Fabrication and characterization of dense YBa2Cu3O7−y superconducting wires by extrusion technique

Materials Science and Engineering B52 (1998) 99 – 104

Fabrication and characterization of dense YBa2Cu3O7 − y superconducting wires by extrusion technique Prakhya Ram *, V.R. Saxena, T.R. Ramamohan Department of Materials Science and Metallurgical Engineering, Indian Institute of Technology, Bombay 400 076, India Received 26 September 1995; accepted 8 September 1997

Abstract YBa2Cu3O7 − y superconducting powders were prepared by solid state reaction method and drawn in to uniform wires of different diameters (0.9–2.5 mm) and lengths up to 10 m by extrusion technique using an appropriate binder – plasticizer–solvent system. The density of the sintered wires was 92% with a high Tc and Jc of 91 K and 2014 A cm − 2 (77 K, zero field) respectively. In the presence of applied magnetic field the transport Jc showed a marginal drop of about 25%. © 1998 Published by Elsevier Science S.A. All rights reserved. Keywords: YBa2Cu3O7 − y wires; Superconductivity; Extrusion technique

1. Introduction High Tc superconducting materials have technological importance for a variety of applications, e.g. magnetic bearings, microwave cavities or persistent current permanent magnets [1]. Superconducting wires wound on a solenoid type geometry are extremely useful in high field magnets [2]. These materials are hard and mechanically brittle and it is difficult to draw them in to fine wires [3]. However, several different approaches for the fabrication of YBa-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O wires were reported [1,4 – 9]. These include metal clad composite wires, metal core composite wires, melt spinning and plasma spraying. Each of these techniques have specific requirements influencing the electrical and mechanical properties of the wires. For example, the choice of a proper cladding metal was found to be critical in the fabrication of metal clad composite wires. The cladding material should not interact with the superconducting core at the processing temperatures. * Corresponding author. Present address: New York State Center for Advanced Technology in Ultrafast Photonic Materials and Devices at The City University of New York, 2900 Bedford Avenue, Brooklyn, New York 11210, USA. Tel.: + 1 718 9515418; fax: +1 718 9514407.

Oxygen supply was also a difficult task to the high Tc core through the metal clad during heat treatments [4,10]. In the metal core composites the coated composite wire required heat treatments to burn the solvent diluted organic binder that lead to microstructural problems [3]. The melt spinning had viscosity and heat treatment related microstructural problems and also because of the high solidification rate there was little chance for texture formation [1,7–9]. In plasma spraying the additional complication was the desired stabilization by a suitable metal coating [1]. While the fabrication of the ceramic superconducting wires was successfully demonstrated, one common problem encountered was the low current carrying capacity, especially in applied magnetic fields, caused by grain boundary weak links. The Jc reported by the above techniques in Y-Ba-Cu-O wires was of the order of 103 A cm − 2 (77 K, zero field) [1,11]. The Jc reported in Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O wires was of the order of 104 A cm − 2 (77 K, zero field) [12,13]. The low Jc in Y-Ba-Cu-O system was because of the weak link problem. Although higher Jc values were reported in metal clad composites using silver sheath [1], the silver is very expensive for drawing the wires over long lengths. In view of the above discussed problems, the fabrication of high Tc superconducting wires using different

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techniques has been the subject of continuous study. Many of the practical difficulties were overcome by using a different technique for making the superconducting wires–tapes [1]. In this, a superconducting slurry with a properly controlled viscosity was extruded through a die into the form of a wire or casted into the shape of tapes [14,15]. Although the mechanical properties of the extruded wires was a crucial task in the development of extruded wires, it is relatively an easy and inexpensive method for fabrication of high Tc superconducting wires. Therefore it is of interest to study the microstructure and improvements in critical current density of YBa2Cu3O7 − y superconducting wires prepared by extrusion technique.

2. Experimental details YBa2Cu3O7 − y superconducting powder was prepared by solid state reaction technique using Y2O3, BaCO3 and CuO in stoichiometric composition [16] and subsequent sintering and annealing at 930°C for 24 h in oxygen atmosphere. The twice sintered pellets were ground uniformly (1.2 mm average particle size) to make the superconducting slurry by mixing an appropriate binder–plasticizer – solvent (B-P-S) system. The slurry was homogeneously milled before being drawn into wires using a lab made extrusion mill. The green wires were wound on a ceramic solenoid and later sintered and annealed in oxygen atmosphere. During sintering schedule the wires were initially subjected to a very slow heating rate of 0.25°C min − 1 until the temperature of the furnace raised to 650°C in oxygen flow. This step was very important to remove the organic contents completely and to avoid the formation of microcracks and blisters. Thereafter the heating was supplied at 1°C min − 1 until the temperature raised to 930°C. At 930°C, the wires were allowed for 24 h in a steady flow of oxygen atmosphere. This step helped to achieve the maximum density and reduce the secondary phases. The samples were cooled slowly at 1°C min − 1 until the temperature came down to 450°C. The samples were annealed at this temperature for 48 h in oxygen atmosphere. This step promoted the oxygen intake of the lattice. From 450°C the wires were al-

Table 1 Binder – plasticizer – solvent systems investigated Binder

Plasticizer

Solvent

1. Polyvinyl chloride (PVC)

1. Polyethylene glycol 2. Glycerin

1. Trichloroethylene 2. Ethanol 3. Benzene

2. Polyvinyl butyral (PVB) 3. Polymethyl methacrylate (PMMA)

4. 5. 6. 7.

Toluene Ethyl ketone Acetone Xylene

lowed to cool down to room temperature at the rate of 2°C min − 1. Small sections of the cooled wires were selected for detailed characterization. X-ray diffraction (XRD), scanning electrom microscopy (SEM) and energy dispersive X-ray analysis (EDXA) data were used to extract the structural information. Tc was measured by four probe methods. AC susceptibility measurements were carried out using a mutual inductance bridge. The Jc was measured using 1 mV cm − 1 criterion at 77 K and in magnetic fields from 0 to 1 T.

3. Results and discussion The rheological and plastic properties of the superconducting slurry were very important in making the wires by extrusion technique. The superconducting slurry should possess the viable conditions to be drawn into the wires through a die. Studies were conducted using different types of polymeric compounds and organic solvents (Table 1) and the superconducting slurry was prepared by mixing them with 123 powder. The slurry was prepared for each of the combinations given in this table in order to select an appropriate binder– plasticizer–solvent (B-P-S) system. The binder, plasticizer and solvent additions were carried out in different weight percentages for the purpose of optimization of the conditions of the superconducting slurry. The solubilities of the various binders were initially tested at

Table 2 Adaptability of the extrusion mixture S. No.

1 2 3

Compound

Polyvinyl butyral (8:10::P:S) Polyethylene glycol (5:10::B:S) Trichloro ethylene (5:8::B:P)

Actition

Binder (B) Plasticizer (P) Solvent (S)

% weight 2

5

8

10

NS NP NE

OM NP NE

HD OM NE

VHD VP OM

NS: Non-sticky; OM: Optimum; HD: Hard; VHD: Very hard; NP: Non-plastic; VP: Very plastic; NE: Non-extrudable.

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Table 3 Properties of the extruded wires S. No.

GT (mm)

ST (mm)

GD (%)

SD (%)

IC (%)

O2

Tc (K)

Tc (K)

Jc (A cm−2)

1 2 3 4

2.9 2.4 1.8 1.2

2.5 2.0 1.5 0.9

65 68 71 75

83 85 88 92

10 8 5 4

6.80 6.85 6.88 6.90

90 90 91 91

8 7 6 4

1249 1382 1640 2014

GT: Green stage thickness; ST: Sintered thickness; GD: Green stage density; SD: Sintered density; IC: Impurity content (after sintering and annealing).

both room temperature and higher temperatures, ranging up to 150°C. For some of the selected choices the binders were able to dissolve after allowing several hours at room temperature or higher temperature. Based on the rheological and plastic properties of the slurry (Table 2), a final selection of the compounds was made with polyvinyl butyral (PVB), ethylene glycol and trichloroethylene as binder, plasticizer and solvent respectively for the extrusion phase of the present experiments. The adaptability of the slurry for extrusion was tested with repetitive initial trials. For uniformity, plasticity and improved microstructure of the wires, it was essential to add an additional amount of the solvent to the extruded plastic mass – wire, mixed it thoroughly, fed back again into the extrusion mill and drew the wires repeatedly. The speed of the extrusion mill wheel was adjusted so as to obtain sufficient density, uniformity and length of the wire. By varying the dimensions of the die, it could be possible to draw the superconducting slurry into fine wires of different diameters. Depending on the plasticity of the slurry, it was also possible over small lengths to subject the wire to an effective thickness change by adjusting the pulling force while extrusion. In the present experiments, the wires drawn were of uniform cross section with the diameter ranging from 2.9 to 1.2 mm (green stage). While the larger thickness wires were drawn in lengths of 1 m, the lower dimensional (0.9 mm) wire was drawn up to 10 m of length. After sintering and annealing schedules as described in the previous section, small portions (3 cm) of the wires were cut along the length at different locations for the basic characterization purposes. The measured values of the superconducting parameters for the present experimental samples are given in Table 3. As seen from the table, the sintered density increased from 83 to 92% (of the theoretical density) as the diameter of the sintered wires was decreased from 2.5 mm to 0.9 mm. The wires were sintered uniformly without any cracks to the visual inspection. Analysis of the XRD data showed orthorhombic structure of the Y-Ba-Cu-O material in the present samples. A typical XRD pattern of the annealed wire (0.9 mm) sample, exhibiting 96% 123 phase content is shown in Fig. 1. The amount of impurity phases present in the samples decreased from about 10% to

about 4% as the thickness of the wires reduced. This reduction was indicative of the high sintered reactions which occurred in thinner wires. The lattice oxygen content present in the 0.9 mm diameter sample was 6.9. Sufficiently high oxygen contents greater than or equal to 6.8 were noticed in all the samples. Fig. 2 shows a typical SEM micrograph taken on the surface of the wire (0.9 mm diameter). The microstructure of this sample revealed a dense structure with no microcracks or pores. The impurity phases were identified by EDX analysis as Y2BaCuO5 (211) and BaCuO2. The sample exhibited approximately 2% of the total impurity content (Table 3) at the grain boundaries. Fig. 3 shows the SEM micrograph for the fractured surface of the same sample. Large and aligned plate like features were observed in the microstructure of the sample. The average dimensions of the plate like grains were about 20 microns. Such large plate like features were reported in bulk high Tc polycrystalline materials possessing high Jc values [17,18]. This type of orderly growth led to the accomplishment of the minimum defects and reduced Josephson type weak links resulting in improved grain connectivity in the structure of the samples [1]. The temperature versus resistivity curves for some typical samples are given in Fig. 4. A high Tc of 91 K was observed for the sample with highest density and smallest diameter. The increase in critical transition width (DTc) from 4 to 8 K with increase in thickness was due to degradation in the microstructure of the samples. A typical SEM micrograph of the surface of the 2.5 diameter wire is shown in Fig. 5. The large DTc was due to grain boundary scattering observable in

Fig. 1. Typical XRD pattern of the sintered, 0.9 mm diameter wire.

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Fig. 4. Resistivity versus temperature graphs of the sintered wires (a) 2.5 mm diameter and (b) 0.9 mm diameter. Fig. 2. Typical SEM micrograph of the surface of the 0.9 mm diameter wire.

polycrystalline samples [19]. This scattering introduces grain boundary resistance and reduces the Jc due to poor connectivity of the grains. This broadening effect also occurs as a consequence of the thermally activated flux creep [20]. The AC susceptibility measurements are in agreement with four probe measurements (Fig. 6). Fig. 7 shows thickness versus critical current density graph of the fabricated wire samples. The transport critical current density measured (77 K and zero field) in 0.9 mm diameter wire was 2014.3 A cm − 2. This is the highest Jc observed among all the present experimental samples. Also, this is the highest Jc reported in bare ceramic wires. Tongo et al. [2] reported on Ag sheathed YBa2Cu3O7 − y wires with Jc exceeding 500 A

cm − 2 (77 K, zero field). Sadakata et al. [11], Uno et al. [21] and Tiefel et al. [3] also reported on Y-Ba-Cu-O wires prepared by similar technique with Jc corresponding to 560, 1000 and 1200 A cm − 2 respectively measured under the above conditions. Tiefel et al. [3] also reported on metal core composite wires of YBa2Cu3O7 − y using powder coating method and achieved a Jc of 800 A cm − 2 (77 K, zero field). Goto and Maruyama [22] fabricated Y-Ba-Cu-O wires using suspension spinning and obtained a Jc of 1000 A cm − 2 at 77 K and zero field. It was reported that in the case of silver sheathed metal clad composite wires, when the sheathing material was etched out before subjecting the wires to furnace sintered, the Jc values were improved abruptly [11]. This etching step was aimed to avoid crack generation in the oxide material during heating

Fig. 3. Typical SEM micrograph of the fractured surface of the 0.9 mm diameter wire.

Fig. 5. Typical SEM micrograph of the surface of the 2.5 diameter wire.

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Fig. 6. Susceptibility versus temperature graphs of the sintered wires (a) 2.5 mm diameter and (b) 0.9 mm diameter.

due to difference in thermal expansion of the superconducting material and silver sheath. For example, Sadakata et al. [11] observed that Jc was changed from 560 to 3930 A cm − 2 (77 K, zero field) in silver sheathed etched Y-Ba-Cu-O wires. The fairly large Jc values obtained in the present wires signify the importance of the inexpensive extrusion technique. Referring to the behaviour of Jc with respect to the changes in thickness of the wires (Fig. 7), it is to be expressed that the evidence of high Jc in thinner wires was indicative of the microstructural influence on the transport current. The large critical currents in thinner wires were possible due to the presence of aligned microstructure and lesser number of intergranular weak links (Figs. 2 and 3). The non-uniform grain growth and high angle grain boundaries (Fig. 5) were ascribed

Fig. 8. Critical current density versus applied magnetic field plot for the 0.9 diameter wire.

as the reasons for the observed low Jc and large DTc in 2.5 mm diameter wire sample. It can be said in the same context that the high amount of oxygen content and lower percentage of secondary phases present in the 0.9 mm diameter samples boosted the Jc to a comparatively high value. It was noticed that the sample to sample variation in critical current density for the same cross sectional thickness was about 10%. The behaviour of transport Jc in the presence of applied magnetic field is shown in Fig. 8. It is seen from the figure that the critical current density was dropped to a low value of 1510 A cm − 2 at 1 T magnetic field. This corresponds to a 25% drop in Jc from the zero field value and this is a moderate drop as compared to the ] 50% drop reported in Ag sheathed high Tc wires [1,12,13]. This marginal drop in the slowly increasing magnetic field indicated the persistent presence of considerable amount of weak links and weak pinning force in the microstructure of the samples. A strong deformation texture that would exhibit much less propensity for flux creep is very essential for improving the Jc (H) characteristics.

4. Conclusions

Fig. 7. Critical current density versus thickness plot for the sintered wires (77 K, zero field).

By means of extrusion technique, it was possible to draw long (up to 10 m) uniform wires of YBa2Cu3O7 − y in varying diameters. Importantly the wires were able to be sintered uniformly with minimum possible microstructural defects. High Jc values greater than 2 × 103 A cm − 2 were achieved in these wires. The present experiments indicated the presence of uniform microstructure with large plate like aligned growth in sintered wire samples. These results emphasize the sig-

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nificance of the extrusion technique in the light of other wire fabrication methods as mentioned in Section 1. It is possible to improve the Jc to further high values by reducing the average particle size of the 123 powder used in the superconducting slurry. The critical current density can also be raised by adding silver or silver oxide to the slurry. These additions improved the mechanical properties, promoted the growth of the superconducting phase and enhanced the grain alignment and grain connectivity [23 – 26]. Further studies incorporating different heat treatment schedules must help achieve a uniform and more aligned microstructure with larger grain size. There is a further possibility to enhance the oxygen intake leading to improvements in the critical current density. By employing flux pinning techniques, such as neutron irradiation, it is possible to reduce the problem of weak links and achieve higher Jc in the presence of magnetic field. Atmospheric protection can be easily achieved on these wires by the application of suitable polymeric coating.

Acknowledgements This work was carried out with the financial assistance from the Council of Scientific and Industrial Research (CSIR), Government of India, New Delhi. The authors are thankful to the same organization for the kind encouragement.

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