Electromagnetic wave absorption properties of graphene modified with carbon nanotube/poly(dimethyl siloxane) composites

By using a catalytic growth procedure, carbon nanotubes (CNTs) are in situ formed on reduced graphene oxide (RGO) sheet at 600 °C. CNTs growing on RGO planes through covalent C–C bond possess lower interfacial contact electrical resistance. As a hybrid structure, the CNTs/graphene (CNT/G) are well dispersed into poly (dimethyl siloxane). The hybrid combining electrically lossy CNTs and RGO, which disperses in electrically insulating matrix, constructs an electromagnetic wave (EM) absorbing material with ternary hierarchical architecture. The interfacial polarization in heterogeneous interface plays an important role in absorbing EM power. When the filler loading is 5 wt.% and thickness of absorber is 2.75 mm, the minimum value of reflection coefficient and the corresponding frequency are −55 dB and 10.1 GHz, and the effective absorption bandwidth reaches 3.5 GHz. Therefore, combining the CNTs and graphene sheet into three-dimensional structures produces CNT/G hybrids that can be considered as an effective route to design light weight and high-performance EM absorbing material, while the effective EM absorption frequency can be designed.     

Singly dispersed carbon nanotube/aluminum composites fabricated by powder metallurgy combined with friction stir processing

Carbon nanotube (CNT)/aluminum composites were fabricated by a combination of powder metallurgy and subsequent friction stir processing (FSP). Microstructural observations indicated that the CNTs were singly dispersed in the composites. The CNTs tended to be dispersed along grain boundaries resulting in a much finer grain size. Although the CNTs were shortened and some Al₄C₃ formed in the matrix, the layer structures of the CNTs were well retained. Raman spectroscopy also showed that the damage to CNTs during FSP was not severe. Compared to that of unreinforced Al, the yield strength of 1 wt.% and 3 wt.% CNT/Al composites increased by 23.9% and 45.0%, respectively. A strength equation relating with load transfer and the grain refinement was proposed to describe the increase of the yield strength of the composites.     

Synthesis and characterisation of silica–carbon nanotube hybrid microparticles and their effect on the electrical properties of poly(vinyl alcohol) composites

Hybrid silica–carbon nanotube (CNT) particles with a radial symmetry were produced by the growth of nanotubes onto spherical, mesoporous silica gel particles using the floating catalyst chemical vapour deposition (FC-CVD) method. Characterisation of the hybrid particles, using electron microscopy, Raman spectroscopy and thermogravimetry showed the geometry and porosity of the silica particles to influence the alignment and density of the CNTs produced. CNT growth initiated in the pores of the gel particles and three hours of CVD growth were required to get extensive surface coverage. In the early stages of growth, the reactants diffused inside the mesoporous silica and consequently the CNTs grew mainly within the silica gel rather than on the surface. Some indication of catalyst templating was observed within the smaller (<10 nm) pores, but this templating did not result in aligned CNTs. Composite films of hybrid silica–CNT particles in poly(vinyl alcohol) were cast and their impedance measured. An electrical percolation threshold of 0.62 wt.% was found for the hybrid particles, of which 0.20 wt.% were CNTs.     

The production of large scale ultrathin aligned CNT films by combining AC electric field with liquid flow

We report a simple and versatile technique combining the use of an AC electric field with a liquid shear force to prepare ultrathin aligned CNT films on solid substrates. Multiwalled carbon nanotubes (MWCNTs), which were synthesized by a template method and acid-treated single walled carbon nanotubes (SWCNTs) were dispersed in water and used for the ultrathin film fabrication. A solid substrate was immersed in the CNT dispersions and withdrawn at constant speed under AC electric field. SEM images of the substrate showed that CNTs were aligned with the AC electric field and the withdrawal direction and formed uniform films with a thickness around 10 nm for SWCNTs and 90 nm for MWCNTs. Repeating the deposition process increases the density and size of the film while also maintaining nanometer-scale thickness. Unidirectional alignment of CNTs was also confirmed by Raman spectra and electric conductivity measurements. It was found that ultrathin films of aligned SWCNTs exhibited very high anisotropic electrical conductivity with conductivity measured parallel to the alignment direction 3.3 × 10⁵ times higher than that measured in the perpendicular direction. We demonstrate that use of the aligned ultrathin SWCNT film for a unidirectional alignment of liquid crystal.     

Aligned sulfur-coated carbon nanotubes with a polyethylene glycol barrier at one end for use as a high efficiency sulfur cathode

High efficient sulfur cathode materials were constructed by the incorporation of aligned sulfur-coated carbon nanotubes (CNTs) and a polyethylene glycol (PEG) barrier at one end. During the charge and discharge of lithium sulfur batteries, high Li ion storage performance can be achieved on the composite electrode, which was benefited from both the aligned CNT structure and the polymer barrier. Aligned CNT framework afforded high conductivity for electron transportation and ordered pores for lithium ion transportation. Meanwhile, the PEG barrier layer greatly suppressed the shuttle of polysulfides. Therefore, this aligned sulfur-coated CNTs with a PEG barrier showed a high initial discharge capacity of 920 and 1128 mAh g⁻¹ in lithium bis(trifluoromethanesulfonyl)imide/1,3-dioxolane/1,2-dimethoxyethane and electrolyte with LiNO₃ additives, respectively. The PEG coated cathode showed high cycle stability that a low degradation with 0.38% per cycle during the 100 cycles at 0.1 C was achieved in LiNO₃-free electrolytes. These Li storage performance was superior to the aligned sulfur-coated CNT electrode without PEG barrier.     

Thermal transport phenomena and limitations in heterogeneous polymer composites containing carbon nanotubes and inorganic nanoparticles

An Off-Lattice Monte Carlo model was developed to investigate effective thermal conductivities (K_eff) and thermal transport limitations of polymer composites containing carbon nanotubes (CNTs) and inorganic nanoparticles. The simulation results agree with experimental data for poly(ether ether ketone) (PEEK) with inclusions of CNTs and tungsten disulfide (WS₂) nanoparticles. The developed model can predict the thermal conductivities of multiphase composite systems more accurately than previous models by taking into account interfacial thermal resistance (R_bd) between the nanofillers and the polymer matrix, and the nanofiller orientation and morphology. The effects of (i) R_bd of CNT–PEEK and WS₂–PEEK (0.0232–115.8 × 10⁻⁸ m²K/W), (ii) CNT concentration (0.1–0.5 wt%), (iii) CNT morphology (aspect ratio of 50–450, and diameter of 2–8 nm), and (iv) CNT orientation (parallel, random and perpendicular to the heat flux) on K_eff of a multi-phase composite are quantified. The simulation results show that K_eff of multiphase composites increases when the CNT concentration increases, and when the R_bd of CNT–PEEK and WS₂–PEEK interfaces decrease. The thermal conductivity of composites with CNTs parallel to the heat flux can be enhanced ∼2.7 times relative to that of composites with randomly-dispersed CNTs. CNTs with larger aspect ratio and smaller diameter can significantly improve the thermal conductivity of a multiphase polymer composite.     

Interplay between exchange interaction and magnetic anisotropy for iron based nanoparticles in aligned carbon nanotube arrays

In this work, we investigate magnetic properties of iron based nanoparticles (NP) intercalated into carbon nanotube (CNT) aligned arrays synthesized by injection chemical vapor deposition. We have analyzed the temperature (T) and the ferrocene concentration (C_F) dependences of the macroscopic magnetic parameters. From these experiments a weaker interaction between magnetic moments of NP was obtained for low C_F values. The random anisotropy model for the experimental data analysis was applied and micromagnetic parameters were evaluated. The law of the approach to magnetic saturation (LAS) was analyzed using the general expression with the correlation function C(r = x/R_a) of magnetic axes, R_a being the magnetic anisotropy correlation length. We obtained that, while for C_F = 0.5% C(r) is a step-like (C(r < 10) = 1, C(r > 10) = 0), for C_F ⩾ 1% C(r) decays rapidly on a short range, (2-3)R_a. Such extended correlations for C_F = 0.5% could be associated with the dominant role of the coherent anisotropy, which is caused by the influence of the alignment of CNT. When the aligned CNTs for C_F = 0.5% are destroyed into powder, the LAS is changed to H^−1/2, which means the dominant role of the exchange mechanism.     

Hierarchical carbon nanotube membrane with high packing density and tunable porous structure for high voltage supercapacitors

We reported the fabrication of a hierarchical carbon nanotube (CNT) membrane by using the 90% granulated double- or triple-walled CNTs and 10% 100 μm long multiwalled CNTs as the linker. The membrane with packing density of 420 kg/m³, excellent electrical conductance and good mechanical strength, functioned as both the electrode and current collector and allowed the weight ratio of CNTs increased up to 45–50% based on the weight of CNT, electrolyte and separator. The granulated double or triple walled CNTs, by the aggregation at high temperature etching using CO₂, simultaneously exhibited high surface area and tunable pore structure and high pore volume, and were favorable for the ion transport of organic electrolyte, due to the effect of opening cap or side wall by the CO₂. The CNT membrane electrode, exhibited the capacitance of 57.9 F/g and the energy density of 35 W h/kg, as operated at 4 V.     

Single stage production of carbon nanotubes using microwave technology

Carbon nanotubes (CNTs) were produced by gas phase single stage tubular microwave chemical vapor deposition (TM–CVD) using ferrocene as a catalyst and acetylene (C₂H₂) and hydrogen (H₂) as precursor gasses. The effect of the process parameters such as microwave power, radiation time, and gas ratio of C₂H₂/H₂ was investigated. The CNTs were characterized using scanning and transmission electron microscopy (TEM), and by thermogravimetric analysis (TGA). Results reveal that the optimized conditions for CNT production were 900 W reaction power, 35 min radiation time, and 0.6 gas ratio of C₂H₂/H₂. TEM analyses revealed that the uniformly dispersed vertical alignment of multiwall carbon nanotubes (MWCNTs) have diameters ranging from 16 to 23 nm. The TGA analysis showed that the purity of CNT produced was 98%.     

Synthesis,morphology and physical properties of multi-walled carbon nanotube/biphenyl liquid crystalline epoxy composites

We have developed multi-walled carbon nanotube/liquid crystalline epoxy composites and studied the effects of incorporation carbon nanotubes (CNTs) on the morphology, thermal and mechanical properties of the composites. The CNTs are functionalized by liquid crystalline (LC) 4,4′-bis(2,3-epoxypropoxy) biphenyl (BP) epoxy resin for the ease of dispersion and the formation of long range ordered structure. The epoxy functionalized CNT (ef-CNT) were dispersed in the LC BP epoxy resin that can be thermal cured with an equivalent of 4,4′-diamino-diphenylsulfone to form composite. The curing process was monitored by polarized optical microscopy. The results indicate the LC resin was aligned along the CNTs to form fiber with dendritic structure initially then further on to obtain micro-sized spherical crystalline along with fibrous crystalline. With homogeneous dispersion and strong interaction between nanotubes and matrix, the composite containing 2.00 wt.% ef-CNT exhibits excellent thermal and mechanical properties. When the amount of ef-CNT exceeds 2.00 wt.%, vitrification stage of curing is fast reached, which lowers the degree of conversion. As compared with the neat resin, the composite containing 2.00 wt.% ef-CNT increases the glass transition temperature by 70.0 °C, the decomposition temperature by 13.8 °C, the storage modulus by 40.9%, and the microhardness by 63.3%.     

Overcoming the quality–quantity tradeoff in dispersion and printing of carbon nanotubes by a repetitive dispersion–extraction process

Dispersion-printing processes are essential for the fabrication of various devices using carbon nanotubes (CNTs). Insufficient dispersion results in CNT aggregates, while excessive dispersion results in the shortening of individual CNTs. To overcome this tradeoff, we propose here a repetitive dispersion–extraction process for CNTs. Long-duration ultrasonication (for 100 min) produced an aqueous dispersion of CNTs with sodium dodecylbenzene sulfonate with a high yield of 64%, but with short CNT lengths (a few μm), and poor conductivity in the printed films (∼450 S cm⁻¹). Short-duration ultrasonication (for 3 min) yielded a CNT dispersion with a very small yield of 2.4%, but with long CNTs (up to 20–40 μm), and improved conductivity in the printed films (2200 S cm⁻¹). The remaining sediment was used for the next cycle after the addition of the surfactant solution. 90% of the CNT aggregates were converted into conductive CNT films within 13 cycles (i.e., within 39 min), demonstrating the improved conductivity and reduced energy/time requirements for ultrasonication. CNT lines with conductivities of 1400–2300 S cm⁻¹ without doping and sub-100 μm width, and uniform CNT films with 80% optical transmittance and 50 Ω/sq sheet resistance with nitric acid doping were obtained on polyethylene terephthalate films.     

Pressureless sintering of carbon nanotube–Al2O3 composites

Alumina ceramics reinforced with 1, 3, or 5 vol.% multi-walled carbon nanotubes (CNTs) were densified by pressureless sintering. Commercial CNTs were purified by acid treatment and then dispersed in water at pH 12. The dispersed CNTs were mixed with Al₂O₃ powder, which was also dispersed in water at pH 12. The mixture was freeze dried to prevent segregation by differential sedimentation during solvent evaporation. Cylindrical pellets were formed by uniaxial pressing and then densified by heating in flowing argon. The resulting pellets had relative densities as high as ～99% after sintering at 1500 °C for 2 h. Higher temperatures or longer times resulted in lower densities and weight loss due to degradation of the CNTs by reaction with the Al₂O₃. A CNT/Al₂O₃ composite containing 1 vol.% CNT had a higher flexure strength (～540 MPa) than pure Al₂O₃ densified under similar conditions (～400 MPa). Improved fracture toughness of CNT–Al₂O₃ composites was attributed to CNT pullout. This study has shown, for the first time, that CNT/Al₂O₃ composites can be densified by pressureless sintering without damage to the CNTs.     

Preparation-microstructure-property relationships in double-walled carbon nanotubes/alumina composites

Double-walled carbon nanotube/alumina composite powders with low carbon contents (2–3 wt.%) are prepared using three different methods and densified by spark plasma sintering. The mechanical properties and electrical conductivity are investigated and correlated with the microstructure of the dense materials. Samples prepared by in situ synthesis of carbon nanotubes (CNTs) in impregnated submicronic alumina are highly homogeneous and present the higher electrical conductivity (2.2–3.5 S cm⁻¹) but carbon films at grain boundaries induce a poor cohesion of the materials. Composites prepared by mixing using moderate sonication of as-prepared double-walled CNTs and lyophilisation, with little damage to the CNTs, have a fracture strength higher (+30%) and a fracture toughness similar (5.6 vs 5.4   MPa m^1/2) to alumina with a similar submicronic grain size. This is correlated with crack-bridging by CNTs on a large scale, despite a lack of homogeneity of the CNT distribution.     

Effect of particle size,heating rate and CNT addition on densification,microstructure and mechanical properties of B4C ceramics

Monolithic B₄C ceramics and B₄C–CNT composites were prepared by spark plasma sintering (SPS). The influence of particle size, heating rate, and CNT addition on sintering behavior, microstructure and mechanical properties were studied. Two different B₄C powders were used to examine the effect of particle size. The effect of heating rate on monolithic B₄C was investigated by applying three different heating rates (75, 150 and 225 °C/min). Moreover, in order to evaluate the effect of CNT addition, B₄C–CNT (0.5–3 mass%) composites were also produced. Fully dense monolithic B₄C ceramics were obtained by using heating rate of 75 °C/min. Vickers hardness value increased with increasing CNT content, and B₄C–CNT composite with 3 mass% CNTs had the highest hardness value of 32.8 GPa. Addition of CNTs and increase in heating rate had a positive effect on the fracture toughness and the highest fracture toughness value, 5.9 MPa m^1/2, was achieved in composite with 3 mass% CNTs.     

Enhanced field emission properties from titanium-coated carbon nanotubes

The effect of titanium (Ti) coating over the surface of carbon nanotubes (CNTs) on field emission characteristics was investigated. Vertically aligned CNTs were grown by inductively-coupled plasma-enhanced chemical vapor deposition (ICP-CVD). In order to reduce the screening effect of electric field due to densely packed CNTs, as-grown CNTs were partly etched back by DC plasma of N₂. Ti with various thicknesses from 5 nm to 150 nm was coated on CNTs by a sputtering method. Since thick Ti coating with thickness of 100 nm or more resulted in the shape of a metal post by merging an individual CNT in a bundle, it was inadequate to a field emission application. On the other hand, thin Ti-coated CNTs with thickness of 10 nm or less showed a lower turn-on field, a higher emission current density, and improved emission uniformity compared with pristine CNTs. The improved emission performance was mainly attributed to the low work function of Ti and the reliable and lower resistance contact between CNTs and substrates.     

Growth of carbon nanotube forests between a bi-metallic catalyst layer and a SiO2 substrate to form a self-assembled carbon–metal heterostructure

A special nanostructure was formed by the growth of carbon nanotubes (CNTs) between a substrate and a thin bi-metallic catalyst layer using a thermal chemical vapor deposition process. The catalyst layer is composed of adjacently disposed Cr and Ni phases formed prior to CNT growth. The Cr/Ni layer serves as a bi-metallic catalyst layer, which is pushed away from the substrate as a thin and continuous nanomembrane with the growth of CNTs. The self-assembled CNT–catalyst heterostructure possesses a smooth surface (RMS = 2.9 nm) with a metallic shine. Directly interlinked to the Cr/Ni layer, dense and vertically aligned multi-walled CNTs are found. Compared to conventional CNT films, the structure has significant advantages for CNT integration. From technology point of view, the structure allows further processing without impact on the CNTs as well as transfer of pristine vertically aligned CNTs to arbitrary substrates. Moreover, the as-grown CNT films provide an interface ideal for further electrical, thermal and mechanical contacting of CNT films. We present structural investigations of this special CNT–metal heterostructure. Furthermore, we discuss possible interface mechanisms during catalyst layer formation and CNT growth.     

Improved mechanical performance of solution-processed MWCNT/Ag nanoparticle composite films with oxygen-pressure-controlled annealing

A method for the synthesis of solution process-based MWCNT/Ag nanoparticle composite thin films as electrode or interconnect materials for flexible electronic devices is presented. The method produces homogeneously-dispersed CNT networks and increases the density of the Ag matrix, which are major factors in determining the mechanical performance of CNT/Ag films. By introducing nanometer-sized Ag particles as a matrix material, the agglomeration of CNTs is suppressed. In addition, the generation of pores during the synthesis procedure is effectively restrained by oxygen-pressure-controlled annealing. The elastic modulus of the pristine Ag films was observed to increase by 34% by adding 5 wt% CNTs. An improvement in the fatigue resistance of the CNTs under cyclic tensile deformation was confirmed. The normalized resistance change ((R ? R_o)/R_o) of the Ag films containing 5 wt% CNTs after fatigue testing was reduced by about 27% compared to that of the pristine Ag films. For industrial application the process has the advantage of relatively low-temperature processing without any high pressure compaction compared to the conventional powder metallurgy techniques normally used.     

Growth and characterization of vertically aligned centimeter long CNT arrays

Record-long (21.7 mm) vertically aligned MWNT arrays were synthesized using water-assisted thermal CVD process. The catalyst lifetime was maintained for 790 min at optimized experimental condition. The growth of the centimeter long CNT was observed by real time photography at different growth conditions. The growth length increased linearly with increasing growth time followed by a sudden growth end. The ratio of ethylene and H₂ concentration as well as the water and ethylene concentration was studied and optimized which led to prolong catalyst lifetime. Transmission electron microscope images confirmed that most CNTs were double wall and the number of wall distribution was uniform along different height position. Raman spectra showed that the I_D/I_G ratio remained constant at the 3 studied positions along the CNT. TGA demonstrated negligible impurity incorporation in the CNT array.     

Physical properties of carbon nanotube sheets drawn from nanotube arrays

We report mechanical, thermal, and electrical properties of novel sheet materials composed of multiwalled carbon nanotubes, drawn from a CNT array. At low loading there is some slippage of CNTs but at higher loading tensile strength σ₀ = 7.9 MPa and Young’s modulus E = 310 MPa. The room-temperature thermal conductivity of the CNT sheet was 2.5 ± 0.5 W m^?1 K^?1, giving a thermal conductivity to density ratio of κ/ρ = 65 W m^?1 K^?1 g^?1 cm³. The heat capacity shows 1D behavior for T > 40 K, and 2D or 3D behavior at lower temperatures. The room-temperature specific heat was 0.83 J g^?1 K^?1. The iV curves above 10 K have Ohmic behavior while the iV curve at T = 2 K is non-Ohmic, and a model to explain both ranges is presented. Negative magnetoresistance was found, increasing in magnitude with decreasing temperature (?15% at T = 2 K and B = 9 T). The tensile strength, Young’s modulus and electrical conductivity of the CNT sheet are low, in comparison with other CNT materials, likely due to defects. Thermal conductivity is dominantly phononic but interfacial resistance between MWCNTs prevents the thermal conductivity from being higher.     

Preparation and electrochemical performance of SnO2@carbon nanotube core–shell structure composites as anode material for lithium-ion batteries

Carbon nanotube-encapsulated SnO₂ (SnO₂@CNT) core–shell composite anode materials are prepared by chemical activation of carbon nanotubes (CNTs) and wet chemical filling. The results of X-ray diffraction and transmission electron microscopy measurements indicate that SnO₂ is filled into the interior hollow core of CNTs and exists as small nanoparticles with diameter of about 6 nm. The SnO₂@CNT composites exhibit enhanced electrochemical performance at various current densities when used as the anode material for lithium-ion batteries. At 0.2 mA cm^?2 (0.1C), the sample containing wt. 65% of SnO₂ displays a reversible specific capacity of 829.5 mAh g^?1 and maintains 627.8 mAh g^?1 after 50 cycles. When the current density is 1.0, 2.0, and 4.0 mA cm^?2 (about 0.5, 1.0, and 2.0C), the composite electrode still exhibits capacity retention of 563, 507 and 380 mAh g^?1, respectively. The capacity retention of our SnO₂@CNT composites is much higher than previously reported values for a SnO₂/CNT composite with the same filling yield. The excellent lithium storage and rate capacity performance of SnO₂@CNT core–shell composites make it a promising anode material for lithium-ion batteries.