Journal of Superconductivity and Novel Magnetism https://doi.org/10.1007/s10948-018-4905-3 ORIGINAL PAPER Calcium-Substituted Y 3 Ba 5 Cu 8 O 18 Ceramics Synthesized via Thermal Treatment Method: Structural and Superconducting Properties Mustafa Mousa Dihom 1 · Abdul Halim Shaari 1 · Hussein Baqiah 1 · Chen Soo Kien 1 · Rabaah Syahidah Azis 1 · Roslan Abd-Shukor 2 · Naif Mohammed Al-Hada 1 · Mohd Mustafa Awang Kechik 1 · Zainal Abidin Talib 1 Received: 2 September 2018 / Accepted: 27 September 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract In this work, Y 3 (Ba 1−x Ca x ) 5 Cu 8 O 18 compounds with x = 0.00, 0.01, 0.02, 0.03 and 0.05 were synthesized by thermal treatment of aqueous solution of metal nitrates and polyvinylpyrrolidone (PVP). The effects of Ca substitution on the crystal structure, electrical resistance and the microstructure of samples were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), four-point probe measurement and AC susceptibility. FESEM analysis showed that there was an increase in the grain size and compactness. The zero-resistance critical temperature decreased from 92 K for the pure sample to 83 K for sample x = 0.05. From AC susceptibility result, the intra- and inter-granular loss peaks shifted toward lower temperature and become wider and broader with the increase of Ca substitution due to the weakening of grain coupling. On the other hand, the inter-granular critical current density (J cm ) was found to increase with Ca substitution and has the highest value J cm = 35 A cm −2 at x = 0.05, indicating that the Ca substitution was associated with the formation of pinning centres in the Y-358 matrix. Keywords Ca substitution · Y-Ba-Cu-O compounds · Critical temperature · Thermal treatment · AC susceptibility 1 Introduction Superconductivity in Y-Ba-Cu-O compounds was discov- ered first by Wu and co-workers [1] in YBa 2 Cu 3 O 7−δ (Y-123) with critical temperature (T c ) about 92 K. Over the last three decades, material scientists have focussed on the study of YBCO family. The superconducting proper- ties of compounds like YBa 2 Cu 3 O 7−δ (Y-123), YBa 2 Cu 4 O 8 (Y-124), Y 2 Ba 4 Cu 7 O 15 (Y-247) and many others were extensively studied. The compounds are being differenti- ated by their number of CuO 2 planes and the CuO chains [2]. A high-temperature YBaCuO-type superconductor is produced using the formula Y 3 Ba 5 Cu 8 O 18 [3]. The reports indicate that to achieve superconductivity at a high T c in YBCO family, more hole carriers should be introduced from the CuO chain structures to the oxygen sites in the CuO 2  Abdul Halim Shaari ahalim@upm.edu.my 1 Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia 2 School of Applied Physics, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia planes to make possible the diagonal charge order [1, 4]. Y-123 has two CuO 2 planes and one CuO chain [5]. Superconductivity in Y-358, a new member of Y-Ba-Cu- O family, was reported by Aliabadi et al. [2], with crystal structure comprised of five CuO 2 planes and three CuO chains per unit cell [6]. The crystal structure is close to that of Y-123 phase, but its c-axis is higher by three times. The high critical temperature reported for Y-358, T c ∼ 100 K, has led researchers to investigate structure, phase formation and superconducting properties [2, 3]. Unfortunately, the crystal structure of Y358 is still under debate. Some group reported that Y-358 has a- and b-axes of Y-123 while the c-axis of Y-358 is almost three times of Y-123. Other groups claimed that crystal structure is similar to that of Y-123 [7–10]. Amado and Sarmago [8] mentioned that Y- 358 is a metastable phase and may form from Y-123 phase. Therefore, more studies about the structural properties of Y-358 are needed. Previous researches were devoted to enhancing and characterizing superconducting properties of Y-358. For instance, current density was improved by sintering it in different atmosphere where the highest critical current was observed by using O 2 /Ar sintering atmosphere [11]. CoFe 2 O 4 introduced to Y-358 compound was associated