Natural inorganic nanotubes reinforced epoxy resin nanocomposites

Natural occurred nanotubes, halloysite nanotubes, were modified by silane and incorporated into epoxy resin to form nanocomposites. The morphology of the nanocomposites was characterized by transmission electron microscopy (TEM). Dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) were performed on the nanocomposites. Flexural property and coefficient of thermal expansion (CTE) of the nanocomposites were also determined. Comparing with the neat resin, about 40% increase in storage modulus at glassy state and 133% at rubbery state were achieved by incorporating 12 wt% modified HNTs into the epoxy matrix. In addition, the nanocomposites exhibited improved flexural strength, char yield and dimensional stability. TEM examination revealed a uniform dispersion of the nanotubes in the epoxy resin. The remarkably positive effects of the HNTs on the performance of the epoxy resin were correlated with the unique characteristics of the HNTs, the uniform dispersion and the possible interfacial reactions between the modified HNTs and the matrix.     

Dielectric relaxation properties of PbTiO3-multiwalled carbon nanotube composites prepared by a sol–gel process

Dielectric composites fabricated by combining multi-walled carbon nanotubes (MWCNT) and PbTiO₃ (PTO) powder were prepared using a sol–gel process. Well-dispersed PTO powder with various volume ratios of MWCNT was compressed to form a pellet, and then silver electrodes were coated on both sides for electrical measurements. The PTO–MWCNT composite with 0.4 vol% MWCNT showed the highest dielectric constant (912 at 1 kHz), which is approximately 25 times larger than that (37 at 1 kHz) of a pure PbTiO₃ pellet. Furthermore, a strong frequency dependence of the dielectric constant in the low frequency range was shown for the PTO–MWCNT composites. Interfacial effects related to dielectric relaxation in composite materials were used to explain an observed increase of the dielectric constant near the percolation threshold.     

Cobalt supported on carbon nanotubes — A promising novel Fischer–Tropsch synthesis catalyst

An extensive study of Fischer–Tropsch synthesis (FTS) on carbon nanotubes (CNT) supported and γ–alumina-supported cobalt catalysts with different amounts of cobalt are reported. Up to 40 wt.% of cobalt is added to the supports by the impregnation method. The effect of the support on the reducibility of the cobalt oxide species, dispersion of the cobalt, average cobalt clusters size, water–gas shift (WGS) activity and activity and selectivity of FTS is investigated. Using carbon nanotubes as cobalt catalyst support was found to cause the reduction temperature of cobalt oxide species to shift to lower temperatures. The strong metal-support interactions are reduced to a large extent and the reducibility of the catalysts improved significantly. CNT aided in well dispersion of metal clusters and average cobalt clusters size decreased. Results are presented showing that the hydrocarbon yield obtained by inventive CNT supported cobalt catalyst is surprisingly much larger than that obtained from cobalt on alumina supports. The maximum concentration of active surface Co° sites and FTS activity for alumina and CNT supported catalysts are achieved at 34 wt.% and 40 wt.% cobalt loading respectively. CNT caused a slight decrease in the FTS product distribution to lower molecular weight hydrocarbons.     

功能化石墨烯/埃洛石纳米管对聚丙烯的协同强韧化改性研究

以功能化氧化石墨烯(GO)-埃洛石纳米管(HNTs)杂化材料(GO@HNTs)为纳米填料,以聚丙烯(PP)为基体,通过熔融共混法制备了不同GO@HNTs 含量的GO@HNTs/PP纳米复合材材料,并对所得杂化填料和PP纳米复合材料的结构与性能进行系统研究。研究结果表明,功能化GO与HNTs之间存在化学相互作用,二者之间形成的“屏障效应”抑制了彼此在PP基体中的团聚。仅添加0.5%GO@HNTs杂化纳米填料后,PP复合材料的拉伸强度和冲击强度分别较纯PP提高了17.5%和80.4%,与单独添加相同含量的GO或HNTs所得复合材料的力学性能相比,GO@HNTs杂化纳米填料对PP基体具有明显的协同增强增韧改性作用。与纯PP相比,GO@HNTs/PP试样表现出更高的储能模量、损耗模量和玻璃化转变峰值。由于GO@HNTs的“异相成核效应”和“物理热阻效应”,有效提高了PP纳米复合材料的结晶温度、熔融温度、结晶度和耐热分解温度。     

A molecular dynamics study of the clustering of implanted potassium in multiwalled carbon nanotubes

We have studied the low energy irradiation of carbon nanotubes (CNT) with K ions using classical molecular dynamics simulations with analytical potentials. The studied CNTs had diameters of about 0.5–1.2 nm and single or multiple walls. The average penetration depth and probabilities to introduce an impurity atom into CNT were studied with simulations on irradiating the CNT with single K ion. The number of potassium clusters, their average sizes and the damage produced into the CNT due to the irradiation were studied using multiple K ion irradiations. We found that the K ions are mobile in CNTs right after the implantation event and that they cluster together. For CNTs with 1–3 coaxial tubes, the highest ratio of K atoms in clusters per total number of K ions was obtained by using an irradiation energy of about 100 eV. Also the least damage per K ion was found to be produced into the CNT with this energy when those energies high enough for the ion to penetrate the outermost wall of the CNT were considered.     

Thermo-electrical properties of PVA-nanotube composite fibers

We present in this work an experimental study of the resistivity of composite nanotube fibers made of polyvinyl alcohol and multiwalled carbon nanotubes. These fibers which exhibit exceptional mechanical properties could be used for new conductive and multifunctional textiles or composites. We report on their electrical properties and draw two main conclusions: (i) when the fibers contain a large fraction of amorphous polymer, a substantial decrease of the resistivity is observed in the vicinity of the glass transition temperature (T_g) of the pure PVA. On the basis of X-ray diffraction characterizations, we believe that this behavior results from the relaxation of stress in the polymer-nanotube composite. Slight structural modifications and partial loss of nanotube alignment at T_g could yield an increase of the density of intertube contacts and thereby to a decrease of the electrical resistivity. (ii) Annealing the fibers at high temperature reduces the fraction of amorphous PVA which becomes more crystalline. As a result, the conductivity becomes more stable and does not exhibit any abrupt variation at T_g. Instead the conductivity is non-metallic with an effective semi-conductor type behavior as observed in other nanotube composites or even in pure nanotube assemblies.     

Stable TiO2 nanotube arrays with high UV photoconversion efficiency

This work is intended to define an optimal methodology for preparing highly ordered TiO₂ nanotube arrays by a 60-V anodization in a glycol ethylene solution. In order to obtain a mechanically stable structure with a high UV photoconversion efficiency, it is necessary to carefully control the growth mechanism through the anodization process. For this reason, the nanotube arrays have to be formed upon a compact titanium dioxide layer with well-defined thickness. Besides, both the fluoride concentration and anodization time are strictly correlated, because elevated concentrations and/or a long anodization time produce unstable structure with low photoconversion efficiency. The best result in the terms of reproducibility has been obtained previously for a three-minute galvanostatic oxide growth on the pickled titanium sheet, and anodic growth in ethylene glycol solution containing 1 wt.% H₂O and 0.20 wt.% NH₄F for a period lower than 4.5 h. The UV photoconversion efficiency was measured and a maximum value of 28.3% has been obtained, which is the highest result in the literature.     

Fracture and progressive failure of defective graphene sheets and carbon nanotubes

An atomistic based finite bond element model for the prediction of fracture and progressive failure of graphene sheets and carbon nanotubes is developed by incorporating the modified Morse potential. The element formulation includes eight degrees of freedom reducing computational cost compared to the 12 degrees of freedom used in other FE type models. The coefficients of the elements are determined based on the analytical molecular structural mechanics model developed by the authors. The model is capable of predicting the mechanical properties (Young’s moduli, Poisson’s ratios and force–strain relationships) of both defect-free and defective carbon nanotubes under different loading conditions. In particular our approach is shown to more accurately predict Poisson’s ratio. The numerical prediction of nonlinear stress–strain relationships for defect-free nanotubes including ultimate strength and strain to failure of nanotubes is identical to our analytical molecular structural mechanics solution. An interaction based mechanics approach is introduced to model the formation of Stone–Wales (5-7-7-5) topological defect. The predicted formation energy is compared with ab initio calculations. The progressive failure of defective graphene sheets and nanotubes containing a 5-7-7-5 defect is studied, and the degradation of Young’s moduli, ultimate strength and failure strains of defective nanotubes is predicted.     

Fabrication of IrO2 decorated vertical aligned self-doped TiO2 nanotube arrays for oxygen evolution in water electrolysis

To fabricate effective and stable OER electrode in water electrolysis, self-doped TiO₂ nanotube arrays which has a higher electrical conductivity than pristine TiO₂ nanotube arrays was used as the support for loading IrO₂. The self-doped TNTA was fabricated by a simple electrochemical reduction of TNTA in neutral electrolyte solution, and then IrO₂ nano particles were deposited by pulse electro-deposition method. The cyclic voltammetric behavior, electrical conductivity and micro structure of self-doped TNTA prepared at different reduction potential were characterized to obtain an optimal performance, and self-doped TNTA prepared under ?1.9 V (vs. Ag/AgCl) shows the best electrochemical performance. After depositing IrO₂, the OER activity and stability of new electrodes were also determined. Due to the enhanced electrical conductivity of support, the mass activity of IrO₂/self-doped TNTA are 50 times higher than IrO₂/TNTA. The OER stability of new electrode was evaluated under constant current of 5 mA/cm², IrO₂/TNTA and IrO₂/Ti were also tested for comparison. A higher stability of IrO₂/self-doped TNTA electrode is observed than the other two electrodes, and XPS studies indicate a lower oxidation state of Ir in IrO₂/self-doped TNTA, this shows a possible interactions between IrO₂ and the new support.     

Synthesis and Characterization of Niobium-doped TiO2 Nanotube Arrays by Anodization of Ti-20Nb Alloys

Well crystallized niobium-doped TiO₂ nanotube arrays（TiNbO-NT） were successfully synthesized via the anodization of titanium/niobium alloy sheets,followed with a heat treatment at 550 C for 2 h.Morphology analysis results demonstrated that both the titanium/niobium alloy microstructure and the dissolution strength of electrolyte played major roles in the formation of nanotube structure.A single-phase microstructure was more favorable to the formation of uniform nanotube arrays,while modulating the dissolution strength of electrolyte was required to obtain nanotube arrays from the alloys with multi-phase microstructures.X-ray diffraction（XRD） and X-ray photoelectron（XPS） analysis results clearly demonstrated that niobium dopants（Nb 5+） were successfully doped into TiO₂ anatase lattice by substituting Ti 4+ in this approach.