Nanotube Suspensions Research Articles

Kinetic Theory Microstructure Modeling in Concentrated Suspensions

When suspensions involving rigid rods become too concentrated, standard dilute theories fail to describe their behavior. Rich microstructures involving complex clusters are observed, and no model allows describing its kinematics and rheological effects. In previous works the authors propose a first attempt to describe such clusters from a micromechanical model, but neither its validity nor the rheological effects were addressed. Later, authors applied this model for fitting the rheological measurements in concentrated suspensions of carbon nanotubes (CNTs) by assuming a rheo-thinning behavior at the constitutive law level. However, three major issues were never addressed until now: (i) the validation of the micromechanical model by direct numerical simulation; (ii) the establishment of a general enough multi-scale kinetic theory description, taking into account interaction, diffusion and elastic effects; and (iii) proposing a numerical technique able to solve the kinetic theory description. This paper focuses on these three major issues, proving the validity of the micromechanical model, establishing a multi-scale kinetic theory description and, then, solving it by using an advanced and efficient separated representation of the cluster distribution function. These three aspects, never until now addressed in the past, constitute the main originality and the major contribution of the present paper.

Synthesis of Porous Materials and Their Microstructural Control through Ice Templating

Ice-templating is a simple, practical kind of template synthesis which consists of sol-gel polycondensation, unidirectional freezing of hydrosols or hydrogels, and pore-preserving drying method, such as freeze drying, thereby resulting in desirable porous materials. The unidirectional freezing may be achieved by either immersion freezing or contact freezing. Freeze-dried materials contain porous microstructures that are replicas of the ice crystals formed during the unidirectional freezing process. Two similar but slightly different applications of ice-templating will be investigated here. Fabrication of macroporous foam materials by unidirectional freezing of an aqueous suspension of carbon nanotubes (CNTs) dispersed by chitosan is a concrete example of functionalization of an electroconductive foam, which is a composite material made from a polymer and CNTs. As expected, their electroconductive properties can be controlled by the fabrication method and condition. Additional examples of porous materials synthesized by ice-templating are porous microfibers and microhoneycombs of silica, titania, silica-alumina, titania-silica, and even carbon. In the preparation, the morphology and porous structure can be controlled by the freezing conditions and synthesis conditions of the corresponding hydrogels, respectively.

Chemical functionalization of Xanthan gum for the dispersion of double-walled carbon nanotubes in water

Achieving stable suspensions of carbon nanotubes (CNTs) in water is still a challenge. Addition of surfactants is desirable as it allows keeping intact the intrinsic properties of the CNTs. However, for different applications, the potential toxicity of the surfactant is an important issue. Polysaccharides are among the best candidates and chemical modification can improve their intrinsic features. They can thus combine the properties of added synthetic counterparts with their intrinsic biocompatibility. In this work, we focused on the synthesis of hydrophobically modified Xanthan (Xan) with three derivatives (Diphenylmaleic anhydride, Phtalic anhydride, Epichlorhydrin-Phenol) to disperse CNTs. The dispersion and the stability against sedimentation of double walled carbon nanotubes (DWCNTs) have been investigated (rheological properties, zeta potential) as a function of pH and Xan concentration. Our results show that stable suspension of DWCNTs for 0.5% (w/w) could be obtained with the three derivatives of modified Xanthan gum in acidic and alkaline media while Xan itself is a very poor dispersing agent for CNTs, giving good evidence of the validity of our approach.

Numerical analysis of percolation formation in carbon nanotube based nanofluids

SUMMARYThe intriguing behaviour of carbon nanotube suspensions shows that thermal conduction cannot be described by conventional approaches. These results led the researchers on the percolation and the interfacial layer resistance (also known as Kapitza resistance) as the main mechanisms governing the effective thermal conductivity enhancement for these nanoparticles suspensions. A numerical simulation on the behaviour of these suspensions, when subjected to a Brownian force field, was conducted to characterize the main factors affecting the dynamic interactions and percolation structures formation. To this end, three different numerical models based on continuum mechanics were developed. The obtained results suggest that the size, shape and aspect ratio of the nanoparticles are main factors controlling the dynamic network formation. On the other hand, the influence of the Brownian motion and the structural flexibility of the nanotubes seem to have a rather negligible effect on the results. The numerical model developed and proposed here may assist in understanding and correlating the experimental thermal conductivity data of nanofluids, contributing to demystify some of the intriguing behaviours reported. Copyright © 2013 John Wiley & Sons, Ltd.

Phase transitions in smectogenic liquid crystal 4-butoxybenzylidene-4′-butylaniline (BBBA) doped by multiwalled carbon nanotubes

The phase transitions in smectogenic liquid crystal BBBA (4-butoxybenzylidene-4′-butylaniline) doped by multi-walled carbon nanotubes (NTs) were studied by methods of optical transmission, differential scanning calorimetry (DSC), measurement of electrical conductivity and analysis of microscopic images. The concentration of NTs was varied within 0–1% wt. Non-monotonous (extremal) changes in temperature, enthalpies and half-width of the DSC peaks of transitions between different phases (smectics, nematic, isotropic) were observed for NT concentrations between 0.05 and 0.1% wt. A noticeable increase of electrical conductivity σ in the same concentration interval evidenced the presence of percolation transition and formation of conductive NT networks. The detailed analysis of σ behavior in the whole concentration interval 0–1% wt revealed the presence of a fuzzy type percolation with multiple thresholds in the studied BBBA + NT suspensions. The percolation behavior was strongly dependent on the temperature, and a noticeable step-like drop of σ in the vicinity of isotropic-nematic transition was observed after the multiple heating–cooling cycles.

Transport and thermal properties of quaternary phosphonium ionic liquids and IoNanofluids

The transport and thermal properties of phosphonium ionic liquids (ILs) and thermally stable suspensions of phosphonium ILs and multiwalled carbon nanotubes (MWCNT) were determined. The thermal conductivity was measured over the temperature range T=(280 to 360)K, and at atmospheric pressure, using the transient hot-wire method with an accuracy estimated as 5% from (0.2 to 2)W·m−1·K−1 and less than ±0.01W·m−1K−1 from (0.02 to 0.2)W·m−1·K−1. The phosphonium ILs studied were trihexyltetradecylphosphonium phosphinate [(C6)3PC14)][Phosph], tretrabutylmethylphosphonium methylsulfate, [(C4)3PC1)][C1SO4], trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide, [(C6)3PC14][NTf2], and trihexyltetradecylphosphonium trispentafluoroethyltrifluorophosphate [(C6)3PC14][FAP]. For all ILs studied, the thermal conductivities were found to lie within the range (0.12 to 0.16)W·m−1·K−1. Our measurements combined with selected values from the open literature allowed us to determine a linear increase of combined properties (thermal conductivity, density and molar mass) with the increasing of molar mass. The addition of only (0.1 to 0.2)wt.% of MWCNT to ILs (IoNanofluids) revealed a slight increasing of the thermal conductivity from 0.5% to 1%. There was also observed a slight decrease of the thermal conductivity with increasing temperature for the ILs alone and respective IoNanofluids. The rheological behaviour of [(C6)3PC14)][Phosph], and of [(C6)3PC14][NTf2] based IoNanofluids was evaluated at T=(298.15 to 333.15)K and for shear rates in the range γ̇=(3 to 30)s−1. All the samples showed shear thinning behaviour. The thermal stability of the IoNanofluids compared to that of ILs alone was studied by High Resolution Thermogravimetric Analysis. It was found that pure [(C6)3PC14)][Phosph] is a thermally stable IL, whose thermal decomposition under the conditions of this study (2K·min−1) and starts above T=513K. It exhibits a one step thermogravimetric profile ([(C6)3PC14][NTf2] that was already investigated in a previous study). A small shift towards lower temperatures was observed in the thermogravimetric curves of IoNanofluids compared to those of the respective ILs alone indicating comparable thermal stabilities. The heat capacities determined by Modulated Differential Scanning Calorimetry within a temperature interval ranging from ca. (310 to 515)K with an estimated uncertainty of less than ±0.012J·K−1·g−1.

An efficient method for the carboxylation of few-wall carbon nanotubes with little damage to their sidewalls

We report a novel method that is able to efficiently functionalize carbon nanotubes (few-walled: from 1 to 6 layers and multiwalled) with a high yield of carboxyl groups, based on treatments with H2O2 in the presence of UV light. The amount of carboxylic groups was quantified by X-ray photoelectron spectroscopy and back-titration, showing both measurements reasonable agreement. According to the zeta potential values and to the amount of suspended nanotubes, we demonstrate that the method is able to produce uniform and stable suspensions of carbon nanotubes in water. With the aid of scanning and transmission electron microscopy, thermogravimetric analysis, and Raman spectroscopy, we show that the surfaces of the tubes are not damaged by the treatment and that the functionalized tubes have an enhanced reactivity toward oxygen. This route is efficient and could now be used to fabricate polymer composites using few-walled and multiwalled carbon nanotubes.

Separation of Single-Walled Carbon Nanotubes by 1-Dodecanol-Mediated Size-Exclusion Chromatography

A simple, single-column, high-throughput fractionation procedure based on size-exclusion chromatography of aqueous sodium dodecyl sulfate suspensions of single-walled carbon nanotubes (SWCNTs) is presented. This procedure is found to yield monochiral or near monochiral SWCNT fractions of semiconducting SWCNTs. Unsorted and resulting monochiral suspensions are characterized using optical absorption and photoluminescence spectroscopy.

Laser modification of optical properties of a carbon nanotube suspension in dimethylformamide

It is shown experimentally that a semitransparent suspension of multiwall carbon nanotubes (MWNTs) in N,N-dimethylformamide (DMF) is irreversibly bleached in a wide optical wavelength range in a field of nanosecond high-power laser radiation with a wavelength of 532 nm. The bleaching is accompanied by a sharp decrease in the nonlinear scattering; the suspension ceases to optically limit power. It is shown that the suspension is bleached due to light-induced chemical reactions between MWNTs and DMF molecules with formation of new hydrocarbon chemical bonds.

Theoretical Studies of “Stabilizing” Behavior About Carbon Nanotubes Under the Electrostatic Force

We study the dispersion and stability of carbon nanotube (CNT) suspensions under the electrostatic interactions. The potential energies of van der Waals (vdW) attractions between the CNT themselves are obtained on the continuum Lennard-Jones (LJ) model. The potential energies of electrostatic repulsions are based upon the Yukawa-segment model. We explore the overall interactions mediated by the vdW force and the electrostatic force between two identical, parallel CNTs. Consequently, we preliminarily confirm the accuracy and reliability of the electrostatic model.

Multi walled carbon nanotubes deposited on metal substrate using EPD technique. A spectroscopic study

The work reports the results of manufacture of carbon coatings on the surface of stainless steel. Multi walled carbon nanotubes (MWCNTs) were used to obtain carbon coatings by electrophoretic deposition technique (EPD). Before deposition MWCNTs were purified and surface functionalised. After purification and functionalization CNTs contained oxygen functional groups on its surface. The experiments were made in two model systems, differing in the type of suspensions of carbon nanotubes used in the electrophoretic deposition process. For the preparation of the suspensions, water and isopropanol were used without any additional components. Both experiments involved the application of the same type of nanotubes, as well as identical processing parameters of EPD. The structural analysis of both types of coatings was performed using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). It was pointed out that the structures of the coatings are different, depending on the type of the applied suspension. The degree of the structure’s arrangement in both analysed layers is different; also the types of oxygen–carbon combinations in both coatings are essentially dissimilar.

Adsorption of Biopolymers on SWCNT: Ordered Poly(rC) and Disordered Poly(rI)

Polymer adsorption onto single-walled carbon nanotubes (SWCNTs) depends on its rigidity/flexibility. The adsorption properties of two related homopolynucleotides poly(rI) and poly(rC) but of different rigidities were compared, employing absorption spectroscopy and molecular dynamics simulation. It was shown that adsorption of the poor base stacked poly(rI) onto the nanotube is less effective than that of the strong base stacked poly(rC), the chain of which is of higher rigidity. Analysis of UV absorption spectra of polymer:nanotube suspension at heating until 90 °C, which leads to partial nanotube aggregation because of the weakly bound polymer sliding from the tube surface, revealed that the percent of precipitated nanotubes in suspension with poly(rI) is larger than that in suspension with poly(rC) (16% vs 7%). This fact indicates the higher stability of SWCNT:poly(rC) hybrid in comparison with SWCNT:poly(rI). Less effective adsorption of poly(rI) is confirmed with a weaker hypochromic effect of nanotubes covered with poly(rI) than with poly(rC), which originates from π-π stacking of nitrogen bases with the nanotube surface. Spontaneous adsorption of oligomers on the nanotube simulated by the molecular dynamics showed that oligomer r(I)25 has a weaker energy of binding to the carbon nanotube surface than r(C)25. The oligomer with ordered bases has a tendency to form the stretched conformation along the nanotube, which provides a higher binding energy, while more flexible r(I)25 forms the stable loop spaced away from the nanotube surface, the stability of which is strengthened with H-bonding between bases.

Mechanism of Electrolyte-Induced Brightening in Single-Wall Carbon Nanotubes

While addition of electrolyte to sodium dodecyl sulfate suspensions of single-wall carbon nanotubes has been demonstrated to result in significant brightening of the nanotube photoluminescence (PL), the brightening mechanism has remained unresolved. Here, we probe this mechanism using time-resolved PL decay measurements. We find that PL decay times increase by a factor of 2 on addition of CsCl as the electrolyte. Such an increase directly parallels an observed near-doubling of PL intensity, indicating the brightening results primarily from changes in nonradiative decay rates associated with exciton diffusion to quenching sites. Our findings indicate that a reduced number of these sites results from electrolyte-induced reorientation of the surfactant surface structure that partially removes pockets of water from the tube surface where excitons can dissociate, and thus underscores the contribution of interfacial water in exciton recombination processes.

Mechanical and morphological properties of highly dispersed carbon nanotubes reinforced cement based materials

Stable homogeneous suspensions of multi-walled carbon nanotubes (MWCNTs) were prepared using gum arabic (GA) as dispersant and were incorporated to Portland cement paste. The dispersion was examined by ultraviolet visible spectroscopy (UV-vis), and the concentration measurement shows that the optimum concentration of GA is 0.45 g · L−1. The dispersibility of the surface-modified MWCNTs in aqueous solution and cement matrix were investigated by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS), and the mechanical properties of the composites were investigated. The results show that the addition of the treated nanotubes can improve both the flexural strength and the compressive strength of the Portland cement composite significantly. The flexural strength of the composite increases up to 43.38% with the MWCNT concentration of 0.08% (by weight of cement). The porosity and pore size distribution of the composites were measured by mercury intrusion porosimetry (MIP), and the results indicate that the cement paste doped with MWCNTs obtained lower porosity and concentrated pore size distribution. The morphological structure was analyzed by field emission scanning electron microscopy (FESEM) and EDS. It is shown that MWCNTs act as bridges and networks across cracks and voids, which transfer the load in case of tension, and the interface bond strength between the nanotubes and matrix is very strong.

Electrochemical characterization and equivalent circuit modeling of single-walled carbon nanotube (SWCNT) coated electrodes

Single-walled carbon nanotube (SWCNT) coated electrodes are fabricated from functionalized suspensions with different pH values. Their electrochemical properties are characterized using cyclic voltammetry and electrochemical impedance spectroscopy. The effects of the pH value of the suspension on electrochemical performances are investigated through examining the microstructures of the SWCNT films and their BET specific areas and pore volume and size distributions. Both these specific areas and pore size distributions are found being important in determining the specific capacitances of SWCNT coated electrodes. An equivalent circuit model is developed for these electrodes using the measured electrochemical impedance data. It is found that the functionalized nanotube suspensions with higher pH values produce more desirable nanostructured electrodes with the increased specific capacitance and reduced electrode resistance. The role of the pseudo-capacitance which arises from the functionalization group is also discussed.

Functional Surfactants for Carbon Nanotubes: Effects of Design

Surfactants are needed to create stable suspensions of carbon nanotubes. Increasingly, these surfactants are given additional functionalities, resulting in bigger and more complex molecules with several subunits. We investigate the effect of assembly of these subunits for a class of perylene-based functional surfactants. The subunits that all surfactants are based on are a perylene core, hydrophilic polyglycerol dendrons, and alkyl chains of different orientations and lengths. The assembly of these subunits affects both the molecules' performance as a surfactant and the efficiency of the energy-transfer complexes formed by the nanotube and surfactant through a π–π stacking mechanism. This results in a best practice guide for designing functional surfactants with π–π stacking cores, and affords more general insights that are applicable to non π–π stacking systems as well.

Experimental Investigation of Rheological Behavior and Pressure Drop of Aqueous Suspensions of Carbon Nanotubes in a Horizontal Tube

A new engineering medium, called nanofluid, have attracted a wide range of researches on many processes in engineering applications. Many experimental and theoretical studies on viscosity of nanofluids are controversial and the shear rate dependent viscosity is frequently neglected. The aim of the present work is to investigate the effect of the rheological behavior of aqueous suspensions of carbon nanotubes on the friction factor and the pressure drop in a horizontal tube under laminar and turbulent flow. The particles weight concentration used here is 0.01%. The rheological behavior was measured for a range of temperature from 0 °C to 40 °C. The results showed that the nanofluids exhibit a shear thinning behavior at very low shear stress. The analysis results of the flow characteristics showed a non-Newtonian behavior under laminar flow and a Newtonian behavior under turbulent flow.

Laser Bleaching of Carbon Nanotubes Suspension in N,N- Dimethylformamide

Laser Bleaching of Carbon Nanotubes Suspension in N,N- Dimethylformamide It is shown experimentally that the multiwall carbon nanotubes (MWNTs) suspension in N,N-dimethylformamide (DMF) irreversibly bleaches in a wide spectrum range under a pulsed nanosecond laser radiation at the wavelength of 532 nm. According to the data of the transmission electron microscopy (TEM), the Raman spectroscopy and the reflection infrared spectrometry the bleaching is shown to be caused by MWNTs degradation, with the formation of new chemical bonds which result from the laserinduced chemical reactions between MWNTs and DMF molecules.

Fast and reversible microscale formation of columns in carbon nanotube suspensions

Homogeneous carbon nanotube suspensions are studied in the semidilute regime, which is permitted by the use of an effective polysiloxane surfactant, PySi70, as a liquid matrix. We study such suspensions under an AC electric field by optical microscopy, transient light transmission and impedance spectroscopy. Optical microscopy shows the formation of long-range microscale columnar order as a result of a fast field-induced phase separation. The aggregation process is found to be reversible and repeatable, with nanotubes re-dispersing into a homogeneous solution, in a diffusive manner, in the seconds following removal of the electric field. The suggested role of the induced dipolar forces is consistent with the relatively fast columnar formation and characteristic concentration of maximum field-response. The equilibrium columnar structure provides fast field-induced control of the transparency and dielectric response, while avoiding electrical breakdown.

0102 急縮小部を通過するカーボンナノチューブ分散流体の速度分布(OS1-1 非ニュートン流体の流動現象,OS1 非ニュートン流体の流動現象)

This report shows the fluid property and the velocity profile of carbon nanotube suspension (CNT). The CNT suspension is complex fluid. Complex fluid is defined as any fluid that does not follow the well-known Newton's law. In this study, two-dimensional velocity profiles of CNT suspension were measured between parallel plates and at the upstream region of the contraction. These results show that the velocity profiles in CNT suspension between parallel plates were in good agreement with Newtonian one. However, the results in CNT suspension at the upstream region of the contraction were not agreement with Newtonian one. This is because the elongational viscosities of CNT suspension effect at the upstream of the contraction.