Repository for natural sciences and technology
The repository is used for the free publication of scientific publications from the fields of natural sciences and technology in the sense of Open Access and is open to all authors. The publications placed in the repository are permanently freely available and citable worldwide. The repository is operated by the Technical Information Library (TIB).
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- ItemDevelopment of Photocatalytically Active Anodized Layers by a Modified Phosphoric Acid Anodizing Process for Air Purification(Basel : MDPI, 2023) Lederer, Stephan; Benfer, Sigrid; Bloh, Jonathan; Javed, Rezan; Pashkova, Aneta; Fuerbeth, WolframOne of the key urban air quality issues is pollution by nitrogen oxides (NOx). To reduce NOx, facade cladding could be provided with photocatalytic properties by incorporating titanium dioxide nanoparticles. For this purpose, a modified phosphoric acid anodizing process (MPAA) was developed for the facade alloy EN AW-5005, in which highly ordered anodized structures with a low degree of arborization and tortuosity were produced. Pore widths between 70 nm and 150 nm and layer thicknesses of about 2–3 μm were obtained. The subsequent impregnation was carried out by dip coating from water-based systems. Depending on the dip-coating parameters and the suspension used, the pores can be filled up to 60% with the TiO2 nanoparticles. Photocatalytic tests according to ISO 22197-1 certify a high photocatalytic activity was obtained with rPCE values > 8 and with rPCE > 2, achieving “photocatalytically active for air purification”. Tests on the corrosion resistance of the anodized coatings with a commercially available aluminum and facade cleaner confirm a protective effect of the anodized coatings when compared with nonanodized aluminum material, as well as with compacted anodized layers.
- ItemPlasma Electrolytic Oxidation on Magnesium AZ31 with Sepiolite as Inhibitor Carrier for Improved Corrosion Protection(Basel : MDPI, 2023) Sottor, Robert; Gruen, Ricarda; Kremmer, Kerstin; Lederer, Stephan; Schneider, Michael; Fuerbeth, WolframPlasma electrolytic oxidation (PEO) in an alkaline silicate electrolyte containing nanosized sepiolite fibers was carried out on magnesium alloy AZ31. The mineral fibers were loaded with different corrosion inhibitors and incorporated in situ during the PEO treatment. The composition and microstructure of the PEO coatings were investigated by SEM. It was shown that the fibers are located on the surface as well as inside the “weak spots” of the coating, i.e., pores and discharge channels. The fixation of the particles is caused by sintering due to the heat developed during the PEO treatment. Investigations using electrochemical impedance spectroscopy and linear sweep voltammetry in 0.01 M NaCl solution confirmed an improvement of the corrosion protection. The use of the inhibitors shifts the critical pitting potential in the anodic direction. Regarding efficiency, cerium-loaded sepiolite showed the best behavior by shifting the pitting potential by +0.9 V.
- ItemMechanisms of Secondary Spreading and Micro Droplet Formation on Steel(Bristol : IOP Publishing, 2024-12) Seeger, Lea; Lobnig, Renate; Schuch, Elmar; Guo, Surong; Fürbeth, WolframA new theory for secondary spreading based on the wetting theory of thin films is presented. It explains how micro droplets within the spreading zone and the primary droplet retain their shape, although connected by a thin electrolyte film and how humidity and salt concentration affect the growth rate of micro droplets. The trigger for secondary spreading, polarization or alkalization, is identified by using droplets of sodium hydroxide solution. Secondary spreading thus occurs on steel from pH 13.5 without corrosion or external polarization. The limiting pH value found explains why secondary spreading on steel only occurs when certain salts are used. The effect of the substrate is investigated by changing the microstructure of the steel. By comparing the sizes of micro droplets and micro structural phases and by scanning electron microscopy/energy-dispersive X-ray analysis measurements of the spreading zone, the existence of an electrolyte film connecting the micro droplets is supported. Ecorr potential profiles of secondary spreading droplets of sodium chloride solution on steel acquired by means of SKP are used to assess the contribution of secondary spreading to the total corrosion current, which is estimated to be low compared to that of the cathodic zone at the edge of the droplet.
- ItemOne-Step Spark Plasma Erosion Processing of Carbon-Coated Sn-Si Nanoparticles for Lithium-Ion Battery Anodes(Basel : MDPI, 2024-09-10) White, Emma Marie Hamilton; Rueschhoff, Lisa M.; Kobayashi, Takeshi; Bloh, Jonathan Z.; Martin, Steve W.; Anderson, Iver E.High density portable energy storage is desirable owing to the energy requirements of portable electronics and electric vehicles. The Li-ion battery’s high energy density could be even further improved through the utilization of alternative materials (instead of carbon) for the anode, such as Sn or Si. Nonetheless, the large volume expansion upon lithiation, up to ~300% for Li22Si5, causes pulverization and rapid capacity degradation during cycling. Sn also forms a Li22Sn5 compound with the equivalent stoichiometric Li capacity but with enhanced ductility. Nano-sized Si and Sn have demonstrated distinctive nanoscale properties, facilitating the retention of higher capacities, particularly when coated with carbon, which improves mechanical stability. To date, the methods of synthesizing coated Si, Sn, or Si-Sn alloyed nanoparticles are complicated, costly, and not readily scalable to meet the demands of cost-effective manufacturing. Spark plasma erosion in a hydrocarbon dielectric has been explored as a one-step process to produce Sn-Si alloy nanoparticles coated with a thin carbon film, offering a scalable and cost-effective processing route. The resulting Sn-Si particles exhibited a bi-modal size distribution at ~5 nm and ~500 nm and were carbon-coated, as intended, from the hydrocarbon dielectric breakdown. The spark-eroded nanoparticles were thoroughly characterized using TEM/EDS, XPS, AES, SSNMR, and TGA, and their improved electrochemical performance was assessed through half-cell experiments.
- ItemThe Influence of Mo Content and Annealing on the Oxidation Behavior of Arc-Melted Cr–xMo–8Si Alloys(Weinheim : Wiley-VCH Verl., 2024-03-21) Koliotassis, Lisa; White, Emma M. H.; Galetz, Mathias C.The influence of Mo (10-40at.%) additions to Cr-8at.%Si was systematically studied by producing arc melted alloys and analyzing the as-cast and annealed microstructures as well as the oxidation resistances. The majority of the alloys contained a solid solution A2 matrix with the intermetallic A15 = (Cr,Mo)3Si phase. The oxidation resistance was tested by thermogravimetric analysis (TGA) at 1200 °C in air for 50 and 100 h. Samples were characterized by XRD, SEM and EPMA (WDS). The studied Cr-xMo-8Si alloy series showed a beneficial effect of Mo on the corrosion behavior of the annealed arc melted alloys exposed at 1200 °C in air. An increased amount of A15 led to the formation of a continuous SiO2 scale beneath the Cr2O3 layer during high temperature exposure. The internal SiO2 layer inhibited further internal oxidation of the A15 phase. Prior annealing also influenced the oxidation behavior of the alloys. Higher annealing temperatures, resulted in an overall reduced fraction of the A15 phase, as well as coarsening, but also showed a more uniform size and distribution of the precipitates, which seems to benefit the formation of a protective duplex oxide scale. Cr-25Mo-8at.%Si annealed at 1350 °C for 100 h, showed the most promising oxidation behavior after exposure in air at 1200 °C for 100 h. For all of the investigated Mo contents, no nitridation was observed when compared to the Cr-8Si reference composition. The oxide scale adhesion also was improved. For Mo >25 at.% no spallation was observed. Evaluation of the TGA data showed it follows the paralinear law, with both the growth rate, kp, and volatilization rate, kv, of Cr2O3 being reduced by adding 10-25 at.% Mo. A binary Cr/Mo-Si phase diagram was generated for the studied alloy series in order to determine accurate annealing conditions and estimate the stability range of the metastable σ phase.
- ItemPhotocatalytic Synthesis of Hydrogen Peroxide from Molecular Oxygen and Water([Cham] : Springer International Publishing, 2023-05-09) Garcia‑Munoz, Patricia; Valenzuela, Laura; Wegstein, Deborah; Schanz, Tobias; Lopez, Girlie Eunice; Ruppert, Agnieszka M.; Remita, Hynd; Bloh, Jonathan Z.; Keller, NicolasHydrogen peroxide is a powerful and green oxidant that allows for the oxidation of a wide span of organic and inorganic substrates in liquid media under mild reaction conditions, and forms only molecular water and oxygen as end products. Hydrogen peroxide is therefore used in a wide range of applications, for which the well-documented and established anthraquinone autoxidation process is by far the dominating production method at the industrial scale. As this method is highly energy consuming and environmentally costly, the search for more sustainable synthesis methods is of high interest. To this end, the article reviews the basis and the recent development of the photocatalytic synthesis of hydrogen peroxide. Different oxygen reduction and water oxidation mechanisms are discussed, as well as several kinetic models, and the influence of the main key reaction parameters is itemized. A large range of photocatalytic materials is reviewed, with emphasis on titania-based photocatalysts and on high-prospect graphitic carbon nitride-based systems that take advantage of advanced bulk and surface synthetic approaches. Strategies for enhancing the performances of solar-driven photocatalysts are reported, and the search for new, alternative, photocatalytic materials is detailed. Finally, the promise of in situ photocatalytic synthesis of hydrogen peroxide for water treatment and organic synthesis is described, as well as its coupling with enzymes and the direct in situ synthesis of other technical peroxides.
- ItemFate and Reactivity of Peroxides Formed over BiVO4 Anodes in Bicarbonate Electrolytes(Washington, DC : American Chemical Society, 2023-02-16) Schanz, Tobias; Burek, Bastien O.; Bloh, Jonathan Z.[no abstract available]
- ItemAnodic peroxide production for advanced oxidation processes with different metal oxide electrodes in carbonate electrolytes(London : Royal Society of Chemistry, 2024-07-19) Schanz, Tobias; Bloh, Jonathan Z.As an alternative to the anthraquinone process that can be used directly on site without storage and transport, electrochemical peroxide synthesis is a promising technology to produce reagents for water remediation via Advanced Oxidation Processes (AOP). The focus of research here is on anodic peroxide production, since cathodic synthesis is already at a high degree of maturity. Different materials and electrolytes have been reported for the anode reactions so far. It has also been shown that some electrolytes such as carbonate-based ones lead to the formation of secondary peroxides such as percarbonates which are well-suited as oxidants for AOP. Herein, these materials and electrolytes are evaluated under different conditions with particular focus on the actual oxidation power of the formed product mixtures.
- ItemAdditively manufactured Ti2Mo for biomedical applications: surface engineering and corrosion properties(London : F1000 Research Ltd, 2024) Zander, Julian P.; Lederer, Stephan; Fürbeth, WolframAdditive manufacturing allows the fast production of small and complex parts with minimum waste for biomedical applications made of titanium alloys. However, the cytotoxicity of certain elements being widely used in these alloys has become a concern, initiating the development of new alloys avoiding their use as alloying elements. In this publication, surface engineering techniques like blasting and plasma electrolytic polishing have been applied to a new Ti2Mo alloy obtained by Selective Laser Melting (SLM) in order to enhance its surface properties. First aim of such surface engineering processes is to develop standardized surfaces for additively manufactured parts, comparable to conventionally produced parts. Residual stresses that arise due to the additive manufacturing process are examined as well as their potential impact on the mechanical and corrosion properties of the alloys. Via suitable heat treatment residual stresses can be lowered and adjusted. Additionally, the corrosion properties are determined using potentiodynamic polarization and tribocorrosion testing.
- ItemCombined anodic and cathodic peroxide production in an undivided carbonate/bicarbonate electrolyte with 144% combined current efficiency(Lausanne : Frontiers Media, 2024-05-17) Schanz, Tobias; Stöckl, Markus; Burek, Bastien O.; Holtmann, Dirk; Bloh, Jonathan Z.In recent years, the electrochemical synthesis of peroxides has attracted renewed interest as a potential environmentally friendly production compared to the established anthraquinone process. In addition, it is possible to produce the peroxides directly on site, eliminating the need for expensive and hazardous transportation and storage. Cathodic production of hydrogen peroxide from oxygen is already quite well developed. Anodic production from water, on the other hand, is still facing significant challenges, despite its historic pioneering role. In this manuscript we show that anodic and cathodic synthesis of peroxides can even be combined to achieve greater than 100% current efficiency (CE) due to the combined effect of both half-reactions. So far, similar devices have always employed different electrolytes for each, which necessitated the use of a membrane and posed contamination risk. However, herein we show that both half-reactions can also employ the same electrolyte. This enables even an undivided cell, omitting the need for the expensive membranes. Despite its simplicity, this setup yielded an outstanding performance with a combined CE of 144%.