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- ItemAspect ratio effects of multi-walled carbon nanotubes on electrical, mechanical, and thermal properties of polycarbonate/MWCNT composites(Hoboken, NJ [u.a.] : Wiley, 2014) Guo, Jiaxi; Liu, Yanjun; Prada-Silvy, Ricardo; Tan, Yongqiang; Azad, Samina; Krause, Beate; Pötschke, Petra; Grady, Brian P.Two multi-walled carbon nanotubes (MWCNTs) having relatively high aspect ratios of 313 and 474 with approximately the same diameter were melt mixed with polycarbonate (PC) in a twin-screw conical micro compounder. The effects of aspect ratio on the electrical, mechanical, and thermal properties of the PC/MWCNT composites were investigated. Electrical conductivities and storage moduli of the filled samples are found to be independent of the starting aspect ratio for these high aspect ratio tubes; although the conductivities and storage moduli are still significantly higher than values of composites made with nanotubes having more commercially common aspect ratios of ∼100. Transmission electron microscopy results suggest that melt-mixing reduces these longer nanotubes to the same length, but still approximately two times longer than the length of commercially common aspect ratio tubes after melt-mixing. Molecular weight measurements show that during melt-mixing the longer nanotubes significantly degrade the molecular weight of the polymer as compared to very similar nanotubes with aspect ratio ∼100. Because of the molecular weight reduction glass transition temperatures predictably show a large decrease with increasing nanotube concentration. © 2013 Wiley Periodicals, Inc.
- ItemElectrical and melt rheological characterization of PC and co-continuous PC/SAN blends filled with CNTs: Relationship between melt-mixing parameters, filler dispersion, and filler aspect ratio(Hoboken, NJ [u.a.] : Wiley, 2018) Liebscher, Marco; Domurath, Jan; Krause, Beate; Saphiannikova, Marina; Heinrich, Gert; Pötschke, PetraElectrical and melt rheological properties of melt-mixed polycarbonate (PC) and co-continuous PC/poly(styrene–acrylonitrile) (SAN) blends with carbon nanotubes (CNTs) are investigated. Using two sets of mixing parameters, different states of filler dispersion are obtained. With increasing CNT dispersion, an increase in electrical resistivity near the percolation threshold of PC–CNT composites and (PC + CNT)/SAN blends is observed. This suggests that the higher mixing energies required for better dispersion also result in a more severe reduction of the CNT aspect ratio; this effect was proven by CNT length measurements. Melt rheological studies show higher reinforcing effects for composites with worse dispersion. The Eilers equation, describing the melt viscosity as function of filler content, was used to fit the data and to obtain information about an apparent aspect ratio change, which was in accordance with measured CNT length reduction. Such fitting could be also transferred to the blends and serves for a qualitatively based discussion. © 2017 Wiley Periodicals
- ItemInfluence of talc with different particle sizes in melt-mixed LLDPE/MWCNT composites(Hoboken, NJ [u.a.] : Wiley, 2013) Müller, Michael Thomas; Dreyße, Janine; Häußler, Liane; Krause, Beate; Pötschke, PetraLinear low-density polyethylene (LLDPE) was melt-mixed with multiwalled carbon nanotubes (MWCNTs) and varying amounts of three different kinds of talc (phyllo silicate), each with a different particle size distribution, to examine the effect of these filler combinations with regards to the electrical percolation behavior. The state of the filler dispersion was assessed using transmission light microscopy and electron microscopy. The use of talc as a second filler during the melt mixing of LLDPE/MWCNT composites resulted in an improvement in the dispersion of the MWCNTs and a decrease of the electrical percolation threshold. Talc with lower particle sizes showed a more pronounced effect than talc with larger particle sizes. However, the improvement in dispersion was not reflected in the mechanical properties. Modulus and stress values increase with both, MWCNT and talc addition, but not in a synergistic manner. The crystallization behavior of the composites was studied by differential scanning calorimetry to determine its potential influence on the electrical percolation threshold. It was found that the crystallinity of the matrix increased slightly with the addition of talc but no further increments were observed with the incorporation of the MWCNTs. © 2013 Wiley Periodicals, Inc.
- ItemHybrid conductive filler/polycarbonate composites with enhanced electrical and thermal conductivities for bipolar plate applications(Manchester, NH : Wiley, 2019) Naji, Ahmed; Krause, Beate; Pötschke, Petra; Ameli, AmirConductive polymer composites (CPCs) with high electrical and thermal conductivities are demanded for bipolar plates of fuel cells. In this work, CPCs of polycarbonate (PC) filled with carbon nanotube (CNT), carbon fiber (CF), graphite (G), and their double and triple hybrids were prepared using solution casting method followed by compression molding. The results showed that the electrical percolation thresholds for the PC-CNT and PC-CF were ~1 wt% and ~10 wt%, respectively, while no clear threshold was found for PC-G composites. Addition of 3–5 wt% CNT improved the electrical conductivity of PC-CF and PC-G systems up to 6 orders of magnitude and enhanced the thermal conductivity as much as 65%. The results of triple hybrid CPCs (with constant loading of 63 wt%) indicated that the combination of highest electrical and thermal conductivities is achieved when the CF and CNT loadings were near their percolation thresholds. Therefore, a triple filler system of 3 wt% CNT, 10 wt% CF, and 50 wt% G resulted in a composite with the through-plane and in-plane electrical conductivity, and thermal conductivity values of 12.8 S/cm, 8.3 S/cm, and 1.7 W/m•K, respectively. The results offer a combination of properties surpassing the existing values and suitable for high-conductivity applications such as bipolar plates. POLYM. COMPOS., 40:3189–3198, 2019. © 2018 Society of Plastics Engineers.
- ItemUltralow percolation threshold in polyamide 6.6/MWCNT composites(Barking : Elsevier, 2015) Krause, Beate; Boldt, Regine; Häußler, Liane; Pötschke, PetraWhen incorporating multiwalled carbon nanotubes (MWCNTs) synthesised by the aerosol-CVD method using different solvents into polyamide 6.6 (PA66) by melt mixing an ultralow electrical percolation threshold of 0.04. wt.% was found. This very low threshold was assigned to the specific characteristic of the MWCNTs for which a very high aspect ratio, a good dispersability in aqueous surfactant dispersions, and relatively low oxygen content near the surface were measured. The investigation of the composites by transmission electron microscopy on ultrathin cuts as well as by scanning electron microscopy combined with charge contrast imaging on compression moulded plates illustrated a good MWCNT dispersion within the PA66 matrix and that the very high aspect ratio of the nanotubes remained even after melt processing. Additionally, the thermal behaviour of the PA66 composites was investigated using differential scanning calorimetry (DSC) showing that the addition of only 0.05. wt.% MWCNT leads to an increase of the onset crystallization temperature of 11. K.
- ItemCellulose-carbon nanotube composite aerogels as novel thermoelectric materials(Amsterdam [u.a.] : Elsevier, 2018) Gnanaseelan, Minoj; Chen, Yian; Luo, Jinji; Krause, Beate; Pionteck, Jürgen; Pötschke, Petra; Qu, HaisongThermoelectric materials based on cellulose/carbon nanotube (CNT) nanocomposites have been developed by a facile approach and the effects of amount (2–10 wt%) and types of CNTs (single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs)) on the morphology (films and aerogels) and the thermoelectric properties of the nanocomposites have been investigated. Composite films based on SWCNTs showed significantly higher electrical conductivity (5 S/cm at 10 wt%) and Seebeck coefficient (47.2 μV/K at 10 wt%) compared to those based on MWCNTs (0.9 S/cm and 11 μV/K, respectively). Lyophilization, leading to development of aerogels with sub-micron sized pores, decreased the electrical conductivity for both types by one order of magnitude, but did not affect the Seebeck coefficient of MWCNT based nanocomposites. For SWCNT containing aerogels, higher Seebeck coefficients than for films were measured at 3 and 4 wt% but significantly lower values at higher loadings. CNT addition increased the thermal conductivity from 0.06 to 0.12 W/(m∙K) in the films, whereas the lyophilization significantly reduced it towards values between 0.01 and 0.09 W/(m∙K) for the aerogels. The maximum Seebeck coefficient, power factor, and ZT observed in this study are 49 μV/K for aerogels with 3 wt% SWCNTs, 1.1 μW/(m∙K2) for composite films with 10 wt% SWCNTs, and 7.4 × 10−4 for films with 8 wt% SWCNTs, respectively.
- ItemInfluence of different carbon nanotubes on the electrical and mechanical properties of melt mixed poly(ether sulfone)-multi walled carbon nanotube composites(Barking : Elsevier, 2012) Chakraborty, Sourav; Pionteck, Jürgen; Krause, Beate; Banerjee, Susanta; Voit, BrigitteCommercial Udel® poly(ether sulfone) (PSU) was filled with three different commercially available multiwalled carbon nanotubes (MWCNTs) by small scale melt mixing. The MWCNTs were as grown NC 7000 and two of its derivatives prepared by ball milling treatment. One of them was unmodified (NC 3150); the other was amino modified (NC 3152). The main difference beside the reactivity was the reduced aspect ratio of NC 3150 and NC 3152 caused by ball milling process. All PSU/MWCNT composites with similar filler content were prepared under fixed processing conditions and comparative analysis of their electrical and mechanical properties were performed and were correlated with their microstructure, characterized by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). A non-uniform MWCNT dispersion was observed in all composites. The MWCNTs were present in form of agglomerates in the size of 10-60. μm whereas the deagglomerated part was homogeneously distributed in the PSU matrix. The differences in the agglomeration states correlate with the variations of properties between different PSU/MWCNT composites. The lowest electrical percolation threshold of 0.25-0.5. wt.% was observed for the shortened non-functionalized MWCNT composites and the highest for amine-modified MWCNT composites (ca. 1.5. wt.%). The tensile behavior of the three composites was only slightly altered with CNT loading as compared to the pure PSU. However, the elongation at break showed a reduction with MWCNT loading and the reduction was least for composite with best MWCNT dispersion. © 2012 Elsevier Ltd.
- ItemImprovement of carbon nanotube dispersion in thermoplastic composites using a three roll mill at elevated temperatures(Barking : Elsevier, 2013) Pötschke, Petra; Krause, Beate; Buschhorn, Samuel T.; Köpke, Ulf; Müller, Michael T.; Villmow, Tobias; Schulte, KarlThe paper reports the effect of using of a three roll mill as additional dispersion step after twin-screw melt extrusion of nanocomposites containing thermoplastic polymers and multiwalled carbon nanotubes. The three roll milling technology was adapted to elevated temperatures of up to 180 °C and examples are shown for its use in processing of different pre-compounded thermoplastic polymer composites based on polypropylene, polycaprolactone and ethylene-vinyl acetate. The aim is to enhance the state of dispersion achieved by the previous melt extrusion step. In particular, depending on the state of dispersion before three roll milling and the adapted conditions, like number of runs and gap sizes, a reduction of number and size of remaining primary nanotube agglomerates was found. This was studied using light microscopy. The resulting improvements in mechanical properties were assessed and could be attributed to the improved dispersion. In some cases agglomerate free samples could be achieved after the three roll milling process. © 2012 Elsevier Ltd.