Tolga Acartürk

Oxygen tracer diffusion in dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ films

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is an interesting material for high temperature oxygen permeation membranes and solid oxide fuel cell cathodes, applications for which oxygen transport properties are crucial. Here oxygen isotope exchange is performed on dense BSCF films prepared by pulsed laser deposition on MgO single crystal substrates. The oxygen isotope profile is analyzed by secondary ion mass spectrometry to extract tracer diffusion coefficients D∗ and effective surface exchange constants k∗. Tracer diffusion has a low activation energy of 0.5 eV. At moderate temperatures, the related oxygen vacancy diffusion coefficient DVO is significantly larger than that for (La,Sr)(Fe,Co)O3−δ perovskites.

Phase Boundary Propagation in Large LiFePO4 Single Crystals on Delithiation

Large single crystals of LiFePO(4) have been chemically delithiated. The relevance of chemical oxidation in comparison with electrochemical delithiation is discussed. Analyses of the Li content and profiles were done by electron energy loss spectroscopy and secondary ion mass spectrometry. The propagation of the FePO(4) phase growing on the surface of the large single crystal was followed by in situ optical microscopy as a function of time. The kinetics were evaluated in terms of linear irreversible thermodynamics and found to be characterized by an induction period followed by parabolic growth behavior of the FePO(4) phase indicating transport control. The growth rate was shown to depend on the crystallographic orientation. Scanning electron microscopy images showed cracks and a high porosity of the FePO(4) layer due to the significant changes in the molar volumes. The transport was found to be greatly enhanced by the porosity and crack formation and hence greatly enhanced over pure bulk transport, a result which is supposed to be very relevant for battery research if coarse-grained powder is used.

Concentration of N and P in SiC Investigated by Time-Of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)

The concentration of nitrogen and phosphorous in SiC bulk material and epitaxial layers was investigated using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The advantage of TOF-SIMS of acquiring a complete mass spectrum in a single run was used to identify the most sensitive atomic ion or ionic cluster for the selected element to be monitored. For the investigation of N with its intrinsic low ionization yield the use of a Cs containing cluster ion is necessary. Selection of a CNCs2 + cluster allows to reach a detection limit of about cN,min » 5×1016 cm-3. In the case of P the elemental ion was used. However, the adjacent mass of 30SiH influences the P peak as well as its background and has to be suppressed. This can be achieved by limiting the residual gas re-adsorption during the measurement resulting in a detection limit of about cP,min » 5×1015 cm-3. These measurement parameters were used to investigate a single crystal SiC bulk sample grown by the modified Lely method with intentional P doping and an N doped epitaxial SiC layer sample.

Interaction of oxygen with halide perovskites

We systematically investigate the consequences of exposing halide perovskites to an oxygen-containing atmosphere, in particular methylammonium lead iodide, the archetypal compound used as photo-absorber in perovskite solar cells. For this purpose, we study oxygen solubility and global reaction with oxygen both in the dark and under light, and we refer to the kinetics in terms of surface reaction and bulk diffusion. As thermodynamics reveals, the material is unstable against oxygen, primarily because of the large driving force of water formation. While under light the material quickly degrades, in the dark the surface reaction kinetics – not the bulk transport – is very sluggish and keeps it metastable. For the same reason, oxygen incorporation into the lattice is negligible in the dark. On illumination, an accelerated oxygen in-diffusion occurs (as shown by 18O incorporation experiments) that severely modifies the electronic and ionic conductivities in the way that is expected for an acceptor dopant. Lastly, we investigate the impact of cation as well as anion mixing on the degradation and stress the necessity of using encapsulation during solar cell operation.

Formation of thin oxide layer on surface of copper caused by implantation of high-energy oxygen ions

Combination of neutron and X-ray reflectometry was used to study the depth profile of 100 nm thin copper films with implanted oxygen ions with energy E = 10–30 keV and fluencies D = (0.2–5.4) ×1016 cm–2. The oxygen ion implantation at 30 keV was shown to lead to formation of 3 nm thick layer on the surface. Density and copper–oxygen stoichiometry of the observed surface layer was close to Cu2O oxide. We attribute the Cu2O oxide formation to highly mobilized copper atoms generated by stimulated ion implantation.Combination of neutron and X-ray reflectometry was used to study the vertical structure of 100 nm-thin copper films with implanted oxygen ions of energy E = [10-30] keV and doses D=[0.2-5.4]x

Silicon-Doped LiFePO4 Single Crystals: Growth, Conductivity Behavior, and Diffusivity

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Mechanism of oxygen interaction with halide perovskites

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Slow CH3NH3+ Diffusion in CH3NH3PbI3 under Light Measured by Solid-State NMR and Tracer Diffusion

. The study shows that oxygen ion implantation with an energy of E = 30 keV leads to the formation of a 3 nm thick layer on the surface. Density and copper/oxygen stoichiometry of the observed surface layer are close to

The LiFePO4/FePO4 Phase Transformation in Large Single Crystals and Small Crystallites of LiFePO4

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