17 results
Search Results
Now showing 1 - 10 of 17
- ItemOn the fundamental relation of laser schlieren deflectometry for temperature measurements in filamentary plasmas(Les Ulis : EDP Sciences, 2015) Schäfer, Jan; Bonaventura, Zdeněk; Foest, RüdigerRecently, laser schlieren deflectometry (LSD) had been successfully employed as a temperature measurement method to reveal the heat convection generated by micro filaments of a self-organized non-thermal atmospheric plasma jet. Based on the theory of the temperature measurements using LSD, in this work, three approaches for an application of the method are introduced: (i) a hyperbolic-like model of refractive index is applied which allows an analytical theory for the evaluation of the deflection angle to be developed, (ii) a Gaussian shape model for the filament temperature is implemented which is analyzed numerically and (iii) an experimental calibration of the laser deflection with a gas mixture of helium and argon is performed. Thus, these approaches demonstrate that a universal relation between the relative maximum temperature of the filament core (T1/T0) and a the maximum deflection angle δ1 of the laser beam can be written as T1/T0=(1 − δ1/δ0)−1, where δ0 is a parameter that is defined by the configuration of the experiment and by the assumed model for the shape of the temperature profile.
- ItemSurface modification of the laser sintering standard powder polyamide 12 by plasma treatments(Weinheim : Wiley-VCH, 2018-6-7) Almansoori, Alaa; Masters, Robert; Abrams, Kerry; Schäfer, Jan; Gerling, Torsten; Majewski, Candice; Rodenburg, CorneliaPolyamide 12 (PA12) powder was exposed for up to 3 h to low pressure air plasma treatment (LP-PT) and several minutes by two different atmospheric pressure plasma jets (APPJ) i.e., kINPen (K-APPJ) and Hairline (H-APPJ). The chemical and physical changes resulting from LP-PT were observed by a combination of Scanning Electron Microscopy (SEM), Hot Stage Microscopy (HSM) and Fourier transform infrared spectroscopy (FTIR), which demonstrated significant changes between the plasma treated and untreated PA12 powders. PA12 exposed to LP-PT showed an increase in wettability, was relatively porous, and possessed a higher density, which resulted from the surface functionalization and materials removal during the plasma exposure. However, it showed poor melt behavior under heating conditions typical for Laser Sintering. In contrast, brief PJ treatments demonstrated similar changes in porosity, but crucially, retained the favorable melt characteristics of PA12 powder.
- ItemPlasma-based VAD process for multiply doped glass powders and high-performance fiber preforms with outstanding homogeneity(Hoboken, NJ : Wiley Interscience, 2020) Trautvetter, Tom; Schäfer, Jan; Benzine, Omar; Methling, Ralf; Baierl, Hardy; Reichel, Volker; Dellith, Jan; Köpp, Daniel; Hempel, Frank; Stankov, Marjan; Baeva, Margarita; Foest, Rüdiger; Wondraczek, Lothar; Wondraczek, Katrin; Bartelt, HartmutAn innovative approach using the vapor axial deposition (VAD), for the preparation of silica-based high-power fiber laser preforms, is described in this study. The VAD uses a plasma deposition system operating at atmospheric pressure, fed by a single, chemically adapted solution containing precursors of laser-active dopants (e.g., Yb2O3), glass-modifier species (e.g., Al2O3), and the silica matrix. The approach enables simultaneous doping with multiple optically active species and overcomes some of the current technological limitations encountered with well-established fiber preform technologies in terms of dopant distribution, doping levels, and achievable active core diameter. The deposition of co-doped silica with outstanding homogeneity is proven by Raman spectroscopy and electron probe microanalysis. Yb2O3 concentrations are realized up to 0.3 mol% in SiO2, with simultaneous doping of 3 mol% of Al2O3.
- ItemInternal Crack Initiation and Growth Starting from Artificially Generated Defects in Additively Manufactured Ti6Al4V Specimen in the VHCF Regime(Basel : MDPI, 2021) Wickmann, Carsten; Benz, Christopher; Heyer, Horst; Witte-Bodnar, Kerstin; Schäfer, Jan; Sander, ManuelaThe aim of the present work was to investigate the ‘fine granular area’ (FGA) formation based on artificially generated internal defects in additively manufactured Ti6Al4V specimens in the early stage of fatigue crack growth in the ‘very high cycle fatigue’ (VHCF) regime. Fatigue tests were performed with constant amplitude at pure tension-compression loading (R = −1) using an ultrasonic fatigue testing setup. Failed specimens were investigated using optical microscopy, high-resolution ‘scanning electron microscopy’ (SEM), and ‘focused ion beam’ (FIB) techniques. Further, the paper introduces alternative proposals to identify the FGA layer beneath the fracture surfaces in terms of the ‘cross section polishing’ (CSP) technique and metallic grindings with special attention paid to the crack origin, the surrounding microstructure, and the expansion of the nanograin layer beneath the fracture surface. Different existing fracture mechanical approaches were applied to evaluate if an FGA formation is possible. Moreover, the results were discussed in comparison to the experimental findings.
- ItemHelixJet: An innovative plasma source for next-generation additive manufacturing (3D printing)(Hoboken, NJ : Wiley Interscience, 2020) Schäfer, Jan; Quade, Antje; Abrams, Kerry J.; Sigeneger, Florian; Becker, Markus M.; Majewski, Candice; Rodenburg, CorneliaA novel plasma source (HelixJet) for use in additive manufacturing (AM)/3D printing is proposed. The HelixJet is a capacitively coupled radio frequency plasma with a double-helix electrode configuration that generates a surprisingly stable and homogeneous glow plasma at low flow rates of argon and its mixtures at atmospheric pressure. The HelixJet was tested on three polyamide powders usually used to produce parts by laser sintering, a powder-based AM process, to form local deposits. The chemical composition of such plasma-printed samples is compared with thermally produced and laser-sintered samples with respect to differences in morphology that result from the different thermal cycles on several length scales. Plasma prints exhibit unique features attributable to the nonequilibrium chemistry and to the high-speed heat exchange.
- ItemSearching for order in atmospheric pressure plasma jets(Bristol : IOP Publ., 2017-11-10) Schäfer, Jan; Sigeneger, Florian; Šperka, Jiří; Rodenburg, Cornelia; Foest, RüdigerThe self-organized discharge behaviour occurring in a non-thermal radio-frequency plasma jet in rare gases at atmospheric pressure was investigated. The frequency of the azimuthal rotation of filaments in the active plasma volume and their inclination were measured along with the gas temperature under varying discharge conditions. The gas flow and heating were described theoretically by a three-dimensional hydrodynamic model. The rotation frequencies obtained by both methods qualitatively agree. The results demonstrate that the plasma filaments forming an inclination angle α with the axial gas velocity uz are forced to a transversal movement with the velocity uφ=tan(α)*uz, which is oriented in the inclination direction. Variations of ${u}_{\phi }$ in the model reveal that the observed dynamics minimizes the energy loss due to convective heat transfer by the gas flow. The control of the self-organization regime motivates the application of the plasma jet for precise and reproducible material processing.
- ItemLiquid assisted plasma enhanced chemical vapour deposition with a non-thermal plasma jet at atmospheric pressure(Amsterdam [u.a.] : Elsevier, 2016) Schäfer, Jan; Fricke, Katja; Mika, Filip; Pokorná, Zuzana; Zajíčková, Lenka; Foest, RüdigerThe present study introduces a process for the synthesis of functional films onto substrates directly from the liquid phase. The reported method is based on the initialization of the synthesis by means of an atmospheric pressure plasma jet operating with argon above a thin liquid film of the starting material. The process is demonstrated by the formation of a thin, solid SiOx film from siloxane-based liquid precursors. Changes in the chemical properties of the precursor were studied in-situ during the polymerization process on the diamond crystal by using Fourier transform infrared spectroscopy The elemental composition of the SiOxCy films was analyzed by X-ray photoelectron spectroscopy (XPS). Furthermore, XPS was applied to study the effect of post-annealing processes on the composition of the films. The obtained deposits exhibit a low concentration of carbon groups. The amount of hydroxyl groups and interstitial water can be reduced significantly by post-process annealing of the films.
- ItemX-ray Ptychographic Imaging and Spectroscopic Studies of Plasma-Treated Plastic Films(Basel : MDPI, 2022) Ravandeh, Mehdi; Mehrjoo, Masoud; Kharitonov, Konstantin; Schäfer, Jan; Quade, Antje; Honnorat, Bruno; Ruiz-Lopez, Mabel; Keitel, Barbara; Kreis, Svea; Pan, Rui; Gang, Seung-gi; Wende, Kristian; Plönjes, ElkePolyethylene terephthalate (PET) is a thermoplastic polyester with numerous applications in industry. However, it requires surface modification on an industrial scale for printing and coating processes and plasma treatment is one of the most commonly used techniques to increase the hydrophilicity of the PET films. Systematic improvement of the surface modification by adaption of the plasma process can be aided by a comprehensive understanding of the surface morphology and chemistry. However, imaging large surface areas (tens of microns) with a resolution that allows understanding the surface quality and modification is challenging. As a proof-of-principle, plasma-treated PET films were used to demonstrate the capabilities of X-ray ptychography, currently under development at the soft X-ray free-electron laser FLASH at DESY, for imaging macroscopic samples. In combination with scanning electron microscopy (SEM), this new technique was used to study the effects of different plasma treatment processes on PET plastic films. The studies on the surface morphology were complemented by investigations of the surface chemistry using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). While both imaging techniques consistently showed an increase in roughness and change in morphology of the PET films after plasma treatment, X-ray ptychography can provide additional information on the three-dimensional morphology of the surface. At the same time, the chemical analysis shows an increase in the oxygen content and polarity of the surface without significant damage to the polymer, which is important for printing and coating processes.
- ItemHighly active heterogeneous hydrogenation catalysts prepared from cobalt complexes and rice husk waste(London : RSC Publ., 2022) Unglaube, Felix; Schlapp, Janina; Quade, Antje; Schäfer, Jan; Mejía, EstebanThe utilization and valorization of agricultural waste is a key strategy for the implementation of a sustainable economy to lessen the environmental footprint of human activities on Earth. This work describes the use of rice husk (RH) from agricultural waste to prepare a highly active catalyst for the reduction of nitro compounds. RH was impregnated with various cobalt complexes bearing N-donor ligands, then pyrolyzed and the resulting composite was etched with a base to remove the silica domains. The composition and morphology of the prepared materials were investigated by IR, AAS, ICP-OES, XRD, BET, XPS and SEM technics. The material showed excellent activity and selectivity in the hydrogenation of nitro groups in aromatic and aliphatic substrates. A remarkable selectivity towards nitro groups was found in the presence of various reactive functionalities, including halogens, carbonyls, borates, and nitriles. Apart from their excellent activity and selectivity, these catalysts showed remarkable stability, allowing their easy recovery and multiple reuse without requiring re-activation.
- ItemAdamant: a JSON schema-based metadata editor for research data management workflows [version 1; peer review: 2 approved](London : F1000 Research Ltd, 2022) Chaerony Siffa, Ihda; Schäfer, Jan; Becker, Markus M.The web tool Adamant has been developed to systematically collect research metadata as early as the conception of the experiment. Adamant enables a continuous, consistent, and transparent research data management (RDM) process, which is a key element of good scientific practice ensuring the path to Findable, Accessible, Interoperable, Reusable (FAIR) research data. It simplifies the creation of on-demand metadata schemas and the collection of metadata according to established or new standards. The approach is based on JavaScript Object Notation (JSON) schema, where any valid schema can be presented as an interactive web-form. Furthermore, Adamant eases the integration of numerous available RDM methods and software tools into the everyday research activities of especially small independent laboratories. A programming interface allows programmatic integration with other software tools such as electronic lab books or repositories. The user interface (UI) of Adamant is designed to be as user friendly as possible. Each UI element is self-explanatory and intuitive to use, which makes it accessible for users that have little to no experience with JSON format and programming in general. Several examples of research data management workflows that can be implemented using Adamant are introduced. Adamant (client-only version) is available from: https://plasma-mds.github.io/adamant.