碳纳米管悬浮液在重力热管中的沸腾特性

为研究纳米流体的沸腾传热效果,进行了以碳纳米管充装的重力热管的沸腾特性实验.以碳纳米管悬浮液为工质的重力热管的起沸温度、温度漂移及蒸发段管壁温度比水工质热管的高,热管的热阻增大,换热性能恶化.通过测定,水中添加碳纳米管颗粒后悬浮液的表面张力增大,使得加热壁面的活化成核点密度、数量、脱离频率、气泡体积、气泡间聚合能力发生改变,气液界面处的温度梯度和浓度梯度引发Marangoni流动,易蒸发组分水在界面处的蒸发产生质扩散效应.几种因素的共同作用使热管蒸发段管壁和热管的启动温度上升,热阻增大,热管的传热性能恶化.     

Polymer/carbon nanotube composite film formation: A fluorescence study

In this study, the effect of multi‐walled Carbon nanotube (MWNT) on film formation behavior of Polystrene (PS) latex film was investigated by using steady state fluorescence technique. Films were prepared by mixing of pyrene (P)‐labeled PS latex with different amounts of MWNTs varying in the range between 0 and 20 wt%. After drying, MWNT containing films were separately annealed above glass transition temperature (T_g) of PS ranging from 100 to 270°C for 10 min. In order to monitor film formation behavior of PS/MWNT composites, Scattered light (I_s) and fluorescence intensities (I_P) from P were measured after each annealing step to monitor the stages of film formation. At 0–20 wt% range of MWNT content films, minimum film formation (T_o), void closure (T_v), and healing, (T_h) temperatures were determined. Void closure and interdiffusion stages were modeled and related activation energies were determined. It was observed that while void closure activation energies increased, backbone activation energies decreased as the percent of MWNT is increased in the composite films. POLYM. COMPOS., 35:817–826, 2014. © 2013 Society of Plastics Engineers     

The role of nitrogen in carbon nanotube formation

To examine the role of nitrogen, Co- and Ni-coated substrates were pretreated with three different gas compositions to compare the pretreated catalyst surfaces; the Fe, Co and Ni foils were subjected to carbon nanotube (CNT) growth experiments with CH₄/H₂ and CH₄/N₂ as source gases; the catalyst pretreatment plus the CNT growth experiments on Co- and Ni-coated Si substrates were carried out using both microwave plasma chemical vapor deposition and electron cyclotron resonance chemical vapor deposition (ECR-CVD) under different nitrogen-containing gases. The results show that the role of nitrogen may be summarized as follows: by comparing with hydrogen plasma, the bombardment energy of nitrogen plasma is greater. Therefore, the presence of nitrogen during CNT growth can keep the front catalyst surface clean and active to prolong surface passivation to enhance carbon bulk diffusion. The higher temperature due to higher bombardment energy of nitrogen plasma can promote agglomeration effects during catalyst pretreatment and the initial stage of CNT growth to produce larger size nano-particles. The presence of nitrogen is a favorable condition for formation of the bamboo-like CNTs, but not a necessary condition. Another favorable condition for formation of the bamboo-like CNTs is to deposit CNTs by ECR-CVD.     

A novel fabrication approach for carbon nanotube lateral field emission devices

A novel fabrication approach for growing carbon nanotubes (CNTs) laterally and selectively on the tip region of lateral micro-fingers with built-in metallic anode utilizing a single-mask microfabrication process is presented. The selective growth of the CNTs was achieved with a two-step microwave plasma-enhanced chemical vapor deposition process involving a pre-growth hydrogen plasma treatment. Without plasma pretreatment, CNTs were found to grow along edges of the sandwiched tri-metal layer including the anode. Interestingly, with plasma pretreatment, CNTs grew selectively near the sharp tip region. Moreover, specific CNTs could be selectively synthesized on the tip region by optimizing the plasma pretreatment and the growth time. In essence, a lateral field emission device having CNT emitters with integrated metallic anode can be fabricated in just a single-mask microfabrication process. This approach can enhance the feasibility of integrating CNTs into vacuum integrated circuits.     

Influence of the gas pressure on single-wall carbon nanotube formation

Experiments and modeling are performed to predict the effect of gas pressure on species distributions and nanotube growth rate under specific conditions of synthesis of single-wall carbon nanotubes (SWCNTs) by arc discharge. Numerical results are compared with experiments in order to find a consistent correlation between the nanotube growth and the pressure. We use argon and helium as buffer gases with a total pressure varied between 0.1 and 1 bar. We experimentally observe that both the anode erosion rate and the Brunauer-Emmett-Teller (BET) surface area of the as-produced nanotube soot material are very sensitive to the total gas pressure in the reactor.     

Aligned millimeter-long carbon nanotube arrays grown on single crystal magnesia

Single crystal magnesium oxide (MgO) was found to be very beneficial to the growth of aligned carbon nanotube (CNT) arrays as long as 2.2 mm by chemical vapor deposition. Before growth, a thin film of catalyst (iron) was coated on the MgO by magnetron sputtering. Scanning electron microscopy was used to study the alignment and length, and transmission electron microscopy was used to exam the wall numbers, diameter, and graphitization. It was found that the number of walls as few as two can be controlled by the catalyst film thickness, whereas the length is a combined result of gas pressure, temperature, and time during growth. Water was found not to be a factor to the length of CNTs grown on MgO, but a significant factor when sapphire was used as the substrates.     

Dual frequency conductivity switching in a carbon nanotube/liquid crystal composite

We report dielectric and conductivity measurements on a composite of carbon nanotubes and a liquid crystal possessing dual frequency switching characteristics. The conductivity increases by two orders of magnitude with respect to that for the host liquid crystal, and achieves negligible temperature dependence. The frequency dependence of the ac conductivity is explained by the extended pair approximation model, although the exponent is slightly higher than generally seen. We demonstrate that the current through the sample can be field-driven between the two anisotropic values (170:1) by simply changing the frequency of the applied voltage, and exhibiting at least a millisecond response.     

Multiwall carbon nanotube elastomeric composites: A review

Nanostructured materials gained great importance in the past decade on account of their wide range of potential applications in many areas. A large interest is devoted to carbon nanotubes that exhibit exceptional electrical and mechanical properties and can therefore be used for the development of a new generation of composite materials. Nevertheless, poor dispersion and poor interfacial bonding limit the full utilization of carbon nanotubes for reinforcing polymeric media.In this paper, recent advances on carbon nanotubes and their composites will be presented through results of the author's research, essentially based on filled elastomeric networks. The intrinsic potential of carbon nanotubes as reinforcing filler in elastomeric materials will be demonstrated. It will be shown that, despite a poor dispersion, small filler loadings improve substantially the mechanical and electrical behaviors of the soft matrix. With the addition of 1 phr of multiwall carbon nanotubes in a styrene-butadiene copolymer, a 45% increase in modulus and a 70% increase in the tensile length are achieved. Straining effects investigated by atomic force microscopy and infrared and Raman spectroscopies, provide interesting results for the understanding of the mechanical behavior of these nanotube-based composites. All the experimental data lead to the belief that the orientation of the nanotubes plays a major role in the mechanical reinforcement. The strong restriction in equilibrium swelling in toluene with the MWNT content is not ascribed to filler-matrix interfacial interactions but to the occlusion of rubber into the aggregates. On the other hand, carbon nanotubes impart conductivity to the insulator matrix. Between 2 and 4 phr, the conductivity increases by five orders of magnitude reflecting the formation of a percolating network. Changes in resistivity under uniaxial extension completed by AFM observations of stretched composites bring new insights into the properties of these composites by highlighting the contribution of orientational effects.     

Effect of carbon nanotube dispersion and network formation on thermal conductivity of thermoplastic polyurethane/carbon nanotube nanocomposites

Carbon nanotubes (CNTs) were dispersed without any solvent in poly(tetramethylene ether glycol), (PTMEG) well above its melting point by ultrasonication in the pulse mode and different times. The polyol/CNT suspensions were used to prepare in situ polymerized thermoplastic polyurethane TPU/CNT nanocomposites with the CNT concentration of ～ 0.05 vol%, much below the CNT geometrical percolation threshold calculated at 0.43 vol%. Results of rotational rheological measurements and ultraviolet–visible (UV‐Vis) spectroscopy analysis revealed improvement in the nanoscale CNT dispersion with sonication time. Moreover, the optical microscopic images and sedimentation behavior for these samples pointed out to the formation of segregated CNT networks with different microstructures at different sonication times. Through‐plane thermal conductivity measurements showed an increase in thermal conductivity of the in‐situ polymerized TPU/CNT nanocomposites from polyol/CNT suspensions with increasing sonication time followed by a decrease at long sonication times. Different models were used to evaluate the role of CNT dispersion state and created microstructure on thermal conductivity of nanocomposites. The formation of a segregated network at medium sonication times consisting of large CNT aggregates and small bundles increased the nanocomposite thermal conductivity up to 99.7%, while at longer sonication times, an increase in interfacial area with a corresponding increase in kapitza boundary resistance, effectively decreased the system thermal conductivity. POLYM. ENG. SCI., 56:394–407, 2016. © 2016 Society of Plastics Engineers     

Ion irradiation for improved graphene network formation in carbon nanotube growth

Ion irradiation of carbon nanotubes very often leads to defect formation. However, we have recently shown that Ar ion irradiation in a limited energy window of 10–25 eV may enhance the initial cap nucleation process, when the carbon network is in contact with the metal nanocatalyst. Here, we employ reactive molecular dynamics simulations to demonstrate that ion irradiation in a higher energy window of 10–35 eV may also heal network defects after the nucleation stage through a non-metal-mediated mechanism, when the carbon network is no longer in contact with the metal nanocatalyst. The results demonstrate the possibility of beneficially utilizing ions in e.g. plasma-enhanced chemical vapour deposition of carbon nanotubes.     

A modified spray-winding approach to enhance the tensile performance of array-based carbon nanotube composite films

A carbon nanotube (CNT) array based spray-winding approach for CNT film fabrication was developed by adding a post hot-pressing process, and an epoxy solution was used to fabricate CNT/epoxy composite film. It showed that the hot-pressing process benefited the load transfer within CNT films by reducing the porosity among CNT bundles and was more efficient in improving the tensile properties of few wall CNT films. The epoxy modified multi-wall CNT film exhibited a tensile strength and modulus of 1540 MPa and 59 GPa, respectively. From the results of scanning electron microscopy and measurements of contact angle, Raman spectroscopy and thermogravimetric analysis, the main mechanism of the improvement was attributed to good wettability of CNT film with epoxy, high degree of CNT alignment, and high CNT load in the CNT film.     

Mechanism of cone-shaped carbon nanotube bundle formation by plasma treatment

Formation of the cone-shaped multi-walled carbon nanotube (MWCNT) bundles was investigated with the consideration of the induced dipole moments of the MWCNTs interaction under the ion irradiation which is accelerated by the applied sheath electric field for the various argon, hydrogen, nitrogen, and oxygen plasmas. Vertically grown MWCNTs were irradiated by energetic ion whose energy and dose were controlled by the sheath formed on the MWCNT substrate. Plasma irradiation was carried out in a downstream region separated from the plasma source region, providing that the irradiated ion density and energy could be controlled precisely with the sheath electric field. In argon and hydrogen plasmas, the cone-shaped MWCNT bundle was not fabricated, while it was formed successfully in nitrogen and oxygen plasmas. Especially, the oxygen plasma was the most effective in the formation of the bundle. The mechanism of the bundle formation could be explained by a model explaining the interaction between the induced dipole moment of the MWCNT and the sheath electric field. For the nitrogen and oxygen plasma irradiated MWCNT, the induced dipole moment could be enhanced by C-N and C-O bonds so the size of the bundle is proportional to the ion irradiation and the sheath electric field.     

Destruction and formation of a conductive carbon nanotube network in polymer melts: In-line experiments

Investigations on electric conductivity and dielectric permittivity have been performed during melt processing of polycarbonate (PC) and polyamide 6 (PA6) containing different amounts of multi-walled carbon nanotubes (MWNT). For the experiments a measurement slit die containing two electrodes in capacitor geometry was flanged to the outlet of a twin-screw extruder. AC conductivity and the related complex permittivity were measured in the frequency range from 21.5 to 10⁶ Hz for different processing conditions (melt temperature and throughput) and after stopping the extruder. It was found that the conductivity dropped down to values typical for the matrix polymer when the extrusion started. After the extruder was stopped the conductivity shows an increase of up to eight orders of magnitude with time. This conductivity recovery in the rest time after mechanical deformation was found to be faster for increasing melt temperature or samples with higher CNT concentration. The increase of the conductivity in the quiescent melt is explained by reorganization of the conductive network-like filler structure, which was - at least partially - destroyed under mechanical deformation. The reformation kinetics of the conductive network after mechanical deformation is considered to be an agglomeration process, which can be approximated by a combination of cluster aggregation and percolation theory.     

A synergistic approach to light-free catalysis using zinc oxide embedded multi-walled carbon nanotube paper

Nanoparticle embedded carbon nanotube papers can produce flexible functional films, whose function is based on the electronic coupling of the nanoparticles with the network of carbon nanotubes. Here we report on the expansion of a synergistic light-free catalyst system that requires both the molecular wire capability of multi-walled carbon nanotubes and the piezoelectric properties of ZnO nanoparticles. The electron–hole pairs can be induced by the piezoelectric effect of ZnO nanoparticles under the effect of pressure. The strong oxidizing radicals interact with the electron–hole pairs and aqueous solution to induce the degradation of methylene blue without the requirement of light. This study highlights the use of nanoparticles for the piezoelectric – light free catalyst system and the limitations of increased nanoparticle loading on the catalytic degradation of methylene blue.     

On-line detection of single-walled carbon nanotube formation during aerosol synthesis methods

Differential electrical mobility (DMA) method for the on-line detection of single-walled carbon nanotubes (SWCNTs) formation was used for the first time. Three different gas-phase synthesis processes were used to produce SWCNTs via CO disproportionation in the presence of catalyst nanoparticles formed either by a hot wire generator method or via thermal decomposition of ferrocene or iron pentacarbonyl. The typical product measured with the DMA method was bundles of SWCNTs, which further agglomerated prior to the measurement. Despite the different product morphology and concentration, the on-line measurement was able to distinguish SWCNT formation in each experimental set-up as an increase in the geometric mean particle diameter and as a decrease in the total particle number concentration. Furthermore, information regarding the relative SWCNT concentration can also be obtained from the DMA measurement. A theoretical approach to the mobility of nonspherical particles in the electric field was successfully developed in order to convert the electrical mobility size of the high aspect ratio SWCNTs measured with DMA to the physical size of the product. Size-selected SWCNTs were studied with transmission electron microscopy in order to find the correlation between the on-line DMA measurement data and the SWCNT morphology.