Modifizierung des ABSE‐Polycarbosilazans mit Multi‐Walled Carbon Nanotubes zur Herstellung spinnfähiger Massen

Modification of the ABSE polycarbosilazane with multi‐walled carbon nanotubes for the creation of spinable masses An inexpensive method has been found to produce ceramic SiCN‐fibres via the precursor route consisting of five processing steps: synthesis of the polymer, preparation of the spinning mass, melt‐spinning, curing via electron beam and subsequent pyrolysis at 1100 °C in a nitrogen atmosphere. A special solid and meltable fibre polymer, the so‐called polycarbosilazane ABSE, has been developed in the last decade for this purpose. Due to its low molecular weight, an adequate catalytic and thermal aftertreatment was necessary to guarantee a stable melt‐spinning process. This article discusses an alternative way to prepare a qualified spinning mass, i.e. the addition of Multi‐Walled Carbon Nanotubes (MWCNTs) to the ABSE melt. For this purpose a homogeneous dispergation of the MWCNTs in the ABSE matrix is necessary. In this study, spinning masses were fabricated in different ways. By optical analysis and comparison of the level of dispergation in these spinning masses an optimized process for integration of the MWCNTs was identified. The influence of the addition of a dispersing agent is investigated as well. In using a dispersing agent, the level of homogeneous dispersion of the MWCNTs increases whereas the interactions between the particles and the precursor melt decrease. In first spinning experiments a good spinability of the masses were noticed. Thus the addition of MWCNTs represents a new way to modify the ABSE precursor for the melt‐spinning process.     

Catalytic performance of acid-treated multi-walled carbon nanotube-supported platinum catalyst for PEM fuel cells

Effects of the surface functional groups of multi-walled carbon nanotubes (MWCNTs) as catalyst support for the durability of polymer electrolyte membrane fuel cells (PEMFCs) were examined at high Pt loading conditions. The amount of oxygen functional groups on the MWCNTs surface was increased as the acid treatment time and temperature increased. We found that more functional groups in MWCNTs improved initial Pt dispersion but deteriorated durability due to the reduced carbon corrosion resistance. The experimental results also showed that despite the surface oxidation, Pt/MWCNT catalysts showed highly improved durability than Pt/C catalysts due to the graphitic nature of MWCNTs. Membrane electrode assembly (MEA) fabricated by 4 h acid treated MWCNTs at 25 °C showed 4 times better durability than commercial Pt/C based MEA at the reverse potential operation generated by fuel starvation conditions. We believe that MWCNTs can be effectively used for PEMFCs even at high loading due to their excellent anti-corrosion properties.     

Deposition of nanocrystalline nonstoichiometric chromium oxide coatings on the surface of multiwalled carbon nanotubes by chromium acetylacetonate vapor pyrolysis

Nanocrystalline coatings of nonstoichiometric chromium oxide have been obtained for the first time on the surface of multiwalled carbon nanotubes (MWCNTs) by the method of metalorganic chemical-vapor deposition using chromium acetylacetonate as a precursor. The new hybrid nanomaterial (Cr₂O_2.4/MWCNT) has been characterized by X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. It is established that oxidation of the hybrid nanomaterial in air under soft conditions (at 380°C) leads to the formation of nanocrystalline chromium oxide (Cr₂O₃) on the surface of MWCNTs.

     

Nanocomposite based on modified multiwalled carbon nanotubes: Fabrication by an oriented spinning process and electrical conductivity

We have produced nanocomposite films consisting of modified multiwalled carbon nanotubes (MWCNTs) and a polymer (MWCNT/polymer weight ratio of 95/5). The nanocomposite has been applied to a paper substrate by an oriented spinning process from a liquid phase. The resistivity of the films has been measured as a function of temperature in the range 20–140°C along and across the preferential orientation direction of the nanotubes in the nanocomposite. The results point to an irreversible transition from semiconducting to metallic behavior of the conductivity of the films.     

Fabrication of 3D carbon nanotube/porous carbon hybrid materials

Three dimensional hybrid carbon materials have been prepared using different biomass-derived porous carbons as catalyst supports for growing multi-walled carbon nanotubes (MWCNTs) via a chemical vapor deposition method. The nickel catalyst-loaded supports before and after growing MWCNTs were characterized by scanning and transmission electron microscopy, Fourier transform infrared spectroscopy spectra, and mercury porosimetry. The results show that the grown MWCNTs microstructures are closely related to the porous structures and surface conditions of the carbon supports. By using bamboo as template, a porous carbon support with a large total pore volume, appropriate pore size, and abundant favorable surface functional groups is obtained, which is found to be an ideal support for growing the MWCNTs. Investigation of growth mechanism demonstrated that the combination of appropriate porous structures and surface conditions plays an essential role in catalyst distribution and MWCNTs growth.     

Preparation and characterization of multi-walled carbon nanotube/hydroxyapatite nanocomposite film dip coated on Ti–6Al–4V by sol–gel method for biomedical applications: An in vitro study

In the present research, the introduction of multi-walled carbon nanotubes (MWCNTs) into the hydroxyapatite (HA) matrix and dip coating of nanocomposite on titanium alloy (Ti–6Al–4V) plate was conducted in order to improve the performance of the HA-coated implant via the sol–gel method. The structural characterization and electron microscopy results confirmed well crystallized HA–MWCNT coating and homogenous dispersion of carbon nanotubes in the ceramic matrix at temperatures as low as 500 °C. The evaluation of the mechanical properties of HA and HA/MWCNT composite coatings with different weight percentages of MWCNTs showed that the addition of low concentrations of MWCNTs (0.5 and 1 wt.%) had improved effect on the mechanical properties of nanocomposite coatings. Moreover, this in vitro study ascertained the biocompatibility of the prepared sol–gel-derived HA/MWCNT composite coatings.     

Quaternäre Keramiken im System Si/B/N/C aus polymeren Carbamidsäurederivaten

Quaternary Ceramics in the System Si/B/N/C from Polymeric Carbamid Acid Derivatives Solid polymeric materials can be formed through reaction of the single-source precursor trichlorosilyl-amino-dichloroborane (TADB) with derivatives of carbamic or formic acid. Pyrolysis of these polymers at 1500 °C results in ceramic materials of the composition SiBN_2.3C_x with X = 0.4 – 2.5. The high-temperature and oxidation behaviours of the ceramics depend on the starting polymer. The ceramic networks consist of tetrahedrally coordinated silicon and trigonally planar coordinated boron. They are homogeneous with respect to the elemental distribution, and remain in the amorphous state up to at least 1500°C.     

Adsorption behavior of carbon nanotubes on polystyrene surfaces

Polystyrene (PS) was prepared using two different polymerization methods (dispersion polymerization and seed polymerization) to investigate the steric stabilizer effect during the adsorption process of carbon nanotubes (CNTs) on the surface of PS microspheres. Experiments with different microsphere diameters and difference types of CNTs were conducted to analyze the curvature effect of the spheres on the adsorption mechanism. The results showed that PS microspheres prepared through dispersion polymerization exhibited preferable adsorption behavior compared to PS spheres prepared through seed polymerization, suggesting that poly(N-vinylpyrrolidone) led to improved adsorption interactions between the CNTs and the PS microspheres in the CNTs dispersion. Additionally, the PS diameter and CNT curvature were examined with respect to the adsorption behavior between the PS microspheres and the CNTs. Multiwalled carbon nanotubes (MWCNTs) were found to be well adsorbed on the surface of PS microspheres measuring 2 microm. However, the MWCNTs were adsorbed much less on the surface of submicron-sized PS microspheres, compared with thinwalled carbon nanotubes (TWCNTs). On the other hand, TWCNTs were found to be suitable for adsorption on submicron-sized PS microspheres. These results also indicate that the curvature of the CNTs and the polymer microspheres are important to the CNT adsorption process.     

Synthesis and properties of magnetic multi-walled carbon nanotubes loaded with Fe4N nanoparticles

High saturation magnetization (>90 emu/g) multi-walled carbon nanotubes (MWCNTs) and Fe₄N nanoparticles composite were successfully synthesized by gas nitriding at 550 °C. Almost all Fe₄N nanoparticles were evenly distributed inside the carbon nanotubes and formed a special composite microstructure. This composite microstructure shows excellent soft magnetic property, structural stability, and chemical stability at room temperature. The investigations of electromagnetic and microwave absorption performances indicate that microwave absorbing capacity of low frequency band of MWCNTs were greatly improved by addition of Fe₄N nanoparticles.     

In situ high-temperature electron microscopy of 3DOM cobalt,iron oxide,and nickel

High-temperature electron microscopy was used to follow how the structure of two specimens of three-dimensionally ordered macroporous (3DOM) materials, also known as inverse opals, and one specimen of a precursor to a 3DOM material changed with temperature. The change in grain size with temperature of 3DOM cobalt and 3DOM iron oxide (as magnetite) was monitored in situ in the TEM by heating in stages to 900 and 1,000 °C, respectively. The two materials studied by TEM showed contrasting grain growth behavior. For 3DOM cobalt, carbon surrounding the nanometer-size grains led to slower grain growth in thinner sample areas than in areas in closer contact with other grains; a bimodal grain-size distribution was observed after heating above 700 °C for 90 min. The grains of the 3DOM iron oxide had no carbon coating and coarsened more evenly to give a unimodal size distribution. Line scans from selected-area diffraction (SAD) patterns were used for phase analysis and showed that traces of cobalt oxide present in the 3DOM cobalt sample at room temperature disappeared when the sample was heated above 500 °C. The transformation of a 3DOM precursor material, nickel(II) oxalate–polystyrene (PS) latex composites, was followed in situ by variable-temperature environmental scanning electron microscopy (ESEM) from room temperature to ca. 700 °C in 0.5–0.7 kPa O₂. The ESEM examination of the 3DOM precursors permitted real-time observation of the polymer template decomposition and the shrinkage that occurs upon calcination of these precursor materials.     

Functionalized Multi‐Wall Carbon Nanotubes for Lipase Immobilization

We examine the immobilization of lipase B from Candida antarctica on functionalized multi‐wall carbon nanotubes (MWCNTs) through physical adsorption. MWCNTs functionalized with carboxyl‐, amine‐ and ester‐ terminal groups on their surface are used as immobilization carriers. Dispersion of the nanotubes and the immobilization procedure take place in aqueous and low‐water media. High enzyme loadings are attained, up to 25% of the weight of the carbon nanotubes. These novel biomaterials are characterized though FT‐IR and Raman spectroscopy. The MWCNT–lipase bioconjugates exhibit high catalytic activity and increased storage and operational stability. The biomaterials retain more than 55% of their initial activity after 6 months at 4 °C, while they retain approximately 25% of their initial activity after 30 d of incubation in hexane at 60 °C. The catalytic behaviour of the immobilized enzyme depends on the terminal group of the carbon nanotubes, the concentration of the enzyme and the immobilization method employed.     

A novel structure for carbon nanotube reinforced alumina composites with improved mechanical properties

Engineering ceramics have high stiffness, excellent thermostability, and relatively low density, but their brittleness impedes their use as structural materials. Incorporating carbon nanotubes (CNTs) into a brittle ceramic might be expected to provide CNT/ceramic composites with both high toughness and high temperature stability. Until now, however, materials fabrication difficulties have limited research on CNT/ceramic composites. The mechanical failure of CNT/ceramic composites reported previously is primarily attributed to poor CNT-matrix connectivity and severe phase segregation. Here we show that a novel processing approach based on the precursor method can diminish the phase segregation of multi-walled carbon nanotubes (MWCNTs), and render MWCNT/alumina composites highly homogeneous. The MWCNTs used in this study are modified with an acid treatment. Combined with a mechanical interlock induced by the chemically modified MWCNTs, this approach leads to improved mechanical properties. Mechanical measurements reveal that only 0.9?vol% acid-treated MWCNT addition results in?27% and?25% simultaneous increases in bending strength (689.6 ± 29.1?MPa) and fracture toughness (5.90 ± 0.27?MPa?m(1/2)), respectively.     

A novel approach to the chemical stabilization of gamma-irradiated ultrahigh molecular weight polyethylene using arc-discharge multi-walled carbon nanotubes

A complete study was made of the stabilization of gamma-irradiated ultrahigh molecular weight polyethylene (UHMWPE) using arc-discharge multi-walled carbon nanotubes (MWCNTs) as inhibitors of the oxidative process. MWCNTs were efficiently incorporated into the polymer matrix by ball milling and thermo-compression processes at concentrations up to 5 wt% and subsequently gamma irradiated at 90 kGy. Raman spectroscopy demonstrated the generation of radicals on the walls of the MWCNTs and that the G/D ratio was altered by their generation. The same spectra showed interactions between the polymer chains as a series of shifts are observed in the UHMWPE bands. The effect of the MWCNTs as inhibitors for the oxidative process of the UHMWPE was evaluated by means of Electron Spin Resonance (ESR) and Fourier Transformed Infrared Spectroscopy (FTIR). ESR detection of the radiation-induced radicals proved the radical scavenger behaviour of MWCNTs. FTIR measurements were performed to ascertain the influence of the irradiation and of the accelerated ageing protocol in the oxidation index of the polymer and the composites. The results pointed to the positive contribution of the MWCNTs in increasing the oxidative stability of the composite when compared to pure UHMWPE. A comparison is made between composites obtained using MWCNTs produced by the carbon vapour deposition and arc-discharge methods.     

Simple method for synthesis of carbon nanotubes over Ni-Mo/Al2O3 catalyst via pyrolysis of polyethylene waste using a two-stage process

Synthesis of valuable multi-walled carbon nanotubes (MWCNTs) by thermal pyrolysis of low-density polyethylene (LDPE) waste was investigated via a two-stage process. The first stage was the thermal pyrolysis of LDPE to gaseous hydrocarbons, and the second stage was the catalytic decomposition of the pyrolysis gases over Ni-Mo/Al₂O₃ catalysts. Two catalysts with the compositions of 5.2%Ni-10.96%Mo/Al₂O₃ and 10%Ni-9.5%Mo/Al₂O₃ were tested for carbon nanotubes (CNTs) formation. The catalyst containing 10%Ni showed better activity in terms of CNTs production. Accordingly, the impact of either pyrolysis or decomposition temperatures was investigated using the 10%Ni-9.5%Mo/Al₂O₃ catalyst. TEM, XRD, Raman spectroscopy, TGA, TPR, and BET analysis tools were used to characterize the fresh catalysts as well as the obtained carbon nanomaterials. TEM images proved that MWCNTs with various morphological structures were obtained at all pyrolysis and decomposition temperatures. Moreover, cup-stacked carbon nanotubes (CS-CNTs) were observed at the decomposition temperature of 600°C. MWCNTs with the best quality were produced at decomposition temperature of 750°C. The optimum pyrolysis and decomposition temperatures in terms of CNTs production were at 700 and 650°C, respectively.     

An innovative method to improve the electrical conductivity of multi-walled carbon nanotubes filled polyamide 6/polystyrene blends

An innovative method has been successfully developed to improve the electrical conductivity of polyamide 6/polystyrene (PA6/PS) blends in this paper. PA6/PS blends containing multi-walled carbon nanotubes (MWCNTs) are prepared by the radical polymerization of styrene in the presence of ?-caprolactam (CL) and MWCNTs, followed by the in-situ anionic ring-opening polymerization of CL. In the resulted PA6/PS blends, MWCNTs are selectively located at the interface of PA6 and PS. Because the interface of 0.5 and 1.0 wt.% MWCNTs filled blends with PA6/PS weight ratio of 70/30 is continuous, a MWCNTs conductive pathway is formed in these two blends, which results in a decrease of volume resistivity by about 9 orders of magnitude.     

Synthesis and characterization of polypyrrole nanotubes/multi-walled carbon nanotubes composites with superior electrochemical performance

Novel polypyrrole nanotubes/multi-walled carbon nanotubes (PPyNTs/MWCNTs) composites have been successfully synthesized via in situ chemical oxidation polymerization with methyl orange as soft template. Scanning electron microscopy and transmission electron microscopy images revealed that MWCNTs intertwined with the PPyNTs and PPyNTs/MWCNTs composites formed in water–ethanol solution. The obtained composites exhibited perfect electrochemical characteristic compared with PPyNTs and MWCNTs owing to the synergetic effect and the specific capacitance of the composites was strongly influenced by the mass ratio of pyrrole to MWCNTs. According to the galvanostatic charge/discharge analysis, the specific capacitance of PPyNTs/MWCNTs composites is up to 352 F g^?1 at a current density of 0.2 A g^?1 in 1 M KCl solution, much higher than that of the PPyNTs (178 F g^?1) and MWCNT (46 F g^?1), suggesting its potential application in supercapacitors.     

Growth of multi-walled carbon nanotubes by catalytic decomposition of acetylene on Ni-supported carbon fibers prepared by the heat-treatment of cellulose fibers

Cellular-type carbon fibers, prepared by heat-treatment of cellulose fibers that ranged from 500 to 2300 °C, were used as catalytic support for the growth of carbon nano-filaments. A comparison of the Raman spectra of products prepared at different heat-treatment temperatures showed significant variation in the carbon structure of the surfaces of the as-prepared carbon fibers. TEM observation clearly revealed that the products heat-treated below 1000 °C had an amorphous phase, and at 1500 °C they were similar to glassy carbon—an example of non-graphitizing carbon. Surface graphitization of the carbon fibers derived by pre-treatment at 1500 °C was accomplished at above 2000 °C. Multi-walled carbon nanotubes (MWCNTs), approximately 15 nm and 8 μm in diameter and length, were synthesized on the surface of the prepared carbon fibers via a tip growth mechanism. TEM images and Raman spectra confirmed that the higher synthesis temperature could lead to the formation of MWCNTs with better crystalline carbon shells.     

Temperature optimisation of CNT synthesis by spray pyrolysis of alpha-pinene as the carbon source

Multi-wall carbon nanotubes (MWCNTs) were prepared by spray-pyrolysis of a botanical hydrocarbon, alpha-pinene and ferrocene as the catalyst at 700–1000°C. The MWCNTs were analysed by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and X-ray diffraction. The microscopy studies show the formation of carbon nanotubes with diameters between 20 and 30 nm and length greater than 100 µm. Raman spectroscopy revealed that the alpha-pinene-grown carbon nanotubes were graphitised showing both the D and G bands at 1330 and 1590 cm^?1, respectively, and that the corresponding intensity band ratio (I _D/I _G) varied with respect to temperature formation.     

Oxidation behavior of green coke-based carbon–ceramic composites incorporating micro- and nano-silicon carbide

The oxidation resistance of the carbon–ceramic composites developed using green coke-based carbon and carbon black as carbon source, boron carbide, and micro- and nano-silicon carbide was carried out in the temperature range of 800 to 1,200 °C. Silicon carbide particulate as such and silicon carbide obtained by the reaction of green coke and silicon provided micro silicon carbide while silicon and carbon black and sol–gel silica and carbon black used as silicon carbide precursors led to the formation of nano-silicon carbide. The oxidation resistance of these composites at 800 to 1,200 °C for 10 h showed that the size of the silicon carbide influenced the oxidation resistance. The weight gain due to protective coating formed on oxidation was higher in composites containing nano-silicon carbide as compared to the composites containing micro silicon carbide.     

Photocatalytic degradation mechanisms of dimethyl phthalate esters by MWCNTs-anatase TiO2 nanocomposites using the UHPLC/Orbitrap/MS technique

Dimethyl phthalate esters (DMPEs) have been identified as endocrine disrupting plastisizers and emerging contaminants which can be released readily upon exposure to the environment. In this study, MWCNTs/TiO₂ nanocomposites, which possess the potential application for the photocatalytic degradation of DMPEs under UV irradiation, were prepared via simple one-pot sol-gel reaction using titanium isopropoxide (TTIP) as titania precursor and multiwalled carbon nanotubes (MWCNTs). The MWCNTs/TiO₂ nanocomposites was calcined in air for 2 h at the temperatures ranging from 350 to 750 °C. As a result, the MWCNTs/TiO₂ nanocomposites synthesized at calcination temperature of 450 °C demonstrated the highest photodegradation efficiency of 97% after 180 min UV irradiation and its degraded products were evaluated using the ultra high performance liquid chromatography (UHPLC) coupled with a high resolution (HR) Orbitrap mass spectrometry (MS). A primary degradation mechanism was proposed and it was noteworthy that some new intermediates were discovered and reported. This work has developed a simple method for qualitative determination of DMPEs based on HPLC with UV detection.