Vol.9 No.3 2017
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Research paper : A super-growth method for single-walled carbon nanotube synthesis (K. Hata)−173−Synthesiology - English edition Vol.9 No.3 (2017) The excellent thermal resistance and carburizing resistant properties of the Ni-Fe-Cr alloy enabled the reuse of substrate material. Regular metals underwent plastic strain and deformation when they were repeatedly exposed to a highly concentrated hydrocarbon atmosphere at 800 ºC and then cooled to room temperature, but these were minimal in the Ni-Fe-Cr alloy. This was one of the extremely important points in establishing the industrial mass production method by the super-growth method.4.1.2 Technological development of the catalystIn order to achieve high-speed synthesis of single-walled CNTs that are vertically aligned on the substrate material using the super-growth method, it is necessary for the catalyst to meet strict conditions. In later research, it was claried that for the single-walled CNTs to grow vertically at millimeter-scale height, there was a sweet spot for the size and spacing of the catalyst[11] (Fig. 10). The sweet spot exists because the region is surrounded by multiple boundaries. That is, multi-walled CNTs are synthesized from large catalysts (multi-walled CNT boundary), while the growth rate of single-walled CNTs that grow from small catalysts are slow (low yield boundary), and CNTs grow laterally from the catalyst with large spacing. The typical catalyst size of the sweet spot is 3 nm, and the catalyst spacing is 15 nm. This catalyst array has to be stably present for at least 10 min in the synthesis temperature of 800 ºC. To the present, the capable catalyst system is only obtainable when the iron thin lm on the alumina catalyst support is subject to hydrogen reduction at high temperature of 800 ºC.The control factor that was crucial to place the catalyst in the sweet spot was the thickness of the alumina catalyst support and the iron catalyst, particularly the thickness of the iron catalyst. As a result of research, a single-walled CNT array could be synthesized only when the thickness of the iron catalyst was between 0.8 nm to 1.3 nm.[12] Therefore, the catalyst lm was initially formed using the sputtering method that had excellent control of lm thickness.① Multi-walled CNT forest⑤ Lateral single-walled CNT agglomerate⑤ Lateral single-walled CNT agglomerateLow yield boundaryCatalyst (CNT) spacing (nm)Catalyst (CNT) size (nm)Lateral growth boundaryLow yield boundaryMulti-walled CNT boundarySWCNT2~3WCNTMWCNT0.00.81.62.43.24.04.85.66.47.28.080706050403020100Silicon substrate (expensive and inappropriate for mass production)Inconel®601NiCr®SUS430SUS304SUS310SYEF426NiYEF50FeCr(50 %)Fig. 9 Development of metal substrateFig. 10 Sweet spot for catalysts[11]

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