1.1‐Millimeter Tall Vertically Aligned‐Carbon Nanotubes Grown by Cold Wall CVD Using a Novel Multiple‐Purge Method

Abstract

These processes still have limitation in achieving high yield and quality, and any commercial use of CNTs is limited due to these challenges encountered in large scale synthesis. [17] According to Shi et al. (2017), [18] "The poor reproducibility in CNT synthesis via CVD has been attributed to fluctuations in ambient and reactor-sorbed water vapor, as well as to carbonaceous deposits inside reactors leading to growth hysteresis." The presence of water or oxygen influences the growth gases and contributes to the formation or removal of carbon coated on the catalyst nanoparticles and avoiding this block deposition. The formation of carbon block coated on the catalyst nanoparticles prevents the CNT forests to grow taller, terminating the growth process. Thus, to obtain taller CNT forests, the lifetime of the catalyst should be prolonged. [17] The abrupt saturation point is observed for CNT growth rate controlled by laser with Meshot and Hart ( 2008). [19] The carbon atoms that reach the catalyst nanoparticles contribute to the CNT growth, but they also coat in form of amorphous carbon on the nanoparticle. Xiang et al. ( 2008) [20] refers to the growth limits of CNT carpets, explaining the diffusion mechanism of precursors for several different methods. Xiang et al. ( 2007) [21] refers the catalyst encapsulation occurs and it can be controlled during the growth changing the saturation point, creating a continuous growth mechanism. This deposition can be controlled by continuously supplying an etchant material that etches out the amorphous carbon coated on the catalyst nanoparticles. [17] The etchant materials generally used are CO 2 , H 2 O, and O 2 , however, excess levels of oxygencontaining compounds can result in etching of the CNTs themselves. [18] Sugime et al. ( 2021) [22] using the hot wire filament grew CNT around 14 cm but in this case the industrial growth is far in terms of reproducibility in quantity.Recently, Machado Jr. and Coelho (2021) [23] reported a novel method to obtain 140 µm tall VA-CNT forests, using a coldwall technique based on a two-phase deposition step. The first phase consists of the flow of NH 3 for a short period of time together with the C 2 H 2 flow to ensure catalyst activation. In the second phase, NH 3 flow is replaced by H 2 flow during the remaining growth time, permitting to obtain tall VA-CNT forests. In this work the saturation point was reached before the 40 min on deposition step in order to get 140 µm. Furthermore, the annealing temperature should be lower than the growth temperature to obtain taller forests. [24] Moreover, adding a third phase of H 2 during the deposition step at a temperature 50 °C above the previous phase, including a purge between each phase, allowed to achieve forests as high and uniform as This paper reports a multi-phase deposition for the growth of carbon nanotubes, intercalating phases of effective growth with purges of the reactor, based on the method recently proposed by the author to grow tall vertically aligned carbon nanotubes (VA-CNT) in a c...

Experimental and theoretical aspects of the growth of vertically aligned CNTs by CCVD on AZO substrate

“…The height at the catalyst composition of 3:2 at 650 °C could not be determined from the SEM image, but it can be assumed that following the trend stated before, the carbon nanotubes at this ratio are higher than at 700 °C. In contrast to other studies 44 – 48 , where the aim was to produce higher VACNTs, in the present work short carbon nanotubes were grown, with the advantage of short carbon nanotubes having better thermal conductivity 49 . It was revealed that the highest VACNTs (9.0 µm and 8.7 µm) were obtained in case of Fe:Co 2:3 at 650 °C and Fe:Co 1:3 at 700 °C, which is an additional difference from the published results is that the highest carbon nanotubes were not formed at the 1:1 catalyst composition 50 .…”