XRD patterns of BNKT-BST mixed powders are shown in Figure . There were no detectable impurities for all compositions. A separation of (110) main peak (2θ at approximately 32°) was not observed for pure BNKT powder. When 5 mol% BST was added, the (110) main peak was slightly asymmetrical and featured a slight splitting of the BST peak. With increasing BST, peaks that belonged to BST were dominantly shown and led to more splitting.
XRD patterns and plots. (a) XRD patterns of BNKT-BST powders. (b) Plots of relative density and sintering temperature.
Plots of relative density as a function of sintering temperature with different BST contents are shown in Figure . The optimum sintering temperature of BNKT-BST ceramics was 1,125°C at which all samples had densities ranging from 5.72 to 5.81 g/cm3, corresponding to at least 98% of their theoretical values (see Table ). Thus, the samples sintered at this temperature were selected for further characterizations.
Physical and electrical properties of (1-x)BNKT-xBST ceramics sintered at 1,125°C
Figure showed XRD patterns of BNKT-BST ceramics. All compositions showed a pure perovskite phase. BST had diffused into the BNKT lattice and formed solid solutions. All peaks were found to shift slightly to a lower angle. The shift scale was increased with the increasing of BST to a maximum value at x
= 0.20. The slight distortion of the XRD patterns was attributed to larger sized Ba2+
(1.42 Å) and Sr2+
(1.26 Å) ions diffused into the BNKT lattice to replace Bi3+
(1.17 Å), Na+
(1.18 Å), and K+
(1.33 Å) [9
], resulting in the enlargement of lattice constant and lattice energy which induced a phase transformation in order to stabilize the structure [10
]. In Figure , Bi0.5
was a mixed phase between BNT rhombohedral and BKT tetragonal, whereas (Ba0.7
was mainly tetragonal in phase. At x
= 0.05, the peak around 46.5° was slightly asymmetrical, and (202) peak started to split into two peaks of (002) and (200). At x
= 0.10, the intensity of (200) peak was found to decrease, while it was gradually increased for (002) peak. Moreover, (200) peak underwent an asymmetric broadening, and (002) peak obviously split into two peaks. This indicated that the addition of a higher tetragonal BST into BNKT at x
= 0.10 became close to the optimum of rhombohedral and tetragonal phases of the BNKT-BST system. As its crystal structure was considered to contain nearly the same amount of coexisting rhombohedral and tetragonal structures in BNKT-0.10BST ceramic, the optimal dielectric and piezoelectric properties should be obtained in this composition. The addition of a BST content greater than 10 mol% led to a wider separation of the (002) and (200) peaks and showed mainly a tetragonal structure, corresponding to an increase in tetragonality as shown in Table .
XRD patterns of BNKT-BST ceramics. The samples were sintered at 1,125°C. (a) 2θ = 10° to 80° and (b) 2θ = 44° to 48°.
SEM images in Figure confirmed that all ceramics were of high quality and densely sintered at 1,125°C. An addition of BST allowed shortening of the sintering duration to attain a dense sintered bulk with similar grain size. The microstructure of a pure BNKT ceramic revealed a larger grain size (0.60 μm) with a relatively wide grain size distribution compared to BST-added samples. The addition of BST, however, slightly inhibited grain growth, as can be seen from a slight drop of grain size from 0.60 μm for pure BNKT to around 0.39 to 0.47 μm for BST-added samples (see Table ).
SEM micrographs of (1-x)BNKT-xBST ceramics. The samples were sintered at 1125°C. (a) x = 0, (b) x = 0.05, (c) x = 0.10, (d) x = 0.15, and (e) x = 0.20.
Dielectric constant and dielectric loss of (1-x
BST ceramics were plotted as a function of temperature shown in Figure . At Tc
, the highest εr
of 5,006 was observed in pure BNKT. For BST-added samples, the maximum εr
of 4,921 was observed in BNKT-0.10BST ceramic. Since the crystalline structure of BNKT-0.10BST was considered to be near optimum composition having a comparable coexistence of rhombohedral and tetragonal phases, the increase in εr
would be expected. The Tc
of pure BNKT was found to be 320°C. It has been shown that an A-site isovalent additive had the effect of lowering the Tc
]. BST is virtually an A-site isovalent additive in which Ba0.7
has an effective charge of +2, which is the same as +2 of Bi0.5
). Moreover, BST has a much lower Tc
(approximately 42°C) [12
] compared with BNKT; a reduction of Tc
was observed in our system. At room temperature, εr
of pure BNKT was found to be 1,419. The addition of 10 mol% BST showed an optimum εr
of 1,609. As free energy of the rhombohedral phase was close to that of the tetragonal phase, these two phases existing at the BNKT-0.10BST composition easily changed to each other when an electric field was applied. This helped promote the movement and polarization of ferroelectric active ions, leading to the increase of εr
]. With a further increasing BST, a slight decrease in εr
was observed. Phase analysis using XRD patterns indicated that the compositions slightly deviated from the optimal composition, and hence, the lowering of εr
values in our samples seemed reasonable.
Plots of temperature dependence on dielectric constant and dielectric loss. The measurement was done at a frequency of 10 kHz for BNKT-BST ceramics and sintered at 1,125°C.
From Figure , the hysteresis loop of pure BNKT showed the maximum Ec
at approximately 31.49 kV/cm, Pr
at approximately 30.48 μC/cm2
, and Rsq
at approximately 1.10. Ferroelectric property was slightly degraded when BST was added, as can be seen from a decreasing trend in Rsq
, and Pr
. Since BST by itself was known to have a low Ec
(approximately 2 kV/cm) and Pr
(approximately 5 μC/cm2
] compared with pure BNKT, this seemed to be the reason for a reduction of both Pr
observed in BST-added samples. Among BST-added samples, the highest Pr
of 28.14 μC/cm2
was observed for BNKT-0.10BST. Besides, an increase of spontaneous polarization directions due to the coexistence of rhombohedral and tetragonal phases (eight directions for rhombohedral phase and six directions for tetragonal phase) was also a reason that gave a high Pr
in BNKT-0.10BST. Moreover, a decrease of Ec
(approximately 22.96 kV/cm) in BNKT-0.10BST in comparison with that in pure BNKT was also observed at this composition. This decrease in Ec
indicated easier ionic motion, and therefore, the improvement of piezoelectricity would be expected for this composition [14
]. The addition of BST content over 10 mol% caused the material to completely transform to a tetragonal phase, resulting in a slight decrease of Pr
. The reduction of Pr
when the crystal structure changed to be more tetragonal in structure was similar to the previous work on BNT-BST system [2
Plots of polarization as an electric field function of (1-x)BNKT-xBST ceramics. The samples were sintered at 1,125°C. (a) x = 0, (b) x = 0.05, (c) x = 0.10, (d) x = 0.15, and (e) x = 0.20.
Piezoelectric coefficients of (1-x
BST ceramics are listed in Table . The d33
of pure BNKT ceramic was 178 pC/N, which was close to the value of 165 pC/N observed earlier by Hiruma et al. [15
]. The highest d33
of 214 pC/N was observed for the BNKT-0.10BST ceramic. As the crystal structure of BNKT-0.10BST was nearly a coexistence of rhombohedral and tetragonal phases, a flexibility increase in the domain wall could effectively occur. Moreover, Ec
of this composition was lower than that of pure BNKT, whereas Pr
was maintained. Thus, it is obvious that the optimal piezoelectric properties would occur in this composition. The d33
decreased with the further increasing BST content of over 10 mol%. This was supported by phase analysis using XRD which indicated a deviation of the composition from the mixed rhombohedral and tetragonal phases of BNKT-BST system to mainly the tetragonal BST phase. In addition, the change in crystal structure to being more tetragonal may also contribute to the reduction in the piezoelectric performance of BNKT-BST ceramics similar to the reduction in d33
observed in the previous work on BNKT-BZT system [13