Abstract
The present investigation focuses on the microstructure and chemical analysis of the in-air processed SmBa 2 Cu 3 O y (Sm-123) sample with 3 wt% BaO 2 and 20 wt% Ag 2 O addition. DC magnetization measurements showed a sharp superconducting transition with onset T c at around 94.5 K. The critical current density (J c ) at 77 K and zero field was 24 kA/cm 2 . SEM analysis indicated dispersion of submicron secondary phase particles in the Sm-123 matrix. SEM by EDX analysis confirmed that matrix chemical ratio at various locations of the pellet was uniform. Trapped magnetic field profile analysis confirmed that samples were single grain. The sample with size of 12 by 12 mm and 5 mm thickness showed the trapped field on top surface 0.26T at liquid nitrogen temperature. The results indicate that content of BaO 2 is crucial for improving utilization parameters of the bulk Sm-123 material melt-processed in air.
Introduction
The light-rare-earth oxygen controlled melt growth "OCMG" LRE-Ba 2 Cu 3 O y (LRE: Nd, Eu, Sm, Gd) "LRE-123" superconductors, both single, binary or ternary, have exhibited currently the highest critical current densities at 77 K in self-field [1] and demonstrated capability of trapping magnetic field as high as 17.5 T at 29 K (about 1.5 T at 77 K) [2] . However, the performance decreases dramatically above 77 K. It is possible to produce high performing superconductors by introducing relatively large (micrometer or sub-micrometer) non-superconducting secondary phase particles, LRE 2 BaCuO 5 "LRE-211", in the LRE-123 material [3] . These particles proved to be effective pinning centres especially at low magnetic fields. Their efficiency is inversely proportional to the average particle size. Thus, reducing the LRE-211 particle size with ball milling, it was possible to improve the flux pinning performance [4] up to very high temperatures. This material showed very high critical current density up to the boiling temperature of liquid oxygen [5] . With this material we could levitate a permanent ericwilliams@example.org K and as a result, a remarkable flux trapping, by an order of magnitude higher than in the best classical hard (ferro) magnets, was achieved [6] . These new materials could be utilized as a new class of high temperature superconducting super-magnets for a wide range of commercial and industrial applications.
Introduction
These results indicate that the ideal material for the superconducting super-magnet would be high-T c LRE-123.
Introduction
However, the OCMG technique regularly used for LRE-123 bulk compounds production is rather complicated and makes the final cost rather high. Therefore, the main challenge for the superconductor technologists' community is to reduce the fabrication cost. For mass production batch process it is essential to optimize the processing conditions, especially in the case of Y-123 and Gd-123 materials [7, 8] . Recently, we successfully produced high performance Gd-123 material in air, enriching it with Ba. The superconducting transition measured at various positions of the batchprocessed single-grain Gd-123 material was sharp (around 1 K width) with the highest onset T c around 93.5 K, similar to the samples melt-processed in a reduced oxygen atmosphere. The self-field J c of 70 kA/cm 2 was achieved in various positions of the pellet at 77 K with H//c-axis. The trapped field observed in the best 45 mm single-grain puck of Gd-123 was in the range of 1.35 T and 0.35 ericwilliams@example.org K and 87.3 K, respectively [8] . Further, SmBa 2 Cu 3 O y "Sm-123" pellet was recently produced in air with combined additions of Ag 2 O and BaO 2 with the aim to collect experience for batch production of Sm-123 without decay of the pinning performance [9] . Magnetization measurements showed the onset T c of 92 K in air-processed Sm-123 sample with 3 wt% BaO 2 and 20 wt% Ag 2 O additions. The critical current density values measured at 77 K were, however, very low compared to those processed by OCMG process. In order to prepare bulk Sm-123 for engineering applications melt-processed in air, a further improvement of flux pinning was essential. For this, microstructural studies and chemical analysis of Sm-123 matrix is crucial.
Introduction
In the present study, we focused on production of high-performance Sm-123 superconducting materials in air using MgO seeds. Magnetization and SEM analysis showed that high quality Sm-123 material can be produced in air. SEM by EDX analysis confirmed that matrix chemical ratio at various locations of the pellet was quite uniform.
Experimental
The powders of Sm 2 O 3 , BaO 2 , and CuO were weighted in the nominal composition of Sm 2 BaCuO 5 "Sm-211" and calcined twice at 870º C and 890º C for 8 h. Finally, the Sm-211 powders were calcined at 910º C for 4 h to control the grain size. In the second stage of the process the Sm-123 and Sm-211 powders were mixed in the molar ratios of Sm123:Sm211=2:1. More details on the Sm-123 processing can be found in Ref. [9] . In order to suppress coarsening of the Sm-211 particles during the melt process, 0.2 wt% Pt and 0.25 wt% CeO 2 were added. Moreover, 3 wt% of BaO 2 was added to suppress Sm/Ba solid state substitution in the superconducting phase composition, similar to our earlier results [9] . 20 wt% of Ag 2 O was also added to improve the mechanical performance of the material. Finally, well-mixed powders were pressed into pellets of 20 mm diameter and 10 mm thickness, which were consolidated by uniaxial pressure of 5 Tone for 5 minutes. DTA measurements were performed in air to determine the peritectic decomposition temperature, T p . This temperature was then used to schedule the heat treatment profile of the melt growth process. Eventually, commercial thin film Nd-123 seeds grown on MgO <100> substrates were used as seed crystals. The Sm-123 pellets were placed on slim Sm-211 rods inside the small box furnace and melt processed in air. The heat treatment profile used in the present experiment was as follows: The sample was heated to the temperature 80 o C above T p within 5 h and held there for 50 min. Then the temperature was reduced during 30 min to 2 o C above T p , and slowly decreased by 25 o C with a cooling rate of 0.20 o C/h. Finally, the temperature was reduced at a cooling rate of 20 o C/h to 100 o C and then the furnace was left to cool down to room temperature. The melt textured samples were annealed at 350-400 o C for 250 hours in flowing pure O 2 gas. Small test specimens in size of 1.5 x 1.5 x 1.0 mm 3 were cut from the bulk samples. Measurements of critical temperature (T c ) and magnetization hysteresis loops (M-H loops) in fields from -2 to +5 T were measured at 77 K using a commercial SQUID magnetometer (Quantum Design, model MPMS5). The magnetic J c values were estimated based on the extended Bean critical state model [10] . The microstructure of these samples was studied by means of scanning electron microscopy (SEM) and chemical composition of the matrix was analyzed by energy dispersive X-ray spectroscopy (EDX). Figure 1 (left) presents the temperature dependence of the dc susceptibility in the field cooled process in magnetic field of 1mT. The sample exhibited a sharp superconducting transition around 94.5K, which is similar to the samples processed by OCMG [12] . The superconducting transition width was less than 1.5 K, indicating that the sample is of a good quality (see Fig., 1 right) . This is good news because now it is possible to make bulk Sm-123 materials processed in air. When Sm-123 material is melt-processed in air, then Sm 1+x Ba 2-x Cu 3 O y clusters are also formed. The compositions with large values of x lead to a destruction of superconductivity due to the decrease in the carrier concentration. The formation of excessive Sm 1+x Ba 2-x Cu 3 O y clusters is avoided by utilizing low p(O 2 ) in OCMG process or employing Ba-rich mixtures in the in-air growth. Recently, we observed that 1 wt% of BaO 2 addition to the Gd-123 system grown in air resulted in superior critical current densities at 77 K, H//c-axis [8] . The critical current density of the present material is in Fig. 1 (right) . One can see on the figure peak effect, similar to the OCMG processed Sm-123 material. The J c reached at 77 K in the self-field 24 kA/cm 2 . This value is still low compared to the OCMG processed Sm-123 material. Further experiments are necessary to optimize the processing parameters and to control the size of Sm-211 secondary phase, to make the in-air processed Sm-123 bulk material processed in batch production competitive also with its electromagnetic performance. Fig. 1 The temperature dependence of dc susceptibility for the Sm-123 bulk sample produced in air (left). The measurements were performed with a FC warming procedure at 10 mT magnetic field. The critical current (Jc) performance of the same material at liquid nitrogen temperature (right). Figure 2 shows the optical micrographs of the polished surface for the bulk Sm-123 material melt processed in air. The distribution of the Sm-211 and Ag particles in the bulks is quite uniform. The scanning electron microscopy results are presented in Fig. 2 (right) . The sample contains fine sub-micron particles along with larger ones. The dispersion of the secondary phase particles in the Sm-123 matrix is uniform (see Fig. 3, right) . Control of the secondary phase size and dispersion in the Sm-123 matrix is evidently crucial for a further improvement of the in-air processed material. Table 1 .
Results and discussion

Superconducting performance

Optical microscopy, scanning electron microscopy (SEM) and EDX analysis.
Further, trapped field was measured on the whole pellets of the Sm-123 material. The measurements showed that even a small sample of Sm123 can trap magnetic field similar to the OCMG processed material. As a result, 12 mm by 12 mm and 5 mm thick samples exhibited trapped field of 0.26T on the sample surface at liquid nitrogen temperature (see Fig. 5 ). Higher values can be reached by increasing the diameter and thickness of the material. In the present stage, 3 wt% of BaO 2 addition was found optimal in Sm-123 system from the point of view of superconducting transition temperature (95 K). Also quite high trapped field values were observed. The results clearly indicate that a refinement of the Sm-211 starting powder is very important to produce the sub-micron Sm-211 particles in the final Sm-123 matrix. The critical current performance at 77 K is still rather low compared to the OCMG processed Sm-123 material. Further experiments are necessary to optimize the processing parameters and to control the size of Sm-211 secondary phase to improve electromagnetic properties of the in-air processed Sm-123 bulk material prepared by batch production. Fig. 4 . Trapped field profile at liquid nitrogen temperature for the bulk melt processed Sm-123 single grain disk 12 mm by 12 mm, fabricated in air by cold seeding method using Nd-123/MgO film as a seed crystal.
Summary
We have studied the superconducting transition, critical current density, and chemical composition of the meltgrown bulk Sm-123 material processed in air. The bulk Sm-123 samples fabricated in air exhibited a sharp superconducting ericwilliams@example.org, showing that the sample was of high quality. The trapped field of 0.26 T was recorded in the sample with 12 mm by 12 mm at liquid nitrogen temperature. The SEM by EDX analysis performed on the sample indicated that the sample was quite uniform. We proved that good quality bulk Sm-123 materials can be produced in air by batch process (in large quantities), reducing thus the cost in an effective manner. The superconducting transition is sharp and high, the trapped field value is high. Some further research is needed to optimize the electromagnetic performance.

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