**1. Introduction**

Yttrium-barium-copper-oxide, YBCO is the first type-II high temperature superconductor (HTS) that has been discovered to be superconducting above the boiling point of liquid nitrogen. Among the family members of YBCO, 123 phase (Y-123) shows the highest onset of superconductivity at around 90 K. In order for Y-123 to be more feasible for a wide range of applications, numerous studies have been undertaken to improve its critical temperature, *T*c and critical current density, *J*c [1–3]. In this regard, addition of chemical dopants is one of the most straight forward strategies. It was shown that the addition of BaZrO3 (BZO) nanoparticles into Y-123 improved *J*c without a ffecting much the *T*c value [4,5]. This is because BZO did not dope into the structure of Y-123. Instead, they either remained unreacted within the matrix or reacted with Y-123 to form nano-precipitates leading to enhanced flux pinning [6].

Intensive research on fabrication of BZO doped YBCO films was conducted in the past [7–11]. However, limited study of BZO doped YBCO bulks has been reported so far [5,12]. From the perspective of large scale applications, it is essential to investigate the e ffects of synthesis route and dopant additions on the superconducting properties of YBCO bulks. There are several methods used to synthesize the materials. For example, solid-state (SS) method [13,14] and wet method such as sol-gel [15], co-precipitation (COP) [16–18], and thermal treatment [19]. In particular, COP is a highly desirable chemical method used to prepare nanomaterials. The powders obtained by COP method show smaller grain size, higher purity, and better homogeneity compared to that obtained using SS method [20,21]. Moreover, multiple calcination and regrinding process is unnecessary to obtain a good superconducting phase for the COP method [17,22]. Previously, we successfully synthesized YBCO added with nano BZO using COP method [23]. Therefore, we are motivated to carry out a comparative study on structural and superconducting properties of BZO nanoparticles added Y-123 prepared via SS method and COP method. COP can be defined as the process of carrying down a precipitate of substances which are normally soluble under the suitable conditions [24] and the starting materials can be metal acetates or metal nitrates [14].

#### **2. Materials and Methods**

Samples with nominal composition of YBa2Cu3O7−<sup>δ</sup> (Y-123) added with *x* mol% of BaZrO3 (BZO) nanoparticles (*x* = 2.0, 5.0, and 7.0) were prepared using solid state (SS) method and co-precipitation (COP) method, respectively.

#### *2.1. Solid-State (SS) Method*

To start with, Y2O3 (99.9%, Alfa Aesar, Ward Hill, Massachusetts, USA), BaCO3 (99.8%, Alfa Aesar, Ward Hill, Massachusetts, USA) and CuO (99.9%, Strem Chemical, Newburyport, Massachusetts, USA) with the stoichiometric ratio of Y:Ba:Cu (1:2:3) were mixed and hand-ground for 1 h using a mortar and pestle. Then, the grounded mixture was calcined in air at 940 ◦C for 12 h. After calcination, the powders were reground for 1 h before adding *x* mol% (*x* = 2.0, 5.0, and 7.0, respectively) of BaZrO3 nanoparticles (BZO, >50 nm, 98.5%, Sigma-Aldrich). The mixed powders were reground again and then pressed into circular pellets (~13-mm diameter and 2-mm thickness) by using a hydraulic press with an applied pressure load of five tons. Finally, the pellets were sintered at 950 ◦C for 12 h and slowly cooled to 450 ◦C for 12 h at the rate of 1 ◦C/min under oxygen flow before further cooling to room temperature. Pure samples (*x* = 0.0) were also prepared according to the same procedure to serve as reference for the purpose of comparison.

#### *2.2. Co-Precipitation (COP) Method*

For COP method, appropriate amounts of Y(CH3COO)3·4H2O (99.9% Alfa Aesar), Ba(CH3COO)2 (≥99% Alfa Aesar), and Cu(CH3COO)2.H2O (≥99% Sigma Aldrich) according to the stoichiometric ratio of Y:Ba:Cu (1:2:3) were dissolved in acetic acid to form solution A. To prepare solution B, oxalic acid was dissolved in a mixture of distilled water: 2-propanol (1:1.5). Both solutions A and B were stirred at 300 rpm for 2 h before being cooled in an ice bath. The mixed solution A and B was filtered and dried at 100 ◦C for 12 h. The obtained dried powders were ground and calcined in air at 900 ◦C for 24 h. After that, appropriate amount of *x* mol% (*x* = 2.0, 5.0, and 7.0, respectively) BaZrO3 nanoparticles (BZO, >50 nm, 98.5%, Sigma-Aldrich) was added to the calcined powders for mixing and grinding. Then, the mixture was pressed into circular pellets (~13-mm diameter and 2-mm thickness) by using a hydraulic press with an applied pressure load of 5 tons. Lastly, the pellets were sintered at 920 ◦C for 15 h and slowly cooled to 650 ◦C for 8 h (annealing process) before further cooling to room temperature. The sintering and annealing were done under a constant oxygen flow. Pure samples (*x* = 0.0) were also prepared according to the same procedure to serve as reference for the purpose of comparison.

#### *2.3. Sample Characterization*

Phase formation and crystal structure of the samples were examined by X-ray di ffraction (XRD) method using the PW 3040/60 MPD X'Pert Pro Panalytical Philips DY 1861 X-ray di ffractometer with Cu-Kα radiation source. Scanning was carried out in 2θ mode over the range of 20◦–80◦ with the increment step size of 0.03◦. The XRD data was analyzed using the X'pert HighScore Plus software. Surface morphology of the pellets was observed using a scanning electron microscope (SEM-LEO 1455 VPSEM). Superconducting properties of the samples were measured using the commercial AC Susceptometer of CryoBIND (cryogenic balanced inductive detector) SR830 at the frequency of 219 Hz and applied field of 0.5 Oe. Uncertainty of the AC Susceptometer is ±0.1 K.
