Papers by Keyword: Critical Current Density

Abstract: The effects of Ni_0.5Zn_0.5Fe₂O₄ nanoparticle addition on the superconducting and transport properties of (Tl_0.85Cr_0.15)Sr₂CaCu₂O₇ (Tl-1212) superconductor were investigated in this paper. The Tl-1212 samples were produced by mixing high purity oxide powders through a solid-state reaction method. Nano Ni_0.5Zn_0.5Fe₂O₄ particles with compositions of 0.001 wt.%, 0.003 wt.%, 0.005 wt.%, 0.01 wt.% and 0.02wt.% with average size of 60 nm were added into the Tl-1212 powders. The transition temperatures (T_c-zero and T_c-onset) were measured using a four-point probe method. The highest T_c-zero recorded was 97 K which was exhibited by the pure Tl-1212 sample. The transport critical current, I_c, of the Tl-1212 samples were found through the 1 µV/cm criterion with temperature ranging from 30 K to 77 K. The sample with a composition of 0.003 wt.% displayed the highest value of J_c at 77 K with a value ranging up to 1780 mA/cm². The Tl-1212 samples were characterised using scanning electron microscopy (SEM), powder X-ray diffraction method (XRD), energy dispersive X-Ray analysis (EDX), electrical resistance measurements and transport critical current density measurements. The J_c of the Tl-1212 superconductor has been improved through the addition of Ni_0.5Zn_0.5Fe₂O₄ nanoparticles but adding an excessive amount has caused its J_c to degrade.

Abstract: A study of the effects of Cerium oxide nanoparticle doped with BSCCO-2223 on the microstructure and superconducting properties was carried out. All samples were synthesized using solid state reaction method. Ce concentration is varied from x = 0.0 up to 0.1 in a general stoichiometry of Bi_1.6Pb_0.4Sr₂Ca_1-xCe_xCu₃O_y. The samples were characterized structurally and electrically by X-Ray Diffraction (XRD) and four-point probe method respectively. XRD analysis shows that both (Bi,Pb)-2212 and (Bi,Pb)-2223 phases coexist in the samples having tetragonal crystal structure but changed to orthorhombic when x=0.10. The values of critical transition temperature, T_C and critical current density, J_C of the samples decreased with the increase in Ce concentration. The possible reasons for the observed degradation in superconducting and structural properties of Bi-2223 due to Ce nanoparticles addition were discussed.

Abstract: Effect of complex magnetic oxide Co_0.5Ni_0.5Fe₂O₄ (CNFO) nanoparticles addition in (Bi_1.6Pb_0.4)Sr₂Ca₂Cu₃O₁₀ (Bi-2223) superconductor tapes was investigated. Ultrafine Bi-2223 powder precursor was prepared via co-precipitation method and was added with 0.01 – 0.05 wt.% Co_0.5Ni_0.5Fe₂O₄ nanoparticles during the final heating stage. The sample with 0.01 wt.% addition, Bi-2223(CNFO)_0.01 was found to have the highest critical current density, J_c. This sample were then chosen to be fabricated into Ag-sheathed superconductor tapes using the powder-in-tube (PIT) method. The tapes were sintered for 50 and 100 h at 845 °C. The phase, microstructure and J_c of the samples were determined by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and four point probe, respectively. J_c of Ag-sheathed Bi-2223(CNFO)_0.01 tapes sintered for 100 h was 19830 A/cm² at 30 K and 3970 A/cm² at 77 K compared to tapes without addition which showed a much lower J_c (6370 A/cm² at 30 K). This study showed that CNFO nanoparticles could act as an effective flux pinning centers to enhance the critical current density in the Bi-2223 superconductor.

Abstract: Our investigations show that the addition of Ti, polyvalent titanium oxides or TiC powders (0.1-40 mm, amount: 10 wt%) can affect the formation of MgB_x (x³4) inclusions and a redistribution of admixed oxygen resulting in the appearance of dispersed inclusions with near MgBO stoichiometry in the MgB₂ matrix and thus influencing the critical current density and the critical magnetic fields of the materials. The highest B_c2 and B_irr were obtained when powdered polyvalent titanium oxides (synthesized by electroerosion dispersion) or powdered titanium were added, but the critical magnetic fields were somewhat lower in the case of Ti additions. We show that Mg diffuses during the synthesis inside the grains of polyvalent titanium oxide, titanium carbide or titanium, which can affect the redistribution of boron and oxygen in the superconducting matrices and thus influence pinning and the superconducting properties.

Abstract: Fault current limiters (FCL) require superconducting (SC) materials which can provide a definite rate of response to a fault event resulting in the SC – normal state reversible transition. The main characteristics determined the material suitability are the critical current density, j_c, thermal conductivity and capacity which are strongly determined by manufacturing technology, in particular, of MgB₂. In the paper we estimate the j_c of bulk MgB₂ samples by the vibrating magnetometer and inductive, contactless transformer, method using ring samples. The bulk MgB₂ samples were produced under 30 MPa (hot pressing) and 2 GPa (quasihydrostatic pressing) at 800-1050 ^оС from different initial ingredients (Mg and B or MgB₂ with and without additions). It is shown that the technology process and initial ingredients strongly influence the distribution of boron-and oxygen-enriched nanosized inhomogenities in MgB₂ matrix, connectivity between SC grains, material porosity and, as result, the SC properties. The transformer method gives the j_c in the range from 1.6·10⁴ up to 6.3·10⁴ A/cm² at about 4 K while using magnetometer measurements the j_c is estimated from 2.24·10⁵ up to 5.1·10⁵ A/cm² at 10 K in self-magnetic fields. The contradictions in the j_c estimated by different methods can be explained by instability of the SC state of MgB₂, caused by variation of the applied magnetic field. Using the transformer method AC losses per a cycle before quenching for the best materials were estimated around 0.75-1 J/cm³, while the power of losses was about 200 W. The FCL model with rings cut out from SC MgB₂ materials prepared using various technologies demonstrated that MgB₂ is a promising material for application in inductive FCLs.

Abstract: The effects of sintering time in high and low density Bi-2223 phase formation have been investigated. The samples were prepared by the solid-state reaction method at various sintering times ranging from 24, 48, 72 and 96 hours. Sucrose was added during palletization and after heated at 400°C for two hours the sucrose was removed and hence low density sample was created. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and critical current density. The transition temperature varies between 102 K and 96 K with increasing of sintering times. The optimal sintering time of the samples Bi-2223 system was found at 850°C for 72 hours. The critical current density, J_C of high density and low density Bi-2223 was measured to be 7.547 A/cm² and 8.333 A/cm² respectively at 77 K under zero magnetic field. The critical current density, J_C and superconductivity transition temperature, T_C of low density were found to be higher than the pure samples. The critical transition temperature increased with a short gap between T_{C onset} and T_{C zero}. The most intense peak in the XRD pattern of sample at sintering time 72 hours belong to the high-T_C phase which also indicates an increase in the volume fraction of the high-T_C phase with optimum sintering time.

Abstract: In this study, as much as 10 and 15 wt.% nanosized silicon and carbon (Si+C) were reacted with (Mg+2B) at 650°C and 850°C, respectively, for 1 hour. The phase formation, surface morphology and superconducting properties of these samples were evaluated. The relative peak intensity as calculated from the XRD patterns indicates the formation of large Mg₂Si volume fraction at low sintering temperature. MgB₄ phase was detected in the samples sintered at high temperature as a result of Mg deficiency. The C substitution level as estimated from the lattice parameters, was shown to increase in the samples reacted with a higher amount of (Si+C) at high temperature. Scanning electron micrograph showed that (Si+C) co-addition had refined the grain size and improved the grain coupling of MgB₂. The superconducting transition temperature was found to decrease with increasing addition level. The superconducting transition width was also broadened because of a large volume fraction of secondary phases. The improved field dependent critical current density at both 5 K and 20 K is accounted to enhanced scattering by C substitution and grain boundary pinning.

Abstract: The bulk samples with the composition Bi_1.8Pb_0.4Sr_1.9Ca_2.1Cu_3.5O_y + x wt% MgO + (5-x) wt% Ag₂O (x = 0, 1, 2, 3, 4 and 5) were prepared by sintering at 840°C for 240 h after partial­melting at 870°C for 1 h. The sample with individual Ag₂O addition shows the lowest melting temperature and the lowest proportion of the Bi-2223 phase, whereas the sample with individual MgO addition shows the highest melting temperature. The highest proportion of the Bi-2223 phase appears in the sample with 1wt% MgO and 4 wt%Ag₂O mix-additions. With increasing MgO content, the width of hysteresis loop ΔM of the bulk samples at 77 K increases (0≦x≦2), and then decreases (2≦x≦5). The largest ΔM appears in the sample with 2 wt% MgO and 3 wt% Ag₂O mix-additions.

Abstract: MgB₂ is a candidate for the fabrication of magnetic coils used in medical applications. Our review indicate that oxide additions based on the rare earth or metalloid elements show improvement of the MgB₂ critical current density (J_c) and the irreversible magnetic field (H_irr) without significantly affecting the critical temperature (T_c) However, the characteristics of the additions and the technological approaches show a strong influence in controlling superconducting properties. Both additions and the technology need a careful and complex optimization in order to enhance the J_c and H_irr.

Abstract: Auger and SEM studies show that with increasing of MgB₂ manufacturing temperature from 600÷800 °C to 1050÷1100 °C the Mg-B-O nanolayers which are present in the MgB₂ matrix transform into distinct dispersed Mg-B-O inclusions. On the other hand the sizes of inclusions of higher magnesium borides (MgB_x, x=7 ÷ 25) which are also present in the MgB₂ matrix. The tendency is observed in a wide range of synthesis pressures (0.1 MPa-2 GPa). The described structural transformations are accompanied by an increase in critical current density, j_c, in low and medium magnetic fields and by transition from the grainboundary to the point pinning. The Ti addition results in a further increase in j_c due to: Ti promotes the formation of higher magnesium boride inclusions and localization (or segregation) of oxygen in MgB₂ matrix, and, hence, facilitates the formation of a homogeneous MgB₂ matrix with lower oxygen content, but with an increased number of Mg-B-O and MgB_x pinning centers. At low synthesis temperature Ti absorbs hydrogen forming titanium hydrides, thus preventing the formation of MgH₂ and provides the material densification. The positive effect of Ti addition is connected with the high ability of Ti to absorb hydrogen, oxygen, and magnesium. The results of the critical current and AC loss study by transformer method using rings from MgB₂ are discussed.