Vol.9 No.3 2017
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Research paper : A super-growth method for single-walled carbon nanotube synthesis (K. Hata)−168−Synthesiology - English edition Vol.9 No.3 (2017) ago, and one cannot say that it has been industrialized. The reason is the low growth efciency of the synthesis of single-walled CNTs. Since the production efficiency is extremely low, the cost is extremely high. The price of the most common commercial single-walled CNT is several tens of thousands of yen per gram, and this price range puts it outside the scope of industrial material. Compared to single-walled CNTs, the multi-walled CNTs have high growth efficiency, are commercially produced, are sold for about ve thousand to ten thousand yen per kilogram, and are distributed widely around the world.1.2 Conventional synthesis methods of CNTs and their issuesSome of the synthesis methods of single-walled CNTs include the laser abrasion method,[3] the electric arc method,[4] and the chemical vapor deposition (CVD) method[5] (Fig. 3). Among these methods, only the CVD method was applied to industrial mass production. For the CVD method, industrial mass production was realized for multi-walled CNTs using the conventional mass production processes that use rotary kilns and fluid bed furnaces, and several commercial plants around the world with production capacity of several hundred tons per year are in operation. However, compared to multi-walled CNTs, single-walled CNTs had narrower diameter and necessitated precise catalyst control. Moreover, the catalyst tended to become deactivated extremely easily, and it was difficult to synthesize at a high yield. When single-walled CNTs were synthesized with the conventional CVD method, the catalyst lifespan was a few minutes, the catalyst activity was several percent, and the growth efciency was extremely low. As a result, the major problem was the large amount of catalyst metal particles that remained in the CNTs as impurities. Therefore, before actually using the single-walled CNTs, they had to undergo renement to remove catalyst impurities. This refinement method consisted of several steps of complicated chemical processes such as oxidation at high temperature and acid treatment, and not only were they expensive, but they also would damage the single-walled CNTs.2 Core technology of the research2.1 Super-growth methodAn innovative CVD method that solved all the technological issues of single-walled CNTs was developed at the National Institute of Advanced Industrial Science and Technology (AIST) in 2004. This was the super-growth method[6] (Fig. 4).The super-growth method is a method where the activity and lifespan of the catalyst are greatly improved by adding trace amount of water to a regular vapor synthesis atmosphere, and thereby greatly increasing the growth efciency. By adding trace amount of water, the catalyst activity that was normally a few percent increased to over 84 %[7] and the catalyst lifespan of several minutes extended to over several tens of minutes to an hour.In the super-growth method, the CNTs can be synthesized most efficiently from iron catalysts on the substrate coated with alumina catalyst supports. It enables the synthesis of “forests” or the long CNT structures aligned vertically on the substrate.With this super-growth method, the improvement of growth efciency of several hundred times was achieved compared to the conventional CNT synthesis methods such as laser ablation, electric arc, HiPCo process, alcohol CVD, or vapor uid methods.For example, the catalyst efficiency of the super-growth method reached 50,000 % by product/catalyst weight ratio, and this was an improvement of several hundred times compared to the conventional CNT synthesis methods (laser ablation 500 %, HiPCo 300 %, alcohol CVD 800 %, and vapor uid 100 %). The dramatic decrease of the amount of catalysts used indicated large manufacturing cost reduction in the future using this growth method.ModerateModerateModerate(2D)ModerateModerateGoodGoodGood(2D)Good(2D)ExcellentExcellentExcellent(3D)ExcellentExcellentExcellentPoorPoorPoorPoorPoorPoorPoorPoorPoorSuper-growthVapor uid methodVapor uid methodSupported catalyst CVD uid bed furnaceLaser ablationElectric arcQualityPurityCostYieldShort Growth Time (e.g. HiPco)Low Density (e.g. eDips)Super-growth is a synthesis method that fullls the demands of yield, cost, purity, and qualitySuper-GrowthCNTMetal catalystSubstrateChemical reactionArgon gasHeating furnaceGas containing trace waterGas exitFig. 3 Comparison of the synthesis methods for single-walled CNTsFig. 4 Super-growth method

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