| Summary: | 碩士 === 國立交通大學 === 材料科學與工程系 === 90 === To enlarge the application areas of the nano-structured materials, such as applications in magnetic recording media, the well-aligned carbon nano-structures encapsulating with magnetic catalyst particles were successfully synthesized on Si wafer by ECR-CVD method with CH4 and H2 as gas sources. The magnetic catalysts, including FePt, CoPt, Nd2Fe14B and Fe thin films, and FeNi thick film, were studied. The main process parameters include hydrogen content in the gas sources, hydrogen plasma catalyst pretreatment, substrate bias, deposition temperature and plasma flow guiding. The magnetic properties, morphologies, microstructures and bonding structures of the magnetic catalyst-assisted carbon nanostructures were characterized by VSM, MFM, AFM, SEM, TEM, HRTEM and Raman spectroscopy. The adhesion properties of nanostructures with the substrates were qualitatively compared by ultrasonic agitation in acetone bath.
Regarding effects of catalyst materials, the results show that at the same deposition conditions, different catalysts can produce carbon nanotubes (CNTs) with different tube number density, tube length, carbon film formation, bonding between catalyst and CNTs, growth mechanism and type of CNTs. These differences in structures or properties may relate to the solubility difference of carbon in catalysts, etching rate difference between CNTs and carbon films by hydrogen plasma. In the present conditions, the maximum tube number density can go up to 134 Gtubes/inch2 for Fe-assisted CNTs. For Nd2Fe14B—assisted CNTs, the longest tube length can reach 2100 nm for 15 min deposition time, which is roughly corresponding to the highest growth rate. For certain applications, if the removal of catalysts from tips of CNTs is required, it can easily be achieved by selecting proper catalyst and combining with ultrasonic agitation in acetone bath.
About effect of plasma flow guiding, the 90°-inclined CNTs was successfully modified to 45°-inclined CNTs by positioning a negatively-biased metal plate above the Si substrate surface to vary the plasma flow pattern. The results also show that the plasma flow guiding may be used to modify the seaweed-like nano-sheets from random orientations to parallel alignment.
For effects of other process parameters, the results indicate that the hydrogen flow rate and substrate bias are essentially the factors governing the differential etching effect to different nanostructures, e.g. carbon film and CNTs. However, the etching effect is more directional for bias, and more isotropic for hydrogen plasma. A lower hydrogen flow rate favors formation of the seaweed-like carbon nanostructures, or CNTs surrounding with other carbon nanostructures. A lower negative bias voltage favors formation of the additional thicker carbon films. The results also show that the hydrogen plasma pretreatment of the catalyst-coated substrates is basically to attack the catalyst film to become the well-distributed nano-particles to act as catalysts of CNTs.
Regard to the magnetic properties of the magnetic metal-encapsulated carbon nanostructures, the grain sizes of the magnetic particles (35 nm, or 10 ~ 100 nm in diameter) are greater than but close to the critical optimum size or single domain size, which favor a higher coercive force. A higher deposition temperature of CNTs results in a greater coercive force due to a smaller catalyst size, and the greatest coercive force can go up to 750 Oe for Fe-assisted CNTs at 715℃ deposition temperature, which is comparable with the reported values in the literature. The process also takes advantages of higher shape and induced anisotropy due to its higher aspect ratio and magnetic annealing effect. The coercive force difference between vertical and horizontal direction can reach 300 Oe in the present conditions. The results also demonstrate the potential applications in magnetic recording media that the isolated and well-distributed magnetic particles in the magnetic metal-encapsulated carbon nanostructures can be imaged by MFM micrographs.
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