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- ItemDirectly Anodized Sulfur-Doped TiO2 Nanotubes as Improved Anodes for Li-ion Batteries(Basel : MDPI, 2020) Sabaghi, Davood; Madian, Mahmoud; Omar, Ahmad; Oswald, Steffen; Uhlemann, Margitta; Maghrebi, Morteza; Baniadam, Majid; Mikhailova, DariaTiO2 represents one of the promising anode materials for lithium ion batteries due to its high thermal and chemical stability, relatively high theoretical specific capacity and low cost. However, the electrochemical performance, particularly for mesoporous TiO2, is limited and must be further developed. Elemental doping is a viable route to enhance rate capability and discharge capacity of TiO2 anodes in Li-ion batteries. Usually, elemental doping requires elevated temperatures, which represents a challenge, particularly for sulfur as a dopant. In this work, S-doped TiO2 nanotubes were successfully synthesized in situ during the electrochemical anodization of a titanium substrate at room temperature. The electrochemical anodization bath represented an ethylene glycol-based solution containing NH4F along with Na2S2O5 as the sulfur source. The S-doped TiO2 anodes demonstrated a higher areal discharge capacity of 95 µAh·cm−2 at a current rate of 100 µA·cm−2 after 100 cycles, as compared to the pure TiO2 nanotubes (60 µAh·cm−2). S-TiO2 also exhibited a significantly improved rate capability up to 2500 µA·cm−2 as compared to undoped TiO2. The improved electrochemical performance, as compared to pure TiO2 nanotubes, is attributed to a lower impedance in S-doped TiO2 nanotubes (STNTs). Thus, the direct S-doping during the anodization process is a promising and cost-effective route towards improved TiO2 anodes for Li-ion batteries.
- ItemLightweight polymer-carbon composite current collector for lithium-ion batteries(Basel : MDPI, 2020) Fritsch, Marco; Coeler, Matthias; Kunz, Karina; Krause, Beate; Marcinkowski, Peter; Pötschke, Petra; Wolter, Mareike; Michaelis, AlexanderA hermetic dense polymer-carbon composite-based current collector foil (PCCF) for lithium-ion battery applications was developed and evaluated in comparison to state-of-the-art aluminum (Al) foil collector. Water-processed LiNi0.5Mn1.5O4 (LMNO) cathode and Li4Ti5O12 (LTO) anode coatings with the integration of a thin carbon primer at the interface to the collector were prepared. Despite the fact that the laboratory manufactured PCCF shows a much higher film thickness of 55 µm compared to Al foil of 19 µm, the electrode resistance was measured to be by a factor of 5 lower compared to the Al collector, which was attributed to the low contact resistance between PCCF, carbon primer and electrode microstructure. The PCCF-C-primer collector shows a sufficient voltage stability up to 5 V vs. Li/Li+ and a negligible Li-intercalation loss into the carbon primer. Electrochemical cell tests demonstrate the applicability of the developed PCCF for LMNO and LTO electrodes, with no disadvantage compared to state-of-the-art Al collector. Due to a 50% lower material density, the lightweight and hermetic dense PCCF polymer collector offers the possibility to significantly decrease the mass loading of the collector in battery cells, which can be of special interest for bipolar battery architectures. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemAuger- and X-ray Photoelectron Spectroscopy at Metallic Li Material: Chemical Shifts Related to Sample Preparation, Gas Atmosphere, and Ion and Electron Beam Effects(Basel : MDPI, 2022) Oswald, SteffenLi-based batteries are a key element in reaching a sustainable energy economy in the near future. The understanding of the very complex electrochemical processes is necessary for the optimization of their performance. X-ray photoelectron spectroscopy (XPS) is an accepted method used to improve understanding around the chemical processes at the electrode surfaces. Nevertheless, its application is limited because the surfaces under investigation are mostly rough and inhomogeneous. Local elemental analysis, such as Auger electron spectroscopy (AES), could assist XPS to gain more insight into the chemical processes at the surfaces. In this paper, some challenges in using electron spectroscopy are discussed, such as binding energy (BE) referencing for the quantitative study of chemical shifts, gas atmospheric influences, or beam damage (including both AE and XP spectroscopy). Carefully prepared and surface-modified metallic lithium material is used as model surface, considering that Li is the key element for most battery applications.
- ItemViscous Flow of Supercooled Liquid in a Zr-Based Bulk Metallic Glass Synthesized by Additive Manufacturing(Basel : MDPI, 2020) Kosiba, Konrad; Deng, Liang; Scudino, SergioThe constraint in sample size imposed by the critical cooling rate necessary for glass formation using conventional casting techniques is possibly the most critical limitation for the extensive use of bulk metallic glasses (BMGs) in structural applications. This drawback has been recently overcome by processing glass-forming systems via additive manufacturing, finally enabling the synthesis of BMGs with no size limitation. Although processing by additive manufacturing allows fabricating BMG objects with virtually no shape limitation, thermoplastic forming of additively manufactured BMGs may be necessary for materials optimization. Thermoplastic forming of BMGs is carried out above the glass transition temperature, where these materials behave as highly viscous liquids; the analysis of the viscosity is thus of primary importance. In this work, the temperature dependence of viscosity of the Zr52.5Cu17.9Ni14.6Al10Ti5 metallic glass fabricated by casting and laser powder bed fusion (LPBF) is investigated. We observed minor differences in the viscous flow of the specimens fabricated by the different techniques that can be ascribed to the higher porosity of the LPBF metallic glass. Nevertheless, the present results reveal a similar overall variation of viscosity in the cast and LPBF materials, which offers the opportunity to shape additively manufactured BMGs using already developed thermoplastic forming techniques.
- ItemStructural and Chemical Hierarchy in Hydroxyapatite Coatings(Basel : MDPI, 2020) Gross, Karlis A.; Petzold, Christiane; Pluduma-LaFarge, Liene; Kumermanis, Maris; HAugen, Håvard J.Hydroxyapatite coatings need similarly shaped splats as building blocks and then a homogeneous microstructure to unravel the structural and chemical hierarchy for more refined improvements to implant surfaces. Coatings were thermally sprayed with differently sized powders (20–40, 40–63 and 63–80 µm) to produce flattened homogeneous splats. The surface was characterized for splat shape by profilometry and Atomic force microscopy (AFM), crystal size by AFM, crystal orientation by X-ray diffraction (XRD) and structural variations by XRD. Chemical composition was assessed by phase analysis, but variations in chemistry were detected by XRD and Raman spectroscopy. The resulting surface electrical potential was measured by Kelvin probe AFM. Five levels of structural hierarchy were suggested: the coating, the splat, oriented crystals, alternate layers of oxyapatite and hydroxyapatite (HAp) and the suggested anion orientation. Chemical hierarchy was present over a lower range of order for smaller splats. Coatings made from smaller splats exhibited a greater electrical potential, inferred to arise from oxyapatite, and supplemented by ordered OH− ions in a rehydroxylated surface layer. A model has been proposed to show the influence of structural hierarchy on the electrical surface potential. Structural hierarchy is proposed as a means to further refine the properties of implant surfaces.
- ItemStress and Microstructure Evolution in Mo Thin Films without or with Cover Layers during Thermal-Cycling(Basel : MDPI, 2020) Park, Eunmi; Seifert, Marietta; Rane, Gayatri K.; Menzel, Siegfried B.; Gemming, Thomas; Nielsch, KorneliusThe intrinsic stress behavior and microstructure evolution of Molybdenum thin films were investigated to evaluate their applicability as a metallization in high temperature microelectronic devices. For this purpose, 100 nm thick Mo films were sputter-deposited without or with an AlN or SiO2 cover layer on thermally oxidized Si substrates. The samples were subjected to thermal cycling up to 900 °C in ultrahigh vacuum; meanwhile, the in-situ stress behavior was monitored by a laser based Multi-beam Optical Sensor (MOS) system. After preannealing at 900 °C for 24 h, the uncovered films showed a high residual stress at room temperature and a plastic behavior at high temperatures, while the covered Mo films showed an almost entirely elastic deformation during the thermal cycling between room temperature and 900 °C with hardly any plastic deformation, and a constant stress value during isothermal annealing without a notable creep. Furthermore, after thermal cycling, the Mo films without as well as with a cover layer showed low electrical resistivity (≤10 μΩ·cm).
- ItemA non-cytotoxic resin for micro-stereolithography for cell cultures of HUVECs(Basel : MDPI, 2020) Männel, Max J.; Fischer, Carolin; Thiele, JulianThree-dimensional (3D) printing of microfluidic devices continuously replaces conventional fabrication methods. A versatile tool for achieving microscopic feature sizes and short process times is micro-stereolithography (µSL). However, common resins for µSL lack biocompatibility and are cytotoxic. This work focuses on developing new photo-curable resins as a basis for µSL fabrication of polymer materials and surfaces for cell culture. Different acrylate-and methacrylate-based compositions are screened for material characteristics including wettability, surface roughness, and swelling behavior. For further understanding, the impact of photo-absorber and photo-initiator on the cytotoxicity of 3D-printed substrates is studied. Cell culture experiments with human umbilical vein endothelial cells (HUVECs) in standard polystyrene vessels are compared to 3D-printed parts made from our library of homemade resins. Among these, after optimizing material composition and post-processing, we identify selected mixtures of poly(ethylene glycol) diacrylate (PEGDA) and poly(ethylene glycol) methyl ethyl methacrylate (PEGMEMA) as most suitable to allow for fabricating cell culture platforms that retain both the viability and proliferation of HUVECs. Next, our PEGDA/PEGMEMA resins will be further optimized regarding minimal feature size and cell adhesion to fabricate microscopic (microfluidic) cell culture platforms, e.g., for studying vascularization of HUVECs in vitro. © 2020 by the authors.
- ItemHydrogel microvalves as control elements for parallelized enzymatic cascade reactions in microfluidics(Basel : MDPI, 2020) Obst, Franziska; Beck, Anthony; Bishayee, Chayan; Mehner, Philipp J.; Richter, Andreas; Voit, Brigitte; Appelhans, DietmarCompartmentalized microfluidic devices with immobilized catalysts are a valuable tool for overcoming the incompatibility challenge in (bio) catalytic cascade reactions and high-throughput screening of multiple reaction parameters. To achieve flow control in microfluidics, stimuli-responsive hydrogel microvalves were previously introduced. However, an application of this valve concept for the control of multistep reactions was not yet shown. To fill this gap, we show the integration of thermoresponsive poly(N-isopropylacrylamide) (PNiPAAm) microvalves (diameter: 500 and 600 µm) into PDMS-on-glass microfluidic devices for the control of parallelized enzyme-catalyzed cascade reactions. As a proof-of-principle, the biocatalysts glucose oxidase (GOx), horseradish peroxidase (HRP) and myoglobin (Myo) were immobilized in photopatterned hydrogel dot arrays (diameter of the dots: 350 µm, amount of enzymes: 0.13-2.3 µg) within three compartments of the device. Switching of the microvalves was achieved within 4 to 6 s and thereby the fluid pathway of the enzyme substrate solution (5 mmol/L) in the device was determined. Consequently, either the enzyme cascade reaction GOx-HRP or GOx-Myo was performed and continuously quantified by ultraviolet-visible (UV-Vis) spectroscopy. The functionality of the microvalves was shown in four hourly switching cycles and visualized by the path-dependent substrate conversion. © 2020 by the authors.
- ItemHydrogel patterns in microfluidic devices by do-it-yourself UV-photolithography suitable for very large-scale integration(Basel : MDPI, 2020) Beck, Anthony; Obst, Franziska; Busek, Mathias; Grünzner, Stefan; Mehner, Philipp J.; Paschew, Georgi; Appelhans, Dietmar; Voit, Brigitte; Richter, AndreasThe interest in large-scale integrated (LSI) microfluidic systems that perform highthroughput biological and chemical laboratory investigations on a single chip is steadily growing. Such highly integrated Labs-on-a-Chip (LoC) provide fast analysis, high functionality, outstanding reproducibility at low cost per sample, and small demand of reagents. One LoC platform technology capable of LSI relies on specific intrinsically active polymers, the so-called stimuli-responsive hydrogels. Analogous to microelectronics, the active components of the chips can be realized by photolithographic micro-patterning of functional layers. The miniaturization potential and the integration degree of the microfluidic circuits depend on the capability of the photolithographic process to pattern hydrogel layers with high resolution, and they typically require expensive cleanroom equipment. Here, we propose, compare, and discuss a cost-efficient do-it-yourself (DIY) photolithographic set-up suitable to micro-pattern hydrogel-layers with a resolution as needed for very large-scale integrated (VLSI) microfluidics. The achievable structure dimensions are in the lower micrometer scale, down to a feature size of 20 µm with aspect ratios of 1:5 and maximum integration densities of 20,000 hydrogel patterns per cm. Furthermore, we demonstrate the effects of miniaturization on the efficiency of a hydrogel-based microreactor system by increasing the surface area to volume (SA:V) ratio of integrated bioactive hydrogels. We then determine and discuss a correlation between ultraviolet (UV) exposure time, cross-linking density of polymers, and the degree of immobilization of bioactive components. © 2020 by the authors.
- ItemSurface modification of silicon nanowire based field effect transistors with stimuli responsive polymer brushes for biosensing applications(Basel : MDPI, 2020) Klinghammer, Stephanie; Rauch, Sebastian; Pregl, Sebastian; Uhlmann, Petra; Baraban, Larysa; Cuniberti, GianaurelioWe demonstrate the functionalization of silicon nanowire based field effect transistors (SiNW FETs) FETs with stimuli-responsive polymer brushes of poly(N-isopropylacrylamide) (PNIPAAM) and poly(acrylic acid) (PAA). Surface functionalization was confirmed by atomic force microscopy, contact angle measurements, and verified electrically using a silicon nanowire based field effect transistor sensor device. For thermo-responsive PNIPAAM, the physicochemical properties (i.e., a reversible phase transition, wettability) were induced by crossing the lower critical solution temperature (LCST) of about 32 C. Taking advantage of this property, osteosarcomic SaoS-2 cells were cultured on PNIPAAM-modified sensors at temperatures above the LCST, and completely detached by simply cooling. Next, the weak polyelectrolyte PAA, that is sensitive towards alteration of pH and ionic strength, was used to cover the silicon nanowire based device. Here, the increase of pH will cause deprotonation of the present carboxylic (COOH) groups along the chains into negatively charged COO- moieties that repel each other and cause swelling of the polymer. Our experimental results suggest that this functionalization enhances the pH sensitivity of the SiNW FETs. Specific receptor (bio-)molecules can be added to the polymer brushes by simple click chemistry so that functionality of the brush layer can be tuned optionally. We demonstrate at the proof-of concept-level that osteosarcomic Saos-2 cells can adhere to PNIPAAM-modified FETs, and cell signals could be recorded electrically. This study presents an applicable route for the modification of highly sensitive, versatile FETs that can be applied for detection of a variety of biological analytes. © 2020 by the authors.