Photon-induced near-field interaction in ultrafast point-projection electron microscopy. - In: CLEO, (2023), JTu2A.133, insges. 2 S.
We report the first study of ultrafast, slow (<100 eV) free electron wavepackets with optical near fields. This interaction is probed in a point-projection-microscope with 50fs temporal resolution using strongly localized fields around a nano-antenna.
Dielectrophoretic actuation in a microfluidic system with coplanar electrode configuration. - In: IEEE Xplore digital library, ISSN 2473-2001, (2023), insges. 4 S.
This paper presents the analysis of the motion characteristics of droplet transport induced by liquid dielectrophoresis (LDEP). Hereby, a novel fluidic system is demonstrated, which enables droplet transport across a planar surface using a coplanar electrode configuration with adjacent electrodes. The droplet displacement is analyzed by high-speed video imaging and digital image processing. It is demonstrated that droplet motion can be initiated through simplified electrode geometries and the experimental results provide deeper insights into LDEP driven droplet manipulation.
Characterization of plastic-metal hybrid composites joined by means of reactive Al/Ni multilayers - evaluation of occurring thermal regime. - In: Engineering for a changing world, (2023), 4.3.056, S. 1-16
Present challenges in material science and joining technology are ever more subject to the desire for lightweight construction and engineering. Plastic-metal composites are suitable material combinations but also require the development and investigation of appropriate joining technologies. A particularly promising approach is the application of reactive multilayer foils. As an innovative method, these foils provide the possibility of flexible and low-distortion joining of dissimilar materials. The underlying reaction mechanism offers fast exothermic reaction propagation with well-known exothermic power output while the energy source is introduced directly into the joining zone. In this work, hybrid lap joints between semi-crystalline polyamide 6 and structured austenitic stainless steel X5CrNi18-10 were joined using reactive Al/Ni multilayer foils. The self-propagating reaction provides immediate temperatures that are well above the melting point of used plastic but decays rapidly after only a few milliseconds. To support ongoing investigations regarding composite formation, analysis of occurring thermal regime is in the focus of this work. Conducted experiments are supported by accompanying thermal simulation in ANSYS Workbench. Besides the estimation regarding sensitivity of thermal material parameters the evaluation of formed melting zone and resulting thermally influenced area is a central topic.
Comparison of the properties of biogenic wine by-products stabilized biocomposites compounded with a miniaturized single-screw extruder and a co-rotating twin-screw extruder. - In: Engineering for a changing world, (2023), 2.3.010, S. 1-13
Bioplastics research is hindered by high material prices and limited availability of biopolymers. For conventional compounding, even on lab-scale, large quantities of material are required. In this study, an alternative process for compounding biocomposites was evaluated to investigate the potential of wine-derived biogenic by-products as functional fillers. Formulations based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and wine grape pomace (WP) with filler contents up to 10 wt.-% were prepared. The materials were processed with a modified miniaturized single-screw extruder (MSE) and compared to a lab-scale twin-screw extruder (TSE). Thermal and rheological properties of the materials were determined using GPC, MFR, DSC, TGA and OIT. The mixing quality of both extruders was evaluated by optical microscopy imaging. The results revealed that the MSE represents an efficient alternative for research purposes, but differences in the dominant degradation mechanisms during processing must be considered. Thermal analysis showed that WP successfully suppressed the thermo-oxidative degradation of PHBV.
Influence of the recycling process parameters on CFRTP waste properties. - In: Engineering for a changing world, (2023), 2.3.002, S. 1-12
Nowadays, the combination of continuous fibers and thermoplastic polymers as the matrix to continuous fiber-reinforced thermoplastics (CFRTP) is receiving increasing attention due to their potential advantages such as excellent weight-specific mechanical properties, short cycle times, storability, repeated meltability, good formability and the use of alternative joining processes enabling automated large volume manufacturing processes which allow various applications in different industries including transportation, construction among others. As the production rate of these materials increases, the amount of waste for disposal increases, for which recycling strategies need to be established to ensure the sustainability of CFRTP. Hence, these recycling strategies must be developed and evaluated economically and ecologically to close the loop and achieve a circular economy to process recycled fiber-reinforced pellets from CFRTP waste to valuable products e.g., by injection molding. This study presents a mechanical recycling approach from CFRTP waste to injection molded test specimens and evaluates the impact of the individual recycling steps along the recycling chain on the fiber length as the fiber length is detrimental to the resulting mechanical properties. First, the CFRTP waste processability is investigated and conditions for size reduction by cutting and shredding into feedstock for extrusion are defined. Second, fiber-reinforced pellets are produced by twin-screw extrusion. The fiber volume content and the process parameters screw speed and temperature during compounding are varied and the influence of these parameters on the fiber length is determined. Third, the extruded pellets are further processed by injection molding. Here, the influence of screw speed, back pressure, and processing temperature as well as the initial fiber length in the extruded granules on the resulting properties is investigated. Quantitative correlations between material properties and processing parameters are presented and suggestions for gentle processing during recycling are given.
Influence of the processing on the properties of continuous fiber reinforced thermoplastic sheets prepared by extrusion. - In: AIP conference proceedings, ISSN 1551-7616, Bd. 2884 (2023), 1, 050005, S. 050005-1-050005-14
Continuous fiber reinforced thermoplastics (CFRT) are composite materials consisting of continuous fibers and a thermoplastic matrix and offer outstanding mechanical properties, low densities, short cycle times and recyclability. CFRT can be classified into unidirectional tapes and sheets utilizing various semi-finished textiles as reinforcement. CFRT sheets are of interest for area measured products or multiaxial loads. Various discontinuous and semi-continuous methods to prepare CFRP sheets are described in the literature. All these methods either feature high cycle times or high investment costs and require double melting of the polymer, e.g., first to produce a polymer film and second to produce the CFRT sheet. An energy efficient alternative to produce CFRT sheets is extrusion, which allows to spare one melting step. A twin-screw extruder melts the polymer, which is then conveyed by a melt pump to the film extrusion dies and applied to both sides of the semi-finished textile, which is wetted and consolidated using a calendar. Due to the high melt viscosity and the line load at the calendar the major challenge is to achieve full void-free impregnation of the semi-finished textile. The mechanical properties of a CFRT sheet are determined by fiber and void volume content. Hence, the influence of the processing conditions on the fiber and void volume content as well as the mechanical properties were examined applying a parametric study of the die temperature, the haul-off speed, and the gap between the calendar rolls. The properties of the extruded CFRT sheets were compared to compression molded sheets. The fiber volume content was directly adjusted by the haul-off speed and the extruder throughput. An increasing die temperature lowers the melt viscosity and results in an increased fiber volume content. Scanning electron microscopy shows complete macro impregnation between the fiber bundles but not completely wetted individual filaments within fiber bundles.
Experimental investigations on the manufacturing of fused silica freeform surfaces by means of fine and ultra-fine grinding. - In: Tenth European Seminar on Precision Optics Manufacturing, (2023), 1275502, S. 1275502-1-1275502-9
The manufacturing of optical freeform surfaces offers a high potential for optical approaches in the future, since they can make new optical systems lighter and more compact or even enable completely new functions, compared to conventional optics. However, the expanded possibilities go hand in hand with higher complexity in production of freeforms for precision optical applications. This leads to high prices and long delivery times. This paper shows an approach to improve manufacturing of freeforms in small batch sizes with a high degree of customization, by a process chain consisting of (ultrasonic-assisted) pre- and fine grinding combined with ultra-fine grinding using resin bond tools. The process chain is suited for efficient fabrication of optical surfaces. A main focus of the experiments is on reduction of low- and mid-spatial frequency surface deviations, as well as surface roughness. Several different influencing factors in a 5-axis CNC grinding process of fused silica freeforms are investigated and their effects on the resulting surface topography (from the low to the high frequency range of surface deviations) are observed using white light interferometry measurement principles. Various optimization approaches can be concluded.
Ruthenium and rhodium vertical interconnect formation using gas phase electrodeposition. - In: 2023 IEEE International Interconnect Technology Conference (IITC) and IEEE Materials for Advanced Metallization Conference (MAM) (IITC/MAM), (2023), insges. 3 S.
This paper presents localized gas phase electrodeposition of ruthenium (Ru) and rhodium (Rh) species into vertical interconnects. A spark discharge generates gas ions and charged species of the desired metal, which are transported by a gas flow and form a plasma jet. Prior lateral nano-bridge growth is further developed and enables the localized metal species deposition into vertical interconnect openings. This approach is additive and saves rare materials during processing. The process allows precise adjustment of the diameter, airgap size, and top finishing bump of the vertical interconnect.
Evaluation of hysteresis response in achiral edges of graphene nanoribbons on semi-insulating SiC. - In: Materials science forum, ISSN 1662-9752, Bd. 1089 (2023), S. 15-22
Hysteresis response of epitaxially grown graphene nanoribbons devices on semi-insulating 4H-SiC in the armchair and zigzag directions is evaluated and studied. The influence of the orientation of fabrication and dimensions of graphene nanoribbons on the hysteresis effect reveals the metallic and semiconducting nature graphene nanoribbons. The hysteresis response of armchair based graphene nanoribbon side gate and top gated devices implies the influence of gate field electric strength and the contribution of surface traps, adsorbents, and initial defects on graphene as the primary sources of hysteresis. Additionally, passivation with AlOx and top gate modulation decreased the hysteresis and improved the current-voltage characteristics.
Extrusion as an energy-efficient manufacturing process for thermoplastic organosheets. - In: Sheet Metal 2023, (2023), 43, S. 345-352
Organosheets combine the advantages of reinforcement fibers and thermoplastic polymers. By pairing these two materials, composites with outstanding mechanical properties and low densities can be produced. These semi-finished products can be further processed into complex and functionalized components by thermoforming or injection molding. There are a number of different manufacturing processes for continuous fiber reinforced thermoplastics (CFRT), however, most of them require long production times and recurrent melting of the polymer resulting in high energy and manufacturing costs. This study presents a novel extrusion process, that enables a continuous production of reinforced thermoplastic sheets with only one melting step. Due to the high energy efficiency and wide range of processible materials, this process shows a high potential for an economical production of CFRT. To investigate the extrusion process in more detail, the influence of the processing and the flow behavior of the polymer on the impregnation quality and the mechanical properties of the composites were studied. The results showed increasing fiber volume contents with lower polymer viscosities. Furthermore, higher die temperatures and pressures resulted in higher fiber volume contents and thus in higher mechanical properties. The experiments also revealed that a complete impregnation can currently not be achieved without an additional small double belt press due to the line load of the calender, the high viscosity of the melt and the short impregnation time.