Florian Kargl

Channel formation and intermediate range order in sodium silicate melts and glasses

We use inelastic neutron scattering and molecular dynamics simulation to investigate the interplay between the structure and the fast sodium ion diffusion in various sodium silicates. With increasing temperature and decreasing density the structure factors exhibit an emerging prepeak around 0.9 A(-1). We show that this prepeak has its origin in the formation of sodium rich channels in the static structure. The channels serve as preferential ion conducting pathways in the relative immobile Si-O matrix. On cooling below the glass transition this intermediate range order is frozen in.

Self diffusion in liquid aluminium

Here we report temperature dependent self-diffusion coefficients of liquid aluminium measured on absolute scale by using incoherent quasielastic neutron scattering at temperatures of 980K, 1020K, and 1060K. Aluminium self-diffusion coefficients follow an Arrhenius law with an activation energy of 280±70 meV. The Sutherland-Einstein equation relating viscosity to the diffusion coefficient well captures the temperature dependence and absolute values of the here reported aluminium self-diffusion coefficients using the covalent radius of aluminium. A comparison to published molecular dynamics simulation data helps to further narrow down the choice of the potential.

Diffusion and Interdiffusion in Binary Metallic Melts

We discuss the dependence of self- and interdiffusion coefficients on temperature and composition for two prototypical binary metallic melts, Al-Ni and Zr-Ni, in molecular-dynamics (MD) computer simulations and the mode-coupling theory of the glass transition (MCT). Dynamical processes that are mainly entropic in origin slow down mass transport (as expressed through self diffusion) in the mixture as compared to the ideal-mixing contribution. Interdiffusion of chemical species is a competition of slow kinetic modes with a strong thermodynamic driving force that is caused by non-entropic interactions. The combination of both dynamic and thermodynamic effects causes qualitative differences in the concentration dependence of self-diffusion and interdiffusion coefficients. At high temperatures, the thermodynamic enhancement of interdiffusion prevails, while at low temperatures, kinetic effects dominate the concentration dependence, rationalized within MCT as the approach to its ideal-glass transition temperature

Does an icosahedral short-range order prevail in glass-forming Zr-Cu melts?

T_c

Atomic dynamics in binary Zr-Cu liquids

. The Darken equation relating self- and interdiffusion qualitatively reproduces the concentration-dependence in both Zr-Ni and Al-Ni, but quantitatively, the kinetic contributions to interdiffusion can be slower than the lower bound suggested by the Darken equation. As temperature is decreased, the agreement with Darkens equation improves, due to a strong coupling of all kinetic modes that is a generic feature predicted by MCT.

Channel diffusion in sodium silicate melts

We report on investigations of the static structure factors of glass-forming Zr-Cu alloy melts by combination of the containerless processing technique of electrostatic levitation with diffraction of neutron and synchrotron radiation. The partial Bhatia-Thornton structure factors SNN and SNC were determined from the two total structure factors. While it is widely assumed in literature that the good glass-forming ability of Zr-Cu is related to an icosahedral short-range order prevailing in the melt, our partial structure factors demonstrate that the liquid Zr-Cu is not characterized by a dominant icosahedral short-range order.

Self and Chemical Diffusion in Liquid Al-Ag

We studied Cu self-dynamics and liquid density of Zr66.7Cu33.3, Zr35.5Cu64.5, Zr36Cu64, and Zr38.2Cu61.8 liquids using a combination of containerless processing techniques and quasielastic neutron scattering. We show that the composition dependence of the Cu self-dynamics is qualitatively controlled by the liquid packing density. It is hence monotonic within a narrow composition range and not sensitive to small composition variations. Similarly, replacing Cu by Ni results in a slower atomic dynamics in the Zr-Ni liquids, as a consequence of the more dense packing. This is in contrast to the strong composition dependence of the glass-forming behaviour, and the better glass-forming ability of the Zr-Cu over Zr-Ni alloys, which usually favours sluggish liquid dynamics. Thus, in the Zr-Cu case, the glass-forming ability is not directly correlated with liquid dynamics and packing density.

ATLAS-M and Batt-M: Development of flight hardware for MAPHEUS sounding rocket

We used quasielastic neutron scattering and molecular dynamics simulations in order to study the interplay of intermediate range order, atomic dynamics and properties of mass transport in sodium silicate melts. Our results show that the sodium ions align in sodium rich channels that disrupt the Si-O structure and that form a percolation network. The sodium ions diffuse along these channels in a relatively immobile Si-O network. The diffusion via these channels results in a decoupling of incoherent from coherent correlations of the sodium relaxation, as evidenced by simulation and experiment. A characteristic length scale for this channel structure is marked by prepeaks in the elastic structure factors at about q = 0.9 A-1, corresponding to length scales of the order of 7 A.

MSL compatible isothermal furnace insert for high temperature shear-cell diffusion experiments

Self-and chemical diffusion coefficients are reported for molten Al-Ag on the Al-rich side of the phase diagram for Ag concentrations of up to 45at% and for pure liquid Ag. Temperature dependent Ag self-diffusion coefficients were obtained using quasi-elastic neutron scattering. Chemical diffusion coefficients were measured in situ by means of X-ray radiography of a long-capillary furnace. A detailed error analysis for the long-capillary experiments is reported. It is shown that perturbing effects can be detected and that accurate chemical diffusion coefficients can be measured with high precision. It is demonstrated based on Al-Ag20at% that the Darken equation appears to be valid for this system with a thermodynamic factor lower than unity. Furthermore, in Al-Ag it appears that Ag self-diffusion for small Ag concentrations is faster than Al-self-diffusion in liquid Al. This contrasts with observations made for other Al-based melts like Al-Ni and Al-Cu.

Thermophysical Property Measurements of Molten Slag and Welding Flux by Aerodynamic Levitator

Two modules ATLAS-M and Batt-M were especially developed for application in the MAPHEUS (MAterialPHysikalische Experimente Unter Schwerelosigkeit) sounding rocket campaign. They were manufactured and built at the Institute of Material Physics in Space at the German Aeropsace Center (DLR). ATLAS-M is a furnace for short time diffusion measurements achieving heating rates of 7K/s and cooling rates of 6K/s. Atlas-M is well suited for investigation of interdiffusion coefficients on a time scale of <200s. Batt-M is a rechargeable battery module, supplying a peak power of 12.6kW with a nominal capacity of 4.6 Ah for operation.