**3. Discussion**

#### *3.1. X-ray Di*ff*raction (XRD) Analysis*

Figure 1 shows the XRD patterns of the samples prepared via SS method and COP method, respectively. The XRD patterns were indexed to Y-123 phase with orthorhombic crystal structure and space group *Pmmm* (ICSD: 01-078-2143). Peaks with the highest intensity for both the SS and COP samples were indexed with the Miller indices of (0 1 3) and (1 0 3). Rietveld refinement was undertaken on the XRD data using the X'pert HighScore Plus software. Accordingly, the refined unit cell lattice parameters indicated that all the samples have an orthorhombic crystal structure. Hence, no structural transformation from orthorhombic to tetragonal phase was occurred as a result of BZO addition. By using the similar refinement method, fraction of di fferent phases (in weight percentage) in the samples was obtained. Compared to SS method, the samples prepared using COP method have higher weight percentage of Y-123 phase in agreemen<sup>t</sup> with previous findings [20,21]. Table 1 indicates the weight percentage of Y-123 phase for the pure SS and COP samples is 93.2% and 97.5%, respectively. However, the weight percentage of Y-123 phase for the SS samples and COP samples started to decrease with the increasing of BZO additions. Some minor peaks of Y2BaCuO5 (Y-211) (ICSD: 01-079-0697) were also observed in the XRD patterns of all the samples. The presence of Y-211 phase may due to the synthesis condition such as the heat treatment process and the preparation of the material itself [25]. A peak corresponding to BaZrO3 (BZO) (ICSD: 01-074-1299) started to appear at 2.0 mol% BZO addition and became more intense with increasing of BZO addition. This observation confirmed the distribution of BZO in the samples [26]. The increased intensity of the peak with increasing BZO addition suggests the saturation of solid solubility of BZO in Y-123 [27].

**Figure 1.** X-ray diffraction patterns of Y-123 + *x* mol% of BZO samples prepared via (**a**) SS method and (**b**) COP method. The Y-211 and BZO are marked with (-) and (♦), respectively.

**Table 1.** Percentages of Y-123, Y-211, and BZO phases for Y-123 + *x* mol% of BZO samples prepared via SS method and COP method.


Table 2 summarizes crystallite size calculated based on the (103) peak of Y-123 using the Scherrer equation [28]:

$$\text{L} = \frac{\text{K}\lambda}{B\_{\text{size}} \text{COS} \Theta} \tag{1}$$

where L is the crystallite size, K is a dimensionless shape factor (0.9), *B*size is line broadening at half of the maximum intensity (FWHM) in radian, λ is the X-ray wavelength for Cu-Kα radiation (1.5406 Å) and θ is Bragg angle in degree. Table 2 shows that the crystallite size of the SS samples is larger than that of the COP samples. Crystallite size of the pure SS and COP samples is 101.61 nm and 63.66 nm, respectively. Upon addition of BZO, the crystallite size increased for the COP samples. The larger crystallite size may be due to the bridging of fine particles that formed the continuous grain boundary networks [29]. Conversely, crystallite size became smaller for the SS samples with addition of BZO. Previously, the crystalline size of CaTiO3 (CTO) was found to decrease possibly due to large difference in ionic radii between the dopant ion and host ion (Ti<sup>4</sup>+) [30]. The nanocrystalline CTO was synthesized using soft chemical method followed by annealing. For our work, however, the same BZO nanoparticles were used for addition into the samples prepared by SS method and COP method. Judging from the data given in Table 2, the decrease in crystalline size for the BZO added SS samples is believed to be due to larger difference of the crystalline size between the BZO and Y-123 compared to that between the BZO and Y-123 prepared using COP method.

**Table 2.** FWHM of (103) peak, crystallite size and average grain size for Y-123 + *x* mol% of BZO samples calculated using Scherrer method. The average grain size of the samples was calculated from the randomly selected 100 grains of the SEM image.


#### *3.2. Microstructure Analysis*

Figure 2 shows the SEM images of the SS and COP samples. All the samples have irregular shape and randomly distributed grains. The SEM images also show the increment in number of pores with BZO addition suggesting more porous structure for both SS and COP samples added with BZO. For calculation of average grain size, 100 grains were selected randomly from each SEM image and measured using the Image-J software. Figure 3 shows the distribution of grain size of the SS and COP samples. The average grain size is 1.24 μm and 0.30 μm, for the pure samples prepared via the SS method and COP method, respectively (Table 2). Addition of BZO into Y-123 sample increased the average grain size of the samples especially for the SS samples. The average grain size increased to 2.71 μm and 0.50 μm for the 7.0 mol% BZO added SS and COP samples, respectively. The increasing grain size is probably due to dispersion of BZO nanoparticles between the grains of Y-123 which promoted grain growth [5,31]. According to Table 2, the powders obtained by the COP method have smaller size and narrower size range (0.30–0.50 μm) in comparison with that obtained via the SS method (1.24–2.71 μm). This may be partly associated with the higher sintering temperature used in the SS method (950 ◦C, compared with 920 ◦C used for the COP method) leading to larger grain growth [21].

**Figure 2.** SEM images of Y-123 added with (**a**) 0.0 mol%, (**b**) 2.0 mol%, (**c**) 5.0 mol%, and (**d**) 7.0 mol% of BZO at 5000× magnification prepared via SS method. Bottom: SEM images of Y-123 added with (**e**) 0.0 mol%, (**f**) 2.0 mol%, (**g**) 5.0 mol%, and (**h**) 7.0 mol% of BZO at 5000× magnification prepared via COP method.

**Figure 3.** *Cont.*

**Figure 3.** Distribution of average grain size for Y-123 added with (**a**) 0.0 mol%, (**b**) 2.0 mol%, (**c**) 5.0 mol%, and (**d**) 7.0 mol% of BZO prepared via SS method and Y-123 added with (**e**) 0.0 mol%, (**f**) 2.0 mol%, (**g**) 5.0 mol%, and (**h**) 7.0 mol% of BZO prepared via COP method.

#### *3.3. Superconducting Properties*

Figure 4 shows the temperature dependence of the real (χ) and imaginary (χ") parts of AC susceptibility (ACS) for all the samples. The real part (χ) shows two transitions labeled as onset critical temperature, *T*c-onset and phase lock-in temperature, *<sup>T</sup>*cj. *T*c-onset is due to transition within the grains (intra-grain) while *T*cj is due to superconducting coupling between grains (inter-grain) [32]. In the temperature range between *T*c-onset and *<sup>T</sup>*cj, the superconducting grains are decoupled and the sample as a whole becomes resistive even if all individual grains are still strongly superconducting. Below *<sup>T</sup>*cj, the grains are coupled or phase-locked in which the phase difference across the intergranular junctions is zero. Consequently, the induced current from the externally applied ac magnetic field gives rise to shielding currents along the outermost surface of the sample. *T*c-onset and *T*cj were determined from the peaks of the derivative of normalized χ, δχ/δT against temperature, T plots (insets). It should also be noted that samples with approximately the same size (cross-section 0.3 × 0.5 cm) were used for measurements under the same conditions for the sake of consistency.

**Figure 4.** Temperature dependence of ACS for Y-123 + *x* mol% of BZO samples prepared via (**a**) SS method (**b**) COP method. Insets show the derivative of normalized χ, δχ/δT against temperature, T.

Table 3 shows that the values of *T*c-onset and *T*cj for the COP samples are close to that of the SS samples. The difference for *T*c-onset and *T*cj is ≤1.6 K and ≤1.2 K, respectively. With increasing of BZO addition from 0.0 mol% to 7.0 mol%, *T*c-onset and *T*cj for the COP samples decreased slightly (0.7 K and 0.8 K for *T*c-onset and *<sup>T</sup>*cj, respectively). Nevertheless, addition of 2 mol% BZO did not change the value of *<sup>T</sup>*cj. The marginal decrease of *T*c-onset is in good agreemen<sup>t</sup> with the previous finding [12,33,34] and could be associated with the reduced hole concentration [23]. However, the values of *T*c-onset and *T*cj for the SS samples showed a larger change compared to the COP samples. For the SS samples, *T*c-onset decreased from 90.3 K for 0.0 mol% to 88.9 K for 7.0 mol% BZO addition (1.4 K). The lower *T*c-onset of the 7.0 mol% BZO added SS sample compared to that of the COP sample may be attributed to inhomogeneity of the former. Meanwhile, the value of *T*cj decreased from 87.8 K for 0.0 mol% to 85.5 K for 7.0 mol% BZO addition (2.3 K). The decrease of *T*cj with the increasing of BZO addition for both COP and SS samples indicates weakening of intergranular coupling perhaps due to the increasing of impurities at the grain boundaries [35,36]. However, the larger decrease of *T*cj in the SS samples implies more severe degradation of grain coupling caused by BZO addition compared to the COP samples.


**Table 3.** Coupling peak temperature, *<sup>T</sup>*p, onset critical temperature, *T*c-onset, phase lock-in temperature, *<sup>T</sup>*cj, and Josephson current, *I*o for Y-123 + *x* mol% of BZO samples in applied AC field of 0.5 Oe.

Imaginary part (χ") of ACS exhibits a broad peak known as coupling peak temperature, *T*p that corresponds to the hysteretic energy dissipation as a result of AC losses at the inter-grain region [37]. The second peak near *T*c (intra-granular peak) for all the samples is not apparent except for the pure sample prepared via the SS method. The absence of the second peak suggests that grain coupling is better for the pure sample prepared via COP method compared to the SS method [37,38]. For the COP samples, addition of 2.0 mol% of BZO did not change the *T*p much compared to the pure sample. For the SS samples, *T*p for the pure sample is 87.3 K compared to 85.8 K for the sample with 2.0 mol% of BZO addition. As shown in Figure 4 and Table 3, *T*p of both the SS and COP samples shifted to lower temperature with the increasing of BZO addition due to the weakening of intergranular coupling between the grains [39]. Other factors that a ffect *T*p also include the changes in microstructure and the amount of impurities at the grain boundaries [40]. It is noteworthy that both *T*p and *T*cj of the 5.0 and 7.0 mol% BZO added samples are quite close to each other for the COP and SS samples. Such a small variation could be related to the saturation of solid solubility of BZO in Y-123 as mentioned in the part of XRD analysis (Section 3.1).

The value of Josephson's current, *I*o was calculated according to the Ambegaokar–Barato ff theory [41] using the following equation assuming that nearest-neighbor grains are coupled by identical Josephson junctions of maximum *I*o:

$$I\_{\rm o} = 1.57 \times 10^{-8} \left( \frac{T\_{\rm c-onset}}{T\_{\rm c-onset} - T\_{\rm cj}} \right) \tag{2}$$

It should be noted that Equation (2) is only valid for s-wave superconductors at temperature near to the superconducting transition temperature, *T*c. Nonetheless, the equation has been adapted for the estimation of maximum Josephson current in d-wave cuprate superconductor [42]. The values of *T*c-onset and *T*cj were estimated from Figure 4 as mentioned before. Due to the granular nature of HTS, grain boundaries of Y-123 act as Josephson junctions. In equation (2), *T*cj represents the Josephson phase-locking temperature which is the crossover temperature between phase-locked and phase fluctuation dominated behavior, corresponding to coupling and decoupling of grains, respectively [41].

Table 3 shows that the *I*o increased for 2.0 mol% samples but decreased for 5.0 mol% and 7.0 mol% BZO addition for both methods. The Josephson current, *I*o = 53.95 μA for 2 mol% BZO added SS sample is the highest. Meanwhile, *I*o for 2.0 mol% BZO added COP sample is 32.08 μA. These values are higher than that *I*o ≈ 5 μA in Bi-Ca-Sr-Cu-O bulk prepared using solid state reaction method [42]. The increasing of Josephson's current, *I*o up to 2.0 mol% indicated better coupling between the grains and thus, stronger Josephson junction. Consequently, the screening current circulated even at higher field causing the increasing tunneling of Josephson current across the grain boundaries [43]. However, further addition of 5.0 mol% and 7.0 mol% BZO in the SS samples, abruptly decreased the *I*o to 28.20 μA and 36.49 μA, respectively. For the COP samples, *I*o decreased to 30.56 μA and 30.54 μA with addition of 5.0 and 7.0 mol% BZO, respectively. Nevertheless, the variation in *I*o with increased BZO addition in the COP samples is smaller probably because of higher homogeneity over the SS samples. Such decrease in *I*o is reflected in the more drastic decrease of *T*cj of the 5.0 and 7.0 mol% BZO added samples (Table 3). This is most probably related to the distribution of BZO particles which are insulating at the grain boundaries. The same reason is believed to be accounted for the lower *I*o of the COP samples compared to that of the SS samples. This can be understood by the presence of more grain boundaries in the COP samples due to smaller grain size (Table 2). Hence, tunneling of Josephine current is hindered due to the distribution of the insulating BZO particles at the grain boundaries.

## **4. Conclusions**

In this work, structural properties of BZO doped Y-123 bulks prepared using COP and SS method, respectively, were studied and their AC susceptibility was compared. The polycrystalline samples with nominal composition of Y-123 added with *x* mol% of BZO nanoparticles (*x* = 0.0, 2.0, 5.0, and 7.0) were synthesized. The XRD patterns showed the formation of Y-123 as the major phase and Y-211 as the secondary phase. The COP samples showed higher weight percentage of Y-123 phase. Refinement of unit cell lattice parameters indicated that all the samples have an orthorhombic crystal structure. Estimation based on the SEM images showed that the COP samples have smaller average grain size than that of the SS samples probably because of lower sintering temperature used for the former. *T*c-onset was found to decrease with addition of BZO as shown by the ACS measurement. The value of Josephson's current, *I*o increased up 53.95 μA and 32.80 μA for 2.0 mol% BZO added SS and COP samples, respectively. The lower *I*o value of the COP samples compared with the SS samples is attributed to the presence of more grain boundaries. Hence, tunneling of *I*o is hindered due to the distribution of insulating BZO particles at the grain boundaries.

**Author Contributions:** Conceptualization: N.M.H., J.K.L., S.K.C., and O.J.L.; methodology: N.M.H., J.K.L., S.K.C., and O.J.L; validation: N.M.H., J.K.L., S.K.C., O.J.L., A.H.S., M.M.A.K., K.P.L., K.B.T., M.M., and M.M.; writing—original draft preparation: N.M.H.; writing—review and editing: J.K.L., S.K.C., and O.J.L; supervision: S.K.C., O.J.L., M.M.A.K., and K.B.T.; funding acquisition: S.K.C., O.J.L., A.H.S., M.M.A.K., K.P.L., M.M., and M.M.

**Funding:** This research was funded by Universiti Putra Malaysia through the Putra-Grant (vote no.: 9552300). N. M. Hapipi would like to acknowledge financial support from the Ministry of Education Malaysia through the MyMaster scholarship, Universiti Putra Malaysia under the Graduate Research Fellowship (GRF). The authors are also grateful to Japan Science and Technology Agency for their financial assistance through SAKURA Exchange Program in Science under Shibaura Institute of Technology.

**Acknowledgments:** We would like to thank Nik Afida Anis Azahari, Kamsiah Alias and Norhaslinda Noruddin for their technical support.

**Conflicts of Interest:** The authors declare no conflict of interest.
