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Influence of oxygen post-annealing on superconducting properties of NdBa2Cu3Oy
_--1
~ ELSEVIER
PHYSICA ® Physica C 282-287 (1997) 771-772
Influence of oxygen post-annealing on superconducting properties of NdBalCuJOy Chuan-Bing Cai, Yue Sun, Shang-Min Gong, Yao-Xian Fu and Hong Zhang Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 20050, China
The onset superconducting transluon temperature(Tc) for oxygen-controlled melt-growth NdBa2Cu30y(Nd123) depended on the post oxygenation temperature(Ta). The fact that the samples with hihger oxygen content exhibit higher Tc implies that oxygen content accounts for the difference of Tc. The higher oxygen content in those samples which grown in the lower oxygen partial pressure P(02), may result from the more second phases and porosities in them, which may provide more passages for oxygen diffusion, especially for those oxygenated at low Ta. The post oxygenation at 250°C seems to be most effective process to obtain high onset Tc, while for Y123, a higher temperature (about 400°C) may be best. The fact may be connected with the lower repulsive energy of oxygen atoms between 0(1) and 0(5) sites in Nd123.
1. INTRODUCTION The melt-processed ReBa2Cu30y(Re123) superconductor with larger atomic radius elements Re( Nd, Sm, Eu etc.) is an attractive subject because it may have higher Tc and irreversibility field as demonstrated in those Nd123 samples, melt grown in reduced oxygen partial pressure P(02)[1]. As well known, Tc greatly depends on the oxygen content(y) for Re123 superconductor. It was reported that Tc and y varied in wide ranges depending on 02/N2 ratio in their mixed gas, and a post-annealing in high P(02) gas lead to a high Tc of 95K with very sharp transition[2]. Obviously, the post oxygenating treatment(Ta) is an important process for obtaining high Tc Nd123, especially for those melt-processed in low P(02) atmosphere. In addition, the differences of the orthorhombic-tetragonal phase transition(O-T) temperature and correspondingy between Nd123 and Y123 imply that there may be different oxygenating process for Nd123 from Y123. Here, we study the dependence of y and Tc on Ta for melt-processed Nd123.
2. EXPERIMENTAL Two groups ofNd123 samples, named A,B, were synthesized by melt growth methods in mixed 0921-4534/97/$17.00 © Elsevier Science B.Y. All rights reserved. PH S0921-4534(97)00404-8
gas(1o/002 + Ar) and air(21o/002 + N2) respectively. All samples were quenched to room temperature at 700°C during the cooling of melt-texture-growth to make samples in orthorhombic phase and 0-T transition occur in the post treatment. The postannealing in flowing oxygen gas was at 250°C, 300°C, 350°C and 400°C respectively. For comparison, the same post-annealing was performed on the melt-processed Y123. X-ray difIraction(XRD) and scanning electron micrograph(SEM) were applied to study the composition and structure of samples. Tc was measured by the ac susceptibility.
3. RESULTS AND DISCUSSION From the y versus c-axis length[2,4] ,y can be determined by the c-axis ofNd123 and Y123 phase, which was calculated from the XRD (001) peak positions. Shown in Table 1 are the onset Tc and y under various Ta. It can be seen that Tc and y for all samples are closely connected with Ta. In range of Ta=250°C to 400°C, Tc and y for Nd123 increase with Ta decreasing and reach maxima at lowest Ta(250°C) , while Tc andy for Y123, decrease with Ta increasing, reach maxima at highest Ta(400°C). The fact that samples with high Tc always contain high y implies that oxygen content accounts for the difference in Tc. Here, samples for Nd123 do not
C.-B. Cai et aU Physica C 282-287 (1997) 771-772
772
Table 1. Onset Tc and oxygen content y under various annealing temperature Ta.
Tc TaCC) 250 300 350 400
A
B
Y123
y A
B
C
83
79
83
6.887
6.869
6.788
75 71
72 60
91 91
6.862 6.848
6.859
6.879 6.874
71
59
92
6.856
6.786 6.777
exhibit Tc>9IK, which may be due to insufficient oxygenating time and pressure. Of two groups of NdI23, group A have higher y and Tc than group B. The phenomena can be explained from the difference in microstructure between them. XRD, SEM experiments show that more second phase Nd211 and porosities in group A[Fig.l.], which may be attributed to more liquid loss owing to lower peritectic temperature under lower P(02) atmosphere. The more Nd211 particles and porosities can provide more passages for oxygen diffusion, especially effectively for oxygenation at low Ta.
o'I""
Fig.I. SEM micrographs of the cleaved plane for two groups of Nd123.
It is generally accepted that oxygen diffusion in Y123 at Ta<300°C is too slow to supply enough oxygen. But NdI23 presents a rather different picture. The highest Tc for Nd123 is obtained at Ta=250°C, 150°C lower than that for YI23, which may result from the difference in lattice parameters between NdI23 and YI23. It is well known that with 0(1) at (0,1/2,0) releasing oxygen and 0(5) at (112,0,0) taking up oxygen, oxygen increases to a
6.886
critical values YO-T, then the O-T transition takes place[3]. Since the repulsion energy Vo between the oxygen ions on the 0(1) and the 0(5) site is an important factor to stabilize the orthorhombic phase, the lower Vo in NdI23 may result in 0-T transition occurring at low oxygenating temperature, it may be the reason for lower optimum temperature in NdI23 than that in Y123. In addition, the lower Vo make it necessary for more oxygen content to stabilize the orthorhombic phase. The higher value of YO-T =6.55 for Nd123 than YO-T =6.36 for YI23 [2] also implies that NdI23 need more oxygen content than YI23 if it exhibits the same Tc. Thus, it is comprehensible that Tc for NdI23, shown in table 1, are lower than 90K, even though the samples contain higher y than that of those Y123 samples with Tc>90K. In summary, compared with Y123, Nd123 has lower O-T transition temperature and higher YO-To In order to obtain high Tc for NdI23, one should pay more attentions to post oxygen annealing, especially for those NdI23 melt grown in the low P(02) atmosphere.
Acknowledgment: The authors are grateful to the National R&D Center on Superconductivity of China for its support.
REFERENCES l.S.I.Yoo et al, Appl.Phys.Lett.65(1994)633 2.H.Shaked et al, Phys.Rev.B.4I(l990)4173 3.J.L.Tallon et al.,Science, 258(1992)781 4.D.Jorgensen, Phys.Rev.B,4I(l990) 1863
~ ELSEVIER
PHYSICA ® Physica C 282-287 (1997) 771-772
Influence of oxygen post-annealing on superconducting properties of NdBalCuJOy Chuan-Bing Cai, Yue Sun, Shang-Min Gong, Yao-Xian Fu and Hong Zhang Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 20050, China
The onset superconducting transluon temperature(Tc) for oxygen-controlled melt-growth NdBa2Cu30y(Nd123) depended on the post oxygenation temperature(Ta). The fact that the samples with hihger oxygen content exhibit higher Tc implies that oxygen content accounts for the difference of Tc. The higher oxygen content in those samples which grown in the lower oxygen partial pressure P(02), may result from the more second phases and porosities in them, which may provide more passages for oxygen diffusion, especially for those oxygenated at low Ta. The post oxygenation at 250°C seems to be most effective process to obtain high onset Tc, while for Y123, a higher temperature (about 400°C) may be best. The fact may be connected with the lower repulsive energy of oxygen atoms between 0(1) and 0(5) sites in Nd123.
1. INTRODUCTION The melt-processed ReBa2Cu30y(Re123) superconductor with larger atomic radius elements Re( Nd, Sm, Eu etc.) is an attractive subject because it may have higher Tc and irreversibility field as demonstrated in those Nd123 samples, melt grown in reduced oxygen partial pressure P(02)[1]. As well known, Tc greatly depends on the oxygen content(y) for Re123 superconductor. It was reported that Tc and y varied in wide ranges depending on 02/N2 ratio in their mixed gas, and a post-annealing in high P(02) gas lead to a high Tc of 95K with very sharp transition[2]. Obviously, the post oxygenating treatment(Ta) is an important process for obtaining high Tc Nd123, especially for those melt-processed in low P(02) atmosphere. In addition, the differences of the orthorhombic-tetragonal phase transition(O-T) temperature and correspondingy between Nd123 and Y123 imply that there may be different oxygenating process for Nd123 from Y123. Here, we study the dependence of y and Tc on Ta for melt-processed Nd123.
2. EXPERIMENTAL Two groups ofNd123 samples, named A,B, were synthesized by melt growth methods in mixed 0921-4534/97/$17.00 © Elsevier Science B.Y. All rights reserved. PH S0921-4534(97)00404-8
gas(1o/002 + Ar) and air(21o/002 + N2) respectively. All samples were quenched to room temperature at 700°C during the cooling of melt-texture-growth to make samples in orthorhombic phase and 0-T transition occur in the post treatment. The postannealing in flowing oxygen gas was at 250°C, 300°C, 350°C and 400°C respectively. For comparison, the same post-annealing was performed on the melt-processed Y123. X-ray difIraction(XRD) and scanning electron micrograph(SEM) were applied to study the composition and structure of samples. Tc was measured by the ac susceptibility.
3. RESULTS AND DISCUSSION From the y versus c-axis length[2,4] ,y can be determined by the c-axis ofNd123 and Y123 phase, which was calculated from the XRD (001) peak positions. Shown in Table 1 are the onset Tc and y under various Ta. It can be seen that Tc and y for all samples are closely connected with Ta. In range of Ta=250°C to 400°C, Tc and y for Nd123 increase with Ta decreasing and reach maxima at lowest Ta(250°C) , while Tc andy for Y123, decrease with Ta increasing, reach maxima at highest Ta(400°C). The fact that samples with high Tc always contain high y implies that oxygen content accounts for the difference in Tc. Here, samples for Nd123 do not
C.-B. Cai et aU Physica C 282-287 (1997) 771-772
772
Table 1. Onset Tc and oxygen content y under various annealing temperature Ta.
Tc TaCC) 250 300 350 400
A
B
Y123
y A
B
C
83
79
83
6.887
6.869
6.788
75 71
72 60
91 91
6.862 6.848
6.859
6.879 6.874
71
59
92
6.856
6.786 6.777
exhibit Tc>9IK, which may be due to insufficient oxygenating time and pressure. Of two groups of NdI23, group A have higher y and Tc than group B. The phenomena can be explained from the difference in microstructure between them. XRD, SEM experiments show that more second phase Nd211 and porosities in group A[Fig.l.], which may be attributed to more liquid loss owing to lower peritectic temperature under lower P(02) atmosphere. The more Nd211 particles and porosities can provide more passages for oxygen diffusion, especially effectively for oxygenation at low Ta.
o'I""
Fig.I. SEM micrographs of the cleaved plane for two groups of Nd123.
It is generally accepted that oxygen diffusion in Y123 at Ta<300°C is too slow to supply enough oxygen. But NdI23 presents a rather different picture. The highest Tc for Nd123 is obtained at Ta=250°C, 150°C lower than that for YI23, which may result from the difference in lattice parameters between NdI23 and YI23. It is well known that with 0(1) at (0,1/2,0) releasing oxygen and 0(5) at (112,0,0) taking up oxygen, oxygen increases to a
6.886
critical values YO-T, then the O-T transition takes place[3]. Since the repulsion energy Vo between the oxygen ions on the 0(1) and the 0(5) site is an important factor to stabilize the orthorhombic phase, the lower Vo in NdI23 may result in 0-T transition occurring at low oxygenating temperature, it may be the reason for lower optimum temperature in NdI23 than that in Y123. In addition, the lower Vo make it necessary for more oxygen content to stabilize the orthorhombic phase. The higher value of YO-T =6.55 for Nd123 than YO-T =6.36 for YI23 [2] also implies that NdI23 need more oxygen content than YI23 if it exhibits the same Tc. Thus, it is comprehensible that Tc for NdI23, shown in table 1, are lower than 90K, even though the samples contain higher y than that of those Y123 samples with Tc>90K. In summary, compared with Y123, Nd123 has lower O-T transition temperature and higher YO-To In order to obtain high Tc for NdI23, one should pay more attentions to post oxygen annealing, especially for those NdI23 melt grown in the low P(02) atmosphere.
Acknowledgment: The authors are grateful to the National R&D Center on Superconductivity of China for its support.
REFERENCES l.S.I.Yoo et al, Appl.Phys.Lett.65(1994)633 2.H.Shaked et al, Phys.Rev.B.4I(l990)4173 3.J.L.Tallon et al.,Science, 258(1992)781 4.D.Jorgensen, Phys.Rev.B,4I(l990) 1863