Young A. Jee

Top-seeded melt growth of Y-Ba-Cu-O superconductor with multiseeding

Y-Ba-Cu-O superconductors were prepared by the top-seeded melt growth process with multiseeding technique. By using several seeds at the same time, large samples could be fabricated in a short time with simple heat treatment. However, the samples fabricated by the normal multiseeding technique show a rapid decrease of trapped magnetic field value across the grain boundaries because of the residual liquid layer. To remove the residual liquid layer, various types of multiseeding were newly suggested. The optimum result was obtained when the seeds were arranged without spacing. The individual grains were combined as a single domain and did not show any deterioration of magnetic properties at the boundary. The formation mechanism of a well-combined single domain by the multiseeding technique is discussed.

Variables affecting the fabrication of single grain YBa2Cu3O7-y superconductors by the top-seeded melt growth process

In this article, we summarize the variables affecting the fabrication of a single grain YBCO superconductor by a top-seeded melt-growth- (TSMG-) process. The important variables are the seed melting and the resolidification which lead to the formation of undesirable subsidiary Y123 grains at the seeded area, and nucleation at both the compact/substrate interfaces and compact surfaces. The undesirable Y123 nucleation acts as the main obstacles for the fabrication of the single grain Y123 superconductor because of its competitive growth with the Y123 grain grown from the seed. The methods of suppressing the formation of the subsidiary Y123 nucleation, and of shortening the entire processing time of the TSMG process were suggested.

Effect of resolidification nature of SmBa2Cu3O7−y seeds on the growth mode of YBa2Cu3O7−y

Abstract SmBa 2 Cu 3 O 7− y (Sm123) seeds dissolved partly or completely during top-seeded melt growth (TSMG) processing of YBa 2 Cu 3 O 7− y . It made a solid solution of (Y 1− x Sm x )Ba 2 Cu 3 O 7− y and (Y 1− x Sm x ) 2 BaCuO 5 around the seed/compact interfaces. The dissolved parts resolidified during peritectic cooling and its resolidification mode affected the growth mode of Y123 grains significantly. The relationship between the growth mode of Y123 grains of TSMG-processed YBCO superconductors and the resolidification nature of Sm123 seeds is reported.

Effects of the seed dimension on the top surface growth mode and the magnetic properties of top-seeded melt growth processed YBCO superconductors

Abstract The growth nature of YBa 2 Cu 3 O 7− y (Y123) grains at the top surfaces of top-seeded melt growth (TSMG) processed YBCO superconductors was studied in respect to the dimension of Sm 1.8 Ba 2.4 Cu 3.4 O x seeds. The top surface growth mode was significantly dependent on the dimensions of the seed, especially on the seed thickness. The use of thin slab-like Sm-seeds led to the nucleation of undesirable subsidiary Y123 grains at the seeded area, while such subsidiary nucleation was hardly observed when thick seeds were used. The formation mechanism of the subsidiary Y123 grains, and the effects on the levitation forces and trapped magnetic field are discussed.

Control of YBCO growth at the compact/substrate interface by bottom seeding and Yb2O3 coating in seeded melt-growth processed YBCO oxides using a MgO substrate

Abstract Effects of a seeding technique and surface coating with Yb 2 O 3 powder on the growth of YBCO grains at the compact/substrate (a MgO single crystal) interfaces were studied. In a single top-seeded sample without Yb 2 O 3 coating, several YBCO grains nucleated at the compact/substrate interface. In a double seeded (top and bottom) sample without Yb 2 O 3 coating, YBCO grains nucleated both at the bottom seed and MgO substrate competitively, resulting in partial texturing of YBCO grains at the bottom. Applying a double seeding technique with Yb 2 O 3 coating both at the compact/MgO interface and sample surfaces inhibited the undesirable subsidiary YBCO nucleation at both the surfaces and interfaces, resulting in the growth of a single YBCO grain. As a result of the suppression of subsidiary Y123 nucleation, a magnetic levitation force of the sample bottom was improved significantly.

Dissolution and resolidification of SmBa/sub 2/Cu/sub 3/O/sub 7-y/ seed during top seeded melt texturing of YBa/sub 2/Cu/sub 3/O/sub 7-y/

During top-seeded melt growth of a YBa/sub 2/Cu/sub 3/O/sub 7-y/ (Y123) single-domain crystal, seed dissolution occurred while the seed was in contact with Ba-Cu-O liquid, and then the dissolved parts were resolidified during cooling. The growth mode of the Y123 crystal was directly affected by the resolidification process of the dissolved part. When the melted part of the seed was resolidified to a polycrystal, Y123 grew also as a polycrystalline form. In contrast, when the dissolved part of the seed completely recovered its mother orientation during cooling, a single-domain Y123 crystal could be obtained. The dissolution behavior of the SmBa/sub 2/Cu/sub 3/O/sub 7-y/ (Sm123) seed is explained in terms of the solubility of samarium in Ba-Cu-O liquid. The resolidification mode of the melted part is discussed in terms of the Sm content within the interdiffusion layer of Sm and Y.

Trapped magnetic field profiles of top-seeded melt growth processed YBCO superconductors

Among various fabrication pwessed developed 50 far, a topseeded melt growth (TSMG) process is known to be an effective way to increase the gain size of YBCO superconductor and hss been widely uxd io fabricate large area single grain YBCO samples [1,2]. In this technique, single grain SmE+Cu,O, (Sm123) and NdBa,Cu,O,,(Ndl23) are placed on the top of the YBCO compact both to provide a nucleation site for a Y123 grain. During slow cooling through a peritectic tcmpezature (TJ of a Yl23 phase, YBCO grains grow at the seeds. In this sNdy, we fabricated TSMG-processed YBCO samples using Sm-seeds and investigated the effect of the growth mode of YBCO grains on trapped magnetic field of TSMG-processed YBCO samples. Surface magnetic flux density profile of samples was measured at 77 K and a field of a 0.4 T Sm-CO permanent magnei and a diameter of 25 mm, at the distancc of 0.5 mm from samples by a hall probe (Lake shore model HGCT 3020). Figure 1 show photos of the top surfaces of YBCO samples TSMG-processed using SmBCO seeds with different seed thickness (t). It can be seen that the growth mode of YBCO grains is dependent on seed thickness. When the seed thickness was small. the Y 123 growth was not easy to be eontmlled. At d . 3 9 and r0.65 mm, the top surfaces of the samples involve many subsidiary gains nucleated mund the seeds. As seed thickness increased, the formation of subsidiary grains was considerably suppressed. At larger f, boundary densities are relatively smaller than that of a sample of smaller f . It seems that there exists critical seed thickness to form subsidiary p i n s : about 1 mm for this study. The boundary of the subsidiary grains was rarely observed for samples with f > I mm. Figure 2 shows surface magnetic flux density of TSMG-processed samples: (a) no seeding, seeds of (b) rO.65 and (c) F3.21 mm. In case of no seeding, several deflections are observed in the two-dimensional field map, due to the presence of grain boundaries of weak link character.

Control of growth rate and texture formation of YBa/sub 2/Cu/sub 3/O/sub 7-y/ single-domain crystal by applying two-step undercooling in TSMG

A fabrication technique for a single-domain Y123 crystal by top-seeded melt processing was developed using two step undercooling. At the first undercooling step, the Y123 nucleus was stabilized, and increased growth rate was intended at the second undercooling step. By the two step undercooling method, the processing time required for the growth of Y123 single-domain crystal was significantly reduced without degradation of the superconducting properties. The microstructures and textures of Y123 samples prepared by two step undercooling were also investigated.

Multi-Seeding of YBCO Superconductors

Due to the low yttrium solubility in Ba-Cu-0 liquid, the growth rate of a YBa2Cu3O7 (Y123) grain in the liquid is low. It takes a few hundred hours to fabricate a large single grain YBCO superconductor. A multi-seeding technique may shorten the processing time needed for the fabrication of large area textured YBCO superconductors. In this study, we tried a top-seeded melt growth process combined with multi-seeding. The processing variables, the growth mode of Y123 grains at the top surfaces, and the levitation forces and trapped magnetic fields of the multi-seeded sample are reported.

Fabrication of a good-quality single grain YBCO sample through the control of seed crystals

We investigated the growth mode of YBa/sub 2/Cu/sub 3/O/sub 7-y/ (Y123) grains and its effect on the levitation forces and trapped magnetic field of top-seeded melt processed YBCO samples. When a slab-like Sm-seed was used, undesirable subsidiary Y123 grains were formed, while the formation of the subsidiary Y123 grains was suppressed and a nearly single Y123 grain grew when a thick and wide seed was used. The formation of the subsidiary Y123 grains decreased the levitation forces and trapped magnetic field, due to the presence of grain boundaries with weak link characteristics.