The interaction of electromagnetic radiation in X and Ka bands with magnetic nanocomposite of disordered carbon nanotubes arrays has been investigated both experimentally and theoretically. Samples were synthesized on the quartz reactor walls by decomposition of ferrocene and xylene which provided random intercalation of iron phase nanoparticles in carbon nanotube array. The exhaustive characterization of the samples by means of the scanning electron microscopy, Raman spectroscopy, and x-ray photoemission spectroscopy was performed. It was found that the absorption of the electromagnetic wave monotonically increases with the frequency. To describe these experimental data, we extended the Bruggeman effective medium theory to a more complex case of a magnetic nanocomposite with randomly distributed spherical ferromagnetic nanoparticles in a conducting medium. The essential feature of the developed model is the consideration of the complex nature of the studied material. In particular, such important parameters as magnetic and dielectric properties of both the carbon nanotube medium and the nanoparticles, the volume concentration and the dimensions of the nanoparticles, the wave impedance of the resistive-capacitive shells of the conductive nanoparticles are explicitly taken into account in our model. Moreover, analysing the experimental results, we were able to obtain the frequency dependencies of permittivity and permeability of the studied nanocomposite.

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(100) oriented niobium (Nb) crystals annealed in the vacuum conditions close to that used in mass production of 1.3 GHz superconducting radio-frequency cavities for linear accelerators, and treated in nitrogen at a partial pressure of 0.04 mbar at temperatures of 800 °C and 900 °C have been studied. The surfaces of the nitrogen-treated samples were investigated by means of various surface-sensitive techniques, including grazing-incidence X-ray diffraction, X-ray photoemission spectroscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy in planar view and on cross-sections prepared by a focused ion beam. The appearance of a dense layer of epitaxial rectangular precipitates has been observed for the Niobium nitrided at 900°C. Increased nitrogen concentration in the near surface region was detected by glow-discharge optical-emission spectroscopy, focused ion-beam cross-sectional images and X-ray photoelectron spectroscopy. Crystalline phases of NbO and β-Nb 2 N were identified by X-ray diffraction. This information was confirmed by X-ray photoelectron measurements, which in addition revealed the presence of Nb 2 O 5 , NbON, NbN and NbN x O y components on the surface. These results establish the near-surface Nb phase composition after high-temperature nitrogen treatment, which is important for obtaining a better understanding of the improved RF cavity performance.

We present results of study of magnetic properties of nanocomposite formed in situ during the synthesis of aligned carbon nanotube (CNT) arrays. CNTs were grown by the atmospheric pressure floating catalyst chemical vapor deposition method. High temperature pyrolysis of ferrocene/xylene solution injected into the quartz tube at high temperature was carried out. By varying the ferrocene content (CF) in the ferrocene/xylene solution we tuned the concentration, location, size, shape and chemical state of iron based catalytic ferromagnetic nanoparticles. The variation of these parameters was reflected in the magnetic properties of the CNT based nanocomposite. In particular, it is shown, that the main interaction mechanism between ferromagnetic nanoparticles for high CF contents is the exchange coupling, while the magnetic anisotropy dominates at low ferrocene concentration. The role of the orientation of the nanotubes is decisive for the observation of magnetic anisotropy. When the alignment is destroyed, the exchange coupling mechanism starts to dominate also for low CF samples. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)