Universität Stuttgart
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Item Open Access A numerical method for the generation of hierarchical Poisson Voronoi microstructures applied in micromechanical finite element simulations : part I: method(2020) Schneider, Y.; Weber, U.; Wasserbäch, W.; Zielke, R.; Schmauder, S.; Tillmann, W.Poisson Voronoi (PV) tessellations as artificial microstructures are widely used in investigations of material deformation behaviors. However, a PV structure usually describes a relative homogeneous field. This work presents a simple numerical method for generating 2D/3D artificial microstructures based on hierarchical PV tessellations. If grains/particles of a phase cover a large size span, the concept of “artificial phases” can be used to create a more realistic size distribution. From case to case, detailed microstructural features cannot be directly achieved by commercial or free softwares, but they are necessary for a deep or thorough study of the material deformation behavior. PV tessellations created in our process can fulfill individual requirements from material designs. Another reason to use PV tessellations is due to the limited experimental data. Concerning the application of PV microstructures, four examples are given. The FE models and results will be presented in consecutive works, i.e. “part II: applications”.Item Open Access Binder-free V2O5 cathode for high energy density rechargeable aluminum-ion batteries(2020) Diem, Achim M.; Fenk, Bernhard; Bill, Joachim; Burghard, ZaklinaNowadays, research on electrochemical storage systems moves into the direction of post-lithium-ion batteries, such as aluminum-ion batteries, and the exploration of suitable materials for such batteries. Vanadium pentoxide (V2O5) is one of the most promising host materials for the intercalation of multivalent ions. Here, we report on the fabrication of a binder-free and self-supporting V2O5 micrometer-thick paper-like electrode material and its use as the cathode for rechargeable aluminum-ion batteries. The electrical conductivity of the cathode was significantly improved by a novel in-situ and self-limiting copper migration approach into the V2O5 structure. This process takes advantage of the dissolution of Cu by the ionic liquid-based electrolyte, as well as the presence of two different accommodation sites in the nanostructured V2O5 available for aluminum-ions and the migrated Cu. Furthermore, the advanced nanostructured cathode delivered a specific discharge capacity of up to ~170 mAh g-1 and the reversible intercalation of Al3+ for more than 500 cycles with a high Coulomb efficiency reaching nearly 100%. The binder-free concept results in an energy density of 74 Wh kg-1, which shows improved energy density in comparison to the so far published V2O5-based cathodes. Our results provide valuable insights for the future design and development of novel binder-free and self-supporting electrodes for rechargeable multivalent metal-ion batteries associating a high energy density, cycling stability, safety and low cost.Item Open Access In vivo shaping of inorganic functional devices using microalgae(2020) Santomauro, Giulia; Stiefel, Michael; Jeurgens, Lars P. H.; Bill, JoachimThe usage of biomineralization processes performed by living microalgae to create 3D nanostructured materials are advantageous compared to conventional synthesis routes. Exploitation of in vivo shaping using living cells leads to inorganic intricate biominerals, produced with low environmental impact. Since biomineralization processes are genetically controlled, the formation of nanostructured materials is highly reproducible. The shells of microalgae, like coccoliths, are particularly of great interest. This study shows the generation of mesoporous highly structured functional materials with induced optoelectronical properties using in vivo processes of the microalga species Emiliania huxleyi. It demonstrates the metabolically driven incorporation of the lanthanide terbium into the coccoliths of E. huxleyi as a route for the synthesis of finely patterned photoluminescent particles by feeding the microalgae with this luminescent element. The resulting green luminescent particles have hierarchical ordered pores on the nano‐ and microscale and may act as powerful tools for many applications; they may serve as imaging probes for biomedical applications, or in microoptics. The luminescent coccoliths combine a unique hierarchical structure with a characteristic luminescence pattern, which make them superior to conventional produced Tb doted material. With this study, the possibility of the further exploitation of coccoliths as advanced functional materials for nanotechnological applications is given.Item Open Access Hydrosilylation of alkynes under continuous flow using polyurethane‐based monolithic supports with tailored mesoporosity(2022) Acikalin, Hande; Panyam, Pradeep K. R.; Shaikh, Abdul Wasif; Wang, Dongren; Kousik, Shravan R.; Atanasova, Petia; Buchmeiser, Michael R.Non‐porous polyurethane‐based monoliths are prepared under solvent‐induced phase separation conditions. They possess low specific surface areas of 0.15 m2 g-1, pore volumes of 1 µL g-1, and a non‐permanent, solvent‐induced microporosity with pore dimensions ≤1 nm. Mesoporosity can be introduced by varying the monomers and solvents. A tuning of the average solubility parameter of the solvent mixture by increasing the macroporogen content results in a decrease in the volume fraction of micropores from 70% to 40% and an increase in the volume fraction of pores in the range of 1.7-9.6 nm from 22% to 41% with only minor changes in the volume fraction of larger mesopores in the range of 9.6–50 nm. The polymeric monoliths are functionalized with quaternary ammonium groups, which allowed for the immobilization of an ionic liquid that contained the ionic Rh‐catalyst [1‐(pyrid‐2‐yl)‐3‐mesityl)‐imidazol‐2‐ylidene))(η4‐1,5‐cyclooctadiene)Rh(I) tetrafluoroborate]. The supported catalyst is used in the hydrosilylation of 1‐alkynes with dimethylphenylsilane under continuous flow using methyl‐tert‐butyl ether as second liquid transport phase. E/Z‐selectivity in hydrosilylation is compared to the one of the analogous biphasic reactions. The strong increase in Z‐selectivity is attributed to a confinement effect provided by the small mesopores.Item Open Access Ab initio surface free energies of tungsten with full account of thermal excitations(2022) Forslund, Axel; Ruban, AndreiThe surface free energies of seven different facets of tungsten (W) are obtained up to the melting point with full account of all the relevant thermal excitations; in particular, thermal atomic vibrations, electronic excitations, and their mutual coupling. The latter is done using ab initio molecular dynamics simulations coupled with the thermodynamic integration technique. In this way, the calculations contain almost no error but the one related to the used exchange-correlation functional, which makes the results truly first principles. The obtained results are compared with previous quasiharmonic calculations for the surface free energies of W and experimental data. The anharmonic contribution is, as expected, important for open surfaces at high temperatures, which leads to a temperature dependence of the surface energy anisotropy. The calculated Wulff shapes and surface energies are in excellent agreement with experimental data close to the melting point, where the crystalline structure of the surface layers is destroyed by a dramatic mobility of the atoms there.Item Open Access Thermally-activated dislocation mobility in bcc metals : an accelerated molecular dynamics study(2021) Grabowski, Blazej; Zotov, NikolayPlastic deformation in metals is controlled by dislocation density and mobility. In bcc metals the mobility of screw dislocations, which takes place by temperature- and stress-driven nucleation of critical kink-pairs, is most essential for deformation. However, the critical resolved shear stresses at low temperatures, as determined from molecular dynamics (MD) simulations performed at constant strain rate, are typically 2–3 times larger than the yield stresses measured experimentally. Here, an accelerated MD procedure is developed and employed to investigate the onset of dislocation mobility in the prototypical system bcc Nb. The method combines constant strain and temperature MD with hyperdynamics, using a bond-boost potential. We demonstrate, with a careful statistical analysis, that the method enables nucleation of critical kink-pairs and the determination of the Gibbs energy of kink-pair formation from accelerated MD simulations at experimentally-measured shear stresses.Item Open Access Adjustable polystyrene nanoparticle templates for the production of mesoporous foams and ZnO inverse opals(2020) Abitaev, Karina; Qawasmi, Yaseen; Atanasova, Petia; Dargel, Carina; Bill, Joachim; Hellweg, Thomas; Sottmann, ThomasThe manifold applications of porous materials, such as in storage, separation, and catalysis, have led to an enormous interest in their cost-efficient preparation. A promising strategy to obtain porous materials with adjustable pore size and morphology is to use templates exhibiting the appropriate nanostructure. In this study, close-packed polystyrene (PS) nanoparticles, synthesized by emulsion polymerization, were used to produce porous PS and ZnO inverse opals. The size and distribution of the polystyrene nanoparticles, characterized by dynamic light scattering (DLS), small-angle neutron scattering (SANS), and scanning electron microscopy (SEM), were controlled via the concentration of sodium dodecyl sulfate (SDS). Systematic measurements of the water/styrene-interfacial tension show that the critical micelle concentration (CMC) of the ternary water–styrene–SDS system, which determines whether monodisperse or polydisperse PS particles are obtained, is considerably lower than that of the binary water–SDS system. The assemblies of close-packed PS nanoparticles obtained via drying were then studied by small-angle X-ray scattering (SAXS) and SEM. Both techniques prove that PS nanoparticles synthesized above the CMC result in a significantly unordered but denser packing of the particles. The polystyrene particles were subsequently used to produce porous polystyrene and ZnO inverse opals. While the former consists of micrometer-sized spherical pores surrounded by extended open-cellular regions of mesopores (Rpore ≈ 25 nm), the latter are made of ZnO-nanoparticles forming a structure of well-aligned interconnected pores.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 Structural and magnetic properties of newly found BaFeO2.667 synthesized by oxidizing BaFeO2.5 obtained via nebulized spray pyrolysis(2021) Wollstadt, Stephan; Ikeda, Yuji; Sarkar, Abhishek; Vasala, Sami; Fasel, Claudia; Alff, Lambert; Kruk, Robert; Grabowski, Blazej; Clemens, OliverA new vacancy-ordered perovskite-type compound Ba3Fe3O8 (BaFeO2.667) was prepared by oxidizing BaFeO2.5 (P21/c) with the latter compound obtained by a spray-pyrolysis technique. The structure of Ba3Fe3O8 was found to be isotypic to Ba3Fe3O7F (P21/m) and can be written as Ba3Fe3+2Fe4+1O8. Mössbauer spectroscopy and ab initio calculations were used to confirm mixed iron oxidation states, showing allocation of the tetravalent iron species on the tetrahedral site and octahedral as well as square pyramidal coordination for the trivalent species within a G-type antiferromagnetic ordering. The uptake and release of oxygen was investigated over a broad temperature range from RT to 1100 °C under pure oxygen and ambient atmosphere via a combination of DTA/TG and variable temperature diffraction measurements. The compound exhibits a strong lattice enthalpy driven reduction to monoclinic and cubic BaFeO2.5 at elevated temperatures.Item Open Access A numerical method to improve the representativeness of real microstructure cut-outs applied in finite element simulations(2021) Schneider, Yanling; Wasserbäch, Werner; Schmauder, Siegfried; Zhou, Zhangjian; Zielke, Reiner; Tillmann, WolfgangTo improve the representativeness of a real microstructural cut-out for modeling purposes, a numerical method named as “boundary pixel color alteration (BPCA)” is presented to modify measured 2D microstructure cut-outs. Its physical background is related to the phase growth. For the application, the precondition is that the representativeness of the microstructure is already satisfied to a certain extent. This method resolves the problem that the phase composition of a small cut-out can have a large discrepancy to the real one. The main idea is to change the pixel color among neighboring pixels belonging to different phases. Our process simultaneously maintains most of the characteristics of the original morphology and is applicable for nearly all kinds of multi-phase or polycrystalline metallic alloys, as well. From our axisymmetric finite element (FE) simulations (ABAQUS ) applied with 2D real microstructures, it shows that the volume ratios of microstructural phases, as a function of the structure position to the symmetric axis, converge to phase area ratios in the 2D cut-out, even though the axisymmetric element volume is position dependent. A mathematical proof provides the reason for the aforementioned convergence. As examples to achieve real compositions and to numerically prove the aforementioned convergence, four different materials including multiphase polycrystals are implemented. An improvement of the predicted FE result is presented for the application of a modified microstructure (with a higher representativeness) compared to the original one.