Vol.9 No.3 2017
62/74

Research paper : A super-growth method for single-walled carbon nanotube synthesis (K. Hata)−175−Synthesiology - English edition Vol.9 No.3 (2017) created several prototypes of the gas supply system and the showerhead that allowed even supply of trace water to the substrate material without turbulence.4.1.4 Technological development of the scalable metal synthesis furnaceThe development of the synthesis furnace itself was a major element in technological development. At the time of publication in Science, we used a quartz furnace. There was a size limitation to a manufacturable quartz furnace, and it was expensive. It was necessary to make a metal furnace. However, in continuous synthesis, the furnace would be exposed continuously to high temperature and highly concentrated hydrocarbon. The specs required for the synthesis furnace were more severe than those for the substrate material.Therefore, a dedicated device was introduced to study the materials that were resistant to the synthesis atmosphere with long-term stability. Using this device, deterioration, carbon adhesion, and carburization of the materials were investigated by exposing several candidate materials for a long time to high temperature and to highly concentrated hydrocarbon. The carbon gases and the reaction of trace water and metals greatly affect the synthesis. The material of the furnace was selected considering the effect on CNT synthesis and the long-term durability in the synthesis environment, and the selected materials were used in the actual small synthesis furnace. The metal showerhead was developed after the synthesis furnace, and we nally succeeded in developing a synthesis furnace that did not use quartz.When the synthesis of CNTs was repeated many times, carbon impurities built up in the synthesis furnace. These carbon impurities had major effects on CNT synthesis by absorbing the water added to the synthesis atmosphere. The furnace had to be cleaned after a certain amount of impurities adhered. Carbon that adhered under high temperature was crystallized and removal was difficult. The easiest method was combustion by introducing oxygen at high temperature, but if that was done, the metal furnace would be damaged by oxidation and its lifespan would decrease. Therefore, we developed a cleaning technology to remove the carbon impurities without damaging the metal synthesis furnace.4.1.5 Large-area synthesis technologyThe technologies were taken further to develop a large-area synthesis furnace that could synthesize on large-area substrate material (A4 size or more) (Fig. 13). When the furnace was upscaled by a batch method, about one hour or more was necessary to heat the furnace to synthesis temperature. However, in continuous synthesis, it was necessary to raise the temperature of the substrate material to synthesis temperature in about 10 min. To alleviate this difference, we developed an extremely special large-area synthesis furnace. In this synthesis device, the large-area substrate material was stored inside the quartz horizontal furnace of 300 φ, and the gas was replaced. A large mufe furnace was placed next to it, and this was maintained at synthesis temperature. The high-temperature mufe furnace moved on rails to envelope the quartz furnace to quickly raise the temperature and heat the substrate material. This format was employed to conduct large-area synthesis at thermal history close to the future continuous synthesis as much as possible. After such technological development, we succeeded in large-area synthesis of A4 size and A3 size.4.1.6 Continuous synthesis technologyThe final, most crucial technological development was the development of continuous synthesis technology. Several continuous syntheses and quasi-continuous methods were considered, and finally, a method in which large substrate material was placed on a belt conveyor and transported continuously to the synthesis furnace was employed (Fig. 14). The characteristic of this method was that there were no shutters or partitions on the synthesis furnace, and it was designed as a complete open system. The substrate that entered the continuous synthesis furnace was transported to different areas. First, it was heated in the heating section, ACS-Nano,3,4164(2009)SubstrateCNT forestShowerheadShowerheadGas3 mm60 mm20 mmYield: 1 g or more (per sheet), same quality as conventional productFig. 12 Development of the showerhead[14]Fig. 13 Technological development of large-area synthesis

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