Effects of grain boundaries on electrical and magnetic properties of melt-processed SmBa2Cu3Ox superconductors

Materials Chemistry and Physics 119 (2010) 182–187

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Effects of grain boundaries on electrical and magnetic properties of melt-processed SmBa2 Cu3 Ox superconductors Ugur Topal a,∗ , M. Eyyuphan Yakinci b a b

TUBITAK-UME (National Metrology Institute), P.K. 54, 41470 Gebze-Kocaeli, Turkey Inonu Universitesi, Fen Edebiyat Fakültesi, Fizik Bölümü, 44280-Malatya, Turkey

a r t i c l e

i n f o

Article history: Received 25 February 2009 Received in revised form 21 July 2009 Accepted 21 August 2009 PACS: 74.25.Fy 74.25.Ha 74.25.Sv 74.81.Bd

a b s t r a c t In the present study, we examine the energy dissipation mechanisms and grain boundary effects on the superconducting properties of high-Tc SmBa2 Cu3 O7−x samples. Sm-123 was synthesized with the top-seeded-melt-growth technique. Grain sizes, as determined by optical microscopy, ranged between 0.5 mm and 1.5 mm. Thus, each specimen, on which the electrical and magnetic measurements were carried out, contained several grains. Some of our findings are as follows: (1) the intragrain Tc is 93.5 K and the intergrain Tc is 87.5 K. Superconducting transition width is narrow and is not affected much by the magnetic field. (2) The intragrain Jc is 110,000 A cm−2 while the intergrain Jc is 210 A cm−2 at 10 K. (3) The intergrain Jc shows a secondary peak with the increase in temperature from 10 K to 30 K. The main reasons behind these observations are discussed in detail. © 2009 Elsevier B.V. All rights reserved.

Keywords: Superconductors Crystal growth Magnetic properties Transport properties

1. Introduction Critical current density is one of the most important properties of superconductors. For most applications of high-Tc superconductors, such as superconducting motors, magnetic bearings, flywheels, wires, etc., a high critical current density Jc is required. For instance, in order to increase the force between a magnet and a superconductor, a superconducting component with maximum induction, which is dependent on Jc and the size of the shielding current loop, is preferred. Various methods have been proposed to achieve higher critical current densities in high Tc s. Different forms of irradiation [1–5], chemical doping [6–8], texturing of grains in preferred orientations [9–11] are some examples of these methods. It is well known that oxide superconductors are very anisotropic and charge carriers can only move along the CuO2 planes. Because of the large anisotropy and short coherence length (a few angstroms), coupling between CuO2 planes becomes weak and super currents cannot flow across the grain boundaries in bulk superconductors. In order to understand the roles of grain boundaries on superconducting properties, many theoretical and experimental studies have been done up to now [12–20]. Due to the

advances in thin film technology, grain boundaries can now be well analyzed in artificially controlled bi-crystalline samples [21–25]. These studies reveal that the weak link behavior of grain boundaries is generally due to the presence of non-superconducting phases and atomic disorder along the boundaries and cracking. It was also found that high-angle grain boundaries might be the reason for weak links. Many researchers have tried to align grains in preferred orientations to minimize the contribution of misorientated angles between grains to link strength. It has been found that texturing is more effectively achieved by melt-processing [9–11]. In the present study, we have prepared SmBa2 Cu3 O7 bulk samples using the Top-Seeded-Melt-Growth (TSMG) technique. Our scan of the literature has shown that data for the Sm-123 system is rather scarce despite its high transition temperature (∼96 K). Intergrain and intragrain critical current densities were determined through electrical and magnetic measurements, respectively. Using the data in literature for other RE-123 systems, we analyze the role of grain boundaries on the superconducting properties of Sm-123 samples; especially on Jc . Energy dissipation mechanisms for these samples were also examined. 2. Experimental procedure

∗ Corresponding author. Tel.: +90 2626795000; fax: +90 2626795001. E-mail address: [email protected] (U. Topal). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.08.039

The precursor bodies were prepared using the solid-state sintering route. High purity powders of Sm2 O3 , BaCO3 and CuO were weighed in the appropriate amounts

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to form a nominal composition of SmBa2 Cu3 O7 . Then powders were mixed well using an agate mortar and pressed into pellets under 2000 kg cm−2 of pressure. The pellets were then sintered at 975 ◦ C for 40 h with intermediate grinding and mixing. Then, the polycrystalline precursors were melt-processed using the TSMG technique [5]. A single MgO (1 0 0) crystal was placed in the center of the upper surface of the sample prior to processing in order to increase the size of the Sm-123 phase domain. Melt-processing was performed under an oxygen partial pressure of 1 vol.% O2 in the flowing nitrogen. The precursor was first heated to 1140 ◦ C and held for 1 h. Then, the temperature was lowered rapidly to 1060 ◦ C, and then slowly at a rate of 0.3 ◦ C h−1 –1030 ◦ C, and afterwards, more rapidly to room temperature. Finally, the melt-processed bulk sample was annealed at 600 ◦ C, 550 ◦ C, 500 ◦ C, 450 ◦ C, and 400 ◦ C during 12 h at each temperature successively in the flow of oxygen. The specimens were then cut into small regular pieces for microstructural, electrical, and magnetic measurements. In order to obtain information about the granular nature of the specimens, optical microscopy and scanning electron microscopy (SEM) were used. For the optical microscopy, the surface of the specimen was polished using different grades of SiC paper and special diamond pastes in a Metaserve2000 polisher. Here, it should be noted that thorough polishing is required in order to remove or eliminate processing induced defects, contaminants, or artifacts. Bar shaped pieces with dimensions of 1 × 1 × 5 mm3 were left for the electrical and magnetic measurements. Electrical measurements were performed in a Quantum Design PhysicalProperty-Measurement-System (PPMS) with a 7 T magnet, using the four-probe technique in which the distance between the probes was 1 mm and contact resistance was in the range of 0.4–0.6 . The transport Jc values were determined from the V–I curves using the 1 ␮V cm−1 criterion in which the current causing 1 ␮V cm−1 potential difference across the specimen is assumed to be the critical current. Magnetization measurements were done in a SQUID magnetometer (MPMS-7) from Quantum Design. Jc values were obtained from magnetic measurements by means of the Bean critical state model; Jc =

20M , a(1 − a/3b)

where M is the width of the magnetization loop in emu cm−3 , a and b are dimensions of the rectangular cross section of the sample perpendicular to the applied field in cm, and Jc is A cm−2 . Magnetic field was applied parallel to the c-axes of the samples in both electrical and magnetic measurements. We note here that, in granular structures, the transport measurements give Jc that passes through grain boundaries (intergrain), while the magnetic measurements give intragrain Jc that circulates inside the grains due to the weak links between the grains.

3. Results and discussion Fig. 1 shows (a) scanning electron micrograph (SEM) and (b) polarized optical micrograph of the TSMG grown SmBa2 Cu3 O7 specimen. As seen from Fig. 1a, it is difficult to determine the grain dimensions from the SEM images. On the other hand, the polarized optical micrograph reveals grain dimensions ranging between 0.5 mm and 1.5 mm. Small precipitates seen over the grains are most probably Sm-211 inclusions. Here, note that different colors in the optical micrograph are due to the different crystallographic orientations of the grains. Fig. 2 shows the resistivity vs. temperature curve of the Sm-123 samples at different magnetic fields between 0 T and 6 T. The inset shows the field-cooled magnetization curve. A characteristic property of these samples is the differences between the intragrain and the intergrain transition temperatures. Intragrain Tc extracted from the M–T curve (Tc-intra = 93.5 ± 0.5 K) is higher than the intergrain Tc (Tc-inter = 87.5 ± 0.9 K) obtained from the resistivity measurements. Such a difference can be attributed to the existence of grain boundaries having lower Tc . The intragrain Tc value, which is very close to the highest value reported in literature (96 K) [26], indicates the high oxygenation of these samples. Note that the highest Tc of 96 K could only be achieved by decreasing the amounts of the naturally induced Sm-211 phase. The M–T curve shows that the transition to superconducting state is not sharp (see Fig. 2 inset). Instead, the transition continues down to the lowest measurement temperature. This is related to the presence of field trapping centers that do not allow flux exclusion (e.g. Sm-211 inclusions).

Fig. 1. (a) Scanning electron microscope and (b) optical microscope images of the sample surface. Different colors in the optical microscope image indicate the different crystal orientations of the grains. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.).

According to the literature, the misorientation angle between grains can be estimated from the electrical measurements. For instance, it was shown by Vanderbemden et al. [27] and Cardwell et al. [21] that in melt-textured high-Tc superconductors, the superconducting transition widths are much affected by the misorientation angle between grains. Their experiments have shown that transition widths are very sharp (T < 1 K) for single domain particles or those having low angle grain boundaries. Where high-

Fig. 2. Resistivity vs. temperature curve in magnetic fields of 0–6 T. Inset: fieldcooled magnetization vs. temperature curve. The Tc (on) values, which were determined from the intersection points of the extrapolations of normal state resistance and the superconducting transition region, have relative uncertainty of 1%. The Tc (on) values, which were determined from the M–T curve, have relative uncertainty of 0.5%.

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Fig. 3. The V–I curves at temperatures (a) 10 K, (b) 20 K, (c) 30 K and (d) 77 K and magnetic fields between 0 T and 3 T.

angle grain boundaries are present, the Josephson coupling energy EJ is rather small. A grain boundary becomes superconducting only when thermal energy kT is smaller than EJ , which is inversely proportional to temperature [27]. Consequently, if the samples are well oxygenated, transition widths may give information about the nature of grain boundaries. As mentioned before, our samples may be assumed to be well oxygenated as a consequence of high intragrain Tc . The transition width of the Sm-123 samples was determined to be ∼3 K from the resistivity vs. temperature curves (see Fig. 2). Besides, it is well known that high-angle grain boundaries behave like weak links, and that weak links between grains result in a strong decrease in Tc (off) values with the application of very small magnetic fields (at the order of several gauss) [19,28]. Fig. 2 infers that Tc (off) values decrease by 3 K even after application of 6 T. Both small transition width and weak magnetic field dependence of Tc (off) values may indicate the presence of low angle grain boundaries or relatively strong coupling between grains of the specimens studied. This idea may also be supported by the value of normal state resistivity  at 0 K (evaluated from the best-fit equation of normal state resistance), which is calculated as −40 ␮ cm. Such negative values were observed in RE-123 single crystals and single domain RE-123 superconductors before [27,29]. However, we believe that it is still early to draw a conclusion about the magnitude of the grain boundary angles. Fig. 3 shows the V–I curve of the samples at temperatures of (a) 10 K, (b) 20 K, (c) 30 K and (d) 77 K at magnetic fields between 0 T and 3 T. As seen, the curve is rather sharp and seems to be linear at low temperatures and in the absence of magnetic field. With increases in the field and temperature, the curves become more rounded, similar to that observed in single-grain YBCO samples [27]. In contrast to the polycrystalline cuprates [19,28], magnetic field seems not to have much of an effect on these samples.

Fig. 4 shows the temperature and magnetic field dependences of the intergrain Jc values extracted from the V–I curves. The intergrain Jc decreases linearly with the increase in magnetic field (Fig. 4a). As mentioned above, weakly connected grains show strong magnetic field dependence. For the studied samples, Jc decreases of about 17–26% in magnitude are observed even under a magnetic field of 1 T at all temperatures. Fig. 4b also reveals quite interesting features. The Jc initially decreases as the temperature increases from 10 K to 20 K. Then, it starts to increase and reaches a secondary maximum value above 20 K. This behavior is encountered in all the Jc –T curves. At higher magnetic fields (2 T–3 T), Jc at 30 K becomes even greater than the Jc at 10 K. At first glance, such a behavior seems strange. Therefore, experiments were repeated several times both on the same piece and on different pieces but the same results have been obtained. Later, our search in literature has shown that such behavior was also encountered in some other studies [22–24,30,31]. In the review article of Hilgenkamp and Mannhart [25], this extraordinary behavior is discussed briefly with the given references. It is understood that this type of local minima or maxima in the Jc –T curves is exactly related to the misorientation of grains. For instance, the Jc of asymmetric 32◦ grain boundaries (37◦ /−5◦ ) shows a minimum below 4.2 K [30]. Ivanov et al. [24] also explained the reasons for such types of behavior through use of the superconducting/semiconducting/superconducting junction models. Later, the presence of midgap states in d-wave superconductors was considered to have a role in such instances of temperature dependency [31]. Furthermore, the theoretical study of Tanaka and Kashiwaya [22] also supports the existence of local minimum regions in the Jc –T curves of symmetric boundaries. They stated a local minimum of the Jc at ∼0.15Tc for a symmetric 36◦ boundary. Taking into account these experimental and theoretical studies, we predict that the misorientation angles of the grains in the samples studied are likely to be symmetric and around 30◦ .

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Fig. 4. Temperature and magnetic field dependences of the intergrain critical current densities Jc .

Fig. 5. The log V vs. log I curves as a function of magnetic field and temperatures.

Table 1 Power exponent ˇ (V ∼ Iˇ ) values at different temperatures and magnetic fields. 10 K

20 K

30 K

77 K

ˇ

ˇ

ˇ

ˇ

0

(1) Region = 52.9 (2) Region = 10.5

(1) Region = 52.9 (2) Region = 9.9

(1) Region = 52.9 (2) Region = 11.7

(1) Region = 10.35 (2) Region = 6.

1T

(1) Region = 28.5 (2) Region = 9.5

(1) Region = 52.9 (2) Region = 9.9

(1) Region = 52.9 (2) Region = 10.0

(1) Region = 10.2 (2) Region = 6.7 (3) Region = 4.5

2T

(1) Region = 22.8 (2) Region = 8.9

(1) Region = 27.9 (2) Region = 9.5

(1) Region = 52.9 (2) Region = 10.0

(1) Region = 10.2 (2) Region = 6.9 (3) Region = 4.5

3T

(1) Region = 10.2 (2) Region = 6.9 (3) Region = 4.7

(1) Region = 10.2 (2) Region = 7.0 (3) Region = 4.7

(1) Region = 10.2 (2) Region = 7.0 (3) Region = 4.7

(1) Region = 10.2 (2) Region = 6.2 (3) Region = 3.3

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Fig. 6. (a) M–H loops at 10 K, 20 K and 30 K (b) Magnetic field dependences of the intragrain critical current densities.

Table 2 Intragrain and Intergrain Jc values for different temperatures and magnetic fields. H (T)

0 1 2 3

Jc-intra (A cm−2 )

Jc-intra (A cm−2 )

Jc-intra (A cm−2 )

Jc- inter (A cm−2 )

Jc- inter (A cm−2 )

Jc- inter (A cm−2 )

10 K

20 K

30 K

10 K

20 K

30 K

109,500 79,740 61,140 50,670

48,060 23,370 17,160 15,240

24,600 9,420 7,200 6,300

212.2 156.3 101.5 46.8

152.5 125.2 91.0 43.4

178.5 144.7 114.2 47.2

In order to understand the dissipation mechanism in high-Tc superconductors, the V–I curves have a special importance. For instance, if the Lorentz force arising from the interaction of transport current with the vortices is larger than the pinning force, then the vortices start to move and dissipate energy, which is linearly proportional to the applied current. On the other hand, in some situations where the Lorentz force is smaller than the pinning force, vortices may also move (or creep) and thus dissipate energy because of thermal activation. For polycrystalline and melttextured high-Tc superconductors, the dissipation mechanism can be explained by means of power law behavior, given by V = Iˇ [16,20,21,28]. Therefore, we plot the V–I data on a logarithmic scale to understand whether our samples obey the power law behavior or not. Fig. 5 shows the log V vs. log I curves as a function of magnetic field and temperature. As it is seen, the log V–I curves may generally be divided into two or three linear parts. At low magnetic fields and temperatures, there are two linear parts but at high fields and temperatures, there are mostly three linear parts in log V–log I curves (see also Table 1). Such linear parts with different slopes have not been observed in polycrystalline cuprates but two linear parts were reported before for melt-textured YBCO samples [20]. The study of Nillson-Melbin et al. [20] on melt-textured YBCO has shown that there is a linear relationship between the critical current and the power exponent ˇ and the Jc takes high values for high ˇ values. In the present study, the slopes of each line, ˇ values, were calculated from the best-fit equations, and are listed in Table 1. The general tendency is towards the presence of a linear relationship between the ˇ values and the Jc values. However, regarding the two or three linear regions, it seems difficult to understand which ˇ value is directly relevant to the critical current density Jc . Fig. 6 shows (a) the M–H loops at 10 K, 20 K, and 30 K and (b) the intragrain Jc values calculated from the M–H curves using Bean’s model [32]. It is remarkable that the Jc initially decreases rapidly but then decreases more gradually with the increase in magnetic field. Here, it is observed that there is no local minimum at 20 K as it was

the case in the transport measurements. It is also seen that there is a large difference between the intragrain Jc values and the intergrain Jc values (see Table 2). For instance, at 10 K and 0 T, while the intragrain Jc is ∼110,000 A cm−2 , the intergrain Jc is just ∼210 A cm−2 . The reason for such a large difference may also be explained by the existence of a high misorientation angle between grains [33]. Many studies have shown that the critical current density Jc decreases significantly as the misorientation angle increases. It was also reported before that the intergrain Jc may decrease over more than three orders of magnitude as the misorientation angle increases from 0◦ to 45◦ [34]. This big difference between the intragrain and intergrain Jc values indicate the presence of high misorientation angles between grains of the specimens in spite of the low concentration of grains (see Fig. 1). As a conclusion, we have examined the intragrain and intergrain critical current densities in Top-Seeded-Melt-Growth SmBa2 Cu3 O7 samples by comparing the results of transport and magnetic measurements. Our measurements reveal that the intragrain Jc is much greater than the intergrain Jc (101 –103 times in magnitude). The reason for this difference was connected to the high misorientation angle of the grains (∼30◦ ). In spite of high-angle grain boundaries, the superconducting properties of the samples have not been affected much by the applied magnetic field. This seems to be due to the low density of grain boundaries or the presence of large grains in the samples. Acknowledgments We would like to thank Mr. Ömer Altan and Dr. Lev Dorosinskii for previewing the article and for their valuable suggestions. References [1] T.J. Shaw, J. Clarke, R.B. Van Dover, L.F. Schneemeyer, A.E. White, Phys. Rev. B 54 (1996) 15411.

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