Universität Stuttgart
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Item Open Access Reinforcement of precursor-derived Si-(B-)C-N ceramics with carbon nanotubes(2005) Katsuda, Yuji; Aldinger, Fritz (Prof. Dr.)Incorporation of carbon nanotubes (CNTs) into the precursor-derived Si-(B-)C-N ceramics has been investigated for the reinforcement of the materials. Different types of CNTs consisting of multi-wall (MW) and single-wall (SW) were examined as the reinforcement of the Si-(B-)C-N ceramics to make a comparison of the effect. Mechanical properties demonstrated in the Si-(B-)C-N/CNT nanocomposites have been discussed in connection with their microstructural features characterized by scanning electron (SEM) and transmission electron microscope (TEM). Other material properties of the nanocomposites as revealed on the thermal stability and the crystallization behavior have been also considered in relation to the microstructural characteristics of the nanocomposites. Dense Si-C-N/CNT nanocomposites containing different types of MWCNTs were successfully prepared by casting of a mixture of MWCNTs and a liquid precursor polymer followed by cross-linking and thermolysis. In these processes, the sonication for deagglomeration and dispersion of CNTs in the precursor polymer as well as the thermolysis condition for ceramization of the cross-linked precursor/CNT nanocomposites was examined to obtain homogeneously CNT distributed Si-C-N ceramics. Fracture toughness behavior of the Si-C-N/CNT nanocomposites has been evaluated by a thermal loading technique on the disc shaped materials. The results reveal a dependence of the fracture toughness on the type of the MWCNTs. The MWCNTs showing high integrity in the tube structure exhibit a remarkable increase in the fracture toughness at the CNT content of 1 – 2 mass %, whereas the other ones possessing amorphous nature exhibits no effect. The microstructural analyses at the fracture surfaces have demonstrated different features of CNTs between both nanocomposites, where pulling out and breaking of CNTs are considered to be reasons for the observed fracture toughness increase. No significant influences observed on the material properties of the Si-C-N/CNT nanocomposites besides the toughening indicates that CNTs can simply work as the reinforcement for the Si-C-N ceramics. SWCNTs incorporation into the Si-C-N materials has revealed toughening effect with similar microstructural features to the MWCNT reinforced Si-C-N nanocomposites. In this system, it was found that the deagglomeration and debundle of the SWCNTs are major issues to make the best use of SWCNTs as the reinforcements. Concerning the Si-B-C-N/CNT nanocomposites, preparation processes via a casting and a warm pressing from different types of boron-containing precursors have been investigated to produce rigid MWCNT nanocomposites. The observed pulling out and breaking CNTs structure at the fracture surfaces of the Si-B-C-N/CNT nanocomposites indicate the toughening effect of CNTs similar to Si-C-N/CNT ceramics. Moreover, the interaction between CNTs and the matrix has appeared to be changed with increasing thermolysis temperature. However, the crystallization of the Si-B-C-N matrix and the deterioration of thermal stability have been disclosed in the Si-B-C-N/CNT nanocomposites. It is revealed that embedded CNTs have an effect to accelerate or to generate nucleation sites for the crystallization of Si-B-C-N matrix.Item Open Access Extremely narrow superconducting band with crystal spin 3/2h in LaH10(2023) Krüger, EkkehardItem Open Access Experimental and computational phase studies of the ZrO2-based systems for thermal barrier coatings(2006) Wang,Chong; Aldinger, Fritz (Prof.)The ZrO2-based materials are practically important as the thermal barrier coatings (TBC) for high temperature gas turbines, due to their low thermal conductivity, high temperature thermal stability and excellent interfacial compatibility. Studies of the phase equilibira, phase transformation, and thermodynamics of the ZrO2-based systems can provide the necessary basic knowledge to develop the next generation TBC materials. In the thesis, the systems ZrO2 - HfO2, ZrO2 - LaO1.5, ZrO2 - NdO1.5, ZrO2 - SmO1.5, ZrO2 - GdO1.5, ZrO2 - DyO1.5, ZrO2 - YbO1.5 and ZrO2 - GdO1.5 - YO1.5 were experimentally studied. The samples were prepared by the chemical co-precipitation method, with aqueous solutions Zr(CH3COO)4, HfO(NO3)2, and RE(NO3)3×xH2O (RE=La, Nd, Sm, Gd, Dy, Yb) as starting materials. Various experimental techniques, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), differential thermal analysis (DTA), and high temperature calorimetry were employed to study the phase transformation, phase equilibria between 1400 and 1700°C, heat content and heat capacity of the materials. A lot of contradictions in the literature were resolved and the phase diagrams were reconstructed.Item Open Access Low temperature sintering additives for silicon nitride(2003) Matovic, Branko; Aldinger, Fritz (Prof. Dr.)Pressureless sintering of Si3N4 with two new additives based on the Li2O-Y2O3 system (LiYO2) and on the Li2O-Al2O3-SiO2 system (LiAlSiO4) were investigated in this study. Experiments were conducted in the areas of powder processing, sintering optimization, phase transformation characterization and microstructural development. Sintered materials were characterized by fracture toughness and thermal diffusivity measurements. The experimental results are summarized in the following: Using three different mechanical mixing processes (attrition milling, ball milling and vibratory milling) for the introduction of additives (LiYO2) into Si3N4 powders, the best results are obtained for attrition milling. This method yields a good dispersion of the additive powder in fine unagglomerated Si3N4 without contamination. It also yields good sintering characteristics. For the LiYO2 system, the densification depends largely on the content of sintering additive. A larger amount of additive means a higher volume of liquid phase, which is favorable for efficient particle rearrangement resulting in higher values of density rate. In case of the LiAlSiO4 additive, it is found that the densification is less dependent on the additive content. The overall sintering kinetics at the low temperatures is less retarded when using the LiYO2 additive in comparison to the LiAlSiO4 additive, resulting in higher densities obtained at lower temperatures and shorter annealing times. With prolonged heating time, the differences in the degree of densification become smaller. The kinetics of phase transformation in the both systems are found to be of first order. In the LiYO2 system, the transformation rate constant increases with increasing additive content. While the opposite behavior is noticed in case of the LiAlSiO4 additive, i.e. the rate constant decreases with higher additive content. The phase transformation is always completed at a later stage than the densification. The lag between the two phenomena in the sintering process is more pronounced with the Li2O-Al2O3-SiO2 additive system. The alpha to beta Si3N4 transformation is accompanied by grain growth. Upon prolongation of the annealing time the grain size and the morphology of the growing beta-Si3N4 particles are significantly changed from equiaxed to elongated. The grain growth becomes anisotropic, leading to rod-like betaƒ{Si3N4 crystals. The growth rate is higher in the LiYO2 system than in the LiAlSiO4 system. At 1600„aC, the microstructure of Si3N4 ceramics sintered with both the additives is characterized by a homogeneous distribution of elongated beta Si3N4 grains and glassy phase located in thin layers at grain boundaries and at triple points. The maximum values obtained for fracture toughness are 6.8 and 6.2 for the materials sintered with LiYO2 and LiAlSiO4 additives, respectively, at 1600„aC for 8 h. The higher value of fracture toughness in the LiYO2 system is attributed to its microstructure with a higher aspect ratio of the elongated beta-Si3N4 grains. Thermal conductivity of the material sintered with the LiYO2 additive is higher in comparison to that sintered with LiAlSiO4 additive. In the LiAlSiO4 system, partial dissolution of Al3+ in the beta-Si3N4 grains results in increasing phonon scattering and hence decreases the thermal conductivity.Item Open Access Finite-temperature interplay of structural stability, chemical complexity, and elastic properties of bcc multicomponent alloys from ab initio trained machine-learning potentials(2021) Gubaev, Konstantin; Ikeda, Yuji; Tasnádi, Ferenc; Neugebauer, Jörg; Shapeev, Alexander V.; Grabowski, Blazej; Körmann, FritzAn active learning approach to train machine-learning interatomic potentials (moment tensor potentials) for multicomponent alloys to ab initio data is presented. Employing this approach, the disordered body-centered cubic (bcc) TiZrHfTax system with varying Ta concentration is investigated via molecular dynamics simulations. Our results show a strong interplay between elastic properties and the structural ω phase stability, strongly affecting the mechanical properties. Based on these insights we systematically screen composition space for regimes where elastic constants show little or no temperature dependence (elinvar effect).Item Open Access Molecular dynamics simulations of screw dislocation mobility in bcc Nb(2021) Zotov, Nikolay; Grabowski, BlazejThe screw dislocation mobility in bcc Nb has been studied by molecular dynamics (MD) simulations at different strain rates and temperatures using an embedded-atom method (EAM) potential. Static properties of the screw dislocation, as determined with the EAM potential, are in agreement with previous density-functional-theory calculations. The elementary slip plane of the screw dislocation remains (110) for all studied strain rates (in the range 6.3 × 107-6.3 × 109 s-1) and temperatures (5 to 550 K). However, the consecutive cross-slip on different symmetry-equivalent (110) planes leads to an effective glide on (112) planes. It is demonstrated that the screw dislocation trajectories, velocities and waviness of the screw dislocation depend on the crystallographic indices, (110) or (112), of the maximum resolved shear stress plane. The waiting time for the start of the screw dislocation motion increases exponentially with decreasing strain rate, substantiating the necessity to apply in future accelerated MD techniques in order to compare with macroscopic stress-strain experiments.Item Open Access High rate electrochemical dissolution of iron-based alloys in NaCl and NaNO3 electrolytes(2002) Wagner, Thomas; Mittemeijer, Eric Jan (Prof. Dr. Ir.)With the investigations presented in this work, the reaction mechanisms and principles of steels upon the high rate electrochemical dissolution in activating NaCl electrolytes and passivating NaNO3 electrolytes are revealed and the role of anodic surface films developing at the substrate surface is included in schematic dissolution models. For the development of accurate dissolution models, mask-less Electrochemical Machining (ECM) experiments with the flow channel cell at high electrolyte flow rates (up to 7 m/s) and current densities up to 70 A/cm2 were carried out in combination with following ex situ surface analysis. On the basis of these experiments a satisfactory dissolution model for heterogeneous steel substrates is presented, with special respect to the influence of local turbulences in the flowing electrolyte. To specify and characterize the electrochemical behaviour of the examined electrolyte / substrate combination, polarization measurements with the rotating cylinder electrode(RCE) are presented.Item Open Access Mechanische Spektroskopie an dünnen Kupferschichten(2001) Hagen, Joachim von der; Arzt, Eduard (Prof. Dr. phil.)In dieser Arbeit wurden erstmalig dünne Kupferschichten, die für die Mikroelektronik von zunehmendem technologischen Interesse sind, systematisch mit Hilfe der mechanischen Spektroskopie untersucht. Dabei handelt es sich um eine empfindliche und zerstörungsfreie Messmethode, mit der man Informationen über Defektstrukturen in der Schicht und in der Substrat/Schicht-Grenzfläche erhalten kann. Darüber hinaus wurden die spektroskopischen Ergebnisse ebenfalls erstmalig vor dem Hintergrund der thermomechanischen Eigenschaften dünner Schichten diskutiert. Die Voraussetzung hierfür wurde durch eine apparative Neuentwicklung geschaffen. Bei den untersuchten Systemen handelte es sich um Kupferschichten auf den Trägermaterialien Silizium und Saphir. Die Messungen beruhen auf der Dämpfung von Eigenschwingungen zwischen 20 bis 530°C. Daneben wird die Eigenfrequenz gemessen, aus der man prinzipiell Rückschlüsse auf den E-Modul von Schicht und Substrat, bzw. auf die Haftung ziehen kann. Es wurden vor allem passivierte und unpassivierte Kupferschichten zwischen 1 und 4 µm auf Siliziumsubstraten untersucht. Kupferschichten auf Silizium-Substraten zeigen ein breites, bei Temperaturzyklen stabiles, Dämpfungsmaximum zwischen 280 und 380°C. Mit zunehmender Schichtdicke wächst dessen Intensität, während sich seine Position zu höheren Temperaturen verschiebt. Auf Grund seiner Aktivierungsenthalpie kann dieses Maximum auf Versetzungsbewegungen zurückgeführt werden. Man nimmt an, dass die Versetzungen thermisch aktivierte, lokale Bewegungen um ihre Gleichgewichtslage ausführen, während sie an ihren Enden fest verankert sind. Als Verankerungspunkte sind vor allem die Grenz-, bzw. die Oberfläche, sowie weitere Versetzungen anzusehen. Die Relaxationsparameter der Dämpfungsmaxima zeigen, dass Einengungseffekte die Mobilität der beweglichen Versetzungssegmente maßgeblich bestimmen, wie es im Zusammenhang mit den hohen inneren Spannungen in dünnen Schichten diskutiert wird.Item Open Access Segregation and phase transformations at interfaces(2004) López, Gabriel Alejandro; Mittemeijer, Eric Jan (Prof. Dr. Ir.)Important properties of metallic materials are strongly influenced by the behavior of interfaces, e.g. grain boundaries and free surfaces. Therefore the understanding of processes, which can change this behavior, have become of great importance from both the theoretical and the practical point of view. To these processes belong the grain boundary and surface segregation, the discontinuous precipitation as well as the grain boundary wetting. A particular goal of this work was to investigate the grain boundary and surface segregation in the Cu-Bi system under exactly the same conditions. The diffusivities of moving grain boundaries was determined in the Cu-In system applying, for the first time, a local analysis of growth kinetics of the discontinuous precipitation reaction. The Al-MG and Al-Zn systems were studied systematically regarding grain boundary wetting, in order to find an explanation for the enormous superplastic behavior of alloys based on these systems. The grain boundary and surface segregation were studied in the Cu-Bi system under identical conditions by using a special sample preparation procedure. After annealing of the samples at temperatures between 1073 and 1223 K segregation of Bi at grain boundaries and internal free surfaces in Cu bicrystals was accomplished. For the first time, Bi segregation at free surfaces was determined under equilibrium conditions. The segregation of Bi at the free surfaces was clearly stronger than at the grain boundaries. The morphology and kinetics of the discontinuous precipitation reaction were comprehensively studied in a Cu-4.5 At,% in alloy. Special attention was given to the determination of the concentration gradients remaining in the solute-depleted matrix. The grain boundary diffusion coefficient was determined applying a local interpretation of growth kinetics and thus the discrepancy between two differently models was eliminated. Finally the wetting behavior in Al-Mg and Al-Zn alloys was studied by a metallographic investigation. With this purpose cross-sections of samples with different compositions, which had been annelaed at different temperatures, were prepared and examined afterwards. The wetting behavior plays a substantial role regarding the mechanical characteristics of these alloys. For the first time, the formation of a second solid layer at the grain boundaries was disccussed in terms of wetting by a solid phase. During the accomplished investigations the possibilities of the analytic transmission electron microscopy were mainly used. Concentration profiles within nm range could be determined thanks to the high resolution of this technique. Furthermore Auger electron spectroscopy, light and scanning electron microscopy, as well as microanalysis and X-ray diffraction analyses were used, in order to achieve the above-mentioned goals of this work.Item Open Access Precipitation of nitrides in iron-based binary and ternary alloys; influence of defects and transformation-misfit stresses(2015) Akhlaghi, Maryam; Mittemeijer, Eric Jan (Prof. Dr. Ir.)The initial microstructure of the unnitrided specimen has a significant influence on the nitriding response of binary Fe-Me (Me: Mo or Al) alloys specimens. This effect was not investigated until now for the case of nitrided ternary Fe-Me1-Me2 alloys, the role of the initial microstructure was studied upon nitriding Fe-4.1 at.% Cr-7.9 at.% Al specimens. To this end, the recrystallized and cold-rolled specimens were nitrided at low nitriding temperature of 400 °C. Upon precipitation of misfitting coherent nitrides during nitriding of thin-foils of binary Fe-Me (Me: Cr and V) alloys, a hydrostatic tensile lattice-stain component results from the elastic accommodation of volume misfit of nitrides and ferrite matrix. The change of the ferrite-matrix lattice parameter can be traced upon precipitation of the nitrides by X-ray diffraction measurements. The theory originally developed for the case of imperfections (by Eshelby) in solids can be applied for quantitatively describing the lattice-parameter changes of the matrix, the nitrides and the aggregate (matrix+ nitrides) as function of volume fraction and type of nitrides.