Helmut Hermann

Short-range order of Zr62−xTixAl10Cu20Ni8 bulk metallic glasses

Abstract The short-range order and crystallization behavior of slowly cooled Zr 62− x Ti x Al 10 Cu 20 Ni 8 bulk metallic glasses have been investigated in terms of the atomic pair correlation function as a function of Ti content x (2≤ x ≤7.5). The structural parameters point to the presence of chemical short-range order in these bulk glasses. An enhanced local excess free volume around the Ti atoms is concluded from density measurements. The first stage of crystallization in Zr 62− x Ti x Al 10 Cu 20 Ni 8 bulk glasses is related to changes in the medium-range order while the first neighborhood is retained. The atomic pair correlation functions of the first crystallization products are similar for all titanium contents. There is no indication of any special atomic arrangement for the particular alloy forming quasicrystals upon heating ( x =3). In case of Zr 54.5 Ti 7.5 Al 10 Cu 20 Ni 8 an ultrafine microstructure consisting of clusters of 2 nm in size is formed as the first step of crystallization.

Metal-organic frameworks as promising candidates for future ultralow-k dielectrics

A class of Zn4O(CO2)6-based metal-organic frameworks (MOFs) is theoretically analyzed with respect to suitability as an interlayer dielectric material for applications in semiconducting devices. The static dielectric constant is calculated using the Clausius–Mossotti approach. For 3 of about 30 of the considered MOFs excellent combinations of ultralow dielectric constant, elastic bulk modulus, and gap energy are found favoring these materials as outstanding candidates for future ultralow-k dielectric materials.

Structural behavior of Pd40Cu30Ni10P20 bulk metallic glass below and above the glass transition

The thermal behavior of the structure of Pd40Cu30Ni10P20 bulk metallic glass has been investigated in situ through the glass transition by means of high-temperature x-ray synchrotron diffraction. The dependence of the x-ray structure factor S(q) of the Pd40Cu30Ni10P20 glass on temperature follows the Debye theory up to the glass transition with a Debye temperature θ=296 K. Above the glass transition temperature Tg, the temperature dependence of S(q) is altered, pointing to a continuous development of structural changes in the liquid with temperature. The atomic pair correlation functions g(r) indicate changes in short-range-order parameters of the first and the second neighborhood with temperature. The temperature dependence of structural parameters is different in glass and in supercooled liquid, with a continuous behavior through the glass transition. The nearest-neighbor distance decreases with temperature, changing the slope at Tg. The interatomic distances of higher coordination shells expand analogo...

A small-angle neutron scattering model for polydisperse spherical particles with diffusion zones and application to soft magnetic metallic glass

An analytical expression for the small-angle neutron scattering intensity of diluted systems of polydisperse spherical particles, with diffusion zones, embedded in a matrix is presented. It is used within a nonlinear regression procedure to analyse small-angle neutron scattering experiments with polarized neutrons on an Fe73.5Si15.5B7CuNb3 alloy. The results for the nuclear and magnetic scattering length densities allow verification of the inhibitor concept introduced for the explanation of the limited sizes of precipitates developing during nanocrystallization. In the case of amorphous Fe73.5Si15.5B7CuNb3 alloy, the observed nanocrystals of the Fe3Si type are surrounded by an Nb-enriched shell, which stops the growth of the precipitates. With the results of polarized neutron scattering experiments, it is shown that magnetic and nuclear small-angle neutron scattering signals have the same origin. Additionally, the precision of the fits is improved by complementary use of polarized neutrons.

Towards controlled production of specific carbon nanostructures— a theoretical study on structural transformations of graphitic and diamond particles

Structural transformations of carbon nanoparticles are studied by means of molecular dynamics using a density-functional-based tight-binding method. The starting particles consist of 64 to 275 atoms arranged on a graphitic or diamond lattice. At elevated temperatures (1400 to 2800 K), the particles transform into spherical or elongated closed cages, concentric shell fullerenes, carbon nanotips, and spiraloidal and irregularly shaped clusters. The type of the final cluster depends essentially on the size and the atomic order of the starting particles, and on the temperature applied. The results show a way to proceed towards controlled preparation of specific carbon nanostructures.

Electronic structure and interband transitions of metallic carbon nanotubes

Band structure of the metallic (n,n) carbon nanotubes (coefficients n are the indices of the two-dimensional primitive lattice vectors of the graphene lattice) is calculated in terms of a linear augmented cylindrical wave method. The results are used to correlate the minimum direct energy gaps E11 between the conduction and valence band singularities with the nanotube diameter d and optical absorption spectra. Significant deviations from the equation E11∼d−1 are observed. The ππ* gap energy increases monotonically with 1/d, whereas the σπ* gap width shows a minimum at n=10. In the (3, 3) tube, the conduction band singularity coincides with the Fermi level, resulting in a drastic increase of the density of states on this level.

Structural behavior of Zr52Ti5Cu18Ni15Al10 bulk metallic glass at high temperatures

The structural behavior of the Zr52Ti5Cu18Ni15Al10 bulk glass-forming alloy has been investigated in situ by means of high-temperature x-ray synchrotron diffraction. The dependence of the structure factor of the glass can be well described with a Debye–Waller factor and a Debye temperature θ=412 K. At the glass transition, the structure factor significantly decreases due to additional thermal excitations. The extrapolation of the structure factor of the supercooled liquid to temperatures above the liquidus curve is in agreement with experimentally determined values of the melt. The short-range order of the glass, of the supercooled liquid state, and of the equilibrium melt at T=1193 K, is found to be quite similar. The formation of complex chemically ordered clusters in the melt is proposed to be essential for the high-glass-forming ability of this alloy.

Ultrafine nanostructure of partially crystallized bulk amorphous Zr54.5Ti7.5Al10Cu20Ni8

Bulk amorphous Zr54.5Ti7.5Al10Cu20Ni8 was investigated by means of small-angle neutron scattering and high-resolution electron microscopy. Partially crystallized states were generated by annealing. The scattering data were analyzed in terms of a model taking into account both properties of the particles and interparticle interference. The mean radius of the particles is 1.3 nm. They are surrounded by a depletion zone with mean thickness of 2.6 nm. The volume fraction of the particles is estimated from the interparticle interference effect; its upper limit after annealing at 653 K for 4 h is 12%. Electron microscopy confirms the size determined from the scattering data and shows that the particles are crystalline.

Structure investigations of thin MoSx films

The influence of process parameters on the formation of microstructure of magnetron-sputtered MoSx-films on silicon wafers was investigated. The MoSx-films were analyzed by means of X-ray diffraction and TEM. The prepared MoSx-films show a microstructure in dimension of some nm without any prefered orientation or texture. The X-ray diffraction measurements indicate clearly differences in the structure for various preparation conditions. Simulations provided a first structural model for the description of the MoSx-films.The MoSx-films consist of two-dimensional nanocrystals and noncrystallographic randomly packed (001) planes. Transitions between these structural states and intermediate states determine the microstructure depending on the preparation conditions.

Structural and magnetic properties of amorphous alloys

Structural and magnetic properties of rapidly quenched amorphous alloys were investigated as a function of the iron content (x = 0 to 20). The observed dependence of the atomic pair correlation functions on the Fe content points to the random incorporation of the iron atoms into the amorphous structure. The magnetic behaviour is dominated by temperature independent Pauli paramagnetism in the range . Magnetic clusters are determined for x = 20. The appearance of these iron clusters is consistent with the observed random distribution of iron atoms in the amorphous alloy. The minimum size of an iron cluster required for the creation of a non-zero magnetic moment amounts to about 100 atoms.