Viorel Sandu

Transport properties of superconducting MgB2 composites with carbon-encapsulated Fe nanospheres

We present the magnetic and transport properties of superconducting composites fabricated by admixing carbon-encapsulated Fe nanospheres and MgB2 powder. The addition of nanoparticles is expected to enhance the critical current density by carbon-doping the MgB2 matrix and by providing artificial pinning sites. Three samples with estimated amounts of 0.35, 0.6, and 1.0 wt. % metallic Fe were prepared using the spark plasma sintering technique. The average size of these nanoparticles is comparable to the superconducting coherence length of MgB2 at approximately 5 nm. We found that the additions do not significantly alter the critical temperature which is very high, close to that of the pure MgB2 samples. We have also observed improved current densities, as high as 1100 kA/cm2 for the samples with 0.35 wt. % metallic Fe at 5 K and 1 T. A core-shell model for explaining the transport data is presented. The field and temperature dependence of the reduced pinning force is described in terms of pinning on grain ...

Magnetic glass-ceramics

We present the magnetic properties of magnetic glass ceramics obtained by crystallization of Fe containing borosilicate glass. Two types of nucleators have been used: Cr2O3 and P2O5. The role of the nucleators proved to be crucial in the size and morphology of the crystallites developed within glassy matrix as well in the magnetic response. The former stimulates the growth of regular single crystals uniformly dispersed within the matrix whereas the latter leads to the formation of grains made of tiny (30 nm), nanocrystals. The magnetic response depends on the amount of Fe ions left dispersed within glassy matrix as paramagnetic ions. Although P2O5 leads to the best structural magnetite, almost 42% of Fe ions are left dispersed in the matrix without magnetic interaction. In the case of Cr2O3, the paramagnetic Fe is decreased to 12% but structural deficiency in the occupancy of the Fe sites of magnetite is revealed by Mössbauer spectroscopy.

The role of radiation damage structure and fine scale precipitation in the pinning improvement of thermal neutron irradiated lithium fluoride-doped YBa2Cu3O7−x

Abstract The improvement of intragranular critical current density, j c , and flux pinning obtained by thermal neutron irradiation (fluences between 10 16 −10 18 neutrons/cm 2 ) in YBa 2 Cu 3 O 7− x doped with 8 mol% lithium fluoride (LiF) (YBCO–LiF) is explained by the occurrence of various irradiation defects (point defect clusters, dislocation loops, stacking faults) observed by transmission electron microscopy (TEM). A twin fading process is also revealed accompanied by a lowering of the orthorhombicity index. At higher irradiation fluences (∼5×10 17 ÷10 18 neutrons/cm 2 ), a new mechanism related to the dispersion of fine Cu 2 O precipitates in the superconducting YBCO–LiF matrix could explain the enhancement of up to eight times obtained for j c values. A good agreement between the dependence of the measured j c values on the applied magnetic field and the calculated curves, j c = f ( B ), derived from a theoretical model, previously proposed for the pinning contribution of fine particles dispersed in YBCO matrix, is obtained. The role of thermal neutron irradiation for the flux pinning improvement by introducing of strong pinning centers (irradiation defects and Cu 2 O microdispersoids) is also discussed.

Fabrication and transport properties of manganite-polyacrylamide-based composites

We present the fabrication and transport properties of a series of composites made of La2/3Sr1/3MnO3 and acrylamide-based copolymers. The most important result is the very narrow transition, of only 27 K, displayed by the peak that appears around the metal-insulator transition of the composites made with poly(acrylamide-vinylacetate). Although the amount of polymer is rather low, different copolymers change drastically the electric transport characteristics.

Synthesis and characterization of star and brush grafted polysiloxanes, obtained by atom transfer radical polymerization

Abstract Star- and brush-polysiloxanes were synthesized via living radical polymerization using two different catalytic systems. The new compounds were characterized by 1H-NMR, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) techniques. CuCl and 2,2’-bipyridyl proved to be the most efficient catalytic system. The type of catalytic system and the reaction conditions control the number of arms for star polymers or the grafted-chain density for brush polymers. The use of chloromethyl styrene as monomer or co-monomer yields homogeneous star- or brush-polysiloxanes, although chloromethyl styrene is an inimer system. The homogeneity of materials was verified by GPC and DSC (TOPEM) methods. For linear grafted polysiloxanes the molecular simulations reflect a tendency of chains to adopt globular conformation for high values of the branching density. The polysiloxanes grafted with styrene/chloromethyl styrene were modified with azobenzene in order to obtain systems able to react to the light stimuli. Rheological behavior of the polymer solutions were evaluated before and after irradiation with UV light. After the UV irradiation a significant viscosity increase was evidenced

Nonmonotonous Variation of the Superconducting Parameters of Neutron Irradiated Li-Doped YBa2Cu3O7−x

The effect of thermal and epithermal neutron irradiation on the superconducting critical temperature and critical current density of some Li-doped YBa2Cu3O7−x samples was studied. The critical temperature exhibits a peak and the critical current density a valley in their dependence on neutron fluence, for moderate dose. A simple model, based on the Van Hove scenario and the kinetics of the defect production, is used to describe both phenomena.

Magnetic properties of glass-ceramics obtained by crystallization of iron-rich borosilicate glasses

The specific dynamic magnetic response and magnetic relaxation phenomena in magnetite-based glass-ceramics by controlled crystallization of Fe-rich borosilicate glasses with 25 wt% Fe2O3, in the presence of two types of nucleating agents, Cr2O3 and P2O5, were investigated. The magnetic response is complex and shows contributions arising from two subsystems: a system with collective characteristics, superspin-glass like, and another one with single particle characteristics (superparamagnetic) with dipolar interaction. The nucleating agents have strong influence on the characteristic temperatures and anisotropy energy.

Charge Transport in Spin-Textured YBa2Cu3O6.25

The electric transport of the charged particles in a spin texture was investigated in a strongly underdoped YBa2Cu3O6.25 single crystal in order to identify the characteristic electrical transport mechanism. The in-plane resistivity revealed three different regimes of charge transport: a chiral 2D VRH regime up to 55 K with a characteristic temperature Td ≈ 12,400 K, an impurity band conduction regime above 55 K, and a metallic-like regime beyond 170 K. The out-of-plane resistivity has only one crossover at 115 K, but the conduction mechanisms controlling the two regimes are not clear.

Fluctuation conductivity in Li-doped YBa2Cu3O7-x

The paraconductivity of Li-doped YBa2Cu3O7−x was measured. We have found that the character of the fluctuation changes as lithium content increases: instead of a crossover from 2D to 3D behavior (D is the dimensionality), as we have observed for a low doping level, a double crossover to the 2D percolative and 3D percolative regimes becomes manifest by increasing the lithium content.

Neutron irradiation of Li-doped YBa2Cu3O7−δ

The effects of neutron irradiation on the critical temperature and critical current density at 77 K and zero magnetic field of high-temperature superconductors was studied in Li-doped YBa2Cu3O7−x. Damage and thermal neutron reactions with Li produce an insignificant increase of the critical temperature for Li inserted as flame-melted Li2CuO2 while for the others it remains nearly unaffected. The critical transport current density of the sintered YBa2Cu3O7−x remains unaffected for samples with Li2CuO2 and shows a decrease for LiFdoped samples.The effects of neutron irradiation on the critical temperature and critical current density at 77K and zero magnetic field of high-temperature superconductors was studied in Li-doped YBa{sub 2}Cu{sub 3}O{sub 7{minus}x}. Damage and thermal neutron reactions with Li produce an insignificant increase of the critical temperature for Li inserted as flame-melted Li{sub 2}CuO{sub 2} while for the others it remains nearly unaffected. The critical transport current density of the sintered YBa{sub 2}Cu{sub 3}O{sub 7{minus}x} remains unaffected for samples with Li{sub 2}CuO{sub 2} and shows a decrease for LiF-doped samples.