M. Jirsa

Optimization of matrix chemical ratio for high flux pinning in ternary (Nd–Eu–Gd)Ba2Cu3Oy

We prepared (Nd, Eu, Gd)Ba2Cu3Oy samples with various Nd: Eu: Gd ratios in the rare earth site. It was found that the three elements contributed to flux pinning in different ways. Nd mainly enhanced flux pinning at low magnetic fields, Eu controlled the second peak position and the irreversibility field, while Gd slightly enhanced intermediate and high-field Jc values. Scaling analyses for the pinning force density as a function of the reduced field h=Ha/Hirr (where Hirr denote the irreversibility field) showed that the highest peak was achieved at h=0.56. This value is even higher than the theoretically predicted highest value of h=0.5. We also show that a maximum flux pinning can be achieved in the whole magnetic field when very fine secondary phase particles are dispersed in a superconducting (Nd, Eu, Gd)Ba2Cu3Oy matrix with an optimum Nd: Eu: Gd ratio.

Pinning characteristics in chemically modified (Nd, Eu, Gd)-Ba-Cu-O superconductors

In melt-processed (Nd0.33Eu0.38Gd0.28)Ba2Cu3Oy (NEG-123) materials we found a new type of nanometer-scale pinning defects. Structural analysis was made with a dynamic force microscope and a scanning tunneling microscope (STM) that both showed the formation of a nanometer-scale lamellar structure. The high magnification STM showed that the nanolamellas are in fact rows (or planes) of aligned NEG-rich clusters 3–4 nm in size. This new pinning medium led to an increase of Birr at 77 K for the H∥ c axis up to 12 T (measured by a superconducting quantum interference device) or 15 T (measured by a vibrating sample magnetometer). A secondary peak as high as 70 kA/cm2 was observed at 4.5 T and decreased to 49 and 22 kA/cm2 at 7 and 10 T, respectively.

Record flux pinning in melt-textured NEG-123 doped by Mo and Nb nanoparticles

We report on a highly improved pinning performance in the melt-textured (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy+35mol% Gd2BaCuO5 (70nm in size) composite doped by tiny amounts of nanometer-sized MoO3 or NbO3 particles. The doping led to a spontaneous formation of clouds with extremely small (⩽10nm) (Nd,Eu,Gd)2BaCuXO5 particles, X=Mo,Nb. With an optimum content of MoO3 and NbO3, the self-field supercurrent at 77K was twofold and threefold higher than before, respectively. At 65K the supercurrent reached 700kA∕cm2 with Mo and 925kA∕cm2 with Nb at both 0 and 4.5T, and the self-field current at 90K was 100A∕cm2 in the Nb-doped sample, which is a value high enough for a safe levitation.

Vortex pinning by mesoscopic defects: A way to levitation at liquid oxygen temperature

The addition of long-term Zr2O3-ball milled Gd2BaCuO5 particles to a standard (Nd,Eu,Gd)Ba2Cu3Oy superconductor led to a spontaneous formation of extremely small (20–50 nm) Zr-rich (Nd,Eu,Gd)–Ba–Cu–O particles during the melt-growth process. This pinning medium, approaching the interesting physical limit of point-like pins from the side of large normal ones, dramatically enhanced supercurrents up to 90 K, both in low and intermediate fields. As a result, levitation experiment could be performed with liquid oxygen as a coolant. The particle size effect resemblance with fast neutron irradiation of RE–Ba–Cu–O single crystals is discussed.

Direct observation and analysis of nanoscale precipitates in (Sm,Eu,Gd)Ba2Cu3Oy

We employed atomic-resolution high-angle annular dark-field scanning transmission electron microscope to characterize the chemical composition and nature of the nanoparticles dispersed in melt-processed (Sm0.33Eu0.33Gd0.33)Ba2Cu3Oy superconductor. We found two types of stable nanometer-scale particles, small LRE-Ba2CuZrOy and (LRE,Zr)BaCuOy(LRE=Sm,Eu,Gd) ones, in the 10–60nm range, and larger particles of Gd2BaCuO5 “Gd-211,” around 100nm in size. A substantial increase of critical current density was observed up to 90K. Experimental data were interpreted in terms of additive pinning by “large” defects and a point-like disorder. The nanosized particles open a new horizon for production of bulk high-Tc superconducting magnets.

Strong pinning in ternary (Nd–Sm–Gd)Ba2Cu3Oy superconductors

We have studied the flux pinning in melt-textured (Nd0.33Sm0.33Gd0.33)Ba2Cu3Oy NSG-123 superconductors with various numbers of Gd2BaCuO5 (Gd-211) particles. Transmission electron microscopy (TEM) showed that submicron Gd-211 particles are uniformly distributed in the superconductive matrix. Dark-field TEM observations further showed that a high density of RE rich RE1+xBa2−xCu3Oy (RE-123ss) clusters 3–10 nm in size were distributed in the NSG-123 matrix. A strongly developed fishtail was observed in the magnetization hysteresis loops of all the samples. A critical current density of 100 kA/cm2 (77 K) was achieved at the secondary peak field of 2 T for the H∥ c axis in the NSG-123 sample with 10 mol % Gd-211. Large grain NSG-123 pellets with 30 mol % Gd-211 and 20 wt % AgO2, 30 mm in diameter and 15 mm in height, exhibited a single-cone profile with a peak value of 1.2 T at 77 K. A higher trapped-field value of 1.5 T was recorded at 2 T, reflecting the secondary peak effect.

Comparison of different approaches to modelling the fishtail shape in RE-123 bulk superconductors

We analyse and compare properties of different models proposed so far for modelling the shapes of FB and JB . curves measured in RE-Ba Cu O RE-123, REs rare earth samples. The formulas following from the discussed 23 7 yd models allow for a direct fit of experimental data in various representations. The models divide in two categories, according to the pinning potential used, namely the power-law and the logarithmic potential. We show that none of the pinning regimes . proposed for low-T superconductors accounts for the fishtail effect FE observed in high- T materials. The conventional c c expressions allow a good fit of high-T experimental data, however, the resulting fitting parameters are much higher than c those predicted by the theory. The logarithmic pinning potential justified in high- T materials by both magnetic and transport c

Novel seeds applicable for mass processing of LRE-123 single-grain bulks

A batch process with cold seeding was developed, which considerably reduces the production cost of various melt-processed LREBa2Cu3Oy (where LRE represents a light rare earth element) compounds. The key prerequisite for successful batch production is novel thin film Nd-123 seeds grown on MgO crystals, compatible with all LRE-123 materials and verified on large Gd-123 and NEG-123 single grains. The samples exhibit a sharp superconducting transition (around 1 K wide) with the onset Tc around 93.5 K. The trapped field measurements showed that all the samples in the batch were single domain and of good pinning performance. The trapped field of nearly 1 and 1.2 T was observed in the best 24 mm single-grain pucks of Gd-123 and NEG-123, respectively. The trapped field in the best 45 mm Gd-123 single-grain pucks reached 1.35 T and 0.35 T at 77 K and 87 K, respectively. Using these batch-processed Gd-123 bulks, we constructed a middle-sized levitation disk. This proved that the present approach leads to production of melt-processed LRE-123 materials with a reasonable cost and high performance suitable for superconducting super-magnets.

Levitation of NEG-123 at the temperature of liquid oxygen (90.2 K)

A long-term Zr2O3 ball milling of Gd2BaCuO5 secondary phase particles dispersed in the melt-textured (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy (NEG-123) compound brought a superb enhancement of electromagnetic performance of this composite at high temperatures: at 77.3 K, Jc reached 192 kA cm−2, at 90.2 K (temperature of liquid oxygen) the remnant Jc was 40 kA cm−2, a value high enough for levitation experiments. Energy dispersive x-ray analysis found small Zr-rich secondary phase particles of only 20–50 nm in size, while the mean particle size after the milling was 70–200 nm, according to the milling time. This new particle size reduction mechanism enables the design of non-contact liquid oxygen pumps for industrial applications.

Influence of combined Pt and CeO2 additions on microstructure and magnetic properties in (Nd, Eu, Gd)-Ba-Cu-O

We studied the effect of the combined addition of Pt and CeO2 on the dispersion of secondary phase Gd2BaCuO5 (Gd-211) particles in melt-processed (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy (NEG-123) by varying the CeO2 content from 0 to 3%. Scanning electron microscopy revealed that submicrometre-sized Gd-211 particles were distributed uniformly throughout the NEG-123 matrix for additions of up to 2 mol% CeO2. The critical current density (Jc) increased with increasing CeO2 content, particularly at low magnetic fields. The zero-field Jc value reached its maximum of 100 kA cm-2 at 77 K for 1 mol% CeO2. An improvement in low-field Jc was attributed to the refinement of Gd-211 particles.