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- ItemMelt mixed composites of polypropylene with singlewalled carbon nanotubes for thermoelectric applications: Switching from p- to n-type behavior by additive addition(Melville, NY : AIP, 2019) Pötschke; Petra; Krause, Beate; Luo, JinjiComposites were prepared with polypropylene (PP) as the matrix and singlewalled CNTs (SWCNTs) of the type TUBALL from OCSiAl Ltd. as the conducting component by melt processing in a small-scale twin-screw compounder. In order to switch the typical p-type behavior of such composites from positive Seebeck coefficients (S) into n-type behavior with negative Seebeck coefficients, a non-ionic surfactant polyoxyethylene 20 cetyl ether (Brij58) was used and compared with a PEG additive, which was shown previously to be able to induce such switching. For PP-2 wt% SWCNT composites Brij58 is shown to result in n-type composites. The negative S values (up to −48.2 µV/K) are not as high as in the case of previous results using PEG (−56.6 µV/K). However, due to the more pronounced effect of Brij58 on the electrical conductivity, the achieved power factors are higher and reach a maximum of 0.144 µW/(m·K2) compared to previous 0.078 µW/(m·K2) with PEG. Dispersion improvement depends on the type of SWCNTs obtained by using varied synthesis/treatment conditions. Solution prepared composites of PEG with SWCNTs also have negative S values, indicating the donation of electrons from PEG to the SWCNTs. However, such composites are brittle and not suitable as thermoelectric materials.
- ItemImpact of synthesis temperature on morphology, rheology and electromagnetic interference shielding of CVD-grown carbon nanotube/polyvinylidene fluoride nanocomposites(Lausanne : Elsevier Sequoia, 2017) Mirkhani, Seyyed Alireza; Arjmand, Mohammad; Sadeghi, Soheil; Krause, Beate; Pötschke, Petra; Sundararaj, UttandaramanEmploying chemical vapor deposition technique, multi-walled carbon nanotubes (CNTs) were synthesized over Fe catalyst at a broad range of temperatures, i.e. 550° C to 950° C (at 100° C intervals). CNTs were melt-mixed into a polyvinylidene fluoride (PVDF) matrix at various loadings, and then compression molded. Surprisingly, despite the ascending trend of CNT powder conductivity with the synthesis temperature, the nanocomposites made with CNT synthesized at 650° C had significantly lower percolation threshold (around 0.4 wt%) and higher electromagnetic interference shielding effectiveness (EMI SE) (20.3 dB over the X-band for 3.5 wt% CNT and 1.1 mm thickness) than the other temperatures. Exhaustive characterization studies were conducted on both CNTs and composites to unveil their morphological and electrical characteristics. Superior EMI shielding of CNT650° C was attributed to a combination of high carbon purity, aspect ratio, crystallinity, and moderate powder conductivity along with decent state of dispersion within the PVDF matrix.
- ItemPolymer - Carbon nanotube composites for thermoelectric applications(Melville, NY : AIP, 2017) Luo, J.; Krause, Beate; Pötschke, PetraThe thermoelectric (TE) performance of electrically conductive thermoplastic composites prepared by melt mixing was investigated. A cost effective widely used in industry polymer, namely polypropylene (PP), was chosen as the matrix to fabricate the composites. Singlewalled carbon nanotubes (SWCNTs), the amount (2 wt%) of which was selected to be above the electrical percolation threshold (< 0.2 wt%), were used to form an electrical conducting network. Besides as-produced SWCNTs plasma modified tubes were employed to study the influence of the functionalization on the morphology, dispersion and TE properties of the PP composites. In addition, melt processing conditions, e.g. temperature, rotation speed, and time during mixing in a small-scale compounder were varied. Furthermore, an ionic liquid (IL, 1-methyl-3-octylimidazolium tetrafluoroborate) was used as a processing additive during melt mixing, which was confirmed to improve the electrical conductivity of the composites. Simultaneous increase in the Seebeck coefficient up to a value of 64 μV/K was recorded, leading to a much better power factor of 0.26 μW/(m·K2) compared to composites without IL. This melt mixing strategy opens new avenues for solvent-free, large scale fabrication of polymer based TE materials.
- ItemDevelopment of a polymer composite with high electrical conductivity and improved impact strength for the application as bipolar plate(Melville, NY : AIP, 2016) Hopmann, C.; Windeck, C.; Cohnen, A.; Onken, J.; Krause, Beate; Pötschke, Petra; Hickmann, T.Bipolar plates constitute the most important structural component in fuel cell stacks. Highly filled thermoplastic composites with high electrical conductivity obtain an increasing importance in the design of bipolar plates as alternative to conventional metallic systems. Thermoplastics (e.g. PP) have suitable properties such as a good processability, chemical resistance, light weight and low production costs. As thermoplastics have low electrical conductivities, conductive fillers have to be included in the matrix. A high content of such fillers (e.g. graphite) in excess of 80 wt.-% is necessary to achieve the desired electrical properties. However, materials with such high filler contents embrittle readily. The workability in injection and compression molding is difficult and the mechanical stability is insufficient in case of strain deformation. As consequence, material failure and an inacceptable amount of damaged goods can be observed during the processing. As no suitable thermoplastic system is available for better mechanical properties, the induction and dispersion of a rubber phase in the thermoplastic matrix can be used to increase the impact strength of the conductive composite. In this research work a ternary composite, based on PP as matrix, EPDM as impact modifier and synthetic graphite as conductive filler, was developed. The material was produced using a 26 mm co-rotating, intermeshing twin-screw extruder. The amounts of PP, EPDM and graphite were varied systematically and a process window was defined that enables improved impact strength and high electrical conductivity of the new material. The results indicate that impact strength can be enhanced by about 99 % with an EPDM content of 30 wt.-% in the PP matrix. The electrical conductivity decreases in a small range with increasing content of EPDM, but the conductivity is still excellent for producing bipolar plates.
- ItemLiquid sensing: Smart polymer/CNT composites(Amsterdam [u.a.] : Elsevier, 2011) Villmow, T.; Pegel, S.; John, A.; Rentenberger, R.; Pötschke, P.Today polymer/carbon nanotube (CNT) composites can be found in sports equipment, cars, and electronic devices. The growth of old and new markets in this area has been stimulated by our increased understanding of relevant production and processing methods, as well as the considerable price reduction of industrial CNT grades. In particular, CNT based electrically conductive polymer composites (CPCs) offer a range of opportunities because of their unique property profile; they demonstrate low specific gravity in combination with relatively good mechanical properties and processability. The electrical conductivity of polymer/CNT composites results from a continuous filler network that can be affected by various external stimuli, such as temperature shifts, mechanical deformations, and the presence of gases and vapors or solvents. Accordingly, CNT based CPCs represent promising candidates for the design of smart components capable of integrated monitoring. In this article we focus on their use as leakage detectors for organic solvents.
- ItemTowards uniform electrochemical porosification of bulk HVPE-grown GaN(Pennington, NJ : Electrochemical Society Inc., 2019) Monaico, E.; Moise, C.; Mihai, G.; Ursaki, V.V.; Leistner, K.; Tiginyanu, I.M.; Enachescu, M.; Nielsch, K.In this paper, we report on results of a systematic study of porous morphologies obtained using anodization of HVPE-grown crystalline GaN wafers in HNO3, HCl, and NaCl solutions. The anodization-induced nanostructuring is found to proceed in different ways on N-and Ga-faces of polar GaN substrates. Complex pyramidal structures are disclosed and shown to be composed of regions with the degree of porosity modulated along the pyramid surface. Depending on the electrolyte and applied anodization voltage, formation of arrays of pores or nanowires has been evidenced near the N-face of the wafer. By adjusting the anodization voltage, we demonstrate that both current-line oriented pores and crystallographic pores are generated. In contrast to this, porosification of the Ga-face proceeds from some imperfections on the surface and develops in depth up to 50 μm, producing porous matrices with pores oriented perpendicularly to the wafer surface, the thickness of the pore walls being controlled by the applied voltage. The observed peculiarities are explained by different values of the electrical conductivity of the material near the two wafer surfaces.
- ItemTransboundary geophysical mapping of geological elements and salinity distribution critical for the assessment of future sea water intrusion in response to sea level rise(Munich : EGU, 2012) Jørgensen, F.; Scheer, W.; Thomsen, S.; Sonnenborg, T.O.; Hinsby, K.; Wiederhold, H.; Schamper, C.; Burschil, T.; Roth, B.; Kirsch, R.; Auken, E.Geophysical techniques are increasingly being used as tools for characterising the subsurface, and they are generally required to develop subsurface models that properly delineate the distribution of aquifers and aquitards, salt/freshwater interfaces, and geological structures that affect groundwater flow. In a study area covering 730 km2 across the border between Germany and Denmark, a combination of an airborne electromagnetic survey (performed with the SkyTEM system), a high-resolution seismic survey and borehole logging has been used in an integrated mapping of important geological, physical and chemical features of the subsurface. The spacing between flight lines is 200–250 m which gives a total of about 3200 line km. About 38 km of seismic lines have been collected. Faults bordering a graben structure, buried tunnel valleys, glaciotectonic thrust complexes, marine clay units, and sand aquifers are all examples of geological structures mapped by the geophysical data that control groundwater flow and to some extent hydrochemistry. Additionally, the data provide an excellent picture of the salinity distribution in the area and thus provide important information on the salt/freshwater boundary and the chemical status of groundwater. Although the westernmost part of the study area along the North Sea coast is saturated with saline water and the TEM data therefore are strongly influenced by the increased electrical conductivity there, buried valleys and other geological elements are still revealed. The mapped salinity distribution indicates preferential flow paths through and along specific geological structures within the area. The effects of a future sea level rise on the groundwater system and groundwater chemistry are discussed with special emphasis on the importance of knowing the existence, distribution and geometry of the mapped geological elements, and their control on the groundwater salinity distribution is assessed.
- ItemWhen Density Functional Approximations Meet Iron Oxides(Washington, DC : Soc., 2016) Meng, Yu; Liu, Xing-Wu; Huo, Chun-Fang; Guo, Wen-Ping; Cao, Dong-Bo; Peng, Qing; Dearden, Albert; Gonze, Xavier; Yang, Yong; Wang, Jianguo; Jiao, Haijun; Li, Yongwang; Wen, Xiao-DongThree density functional approximations (DFAs), PBE, PBE+U, and Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE), were employed to investigate the geometric, electronic, magnetic, and thermodynamic properties of four iron oxides, namely, α-FeOOH, α-Fe2O3, Fe3O4, and FeO. Comparing our calculated results with available experimental data, we found that HSE (a = 0.15) (containing 15% "screened" Hartree-Fock exchange) can provide reliable values of lattice constants, Fe magnetic moments, band gaps, and formation energies of all four iron oxides, while standard HSE (a = 0.25) seriously overestimates the band gaps and formation energies. For PBE+U, a suitable U value can give quite good results for the electronic properties of each iron oxide, but it is challenging to accurately get other properties of the four iron oxides using the same U value. Subsequently, we calculated the Gibbs free energies of transformation reactions among iron oxides using the HSE (a = 0.15) functional and plotted the equilibrium phase diagrams of the iron oxide system under various conditions, which provide reliable theoretical insight into the phase transformations of iron oxides.
- ItemNanofiller dispersion, morphology, mechanical behavior, and electrical properties of nanostructured styrene-butadiene-based triblock copolymer/CNT composites(Basel : MDPI, 2019) Staudinger, Ulrike; Satapathy, Bhabani K.; Jehnichen, DieterA nanostructured linear triblock copolymer based on styrene and butadiene with lamellar morphology is filled with multiwalled carbon nanotubes (MWCNTs) of up to 1 wt% by melt compounding. This study deals with the dispersability of the MWCNTs within the nanostructured matrix and its consequent impact on block copolymer (BCP) morphology, deformation behavior, and the electrical conductivity of composites. By adjusting the processing parameters during melt mixing, the dispersion of the MWCNTs within the BCP matrix are optimized. In this study, the morphology and glass transition temperatures (Tg) of the hard and soft phase are not significantly influenced by the incorporation of MWCNTs. However, processing-induced orientation effects of the BCP structure are reduced by the addition of MWCNT accompanied by a decrease in lamella size. The stress-strain behavior of the triblock copolymer/MWCNT composites indicate higher Young’s modulus and pronounced yield point while retaining high ductility (strain at break ~ 400%). At a MWCNT content of 1 wt%, the nanocomposites are electrically conductive, exhibiting a volume resistivity below 3 × 103 Ω·cm. Accordingly, the study offers approaches for the development of mechanically flexible functional materials while maintaining a remarkable structural property profile.
- ItemEstablishment, morphology and properties of carbon nanotube networks in polymer melts(Amsterdam [u.a.] : Elsevier, 2012) Alig, I.; Pötschke, P.; Lellinger, D.; Skipa, T.; Pegel, S.; Kasaliwal, G.R.; Villmow, T.As for nanofillers in general, the properties of carbon nanotube (CNT) -polymer composites depend strongly on the filler arrangement and the structure of the filler network. This article reviews our actual understanding of the relation between processing conditions, state of CNT dispersion and structure of the filler network on the one hand, and the resulting electrical, melt rheological and mechanical properties, on the other hand. The as-produced rather compact agglomerates of CNTs (initial agglomerates, >1 μm), whose structure can vary for different tube manufacturers, synthesis and/or purification conditions, have first to be well dispersed in the polymer matrix during the mixing step, before they can be arranged to a filler network with defined physical properties by forming secondary agglomerates. Influencing factors on the melt dispersion of initial agglomerates of multi-walled CNTs into individualized tubes are discussed in context of dispersion mechanisms, namely the melt infiltration into initial agglomerates, agglomerate rupture and nanotube erosion from agglomerate surfaces. The hierarchical morphology of filler arrangement resulting from secondary agglomeration processes has been found to be due to a competition of build-up and destruction for the actual melt temperature and the given external flow field forces. Related experimental results from in-line and laboratory experiments and a model approach for description of shear-induced properties are presented.