Michael J. Hoffmann

Ferroelectric domains in methylammonium lead iodide perovskite thin-films

We explore the ferroic properties of methylammonium lead iodide perovskite solar cells by piezoresponse force microscopy (PFM). In vertical and horizontal PFM imaging, we find domains of alternating polarization with a width of 90 nm which we identify as polarized ferroelectric domains. High-resolution photo-conductive atomic force micrographs under illumination also show alternating charge carrier extraction patterns which we attribute to the local vertical polarization components within the ferroelectric domains. The correlation of the sample properties with atomic force and kelvin probe force micrographs evidence the ferroelectric nature of the domains.

Microstructure and mechanical properties of Li0.33La0.567TiO3

Mechanical properties of conventional sintered Li0.33La0.567TiO3 (LLTO) are presented with focus on the correlation with the microstructural appearance. Variation in density, grain size, and second phase are achieved by changing the lithium to lanthanum ratio and sintering conditions. All samples show varying amounts of a second phase which is identified as TiO2. These inert particles have no effect on the measured mechanical properties. In contrast, a high sensitivity to changes in the microstructural evolution is found. Therefore, density and grain size are the important microstructural features to control both electrical and mechanical properties. For stoichiometric, a dense LLTO a Young’s modulus of 200xa0GPa, KIC of 1.2xa0MPam0.5, and a Vickers hardness of 8.4xa0GPa are measured. For all dense samples, the characteristic bending strength shows values around 150xa0MPa and Weibull modulus of mxa0=xa07–9. Deviations from these results are explained by microstructural events like second phases, density, or grain size.

Investigation of the wetting behavior of Na and Na alloys on uncoated and coated Na-β”-alumina at temperatures below 150 °C

Insufficient wetting of Na on Na-β”-alumina is responsible for an increased interfacial polarization in Na-batteries if the operating temperature is below 270–300xa0°C. In this study, Na alloys (Na–Sn, Na–Bi and Na–In) on the ceramic surface are examined on their capability to improve the wetting with molten sodium at medium temperatures and thereby enabling operation of the cell at temperatures in the range of 105 to 150xa0°C. Metallic coatings with Bi or Sn confirm this improvement at a temperature of 115xa0°C. The overpotential of a symmetrical Na│Na-β”-alumina│Na cell is determined by galvanostatic cycling and electrochemical impedance spectroscopy and then associated to contact angle measurements. As a result, both the alloys and the metallic coatings have a positive impact on wetting and reduction of the overpotential during galvanostatic cycling.

Additive Manufacturing of a lightweight rotor for a permanent magnet synchronous machine

Additive Manufacturing (AM), also known as 3D printing, is a relatively new technology which enables the toolless production of components and entire assemblies directly from a CAD file. Today, the technology is still not widely used in industrial production. It is mainly limited to special applications, although it shows great potential. In this paper, first approaches are shown to apply AM to the production of rotors for permanent magnet synchronous machines (PMSM). The possibilities of a lightweight design with a low moment of inertia as well as the influence on the magnetic anisotropy for an improved sensorless control of PMSM are pointed out. The results clearly demonstrate the great potential of additive manufacturing in electrical engineering applications.

Process Development for the Ceramic Injection Molding of Oxide Chopped Fiber Reinforced Aluminum Oxide

In this study, it was tried to develop a process chain for ceramic injection molding of Al2O3-chopped-fiber reinforced oxide-ceramic-matrix-composite. The feedstocks are compounded at 50 Vol. % filling degree of solid (Al2O3 μ-powder (Taimei Chemicals Co. Ltd.) and 3,2 mm chopped fibers (3M)), in which fiber content varies from 0 Vol. % to 100 Vol. %. As binder system, PE + Paraffin Wax + Stearic Acid are used. The ingredients are compounded in a kneader (Brabender) at 125°C and after the viscosity measurement in the high pressure capillary rheometer at 160°C and certain shear rates, the feedstock is injection molded (Battenfeld) at 160°C, which is followed by debinding process, including chemical (in n-Hexane) and thermal steps, and 2h sintering at different temperatures. Flow paths in the machinery parts, rheological properties of binding system, fiber content and the fiber orientation have significant effect on the flow behavior of the feedstock, fiber -orientation, -distribution & -length, which are crucial to understand the properties of end-parts like mechanical reinforcement of the fibers. The fibers in the sintered parts are ca. 200 μm in average length. The fibers in the feedstock show different orientations depending on the part-geometry and the green bodies have different densities depending on sintering temperature, amount of dispersant and fiber orientation.