不同参数多壁碳纳米管的拉曼光谱研究

拉曼光谱是进行碳材料结构与性质研究的有力手段,为了研究多壁碳纳米管(MWCNT)的管径和长度对其拉曼光学性质的影响,本研究对一系列不同管径和长度的多壁碳纳米管进行拉曼光谱的测试和分析。研究发现: 与高取向的石墨相比,多壁碳纳米管一阶拉曼光谱的G峰中心和D峰中心都会向低波数发生不同程度的红移;MWCNT两个主要特征峰(G峰和D峰)峰强在其他条件相同的情况下,与MWCNT的管径成正比,与长度成反比;G峰的频移与MWCNT的管径和长度两个因素密切相关,与管径成反比关系(这与单壁碳纳米管的径向呼吸模有着一致的结果),与管长成正比关系,而D 峰的频移受MWCNT的管径和长度的影响很弱,并对此现象进行了初步分析。在此基础上,我们以MWCNT的长径比为横坐标,G峰频移为纵坐标作图,进行线性拟合,得到了G峰频移与长径比成一定的线性递增关系。采用同样的分析方法,我们将G峰和D峰强度分别对MWCNT的长径比作图,进行线性拟合,得到了G峰和D峰强度分别与MWCNT的长径比成一定的线性递减关系。     

Aerosol generation and measurement of multi-wall carbon nanotubes

Mass production of some kinds of carbon nanotubes (CNT) is now imminent, but little is known about the risk associated with their exposure. It is important to assess the propensity of the CNT to release particles into air for its risk assessment. In this study, we conducted aerosolization of a multi-walled CNT (MWCNT) to assess several aerosol measuring instruments. A Palas RBG-1000 aerosol generator applied mechanical stress to the MWCNT by a rotating brush at feed rates ranging from 2 to 20 mm/h, which the MWCNT was fed to a two-component fluidized bed. The fluidized bed aerosol generator was used to disperse the MWCNT aerosol once more. We monitored the generated MWCNT aerosol concentrations based on number, area, and mass using a condensation particle counter and nanoparticle surface area monitor. Also we quantified carbon mass in MWCNT aerosol samples by a carbon monitor. The shape of aerosolized MWCNT fibers was observed by a scanning electron microscope (SEM). The MWCNT was well dispersed by our system. We found isolated MWCNT fibers in the aerosols by SEM and the count median lengths of MWCNT fibers were 4–6 μm. The MWCNT was quantified by the carbon monitor with a modified condition based on the NIOSH analytical manual. The MWCNT aerosol concentration (EC mass base) was 4 mg/m³ at 2 mm/h in this study.     

Structural Analysis of Collapsed, and Twisted and Collapsed, Multiwalled Carbon Nanotubes by Atomic Force Microscopy

A fully collapsed multiwalled carbon nanotube (MWCNT1) section and a different twisted and fully collapsed MWCNT were observed with tapping-mode atomic force microscopy. The collapsed section of MWCNT1 was significantly more flexible than the uncollapsed sections, and advanced 120 nm within 1 month. The collapse of MWCNT1 was most likely initiated by its interaction with the surface, and possibly a water meniscus. The ability of carbon nanotubes to radially deform under the influence of surface interactions is in striking contrast with their extremely high axial rigidity.     

A comprehensive study of sound pressure in a finite-length fluid-filled multi-walled carbon nanotube

The aim of this paper is to analyze vibrational behavior and the sound wave propagation in the finite-length fluid-filled multi-walled carbon nanotubes (MWCNTs) and to determine the exact sound pressure load effect on it, and compare it to what has been used by the other researchers. For this purpose, the solution of the modified complex Helmholtz equation is derived by considering the non-rigidity of the CNT and the wave reflections at the open ends of the MWCNT. These investigations are very important for potential application of CNT-filled polymeric foam that is used as sound absorber. In this paper, in formulating the sound pressure load exerted on the innermost tube of the finite-length fluid-filled MWCNT, the following points have been studied for the first time: (i) the energy loss in the fluid, which cannot be ignored in the high frequency analysis; (ii) the non-rigidity of the MWCNT through considering finite acoustical impedance for its walls; (iii) the wave reflections at the open ends of the finite-length MWCNT to calculate the sound pressure load term which is coupled with the dynamic equations of motion for the finite-length fluid-filled MWCNT. The results show that ignoring the mentioned points would cause errors in the prediction of the sound pressure load exerted on the finite-length fluid-filled MWCNT.     

Milling and dispersion of multi-walled carbon nanotubes in texanol

Rheological results were used to determine the optimum type of dispersant and its concentration for six commercial dispersants for the dispersion of multi-walled carbon nanotube (MWCNT) agglomerates in texanol. An unsaturated polycarboxylic acid copolymer (BYK P-104) exhibited the optimum performance with the lowest MWCNT slurry viscosity in texanol. The cutting and dispersion efficiencies of MWCNTs with 20 wt.% of BYK P-104 dispersant were compared using conventional ball milling and high energy milling, whereby the latter was found to be more effective. High energy milling for 2 h produced a large portion of MWCNT agglomerates smaller than 150 nm, showing a drastic increase in slurry viscosity due to the dispersion into individual CNTs. On the other hand, 120 h ball milling was required to achieve the agglomerate size of 300 nm with less viscosity increase upon milling. Decrease in the degree of MWCNT crystallinity was observed by both milling, even though 2 h high energy milling showed slightly less damage than 120 h ball milling based on XRD and Raman spectroscopy results.     

Mapping the resonance wavelengths of MWCNT as an optical nanoantenna

We investigated the surface plasmon resonances of multi-wall carbon nanotube (MWCNT) for applications as the optical antenna. We calculated the near-field and far-field response of MWCNT using finite integral technique. In addition, the effect of shape and dimensions on the optical response of MWCNT was studied. Also, the dielectric properties of MWCNT obtained from the experimental results in the literature were fitted with a Drude–Lorentz model. Finally, a full mapping of the geometry (length and radius) dependence for MWCNT was presented and discussed.     

多壁碳纳米管阵列场发射研究

研究了Ar离子束轰击及温度对多壁碳纳米管阵列场发射性能的影响.经Ar离子轰击35min后，发现阵列顶端的Fe催化剂颗粒明显减少，弯曲的顶部被轰击掉，使碳纳米管的场发射电流明显减小而场发射像无明显改变.温度的增加引起碳纳米管的场发射电流也随之增加.还研 究了在透明阳极技术中涂在阳极的荧光粉对场发射电流的影响.对同一碳纳米管阵列样品，发现涂有荧光粉的透明阳极使测量到的场发射电流大幅度减小，只是未涂荧光粉阳极电流的 1/30左右.直接用二氧化锡导电膜作阳极时，测得样品的开启场强为1.0V/μm.沉积了荧光粉的二 关键词： 多壁碳纳米管 场发射     

碳纳米管端口的场蒸发

将单根多壁碳纳米管(multi-walled carbon nanotube,MWCNT)组装在W针尖上并送入超高真空场发射/场离子显微镜(Ultrahigh Vacuum Field-emission/Field-Ion microscope,UHV-FEM/FIM)进行场蒸发及场发射研究.结果表明,场蒸发可以降低MWCNT的逸出功,从而增强其场发射能力.估算MWCNT的蒸发场低于1.3×10⁸V·cm^-1,且在此场强下的平均蒸发速率为9.4nm·min^-1.定性讨论了MWCNT的蒸发场大大低于C的理论值的原因.首先,通过场解吸获得的清洁端口上有较多悬挂键,平均每个C原子的配位数较小,所以升华热较低.其次,可能存在于MWCNT中的H原子会在强场下碰撞端口的C原子,使其更易蒸发.以上结果显示了利用场蒸发剪短碳纳米管从而改善其场发射特性的可行性. 关键词： 碳纳米管 场蒸发 场发射     

Fabricating and improving properties of copper matrix nanocomposites by electroless copper-coated MWCNTs

In this study, copper (Cu) nanocomposites reinforced by coated multiwall carbon nanotubes (MWCNTs) have been fabricated with different weight fractions of MWCNT. In the first step, the as-received MWCNTs were coated with Cu using electroless deposition process. In the next step, combination of sonication and ball milling (with two milling time of 1.5 and 3 h) was used for preparing MWCNT/Cu composite powders. Finally, the disk-shaped specimens were sintered by hot-press sintering machine. Characterization of sintered nanocomposites revealed that increasing milling time led to improved mechanical properties, but higher defect density on the MWCNT sidewalls is obtained which is especially undesirable for electrical properties of nanocomposite. Our results indicated that simultaneous improvements of interface reactions and distribution uniformity of MWCNTs and Cu are key factors for obtaining enhanced mechanical properties. Accordingly, enhancement of up to ~150 and ~86 % in microhardness compared to pure Cu and 1 wt% as-received MWCNT/Cu was achieved by addition of 1 wt% Cu-coated MWCNT. On the contrary, existence of oxygen atoms in the Cu and coated MWCNT interface (from functional groups and deposited copper oxide) obstructs considerable improvement of electrical resistivity compared to as-received MWCNT/Cu nanocomposites.     

聚苯胺/碳纳米管复合体的制备及其三阶非线性光学性能研究

通过原位吸附-受限生长聚合法成功制备了聚苯胺/碳纳米管(PANI/MWCNT)复合体.红外光谱和拉曼光谱证实了在碳纳米管(MWCNT)表面的包覆层为聚苯胺(PANI).紫外—可见吸收光谱表明随着MWCNT含量的增加PANI的吸收发生红移且强度提高.扫描电子显微镜和透射电子显微镜观察发现，PANI/MWCNT复合体直径为40—70nm，其中PANI的包覆层厚度约为15—20nm.利用四波混频方法测试它们的三阶非线性光学系数，结果发现PANI/MWCNT复合体的三阶非线性光学系数比纯PANI大，这说明在MWC 关键词： 碳纳米管 聚苯胺 复合体 三阶非线性光学系数     

Facile synthesis of a sulfur/multiwalled carbon nanotube nanocomposite cathode with core–shell structure for lithium rechargeable batteries

A novel sulfur/multiwalled carbon nanotube nanocomposite (S/MWCNT) was prepared by a facile quasi-emulsion template method in an O/W system. Transmission and scanning electronic microscopy show the formation of a highly developed core–shell tubular structure consisting of S/MWCNT composite with uniform sulfur coating on its surface. The homogenous dispersion and integration of MWCNT in the S/MWCNT composite create a highly conductive and mechanically flexible framework, enhancing the electronic conductivity and consequently the rate capability of the material. The S/MWCNT composite cathode could deliver a stable discharge (the fifth cycle) capacity of about 903 mAh g^?1 at 0.1 C, 751 mAh g^?1 at 0.5 C, and 631 mAh g^?1 at 1 C.     

Effect of dispersion state of carbon nanotube on the thermal conductivity of poly(dimethyl siloxane) composites

Herein, the effect of dispersion uniformity of multi-walled carbon nanotube (MWCNT) on the thermal conductivity of poly(dimethyl siloxane) (PDMS) composites was investigated by comparing experimentally obtained and calculated results based on simple models. Two different MWCNTs, i.e., raw and oxidized/masterbatched MWCNTs, were used and compared. For raw MWCNT, the dispersion in PDMS was poor, resulting in the significant reduction in the aspect ratio of MWCNT. However, for composites using masterbatched MWCNT, the thermal conductivity was always about 10% greater than those prepared with raw MWCNT and the aspect ratio calculated by the model equation was also 1.7 times greater. Above 1.5 phr masterbatched MWCNT concentrations, the aspect ratio of 430 was maintained. Finally, the results suggest that the thermal conductivity can be correlated with the degree of dispersion and aspect ratio obtained from the model equation used.     

Experimental study on enhanced heat transfer of nanocomposite phase change materials

Nanocomposite phase change materials (NCPCMs) containing different mass fractions (nanomaterial concentration) and different copper nanoparticle (CN)/multi-walled carbon nanotubes (MWCNT) mass ratios were prepared and experimentally studied. Latent heat and thermal conductivity of the NCPCMs were studied and calculated by using the T-history method. The results revealed that addition of CN or MWCNT to the phase change material (PCM) resulted in NCPCMs with enhanced thermal conductivity and charge rates, respectively. However, when both nanoparticle materials were added to the PCM simultaneously, the resulting NCPCMs improved their thermal performance below expectations as a result of agglomeration and sedimentation between the two additives. Thus, the NCPCMs containing only CN or MWCNT showed superior thermophysical properties than the pure PCM, while the NCPCM containing both CN and MWCNT did not improve the thermal characteristic of PCM significantly.     

Electrical,optical and fluorescence percolations in P(VAc-co-BuA)/MWCNT composite films

Effects of multiwall carbon nanotube (MWCNT) addition on the electrical conductivities, optical transparencies and fluorescence emissions of poly(vinyl acetate-co-butyl acrylate) (P(VAc-co-BuA))/MWCNT composite films were studied. Optical transmission, fluorescence emission and two point probe resistivity techniques were used to determine the variations of the optical, fluorescence and electrical properties of the composites, respectively. Transmitted photon intensity (I _tr), fluorescence emission intensity (I _fl) and surface resistivity (ρ _s) of the composite films were monitored as a function of MWCNT mass fraction (M) at room temperature. All these measured quantities of the composites were decreased by increasing the content of MWCNT in the composite. The conductivity and the optical results were attributed to the classical and site percolation theories, respectively. The fluorescence results, however, possessed both the site and classical percolation theories at low and high MWCNT content regions, respectively.     

Characterisation of carbonaceous materials using Raman spectroscopy: a comparison of carbon nanotube filters,single‐ and multi‐walled nanotubes,graphitised porous carbon and graphite

Multi‐walled carbon nanotube (MWCNT) filters have been recently synthesised which have specific molecular filtering capabilities and good mechanical strength. Optical and scanning electron microscopy (SEM) reveals the formation of highly aligned arrays of bundles of carbon nanotubes having lengths up to 500 µm. The Raman spectra of this material along with four other carbonaceous materials, commercially available single‐walled carbon nanotubes (SWCNTs) and MWCNTs, graphitised porous carbon (Carbotrap) and graphite have been recorded using two‐excitation wavelengths, 532 and 785 nm, and analysed for band positions and shape with special emphasis paid to the D‐, G‐ and G′‐bands. A major difference between the different MWCNT varieties analysed is that G‐bands in the MWCNT filters exhibit almost no dispersion, whereas the other MWCNTs show a noticeable dispersive behaviour with a change in the excitation wavelength. Spectral features similar to those of the MWCNT filter varieties were observed for the Carbotrap material. From the line shape analysis, the intensity ratio, I_D/I_G, of the more ordered MWCNT filter material using the integral G‐band turns out to be two times lower than that of the less ordered MWCNT filter product at both excitation wavelengths. This parameter can, therefore, be used as a measure of the degree of MWCNT alignment in filter varieties, which is well supported also by our SEM study. Copyright © 2008 John Wiley & Sons, Ltd.     

Enhanced rate performance of polypyrrole-coated sulfur/MWCNT cathode material: a kinetic study by electrochemical impedance spectroscopy

Lithium-sulfur batteries have a poor cyclability and inferior rate capability due to the shuttle effect of lithium polysulfides. To solve these problems, a sulfur-coated MWCNT composite (S/MWCNT) was coated with conductive polypyrrole (PPy) to trap the polysulfides and facilitate charge and lithium ion transport. From the contact angle measurement, it is found that the PPy coating improves the wettability of the S/MWCNT composite. Compared with the bare S/MWCNT composite, the PPy-coated S/MWCNT composite cathode exhibited improved cycle stability and high-rate performance. A reversible discharge capacity of 671 mAh g^?1 was maintained after 50 cycles at 3 C for the PPy-coated composite. The effect of PPy coating on kinetic property was investigated by electrochemical impedance spectroscopy (EIS). The electrolyte resistance, surface film resistance, charge transfer resistance, lithium ion diffusion coefficient, and exchange current density were evaluated from the EIS measurements. The EIS results reveal that the PPy coating increases both Li ion diffusion into the cathode and exchange current density. The as-prepared PPy-coated S/MWCNT composite can be considered to be a promising candidate for high capacity and high-rate performance cathode material.     

Evaluation of thermal and flame properties of HDPE-MWCNT-SiO2 nanocomposites

High-density polyethylene (HDPE) composites reinforced with multiwalled carbon nanotubes (MWCNTs) and nano-silicon dioxide (SiO₂) fillers were evaluated for flame retardancy and thermal properties for cable and wire applications. In this study, the filler percentages of MWCNT and nano-SiO₂ have varied from 0 to 5 wt% in HDPE composite with polyethylene-grafted glycidyl methacrylate compatibilizer and 3-aminopropyl triethoxy silane coupling agent. Addition of MWCNT’s and nano-SiO₂ to the HDPE composite is observed to enhance the limiting oxygen index and char formation. Cone calorimeter results also show a 53% reduction in the peak heat release rate of the HDPE composite with 5 wt% of MWCNT. The existence of synergism between the uniformly dispersed MWCNT and nano-SiO₂ has been verified using Finite Element Method (FEM)-based thermal simulations.     

Synthesis and elevated temperature performance of a polypyrrole-sulfur-multi-walled carbon nanotube composite cathode for lithium sulfur batteries

A sulfur-multi-walled carbon nanotube composite (S/MWCNT) was prepared using a two-step procedure of liquid-phase infiltration and melt diffusion. Polypyrrole (PPy) conductive polymer was coated on the surface of the as-prepared S/MWCNT composite by in situ polymerization of pyrrole monomer to obtain PPy/S/MWCNT composite. The composite materials were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The electrochemical performance of the as-prepared cathode material was investigated at 25, 40, and 70 °C at various rates. It was found that temperature has dual effects on the performance of Li/S cells. Increasing the temperature, on one hand, facilitates the lithium ion transport through the cathode and, on the other hand, leads to faster dissolution of active material into the electrolyte. The PPy coating can effectively trap polysulfides in its porous structure, even at elevated temperatures, leading to the improvement of the discharge capacity, the cycle stability, and the coulombic efficiency. The electrochemical impedance spectroscopy (EIS) results reveal that the PPy coating reduces the formation of passive layer on the cathode surface, even at high temperatures, resulting in a better elevated temperature performance. A high reversible capacity of 945 mAh g^?1 was maintained after 50 cycles for the PPy/S/MWCNT composite at 70 °C at a rate of 0.5 C.     

Electrical Properties of Isotactic Polypropylene/Multiwalled Carbon Nanotubes Composites Prepared by Vibration Injection Molding

To study the effect of vibration field on the electrical conductivity properties of nanocomposites, isotactic polypropylene (iPP)/multiwalled carbon nanotubes (MWCNT) composites were prepared by conventional injection molding and vibration injection molding. Results showed that the electrical conductivity of iPP/MWCNT composites was significantly promoted by vibration injection molding. Vibration injection molded samples had a percolation threshold of about 2.7 wt% compared with the threshold of about 4.5 wt% for conventional injection molded samples. The effects of test locations and vibration frequency on the electrical conductivity of composites were investigated. The samples exhibited an inhomogeneity along the injection direction. The electrical conductivity of the samples was different at different test locations and increased with increasing vibration frequency. Polarized light microscopy (PLM) results indicated that vibration injection molding can induce MWCNT aggregates to be stretched and oriented along the flow direction, which could form conductive networks and greatly enhance the electrical conductivity of iPP/MWCNT composites.     

Facile in situ hydrothermal synthesis of BiVO4/MWCNT nanocomposites as high performance visible-light driven photocatalysts

Visible-light responsive monoclinic BiVO₄/MWCNT nanocomposites were facilely prepared via an in situ hydrothermal method by using sodium dodecyl sulfonate (SDS) as a guiding surfactant. The as-prepared BiVO₄/MWCNT nanocomposites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), the Fourier transform infrared spectroscopy (FTIR) and UV–vis diffuse reflectance spectroscopy. The results showed that the hydrothermal temperature and adding SDS had significant influence on the morphology and size of BiVO₄. The photocatalytic activities of BiVO₄/MWCNT nanocomposites were investigated by degrading methylene blue (MB) under visible-light irradiation. Remarkable enhancement in photodecomposition of MB was observed with BiVO₄/MWCNT composite compared with bare BiVO₄ particles. This improvement of photocatalytic was attributed to the effective charge transfer from BiVO₄ nanocrystals to MWCNT, which promoted the migration efficiency of photogenerated electron–hole. Furthermore, a possible mechanism for the photocatalytic oxidative degradation was also discussed.