Single-walled Carbon Nanotube Buckypaper Research Articles

Multifunctional Electroactive Nanocomposites Based on Piezoelectric Boron Nitride Nanotubes.

Space exploration missions require sensors and devices capable of stable operation in harsh environments such as those that include high thermal fluctuation, atomic oxygen, and high-energy ionizing radiation. However, conventional or state-of-the-art electroactive materials like lead zirconate titanate, poly(vinylidene fluoride), and carbon nanotube (CNT)-doped polyimides have limitations on use in those extreme applications. Theoretical studies have shown that boron nitride nanotubes (BNNTs) have strength-to-weight ratios comparable to those of CNTs, excellent high-temperature stability (to 800 °C in air), large electroactive characteristics, and excellent neutron radiation shielding capability. In this study, we demonstrated the experimental electroactive characteristics of BNNTs in novel multifunctional electroactive nanocomposites. Upon application of an external electric field, the 2 wt % BNNT/polyimide composite was found to exhibit electroactive strain composed of a superposition of linear piezoelectric and nonlinear electrostrictive components. When the BNNTs were aligned by stretching the 2 wt % BNNT/polyimide composite, electroactive characteristics increased by about 460% compared to the nonstretched sample. An all-nanotube actuator consisting of a BNNT buckypaper layer between two single-walled carbon nanotube buckypaper electrode layers was found to have much larger electroactive properties. The additional neutron radiation shielding properties and ultraviolet/visible/near-infrared optical properties of the BNNT composites make them excellent candidates for use in the extreme environments of space missions.

3D chemically cross-linked single-walled carbon nanotube buckypapers

Single-walled carbon nanotubes (SWCNTs) covalently modified with OH functional groups were assembled into buckypapers through solvent dispersion and vacuum filtration. These SWCNT-OH buckypaper sheets were subsequently crosslinked by wetting with bifunctional linkers followed by hot compression causing reaction between the functional groups of the reagent and OH functional groups on the side-walls of SWCNTs to create three-dimensional (3D) covalently cross-linked buckypapers. Cross-linking also was performed using SWCNTs encapsulated with a functionalized polymer wrapping in a core–shell structure, where OH or/and NH2 groups are available on the surface of the polymeric shell for reaction. The 3D cross-linked SWCNT buckypapers retain the porous character typical of buckypaper, and were characterized for their tensile properties and thermal and electrical conductivities. Several cross-linking approaches dramatically improved the mechanical properties. The strongest and stiffest papers (32 MPa, E = 3.1 GPa), which approach 10× stronger and stiffer than the pristine non-crosslinked buckypaper, were obtained at the expense of a loss of electrical conductivity. In other cases, such as cross-linking using a high-performance epoxy resin monomer, improvements in strength and stiffness of ∼5× were obtained while retaining electrical and thermal conductivity. Therefore, the optimal cross-linking approach would be determined by the desired, multifunctional properties. Additionally, the approach can be used in the preparation buckypaper composites and it is demonstrated that cross-linking using a multifunctional epoxy resin prior to impregnation with the same epoxy resin results in substantially better mechanical properties in comparison to just epoxy-impregnation of pristine buckypaper.

Electrochemical behaviour of different redox probes on single wall carbon nanotube buckypaper-modified electrodes

In the present work, the electrochemical properties of single-walled carbon nanotube buckypapers (BPs) were examined in terms of carbon nanotubes nature and preparation conditions. The performance of the different free-standing single wall carbon nanotube sheets was evaluated via cyclic voltammetry of several redox probes in aqueous electrolyte. Significant differences are observed in the electron transfer kinetics of the buckypaper-modified electrodes for both the outer- and inner-sphere redox systems. These differences can be ascribed to the nature of the carbon nanotubes (nanotube diameter, chirality and aspect ratio), surface oxidation degree and type of functionalities. In the case of dopamine, ferrocene/ferrocenium, and quinone/hydroquinone redox systems the voltammetric response should be thought as a complex contribution of different tips and sidewall domains which act as mediators for the electron transfer between the adsorbate species and the molecules in solution. In the other redox systems only nanotube ends are active sites for the electron transfer. It is also interesting to point out that a higher electroactive surface area not always lead to an improvement in the electron transfer rate of various redox systems.In addition, the current densities produced by the redox reactions studied here are high enough to ensure a proper electrochemical signal, which enables the use of BPs in sensing devices.

Influence of lengths of millimeter-scale single-walled carbon nanotube on electrical and mechanical properties of buckypaper

The electrical conductivity and mechanical strength of carbon nanotube (CNT) buckypaper comprised of millimeter-scale long single-walled CNT (SWCNT) was markedly improved by the use of longer SWCNTs. A series of buckypapers, fabricated from SWCNT forests of varying heights (350, 700, 1,500 μm), showed that both the electrical conductivity (19 to 45 S/cm) and tensile strength (27 to 52 MPa) doubled. These improvements were due to improved transfer of electron and load through a reduced number of junctions for longer SWCNTs. Interestingly, no effects of forest height on the thermal diffusivity of SWCNT buckypapers were observed. Further, these findings provide evidence that the actual SWCNT length in forests is similar to the height.

Single-walled carbon nanotube buckypapers as electrocatalyst supports for methanol oxidation

This work studies the use of various single-walled carbon nanotube (SWCNT) buckypapers as catalyst supports for methanol electro-oxidation in acid media. Buckypapers were obtained by vacuum filtration from pristine and oxidized SWCNT suspensions in different liquid media. Pt–Ru catalysts supported on the buckypapers were prepared by multiple potentiostatic pulses using a diluted solution of Pt and Ru salts (2 mM H2PtCl6 + 2 mM RuCl3) in acid media. The resulting materials were characterized via SEM, TEM, EDX and ICP-OES analysis. Well dispersed rounded nanoparticles between 2 and 15 nm were successfully electrodeposited on the SWCNT buckypapers. The ruthenium content in the bimetallic deposits was between 32 and 48 at. %, while the specific surface areas of the catalysts were in the range of 72–113 m2 g−1. It was found that the solvent used to prepare the SWCNT buckypaper films has a strong influence on the catalyst dispersion, particle size and metal loading. Cyclic voltammetry and chronoamperometry experiments point out that the most active electrodes for methanol electro-oxidation were prepared with the buckypaper supports that were obtained from SWCNT dispersions in N-methyl-pyrrolidone.

Ionic liquid coated single-walled carbon nanotube buckypaper as supercapacitor electrode

Effect of stacking structure of single-walled carbon nanotubes (SWCNTs) on its performance as electrode of supercapacitor was investigated in the present work. Considering SWCNTs easily formed bundles due to strong van de Waals attraction between tubes, we proposed first dispersion of SWCNTs by ionic liquids (ILs) of 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMIMBF4), followed by fabrication of buckypaper by compression. The debundling effect of ILs on SWCNTs increased the interface between electrode and electrolyte, decreased electrical resistance, and, consequently, increased performance of the supercapacitor. Since ILs, used to disperse SWCNTs, also functioned as electrolyte in supercapacitor, our method is a simple way to prepare buckypaper electrode with high performance.

Bacterial Filtration Using Carbon Nanotube/Antibiotic Buckypaper Membranes

The preparation of free‐standing carbon nanotube “buckypaper” (BP) membranes consisting of either single‐walled carbon nanotubes (SWNTs) or multi‐walled carbon nanotubes (MWNTs), and the antibiotic ciprofloxacin (cipro), is reported. The electrical, mechanical and morphological properties of these membranes have been characterised and are compared to those of the corresponding buckypaper membranes containing the surfactant Triton X‐100 (Trix). Analysis of scanning electron microscopic images of the surfaces of SWNT/cipro and SWNT/Trix (Trix = Triton X‐100) buckypapers revealed that the diameter of their surface pores was significantly smaller than that of the corresponding materials prepared using MWNTs. Similarly, the average internal pore diameter of both SWNT buckypapers was found to be smaller than that of their MWNT counterparts, after analysis of binding isotherms derived from nitrogen adsorption/desorption measurements performed on the materials. All four buckypaper membranes examined were found to be >99% effective for removing Escherichia coli (E. coli) from aqueous suspensions. However, buckypapers containing ciprofloxacin outperformed their counterparts containing the surfactant. Both MWNT buckypapers were more effective at preventing passage of E. coli than their analogues containing SWNTs, while fluorescence microscopic examination of stained membrane surfaces demonstrated that buckypapers composed of SWNTs had greater bactericidal properties.

Synthesis, properties and water permeability of SWNT buckypapers

The ability of macrocyclic ligands to facilitate formation of dispersions of single-walled carbon nanotubes (SWNTs) was investigated using a combination of absorption spectrophotometry and optical microscopy. Vacuum filtration of aqueous dispersions containing SWNTs and various macrocyclic ligands (derivatised porphyrin, phthalocyanine, cyclodextrin and calixarene) afforded self-supporting membranes known as buckypapers. Microanalytical data and energy dispersive X-ray spectra were obtained for these buckypapers and provided evidence for retention of the macrocyclic ligands within the structure of the membranes. The electrical conductivities of the membranes varied between 30 ± 20 and 220 ± 60 S cm−1, while contact angle analysis revealed they all possessed hydrophilic surfaces. The mechanical properties of buckypapers prepared using macrocyclic ligands as dispersants were shown to be comparable to that of a benchmark material prepared using the surfactant Triton X-100 (Trix). Incorporation of the macrocyclic ligands into SWNT buckypapers was found to increase their permeability up to ten-fold compared to buckypapers prepared using Trix. No correlation was observed between the water permeability of the membranes and the average size of either their surface or internal pores. However, the water permeability of the membranes was found to be inversely dependent on their surface area.

An interaction stress analysis of nanoscale elastic asperity contacts

A new contact mechanics model is presented and experimentally examined at the nanoscale. The current work addresses the well-established field of contact mechanics, but at the nanoscale where interaction stresses seem to be effective. The new model combines the classic Hertz theory with the new interaction stress concept to provide the stress field in contact bodies with adhesion. Hence, it benefits from the simplicity of non-adhesive models, while offering the same applicability as more complicated models. In order to examine the model, a set of atomic force microscopy experiments were performed on substrates made from single-walled carbon nanotube buckypaper. The stress field in the substrate was obtained by superposition of the Hertzian stress field and the interaction stress field, and then compared to other contact models. Finally, the effect of indentation depth on the stress field was studied for the interaction model as well as for the Hertz, Derjaguin-Muller-Toporov, and Johnson-Kendall-Roberts models. Thus, the amount of error introduced by using the Hertz theory to model contacts with adhesion was found for different indentation depths. It was observed that in the absence of interaction stress data, the Hertz theory predictions led to smaller errors compared to other contact-with-adhesion models.

A theoretical evaluation of the effects of carbon nanotube entanglement and bundling on the structural and mechanical properties of buckypaper

Structural formation mechanisms of carbon nanotube (CNT) buckypaper and their effects on its mechanical properties are studied with numerical simulations. A bond swap algorithm, resulting from coupling the molecular dynamics and Monte Carlo methods, has been developed to equilibrate initial structures of buckypaper, generated by a random walk approach. Entanglement and bundling mechanisms are found to affect major structural features of buckypaper. Both mechanisms are evaluated quantitatively by calculating the entanglement network and pore size of buckypaper. Compared with (8,8)-(12,12) double-walled CNT, the structure of (5,5) single-walled CNT buckypaper is mainly dominated by entanglement, due to its smaller adhesion energy. We show that the pore size of modeled buckypaper, containing both types of CNTs, can be tuned from 7nm to 50nm by increasing the double-walled CNT content from 0wt% to 100wt%, due to the transformation from entanglement-dominated to bundling-dominated structures. Such an observation agrees exceptionally well with experimental results. Both entanglement and bundling mechanisms are also found to play important roles in the mechanical properties of buckypaper. The findings open a way to tailor both structural and mechanical properties of buckypaper, such as Young’s modulus or Poisson’s ratio, by using different CNTs and their mixtures.

Emitter spacing effects on field emission properties of laser-treated single-walled carbonnanotube buckypapers

Carbon nanotube (CNT) emitters on buckypaper were activated by laser treatment andtheir field emission properties were investigated. The pristine buckypapers and CNTemitters’ height, diameter, and spacing were characterized through optical analysis. Theemitter spacing directly impacted the emission results when the laser power and treatmenttimes were fixed. The increasing emitter density increased the enhanced fieldemission current and luminance. However, a continuous and excessive increase ofemitter density with spacing reduction generated the screening effect. As a result,the extended screening effect from the smaller spacing eventually crippled thefield emission effectiveness. Luminance intensity and uniformity of field emissionsuggest that the highly effective buckypaper will have a density of 2500 emissionspots cm − 2, which presents an effective field enhancement factor of 3721 and a moderated screeningeffect of 0.005. Proper laser treatment is an effective post-treatment process for optimizingfield emission, luminance, and durability performance for buckypaper cold cathodes.

Single-walled carbon nanotube buckypaper and mesophase pitch carbon/carbon composites

Carbon/carbon composites consisting of single-walled carbon nanotube (SWCNT) buckypaper (BP) and mesophase pitch resin have been produced through impregnation of BP with pitch using toluene as a solvent. Drying, stabilization and carbonization processes were performed sequentially, and repeated to increase the pitch content. Voids in the carbon/carbon composite samples decreased with increasing impregnation process cycles. Electrical conductivity and density of the composites increased with carbonization by two to three times that of pristine BP. These results indicate that discontinuity and intertube contact barriers of SWCNTs in the BP are partially overcome by the carbonization process of pitch. The temperature dependence of the Raman shift shows that mechanical strain is increased since carbonized pitch matrix surrounds the nanotubes.

Correlation between Young’s modulus and impregnation quality of epoxy-impregnated SWCNT buckypaper

Carbon nanotube (CNT) sheets, also known as buckypaper, have high potential for structural applications due to their high volume fraction of CNT, the strongest and stiffest materials known. In this work, two different techniques, one based on positive pressure and another based on vacuum infiltration, are utilized to impregnate single-walled carbon nanotube (SWCNT) buckypaper sheets of 50–70 μm in thickness, resulting in a Young’s modulus of up to 15.4 GPa. Scanning electron microscopy demonstrates that the vacuum-based technique results in more effective impregnation of the buckypaper than the positive pressure technique. Thermogravimetry analysis of vacuum-impregnated specimens indicated a void content ranging from 5% to 32%. An advanced Mori–Tanaka-based micromechanics technique is also utilized to predict the effect of SWCNT volume fraction and void content on Young’s modulus of nanocomposites. These calculations suggest a higher void content of around 40% for the vacuum-impregnated composites.

Study of fire retardant behavior of carbon nanotube membranes and carbon nanofiber paper in carbon fiber reinforced epoxy composites

Single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) membranes (buckypaper) and carbon nanofiber (CNF) paper were incorporated onto the surface of epoxy carbon fiber composites, as proposed fire shields. Their flammability behaviors were investigated by a cone calorimeter. SWCNT buckypaper and CNF paper did not show notable improvement on fire retardancy. However, MWCNT buckypaper acted as an effective flame-retardant shield, reducing the peak heat release rate by more than 60% and reducing smoke generation by 50% during combustion. The pore structures of buckypapers and CNF paper were characterized by scanning electron microscopy (SEM), mercury intrusion porosimetry, and N 2 adsorption isotherms. Gas permeability of buckypaper and carbon nanofiber paper was measured. The correlation between buckypaper and CNF paper properties and their fire retardancy was discussed.

Surfactant-free assembling of functionalized single-walled carbon nanotube buckypapers

The electrical and textural properties of single-walled carbon nanotube buckypapers were tunned through chemical functionalization processes. Single-walled carbon nanotubes (SWCNTs) were covalently functionalized with three different chemical groups: Carboxylic acids (–COOH), benzylamine (–Ph–CH2–NH2), and perfluorooctylaniline (–Ph–(CF2)7–CF3). Functionalized SWCNTs were dispersed in water or dimethylformamide (DMF) by sonication treatments without the addition of surfactants or polymers. Carbon nanotube sheets (buckypapers) were prepared by vacuum filtration of the functionalized SWCNT dispersions. The electrical conductivity, textural properties, and processability of the functionalized buckypapers were studied in terms of SWCNT purity, functionalization, and assembling conditions. Carboxylated buckypapers demonstrated very low specific surface areas (<1m2/g) and roughness factor (Ra=14nm), while aminated and fluorinated buckypapers exhibited roughness factors of around 70nm and specific surface areas of 160–180m2/g. Electrical conductivity for carboxylated buckypapers was higher than for as-grown SWCNTs, but for aminated and fluorinated SWCNTs it was lower than for as-grown SWCNTs. This could be interpreted as a chemical inhibition of metallic SWCNTs due to the specificity of the diazonium salts reaction used to prepare the aminated and fluorinated SWCNTs. The utilization of high purity as-grown SWCNTs positively influenced the mechanical characteristics and the electrical conductivity of functionalized buckypapers.

Effects of surfactants and alignment on the physical properties of single-walled carbon nanotube buckypaper

Single-walled carbon nanotubes were dispersed in an aqueous medium using surfactants and filtered to make entangled networks, called buckypaper (BP), and the Raman spectra of BP samples revealed the degree of entanglement and residual surfactant content. The temperature dependence of the G-band peak shift in the BP was found to depend on the reduction in residual surfactant and nanotube oxidation. The electrical conductivity was improved after removing the surfactant and increasing the nanotube alignment, although the temperature dependence of electrical resistivity still followed a variable range hopping conduction behavior. The mechanical properties were affected by the degree of entanglement, alignment, and residual surfactant content, and tensile properties were found to improve with the reduction in surfactant and enhancement of alignment.

The effect of preparation conditions and biopolymer dispersants on the properties of SWNT buckypapers

The effect of varying preparation conditions on the properties of buckypapers made using single-walled carbon nanotubes (SWNTs) was systematically investigated. Changing the sonication time, final dispersion volume or membrane filter used to prepare SWNT–Triton X-100 buckypapers all generally had only a small influence on the density, thickness, contact angle and electrical conductivity of the resulting material. More significant changes were noted when the effects of variations in the above preparation conditions on the mechanical properties and surface morphology of SWNT–Triton X-100 buckypapers were investigated. However, the largest changes in properties were found when various biopolymers (bovine serum albumin, lysozyme, gellan gum and chitosan) were used instead of Triton X-100 to prepare dispersions from which the buckypapers were made. It was observed that the electrical conductivity of the buckypapers decreased as the molecular mass of the biopolymer dispersant used increased, whereas the mechanical properties of the materials showed significant improvements.

Structural changes and Raman analysis of single-walled carbon nanotube buckypaper after high current density induced burning

Single-walled carbon nanotube (SWCNT) buckypaper (BP) was exposed to high temperatures with electrical current-driven thermal heating either in the air or a vacuum. High electrical currents generate Joule heating and then cause breakdown of the BP in the air at over 400 °C due to rapid oxidation. In the vacuum, electrical resistive heating can generate temperatures of more than 2000 °C for the samples. Structural changes of SWCNTs after electrical current heating were observed using electron microscopy and Raman spectra. After breakdown of BP, the disorder-induced D-band increased and a smaller diameter related radial-breathing mode was reduced in the high temperature region. Structural transformations of SWCNT to other carbon nanostructures were observed after current-driven high-temperature treatment in the vacuum. In addition, surface-enhanced Raman scattering with intensity enhancement more than ten times was observed in the BP with agglomerated Fe or Ti particles.

A carbon nanotube strain sensor for structural health monitoring

A carbon nanotube polymer material was used to form a piezoresistive strain sensor forstructural health monitoring applications. The polymer improves the interfacial bondingbetween the nanotubes. Previous single walled carbon nanotube buckypaper sensorsproduced distorted strain measurements because the van der Waals attraction force allowedaxial slipping of the smooth surfaces of the nanotubes. The polymer sensor uses largermulti-walled carbon nanotubes which improve the strain transfer, repeatability andlinearity of the sensor. An electrical model of the nanotube strain sensor was derived basedon electrochemical impedance spectroscopy and strain testing. The model is useful fordesigning nanotube sensor systems. A biomimetic artificial neuron was developedby extending the length of the sensor. The neuron is a long continuous strainsensor that has a low cost, is simple to install and is lightweight. The neuron has alow bandwidth and adequate strain sensitivity. The neuron sensor is particularlyuseful for detecting large strains and cracking, and can reduce the number ofchannels of data acquisition needed for the health monitoring of large structures.

Effect of Chemical Treatment on Electrical Conductivity, Infrared Absorption, and Raman Spectra of Single-Walled Carbon Nanotubes

We investigate the magnitude and temperature dependence of electrical conductivity, the optical and infrared absorption, and the Raman spectra of single-walled carbon nanotube (SWNT) bucky-paper after chemical treatment and determine the correlations between the changes in these properties. Ionic-acceptor doping of the SWNT bucky-paper (with SOCl(2), iodine, H(2)SO(3), etc.) causes an increase of electrical conductivity that correlates with an increase of the absorbance in the far-IR region and an increase in the frequency of Raman spectral lines. Conversely, treatment with other molecules (e.g., aniline, PyPhF(5), PhCH(2)Br, etc.) leads to a decrease in both conductivity and far-IR absorption. The temperature dependence of the conductivity gives a good indication of the presence of metallic charge carriers and is in agreement with the model of interrupted metallic conduction.