Roland Würschum

OXYGEN DIFFUSION IN ULTRAFINE GRAINED MONOCLINIC ZRO2

The diffusion of oxygen in ultrafine grained, undoped monoclinic ZrO2 was studied using 18O as tracer and secondary ion mass spectroscopy profiling. Samples with a relative mass density of 97%–99% and average crystallite sizes of 80 or 300 nm were prepared from Zr by sputtering, inert-gas-condensation, oxidation, in situ consolidation of nanocrystalline (n-)ZrO2 powder and subsequent pressureless sintering at 950 or 1050 °C in vacuum. Volume and interface diffusivities were directly determined from the 18O diffusion profiles in n-ZrO2 in the type B and type A regime of interface diffusion. The diffusion of 18O in interfaces in undoped n-ZrO2 is 103–104 times faster than in the bulk of the crystallites throughout the temperature range of 450 to 950 °C studied. These diffusivities are independent of the crystallite size in the range of 70–300 nm. The activation energies QV=2.29 eV and QB=1.95 eV for the volume (QV) and interface diffusion (QB) are considerably higher than the diffusion activation energies f...

Spin-glass freezing of maghemite nanoparticles prepared by microwave plasma synthesis

Magnetic properties of 6 nm maghemite nanoparticles (prepared by microwave plasma synthesis) have been studied by ac and dc magnetic measurements. Structural characterization includes x-ray diffraction and transmission electron microscopy. The temperature scans of zero field cooled/field cooled (ZFC/FC) magnetization measurements show a maximum at 75 K. The ZFC/FC data are fitted to the Brown-Neel relaxation model using uniaxial anisotropy and a log-normal size-distribution function to figure out the effective anisotropy constant Keff. Keff turns out to be larger than the anisotropy constant of bulk maghemite. Fitting of the ac susceptibility to an activated relaxation process according to the Arrhenius law provides unphysical values of the spin-flip time and activation energy. A power-law scaling shows a satisfactory fit to the ac susceptibility data and the dynamic critical exponent (zv ≈ 10) takes value between 4 and 12 which is typical for the spin-glass systems. The temperature dependence of coercivity and exchange bias shows a sharp increase toward low temperatures which is due to enhanced surface anisotropy. The source of this enhanced magnetic anisotropy comes from the disordered surface spins which get frozen at low temperatures. Memory effects and thermoremanent magnetization experiments also support the existence of spin-glass behaviour. All these magnetic measurements signify either magnetic blocking or surface spin-glass freezing at high and low temperatures, respectively.Magnetic properties of 6 nm maghemite nanoparticles (prepared by microwave plasma synthesis) have been studied by ac and dc magnetic measurements. Structural characterization includes x-ray diffraction and transmission electron microscopy. The temperature scans of zero field cooled/field cooled (ZFC/FC) magnetization measurements show a maximum at 75 K. The ZFC/FC data are fitted to the Brown-Neel relaxation model using uniaxial anisotropy and a log-normal size-distribution function to figure out the effective anisotropy constant Keff. Keff turns out to be larger than the anisotropy constant of bulk maghemite. Fitting of the ac susceptibility to an activated relaxation process according to the Arrhenius law provides unphysical values of the spin-flip time and activation energy. A power-law scaling shows a satisfactory fit to the ac susceptibility data and the dynamic critical exponent (zv ≈ 10) takes value between 4 and 12 which is typical for the spin-glass systems. The temperature dependence of coercivi...

Adsorption-driven tuning of the electrical resistance of nanoporous gold

The electrical resistance of nanoporous gold prepared by dealloying is tuned by charging the surfaces of the porous structure in an electrolyte. Reversible variations in the resistance up to approximately 4% and 43% occur due to charging in the regimes of double layer charging and specific adsorption, respectively. Charging-induced variations in the electron density or of the volume cannot account for the resistance variation, indicating that this variation is primarily caused by charge-induced modifications of the charge carrier scattering at the solid-electrolyte interface. The relative resistance variation in nanoporous Au with surface charging is found to be much higher than reported for porous nanocrystalline Pt. This is due to the lesser resistance contribution from internal grain boundaries. The resistance variation in nanoporous Au is also higher than that found in thin films owing to the stronger surface scattering in the ligament structure compared to plan surfaces. We argue that the strong resi...

Tuneable magnetic susceptibility of nanocrystalline palladium

A charge-induced reversible variation of the paramagnetic susceptibility χ of Pd is reported. In situ charging was performed by means of a nanocrystallite-electrolyte composite. The charge-induced variation of χ is analyzed taking into account the modification of the charge carrier density by surface charging and the effect of charge-induced pressure on the nanocrystallites. The present studies may open up the way to switchable ferromagnetism.

High temperature Mössbauer effect study of Fe90Zr7B3 nanocrystalline alloy

Fe90Zr7B3 NANOPERM alloy is investigated in as-quenched and nanocrystalline forms by means of high temperature (up to 1040 K) Mossbauer spectroscopy. These studies are aimed at revealing the relationship of microstructure to magnetic properties for57Fe phases and their temperature dependences in NANOPERM-type ternary alloy at temperatures exceeding the onset of the second crystallization. For this purpose the nanocrystalline sample was prepared by annealing an amorphous precursor at 893 K for 1 h providing 54% of bcc α-Fe nanocrystalline grains. At this stage the first crystallization is almost completed. Because of the progress of the crystallization process during the acquisition of Mossbauer spectra beyond the temperature of the first crystallization, the results obtained are discussed for three temperature intervals: below the first crystallization (782 K), between the first and the second crystallization, and above the second crystallization temperature (931 K). Conclusions related to the evolution of the crystalline fraction, interfacial regions and the amorphous residual phase are derived by comparing spectral parameters obtained from the in situ high temperature Mossbauer effect measurements with those from room temperature Mossbauer spectra acquired immediately after each high temperature experiment. The latter revealed structural modifications imposed during Mossbauer spectroscopy at high temperatures, whereas the in situ experiments identify thermally induced dynamic processes.

Dilatometry: a powerful tool for the study of defects in ultrafine-grained metals

Vacancies, dislocations, and interfaces are structural defects that are deliberately introduced into solids during grain refinement processes based on severe plastic deformation (SPD). Specific combinations of these defects determine the improved mechanical properties of the obtained ultrafine-grained materials. High-precision, non-equilibrium dilatometry, i.e., measurement of the irreversible macroscopic length change upon defect annealing, provides a powerful technique for the characterization and the study of the kinetics of these defects. It is applied to determine absolute concentrations of vacancies, to characterize dislocation processes, and to assess grain boundary excess volume in pure, FCC and BCC ultrafine-grained metals processed by SPD.

Interface diffusion and amorphous intergranular layers in nanocrystalline Fe90Zr7B3

Iron tracer diffusion was studied in soft-magnetic nanocrystalline Fe90Zr7B3 without any influence of porosity, relaxation, or grain growth. The interfacial diffusion characteristics differ substantially from grain boundaries in metals due to the presence of an intergranular amorphous phase. The reduced diffusivity in thin amorphous layers compared to in the initial amorphous phase indicates the effect of confinement. The indication of a second, fast interfacial diffusion path is found and quantitatively analyzed within the framework of a two interface-type model.

Chemically sensitive free-volume study of amorphization of Cu60Zr40 induced by cold rolling and folding

With the aim to contribute to a microscopical understanding of the processes of solid-state amorphization, the chemically sensitive technique of background—reduced Doppler broadening of positron-electron annihilation radiation in combination with positron lifetime spectroscopy and microstructural characterization is applied to a free volume study of the amorphization of Cu60Zr40 induced by consecutive folding and rolling. Starting from the constituent pure metal foils, a nanosale multilayer structure of elemental layers and amorphous interlayers develops in an intermediate state of folding and rolling, where free volumes with a Zr-rich environment occur presumably located in the hetero-interfaces between the various layers or in grain boundaries of the Cu layers. After complete intermixing and amorphization, the local chemical environment of the free volumes reflects the average chemical alloy composition. In contrast to other processes of amorphization, free volumes of the size of few missing atoms occur...

Charging-induced defect formation in LixCoO2 battery cathodes studied by positron annihilation spectroscopy

Charging-induced formation of vacancy-type defects in LixCoO2 battery cathodes was studied by the defect-specific techniques of positron lifetime spectroscopy and Doppler broadening of positron–electron annihilation radiation. The regime of reversible charging is dominated by vacancy-type defects on the Li+-sublattice the size of which increases with increasing Li+-extraction. Indication is found that Li+-reordering which occurs at the limit of reversible Li+-extraction (x = 0.55) causes a transition from two-dimensional agglomerates into one-dimensional vacancy chains. Degradation upon further Li+-extraction is accompanied by the formation of vacancy complexes on the Co- and anion sublattice.

Electrochemical cell for in situ electrodeposition of magnetic thin films in a superconducting quantum interference device magnetometer

An electrochemical cell is designed and applied for in situ electrodeposition of magnetic thin films in a commercial SQUID magnetometer system. The cell is constructed in such a way that any parasitic contribution of the cell and of the substrate for electrodeposition to the magnetic moment of the deposited film is reduced to a minimum. A remanent minor contribution is readily taken into account by a proper analysis of the detected signal. Thus, a precise determination of the absolute magnetic moment of the electrodeposited magnetic film during its growth and dissolution is achieved. The feasibility of the cell design is demonstrated by performing Co electrodeposition using cyclic voltammetry. For an average Co film thickness of (35.6 ± 3.0) atomic layers, a magnetic moment per Co atom of (1.75 ± 0.11) μ(B) was estimated, in good agreement with the literature bulk value.