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- ItemThe complexity of surface acoustic wave fields used for microfluidic applications(Amsterdam [u.a.] : Elsevier, 2020) Weser, R.; Winkler, A.; Weihnacht, M.; Menzel, S.; Schmidt, H.Using surface acoustic waves (SAW) for the agitation and manipulation of fluids and immersed particles or cells in lab-on-a-chip systems has been state of the art for several years. Basic tasks comprise fluid mixing, atomization of liquids as well as sorting and separation (or trapping) of particles and cells, e.g. in so-called acoustic tweezers. Even though the fundamental principles governing SAW excitation and propagation on anisotropic, piezoelectric substrates are well-investigated, the complexity of wave field effects including SAW diffraction, refraction and interference cannot be comprehensively simulated at this point of time with sufficient accuracy. However, the design of microfluidic actuators relies on a profound knowledge of SAW propagation, including superposition of multiple SAWs, to achieve the predestined functionality of the devices. Here, we present extensive experimental results of high-resolution analysis of the lateral distribution of the complex displacement amplitude, i.e. the wave field, alongside with the electrical S-parameters of the generating transducers. These measurements were carried out and are compared in setups utilizing travelling SAW (tSAW) excited by single interdigital transducer (IDT), standing SAW generated between two IDTs (1DsSAW, 1D acoustic tweezers) and between two pairs of IDTs (2DsSAW, 2D acoustic tweezers) with different angular alignment in respect to pure Rayleigh mode propagation directions and other practically relevant orientations. For these basic configurations, typically used to drive SAW-based microfluidics, the influence of common SAW phenomena including beam steering, coupling coefficient dispersion and diffraction on the resultant wave field is investigated. The results show how tailoring of the acoustic conditions, based on profound knowledge of the physical effects, can be achieved to finally realize a desired behavior of a SAW-based microacoustic-fluidic system. © 2020 Elsevier B.V.
- ItemAcoustic resonance effects and cavitation in SAW aerosol generation(Amsterdam [u.a.] : Elsevier, 2023) Roudini, Mehrzad; Manuel Rosselló, Juan; Manor, Ofer; Ohl, Claus-Dieter; Winkler, AndreasThe interaction of surface acoustic waves (SAWs) with liquids enables the production of aerosols with adjustable droplet sizes in the micrometer range expelled from a very compact source. Understanding the nonlinear acousto-hydrodynamics of SAWs with a regulated micro-scale liquid film is essential for acousto-microfluidics platforms, particularly aerosol generators. In this study, we demonstrate the presence of micro-cavitation in a MHz-frequency SAW aerosol generation platform, which is touted as a leap in aerosol technology with versatile application fields including biomolecule inhalation therapy, micro-chromatography and spectroscopy, olfactory displays, and material deposition. Using analysis methods with high temporal and spatial resolution, we demonstrate that SAWs stabilize spatially arranged liquid micro-domes atop the generator's surface. Our experiments show that these liquid domes become acoustic resonators with highly fluctuating pressure amplitudes that can even nucleate cavitation bubbles, as supported by analytical modeling. The observed fragmentation of liquid domes indicates the participation of three droplet generation mechanisms, including cavitation and capillary-wave instabilities. During aerosol generation, the cavitation bubbles contribute to the ejection of droplets from the liquid domes and also explain observed microstructural damage patterns on the chip surface eventually caused by cavitation-based erosion.
- ItemAdvanced thermal stability investigations of the Mn–Al-Ga system(Amsterdam [u.a.] : Elsevier, 2020) Mix, T.; Woodcock, T.G.A ternary Mn–Al-Ga alloy with the nominal composition Mn55Al38.57Ga6.43 was produced by arc melting. After homogenisation, the alloy consisted of the ε and γ2 phases. Appropriate heat treatments were used to transform each of these into a phase with the L10 structure. These two L10 phases had different compositions, lattice parameters and magnetic properties. In order to test the stability of the L10 phases against decomposition, heat treatments were carried out at 700 °C for durations of up to 14 days. The results showed that the decomposition started with formation of the β-Mn phase and subsequent appearance of the γ2 phase. The resulting diffusion gradients resulted in composition changes in the L10 phases and after 7 days, only a single, intermediate composition remained. After 14 days, the decomposition was almost complete. The decomposition of the L10 phases in the ternary Mn–Al-Ga alloy was significantly slower than in binary Mn–Al alloys.
- ItemNaOH protective layer for a stable sodium metal anode in liquid electrolytes(Amsterdam [u.a.] : Elsevier, 2024) Thomas, Alexander; Pohle, Björn; Schultz, Johannes; Hantusch, Martin; Mikhailova, DariaSodium is known as a soft metal that can easily change its particle morphology. It can form outstretched and rolled fibers with plastic or brittle behavior, and cubes. In Na-batteries, metallic Na anodes demonstrate a high reactivity towards the majority of electrolyte solutions, volume change and a random deposition process from the electrolyte, accompanied by dendrite formation. In order to smooth the electrochemical Na deposition, we propose NaOH as a simple artificial protective layer for sodium, formed by its exposure to ambient conditions for a certain period of time. The formed NaOH layer on top of the metallic sodium suppresses the volume change and dendrite growth on the sodium surface. Additionally, the protected sodium does not change its morphology after a prolonged contact with carbonate-based electrolytes. In symmetric Na-batteries, the NaOH layer increases the lifetime of the electrochemical cell by eight times in comparison to non-protected Na. In the full-cell with a layered sodium oxide cathode, the NaOH-protected sodium anode also leads to a high cycling stability, providing 81 % of the initial cell capacity after 500 cycles with a 1C current rate. In contrast, batteries with a non-protected Na-anode reach only 20 % of their initial capacity under the same conditions. Therefore, the main benefits of the NaOH artificial layer are the chemical compatibility with the carbonate-based electrolytes, the protection of Na metal against reaction with the electrolyte solution, the rapid Na-ion diffusion through the layer and the formation of a mechanical barrier, mitigating Na-dendrite growth. This work presents an easily scalable method to protect sodium without any additional chemicals or a special environment for this reaction.
- ItemProgress and challenges in using sustainable carbon anodes in rechargeable metal-ion batteries(Amsterdam [u.a.] : Elsevier, 2021) Soltani, Niloofar; Bahrami, Amin; Giebeler, Lars; Gemming, Thomas; Mikhailova, DariaRechargeable lithium-ion batteries (LIBs) are one of the most promising alternatives to effectively bypass fossil fuels. However, long-term energy application of LIBs could be restricted in the future due to the increased production cost of LIB arising from the shortage and inaccessibility of Li in the Earth's crust. Na or K have been considered as substitutes for Li but in spite of their natural abundance, they suffer from low gravimetric/volumetric energy density. An alternative to increase the efficiency of sodium-ion battery (SIBs) and potassium-ion battery (KIBs) is to focus on finding the high‐performing negative electrode, the anode. The large volume changes of alloying and conversion type anodes for KIBs and SIBs make hard carbons to a better option on this regard than usual graphitic carbons, but a key obstacle is the reliance on unsustainable sources. Thus, biomass-derived carbon could offer a promising alternative, and it has indeed been in the focus of much recent work. This review highlights the recent advances in using carbon extracted from various biomass sources in rechargeable Li-, Na-, and K-ion batteries. Maximizing the energy and power densities as well as the lifetime of carbon anodes require an exploration of the right balance between carbon structures, pore morphology, chemical composition and alkali metal-ion storage. Thus, in this review, first, we take stock of key challenges and opportunities to extract carbon from various plants structural components and identify the extracted carbon structure compared to graphite-like structure. Then, we provide an overview on morphological and structural modification of the extracted carbons. Finally, we show how the physicochemical properties, structural alignment and morphological variation of the biomass-derived carbon can affect the storage mechanism and electrochemical performance. The extensive overview of this topic provided here is expected to stimulate further work on environmentally friendly battery design and towards the optimization of the battery performance. Electrode materials in alkali-metal-ion batteries that are based on biomass-derived carbon may allow not only a technical breakthrough, but also an ethically and socially acceptable product.
- ItemThe effect of Ti or Zr additions on the microstructure and magnetic properties of MnAl-C alloys(Amsterdam [u.a.] : Elsevier, 2021) Feng, L.; Nielsch, K.; Woodcock, T.As-transformed and hot-deformed samples of MnAl-C alloys with Ti or Zr additions have been produced and characterized using magnetometry, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Both Ti and Zr additions in MnAl-C alloys form carbide primary phases, TiC and ZrC, which consume the carbon meant to be dissolved in the metastable τ-phase to stabilize it against decomposition. With these two additions, the Curie temperature of τ-phase increases while its stability against decomposition decreases. After hot deformation, the MnAl-C alloys with Ti or Zr additions have lower polarisation and remanence due to the reduced stability of the τ-phase. Adding extra carbon along with Ti to a MnAl-C alloy in order to compensate for the C lost on formation of TiC restored the original stability of the τ-phase. After hot-deformation, this alloy exhibited a lower polarisation and remanence owing to the unexpected formation of the γ2-phase.