03 Fakultät Chemie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/4
Browse
8 results
Search Results
Item Open Access The Fermi energy as common parameter to describe charge compensation mechanisms : a path to Fermi level engineering of oxide electroceramics(2023) Klein, Andreas; Albe, Karsten; Bein, Nicole; Clemens, Oliver; Creutz, Kim Alexander; Erhart, Paul; Frericks, Markus; Ghorbani, Elaheh; Hofmann, Jan Philipp; Huang, Binxiang; Kaiser, Bernhard; Kolb, Ute; Koruza, Jurij; Kübel, Christian; Lohaus, Katharina N. S.; Rödel, Jürgen; Rohrer, Jochen; Rheinheimer, Wolfgang; De Souza, Roger A.; Streibel, Verena; Weidenkaff, Anke; Widenmeyer, Marc; Xu, Bai-Xiang; Zhang, HongbinChemical substitution, which can be iso- or heterovalent, is the primary strategy to tailor material properties. There are various ways how a material can react to substitution. Isovalent substitution changes the density of states while heterovalent substitution, i.e. doping, can induce electronic compensation, ionic compensation, valence changes of cations or anions, or result in the segregation or neutralization of the dopant. While all these can, in principle, occur simultaneously, it is often desirable to select a certain mechanism in order to determine material properties. Being able to predict and control the individual compensation mechanism should therefore be a key target of materials science. This contribution outlines the perspective that this could be achieved by taking the Fermi energy as a common descriptor for the different compensation mechanisms. This generalization becomes possible since the formation enthalpies of the defects involved in the various compensation mechanisms do all depend on the Fermi energy. In order to control material properties, it is then necessary to adjust the formation enthalpies and charge transition levels of the involved defects. Understanding how these depend on material composition will open up a new path for the design of materials by Fermi level engineering.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 Asymmetric Rh diene catalysis under confinement : isoxazole ring‐contraction in mesoporous solids(2024) Marshall, Max; Dilruba, Zarfishan; Beurer, Ann‐Katrin; Bieck, Kira; Emmerling, Sebastian; Markus, Felix; Vogler, Charlotte; Ziegler, Felix; Fuhrer, Marina; Liu, Sherri S. Y.; Kousik, Shravan R.; Frey, Wolfgang; Traa, Yvonne; Bruckner, Johanna R.; Plietker, Bernd; Buchmeiser, Michael R.; Ludwigs, Sabine; Naumann, Stefan; Atanasova, Petia; Lotsch, Bettina V.; Zens, Anna; Laschat, SabineCovalent immobilization of chiral dienes in mesoporous solids for asymmetric heterogeneous catalysis is highly attractive. In order to study confinement effects in bimolecular vs monomolecular reactions, a series of pseudo‐C2‐symmetrical tetrahydropentalenes was synthesized and immobilized via click reaction on different mesoporous solids (silica, carbon, covalent organic frameworks) and compared with homogeneous conditions. Two types of Rh‐catalyzed reactions were studied: (a) bimolecular nucleophilic 1,2‐additions of phenylboroxine to N‐tosylimine and (b) monomolecular isomerization of isoxazole to 2H‐azirne. Polar support materials performed better than non‐polar ones. Under confinement, bimolecular reactions showed decreased yields, whereas yields in monomolecular reactions were only little affected. Regarding enantioselectivity the opposite trend was observed, i. e. effective enantiocontrol for bimolecular reactions but only little control for monomolecular reactions was found.Item Open Access 3D sub-nanometer analysis of glucose in an aqueous solution by cryo-atom probe tomography(2021) Schwarz, T. M.; Dietrich, C. A.; Ott, J.; Weikum, E. M.; Lawitzki, R.; Solodenko, H.; Hadjixenophontos, E.; Gault, B.; Kästner, J.; Schmitz, G.; Stender, P.Atom Probe Tomography (APT) is currently a well-established technique to analyse the composition of solid materials including metals, semiconductors and ceramics with up to near-atomic resolution. Using an aqueous glucose solution, we now extended the technique to frozen solutions. While the mass signals of the common glucose fragments CxHy and CxOyHz overlap with (H2O)nH from water, we achieved stoichiometrically correct values via signal deconvolution. Density functional theory (DFT) calculations were performed to investigate the stability of the detected pyranose fragments. This paper demonstrates APT’s capabilities to achieve sub-nanometre resolution in tracing whole glucose molecules in a frozen solution by using cryogenic workflows. We use a solution of defined concentration to investigate the chemical resolution capabilities as a step toward the measurement of biological molecules. Due to the evaporation of nearly intact glucose molecules, their position within the measured 3D volume of the solution can be determined with sub-nanometre resolution. Our analyses take analytical techniques to a new level, since chemical characterization methods for cryogenically-frozen solutions or biological materials are limited.Item Open Access Relationship between phase fractions and mechanical properties in heat‐treated laser powder‐bed fused co‐based dental alloys(2020) Kobylinski, Jonas von; Hitzler, Leonhard; Lawitzki, Robert; Krempaszky, Christian; Öchsner, Andreas; Werner, EwaldMetal additive manufacturing of dental prostheses consisting of cobalt−chromium−tungsten (Co-Cr-W) alloys poses an alternative to investment casting. However, metal additive manufacturing processes like Laser Powder‐Bed Fusion (LPBF) can impact the elastic constants and the mechanical anisotropy of the resulting material. To investigate the phase compositions of mechanically different specimens in dependence of their postprocessing steps (e. g. heat treatment to relieve stress), the current study uses X‐ray Diffraction (XRD), Electron BackScatter Diffraction (EBSD), and Transmission Electron Microscopy (TEM) for phase identification. Our studies connect plastic deformation of Remanium star CL alloy with the formation of the hexagonal ϵ‐phase and heat treatment with the formation of the D024‐phase, while partially explaining previously observed differences in Young's moduli.Item Open Access One‐step thermal gradient‐ and antisolvent‐free crystallization of all‐inorganic perovskites for highly efficient and thermally stable solar cells(2022) Byranvand, Mahdi Malekshahi; Kodalle, Tim; Zuo, Weiwei; Magorian Friedlmeier, Theresa; Abdelsamie, Maged; Hong, Kootak; Zia, Waqas; Perween, Shama; Clemens, Oliver; Sutter‐Fella, Carolin M.; Saliba, MichaelAll‐inorganic perovskites have emerged as promising photovoltaic materials due to their superior thermal stability compared to their heat‐sensitive hybrid organic–inorganic counterparts. In particular, CsPbI2Br shows the highest potential for developing thermally‐stable perovskite solar cells (PSCs) among all‐inorganic compositions. However, controlling the crystallinity and morphology of all‐inorganic compositions is a significant challenge. Here, a simple, thermal gradient‐ and antisolvent‐free method is reported to control the crystallization of CsPbI2Br films. Optical in situ characterization is used to investigate the dynamic film formation during spin‐coating and annealing to understand and optimize the evolving film properties. This leads to high‐quality perovskite films with micrometer‐scale grain sizes with a noteworthy performance of 17% (≈16% stabilized), fill factor (FF) of 80.5%, and open‐circuit voltage (VOC) of 1.27 V. Moreover, excellent phase and thermal stability are demonstrated even after extreme thermal stressing at 300 °C.Item Open Access PEO infiltration of porous garnet-type lithium-conducting solid electrolyte thin films(2021) Waidha, Aamir Iqbal; Vanita, Vanita; Clemens, OliverComposite electrolytes containing lithium ion conducting polymer matrix and ceramic filler are promising solid-state electrolytes for all solid-state lithium ion batteries due to their wide electrochemical stability window, high lithium ion conductivity and low electrode/electrolyte interfacial resistance. In this study, we report on the polymer infiltration of porous thin films of aluminum-doped cubic garnet fabricated via a combination of nebulized spray pyrolysis and spin coating with subsequent post annealing at 1173 K. This method offers a simple and easy route for the fabrication of a three-dimensional porous garnet network with a thickness in the range of 50 to 100 µm, which could be used as the ceramic backbone providing a continuous pathway for lithium ion transport in composite electrolytes. The porous microstructure of the fabricated thin films is confirmed via scanning electron microscopy. Ionic conductivity of the pristine films is determined via electrochemical impedance spectroscopy. We show that annealing times have a significant impact on the ionic conductivity of the films. The subsequent polymer infiltration of the porous garnet films shows a maximum ionic conductivity of 5.3 × 10-7 S cm-1 at 298 K, which is six orders of magnitude higher than the pristine porous garnet film.Item Open Access Nitriding of Fe-Cr-Al alloys : nitride precipitation and phase transformations(2008) Clauß, Arno Rainer; Mittemeijer, Eric J. (Prof. Dr. Ir.)Nitriding of the ternary iron-based alloy Fe–1.5wt.%Cr–1.5wt.%Al (i.e. Fe–1.6at.%Cr–3.1at.%Al) leads to the precipitation of mixed, cubic, rock-salt structure type Cr1-xAlxN precipitates. These precipitates are not in thermodynamic equilibrium, which would involve precipitation of cubic, rock-salt structure type CrN and hexagonal, wurtzite structure type AlN. The mixed, cubic, rock-salt structure type Cr1-xAlxN precipitates develop because diffusion of Cr and Al in ferrite, as compared to diffusion of N in ferrite, is very slow and precipitation of hexagonal, wurtzite structure type AlN is a process with difficult nucleation. The precipitation of mixed cubic, rock-salt structure type Cr1-xAlxN already releases a considerable (although not maximal) amount of energy. The Cr1-xAlxN precipitates develop as platelets initially coherent with the ferrite matrix according to the Bain orientation relationship. At this initial stage the nitride platelets diffract coherently with the matrix and separate nitride reflections do not occur in the X-ray diffractogram. More and finer nitride precipitates occur near the surface than at larger depths beneath the surface, because the driving force for nitride precipitation is largest near the surface. For coarser nitride platelets (i.e. at larger depths; see above) the Bain orientation relationship is no longer fulfilled exactly. The precipitates “break up” (contrast variation along the platelets in transmission electron micrographs and splitting of electron diffraction spots). Large amounts of excess nitrogen are taken up upon precipitation of the Cr1-xAlxN precipitates, as a result of the elastic accommodation of the precipitate/matrix misfit. The excess nitrogen uptake is largest near the surface, because the finest precipitates occur there (see above) and are subjected to (almost) full elastic accommodation of the precipitate/matrix misfit. Excess nitrogen, which remains in the specimen after denitriding is ascribed to nitrogen atoms strongly bonded at the precipitate platelet surfaces to in particular the Al atoms in the mixed nitride at the platelet surfaces. Upon nitriding the ferritic iron-based alloy Fe–1.5wt.%Cr–1.5wt.%Al, mixed, metastable cubic, rock-salt structure type Cr1-xAlxN nitrides develop in the nitrided zone, which contains considerably more nitrogen than necessary to precipitate all Cr and all Al (i.e. excess nitrogen; see above). Subsequent annealing at higher temperature than the nitriding temperature leads to the development of the equilibrium precipitates CrN and AlN. Cubic, rock-salt structure type and hexagonal, wurtzite structure type particles occur after annealing in the nitrided zone, which exhibit a Bain-type orientation relationship and a Pitsch–Schrader orientation relationship, respectively, with the ferrite matrix, and which correspond with CrN and AlN, respectively. Part of the cubic, rock-salt structure type particles is (still) mixed Cr1-xAlxN nitride, however containing less Al than initially present. Transformation of the initial mixed Cr1-xAlxN nitrides proceeds by their Al depletion. The subsequent precipitation of AlN occurs in the interior and at grain boundaries of the matrix. A coarser microstructure results. The precipitates no longer exhibit strong coherency with the ferrite matrix, as reflected by the strong decrease of the broadening of the XRD ferrite-matrix reflections and the distinct decrease of hardness. Annealing leads to the presence of nitrogen in the originally unnitrided core by diffusion of mobile excess nitrogen from the nitrided zones. This nitrogen immediately precipitates as relatively coarse CrN at grain boundaries and as smaller cubic, rock-salt structure type CrN and hexagonal, wurtzite structure type AlN in the interior of the grains. Mixed Cr1-xAlxN nitride does not develop.