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The actual limitations for the trapped field in YBa2Cu3O7−δ (YBCO) monoliths are discussed. The influence of the sample geometry and of the critical current density on the trapped field is investigated by numerical calculations. The field dependence of the critical current density strongly influences the trapped field. A nonlinear relationship between the sample size, the critical current density and the resulting trapped field is derived. The maximum achievable trapped field in YBCO at 77 K is found to be around 2.5 T. This limit is obtained for reasonable geometries and high but realistic critical current densities. Such high fields have not been reached experimentally so far, due to non-optimized flux pinning and material inhomogeneities. These inhomogeneities can be directly assessed by the magnetoscan technique, and their influence is discussed. Significant differences between the a- and the c-growth sectors were found. Limitations due to cracks and non-superconducting inclusions (e.g. 211 particles) are estimated and found to be candidates for variations of Jc on a millimetre length scale, as observed in experiments.

Section: Numerical Calculationsmentioning

confidence: 99%

Limitations for the trapped field in large grain YBCO superconductors

Eisterer

Haindl

Zehetmayer

et al. 2006

“…The improvement of J c -B properties will enhance the field trapping capability of bulk samples. We have already succeeded in fabricating single domain Gd-Ba-Cu-O/Ag bulk with a diameter of 32 mm by the hot-seeding method [22,27] using Nd123 crystal as a seed (figure 10). In the present study, the Gd211 powders calcined at 900 • C were used as a precursor for the growth of large bulk samples.…”

Section: Fabrication Of a Large Bulk Sample And Its Trapped Fieldmentioning

confidence: 99%

Influence of the size of Gd211 starting powder on the critical current density of Gd-Ba-Cu-O bulk superconductor

Nariki¹,

Seo²,

Sakai³

et al. 2000

101

The relationship between the particle size of Gd211 powder in the precursor and the particle size of Gd211 inclusions in Gd-Ba-Cu-O bulk has been investigated. Gd211 starting powders with various diameters were prepared by the calcination of Gd 2 O 3 , BaO 2 and CuO powders at different temperatures between 800 and 1000 • C. The particle size of Gd211 in the melt-grown bulk was proportional to the particle size of the initial Gd211 powder. In conclusion, the employment of fine Gd211 powder led to a size reduction of 211 particles in the bulk, while largely enhancing the J c values in low magnetic fields. A large Gd-Ba-Cu-O/Ag bulk sample, 32 mm in diameter, could also be fabricated by the hot-seeding method. The maximum trapped field value revealed 1.5 T at 77 K.

“…Only an asymmetric sigmoidal function can express the histogram from small size voids to big ones. Equation (1) shows the function and the parameters in this case. Each parameter in the function required to modify the shape of the curve has no physical meaning.…”

Section: Observations Of the Samplementioning

confidence: 99%

“…The tensile strengths obtained varied widely from 29 to 150 MPa. Since the size of the voids depends on the parameters in equation (1), this scatter of the tensile strengths is thought to be correlated with them.…”

Section: Observations Of the Samplementioning

confidence: 99%

“…Since the size and the location of the voids in a bulk are distributed on probability, the ultimate tensile strength of the specimens are also distributed on probability. Thus many experimental data to obtain the average and the deviation of the strengths are required for each bulk sample [1][2][3][4][5][6]. Decrease of the voids in the bulks contributes much to the improvements of the strength.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Ultimate tensile strength of an artificial bulk superconductor with voids

Kasaba

Katagiri

Hokari

2006

Rare earth based bulk superconductors originally have many voids and cracks. Thus their ultimate tensile strengths are not material constants but apparent ones depending on the voids and cracks in each bulk. In this study, based on the observations of the surface of the actual specimens, artificial samples with voids were generated for numerical stress calculations. The sizes of voids in an artificial specimen were assumed by the probability distribution function expressing the results of the observations, while the locations of voids were randomly assumed. Numerical calculations were repeatedly conducted to obtain the statistical results of the stress concentration factors around voids. The scatter of experimental strength and Young’s modulus among the specimens having linear stress–strain curves can be predicted by the evaluations proposed in this study.