Improved critical current density in Zn doped YBCO single crystals

Magnetic measurements were made using pure YBCO and Zn doped YBa₂(Cu_1?xZn_x)₃O_7?σ. Single crystals with Zn concentration of 0.5%, 1.5%, 3.0% and 4.3%. The magnetic hysteresis loops for these samples were measured in the temperature range 0.1 ? T/T_c ? 0.96 under magnetic fields of 5 T using SQUID. It was found that the critical current density J_c increased for low Zn content samples up to 3% Zn concentration compared to pure YBCO sample and decreased for the higher Zn content samples. These values varied consistently when compared at magnetic fields of 1 T and 3 T. Moreover Zn doped samples showed significant values of J_c in the temperature range of 0.7–0.9T_c, close to critical temperature compared to pure YBCO sample. The irreversibility field H_irr was also enhanced in this temperature range showing consistent decrease with increase of Zn concentration. The peak field H_p above H_c1 and irreversibility field H_irr, both show power law dependence of the form H = m₁(1 ? T/T_c)^m2 in the temperature range of 0.75–0.96T_c. The values of parameter m₂ increased from 1.44 to 1.95 for the samples up to 3% Zn content and decreased to 1.37 for higher Zn contents. The ratio H_irr/H_p was found to be 3–4 for the lower Zn content samples and was 7–8 for the sample with high Zn content indicating more disorder for higher Zn content samples. The region between peak field H_p and irreversibility field H_irr was broadened with the increase of Zn concentration. The strong effect of Zn substitution in modifying behavior of these samples even at elevated temperatures is possibly due to the changes in the anisotropy of our samples with the increase of Zn concentration and also due to the locally induced changes in magnetic moments by Zn substitution.     

Influence of Zr and Ce doping on electromagnetic properties of (Gd,Y)–Ba–Cu–O superconducting tapes fabricated by metal organic chemical vapor deposition

(Gd,Y)Ba₂Cu₃O_x tapes have been fabricated by metal organic chemical vapor deposition (MOCVD) with Zr-doping levels of 0–15 mol.% and Ce doping levels of 0–10 mol.% in 0.4 μm thick films. The critical current density (J_c) of Zr-doped samples at 77 K, 1 T applied in the orientation of H 6 c is found to increase with Zr content and shows a maximum at 7.5% Zr doping. The 7.5% Zr-doped sample exhibits a critical current density (J_c) of 0.95 MA/cm² at H 6 c which is more than 70% higher than the J_c of the undoped sample. The peak in J_c at H 6 c is 83% of that at H 6 a–b in the 7.5% Zr-doped sample which is more than twice as that in the undoped sample. Superconducting transition temperature (T_c) values as high as about 89 K have been achieved in samples even with 15% Zr and 10% Ce. Ce-doped samples with and without Ba compensation are found to exhibit substantially different J_c values as well as angular dependence characteristics.     

The effect of sintering temperature on the intergranular properties of Bi2223 superconductors

A systematic study of the intergranular properties of (Bi,Pb)₂Sr₂Ca₂Cu₃O_y (Bi2223) polycrystalline samples has been done using the electrical resistivity and Ac susceptibility techniques. In this project, we have prepared a series of Bi2223 samples with different sintering temperature. The XRD results show that by increasing sintering temperature up to 865 °C the Bi2212 phase fraction decrease. It was found that the Bi2212 phase on the grain boundaries is likely to play the role of weak links and consequently reduces the intergranular critical current densities. Analysis of the temperature dependence of the Ac susceptibility near the transition temperature (T_c) has been done employing Bean's Critical State Model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the weak links are changed from superconductor–normal metal–superconductor (SNS) for well-coupled sample to superconductor–insulator–superconductor (SIS) type of junctions for the sample with high Bi2212 phase fraction.     

Strongly enhanced flux pining of GdBCO coated conductors with BaSnO3 nanorods by pulsed laser deposition

We report the effects of BSO addition on the crystallinity, texture, and the field dependency of critical current density (J_c) of GdBCO coated conductors (CCs) prepared by pulsed laser deposition (PLD). Undoped and BSO-doped GdBCO films showed only c-axis oriented growth, and the incorporated BSO nanorods exhibited epitaxial relationship with the GdBCO matrix. In comparison with undoped film, BSO-doped GdBCO film exhibited greatly enhanced J_c and higher pinning force densities in the entire field region of 0–5 T (H//c) at 77 and 65 K. The BSO-doped GdBCO film showed the maximum pinning force densities (F_p) of 6.5 GN/m³ (77 K, H//c) and 32.5 GN/m³ (65 K, H//c), ～2.8 times higher than those of the undoped sample. Cross-sectional TEM analyses exhibited nano-structured BSO nanorods roughly aligned along the c-axis of the GdBCO film, which are believed effective flux pinning centers responsible for strongly improved critical current densities in magnetic fields.     

Pinning,thermally activated depinning and their importance for tuning the nanoprecipitate size and density in high Jc YBa2Cu3O7−x films

YBa₂Cu₃O_7?x (Y123) films with quantitatively controlled artificial nanoprecipitate pinning centers were grown by pulsed laser deposition (PLD) and characterized by transport over wide temperature (T) and magnetic field (H) ranges and by transmission electron microscopy (TEM). The critical current density J_c was found to be determined by the interplay of strong vortex pinning and thermally activated depinning (TAD), which together produced a non-monotonic dependence of J_c on c-axis pin spacing d_c. At low T and H, J_c increased with decreasing d_c, reaching the very high J_c ～ 48 MA/cm² ～20% of the depairing current density J_d at 10 K, self-field and d_c ～ 10 nm, but at higher T and H when TAD effects become significant, J_c was optimized at larger d_c because longer vortex segments confined between nanoprecipitates are less prone to thermal fluctuations. We conclude that precipitates should extend at least several coherence lengths along vortices in order to produce irreversibility fields H_irr(77 K) greater than 7 T and maximum bulk pinning forces F_p,max(77 K) greater than 7–8 GN/m³ (values appropriate for H parallel to the c-axis). Our results show that there is no universal pin array that optimizes J_c at all T and H.     

Effects of nanoscale defects on critical current density of (Y1−xEux)Ba2Cu3O7−δ thin films

In pulsed laser deposition of YBa₂Cu₃O_7?δ films, defect introduction into the films tends to anisotropically improve the pinning along the H||c direction due to the columnar growth mode of the process. In Eu-substituted samples, however, even though an increase in critical current density (J_c) in the H||c direction was observed for low fields (H = 0.2 T), the improvement was more notable for the H||ab-plane at both low and higher fields. Herein we present detailed TEM microstructural studies to understand these new trends in J_c(H), which are markedly different than flux pinning increases achieved with other methods, for example, with nanoparticle additions. Threading dislocations, observed in the Eu-substituted samples along the c-axis, account for J_c enhancement with H||c at low field. The enhanced ab-planar pinning in the Eu-substituted samples is attributed to the extensive bending of the {0 0 1} lattice planes throughout the film, and the crystal lattice defects with excess Cu–O planes, that were effective in increasing the J_c for H||ab at both low and high fields.     

Pinning potential in thick PrBa2Cu3Ox/YBa2Cu3O7−δ quasi-multilayers

Using Pulsed Laser Deposition we have fabricated thick quasi-multilayers composed of incomplete layers of PrBa₂Cu₃O_x (PrBCO) nano-dots and layers of YBa₂Cu₃O_7?δ (YBCO). The number of such sequences was between 2 and 6, with the thickness of individual YBCO layers between 565 and 885 nm, and total thickness between 1.13 and 5.31 μm. For the thinner quasi-multilayer, DC magnetization studies showed an increase in the critical current density J_c at all fields in comparison with a pure YBCO reference sample, while the thicker samples showed an increased J_c only in high fields. We have also investigated the frequency dependence of J_c from AC susceptibility studies and found that the pinning potential is well described by a logarithmic dependence on current density. Pinning potentials in PrBCO/YBCO quasi-multilayers also proved to be higher than in the reference sample at high fields. From angle-dependent transport measurements we have found indications of strong pinning centres induced by the (PrBCO) nano-dots, both isotropic and c-axis correlated.     

Effect of Boron substitution on the superconductivity of non-oxide perovskite MgCNi3

We report synthesis, structural and magnetic (DC and AC) properties of Boron substituted MgCNi₃ superconductor. A series of polycrystalline bulk samples Mg_1.2C_1.6?xB_xNi₃ (x=0.0, 0.08 and 0.16) is synthesized through standard solid-state reaction route, which are found to crystallize in cubic perovskite structure with space group Pm3m. Rietveld analysis of observed XRD data show that lattice parameters expand from a=3.8106 (4) Å for pure, to 3.8164 (2) Å and 3.8173 (5) Å for 5% and 10% Boron substituted samples respectively. DC magnetization exhibited superconducting transition (T_c) at around 7.3 K for pure sample, and the same decreases slightly with Boron substitution. The lower critical field (H_c1) at 2 K is around 150 Oe for pure sample, which increases slightly with Boron substitution. For pure sample the upper critical field (H_c2) being determined from AC susceptibility measurements is 11.6 kOe and 91.70 kOe with 50% and 90% diamagnetism criteria respectively, which decreases to 5.57 kOe and 42.5 kOe respectively for 10% Boron substituted sample. 10% Boron substitution at Carbon site has decreased both the H_c2 and T_c. On the other hand lower critical field (H_c1) at 2 K is slightly increased from around 150 Oe for pure sample, to 200 Oe for 10% Boron substituted sample. Seemingly, the Carbon site Boron substitution induced disorder though has increased slightly the H_c1 but with simultaneous decrease in superconducting transition temperature (T_c) and upper critical field (H_c2). The high relative proportion of Ni in studied MgCNi₃ suggests that magnetic interactions are important and non-oxide perovskite structure make it interesting.     

A new method for estimating the critical current density of a superconductor from its hysteresis loop

The critical current density J_c of some of the superconducting samples, calculated on the basis of the Bean’s model, shows negative curvature for low magnetic field with a downward bending near H = 0. To avoid this problem Kim’s expression of the critical current density, J_c = k/(H₀ + H), where J_c has positive curvature for all H, has been employed by connecting the positive constants k and H₀ with the features of the hysteresis loop of a superconductor. A relation between the full penetration field H_p and the magnetic field H_min, at which the magnetization is minimum, is obtained from the Kim’s theory. Taking the value of J_c at H = H_p according to the actual loop width, as in the Bean’s theory, and at H = 0 according to an enhanced loop width due to the local internal field, values of k and H₀ are obtained in terms of the magnetization values M⁺(?H_min), M^?(H_min), M⁺(H_p) and M^?(H_p). The resulting method of estimating J_c from the hysteresis loop turns out to be as simple as the Bean’s method.     

Critical current density in MgB2 bulk samples after co-doping with nano-SiC and poly zinc acrylate complexes

The co-doped MgB₂ bulk samples have been synthesized using an in situ reaction processing. The additives is 8 wt.% SiC nano powders and 10 wt.% [(CH₂CHCOO)₂Zn]_n poly zinc acrylate complexes (PZA). A systematic study was performed on samples doped with SiC or PZA and samples co-doped with both of them. The effects of doping and co-doping on phase formation, microstructure, and the variation of lattice parameters were studied. The amount of substituted carbon, the critical temperature (T_c) and the critical current density (J_c) were determined. The calculated lattice parameters show the decrease of the a-axis, while no obvious change was detected for c-axis parameter in co-doped samples. This indicates that the carbon was substituted by boron in MgB₂. The amount of substituted carbon for the co-doped sample shows an enhancement compared to that of the both single doped samples. The co-doped samples perform the highest J_c values, which reaches 3.3 × 10⁴ A/cm² at 5 K and 7 T. It is shown that co-doping with SiC and organic compound is an effective way to further improve the superconducting properties of MgB₂.     

Effect of processing temperature on the superconducting properties of acetone doped MgB2 tapes

Correlation of phase formation, critical transition temperature T_c, microstructure, and critical current density J_c with sintering temperature has been studied for acetone doped MgB₂/Fe tapes. Sintering was performed at 600–850 °C for 1 h in a flowing Ar atmosphere. High boron substitution by carbon was obtained with increasing the sintering temperature; however, the acetone doped samples synthesized at 800 °C contain large size MgB₂ grains and more MgO impurities. Incomplete reaction for the acetone doped samples heated at 600 °C result in bad intergrain connectivity. At 4.2 K, the best J_c value was achieved in the acetone doped sample sintered at 700 °C, which reached 24,000 A/cm² at 10 T and 10,000 A/cm² at 12 T, respectively. Our results indicate that the small grain size and less impurity were also important for the improvement of J_c–B properties besides the substitutions of B by C.     

Increase of critical current density with doping carbon nano-tubes in YBa2Cu3O7−δ

The effects of carbon nano-tubes (CNTs) on the crystal structure and superconducting properties of YBa₂Cu₃O_7?δ (Y-123) compound were studied. Samples were synthesized using standard solid-state reaction technique by adding CNT up to 1 wt% and X-ray diffraction data confirm the single phase orthorhombic structure for all the samples. Current–voltage measurements in magnetic fields up to 9 T were used to study the pinning energy U_J and critical current density J_c as a function of magnetic field at fixed temperature. We find that while T_c does not change much with the CNT doping (91–92 K), both U_J and J_c increase systematically up to 0.7 wt% CNT doping in a broad magnetic field ranges between 0.1 and 9 T and J_c in the 0.7 wt% CNT doped sample is at least 10 times larger than that of the pure Y-123. The scanning electron microscope image shows that CNTs are forming an electrical-network between grains. These observations suggest that the CNT addition to the Y-123-compounds improve the electrical connection between superconducting grains to result in the J_c increase.     

Effects of doping with nanoscale Co3O4 particles on the superconducting properties of powder-in-tube processed MgB2 tapes

Nanoscale Co₃O₄ particles were doped into MgB₂ tapes with the aim of developing superconducting wires with high-current-carrying capacity. Fe-sheathed MgB₂ tapes with a mono-core were prepared using the in situ powder-in-tube (PIT) process with the addition of 0.2–1.0 mol% Co₃O₄. The critical temperature decreased monotonically with an increasing amount of doped Co₃O₄ particles for all heat-treatment temperatures from 600 to 900 °C. However, the transport critical current density (J_c) at 4.2 K varied with the heat-treatment temperatures. The J_c values in magnetic fields ranging from 7 to 12 T decreased monotonically with increasing Co₃O₄ doping level for a heat-treatment temperature of 600 °C. In contrast, some improvements on the J_c values of the Co₃O₄ doped tapes were observed in the magnetic fields below 10 T for 700 and 800 °C. Furthermore, J_c values in all the fields measured increased as the Co₃O₄ doping level increase from 0 to 1 mol% for 900 °C. This heat-treatment temperature dependence of the J_c values could be explained in terms of the heat-treatment temperature dependence of the irreversibility field with Co₃O₄ doping.     

AC response of YBa2Cu3O7 thin film superconductor

Low-field ac measurements of magnetic susceptibility of YBa₂Cu₃O₇ high-temperature superconducting thin film were carried out over a wide range of temperatures and ac magnetic field amplitudes. A strong field dependence of the real χ′ and imaginary χ″ components was observed. The field dependence of the imaginary component is used to extract the temperature dependence of the critical current density in the sample using the critical state model. The exponent β of the power law relation J_c ～ (1 ? T/T_c)^β was determined from ac-susceptibility data and different values were found. Experimental results are compared with predictions of some existing theoretical models describing the ac response of superconducting thin film in perpendicular ac field.     

Effect of W-addition on pinning property of MgB2

The 5d transition metal W was added into the MgB₂ superconductor. The Mg, B and W were sintered at 1173 K for 30 min under H₂/Ar atmosphere in the electric furnace. The W_x(MgB₂)_1?x samples were prepared in the W concentration range of 0 ? x ? 0.05. Temperature and field dependences of magnetization were measured by the SQUID magneto-meter. The field and x dependences of J_c at 20 K were analyzed by the extended critical state model. The enhancement of J_c became maximum for the x = 0.02 sample.     

The effect of pinning centers in Zn-doped CuBa2Ca3Cu4O12−y high-temperature superconductors

CuBa₂Ca₃Cu₄O_12−y (Cu:1234) high-temperature superconductors (HTS) doped with up to 2% Zn were grown using the high-pressure synthesis technique. Magnetization loops of the samples were measured at various temperatures between 5 and 77.3 K and magnetic fields up to 14 T. Critical current densities J_c of the samples were estimated using the critical state model. The results show that Zn-induced pinning centers increase J_c of Cu:1234 several times, depending on field and temperature. From the experimentally determined field-temperature region in which a higher Zn concentration lead to a higher J_c, we suggest the existence of a cross-over from quite efficient, extended (in the c-axes direction) pinning centers to point-like (inefficient) pinning centers at a certain temperature, depending on field. This effect can be attributed to the fact that, unlike other HTS, in Cu:1234 there is a second critical temperature T_c2 of about 70–80 K (in zero field, and 50–60 K in 15 T), related to the over-doping of pyramidal basal plane (outer CuO₂ plane).     

Intrinsic pinning properties of FeSe0.5Te0.5

The intrinsic pinning properties of FeSe_0.5Te_0.5, which is a superconductor with a critical temperature T_c of approximately 14 K, were studied through the analysis of magnetization curves obtained using an extended critical state model. For the magnetization measurements carried out with a superconducting quantum interference device (SQUID), external magnetic fields were applied parallel and perpendicular to the c-axis of the sample. The critical current density J_c under the perpendicular magnetic field of 1 T was estimated using the Kimishima model to be equal to approximately 1.6 × 10⁴, 8.8 × 10³, 4.1 × 10³, and 1.5 × 10³ A/cm² at 5, 7, 9, and 11 K, respectively. Furthermore, the temperature dependence of J_c was fitted to the exponential law of J_c(0) × exp(?αT/T_c) up to 9 K and the power law of J_c(0) × (1 ? T/T_c)ⁿ near T_c.     

Doping dependence of irreversibility line in Y1−xCaxBa2Cu3O7−δ

The irreversibility lines (IL) for series of Y_1?xCa_xBa₂Cu₃O_7?δ (x = 0.025; 0.10 and 0.20) polycrystalline samples with different overdoping were investigated. The irreversibility fields were determined from measurements of third harmonics AC susceptibility as a function of DC field at constant temperature. For the weakly overdoped sample (with x = 0.025) H_irr(77 K) is about 7 T, which is higher than the previously reported for the non-substituted one. The irreversibility line behavior is typical for glass–liquid phase transition and this is confirmed by transport measurements. On increasing the overdoping the irreversibility fields were shifted towards lower temperatures. The behavior of H_irr(T) for the highly overdoped sample (with x = 0.20) is influenced by the surface barrier effect. It is supposed that in highly overdoped specimen the process of phase separation is enhanced and the Fermi clusters grow in size. This leads to a suppression of the bulk pinning and to a domination of the surface barrier effects and flux creep as well. As a confirmation, the obtained quadratic J_c(T) dependences were presented demonstrating the existence of S–N–S type inter-grain joints in the highly overdoped samples.     

Relaxation process and ferromagnetic resonance investigation of ferrofluids with Mn–Zn and Mn–Fe mixed ferrite particles

The magnetic relaxation processes in two ferrofluids with Mn_0.4Zn_0.6Fe₂O₄ (sample F1) and Mn_0.6Fe_0.4Fe₂O₄ (sample F2) mixed ferrite particles, dispersed in n-decan and kerosene, respectively, are investigated through the determination of components χ′ and χ′′ of the complex magnetic susceptibility in the range of (2–30) MHz. The values of the saturation magnetization of the two ferrofluids are M_∞=5.28 kA/m for sample F1 and M_∞=10.99 kA/m for sample F2. A maximum of the imaginary component χ′′ was observed for both samples at frequencies of tens MHz. This maximum was assigned to relaxation processes of Néel type.The effective anisotropy constant K of the particles from the studied samples was evaluated, using both static and dynamic measurements and the values were found to be K₁=6.12×10³ J m⁻³ for the ferrofluid F1, and K₂=5.60×10³ J m⁻³ for the ferrofluid F2. From ferromagnetic resonance measurements, and based on the theoretical values computed for the Lande factor (g), the effective anisotropy constants for the mixed ferrite particles in the studied ferrofluids and the anisotropy field values were determined using a new method. The values obtained in this way for the anisotropy constants K₁ and K₂ are compared to the ones determined from magnetic relaxation measurements.     

Preparation of solution-based YBCO films with BaSnO3 particles

The YBCO films with BaSnO₃ (BSO) particles were prepared on LAO (0 0 1) substrates by metal organic deposition using trifluoroacetates (TFA-MOD) via introducing SnCl₄ powders into the YBCO precursor solution. It was found that with the increase of the SnCl₄ contents, the slower decomposition and higher temperature for nucleation during the reaction were requested compared to that of pure YBCO film. The YBCO films with different contents of Sn with dense surface and well c-alignment were obtained under optimized heat treatment, and the BaSnO₃ phases were detected by XRD analysis. Litter effect of BSO particles on the T_c and J_c values of YBCO films was found. All YBCO films with BSO particles had T_c values over 90 K and J_c values over 1 MA/cm². A significant enhancement of J_c was observed for YBCO films with BSO particles compared to that of pure YBCO film by the field dependence of J_c values. The best property was obtained for YBCO film with 6 mol.% Sn at 77 K under magnetic field. The results showed that the J_c value of YBCO film with 6 mol.% Sn was enhanced by a factor of 2 in 2 T, and over a factor of 10 beyond 4 T compared to that of pure YBCO film.