Vol.9 No.3 2017

Research paper : A super-growth method for single-walled carbon nanotube synthesis (K. Hata)−176−Synthesiology - English edition Vol.9 No.3 (2017) exposed to hydrogen atmosphere in the catalyst formation section, and the iron film was reduced to fine catalyst particles. The CNTs were synthesized in the synthesis section, and the temperature of the substrate material was decreased in the cooling section. The substrate material transported on the belt conveyor in the synthesis furnace underwent several processes continuously. To realize this in the synthesis furnace without shutters or partitions, gas showers were set between each section, to establish sections where the gases were separated although they were connected spatially. The continuous synthesis furnace introduced showed growth of CNTs from the very rst substrate, and I was thoroughly impressed by the engineering capability of the company.4.1.7 Integration of the elemental technologiesWhile developing the elemental technologies such as substrate material, a catalyst, large-area synthesis technology, and continuous synthesis technology, these technologies were integrated at the same time. Ultimately, unless all elemental technologies were integrated, we would not have a mass production process. However, when the elemental technologies were integrated, the complexity of the technology increased and new technical issues arose.For example, silicon substrate material was used at rst, and the technological development was conducted to replace it with a metal substrate. The size considered was 2 cm square. Several substrate materials were tried to see which 2 cm square was optimal for CNT synthesis. As a result, it was found that Inconel® was optimal. However, since Inconel® contained high amount of nickel, the nickel content had to be reduced to a minimum to reduce the cost. Next, the metal was upscaled to A4 size and growth took place in the large-area synthesis furnace. It was then found that heat distortion occurred when thermal history was added to the large-area metal substrate material, although this was not a problem in small size material. Of course, thermal distortion could be prevented if thicker substrate material was used, but this became costly. Also, the added weight of large-area material made the handling more difficult. Moreover, time was required for heating and cooling, and it could not be used in the continuous synthesis furnace. Therefore, we had to seek a solution that simultaneously fullled these conicting factors. The integration of technology continued. In fact, sputtered catalysts were used in all the research described above. Therefore, we shifted to wet catalysts and the same considerations were repeated. The density and evenness of the wet catalysts were inferior to the sputtered catalysts, and thermal distortion occurred. When thermal distortion occurred in large-area substrate material, the coating with catalysts became difcult.Next, we had to reuse the substrate material. It had to be reusable many times or else the cost of the substrate would be added to the cost of the final product. However, if the substrate material was reused, the material would have double, triple, and quadruple thermal history, thermal distortion would accumulate and increase, and it would become more difficult to fulfill all other technological elements at the same time. We had to strike a balance among such diverse factors, and at the same time develop technology to inhibit the negative factors such as thermal distortion. Each elemental technology was integrated to complete the mass production process.5 Current status of the research and its future5.1 Commercialization by Zeon Corporation and future prospectWhen we succeeded in continuous synthesis, Mr. Arakawa started to consider practical use. In terms of the “black box strategy,” we had no more technical issues that could not be solved, and the mass production technology would be completed if sufcient money and manpower were invested. However, the bankruptcy of Lehman Brothers occurred, and the economic situation turned difficult for companies to invest in facilities, and the move toward realization stopped. At that time, the director of METI came for a visit, and advised us to go for product realization using the supplementary budget.Through such course of events, the pilot plant was built on AIST grounds using the facility funds of the supplementary budget. This plant was managed jointly by AIST and Zeon. The pilot plant was 12 m in length, the muffle furnace was an open system on both ends, the gas sections were created using multiple gas showers, and the super-growth single-walled CNTs were manufactured continuously on a substrate of 50 cm square transported on a belt conveyor. !"#$%&'(%)*'+#",!-.-$%)'&750 ℃750 ℃RT→750 ℃750 ℃→RTCooling zoneReaction pathReduction furnaceHeating zoneFig. 14 Technological development of continuous synthesis

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