Additive manufacturing of CNTs/PLA composites and the correlation between microstructure and functional properties

Owing to the facile,low cost,rapid,personalization characters,3D printing method has been one of the most attractive additive manufacturing processes in medicine,airplane,packaging and printing areas.In this work,a series of carbon nanotubes/polylactic acid(CNTs/PLA) composites were prepared through the combination of molten co-extrusion and 3D printing processes.The orientation and dispersion of CNTs in PLA matrix were investigated to explore the impact of 3D printing process on the morphology of CNTs/PLA composites via transmission electron microscopy,field emission scanning electron microscopy and Raman spectroscopy.X-ray diffractometer,differential scanning calorimetry,and thermal gravity analysis were employed to study the crystal structure and thermal properties of the composites.In addition,the electrical conductivity of the prepared specimen revealed that the orientation of CNTs in PLA might enhance the conductivity of the composite.It was found that 3D printing process was beneficial to increasing the purity of CNTs,electrical conductivity and mechanical properties of CNTs/PLA composites.     

Robust reduction of graphene fluoride using an electrostatically biased scanning probe

We report a novel and easily accessible method to chemically reduce graphene fluoride （GF） sheets with nanoscopic precision using high electrostatic fields generated between an atomic force microscope （AFM） tip and the GF substrate. Reduction of fluorine by the electric field produces graphene nanoribbons （GNR） with a width of 105-1,800 nm with sheet resistivity drastically decreased from 〉1 TΩ.sq.^-1 （GF） down to 46 kΩ.sq.^-1 （GNR）. Fluorine reduction also changes the topography, friction, and work function of the GF. Kelvin probe force microscopy measurements indicate that the work function of GF is 180-280 meV greater than that of graphene. The reduction process was optimized by varying the AFM probe velocity between 1.2 μm.s^-1 and 12 μm.s^-1 and the bias voltage applied to the sample between -8 and -12 V. The electrostatic field required to remove fluorine from carbon is -1.6 V.nm-1. Reduction of the fluorine may be due to the softening of the C-F bond in this intense field or to the accumulation and hydrolysis of adventitious water into a meniscus.     

Functionally Graded Dual-nanoparticulate-reinforced Aluminium Matrix Bulk Materials Fabricated by Spark Plasma Sintering

Functionally graded（FG） carbon nanotubes（CNT） and nano-silicon carbide（nSiC） reinforced aluminium（Al）matrix composites have been successfully fabricated using high-energy ball milling followed by solid-state spark plasma sintering processes.The CNTs were well-dispersed in the Al particles using the nSiC as a solid mixing agent.Two different types of multi-walled CNTs were used to add different amounts of CNTs in the same volume.The ball milled Al—CNT—nSiC and Al—CNT powder mixtures were fully densified and demonstrated good adhesion with no serious microcracks and pores within an FG multilayer composite.Each layer contained different amounts of the CNTs,and the nSiC additions showed different microstructures and hardness.It is possible to control the characteristics of the FG multilayer composite through the efficient design of an Al—CNT—nSiC gradient layer.This concept offers a feasible approach for fabricating the dualnanoparticulate-reinforced Al matrix nanocomposites and can be applied to other scenarios such as polymer and ceramic systems.     

High-detectivity InAs nanowire photodetectors with spectral response from ultraviolet to near-infrared

InAs is a direct, narrow band gap （0.354 eV） material with ultrahigh electron mobility, and is potentially a good optoelectronic device candidate in the wide UV-visible-near-infrared region. In this work we report the fabrication of InAs nanowire-based photodetectors, which showed a very high photoresponse over a broad spectral range from 300 to 1,100 nm. The responsivity, external quantum efficiency and detectivity of the device were respectively measured to be 4.4 × 103 AW , 1.03 × 106%, and 2.6 × 1011 Jones to visible incident light. Time dependent measurements at different wavelengths and under different light intensities also demonstrated the fast, reversible, and stable photoresponse of our device. Theoretical calculations of the optical absorption and the electric field component distribution were also performed to elucidate the mechanism of the enhanced photoresponse. Our results demonstrate that the single-crystalline InAs NWs are very promising candidates for the design of high sensitivity and high stability nanoscale photodetectors with a broad band photoresponse.     

Effects of Pressure on the Performance of CNTs-Supported Catalyst in a Fischer-Tropsch Reaction

Fischer-Tropsch （FT） reaction involves conversion of syngas （a mixture of carbon monoxide and hydrogen） into higher hydrocarbons in the presence of an active catalyst. The syngas can be derived from non-petroleum feedstocks such as coal, biomass and natural gas, thus the FT reaction provides an alternative route for production of clean fuels. The FT process has received growing interest in recent years due to uncertainty in the Middle East, fast depletion of fossil fuel and environmental concern. This paper reports the synthesis, physicochemical properties and catalytic performance of cobalt-based catalyst in the FT reaction. The catalysts comprised metal nanoparticles supported on carbon nanotubes （CNTs） which were synthesized via a wet impregnation method. The catalysts were characterized using transmission electron microscopy （TEM）, temperature-programmed reduction/desorption （TPR/TPD） and X-ray photoelectron spectroscopy （XPS）. The performance of the cobalt-based catalyts in a FT reaction was evaluated in a fixed bed microreactor equipped with an on-line gas chromatograph for analyses of hydrocarbon products. The catalysts investigated in this work were Co/CNTs, 70Co30Mn/CNTs, 0.06%K/70Co30Fe/CNTs and 0.04%Nb/70Co30Fe/CNTs. TEM analyses revealed that the average sizes of the metal nanoparticles were 4-5 nm. Based on TPD analyses, the dispersion of these nanoparticles on CNTs were greater than 90%. The presence of both Co2＋ and Co3＋ ions were confirmed by XPS analysis. The 0.04% Nb/70Co30Fe/CNTs catalyst performed better than other catalysts in the FT reaction where it resulted in CO conversion of 35% and 16% C5＋ selectivity at pressure of 1 bar, 220 ~C and H2：CO of 2：1. Using the same catalyst, the CO conversion and C5＋ selectivity increased to 60% and 57%, respectively when the pressure was increased to 20 bar.     

Low-temperature Synthesis of Large-area Films of Molybdenum Trioxide Microbelts in Air and the Dependence of Their Field Emission Performance on Growth Conditions

A simple method is introduced for the preparation of large-area films of molybdenum trioxide (MoO3) microbelts. It is found that such films can be grown on indium tin oxide (ITO) glasses or silicon substrates at low temperatures by thermal evaporation deposition in air without using catalyst. Field emission measurements show that the turn-on field of the MoO3 microbelts is as low as 2.2 V/μm required to obtain a current density of 10 μA/cm2. The combination of the simplicity of the growth method and the att...     

Mono-disperse SrMoO_4 nanocrystals: Synthesis,luminescence and photocatalysis

Well-dispersed uniform SrMoO_4 nanocrystals were synthesized by thermal decomposition of a metalorganic salt in the organic solvent under different temperatures(80, 100, 120, 140, and 160?C). The smallest diameter of these SrMoO_4 nanocrystals is only about 2 nm, which is regarded as the smallest values to date. The UV–vis absorbance spectra present that the larger absorption of our samples is mainly distributed in the visible light region and UV light region. The lowest energy gap is found to be 2.71 e V.Such a small gap is ascribed to the introduction of intermediate energy levels, which are due to the surface defects with decreasing the size of nanostrcutrues. The photoluminescence measurement suggests that all these samples exhibit a board and strong emission band in the range from 500 to 700 nm. Through the deconvolution of the photoluminescence spectra, the emission profiles are found to be associated with three various components(green, yellow, and red). Moreover, the photodegration of methyl blue over our SrMoO_4 samples reaches nearly 100% in 120 min. Such a high photodegration may be related to the following aspects. One is related to the size and morphology. Larger surface area leads to more absorption of methyl blue, and the small size nanoparticles lead to the efficient separation of these photogenerated electron-hole pairs. The other is related to the narrow band gap. The small gap is beneficial to more electrons to be excited from the valence band to the conduction band, and eventually more electron-hole pairs are created. Our investigations clearly suggest that thermal decomposition of one metal-organic salt in organic solvent will be a good choice to synthesize the nanoparticles with small size and uniform distribution. Our results also indicate that these SrMoO_4 nanoparticles possibly have great potential utilities in photocatalysts.     

Morphological,Optical and DC Conduction Properties of a-GaSe Semiconductor Nanoparticle Thin Films

Amorphous gallium selenide （a-GaSe） semiconductor nanoparticle thin films were deposited onto well cleaned glass substrates by inert gas condensation （IGC） technique under a vacuum of 400 x 10-6 Pa （3 x 10-6 Torr）. The films were characterized by different structural and optical techniques, including X-ray diffraction, field emission scanning electron microscopy （FESEM）, field emission transmission electron microscopy （FETEM）, UV-visible absorption spectroscopy and I-V measurements. The particle size and size distribution were determined by TEM images which show the presence of spherical particles in the range of 5-50 nm in size. SEM images indicate that the a-GaSe film grown on glass substrate is almost smooth and dense. The optical properties of a-GaSe nanoparticle thin films were determined by optical absorption spectra. The optical bandgap of the film was estimated to be 2.19 eV and the transitions are allowed direct type. The electrical conductivity of the deposited films has been studied as a function of temperature. In the higher temperature range the dominance of thermally activated band conduction was observed; whereas in the low temperature ranqe the hoppinQ conduction in the band tails of localized states was found to be dominated.     

Tunable order in colloids of hard magnetic hexaferrite nanoplatelets

Structural ordering in the concentrated magnetic colloids containing 50×5 nm hard magnetic disc-like SrFe₁₂O₁₉ nanoparticles was investigated by cryogenic scanning electron microscopy,optical microscopy,magnetic measurements,and small-angle X-ray scattering.It was revealed that macroscopically homogeneous magnetic liquid consists of dynamic threads of stacked nanoparticles.The threads align into quasiperiodic arrays with the distances between individual threads of a few micrometers.They also can form pseudodomain structures with -90°domain boundaries realized through T-type thread interconnects.The effects of magnetic attraction and electrostatic repulsion on the equilibrium interplatelet distance in the threads were studied.It was demonstrated that this distance can be tuned by the control of the particles charge and electric double layer screening from Stern layer thickness(-1 nm)to tens of nanometers.It was shown that the permanent magnetic field is not able to cause any structural changes in the ordered magnetic liquid phase,while alternating field draws particles apart by their vibrations.External variation of interparticle distance up to 6%was achieved using an alternating magnetic field of low intensity.Experimental data were complemented by the theoretical models of screened electrostatic interactions between spherical and platelike magnetic particles.The last model provides good predictive power and correlates with the experimental data.The stabilization energy of the condensed phase in the order of 1-10 kBT was derived from the model.An approach allows controlling of an equilibrium interparticle distance and interparticle distance distribution by adjusting the magnetization and surface charge of the particles as well as the ionic strength of the solvent.     

Hydrogen Storage Alloy and Carbon Nanotubes Mixed Catalyst in a Direct Borohydride Fuel Cell

In this study, carbon nanotubes (CNTs) were mixed with AB5-type hydrogen storage alloy (HSA), as catalyst for an anode in a direct borohydride fuel cell (DBFC). As comparision, a series of traditional carbon materials, such as acetylene black, Vulcan XC-72R, and super activated carbon (SAC) were also employed. Electrochemical measurements showed that the electrocatalytic activity of HSA was improved greatly by CNTs. The current density of the DBFC employing the HSA/CNTs catalytic anode could reach 1550 mA·cm-2 (at -0.6 V vs the Hg/HgO electrode) and the maximum power density of 65 mW·cm-2 for this cell could be achieved at room temperature. Furthermore, the life time test lasting for 60 h showed that the cell displayed a good stability.     

Enhanced field emission characteristics of nitrogen-doped carbon nanotube films grown by microwave plasma enhanced chemical vapor deposition process

Nitrogen-doped carbon nanotube (CNT) films have been synthesized by simple microwave plasma enhanced chemical vapor deposition technique. The morphology and structures were investigated by scanning electron microscopy and high resolution transmission electron microscopy. Morphology of the films was found to be greatly affected by the nature of the substrates. Vertically aligned CNTs were observed on mirror polished Si substrates. On the other hand, randomly oriented flower like morphology of CNTs was found on mechanically polished ones. All the CNTs were found to have bamboo structure with very sharp tips. These films showed very good field emission characteristics with threshold field in the range of 2.65-3.55 V/μm. CNT film with flower like morphology showed lower threshold field as compared to vertically aligned structures. Open graphite edges on the side surface of the bamboo-shaped CNT are suggested to enhance the field emission characteristics which may act as additional emission sites.     

碳纳米管表面化学镀银及场发射性能研究

利用化学镀方法对碳纳米管(carbon nano-tubes,CNTs)表面金属化镀银,研究表面化学镀银碳纳米管的场发射性能。碳纳米管经氧化处理后,表面存在一些羰基(CO)、羧基(—COOH)和羟基(—OH)等活性基团,经敏化、活化处理后,形成金属钯活化中心,进而还原金属银离子,从而获得表面化学镀银的碳纳米管。表面化学镀银碳纳米管阴极的开启电场约为0.19V/μm,当电场强度为0.37V/μm时,最大发射电流达6mA/cm2,场增强因子约为25565。实验结果表明,化学镀银层可以提高碳纳米管的电子传输和热传输能力,提高碳纳米管的场发射电流和发射稳定性,有利于碳纳米管在场发射平板显示领域的应用。     

Efficient fabrication of carbon nanotube micro tip arrays by tailoring cross-stacked carbon nanotube sheets

Carbon nanotube (CNT) micro tip arrays with hairpin structures on patterned silicon wafers were efficiently fabricated by tailoring the cross-stacked CNT sheet with laser. A blade-like structure was formed at the laser-cut edges of the CNT sheet. CNT field emitters, pulled out from the end of the hairpin by an adhesive tape, can provide 150 μA intrinsic emission currents with low beam noise. The nice field emission is ascribed to the Joule-heating-induced desorption of the emitter surface by the hairpin structure, the high temperature annealing effect, and the surface morphology. The CNT emitters with hairpin structures will greatly promote the applications of CNTs in vacuum electronic devices and hold the promises to be used as the hot tips for thermochemical nanolithography. More CNT-based structures and devices can be fabricated on a large scale by this versatile method.     

碳纳米管/聚四氟乙烯复合材料场发射特性研究

通过催化热解法制备了碳纳米管,采用机械共混法制作了碳纳米管/聚四氟乙烯复合材料场发射阴极,研究了不同碳纳米管质量分数对复合材料阴极场发射特性的影响,通过制作的封装结构,对比研究了真空环境下碳纳米管/聚四氟乙烯与碳纳米管/环氧树脂复合材料的场发射特性,证明碳纳米管/聚四氟乙烯复合材料更适用于场发射阴极的真空环境中,可以满足场发射显示器件的要求.     

Single- and few-walled carbon nanotubes grown at temperatures as low as 450 degrees c: electrical and field emission characterization

Single-wall (SW-) and few-walled (FW-) carbon nanotubes (CNTs) were synthesized on aluminum/ cobalt coated silicon at temperatures as low as 450 degrees C by plasma enhanced chemical vapor deposition technique (PECVD). The SWCNTs and FWCNTs grow vertically oriented and well separated from each other. The cold field emission studies of as-grown SWCNTs and FWCNTs showed low turn-on field emission threshold voltages, strongly dependent of the nanotubes morphology. Current-voltage curves of individual CNTs, measured by conductive atomic force microscopy (CAFM), showed an electrical resistance of about 90 Komega, that is attributed mainly to the resistance of the contact between the CNTs and the conductive CAFM tip (Au and Pt).     

Novel ring structure for minimisation of screening effect in carbon nanotube based field emitters

Carbon Nanotubes (CNTs) are promising candidates for cold cathodes because of their high aspect ratio and robustness. However, the major hindrance in cold cathode based applications is the screening effect, which reduces the effective field at the tip and thereby the current density. The emission current can be improved by minimising the screening effect. The adverse effect of screening can be addressed by either controlling the growth density or by optimising the patterns of CNT cathodes. Here, novel patterns have been used to increase edge length per unit area in planar vertically aligned CNT bundles. Our motive was to increase the number of effective emitters, since the CNT at the edges are less screened by the proximal CNTs. By varying geometry and spacing of solid CNT dot patterns and by introducing the square ring structures; we could successfully enhance the effective emitters at the edges. It has been observed that an enhancement of edge length from 0.032 per micron to 0.2 per micron increases the current density from 0.71mA/cm² to 16.2 mA/cm² at a field of 4.5 V/μm. CNTs in dotted structure with high value of edge length per unit area emit very high current density as compared to other dotted structures with low value of edge length per unit area Simulation studies confirms our argument that CNTs at the corners are the least screened and have the maximum local electric field.     

碳纳米管非金属掺杂对结构和性能影响的研究

通过讨论氮、硼、硅、氟等非金属原子掺杂的碳纳米管,对场电子发射特性的影响。介绍了掺杂在场电子发射、能源电池、气体传感器等领域的研究和应用。掺杂可以增加碳纳米管的缺陷,改变其电子结构。掺杂可使碳纳米管转变为n型半导体或是金属性导体,将提高场发射性能。同时,掺杂亦可使碳纳米管向P型半导体转变,这将不利于场发射性能改善。当场发射性能随着掺杂浓度升高而提高时,存在最佳掺杂浓度值,一旦超出,则场发射性能逐渐下降。因此,研究碳纳米管非金属掺杂具有重要的应用价值。     

Growth and characterization of carbon nanofibers by a technique of polymer doped catalyst and hot-filament chemical vapor deposition

Carbon nanostructures have been prepared from the catalytic conversion of polyethylene glycol using a rapid immersion in hot-filament system fed with ethanol, hydrogen and argon. Fiber structures of external diameter about 30 nm have been observed by field emission scanning electron microscopy (FESEM). Raman measurements indicate high degree of C-C sp² ordering which suggests that the samples correspond to CNTs of good tube crystallinity. The samples presented remarkable field emission properties. Lowest threshold field achieved for electron emission was 1.0 V/μm.     

Electron field emission characteristics of different surface morphologies of ZnO nanostructures coated on carbon nanotubes

The optimal carbon nanotube (CNT) bundles with a hexagonal arrangement were synthesized using thermal chemical vapor deposition (TCVD). To enhance the electron field emission characteristics of the pristine CNTs, the zinc oxide (ZnO) nanostructures coated on CNT bundles using another TCVD technique. Transmission electron microscopy (TEM) images showed that the ZnO nanostructures were grown onto the CNT surface uniformly, and the surface morphology of ZnO nanostructures varied with the distance between the CNT bundle and the zinc acetate. The results of field emissions showed that the ZnO nanostructures grown onto the CNTs could improve the electron field emission characteristics. The enhancement of field emission characteristics was attributed to the increase of emission sites formed by the nanostructures of ZnO grown onto the CNT surface, and each ZnO nanostructure could be regarded as an individual field emission site. In addition, ZnO-coated CNT bundles exhibited a good emission uniformity and stable current density. These results demonstrated that ZnO-coated CNTs is a promising field emitter material.     

Clean carbon nanotube field emitters aligned horizontally

Horizontally aligned carbon nanotube (CNT) field emitters, which strongly adhere to the substrate and show good field emission properties, were fabricated by electrophoresis deposition and fissure formation techniques. A thin film of CNTs was deposited on a substrate, by electrophoresis, from an aqueous mixture of CNT and detergent, and then the detergent was deposited also by electropholysis. CNTs with a clean surface were exposed in the fissures produced by firing. The field emission was increased significantly due to the additional deposition of the detergent. When the CNTs were cut by increasing the firing time, the field emission increased significantly, while their stability decreased considerably. Our method does not require any further treatment for field emission.