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- ItemA promising approach to low electrical percolation threshold in PMMA nanocomposites by using MWCNT-PEO predispersions(Oxford : Elsevier Science, 2016) Mir, Seyed Mohammad; Jafari, Seyed Hassan; Khonakdar, Hossein Ali; Krause, Beate; Pötschke, Petra; Taheri Qazvini, NaderElectrical conductive poly(methyl methacrylate) (PMMA) nanocomposites with low percolation threshold are very challenging to be prepared. Here, we show that the miscibility between poly(ethylene oxide) (PEO) as matrix for predispersions of multi-walled carbon nanotubes (MWCNTs) and PMMA represents an efficient approach to achieve very low electrical percolation threshold. PMMA/PEO-MWCNTs nanocomposites were prepared by a two-step solution casting method involving pre-mixing of MWCNTs with PEO and then mixing of PEO-MWCNTs with PMMA, resulting in a PMMA/PEO ratio of 80/20 wt%. The electrical percolation threshold (EPT) value was determined to be ~ 0.07 wt% which is significantly lower than most of the reported EPT values in the literature for PMMA/CNT composites. The very low electrical percolation threshold was attributed to the effectual role of PEO in self-assembly of secondary structures of nanotubes into an electrically conductive network. This was further confirmed by transmission electron microscopy and by comparing the obtained EPT value with the prediction of the excluded volume model in which statistical percolation threshold is defined based on uniform distribution of high-aspect ratio sticks in a matrix. Moreover, based on UV–Vis measurements and linear viscoelastic rheological measurements, optical and rheological percolation thresholds were obtained at nearly 0.01 wt% and 0.5 wt%, respectively.
- ItemTemperature-dependent dynamic compressive properties and failure mechanisms of the additively manufactured CoCrFeMnNi high entropy alloy(Oxford : Elsevier Science, 2022) Chen, Hongyu; Liu, Yang; Wang, Yonggang; Li, Zhiguo; Wang, Di; Kosiba, KonradCoCrFeMnNi high entropy alloy (HEA) parts were fabricated by laser powder bed fusion (LPBF), and their dynamic compressive properties at different temperatures as well as the resulting microstructures were analyzed. The HEAs showed an unprecedented strength-ductility combination, especially at a cryogenic temperature of 77 K and a high strain rate of 3000 s−1. Under this testing condition, the yield strength (YS) of the HEAs amounted to 665 MPa. Regardless of the testing temperature, the deformation mechanism of all investigated HEAs was dominated by a synergistic effect consisting of deformation twinning and dislocation pile-up around twins. The fraction of twin boundaries and dislocation density within the deformed microstructure of the HEA correlated with the test temperature. At 77 K, the formation of nanotwins together with dislocation slip prevailed and contributed to pronounced twin-twin and twin-dislocation interactions which effectively restricted the dislocation movement and, hence, contributed to a higher YS as well as strain hardening rate in comparison to that of the HEAs at room temperature of 298 K. The LPBF-fabricated HEAs showed unpronounced thermal softening even at a high testing temperature of 1073 K. Continuous dynamic recrystallization was restricted in the HEA because of its inherent sluggish dislocation kinetics and low stacking fault energy.
- ItemPhase and grain size engineering in Ge-Sb-Te-O by alloying with La-Sr-Mn-O towards improved material properties(Oxford : Elsevier Science, 2020) Kraft, Nikolas; Wang, Guoxiang; Bryja, Hagen; Prager, Andrea; Griebel, Jan; Lotnyk, AndriyGe-Sb-Te alloys are promising materials for non-volatile memory applications. Alloying of the materials with various elements is considered as prospective approach to enhance material properties. This work reports on the preparation and characterization of pure Ge-Sb-Te-O (GSTO) and alloyed with La-Sr-Mn-O (LSMO) thin films. Thermal heating of amorphous thin films to different temperatures show distinct crystallization behavior. A general trend is the decrease in the size of GSTO crystallites and the suppression in the formation of stable trigonal GSTO phase with increasing content of LSMO. Microstructural studies by transmission electron microscopy show the formation of metastable GSTO nanocrystallites dispersed in the amorphous matrix. Analysis of local chemical bonding by X-ray spectroscopy reveal the presence of different oxides in the GSTO-LSMO composites. Moreover, the composites with a high LSMO content exhibit higher crystallization temperature and significant larger sheet resistance in amorphous and crystalline phase, while a memory device made of GSTO-LSMO alloy reveals bipolar switching and synaptic behavior. In addition, the amount of LSMO in GSTO-LSMO thin films influences their optical properties and band gap. Overall, the results of this work reveal the highly promising potential of GSTO-LSMO nanocomposites for data storage and reconfigurable photonic applications as well as neuro-inspired computing.
- ItemPolypropylene-based melt mixed composites with singlewalled carbon nanotubes for thermoelectric applications: Switching from p-type to n-type by the addition of polyethylene glycol(Oxford : Elsevier Science, 2017) Luo, Jinji; Cerretti, Giacomo; Krause, Beate; Zhang, Long; Otto, Thomas; Jenschke, Wolfgang; Ullrich, Mathias; Tremel, Wolfgang; Voit, Brigitte; Pötschke, PetraThe thermoelectric properties of melt processed conductive nanocomposites consisting of an insulating polypropylene (PP) matrix filled with singlewalled carbon nanotubes (CNTs) and copper oxide (CuO) were evaluated. An easy and cheap route to switch p-type composites into n-type was developed by adding polyethylene glycol (PEG) during melt mixing. At the investigated CNT concentrations of 0.8 wt% and 2 wt% (each above the electrical percolation threshold of ∼0.1 wt%), and a fixed CuO content of 5 wt%, the PEG addition converted p-type composites (positive Seebeck coefficient (S)) into n-type (negative S). PEG was also found to improve the filler dispersion inside the matrix. Two composites were prepared: P-type polymer/CNT composites with high S (up to 45 μV/K), and n-type composites (with S up to −56 μV/K) through the addition of PEG. Two prototypes with 4 and 49 thermocouples of these p- and n-type composites were fabricated, and delivered an output voltage of 21 mV and 110 mV, respectively, at a temperature gradient of 70 K.
- ItemComparative study of singlewalled, multiwalled, and branched carbon nanotubes melt mixed in different thermoplastic matrices(Oxford : Elsevier Science, 2018) Krause, Beate; Barbier, Carine; Kunz, Karina; Pötschke, PetraIn this contribution, three different types of CNTs, namely single-walled (SWCNT), multi-walled (MWCNT) and branched MWCNTs were melt mixed in amounts of 0.1–10 wt.-% in polypropylene (PP), polycarbonate (PC) and poly(vinylidene fluoride) (PVDF) using a small-scale microcompounder. The filler dispersion of compression-moulded samples was characterized using light and electron microscopy, and the electrical and thermal properties were measured. The lowest electrical percolation thresholds were found for composites of PP/SWCNT, PP/branched MWCNT and PC/branched MWCNT, which percolated already at <0.1 wt.-% CNT loading. Low values of electrical volume resistivity of about 3 Ohm·cm (PVDF), 7 Ohm·cm (PP) and 2 Ohm·cm (PC) could be reached when loading with 2 wt.-% branched MWCNT. A homogeneous dispersion in the macro- and microlevel was observed especially for composites containing branched MWCNTs. For all CNT types, a matrix nucleation effect was found in PP and PVDF using differential scanning calorimetry.
- ItemMelt mixed SWCNT-polypropylene composites with very low electrical percolation(Oxford : Elsevier Science, 2016) Krause, Beate; Pötschke, Petra; Ilin, Evgeniy; Predtechenskiy, MikhailSinglewalled carbon nanotube material of the type TUBALL™ (OCSiAl) was used to prepare composites with polypropylene by melt mixing using a conical twin screw micro-compounder. The compression moulded composites showed electrical percolation between 0.075 and 0.1 wt % and achieved volume resistivity values lower than 1 kOhm-cm already at 0.8 wt % loading. Light microscopy and scanning electron microscopy revealed good distribution and dispersion into small diameter bundles as well as retained high nanotube length. In connection with the very low percolation threshold this indicates that the SWCNT material shows an exceptionally good dispersibility which may be due to relatively high nanotube diameters with a mean value of 1.6 nm. In tensile tests already 0.1 wt % nanotube additions resulted in slight increase in Young's modulus and maximum stress. Tuball™ SWCNT material seems to be very promising for conductivity enhancement.