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Y2Ba5(Sn3−y−zCuyPtz)Ox in textured YBCO superconductors
Physica C 312 Ž1999. 261–268
Y2 Ba 5 žSn 3yyyzCu y Pt z /O x in textured YBCO superconductors Z.H. He
a,b,),1
, M.Z. Wu b, G. Bruchlos b, X.M. Xiong a , Y.Y. Luo a , W. Gawalek b
b
a Department of Physics, Zhongshan UniÕersity, Guangzhou, 510275, China Institut fur ¨ Physikalische Hochtechnologie, Postfach 100239, Jena, D-07702, Germany
Received 20 November 1998; accepted 18 December 1998
Abstract Ultrafine SnO 2 powder was made with grain size down to 20 nm. Such powder was introduced into the textured growth of Y1qx Ba 2 Cu 3 O 7q d ŽYBCO. superconductors. Crystalline inclusions, which were cubic in shape and from micrometer to submicrometer in size, were found trapped inside such grown YBCO superconductors. Determined by energy dispersive spectroscopy ŽEDS., and further confirmed by X-ray diffraction, the composition of the inclusions was Y2 Ba 5ŽSn 3yyyz Cu y Pt z .O x . This Sn-based phase belongs to double perovskite structures. They were not trapped uniformly inside the YBa 2 Cu 3 O 7y d Ž123., possibly caused by the agglomeration of the SnO 2 powder at the very beginning. Nevertheless, it is possible for fine Sn-based inclusions down to a few hundred nanometers to be trapped inside the 123. The addition of the ultrafine SnO 2 to YBCO textured superconductors without Pt did not seem to affect the size of Y2 BaCuO5 Ž211. particles. For comparison, Y2 Sn 2 O 7 was also introduced to YBCO textured superconductor. In this case, the so-called f phase YBa 3 Sn 2yx Cu xO 9y d was found. q 1999 Elsevier Science B.V. All rights reserved. PACS: 7470; 6150 Keywords: Ultrafine SnO 2 ; YBCO; Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x ; Double perovskite; Textured high Tc superconductors
1. Introduction As the melt texturing of YBCO improves the superconducting coupling between superconducting grains, and pushes the critical current density Jc up to an order of magnitude of 10 4 Arcm2 at 77 K in a magnetic field of several Tesla w1x, several potential applications, such as superconducting motors w2x, levitation transporting system w1x, and superconducting flywheels w1x, are considered promising. One of )
Corresponding author. Institut fur ¨ Physikalische Hochtechnologie, Postfach 100239, Jena, D-07702, Germany. E-mail: [email protected] 1 Current address in Germany is valid until July 30, 1999.
the obstacles to the applications is its relatively low Jc compared to that of YBCO thin films. One of the attempts to increase Jc is by doping, especially with Sn-based compounds w3,4x. Compared to the other element doping, the addition of Sn or Sn-based compounds has the advantage of not reducing the superconducting transition temperature Tc . It is also beneficial to the textured growth of 123 w5x and the superconducting properties w6x. There are a few arguments about the mechanism of such improvement, which are related to the reacted products between the Sn-based compounds and the 123. Reacted products of BaSnO 3 w7x, YBa 2 SnO5.5 w8x, the so-called ‘tinphase’ YBa 2.6 SnCu 0.6 Pt 0.1O x w9x, and the so-called f phase YBa 3 Sn 2yx Cu xO 9y d w10,11x were reported.
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 7 1 3 - 8
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Another attempt is to introduce nanosized nonsuperconducting inclusions inside 123 as flux pinning centers w12x. In this paper, we introduced nanosize powder SnO 2 to the YBCO textured growth. In this way, we found inclusions of a Sn-based phase Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x . We also found that some of such inclusions down to a few hundred nanometers were trapped inside the 123 crystal. However, such inclusions up to now do not distribute uniformly inside the material.
2. Experimental
croscope ŽTEM. image, Žsee Fig. 1. the average particle size of such made powders was about 20 nm. To destroy the agglomeration of the SnO 2 ultrafine particles to be mixed with the 123 powder, a beaker of acetone or alcohol was employed for suspension of the powder, which was then laid into a strong ultrasonic bath for at least 30 min. As the photograph of TEM shows, that is effective, but not thorough, for destroying the agglomeration. The precursor powders were added to the vibrating SnO 2 suspension for homogeneous mixing. The precursor powders could then separate the SnO 2 fine particles during drying and thus prevent them from agglomerating again.
2.1. Preparation of ultrafine SnO2 powder The SnO 2 powder was prepared by employing sol–gel method based on the following reaction: SnCl 2 q 2NH 4 OH ™ Sn Ž OH . 2 x q 2NH 4 Cl, 2Sn Ž OH . 2 q O 2 ™ 2SnO 2 q 2H 2 O.
Ž 1. Ž 2.
Stoichiometric SnCl 2 P 2H 2 O was dissolved in ethylene alcohol ŽEA.. The solution was then poured into the stirring ammonia. The pH value of the mixed solution was controlled between 9 and 10. The mixed solution was filtered after being stirred for 10 min. The gel subsequently obtained, white in color, was washed by EA for five times, dried and heated at 5008C for 1 h. Determined by both the width of the X-ray diffraction peaks and the transmitting electronic mi-
2.2. Preparation of Y2 Sn 2 O 7 The Y2 Sn 2 O 7 powder was prepared by employing sol–gel method based on the following reaction: SnCl 2 q YCl 3 q 5NH 4 OH™Sn Ž OH . 2 x q Y Ž OH . 3 x q 5NH 4 Cl.
Ž 3.
2Sn Ž OH . 2 q 2Y Ž OH . 3 q O 2 ™ Y2 Sn 2 O 7 q 5H 2 O. Ž 4. Stoichiometric SnCl 2 solution and YCl 3 solution were mixed. The solution was then poured into the stirring ammonia. The pH value of the mixed solution was controlled in a range between 9 and 10. The mixed solution was filtered after being stirred for 10 min. The gel obtained, white in color, was washed by water first and then by EA for five times. It was then dried and sintered at 10008C for 5 h. Such prepared powder was labeled YSO. The X-ray diffraction patterns for the final product showed that a Y2 Sn 2 O 7 phase formed! However, SnO 2 diffraction lines appears. The SnO 2 is estimated to be about 10%. The extra SnO 2 should be caused by the washing loss of YŽOH. 3 , which is more serious than that of SnŽOH. 2 . 2.3. Preparation of the high Tc textured YBCO superconductors
Fig. 1. The bright field TEM graph of the SnO 2 powder.
Three textured YBCO samples, containing Snbased oxides, were investigated in this paper. The first, Z78, has initial materials of YBa 2 Cu 3 O x q
Z.H. He et al.r Physica C 312 (1999) 261–268
0.25Y2 O 3 q 1 wt.% PtO 2 q 1 wt.% SnO 2 . The well-mixed powder was pressed into a pellet with diameter of 30 mm. A standard process was applied for the seeded melt-textured growth w13x. The second, 970813B, and the third, L9708042, has respectively initial materials of YBa 2 Cu 3 O x q 0.1ŽY2 O 3 q 2SnO 2 . and YBa 2 Cu 3 O x q 0.1Y2 Sn 2 O 7 ŽYSO.. The well-mixed powders were pressed into a bar of 30.0 = 5.0 = 1.5 mm3. Both the nominal compositions and the growth processes w14x are the same for the second and third sample. The YBa 2 Cu 3 O x powder used in Z78 were offered by Solvay, while those for 970813B and L9708042 were homemade in Zhongshan University.
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295 mT; and the critical current density Jc was 3 = 10 4 Arcm2 . Although the trapped field was not high and no indication of significant increase of Jc , the levitation force was the highest among almost one thousand textured samples. From the overlook composition images of the sample’s top surface, one can find that the Sn distribution is not microscopically uniform. These Snbased phase tended to accumulate in liquid phase regions and interfaces. One can also find it inside the 123 phase. Close-look composition images show clearly bright cubic inclusions containing Sn. They look similar to those reported by Monot et al. w9x, but better crystallized Žsee Fig. 2a.. The inclusion size
2.4. Preparation of the Y2 Ba5 Sn 2 .5 Cu 0 .5 O13.5 phase samples for X-ray diffraction analysis According to the compositions determined with EDS, two samples were synthesized with nominal com position of Y 2 Ba 5 Sn 2 .5 Cu 0 .5 O 1 3 .5 and Y2 Ba 5 Sn 1.5 PtCu 0.5 O 13.5 . The standard solid state reaction was employed for the synthesis. Stoichiometric amounts of Y2 O 3 , BaCO 3 , CuO, SnO 2 Žabout 0.3 mm in size., and PtO 2 Žif applied. were mixed and ground for 30 min. The mixture was heated to 10008C, kept overnight, and then sintered at 14008C for 5 h. 2.5. Characterizations A JeoL JXA-8800L scanning electronic microscope ŽSEM., equipped with an EDS analysis system, was employed for micrographic investigation and composition analysis. The X-ray diffraction was performed on a Philips X’pert system.
3. Results 3.1. SEM for sample Z78 Compared to the other standard textured samples, 1 wt.% of SnO 2 did not change the superconducting properties very much. At the temperature of 77 K, 0.5 mm away from the top of the sample, the levitation force Žzero field cooling. was 66 N; the maximum trapped magnetic field reached as high as
Fig. 2. The SEM composition graphs for the top surface of Z78. D1: 123 phase, D2: 211 phase. Bright areas, labeled as D3, are Sn-based phases. Ža. Caves inside the crystals are clearly seen; Žb. the size of the Sn-based inclusions is comparable to that of the 211 particle.
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Table 1 Compositions of the cubic shape crystals, 123 phase and 211 phase on the top surface of Z78 Site
r ŽY.
r ŽBa.
r ŽCu.
r ŽPt.
r ŽSn.
Ýa
ErrorŽY. Ž%.
ErrorŽBa. Ž%.
ErrorŽÝa . Ž%.
3-1 3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 3-10 4-0 4-1 4-2 4-3 4-4 Average 123-1 123-2 211-1 211-2
22.7 21.4 22.1 21.8 21.6 21.3 21.9 21.4 22.0 20.6 22.1 22.7 19.4 18.2 20.3 21.3 18.0 17.8 50.2 51.2
49.2 48.8 49.4 49.9 49.6 49.1 52.3 48.2 47.9 48.5 49.7 48.7 49.1 49.1 49.3 49.3 32.8 33.1 24.5 24.7
9.5 5.6 5.1 5.2 5.3 5.9 1.0 6.6 4.6 5.7 2.8 3.0 4.4 4.5 4.8 4.9 49.1 49.1 25.3 24.1
8.4 13.1 14.7 13.8 13.5 13.9 14.0 13.0 10.0 8.8 7.2 5.8 2.8 2.8 3.3 9.7 – – – –
10.2 11.1 8.8 9.3 10.0 9.9 10.8 10.9 15.5 16.4 18.8 19.9 24.4 25.5 22.3 14.9 – – – –
28.1 29.8 28.6 28.3 28.8 29.7 25.8 30.5 30.1 30.9 28.8 28.7 31.6 32.8 30.4 29.5 – – – –
6.6 0.5 3.8 2.3 1.4 0.0 2.8 0.5 3.3 y3.3 3.8 6.6 y8.9 y14.6 y4.7
y0.20 y1.01 0.20 1.22 0.61 y0.41 6.09 y2.23 y2.84 y1.62 0.81 y1.22 y0.41 y0.41 0.00
y4.7 1.0 y3.1 y4.1 y2.4 0.7 y12.5 3.4 2.0 4.7 y2.4 y2.7 7.1 11.2 3.1
a
Ý s r ŽCu. q r ŽPt. q r ŽSn..
distributes from submicrometer to 10 mm, which is comparable to the size of 211 particles Žsee Fig. 2b.. Determined with EDS, the compositions of such inclusions are listed in Table 1. Different site label in the table represents different inclusion suitable for EDS analysis. ŽTo judge the reliability of the compositions determined with EDS, some of those of 123 and 211 crystals are also listed out for reference.. As one can see, though the contents of Cu, Pt and Sn vary in a wide range, their sum is almost unchanged. ŽThe relative errors, defined by < rd y r
tens of micrometers in size. The compositions of such inclusions, obtained in the same way, are listed in Table 2. In those regions containing Sn, they were slightly contaminated by both Cl and Al. The 123 phase and the 211 phase remain pure. The Cl con-
3.2. SEM for sample 970813B The distribution of Sn-based phase is similar to that for sample Z78. The average size of such inclusions is also approximately the same Žsee Fig. 3.. In contrast, the 211 particles ranged from micrometer to
Fig. 3. The SEM composition graph for the surface of 970813B cubic crystals in liquid phase region and the areas in which no identified crystal was seen in the resolution limit. D1: 123 phase, D2: 211 phase. Bright areas, labeled as D3, are Sn-based phases.
Z.H. He et al.r Physica C 312 (1999) 261–268 Table 2 Compositions of the cubic shape crystals, 123 phase and 211 phase on the surface of 970813B Site
r ŽY. r ŽBa. r ŽCu. r ŽSn. r ŽAl. r ŽCl. r ŽCuqSn.
5-1 Sn1 5-1 Sn2 5-1 Sn3 5-1 Sn4 5-1 Sn5 5-2 Sn1 5-2 Sn2 Average 5-1 ‘123’ 5-2 ‘123’ 5-1 ‘211’
19.4 17.7 18.7 18.6 18.2 20.1 18.2 18.7 18.2 17.3 50.5
49.7 50.0 49.1 49.6 50.6 47.5 49.6 49.4 33.7 34.0 25.0
3.1 3.5 6.8 3.9 3.2 5.7 4.0 4.3 48.1 48.7 24.6
25.0 24.6 23.2 24.6 22.7 24.1 23.4 23.9 – – –
1.6 3.0 1.5 2.5 4.3 1.6 2.5 2.4 – – –
1.3 1.2 0.8 0.9 1.1 1.1 1.1 1.1 – – –
28.1 28.1 30.0 28.5 25.9 29.8 27.4 28.3 – – –
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Y2 Ba 5 Sn 2.5 Cu 0.5 O13.5 . They were too small to be observed within the resolution of the SEM. Assuming the area consists of Y2 Ba 5 Sn 2.5 Cu 0.5 O 13.5 or the f phase, BaCuO 2 and CuO, we can estimate their ratio to be 1:3:6.5. High resolution SEM is called for further judgment. 3.3. SEM for sample L9708042 Again, such inclusions are not uniformly distributed inside the sample. Furthermore, such inclusions did not accumulate uniformly in every interface or liquid phase region Žsee Fig. 4a.. In the neighborhoods of some other regions, not even a
tamination may come from the preparation of SnO 2 nanopowder, in which Cly1 ions were not washed away completely. The Al contamination, on the other hand, should come from the reaction between the 123 phase and the alumina crucibles. Assuming the Al would substitute the Y site, the average composition of 970813B gives a quite similar formula as that of Z78: Y2y z Al z Ba 5 ŽSn 3yy Cu y .O x . It can belong to the Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x phase with z s 0. The assumption is not unreasonable because both Al ion and Y ion possess the same valence of q3, though their radii are quite different. If the Al does not occupy the Y site, the composition of the inclusions will shift, but not get to that of the f phase w10x or f 2 phase suggested in Ref. w11x. In addition to such Sn-based cubic inclusions, there were some other Sn-based areas in which no identified crystal was seen Žalso see Fig. 3, the composition for such areas is given in Table 3.. It is possible, however, that some Sn-based little crystals exist with the similar composition of f phase or
Table 3 Compositions of areas containing Sn, in which no crystal was distinguished Site
r ŽY. r ŽBa. r ŽCu. r ŽSn. r ŽAl. r ŽCl. r ŽCuqSn.
5-1 SnCu1 5-3 SnCu1 5-3 SnCu2 Average
6.7 8.1 7.4 7.4
39.9 39.0 40.7 39.9
41.0 39.3 38.7 39.7
10.9 12.0 11.9 11.6
1.0 1.0 1.1 1.0
0.6 0.6 0.3 0.5
51.9 51.3 50.6 51.3
Fig. 4. The SEM composition graphs for the surface of L9708042. Ža. Nonuniform distribution of Sn-based inclusions; Žb. inclusions down to few hundred nanometers are observed. D1: 123 phase, D2: 211 phase. Bright areas, labeled as D3, are Sn-based phases.
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single piece of Sn-based phase could be found. Some of the inclusions can be found even inside the 211 particles that are much larger Žboth Fig. 4a and b.. The noncrystal areas are similar to those of 970813B. A closer look shows that the Sn-based inclusions were submicrometer in size Žsee Fig. 4b.. They were much smaller than those in 970813B though the distribution of Sn is similar. This is clear evidence that Sn-based nanosize inclusions can be trapped inside 123 phase. The composition of such inclusions is given in Table 4. It seems more close to that of the f phase YBa 3 Sn 2yx Cu xO 9y d w10x than to that of Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x phase in Z78. However, the shape is cubic rather than needle-like as reported in Ref. w10x. 3.4. X-ray diffraction analysis for the Sn-based phase According to the compositions determined by the EDS, two samples were synthesized for X-ray diffraction. The powder of Y2 Ba 5 Sn 1.5 PtCu 0.5 O 13.5 is black in color while that of Y2 Ba 5 Sn 2.5 Cu 0.5 O13.5 is brown. They both form a cubic double perovskite structure, as shown in Fig. 5. Determined by least squares refinement, the lattice parameter a is respec˚ and 8.472 A˚ for Y2 Ba 5 Sn 1.5 PtCu 0.5tively 8.392 A O 13.5 and Y2 Ba 5 Sn 2.5 Cu 0.5 O13.5 . Their diffraction patterns are similar to that of YBa 2 SnO5.5 phase w8x and that of f phase YBa 3 Sn 2yx Cu xO 9y d w10x. This
Fig. 5. X-ray diffraction patterns for Ža. Y2 Ba 5 Sn 1.5 PtCu 0.5 O13.5 and Žb. Y2 Ba 5 Sn 2.5 Cu 0.5 O13.5 .
suggests that, though the composition differs from each other greatly, the three phases belong to the same structure. This structure is similar to the double perovskite ŽU0.6 Pt 0.4 .YBa 2 O6 , reported by Sawh et al. w15x the particles of which act as effective flux pinning centers in YBCO textured superconductor.
4. Discussion and conclusions Table 4 Compositions of the Sn-based inclusions, 123 phase and 211 phase on the surface of L9708042 Site
r ŽY.
r ŽBa.
r ŽCu.
r ŽSn.
r ŽCl.
r ŽCuqSn.
4-1 Sn1 4-2 Sn2 4-3 Sn3 4-3 Sn4 4-4 Sn5 4-5 Sn6 4-6 Sn7 4-6 Sn8 4-6 Sn9 4-6 Sn10 4-7 Sn11 Average 4-2 ‘123’ 4-2 ‘211’
13.5 16.6 17.1 15.7 17.7 17.8 18.2 19.0 17.4 17.7 16.5 17.0 17.6 48.9
48.9 46.8 49.7 45.5 48.5 47.3 48.7 48.3 49.6 49.7 48.1 48.3 33.8 25.1
12.6 16.6 4.1 12.5 9.8 12.3 6.7 6.8 6.8 5.4 8.7 9.3 48.6 24.9
24.1 19.4 27.7 24.8 22.4 20.2 24.6 24.0 24.6 25.8 22.0 23.6 – –
1.0 0.6 1.3 1.4 1.6 2.4 1.9 1.9 1.6 1.4 4.8 1.8 – 1.1
36.7 36.0 31.8 37.3 32.2 32.5 31.3 30.8 31.4 31.2 30.7 32.9 – –
4.1. The formation of Y2 Ba5 (Sn 3 y y y z Cu y Pt z )Ox phase The composition variation, as indicated in Table 1, should depend on the local supply of Sn, Cu, and Pt. Such composition differs from both the YBa 2SnO5.5 phase w8x and the f phase YBa 3 Sn 2yxCu xO 9y d w10x. Within the common experimental errors of EDS, it is reasonable to suggest the ‘tinphase’ YBa 2.6 SnCu 0.6 Pt 0.1O x w9x be one of the Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x phases. It is interesting to note that YBa 2 SnO5.5 phase is generated for the addition of SnO 2 ; the so-called ‘tin-phase’ YBa 2.6 SnCu 0.6 Pt 0.1O x and the f phase YBa 3 Sn 2yx Cu xO 9y d are generated for the addition of BaSnO 3 ; while in this work, Y2 Ba 5 ŽSn 3yyyzCu y Pt z .O x was found for the addition of ultrafine
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SnO 2 powder but the f phase was also found for the addition of Y2 Sn 2 O 7 . It is difficult to relate the different reacted products to the different initial materials or composition, but the local element concentration, together with the existence of Pt, should play an important role in the formation of the final Snbased product. Based on the X-ray diffraction analysis, we assume that all the three phases belong to the same double perovskite structure but with different composition caused by element substitution.
particles in a similar way to that reported by Varanasi et al. w16x. Although quite close to those large crystals, some small Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x crystals down to a few hundred nanometers still survive from being devoured by those large Žsee Fig. 2a.. A flat facet also indicates a nearly zero interface energy, or negligible interface energy comparing to its bulk energy. The crystal growing mechanism and the interface energy of Y2 Ba 5 Sn 2.5 Cu 0.5 O 13.5 deserve to be investigated furthermore.
4.2. The effect of Y2 Ba5 Sn 2 .5 Cu 0 .5 O13.5 on the size of 211 phase
4.4. The distribution of Sn-based inclusions
As can be seen from Fig. 3 to Fig. 4a and b, the existence of Y2 Ba 5 Sn 2.5 Cu 0.5 O 13.5 , or the addition of ultrafine SnO 2 , does not reduce the size of 211 particles. This is in contrast to the effect of BaSnO 3 that reduces the size of 211 w6x. For the sample containing Pt, the 211 particles are much smaller Žsee Fig. 2b.. In this case, it is difficult to separate the effect of PtO 2 on the 211 phase from that of Y2 Ba 5 Sn 1.5 PtCu 0.5 O 13.5 if there is any. It is reasonable to believe that Y2 Ba 5 Sn 1.5 PtCu 0.5 O 13.5 plays a similar role as Y2 Ba 5 Sn 2.5 Cu 0.5 O 13.5 does since their structures are the same and their compositions are similar. As to the influence of processes on reducing the size of 211 particles, it is not easy to conclude because the textured growth process for Z78 differed from that for 970813B and L9708042. However, it is well known that PtO 2 plays a role more crucial than processes do. We thus infer that the existence of Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x does not hinder the PtO 2 from doing its job. 4.3. The Y2 Ba5 (Sn 3 y y y z Cu y Pt z )Ox crystals For those inclusions that can be seen clearly, they are cubic in shape and have flat facets, no matter whether they are inside 123 or liquid phase regions. This shape is quite different from the sphere shape of 211, indicating a different crystal-growing mechanism from that of 211. It differs from the needleshape f phase particles too w10x. On the other hand, we found caves in some of the Sn-based crystals ŽFig. 2a.. This suggests that those crystals did not grow from a single nucleus. It is quite possible that they developed from a group of agglomerative SnO 2
Although there is clear evidence about trapped nanocrystals inside the 123 phase, many other Snbased inclusions are not distributed uniformly. This might be caused by the nonuniform distribution of SnO 2 at the beginning, due to their agglomeration. This explanation is supported by the microstructure, observed under a polarized optical microscope, for a sample quenched in air from 11008C, in which accumulated square inclusions were found. We cannot rule out another possibility that, when the growing rate is low enough, the Sn-based phase is pushed out of the 123 grains by the growth front, so that such nonuniform distribution forms during the growth of 123 grains. The 123 precursor powder with large, and different size can also lead to the nonuniform distribution of the Sn-based inclusions. The three mechanisms are applicable to the formation of the Sn-based non crystal areas. Whichever mechanism works, the accumulation of Sn-based phases in grain boundaries would weaken the superconducting coupling between the grains. Nevertheless, when the problems of the agglomeration and the nonuniform mixing are solved, it is possible for fine Sn-based inclusions to be uniformly trapped inside the 123 crystals of YBCO textured superconductors.
Acknowledgements We would like to appreciate the help of Mrs. C. Schmidt and Dr. R. Hergt for the X-ray diffraction measurement and the discussion thereafter. The work was partially supported by the German BMBF project Žcontract number 13N6854., the National Nature Science Foundation of China, the Nature Science
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Foundation of Guangdong Province ŽP.R. China., and the Alexander von Humboldt Foundation.
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Y2 Ba 5 žSn 3yyyzCu y Pt z /O x in textured YBCO superconductors Z.H. He
a,b,),1
, M.Z. Wu b, G. Bruchlos b, X.M. Xiong a , Y.Y. Luo a , W. Gawalek b
b
a Department of Physics, Zhongshan UniÕersity, Guangzhou, 510275, China Institut fur ¨ Physikalische Hochtechnologie, Postfach 100239, Jena, D-07702, Germany
Received 20 November 1998; accepted 18 December 1998
Abstract Ultrafine SnO 2 powder was made with grain size down to 20 nm. Such powder was introduced into the textured growth of Y1qx Ba 2 Cu 3 O 7q d ŽYBCO. superconductors. Crystalline inclusions, which were cubic in shape and from micrometer to submicrometer in size, were found trapped inside such grown YBCO superconductors. Determined by energy dispersive spectroscopy ŽEDS., and further confirmed by X-ray diffraction, the composition of the inclusions was Y2 Ba 5ŽSn 3yyyz Cu y Pt z .O x . This Sn-based phase belongs to double perovskite structures. They were not trapped uniformly inside the YBa 2 Cu 3 O 7y d Ž123., possibly caused by the agglomeration of the SnO 2 powder at the very beginning. Nevertheless, it is possible for fine Sn-based inclusions down to a few hundred nanometers to be trapped inside the 123. The addition of the ultrafine SnO 2 to YBCO textured superconductors without Pt did not seem to affect the size of Y2 BaCuO5 Ž211. particles. For comparison, Y2 Sn 2 O 7 was also introduced to YBCO textured superconductor. In this case, the so-called f phase YBa 3 Sn 2yx Cu xO 9y d was found. q 1999 Elsevier Science B.V. All rights reserved. PACS: 7470; 6150 Keywords: Ultrafine SnO 2 ; YBCO; Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x ; Double perovskite; Textured high Tc superconductors
1. Introduction As the melt texturing of YBCO improves the superconducting coupling between superconducting grains, and pushes the critical current density Jc up to an order of magnitude of 10 4 Arcm2 at 77 K in a magnetic field of several Tesla w1x, several potential applications, such as superconducting motors w2x, levitation transporting system w1x, and superconducting flywheels w1x, are considered promising. One of )
Corresponding author. Institut fur ¨ Physikalische Hochtechnologie, Postfach 100239, Jena, D-07702, Germany. E-mail: [email protected] 1 Current address in Germany is valid until July 30, 1999.
the obstacles to the applications is its relatively low Jc compared to that of YBCO thin films. One of the attempts to increase Jc is by doping, especially with Sn-based compounds w3,4x. Compared to the other element doping, the addition of Sn or Sn-based compounds has the advantage of not reducing the superconducting transition temperature Tc . It is also beneficial to the textured growth of 123 w5x and the superconducting properties w6x. There are a few arguments about the mechanism of such improvement, which are related to the reacted products between the Sn-based compounds and the 123. Reacted products of BaSnO 3 w7x, YBa 2 SnO5.5 w8x, the so-called ‘tinphase’ YBa 2.6 SnCu 0.6 Pt 0.1O x w9x, and the so-called f phase YBa 3 Sn 2yx Cu xO 9y d w10,11x were reported.
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 7 1 3 - 8
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Another attempt is to introduce nanosized nonsuperconducting inclusions inside 123 as flux pinning centers w12x. In this paper, we introduced nanosize powder SnO 2 to the YBCO textured growth. In this way, we found inclusions of a Sn-based phase Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x . We also found that some of such inclusions down to a few hundred nanometers were trapped inside the 123 crystal. However, such inclusions up to now do not distribute uniformly inside the material.
2. Experimental
croscope ŽTEM. image, Žsee Fig. 1. the average particle size of such made powders was about 20 nm. To destroy the agglomeration of the SnO 2 ultrafine particles to be mixed with the 123 powder, a beaker of acetone or alcohol was employed for suspension of the powder, which was then laid into a strong ultrasonic bath for at least 30 min. As the photograph of TEM shows, that is effective, but not thorough, for destroying the agglomeration. The precursor powders were added to the vibrating SnO 2 suspension for homogeneous mixing. The precursor powders could then separate the SnO 2 fine particles during drying and thus prevent them from agglomerating again.
2.1. Preparation of ultrafine SnO2 powder The SnO 2 powder was prepared by employing sol–gel method based on the following reaction: SnCl 2 q 2NH 4 OH ™ Sn Ž OH . 2 x q 2NH 4 Cl, 2Sn Ž OH . 2 q O 2 ™ 2SnO 2 q 2H 2 O.
Ž 1. Ž 2.
Stoichiometric SnCl 2 P 2H 2 O was dissolved in ethylene alcohol ŽEA.. The solution was then poured into the stirring ammonia. The pH value of the mixed solution was controlled between 9 and 10. The mixed solution was filtered after being stirred for 10 min. The gel subsequently obtained, white in color, was washed by EA for five times, dried and heated at 5008C for 1 h. Determined by both the width of the X-ray diffraction peaks and the transmitting electronic mi-
2.2. Preparation of Y2 Sn 2 O 7 The Y2 Sn 2 O 7 powder was prepared by employing sol–gel method based on the following reaction: SnCl 2 q YCl 3 q 5NH 4 OH™Sn Ž OH . 2 x q Y Ž OH . 3 x q 5NH 4 Cl.
Ž 3.
2Sn Ž OH . 2 q 2Y Ž OH . 3 q O 2 ™ Y2 Sn 2 O 7 q 5H 2 O. Ž 4. Stoichiometric SnCl 2 solution and YCl 3 solution were mixed. The solution was then poured into the stirring ammonia. The pH value of the mixed solution was controlled in a range between 9 and 10. The mixed solution was filtered after being stirred for 10 min. The gel obtained, white in color, was washed by water first and then by EA for five times. It was then dried and sintered at 10008C for 5 h. Such prepared powder was labeled YSO. The X-ray diffraction patterns for the final product showed that a Y2 Sn 2 O 7 phase formed! However, SnO 2 diffraction lines appears. The SnO 2 is estimated to be about 10%. The extra SnO 2 should be caused by the washing loss of YŽOH. 3 , which is more serious than that of SnŽOH. 2 . 2.3. Preparation of the high Tc textured YBCO superconductors
Fig. 1. The bright field TEM graph of the SnO 2 powder.
Three textured YBCO samples, containing Snbased oxides, were investigated in this paper. The first, Z78, has initial materials of YBa 2 Cu 3 O x q
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0.25Y2 O 3 q 1 wt.% PtO 2 q 1 wt.% SnO 2 . The well-mixed powder was pressed into a pellet with diameter of 30 mm. A standard process was applied for the seeded melt-textured growth w13x. The second, 970813B, and the third, L9708042, has respectively initial materials of YBa 2 Cu 3 O x q 0.1ŽY2 O 3 q 2SnO 2 . and YBa 2 Cu 3 O x q 0.1Y2 Sn 2 O 7 ŽYSO.. The well-mixed powders were pressed into a bar of 30.0 = 5.0 = 1.5 mm3. Both the nominal compositions and the growth processes w14x are the same for the second and third sample. The YBa 2 Cu 3 O x powder used in Z78 were offered by Solvay, while those for 970813B and L9708042 were homemade in Zhongshan University.
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295 mT; and the critical current density Jc was 3 = 10 4 Arcm2 . Although the trapped field was not high and no indication of significant increase of Jc , the levitation force was the highest among almost one thousand textured samples. From the overlook composition images of the sample’s top surface, one can find that the Sn distribution is not microscopically uniform. These Snbased phase tended to accumulate in liquid phase regions and interfaces. One can also find it inside the 123 phase. Close-look composition images show clearly bright cubic inclusions containing Sn. They look similar to those reported by Monot et al. w9x, but better crystallized Žsee Fig. 2a.. The inclusion size
2.4. Preparation of the Y2 Ba5 Sn 2 .5 Cu 0 .5 O13.5 phase samples for X-ray diffraction analysis According to the compositions determined with EDS, two samples were synthesized with nominal com position of Y 2 Ba 5 Sn 2 .5 Cu 0 .5 O 1 3 .5 and Y2 Ba 5 Sn 1.5 PtCu 0.5 O 13.5 . The standard solid state reaction was employed for the synthesis. Stoichiometric amounts of Y2 O 3 , BaCO 3 , CuO, SnO 2 Žabout 0.3 mm in size., and PtO 2 Žif applied. were mixed and ground for 30 min. The mixture was heated to 10008C, kept overnight, and then sintered at 14008C for 5 h. 2.5. Characterizations A JeoL JXA-8800L scanning electronic microscope ŽSEM., equipped with an EDS analysis system, was employed for micrographic investigation and composition analysis. The X-ray diffraction was performed on a Philips X’pert system.
3. Results 3.1. SEM for sample Z78 Compared to the other standard textured samples, 1 wt.% of SnO 2 did not change the superconducting properties very much. At the temperature of 77 K, 0.5 mm away from the top of the sample, the levitation force Žzero field cooling. was 66 N; the maximum trapped magnetic field reached as high as
Fig. 2. The SEM composition graphs for the top surface of Z78. D1: 123 phase, D2: 211 phase. Bright areas, labeled as D3, are Sn-based phases. Ža. Caves inside the crystals are clearly seen; Žb. the size of the Sn-based inclusions is comparable to that of the 211 particle.
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Table 1 Compositions of the cubic shape crystals, 123 phase and 211 phase on the top surface of Z78 Site
r ŽY.
r ŽBa.
r ŽCu.
r ŽPt.
r ŽSn.
Ýa
ErrorŽY. Ž%.
ErrorŽBa. Ž%.
ErrorŽÝa . Ž%.
3-1 3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 3-10 4-0 4-1 4-2 4-3 4-4 Average 123-1 123-2 211-1 211-2
22.7 21.4 22.1 21.8 21.6 21.3 21.9 21.4 22.0 20.6 22.1 22.7 19.4 18.2 20.3 21.3 18.0 17.8 50.2 51.2
49.2 48.8 49.4 49.9 49.6 49.1 52.3 48.2 47.9 48.5 49.7 48.7 49.1 49.1 49.3 49.3 32.8 33.1 24.5 24.7
9.5 5.6 5.1 5.2 5.3 5.9 1.0 6.6 4.6 5.7 2.8 3.0 4.4 4.5 4.8 4.9 49.1 49.1 25.3 24.1
8.4 13.1 14.7 13.8 13.5 13.9 14.0 13.0 10.0 8.8 7.2 5.8 2.8 2.8 3.3 9.7 – – – –
10.2 11.1 8.8 9.3 10.0 9.9 10.8 10.9 15.5 16.4 18.8 19.9 24.4 25.5 22.3 14.9 – – – –
28.1 29.8 28.6 28.3 28.8 29.7 25.8 30.5 30.1 30.9 28.8 28.7 31.6 32.8 30.4 29.5 – – – –
6.6 0.5 3.8 2.3 1.4 0.0 2.8 0.5 3.3 y3.3 3.8 6.6 y8.9 y14.6 y4.7
y0.20 y1.01 0.20 1.22 0.61 y0.41 6.09 y2.23 y2.84 y1.62 0.81 y1.22 y0.41 y0.41 0.00
y4.7 1.0 y3.1 y4.1 y2.4 0.7 y12.5 3.4 2.0 4.7 y2.4 y2.7 7.1 11.2 3.1
a
Ý s r ŽCu. q r ŽPt. q r ŽSn..
distributes from submicrometer to 10 mm, which is comparable to the size of 211 particles Žsee Fig. 2b.. Determined with EDS, the compositions of such inclusions are listed in Table 1. Different site label in the table represents different inclusion suitable for EDS analysis. ŽTo judge the reliability of the compositions determined with EDS, some of those of 123 and 211 crystals are also listed out for reference.. As one can see, though the contents of Cu, Pt and Sn vary in a wide range, their sum is almost unchanged. ŽThe relative errors, defined by < rd y r
tens of micrometers in size. The compositions of such inclusions, obtained in the same way, are listed in Table 2. In those regions containing Sn, they were slightly contaminated by both Cl and Al. The 123 phase and the 211 phase remain pure. The Cl con-
3.2. SEM for sample 970813B The distribution of Sn-based phase is similar to that for sample Z78. The average size of such inclusions is also approximately the same Žsee Fig. 3.. In contrast, the 211 particles ranged from micrometer to
Fig. 3. The SEM composition graph for the surface of 970813B cubic crystals in liquid phase region and the areas in which no identified crystal was seen in the resolution limit. D1: 123 phase, D2: 211 phase. Bright areas, labeled as D3, are Sn-based phases.
Z.H. He et al.r Physica C 312 (1999) 261–268 Table 2 Compositions of the cubic shape crystals, 123 phase and 211 phase on the surface of 970813B Site
r ŽY. r ŽBa. r ŽCu. r ŽSn. r ŽAl. r ŽCl. r ŽCuqSn.
5-1 Sn1 5-1 Sn2 5-1 Sn3 5-1 Sn4 5-1 Sn5 5-2 Sn1 5-2 Sn2 Average 5-1 ‘123’ 5-2 ‘123’ 5-1 ‘211’
19.4 17.7 18.7 18.6 18.2 20.1 18.2 18.7 18.2 17.3 50.5
49.7 50.0 49.1 49.6 50.6 47.5 49.6 49.4 33.7 34.0 25.0
3.1 3.5 6.8 3.9 3.2 5.7 4.0 4.3 48.1 48.7 24.6
25.0 24.6 23.2 24.6 22.7 24.1 23.4 23.9 – – –
1.6 3.0 1.5 2.5 4.3 1.6 2.5 2.4 – – –
1.3 1.2 0.8 0.9 1.1 1.1 1.1 1.1 – – –
28.1 28.1 30.0 28.5 25.9 29.8 27.4 28.3 – – –
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Y2 Ba 5 Sn 2.5 Cu 0.5 O13.5 . They were too small to be observed within the resolution of the SEM. Assuming the area consists of Y2 Ba 5 Sn 2.5 Cu 0.5 O 13.5 or the f phase, BaCuO 2 and CuO, we can estimate their ratio to be 1:3:6.5. High resolution SEM is called for further judgment. 3.3. SEM for sample L9708042 Again, such inclusions are not uniformly distributed inside the sample. Furthermore, such inclusions did not accumulate uniformly in every interface or liquid phase region Žsee Fig. 4a.. In the neighborhoods of some other regions, not even a
tamination may come from the preparation of SnO 2 nanopowder, in which Cly1 ions were not washed away completely. The Al contamination, on the other hand, should come from the reaction between the 123 phase and the alumina crucibles. Assuming the Al would substitute the Y site, the average composition of 970813B gives a quite similar formula as that of Z78: Y2y z Al z Ba 5 ŽSn 3yy Cu y .O x . It can belong to the Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x phase with z s 0. The assumption is not unreasonable because both Al ion and Y ion possess the same valence of q3, though their radii are quite different. If the Al does not occupy the Y site, the composition of the inclusions will shift, but not get to that of the f phase w10x or f 2 phase suggested in Ref. w11x. In addition to such Sn-based cubic inclusions, there were some other Sn-based areas in which no identified crystal was seen Žalso see Fig. 3, the composition for such areas is given in Table 3.. It is possible, however, that some Sn-based little crystals exist with the similar composition of f phase or
Table 3 Compositions of areas containing Sn, in which no crystal was distinguished Site
r ŽY. r ŽBa. r ŽCu. r ŽSn. r ŽAl. r ŽCl. r ŽCuqSn.
5-1 SnCu1 5-3 SnCu1 5-3 SnCu2 Average
6.7 8.1 7.4 7.4
39.9 39.0 40.7 39.9
41.0 39.3 38.7 39.7
10.9 12.0 11.9 11.6
1.0 1.0 1.1 1.0
0.6 0.6 0.3 0.5
51.9 51.3 50.6 51.3
Fig. 4. The SEM composition graphs for the surface of L9708042. Ža. Nonuniform distribution of Sn-based inclusions; Žb. inclusions down to few hundred nanometers are observed. D1: 123 phase, D2: 211 phase. Bright areas, labeled as D3, are Sn-based phases.
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single piece of Sn-based phase could be found. Some of the inclusions can be found even inside the 211 particles that are much larger Žboth Fig. 4a and b.. The noncrystal areas are similar to those of 970813B. A closer look shows that the Sn-based inclusions were submicrometer in size Žsee Fig. 4b.. They were much smaller than those in 970813B though the distribution of Sn is similar. This is clear evidence that Sn-based nanosize inclusions can be trapped inside 123 phase. The composition of such inclusions is given in Table 4. It seems more close to that of the f phase YBa 3 Sn 2yx Cu xO 9y d w10x than to that of Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x phase in Z78. However, the shape is cubic rather than needle-like as reported in Ref. w10x. 3.4. X-ray diffraction analysis for the Sn-based phase According to the compositions determined by the EDS, two samples were synthesized for X-ray diffraction. The powder of Y2 Ba 5 Sn 1.5 PtCu 0.5 O 13.5 is black in color while that of Y2 Ba 5 Sn 2.5 Cu 0.5 O13.5 is brown. They both form a cubic double perovskite structure, as shown in Fig. 5. Determined by least squares refinement, the lattice parameter a is respec˚ and 8.472 A˚ for Y2 Ba 5 Sn 1.5 PtCu 0.5tively 8.392 A O 13.5 and Y2 Ba 5 Sn 2.5 Cu 0.5 O13.5 . Their diffraction patterns are similar to that of YBa 2 SnO5.5 phase w8x and that of f phase YBa 3 Sn 2yx Cu xO 9y d w10x. This
Fig. 5. X-ray diffraction patterns for Ža. Y2 Ba 5 Sn 1.5 PtCu 0.5 O13.5 and Žb. Y2 Ba 5 Sn 2.5 Cu 0.5 O13.5 .
suggests that, though the composition differs from each other greatly, the three phases belong to the same structure. This structure is similar to the double perovskite ŽU0.6 Pt 0.4 .YBa 2 O6 , reported by Sawh et al. w15x the particles of which act as effective flux pinning centers in YBCO textured superconductor.
4. Discussion and conclusions Table 4 Compositions of the Sn-based inclusions, 123 phase and 211 phase on the surface of L9708042 Site
r ŽY.
r ŽBa.
r ŽCu.
r ŽSn.
r ŽCl.
r ŽCuqSn.
4-1 Sn1 4-2 Sn2 4-3 Sn3 4-3 Sn4 4-4 Sn5 4-5 Sn6 4-6 Sn7 4-6 Sn8 4-6 Sn9 4-6 Sn10 4-7 Sn11 Average 4-2 ‘123’ 4-2 ‘211’
13.5 16.6 17.1 15.7 17.7 17.8 18.2 19.0 17.4 17.7 16.5 17.0 17.6 48.9
48.9 46.8 49.7 45.5 48.5 47.3 48.7 48.3 49.6 49.7 48.1 48.3 33.8 25.1
12.6 16.6 4.1 12.5 9.8 12.3 6.7 6.8 6.8 5.4 8.7 9.3 48.6 24.9
24.1 19.4 27.7 24.8 22.4 20.2 24.6 24.0 24.6 25.8 22.0 23.6 – –
1.0 0.6 1.3 1.4 1.6 2.4 1.9 1.9 1.6 1.4 4.8 1.8 – 1.1
36.7 36.0 31.8 37.3 32.2 32.5 31.3 30.8 31.4 31.2 30.7 32.9 – –
4.1. The formation of Y2 Ba5 (Sn 3 y y y z Cu y Pt z )Ox phase The composition variation, as indicated in Table 1, should depend on the local supply of Sn, Cu, and Pt. Such composition differs from both the YBa 2SnO5.5 phase w8x and the f phase YBa 3 Sn 2yxCu xO 9y d w10x. Within the common experimental errors of EDS, it is reasonable to suggest the ‘tinphase’ YBa 2.6 SnCu 0.6 Pt 0.1O x w9x be one of the Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x phases. It is interesting to note that YBa 2 SnO5.5 phase is generated for the addition of SnO 2 ; the so-called ‘tin-phase’ YBa 2.6 SnCu 0.6 Pt 0.1O x and the f phase YBa 3 Sn 2yx Cu xO 9y d are generated for the addition of BaSnO 3 ; while in this work, Y2 Ba 5 ŽSn 3yyyzCu y Pt z .O x was found for the addition of ultrafine
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SnO 2 powder but the f phase was also found for the addition of Y2 Sn 2 O 7 . It is difficult to relate the different reacted products to the different initial materials or composition, but the local element concentration, together with the existence of Pt, should play an important role in the formation of the final Snbased product. Based on the X-ray diffraction analysis, we assume that all the three phases belong to the same double perovskite structure but with different composition caused by element substitution.
particles in a similar way to that reported by Varanasi et al. w16x. Although quite close to those large crystals, some small Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x crystals down to a few hundred nanometers still survive from being devoured by those large Žsee Fig. 2a.. A flat facet also indicates a nearly zero interface energy, or negligible interface energy comparing to its bulk energy. The crystal growing mechanism and the interface energy of Y2 Ba 5 Sn 2.5 Cu 0.5 O 13.5 deserve to be investigated furthermore.
4.2. The effect of Y2 Ba5 Sn 2 .5 Cu 0 .5 O13.5 on the size of 211 phase
4.4. The distribution of Sn-based inclusions
As can be seen from Fig. 3 to Fig. 4a and b, the existence of Y2 Ba 5 Sn 2.5 Cu 0.5 O 13.5 , or the addition of ultrafine SnO 2 , does not reduce the size of 211 particles. This is in contrast to the effect of BaSnO 3 that reduces the size of 211 w6x. For the sample containing Pt, the 211 particles are much smaller Žsee Fig. 2b.. In this case, it is difficult to separate the effect of PtO 2 on the 211 phase from that of Y2 Ba 5 Sn 1.5 PtCu 0.5 O 13.5 if there is any. It is reasonable to believe that Y2 Ba 5 Sn 1.5 PtCu 0.5 O 13.5 plays a similar role as Y2 Ba 5 Sn 2.5 Cu 0.5 O 13.5 does since their structures are the same and their compositions are similar. As to the influence of processes on reducing the size of 211 particles, it is not easy to conclude because the textured growth process for Z78 differed from that for 970813B and L9708042. However, it is well known that PtO 2 plays a role more crucial than processes do. We thus infer that the existence of Y2 Ba 5 ŽSn 3yyyz Cu y Pt z .O x does not hinder the PtO 2 from doing its job. 4.3. The Y2 Ba5 (Sn 3 y y y z Cu y Pt z )Ox crystals For those inclusions that can be seen clearly, they are cubic in shape and have flat facets, no matter whether they are inside 123 or liquid phase regions. This shape is quite different from the sphere shape of 211, indicating a different crystal-growing mechanism from that of 211. It differs from the needleshape f phase particles too w10x. On the other hand, we found caves in some of the Sn-based crystals ŽFig. 2a.. This suggests that those crystals did not grow from a single nucleus. It is quite possible that they developed from a group of agglomerative SnO 2
Although there is clear evidence about trapped nanocrystals inside the 123 phase, many other Snbased inclusions are not distributed uniformly. This might be caused by the nonuniform distribution of SnO 2 at the beginning, due to their agglomeration. This explanation is supported by the microstructure, observed under a polarized optical microscope, for a sample quenched in air from 11008C, in which accumulated square inclusions were found. We cannot rule out another possibility that, when the growing rate is low enough, the Sn-based phase is pushed out of the 123 grains by the growth front, so that such nonuniform distribution forms during the growth of 123 grains. The 123 precursor powder with large, and different size can also lead to the nonuniform distribution of the Sn-based inclusions. The three mechanisms are applicable to the formation of the Sn-based non crystal areas. Whichever mechanism works, the accumulation of Sn-based phases in grain boundaries would weaken the superconducting coupling between the grains. Nevertheless, when the problems of the agglomeration and the nonuniform mixing are solved, it is possible for fine Sn-based inclusions to be uniformly trapped inside the 123 crystals of YBCO textured superconductors.
Acknowledgements We would like to appreciate the help of Mrs. C. Schmidt and Dr. R. Hergt for the X-ray diffraction measurement and the discussion thereafter. The work was partially supported by the German BMBF project Žcontract number 13N6854., the National Nature Science Foundation of China, the Nature Science
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Foundation of Guangdong Province ŽP.R. China., and the Alexander von Humboldt Foundation.
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