Quantitative TEM and STEM Study of Pt-Nanoparticles Coarsening and Ge(Mn)-based Ferromagnetic Nanostructures

In this work, different systems have been studied using transmission electron microscopy (TEM) methods: Pt nanoparticles on amorphous carbon, Ge quantum dots, Mn incorporation in Ge QDs and GeMn nanocolumns embedded in Ge or GeSn matrix. The structural and chemical observation have been correlated with physical properties

Oxygen reduction at thin dense La0.52Sr0.48Co0.18Fe0.82O3- δ electrodes: Part I: Reaction model and faradaic impedance

The faradaic impedance of oxygen reduction has been simulated for thin dense two-dimensional $${\text{La}}_{{0.52}} {\text{Sr}}_{{0.48}} {\text{Co}}_{{0.18}} {\text{Fe}}_{{0.82}} {\text{O}}_{{3 - \delta }}$$ electrodes in air at 600°C. The reaction model accounts for the defect chemistry of the ceramic films and includes bulk and surface pathways. It was demonstrated that the contribution of the surface pathway to the reaction was negligible due to the small length of triple phase boundary gas/electrode/electrolyte. The diffusion of oxygen in the bulk of $${\text{La}}_{{0.52}} {\text{Sr}}_{{0.48}} {\text{Co}}_{{0.18}} {\text{Fe}}_{{0.82}} {\text{O}}_{{3 - \delta }}$$ (LSCF) can be evidenced by measuring the polarization resistance as a function of the electrode thickness that ranged between 10 and 800nm. When recorded as a function of the electrode potential and thickness, the frequency response exhibited features that were specific to the rate-determining steps of the reaction. The oxygen reduction mechanism and kinetics can therefore be identified by means of impedance spectroscopy. The faradaic impedances calculated for realistic values of the rate constants exhibited a noteworthy large faradaic capacitanc

Enantioselective γ-lactam synthesis via palladium-catalyzed intramolecular asymmetric allylic alkylation

A Pd(0)-catalyzed intramolecular allylic alkylation in the presence of (R)-3,5-t-Bu-MeOBIPHEP takes place in up to 92:8 er in agreement with DFT calculations and provides easy access to enantioenriched disubstituted γ-lactams

Oxygen reduction at thin dense La0.52Sr0.48Co0.18Fe0.82O3- δ electrodes: Part II: Experimental assessment of the reaction kinetics

The mechanism and kinetics of oxygen reduction at thin dense two-dimensional $${\text{La}}_{{0.52}} {\text{Sr}}_{{0.48}} {\text{Co}}_{{0.18}} {\text{Fe}}_{{0.82}} {\text{O}}_{{3 - \delta }}$$ (LSCF) electrodes have been investigated in air between 500 and 700 °C with electrochemical impedance spectroscopy and steady-state voltammetry. Dense and geometrically well-defined LSCF films with various thicknesses ranging between 16 and 766nm have been prepared on cerium gadolinium oxide substrates by pulsed laser deposition and structured with photolithography. The current collection was ensured by a porous LSCF layer. A good agreement was found between the experimental data and the impedance of the reaction model calculated with state-space modelling for various electrode potentials and thicknesses. It was evidenced that oxygen adsorption, incorporation into the LSCF and bulk diffusion are rate-determining while charge transfer at the electrode/electrolyte interface remains at quasi-equilibrium. The 16 and 60nm thin dense LSCF electrodes appear to be more active towards oxygen reduction than thicker layers and porous films at 600 and 700°

Interface-driven phase separation in multifunctional materials: the case of GeMn ferromagnetic semiconductor

We use extensive first principle simulations to show the major role played by interfaces in the mechanism of phase separation observed in semiconductor multifunctional materials. We make an analogy with the precipitation sequence observed in over-saturated AlCu alloys, and replace the Guinier-Preston zones in this new context. A new class of materials, the $\alpha$ phases, is proposed to understand the formation of the coherent precipitates observed in the GeMn system. The interplay between formation and interface energies is analyzed for these phases and for the structures usually considered in the literature. The existence of the alpha phases is assessed with both theoretical and experimental arguments

Thermoelectric properties of high quality nanostructured Ge:Mn thin films

We report on the elaboration of germanium manganese nanostructured thin films and the measurement of their thermoelectric properties. We investigate the growth of Ge:Mn layers along with a thorough structural characterization of this materials at the nanoscale. The room temperature thermoelectric properties of these layers containing spherical inclusions are discussed regarding their potential as a model of "electron crystal phonon glass material". We show that the thermal conductivity can be decreased by a factor of 30, even if the electronic properties can be conserved as in the bulk. The thermoelectric performance ZT of such material is as high as 0.15 making them a promising thermoelectric p-type material for Ge related application

The influence of constrictivity on the effective transport properties of porous layers in electrolysis and fuel cells

The aim of the present investigation is to define microstructure parameters, which control the effective transport properties in porous materials for energy technology. Recent improvements in 3D-imaging (FIB-nanotomography, synchrotron X-ray tomography) and image analysis (skeletonization and graph analysis, transport simulations) open new possibilities for the study of microstructure effects. In this study, we describe novel procedures for a quantitative analysis of constrictivity, which characterizes the so-called bottleneck effect. In a first experimental part, methodological tests are performed using a porous (La,Sr)CoO3 material (SOFC cathode). The tests indicate that the proposed procedure for quantitative analysis of constrictivity gives reproducible results even for samples with inhomogeneous microstructures (cracks, gradient of porosity). In the second part, 3D analyses are combined with measurements of ionic conductivity by impedance spectroscopy. The investigations are preformed on membranes of electrolysis cells with porosities between 0.27 and 0.8. Surprisingly, the tortuosities remain nearly constant (1.6) for the entire range of porosity. In contrast, the constrictivities vary strongly and correlate well with the measured transport resistances. Hence, constrictivity represents the dominant microstructure parameter, which controls the effective transport properties in the analysed membrane materials. An empirical relationship is then derived for the calculation of effective transport properties based on phase volume fraction, tortuosity, and constrictivit

Electrical and thermal spin accumulation in germanium

In this letter, we first show electrical spin injection in the germanium conduction band at room temperature and modulate the spin signal by applying a gate voltage to the channel. The corresponding signal modulation agrees well with the predictions of spin diffusion models. Then by setting a temperature gradient between germanium and the ferromagnet, we create a thermal spin accumulation in germanium without any tunnel charge current. We show that temperature gradients yield larger spin accumulations than pure electrical spin injection but, due to competing microscopic effects, the thermal spin accumulation in germanium remains surprisingly almost unchanged under the application of a gate voltage to the channel.Comment: 7 pages, 3 figure