m ulk Hiroyuki Fujimoto *, Hitoshi Ozaku superconductors have a high critical current den- sity, Jc at 77 K, and high magnetic field [1–4]. In the case of light rare-earth (LRE: e.g. Nd, Sm, Eu, Gd)–Ba–Cu–O bulk superconductors, melt-pro- A high Jc and a large current loop size are the essential requirements for large scale engineering applications of REBa2Cu3Oy (RE123) supercon- ducting bulks [1–5]. However, there still exist some problems to prevent the achievement of stable production of large single-domain RE123 bulks. (2004 * Corresponding author. Tel.: +81-42-573-7297; fax: +81-42- Applied Superconductivity Laboratory, Materials Technology Division, Railway Technical Research Institute, 2-8-38 Hikari-cho, Kokubunji-shi, Tokyo 185-8540, Japan Received 29 October 2003; accepted 19 January 2004 Available online 2 July 2004 Abstract Melt-processed REBaCuO (RE: rare-earth) superconductors have a high Jc at 77 K and high magnetic field, and light rare-earth (LRE) BaCuO has a larger Jc than Y123 in high magnetic field. These materials are expected to be applied to high field applications such as a superconducting permanent magnet with the liquid nitrogen refrigeration, flywheels, current leads, and so on. In this study, we discuss SmBaCuO bulks melt-processed in air and their charac- teristic superconducting and mechanical properties. Isothermal undercooling growth in air with oxygen annealing and Nd123/MgO thin film cold seeding technique were applied in SmBaCuO/Ag system to seek the high efficiency of process, homogeneity of composition, and feasibility of batch production. Single-domain growth of superconducting phases of a square larger than 15 mm on a side and 10 mm in thickness was achieved using this technique. The result implies that melt-processed in air applying isothermal method and thin film seeding in Sm system is feasible for pro- ducing larger bulks in large scale applications. � 2004 Published by Elsevier B.V. PACS: 74.72.Bk Keywords: Sm–Ba–Cu–O; Melt-process; Isothermal solidification; Nd123/MgO thin film; Single-domain 1. Introduction Melt-processed rare-earth (RE: Y or rare-earth elements)–Ba–Cu–O high critical temperature, Tc cessed in a reduced oxygen atmosphere, named oxygen-controlled-melt-growth (OCMG) process, Tc and Jc are reported to be higher than those of RE–Ba–Cu–O superconductors [3,4]. Superconducting and of Sm–Ba–Cu–O b Physica C 412–414 573-7360. E-mail address:
[email protected] (H. Fujimoto). 0921-4534/$ - see front matter � 2004 Published by Elsevier B.V. doi:10.1016/j.physc.2004.01.085 echanical properties superconductors ) 657–661 www.elsevier.com/locate/physc LRE123 system has a wide range of RE–Ba solid solution, the substitution of Ba by rare-earth the mechanical properties of the Sm123 bulks, crosshead speed of 0.05 mm/s by the material ysica such as coefficient of thermal expansion, bending strength, hardness, and surface roughness. 2. Experimental � 20–30 mm · 5–10 mm size cylindrical precur- sor pellets were prepared with nominal composi- tion of SmBa2Cu3Oy, 5–40 mol% Sm2BaCuO5 (Sm211), 10 wt% Ag2O, and 0.5 wt% Pt, using Sm2O3, BaCO3, CuO raw powders. The (0 0 1)Nd123/MgO thin film was placed on the top of the pellets at room temperature before a ther- mal cycle, i.e., a cold seeding method was used. A maximum temperature (Tmax) of 1333–1373 K and a holding temperature (Th) of e.g., 1293 K, corre- sponding to an undercooling degree of DT ¼ 5 K was used. After a given holding time (th) of 5–20 h, the sample was quenched or rapidly cooled to room temperature. The c-axis of the crystal is al- most perpendicular to the surface of the bulk sample [13]. Trapped magnetic field distribution of the Sm123/Ag samples was measured at 77 K after the field cooling process, using a scanning Hall sensor. elements [3], and RE–Ba substitution deteriorates superconducting properties, and it should be sup- pressed. We pay our attention to the isothermal solidification growth. While the isothermal solidi- fication growth process provides us with the opportunity to obtain homogeneous composition, good electromagnetic and mechanical properties as well as the feasibility of batch production, it requires a challenge to control spontaneous nucleation and multiple growths [6–9]. We exam- ined the Nd123/MgO thin film seeding technique to seek stable production of bulk superconductors and to obtain a large single-domain bulk [1,5,10]. In this study, Ag added Sm123 textured single- domain superconducting bulks were fabricated to seek stable production, using Nd123/MgO thin film as a cold seeding [11,12], by processing with oxygen annealing after an isothermal melt-process in air. Then superconducting properties of the samples, such as Tc, Jc, and trapped magnetic field at 77 K, were characterized. We also characterized 658 H. Fujimoto, H. Ozaku / Ph We applied a specially treated micro Hall probe. testing machine. The hardness was obtained by using the Vickers hardness machine in accordance with JIS-Z-2244 [15]. Further the surface rough- ness was obtained by following JIS-B-0601 [16]. Mechanical properties such as porosity and den- sity were also measured. All the measurements were conducted at room temperature in air, except the thermal expansion. 3. Results and discussion 3.1. Sm123/Ag superconductors We investigated the crystal growth rate of the samples, changing the Sm211 mol% for the start- ing composition with 10 wt% Ag2O. For SmB- aCuO, the crystal growth rates in the a or b-axis direction of the crystal with 10 wt% Ag2O as a starting composition are around 1.6 mm/h for Sm1:4Ba2:2Cu3:2Oy and Sm1:8Ba2:4Cu3:4Oy at Tmax ¼ 1373 K, Th ¼ 1291 K, and DT ¼ 7 K. It is obvious that the crystal growth rate of SmBaCuO is larger than that of YBaCuO. With increasing the Sm211 contents, the growth rates of Sm123 bulks increase [13]. Fig. 1 shows the optical image for Nd123/MgO thin film seeded single-domain Sm123 samples; The active area of the sensor is 50 lm · 50 lm. Then the critical temperature, Tc and the critical current density, Jc of the samples were character- ized by a superconducting quantum interference devise (SQUID) magnetometer. Jc was calculated using magnetic hysteresis obtained by magnetiza- tion measurements. Mechanical properties of the Sm123/Ag sam- ples, such as the thermal expansion, the bending strength, the hardness, and the surface roughness were also measured. The coefficient of thermal expansion was measured at between 173 and 373 K, using the Thermomechanical Analyzer, TMA (Rigaku). Then the flexural strength of the sample was measured by the three-point flexure test, par- tially following JIS-R-1601 [14]. The JIS is abbreviation of the Japanese Industrial Standards. Three-point flexure measurements were made at a C 412–414 (2004) 657–661 Sm1.7, when changing the Sm211 volume Fig. 2. Trapped magnetic field distribution at 77 K of Nd123/ MgO thin film seeded single-domain Sm123 samples; Sm211 ¼ 35 mol%, Tmax ¼ 1373 K, Th ¼ 1293 K. 30000 40000 50000 60000 c (A /c m 2 ) Hex // c-axis 66 K ysica fractions. In this case, Sm211 ¼ 35 mol%, Tmax ¼ 1373 K, Th ¼ 1293 K, DT ¼ 5 K, and the holding period ðthÞ ¼ 10–20 h. Fig. 1. Optical images of Nd123/MgO thin film seeded single- domain Sm123 samples, Sm211 ¼ 35 mol%, Tmax ¼ 1373 K, Th ¼ 1293 K. H. Fujimoto, H. Ozaku / Ph 3.2. Electromagnetic properties of Sm123/Ag Fig. 2 shows the trapped magnetic field distri- bution of the Sm123 sample with 35 mol% Sm211 with 10 wt% Ag2O(Sm1.7). The size of the crystal is around 13–15 mm side length, and the maximum trapped magnetic field is around 2000 G. The profiles of contour lines are circular and smooth. This implies that the Sm1.7 sample consist of one domain. The temperature dependence of the magnetic susceptibility of the Sm1.7 sample at zero-field- cooling (ZFC) process was measured in a field of 10 Oe, indicating Tc;onset and Tc;zero are 94 and 90 K, respectively. We also measured the magnetic field dependence of Jc in the Sm1.7 sample. Concerning about the direct current (DC) magnetization, we can analyze magnetization on the assumption that on macroscopic scale, the critical state model works with a good approximation. The reason is that we can measure magnetization after the relaxation process of the internal flux density dis- 10 30 0 500 1000 1500 2000 2500 B tr ap , G au ss 0 20 0 1 0 2 0 3 0 Sample size 20 mm C 412–414 (2004) 657–661 659 tribution almost converges. In this case, we need relations between magnetization and Jc, based on the extended Bean’s model to obtain Jc of the bulk samples. We can apply general two-dimensional geometric relations between magnetization and Jc [17]. The profiles of the magnetic field dependence of Jc in the Sm1.7 sample with 10 wt% Ag2O at 66 and 77 K show a typical peak effect in the LRE system, as shown in Fig. 3. The maximum Jc of the sample is 1–2 · 104 A/cm2 at around 1–2 T. 3.3. Mechanical properties of Sm123/Ag Mechanical properties of the SmBaCuO/Ag sample were measured, and all the specimens for 0 10000 20000 0 1 2 3 4 5 6 7 Magnetic Field, Hex (T) J 77 K Fig. 3. Magnetic field dependence of the critical current den- sity, Jc-B of the Sm1.7 superconductors. ysica the measurements were machined from the iden- tified sample, Sm1.7 for the trapped field and the SQUID measurements. Obtained values of the mechanical properties are shown in Table 1. First, the coefficient of thermal expansion of Sm1.7 is 9.44 · 10�6 K�1, parallel to the (ab) plane, and 1.44 · 10�5 K�1, parallel to the c-axis between 173 and 273 K. A compressive load method was applied, and a standard sample of SiO2 with L ¼ 10–20 mm, t ¼ 5 mm was used. Temperatures for the measurement are between 173 and 373 K with a rate of 5.0 K/min. The sample for the measurement is 7.4 mm · 7.0 mm · 1.6 mm (par- allel to the c-axis). Second, the three-point flexural strength of the sample was measured by following JIS-R-1601 at room temperature in air [18,19]. The size of the specimen is 2 mm · 1 mm · 7 mm (width, thick- ness, length), and the number of specimens is four. Table 1 Mechanical properties of SmBaCuO bulk, Sm1.7; thermal expansion, flexural stress, hardness, Hv, and surface roughness Mechanical properties Values Coefficient of thermal expansion: at 173–373 K 9.44· 10�6 K�1 (ab) plane, at 173–273 K 1.44· 10�5 K�1 c-axis, at 173–273 K Bending strength: three- point at room temperature 60–110 MPa, load ? (ab) plane Hardness: Vickers, Hv at room temperature 200–450, on the (ab) plane Surface roughness: Ra 2–4 lm 660 H. Fujimoto, H. Ozaku / Ph In this study, the bending strength in the case of the c-axis of the crystal ? the 2 mm · 7 mm surface was obtained. Flexural stress was obtained using the equation: 3PL=2bh2 (MPa), where P is load at fracture, (N), L is the distance of supporting span, (m), b is the width of specimen, (m), and h is the thickness of specimen, (m). The result shows that the bending strength by three-point test at room temperature is between 60 and 110 MPa, for the load ? the (ab) plane. Third, the Vickers hardness, Hv was measured at room temperature in air, following JIS-Z-2244 [19]. Load for the measurements is between 5 and 10 kf, a moving rate is 50 mm/s. Duration time which is defined by the time after indentation is 4. Conclusions Isothermal solidification growth with a Nd123/ MgO thin film seed is used to fabricate textured Ag-doped Sm–Ba–Cu–O superconducting bulks in order to ensure stable production of large single- domain bulks. The effects of Tmax, Th and Sm211 contents on the crystal growth were investigated. The result shows that single-domain growth can be achieved with increasing the Tmax. Single-domain growth of superconducting phases of a square larger than 15 mm on a side and 5–10 mm in thickness was achieved using this technique. We also discuss the characteristic supercon- ducting and mechanical properties of Sm123/Ag bulks. The trapped magnetic field distribution, Tc and Jc of the Sm123 samples are shown, and mechanical properties of the Sm123/Ag sample such as thermal expansion, bending strength, hardness, Hv, and surface roughness were mea- sured. The result implies that, for producing larger bulks in large scale applications [20–23], the melt- made, is 15 s. The number of the samples is eight. The number of the indentations is 5–10 per spec- imen. Hv was obtained by using the equation: P=S ¼ 1:8544 P=d2, where P is the indentation load in kgf, S is the area of the indentation in mm2, and d is the average length of the indentation diagonals in mm. The result shows that Hv on the (ab) plane is between 200 and 450. Forth, the surface roughness of Ra (average roughness) of the sample was measured by a pro- file method using a contact stylus, following JIS-B- 0601 at room temperature in air [19]. Conditions for the measurements are as follows: evaluation length is 8.00 mm, reference length is 0.25–0.80 mm, sampling length is 0.8 mm, longitudinal magnification is 5000, lateral magnification is 10 or 50, and drive speed is 0.5 mm/s. The number of the samples is four, and the number of measurements is five per specimen. Ra;average is 2 lm for the top and bottom surfaces of the Sm1.7 bulk, on the other hand, 4 lm for the side surfaces of the samples. C 412–414 (2004) 657–661 process in air by isothermal solidification method and thin film seeding in Sm system is effective as well as YBaCuO. References [1] M. Murakami, Melt Processed High-Temperature Super- conductors, World Scientific, Singapore, 1992, p. 21, p. 68, p. 211, p. 309, p. 334. [2] H. Fujimoto, M. Murakami, S. Gotoh, N. Koshizuka, T. 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[22] H. Fujimoto, H. Kamijo, T. Higuchi, Y. Nakamura, K. Nagashima, M. Murakami, S.I. Yoo, IEEE Trans. Appl. Supercond. 9 (2) (1999) 301. [23] H. Fujimoto, 2001 National Convention Record, The Institute of Electrical Engineers of Japan (IEEJ), (3.2001), S18-6, 2001 (in Japanese). Superconducting and mechanical properties of Sm-Ba-Cu-O bulk superconductors Introduction Experimental Results and discussion Sm123/Ag superconductors Electromagnetic properties of Sm123/Ag Mechanical properties of Sm123/Ag Conclusions References