Synthesis and characterization of thermotropic liquid crystalline polyester/multi-walled carbon nanotube nanocomposites

Thermotropic liquid crystalline polyester (TLCP) was synthesized via low-temperature solution polycondensation from 1,4-Bis(4-Hydroxybenzoyloxy)butane and terephthaloyl dichloride. Polymer nanocomposites based on a small quantity of multi-walled carbon nanotubes (MWNTs) were prepared by in situ polymerization method. The wide-angle X-ray diffraction (WAXD) results suggested that the addition of MWNTs to TLCP matrix did not significantly change the crystal structure of TLCP. The interactions between the molecules of the TLCP host phase and the carbon nanotubes were investigated through Raman spectroscopy investigations. We detected a distinct wave number shift of the radial breathing modes, confirming the carbon nanotubes interacted with the surrounding liquid crystal molecules, most likely through aromatic interactions (π-stacking). The interactions between liquid crystal host and nanotube guests were also evident from a polarizing microscopy (POM) study of the liquid crystal-isotropic phase transition in the proximity of nanotubes. The thermal properties and the morphological properties of the TLCP/MWNTs nanocomposites were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). TGA data demonstrated the addition of a small amount of MWNTs into TLCP matrix could improve the thermal stability of TLCP matrix. DSC results revealed that melt transition temperatures and isotropic transition temperatures of the hybrids were enhanced.     

Synthesis, characterization and cytotoxicity of surface amino-functionalized water-dispersible multi-walled carbon nanotubes

Surface functionalization of multi-walled carbon nanotubes (MWCNTs), with amino groups via chemical modification of carboxyl groups introduced on the nanotube surface, using O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (N-HATU) and N,N-diisopropylethylamine (DIEA) is reported. The N-HATU coupling agent provides faster reaction rate and the reaction occurs at lower temperature compared to amidation and acylation-amidation chemistry. The amines, 1,6-hexanediamine (HDA), diethylenetriamine (DETA), triethylenetetramine (TETA) and 1,4-phenylenediamine (PDA) were used. The resulting materials were characterized with different techniques such as FTIR, XRD, elemental analysis, TGA, TEM, UV-vis spectroscopy and cyclic voltammetry. MWCNTs functionalized with PDA posses the best dispersibility and electron transfer properties in comparison to the others amines. Functionalized MWCNTs, at the concentrations between 1 and 50 μg ml⁻¹, were not cytotoxic for the fibroblast L929 cell line. However, the concentrations of MWCNTs higher of 10 μg ml⁻¹ reduced cell growth and this effect correlated positively with the degree of their uptake by L929 cells.     

A comparison between the mechanical and thermal properties of single-walled carbon nanotubes and boron nitride nanotubes

Carbon nanotubes (CNTs) are semimetallic while boron nitride nanotubes (BNNTs) are wide band gap insulators. Despite the discrepancy in their electrical properties, a comparison between the mechanical and thermal properties of CNTs and BNNTs has a significant research value for their potential applications. In this work, molecular dynamics simulations are performed to systematically investigate the mechanical and thermal properties of CNTs and BNNTs. The calculated Young’s modulus is about 1.1 TPa for CNTs and 0.72 TPa for BNNTs under axial compressions. The critical bucking strain and maximum stress are inversely proportional to both diameter and length-diameter ratio and CNTs are identified axially stiffer than BNNTs. Thermal conductivities of (10, 0) CNTs and (10, 0) BNNTs follow similar trends with respect to length and temperature and are lower than that of their two-dimensional counterparts, graphene nanoribbons (GNRs) and BN nanoribbons (BNNRs), respectively. As the temperature falls below 200 K (130 K) the thermal conductivity of BNNTs (BNNRs) is larger than that of CNTs (GNRs), while at higher temperature it is lower than the latter. In addition, thermal conductivities of a (10, 0) CNT and a (10, 0) BNNT are further studied and analyzed under various axial compressive strains. Low-frequency phonons which mainly come from flexure modes are believed to make dominant contribution to the thermal conductivity of CNTs and BNNTs.     

Synthesis of multi-walled carbon nanotubes for NH3 gas detection

It has been recently demonstrated that carbon nanotubes (CNTs) represent a new type of chemical sensor capable of detecting a small concentration of molecules such as CO, NO₂, NH₃.In this work, CNTs were synthesized by chemical vapor deposition (CVD) on the SiO₂/Si substrate by decomposition of acetylene (C₂H₂) on sputtered Ni catalyst nanoparticles. Their structural properties are studied by atomic force microscopy, high-resolution scanning electron microscopy (HRSEM) and Raman spectroscopy. The CNTs grown at 700 °C exhibit a low dispersion in size, are about 1 μm long and their average diameter varies in the range 25–60 nm as a function of the deposition time. We have shown that their diameter can be reduced either by annealing in oxygen environment or by growing at lower temperature (less than 600 °C).We developed a test device with interdigital Pt electrodes on an Al₂O₃ substrate in order to evaluate the CNTs-based gas sensor capabilities. We performed room temperature current–voltage measurements for various gas concentrations. The CNT films are found to exhibit a fast response and a high sensitivity to NH₃ gas.     

Preparation and characterization of amphiphilic multi-walled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs, average diameter 8 nm) functionalized by N-vinyl pyrrolidone (NVP) were synthesized by radical polymerization and characterized by Fourier transform infrared and Raman spectroscopies, thermogravimetric analysis and transmission electron microscopy. These NVP–MWCNTs exhibit remarkable solubility in water, ethanol and dimethyl formamide. The polyvinyl pyrrolidone can be attached onto the surface of the MWCNTs and the degree of functionalization is affected by NVP content. The functionalization causes possible grafting reaction and solid physical coating between MWCNTs and PVP.     

Synthesis and characterization of carbon nanotubes on carbon microfibers by floating catalyst method

In this paper, carbon nanotubes were synthesized on carbon microfibers by floating catalyst method with the pretreatment of carbon microfibers at the temperature of 1023 K, using C₂H₂ as carbon source and N₂ as carrier gas. The morphology and microstructure of carbon nanotubes were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The composition of carbon nanotubes was determined by energy dispersive X-ray spectroscopy (EDX). The results showed that the surface of treated carbon microfibers was thickly covered by carbon nanotubes with diameters of about 50 nm. EDX image indicated that the composition of carbon nanotubes was carbon. In comparison with the sample grown on untreated carbon microfibers surface, it was found that after carbon microfibers were boiled in the solution of sulfur acid and nitric acid (VH₂SO₄:VHNO₃ = 1:3) and immersed in the solution of iron nitrate and xylene, carbon nanotubes with uniform density can be grown on carbon microfibers surface. Based on the results, we concluded that the pretreatment of carbon microfibers had great effect on the growth of carbon nanotubes by floating catalyst method.     

A novel carbon nanotubes/Fe3O4 inorganic hybrid material: Synthesis, characterization and microwave electromagnetic properties

Magnetite particles with nanoscale sizes were self-assembled along multiwalled carbon nanotubes through a simple, effective and reproducible solvothermal method. The morphology, composition and phase structure of as-prepared hybrid materials were characterized by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The results showed that denseness, size and crystallinity of magnetite can be altered by controlling the reaction parameters. Magnetization measurement indicated that both coercivity and saturation magnetization increased linearly with increasing magnetite concentration in the hybrid materials. Electromagnetic properties of the carbon nanotubes/magnetite inorganic hybrid materials were measured at 1∼18 GHz. The magnetic loss was caused mainly by ferromagnetic natural resonance, which is in agreement with the Kittel equation. The as-prepared inorganic hybrid materials are believed to have potential applications in the microwave absorbing area.     

Electronic thermal conductivity of armchair graphene nanoribbons and zigzag carbon nanotubes

Through the Green's function formalism and tight-binding Hamiltonian model calculations, the temperature dependent electronic thermal conductivity (TC) for different diameters of zigzag carbon nanotubes and their corresponding unzipped armchair graphene nanoribbons is calculated. All functional temperature dependencies bear crossovers, for which, at higher temperatures, nanotubes have a slightly higher TC than their derived nanoribbons, while below that crossover, both systems exhibit a significant coincidence over a moderate range of lower temperatures. Noticeably, TC decreases with increasing the width or diameter of the corresponding systems. Also, at low temperatures TC is proportional to the density of states around the Fermi level, and thus increasing for metal or semiconductors of narrower gap cases.     

Mechanical properties of nickel-coated single-walled carbon nanotubes and their embedded gold matrix composites

The effects of nickel coating on the mechanical behaviors of armchair single-walled carbon nanotubes (SWCNTs) and their embedded gold matrix composites under axial tension are investigated using molecular dynamics (MD) simulation method. The results show that the Young's moduli and tensile strength of SWCNTs obviously decrease after nickel coating. For armchair SWCNTs, the decreased ratio of the Young's moduli of SWCNTs with smaller radius is larger than that of SWCNTs with larger radius. A comparison is made between the response to Young's modulus of a composite with parallel embedded nanotube and the response of a composite with vertically embedded nanotube. The results show that the uncoated SWCNT can enhance the Young's modulus of composite under the condition of parallel embedment, but such improvement disappears under the condition of vertical embedment because the interaction between SWCNT and gold matrix is too weak for effective load transfer. However, the nickel-coated SWCNT can indeed significantly improve the composite behavior.     

The influence of treatment duration on multi-walled carbon nanotubes functionalized by H2SO4/HNO3 oxidation

Variation in the nature of multi-walled carbon nanotubes (MWCNTs) subjected to different degrees of oxidation was investigated. The microstructure was determined by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) methods, and the surface chemistry was evaluated in terms of the functional groups determined by X-ray photoelectron spectroscopy (XPS) and thermal analysis-mass spectroscopy (TA-MS). In addition, TGA was used to indicate the thermal stability of the nanotubes. Results demonstrate that the graphitic structure of nanotubes oxidized with a mild mixture of H₂SO₄/HNO₃ was preserved. Decrease in the degree of crystallinity started with widening of the C(0 0 2) XRD diffraction peak, followed by this peak shifting towards lower angles. The oxygen content increased with increasing treatment time. A defect peak incorporated in deconvolution of XPS C1s spectra was helpful for detecting the generation of defect sites. The predominant surface functionalities of the nanotubes have been changed from basic to acidic groups after treatment for one day. The samples oxidized for two days had the most abundant surface -COOH and the highest oxidation resistance. The oxidation mechanism of MWCNTs in mild H₂SO₄/HNO₃ mixture was proposed, which was a successive and iterative process, including the initial attack on active sites, and next the hexagon electrophilic attack generating new defects and introducing more oxygen, and then the tubes becoming thinner and shorter.     

Thermal conductivity and thermal rectification in carbon nanotubes with geometric variations of doped nitrogen: Non-equilibrium molecular dynamics simulations

The thermal conductivity of carbon nanotubes with geometric variations of doped nitrogen is investigated. The phenomenon of thermal rectification shows that the heat transport is preferably in one direction. The asymmetric heat transport of the triangular single-nitrogen-doped carbon nanotubes (SNDCNTs) is larger than that of the parallel various-nitrogen-doped carbon nanotubes (VNDCNTs).     

Preparation and characterization of aligned carbon nanotubes/polylactic acid composite fibers

Aligned functionalized multiwalled carbon nanotubes/polylactic acid (MWNTs-PCL/PLA) composite fibers were successfully prepared by electrospinning processing. The MWNTs bonded with the polycaprolactone chains exhibited excellent uniform dispersion in PLA solution by comparing with the acid-functionalized MWNTs and amino-functionalized MWNTs. Optical microscopy was used to study the aligned degree of the fibers and to investigate the influences of the electrodes distance on the alignment and structure of the fibers, and results showed that the best quality of aligned fibers with dense structure and high aligned degree were obtained at an electrodes distance of 3 cm. Moreover, the MWNTs embedded inside the MWNTs-PCL/PLA fibers displayed well orientation along the axes of the fibers, which was demonstrated by field emission scanning electron microscopy, transmission electron microscopy and Raman spectroscopy.     

Influence of filler alignment in the mechanical and electrical properties of carbon nanotubes/epoxy nanocomposites

In this work, we report the mechanical and electrical properties of carbon nanotubes/epoxy composites prepared with aligned and randomly oriented nanotubes as filler. The samples are disks of 30 mm in diameter and 3 mm in thickness. To obtain the carbon nanotubes alignment, an external electric field (250 VAC; 50 Hz) was applied through the thickness of the sample during all the cure process. The AC electrical current was measured, during the cure, as a strategy to determine the optimum time in which the alignment reaches the maximum value. DC conductivity measured after the cure shows a percolation threshold in the filler content one order of magnitude smaller for composites with aligned nanotubes than for composites with randomly oriented filler (from 0.06 to 0.5 wt%). In the percolation threshold, the achieved conductivity was 1.4×10⁻⁵ Sm⁻¹. In both cases, aligned and randomly distributed carbon nanotube composites, the wear resistance increases with the addition of the filler while the Rockwell hardness decreases independently of the nanotubes alignment.     

Thermal and electrical conductivity of poly(l-lactide)/multiwalled carbon nanotube nanocomposites

Multiwalled carbon nanotubes (MWCNTs) are considered to be the ideal reinforcing agent for high-strength polymer composites, because of their fantastic mechanical strength, high electrical and thermal conductivity and high aspect ratio. Polymer/MWCNTs composites are easily molded, and the resulting shaped plastic articles have a perfect surface appearance compared with polymer composites made using usual carbon or glass fibers. Good interfacial adhesion between the MWCNTs and the polymer matrix is essential for efficient load transfer in the composite. The ultrahigh strength polymer composites demand the uniform dispersion of the MWCNTs in the polymer matrix without their aggregation and the good miscibility between MWCNT and polymer matrix. This approach can also be applied to biodegradable synthetic aliphatic polyesters such as poly(l-lactide) (PLLA), which has received a great deal of attention due to environmental concerns. In this study, PLLA was melt-compounded with MWCNTs. A high degree of dispersion of the MWCNTs in the composites was obtained by grafting PLLA onto the MWCNTs (PLLA-g-MWCNTs). After oxidizing the MWCNTs by treating them with strong acids, they were reacted with l-lactide to produce the PLLA-g-MWCNTs. The mechanical properties of the PLLA/PLLA-g-MWCNT composite were higher than those of the PLLA/MWCNT composite. The electrical conductivity of the composites was determined by measuring the volume resistivity, which is a value of the resistance expressed in a unit volume by two-probe method. The thermal diffusivity and heat capacity of composites was measured by laser flash method, and the effects of modification of the MWCNT in PLLA matrix are discussed.     

Preparation, magnetic and electromagnetic properties of polyaniline/strontium ferrite/multiwalled carbon nanotubes composite

Strontium ferrite particles were firstly prepared by sol-gel method and self-propagating synthesis, and then the polyaniline/strontium ferrite/multiwalled carbon nanotubes composites were synthesized through in situ polymerization approach. Structure, morphology and properties of the composite were characterized by various instruments. XRD analysis shows that the output of PANI increases with the increase of the content of MWCNTs, due to the large surface area of MWCNTs. Because of the coating of PANI, the outer diameter of MWCNTs increases from 10 nm to 20-40 nm. The electrical conductivity of the composites increases with the amount increase of MWCNTs and reaches 7.2196 S/cm in the presence of 2 g MWCNTs. The coercive force of the composites prepared with 2 g MWCNTs is 7457.17 Oe, which is much bigger than that of SrFe₁₂O₁₉ particles 6145.6 Oe, however, both the saturation magnetization and the remanent magnetization of the composite become much smaller than those of SrFe₁₂O₁₉ particles. The electromagnetic properties of the composite are excellent in the frequency range of 2-18 GHz, which mainly depend on the dielectric loss in the range of 2-9 GHz, and mainly on the magnetic loss in the range of 9-18 GHz.     

Optimizing mechanical and thermal expansion properties of carbon/carbon composites by controlling textures

The present study explored the effect of medium texture (MT) content on flexural properties and thermal expansion coefficients (CTE_S) of carbon/carbon (C/C) composites with multilayered pyrolytic carbon. The specimen with 39% MT exhibited maximum flexural strength of 221.55 MPa, increasing by 52% compared with pure high texture. While the flexural strength decreased when the MT content exceeded 39%. The excellent strength can be attributed to crack deflection between multilayered texture and the strong interface bonding between fibers and matrix. Moreover, the four specimens expressed a similar trend of CTE_S in the direction of XY and Z. In the direction of XY, the specimen with 39% MT had the lowest CTE_S from 800 °C to 2100 °C. Therefore, the C/C composites with 39% MT have the best mechanical and thermal expansion properties, which means that the properties of C/C composites can be optimized by controlling the texture.     

Synthesis, characterization and electrochemical behavior of polypyrrole/carbon nanotube composites using organometallic-functionalized carbon nanotubes

Thorn-like, organometallic-functionalized carbon nanotubes were successfully developed via a novel microwave hydrothermal route. The organometallic complex with methyl orange and iron (III) chloride served as reactive seed template, resulting in the oriented polymerization of pyrrole on the modified carbon nanotubes without the assistance of other oxidants. Morphological and structural characterizations of the carbon nanotube/methyl orange-iron (III) chloride and polypyrrole/carbon nanotube composites were examined using transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), infrared spectroscopy and X-ray diffraction (XRD). The electrochemical property of the polypyrrole/carbon nanotube composite was elucidated by cyclic voltammetry and galvanostatic charge-discharge. A specific capacitance of 304 F g⁻¹ was obtained within the potential range of −0.5-0.5 V in 1 M KCl solution.     

Synthesis,characterization, and electrochemical properties of imidazole derivatives functionalized single‐walled carbon nanotubes

The imidazole derivatives functionalized single‐walled carbon nanotubes (SWNTs) were synthesized by a diazonium‐based reaction. We have designed and synthesized two imidazole derivatives to modify SWNTs. The resulting products were characterized by Fourier transform infrared (FT‐IR) spectroscopy, Raman spectroscopy, ultraviolet visible (UV/Vis) spectroscopy, thermo gravimetric analysis (TGA), energy dispersive X‐ray spectroscopy (EDX), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Electrochemical measurements via a cyclic voltammetry method revealed that the weak intramolecular electronic interactions presented between the attached imidazole derivatives groups and the nanotubes. Copyright © 2008 John Wiley & Sons, Ltd.     

Physical properties,thermal stability,and glass transition temperature of multi-walled carbon nanotube/polypyrrole nanocomposites

Polypyrrole (PPy) was synthesized and doped with 1, 2, 4, and 8?wt.% of functionalized multi-wall carbon nanotubes (MWCNTs) by in situ polymerization. TGA/DTA analysis of nanocomposites revealed an increase in thermal stability by increasing the CNTs wt.%. Measurement of electrical resistivity showed a reduction in the resistivity of the composites at all temperatures. The glass transition temperature (T_g) of the samples was measured using electrical resistivity measurements and showed that by increasing the amount of functionalized MWCNTs in PPy, its T_g was increased. Temperature dependence of resistivity of pressed pure PPy showed that by increasing the pelletization pressure, the T_g increased. Also the hardness of nanocomposites was increased by increasing the MWCNTs wt.%.     

Low temperature crystallization behavior of multi-walled carbon nanotubes/Pb(Zr0.52Ti0.48)O3 nanocomposite thin films through annealing in various atmosphere and duration control

We report a successful fabrication of 300 nm thick carbon nanotubes and Pb(Zr_0.52Ti_0.48)O₃ (CNT–PZT) nanocomposite thin films with annealing temperature as low as 500 °C in H₂/N₂ atmosphere. Realizing the thickness of CNT–PZT nanocomposite thin films down to few hundred nanometers is one way to reduce the operating voltage of its application to micro- or nano-electromechanical system. The field emission scanning electron microscopic and atomic microscopic analysis revealed that the nanocomposite thin films annealed in H₂/N₂ atmosphere exhibits the most favorable surface morphology with adequate perovskite (111) reflection of PZT based on X-ray diffraction analysis. The measured dielectric constant and loss tangent of the nanocomposite thin films show that the annealing duration of 30 min promotes the optimum dielectric properties of the nanocomposite thin films. Our observations suggest that the annealing atmosphere and duration are important parameters in controlling the crystallization behavior hence the dielectric properties of the nanocomposite thin films, which can be readily applicable to other nanocomposite thin films.