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Effect of Ag addition on the microstructures and superconducting properties of bulk YBCO fabricated by Directionally Solidified Preform Optimized Infiltration Growth Process
Physica C xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Effect of Ag addition on the microstructures and superconducting properties of bulk YBCO fabricated by Directionally Solidified Preform Optimized Infiltration Growth Process N. Devendra Kumar a,⇑, P. Missak Swarup Raju a, S. Pavan Kumar Naik a, T. Rajasekharan b,1, V. Seshubai a a b
School of Physics, University of Hyderabad, Hyderabad, India Rajiv Gandhi University of Knowledge Technologies, Vindhya C4, IIIT Campus, Hyderabad, India
a r t i c l e
i n f o
Article history: Received 28 February 2013 Received in revised form 30 May 2013 Accepted 3 June 2013 Available online xxxx Keywords: YBCO superconductor Refinement of Y2BaCuO5 YBCO/Ag Infiltration Growth Directional solidification
a b s t r a c t A bulk YBa2Cu3O7d (YBCO)/Ag superconducting composite with a homogenous distribution of fine and spherical particles of metallic Ag and Y2BaCuO5 (Y-211) has been fabricated by employing Directionally Solidified Preform Optimized Infiltration Growth Process (DS-POIGP). The effect of adding Ag into the liquid phase source (YBa2Cu3O7d) placed above the Y-211 preform on the microstructures and current densities is investigated. The addition of Ag led to a significant refinement of Y-211 particle size and, hence, enabled the enhancement of current densities at low fields. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Bulk YBa2Cu3O7d (YBCO/Y-123) high-temperature superconductors (HTSC) have been widely investigated, as they show great potential in several engineering applications such as levitation platforms, trapped field magnets, fault current limiters [1–4]. Melt Growth (MG) and Infiltration Growth (IG) are two well-known processing techniques that enable the fabrication of bulk products exhibiting large critical current densities (Jc) [5–13]. In brief, both these processing techniques involve heating the compound above its peritectic decomposition temperature (Tp; for instance Tp of YBCO is 1008 °C) followed by slow cooling through the Tp [8,11]. The cooling rates through the critical temperature window (generally from 1010 °C to 980 °C) are as slow as 0.2–0.5 °C/h, which means that it can take several days to produce materials of reasonable size (e.g., discs of 10–30 mm diameter). In contrast, Directional Solidification (DS) facilitates the production of (RE)BCO products over short time durations [14–16]. To make use of the advantages associated with DS, attempts have been made to integrate directional solidification with MG and IG processes. These integrated processes have been referred to as DS-MG [17,18] and ⇑ Corresponding author. Present address: S.U.P.R.A.T.E.C.S., University of Liège, Liège, Belgium. Tel.: +32 4366 3543. E-mail address: [email protected] (N. Devendra Kumar). 1 Previously with the Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, India.
DS-IG [19,20]. In the context of developing HTSC wires by the powder-in-tube method [21,22], these recent developments are quite appealing. It has been demonstrated that Preform Optimization in Infiltration Growth Process (POIGP) enables fabrication of high Jc YBCO superconducting composites with having good homogeneity in the distribution of Y2BaCuO5 (Y-211) particles, causing spatial uniformity in the field dependence of Jc ‘Jc(H)’ across the volume of the samples [23,24]. Recently, we integrated POIGP with directional solidification with the aim of achieving uniformly high Jc bulk products in short time durations [25]. This integrated process has been referred to as Directionally Solidified POIGP (DS-POIGP). In the present work, the effects of adding Ag into the liquid phase source in YBCO samples fabricated by DS-POIGP are discussed. The influence of Ag on the particle size of Y-211 and, hence, on the Jc (H) is studied and discussed.
2. Experimental To prepare precursor powders of Y-123 and Y-211, high purity (99.9%) Y2O3 (Indian Rare Earths Ltd.), BaCO3 and CuO (E. Merck) were used. Y-123 and Y-211 powders with particle sizes of 0.5– 1 lm were prepared following the chemical route (citrate method) as discussed elsewhere [23,26]. Two Y-211 preform pellets were prepared with 16 16 mm square cross-section and 8–9 mm height in a steel die under an applied uniaxial compaction pressure of 460 MPa; this value was optimized previously, and details are
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Please cite this article in press as: N. Devendra Kumar et al., Effect of Ag addition on the microstructures and superconducting properties of bulk YBCO fabricated by Directionally Solidified Preform Optimized Infiltration Growth Process, Physica C (2013), http://dx.doi.org/10.1016/j.physc.2013.06.013
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reported elsewhere [23,27]. With regard to the liquid phase source, a pure Y-123 pellet was used for one of the samples. This sample is hereafter referred to as Sample-A. For the other sample, which is termed Sample-B, Y-123 + 20 wt.% Ag was used as the liquid phase source. To create this sample, weighed amount of Ag powder was mixed thoroughly with Y-123 using an agate mortar, and this mixture was compacted into a pellet. In the present experiments, the ratio of the liquid phase (Y-123 or Y-123 + Ag) pellet to the Y211 preform pellet was maintained at 2:1 by weight. These pellets were supported with thin layers of Y2O3, yttria-stabilized zirconia (YSZ) and alumina (Al2O3) to prevent the outflow of the liquid phases during heat treatment. NdBa2Cu3O7d (Nd-123) seeds were used to promote the textured growth of Y-123. No Y-211 grainrefining compounds such as Pt, PtO2 or CeO2 were used in the present work. A schematic of the arrangement followed for fabricating samples (Sample-A and Sample-B) by DS-POIGP is shown in Fig. 1. The samples arranged as described above were moved through a vertical tubular muffle furnace using a dc motor and pulley-gear arrangement. A schematic of the translation arrangement through the muffle of the furnace is shown in Fig. 2a. The translation rate of the sample assembly within the furnace was 4 mm/h. The temperature of the furnace was controlled using a temperature controller (model 2404, Eurotherm) and thyristor (model TE10A, Eurotherm). The vertical furnace had a uniform hot zone of 60 mm. The temperature profile recorded maintaining the furnace at an operating temperature of 1100 °C is shown in Fig. 2b. Both Y-211 preform pellets were pre-sintered at 950 °C for 4 h prior to infiltration of the liquid phases to improve their mechanical rigidity. In both experiments, the sample assemblies were placed at the center of the hot-zone of the furnace. Then the temperature of the furnace was increased and held at 1100 °C for 1 h to ensure complete infiltration of the liquid phases (BaCuO2 and CuO) into the pre-sintered Y-211 preforms. The assembly was then lowered at a rate of 4 mm/h. This procedure enabled the samples to be fabricated in time duration of 30 h. The resulting samples (Sample-A and Sample-B) were oxygenated in a tubular furnace at 460 °C in flowing oxygen for 100 h. A low speed saw (Isomet1000, Buehler) was used to slice thin sections from the samples as required to record magnetic hysteresis (M–H) loops and microstructures. Specimens used for magnetization measurements were 5 mm long with a cross-sectional area of 1.5 2 mm2. These specimens were collected from the center region of the DS-POIG processed samples. M–H loops were recorded on the samples at 77 K by applying a magnetic field of 10 T using a Physical Property Measurement System (PPMS, Quantum Design). Jc was calculated by following the extended Bean’s critical state model [28,29] with the formula J c ¼ 20 DdM A/cm2. Here, (DM) (in emu/cc) is the difference in the hysteretic magnetization between the curves obtained while increasing and decreasing the magnetic fields, and ‘d’ is the reduced dimension where ‘a’ and ‘b’ are the planar dimensions (in cm) of the sample with a > b.
To obtain microstructural information, the sectioned specimens from each of the samples were mounted in bakelite (using a hot press) and were polished down to 0.25 lm (using a Buehler autopolisher). The polished specimens were observed under a Field-Emission Scanning Electron Microscope ‘FE-SEM’ (Zeissmake, Ultra 55 model). In the FE-SEM, an in-lens detector was used to observe Y-211 particles in the Y-123 matrix, and a secondary electron (SE) detector was used to observe macro-defects such as pores and cracks. A statistical analysis of the characteristics of the Y-211 particles was carried out using AxioVision software.
3. Results and discussion The secondary electron and in-lens images obtained employing an FE-SEM for Sample-A and Sample-B are shown in Fig. 3. The micrographs shown in Fig. 3a and b correspond to Sample-A, while those in Fig. 3c and d correspond to Sample-B. It is evident from Fig. 3a and c that the macro-cracks that formed in the Ag-free Sample-A were successfully fused with the addition of Ag as in SampleB. The addition of Ag increased the average distance between the macro-cracks to 170 lm. The Ag-added Sample-B was also found to be free from porosity (<0.2%) unlike the Ag-free Sample-A where the porosity was 4%. Sample-A was found to contain coarse Y-211 as observed in Fig. 3c. On the other hand, the addition of Ag enabled significant refinement in the size of Y-211, as observed in Fig. 3d. Electron micrographs were recorded under similar magnifications using an in-lens detector for both Sample-A and Sample-B. Y-211 particle sizes were estimated for each of these micrographs using the AxioVision software. The corresponding histograms, which depict the distributions in the particle size of Y-211 for both the samples are shown in Fig. 4. The addition of Ag refined the size of the Y-211 particles and centered its distribution at approximately 0.75 lm in comparison with the 2-lm-size particles present in the Ag-free Sample-A fabricated by DS-POIGP. Additionally, very fine Y-211 particles of <0.5 lm are also seen in the Ag-added sample (Sample-B). In addition to the refinement of the Y-211 particles, Ag particles were also found to be well refined in Sample-B. Fig. 5 shows a secondary electron micrograph obtained from Sample-B. The figure shows that the Ag particles are uniformly distributed in the end product and are in the size range from 1 to 4 lm, which is quite noteworthy compared to the values reported in the literature [30–35]. The field dependence of Jc (determined from the corresponding magnetic hysteresis loops) obtained in the Sample-B at 77 K is compared in Fig. 6 with that of Ag-free sample (Sample-A). It can be observed from Fig. 6 that Jc(H) of the YBCO/Ag composite (Sample-B) showed a visible improvement over the performance of the Ag-free sample (Sample-A) in the low-field region (<3 T). The performance of both samples is nearly the same in the high field region (>3 T). The enhancement in Jc(H) at low fields is attributed to the improved microstructural features discussed above; in par-
Fig. 1. Schematic of the sample arrangement followed for fabricating Sample-A and Sample-B by DS-POIGP. Sample-A is an Ag-free sample, while for Sample-B, metallic Ag (20 wt.% w.r.t. Y-211 preform) was added to the liquid phase source (Y-123).
Please cite this article in press as: N. Devendra Kumar et al., Effect of Ag addition on the microstructures and superconducting properties of bulk YBCO fabricated by Directionally Solidified Preform Optimized Infiltration Growth Process, Physica C (2013), http://dx.doi.org/10.1016/j.physc.2013.06.013
N. Devendra Kumar et al. / Physica C xxx (2013) xxx–xxx
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Fig. 2. (a) Schematic sketch of the vertical furnace employed for fabricating YBCO samples by DS-POIGP. (b) Temperature profile obtained in the vertical furnace.
Fig. 3. (a and c) Secondary electron images and (b and d) in-lens images obtained from the samples (Sample-A and Sample-B) fabricated by DS-POIGP.
Fig. 4. Histograms showing the size distribution of Y-211 particles in (a) Ag-free YBCO sample (Sample-A) and (b) Ag-added YBCO sample (Sample-B).
Please cite this article in press as: N. Devendra Kumar et al., Effect of Ag addition on the microstructures and superconducting properties of bulk YBCO fabricated by Directionally Solidified Preform Optimized Infiltration Growth Process, Physica C (2013), http://dx.doi.org/10.1016/j.physc.2013.06.013
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the XI plan for FE-SEM is gratefully acknowledged. A grant from UGC-CAS for a continuous flow cryostat is acknowledged. TR thanks RGUKT, IIIT, Hyderabad for permission to publish this paper.
References
Fig. 5. Secondary electron micrograph obtained from the YBCO/Ag composite (Sample-B) showing a good distribution of fine sized Ag particles in the matrix of Y123.
Fig. 6. Field dependence of Jc at 77 K for YBCO/Ag sample (Sample-B) in comparison with Ag-free sample (Sample-A).
ticular, the refinement in the Y-211 particle size and the associated rise in surface area of the Y-211 particles which caused an effective increase in the interfacial defect density. It is interesting to note that both Sample-A and Sample-B showed irreversibility fields (defined as the field at which Jc is better than 100 A/cm2 [36]) better than 5 T at 77 K. 4. Summary and conclusions YBCO and YBCO/Ag bulk superconducting composites with irreversibility fields better than 5 T at 77 K were achieved through Directionally Solidified Preform Optimized Infiltration Growth Process (DS-POIGP) in a relatively short time duration as compared to the conventional MG and IG processes. Adding Ag to the liquid phase source (Y-123) resulted in fine-sized Y-211 and Ag particles in the end product. The addition of Ag enhanced the superconducting properties in the low field regime (<3 T). This improvement in the superconducting properties may be attributed to the enhanced defect density associated with a larger Y-211/Y-123 interface. Because the DS-POIGP is capable of producing YBCO superconducting composites over relatively short time durations with enhanced Jc performance, the importance of this process becomes much more interesting from the view point of wire fabrication Acknowledgments NDK and VS acknowledge the Department of Science and Technology (DST) for financial assistance in the form of research projects (SR/S2/CMP-47/2004 and IR/S5/IU-01/2006). A grant from
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Please cite this article in press as: N. Devendra Kumar et al., Effect of Ag addition on the microstructures and superconducting properties of bulk YBCO fabricated by Directionally Solidified Preform Optimized Infiltration Growth Process, Physica C (2013), http://dx.doi.org/10.1016/j.physc.2013.06.013
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Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Effect of Ag addition on the microstructures and superconducting properties of bulk YBCO fabricated by Directionally Solidified Preform Optimized Infiltration Growth Process N. Devendra Kumar a,⇑, P. Missak Swarup Raju a, S. Pavan Kumar Naik a, T. Rajasekharan b,1, V. Seshubai a a b
School of Physics, University of Hyderabad, Hyderabad, India Rajiv Gandhi University of Knowledge Technologies, Vindhya C4, IIIT Campus, Hyderabad, India
a r t i c l e
i n f o
Article history: Received 28 February 2013 Received in revised form 30 May 2013 Accepted 3 June 2013 Available online xxxx Keywords: YBCO superconductor Refinement of Y2BaCuO5 YBCO/Ag Infiltration Growth Directional solidification
a b s t r a c t A bulk YBa2Cu3O7d (YBCO)/Ag superconducting composite with a homogenous distribution of fine and spherical particles of metallic Ag and Y2BaCuO5 (Y-211) has been fabricated by employing Directionally Solidified Preform Optimized Infiltration Growth Process (DS-POIGP). The effect of adding Ag into the liquid phase source (YBa2Cu3O7d) placed above the Y-211 preform on the microstructures and current densities is investigated. The addition of Ag led to a significant refinement of Y-211 particle size and, hence, enabled the enhancement of current densities at low fields. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Bulk YBa2Cu3O7d (YBCO/Y-123) high-temperature superconductors (HTSC) have been widely investigated, as they show great potential in several engineering applications such as levitation platforms, trapped field magnets, fault current limiters [1–4]. Melt Growth (MG) and Infiltration Growth (IG) are two well-known processing techniques that enable the fabrication of bulk products exhibiting large critical current densities (Jc) [5–13]. In brief, both these processing techniques involve heating the compound above its peritectic decomposition temperature (Tp; for instance Tp of YBCO is 1008 °C) followed by slow cooling through the Tp [8,11]. The cooling rates through the critical temperature window (generally from 1010 °C to 980 °C) are as slow as 0.2–0.5 °C/h, which means that it can take several days to produce materials of reasonable size (e.g., discs of 10–30 mm diameter). In contrast, Directional Solidification (DS) facilitates the production of (RE)BCO products over short time durations [14–16]. To make use of the advantages associated with DS, attempts have been made to integrate directional solidification with MG and IG processes. These integrated processes have been referred to as DS-MG [17,18] and ⇑ Corresponding author. Present address: S.U.P.R.A.T.E.C.S., University of Liège, Liège, Belgium. Tel.: +32 4366 3543. E-mail address: [email protected] (N. Devendra Kumar). 1 Previously with the Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, India.
DS-IG [19,20]. In the context of developing HTSC wires by the powder-in-tube method [21,22], these recent developments are quite appealing. It has been demonstrated that Preform Optimization in Infiltration Growth Process (POIGP) enables fabrication of high Jc YBCO superconducting composites with having good homogeneity in the distribution of Y2BaCuO5 (Y-211) particles, causing spatial uniformity in the field dependence of Jc ‘Jc(H)’ across the volume of the samples [23,24]. Recently, we integrated POIGP with directional solidification with the aim of achieving uniformly high Jc bulk products in short time durations [25]. This integrated process has been referred to as Directionally Solidified POIGP (DS-POIGP). In the present work, the effects of adding Ag into the liquid phase source in YBCO samples fabricated by DS-POIGP are discussed. The influence of Ag on the particle size of Y-211 and, hence, on the Jc (H) is studied and discussed.
2. Experimental To prepare precursor powders of Y-123 and Y-211, high purity (99.9%) Y2O3 (Indian Rare Earths Ltd.), BaCO3 and CuO (E. Merck) were used. Y-123 and Y-211 powders with particle sizes of 0.5– 1 lm were prepared following the chemical route (citrate method) as discussed elsewhere [23,26]. Two Y-211 preform pellets were prepared with 16 16 mm square cross-section and 8–9 mm height in a steel die under an applied uniaxial compaction pressure of 460 MPa; this value was optimized previously, and details are
0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.06.013
Please cite this article in press as: N. Devendra Kumar et al., Effect of Ag addition on the microstructures and superconducting properties of bulk YBCO fabricated by Directionally Solidified Preform Optimized Infiltration Growth Process, Physica C (2013), http://dx.doi.org/10.1016/j.physc.2013.06.013
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reported elsewhere [23,27]. With regard to the liquid phase source, a pure Y-123 pellet was used for one of the samples. This sample is hereafter referred to as Sample-A. For the other sample, which is termed Sample-B, Y-123 + 20 wt.% Ag was used as the liquid phase source. To create this sample, weighed amount of Ag powder was mixed thoroughly with Y-123 using an agate mortar, and this mixture was compacted into a pellet. In the present experiments, the ratio of the liquid phase (Y-123 or Y-123 + Ag) pellet to the Y211 preform pellet was maintained at 2:1 by weight. These pellets were supported with thin layers of Y2O3, yttria-stabilized zirconia (YSZ) and alumina (Al2O3) to prevent the outflow of the liquid phases during heat treatment. NdBa2Cu3O7d (Nd-123) seeds were used to promote the textured growth of Y-123. No Y-211 grainrefining compounds such as Pt, PtO2 or CeO2 were used in the present work. A schematic of the arrangement followed for fabricating samples (Sample-A and Sample-B) by DS-POIGP is shown in Fig. 1. The samples arranged as described above were moved through a vertical tubular muffle furnace using a dc motor and pulley-gear arrangement. A schematic of the translation arrangement through the muffle of the furnace is shown in Fig. 2a. The translation rate of the sample assembly within the furnace was 4 mm/h. The temperature of the furnace was controlled using a temperature controller (model 2404, Eurotherm) and thyristor (model TE10A, Eurotherm). The vertical furnace had a uniform hot zone of 60 mm. The temperature profile recorded maintaining the furnace at an operating temperature of 1100 °C is shown in Fig. 2b. Both Y-211 preform pellets were pre-sintered at 950 °C for 4 h prior to infiltration of the liquid phases to improve their mechanical rigidity. In both experiments, the sample assemblies were placed at the center of the hot-zone of the furnace. Then the temperature of the furnace was increased and held at 1100 °C for 1 h to ensure complete infiltration of the liquid phases (BaCuO2 and CuO) into the pre-sintered Y-211 preforms. The assembly was then lowered at a rate of 4 mm/h. This procedure enabled the samples to be fabricated in time duration of 30 h. The resulting samples (Sample-A and Sample-B) were oxygenated in a tubular furnace at 460 °C in flowing oxygen for 100 h. A low speed saw (Isomet1000, Buehler) was used to slice thin sections from the samples as required to record magnetic hysteresis (M–H) loops and microstructures. Specimens used for magnetization measurements were 5 mm long with a cross-sectional area of 1.5 2 mm2. These specimens were collected from the center region of the DS-POIG processed samples. M–H loops were recorded on the samples at 77 K by applying a magnetic field of 10 T using a Physical Property Measurement System (PPMS, Quantum Design). Jc was calculated by following the extended Bean’s critical state model [28,29] with the formula J c ¼ 20 DdM A/cm2. Here, (DM) (in emu/cc) is the difference in the hysteretic magnetization between the curves obtained while increasing and decreasing the magnetic fields, and ‘d’ is the reduced dimension where ‘a’ and ‘b’ are the planar dimensions (in cm) of the sample with a > b.
To obtain microstructural information, the sectioned specimens from each of the samples were mounted in bakelite (using a hot press) and were polished down to 0.25 lm (using a Buehler autopolisher). The polished specimens were observed under a Field-Emission Scanning Electron Microscope ‘FE-SEM’ (Zeissmake, Ultra 55 model). In the FE-SEM, an in-lens detector was used to observe Y-211 particles in the Y-123 matrix, and a secondary electron (SE) detector was used to observe macro-defects such as pores and cracks. A statistical analysis of the characteristics of the Y-211 particles was carried out using AxioVision software.
3. Results and discussion The secondary electron and in-lens images obtained employing an FE-SEM for Sample-A and Sample-B are shown in Fig. 3. The micrographs shown in Fig. 3a and b correspond to Sample-A, while those in Fig. 3c and d correspond to Sample-B. It is evident from Fig. 3a and c that the macro-cracks that formed in the Ag-free Sample-A were successfully fused with the addition of Ag as in SampleB. The addition of Ag increased the average distance between the macro-cracks to 170 lm. The Ag-added Sample-B was also found to be free from porosity (<0.2%) unlike the Ag-free Sample-A where the porosity was 4%. Sample-A was found to contain coarse Y-211 as observed in Fig. 3c. On the other hand, the addition of Ag enabled significant refinement in the size of Y-211, as observed in Fig. 3d. Electron micrographs were recorded under similar magnifications using an in-lens detector for both Sample-A and Sample-B. Y-211 particle sizes were estimated for each of these micrographs using the AxioVision software. The corresponding histograms, which depict the distributions in the particle size of Y-211 for both the samples are shown in Fig. 4. The addition of Ag refined the size of the Y-211 particles and centered its distribution at approximately 0.75 lm in comparison with the 2-lm-size particles present in the Ag-free Sample-A fabricated by DS-POIGP. Additionally, very fine Y-211 particles of <0.5 lm are also seen in the Ag-added sample (Sample-B). In addition to the refinement of the Y-211 particles, Ag particles were also found to be well refined in Sample-B. Fig. 5 shows a secondary electron micrograph obtained from Sample-B. The figure shows that the Ag particles are uniformly distributed in the end product and are in the size range from 1 to 4 lm, which is quite noteworthy compared to the values reported in the literature [30–35]. The field dependence of Jc (determined from the corresponding magnetic hysteresis loops) obtained in the Sample-B at 77 K is compared in Fig. 6 with that of Ag-free sample (Sample-A). It can be observed from Fig. 6 that Jc(H) of the YBCO/Ag composite (Sample-B) showed a visible improvement over the performance of the Ag-free sample (Sample-A) in the low-field region (<3 T). The performance of both samples is nearly the same in the high field region (>3 T). The enhancement in Jc(H) at low fields is attributed to the improved microstructural features discussed above; in par-
Fig. 1. Schematic of the sample arrangement followed for fabricating Sample-A and Sample-B by DS-POIGP. Sample-A is an Ag-free sample, while for Sample-B, metallic Ag (20 wt.% w.r.t. Y-211 preform) was added to the liquid phase source (Y-123).
Please cite this article in press as: N. Devendra Kumar et al., Effect of Ag addition on the microstructures and superconducting properties of bulk YBCO fabricated by Directionally Solidified Preform Optimized Infiltration Growth Process, Physica C (2013), http://dx.doi.org/10.1016/j.physc.2013.06.013
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Fig. 2. (a) Schematic sketch of the vertical furnace employed for fabricating YBCO samples by DS-POIGP. (b) Temperature profile obtained in the vertical furnace.
Fig. 3. (a and c) Secondary electron images and (b and d) in-lens images obtained from the samples (Sample-A and Sample-B) fabricated by DS-POIGP.
Fig. 4. Histograms showing the size distribution of Y-211 particles in (a) Ag-free YBCO sample (Sample-A) and (b) Ag-added YBCO sample (Sample-B).
Please cite this article in press as: N. Devendra Kumar et al., Effect of Ag addition on the microstructures and superconducting properties of bulk YBCO fabricated by Directionally Solidified Preform Optimized Infiltration Growth Process, Physica C (2013), http://dx.doi.org/10.1016/j.physc.2013.06.013
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the XI plan for FE-SEM is gratefully acknowledged. A grant from UGC-CAS for a continuous flow cryostat is acknowledged. TR thanks RGUKT, IIIT, Hyderabad for permission to publish this paper.
References
Fig. 5. Secondary electron micrograph obtained from the YBCO/Ag composite (Sample-B) showing a good distribution of fine sized Ag particles in the matrix of Y123.
Fig. 6. Field dependence of Jc at 77 K for YBCO/Ag sample (Sample-B) in comparison with Ag-free sample (Sample-A).
ticular, the refinement in the Y-211 particle size and the associated rise in surface area of the Y-211 particles which caused an effective increase in the interfacial defect density. It is interesting to note that both Sample-A and Sample-B showed irreversibility fields (defined as the field at which Jc is better than 100 A/cm2 [36]) better than 5 T at 77 K. 4. Summary and conclusions YBCO and YBCO/Ag bulk superconducting composites with irreversibility fields better than 5 T at 77 K were achieved through Directionally Solidified Preform Optimized Infiltration Growth Process (DS-POIGP) in a relatively short time duration as compared to the conventional MG and IG processes. Adding Ag to the liquid phase source (Y-123) resulted in fine-sized Y-211 and Ag particles in the end product. The addition of Ag enhanced the superconducting properties in the low field regime (<3 T). This improvement in the superconducting properties may be attributed to the enhanced defect density associated with a larger Y-211/Y-123 interface. Because the DS-POIGP is capable of producing YBCO superconducting composites over relatively short time durations with enhanced Jc performance, the importance of this process becomes much more interesting from the view point of wire fabrication Acknowledgments NDK and VS acknowledge the Department of Science and Technology (DST) for financial assistance in the form of research projects (SR/S2/CMP-47/2004 and IR/S5/IU-01/2006). A grant from
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