U. Herr

Investigation of nanocrystalline iron materials by Mössbauer spectroscopy

Nanocrystalline materials, which have been proposed to represent a new solid state structure, are investigated by Mossbauer spectroscopy. Nanocrystalline materials are polycrystals with a crystal size of typically 1–10 nm. These materials consist of two components of comparable volume fractions: a crystalline component and an interfacial component, formed by the atoms located either in the crystals or in the interfacial regions between them. As the atomic configurations of both components are different, two kinds of Mossbauer spectra are expected. Iron nanocrystalline material is found to exhibit a two‐component Mossbauer spectrum, consisting of a crystalline component and a second one with different Mossbauer parameters. The Mossbauer parameters of the second subspectrum are consistent with the model of the interfacial component of a nanocrystalline material.

CHEMICAL VAPOR SYNTHESIS AND LUMINESCENCE PROPERTIES OF NANOCRYSTALLINE CUBIC Y2O3:EU

Nanocrystalline europium doped yttria was synthesized using a chemical vapor technique. The powder was characterized by x-ray diffraction, transmission electron microscopy, and ultraviolet spectroscopy. For the first time it was possible to obtain single phase Y2O3:Eu nanoparticles crystallized in the cubic structure with an average particle size of only 10 nm. The reflection, excitation, and emission spectra were studied. The nanoparticles show blue shifted absorption bands with respect to coarse grained material.

Ball milling of systems with positive heat of mixing: Effect of temperature in Ag-Cu

Abstract Ag50Cu50 alloys prepared by ball milling were subsequently milled at different temperatures ranging from about 85 to 473 K and investigated by X-ray diffraction and differential scanning calorimetry. Milling at subambient temperatures led to a homogeneous f.c.c. solid solution, while milling at 473 K led to a fully decomposed, two-phase mixture. Partially decomposed systems were obtained during milling at intermediate temperatures with three co-existing phases all present. The milling-induced decomposition process had a very low apparent activation enthalpy, ≈ 0.11 eV, which is attributed to a first-order kinetic process involving non-equilibrium vacancies. The results are discussed in terms of competing mixing and de-mixing reaction rates.

Formation of supersaturated solid solutions in the immiscible Ni–Ag system by mechanical alloying

The mechanical alloying process in the immiscible Ni–Ag system with a positive heat of mixing was investigated by x‐ray diffraction and differential scanning calorimetry. High energy ball milling of mixed elemental powders, with nominal composition NixAg100−x (x=95, 90, 70, 50, and 30), results in the formation of mixtures of supersaturated, nanocrystalline Ni‐rich and Ag‐rich solid solutions. The solubilities and final grain sizes of these phases depend on the nominal composition of the powder. The maximum solubilities were determined using Vegard’s law to be 4.3 at. % Ni in Ag and 6.6 at. % Ag in Ni for samples milled at room temperature. The effect of milling temperature on mechanical alloying was examined in the range −195 to 250 °C. Lower temperature milling leads to a larger solubility of Ni in the Ag‐rich samples, up to 7.1 at. % for the Ni30Ag70 composition. Indications for the existence of a concentrated solid solution (Ni36Ag64‐Ni44Ag56) were also found. Milling at higher temperatures leads to l...

Luminescence of bulk and nanocrystalline cubic yttria

The luminescence of nanocrystalline cubic yttria has been measured for 300 and 80 K. A blueshift and a broadening of the absorption edge, both depending on the particle size were found. The energy of the radiation emitted due to the recombination of a bound exciton does not depend on particle size. The observations are not caused by quantum or phonon confinement. Using a quantum mechanical configurational coordinate diagram an explanation can be given. Hydrostatic pressure increasing the phonon energy of the excited state of the bound exciton, or an increase of the exciton-phonon coupling may generate the size dependent optical properties of nanocrystalline yttria.

Alloy effects in consolidated binary mixtures of nanometer-sized crystals investigated by Mössbauer spectroscopy

Abstract In this paper, investigations by Mossbauer spectroscopy of binary mixtures of Fe crystals and Cu or Ag crystals are reported; the crystal sizes of the constituents are in the range of 6–20 nm. In the case of the Fe/Cu system, samples containing small iron concentrations (about 1 at %) as well as samples with about 30 at % Fe were examined; in the case of Ag/Fe, results for 30 at % Fe are reported. The principal finding is that the Mossbauer spectra show components different from α-Fe which may be identified, by comparison with metastable Cu/Fe or Ag/Fe alloys generated by co-sputtering, as alloy phases of the constituent elements although Cu/Fe as well as Ag/Fe alloys exhibit little or no solubility. The formation of alloy phases is proposed to occur at the grain or interphase boundaries between the crystals and seems to be based on the enhanced free volume and/or local strain fields at the interfaces.

Vibrational behaviour of nanocrystalline Ni

Abstract The phonon density of states (DOS) of nanocrystalline Ni has been investigated by means of inelastic neutron scattering. Compacted as well as non-compacted samples were prepared by inert gas condensation. With respect to a reference sample of coarse grained polycrystalline Ni, the DOS of the compacted sample is strongly enhanced at frequencies below 15 me V. The specific heat calculated from this DOS reproduces the excess specific heat well, as measured in similar samples. Attributing tentatively the observed enhancement of the DOS io the grain-boundary (GB) component, its own GB-DOS is obtained. This GB-DOS shows an almost linear slope at low frequencies in contrast to what is found e.g. in amorphous metals. The non-compacted nanocrystalline powder shows a less pronounced enhancement of the low energy DOS. This supports our view that the excess vibrations originate from the grain-boundaries and are not due to a large fraction of inner surfaces.

Structural changes of the metallic glass Zr65Al7.5Cu27.5 during glass transition and in the undercooled liquid region

The metallic glass Zr65Al7.5Cu27.5 exhibits a wide temperature range in the undercooled liquid region. This offers the possibility of examining the phenomena that occur during glass transition. We investigated this glass by x-ray diffraction and x-ray absorption spectroscopy (XAS) at room temperature and at elevated temperatures below and above the glass transition temperature. The total number of nearest neighbors determined by x-ray diffraction remained unchanged. Using XAS irreversible changes in the short range order (SRO) towards the SRO of crystalline Zr2Cu is observed in the nearest neighbor shell which is consistent with the x-ray diffraction results. The long range order was not affected by these changes. From the temperature dependence of the atomic mean-square relative displacements σ2, Debye temperatures were determined and they compare well with low temperature specific heat measurements. Above Tg the slope of σ2 increases drastically and is attributed to anharmonic effects.

Magnetic domains and annealing-induced magnetic anisotropy in nanocrystalline soft magnetic materials

The magnetic domains of nanocrystalline Fe84Nb6B10 annealed under static and rotating magnetic fields have been investigated by means of magneto-optical Kerr effect (MOKE) microscopy in order to clarify the origin of the dramatic magnetic softening brought about by rotating field annealing. The coercivity (Hc) values after static- and rotating-magnetic field annealings are 5.9 and 3.0A∕m, respectively. The MOKE image after static field annealing implies a highly coherent uniaxial anisotropy (Ku) in the sample whereas no sign of such a strong Ku is evident in the domain configuration after rotating field annealing. Our analytical solution of the random anisotropy model with additional Ku predicts that the fluctuating amplitude of the effective anisotropy (δK) in nanocrystalline Fe84Nb6B10 decreases from 20to11J∕m3 by removing Ku. The observed reduction of Hc may be attributed to this decrease in δK induced by rotating field annealing.

Response of the potential of a gold electrode to elastic strain

We describe the measurement of the response, sigma, of the potential of a metal electrode to elastic strain under open circuit conditions. The experimental response exhibits a frequency dependence due to Faraday loss currents, which become negligible beyond 30 Hz. For a (111)-textured gold film the experimental value at higher frequency, sigma = -1.83 V, compares well with the intrinsic value of Au(111) predicted by recent ab initio computation.