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Evolution of melting point of the catalyst (solid triangles) measured by DSC and carbon uptake (solid circles) with synthesis duration. The values of the melting points after heat treatment are indicated by open squares.
Raman spectra of the samples after synthesis for 1.5, 2, 3, 5, 7, 20, and , using a laser excitation of . The insets show the variation of the ratio with the synthesis duration.
Scheme of the iron-carbon phase diagram. , , , , , and represent the melting temperatures determined by DSC for pristine catalyst, and the catalyst after SWNTs synthesis for 3, 5, 7, 20, and , respectively. All points are placed with the assumption that liquefaction and solidification of the particles are homogeneous. For comparison, the liquid lines for the nickel-carbon and cobalt-carbon phases are shown. The ratios between the characteristic temperatures, as well as the carbon contents for the iron, nickel, and cobalt lines, have been kept as in the corresponding bulk metals. The carbon content at the eutectic points of cobalt-carbon and nickel-carbon phases is approximately two times lower and the temperature significantly higher than for the iron-carbon phase. Additionally, the solidification of cobalt-carbon and nickel-carbon phases occurs at relatively low concentration of carbon, at a given temperature. These can terminate the growth of SWNTs at early stages. Therefore, cobalt and nickel catalysts will require higher synthesis temperatures, as in the cases of laser ablation or arc-discharge methods.
Raman spectra of the samples after synthesis at 695, 700, 730, 750, 760, and , using a laser excitation of . The insets show the variation of the ratio with the temperature.
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