Improvement of MTG Process for Preparation and its property of YBCO Superconductor

To improve the superconductivity of high temperature YBCO superconductor, it was prepared and examined by MTG (melt-textured growth) process with use of SmBaCuO seed crystal for orientation, additives of Ag and 211 phase for current density and flux pinning, and heat treatment for oxygen absorption. SmBaCuO single crystals were prepared by MTG process with oxygen absorption and analyzed by XRD and Laue technique. It was estimated that optimum value of Y2BaCuO5 (211 phase) contents in the textured YBCO superconductor as flux pinning center was about 20wt.%. Ag contents have no influence on the critical temperature but have large influence on the critical current density of the


Introduction
In recent years, it was investigated on the improvement of critical current density for the high temperature superconductors of YBCO, SmBCO and YbBCO, respectively in the Lab. and presented in the conventional journal, but it would be discussed for the improvement of MTG process limited only for the YBCO superconductor.
There are some extra non-superconducting 211 phase of Y 2 BaCuO 5 in the melt-condensed process due to the loss of the liquid phase (BaCuO 2 +CuO), which was caused by the slow cooling rate in MTG process. Some works show that the 211 phase of fine particles, as an additive material, are effective in flux pinning center of YBCO superconductor [1,2], and the critical current density J c could be increased with the increase of 211 phase content [3]. But there are some opposite points of view which say that either J c has no relation with 211 *Corresponding author. Tel : +82-335-330-6371, Fax : +82-335-3367244 E-mail : dwhsoh@wh.myongji.ac.kr phase or it declines with increasing 211 phase contents [4,5].
And the research for the effect of Ag in the sintered YBCO shows that the correct amount of Ag can eliminate the micro-cracks in the YBCO and improve the weak link of grain boundaries [6,7]. For the textured YBCO made by MTG, it was found in this study that 10 15wt.% of Ag was useful added in the YBCO matrix [8,9]. It was also estimated that J c increased with Ag contents in YBCO, but there was a maximum value of Ag contents in which J c would be start to decrease with increasing Ag contents [10]. The solubility of Ag in the textured YBCO was determined as about 15wt.% in the experiment and it did not have any influence on the critical temperature (T c ) of YBCO, but did on the critical current density (J c ) effectively.
In this paper, it was measured and experienced that the critical current density of textured YBCO with additives in different contents of 211 phase and Ag, tested the regularity of the effects on the additives of 211 phase and Ag on the superconductivity. It is very difficult to grow the textured YBCO when the Ag content is more than 4wt.% in the composition [11]. So the small pieces of the bulk SmBaCuO single crystal were prepared to be used as the seeds, which could induce the textured growth of YBCO in the melted process. The oxygen absorption process was also studied in various temperatures and times which is very important for the superconductivity of YBCO.

Growth of SmBa 2 Cu 3 O x crystals
It was also prepared by melt-condensed method for use of seed crystals SmBaCuO and NdBaCuO for the MTG process of YBCO but it was more difficult to make large pieces of single crystal with NdBaCuO than SmBaCuO [12,13].
The melting point of SmBaCuO is 50 °C higher than that of YBCO. Therefore the single crystal of SmBaCuO can be used enough in temperature as the seed crystal for the growth of the textured YBCO. The 5×5×2 mm 3 single crystal pieces of SmBaCuO can be obtained from the grown bulk crystal by cleaving.

Preparation of textured YBCO by MTG method
The MTG process for YBCO was taken place in a Ni-Cr resistance wire furnace with a constant temperature zone of 60mm, which was controlled by a Pt-Pt/Rh thermocouple. The mixture of YBCO(80wt.%), Y 2 BaCuO 5 (10wt.%) and Ag(10wt.%) was placed on the Alumina substrate of high quality. The sample was heated to 1080 °C in 2h, held at 1080 °C for 1h, reduced to 1030 °C in 20 min., and reduced to 980 °C at the rate of 1-2 °C per hour, then reduced to 400°C at the cooling rate of 100 °C per hour, The oxygen absorption was taken place at 400 °C under O 2 environment for about 24 hours.

Determination of 211 phase contents in the textured YBCO
X-ray diffraction analyses were performed for six samples of the sintered YBCO with 5, 10, 15, 20, 30, 40wt.% of 211 phase contents under the same test condition. <110> and <013> peaks of 123 phase, and <311> peak of 211 phase were selected to be characteristic peaks. The curve of I 211 / I 211 +I 123 versus 211 phase contents was drawn in Fig.1, in which I 211 and I 123 were the integrated peak intensities of above selected peaks, and the relation of J c and 211 phase of contents was shown in Fig. 2. The relation in Fig. 1 will be used as the reference for the calculation of 211 phase contents in textured YBCO.

Properties of Ag in YBCO
The black Ag 2 O powder, produced from the decomposition of Ag 2 CO 3 , was mixed in YBCO powder with several mixing ratio of contents before MTG process. It was observed that the melting point of Ag doped YBCO decreased with increasing Ag contents and the solubility of Ag in YBCO is about 15wt.% which were important references of the MTG program designing for preparation of textured YBCO. The particle size and the distribution in the textured YBCO was observed by SEM and the Ag contents were measured by Electronic Energy Spectroscopy.

The measurement of superconductivity
The magnetic hysteresis curves of the samples were measured by the Vibrating Sample Magnetometer at 77K and the critical current densities were calculated by means of Bean's model with respect to the preparing conditions. The critical temperatures were also determined by measuring the AC susceptibility of the samples.

Growth of SmBaCuO seed crystal
To induce the textured YBCO in MTG process, the pieces of SmBa 2 Cu 3 O x single crystals with 5×5×2mm 3 were also prepared by MTG process and it was proved by X-ray diffraction and X-ray back reflection Laue technique. The oxygen absorption in SmBa 2 Cu 3 O x was studied at several constant temperatures and zero resistance superconductivity at 90K was obtained after heat treated in oxygen flow at 400 °C for 96 hours.
In order to improve the superconductivity of large bulk YBCO superconductor, it should be useful taking a seed in YBCO. As the seed material, it is necessary of particular features: (1) its crystal structure should be same with YBCO, and (2) the melting point must be higher than YBCO. SmBa 2 Cu 3 O x and NdBa 2 Cu 3 O x crystals have the above particular features of ideal materials as the seed. But for the small grains of NdBa 2 Cu 3 O x than SmBa 2 Cu 3 O x by MTG process , SmBa 2 Cu 3 O x is more suitable to introduce large bulk YBCO growth in MTG process. The prepared SmBa 2 Cu 3 O x samples were well oriented with a-b planes that the all peaks of XRD patterns were 00l planes (quadratic) and confirmed by X-ray backreflection Laue technique.
It is difficult to make the SmBa 2 Cu 3 O x single crystal with 90K zero resistance by heat treatment in oxygen flow. For the SmBa 2 Cu 3 O x single crystal, the suitable temperature of oxygen absorption was confirmed by thermo-gravimetry at 400 °C. The experimental result is shown in Fig 3. From the figure, it could be seen that the process of oxygen absorption of SmBa 2 Cu 3 O x single crystal was accomplished slower than YBCO crystal, and the SmBa 2 Cu 3 O x sample was not saturated in oxygen content until it was heat treated for 72 hours. To determine the best temperature for oxygen absorption and investigate the effect of heat treatment on the critical temperature, it was heat treated in oxygen flow at 400 °C, 450 °C and 500°C for 96 hours. The critical temperature of the samples was measured by the way of magnetic susceptibility, the results are shown in Fig. 4   In Fig.5, it can be seen that the critical temperatures of SmBa 2 Cu 3 O x single crystals which were heat treated at 400 °C, 450 °C and 500 °C for 96 hours were 91K, 86K and 77K, and it was decreased with increasing the heat treatment temperature. When the heat treatment temperature was increased more than 550 °C and 600 °C, the critical temperature could not be measured in liquid nitrogen.

The Orientation of YBCO
In the test, the oriented YBCO was induced by SmBaCuO seed from the molten YBCO. Therefore the position of SmBaCuO seed is quite important for the orientation of YBCO. The a-b plane (001) of the SmBaCuO seed which was carefully cloven from the bulk crystal should be paralleled with the surface of the melted YBCO. Fig.6(a) is the X-ray diffraction pattern of SmBaCuO seed, it is perfectly C-axis orientation.
The seed of SmBaCuO was put on the surface of melted YBCO at 1030-1050 °C, at which YBCO was stayed in melted state and SmBaCuO seed was still stayed in solid state. When the temperature of the system reached about 1010 °C, the melting point of YBCO, it would be started to crystallize around the SmBaCuO seed with the same orientation. The orientation of YBCO was tremendously in agreement with that of SmBaCuO seed, which was proved by the comparison of XRD patterns of oriented YBCO made by MTG in Fig.6(b), and of SmBaCuO seed in Fig.6(a).

The optimum value of 211 phase contents in the textured YBCO
Samples with dimension of about 10×5×1 mm 3 which were cut from textured YBCO bars were heated at 450 °C for 50 hours under flowing oxygen to absorb it. After the heat treatment, several 10 milligrams of YBCO powder were taken from each textured YBCO sample for the Xray diffraction analysis and the 211 phase contents were calculated using the curve in Fig.1. Also YBCO samples (10×5×1 mm 2 ) were used to measure the critical current densities. The 211 content and critical current density were interrelated with each other as shown in Fig.2. It can be seen that the optimum value of its contents in the textured YBCO is about 20wt.%, and before this value J c increases with increasing 211 contents, but after that value J c decreases with increasing 211 contents. It seems that as the flux pinning centers, 211 particles could make critical current density increase with 211 contents, but as the nonsuperconductive phase of 211 particles could make critical current density decrease when the volume ratio of 211 phase was too large.

Ag behavior in the textured YBCO The solubility of Ag in condensed YBCO
The added Ag contents in the sintered YBCO and the analyzed Ag contents in the condensed YBCO were shown in table 1. The analyzed values of Ag contents in condensed YBCO were in accord with the added Ag contents, if the values of added Ag contents were less than 14wt.%. But the analyzed values of Ag contents did not increase and remain as constant value of 15wt.% when the added values of Ag contents were larger than 16wt.%. This suggests that Ag contents in YBCO would be saturated, or the solubility of Ag in condensed YBCO would be 15wt.%. When Ag additions were over the solubility (15wt.%) of Ag in condensed YBCO, the extra Ag would migrate or escape from the YBCO sample into slag on the substrate. The relation between Ag contents analyzed in textured YBCO and added in sintered YBCO before melting process was shown in Fig.7. Fig.7 The relation between Ag contents analyzed in Fig.8 AC susceptibility of textured YBCO textured YBCO and Ag addition

Effect of Ag on the superconductivity of textured YBCO
The critical temperatures were determined by AC susceptibility of textured YBCO samples with different Ag contents (0, 4, 8, 12, 16, 18wt.%) as shown in Fig.8. The Ag contents did not influence on critical temperature of YBCO in the experiment, which was showed in good agreement with the reported results [14] for the sintered YBCO sample. But Ag contents had large influence on the critical current density as shown in Fig.9, in which J c was increased with increasing Ag contents up to 15wt.%, at which Jc showed a constant value of (7-8) × 10 4 A/cm 2 (0.01T,77K). The result was corresponded with the Ag solubility (15wt.%) in condensed YBCO.

Ag inclusions in YBCO superconductors
When Ag contents in YBCO was larger than 6wt.%, the diffraction peaks of Ag contents could be found in the X-ray diffraction patterns. Fig.6 shows the X-ray diffraction pattern of textured YBCO with 18wt.% of Ag contents. The intensity of Ag pattern increased with increasing Ag contents in YBCO. It means the doped Ag did not react with YBCO, but existed in the form of the inclusions in the YBCO matrix. SEM photographs [ Fig.11.(a) and (b)] of textured YBCO were observed with Ag contents of 14wt.% and 18wt.% respectively. In the pictures the Ag inclusions were distributed in YBCO matrix as particles in the range of 10-50 °C in diameters.

Oxygen absorption and current density of textured YBCO
The oxygen absorption of the oriented bulk YBCO has some different behavior due to its large volume and high density, in which oxygen diffusion is more difficult than that in small YBCO sample [10]. In the test, oriented YBCO sample of 9.76g was used under flowing O 2 . The temperatures for oxygen absorption are 380°C , 400 °C, 450 °C, 500 °C and 550 °C respectively. An electronic balance with 10 -4 g accuracy was used to record the weight change in oxygen absorption process. The best temperature for oxygen absorption was 400 °C among the results of test temperatures as shown in Fig.12. It seems that after 1300 minutes (22 hours), the oxygen absorbed in YBCO was still unsaturated. Recently, more than 100 hours was used for oxygen absorption [15]. If the time of oxygen absorption of the textured YBCO was over 100 hours, the critical current density would be larger than 7×10 4 A/cm 2 (77K, 0.01T) as shown in Fig.9.

Critical current density of the oriented YBCO
The hysteresis loops of two samples of 8.34×4.00×1.62mm 3 and 9.76×4.44×2.16mm 3 after heat treatment at 400 °C under O 2 for 24hours were measured by a Vibrating Sample Magnetometer (see Fig13). According to bean's model, J c =20⋅∆Md -1 was used to calculate the critical current density (J c ).
From the hysteresis loops of oriented YBCO after heat treatment at 400 °C under O 2 flow for 23hours, J c of both samples were calculated with 7×10 A cm -2 (77K, 0.01T). The magnetic levitation force of oriented YBCO sample, 21mm in diameter, 10mm in height, is about 4.6N⋅cm -2 in liquid nitrogen.

Conclusions
To improve the superconductivity of the high Tc superconductor, YBaCuO superconductor was prepared by MTG process with use of SmBaCuO seed crystal and additives of Ag, Y 2 BaCuO 5 (211 phase) to eliminate the weak links of the grain boundaries and micro-cracks, to increase the flux pinning effect in the bulk YBCO.
From the investigation and analysis of the experimental results, it was concluded as follows ; 20wt.% of 211 contents in textured YBCO was the optimum value in YBCO for the flux pinning center.
The solubility of Ag in condensed YBCO was about 15wt.% and J c of textured YBCO was increased with increasing Ag contents. But the Ag contents was more than 15wt.%, the J c of YBCO was tended to a stable value.
When the SmBaCuO single crystals were used as seeds to induce the growth of YBCO, the orientation of YBCO was the same as SmBaCuO seeds.
The optimum condition of heat treatment for oxygen absorption of bulk condensed YBCO was at 400 °C in O 2 flow for more than 24 hours.
The critical current density of a typical sample of oriented YBCO, which was included 20wt.% of 211 phase and 10wt.% of Ag, was obtained with the value of 7×10 4 A⋅cm -2 (77K, 0.01T).