Raman study of carbon nanotube purification using atmospheric pressure plasma

Multiwalled carbon nanotubes (MWCNTs) were treated with an atmospheric pressure plasma source using an argon/water mixture. Optical emission diagnostics has shown that hydroxyl radicals (OH) were the major reactive species in the plasma. The structural changes in MWCNTs were monitored by micro-Raman spectroscopy. The observed variation of the D and G band intensity ratio and position dispersion with plasma treatment time was ascribed to the change in structural disorder on MWCNT surfaces. Scanning electron microscopic study showed that some defects can be induced in MWCNTs during plasma treatment. Results of thermogravimetric analysis indicated that atmospheric pressure OH plasma is as effective as traditional wet methods for purifying MWCNTs.     

A carbon nanotube film as a radio frequency filter

This paper proposes a new concept for the application of carbon nanotubes to electronic devices. A carbon nanotube (150-200 nm width, few μm length) film grown on a metal sensitized quartz surface was modeled using a two-pole lumped element equivalent circuit consisting of a capacitor, an inductor, and two resistors. The capacitor was in series with the inductor resulting in band-stop filter characteristics with a central frequency of 18 MHz. The reactive subcircuit was in parallel with one resistor and in series with the other. The magnitude of the parallel resistance had a large influence on the efficiency of the reactive elements and the filter quality factor. A two-dimensional carbon nanotube film is expected to be suitable in the design of miniaturized RF filters.     

Functionalization of carbon nanotube yarn by acid treatment

Carbon nanotube (CNT) yarn was functionalized using sulfuric and nitric acid solutions in 3:1 volumetric ratio. Successful functionalization of CNT yarn with carboxyl and hydroxyl groups (e.g., COOH, COO–, OH, etc.) was confirmed by attenuated total reflectance spectroscopy. X-ray diffraction revealed no significant change to the atomic in-plane alignment in the CNTs; however, the coherent length along the diameter was significantly reduced during functionalization. A morphology change of wavy extensions protruding from the surface was observed after the functionalization treatment. The force required to fracture the yarn remained the same after the functionalization process; however, the linear density was increased (310%). The increase in linear density after functionalization reduced the tenacity. However, the resistivity density product of the CNT yarn was reduced significantly (234%) after functionalization.     

Synthesis of carbon nanosheets and carbon nanotubes by radio frequency plasma enhanced chemical vapor deposition

Carbon nanosheets are a two-dimensional carbon nanostructure consisting of free-standing graphite sheets (∼ 1 nm thick) which we deposit using an inductively coupled RF PECVD method. Our previous reports have shown that carbon nanosheets can be grown on a variety of substrates, without catalyst, and under various growth conditions. In this work we describe process details and plasma control parameters to permit rapid changeover from nanotube to nanosheet deposition in the same system. We present a thorough characterization of the nanosheet layer that is produced under these circumstances. We also propose plausible mechanisms to account for the different mode of layer growth leading to the observed topography and conformation of the nanosheets. The experimental evidence suggests that a combination of high plasma electron density and large atomic hydrogen density of the inductively coupled plasma is the reason for nanosheet formation.     

The mechanism of air/oxygen/helium atmospheric plasma action on PVA

The effect of the air/oxygen/helium atmospheric plasma treatment on desizing polyvinyl alcohol (PVA) on cotton fabric was discussed as compared with the conventional H2O2 desizing. The possible change mechanism of PVA during atmospheric plasma exposure was induced through a combination of weight loss of PVA after plasma, PVA dissolving rate in water at room temperature, X‐ray photoelectron spectroscopy, and Fourier Transform infrared spectroscopy. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Effect of atmospheric pressure helium plasma on felting and low temperature dyeing of wool

Wool fabrics were treated with atmospheric pressure helium glow discharge plasma in an attempt to improve felting and dyeing behavior with cold brand reactive dyes using cold pad‐batch method at neutral pH. On glow plasma treatment, the hydrophilicity of wool surface and its resistance toward felting was greatly improved without any significant damage to the cuticle layer. The color strength of the plasma treated dyed wool on the surface (in terms of K/S) was found to be nearly double of the color strength of dyed untreated wool fabric. However, the corresponding total dye uptake of the treated wool increased by a much lower value of 40%–50%. The reason behind this altered dyeing behavior was investigated by studying the dye kinetics using infinite bath and surface characteristics using SEM and SIMS. It was found that the glow plasma treatment greatly transformed the chemical surface of the wool fibers. It resulted in uniform removal of hydrophobic cuticular layer, which resulted in better diffusion of the dye molecules into the fiber, and formation of hydrophilic ? NH₂ groups near the surface, which helped in anchoring the dye molecules close to the surface giving higher color strength than expected. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Atmospheric pressure helium + oxygen plasma treatment of ultrahigh modulus polyethylene fibers

Ultrahigh modulus polyethylene fibers were treated with atmospheric pressure helium + oxygen plasma in a capacitively coupled device at a frequency of 7.5 kHz. The fibers were treated for 0, 0.5, 1, 1.5, and 2 min. The surfaces of the fibers treated with He + O₂ plasma were etched and micro-cracks were formed. XPS analysis showed a 65ndash213% increase in oxygen content on the surfaces of all plasma-treated fibers, except for the 1.5 min group. An increase in the concentration of C—O and the appearance of C=O bonds on the surfaces of plasma-treated fibers were observed. In the micro-bond test, He + O2 plasma-treated groups had a 65–104% increase in interfacial shear strength over that of the control. The tensile strength of the fibers was either unchanged or decreased by 10–13% by the plasma treatments.     

Structure changes during tensile deformation and mechanical properties of a twisted carbon nanotube yarn

The small-angle X-ray scattering measurements during tensile deformation have been performed for studying the structure and mechanical property relationships of twisted carbon nanotube (CNT) yarns. The tensile strength distribution and the diameter changes during tensile deformation have also been measured. The orientation distribution of the CNTs in the yarn has been determined and its changes during tensile deformation have been related to the variation of the tensile modulus with the twist angle. The tensile modulus and Poisson’s ratio of the yarns decreased with increasing twist angle, whereas the tensile strength of the yarn showed a maximum at the twist angle of 25°. At this twist angle, the distribution width of the tensile strength was minimum indicating the higher uniformity of the yarn structure.     

Synthesis and characterization of carbon nanowalls on different substrates by radio frequency plasma enhanced chemical vapor deposition

A radio frequency plasma enhanced chemical vapor deposition system was used for the successful growth of thin vertical carbon nanowalls, also known as vertical graphene, on various substrates. Transmission electron microscopy studies confirmed the presence of vertical graphene walls, which are tapered, typically consisting of 10 layers at the base tapering off to 2 or 3 layers at the top. The sides of the walls are facetted at quantized angles of 30° and the facetted sides are usually seamless. Growth occurs at the top open edge which is not facetted. Hydrogen induced etching allows for nucleation of branch walls apparently involving a carbon onion-like structure at the root base. Characterization by a superconducting quantum interference device showed magnetic hysteresis loops and weak ferromagnetic responses from the samples at room temperature and below. Temperature dependence of the magnetization revealed a magnetic phase transition around T = 50 K highlighting the coexistence of antiferromagnetic interactions as well as ferromagnetic order.     

Activation of carbon fibres from pitch by atmospheric oxygen

Oxidative etching of carbon fibres obeys the principles of polychromatic kinetics of polymer reactions. The differences in the efficiency of activation of carbon fibres based on different raw materials by thermooxidation in air are due to the morphological features of their structure. The possibility of obtaining activated carbon fibres by oxidation of the starting carbon fibres from isotropic pitch with atmospheric oxygen at high temperature was demonstrated.Translated from Khimicheskie Volokna, No. 6, pp. 40–43, November–December, 1994.     

Effects of atmospheric pressure helium/air plasma treatment on adhesion and mechanical properties of aramid fibers

In order to investigate the effect of atmospheric pressure plasmas on adhesion between aramid fibers and epoxy, aramid fibers were treated with atmospheric pressure helium/air for 15, 30 and 60 s on a capacitively-coupled device at a frequency of 5.0 kHz and He outlet pressure of 3.43 kPa. SEM analysis at 10 000× magnification showed no significant surface morphological change resulted from the plasma treatments. XPS analysis showed a decrease in carbon content and an increase in oxygen content. Deconvolution analysis of C1s, N1s and O1s peaks showed an increase in surface hydroxyl groups that can interact with epoxy resin. The microbond test showed that the plasma treatment for 60 s increased interfacial shear strength by 109% over that of the control (untreated). The atmospheric pressure plasma increased single fiber tensile strength by 16-26%.     

Low-temperature synthesis of carbon nanotubes using corona discharge plasma at atmospheric pressure

A new method, which combines non-equilibrium plasma reaction with template-controlled growth technology, has been developed for synthesizing aligned carbon nanotubes at atmospheric pressure and low temperature. Multiwall carbon nanotubes with diameters of approximately 40 nm were restrictedly synthesized in the channels of anodic aluminum oxide template from a methane/hydrogen mixture gas by a.c. corona discharge plasma reaction at a temperature below 200 °C. It was observed that amorphous carbon deposited on the tops of the carbon nanotubes. The formation mechanism of carbon nanotubes synthesized by corona discharge plasma method was discussed.     

Mechanism of cone-shaped carbon nanotube bundle formation by plasma treatment

Formation of the cone-shaped multi-walled carbon nanotube (MWCNT) bundles was investigated with the consideration of the induced dipole moments of the MWCNTs interaction under the ion irradiation which is accelerated by the applied sheath electric field for the various argon, hydrogen, nitrogen, and oxygen plasmas. Vertically grown MWCNTs were irradiated by energetic ion whose energy and dose were controlled by the sheath formed on the MWCNT substrate. Plasma irradiation was carried out in a downstream region separated from the plasma source region, providing that the irradiated ion density and energy could be controlled precisely with the sheath electric field. In argon and hydrogen plasmas, the cone-shaped MWCNT bundle was not fabricated, while it was formed successfully in nitrogen and oxygen plasmas. Especially, the oxygen plasma was the most effective in the formation of the bundle. The mechanism of the bundle formation could be explained by a model explaining the interaction between the induced dipole moment of the MWCNT and the sheath electric field. For the nitrogen and oxygen plasma irradiated MWCNT, the induced dipole moment could be enhanced by C-N and C-O bonds so the size of the bundle is proportional to the ion irradiation and the sheath electric field.     

Effect of using oxygen,carbon dioxide,and carbon monoxide as active gases in the atmospheric plasma treatment of fiber-reinforced polycyanurate composites

A study was undertaken to address the effect of using different active gases during the atmospheric plasma treatment of composite specimens for adhesive bonding. The effect of using oxygen, carbon dioxide, or carbon monoxide on the surface chemistry, morphology, and mechanical properties of cyanate ester composites was investigated. CO treatment resulted in a surface profile that could be tailored to create an oxygen/carbon ratio as high as 0.71 with a negligible degree of polymer degradation as verified by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy. On the other hand, CO₂ and O₂ treatments resulted in a fairly high degree of chain scission and degradation using otherwise similar treatment conditions. However, significant bond strength improvement (>75%) over conventional abrasion surface preparation techniques was achieved for all three types of gases. XPS of CO-treated specimens showed a large increase in carbonyl species formation in comparison with the weakly bonded carbonates (ash) formed when treating the same composites with CO₂ and O₂ gas suggesting a different mechanism. These results present a method by which sensitive carbon-based, hydrophobic surfaces can be modified without damaging the underlying substrate as well as improving bond performance over conventional surface preparation methods. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The effect of atmospheric pressure helium plasma treatment on the surface and mechanical properties of ultrahigh-modulus polyethylene fibers

Ultrahigh-modulus polyethylene fibers were treated with atmospheric pressure He plasma on a capacitively coupled device at a frequency of 7.5 kHz and a He partial vapor pressure of 3.43 × 10³ Pa. The fibers were treated for 0, 1, and 2 min. Microscopic analysis showed that the surfaces of the fibers treated with He plasma were etched and that the 2-min He plasma-treated group had rougher surfaces than the 1-min He plasma-treated group. XPS analysis showed a 200% increase in the oxygen content and a 200% increase in the concentration of C—O bonds (from 11.4% to 31%) and the appearance of C=O bonds (from 0% to 7.6%) on the surface of plasma-treated fibers for the 2-min He plasma-treated group. In the microbond test, the 2-min He plasma-treated group had a 100% increase of interfacial shear strength over that of the control group, while the 1-min He plasma-treated group did not show a significant difference from the control group. The 2-min He plasma-treated group also showed a 14% higher single-fiber tensile strength than the control group.     

Rapid and low temperature spark plasma sintering synthesis of novel carbon nanotube reinforced titanium matrix composites

Multi–walled carbon nanotube (MWCNT) reinforced titanium matrix composites were synthesized using a spark plasma sintering method at a low sintering temperature of 550 °C. The effects of the weight fraction of MWCNTs on the microstructures and the mechanical and thermal properties of the composites were investigated. No reaction products were detected in the composites, indicating that the MWCNTs in the composites maintained their structural integrity after sintering, and thus, because of their advantageous properties, could reinforce the titanium matrix. As a result, the compressive strength of the composite containing 0.4 wt.% MWCNTs reached 1106 MPa, which was an increase of 61.5% compared to that of pure titanium under at the same conditions. In addition, the results revealed that compressive strength of the bulk compacts increased initially and then decreased with an increase in weight fraction of MWCNTs. However, compressive strain of the sintered composites continued to fall at a slow rate. The microhardness and thermal diffusivity of the composites rose steadily with an increasing content of MWCNTs. When the weight fraction of MWCNTs in the composites exceeded 0.8%, the compressive strength of the composites declined significantly due to the increasing aggregation of the MWCNTs.     

Fabrication of vertically aligned single-walled carbon nanotubes in atmospheric pressure non-thermal plasma CVD

A fabrication technique of high-purity vertically aligned single-walled carbon nanotubes (VA-SWCNTs) using atmospheric pressure plasma enhanced chemical vapor deposition is presented. Although densely mono-dispersed Fe-Co catalysts of a few nanometers is primarily responsible for VA-SWCNT growth, carbon precipitation was virtually absent in the thermal CVD regime at 700 °C. On the other hand, high-purity VA-SWCNTs without measurable defects were grown at 4 μm min⁻¹ by applying atmospheric pressure radio-frequency discharge (APRFD) which has been previously developed for this purpose. The results proved that cathodic ion sheath adjacent to the substrates, where a large potential drop exists, also plays an essential role for the controlled growth of SWCNTs, while ion damage to the VA-SWCNTs is inherently avoided due to high collision frequency among molecules in atmospheric pressure. Operation regime of APRFD and tentative reaction mechanisms for VA-SWCNT growth are discussed along with optical emission spectroscopy of near substrate region.     

Fabrication of a multifunctional carbon nanotube "cotton" yarn by the direct chemical vapor deposition spinning process

A continuous cotton-like carbon nanotube fiber yarn, consisting of multiple threads of high purity double walled carbon nanotubes, was fabricated in a horizontal CVD gas flow reactor with water vapor densification by the direct chemical vapor deposition spinning process. The water vapor interaction leads to homogeneous shrinking of the CNT sock-like assembly in the gas flow. This allows well controlled continuous winding of the dense thread inside the reactor. The CNT yarn is quite thick (1-3 mm), has a highly porous structure (99%) while being mechanically strong and electrically conductive. The water vapor interaction leads to homogeneous oxidation of the CNTs, offering the yarn oxygen-functionalized surfaces. The unique structure and surface of the CNT yarn provide it multiple processing advantages and properties. It can be mechanically engineered into a dense yarn, infiltrated with polymers to form a composite and mixed with other yarns to form a blend, as demonstrated in this research. Therefore, this CNT yarn can be used as a "basic yarn" for various CNT based structural and functional applications.     

The influence of the carbon precursor,carbon feed rate and helium gas flow rate on the synthesis of fullerenes from carbon powder in an entrained flow 3-phase AC plasma reactor operating at atmospheric pressure

We studied an entrained flow 3-phase AC plasma reactor operating at atmospheric pressure with helium for the synthesis of fullerenes from different carbon powder precursors through an evaporation–condensation process. A parametric study focusing on gas flow rate and carbon powder feed rate was carried out using three commercial carbon materials: Y50A acetylene black from SN2A, E 250 carbon black from TIMCAL, and KS 4 graphite powder from TIMCAL. This study revealed a strong dependence of these parameters with fullerene yield and rate of fullerene production. It also revealed the key role of the carbon black purity. The best results were obtained using acetylene black Y50 A, for which a rate of fullerene production of the order of 17 g h^?1 corresponding to a 480 g h^?1 carbon feed rate at 3.6% fullerene content (C60 + C70) were obtained. The specific energy input defined as the plasma power related to carbon precursor feed rate and rate of fullerene production were estimated to 0.039 kWh g^?1 and 1.11 kWh g^?1 respectively.     

Continuous and rapid stabilization of polyacrylonitrile fiber bundles assisted by atmospheric pressure plasma for fabricating large-tow carbon fibers

A bundle of carbon fibers (CFs) with 24,000 filaments was prepared using atmospheric pressure plasma and heat simultaneously for only 30 min, followed by the carbonization process. Conventional thermal stabilization process takes more than few hours to prevent the ignition of the polyacrylonitrile (PAN) fiber when large-tow PAN fibers are stabilized. The CFs produced using the plasma stabilization process had a tensile strength as high as 2.6 GPa. This strength level, which is slightly higher than that of CFs stabilized by the conventional process for 120 min, satisfies the demands in automobile applications. It is believed that the plasma-based stabilization process provides a potential solution not only for shortening the process time but also for providing continuous stabilization of large-tow carbon fibers.