Fusion of carbon nanotubes for fabrication of field emission cathodes

doi:10.1016/j.carbon.2011.07.058

Carbon

Volume 50, Issue 2, February 2012, Pages 356-361

https://doi.org/10.1016/j.carbon.2011.07.058 Get rights and content

Abstract

Consolidated carbonaceous samples prepared by spark plasma sintering of multi-walled carbon nanotubes are analyzed, and the effect of the heating regime on their morphology, density, thermal stability, electron field emission and adhesive behavior studied. The trend in the field emission properties of these samples is explained by the changes in the mobility of the nanotube tips. The effect of such changes in the number of free nanotube tips is also deduced from micro-adhesion data, obtained from pull-off tests using atomic force microscopy.

Introduction

One of the new techniques that have recently been reported for the preparation of advanced functional materials based on bulky networks of carbon nanotubes is spark plasma sintering (SPS). SPS is a relatively new sintering process, which uses a pulsed DC current passed directly through both a graphite die and compacted powder sample contained therein. To date, it has mainly been used in sintering metallic or ceramic powders. The difference between SPS and other hot pressing methods is that in SPS the heat is generated internally. Allowing a very high heating rate to be achieved, and the sintering process can be performed in a very short time. To date, only a few papers has discussed the results of SPS processing of carbon nanotubes, their structural and the physical properties and the potential applications of the prepared bulky carbon nanotube samples [1], [2], [3], [4], [5].

Zhang et al. have prepared bulk nanotube samples using the SPS method [3]. They have shown that, in addition to welding of carbon nanotubes together, spark plasma sintering process can partly convert carbon nanotubes to diamond. They have reported that this conversion starts at 1200 °C and well-crystallized diamond crystals with a size greater than 10 μm can be obtained at 1500 °C [1]. It has been suggested that the presence of metallic catalyst in the carbon nanotubes can decrease the energy barrier for diamond nucleation, and that the presence of amorphous carbon impurities can reduce the efficiency of the process [2].

Fused networks of nanotubes prepared by SPS technique have great potential in applications such as supercapacitors, hydrogen storage, electron field emission cathodes and as electrodes for rechargeable batteries. In this paper, we report the preparation of spark plasma sintered carbon nanotubes processed at different sintering temperatures, and we study the effect of such variation in processing on their physical and morphological properties. We investigate one of the potential applications of these materials, their field emission properties. In addition, by using force–distance curves obtained from an atomic force microscope (AFM) with a modified cantilever, we have measured the pull-off force of this probe from the surface, which allows us to obtain a better understanding of the van der Waals forces and the degree of motion of the nanotube tips on the surface of the samples. We propose that there is a link in the understanding of both their electron emission and adhesive properties, based on the nanotube tip mobility.

Section snippets

Spark plasma sintering process

In order to prepare bulky carbon nanotube networks by sintering and fusing nanotubes a Dr. Sinter 925SPS apparatus (SPS SYNTEX INC) was used. 0.2 g of carbon nanotubes (Nanocyl 3150, Nanocyl S.A, Belgium) with an average diameter 10 nm and length of 1 μm were contained within 20 mm graphite moulds and then placed in the SPS chamber. The SPS temperature was monitored using an infrared camera through a hole at the middle of the graphite mould. The sintering process was performed under a vacuum of 6

Results and discussion

Four samples were prepared by spark plasma sintering of the carbon nanotubes. In each experiment 200 mg of carbon nanotube were sintered under a pressure of 50 MPa. The sintering temperature was changed in each sample, and samples with a sintering temperature of 450, 1000, 1500 and 2000 °C were prepared and labeled as SPS450, SPS1000, SPS1500 and SPS2000, respectively. Fig. 1a shows the heating diagram of each of these three samples, it can be seen in this figure that the holding time at the

Conclusion

Spark plasma sintering technique was successfully used for the preparation of carbon nanotube network samples. The analysis of prepared networks showed that by increasing the sintering temperature from 1000 to 2000 °C, the nanotube network becomes more packed and dense. The nanotube networks prepared exhibit good field emission and adhesion performance. Although increasing the sintering temperature results in better graphitic samples, the fusion of nanotubes in such high sintering temperature

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In this work, silicon carbide (SiC) was utilized as a binding agent to fuse carbon nanotubes (CNTs) into highly tough dense CNT bulk nanocomposites through spark plasma sintering (SPS) method. Phase studies were performed using x-ray diffraction analysis (XRD) and field-emission scanning electron microscopy (FESEM) and obtained results were verified by the microstructural evolution. Also, the optimum processing temperature was determined as 1600 °C at which the undesired allotropic phase transformation of SiC (β) → SiC (α) was avoided and single-walled CNTs (SWCNTs) were structurally preserved. Fracture toughness of the nanocomposite synthesized at optimum processing conditions was determined by Vickers indentation method equal to 13.3 MP·m^1/2. Toughening mechanisms were directly observed throughout the microstructure among which crack-wake bridging by SWCNTs and crack deflection by SiC particles were the most dominant.
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Carbon nanomaterials such as carbon nanotubes (CNT) [1–9] and graphene [10–13] are of great interest for their field emission (FE) characteristics but fabricating them into stable, efficient, macro-scale cathodes is proving to be very difficult.
Two methods for increasing the number of free carbon nanotube (CNT) tips in carbon nanotube webs (CNTW) and improving their field emission (FE) performance are proposed. It was observed that by laterally compressing samples by 35 percent it is possible to improve FE performance to some extent, with further compression leading to a loss of FE properties due to folding of webs and adhesion between CNT tips. Multilayered samples were also studied and it was found that samples with successive layers in which the orientation of the nanofibers in successive layers was the same, increased inter-bundle van der Waals forces, leading to more compact webs with no improvement in FE performance. However, it was found that if the fibers in successive layers were perpendicular to each other the inter-bundle attraction was minimized, maximizing the number of active CNT tips and yielding materials that exhibit the best FE performance reported for any material to date.
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The webs are of great interest for a range of potential applications both as spun, such as in polarized incandescent light sources, microwave welding of plastics and actuators [2,7] or twisted into yarns [1]. There has been increasing interest in the development of field emission (FE) devices based on carbon nanotubes [8–13] in the last years. Very recently a few reports have appeared describing some initial studies on the use of spinnable nanotube webs and fibres as cathodes for FE, with promising results [6,8,14,15].
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Fusion of carbon nanotubes for fabrication of field emission cathodes

Abstract

Introduction

Section snippets

Spark plasma sintering process

Results and discussion

Conclusion

Carbon

Carbon

Carbon

Carbon

Carbon

Carbon

Carbon

Carbon

Carbon

Solid-State Electron

Spark plasma sintering and thermal conductivity of carbon nanotube bulk materials

J Appl Phys

Calibration of rectangular atomic force microscope cantilevers

Rev Sci Instrum