in the strained matrix, dissolved excess nitrogen), the diffusion coefficient of nitrogen in the matrix and the solubility product of the nitride precipitates.
  • Thumbnail Image
    ItemOpen Access
    Electrochemical studies of MBE-grown CaF2/BaF2 heterolayers
    (2007) Matei, Ion; Maier, Joachim (Prof. Dr.)
    Ionic conductors, materials in which specific ions can migrate preferentially with high mobility, are of prime importance for electrochemical measurements, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors. This research study is focused on the dynamics of ion-conducting superlattices synthesized by molecular beam epitaxy (MBE) in which the interfaces are artificially tuned, with the aim of designing superior ionic conductors by controlling their interfaces. The interface is also expected to introduce lattice strain due to lattice mismatches and/or to change space charge distribution at the interfaces when superlattices of different ionic conductors are fabricated with a period of a few to a few hundred nanometres. Since the superlattice structure enables to tune the crystal structure to some extent, the ionic conductivity dependence on the structural parameters will also be investigated in this study. A qualitatively different conductivity behaviour is expected when the interface spacing is comparable to or smaller than the width of the space charge regions in comparatively large crystals: single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated. These results demonstrate mesoscopic ion conductivity effect in nanosystems (extremely thin films, nanocrystalline materials). In order to obtain more fundamental insight into the conductivity effects, some points still need to be clarified and are addressed in this study: (1) the detailed understanding of the defect chemical situation and the conductivity effects in parallel and in perpendicular direction to the interfaces; (2) the annealing effects; (3) theoretical model and numerical evaluation in periods of the mesoscopic situation (thinner than 50nm). To understand these effects in depth, electrical measurements on parallel (along the interfaces) and perpendicular (to the interfaces) configuration of the heterostructures as well as thermodynamic modelling are performed. Multilayers of CaF2/BaF2 have been prepared by molecular beam epitaxy on different substrates (Al2O3, Si, Nb-doped SrTiO3), with highly defined geometry, periodicity, interfacial spacings and layer sequence. The measured effective parallel conductivity (i.e. derived from the measurement of parallel conductance via the total thickness ~400nm) progressively increases with interfacial density. The purpose of the annealing experiment is to determine the anomalous decrease of the parallel conductivity of the sample as the annealing temperature increases. This can be associated with the existence of unstable dislocations not only at the interface, but also inside the layers that can be annealed out by thermal treatment. This results in a clear picture: in annealed samples there is a fluorite ( -ions) transfer from one phase to the other. In a non-annealed samples this is superimposed by charging of dislocations leaving vacancies in the vicinity. The heterostructures on conductive substrates were also prepared and allow us to carry out the conductivity measurement in the perpendicular direction to the interfaces. Mesoscopic size effects predict a decrease in the difference between parallel and perpendicular conductivity with the increase in the number of interfaces. This is very satisfactory as a parallel conductivity pronounces the highly conductive regions, while the perpendicular one emphasizes the less conductive regions. In this study, the thickness dependence of the layer conductivities is numerically calculated using both the Gouy-Chapman and the Mott-Schottky modes. The calculated concentration profile turns only out to be consistent with the charge density of the Mott-Schottky model if the frozen-in impurity profile is assumed to be moderately increased. In summary: 1. Heterolayers of CaF2/BaF2 have been prepared by molecular beam epitaxy (MBE) on different substrates (Al2O3, Nb-doped SrTiO3), with highly defined geometry, periodicity, interfacial spacings and layer sequence. 2. XRD and AFM measurements demonstrate that defined highly oriented heterostructures of CaF2/BaF2… can be prepared on different substrates. 3. The conductivity effects can be understood in terms of ionic space charge effects occurring as a consequence of a thermodynamic redistribution at equilibrium. 4. The influence of annealing effects on the resistance of the sample has been studied and analysed. Unstable dislocations appear to be charged by adsorption. 5. In this study, the thickness dependence of the layer conductivities is numerically calculated using both the Gouy-Chapman, and the Mott-Schottky models. In direct comparison to the experimental data, the modified Mott-Schottky mode (impurity profile with a gradient close to the interface) can reproduce the features of the experiments even in the mesoscopic range.