In this study, a range of sintering temperatures was used to fabricate the tested composites to determine the optimum sintering temperature which provided the optimum properties. For pure BZT93 ceramics, the optimum sintering temperature was 1,450°C, while for the BZT93-NiO composites, 1,300°C. This lower sintering temperature is due to the mismatch between the different components, leading to an inhibition of the sintering ability. The optimum sintering temperature samples were selected for characterization. The phase formation of the pure BZT93 ceramic and composites sintered at an optimum sintering temperature was determined using the XRD technique at room temperature. The XRD results are shown in Figure 1. For the pure BZT93, the XRD pattern corresponded to a pure orthorhombic perovskite phase 9 10 . In the case of the composites, the XRD peaks at 2θ ~ 37° and 44° indicated an impurity phase. The impurity peaks were identified as NiO, corresponding to the JCPDS file no. 044-1159, confirming a formation of the composites.

Figure 2 shows the M-H magnetic hysteresis loops of the samples measured at room temperature. The 1 vol.% sample exhibited a weak magnetic behavior. However, an improvement in magnetic properties was clearly observed for the composites containing NiO > 1.0 vol.%. The values of the coercive magnetic field [H _c] and remnant magnetization [M _r] of the samples are listed in Table 1. Figure 3 shows the P-E ferroelectric hysteresis loops (at room temperature) with different NiO contents. The shape of the hysteresis loop for the pure BZT93 ceramics indicates a normal ferroelectric behavior. For samples with higher NiO concentrations, however, the hysteresis loop became more slanted. Furthermore, a lossy capacitor hysteresis loop was clearly observed for the 3 vol.% sample. This may be due to the NiO additive producing a higher electrical conductivity or higher leakage characteristic in the samples. The ferroelectric properties such as remanent polarization [P _r] and coercive field [E _c] are shown in Table 1. Based on the results, the 1 vol.% samples showed the optimum properties combining between the ferroelectric and ferromagnetic properties (M _r = 0.02 emu/g, H _c = 4.51 kOe, P _r = 13.1 μC/cm², and E _c = 9.9 kV/cm) of this composite system. These ferromagnetic and ferroelectric properties were considerably high for single-phase multiferroic materials 11 12 and other multiferroic composites 13 14 .

Figure 4 shows plots of the relative permittivity and loss tangent as a function of temperature at various NiO concentrations. Two phase transition peaks in the permittivity curve were observed for the pure BZT93. The relative permittivity and loss tangent curves for the pure BZT93 ceramic are similar to those reported in a previous work 8 15 . Furthermore, all samples showed a weak frequency dispersion of the relative permittivity. However, an obvious change in the relative permittivity curve was observed when NiO was added to the samples. The transition temperature [T _m] at maximum relative permittivity [ε _{r, max}] decreased from 105°C for the pure BZT93 ceramics to 60°C for the 1.0 vol.% sample, then gradually decreased to 57°C for the 3.0 vol.% sample. Moreover, the maximum relative permittivity decreased from 12,000 for the pure BZT93 ceramics to 3,200 for the 3.0 vol.% samples. In addition, the two phase transition temperatures merged into a single diffuse phase transition at higher NiO contents (Figure 4d). To check the effect of NiO on the degree of the diffuse phase transition, diffuseness parameter [δ_γ ] was determined using the following expression:

ε r,max ε r = exp ( T - T m ) 2 2 δ γ 2

The value of δ_γ was determined from a plot of ln (ε _{r, max} /ε_r ) versus the (T - T _m)² 16 . The values of δ_γ as a function of NiO content are shown in Table 1. The parameter δ_γ increased with increasing NiO content, confirming that the addition of NiO promoted the diffuse phase transition of the composites.

Huang and Tuan proposed that Ni ions could substitute the Ti ions in BaTiO₃ lattices 17 . It has also been reported that La³⁺ doped at the Ti site of BaTiO₃ ceramics exhibits a change in the transition temperature as well as a pronounced diffuseness transition 18 19 20 21 22 . The La ions are effective in breaking the long-range order and produce Ti vacancies. This breakage of long-range ordering leads to a reduction of the ferroelectric characteristics and enhances the diffuse phase transition. In our present work, unit cell volume was calculated from XRD diffraction patterns, and the calculation result is listed in Table 1. The calculation result indicated an increase in the unit cell volume after adding NiO. This increase may be due to the Ni ions substituting the Ti ions (at the B site). Therefore, substitution of the Ni ions at the B site may result in breaking the long-range ordering, resulting in a reduction of the ferroelectric behavior with the transition becoming more diffuse 23 . Further, with increasing NiO content, the structure of the composites became more heterogeneous. This may contribute to the diffuse phase transition of the samples. From Figure 4, the increase of loss tangent with NiO content implies a higher electrical conductivity of the composites. However, the highest loss tangent in the present work was lower than 0.035, indicating that the present composites still have a potential for capacitor applications. This result also supports the reason for the presence of the lossy capacitor hysteresis behavior of the composites.