Stability Of Multi-walled Carbon Nanotubes Research Articles

Effect of primary particle size on colloidal stability of multiwall carbon nanotubes

Multiwall carbon nanotubes (MWCNTs) are one type of nanoparticles that have various special properties and potential applications. Due to their increasing production and potential toxicological effects, the environmental behavior and transport of MWCNTs have become important research topics. Particle size is one of the important properties of nanoparticles, yet its effects on MWCNT environmental behaviors have not been fully investigated. In this study, we tested how the length of MWCNTs influenced their settling in the presence of Na(+) and/or natural organic matter (NOM), and postulated the governing mechanisms. The results showed that when adding Na(+) the shorter MWCNTs exhibited preferential aggregation and settling. One possible reason could be that shorter MWCNTs possess larger specific surface area and consequently stronger attraction forces. However, NOM strongly mitigated such aggregation, and helps to disperse MWCNTs regardless of their length and aqueous conditions.

Uniform Dispersion of Multiwalled Carbon Nanotubes in Copper Matrix Nanocomposites Using Metal Injection Molding Technique

This work presents a novel fabrication approach of multiwalled carbon nanotubes (MWNTs) reinforced copper (Cu) matrix nanocomposites. A combination of nanoscale dispersion of functionalized MWNTs in low viscose media of dissolved paraffin wax under sonication treatment followed by metal injection molding (MIM) technique was adopted. MWNTs contents were varied from 0 to 10 vol.%. Information about the degree of purification and functionalization processes, evidences on the existence of the functional groups, effect of sonication time on the treated MWNTs, and microstructural analysis of the fabricated Cu/MWNTs nanocomposites were determined using TEM, EDX, FESEM, and Raman spectroscopy analysis. The results showed that the impurities of the pristine MWNTs such as Fe, Ni catalyst, and the amorphous carbon have been significantly removed after purification process. Meanwhile, FESEM and TEM observations showed high stability of MWNTs at elevated temperatures and uniform dispersion of MWNTs in Cu matrix at different volume fractions and sintering temperatures (950, 1000 & 1050°C). The experimentally measured thermal conductivities of Cu/MWNTs nanocomposites showed remarkable increase (11.25% higher than sintered pure Cu) with addition of 1 vol.% MWNTs, and slight decrease below the value of sintered Cu at 5 and 10 vol.% MWNTs.

Tuning the adsorption capability of multi-walled carbon nanotubes to polar and non-polar organic compounds by surface oxidation

This current study addresses the inter-relationship between the surface oxidation of multi-walled carbon nanotubes (MWCNTs) and the polarity of organic solutes and its effect on the adsorption reaction between them. Results indicated that pristine MWCNTs exhibited a stronger affinity toward non-polar compounds such as phenanthrene compared to that of ozone-oxidized MWCNTs (O-MWCNTs). On the contrary, the adsorption of polar compounds, such as phenol, on MWCNTs occurred to a much less extent compared to that on O-MWCNTs as indicated by a smaller Freundlich affinity coefficient. The adsorption capability of various organic compounds on MWCNTs could be tuned. It was found that the stability of MWCNTs did not control the adsorption capacity. The solution chemistry only had minimal influence on the adsorption of phenanthrene on MWCNTs. Results demonstrated the importance of surface chemistry and the properties of adsorbates on the adsorption process. Overlooking either aspect might lead to a misinterpretation of the adsorption reaction. Understanding the interactive nature of MWCNTs adsorption provides information on the preparation of green and “smart” adsorbents according to the nature of adsorbates.

Dispersion stability in carbon nanotube modified polymers and its effect on the fracture toughness

In this paper, the dispersion stability of multiwall carbon nanotubes (MWNTs) mixed with an epoxy resin is studied. An instrumented optical microscope with a hot stage was used to study the evolution of the carbon nanotubes (CNTs) dispersion during the cure of the resin. A new image processing approach is then introduced to quantify dispersion and identify the source of dispersion degradation during the cure. The results showed that the reduction of the resin viscosity at temperatures greater than 100 °C caused an irreversible re-agglomeration of the CNTs in the matrix. It was shown that the fine-tuning of the ratio and type of curing agent as well as the curing temperature directly affect the dispersion stability of MWNTs in the epoxy polymer. The dispersion quality was then directly correlated to the fracture toughness of the modified resin and a maximum of 20% improvement was achieved.

Enhanced thermal stability of multi-walled carbon nanotubes after coating with polyaniline salt

Multi-walled carbon nanotubes (MWCNT) were coated by polyaniline (PANI) in situ during the oxidation of aniline with ammonium peroxydisulfate in a mixture of ethanol (50 vol.%)–water. PANI alone and PANI-coated MWCNT were carbonized under nitrogen atmosphere at 650 °C and the structural changes were studied by infrared spectroscopy and Raman scattering using 633 and 785 nm excitation lines. MWCNT coated with carbonized PANI salt exhibit a Raman spectrum close to that of the pristine MWCNT before heating. In contrast, the spectra of MWCNT and MWCNT coated with PANI base correspond after exposure to 650 °C to a more disordered carbon-like material according to their Raman spectrum. This means that the coating with PANI salt has a protective effect on the structure of MWCNT when exposed to elevated temperature. During heating, the overlayer of PANI salt thermally deprotonates to PANI base at first, and subsequently converts to a carbonized PANI coating. The integrity of the coating is not damaged. When the PANI coating was chemically deprotonated to PANI base before heating, the accompanying volume contraction reduced the contact between MWCNT and PANI base coating and introduced defects, such as cracks. The protective effect of PANI coating was then lost. The possible interactions between PANI and MWCNT are discussed on the basis of Raman spectra.

Effect of heat treatment on the structure and stability of multiwalled carbon nanotubes produced by catalytic chemical vapor deposition technique

The multiwalled carbon nanotubes (MWCNTs) produced by catalytic chemical vapor deposition (CCVD) route were heat treated to 2500°C to improve the structure, morphology and purity level. The process has lead to substantial reduction in the catalytic impurity along with an improved thermal stability and degree of graphitization of these tubes that can possibly lead to its better utilization in various applications. The structural changes following heat treatment have been correlated using various characterization techniques such as Raman spectroscopy, X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, thermo gravimetric analysis and electron paramagnetic resonance spectroscopy. The electrical and mechanical properties of the polymer composites prepared with heat treated MWCNT show improved properties over the one prepared by as produced MWCNT.

The effect of dispersants on the tensile properties of carbon nanotube/vinyl ester composites

AbstractThe effects of eight different dispersants on the stability of multiwalled carbon nanotubes (MWCNTs) in vinyl ester (VE) resin systems have been systematically analyzed. Suspensions incorporating different amounts of dispersants relative to the concentration of MWCNTs in the VE have been prepared. The resistance to sedimentation of MWCNTs in the resin was investigated by using a centrifugation technique. One dispersing agent (B60H) was found to impart long‐term stability to suspensions of MWCNTs in VE. VE‐based composites reinforced with MWCNTs were prepared using different amounts of the dispersing agent. Tensile tests were performed to analyze the effect of different concentrations of MWCNTs and the use of the dispersing agent in the composites. Fracture surfaces of VE nanocomposites have been analyzed using scanning electron microscopy (SEM) to elucidate the reinforcing effect of the CNTs. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers

Effect of block-copolymer dispersants on properties of carbon nanotube/epoxy systems

The effects of the addition of eight different block copolymers on the dispersion stability of multi-walled carbon nanotubes (MWCNTs) are reported. Suspensions of CNTs in different components of an epoxy system have been prepared using a tip sonicator and different amounts of block copolymers. The resistance to sedimentation of MWCNTs in various media was systematically investigated by using a centrifugation technique. Block copolymers that result in dispersions of MWCNTs in epoxy and hardener stable for more than 1week have been obtained. Dispersions using a single or a combination of two different dispersing agents have been used for the fabrication of MWCNT nanocomposites. The effect of different preparation routes and use of block copolymers on the tensile properties and surface resistivity of the composites have been evaluated. The results obtained have been related with the dispersion stability of the MWCNTs in the epoxy components.

Stability of multi-walled carbon nanotubes in commonly used acidic media

Multi-walled carbon nanotubes (MWCNT) were treated in five commonly used acidic media with different p K a values. The stability of the MWCNT was evaluated by using various characterization techniques. The results indicated that oxidative damage of MWCNT was not closely related to acid strength, and that the MWCNT was more sensitive to the oxidizing nature of the acid. As a result, the MWCNT treated with the strongest acid, trifluoromethanesulfonic acid (p K a = −15.9), only reached oxygen content of 1.8%, while nitric acid (p K a = −1.5)/sulfuric acid (p K a = −3.0) treated MWCNT showed oxygen content as high as 19% after 24 h treatment. The remaining acidic media did not noticeably increase the oxygen content of the MWCNT up to 24 h treatment.

Dispersion Stability of Fluorinated Multi-Walled Carbon Nanotubes in FC-27 Refrigerant

In order to achieve the dispersion stability of multi-walled carbon nanotubes (MWCNT) in a fluorinated refrigerant (FC-72) used in various cooling purposes, fluorinated MWCNT (MWCNT-F) was prepared by a combined process of oxidation and fluorination. As a fluorine source, (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane was used to react with hydroxyl groups on MWCNT (MWCNT-OH) generated by chemical oxidation. Pristine MWCNT, MWCNT-OH, and MWCNT-F were dispersed in FC-72 and MWCNT-F was also dispersed in polar and nonpolar solvents. The MWCNT-F has excellent colloidal stability in FC-72 because of the chemical affinity between FC-72 and functional groups (-CFn) on the side walls of MWCNT. Through surface modifications, we could obtain the enhanced dispersion stability of MWCNT in a refrigerant. This homogenous MWCNT solution in FC-72 may be used to increase the heat transfer in FC-72 based nanofluids.

Thermal stability of carbon nanotubes probed by anchored tungsten nanoparticles

The thermal stability of multiwalled carbon nanotubes (CNTs) was studied in high vacuum using tungsten nanoparticles as miniaturized thermal probes. The particles were placed on CNTs inside a high-resolution transmission electron microscope equipped with a scanning tunneling microscope unit. The setup allowed manipulating individual nanoparticles and heating individual CNTs by applying current to them. CNTs were found to withstand high temperatures, up to the melting point of 60-nm-diameter W particles (∼3400 K). The dynamics of W particles on a hot CNT, including particle crystallization, quasimelting, melting, sublimation and intradiffusion, were observed in real time and recorded as a video. Graphite layers reel off CNTs when melted or premelted W particles revolve along the tube axis.

Surface oxidation and effect of electric field on dispersion and colloids stability of multiwalled carbon nanotubes

Chemical functionalization is a procedure to oxidize the surface of materials. Multiwalled carbon nanotubes (MWCNTs) were purified and functionalized using oxidizing mixture acids (sulfuric and nitric acids). The effect of purification and functionalization on the decrease of MWCNTs length and solubility in polar solvents was studied. The length of MWCNTs decreased when the purification and functionalization time was increased. The functionalization of MWCNTs was confirmed by Raman spectroscopy measurements. It was found that, by functionalization the solubility and colloidal stability of MWCNTs were increased in polar media like water and methanol. We found also dispersion status of functionalized MWCNTs in water is evidently improved by applying an AC electric field of 25 V at low frequency and proper applying time.

Fabrication of Multi-Walled Carbon Nanotube/Vinyl Ester Nanocomposites: Dispersion and Stabilization

Excellent physical and mechanical properties of carbon nanotubes (CNTs) make them outstanding candidate as fillers to fabricate multi-functional polymer composites. It is assumed that a high level of dispersion in the preparation stage may lead to a more effective nanocomposite. In this research, the dispersion state of multi-walled carbon nanotubes (MWNTs) at various contents in an unsaturated vinyl ester resin is investigated during fabrication by on-line monitoring the viscosity of suspensions as a function of sonication time and energy introduced. The results show that initial viscosities of suspensions increase by adding more MWNTs to the resin. The viscosities gradually increase during the sonication and reach to maximum values, when it is assumed that the dispersion is completed. After this step the viscosity subsequently decreases. The energy density required to achieve a good dispersion of MWNTs in vinyl ester is obtained. The qualities of dispersion in cured composites are characterized by examining the sections using the scanning electron microscope (SEM) to confirm the results of viscosity measurements. The stabilization of MWNTs is achieved by adding a commercial dispersant and stabilizer, BYK-P 104S, by 0.0375 wt% . The results indicate that adding more surfactant to the suspension makes it unstable and leads to flocculation. The stabilization of suspensions is investigated by using viscosity measurement. Also, FT-IR is used to determine the possible mechanism of surfactant to stabilize the MWNTs in vinyl ester.

Catalytic activity and stability of multiwalled carbon nanotubes in catalytic wet air oxidation of oxalic acid: The role of the basic nature induced by the surface chemistry

Different chemical and thermal treatments (nitric acid at boiling temperature, liquid-phase urea treatment at 200°C, and gas-phase thermal treatment with nitrogen at 600°C) were applied to multiwalled carbon nanotubes (MWCNTs) in order to produce materials with different textural and chemical properties. Nitrogen adsorption isotherms, temperature programmed desorption, pHpzc (point of zero charge), elemental analysis and X-ray photoelectron spectroscopy were used to characterize these materials. The chemical and thermal treatments have influence on the materials pHpzc, which decreases upon nitric acid treatment and increases with urea or gas-phase thermal treatments. The original and modified MWCNTs, without any impregnated metal, were investigated as catalysts in the catalytic wet air oxidation (CWAO) process, using oxalic acid as model compound at 140°C and 40bar of total pressure. Oxalic acid is very stable under non-catalytic conditions but can be totally degraded in less than 30min in the presence of MWCNTs. The rate of oxidation of oxalic acid depends on the chemical properties of MWCNTs, the apparent initial first-order rate constants being lower for the MWCNTs with a marked acid character. The textural properties of MWCNTs are stable in cyclic CWAO experiments, but a decrease of their basic character leads to the reduction of their catalytic activity, even if this activity is still high with reused catalysts (nearly total oxalic acid degradation in 45min and complete mineralization in 120min). Therefore, MWCNTs without any impregnated metal, especially those of more basic character, are very active catalysts for CWAO, their activity strongly depending on the stability of their surface chemistry.

Effect of Surfactants on the Dispersion of Multi-Walled Carbon Nanotubes in Epoxy Resin

In order to improve the dispersion homogeneity and stabilization of multi-walled carbon nanotubes (MWNT) in epoxy resin, the effect of different surfactants such as hexadecyl trimethyl ammonium bromide (HTAB), sodium dodecyl sulfate (SDS), oleic acid, Triton X-100 and BYK-9077 on the dispersion of MWNT were investigated. Suspensions of MWNTs/epoxy resin with various surfactants were prepared by ultrasonic agitation. Dispersion homogeneity of MWNT in epoxy resin was evaluated by transmission electron microscopy (TEM), and dispersion stabilization was evaluated by stationary observation. The results show that dispersion properties were not improved by HTAB, while they were improved incoordinately by SDS, oleic acid, Triton X-100 and BYK-9077. Suspensions of MWNTs/epoxy resin with HTAB, SDS, oleic acid and Triton X-100 were all delaminated quickly, it means, dispersion stabilization were not improved by these surfactants. BYK-9077 was the only surfactant which enhanced the dispersion homogeneity and stabilization of MWNTs/epoxy resin significantly. Based on this research, various weight ratios of surfactant BYK-9077 to MWNT were studied. The experimental results show that the dispersion system could reach a balance state when the ratio of surfactant BYK-9077 to MWN was 2.

Stability of multi-wall carbon nanotubes in air

Multi-walled carbon nanotubes (MWCNTs) have been synthesized by chemical vapor deposition using xylenes as carbon precursors and ferrocene as a catalyst. The stability of the purified MWCNTs was investigated, their properties were characterized by SEM and volume electric resistivity. Results showed that MWCNTs exposed to air under ambient temperature will be slowly oxidized and shortened. The extent of oxidation increases with time, and MWCNTs were almost completely transformed into amorphous carbon after 15 d. The volume electric resistivity also varied as a result of oxidation. These results indicate that asprepared MWCNTs should be kept in a wellclosed container to prevent them from oxidization.

Oxidation Stability of Multiwalled Carbon Nanotubes for Catalytic Applications

The oxidation of multiwalled carbon nanotubes (CNTs) was investigated with regard to a detailed prediction of the lifetime of this material as a catalyst for oxidative dehydrogenations. A power-law kinetics is found to be adequate for the description of CO2 formation in the temperature range of 623−823 K and under O2 partial pressures of 0.025−0.6 bar. The stability against oxidation can be enhanced by passivation with B2O3 or P2O5 and by high temperature treatment. The progress of oxidative degradation was monitored by TEM and Raman spectroscopy. A mechanistic study supported by high pressure XPS and SSITKA reveals full agreement with the established model of the oxidation of conventional carbon materials; however, the theory of sequential layer degradation as observed for single crystal graphite is not transferable to a technical grade CNT material, and instead, various modes of propagation of combustion sites are identified.

Enhancement of thermal stability of multiwalled carbon nanotubes via different silanization routes

This work presents an effect of two different silanization procedures on thermal and structural properties of oxidized and oxidized followed by sodium borohydrate (NaBH 4) reduction of multiwalled carbon nanotubes (MWCNTs). Purified sample was oxidized in a mixture of nitric and sulfuric acids in a reflux. An oxidized material was divided into two batches. The first batch underwent a silanization procedure directly, while the second batch was reduced by NaBH 4 treatment prior to the silanization. The silanization experiments were performed: (A) with γ-aminopropyltriethoxysilane (APTES) at room temperature in acetone (pH ∼7) and (B) with condensated γ-aminopropyltriethoxysilane at 40 °C in water (pH 4). The extent of the functionalization of the samples after each procedure was examined by Raman spectroscopy. The vibrational properties of the materials were studied via Fourier transform infrared spectroscopy. Boehms titration technique was applied to quantify the amount of the functional groups on MWCNTs. The morphology of the pristine and functionalized carbon nanotubes was exposed to high-resolution transmission electron microscopy analysis. The energy dispersive X-ray (EDX) analysis was used to characterize the elemental composition of each sample. The effect of the silanization process on the thermal properties of MWCNTs was investigated by thermogravimetry analysis. Interestingly, the significant increase of the thermal stability of silanized MWCNTs samples in respect to the pristine MWCNTs was observed.

Structural and chemical stability of multiwall carbon nanotubes in sintered ceramic nanocomposite

The structural and chemical stability of multiwall carbon nanotubes (MWNTs) in ceramic nanocomposites prepared by spark plasma sintering was studied. High resolution electron microscopy, X-ray diffraction and Raman spectroscopy were used to evaluate any degradation of the MWNTs. They were found to be well preserved in alumina after sintering up to 1900°C/100 MPa/3 min. In boron carbide, structural degradation of MWNTs started from ∼1600°C when sintered for 20 min. Multiwall carbon nanotubes maintained their high aspect ratio and fibrous nature even after being sintered in boron carbide at 2000°C for 20 min. However, no Raman vibrations of MWNTs were observed for nanocomposites processed at temperatures <2000°C, which indicates that they were severely degraded. Structural preservation of MWNTs in ceramic nanocomposites depends on the ceramic matrix, sintering temperature and dwell time. Multiwall carbon nanotubes were not preserved for matrices that require high sintering temperatures (>1600°C) and longer processing times (>13 min).

Different stabilities of multiwalled carbon nanotubes in fresh surface water samples

The stability of multiwalled carbon nanotubes (MWNTs) in particulate aggregates and surfactant-facilitated suspensions after being mixed into eight types of fresh surface water samples was investigated. MWNTs in particulate aggregates could not be stabilized in any of the water samples except for the one having relatively high content of dissolved organic matter with the aid of sonication. Sodium dodecyl benzenesulfonate (SDBS), polyethylene glycol octylphenyl ether (TX100) and cetyltrimethyl ammonium bromide (CTAB) were used to prepare MWNT suspensions. SDBS- and TX100-stabilized MWNTs could partly remain suspending after being mixed into the water samples, whereas CTAB-stabilized MWNTs readily sedimentated due to the surface charge neutralization by the surface water contained negatively-charged anions and colloids. This is a first systematic study examining the stability of carbon nanotubes in natural surface waters, the results from which will be useful for understanding the transport, fate and ecological effect of carbon nanotubes in the aqueous environment.