K. Biedermann

Enhanced magnetism in Fe-filled carbon nanotubes produced by pyrolysis of ferrocene

By optimization of the synthesis of ferromagnetic-filled carbon nanotube ensembles on Si substrates (catalytic decomposition of ferrocene) and following annealing at 645°C, marked hysteresis loops can be measured by the alternating-gradient method. Unusually high coercivities and strong anisotropies with an easy magnetic axis parallel to the alignment of the nanotubes are observed from the as-grown samples, whereas an enhanced magnetic saturation moment (up to a factor of 2) and a decreased anisotropy are realized after annealing at 645°C. The increase of the magnetic saturation moment of the Fe-filled carbon nanotube ensembles is caused by the entire transformation within the tubes of the γ-Fe and Fe3C phases to ferromagnetic α-Fe and graphite. X-ray diffraction with different glancing incidence shows that the γ-Fe is predominantly at the tips of the nanotubes, while the iron carbide resides closer to the substrate. However, after the annealing process only α-Fe is found. At an annealing temperature of 6...

Bulk synthesis of carbon-filled silicon carbide nanotubes with a narrow diameter distribution

We report on a simple and low-cost route to produce SiC nanotubes without the need for the more usual oxide based reactions using a high temperature substitution reaction between multiwall carbon nanotubes as a frame and Si powder. Local scale studies using transmission and scanning electron microscopy are also presented. The SiC nanotubes are carbon filled and open ended. Both their mean diameter and diameter distribution are smaller than previously reported for SiC nanotubes. Bulk scale studies using Raman and infrared as a probe showed the sample to be comprised of multiple polytypes and that finite size effects are present.

On the diffusion-controlled growth of multiwalled carbon nanotubes

Multiwalled carbon nanotubes were deposited by microwave chemical-vapor deposition on Fe, Co, and Ni catalyst layers. By adjusting the deposition temperature and time predominantly either a tubular structure or a bamboolike structure was obtained. The corresponding growth rates of these forms were determined. The activation energy of diffusion and the diffusion coefficient of carbon in the catalyst particles were evaluated to be as high as 0.3–0.6eV and (0.4–1)×10−10m2∕s for the tubular structure and 1.4–1.6eV and (0.6–1.8)×10−11m2∕s for the bamboolike structure. These data prove the occurrence of a liquid catalyst state during tubular growth and of a solid state in the case of bamboolike growth. It is likely that surface melting of the catalyst particles can also occur during the growth process. The tube growth is limited by time due to the metastable state of the catalyst particles.

Synthesis of SiC Nanorods by Chemical Vapor Deposition

Abstract In this paper we describe a method for the synthesis of SiC‐nanorods on different substrates by using chemical vapor deposition. Similar to the growth of single‐ and multi‐walled carbon nanotubes, SiC‐nanorods grow about a catalytically controlled process. Suitable catalysts are Fe, Ni, or Co. For the deposition we used a ferrocene [Fe(C5H5)2], a hydrocarbon (C6H6 or C6H14), and as the precursor for silicon, the liquid SiCl4. HRTEM‐investigations with EEL‐ and EDX‐spectroscopy show the well‐developed SiC‐nanorods. The yield of SiC is dependent on the gas phase composition, especially the concentration of ferrocene and the hydrogen excess. During on Si‐substrates branched nanorods are always formed with the catalyst particle on the tip; on WC/Co hardmetals an interaction can be observed between the catalyst Co and the SiCl4 (Co‐silicides).

Thermally Induced Templated Synthesis for the Formation of SiC Nanotubes and more

A thermally induced templated synthesis for SiC nanotubes and nanofibers using ammonia or nitrogen as a carrier gas, single wall carbon nanotubes (SWCNT) as templates as well as gaseous Si is presented. The bundles of SWCNT act as both the carbon source and as a nanoframe from which SiC structures form. Depending on the duration of the thermally induced templated reaction, for a fixed temperature, carrier gas, and gas pressure, various SiC nanostructures are obtained. These structures include SiC nanorods coated in C, SiC nanorods, SiC nanotubes and SiC nanocrystals. From our analysis using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), electron energy‐loss spectroscopy (EELS), electron diffraction (EDX), optical absorption spectroscopy and Raman spectroscopy as probes we prove that H has a key role on the morphology and stochiometry of the different SiC nanostructures.