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- 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.
- ItemMixed Carbon Nanomaterial/Epoxy Resin for Electrically Conductive Adhesives(Basel : MDPI, 2020) Lopes, Paulo E.; Moura, Duarte; Hilliou, Loic; Krause, Beate; Pötschke, Petra; Figueiredo, Hugo; Alves, Ricardo; Lepleux, Emmanuel; Pacheco, Louis; Paiva, Maria C.The increasing complexity of printed circuit boards (PCBs) due to miniaturization, increased the density of electronic components, and demanding thermal management during the assembly triggered the research of innovative solder pastes and electrically conductive adhesives (ECAs). Current commercial ECAs are typically based on epoxy matrices with a high load (>60%) of silver particles, generally in the form of microflakes. The present work reports the production of ECAs based on epoxy/carbon nanomaterials using carbon nanotubes (single and multi-walled) and exfoliated graphite, as well as hybrid compositions, within a range of concentrations. The composites were tested for morphology (dispersion of the conductive nanomaterials), electrical and thermal conductivity, rheological characteristics and deposition on a test PCB. Finally, the ECA’s shelf life was assessed by mixing all the components and conductive nanomaterials, and evaluating the cure of the resin before and after freezing for a time range up to nine months. The ECAs produced could be stored at −18 °C without affecting the cure reaction.
- ItemModelling and Experimental Investigation of Hexagonal Nacre-Like Structure Stiffness(Basel : MDPI, 2020) Rouhana, Rami; Stommel, MarkusA highly ordered, hexagonal, nacre-like composite stiffness is investigated using experiments, simulations, and analytical models. Polystyrene and polyurethane are selected as materials for the manufactured specimens using laser cutting and hand lamination. A simulation geometry is made by digital microscope measurements of the specimens, and a simulation is conducted using material data based on component material characterization. Available analytical models are compared to the experimental results, and a more accurate model is derived specifically for highly ordered hexagonal tablets with relatively large in-plane gaps. The influence of hexagonal width, cut width, and interface thickness are analyzed using the hexagonal nacre-like composite stiffness model. The proposed analytical model converges within 1% with the simulation and experimental results
- ItemPolydopamine-Coated Paraffin Microcapsules as a Multifunctional Filler Enhancing Thermal and Mechanical Performance of a Flexible Epoxy Resin(Basel : MDPI, 2020) Fredi, Giulia; Zimmerer, Cordelia; Scheffler, Christina; Pegoretti, AlessandroThis work focuses on flexible epoxy (EP) composites containing various amounts of neat and polydopamine (PDA)-coated paraffin microcapsules as a phase change material (PCM), which have potential applications as adhesives or flexible interfaces with thermal management capability for electronics or other high-value-added fields. After PDA modification, the surface of PDA-coated capsules (MC-PDA) becomes rough with a globular appearance, and the PDA layer enhances the adhesion with the surrounding epoxy matrix, as shown by scanning electron microscopy. PDA deposition parameters have been successfully tuned to obtain a PDA layer with a thickness of 53 ± 8 nm, and the total PDA mass in MC-PDA is only 2.2 wt %, considerably lower than previous results. This accounts for the fact that the phase change enthalpy of MC-PDA is only marginally lower than that of neat microcapsules (MC), being 221.1 J/g and 227.7 J/g, respectively. Differential scanning calorimetry shows that the phase change enthalpy of the prepared composites increases with the capsule content (up to 87.8 J/g) and that the enthalpy of the composites containing MC-PDA is comparable to that of the composites with MC. Dynamic mechanical analysis evidences a decreasing step in the storage modulus of all composites at the glass transition of the EP phase, but no additional signals are detected at the PCM melting. PCM addition positively contributes to the storage modulus both at room temperature and above Tg of the EP phase, and this effect is more evident for composites containing MC-PDA. As the capsule content increases, the mechanical properties of the host EP matrix also increase in terms of elastic modulus (up to +195%), tensile strength (up to +42%), Shore D hardness (up to +36%), and creep compliance (down to −54% at 60 min). These effects are more evident for composites containing MC-PDA due to the enhanced interfacial adhesion.
- ItemDoes the Type of Polymer and Carbon Nanotube Structure Control the Electromagnetic Shielding in Melt-Mixed Polymer Nanocomposites?(Basel : MDPI, 2020-1-15) Biswas, Sourav; Muzata, Tanyaradzwa S.; Krause, Beate; Rzeczkowski, Piotr; Pötschke, Petra; Bose, SuryasarathiA suitable polymer matrix and well dispersed conducting fillers forming an electrically conducting network are the prime requisites for modern age electromagnetic shield designing. An effective polymer-based shield material is designed that can attenuate 99.9% of incident electromagnetic (EM) radiation at a minimum thickness of <0.5 mm. This is accomplished by the choice of a suitable partially crystalline polymer matrix while comparing non-polar polypropylene (PP) with polar polyvinylidene fluoride (PVDF) and a best suited filler nanomaterial by comparing different types of carbon nanotubes such as; branched, single-walled and multi-walled carbon nanotubes, which were added in only 2 wt %. Different types of interactions (polar-polar and CH-π and donor-acceptor) make b-MWCNT more dispersible in the PVDF matrix, which together with high crystallinity resulted in the best electrical conductivity and electromagnetic shielding ability of this composite. This investigation additionally conceals the issues related to the thickness of the shield material just by stacking individual thin nanocomposite layers containing different carbon nanotube (CNT) types with 0.3 mm thickness in a simple manner and finally achieves 99.999% shielding efficiency at just 0.9 mm thickness when using a suitable order of the different PVDF based nanocomposites.