Dóra Fejes

A Review of the Properties and CVD Synthesis of Coiled Carbon Nanotubes

The CVD route for carbon nanotube production has become a popular method to make large amounts of multiwall carbon nanotubes. The structure, morphology and size of carbon materials depend critically on the catalyst preparation and deposition conditions. According to current knowledge, CVD method is the only process which can produce carbon nanocoils. These nanocoils are perfect candidates for nanotechnology applications. One might indeed hope that these coils would have the extraordinary stiffness displayed by straight nanotubes. Based on theoretical studies, regular coiled nanotubes exhibit exceptional mechanical, electrical, and magnetic properties due to the combination of their peculiar helical morphology and the fascinating properties of nanotubes. In spite of its technological interest, relatively low attention has been paid to this special field. In this paper we attempt to summarize results obtained until now.

MnO nanoparticles as the cause of ferromagnetism in bulk dilute Mn-doped ZnO

We show that the observed ferromagnetic behavior of ZnO lightly doped with Mn coincides with the presence of MnO nanoparticles, whereas cluster-free Mn doped ZnO behaves paramagnetically. This conclusion is reached by a study of the structural and magnetic properties of powdered samples of (Mnx,Zn1–x)O with x ≤ 0.033 using polarized neutron scattering. Two types of samples were synthesized via, respectively, a solid state method and the decomposition of hydrozincite. Further characterization has been performed using standard X-ray diffraction and magnetization measurements. The results show evidence for the formation of MnO nanoparticles in the highest doped samples for both synthesis methods, with a ferromagnetic behavior attributed to uncompensated Mn2+ in the MnO nanoparticles. The lower Mn-doped samples showed no evidence for structural segregation or magnetic correlations and showed only a paramagnetic behaviour.

CCVD preparation of highly uniform carbon micro- and nanocoil fibers

Micro- and nanosized carbon fibers with special helical structure have many promising applications. These structures can be used as reinforcing materials in polymer composites, wave absorption or tactile sensor elements. In this fiber preparation method the applied catalyst precursor were nickel (II) oxide nanoparticles. Sulfuric modification of nickel containing catalyst grain yields carbon fibers with special coiled structures in acetylene decomposition. The preparation of carbon fiber coils by the CCVD method was carried out between 700- 800 C using thiophene impurity. The catalyst particles and the observed structures were studied by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The lengths of fibers can even reach millimeter size and the average fiber diameter is 1 μm. The diameter of coils is 1-20 μm, the coil pitch is 0.5-1.5 μm. Carbon microcoils might have interesting applications in the near future.