Controllable purification, cutting and unzipping of multi-walled carbon nanotubes with a microwave method

A rapid microwave-assisted method was developed for the purification, cutting and unzipping of arrays of multi-walled carbon nanotubes (MWCNTs) using a mixture of KMnO₄ and H₂SO₄. To harness the extent of treatment, MWCNT products were fully characterized at different reaction times by UV-visible and Raman spectroscopies as well as scanning and transmission electron microscopies. The results show that the carbon nanoparticles and the amorphous carbon which coated the MWCNTs were removed after about 10 minutes. The excessive oxidation of MWCNTs then leads to cutting and unzipping of graphitic walls. Moreover, while the catalyst residues outside the MWCNTs were rapidly extracted up to 10 minutes, the removal of catalyst residues inside the MWCNTs did not begin before 20 minutes. This method can be considered as an efficient route for the purification, cutting and unzipping of MWCNTs due to its fast and controllable procedure.     

Influence of disorder on microcavity polariton linewidths

Polariton linewidths have been measured in a series of microcavities with different excitonic and cavity inhomogeneous broadening in the weak-disorder regime. We show experimentally that the behaviour of the polariton linewidths as a function of the detuning depends on the asymmetric line shape of an inhomogeneously broadened exciton line and particularly the disorder effect can be modulated and cancelled around resonance. When the disorder contribution is minimal, the behaviour of the cavity polariton linewidths tends to one of the homogeneous broadening system.     

Experimental Study of Sediment Flow Discharge in New System of Bottom Intakes with Porous Media

Bottom rack intake is one of the most popular structures for diverting water in steep rivers. The problems of corrosion, deformity, and clogging of the bottom racks in long term inspire a new system of bottom intake in which a filled trench of porous media replaces the bottom racks. Diversion of a specified amount of water through the porous media, when the void space in the granular material is filled with fine sediments in comparison to the bottom racks, requires much larger structure. For the proposed water intake, lower cost of construction and maintenance and higher compatibility with the river morphology are considered as major advantages. This research deals with an experimental model with two-story channel, the lower one is used to convey diverted water through the porous media and the upper one is used to carry the remained flow to downstream. Measurements of the diverted discharge were performed for different rates of flow, grain, and sediment size distributions as well as surface slopes of intake. Results show that despite clogging of the suspended sediments in porous media, using appropriate grain size with a surface slope of the porous media increases the discharge coefficient of system. In comparison to the clear water flow, the diverted discharge reduction induced by clogging is not significant and the efficiency of system in long term with no operation effort is confirmed. An empirical formulation has been proposed based on the concept of Darcy’s law for surface infiltration. The equation is found to be dependent on size ratio, Reynolds number, and hydraulic gradient. The theoretical predictions in comparison with the experimental results have shown a good consistency.     

Control parameter estimation in a semi-linear parabolic inverse problem using a high accurate method

Parabolic inverse problems have an important role in many branches of science and technology. The aim of this research work is to solve these classes of equations using a high order compact finite difference scheme. We consider the following inverse problem for finding u(x, t) and p(t) governed by u_t = u_xx + p(t)u + φ(x, t) with an over specified condition inside the domain. Spatial derivatives are approximated using central difference scheme. The time advancement of the simulation is performed using a “third order compact Runge-Kutta method”. The convergence orders for the approximation of both u and p are of o(k³ + h²) which improves the results obtained in the literature. An exact test case is used to evaluate the validity of our numerical analysis. We found that the accuracy of the results is better than that of previous works in the literature.     

Forced Convection Heat Transfer of Nanofluids in a Porous Channel

This article is concerned with the effects of flow and migration of nanoparticles on heat transfer in a straight channel occupied with a porous medium. Investigation of force convective heat transfer of nanofluids in a porous channel has not been considered completely in the literature and this challenge is generally considered to be an open research topic that may require more study. The fully developed flow and steady Darcy?CBrinkman?CForchheimer equation is employed in porous channel. The thermal equilibrium model is assumed between nanofluid and solid phases. It is assumed that the nanoparticles are distributed non-uniformly inside the channel. As a result the volume fraction distribution equation is also coupled with governing equations. The effects of parameters such as Lewis number, Schmidt number, Brownian diffusion, and thermophoresis on the heat transfer are completely studied. The results show that the local Nusselt number is decreased when the Lewis number is increased. It is observed that as the Schmidt number is increased, the wall temperature gradient is decreased and as a consequence the local Nusselt number is decreased. The effects of Lewis number, Schmidt number, and modified diffusivity ratio on the volume fraction distribution are also studied and discussed.     

Microwave-induced high surface functionalization of multi-walled carbon nanotubes for long-term dispersion in water

Multi-walled carbon nanotubes (MWCNTs) were functionalized by monoethanolamine (MEA), diethanolamine (DEA) and triethanolamine (TEA) by a rapid microwave-assisted method. Surface functional groups and morphology of MWCNTs were analyzed by ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, Raman spectroscopy and transmission electron microscopy. The water dispersibility of samples was also measured over time and compared with that of MWCNT functionalized by ethylenediamine (EDA). Accordingly, the dispersibility was in the sequence MEA-MWCNTs > DEA-MWCNTs > TEA-MWCNTs > EDA-MWCNTs > pristine MWCNTs. The significant long-term dispersibility of MEA-MWCNTs was related to its exceptionally high surface functionalization (89.3 %) and lack of chemical bridging between adjacent MWCNTs. This strategy, along with the low-cost functionalizing agents, could pave the way for large-scale dispersion of CNTs in the polar solvents.     

Detailed profiling of CNTs arrays along the growth window in a floating catalyst reactor

We report a detailed longitudinal and depth profiles of multi-wall carbon nanotubes (CNTs) arrays synthesized using xylene and ferrocene in a floating catalyst reactor. Point to point analyses of the CNTs grown in a “growth window” with CNTs arrays longer than 0.5 mm were performed using optical microscopy, Raman spectroscopy, FESEM, high-resolution TGA/DTA, and TEM techniques. The heights of the CNTs arrays show a maximum at a mid point of the growth window, while a reverse trend of minimum is observed for iron-to-CNTs atomic ratios. The ratio of amorphous carbon to CNTs sharply increases along the growth window and from the bottom to top of CNTs arrays. The CNTs diameter also increases along the growth window, due to deposition of the amorphous carbon, which can be almost removed by temperature programmed oxidation up to around 500 °C. A base growth mechanism, the variations of catalyst content, residence time and temperature profile along the growth window, the adsorption and decomposition of polycyclic aromatic hydrocarbons to amorphous carbon, and a limited diffusion of hydrocarbon species through the arrays covered by excessive amorphous carbon may explain the results.     

Alignment of carbon nanotubes in bulk epoxy matrix using a magnetic-assisted method: Solenoid magnetic field

Alignment of multi-walled carbon nanotubes (MWCNTs) in a bulk epoxy matrix was studied using a magnetic-assisted method. In this study, the applicability of solenoid magnetic field was compared to that of static magnetic field, which has recently been published. It has been found that solenoid magnetic field is able to provide better alignment of MWCNTs due to the uniform magnetic field along the axis of solenoid. Remarkable improvements were achieved in the mechanical properties of aligned MWCNT-reinforced epoxy composites. Alignment of MWCNTs was clearly observed in all regions of the composites by scanning electron microscopy and transmission electron microscopy.     

CFD modeling of catalyst pellet for oxidative coupling of methane: Heat transfer and reaction

This study deals with the phenomena occuring at single-pellet catalyst scale for the oxidative coupling of methane where heat transfer plays an important role. Computational fluid dynamics (CFD) is used for obtaining detailed rate and temperature profiles through the porous catalytic pellet where reaction and diffusion compete. Intra-particle temperature and concentration gradients were taken into account by solving heat transfer coupled with continuity equations in the catalyst pellet. In heat transfer, the energy term due to highly exothermic reaction was considered. Two external programs were successfully implemented into the CFD-code as kinetic and heat of reaction terms. Simulation results showed that reaction was favored at the beginning for the pellet, followed by diffusion predomination. The results of CFD simulation indicate that temperature variation within the catalyst pellet is <2 K due to exothermic oxidation. The results showed further that exothermic oxidation reactions occurred prior to endothermic coupling reaction in the pellet.