The XRD patterns of 0.98NKN-0.02BZT ceramics with sintering temperatures were varied from 1,080°C to 1,120°C as shown in Figure . As seen from these XRD patterns, the 0.98NKN-0.02BZT phase sintered at various sintering temperatures was well developed without a second phase. It can be seen clearly in Figure that the 0.98NKN-0.02BZT ceramic had an orthorhombic phase that was not changed for all samples. The orthorhombic phases are characterized by (200) and (020) peaks splitting at approximately 45.5°, and when the sintering temperature was increased, the peak form is almost the same. These results indicated that the 0.98NKN-0.02BZT ceramics with various sintering temperatures are regarded to have an orthorhombic structure. However, the degree of crystallization of all samples is completely different. The 0.98NKN-0.02BZT ceramics were well crystallized with increasing sintering temperature. However, as the sintering temperature was increased above 1,090°C, the peak shape became flatter than that of 0.98NKN-0.02BZT ceramics sintered at 1,090°C. It can be inferred that 0.98NKN-0.02BZT ceramics sintered above 1,090°C lost their well-developed orthorhombic structure. These structural results cause a decline of electrical properties such as d33 and dielectric constant.
XRD patterns of 0.98NKN-0.02BZT ceramics.
XRD patterns of 0.98NKN-0.02BZT ceramics near the (020) and (200) planes.
Figure shows the SEM images of the 0.98NKN-0.02BZT ceramics sintered at various sintering temperatures. As shown in Figure , 0.98NKN-0.02BZT ceramics sintered at 1,080°C showed a small average grain size, although a dense microstructure was formed. It can be inferred that the grain growth was not completed due to low sintering temperature. Figure exhibits the SEM images of 0.98NKN-0.02BZT ceramics sintered at 1,090°C. The cavities have been reduced, and the sample turns into a higher-density microstructure with an increased average grain size. This is according to the kinetic grain growth equation expressed as [13
SEM images of 0.98NKN-0.02BZT ceramics. (a) 1,080°C, (b) 1,090°C, (c) 1,100°C, (d) 1,110°C, and (e) 1,120°C.
where G is the average grain size at the time, n, the kinetic grain growth exponent, t, the sintering time, K0, a constant, Q, the apparent activation energy, R, the gas constant, and T, the absolute temperature. It can be inferred that increasing sintering temperature improves the grain growth. However, with an increasing sintering temperature above 1,090°C, the microstructure was inhomogeneous and the grain size becomes exceptionally huge. These can be the reason for the deterioration of the relative bulk density over 1,090°C as shown in the SEM images.
Figure shows the temperature dependence of the dielectric constant as a function of the temperature for 0.98NKN-0.02BZT ceramics sintered at various sintering temperatures. All samples show transitional peaks and one-transition temperatures at Curie temperature [Tc] of the 0.98NKN-0.02BZT ceramics. The Tc slightly, but not rapidly, decreased with increasing sintering temperature. The Tc for all samples sintered at 1,080°C, 1,090°C, 1,100°C, and 1,110°C is 406°C, 411°C, 417°C, and 421°C, respectively. The Tc increased with increasing sintering temperature owing to considerably increase the K ratio in the A-site of NKN ceramics. This phenomenon of gradually increasing Tc for the 0.98NKN-0.02BZT ceramics is similar to that of the NKN system with an increasing K ratio. The dielectric constants enhanced with increasing sintering temperature. However, when increasing the sintering temperature above 1,090°C, the dielectric constant decreased. From this decreased dielectric constant, it can be inferred that volatile Na and K ions were evaporated at the high sintering temperature and relative bulk density was decreased. The maximum dielectric constant of 0.98NKN-0.02BZT ceramics is 1,951 sintered at 1,090°C.
Temperature-dependent dielectric properties of 0.98NKN-0.02BZT ceramics.
Figure shows the d33
, and relative density of the poled 0.98NKN-0.02BZT ceramics sintered at various sintering temperatures. It is obvious that d33
, and relative density have a similar tendency as a function of sintering temperature. The d33
, and relative density of 0.98NKN-0.02BZT ceramics sintered at 1,080°C are 201 pC/N, 0.33, and 88%, respectively and peaked their maximum values, which are 217 pC/N, 0.41, and 97%, respectively. It can be concluded that the promotion can be attributed to the increase in bulk density, lowering the leakage current, and improving the poling process. With a further increasing sintering temperature above 1,090°C, the piezoelectric properties and relative density decreased. It can be explained that the samples start to heavily volatilize Na and K of 0.98NKN-0.02BZT ceramics. The kp
is calculated by the following equation [14
Piezoelectric properties and relative density of 0.98NKN-0.02BZT ceramics.
where fr is the resonance frequency, fa is the antiresonance frequency, a = 0.395, and b = 0.674 for a planar (kp) mode. Figure shows the ferroelectric properties of the 0.98NKN-0.02BZT ceramics sintered at various temperatures. The Pr is 5.8 μC/cm2 and the coercive electric field [Ec] is 5.4 kV/cm for 0.98NKN-0.02BZT ceramics sintered at 1,080°C. When the sintering temperature was increased up to 1,090°C, the well-saturated ferroelectric properties were obtained, and the values of Pr and Ec of samples sintered at 1,090°C were 10.3 μC/cm2 and 7.2 kV/cm, respectively. Continuously increasing the sintering temperature above 1,090°C, the ferroelectric properties decreased due to heavy volatilization of Na and K at high sintering temperature. The ferroelectric properties of 0.98NKN-0.02BZT ceramics have a similar tendency as with the piezoelectric and dielectric properties. The increase of ferroelectric properties might be caused by the increase of the relative bulk density that reduces the leakage current, promoting the polarization process.
P-E hysteresis loops of 0.98NKN-0.02BZT ceramics.