Effect of temperature on dielectric, ferroelectric, and piezoelectric properties of knlns - bnkz ceramics

re measured by Archimedes’ principle. The crystal 
phase structure and microstructure of the samples were examined at room temperature by 
X-ray diffraction (XRD; Bruker D8 ADVANCE) and scanning electron microscope 
(SEM; Hitachi S-4800). To measure electrical properties, both surfaces of the samples 
were polished and coated with silver paste and heated at 500 °C for 15 min. The 
temperature dependence of the dielectric constant was determined using an automatic 
RLC Hioki 3532 impedance analyzer. The samples were poled in a silicone oil bath at 
60 °C by applying an electric field of 35 kV/cm for 20 min and then aged for 24 hours. 
The temperature dependence of the piezoelectric properties of the ceramic samples was 
determined using an impedance analyzer (HP 4193A and RLC Hioki 3532) to measure 
the spectrum of radial resonant vibration of the KNLNS-BNKZ samples at different 
temperatures. The ferroelectric hysteresis loops were examined by the Sawyer-Tower 
method. 
3. RESULTS AND DISCUSSION 
3.1. Phase structure and microstructure of KNLNS-BNKZ ceramics 
Figure 1 shows the XRD pattern in the 2θ range of 20°-80° measured at room 
temperature with Cu-K radiation of wavelength 1.5405 Å for the KNLNS-BNKZ 
ceramics. The inset in Figure 1 is an enlarged view of the XRD pattern simulated with 
Gaussian fitting functions of the KNLNS-BNKZ ceramic sample at 2θ 45.5°. 
Figure 1. XRD pattern of a KNLNS-BNKZ ceramic sample. The inset is an enlarged 
view of the XRD pattern simulated with Gaussian fitting functions at 2θ 45.5° 
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Figure 1 shows that the KNLNS-BNKZ ceramic sample exhibits a pure perovskite 
phase. No secondary phase was detected, indicating that BNKZ diffused into the KNN 
matrix to create a homogeneous KNLNS-BNKZ solid solution. The simulation of 
diffraction peaks at 2 45.5° by Gaussian functions confirmed that the phase structure 
of the ceramic sample is a mixed phase of rhombohedral and tetragonal (R-T) phases, 
characterized by the overlap of (002)T, (200)T, and (200)R diffraction peaks with an 
extended diffraction peak shape (Qin et al., 2016). This result is consistent with the 
research of Zhang et al. (2018) on the effect of sintering temperature on the phase 
structure of 0.9625(K0.48Na0.52)(Nb0.6Sb0.4)O3-0.0375Bi0.5(Na0.82K0.18)0.5ZrO3 ceramics. It 
showed that at the sintering temperature of 1,140 °C, the phase structure of the ceramic 
sample is a mixture of two rhombohedral-tetragonal phases (R-T). 
The density of the ceramic samples was determined by the Archimedes method. 
The mass of the sample when weighed in air is m1, and m2 is the mass of the sample when 
totally submerged in ethanol. Thus, the D density of the sample is calculated by Equation (1): 
EthanolD
mm
m
D
21
1
−
=
where Dethanol = 0.791 g/cm
3 is the density of ethanol. 
Based on Equation (1), the densities of the KNLNS-BNKZ ceramic samples with 
sintering temperature of 1,060 °C were determined, as shown in Table 1. 
Table 1. The density of KNLNS-BNKZ ceramic samples 
Sample m1 (g) m2 (g) m1 - m2 (g) Density, D (g/cm3) D (g/cm3) 
1 0.5704 0.4677 0.1027 4.39 
4.39 
2 0.5658 0.4636 0.1022 4.38 
3 0.5780 0.4739 0.1041 4.39 
4 0.5394 0.4421 0.0973 4.38 
5 0.6127 0.5023 0.1104 4.39 
Table 1 shows that the average density of the KNLNS-BNKZ ceramic has a 
relatively high value of 4.39 g/cm3. This result is consistent with the microstructure image 
(SEM) of the ceramic shown in Figure 2. 
Figure 2 shows an SEM image of a KNLNS-BNKZ ceramic sample fabricated by 
the two-step sintering technique with the sintering temperature T2 = 1,060 °C. We used 
the linear cutting method to evaluate the average grain size of the ceramics. It can be seen 
that the ceramics show cubic-like grains, which is the characteristic shape of KNN-based 
ceramics (Ramajo et al., 2014). The microstructure of the ceramic is relatively dense with 
closely packed grains and few pores, consistent with its high density (4.39 g/cm3). The 
average grain size calculated with the linear cutting method has a value of 1.3 m. 
(1) 
Phan Dinh Gio and Bui Thi Tuyet Nhung 
38 
Figure 2. SEM micrograph of a KNLNS-BNKZ ceramic sample sintered at 
T2 = 1060 °C for 5 hours 
3.2. The temperature dependence of the dielectric properties of KNLNS-BNKZ 
ceramics 
To determine the dependence of the dielectric properties of KNLNS-BNKZ 
ceramics on temperature and frequency, the automatic RLC Hioki 3532 analyzer was 
used to measure capacitance Cs and dielectric loss tan versus temperature at different 
frequencies. Samples were placed in an oven and measured at frequencies of 1 kHz, 10 kHz, 
100 kHz, and 1,000 kHz. The results are shown in Figure 3. 
Figure 3. Temperature dependence of dielectric constant ε (a) and dielectric loss 
tanδ (b) of KNLNS-BNKZ ceramics measured at frequencies of 1 kHz, 10 kHz, 
100 kHz, and 1,000 kHz 
(a) (b) 
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Figure 3(a) shows the temperature-dependent dielectric constant (-T) curves of a 
KNLNS-BNKZ sample measured at frequencies from 1 kHz to 1,000 kHz. As shown, 
two dielectric peaks were clearly observed over the measurement temperature range. One 
peak appeared near room temperature and may be related to the rhombohedral-tetragonal 
ferroelectric phase transition (TR-T) (Wang et al., 2014). This result is consistent with the 
structural phase evolution mentioned above. The other peak in the higher temperature 
range corresponds to the transition from the tetragonal ferroelectric phase to the cubic 
paraelectric phase (TC Curie temperature). This peak is not as sharp as normal for 
ferroelectric materials, but is broad, which is typical for a diffuse transition. This is one 
of the characteristics of relaxor ferroelectrics with disordered perovskite; therefore, the 
temperature corresponding to the TC peak is often denoted Tm (Tm is an average value of 
TC). At the measurement frequency of 1 kHz, the Tm value of the ceramics is determined 
to be 182 °C. The results also show that when the measurement frequency is increased, 
the maximum of εmax decreases and the temperature Tm corresponding to the maximum 
εmax shifts toward higher temperatures. This is contrary to the normal ferroelectric 
behavior of KNN, where the position of the (-T) peak remains nearly constant versus 
temperature as the frequency increases. The above results show that the dielectric 
properties of the ceramics strongly depend on the frequency of the external field, meaning 
that there is dielectric dispersion. This is an important characteristic of relaxor 
ferroelectrics (Xu, 1991). 
Figure 3(b) shows the dependence of dielectric loss tan on the temperature of the 
KNLNS-BNKZ ceramic sample measured at the frequencies of 1 kHz, 10 kHz, 100 kHz, 
and 1,000 kHz. The results show that the (tan -T) curves have low loss values and are 
stable over a wide temperature range from room temperature to about 200 °C. When the 
temperature rises above 200 °C, tan increases sharply due to the conductive mechanism 
(Zhao et al., 2011). 
3.3. The temperature dependence of the piezoelectric properties of KNLNS-BNKZ 
ceramics 
Figure 4 shows the radial resonant vibration spectra of a KNLNS-BNKZ ceramic 
sample, representing the frequency dependence of the impedance Z, measured at different 
temperatures from 30 °C to 160 °C. Based on these vibration spectra, we use the following 
equations from the IRE-61 Standards (Jaffe et al., 1961) to calculate the planar 
electromechanical coupling factor kp and the d31 piezoelectric coefficient: 
2
1
)(574.0395.0 
−+
−
=
psp
ps
p
fff
ff
k
ET skd 1133
12
3131 ..1085.8 
− =
(3) 
 (2) 
Phan Dinh Gio and Bui Thi Tuyet Nhung 
40 
Figure 4. The radial resonant vibration spectra of KNLNS-BNKZ ceramic 
measured at different temperatures from 30 °C to 160 °C 
The temperature dependence of the electromechanical coupling factor kp and the 
d31 piezoelectric coefficient of the KNLNS-BNKZ ceramics were determined from 
Equations (2) and (3). As shown in Figure 5, when the temperature increases from room 
temperature to 100 °C, both kp and d31 remain almost constant, fluctuating about 0.47-0.45 
and 145-125 pC/N, respectively. They then decrease sharply as the temperature increases 
to near the Curie temperature of the ceramics (182 °C). This indicates that near 100 °C 
the orientation of the after poling domains is very stable (Jaffe et al., 1971). 
Figure 5. The kp and d33 values of KNLNS-BNKZ ceramics as a function of 
temperature 
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3.4. The temperature dependence of the ferroelectric properties of KNLNS-BNKZ 
ceramics 
The ferroelectric properties of a KNLNS-BNKZ ceramic sample were determined 
at different temperatures by the Sawyer-Tower method. Figure 6 shows the shapes of the 
P-E ferroelectric hysteresis loops of a ceramic sample for temperatures from 30 °C to 160 °C. 
As shown in Figure 6, the hysteresis loops of the KNLNS-BNKZ ceramic sample 
measured at different temperatures have the characteristic shape of ferroelectric material: 
well-saturated P-E hysteresis loops. When the temperature increases from room 
temperature to 100 °C, the shape and size of the hysteresis loop remains almost the same; 
however, when the temperature rises above 120 °C, the shape of the hysteresis loop 
becomes narrower, and especially at 160 °C, the hysteresis loop is very thin and almost 
straight. As determined above, the Curie temperature of the ceramic sample is 182 °C; 
thus, above the TC temperature, the ceramics will exist in the paraelectric phase, and the 
relationship between the polarization P and the electric field E will be linear (Xu, 1991). 
The remanent polarization Pr and the coercive field EC were determined at 
different temperatures from the shape of the ferroelectric hysteresis loops. Figure 7 shows 
the temperature dependence of Pr and EC for the ceramics. It can be seen that the change 
in the remanent polarization Pr is very small (Pr 18.9-17.0 C/cm2) over the temperature 
range 30-100 °C, then decreases sharply from 17.0 C/cm2 to 1.2 C/cm2 when the 
temperature increases from 100 °C to 160 °C (Tc = 182 °C). In addition, the coercive field 
EC decreased slightly from 5.24 kV/cm to 4.00 kV/cm when the temperature increased 
from room temperature to 160 °C, possibly due to increased mobility of the domain wall 
when temperature increases (Xu, 1991). 
Thus, similar to piezoelectric properties in the range from room temperature to 
100 °C, the ferroelectric properties of the KNLNS-BNKZ ceramic are largely stable. 
Figure 6. The P-E hysteresis loops of a KNLNS-BNKZ ceramic sample measured 
at temperatures from 30 °C to 160 °C 
Phan Dinh Gio and Bui Thi Tuyet Nhung 
42 
Figure 7. The Pr and EC values of KNLNS-BNKZ ceramics as a function of 
temperature 
4. CONCLUSIONS 
The 0.96(K0.48Na0.48Li0.04)(Nb0.95Sb0.05)O3-0.04Bi0.5(Na0.82K0.18)0.5ZrO3 lead-free 
piezoceramics were successfully fabricated by a two-step sintering method with the 
second step sintering temperature, T2 = 1,060 °C for 5 hours. The ceramics possess high 
density (4.39 g/cm3) and the phase structure is the rhombohedral-tetragonal (R-T) mixed 
pure perovskite phase. The temperature dependence of the dielectric, ferroelectric, and 
piezoelectric properties were determined. Changes in the electromechanical coupling 
factor, kp, the d31 piezoelectric coefficient, and the remanent polarization Pr are very small 
over the temperature range from room temperature to 100 °C. With the above properties, 
KNLNS-BNKZ ceramics can be used to fabricate ultrasonic transducers. 
ACKNOWLEDGMENTS 
 This research is funded by Vietnam National Foundation for Science and 
Technology Development (NAFOSTED) under grant number 103.02-2019.08. 
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