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The possibility of negative substitution (x) in melt-processed Gd1+xBa2−xCu3O7−δ GdBCO bulk superconductors
Physica C 468 (2008) 1408–1410
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
The possibility of negative substitution (x) in melt-processed Gd1+xBa2 xCu3O7 GdBCO bulk superconductors
d
Y. Shi a,*, N.H. Babu b, K. Iida a, D.A. Cardwell a a b
IRC in Superconductivity, Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK BCAST, Brunel University, Uxbridge UB8 3PH, UK
a r t i c l e
i n f o
Article history: Available online 28 May 2008 PACS: 74.72.Bk 74.81.Bd 74.25.Ha Keywords: Gd-163 Solid solution Gd1+xBa2 xCu3O7 Negative x EPMA
a b s t r a c t We have reported previously the successful fabrication of single grain Gd–Ba–Cu–O (GdBCO) bulk superconductors in air by top seeded melt growth (TSMG) using a generic Mg–Nd–Ba–Cu–O (Mg–NdBCO) seed crystal. Here, we report a new processing route that enables the fabrication of bulk GdBCO single grains in air using precursors with higher Ba content than has been achieved in previous studies. In particular, the composition of the superconducting matrix of single grains prepared with high, but varying, levels of Ba is analyzed by EPMA. The degree of substitution Ba for Gd (corresponding to a negative value of x in the composition formulation), in GdBCO is proposed for the first time based on the superconducting, compositional, chemical and structural properties of the Gd-123ss (Gd1+xBa2 xCu3O7 d) phase matrix observed this study. Ó 2008 Elsevier B.V. All rights reserved.
d
1. Introduction It is well known that Gd–Ba–Cu–O (GdBCO) single grains fabricated in air tend to form a Gd1+xBa2 xCu3O7 d (x > 0) type solid solution (Gd-123ss) phase, since the ionic radii of Gd and Ba elements are relatively close [1]. However, the superconducting properties of samples with a higher value of x are decreased significantly compared to the unsubstituted compound. As a result, Ba-rich compounds such as BaO2, BaCuO2 and BaCO3 [2–6] have been added to precursor compositions in an attempt to suppress the extent of Gd/Ba substitution during melt processing of GdBCO single grains in air. To date it has not been possible to study the effects of adding high concentrations of Ba on the properties of bulk GdBCO, since it has proved difficult to grow a single grain of sufficient size due to the relatively slow growth rate of the superconducting Gd-123ss phase (which decreases with increasing Ba content in the precursor) [6]. In this paper, we report a new processing route that enables GdBCO single grains to be fabricated readily with a high Ba content in the precursor composition [7]. The effects of the addition of these high levels of Ba to the precursor on the properties of GdBCO single grains are reported. The possibility of the existence of a second phase in the superconducting * Corresponding author. Tel.: +44 1223 330287; fax: +44 1223 337074. E-mail address: [email protected] (Y. Shi). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.126
Gd-123 phase matrix and the formation of a novel Gd1+xBa 2 xCu3O7 d solid solution with x < 0 for single grains fabricated from precursors containing high concentrations of Ba are discussed using results of analyses of the superconducting, compositional, chemical and structural properties of the Gd-123ss matrix. 2. Experimental A novel processing route was employed in this study to fabricate GdBCO single grain samples from precursors of high Ba content [7]. This involved using GdBa4Cu3O8 d (Gd-143) and GdBa6Cu3O10 d (Gd-163) (Ba-rich compounds) as starting materials, rather than BaO2 in an otherwise conventional top seeded melt growth (TSMG) process. Gd-163 powder was synthesised by conventional solid-state reaction between Gd2O3, BaO2 and CuO in air at 950 °C, as described in Ref. [7]. Precursor powders with starting compositions 70 wt% Gd-123 + (30 X) wt% Gd-211 + X wt% Gd-163 + 0.1 wt% Pt, where X = 5, 10, 15 and 30, were mixed thoroughly using a mortar and pestle and pressed uniaxially into pellets. A generic seed of NdBCO doped with 1 wt% MgO fabricated by melt processing [8] was placed on the top surface of the pellet prior to melt processing (i.e. the cold-seeding method). A conventional TSMG thermal profile was used in this study, as described in Ref. [7]. The as-processed samples, fabricated from precursor powders of different Ba composition, consisted predominantly of single
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grains with 13 mm in diameter and were annealed at 440 °C– 360 °C for 150 h in flowing oxygen gas. The individual single grains were cut into small specimens of approximately 1.0 1.2 0.5 mm3, as illustrated schematically in Fig. 1 (inset). Magnetisation measurements on these small samples were performed using a commercial SQUID magnetometer to determine their superconducting transition temperature (Tc). Some of the fully processed single grains were crushed into powders for DTA and XRD analysis. Detailed analyses of the chemical composition of the various Gd-123ss superconducting phases prepared from different Ba-rich precursor powders were carried out using electron probe microanalysis (EPMA) to understand further the extent of Gd/Ba substitution in these samples. The composition of each Gd-123ss phase was estimated by taking the average value of the composition measured at least 15, randomly selected, points on the surface of each sample. The electron beam was focused predominantly within the vicinity of the seed crystal, which was almost free of Gd2BaCuO5 (Gd-211) phase, in order to minimize errors in determination of the Gd-123ss composition. A standard GdBCO, single grain sample of composition Gd-123 + 0.33 mol Gd-211 + 0.5 wt% Pt processed in 1% O2 in N2 was used as a reference sample in the EPMA analysis. The precursor compositions of the samples analyzed were; 70 wt% Gd-123 + 30 wt% Gd-211 + Y wt% BaO2 + 0.1 wt% Pt, where Y = 0, 1, 2, 4 and 70 wt% Gd123 + (30 X) wt% Gd-211 + X wt% Gd-163 + 0.1 wt% Pt, where X = 5, 10, 15 and 30. 3. Results and discussion 3.1. Superconducting transition temperatures It is well known that superconducting properties of (LRE)BCO (where LRE, light rare earth element) bulk samples are very sensitive to the degree of LRE/Ba substitution in the superconducting phase matrix (Gd tends to substitute for Ba to form Gd1+xBa2 xCu3O7 d, in which x is larger than zero). Adding a small amount of BaO2 can suppress the Gd/Ba substitution and lead to good superconducting properties of the Gd-123 phase (i.e. with an x value of zero). However, the addition of a higher concentration of BaO2 to the precursor decreases the onset Tc of the fully processed samples [6]. This is consistent with the results shown in Fig. 1, which indicate that the onset Tc of samples cut from different posi-
tions within the single GdBCO grains fabricated in this study decreases with increasing Gd-163 concentration in the precursor powder. The reason for this effect remains unclear, although it may be related to the possibility of Ba substituting for Gd under processing conditions of high Ba concentration, given that the ionic radii of Gd and Ba elements are relatively close. Gd1+xBa2 xCu3O7 d (x < 0) phase, therefore, might exist in the single grain if the GdBCO growth process is similar to that of the NdBCO system [9]. 3.2. Peritectic decomposition temperatures Fig. 2 shows the peritectic decomposition temperatures of both the precursors and the crushed single grains with same composition. It can be seen that the peritectic temperatures of the precursors are different with that of the single grains. DTA traces 1–3 are about same, the peritectic temperature becomes lower when there is more Ba (Gd-163) in the precursor. For the single grains, DTA traces 4 and 5 in this figure are almost identical to that of the pure Gd-123 phase; however, the Tp of single grains fabricated from precursor powder containing more than 10 wt% of Gd-163 is observed to increase by between 5 °C and 14 °C (DTA traces 6 and 7 in Fig. 2) compared with that of pure Gd-123. In addition, the crystal structures of these single grains, identified from XRD, are identical to that of Gd-123. A plausible interpretation of these data is that the presence of a large amount of Ba in the precursor powder changes the chemical composition of the Gd-123 single grain matrix. This implies that Ba from the Gd-163 precursor phase dissolves in the Gd-123 lattice and forms Gd1+xBa2 xCu3O7 d (x < 0). The peritectic decomposition temperature of such a solid solution may, evidently, be higher than that of pure Gd-123. 3.3. Chemical composition analyses EPMA measurements of the composition of the Gd-123ss (Gd1+xBa2 xCu3O7 d) superconducting phases of single grains fabricated from different Gd-163 in the precursor powder provide further evidence to support the formation of a Gd1+xBa2 xCu3O7 d (x < 0) phase. It can be seen from Fig. 3 that the composition distribution for all the samples is consistent, with the exception of sample 7. Measured composition values of Gd and Ba of each sample stay constant and their variations are within 0.5 mol% and 1.0 mol%, respectively, which is within the error of EPMA. (The
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1ta 2ta3ta4ta5ta 1tb . 1tc 1td 3td
a axis
1
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5wt% Gd-163 10wt% Gd-163 15wt% Gd-163 30wt% Gd-163 Gd-
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900
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Temperature (ºC)
Positions from the seed Fig. 1. The spatial variation of Tc along the a-axis for single GdBCO grains fabricated in air with starting composition 70 wt% Gd-123 + (30 X) wt% Gd-211 + X wt% Gd163 + 0.1 wt% Pt, where X = 5, 10, 15 and 30.
Fig. 2. DTA traces for precursor compositions of 70 wt% Gd-123 + (30 X) wt% Gd211 + X wt% Gd-163 + 0.1 wt% Pt with X = 5, 10 and 15 (traces 1–3). Traces 4–7 are for single grains grown from precursor powder with starting composition 70 wt% Gd-123 + (30 X) wt% Gd-211 + X wt% Gd-163 + 0.1 wt% Pt with X = 5, 10, 15 and 30.
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concentration in the precursor powder exceeds 35. Especially, the x values for samples 7 and 8 are negative in the range of measurement error, which means formation of Gd1+xBa2 xCuO7 d solid solution with x < 0 is possible. The observed range in x from the EPMA data, spanning both positive and negative values, indicates that Gd and Ba can substitute one another in the Gd-123ss lattice under appropriate chemical and thermal conditions.
18 1 2 3 4 5 6 7 8 S
16 Gd mol.% in matrix
Gd mol.% in matrix
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15
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sample 7
3.4. Second phase presence in the Gd-123 matrix
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0 32
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Ba mol.% in matrix
13 31
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Ba mol.% in matrix Fig. 3. The concentration distribution of Gd and Ba in the various single grain samples measured by EPMA. Samples 1–4 relate to compositions of 70 wt% Gd123 + 30 wt% Gd-211 + Y wt% BaO2 + 0.1 wt% Pt, where Y = 0, 1, 2 and 4. Samples 5– 8 relate to compositions of 70 wt% Gd-123 + (30 X) wt% Gd-211 + X wt% Gd163 + 0.1 wt% Pt, where X = 5, 10, 15 and 30. The inset shows the measured EPMA data for sample 7, which exhibit significant scatter.
x Value in Gd 1+xBa2-xCu3O7-δ
0.15 1: 0wt%BaO2 2: 1wt%BaO2 3: 2wt%BaO2 4: 4wt%BaO2 5: 5wt%Gd-163 6: 10wt%Gd-163 7: 15wt%Gd-163 8: 30wt%Gd-163
0.10
2 0.05
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-0.05
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4. Conclusions
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-0.10
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The EPMA data for the chemical constitution of the bulk matrix is rather scattered for sample 7, which was fabricated from an initial precursor composition of 70 wt% Gd-123 + 15 wt% Gd211 + 15 wt% Gd-163 + 0.1 wt% Pt (the inset of Fig. 3 shows the full range of the data). However, it is possible to separate these data generally into two distinct regions. The first (circle 1 in Fig. 3) has a composition close to Gd-123 and has been used to calculate the value of x for this sample. The other region (circle 2 in Fig. 3) is much broader and includes the composition corresponding to the high temperature liquid phase(s) (i.e. in which the Gd concentration is zero). This indicates that sample 7 contains at least two phase components, including a Gd-123ss phase and the liquid phase(s). The reason for the scattered data in the second region may be due to the inhomogeneous composition of the liquid during solidification. This residual liquid phase(s) was not apparent from the electron microscope image during the EPMA measurements, suggesting that a liquid-like second phase exists in the superconducting matrix on a sub-micron scale when the samples are processed from precursors containing high concentrations of Ba. This may be significant for enhanced flux pinning in the GdBCO system, although further studies of the superconducting and chemical properties of these materials are necessary.
42
Ba concentration in precursor (mol.%) Fig. 4. Values of x in Gd1+xBa2 xCu3O7 d for the various single grain Gd-123ss phases fabricated from precursor powders containing different concentrations of Ba.
The effects of high Ba content in the precursor powder on the properties of GdBCO single grains has been studied via a new TSMG processing route for this material. Studies of the compositional, chemical, structural and superconducting properties of single grains grown with different amounts of novel, Ba-rich phases ((Gd-143) and (Gd-163)) in the precursor have indicated that the formation of Gd1+xBa2 xCuO7 d solid solution with x < 0 is possible during the melt growth process. Finally, samples fabricated from precursor compositions with a high Ba content may containing second phase inclusions that could enhance flux pinning in bulk GdBCO materials. Acknowledgement
composition of the standard is marked as in Fig. 3.) Data of such consistency can be used meaningfully to calculate the x value in the solid solution phase, Gd1+xBa2 xCu3O7 d. Fig. 4 shows the average value of x in the Gd-123ss phase for each sample as a function of Ba molar concentration. The value of x for sample 7 is obtained from the data ‘cluster’ at low mol% values of Ba, shown in the inset in Fig. 3. The error bars are displayed in the Fig. 4 to show the error caused by EPMA. The error in the measured EPMA data caused by standard sample is very small, even though the exact composition of the standard sample is unknown. It is reasonable, however, to assume that the value of x in the standard sample is very close to 0, since the sample has a high Tc of around 94 K and exhibits a sharp superconducting transition width of less than 1 K. The dashed line in Fig. 4 indicates the x value of the standard sample. Significantly, the value of x becomes negative when the Ba molar
Authors would like to acknowledge EPSRC for financial support. References [1] M. Murakami, N. Sakai, T. Higuchi, S.I. Yoo, Supercond. Sci. Technol. 9 (1996) 1015. [2] J.Q. Dai, Z.X. Zhao, J.W. Xiong, Supercond. Sci. Technol. 16 (2003) 815. [3] N.H. Babu, K. Iida, Y. Shi, D.A. Cardwell, Appl. Phys. Lett. 87 (2005) 202506. [4] C. Xu, A. Hu, N. Sakai, M. Izumi, I. Hirabayashi, Supercond. Sci. Technol. 18 (2005) 229. [5] Y. Shi, N.H. Babu, K. Iida, D.A. Cardwell, Supercond. Sci. Technol. 20 (2007) 38. [6] Y. Shi, N.H. Babu, K. Iida, D.A. Cardwell, IEEE Trans. Appl. Supercond. 17 (2007) 2984. [7] Y. Shi, N.H. Babu, K. Iida, D.A. Cardwell, J. Phys.: Conf. Ser. 97 (2008) 012250. [8] Y. Shi, N.H. Babu, D.A. Cardwell, Supercond. Sci. Technol. 18 (2005) L13. [9] G. Osabe, T. Takizawa, S.I. Yoo, N. Sakai, T. Higuchi, M. Murakami, Mater. Sci. Eng. B 65 (1999) 11.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
The possibility of negative substitution (x) in melt-processed Gd1+xBa2 xCu3O7 GdBCO bulk superconductors
d
Y. Shi a,*, N.H. Babu b, K. Iida a, D.A. Cardwell a a b
IRC in Superconductivity, Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK BCAST, Brunel University, Uxbridge UB8 3PH, UK
a r t i c l e
i n f o
Article history: Available online 28 May 2008 PACS: 74.72.Bk 74.81.Bd 74.25.Ha Keywords: Gd-163 Solid solution Gd1+xBa2 xCu3O7 Negative x EPMA
a b s t r a c t We have reported previously the successful fabrication of single grain Gd–Ba–Cu–O (GdBCO) bulk superconductors in air by top seeded melt growth (TSMG) using a generic Mg–Nd–Ba–Cu–O (Mg–NdBCO) seed crystal. Here, we report a new processing route that enables the fabrication of bulk GdBCO single grains in air using precursors with higher Ba content than has been achieved in previous studies. In particular, the composition of the superconducting matrix of single grains prepared with high, but varying, levels of Ba is analyzed by EPMA. The degree of substitution Ba for Gd (corresponding to a negative value of x in the composition formulation), in GdBCO is proposed for the first time based on the superconducting, compositional, chemical and structural properties of the Gd-123ss (Gd1+xBa2 xCu3O7 d) phase matrix observed this study. Ó 2008 Elsevier B.V. All rights reserved.
d
1. Introduction It is well known that Gd–Ba–Cu–O (GdBCO) single grains fabricated in air tend to form a Gd1+xBa2 xCu3O7 d (x > 0) type solid solution (Gd-123ss) phase, since the ionic radii of Gd and Ba elements are relatively close [1]. However, the superconducting properties of samples with a higher value of x are decreased significantly compared to the unsubstituted compound. As a result, Ba-rich compounds such as BaO2, BaCuO2 and BaCO3 [2–6] have been added to precursor compositions in an attempt to suppress the extent of Gd/Ba substitution during melt processing of GdBCO single grains in air. To date it has not been possible to study the effects of adding high concentrations of Ba on the properties of bulk GdBCO, since it has proved difficult to grow a single grain of sufficient size due to the relatively slow growth rate of the superconducting Gd-123ss phase (which decreases with increasing Ba content in the precursor) [6]. In this paper, we report a new processing route that enables GdBCO single grains to be fabricated readily with a high Ba content in the precursor composition [7]. The effects of the addition of these high levels of Ba to the precursor on the properties of GdBCO single grains are reported. The possibility of the existence of a second phase in the superconducting * Corresponding author. Tel.: +44 1223 330287; fax: +44 1223 337074. E-mail address: [email protected] (Y. Shi). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.126
Gd-123 phase matrix and the formation of a novel Gd1+xBa 2 xCu3O7 d solid solution with x < 0 for single grains fabricated from precursors containing high concentrations of Ba are discussed using results of analyses of the superconducting, compositional, chemical and structural properties of the Gd-123ss matrix. 2. Experimental A novel processing route was employed in this study to fabricate GdBCO single grain samples from precursors of high Ba content [7]. This involved using GdBa4Cu3O8 d (Gd-143) and GdBa6Cu3O10 d (Gd-163) (Ba-rich compounds) as starting materials, rather than BaO2 in an otherwise conventional top seeded melt growth (TSMG) process. Gd-163 powder was synthesised by conventional solid-state reaction between Gd2O3, BaO2 and CuO in air at 950 °C, as described in Ref. [7]. Precursor powders with starting compositions 70 wt% Gd-123 + (30 X) wt% Gd-211 + X wt% Gd-163 + 0.1 wt% Pt, where X = 5, 10, 15 and 30, were mixed thoroughly using a mortar and pestle and pressed uniaxially into pellets. A generic seed of NdBCO doped with 1 wt% MgO fabricated by melt processing [8] was placed on the top surface of the pellet prior to melt processing (i.e. the cold-seeding method). A conventional TSMG thermal profile was used in this study, as described in Ref. [7]. The as-processed samples, fabricated from precursor powders of different Ba composition, consisted predominantly of single
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grains with 13 mm in diameter and were annealed at 440 °C– 360 °C for 150 h in flowing oxygen gas. The individual single grains were cut into small specimens of approximately 1.0 1.2 0.5 mm3, as illustrated schematically in Fig. 1 (inset). Magnetisation measurements on these small samples were performed using a commercial SQUID magnetometer to determine their superconducting transition temperature (Tc). Some of the fully processed single grains were crushed into powders for DTA and XRD analysis. Detailed analyses of the chemical composition of the various Gd-123ss superconducting phases prepared from different Ba-rich precursor powders were carried out using electron probe microanalysis (EPMA) to understand further the extent of Gd/Ba substitution in these samples. The composition of each Gd-123ss phase was estimated by taking the average value of the composition measured at least 15, randomly selected, points on the surface of each sample. The electron beam was focused predominantly within the vicinity of the seed crystal, which was almost free of Gd2BaCuO5 (Gd-211) phase, in order to minimize errors in determination of the Gd-123ss composition. A standard GdBCO, single grain sample of composition Gd-123 + 0.33 mol Gd-211 + 0.5 wt% Pt processed in 1% O2 in N2 was used as a reference sample in the EPMA analysis. The precursor compositions of the samples analyzed were; 70 wt% Gd-123 + 30 wt% Gd-211 + Y wt% BaO2 + 0.1 wt% Pt, where Y = 0, 1, 2, 4 and 70 wt% Gd123 + (30 X) wt% Gd-211 + X wt% Gd-163 + 0.1 wt% Pt, where X = 5, 10, 15 and 30. 3. Results and discussion 3.1. Superconducting transition temperatures It is well known that superconducting properties of (LRE)BCO (where LRE, light rare earth element) bulk samples are very sensitive to the degree of LRE/Ba substitution in the superconducting phase matrix (Gd tends to substitute for Ba to form Gd1+xBa2 xCu3O7 d, in which x is larger than zero). Adding a small amount of BaO2 can suppress the Gd/Ba substitution and lead to good superconducting properties of the Gd-123 phase (i.e. with an x value of zero). However, the addition of a higher concentration of BaO2 to the precursor decreases the onset Tc of the fully processed samples [6]. This is consistent with the results shown in Fig. 1, which indicate that the onset Tc of samples cut from different posi-
tions within the single GdBCO grains fabricated in this study decreases with increasing Gd-163 concentration in the precursor powder. The reason for this effect remains unclear, although it may be related to the possibility of Ba substituting for Gd under processing conditions of high Ba concentration, given that the ionic radii of Gd and Ba elements are relatively close. Gd1+xBa2 xCu3O7 d (x < 0) phase, therefore, might exist in the single grain if the GdBCO growth process is similar to that of the NdBCO system [9]. 3.2. Peritectic decomposition temperatures Fig. 2 shows the peritectic decomposition temperatures of both the precursors and the crushed single grains with same composition. It can be seen that the peritectic temperatures of the precursors are different with that of the single grains. DTA traces 1–3 are about same, the peritectic temperature becomes lower when there is more Ba (Gd-163) in the precursor. For the single grains, DTA traces 4 and 5 in this figure are almost identical to that of the pure Gd-123 phase; however, the Tp of single grains fabricated from precursor powder containing more than 10 wt% of Gd-163 is observed to increase by between 5 °C and 14 °C (DTA traces 6 and 7 in Fig. 2) compared with that of pure Gd-123. In addition, the crystal structures of these single grains, identified from XRD, are identical to that of Gd-123. A plausible interpretation of these data is that the presence of a large amount of Ba in the precursor powder changes the chemical composition of the Gd-123 single grain matrix. This implies that Ba from the Gd-163 precursor phase dissolves in the Gd-123 lattice and forms Gd1+xBa2 xCu3O7 d (x < 0). The peritectic decomposition temperature of such a solid solution may, evidently, be higher than that of pure Gd-123. 3.3. Chemical composition analyses EPMA measurements of the composition of the Gd-123ss (Gd1+xBa2 xCu3O7 d) superconducting phases of single grains fabricated from different Gd-163 in the precursor powder provide further evidence to support the formation of a Gd1+xBa2 xCu3O7 d (x < 0) phase. It can be seen from Fig. 3 that the composition distribution for all the samples is consistent, with the exception of sample 7. Measured composition values of Gd and Ba of each sample stay constant and their variations are within 0.5 mol% and 1.0 mol%, respectively, which is within the error of EPMA. (The
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Tc (K)
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1ta 2ta3ta4ta5ta 1tb . 1tc 1td 3td
a axis
1
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-20
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5wt% Gd-163 10wt% Gd-163 15wt% Gd-163 30wt% Gd-163 Gd-
4
850
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1100
Temperature (ºC)
Positions from the seed Fig. 1. The spatial variation of Tc along the a-axis for single GdBCO grains fabricated in air with starting composition 70 wt% Gd-123 + (30 X) wt% Gd-211 + X wt% Gd163 + 0.1 wt% Pt, where X = 5, 10, 15 and 30.
Fig. 2. DTA traces for precursor compositions of 70 wt% Gd-123 + (30 X) wt% Gd211 + X wt% Gd-163 + 0.1 wt% Pt with X = 5, 10 and 15 (traces 1–3). Traces 4–7 are for single grains grown from precursor powder with starting composition 70 wt% Gd-123 + (30 X) wt% Gd-211 + X wt% Gd-163 + 0.1 wt% Pt with X = 5, 10, 15 and 30.
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concentration in the precursor powder exceeds 35. Especially, the x values for samples 7 and 8 are negative in the range of measurement error, which means formation of Gd1+xBa2 xCuO7 d solid solution with x < 0 is possible. The observed range in x from the EPMA data, spanning both positive and negative values, indicates that Gd and Ba can substitute one another in the Gd-123ss lattice under appropriate chemical and thermal conditions.
18 1 2 3 4 5 6 7 8 S
16 Gd mol.% in matrix
Gd mol.% in matrix
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15
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sample 7
3.4. Second phase presence in the Gd-123 matrix
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Ba mol.% in matrix
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Ba mol.% in matrix Fig. 3. The concentration distribution of Gd and Ba in the various single grain samples measured by EPMA. Samples 1–4 relate to compositions of 70 wt% Gd123 + 30 wt% Gd-211 + Y wt% BaO2 + 0.1 wt% Pt, where Y = 0, 1, 2 and 4. Samples 5– 8 relate to compositions of 70 wt% Gd-123 + (30 X) wt% Gd-211 + X wt% Gd163 + 0.1 wt% Pt, where X = 5, 10, 15 and 30. The inset shows the measured EPMA data for sample 7, which exhibit significant scatter.
x Value in Gd 1+xBa2-xCu3O7-δ
0.15 1: 0wt%BaO2 2: 1wt%BaO2 3: 2wt%BaO2 4: 4wt%BaO2 5: 5wt%Gd-163 6: 10wt%Gd-163 7: 15wt%Gd-163 8: 30wt%Gd-163
0.10
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4. Conclusions
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The EPMA data for the chemical constitution of the bulk matrix is rather scattered for sample 7, which was fabricated from an initial precursor composition of 70 wt% Gd-123 + 15 wt% Gd211 + 15 wt% Gd-163 + 0.1 wt% Pt (the inset of Fig. 3 shows the full range of the data). However, it is possible to separate these data generally into two distinct regions. The first (circle 1 in Fig. 3) has a composition close to Gd-123 and has been used to calculate the value of x for this sample. The other region (circle 2 in Fig. 3) is much broader and includes the composition corresponding to the high temperature liquid phase(s) (i.e. in which the Gd concentration is zero). This indicates that sample 7 contains at least two phase components, including a Gd-123ss phase and the liquid phase(s). The reason for the scattered data in the second region may be due to the inhomogeneous composition of the liquid during solidification. This residual liquid phase(s) was not apparent from the electron microscope image during the EPMA measurements, suggesting that a liquid-like second phase exists in the superconducting matrix on a sub-micron scale when the samples are processed from precursors containing high concentrations of Ba. This may be significant for enhanced flux pinning in the GdBCO system, although further studies of the superconducting and chemical properties of these materials are necessary.
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Ba concentration in precursor (mol.%) Fig. 4. Values of x in Gd1+xBa2 xCu3O7 d for the various single grain Gd-123ss phases fabricated from precursor powders containing different concentrations of Ba.
The effects of high Ba content in the precursor powder on the properties of GdBCO single grains has been studied via a new TSMG processing route for this material. Studies of the compositional, chemical, structural and superconducting properties of single grains grown with different amounts of novel, Ba-rich phases ((Gd-143) and (Gd-163)) in the precursor have indicated that the formation of Gd1+xBa2 xCuO7 d solid solution with x < 0 is possible during the melt growth process. Finally, samples fabricated from precursor compositions with a high Ba content may containing second phase inclusions that could enhance flux pinning in bulk GdBCO materials. Acknowledgement
composition of the standard is marked as in Fig. 3.) Data of such consistency can be used meaningfully to calculate the x value in the solid solution phase, Gd1+xBa2 xCu3O7 d. Fig. 4 shows the average value of x in the Gd-123ss phase for each sample as a function of Ba molar concentration. The value of x for sample 7 is obtained from the data ‘cluster’ at low mol% values of Ba, shown in the inset in Fig. 3. The error bars are displayed in the Fig. 4 to show the error caused by EPMA. The error in the measured EPMA data caused by standard sample is very small, even though the exact composition of the standard sample is unknown. It is reasonable, however, to assume that the value of x in the standard sample is very close to 0, since the sample has a high Tc of around 94 K and exhibits a sharp superconducting transition width of less than 1 K. The dashed line in Fig. 4 indicates the x value of the standard sample. Significantly, the value of x becomes negative when the Ba molar
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