68 research outputs found
Effects of High Intensity Ultrasound on BSCCO-2212 Superconductor
Slurries of powdered Bi2Sr2CaCu2O8+x superconductor in low volatility alkanes
were treated with intense ultrasound. Significant enhancements of magnetic
irreversibility as well as transport critical current are reported. The effects
are dependent on the concentration of the slurry and are optimal for 1.5% wt.
slurry loading. Electron microscopy shows that ultrasonic treatment leads to a
change in grain morphology and intergrain welding. The observed enhancement of
superconducting properties is consistent with the limitations in critical
currents in BSCCO superconductor being due to intergrain coupling rather than
intragrain pinning strength.Comment: 3 page
Superconducting Nanocomposites: Enhancement of Bulk Pinning and Improvement of Intergrain Coupling
Heterogeneous sonochemical method was applied for synthesis of novel
superconducting nanocomposites consisting of magnetic (and/or nonmagnetic)
nanoparticles embedded into the bulk of ceramic superconductors. In addition to
in-situ production of the efficient pinning centers, this synthesis method
considerably improves the interbrain coupling. Significant enhancement of the
magnetic irreversibility is reported for Fe2O3 nanoparticles embedded into the
bulk of MgB2 superconductor. Nonmagnetic Mo2O5 nanoparticles also increase
pinning strength, but less than magnetic Fe2O3. Detailed magnetization and
electron microscopy characterization is presented. Theory of bulk magnetic
pinning due to ferromagnetic nanoparticles of finite size embedded into the
superconducting matrix is developed
Sonochemical Modification of the Superconducting Properties of MgB2
Ultrasonic irradiation of magnesium diboride slurries in decalin produces
material with significant inter-grain fusion. Sonication in the presence of
Fe(CO)5 produces magnetic Fe2O3 nanoparticles embedded in the MgB2 bulk. The
resulting superconductor-ferromagnet composite exhibits considerable
enhancement of the magnetic hysteresis, which implies an increase of vortex
pinning strength due to embedded magnetic nanoparticles
Effective collective barrier for magnetic relaxation in frozen ferrofluids
Magnetic relaxation and frequency response were measured in frozen
ferrimagnetic colloids of different concentrations. A crossover from reversible
to irreversible behavior is observed for concentrated colloids. In irreversible
state, magnetic relaxation is time-logarithmic over seven orders of magnitude
of experimental time windows. A master curve construction within mean field
phenomenological model is applied to extract effective collective barrier as a
function of the irreversible magnetization. The barrier logarithmically
diverges, providing evidence for self-organized critical behavior during
magnetic relaxation in frozen ferrofluids
Comparative Study of Magnetic Properties of Nanoparticles by High-Frequency Heat Dissipation and Conventional Magnetometry
The rate of heating of 15 nm uniformly-sized magnetic aqueous nanoparticles suspension by high-amplitude and high-frequency ac magnetic field induced by the resonating LC circuit was measured. The results are analyzed in terms of specific energy absorption rate (SAR). Fitting field amplitude and frequency dependences of SAR to the linear response theory, magnetic moment per particles was extracted. The value of magnetic moment was independently evaluated from dc magnetization measurements of a frozen colloid by fitting field-dependent magnetization to a Langevin function. The two methods produced similar results, which are compared to the theoretical expectation for this particle size. Additionally, analysis of SAR curves yielded effective relaxation time
Direct Observation of Early Stages of Growth of Multilayered DNA-Templated Au-Pd-Au Core-Shell Nanoparticles in Liquid Phase
We report here on direct observation of early stages of formation of multilayered bimetallic Au-Pd core-shell nanocubes and Au-Pd-Au core-shell nanostars in liquid phase using low-dose in situ scanning transmission electron microscopy (S/TEM) with the continuous flow fluid cell. The reduction of Pd and formation of Au-Pd core-shell is achieved through the flow of the reducing agent. Initial rapid growth of Pd on Au along <111> direction is followed by a slower rearrangement of Pd shell. We propose the mechanism for the DNA-directed shape transformation of Au-Pd core-shell nanocubes to adopt a nanostar-like morphology in the presence of T30 DNA and discuss the observed nanoparticle motion in the confined volume of the fluid cell. The growth of Au shell over Au-Pd nanocube is initiated at the vertices of the nanocubes, leading to the preferential growth of the {111} facets and resulting in formation of nanostar-like particles. While the core-shell nanostructures formed in a fluid cell in situ under the low-dose imaging conditions closely resemble those obtained in solution syntheses, the reaction kinetics in the fluid cell is affected by the radiolysis of liquid reagents induced by the electron beam, altering the rate-determining reaction steps. We discuss details of the growth processes and propose the reaction mechanism in liquid phase in situ
Magnetic nanoparticles as efficient bulk pinning centers in type-II superconductors
Enhancement of flux pinning by magnetic nanoparticles embedded into the bulk
of type-2 superconductor is studied both theoretically and experimentally.
Magnetic part of the pinning force associated with the interaction between a
spherical magnetic inclusion and an Abrikosov vortex was calculated in the
London approximation. Calculations are supported by the experimental results
obtained on sonochemically modified MgB2 superconductor with embedded magnetic
Fe2O3 nanoparticles and compared to MgB2 with nonmagnetic Mo2O5 pinning centers
of similar concentration and particle size distribution. It is shown that
ferromagnetic nanoparticles result in a considerable enhancement of vortex
pinning in large-kappa type-2 superconductors.Comment: PDF, 14 page
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