E. Mogilko

Synthesis of Carbon Nanomaterials by a Catalytic Disproportionation of Carbon Monoxide

Abstract Low‐temperature synthesis of multi‐walled carbon nanofibers and nanocapsules using catalytic disproportionation of carbon monoxide is reported. The reaction products were studied by TEM, HRTEM, and XRD. In addition, we demonstrate that hot isostatic pressing of our carbon nanopowder furnishes a promising route for the controlled fabrication of novel graphite‐like materials reinforced with multi‐walled carbon nanofibers and nanocapsules.

Coiled to Diffuse: Brownian Motion of a Helical Bacterium

We employ real-time three-dimensional confocal microscopy to follow the Brownian motion of a fixed helically shaped Leptospira interrogans (LI) bacterium. We extract from our measurements the translational and the rotational diffusion coefficients of this bacterium. A simple theoretical model is suggested, perfectly reproducing the experimental diffusion coefficients, with no tunable parameters. An older theoretical model, where edge effects are neglected, dramatically underestimates the observed rates of translation. Interestingly, the coiling of LI increases its rotational diffusion coefficient by a factor of 5, compared to a (hypothetical) rectified bacterium of the same contour length. Moreover, the translational diffusion coefficients would have decreased by a factor of ~1.5, if LI were rectified. This suggests that the spiral shape of the spirochaete bacteria, in addition to being employed for their active twisting motion, may also increase the ability of these bacteria to explore the surrounding fluid by passive Brownian diffusion.

Analysis of the excess resistivity induced by a low magnetic field in YBCO and BSCCO compounds

The results of a (weak) DC magnetic field induced excess resistivity in granular high-temperature superconductors are analyzed. Assuming a random mixture of superconducting and normal regions, the two characteristic excess resistivity peaks are interpreted as originating from two different mechanisms: the higher temperature peak in excess resistivity is attributed to a distribution of the critical temperature in individual grains affected by the applied magnetic field, this results in a quasi-percolative normal current energy dissipation. The lower temperature wide peak is attributed to an interplay between strong pinning due to the spatial inhomogeneity of the condensation energy on one hand and to the onset of proximity effect with decreasing temperature, on the other. The results obtained with various concentrations of Ag admixture and under varying magnetic field are consistent with the predictions of this model.

Anomalous quenching of the remanent resistivity in YBCO ceramics

Abstract We report an anomalous reduction of the resistivity in certain YBCO ceramics, upon applying and consequently removing a low DC magnetic field. This phenomenon is observed just below Tc, while at lower temperatures the observed remanent resistivity is higher than the pristine, similar to the characteristic “field-on” resistivity hump. We show that this effect can be interpreted as originating in an intricate interplay of the frozen-in screening currents and the superconducting percolating current in the presence of the complex intergranular boundary system.

The effect of Ag concentration on the vortex pinning at the grain boundaries in the Ag/BSSCO composites

Abstract The BSSCO/Ag compounds prepared via the AgNO 3 route exhibit a complex magnetic field induced excess resistance. We argue that this is related to several different types of pinning centers. The pinning strength depends strongly on the Ag content and on its distribution in the BSSCO matrix.

Estimation of the Josephson critical current of a single grain: percolation model of the resistive transition

Abstract The granular HTS is treated here as a S-N mixture. Experimental data are used to determine the percolation threshold f0 (the volume fraction of superconducting grains at zero resistance) and fp (corresponding to the appearance of the first spanning superconducting cluster). The latter consists of percolating channels, each carrying the Josephson critical current Ich. We demonstrate that, knowing f0 and fp as well as the morphology and orientation of the grains, one can derive realistic estimates of Ich. This is realized by assuming a parallel resistive combination, one resistor being the spanning superconducting cluster, the other the nonspanning network. The former is treated as a percolation problem while the later is described within the effective-medium theory.

Characterization of superconducting YBCO/polyacrylonitrile composites

Abstract The elasto-mechanical and transport properties of HTSC YBCO/polyacrylonitrile (PAN) composites have been investigated. The results indicate clearly the effect of PAN presence on the porosity, microstructure, crack-stability and toughness, and grain-surface pinning strength. At 35% PAN fraction an extensive bridgelike linking network develops, which results in a dramatic increase of the crack-stability and an increased fluxon mobility. In contrast to other reported SC/polymer systems, the YBCO/ PAN composite exhibits a transport current resistive transition even at relatively high PAN concentration. A comparison of finegrain and coarse-grain YBCO ceramic characteristics allows a better understanding of the role played by the PAN host.

Carbon Encapsulated Magnetic Nanoparticles Produced by a Catalytic Disproportionation of Carbon Monoxide

Carbon nanocapsules with a ferromagnetic core of single-crystalline Fe3O4 are demonstrated to be effectively synthesized and collected separately from the other nano-carbon products of the low-temperature reaction of catalytic disproportionation of carbon monoxide. HRTEM demonstrated a defect-free crystalline structure of the Fe3O4 nanoparticles. The encapsulating carbon shells of the Fe3O4 nanoparticles are stable in air at room temperature, but do not prevent them at high temperatures. Accordingly, these nanoparticles may also act as catalysts for the corresponding production of carbon nanomaterials via carbon monoxide disproportionation. In particular, we demonstrate the corresponding transformation of a Fe3O4 core to an iron carbide nanoparticle with simultaneous formation of additional encapsulating carbon layers. Characterization of the synthesized materials by DC magnetization represents clearly resolved hysteresis loops. However characteristic S-shape of the loops (magnetization is still not saturated at 16 kOe) points out some superparamagnetic effects driven by the nano-size origin of the samples. Analysis of the sample’s EPR spectra provides an additional insight to the coexistence of several magnetic phases in the synthesized nanomaterials.

Anisotropic characteristics of the energy dissipation in Bi(2223) ceramics and tapes in weak magnetic fields

Abstract For practical application of HTSC tapes as current leads, their behavior in realistic conditions, in the presence of magnetic field, becomes crucially important. We have investigated in detail the resistivity transition and the excess resistivity induced by low magnetic fields (1–750 Oe) in highly aligned composite structures of Bi(2223) based tapes and also, for comparison, in bulk ceramics of the same composition. We demonstrate the strong dependence of the electrical and magnetic properties on the microstructure of the composite ceramics. We observed also a dependence of the energy dissipation on the direction of the transport current with respect to the grain orientation. Percolation theory and the effective medium approximation are applied to analyze the anisotropy in the resistive transition of these systems.