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Fabrication of YBa2Cu3O7−x (YBCO) superconductor bulk structures by extrusion freeforming
Ceramics International 42 (2016) 15836–15842
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Fabrication of YBa2Cu3O7 x (YBCO) superconductor bulk structures by extrusion freeforming Xiangxia Wei, Ragavendran Sundaram Nagarajan, Erwin Peng, Junmin Xue, John Wang, Jun Ding n Department of Materials Science and Engineering, National University of Singapore, 117574, Singapore
art ic l e i nf o
a b s t r a c t
Article history: Received 13 April 2016 Received in revised form 17 June 2016 Accepted 8 July 2016 Available online 9 July 2016
Practical applications of high temperature superconductors may require them to be processed into complex geometries. In this work, slurry-based extrusion freeforming coupled with high temperature treatment was attempted for the fabrication of bulk YBa2Cu3O7 x (YBCO) superconducting structures. YBCO parts with approximately 93% of the theoretical density were successfully fabricated after sintering at 940 °C for 60 h, with the obtained constituent phases strongly dependent on the heat treatment temperature and duration. A high critical transition temperature (TC ¼92 K) and good magnetic levitation ability could be obtained after optimization of the heat treatment conditions. Overall, the experimental results demonstrate that extrusion freeforming is a feasible and effective technique for fabricating YBCO superconductors that have desirable configurations and good superconductivity properties. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: 3D printing Extrusion freeforming Bulk YBCO superconductor Complex shapes
1. Introduction In the late 1980s, YBa2Cu3O7 x (YBCO) was the first superconductor discovered and has a critical transition temperature above the boiling point of liquid nitrogen [1]. With the ability to trap magnetic fields as high as 17 T, bulk YBCO has many electromagnetic applications [2–7]. To date, single-domain bulk YBCO has been fabricated by various melt-texturing techniques aided by the presence of the liquid phase [8–10], such as the top-seed melting texturing method, melt texture growth method, and powder melting process. However, most of these techniques are only capable of fabricating disk or rectangular bulk YBCO shapes due to the lack of bulk processing techniques and inherent brittleness of YBCO. Thus, fabrication of YBCO superconductors with complicated geometries as well as uniform chemical compositions and good superconducting properties is still highly challenging [11]. Recently, additive manufacture (or 3D printing) has attracted significant attention due to its great potential for use in commercial applications [12]. And it can be classified into various technologies, including stereo lithography, selective laser sintering or melting, fused deposition modeling, laminated object manufacturing, inkjet printing, and extrusion freeforming [13–20]. Among these techniques, extrusion freeforming has been widely n
Corresponding author. E-mail address: [email protected] (J. Ding).
http://dx.doi.org/10.1016/j.ceramint.2016.07.052 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
studied due to its advantages, such as flexibility and low cost [21– 23]. However, research on the use of 3D printing techniques for the fabrication of bulk YBCO materials has been rarely reported. For instance, selective laser sintering was studied by Agarwala et al. for fabricating YBCO samples [24]. However, it was found that the impurity phases, such as Y2BaCuO5, BaCuO2 and BaCuO2 x, still co-existed with the produced YBCO bulk samples. In another study, Ponnusamy et al. successfully produced YBa2Cu3O7 x with simple shapes, e.g., wires and rods, using the plastic extrusion process [25]. Additionally, the techniques for the fabrication of superconducting YBCO samples with simple shapes were also studied by other researchers [26,27]. However, to the best of our knowledge, bulk polycrystalline YBCO with arbitrary shapes that are fabricated by paste extrusion freeforming has not been reported previously. Accordingly, this work aims to provide a relatively simple additive manufacturing process to fabricate dense multifunctional YBCO bulk materials with desired shapes and uniform structures through extrusion freeforming coupled with a post-sintering process. The relationship between the heat treatment conditions, such as the sintering temperature and duration of crystal growth, and the characteristics of the materials were evaluated. Scanning electron microscopy and X-ray diffraction were used to characterize the grain morphologies and phases of the samples, respectively. Finally, different 3D-printed YBCO structures were levitated, showing their potential in magnetic levitation
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was extruded through a nozzle, as shown in Fig. 1. The extruder was free to move in the XY-plane, following the contour according to the given CAD model, while extruding material immediately on the working surface. After one layer was finished, the Z-axis moved down by a distance equal to the layer thickness. This process was performed repeatedly until the final ceramic green-body was obtained.
applications.
2. Experimental 2.1. Materials In this work, YBCO samples were prepared by a solid-state reaction method as described elsewhere [28]. High-purity (99.99%) powders of barium carbonate, copper oxide and yttrium oxide obtained from Sigma–Aldrich were mixed with stoichiometric molar ratios in an agate mortar, as shown in Fig. 1. Then, the powders were used as precursors and were mixed with the polyethylene glycol (PEG, obtained from Sigma-Aldrich), Solsperse 20,000 (supplied by Lubrizol), polyvinyl alcohol (PVA) and deionized water to form the paste for extrusion freeforming. 2.2. Paste preparation To prepare an extrudable paste with a high solid loading of YBCO precursor powder, PVA was first dissolved in water with a weight ratio of 8:1 for use as a binder in this work. Subsequently, an appropriate amount of plasticizer PEG-400 and dispersant Solsperse 20,000 (binder/plasticizer/dispersant weight ratio¼15:5:4) was directly added to the PVA solution, thus obtaining the binder solution. Then, the YBCO precursor powder was added to the binder solution with an optimal weight fraction of 71.5%, and the powder was thoroughly dispersed in the solution by an agate mortar with a pestle to form a homogeneous paste for printing. Finally, the obtained homogeneous paste was immediately transferred from the mortar into a 10-mL plastic syringe to avoid possible drastic changes in the paste viscosity due to evaporation. 2.3. Extrusion free-forming fabrication of YBCO precursors Using a 3Dison Multi printer equipped with a ROKIT. Inc. extruder, the homogeneous aqueous paste inside the plastic syringe
2.4. Post-sintering process Initially, the fabricated ceramic green body was solidified by water evaporation at 70 °C in air. Then, the green-body compacts experienced de-binding and sintering, which were accomplished in one step in a high temperature furnace using Carbolite Laboratory Chamber 1100 at different annealing temperatures in the range of 600–1100 °C for 20 h with a constant heating rate of 1 °C/ min. To study the influence of the annealing period, the samples were also sintered with durations of 5, 20, 40 and 60 h at 940 °C. All samples were then further annealed at 550 °C for 10 h to ensure oxygenation. Finally, bulk YBCO samples without cracking and deformations were obtained after cooling naturally to room temperature. 2.5. Characterization The gravimetric method was used for density measurements. The dimensions were measured using a Vernier caliper or propeller micrometer with an accuracy of 0.01 mm, and the specimens were weighted accurately to 1 mg. The relative density of the un-sintered (“green”) and sintered specimens is defined as the ratio of bulk density/theoretical density of superconductor (ca. 6.30 g/cm3) [29]. Additionally, the morphologies of the sintered surface were examined by scanning electron microscopy (SEM, Zeiss Supra 40 FESEM) with a 5 kV acceleration voltage for all samples. Chemical analysis was performed using the energy dispersive X-ray spectrum (EDX) system attached to the SEM. The structure and phase identification of the sintered samples were
Fig. 1. Schematic diagram of (i) the extrusion paste preparation and (ii) 3D printing/post-sintering process.
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obtained by X-ray powder diffraction using Bruker D8 Advanced Diffractometer System with Cu Kα source. Meanwhile, TC measurements were conducted on the sintered samples using a superconducting quantum interference device (SQUID). Additionally, the magnetic levitation experiment was performed simply by immersing the YBCO bulk sintered samples into liquefied nitrogen for approximately 3 min, followed by the transfer of the cooled down YBCO samples onto the top of a rare-earth magnet.
3. Results and discussion 3.1. Optimization of solid loading of YBCO precursors In general, polymer-ceramic paste properties are strongly dependent on the relative contents of the polymer, ceramic and solvent that can simply be determined by experimental tests. As shown in Fig. 2, the YBCO precursor powders with different initial fractions ranging from 50 wt% to 76 wt% were dispersed in the binder solutions. At a lower ceramic loading weight, the pastes displayed a relatively lower yield strength and could not maintain their shapes, which tended to slump over time. On the other hand, when the ceramic loading was higher than 71.5 wt%, the aqueous pastes required higher pressure to be squeezed out and, ultimately, became non-extrudable. Apparently, the paste with approximately 71.5 wt% ceramic loading was optimal and suitable for continuous extrusion from the printer nozzle, enabling smooth fabrication of ceramic green body. 3.2. Effect of sintering temperature and duration The sintering temperature and duration were modified for several identical cubically–shaped printed samples to study their relationships with the relative density. These cubic samples were annealed at different temperatures for the fixed 20 h. As seen from
Fig. 2. Influence of the weight fraction of YBCO precursors in the binder solution.
Fig. 3a, the green part before sintering was nearly 45% of the YBCO theoretical density. After the sintering treatment at 600 °C, the sample density increased to approximately 50%, which was mainly due to the burnout of the organic content as well as the complete solvent evaporation [30]. Upon the increase in the sintering temperature from 800 °C to 1100 °C, there was a significant enhancement in the relative density of the samples from approximately 63% to 94%. The 94% relative density obtained at 1100 °C can be attributed to the complete inter-particle bonding in the liquid phase sintering process. This result indicates that high density YBCO could easily be produced by a solid-state method at a higher sintering temperature with the same treatment time. Fig. 3b shows the effect of sintering durations on the densification of printed YBCO samples. At 940 °C, the sample density increased linearly from approximately 70% to nearly 91% with the increase in the sintering duration from 5 to 60 h. This trend obviously suggested that the extended dwelling period would lead to a significant enhancement in the final density. It is important to note that the significance of the nearly fully dense ceramic structure cannot be overemphasized, particularly for such a pressureless fabrication technique. Furthermore, based on the results reported by Suasmoro et al. [31], a high critical current density can be achieved in porous YBCO ceramics with a relative density of approximately 88% with full oxygenation and clean grain boundaries. Therefore, fully dense YBCO samples are not strongly encouraged due to the need for oxygenation process. The surface morphologies of bulk YBCO samples under different sintering temperatures and durations were characterized by SEM, with the obtained images shown in Fig. 4. Examination of Fig. 4a shows that the YBCO precursor powder appeared to be homogeneous and the average particulate dimension was approximately 1–2 mm. Meanwhile, the SEM micrographs of YBCO samples after sintering at 800 °C, 940 °C and 1100 °C for 20 h, as shown in Fig. 4b–d, indicated that the porosity decreased with the increasing temperature, and the randomly oriented grains became larger. The pores gradually diminished during the sintering diffusion process, leading to the significant densification of the YBCO part, consistent with the densification enhancement mentioned above. Similarly, Fig. 4e–h show the surface microstructures of the bulk YBCO samples after sintering at 940 °C with different holding times ranging from 5 to 60 h. It appears that an extended period of sintering is necessary to allow sufficient atom diffusion, resulting in the sample with smaller porosity and larger grains compared to the sample treated for a shorter duration. To summarize, the overall decrease of porosity during the sintering process is strongly affected by both the temperature and duration of sintering. Additionally, the EDX analysis performed on the YBCO precursor powder confirmed the presence of Y, Ba, Cu and O as shown in Fig. 4i. The chemical composition is illustrated in the inset of Fig. 4i. In the quantitative analyses the oxygen was omitted, but the proportions of Y/Ba/Cu showed close similarity to the stoichiometric composition of YBa2Cu3O7 x with a small deviation of the barium content. To confirm the formation of superconducting YBCO phases, the samples were further analyzed by X-ray diffraction. The XRD patterns of bulk YBCO parts after sintering at various temperatures for 20 h are shown in Fig. 5a. As can be clearly observed from Fig. 5a, all of the peaks were attributed to the YBCO precursors, namely Y2O3, BaCO3 and CuO, without the presence of the YBCO 123 orthorhombic phase after sintering at 600 °C. Even at 800 °C, the main peak represented the YBCO orthorhombic phase, with a large amount of secondary phases belonging to its precursors. Moreover, examination of the XRD pattern revealed that a nearly pure orthorhombic phase of YBCO was successfully formed by sintering at 940 °C, as could was demonstrated by the X-ray
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Fig. 3. (a) Density of printed YBCO sintered for 20 h at different temperatures; (b) density of printed YBCO sintered at 940 °C at different times.
Fig. 4. (a–d): SEM microstructures of the printed bulk YBCO sintered for 20 h at different temperatures. (a) YBCO precursor powder, (b) samples sintered at 800 °C, (c) samples sintered at 940 °C, (d) samples sintered at 1100 °C. (e–h): SEM microstructures of printed bulk YBCO sintered at 940 °C for different time. (e) Samples sintered for 5 h, (f) samples sintered for 20 h, (g) samples sintered for 40 h, (h) samples sintered for 60 h. (Bars ¼ 2 mm for all of the samples). (i) EDX spectrum taken from the precursor powder shown in (a): inset is quantitative chemical composition.
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ƽ) CuO, (
Fig. 5. (a) XRD spectra of printed YBCO sintered for 20 h at different temperatures. Phases present are indicated as: (○) Y123, (□) BaCO3, (∇) Y2O3, ( ( ) Y2BaCuO5. (b) XRD spectra of printed YBCO sintered at 940 °C for different time.
ͩ
) BaCuO2,
Fig. 6. (a) Zero field cooling (ZFC) and field cooling (FC) curves of bulk YBCO at an applied field of 100 Oe (inset: point of superconducting transition). (b) Temperature dependent magnetic hysteresis loops of the YBCO sample sintered at 940 °C for 40 h.
reflection peaks at 2θ of 32° and 58°. However, when the YBCO samples were further heated to 1100 °C, significantly above the peritectic temperature of YBCO materials (TP ¼1015 °C) [32], the YBCO 123 phase was partially dissociated into the Y2BaCuO5 (Y211) phase and some liquid phases, such as BaCuO2. The XRD patterns of bulk YBCO sintered at 940 °C with different durations, such as 5, 20, 40 and 60 h, are shown in Fig. 5b. The characteristic peaks of the orthorhombic structure at (103), (013) and (116) appeared in the diffraction pattern. Some peaks along the c-axis were also present. The peaks were clean and strong without any indication of the presence of impurity phases, further demonstrating the good crystallinity of the YBCO sample. Additionally, the YBCO (003) intensity increased with the extending annealing time, indicating that a longer annealing duration would lead to the compaction or densification of the YBCO samples. This trend was consistent with the SEM images that showed enhanced densification with the increasing sintering duration. 3.3. Superconducting properties of the printed YBCO with different geometries Fig. 6a shows the magnetic moment as a function of the temperature with an applied magnetic field of 100 Oe under zero field
cooled (ZFC) and field cooled (FC) conditions to characterize the superconducting properties, such as the critical temperature (TC) of the samples. The ZFC, also called the diamagnetic shielding signal, was measured by cooling the sample in zero fields and then applying a small field at the lowest measurement temperature. The FC condition corresponded to subsequently cooling the sample in the same field from above TC to obtain the Meissner signal [31]. Examination of Fig. 6a shows that a large diamagnetic signal was present below 92 K for the bulk YBCO sintered for 40 h at 940 °C, indicating the TC that can be better observed in the inset of the figure. Furthermore, magnetic hysteresis loops were measured at temperatures of 4.2 K, 50 K and 77 K with applied fields ranging from 70 kOe to 70 kOe, as shown in Fig. 6b. At 77 K, the magnetization appeared to be quite sensitive to the applied field, and higher fields were trapped by decreasing the temperature. This result could be attributed to the increased flux pinning at lower temperatures inside the sample [33]. In addition to the method described above, the superconductivity of YBCO can also be verified by its Meissner effect, where the superconducting material can levitate on top of a permanent magnet, as shown in Fig. 7a. To achieve this phenomenon, we first cooled the high temperature superconductor YBCO below its critical temperature (TC ¼92 K) using liquid nitrogen.
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Fig. 7. (a) Magnetic levitation experiment of printed YBCO samples, inclusive of hollow cylinder levitation, 3-sided gengon levitation, and U-shaped box levitation. (b) Printed YBCO with various shapes, such as cube, cone, hollow cylinder and box, and zig-zag wall. (Bars ¼1 cm for all of the samples).
Subsequently, after placing the YBCO samples on top of rare-earth magnets, the phenomenon of magnetic levitation could be observed due to the interaction between the permanent magnet and the high TC superconductor. This material not only easily levitated in a magnetic field but also showed strong diamagnetic flux exclusion above the liquid-nitrogen temperature. Furthermore, bulk YBCO with various shapes, e.g., hollow cylinder, gengon and U-shaped box, were fabricated, and their levitation effects were also demonstrated. Additionally, much more complex structures, such as a cone and a zig-zag wall, could also be easily obtained by 3D printing, as shown in Fig. 7b. Compared with other conventional fabrication methods of bulk YBCO, such as top seeded melt growth, seeded infiltration and growth, the extrusion freeforming possesses several advantages. Firstly, the value of TC for conventional samples was around 90 K [34], while the TC was observed to be 92 K for YBCO obtained from extrusion freeforming in this work. The higher TC may be contributed to the sufficient oxygen diffusion inside bulk YBCO due to the absence of pressing treatment, which was a common process in the conventional techniques. Moreover, other fabrication methods only produce simple components by cutting the pressed pellets, but extrusion freeforming provides an important strategy to produce YBCO materials with nearly unlimited design freedom. Finally, extrusion freeforming has low cost, short processing time and simplicity without the need of high quality seeds. In summary, extrusion freeforming is a promising and versatile approach for the production of bulk YBCO parts for various practical applications with more degrees of freedom in the shape design and good superconductivity.
heating temperature and duration. Interestingly, YBCO specimens that had a relative density of approximately 93% without any cracks or deformations were produced after pressure-less sintering at 940 °C for 60 h. At the same time, a higher sintering temperature and longer duration could decrease the porosity of the final products. According to the XRD results, sintering at 940 °C in air facilitated the formation of pure 123 orthorhombic YBCO phase without the secondary phase. Moreover, the YBCO parts that had desirable shapes exhibited a high transition temperature (92 K) and good superconducting properties. Overall, these results show that the extrusion freeforming process is capable of handling the starting ceramic powders and thus can be up-scaled for efficient production of a bulk YBCO superconductor for various practical applications.
Notes The authors declare no competing financial interests.
Acknowledgment The project is financially supported by NUS Strategic Research Fund R261509001646 and R261509001733. The first author is also grateful to the scholarship from China Scholarship Council (CSC No. 201406320189).
References 4. Conclusions YBCO bulk structures were successfully fabricated by using 3D extrusion freeforming coupled with a solid-state reaction process. Optimization of the annealing step revealed that the density, microstructure and phase were significantly influenced by the
[1] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y. Q. Wang, C.W. Chu, Superconductivity at 93 K in a new mixed-phase Y-Ba-CuO compound system at ambient pressure, Phys. Rev. Lett. 58 (1987) 908–910. [2] M. Tomita, M. Murakami, High-temperature superconductor bulk magnets that can trap magnetic fields of over 17 T at 29 K, Nature 421 (2003) 517–520. [3] M. Murakami, Electromagnetic applications of melt-processed YBCO, Ceram. Int. 23 (1997) 203–207. [4] N. Koshizuka, R&D of superconducting bearing technologies for flywheel
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X. Wei et al. / Ceramics International 42 (2016) 15836–15842
energy storage systems, Physica C 445–448 (2006) 1103–1108. [5] P. Tixador, X. Obradors, R. Tournier, T. Puig, D. Bourgault, X. Granados, J. M. Duval, E. Mendoza, X. Chaud, E. Varesi, E. Beaugnon, D. Isfort, Quench in bulk HTS materials- application to the fault current limiter, Supercond. Sci. Technol. 13 (2000) 493–497. [6] S.I. Yoo, H. Fujimoto, N. Sakai, M. Murakami, Melt-processed LRE-Ba-Cu-O superconductors and prospects for their applications, J. Alloy. Compd. 250 (1997) 439–448. [7] D.U. Gubser, Superconducting motors and generators for naval applications, Physica C 392–396 (2003) 1192–1195. [8] S. Marinel, J. Wang, I. Monot, M.P. Delamare, J. Provost, G. Desgardin, Topseeding melt texture growth of single-domain superconducting YBa2Cu3O7 δ pellets, Supercond. Sci. Technol. 10 (1997) 147–155. [9] W.M. Yang, L. Zhou, Y. Feng, P.X. Zhang, M.Z. Wu, C.P. Zhang, J.R. Wang, Z. H. Du, F.Y. Wang, Z.M. Yu, X.Z. Wu, W. Gawalek, P. Gornert, The effect of excess Y2O3 addition on the levitation force of melt processed YBCO bulk superconductors, Physica C 305 (1998) 269–274. [10] L. Zhou, S.K. Chen, K.G. Wang, X.Z. Wu, P.X. Zhang, Y. Feng, Synthesis of ultrafine Y2BaCuO5 powder and its incorporation into YBCO bulk by powder melting process, Physica C 363 (2001) 99–106. [11] T. Yamashita, J.A. Alarco, J. Riches, A. Bhargava, J.C. Barry, I.D.R. Mackinnon, Binder effect on microstructure and properties of YBa2Cu3O7 x extruded wires, Physica C 298 (1998) 159–165. [12] N.N. Guo, M.C. Leu, Additive manufacturing: technology, applications and research needs, Front. Mech. Eng. 8 (2013) 215–243. [13] G.M. Lous, I.A. Cornejo, T.F. McNulty, A. Safari, S.C. Danforth, Fabrication of piezoelectric ceramic/polymer composite transducers using fused deposition of ceramics, J. Am. Ceram. Soc. 83 (2000) 124–128. [14] M. Allahverdi, S.C. Danforth, M. Jafari, A. Safari, Processing of advanced electroceramic components by fused deposition technique, J. Eur. Ceram. Soc. 21 (2001) 1485–1490. [15] Z.S. Macedo, M.H. Lente, J.A. Eiras, A.C. Hernandes, Dielectric and ferroelectric properties of Bi4Ti3O12 ceramics produced by a laser sintering method, J. Phys.: Condens. Matter 16 (2004) 2811–2818. [16] H. Yves-Christian, W. Jan, M. Wilhelm, W. Konrad, P. Reinhart, Net shaped high performance oxide ceramic parts by selective laser melting, Phys. Procedia 5 (2010) 587–594. [17] T. Chartier, C. Chaput, F. Doreau, M. Loiseau, Stereolithography of structural complex ceramic parts, J. Mater. Sci. 37 (2002) 3141–3147. [18] Y. Zhang, X. He, S. Du, J. Zhang, Al2O3 ceramics preparation by LOM (Laminated Object Manufacturing), Int. J. Adv. Manuf. Technol. 17 (2001) 531–534. [19] E.B. Secor, P.L. Prabhumirashi, K. Puntambekar, M.L. Geier, M.C. Hersam, Inkjet printing of high conductivity, flexible graphene patterns, J. Phys. Chem. Lett. 4
(2013) 1347–1351. [20] X. Lu, Y. Lee, S. Yang, Y. Hao, R. Ubic, J.R.G. Evans, C.G. Parini, Fabrication of electromagnetic crystals by extrusion freeforming, Metamaterials 2 (2008) 36–44. [21] T. Huang, M.S. Mason, X. Zhao, G.E. Hilmas, M.C. Leu, Aqueous-based freezeform extrusion fabrication of alumina components, Rapid Prototyp. J. 15/2 (2009) 88–95. [22] J. Powell, S. Assabumrungrat, S. Blackburn, Design of ceramic paste formulations for co-extrusion, Powder Technol. 245 (2013) 21–27. [23] M.S. Mason, T. Huang, R.G. Landers, M.C. Leu, G.E. Hilmas, Aqueous-based extrusion of high solids loading ceramic pastes: process modeling and control, J. Mater. Process. Technol. 209 (2009) 2946–2957. [24] M. Agarwala, D.L. Bourell, A. Manthiram, B.R. Birmingham, H.L. Marcus, High Tc dual phase Ag-YBa2Cu3O7 x composites prepared by selective laser sintering and infiltration, J. Mater. Sci. 30 (1995) 459–464. [25] D. Ponnusamy, K. Ravi-Chandar, Extrusion of superconducting wires of YBa2Cu3O7 x, J. Mater. Res. 8 (1993) 268–273. [26] R. Enomoto, M. Tamaki, S. Ohno, M. Fuwa, H. Takei, Fabrication of Y-Ba-Cu-O superconducting wire by the powder-sol method, Jpn. J. Appl. Phys. 28 (1989) 1207–1209. [27] H. Varma, P. Mukundan, K.G. Warrier, A.D. Darmodoran, U. Balachandran, Nonaqueous cold extrusion as a route for yttrium barium copper oxide wires, J. Am. Ceram. Soc. 73 (1990) 3100–3102. [28] H.J. Scheel, F. Licci, Crystal growth of YBa2Cu3O7 x, J. Cryst. Growth 85 (1987) 607–614. [29] C. Vipulanandan, S. Salib, Mechanical and physical properties of sintered YBCO-metal bulk composites with silver powder and fibres, J. Mater. Sci. 30 (1995) 763–769. [30] M.A.A. Moussa, S.M. El-Hefnawy, M.M. Abou-Alkheer, H.I.A. Al-Nattar, Melt processing of YBCO thick films fabricated by sol gel route powder on YSZ, J. Supercond. Nov. Magn. 22 (2009) 479–485. [31] S. Suasmoro, M.F. Khalfi, A. Khalfi, G. Trolliard, D.S. Smith, J.P. Bonnet, Microstructural and electrical characterization of bulk YBa2Cu3O7 δ ceramics, Ceram. Int. 38 (2012) 29–38. [32] T. Aselage, K. Keefer, Liquidus relations in Y–Ba–Cu oxides, J. Mater. Res. 3 (1988) 1279–1291. [33] G. Blatter, M.V. Feigel'man, V.B. Geshkenbein, A.I. Larkin, V.M. Vinokur, Vortices in high-temperature superconductors, Rev. Mod. Phys. 66 (1994) 1125–1388. [34] K. Iida, N.H. Babu, Y. Shi, D.A. Cardwell, Seeded infiltration and growth of large, single domain Y–Ba–Cu–O bulk superconductors with very high critical current densities, Supercond. Sci. Technol. 18 (2005) 1421–1427.
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Fabrication of YBa2Cu3O7 x (YBCO) superconductor bulk structures by extrusion freeforming Xiangxia Wei, Ragavendran Sundaram Nagarajan, Erwin Peng, Junmin Xue, John Wang, Jun Ding n Department of Materials Science and Engineering, National University of Singapore, 117574, Singapore
art ic l e i nf o
a b s t r a c t
Article history: Received 13 April 2016 Received in revised form 17 June 2016 Accepted 8 July 2016 Available online 9 July 2016
Practical applications of high temperature superconductors may require them to be processed into complex geometries. In this work, slurry-based extrusion freeforming coupled with high temperature treatment was attempted for the fabrication of bulk YBa2Cu3O7 x (YBCO) superconducting structures. YBCO parts with approximately 93% of the theoretical density were successfully fabricated after sintering at 940 °C for 60 h, with the obtained constituent phases strongly dependent on the heat treatment temperature and duration. A high critical transition temperature (TC ¼92 K) and good magnetic levitation ability could be obtained after optimization of the heat treatment conditions. Overall, the experimental results demonstrate that extrusion freeforming is a feasible and effective technique for fabricating YBCO superconductors that have desirable configurations and good superconductivity properties. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: 3D printing Extrusion freeforming Bulk YBCO superconductor Complex shapes
1. Introduction In the late 1980s, YBa2Cu3O7 x (YBCO) was the first superconductor discovered and has a critical transition temperature above the boiling point of liquid nitrogen [1]. With the ability to trap magnetic fields as high as 17 T, bulk YBCO has many electromagnetic applications [2–7]. To date, single-domain bulk YBCO has been fabricated by various melt-texturing techniques aided by the presence of the liquid phase [8–10], such as the top-seed melting texturing method, melt texture growth method, and powder melting process. However, most of these techniques are only capable of fabricating disk or rectangular bulk YBCO shapes due to the lack of bulk processing techniques and inherent brittleness of YBCO. Thus, fabrication of YBCO superconductors with complicated geometries as well as uniform chemical compositions and good superconducting properties is still highly challenging [11]. Recently, additive manufacture (or 3D printing) has attracted significant attention due to its great potential for use in commercial applications [12]. And it can be classified into various technologies, including stereo lithography, selective laser sintering or melting, fused deposition modeling, laminated object manufacturing, inkjet printing, and extrusion freeforming [13–20]. Among these techniques, extrusion freeforming has been widely n
Corresponding author. E-mail address: [email protected] (J. Ding).
http://dx.doi.org/10.1016/j.ceramint.2016.07.052 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
studied due to its advantages, such as flexibility and low cost [21– 23]. However, research on the use of 3D printing techniques for the fabrication of bulk YBCO materials has been rarely reported. For instance, selective laser sintering was studied by Agarwala et al. for fabricating YBCO samples [24]. However, it was found that the impurity phases, such as Y2BaCuO5, BaCuO2 and BaCuO2 x, still co-existed with the produced YBCO bulk samples. In another study, Ponnusamy et al. successfully produced YBa2Cu3O7 x with simple shapes, e.g., wires and rods, using the plastic extrusion process [25]. Additionally, the techniques for the fabrication of superconducting YBCO samples with simple shapes were also studied by other researchers [26,27]. However, to the best of our knowledge, bulk polycrystalline YBCO with arbitrary shapes that are fabricated by paste extrusion freeforming has not been reported previously. Accordingly, this work aims to provide a relatively simple additive manufacturing process to fabricate dense multifunctional YBCO bulk materials with desired shapes and uniform structures through extrusion freeforming coupled with a post-sintering process. The relationship between the heat treatment conditions, such as the sintering temperature and duration of crystal growth, and the characteristics of the materials were evaluated. Scanning electron microscopy and X-ray diffraction were used to characterize the grain morphologies and phases of the samples, respectively. Finally, different 3D-printed YBCO structures were levitated, showing their potential in magnetic levitation
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was extruded through a nozzle, as shown in Fig. 1. The extruder was free to move in the XY-plane, following the contour according to the given CAD model, while extruding material immediately on the working surface. After one layer was finished, the Z-axis moved down by a distance equal to the layer thickness. This process was performed repeatedly until the final ceramic green-body was obtained.
applications.
2. Experimental 2.1. Materials In this work, YBCO samples were prepared by a solid-state reaction method as described elsewhere [28]. High-purity (99.99%) powders of barium carbonate, copper oxide and yttrium oxide obtained from Sigma–Aldrich were mixed with stoichiometric molar ratios in an agate mortar, as shown in Fig. 1. Then, the powders were used as precursors and were mixed with the polyethylene glycol (PEG, obtained from Sigma-Aldrich), Solsperse 20,000 (supplied by Lubrizol), polyvinyl alcohol (PVA) and deionized water to form the paste for extrusion freeforming. 2.2. Paste preparation To prepare an extrudable paste with a high solid loading of YBCO precursor powder, PVA was first dissolved in water with a weight ratio of 8:1 for use as a binder in this work. Subsequently, an appropriate amount of plasticizer PEG-400 and dispersant Solsperse 20,000 (binder/plasticizer/dispersant weight ratio¼15:5:4) was directly added to the PVA solution, thus obtaining the binder solution. Then, the YBCO precursor powder was added to the binder solution with an optimal weight fraction of 71.5%, and the powder was thoroughly dispersed in the solution by an agate mortar with a pestle to form a homogeneous paste for printing. Finally, the obtained homogeneous paste was immediately transferred from the mortar into a 10-mL plastic syringe to avoid possible drastic changes in the paste viscosity due to evaporation. 2.3. Extrusion free-forming fabrication of YBCO precursors Using a 3Dison Multi printer equipped with a ROKIT. Inc. extruder, the homogeneous aqueous paste inside the plastic syringe
2.4. Post-sintering process Initially, the fabricated ceramic green body was solidified by water evaporation at 70 °C in air. Then, the green-body compacts experienced de-binding and sintering, which were accomplished in one step in a high temperature furnace using Carbolite Laboratory Chamber 1100 at different annealing temperatures in the range of 600–1100 °C for 20 h with a constant heating rate of 1 °C/ min. To study the influence of the annealing period, the samples were also sintered with durations of 5, 20, 40 and 60 h at 940 °C. All samples were then further annealed at 550 °C for 10 h to ensure oxygenation. Finally, bulk YBCO samples without cracking and deformations were obtained after cooling naturally to room temperature. 2.5. Characterization The gravimetric method was used for density measurements. The dimensions were measured using a Vernier caliper or propeller micrometer with an accuracy of 0.01 mm, and the specimens were weighted accurately to 1 mg. The relative density of the un-sintered (“green”) and sintered specimens is defined as the ratio of bulk density/theoretical density of superconductor (ca. 6.30 g/cm3) [29]. Additionally, the morphologies of the sintered surface were examined by scanning electron microscopy (SEM, Zeiss Supra 40 FESEM) with a 5 kV acceleration voltage for all samples. Chemical analysis was performed using the energy dispersive X-ray spectrum (EDX) system attached to the SEM. The structure and phase identification of the sintered samples were
Fig. 1. Schematic diagram of (i) the extrusion paste preparation and (ii) 3D printing/post-sintering process.
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obtained by X-ray powder diffraction using Bruker D8 Advanced Diffractometer System with Cu Kα source. Meanwhile, TC measurements were conducted on the sintered samples using a superconducting quantum interference device (SQUID). Additionally, the magnetic levitation experiment was performed simply by immersing the YBCO bulk sintered samples into liquefied nitrogen for approximately 3 min, followed by the transfer of the cooled down YBCO samples onto the top of a rare-earth magnet.
3. Results and discussion 3.1. Optimization of solid loading of YBCO precursors In general, polymer-ceramic paste properties are strongly dependent on the relative contents of the polymer, ceramic and solvent that can simply be determined by experimental tests. As shown in Fig. 2, the YBCO precursor powders with different initial fractions ranging from 50 wt% to 76 wt% were dispersed in the binder solutions. At a lower ceramic loading weight, the pastes displayed a relatively lower yield strength and could not maintain their shapes, which tended to slump over time. On the other hand, when the ceramic loading was higher than 71.5 wt%, the aqueous pastes required higher pressure to be squeezed out and, ultimately, became non-extrudable. Apparently, the paste with approximately 71.5 wt% ceramic loading was optimal and suitable for continuous extrusion from the printer nozzle, enabling smooth fabrication of ceramic green body. 3.2. Effect of sintering temperature and duration The sintering temperature and duration were modified for several identical cubically–shaped printed samples to study their relationships with the relative density. These cubic samples were annealed at different temperatures for the fixed 20 h. As seen from
Fig. 2. Influence of the weight fraction of YBCO precursors in the binder solution.
Fig. 3a, the green part before sintering was nearly 45% of the YBCO theoretical density. After the sintering treatment at 600 °C, the sample density increased to approximately 50%, which was mainly due to the burnout of the organic content as well as the complete solvent evaporation [30]. Upon the increase in the sintering temperature from 800 °C to 1100 °C, there was a significant enhancement in the relative density of the samples from approximately 63% to 94%. The 94% relative density obtained at 1100 °C can be attributed to the complete inter-particle bonding in the liquid phase sintering process. This result indicates that high density YBCO could easily be produced by a solid-state method at a higher sintering temperature with the same treatment time. Fig. 3b shows the effect of sintering durations on the densification of printed YBCO samples. At 940 °C, the sample density increased linearly from approximately 70% to nearly 91% with the increase in the sintering duration from 5 to 60 h. This trend obviously suggested that the extended dwelling period would lead to a significant enhancement in the final density. It is important to note that the significance of the nearly fully dense ceramic structure cannot be overemphasized, particularly for such a pressureless fabrication technique. Furthermore, based on the results reported by Suasmoro et al. [31], a high critical current density can be achieved in porous YBCO ceramics with a relative density of approximately 88% with full oxygenation and clean grain boundaries. Therefore, fully dense YBCO samples are not strongly encouraged due to the need for oxygenation process. The surface morphologies of bulk YBCO samples under different sintering temperatures and durations were characterized by SEM, with the obtained images shown in Fig. 4. Examination of Fig. 4a shows that the YBCO precursor powder appeared to be homogeneous and the average particulate dimension was approximately 1–2 mm. Meanwhile, the SEM micrographs of YBCO samples after sintering at 800 °C, 940 °C and 1100 °C for 20 h, as shown in Fig. 4b–d, indicated that the porosity decreased with the increasing temperature, and the randomly oriented grains became larger. The pores gradually diminished during the sintering diffusion process, leading to the significant densification of the YBCO part, consistent with the densification enhancement mentioned above. Similarly, Fig. 4e–h show the surface microstructures of the bulk YBCO samples after sintering at 940 °C with different holding times ranging from 5 to 60 h. It appears that an extended period of sintering is necessary to allow sufficient atom diffusion, resulting in the sample with smaller porosity and larger grains compared to the sample treated for a shorter duration. To summarize, the overall decrease of porosity during the sintering process is strongly affected by both the temperature and duration of sintering. Additionally, the EDX analysis performed on the YBCO precursor powder confirmed the presence of Y, Ba, Cu and O as shown in Fig. 4i. The chemical composition is illustrated in the inset of Fig. 4i. In the quantitative analyses the oxygen was omitted, but the proportions of Y/Ba/Cu showed close similarity to the stoichiometric composition of YBa2Cu3O7 x with a small deviation of the barium content. To confirm the formation of superconducting YBCO phases, the samples were further analyzed by X-ray diffraction. The XRD patterns of bulk YBCO parts after sintering at various temperatures for 20 h are shown in Fig. 5a. As can be clearly observed from Fig. 5a, all of the peaks were attributed to the YBCO precursors, namely Y2O3, BaCO3 and CuO, without the presence of the YBCO 123 orthorhombic phase after sintering at 600 °C. Even at 800 °C, the main peak represented the YBCO orthorhombic phase, with a large amount of secondary phases belonging to its precursors. Moreover, examination of the XRD pattern revealed that a nearly pure orthorhombic phase of YBCO was successfully formed by sintering at 940 °C, as could was demonstrated by the X-ray
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Fig. 3. (a) Density of printed YBCO sintered for 20 h at different temperatures; (b) density of printed YBCO sintered at 940 °C at different times.
Fig. 4. (a–d): SEM microstructures of the printed bulk YBCO sintered for 20 h at different temperatures. (a) YBCO precursor powder, (b) samples sintered at 800 °C, (c) samples sintered at 940 °C, (d) samples sintered at 1100 °C. (e–h): SEM microstructures of printed bulk YBCO sintered at 940 °C for different time. (e) Samples sintered for 5 h, (f) samples sintered for 20 h, (g) samples sintered for 40 h, (h) samples sintered for 60 h. (Bars ¼ 2 mm for all of the samples). (i) EDX spectrum taken from the precursor powder shown in (a): inset is quantitative chemical composition.
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ƽ) CuO, (
Fig. 5. (a) XRD spectra of printed YBCO sintered for 20 h at different temperatures. Phases present are indicated as: (○) Y123, (□) BaCO3, (∇) Y2O3, ( ( ) Y2BaCuO5. (b) XRD spectra of printed YBCO sintered at 940 °C for different time.
ͩ
) BaCuO2,
Fig. 6. (a) Zero field cooling (ZFC) and field cooling (FC) curves of bulk YBCO at an applied field of 100 Oe (inset: point of superconducting transition). (b) Temperature dependent magnetic hysteresis loops of the YBCO sample sintered at 940 °C for 40 h.
reflection peaks at 2θ of 32° and 58°. However, when the YBCO samples were further heated to 1100 °C, significantly above the peritectic temperature of YBCO materials (TP ¼1015 °C) [32], the YBCO 123 phase was partially dissociated into the Y2BaCuO5 (Y211) phase and some liquid phases, such as BaCuO2. The XRD patterns of bulk YBCO sintered at 940 °C with different durations, such as 5, 20, 40 and 60 h, are shown in Fig. 5b. The characteristic peaks of the orthorhombic structure at (103), (013) and (116) appeared in the diffraction pattern. Some peaks along the c-axis were also present. The peaks were clean and strong without any indication of the presence of impurity phases, further demonstrating the good crystallinity of the YBCO sample. Additionally, the YBCO (003) intensity increased with the extending annealing time, indicating that a longer annealing duration would lead to the compaction or densification of the YBCO samples. This trend was consistent with the SEM images that showed enhanced densification with the increasing sintering duration. 3.3. Superconducting properties of the printed YBCO with different geometries Fig. 6a shows the magnetic moment as a function of the temperature with an applied magnetic field of 100 Oe under zero field
cooled (ZFC) and field cooled (FC) conditions to characterize the superconducting properties, such as the critical temperature (TC) of the samples. The ZFC, also called the diamagnetic shielding signal, was measured by cooling the sample in zero fields and then applying a small field at the lowest measurement temperature. The FC condition corresponded to subsequently cooling the sample in the same field from above TC to obtain the Meissner signal [31]. Examination of Fig. 6a shows that a large diamagnetic signal was present below 92 K for the bulk YBCO sintered for 40 h at 940 °C, indicating the TC that can be better observed in the inset of the figure. Furthermore, magnetic hysteresis loops were measured at temperatures of 4.2 K, 50 K and 77 K with applied fields ranging from 70 kOe to 70 kOe, as shown in Fig. 6b. At 77 K, the magnetization appeared to be quite sensitive to the applied field, and higher fields were trapped by decreasing the temperature. This result could be attributed to the increased flux pinning at lower temperatures inside the sample [33]. In addition to the method described above, the superconductivity of YBCO can also be verified by its Meissner effect, where the superconducting material can levitate on top of a permanent magnet, as shown in Fig. 7a. To achieve this phenomenon, we first cooled the high temperature superconductor YBCO below its critical temperature (TC ¼92 K) using liquid nitrogen.
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Fig. 7. (a) Magnetic levitation experiment of printed YBCO samples, inclusive of hollow cylinder levitation, 3-sided gengon levitation, and U-shaped box levitation. (b) Printed YBCO with various shapes, such as cube, cone, hollow cylinder and box, and zig-zag wall. (Bars ¼1 cm for all of the samples).
Subsequently, after placing the YBCO samples on top of rare-earth magnets, the phenomenon of magnetic levitation could be observed due to the interaction between the permanent magnet and the high TC superconductor. This material not only easily levitated in a magnetic field but also showed strong diamagnetic flux exclusion above the liquid-nitrogen temperature. Furthermore, bulk YBCO with various shapes, e.g., hollow cylinder, gengon and U-shaped box, were fabricated, and their levitation effects were also demonstrated. Additionally, much more complex structures, such as a cone and a zig-zag wall, could also be easily obtained by 3D printing, as shown in Fig. 7b. Compared with other conventional fabrication methods of bulk YBCO, such as top seeded melt growth, seeded infiltration and growth, the extrusion freeforming possesses several advantages. Firstly, the value of TC for conventional samples was around 90 K [34], while the TC was observed to be 92 K for YBCO obtained from extrusion freeforming in this work. The higher TC may be contributed to the sufficient oxygen diffusion inside bulk YBCO due to the absence of pressing treatment, which was a common process in the conventional techniques. Moreover, other fabrication methods only produce simple components by cutting the pressed pellets, but extrusion freeforming provides an important strategy to produce YBCO materials with nearly unlimited design freedom. Finally, extrusion freeforming has low cost, short processing time and simplicity without the need of high quality seeds. In summary, extrusion freeforming is a promising and versatile approach for the production of bulk YBCO parts for various practical applications with more degrees of freedom in the shape design and good superconductivity.
heating temperature and duration. Interestingly, YBCO specimens that had a relative density of approximately 93% without any cracks or deformations were produced after pressure-less sintering at 940 °C for 60 h. At the same time, a higher sintering temperature and longer duration could decrease the porosity of the final products. According to the XRD results, sintering at 940 °C in air facilitated the formation of pure 123 orthorhombic YBCO phase without the secondary phase. Moreover, the YBCO parts that had desirable shapes exhibited a high transition temperature (92 K) and good superconducting properties. Overall, these results show that the extrusion freeforming process is capable of handling the starting ceramic powders and thus can be up-scaled for efficient production of a bulk YBCO superconductor for various practical applications.
Notes The authors declare no competing financial interests.
Acknowledgment The project is financially supported by NUS Strategic Research Fund R261509001646 and R261509001733. The first author is also grateful to the scholarship from China Scholarship Council (CSC No. 201406320189).
References 4. Conclusions YBCO bulk structures were successfully fabricated by using 3D extrusion freeforming coupled with a solid-state reaction process. Optimization of the annealing step revealed that the density, microstructure and phase were significantly influenced by the
[1] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y. Q. Wang, C.W. Chu, Superconductivity at 93 K in a new mixed-phase Y-Ba-CuO compound system at ambient pressure, Phys. Rev. Lett. 58 (1987) 908–910. [2] M. Tomita, M. Murakami, High-temperature superconductor bulk magnets that can trap magnetic fields of over 17 T at 29 K, Nature 421 (2003) 517–520. [3] M. Murakami, Electromagnetic applications of melt-processed YBCO, Ceram. Int. 23 (1997) 203–207. [4] N. Koshizuka, R&D of superconducting bearing technologies for flywheel
15842
X. Wei et al. / Ceramics International 42 (2016) 15836–15842
energy storage systems, Physica C 445–448 (2006) 1103–1108. [5] P. Tixador, X. Obradors, R. Tournier, T. Puig, D. Bourgault, X. Granados, J. M. Duval, E. Mendoza, X. Chaud, E. Varesi, E. Beaugnon, D. Isfort, Quench in bulk HTS materials- application to the fault current limiter, Supercond. Sci. Technol. 13 (2000) 493–497. [6] S.I. Yoo, H. Fujimoto, N. Sakai, M. Murakami, Melt-processed LRE-Ba-Cu-O superconductors and prospects for their applications, J. Alloy. Compd. 250 (1997) 439–448. [7] D.U. Gubser, Superconducting motors and generators for naval applications, Physica C 392–396 (2003) 1192–1195. [8] S. Marinel, J. Wang, I. Monot, M.P. Delamare, J. Provost, G. Desgardin, Topseeding melt texture growth of single-domain superconducting YBa2Cu3O7 δ pellets, Supercond. Sci. Technol. 10 (1997) 147–155. [9] W.M. Yang, L. Zhou, Y. Feng, P.X. Zhang, M.Z. Wu, C.P. Zhang, J.R. Wang, Z. H. Du, F.Y. Wang, Z.M. Yu, X.Z. Wu, W. Gawalek, P. Gornert, The effect of excess Y2O3 addition on the levitation force of melt processed YBCO bulk superconductors, Physica C 305 (1998) 269–274. [10] L. Zhou, S.K. Chen, K.G. Wang, X.Z. Wu, P.X. Zhang, Y. Feng, Synthesis of ultrafine Y2BaCuO5 powder and its incorporation into YBCO bulk by powder melting process, Physica C 363 (2001) 99–106. [11] T. Yamashita, J.A. Alarco, J. Riches, A. Bhargava, J.C. Barry, I.D.R. Mackinnon, Binder effect on microstructure and properties of YBa2Cu3O7 x extruded wires, Physica C 298 (1998) 159–165. [12] N.N. Guo, M.C. Leu, Additive manufacturing: technology, applications and research needs, Front. Mech. Eng. 8 (2013) 215–243. [13] G.M. Lous, I.A. Cornejo, T.F. McNulty, A. Safari, S.C. Danforth, Fabrication of piezoelectric ceramic/polymer composite transducers using fused deposition of ceramics, J. Am. Ceram. Soc. 83 (2000) 124–128. [14] M. Allahverdi, S.C. Danforth, M. Jafari, A. Safari, Processing of advanced electroceramic components by fused deposition technique, J. Eur. Ceram. Soc. 21 (2001) 1485–1490. [15] Z.S. Macedo, M.H. Lente, J.A. Eiras, A.C. Hernandes, Dielectric and ferroelectric properties of Bi4Ti3O12 ceramics produced by a laser sintering method, J. Phys.: Condens. Matter 16 (2004) 2811–2818. [16] H. Yves-Christian, W. Jan, M. Wilhelm, W. Konrad, P. Reinhart, Net shaped high performance oxide ceramic parts by selective laser melting, Phys. Procedia 5 (2010) 587–594. [17] T. Chartier, C. Chaput, F. Doreau, M. Loiseau, Stereolithography of structural complex ceramic parts, J. Mater. Sci. 37 (2002) 3141–3147. [18] Y. Zhang, X. He, S. Du, J. Zhang, Al2O3 ceramics preparation by LOM (Laminated Object Manufacturing), Int. J. Adv. Manuf. Technol. 17 (2001) 531–534. [19] E.B. Secor, P.L. Prabhumirashi, K. Puntambekar, M.L. Geier, M.C. Hersam, Inkjet printing of high conductivity, flexible graphene patterns, J. Phys. Chem. Lett. 4
(2013) 1347–1351. [20] X. Lu, Y. Lee, S. Yang, Y. Hao, R. Ubic, J.R.G. Evans, C.G. Parini, Fabrication of electromagnetic crystals by extrusion freeforming, Metamaterials 2 (2008) 36–44. [21] T. Huang, M.S. Mason, X. Zhao, G.E. Hilmas, M.C. Leu, Aqueous-based freezeform extrusion fabrication of alumina components, Rapid Prototyp. J. 15/2 (2009) 88–95. [22] J. Powell, S. Assabumrungrat, S. Blackburn, Design of ceramic paste formulations for co-extrusion, Powder Technol. 245 (2013) 21–27. [23] M.S. Mason, T. Huang, R.G. Landers, M.C. Leu, G.E. Hilmas, Aqueous-based extrusion of high solids loading ceramic pastes: process modeling and control, J. Mater. Process. Technol. 209 (2009) 2946–2957. [24] M. Agarwala, D.L. Bourell, A. Manthiram, B.R. Birmingham, H.L. Marcus, High Tc dual phase Ag-YBa2Cu3O7 x composites prepared by selective laser sintering and infiltration, J. Mater. Sci. 30 (1995) 459–464. [25] D. Ponnusamy, K. Ravi-Chandar, Extrusion of superconducting wires of YBa2Cu3O7 x, J. Mater. Res. 8 (1993) 268–273. [26] R. Enomoto, M. Tamaki, S. Ohno, M. Fuwa, H. Takei, Fabrication of Y-Ba-Cu-O superconducting wire by the powder-sol method, Jpn. J. Appl. Phys. 28 (1989) 1207–1209. [27] H. Varma, P. Mukundan, K.G. Warrier, A.D. Darmodoran, U. Balachandran, Nonaqueous cold extrusion as a route for yttrium barium copper oxide wires, J. Am. Ceram. Soc. 73 (1990) 3100–3102. [28] H.J. Scheel, F. Licci, Crystal growth of YBa2Cu3O7 x, J. Cryst. Growth 85 (1987) 607–614. [29] C. Vipulanandan, S. Salib, Mechanical and physical properties of sintered YBCO-metal bulk composites with silver powder and fibres, J. Mater. Sci. 30 (1995) 763–769. [30] M.A.A. Moussa, S.M. El-Hefnawy, M.M. Abou-Alkheer, H.I.A. Al-Nattar, Melt processing of YBCO thick films fabricated by sol gel route powder on YSZ, J. Supercond. Nov. Magn. 22 (2009) 479–485. [31] S. Suasmoro, M.F. Khalfi, A. Khalfi, G. Trolliard, D.S. Smith, J.P. Bonnet, Microstructural and electrical characterization of bulk YBa2Cu3O7 δ ceramics, Ceram. Int. 38 (2012) 29–38. [32] T. Aselage, K. Keefer, Liquidus relations in Y–Ba–Cu oxides, J. Mater. Res. 3 (1988) 1279–1291. [33] G. Blatter, M.V. Feigel'man, V.B. Geshkenbein, A.I. Larkin, V.M. Vinokur, Vortices in high-temperature superconductors, Rev. Mod. Phys. 66 (1994) 1125–1388. [34] K. Iida, N.H. Babu, Y. Shi, D.A. Cardwell, Seeded infiltration and growth of large, single domain Y–Ba–Cu–O bulk superconductors with very high critical current densities, Supercond. Sci. Technol. 18 (2005) 1421–1427.