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- ItemMicroscopic Softening Mechanisms of an Ionic Liquid Additive in an Electrically Conductive Carbon-Silicone Composite(Weinheim : Wiley, 2022) Zhang, Long; Schmidt, Dominik S.; González‐García, Lola; Kraus, TobiasThe microstructural changes caused by the addition of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to polydimethylsiloxane (PDMS) elastomer composites filled with carbon black (CB) are analyzed to explain the electrical, mechanical, rheological, and optical properties of IL-containing precursors and composites. Swelling experiments and optical analysis indicate a limited solubility of the IL in the PDMS matrix that reduces the cross-linking density of PDMS both globally and locally, which reduces the Young's moduli of the composites. A rheological analysis of the precursor mixture shows that the IL reduces the strength of carbon–carbon and carbon–PDMS interactions, thus lowering the filler–matrix coupling and increasing the elongation at break. Electromechanical testing reveals a combination of reversible and irreversible piezoresistive responses that is consistent with the presence of IL at microscopic carbon–carbon interfaces, where it enables re-established electrical connections after stress release but reduces the absolute conductivity.
- ItemPreparation and characterization of Na2O-Al2O3-B2O3 sol-gel glasses with aluminum lactate and formiate as precursors(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2005) Hoyer, Lars P.; Helsch, Gundula; Frischat, Günther Heinz; Zhang, Long; Eckert, HellmutIn this work aluminum lactate and aluminum formiate have been used as precursors to obtain room temperature stable sols and gels and after annealing homogeneous glasses in the System Na2O-Al2O3-B2O3. The local environments and connectivities of boron and aluminum have been investigated by 11B and 27Al solid-state nuclear magnetic resonance (NMR). It was found that the local atomic structures of the sol-gel glasses markedly depend on the precursors and the preparation routes and are also dissimilar to those of melt quenched glasses of the same compositions. Thus, for example, the fractions of BO3/2 and BO4/2 units differ and it is interesting to note that there are no asymmetric ΒO2/2(O-) units present in the sol-gel materials. The 27Al spectra show AI in four-, five- and sixfold coordination, whose relative abundance in a given glass composition is also dependent on the preparadon route. Rotational echo double resonance (REDOR) 11B {27Al} and 27Al {11B} resuhs indicate that the extent of B-O-Al connectivity is diminished in the gel prepared glasses when compared to the melt cooled glasses. Element distributions are reported on the basis of secondary neutral mass spectrometry (SNMS) data, and the nanostructures of surfaces have been characterized by atomic force microscopy (AFM).
- ItemTowards the better: Intrinsic property amelioration in bulk metallic glasses(London : Nature Publishing Group, 2016) Sarac, Baran; Zhang, Long; Kosiba, Konrad; Pauly, Simon; Stoica, Mihai; Eckert, JürgenTailoring the intrinsic length-scale effects in bulk metallic glasses (BMGs) via post-heat treatment necessitates a systematic analyzing strategy. Although various achievements were made in the past years to structurally enhance the properties of different BMG alloys, the influence of short-term sub-glass transition annealing on the relaxation kinetics is still not fully covered. Here, we aim for unraveling the connection between the physical, (thermo)mechanical and structural changes as a function of selected pre-annealing temperatures and time scales with an in-house developed Cu46Zr44Al8Hf2 based BMG alloy. The controlled formation of nanocrystals below 50 nm with homogenous distribution inside the matrix phase via thermal treatment increase the material’s resistance to strain softening by almost an order of magnitude. The present work determines the design aspects of metallic glasses with enhanced mechanical properties via nanostructural modifications, while postulating a counter-argument to the intrinsic property degradation accounted for long-term annealing.
- ItemPercolation of rigid fractal carbon black aggregates(Melville, NY : American Institute of Physics, 2021) Coupette, Fabian; Zhang, Long; Kuttich, Björn; Chumakov, Andrei; Roth, Stephan V.; González-García, Lola; Kraus, Tobias; Schilling, TanjaWe examine network formation and percolation of carbon black by means of Monte Carlo simulations and experiments. In the simulation, we model carbon black by rigid aggregates of impenetrable spheres, which we obtain by diffusion-limited aggregation. To determine the input parameters for the simulation, we experimentally characterize the micro-structure and size distribution of carbon black aggregates. We then simulate suspensions of aggregates and determine the percolation threshold as a function of the aggregate size distribution. We observe a quasi-universal relation between the percolation threshold and a weighted average radius of gyration of the aggregate ensemble. Higher order moments of the size distribution do not have an effect on the percolation threshold. We conclude further that the concentration of large carbon black aggregates has a stronger influence on the percolation threshold than the concentration of small aggregates. In the experiment, we disperse the carbon black in a polymer matrix and measure the conductivity of the composite. We successfully test the hypotheses drawn from simulation by comparing composites prepared with the same type of carbon black before and after ball milling, i.e., on changing only the distribution of aggregate sizes in the composites.