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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.
- ItemEffect of Ge-doping on the short-wave, mid- and far-infrared intersubband transitions in GaN/AlGaN heterostructures(Bristol : IOP, 2017) Lim, Carolin B.; Ajay, Akhil; Lähnemann, Jonas; Bougerol, Catherine; Monroy, EvaThis paper assesses the effects of Ge-doping on the structural and optical (band-to-band and intersubband (ISB)) properties of GaN/AlGaN multi-quantum wells (QWs) designed to display ISB absorption in the short-wave, mid- and far-infrared ranges (SWIR, MIR, and FIR, respectively). The standard c-plane crystallographic orientation is considered for wells absorbing in the SWIR and MIR spectral regions, whereas the FIR structures are grown along the nonpolar m-axis. In all cases, we compare the characteristics of Ge-doped and Si-doped samples with the same design and various doping levels. The use of Ge appears to improve the mosaicity of the highly lattice-mismatched GaN/AlN heterostructures. However, when reducing the lattice mismatch, the mosaicity is rather determined by the substrate and does not show any dependence on the dopant nature or concentration. From the optical point of view, by increasing the dopant density, we observe a blueshift of the photoluminescence in polar samples due to the screening of the internal electric field by free carriers. In the ISB absorption, on the other hand, there is a systematic improvement of the linewidth when using Ge as a dopant for high doping levels, whatever the spectral region under consideration (i.e. different QW size, barrier composition and crystallographic orientation).
- 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.
- ItemElectrically conductive and piezoresistive polymer nanocomposites using multiwalled carbon nanotubes in a flexible copolyester: Spectroscopic, morphological, mechanical and electrical properties(Amsterdam [u.a.] : Elsevier, 2022) Dhakal, Kedar Nath; Khanal, Santosh; Krause, Beate; Lach, Ralf; Grellmann, Wolfgang; Le, Hai Hong; Das, Amit; Wießner, Sven; Heinrich, Gert; Pionteck, Jürgen; Adhikari, RameshwarNanocomposites of multiwalled carbon nanotubes (MWCNTs) with poly(butylene adipate-co-terephthalate) (PBAT), a flexible aromatic–aliphatic copolyester, were prepared by melt mixing followed by compression moulding to investigate their spectroscopic, morphological, mechanical and electrical properties. A comparison of the Fourier transform infrared (FTIR) spectra of the neat polymer matrix and the composites showed no difference, implying a physical mixing of the matrix and the filler. A morphological investigation revealed the formation of a continuous and interconnected MWCNT network embedded in the polymer matrix with partial agglomeration. Increasing Martens hardness and indentation modulus and decreasing maximum indentation depth with increasing filler concentration demonstrated the reinforcement of the polymer by the MWCNTs. A volume resistivity of 4.6 × 105 Ω cm of the materials was achieved by the incorporation of only 1 wt.-% of the MWCNTs, which confirmed a quite low percolation threshold (below 1 wt.-%) of the nanocomposites. The electrical volume resistivity of the flexible nanocomposites was achieved up to 1.6 × 102 Ω cm, depending on the filler content. The elongation at the break of the nanocomposites at 374% and the maximum relative resistance changes (ΔR/R0) of 20 and 200 at 0.9 and 7.5% strains, respectively, were recorded in the nanocomposites (3 wt.-% MWCNTs) within the estimated volume resistivity range. A cyclic strain experiment shows the most stable and reproducible ΔR/R0 values in the 2%–5% strain range. The electrical conductivity and piezoresistivity of the investigated nanocomposites in correlation with the mechanical properties and observed morphology make them applicable for low-strain deformation-sensing.
- 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