Klaus Rätzke

Diffusion in metallic glasses and supercooled melts

Amorphous metallic alloys, also called metallic glasses, are of considerable technological importance.The metastability of these systems, which gives rise to various rearrangement processes at elevatedtemperatures, calls for an understanding of their diffusional behavior. From the fundamental point ofview, these metallic glasses are the paradigm of dense random packing. Since the recent discovery ofbulk metallic glasses it has become possible to measure atomic diffusion in the supercooled liquid stateand to study the dynamics of the liquid-to-glass transition in metallic systems. In the present article theauthors review experimental results and computer simulations on diffusion in metallic glasses andsupercooled melts. They consider in detail the experimental techniques, the temperature dependenceof diffusion, effects of structural relaxation, the atom-size dependence, the pressure dependence, theisotope effect, diffusion under irradiation, and molecular-dynamics simulations. It is shown thatdiffusion in metallic glasses is significantly different from diffusion in crystalline metals and involvesthermally activated, highly collective atomic processes. These processes appear to be closely related tolow-frequency excitations. Similar thermally activated collective processes were also found to mediatediffusion in the supercooled liquid state well above the caloric glass transition temperature. Thisstrongly supports the mode-coupling scenario of the glass transition, which predicts an arrest ofliquidlike flow already at a critical temperature well above the caloric glass transition temperature.

Liquids with permanent porosity

Porous solids such as zeolites and metal–organic frameworks are useful in molecular separation and in catalysis, but their solid nature can impose limitations. For example, liquid solvents, rather than porous solids, are the most mature technology for post-combustion capture of carbon dioxide because liquid circulation systems are more easily retrofitted to existing plants. Solid porous adsorbents offer major benefits, such as lower energy penalties in adsorption–desorption cycles, but they are difficult to implement in conventional flow processes. Materials that combine the properties of fluidity and permanent porosity could therefore offer technological advantages, but permanent porosity is not associated with conventional liquids. Here we report free-flowing liquids whose bulk properties are determined by their permanent porosity. To achieve this, we designed cage molecules that provide a well-defined pore space and that are highly soluble in solvents whose molecules are too large to enter the pores. The concentration of unoccupied cages can thus be around 500 times greater than in other molecular solutions that contain cavities, resulting in a marked change in bulk properties, such as an eightfold increase in the solubility of methane gas. Our results provide the basis for development of a new class of functional porous materials for chemical processes, and we present a one-step, multigram scale-up route for highly soluble ‘scrambled’ porous cages prepared from a mixture of commercially available reagents. The unifying design principle for these materials is the avoidance of functional groups that can penetrate into the molecular cage cavities.

Aging and free volume in a polymer of intrinsic microporosity (PIM-1)

There is a growing market for polymeric gas separation membranes for applications such as air separation and carbon dioxide capture. One of the key properties dominating transport is the free volume between atoms, allowing gas diffusion. However, thin films, in particular, undergo aging, decreasing free volume, and, hence, performance with time. We have measured the change in free volume during aging of thin films of a polymer of intrinsic microporosity (PIM-1) by depth-resolved positron annihilation lifetime spectroscopy. For films with thickness, d, smaller than 1 μm, aging is nearly complete after 3 months, whereas for films with d > 1 μm aging continues even after several months. Aging is thickness-and time-dependent and the free volume diffuses through the film to the free surface.

Radiotracer measurements as a sensitive tool for the detection of metal penetration in molecular-based organic electronics

The metallization of organic thin films is a crucial point in the development of molecular electronics. However, there is no method established yet to detect trace amounts of metal atoms in those thin films. Radiotracer measurements can quantify even very small amounts of material penetrating into the bulk, in our case less than 0.01% of a monolayer. Here, the application of this technique on two different well-characterized organic thin film systems (diindenoperylene and pentacene) is demonstrated. The results show that Ag is mainly adsorbed on the surface, but indicate that already at moderate deposition temperatures Ag can penetrate into the organic thin films and agglomerate at the film/substrate interface.

Molecular dynamics approach to structure–property correlation in epoxy resins for thermo-mechanical lifetime modeling

This paper addresses the potential of molecular dynamics simulation for structure-property correlations in epoxy-resins. This is an important topic within a multi-scale framework to lifetime prediction in electronic packaging. For that purpose, epoxy-resins with small systematic variations in chemical structure have been synthesised and then characterised by various thermo-mechanical testing methods. It was found that moisture diffusion showed the greatest response with respect to material and loading parameters such as polarity, free volume, moisture concentration and temperature. Based on a parametric study, modeling approaches of various complexity have been able to show f rst qualitative but then also quantitative agreement. The paper comments further on the accuracy and limits of the method and correlates the calculations with experimental structural analysis results.

Absence of Isotope Effect of Diffusion in a Metallic Glass

The isotope effect E = d ln(D)/d ln(1/√m) of Co diffusion in structurally relaxed Co86Zr14 and Co81Zr19 glasses has been measured by means of a radiotracer technique. Within experimental accuracy no isotope effect was detected (E < 0.04). This suggests a highly cooperative diffusion mechanism. The connection between diffusion and collective low-frequency relaxations in glasses is discussed.

Free Volume and Swelling in Thin Films of Poly(N‐isopropylacrylamide) End‐Capped with n‐Butyltrithiocarbonate

The free volume in thin films of poly(N-isopropylacrylamid) end-capped with n-butyltriocarbonate (nbc-PNIPAM) is probed with positron annihilation lifetime spectroscopy (PALS). The PALS measurements are performed as function of energy to obtain depth profiles of the free volume of nbc-PNIPAM films. The range of nbc-PNIPAM films with thicknesses from 40 to 200 nm is focused. With decreasing film thickness the free volume increases in good agreement with an increase in the maximum swelling capability of the nbc-PNIPAM films. Thus in thin hydrogel films the sorption and swelling behavior is governed by free volume.

Molecular dynamics approach to structure-property correlation in epoxy resins for thermo-mechanical lifetime modeling

This paper addresses the potential of molecular dynamics simulation for structure-property correlations in epoxy-resins. This is an important topic within a multi-scale framework to lifetime prediction in electronic packaging. For that purpose, epoxy-resins with small systematic variations in chemical structure have been synthesised and then characterised by various thermo-mechanical testing methods. It was found that moisture diffusion showed the greatest response with respect to material and loading parameters such as polarity, free volume, moisture concentration and temperature. Based on a parametric study, modeling approaches of various complexity have been able to show f rst qualitative but then also quantitative agreement. The paper comments further on the accuracy and limits of the method and correlates the calculations with experimental structural analysis results.

7Be tracer diffusion in a deeply supercooled Zr46.7Ti8.3Cu7.5Ni10Be27.5 melt

Beryllium self-diffusion in the deeply supercooled state of a Zr46.7Ti8.3Cu7.5Ni10Be27.5 alloy has been studied by a radiotracer technique by employing the isotope 7Be in combination with ion-beam sectioning. The temperature dependence of the diffusivity D is Arrhenius like with D(T)=102.6 m2 s−1 exp(−2.6 eV/kBT). The tracer diffusivities consistently extend the size dependence of the diffusion observed in the supercooled liquid state of this alloy to the smallest constituent, but are at variance with previously published results of chemical Be diffusion.

Codiffusion of P32 and Co57 in glass-forming Pd43Cu27Ni10P20 alloy and its relation to viscosity

In general, the Stokes-Einstein (SE) equation is well accepted for melts. In the supercooled melt a decoupling of diffusivity and viscosity around the critical temperature Tc of the mode coupling theory is observed. The authors measured simultaneously the P32 and Co57 diffusion in the supercooled melt of the Pd43Cu27Ni10P20 alloy from 573 up to 640K using the radiotracer technique. They found that P and Co have similar diffusivities and compared them to viscosity data using SE equation. This shows that the time scales of Co and P diffusions are decoupled by up to two orders of magnitude from time scales of viscous flow in the supercooled state.