Mass production of aligned carbon nanotube arrays by fluidized bed catalytic chemical vapor deposition
Introduction
Carbon nanotubes (CNTs) have become one of the most famous nanocarbon. Various potential commercial applications have been found, including engineering materials to enhance the electrical and electrostatic properties or tenacity of polymers, metals, cerement, and coatings [1], [2]; catalyst for oxidative dehydrogenation [3], [4], [5], [6]; catalysis carriers for noble metals in hydrogenation, Fischer–Tropsch synthesis, and others [3], [7], [8]; energy conversion components of supercapacitors, fuel cells or lithium ion secondary batteries [9], [10], [11], [12]; field emission display [13], [14]; buckypaper [15]; transparent conductive film [16]; sensors, and electronics, devices [17], and so on. Some applications have been achieved on the basis of the mass production of multi-walled CNTs (MWCNTs) or single-walled CNTs (SWCNTs), which is the first step for those fascinating applications. Recently, vertically aligned CNT (VACNT) arrays, with the advantages of good orientation, large aspect ratio, and high purity, are attractive because of their wide potential applications, i.e. in field emission devices, anisotropic conductive materials, membrane filtration materials, the super springs, filaments, super strong fibers, nano-brushes, sensors, and so on [14], [18], [19], [20], [21]. However, based on the current worldwide CNT market status, some obstacles appeared such as high prices (10 $/g) and poor availability (around 1 kg/year). Even in the literature, the best yields of MWCNT arrays were just 1.0 g/h and 200 g/h for the synthesis on a flat quartz plate as substrate [22] and on spherical substrates [23], respectively. Compared with agglomerated CNTs, it is critical to develop a method for the mass production of VACNT arrays.
Chemical vapor deposition (CVD) is the most powerful way for controllable growth of CNTs. One of the most popular ways for the mass production of CNTs is fluidized bed catalytic chemical vapor deposition (CVD) [24], [25], [26]. The fluidized bed reactor has great advantages in terms of providing enough space for CNT growth, excellent diffusion and heat transfer, easy scaling up and continuous operation, and so on. A pilot mass production of agglomerated multi-walled CNTs (MWCNTs) with high yield (15 kg/h) was realized in a fluidized bed reactor in 2002 [27]. Up to now, agglomerated single/double/few-walled CNTs can also be mass produced in a fluidized bed reactor. The researches on the production of CNTs have been reported by many groups, including Wei [26], [27], [28], [29], Serp [24], [30], [31], [32], Windle [33], See and Harris [25], [34], [35], Hee [36], Xu and Zhu [37], Rakov [38], Yang [39], Heish [40], [41], and others. The products of the fluidized bed process mentioned above were agglomerated CNTs. While for VACNT arrays, they were commonly synthesized on a flat substrate using various CVD methods in a fixed bed [14], [18], [19], [20], [21], [22]. However, the flat substrates always possess low specific surface area (<0.5 m2/g), which limits the mass production of VACNT arrays. Besides, it is difficult to suspend or move the flat substrate by gas flow or other methods because of the anisotropic morphology of a flat substrate, which results in its poor mobility. Thus, the yield of VACNT arrays grown on a flat substrate is very limited. Later, various particles with larger surface area, such as spheres [23], [42], [43], fibers [44], flakes [45], were used for the growth of VACNT arrays in large quantities. Recently, a strategy for VACNT arrays grown among lamellar catalyst was developed [46]. Compared with flat substrate (about 4 cm2/g for wafer with a thickness of 0.1 mm) or spherical substrate (about 400 cm2/g for spheres with a diameter of 0.7 mm) with limited surface area, the lamellar catalysts were with much larger specific surface area (higher than 30,000 cm2/g), providing enough surface area for the growth of VACNT arrays. Meanwhile, the size of the lamellar catalysts can be limited into A particles according to Geldart particles classification to simplify the operation in the fluidized bed. It shows a potential way for the mass production of VACNT arrays in a fluidized bed reactor [47]. However, the effects of various key parameters on the growth behavior of CNT arrays intercalatedly grown on lamellar compounds were still unclear.
Herein, a fluidized bed CVD process was comprehensively developed for the mass production of VACNT arrays intercalatedly grown among vermiculites. Various key parameters were investigated, including the catalyst loading amount, catalyst reduction time, growth temperature, space velocity, and apparent gas velocity. Based on the parameter study, VACNT arrays with a yield of 3.0 kg/h were produced under optimized conditions in a pilot plant fluidized bed reactor.
Section snippets
Experimental
Vermiculite, a clay mineral which is a group of micaceous hydrated silicate minerals related to the chlorites and used in heat-expanded form as insulation and as a planting medium, was used as carrier of the catalyst. The vermiculite used in our experiment was mined in Lingshou, Hebei Province of China. In brief, vermiculite powder with a size of 100–250 μm (bulk density of about 160 kg/m3) was suspended in distilled water to form a uniform suspension through strong stirring at 80 °C.
Results and discussion
Natural vermiculites with a size of 100–250 μm, were selected as the substrate for CNT arrays. As reported in the previous report [47], they are easily to be fluidized with a gas velocity ranging from 7 to 24 cm/s. After the impregnation and drying, the active phase can be intercalated among the catalyst layers. The specific surface area of the vermiculite substrate used is about 4.5 m2/g. In order to evaluate the performance of the lamellar Fe/Mo/vermiculite catalyst on the growth of VACNT array,
Conclusions
Lamellar Fe/Mo/vermiculites with a diameter of 100–250 μm were used as catalyst carriers for the scaled production of VACNTs. Various parameters, including catalyst loading amount, catalyst reduction conditions, growth temperature, space velocity, and apparent gas velocity were investigated to develop the process for the mass production of VACNT arrays. The main conclusions that can be drawn from the present work are:
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The temperature influences significantly the structure of the CNTs as well as
Acknowledgements
The work was supported by the Natural Scientific Foundation of China (Nos. 20606020, 20736004, 20736007, 2007AA03Z346), the China National Program (No. 2006CB0N0702).
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Current address: Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA.