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- ItemMelt-mixed PP/MWCNT composites: Influence of CNT incorporation strategy and matrix viscosity on filler dispersion and electrical resistivity(Basel : MDPI, 2019) Pötschke, Petra; Mothes, Fanny; Krause, Beate; Voit, BrigitteSmall-scale melt mixing was performed for composites based on polypropylene (PP) and 0.5–7.5 wt % multiwalled carbon nanotubes (MWCNT) to determine if masterbatch (MB) dilution is a more effective form of nanofiller dispersion than direct nanotube incorporation. The methods were compared using composites of five different PP types, each filled with 2 wt % MWCNTs. After the determination of the specific mechanical energy (SME) input in the MB dilution process, the direct-incorporation mixing time was adjusted to achieve comparable SME values. Interestingly, the electrical resistivity of MB-prepared samples with 2 wt % MWCNTs was higher than that of those prepared using direct incorporation—despite their better dispersion—suggesting more pronounced MWCNT shortening in the two-step procedure. In summary, this study on PP suggests that the masterbatch approach is suitable for the dispersion of MWCNTs and holds advantages in nanotube dispersion, albeit at the cost of slightly increased electrical resistivity.
- ItemCharacterization of highly filled PP/graphite composites for adhesive joining in fuel cell applications(Basel : MDPI, 2019) Rzeczkowski, Piotr; Krause, Beate; Pötschke, PetraIn order to evaluate the suitability of graphite composite materials for use as bipolar plates in fuel cells, polypropylene (PP) was melt compounded with expanded graphite as conductive filler to form composites with different filler contents of 10–80 wt %. Electrical resistivity, thermal conductivity, and mechanical properties were measured and evaluated as a function of filler content. The electrical and thermal conductivities increased with filler content. Tensile and flexural strengths decreased with the incorporation of expanded graphite in PP. With higher graphite contents, however, both strength values remained more or less unchanged and were below the values of pure PP. Young’s-modulus and flexural modulus increased almost linearly with increasing filler content. The results of the thermogravimetric analysis confirmed the actual filler content in the composite materials. In order to evaluate the wettability and suitability for adhesive joining of graphite composites, contact angle measurements were conducted and surface tensions of composite surfaces were calculated. The results showed a significant increase in the surface tension of graphite composites with increasing filler content. Furthermore, graphite composites were adhesively joined and the strength of the joints was evaluated in the lap-shear test. Increasing filler content in the substrate material resulted in higher tensile lap-shear strength. Additionally, the influence of surface treatment (plasma and chemical) on surface tension and tensile lap-shear strength was investigated. The surface treatment led to a significant improvement of both properties.
- 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.
- ItemDevelopment of joining methods for highly filled Graphite/PP composite based bipolar plates for fuel cells: Adhesive joining and welding(Melville, NY : AIP, 2019) Rzeczkowski, P.; Lucia, M.; Müller, A.; Facklam, M.; Cohnen, A.; Schäfer, P.; Hopmann, C.; Hickmann, T.; Pötschke, Petra; Krause, BeateNovel material solutions for bipolar plates in fuel cells require adapted ways of joining and sealing technologies. Safe and life time enduring leak-tight contacts must be achieved by automatic processes using reasonable joint forces. A proper sealing should manage such challenges as good ageing properties, excellent leaktightness, high thermal conductivity and low gas permeability. Hence in this work, adhesive bonding and welding are considered as suitable methods, which can fulfill the requirements mentioned above. Adhesive systems seem to be more easy to apply than conventional sealing (hand layed-up rubber gaskets), e.g. with automatic dispensers. Additionally, the properties of an adhesive joint can be enhanced by a process-specific surface pre-treatment. This work focuses on the characterization of adhesive systems and their joints with highly filled graphite composites. Mechanical properties of the joints were characterized through lap-shear tests. The influence of ageing caused by humidity or acidic solvent at increased temperature on the bond line properties as well as neat adhesive was examined. The thermal conductivities of neat adhesives and through the entire joint were examined. In order to improve above conductivities, roughening, substrate pre-heating, post-curing and various contact pressure weights were applied. Plasma treatment was chosen as surface pre-treatment method for improving substrate's surface energy. An alternative to bonding is plastic welding, which does not require the use of sealants and adhesives. Based on former study of influences of filler content on the welding process using ultrasonic, hot plate or infrared welding, a welding method for joining the graphite compounds was derived.
- ItemNitrogen-Doped Carbon Nanotube/Polypropylene Composites with Negative Seebeck Coefficient(Basel : MDPI, 2020) Krause, Beate; Konidakis, Ioannis; Arjmand, Mohammad; Sundararaj, Uttandaraman; Fuge, Robert; Liebscher, Marco; Hampel, Silke; Klaus, Maxim; Serpetzoglou, Efthymis; Stratakis, Emmanuel; Pötschke, PetraThis study describes the application of multi-walled carbon nanotubes that were nitrogen-doped during their synthesis (N-MWCNTs) in melt-mixed polypropylene (PP) composites. Different types of N-MWCNTs, synthesized using different methods, were used and compared. Four of the five MWCNT grades showed negative Seebeck coefficients (S), indicating n-type charge carrier behavior. All prepared composites (with a concentration between 2 and 7.5 wt% N-MWCNTs) also showed negative S values, which in most cases had a higher negative value than the corresponding nanotubes. The S values achieved were between 1.0 µV/K and −13.8 µV/K for the N-MWCNT buckypapers or powders and between −4.7 µV/K and −22.8 µV/K for the corresponding composites. With a higher content of N-MWCNTs, the increase in electrical conductivity led to increasing values of the power factor (PF) despite the unstable behavior of the Seebeck coefficient. The highest power factor was achieved with 4 wt% N-MWCNT, where a suitable combination of high electrical conductivity and acceptable Seebeck coefficient led to a PF value of 6.1 × 10−3 µW/(m·K2). First experiments have shown that transient absorption spectroscopy (TAS) is a useful tool to study the carrier transfer process in CNTs in composites and to correlate it with the Seebeck coefficient.
- ItemElucidating the chemistry behind the reduction of graphene oxide using a green approach with polydopamine(Basel : MDPI, 2019) Silva, Cláudia; Simon, Frank; Friedel, Peter; Pötschke, Petra; Zimmerer, CordeliaA new approach using X-ray photoelectron spectroscopy (XPS) was employed to give insight into the reduction of graphene oxide (GO) using a green approach with polydopamine (PDA). In this approach, the number of carbon atoms bonded to OH and to nitrogen in PDA is considered and compared to the total intensity of the signal resulting from OH groups in polydopamine-reduced graphene oxide (PDA-GO) to show the reduction. For this purpose, GO and PDA-GO with different times of reduction were prepared and characterized by Raman Spectroscopy and XPS. The PDA layer was removed to prepare reduced graphene oxide (RGO) and the effect of all chemical treatments on the thermal and electrical properties of the materials was studied. The results show that the complete reduction of the OH groups in GO occurred after 180 min of reaction. It was also concluded that Raman spectroscopy is not well suited to determine if the reduction and restoration of the sp2 structure occurred. Moreover, a significant change in the thermal stability was not observed with the chemical treatments. Finally, the electrical powder conductivity decreased after reduction with PDA, increasing again after its removal. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemThermal annealing to influence the vapor sensing behavior of co-continuous poly(lactic acid)/polystyrene/multiwalled carbon nanotube composites(Amsterdam [u.a.] : Elsevier Science, 2020) Li, Yilong; Pionteck, Jürgen; Pötschke, Petra; Voit, BrigitteWith the main purpose of being used as vapor leakage detector, the volatile organic compound (VOC) vapor sensing properties of conductive polymer blend composites were studied. Poly(lactic acid)/polystyrene/multi-walled carbon nanotube (PLA/PS/MWCNT) based conductive polymer composites (CPCs) in which the polymer components exhibit different interactions with the vapors, were prepared by melt mixing. CPCs with a blend composition of 50/50 wt% resulted in the finest co-continuous structure and selective MWCNT localization in PLA. Therefore, these composites were selected for sensor tests. Thermal annealing was applied aiming to maintain the blend structure but improving the sensing reversibility of CPC sensors towards high vapor concentrations. Different sensing protocols were applied using acetone (good solvent for PS and PLA) and cyclohexane (good solvent for PS but poor solvent for PLA) vapors. Increasing acetone vapor concentration resulted in increased relative resistance change (Rrel) of CPCs. Saturated cyclohexane vapor resulted in lower response than nearly saturated acetone vapor. The thermal annealing at 150 °C did not change the blend morphology but increased the PLA crystallinity, making the CPC sensors more resistant to vapor stimulation, resulting in lower Rrel but better reversibility after vapor exposure.
- ItemThe influence of the blend ratio in PA6/PA66/MWCNT blend composites on the electrical and thermal properties(Basel : MDPI, 2019) Krause, Beate; Kroschwald, Lisa; Pötschke, PetraIt is known that the percolation threshold of polyamide 6 (PA6)/multiwalled carbon nanotube (MWCNT) composites is higher than that of PA66/MWCNT composites under the same mixing conditions and melt viscosity. A series of blends of PA6 and PA66 containing 1 wt % MWCNTs have been prepared to investigate this phenomenon. At contents up to 20 wt % PA66, the blends were not electrically conductive. The electrical resistivity dropped to 109 Ohm∙cm for PA66/PA6 30/70 blends. The resistivity was 105 Ohm∙cm at higher PA66 contents. Differential scanning calorimetry was used to investigate the thermal behavior of blends. The glass transition temperature was almost constant for all blend compositions, indicating that the amorphous phases are miscible. The MWCNT addition influenced the crystallization of PA66 much more than the PA6 crystallization. A heterogeneous crystallization of the polyamide in PA66/PA6 blends took place, and the MWCNTs were mainly localized in the earlier crystallizing PA66 phase. Thus, the formation of the nanotube network and thus the electrical volume resistivity of the PA6/PA66 blends with 1 wt % MWCNTs is significantly influenced by the crystallization behavior. In PA66/PA6 blends up to 60 wt %, the more expensive PA66 can be replaced by the cheaper PA6 while retaining its electrical properties
- ItemEffect of filler synergy and cast film extrusion parameters on extrudability and direction-dependent conductivity of PVDF/carbon nanotube/carbon black composites(Basel : MDPI, 2020) Krause, Beate; Kunz, Karina; Kretzschmar, Bernd; Kühnert, Ines; Pötschke, PetraIn the present study, melt-mixed composites based of poly (vinylidene fluoride) (PVDF) and fillers with different aspect ratios (carbon nanotubes (CNTs), carbon black (CB)) and their mixtures in composites were investigated whereby compression-molded plates were compared with melt-extruded films. The processing-related orientation of CNTs with a high aspect ratio leads to direction-dependent electrical and mechanical properties, which can be reduced by using mixed filler systems with the low aspect ratio CB. An upscaling of melt mixing from small scale to laboratory scale was carried out. From extruded materials, films were prepared down to a thickness of 50 µm by cast film extrusion under variation of the processing parameters. By combining CB and CNTs in PVDF, especially the electrical conductivity through the film could be increased compared to PVDF/CNT composites due to additional contact points in the sample thickness. The alignment of the fillers in the two directions within the films was deduced from the differences in electrical and mechanical film properties, which showed higher values in the extrusion direction than perpendicular to it.
- ItemGraphite modified epoxy-based adhesive for joining of aluminium and PP/graphite composites(New York, NY [u.a.] : Taylor & Francis, 2020) Rzeczkowski, P.; Pötschke, Petra; Fischer, M.; Kühnert, I.; Krause, BeateA graphite-modified adhesive was developed in order to simultaneously enhance the thermal conductivity and the strength of an adhesive joint. The thermal conductivity through the joint was investigated by using highly filled PP/graphite composite substrates, which were joined with an epoxy adhesive of different layer thicknesses. Similar measurements were carried out with a constant adhesive layer thickness, whilst applying an epoxy adhesive modified with expanded graphite (EG) (6, 10, and 20 wt%). By reducing the adhesive layer thickness or modifying the adhesive with conductive fillers, a significant increase of the thermal conductivity through the joint was achieved. The examination of the mechanical properties of the modified adhesives was carried out by tensile tests (adhesive only), lap-shear tests, and fracture energy tests (mode 1) with aluminium substrates. Modification of the adhesive with EG led to an increase of the tensile lap-shear strength and the adhesive fracture energy (mode 1) of the joint. In addition, burst pressure tests were performed to determine the strength of the joint in a complex component. The strength of the joint increased with the graphite content in the PP substrate and in the epoxy adhesive.
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