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Institute: search.filters.UBSInstitut.Institut für MaterialwissenschaftStart date: 2007End date: 2009

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    ItemOpen Access
    The effect of substrate orientation on the kinetics and thermodynamics of initial oxide-film growth on metals
    (2007) Reichel, Friederike; Mittemeijer, Eric J. (Prof. Dr. Ir.)
    This thesis addresses the effect of the parent metal-substrate orientation on the thermodynamics and kinetics of ultra-thin oxide-film growth on bare metals upon their exposure to oxygen gas at low temperatures (up to 650 K). For such thin oxide overgrowths on their metals, the resulting oxide-film microstructures often differ from those predicted by bulk thermodynamics, because of the relatively large contributions of interface and surface energies to the total energetics of the various metal-substrate/oxide-film systems. To this end, a model description has been developed to predict the thermodynamically stable microstructure of a thin oxide film grown on its bare metal substrate as function of the oxidation conditions and the substrate orientation. An amorphous state for ultra-thin oxide films grown on e.g. Al, Ti, Zr or Si can be thermodynamically, instead of kinetically, preferred up to a certain critical oxide-film thickness, because of the lower sum of surface and interface energies as compared to the corresponding crystalline modification. Beyond this critical oxide-film thickness, bulk thermodynamics will strive to stabilize the competing crystalline oxide phase. For Mg and Ni, the critical oxide-film thickness is less than 1 oxide monolayer and therefore the initial development of an amorphous oxide phase on these metal substrates is unlikely. Finally, for Cu and densely packed Cr and Fe metal surfaces, oxide overgrowth is predicted to proceed by the direct formation and growth of a crystalline oxide phase. Further, polished Al single-crystals with {111}, {100} and {110} surface orientations were introduced in an ultra-high vacuum system for specimen processing and analysis. After surface cleaning and annealing, the bare Al substrates have been oxidized by exposure to pure oxygen gas. During the oxidation, the oxide-film growth kinetics has been established by real-time in-situ spectroscopic ellipsometry. After the oxidation, the oxide-film microstructures were investigated by angle-resolved X-ray photoelectron spectroscopy and low energy electron diffraction. Finally, high-resolution transmission electron microscopic analysis was applied to study the microstructure and morphology of the grown oxide films on an atomic scale. The stoichiometric (i.e. Al2O3) oxide films grown on Al{111} are amorphous up to 450 K, whereas at higher temperatures epitaxial crystalline oxide films with a coherent metal/oxide interface develop. The oxide films grown on Al{100} are also overall stoichiometric, have uniform thicknesses and atomically flat metal/oxide interfaces. They are amorphous up to 400 K, but are transformed into crystalline gamma-Al2O3 upon annealing beyond a critical thickness. At more elevated temperatures (> 400 K), a crystalline Al2O3 film with a semi-coherent metal/oxide interface develops. For the crystalline gamma-Al2O3 overgrowth on Al{100}, an unexpected high lattice mismatch (> 15%) between the Al{100} substrate and the gamma-Al2O3 overgrowth is found with a semi-coherent metal/oxide interface. The oxide films grown on Al{110} for temperatures smaller than 550 K are also overall stoichiometric and amorphous. At more elevated temperatures (> 550 K), the original bare Al{110} surface becomes reconstructed at the onset of oxidation and {111}-facets develop. The kinetics of the oxide-film growth on the bare Al{100} and Al{110} substrates can be subdivided into a initial, very fast and a subsequent, very slow oxidation stage. For the oxidation of the bare Al{111} substrate up to 450 K, a distinction between an initial, very fast and a subsequent, very slow oxidation stage cannot be made. Instead, the initial oxide-film growth rate on Al{111} decreases only gradually with increasing oxidation time. The experimental growth curves for the thermal oxidation of Al single-crystals in the temperature regime of 350 – 600 K can be accurately described by considering the coupled currents of Al3+ cations and electrons in an uniform surface-charge field. As such, a gradual transformation of the initial amorphous oxide film on Al{100} into gamma-Al2O3, was observed with increasing oxidation temperature in the range of 350 – 600 K for Al{100}, as well as up to 450 K for Al{110}. Whereas, on Al{111}, the corresponding amorphous-to-crystalline transition was found to be more abrupt.
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    ItemOpen 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).
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    ItemOpen Access
    Interface stability in solid oxide fuel cells for intermediate temperature applications
    (2007) Solak, Nuri; Aldinger, Fritz (Prof. Dr. rer. nat)
    Strontium- and magnesium-doped lanthanum gallate (LSGM) perovskite-type compounds and doped ceria-based materials have recently been considered the most promising solid electrolytes for intermediate temperature solid oxide fuel cell (IT-SOFC) applications. While nickel metal is commonly used for the fabrication of cermet-type anodes, the rare earth nickelates, such as Sr-doped La2NiO4 (LSN), are recently developed high-performance cathode materials. For successful implementation in IT-SOFC, it is therefore essential to know the phase equilibria and thermodynamic properties for systems representing the solid electrolyte and electrode materials across their various combinations. This thesis aims to determine the phase equilibria and the thermodynamics of the relevant phases in the systems La-Sr-Ga-Mg-Ni-O, Ce-Gd-Sr-Ni-O, and Ce-Gd-La-Ni-O. Subsystems of these multi-component systems were thermodynamically modeled, based on the available literature and experimental data obtained from this work. The experimental studies were designed based on the calculated phase diagrams. A minimum number of compositions was chosen strategically to obtain a preliminary prediction of the phases in equilibrium in each constituent subsystem. Finally, the experimental and computational results were used to predict the compatibility/reactivity of IT-SOFC components under fabrication and/or operation conditions. Various experimental techniques were employed for determination of the phase equilibria such as Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX), X-ray Diffraction (XRD), Differential Scanning and Adiabatic Calorimetry, and Mass Spectrometry (MS). The CALPHAD-method (CALculation of PHAse Diagrams) and THERMOCALC software were used to obtain self-consistent sets of Gibbs energy functions. The following systems were investigated experimentally: La-Ni-O, La-Ga-Ni-O, La-Sr-Ni-O, La-Mg-Ni-O, La-Ga-Mg-Ni-O, La-Sr-Ga-Ni-O, La-Sr-Ga-Mg-Ni-O, Ce-Ni-O, Ce-Sr-O, Gd-Ni-O, Gd-Sr-O, Ce-Gd-Ni-O, Ce-Gd-Sr-O, Ce-Sr-Ni-O, Gd-Sr-Ni-O, Ce-Gd-Sr-Ni-O and Ce-Gd-La-Ni-O. Using results from this experimental work and data from the literature, the following systems were thermodynamically modeled: La-Ni-O, La-Ga-Ni-O, La-Sr-Ni-O, La-Mg-Ni-O, Ce-Ni-O, Ce-Sr-O, Gd-Ni-O and Gd-Sr-O. The systems, La-Ga-Mg-Ni-O, La-Sr-Ga-Ni-O, and Ce-Gd-Ni-O were extrapolated using parameters optimized from the constituent lower-order systems. In the La-Ni-O system, the enthalpy of formation, entropy and heat capacity of La3Ni2O7, La4Ni3O10, and LaNiO3, were determined experimentally for the first time using equilibration with the gas phase, adiabatic calorimetry and differential scanning calorimetry. In the La-Ga-Ni-O, La-Sr-Ni-O and La-Mg-Ni-O systems, extended solid solutions of La(Ga,Ni)O3, La2(Ni,Ga)O4, La4(Ni,Ga)3O10, (La,Sr)2NiO4, and La2(Ni,Mg)O4 were found, and the limits of their homogeneity ranges have been established for the first time. In addition, the compound LaNiGa11O19, with a magnetoplumbite-type structure was identified, which has not been reported in the literature to date. In the La-Ga-Mg-Ni-O system, the temperature dependence of the quasi-quaternary homogeneity range of La(Ga,Mg,Ni)O3 was determined. In the La-Sr-Ga-Ni-O system, a reaction was observed between LaGaO3 and LaSrNiO4 that formed a melilite-type La1-xSr1+xGa3O7+z, LaGaSrO4 and NiO phase. Similar reaction mechanisms were observed in the La-Sr-Ga-Mg-Ni-O system. Experiments in the Ce-Ni-O system were conducted in air as well as in a reducing atmosphere. It has been found that NiO does not react with CeO2. In the Ce-Sr-O system, the entropy and heat capacity of Sr2CeO4 were experimentally determined for the first time. In the Gd-Ni-O system a eutectic reaction was observed (liquid <=> B-Gd2O3 + NiO). The Gd-Sr-O system was modeled thermodynamically based on data from the literature and the experimentally determined homogeneity range on the Gd2O3-rich site. In the Ce-Sr-Ni-O system the solid solution of (Ce,Sr)2NiO4-z was determined. No reaction between NiO and SrCeO3 / Sr2CeO4 was found. Similarly, in the Ce-Gd-Ni-O system, no reaction was observed between (Ce,Gd)O2-z and NiO. In contrast, solid solutions of Sr(Ce,Gd)O3, Sr2(Ce,Gd)O4 and (Gd,Sr)2(Sr,Ce)O4 were determined in the Ce-Gd-Sr-O system. Also, an extended solid solution of (Gd,Sr)2NiO4 was found in the Gd-Sr-Ni-O system that does not exist in the quasi-binary sections, but is stable in higher-order systems only because a solid solution is formed. It has been also found that there is no NiO solubility in the Gd2SrO4 phase. It could be concluded that doped ceria-based materials are chemically compatible with NiO during conditions typical for both the fabrication and the operation of IT-SOFC’s, whereas LSGM-type electrolytes react with NiO under the fuel cell fabrication conditions. Moreover, although La2NiO4 is a high-performance cathode, it cannot be used in combination with LSGM- or CGO-type electrolytes, due to its reactivity with both of these materials under fabrication conditions.
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    ItemOpen Access
    Deposition of metal oxide thin films from solutions containing organic additives
    (2007) Lipowsky, Peter; Aldinger, Fritz (Prof. Dr.)
    In bio-inspired materials synthesis the principles of biomineralization are employed for the fabri­cation of materials with favourable functional properties at near-ambient temperature and with little expenditure: Organic templates direct the formation of inorganic matter. In aqueous so­lu­tion, zinc compounds with manifold morphologies are produced by ther­mal hy­dro­ly­sis of zinc nitrate in the presence of biomolecules like amino acids and dipeptides. In methanol, ZnO films are deposited by hydro­lysis of zinc acetate in the presence of polymers like poly­vi­nyl­pyrro­li­done (PVP) and poly­ethylene glycol. With PVP, particularly smooth, uniform and stable films are fa­bri­cated. Their thickness is determined by the deposition time and the polymer concen­tration. Various microscopic and spec­tro­scopic mea­sure­ments prove that the films consist of textured na­no­cry­stal­line zinc oxide. Selected properties of the films, such as their photo­lumi­nescence, are in­ve­sti­gated. Film de­po­si­tion is possible on substrates with organic coatings bearing certain func­tio­nal groups. Pat­terned films can be de­po­si­ted after local de­com­po­si­tion of the or­ga­nic coating by UV light. The mecha­nism of film formation is treated in detail. Like in bio­mineralization, an amor­phous transient state of mat­ter occurs before crystallization. This state suc­cumbs to ZnO nano­crystals, which either aggregate in solution or adsorb to the substrate. It is de­mon­stra­ted in what way the additive controls the reaction. Sulfonate-mo­di­fied po­­ly­­sty­­rene beads are coa­ted with zinc oxide and used as sacrificial temp­lates for the fabrication of zinc oxide hollow spheres. La­mi­nates of alternating layers of zinc oxide and poly(amino acids) are deposited and ex­hibit an im­proved mechanical per­for­mance com­pared to the monolithic zinc oxide.
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    Interrelationships of microstructure, stress and diffusion
    (2008) Kuru, Yener; Mittemeijer, Eric Jan (Prof. Dr. Ir.)
    Extensive research has been performed on thin metal films due to their interesting mechanical, electrical and magnetic properties. They can exhibit very high residual, internal stresses arising from the film growth and/or external effects. Apart from direct mechanical consequences, several processes such as grain growth and diffusion can be affected by these stresses and their gradients. As a result, it is of cardinal importance to measure and control the residual stresses in thin films. X-ray diffraction (XRD) is one of the most frequently used approaches for (residual) stress measurement. It is non-destructive, highly accurate (stress (variation) of some MPa can be detected) and the stress states of all crystalline phases in a layered structure can be obtained separately. Moreover, additional microstructural information, as the crystallographic texture, the density of crystalline defects, such as dislocations, and the crystal size can be acquired from the collected XRD data. This thesis is dedicated to the investigation of microstructural changes, residual stresses and interdiffusion in thin films by in-situ XRD. A focal point of interest is methodological aspects of in-situ measurements, which are discussed in detail in Chapter 2 and come to application in the following Chapters 3 and 4.
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    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.
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    The strength limits of ultra-thin copper films
    (2007) Wiederhirn, Guillaume; Arzt, Eduard (Prof. Dr.)
    Elucidating size effects in ultra-thin films is essential to ensure the performance and reliability of MEMS and electronic devices. In this dissertation, the influence of a capping layer on the mechanical behavior of copper (Cu) films was analyzed. Passivation is expected to shut down surface diffusion and thus to alter the contributions of dislocation- and diffusion-based plasticity in thin films. Experiments were carried out on 25 nm to 2 µm thick Cu films magnetron-sputtered onto amorphous-silicon nitride coated silicon (111) substrates. These films were capped with 10 nm of aluminum oxide or silicon nitride passivation without breaking vacuum either directly after Cu deposition or after a 500 °C anneal. The evolution of thermal stresses in these films was investigated mainly by the substrate curvature method betweeen -160 °C and 500 °C. Negligible differences were detected for the silicon nitride vs. the aluminum oxide passivated Cu films. The processing parameters associated with the passivation deposition also had no noticeable effect on the stress-temperature behavior of the Cu. However, the thermomechanical behavior of passivated Cu films strongly depended on the Cu film thickness. For films in the micrometer range, the influence of the passivation layer was not significant, which suggests that the Cu deformed mainly by dislocation plasticity. However, diffusional creep plays an increasing role with decreasing film thickness since it becomes increasingly difficult to nucleate dislocations in smaller grains. Size effects were investigated by plotting the stress at room temperature after thermal cycling as a function of the inverse film thickness. Between 2 µm and 200 nm, the room temperature stress was inversely proportional to the film thickness. The passivation exerted a strong effect on Cu films thinner than 100 nm by effectively shutting down surface diffusion mechanisms. Since dislocation processes were also shut off in these ultra-thin films, they exhibited purely elastic behavior in the measured temperature range. Their lack of plasticity was confirmed by in-situ TEM analysis, which revealed the presence of sessile parallel glide dislocations during thermal cycling. The stress plateau reported for films thinner than 100 nm was attributed to the fact that the thermal strain applied was insufficient to induce yielding. The highest stress value of 1.7 GPa measured at -150 °C is therefore a lower limit for the actual flow stress since even at this high stress the films remained elastic.
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    Wet chemical synthesis and characterization of organic/TiO2 multilayers
    (2008) Tucic, Aleksandar; Aldinger, Fritz (Prof. Dr.)
    The low-temperature deposition of oxide-base thin films from solution induced by organic templates is inspired by the process of biomineralization. Biominerals, i.e. inorganic materials synthesized by living organisms, show highly controlled micro- and nanostructures and in many cases physical properties superior to their manmade counterparts. In bio-inspired processes thin oxide films can be deposited from aqueous solutions on organic self-assembled monolayers or polyelectrolytes (PE). Liquid flow deposition (LFD) for the synthesis of TiO2 is based on the continuous flow of a precursor solution along the substrate. Whereas the concentration of the precipitating species within the reaction solution decreases with increasing deposition time, LFD provides a means to keep the concentration within the solution constant. Consequently, also the growth rate of the film is not affected by such aging effects. The deposition technique for the synthesis of PE layers is based on the electrostatic attraction between oppositely charged polyions layer by layer. The spontaneous sequential adsorption of dissolved anionic and cationic polyelectrolytes leads to the formation of ordered multilayer assemblies on a solid substrate. In this work, both techniques were combined in order to synthesize composite, multilayer PE/TiO2 thin films by wet chemical processing is investigated. The main aim is to mimic the architecture of nacre, which is present for instance in sea-shells in order to achive ceramic-based system with enhanced mechanical performances. The properties of the deposited films were characterized by means of SEM, XRD, AFM in order to establish the optimum parameters of the reaction process concerning the film homogeneity, thickness, structure and surface roughness. Polyelectrolyte (PE) films were synthesized applying the layer-by-layer deposition technique. The film thickness was determined applying AFM, TEM cross-sections and the quartz-crystal microbalance (QCM) technique. AFM investigations of the surface morphology of the PE layers showed densely packed globular aggregates of the deposited polymer. In order to investigate the dependence of the morphology and structure of the TiO2 films on the surface modification, depositions on Si substrates modified with polyelectrolytes (PE) were carried on. Thickness of the deposited TiO2 films, estimated by SEM cross-sections, is slightly higher than that of films deposited unmodified Si. Homogeneous films with the same microstructure were deposited on unmodified Si substrate and on PE-covered silicon substrates. Composite PE/TiO2 films were synthesised by applying the layer-by-layer deposition technique for the synthesis of PE layers and the static deposition techniques for synthesis of TiO2 layers. Auger electron spectroscopy (AES) was used to determine the concentrations of Ti and O (as the main constituent of the inorganic phase), C (as the main constituent of the organic phase) and Si (substrate), as a function of depth below the film surface. The AES profile clearly demonstrates the presence of a multilayered structure of alternating TiO2-enriched and C-enriched layers; i.e. it provides proof for the existence of an ordered composite structure of well-defined inorganic and organic layers. SEM, TEM and STEM cross-sections were used for the characterization of the microstructure of the multilayers. Analytical TEM investigations were performed using the energy-dispersive X-ray spectroscopy (EDX) and electron-energy loss spectroscopy (EELS) method. The nanoindentation technique was employed to determine the mechanical properties of obtained composite films, with the emphasis on their hardness and Young’s modulus. The comparison of the nanoindentation data obtained from the TiO2 single layer and the (PE/TiO2)2 and (PE/TiO2)3 multilayer samples reveals that the incorporation of organic layers improves the mechanical properties of CBD-derived TiO2 films. This enhanced mechanical performances can be attributed to the hardening by the differences in shear modulus between the organic and the inorganic phase and the interaction between the incorporated TiO2 particles and the PE, which is stronger then the one between the TiO2 particles within the oxide layers.
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    Thermodynamic optimization of the PbO-ZrO2-TiO2 (PZT) system and its application to the processing of composites of PZT ceramics and copper
    (2007) Cancarevic, Marija; Aldinger, Fritz (Prof. Dr. rer. nat.)
    PbZrxTi1-xO3 (PZT) or modified PZT solid solutions are of great interest for technological applications, which result from their piezoelectric, ferroelectric and pyroelectric properties. Although extensive experimental studies of the PZT system have been carried out in the past little attention was paid to phase equilibria and thermodynamics of the system Pb-Zr-Ti-O, which are important for the optimization of manufacturing and sintering conditions of the PZT ceramics as well as for tailoring their physical properties. In view of using copper for conductor lines in hybrid systems the compatibility between Cu and PZT become of special interest which requires an understanding also the multicomponent Cu-Pb-Zr-Ti-O system. The aim of this thesis was to obtain a consistent set of thermodynamic data for the Cu-Pb-Zr-Ti-O system, by means of the CALPHAD method, and then to calculate phase equlibria and chemical potential diagrams, which are relevant to the processing of actuators based on PZT ceramics and copper. The thermodynamic properties were described using the compound energy formalism (CEF) as well as the substitutional solution model for various solid phases and the associate model for the liquid phase, while the Redlich-Kister series were used to account for the interactions between species. Associate solution model adopted for the description of the liquid phase in the multicomponent Cu-Pb-Zr-Ti-O system was found to be superior for calculating the relevant phase equilibria in comparison with the two-sublattice ionic model, although both models can be successfully applied to the binary systems (Zr-O, Ti-O, Cu-O, Pb-O). The ternary Cu-Pb-O, Cu-Ti-O and Cu-Zr-O systems were assessed for the first time. The binary Cu-O and Pb-O systems were taken from literature with some modifications of the Pb-O system in the liquid phase region, while the binary Ti-O system was completely reassessed in the present work. The ternary Cu-Pb-O system shows a large liquid miscibility gap of peculiar shape, which connects to the miscibility gaps in each of the binary sub-systems. The ternary compound Cu2PbO2 was modelled as a stoichiometric compound. Its thermodynamic properties were estimated by experiments. In the modelling of the ternary Cu-Ti-O system the three ternary compounds, Cu3Ti3O, Cu2Ti4O and Cu3TiO4 were taken as stoichiometric compounds. In the Cu-Zr-O system the literature data show no existence of ternary compounds and it was experimentally proved in this work. The quasibinary PbO-ZrO2, PbO-TiO2 and ZrO2-TiO2 systems, the edges of the quasiternary PbO-ZrO2-TiO2 (PZT) system, were reassessed on the basis of most recent literature data. Thermodynamic properties of the end-members, ZrO2 and PbO have been taken from the literature, while those of TiO2 were evaluated in the present work. Due to limited experimental information, PbTiO3 (tetragonal and cubic forms) and PbZrO3 (cubic form) were considered as stoichiometric compounds in the PbO-TiO2 and PbO-ZrO2 systems, while the tetragonal and orthorhombic PbO solid solutions were described by a substitutional model. The perovskite solid solution series, PbZrxTi1-xO3 was modelled as high temperature cubic form using the substitutional model. Calculated phase diagrams, i.e., predicted phase relations in the multicomponent Cu-Pb-Zr-Ti-O system (isobaric-isothermal sections and chemical potential diagrams) were checked experimentally. Experimental points were chosen based on CALPHAD approach and all compositions were prepared by solid state reaction. For the verification of the phase relations and invariant reactions in the oxide rich part (Cu2O-CuO-PbO-TiO2 and Cu2O-CuO-PbO-ZrO2 isotherms) experiments were done in air. The investigation of the reactivity between Cu and PbTiO3, PbZrO3 or PZT in the solid state was performed at the carefully controlled partial pressure of oxygen using different kinds of buffers (Ni/NiO, Cu/Cu2O) at 1073 and 1173 K. Microstructural characterization of the samples was done by X-ray, DTA, SEM and EDX analysis. The database evaluated in this thesis is reliable for extrapolating calculations in the oxygen-rich part of the multicomponent Cu-Pb-Zr-Ti-O system, i.e., in the PbO-ZrO2-TiO2, Cu2O-CuO-PbO-ZrO2, Cu2O-CuO-PbO-TiO2 and Cu2O-CuO-ZrO2-TiO2 sub-systems. In addition, it can be used for prediction of reactions between metals and oxides, i.e. between Cu and PZT ceramics (Cu-PbO-ZrO2-TiO2 system), for which purpose it was mainly developed, but not for calculations in the Cu-Pb-Zr-Ti system and corresponding subsystems. The phase diagrams of the multicomponent Cu-Pb-Zr-Ti-O system calculated in this thesis were found to be well consistent with the experimentally obtained results, which clearly show the chemical stability of copper and PZT ceramic at temperatures below 1273 K in reducing atmosphere. The reactivity between copper and PbTiO3, PbZrO3 or PbZrxTi1-xO3 (x=0.44, 0.5) was not observed using the characterization methods applied in this work. Detailed investigation of the feasibility of using copper in piezoelectric actuators based on PZT solid solutions requires the application of additional characterization methods such as measurements of electrical and polarization properties. In addition, detailed investigation of the kinetics of PbO evaporation seems to be of fundamental importance in optimization of the processing conditions (temperature and time) for PZT-based ceramics.
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    Nitriding of iron-based alloys : residual stresses and internal strain fields
    (2007) Vives Díaz, Nicolás; Mittemeijer, Eric (Prof. Dr. Ir.)
    Different iron-chromium alloys (4, 8, 13 and 20 wt.% Cr) were nitrided in NH3/H2 gas mixtures at 580 ºC. The nitrided microstructure was investigated by X-ray diffraction, light microscopy, hardness measurements and scanning electron microscopy. Composition depth-profiles of the nitrided zone were determined by electron probe microanalysis. Various microstructures develop, depending on the nitriding conditions and the alloy composition (chromium content). The initial development of coherent, sub-microscopical CrN nitrides leads to a state of hydrostatic stress allowing the uptake of excess nitrogen dissolved in the ferrite matrix. It is shown that the outcome of the subsequent discontinuous coarsening process, which leads to a lamellar microstructure, has a close relation to the nitrogen supersaturation. As a result, the occurrence of a distinct gradient in hardness across the nitrided zone can be understood. Residual stress-depth profiles of the nitrided specimens were measured using the (X-ray) diffraction sin^2 (psi) method in combination with cumulative sublayer removals and correction for corresponding stress relaxations. Unusual, nonmonotonous changes of stress with depth could be related to the microstructure of the nitrided zone. A model description of the evolution of the residual stress as function of depth and nitriding time is given. Specimens of Fe-2.23 at.% V alloy were nitrided in a NH3/H2 gas mixture at 580 ºC. The nitrided microstructure was investigated by X-ray diffraction, and (conventional and high resolution) transmission electron microscopy. For specimens homogeneously nitrided during relatively short times no separate VN reflections developed but instead sidebands associated with ferrite reflections, most pronouncedly for the Fe-200 reflection, appeared. The diffractograms measured for the different specimens were interpreted as the result of coherent diffraction of the nitride platelets with the surrounding ferrite matrix, which is tetragonally distorted: the distorted ferrite matrix and the nitride platelets are represented by a single b.c.t. lattice, whereas the remaining part of the ferrite is described by a b.c.c. lattice. Analysis of the microstructure of the nitrided specimens using high resolution transmission electron microscopy investigations confirmed the existence of very tiny VN platelets, coherent with the surrounding matrix. Annealing at elevated temperatures (up to 750 ºC) after nitriding led to (moderate) coarsening of the nitride precipitates. The coarsening is associated with the occurrence of local disruptions/bending of lattice planes in the VN platelet. This effect causes that the VN platelets appear segmented in the diffraction contrast images. The specific changes in the X-ray diffractograms, as function of the stage of aging, could be consistently described as consequence of the transition from coherent to incoherent diffraction of the nitride platelets with reference to the surrounding ferrite matrix.