- Email: [email protected]
Top surface morphologies of melt growth processed Y1.5Ba2Cu3O7−y bulk superconductors with corner or edge seeding
Physica C 495 (2013) 225–228
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Top surface morphologies of melt growth processed Y1.5Ba2Cu3O7y bulk superconductors with corner or edge seeding C.-J. Kim a,⇑, S.A. Jung a, H.-W. Park b, B.-H. Jun a, S.-D. Park a a b
Neutron Science Division, Korea Atomic Energy Research Institute, Daedoek-Daero 1045, Yuseong, Daejeon 305-353, Republic of Korea College of Energy, Material and Chemical Engineering, Korea University of Technology and Education, 1600 Chungjeol-ro, Chunan, Chungnam 330-708, Republic of Korea
a r t i c l e
i n f o
Article history: Received 22 August 2013 Received in revised form 9 September 2013 Accepted 22 September 2013 Available online 1 October 2013 Keywords: Corner or edge seeding Y1.5Ba2Cu3O7y Top surface morphology Magnetic flux density Magnetic levitation force
a b s t r a c t A corner or edge seeding was attempted to control top surface morphologies (facet lines) of top-seeded melt growth (TSMG) processed Y1.5Ba2Cu3O7y (Y1.5) bulk superconductors. The orientation and numbers of facet lines were successfully modified using the corner/edge seeding with adjusted seed orientations. Most of the facet lines developed on the top surfaces were nearly straight, whereas some of them often had curvatures when the facet lines met the edges with high angles. The size of the growth area of Y123 on the top surfaces was dependent not only on the seeding method but also on the seed orientation. The unreacted regions were often observed on the local parts of the top surfaces, which are attributed to the difference in a growth rate among growth planes. The top surface with the corner seeding where the h1 1 0i growth direction is parallel to the diagonal of the Y123 compact showed the highest magnetic flux density and magnetic levitation forces owing to the largest growth area of Y123. Ó 2013 Published by Elsevier B.V.
1. Introduction The top-seeded melt growth (TSMG) process is widely used for the fabrication of single grain REBa2Cu3Ox (RE123, RE: rare-earth elements) bulk superconductors with a high magnetic flux density and levitation performance [1–5]. In the conventional TSMG process, a seed is placed at the centre of the top surface of RE123 powder compacts prior to the heat treatment. The seeded RE123 compact is subjected to melt growth heating cycles. When the nucleation of RE123 grains is strictly limited, a RE123 grain grows at the centre seed through a peritectic reaction. During the prolonged isothermal holding just below a peritectic reaction temperature (Tp, 1005 °C) or slow cooling through Tp, only a single RE123 grain grows at the seed without subsidiary RE123 nucleation. Large single grain RE123 bulk superconductors of several cm in diameter can be fabricated using the TSMG process [1,2,5]. The facet lines always developed on the top surface of the single grain RE123 bulk compacts, which is related to the equilibrium shape of RE123 grains in liquid. They begin at the seed and develop along the h1 1 0i directions of RE123. The facet lines correspond to specific crystallographic planes in 3-dimensional space, which determine the segregation of RE2BaCuO5 (RE211) particles inside the RE123 grains [6–8]. The Y2BaCuO5 (Y211) particle segregation in a YBa2Cu3Ox (Y123) system is classified into two patterns. One is 1-dimensional linear tracks and the other is 2-dimensional seg⇑ Corresponding author. Tel.: +82 42 868 8908; fax: +82 42 868 8275. E-mail address: [email protected] (C.-J. Kim). 0921-4534/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.physc.2013.09.012
regation patterns on the polished surfaces. The linear Y211 tracks form in a stoichiometric Y123 system [6], whereas the planar Y211 segregations form in a Y211 excessive Y123 system [8]. In the former case, Y211 particles were trapped along the h1 1 0i directions, making x-like tracks [6]. In the latter case, Y211 particles are trapped within specific crystallographic planes bounded by the (1 1 0) planes and are free in other neighbouring planes, making butterfly-like patterns [7,8]. Since Y211 particles act as flux pinning centres of Y123 [9], the current properties of the regions with more Y211 particles will be higher than those of the regions with less Y211 particles. It is therefore necessary to control the top surface morphologies of TSMG processed RE123 samples to have a uniform Y211 distribution within the Y123 grains. In this study, we attempted corner or edge seeding with controlled seed orientations to modify the growth morphologies of top surfaces of TSMG-processed Y123 bulk superconductors. Based on the top surface morphologies of the TSMG-processed Y123 samples with corner or edge seeding, the magnetic flux density and magnetic levitation forces of the top surfaces are reported. 2. Experimental procedure The precursor powder used in this study was a mixture of 1 mol Y123 (Solvay Germany, 99.9% purity, 2–3 lm in size) and 0.25 mol Y2O3 (BM-CHEM HI-TECH Co., Ltd, China, 99.99% purity, 0.2–3 lm in size) powder, whose nominal composition is Y1.5Ba2Cu3O7y (Y1.5). 1 wt.% CeO2 powder was added to the powder mixture to refine the Y211 particles [10]. The powder mixture was ball-milled
226
C.-J. Kim et al. / Physica C 495 (2013) 225–228
for 24 h using ZrO2 balls. 15 g of the ball-milled Y1.5 powder was uniaxially pressed in a rectangular steel mould (20 mm 20 mm) into a compact. A Sm123 seed with adjusted seed orientations was placed at the corner or edge of the top surfaces of Y1.5 compacts (see Fig. 1). Tetragons marked by ‘‘a’’ and ‘‘b’’ correspond to corner seeding with orientation relationships of h1 0 0iseed//sidecompact and h1 1 0iseed//sidecompact, respectively. Tetragons marked by ‘‘c’’ and ‘‘d’’ correspond to edge seeding with h1 1 0iseed//sidecompact and h1 0 0iseed//sidecompact, respectively. From the schematic views, the numbers and orientations of the facet lines that will be developed on the top surfaces can be predicted. The seeded Y1.5 compacts were subjected to the heating cycles of the melt growth process for the fabrication of single grain Y123 bulk superconductors. The seeded Y1.5 compacts were located at the centre of a box furnace, heated to 1040 °C in air at a heating rate of 100 °C/h held at this temperature for 1 h, cooled to 1020 °C at a rate of 10 °C/h, cooled again to 970 °C at a rate of 0.3 °C/h and finally cooled down to room temperature at a rate of 100 °C/h. For oxygenation, the TSMG processed Y1.5 samples were heated to 500 °C/h at a rate of 100 °C/h in flowing oxygen, held at this temperature for 50 h, cooled to 450 °C at a rate of 100 °C/h, held at this temperature for 100 h, and then cooled to room temperature at a rate of 200 °C/h. The TSMG processed Y1.5 samples were field-cooled to 77 K using a Nd–B–Fe permanent magnet with a surface field of 0.52 Tesla (T), and the magnetic flux density distribution at 77 K was measured for the top surfaces using a Hall probe. Force–distance (F–d) curves were measured for field-cooled or zero field-cooled samples to 77 K using a Nd–B–Fe permanent magnet with a diameter of 30 mm and surface magnetic field of 0.5 T. The magnetic levitation forces at 77 K were estimated from F–d curves.
3. Results and discussion Fig. 2(a)–(d) shows the top surface views of TSMG processed Y1.5 samples with corner seeding (a and b) and edge seeding (c and d). After TSMG processing, the size of Y1.5 compacts was reduced from 20 mm to 15 mm owing to the volume contraction. It can be seen in all samples that the facet lines are developed on the top surfaces and the shapes are almost similar to those predicted in the schematic view of Fig. 1. One diagonal facet line, which began from a seed and grew to the corner of the compact, is observed at the top surface of sample (a). The entire top surface of sample (a) is covered with a grown Y123 grain without unreacted parts owing to the fast growth along the h1 1 0i direction. The top surface morphology with one diagonal facet line corresponds to a quarter of the top surface view of the conventional TSMG processed Y123 superconductors with centre seeding [2], and is similar to that observed in the buffer bridge processed Y123 samples [11]. On the other hand, two facet lines with deviation angles of 3–5° from the sides of the compact are developed
a
c [010]
b d
[110]
[100]
Fig. 1. Schematic view of corner seeding (a and b) and edge seeding (c and d).
(see the lines marked by arrows) on the top surface of sample (b). Unlike the top surface of sample (a), the unreacted region is present at the right upper corner (see the region marked by ‘‘r’’) of the top surface. This is because the growth rate (RGh1 0 0i) along the h1 0 0i direction of Y123 was lower than that along the h1 1 0i direction (RGh1 1 0i). Meanwhile, three facet lines are observed on the top surface of sample (c); one line is almost normal to the side of the compact, and two lines are parallel. Similar to those observed in sample (b), the unreacted regions are also present at the two upper corners of the top surface. On the other hand, two facet lines are observed on the top surfaces of sample (d). One facet line finishes at the corner of the compact and another facet line finishes at the intermediate point of the side. Interestingly, the facet lines have large curvatures, which are comparable to the almost straight facet lines observed in the other samples. The formation of the facet lines with large curvatures appears to be attributed to the unbalanced mass transfer for the growth of Y123 grains in the edge regions. The top surface morphologies (tracks of facet lines and growth fronts) of Fig. 2 are schematically shown in Fig. 3(a)–(d). The size of the growth area of Y123 on the top surfaces is dependent on the seeding method and seed orientation. The entire top surface was covered with a Y123 grain when the h1 1 0i facet line is parallel to the diagonal of the compact (see Fig. 3(a)). The orientation relationship of the corner-seeded Y123 sample is h1 0 0iseed//sidecompact. Except sample (a), unreacted regions are observed in the top surfaces of other samples owing to the slower growth rate along h1 0 0i directions. Fig. 4(a) and (b) shows the typical side views (surface morphology) of (a) edge-seeded Y123 and (b) corner seeded Y123 sample after a melt growth process. The side view of sample (a) is divided into a dark isosceles triangle region (c-growth sector) and two upper regions marked by arrows (a/b growth sectors). The cgrowth sector is symmetrical in shape because of the equivalent growth of Y123 toward both (left and right) directions from the seed. On the other hand, the c-growth sector of sample (b) is unsymmetrical in shape. It is likely that the Y123 growth to the direction opposite to the corner is easy, whereas the Y123 growth toward the corner is limited. From the top surface views of Fig. 3 and side views of Fig. 4, the growth nature of Y123 grains in the corner- or edge-seeded Y123 samples was well understood. The two important growing plane/ direction families in determining the top surface morphologies are {1 0 0}/h1 0 0i and {1 1 0}/h1 0 0i. The growth rate relationship between the two directions is given by
RGh1 1 0i ¼
pffiffiffi 2RGh1 0 0i
ð1Þ
where RGh1 1 0i and RGh1 0 0i are the growth rate of Y123 along a h1 1 0i direction and a h1 0 0i direction, respectively. This orientation relationship indicates that the h1 1 0i direction should be utilized to obtain a lager Y123 area in the corner or edge seeding. Fig. 5 shows magnetic flux density (B) maps of TSMG processed Y1.5 samples with corner seeding (a and b) or edge seeding (c and d), field-cooled at 77 K using a Nd–B–Fe permanent magnet. The B maps of all samples show a single grain flux contour with a single peak near the centre of the maps. No deep valleys or disconnected parts owing to the weakly linked grain boundaries or cracks are observed in all B maps. It is pointed out that the B maps of samples (a) and (d) are nearly symmetric in shape, whereas the B maps of samples (b) and (c) are non-symmetric. The non-symmetric B contours of samples (b) and (c) are attributed to the presence of unreacted non-superconducting regions at the upper parts of samples (a) and (d), as already observed in Fig. 2. The maximum B values at the peak points of samples (a)–(d) are 2.71 kG, 2.29 kG, 2.24 kG and 1.88 kG, respectively. The results of the magnetic flux density
227
C.-J. Kim et al. / Physica C 495 (2013) 225–228
(a)
(b) r
s
s
(c)
(d) r r
s
s
10 mm Fig. 2. Top surface views of TSMG processed Y1.5 samples with corner seeding (a and b) or edge seeding (c and d). ‘‘s’’ and ‘‘r’’ denote a seed and an unreacted region, respectively.
<110> directions
<110>
Y123 area <110>
(a)
Seed
a/b Facet line
Seed
c
Fig. 3. Schematics of the growth areas of Y123 on the top surfaces of Fig. 2.
measurement for the field-cooled samples are consistent with the top surface morphologies of Fig. 2. Fig. 6 shows F–d curves at 77 K of TSMG processed Y1.5 samples with corner seeding (a and b) or edge seeding (c and d), (a) zerofield cooled and (b) field cooled using a Nd–B–Fe permanent magnet. The F–d curves of the zero-field cooled samples show typical hysteresis characteristics by a strong Meissner repulsion. In all F– d curves in Fig. 6(a), the attractive force terms are zero. The maximum repulsive forces (FRep.) at a 1 mm gap of samples (a)–(d) are 44.6 N, 37.9 N, 44.4 N and 39.7 N, respectively. Sample (a) shows the highest FRep., whereas sample (b) shows the lowest FRep.. Since a magnetic levitation force is dependent on the size (area) of a superconductor facing a permanent magnet, it is reasonable that the top surface of sample (b) having an unreacted non-superconducting area showed the lowest FRep.. On the other hand, the F–d curves of the field-cooled samples show hysteresis characteristics having larger attractive forces (FAttr.) and relatively smaller repulsive forces in comparison to those of the zero field-cooled samples. The large FAttr. is attributed to the magnetic fields trapped within the superconductors by field cooling. The max. FAttr.s are 16.2 N,
(b)
Seed
a/b c
10 mm Fig. 4. Side views of (a) edge-seeded and (b) corner-seeded Y123 sample after the melt growth process. The grown Y123 areas in both samples are marked by a rectangular box.
C.-J. Kim et al. / Physica C 495 (2013) 225–228
(d)
2.5
2.5
2.5
2.5
2.0
2.0
2.0
2.0
2 4 6 8 10 12 14 16 18 20
X Axis (mm)
3.000 2.500 2.000 1.500 1.000 0.5000 0.2500 0 -0.3000
20 18 16 14 12 10 8 6 4 2 2 4 6 8 10 12 14 16 18 20
2
4
6
(m m)
Field (kG
Field (kG
Axi 14 16 s (m 18 20 m)
0.5 0.0 2
20 18 16 14 12 10 8 6 4 2 2 4 6 8 10 12 14 16 18 20
X Axis (mm)
X Axis (mm)
4 6 8 10
X A 12 14 16 xis (mm 18 20 )
2
4
6
20 18 16 14 12 10
Ax is (m m)
6
8
1.0
8
Y
4
0.0 2 4 6 8 X 10 12
20 18 16 14 12 10
Y Axis (mm)
Y Axis (mm)
20 18 16 14 12 10 8 6 4 2
2
0.5
1.5
Ax is
XA 14 xis 16 18 20 (mm )
8
1.0
Y
(m
is
6
0.0 2 4 6 8 10 12
20 18 16 14 12 10
1.5
Y Axis (mm)
2
0.5
Ax is (m m)
4 20
8
Ax
2 4 6 8
X A 10 12 14 16 xis (mm 18 )
Y Axis (mm)
20 18 16 14 12 10
1.0
Y
0.0
m)
0.5
1.5
Y
1.0
Field (kG
1.5
)
(c) )
(b) )
(a) Field (kG
)
228
20 18 16 14 12 10 8 6 4 2 2 4 6 8 10 12 14 16 18 20
X Axis (mm)
Fig. 5. Magnetic flux density maps of the TSMG processed Y1.5 samples with corner seeding (a and b) or edge seeding (c and d), field-cooled at 77 K using a Nd–B–Fe permanent magnet.
4. Conclusions A corner or edge seeding has been attempted to control the top surface morphologies (facet lines) of TSMG processed Y1.5 bulk superconductors. The facet lines, which are developed on the top surface of the TSMG processed Y1.5 samples, were successfully controlled using the corner or edge seeding with adjusted seed orientations. Most of the facet lines were nearly straight, whereas some of them had a curvature. The formation of the curved facet lines seems to be ascribed to the unbalanced mass transfer for the growth of Y123 grains in the edge regions of the compacts. The top surface morphology was dependent not only on the seeding method but also on the seed orientation. The unreacted non-superconducting regions were observed in parts of the top surfaces, owing to the slow growth rate along the h1 0 0i directions. The top surface with corner seeding where the h1 1 0i growth direction is parallel to the diagonal of the Y123 compact showed the highest magnetic flux density and magnetic levitation forces owing to the largest growth area of Y123. Acknowledgement This work was financially supported by the National R&D Program of the Ministry of Education, Science and Technology (MEST), Republic of Korea. References
Fig. 6. Force–distance curves at 77 K of TSMG-processed Y1.5 samples with corner seeding (a and b) or edge seeding (c and d): (a) zero-field cooled and (b) field cooled using a Nd–B–Fe permanent magnet.
12.5 N, 16.5 N, and 14 N, respectively. The max. FRep. of samples (a)–(d) are 24.3 N, 23.2 N, 22.0 N and 21.0 N, respectively. This result of the F–d measurement agrees with the result of the B measurement shown in Fig. 5.
[1] M. Murakami, N. Sakai, T. Higuchi, S.I. Yoo, Supercond. Sci. Technol. 9 (1996) 1015. [2] C.J. Kim, Y.A. Jee, G.W. Hong, Supercond. Sci. Technol. 13 (2000) 709. [3] T. Meignan, P.J. McGinn, C. Varanasi, Supercond. Sci. Technol. 10 (1997) 109. [4] V. Antal et al., Supercond. Sci. Technol. 22 (2009) 105001. [5] D.A. Cardwell et al., Supercond. Sci. Technol. 23 (2010) 034008. [6] C.J. Kim, K.B. Kim, H.W. Park, I.H. Kuk, G.W. Hong, J. Mater. Sci. 32 (1997) 4701. [7] Y.A. Jee, S.J.L. Kang, H. Chung, J. Mater. Res. 13 (1998) 583. [8] C.J. Kim, K.B. Kim, I.H. Kuk, G.W. Hong, J. Mater. Res. 13 (1998) 269. [9] M. Murakami et al., Supercond. Sci. Technol. 4 (1991) S43. [10] C.J. Kim, H.W. Park, K.B. Kim, G.W. Hong, Supercond. Sci. Technol. 8 (1995) 652. [11] J.H. Lee et al., Supercond. Sci. Technol. 24 (2011) 055019.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Top surface morphologies of melt growth processed Y1.5Ba2Cu3O7y bulk superconductors with corner or edge seeding C.-J. Kim a,⇑, S.A. Jung a, H.-W. Park b, B.-H. Jun a, S.-D. Park a a b
Neutron Science Division, Korea Atomic Energy Research Institute, Daedoek-Daero 1045, Yuseong, Daejeon 305-353, Republic of Korea College of Energy, Material and Chemical Engineering, Korea University of Technology and Education, 1600 Chungjeol-ro, Chunan, Chungnam 330-708, Republic of Korea
a r t i c l e
i n f o
Article history: Received 22 August 2013 Received in revised form 9 September 2013 Accepted 22 September 2013 Available online 1 October 2013 Keywords: Corner or edge seeding Y1.5Ba2Cu3O7y Top surface morphology Magnetic flux density Magnetic levitation force
a b s t r a c t A corner or edge seeding was attempted to control top surface morphologies (facet lines) of top-seeded melt growth (TSMG) processed Y1.5Ba2Cu3O7y (Y1.5) bulk superconductors. The orientation and numbers of facet lines were successfully modified using the corner/edge seeding with adjusted seed orientations. Most of the facet lines developed on the top surfaces were nearly straight, whereas some of them often had curvatures when the facet lines met the edges with high angles. The size of the growth area of Y123 on the top surfaces was dependent not only on the seeding method but also on the seed orientation. The unreacted regions were often observed on the local parts of the top surfaces, which are attributed to the difference in a growth rate among growth planes. The top surface with the corner seeding where the h1 1 0i growth direction is parallel to the diagonal of the Y123 compact showed the highest magnetic flux density and magnetic levitation forces owing to the largest growth area of Y123. Ó 2013 Published by Elsevier B.V.
1. Introduction The top-seeded melt growth (TSMG) process is widely used for the fabrication of single grain REBa2Cu3Ox (RE123, RE: rare-earth elements) bulk superconductors with a high magnetic flux density and levitation performance [1–5]. In the conventional TSMG process, a seed is placed at the centre of the top surface of RE123 powder compacts prior to the heat treatment. The seeded RE123 compact is subjected to melt growth heating cycles. When the nucleation of RE123 grains is strictly limited, a RE123 grain grows at the centre seed through a peritectic reaction. During the prolonged isothermal holding just below a peritectic reaction temperature (Tp, 1005 °C) or slow cooling through Tp, only a single RE123 grain grows at the seed without subsidiary RE123 nucleation. Large single grain RE123 bulk superconductors of several cm in diameter can be fabricated using the TSMG process [1,2,5]. The facet lines always developed on the top surface of the single grain RE123 bulk compacts, which is related to the equilibrium shape of RE123 grains in liquid. They begin at the seed and develop along the h1 1 0i directions of RE123. The facet lines correspond to specific crystallographic planes in 3-dimensional space, which determine the segregation of RE2BaCuO5 (RE211) particles inside the RE123 grains [6–8]. The Y2BaCuO5 (Y211) particle segregation in a YBa2Cu3Ox (Y123) system is classified into two patterns. One is 1-dimensional linear tracks and the other is 2-dimensional seg⇑ Corresponding author. Tel.: +82 42 868 8908; fax: +82 42 868 8275. E-mail address: [email protected] (C.-J. Kim). 0921-4534/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.physc.2013.09.012
regation patterns on the polished surfaces. The linear Y211 tracks form in a stoichiometric Y123 system [6], whereas the planar Y211 segregations form in a Y211 excessive Y123 system [8]. In the former case, Y211 particles were trapped along the h1 1 0i directions, making x-like tracks [6]. In the latter case, Y211 particles are trapped within specific crystallographic planes bounded by the (1 1 0) planes and are free in other neighbouring planes, making butterfly-like patterns [7,8]. Since Y211 particles act as flux pinning centres of Y123 [9], the current properties of the regions with more Y211 particles will be higher than those of the regions with less Y211 particles. It is therefore necessary to control the top surface morphologies of TSMG processed RE123 samples to have a uniform Y211 distribution within the Y123 grains. In this study, we attempted corner or edge seeding with controlled seed orientations to modify the growth morphologies of top surfaces of TSMG-processed Y123 bulk superconductors. Based on the top surface morphologies of the TSMG-processed Y123 samples with corner or edge seeding, the magnetic flux density and magnetic levitation forces of the top surfaces are reported. 2. Experimental procedure The precursor powder used in this study was a mixture of 1 mol Y123 (Solvay Germany, 99.9% purity, 2–3 lm in size) and 0.25 mol Y2O3 (BM-CHEM HI-TECH Co., Ltd, China, 99.99% purity, 0.2–3 lm in size) powder, whose nominal composition is Y1.5Ba2Cu3O7y (Y1.5). 1 wt.% CeO2 powder was added to the powder mixture to refine the Y211 particles [10]. The powder mixture was ball-milled
226
C.-J. Kim et al. / Physica C 495 (2013) 225–228
for 24 h using ZrO2 balls. 15 g of the ball-milled Y1.5 powder was uniaxially pressed in a rectangular steel mould (20 mm 20 mm) into a compact. A Sm123 seed with adjusted seed orientations was placed at the corner or edge of the top surfaces of Y1.5 compacts (see Fig. 1). Tetragons marked by ‘‘a’’ and ‘‘b’’ correspond to corner seeding with orientation relationships of h1 0 0iseed//sidecompact and h1 1 0iseed//sidecompact, respectively. Tetragons marked by ‘‘c’’ and ‘‘d’’ correspond to edge seeding with h1 1 0iseed//sidecompact and h1 0 0iseed//sidecompact, respectively. From the schematic views, the numbers and orientations of the facet lines that will be developed on the top surfaces can be predicted. The seeded Y1.5 compacts were subjected to the heating cycles of the melt growth process for the fabrication of single grain Y123 bulk superconductors. The seeded Y1.5 compacts were located at the centre of a box furnace, heated to 1040 °C in air at a heating rate of 100 °C/h held at this temperature for 1 h, cooled to 1020 °C at a rate of 10 °C/h, cooled again to 970 °C at a rate of 0.3 °C/h and finally cooled down to room temperature at a rate of 100 °C/h. For oxygenation, the TSMG processed Y1.5 samples were heated to 500 °C/h at a rate of 100 °C/h in flowing oxygen, held at this temperature for 50 h, cooled to 450 °C at a rate of 100 °C/h, held at this temperature for 100 h, and then cooled to room temperature at a rate of 200 °C/h. The TSMG processed Y1.5 samples were field-cooled to 77 K using a Nd–B–Fe permanent magnet with a surface field of 0.52 Tesla (T), and the magnetic flux density distribution at 77 K was measured for the top surfaces using a Hall probe. Force–distance (F–d) curves were measured for field-cooled or zero field-cooled samples to 77 K using a Nd–B–Fe permanent magnet with a diameter of 30 mm and surface magnetic field of 0.5 T. The magnetic levitation forces at 77 K were estimated from F–d curves.
3. Results and discussion Fig. 2(a)–(d) shows the top surface views of TSMG processed Y1.5 samples with corner seeding (a and b) and edge seeding (c and d). After TSMG processing, the size of Y1.5 compacts was reduced from 20 mm to 15 mm owing to the volume contraction. It can be seen in all samples that the facet lines are developed on the top surfaces and the shapes are almost similar to those predicted in the schematic view of Fig. 1. One diagonal facet line, which began from a seed and grew to the corner of the compact, is observed at the top surface of sample (a). The entire top surface of sample (a) is covered with a grown Y123 grain without unreacted parts owing to the fast growth along the h1 1 0i direction. The top surface morphology with one diagonal facet line corresponds to a quarter of the top surface view of the conventional TSMG processed Y123 superconductors with centre seeding [2], and is similar to that observed in the buffer bridge processed Y123 samples [11]. On the other hand, two facet lines with deviation angles of 3–5° from the sides of the compact are developed
a
c [010]
b d
[110]
[100]
Fig. 1. Schematic view of corner seeding (a and b) and edge seeding (c and d).
(see the lines marked by arrows) on the top surface of sample (b). Unlike the top surface of sample (a), the unreacted region is present at the right upper corner (see the region marked by ‘‘r’’) of the top surface. This is because the growth rate (RGh1 0 0i) along the h1 0 0i direction of Y123 was lower than that along the h1 1 0i direction (RGh1 1 0i). Meanwhile, three facet lines are observed on the top surface of sample (c); one line is almost normal to the side of the compact, and two lines are parallel. Similar to those observed in sample (b), the unreacted regions are also present at the two upper corners of the top surface. On the other hand, two facet lines are observed on the top surfaces of sample (d). One facet line finishes at the corner of the compact and another facet line finishes at the intermediate point of the side. Interestingly, the facet lines have large curvatures, which are comparable to the almost straight facet lines observed in the other samples. The formation of the facet lines with large curvatures appears to be attributed to the unbalanced mass transfer for the growth of Y123 grains in the edge regions. The top surface morphologies (tracks of facet lines and growth fronts) of Fig. 2 are schematically shown in Fig. 3(a)–(d). The size of the growth area of Y123 on the top surfaces is dependent on the seeding method and seed orientation. The entire top surface was covered with a Y123 grain when the h1 1 0i facet line is parallel to the diagonal of the compact (see Fig. 3(a)). The orientation relationship of the corner-seeded Y123 sample is h1 0 0iseed//sidecompact. Except sample (a), unreacted regions are observed in the top surfaces of other samples owing to the slower growth rate along h1 0 0i directions. Fig. 4(a) and (b) shows the typical side views (surface morphology) of (a) edge-seeded Y123 and (b) corner seeded Y123 sample after a melt growth process. The side view of sample (a) is divided into a dark isosceles triangle region (c-growth sector) and two upper regions marked by arrows (a/b growth sectors). The cgrowth sector is symmetrical in shape because of the equivalent growth of Y123 toward both (left and right) directions from the seed. On the other hand, the c-growth sector of sample (b) is unsymmetrical in shape. It is likely that the Y123 growth to the direction opposite to the corner is easy, whereas the Y123 growth toward the corner is limited. From the top surface views of Fig. 3 and side views of Fig. 4, the growth nature of Y123 grains in the corner- or edge-seeded Y123 samples was well understood. The two important growing plane/ direction families in determining the top surface morphologies are {1 0 0}/h1 0 0i and {1 1 0}/h1 0 0i. The growth rate relationship between the two directions is given by
RGh1 1 0i ¼
pffiffiffi 2RGh1 0 0i
ð1Þ
where RGh1 1 0i and RGh1 0 0i are the growth rate of Y123 along a h1 1 0i direction and a h1 0 0i direction, respectively. This orientation relationship indicates that the h1 1 0i direction should be utilized to obtain a lager Y123 area in the corner or edge seeding. Fig. 5 shows magnetic flux density (B) maps of TSMG processed Y1.5 samples with corner seeding (a and b) or edge seeding (c and d), field-cooled at 77 K using a Nd–B–Fe permanent magnet. The B maps of all samples show a single grain flux contour with a single peak near the centre of the maps. No deep valleys or disconnected parts owing to the weakly linked grain boundaries or cracks are observed in all B maps. It is pointed out that the B maps of samples (a) and (d) are nearly symmetric in shape, whereas the B maps of samples (b) and (c) are non-symmetric. The non-symmetric B contours of samples (b) and (c) are attributed to the presence of unreacted non-superconducting regions at the upper parts of samples (a) and (d), as already observed in Fig. 2. The maximum B values at the peak points of samples (a)–(d) are 2.71 kG, 2.29 kG, 2.24 kG and 1.88 kG, respectively. The results of the magnetic flux density
227
C.-J. Kim et al. / Physica C 495 (2013) 225–228
(a)
(b) r
s
s
(c)
(d) r r
s
s
10 mm Fig. 2. Top surface views of TSMG processed Y1.5 samples with corner seeding (a and b) or edge seeding (c and d). ‘‘s’’ and ‘‘r’’ denote a seed and an unreacted region, respectively.
<110> directions
<110>
Y123 area <110>
(a)
Seed
a/b Facet line
Seed
c
Fig. 3. Schematics of the growth areas of Y123 on the top surfaces of Fig. 2.
measurement for the field-cooled samples are consistent with the top surface morphologies of Fig. 2. Fig. 6 shows F–d curves at 77 K of TSMG processed Y1.5 samples with corner seeding (a and b) or edge seeding (c and d), (a) zerofield cooled and (b) field cooled using a Nd–B–Fe permanent magnet. The F–d curves of the zero-field cooled samples show typical hysteresis characteristics by a strong Meissner repulsion. In all F– d curves in Fig. 6(a), the attractive force terms are zero. The maximum repulsive forces (FRep.) at a 1 mm gap of samples (a)–(d) are 44.6 N, 37.9 N, 44.4 N and 39.7 N, respectively. Sample (a) shows the highest FRep., whereas sample (b) shows the lowest FRep.. Since a magnetic levitation force is dependent on the size (area) of a superconductor facing a permanent magnet, it is reasonable that the top surface of sample (b) having an unreacted non-superconducting area showed the lowest FRep.. On the other hand, the F–d curves of the field-cooled samples show hysteresis characteristics having larger attractive forces (FAttr.) and relatively smaller repulsive forces in comparison to those of the zero field-cooled samples. The large FAttr. is attributed to the magnetic fields trapped within the superconductors by field cooling. The max. FAttr.s are 16.2 N,
(b)
Seed
a/b c
10 mm Fig. 4. Side views of (a) edge-seeded and (b) corner-seeded Y123 sample after the melt growth process. The grown Y123 areas in both samples are marked by a rectangular box.
C.-J. Kim et al. / Physica C 495 (2013) 225–228
(d)
2.5
2.5
2.5
2.5
2.0
2.0
2.0
2.0
2 4 6 8 10 12 14 16 18 20
X Axis (mm)
3.000 2.500 2.000 1.500 1.000 0.5000 0.2500 0 -0.3000
20 18 16 14 12 10 8 6 4 2 2 4 6 8 10 12 14 16 18 20
2
4
6
(m m)
Field (kG
Field (kG
Axi 14 16 s (m 18 20 m)
0.5 0.0 2
20 18 16 14 12 10 8 6 4 2 2 4 6 8 10 12 14 16 18 20
X Axis (mm)
X Axis (mm)
4 6 8 10
X A 12 14 16 xis (mm 18 20 )
2
4
6
20 18 16 14 12 10
Ax is (m m)
6
8
1.0
8
Y
4
0.0 2 4 6 8 X 10 12
20 18 16 14 12 10
Y Axis (mm)
Y Axis (mm)
20 18 16 14 12 10 8 6 4 2
2
0.5
1.5
Ax is
XA 14 xis 16 18 20 (mm )
8
1.0
Y
(m
is
6
0.0 2 4 6 8 10 12
20 18 16 14 12 10
1.5
Y Axis (mm)
2
0.5
Ax is (m m)
4 20
8
Ax
2 4 6 8
X A 10 12 14 16 xis (mm 18 )
Y Axis (mm)
20 18 16 14 12 10
1.0
Y
0.0
m)
0.5
1.5
Y
1.0
Field (kG
1.5
)
(c) )
(b) )
(a) Field (kG
)
228
20 18 16 14 12 10 8 6 4 2 2 4 6 8 10 12 14 16 18 20
X Axis (mm)
Fig. 5. Magnetic flux density maps of the TSMG processed Y1.5 samples with corner seeding (a and b) or edge seeding (c and d), field-cooled at 77 K using a Nd–B–Fe permanent magnet.
4. Conclusions A corner or edge seeding has been attempted to control the top surface morphologies (facet lines) of TSMG processed Y1.5 bulk superconductors. The facet lines, which are developed on the top surface of the TSMG processed Y1.5 samples, were successfully controlled using the corner or edge seeding with adjusted seed orientations. Most of the facet lines were nearly straight, whereas some of them had a curvature. The formation of the curved facet lines seems to be ascribed to the unbalanced mass transfer for the growth of Y123 grains in the edge regions of the compacts. The top surface morphology was dependent not only on the seeding method but also on the seed orientation. The unreacted non-superconducting regions were observed in parts of the top surfaces, owing to the slow growth rate along the h1 0 0i directions. The top surface with corner seeding where the h1 1 0i growth direction is parallel to the diagonal of the Y123 compact showed the highest magnetic flux density and magnetic levitation forces owing to the largest growth area of Y123. Acknowledgement This work was financially supported by the National R&D Program of the Ministry of Education, Science and Technology (MEST), Republic of Korea. References
Fig. 6. Force–distance curves at 77 K of TSMG-processed Y1.5 samples with corner seeding (a and b) or edge seeding (c and d): (a) zero-field cooled and (b) field cooled using a Nd–B–Fe permanent magnet.
12.5 N, 16.5 N, and 14 N, respectively. The max. FRep. of samples (a)–(d) are 24.3 N, 23.2 N, 22.0 N and 21.0 N, respectively. This result of the F–d measurement agrees with the result of the B measurement shown in Fig. 5.
[1] M. Murakami, N. Sakai, T. Higuchi, S.I. Yoo, Supercond. Sci. Technol. 9 (1996) 1015. [2] C.J. Kim, Y.A. Jee, G.W. Hong, Supercond. Sci. Technol. 13 (2000) 709. [3] T. Meignan, P.J. McGinn, C. Varanasi, Supercond. Sci. Technol. 10 (1997) 109. [4] V. Antal et al., Supercond. Sci. Technol. 22 (2009) 105001. [5] D.A. Cardwell et al., Supercond. Sci. Technol. 23 (2010) 034008. [6] C.J. Kim, K.B. Kim, H.W. Park, I.H. Kuk, G.W. Hong, J. Mater. Sci. 32 (1997) 4701. [7] Y.A. Jee, S.J.L. Kang, H. Chung, J. Mater. Res. 13 (1998) 583. [8] C.J. Kim, K.B. Kim, I.H. Kuk, G.W. Hong, J. Mater. Res. 13 (1998) 269. [9] M. Murakami et al., Supercond. Sci. Technol. 4 (1991) S43. [10] C.J. Kim, H.W. Park, K.B. Kim, G.W. Hong, Supercond. Sci. Technol. 8 (1995) 652. [11] J.H. Lee et al., Supercond. Sci. Technol. 24 (2011) 055019.