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Study of morphology and control of electric structure for carbon based hetero-nanotubes synthesized under high magnetic field

强磁场下合成碳基异质纳米管形貌及电结构控制研究

基本信息

批准号：
12640318
负责人：
YOKOMICHI Haruo
金额：
$ 2.43万
依托单位：
Toyama Prefectural University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2000
资助国家：
日本
起止时间：
2000 至 2001
项目状态：
已结题

项目摘要

Carbon nanotubes were synthesized by thermal CVD under high magnetic fields of 0T,1T,5T and 10T on the SiO_2 and Si substrates using a magnetic-field-proof furnace. The C_2H_2 gas pressure and temperature were 800℃ and 30 Torr, respectively. Although subtle differences between 0T and the presence of magnetic field were discernible in the SEM images, it was difficult by SEM to precisely evaluate changes of tube length and production yield under magnetic field. The growth directions of the nanotubes and nanowires were randomly oriented in all the cases. Differences in the presence of magnetic field were scarcely discernible by SEM for the deposits under low gas pressure (10 Torr) and low temperature (10 Torr and 700℃) using Ni Although an increase of onion-like structures was discernible in TEM images for the deposits under the magnetic field, obvious magnetic effects were not observed. Accordingly, synthesis was carried out under 700℃ and 10 Torr using the Fe catalyst. The production yield of the deposits under the magnetic field decreased in the case of Fe as the catalyst, whereas the Ni catalyst did not cause such a magnetic effect. A difference between Ni and Fe is Curie temperature, which is 358℃ for Ni and 769℃ for Fe. Since the synthesis temperature was 700℃, ferromagnetism of the Fe catalyst should be maintained at the synthesis temperature. It was suggested that the magnetic field influences on formation processes of nanotubes via magnetic effects on metal catalysts. For instance, a radical pair mechanism, is possible : a singlet-triplet(S-T) spin transition for precursors generated by thermal decomposition may occur under high magnetic field. This mechanism results in the magnetic effects due to the anisotropies in g-values and hyperfine interaction constants of radicals.

采用热化学气相沉积法，在0 T、1 T、5 T和10 T的强磁场下，用防磁场炉在SiO_2和Si衬底上制备了碳纳米管。C_2H_2气体压力和温度分别为800℃和30 Torr。虽然在SEM图像中可以辨别出0 T和磁场存在之间的细微差异，但是通过SEM难以精确地评估磁场下的管长度和生产产率的变化。在所有的情况下，纳米管和纳米线的生长方向是随机取向的。在低气压（10 Torr）和低温（10 Torr和700℃）下，用SEM观察到磁场的存在几乎没有差别。虽然在磁场下，沉积物的TEM图像中可以观察到洋葱状结构的增加，但没有观察到明显的磁效应。因此，使用Fe催化剂在700℃和10 Torr下进行合成。在Fe作为催化剂的情况下，在磁场下沉积物的产率降低，而Ni催化剂没有引起这样的磁效应。Ni和Fe之间的差别是居里温度，Ni的居里温度为358℃，Fe的居里温度为769℃。由于合成温度为700℃，因此铁催化剂的铁磁性应在合成温度下保持。研究表明，磁场通过对金属催化剂的磁效应影响纳米管的形成过程。例如，自由基对机制是可能的：由热分解产生的前体的单重态-三重态（S-T）自旋跃迁可以在高磁场下发生。这一机制导致了自由基的g值和超精细相互作用常数的各向异性所引起的磁效应。