Quantification of the Level of Samarium/Barium Substitution in the Ag-Sm1+xBa2-xCu3O7-δ System

The high-temperature SmBa2Cu3O7-δ (Sm-123) superconducting system, which is characterised by a high critical transition temperature (Tc) and a high critical current density (Jc), suffers severely from the effects of Sm/Ba substitution in the superconducting Sm-123 phase matrix, and especially so for large, single grains grown in air, resulting in a significant variation in Tc at different positions within a single grain. As a result, the suppression of Sm/Ba substitution in the Sm1+xBa2-xCu3O7-δ phase matrix (SmBCO, where x represents the Sm/Ba substitution level in the SmBCO system) is critical to achieving good superconducting properties in this material. Here we report the use of Electron Probe Micro-Analysis (EPMA) to investigate, adjust and optimise the composition of mechanically-stabilised standard Ag-SmBCO bulk single grains. We show that the substitution levels within these samples changes linearly within increasing distance from the vicinity of a single crystal seed used to nucleate the single grain growth process. In addition, we identify a constant value of x of – 0.080 for the composition-adjusted Ag-SmBCO bulk single grain. This is the first time that the quantification of the Sm/Ba substitution level in the SmBCO system has been measured accurately and directly using EPMA, and 2 suggests clearly that the Sm/Ba substitution can be suppressed effectively in air. This research will provide significant insight into the development of a process to suppress Sm/Ba substitution even further in superconducting SmBCO single grains in the future.


Introduction
SmBa 2 Cu 3 O 7-δ (Sm-123, or SmBCO) is a member of the (RE)Ba 2 Cu 3 O 7-δ (RE-123) family (where RE is rare earth element or Y) of high-temperature superconductors (HTS). SmBCO fabricated in the form of large, single grains by melt processing has significant potential for use in practical applications due to its high critical transition temperature (T c ), high critical current density (J c ), the so-called "peak effect" characteristic present in its magnetic hysteresis (M-H) behaviour in high applied magnetic field and high irreversibility field. SmBCO bulk superconductors are capable of supporting macroscopic currents at temperatures above the boiling point of liquid nitrogen (77 K), and can, therefore, be used potentially in a variety of high field, quasi-permanent magnet applications such as magnetic bearings [1] and flywheel energy storage systems [2]. It is necessary to process SmBCO materials in the form of large, single grains and avoid the presence of grain boundaries, however, if the SmBCO bulk superconductor is to carry large currents on the length scale of the sample, which is essential for the generation of magnetic fields that are much larger than those produced by conventional permanent magnet materials [3]. The growth of superconducting (RE)BCO single grains has been developed systematically over the past 30 years, and processes to achieve the stable growth of large SmBCO single grains has been achieved. In general, however, the processing conditions required for SmBCO bulk superconductors are more complicated than those for YBa 2 Cu 3 O 7-δ (YBCO), which has been investigated more extensively, due primarily to the high melting temperature of the precursor powders, rapid growth rate, which is difficult to control, and the need to process SmBCO under reduced oxygen partial pressure in order to inhibit the substitution of samarium (Sm) on the barium (Ba) site in the superconducting Sm-123 phase matrix [4]. Reducing the substitution level of Ba by Sm in the SmBCO system in a practical processing environment represents a particular challenge due primarily to the severity of this effect in air, which leads directly to vastly inferior superconducting properties compared even with the properties of YBCO single grains. As a result, extensive research performed worldwide on the suppression of Sm/Ba substitution in SmBCO has identified that the effects of substitution can be reduced by the addition of a small amount of BaO 2 or BaCuO 2 to the SmBCO precursor powders prior to melt processing [5]. Although the Sm/Ba substitution can be suppressed to some extent by this technique, the inherent inhomogeneity of the superconducting SmBCO single grains grown subsequently in air limits the effectiveness of this approach, since different amounts of the additional Ba are required throughout the SmBCO single grain growth process to uniformly suppress the extent of Sm/Ba substitution. Furthermore, Sm/Ba substitution occurs in Sm-123 matrix containing embedded, discrete Sm 2 BaCuO 5 (Sm-211) inclusions 4 throughout the entire cross section of the sample. As a result, the detectable microscopic length over which the local Sm-123 composition can be measured reliably (i.e. without impingement of Sm-211) is confined, roughly, to 2-10 μm.
Beyond that, the Sm/Ba substitution levels at different positions within the single grain are extremely difficult to detect, which presents a significant challenge to energy dispersive x-ray spectrometry (EDX), which is the characterisation technique used most commonly for the characterisation of chemical composition in refractory metal oxides. This is another reason why the precise measurement of the Sm/Ba substitution level in the SmBCO system is rarely possible.
The distribution and extent of Sm/Ba substitution in the SmBCO system has yet to be studied systematically, despite its fundamental influence on the superconducting properties of SmBCO single grains. In this paper, we report for the first time the precise quantification of the Sm/Ba substitution level in the SmBCO system using an Electron Probe Micro-analyser (EMPA). Single grain Ag-SmBCO bulk samples grown by top seeded melt growth (TSMG) with and without the presence of a YBa 2 Cu 3 O 7−δ (Y-123) layer, added in order to modify the composition to confirm the effectiveness of the method, were measured and analysed systematically as part of this study. The substitution level within the bulk SmBCO samples has been quantified extensively from the top surface to the bottom of the samples, discussed in detail and correlated with the results of their measured superconducting properties, demonstrating unequivocally that EPMA is an effective method for identifying accurately the Sm/Ba substitution level in the SmBCO system. Finally, the addition of 5 a layer of Y-123 at the bottom of the pressed SMBCO pre-forms prior to melt processing leads directly to a more uniform matrix composition in the fully processed SmBCO single grains, which, in turn, results in greater control of the extent of Sm/Ba substitution.

2.1
Production of Ag-SmBCO Single Grains in Air with and without a

Y-123 Layer
Commercial Sm-123 (TOSHIMA, average particle size 2-3 μm) and Sm-211 (TOSHIMA, average particle size 1-2 μm) precursor powders in a weight ratio  [9] was used to nucleate and grow the required grain orientation with an associated buffer layer, the preparation and properties of which are reported elsewhere [10]. At high temperatures, the seed melts partially at its interface with the SmBCO precursor pellet, so contact with the bulk pre-form may lead to contamination of the seeding material and consequently lower the melting temperature of the seed, which results in dissolving and/or the deterioration of the seed. A minimum thickness of 100 µm is required, therefore, for the seed crystal to retain its integrity [11] to avoid such a problem. To further avert the potential of the contaminations from the seed, a buffer layer is additionally added between the generic seed and the Ag-SmBCO perform in this research. The seed, the buffer layer and the Ag-SmBCO pre-form (with and without Y-123 and Yb 2 O 3 layers) were aligned before melt processing to yield the required orientation of the single grain, as illustrated schematically in Figure 1 (a). The arrangement was placed on an alumina plate in a box furnace prior to top seeded melt growth (TSMG) using the heating profile shown schematically in Figure 1 (b). The TSMG process is based on a peritectic solidification reaction that occurs at the peritectic temperature, T p (1045 °C for the starting powder composition in Ag-SmBCO system in this paper), during which solid Sm-123 is formed from solid Sm-211, a Ba-Cu-O based liquid phase (L) and oxygen gas (G) [12]; The Ag-SmBCO single grain growth process involved ramping the temperature initially to 1070 °C, holding at this temperature for 20 minutes to allow thorough decomposition of the precursor powders and cooling the furnace to 1037 °C at a rate of 50 °C · h −1 , at which the growth of the bulk single grain begins. The cooling rate was then decreased to 1 °C · h −1 at this temperature, then further to 0.5 °C · h −1 over the temperature range between 1024 °C and 1010 °C and then slow cooled at a rate of 0.3 °C · h −1 to 1004 °C, to complete growth of the single grain. Finally, the sample was furnace-cooled to room temperature. The as-grown samples were oxygenated subsequently at 360 °C for fourteen days to drive the non-superconducting, tetragonal Sm-123 phase to the desired orthorhombic, superconducting phase. 8

Processed with and without a Y-123 Layer
The spatial variation in T c and J c throughout the single grain bulk samples was measured using a Superconducting Quantum Interference Device (SQUID) MPMS XL magnetometer. The MPMS XL SQUID system is capable of measuring very small magnetic moments with a nominal sensitivity of 1×10 −11 A· m 2 . The range of the measurement temperature was from 1.9 K to 300 K with a magnetic field induction of The samples to be measured were cut into slices across their centre, with each slice being cut further into smaller specimens of size approximately 1.5 mm × 2.0 mm × 1.2 mm at different positions within the bulk single grain, as shown schematically in Figure 1 (c). A field of 0.002 T was applied to the samples after zero-field-cooling prior to the measurement of T c . The extended Bean critical state model [13] was used to calculate J c at 77 K from the measured magnetic hysteresis loops of the sub-specimens (i.e. from the measured M-H loops).

Optical Microscopy of Ag-SmBCO Single Grains Processed with and without a Y-123 Layer
Optical microscopy was used to examine the size and distribution of Sm-211 particles in the as-grown Ag-SmBCO single grains with and without a Y-123 layer along c-axis. The as-prepared superconducting pellets were cut into two halves along the c-axis through the seed and the exposed cross-section was polished sequentially using 120, 220, 320, 800, 1000, 1200 and 2400 grit SiC papers. Further polishing was achieved by using 3 μm and 1 μm diamond spray. A Nikon Eclipse ME600 optical microscope was used to observe the microstructures of the exposed cross-sections.

Chemical Composition Analysis of Ag-SmBCO Bulk Single Grains with and without a Y-123 Layer
The SmBa 2 Cu 3 O 7−δ (δ = 0-1), or Sm-123, superconducting phase is characterised crystallographically by a distorted oxygen-deficient perovskite structure, as shown in

Superconducting Properties: T c and J c
The substitution of Ba by Sm in the SmBCO system is responsible partly for the appearance of the so-called secondary "fishtail" peak effect in the M-H. This peak effect is believed to originate from the formation of local, oxygen-deficient regions in the Sm-123 microstructure, which have been attributed to effective field-induced pinning. When exposed to an external magnetic field, the Sm-123ss in the lattice may disturb the superconducting order parameter at the nanoscale level, therefore providing a pinning force to the motion of magnetic flux [16]. layer exhibits an inhomogeneous distribution throughout the parent single grain, which is not favourable for trapping magnetic field, such as that required for macro-scale practical applications [17]. In general, the Ag-SmBCO sub-specimens at positions 1b and 1c exhibit a more pronounced peak effect and higher irreversibility field than at positions 1d and 1e. In summary, J c exhibits a smaller irreversibility field and various secondary peak effects with different values of J c0 , which is J c at zero field in this paper, over the entire cross-section of the sample.

Ag-SmBCO Samples Processed with a Y-123 Layer
To further confirm the effectiveness and accuracy of the EMPA measurements on quantifying the Sm/Ba substitution level in the Ag-SmBCO system, bulk samples were prepared with an additional Y-123 layer beneath the pre-form to enrich the liquid phase composition and, therefore, to adjust the Ag-SmBCO composition during the TSMG process [19]. Zhou et al. [19] have performed extensive research on the function of the Y-123 layer on the growth and superconducting properties of    such as micro-cracks, RE-211 particle inclusions and twin planes [3]. Particularly, the particle size and distribution of RE-211 are directly correlated to J c [20].  As indicated previously, only Sm and Ba require specific consideration when calculating x in the Sm 1+x Ba 2−x Cu y O z superconducting phase formula, whereas the amounts of Cu and O have been observed from EPMA measurements to be constant, as illustrated in Figure 6 (b).
The average value of x in Sm 1+x Ba 2−x Cu y O z , which was estimated from the data shown in Figure 6 (c), indicates that the value of x varies more gently for the single grain processed with a Y-123 layer, which indicates that the presence of a Y-123 layer is beneficial to processing a more uniform Ag-SmBCO bulk single grain. The value of x in Sm 1+x Ba 2−x Cu 3 O 7−δ can be determined reliably, since the data collected are reasonably constant and their variation is within 1 mol. %, which is within the error of EPMA. The calculated average value of x is − 0.080 for samples grown with a Y-123 layer. This indicates that the substitution of Sm on the Ba site is enhanced by adding a Y-123 layer, which is due to the increase of the relative concentration of Sm ions in the liquid during TSMG. In addition, such enhanced Sm/Ba substitution accounts for the inferior irreversibility field and J c0 in Figure 5 (c).

Conclusions
In summary, we have reported for the first time that EPMA measurements can determine precisely the Sm/Ba substitution level in the Ag-SmBCO system fabricated in air. This method is even sensitive to subtle changes applied to the system, in the case in this study, by adding a Y-123 layer to the bottom of the bulk pre-form prior to melt processing in an attempt to provide a more homogenous Ag-SmBCO single grain with a controlled level of solid state substitution. The substitution levels of the Ag-SmBCO bulk single grains grown with and without a Y-123 layer have been measured, and demonstrate clearly that the substitution level of the Ag-SmBCO bulk 22 single grain grown without a Y-123 layer changes linearly with increasing distance from the seed. The value of x value measured for the composition-adjusted Ag-SmBCO bulk single grain, on the other hand, is relatively constant at -0.080, which suggests that Sm/Ba substitution for single grains processed in air can be detected accurately and controlled effectively. These results of this research have considerable potential for enabling further modification and development of the Ag-SmBCO system processed in air and optimisation of its superconducting properties, given that the level of Sm/Ba substitution determines critically these properties.