3.1. Properties of the ring-shaped KNN-BNKLZ-BS ceramic samples
The room-temperature XRD patterns revealed that KNN-0.03BNKLZ-yBS ceramic samples (y = 0 − 0.015) were of a pure perovskite structure without second phases, indicating homogeneous solid solutions in the investigated dopant concentrations (Fig. 2 (a)). However, there was a significant change of phase structure as a function of the BS concentration, as shown in the amplified XRD patterns (2θ = 44° to 47°) (Fig. 2 (a)). The detailed structural properties such as constituent phases and crystal symmetry, cell parameters and actual phase fractions are shown in the Rietveld refinement results (Figs. 2 (b) and S3). Both the low Rwp of less than 10% and the S value close to 1 prove high reliability of the resulting data (Table 1) [23]. It can be seen that three polar phases of rhombohedral (R), orthorhombic (O), and tetragonal (T) were present in different ratios as a function of BS concentration. In the absence of BS doping, the KNN-0.03BNKLZ ceramic exhibited a coexistence of R, O and T phases with a fair proportion of O phase(63 %). From the fact that a virgin KNN displays an O symmetry at room temperature, the coexistence of R, O, and T phases suggests that the addition of 0.03 mol% BNKLZ induced the shift of two polymorphic phase transition temperatures of R-O (TR-O) and O-T (TO-T) toward the room temperature. As the BS was added, the volume fraction of the O phase decreased and at the same time the R or T phases increased. Finally, the O phase disappeared at y = 0.01, leaving the R and T phases. Upon further increasing y, the R phase became enriched (y = 0.015). As a result, the addition of BS is also found to be effective to move the TR-O and TO-T into the room temperature region and thus to form the R-T structure.
Table 1
Structural characteristics of KNN-0.03BNKLZ-yBS ceramic samples (y = 0 – 0.015) analyzed by Rietveld refinement method.
BS (mol%)
|
Phase structure
|
Cell parameters
|
Phase fraction (%)
|
Rwp(%)
|
S
|
a (Å)
|
b (Å)
|
c (Å)
|
|
0
|
R-O-T
|
3.9792
|
3.9792
|
3.9792
|
89.7972
|
21.1(R3m:R)
|
9.30
|
1.02
|
|
|
3.9574
|
5.6537
|
5.6586
|
90
|
63.0(Amm2)
|
|
|
|
|
3.9599
|
3.9599
|
4.0103
|
90
|
15.9(P4mm)
|
|
|
0.25
|
R-O-T
|
3.9805
|
3.9805
|
3.9805
|
89.7495
|
31.5(R3m:R)
|
9.75
|
1.08
|
|
|
3.9551
|
5.6170
|
5.7252
|
90
|
44.1(Amm2)
|
|
|
|
|
3.9615
|
3.9615
|
4.0049
|
90
|
24.4(P4mm)
|
|
|
0.5
|
R-O-T
|
4.0070
|
4.0070
|
4.0070
|
89.7434
|
14.9(R3m:R)
|
9.37
|
1.03
|
|
|
3.9577
|
5.6278
|
5.6359
|
90
|
19.1(Amm2)
|
|
|
|
|
3.9670
|
3.9670
|
4.0099
|
90
|
66.0(P4mm)
|
|
|
0.75
|
R-O-T
|
3.9926
|
3.9926
|
3.9926
|
89.8112
|
16.0(R3m:R)
|
5.64
|
1.39
|
|
|
3.9531
|
5.6533
|
5.6847
|
90
|
16.9(Amm2)
|
|
|
|
|
3.9694
|
3.9694
|
4.0117
|
90
|
67.1(P4mm)
|
|
|
1.0
|
R-T
|
3.9984
|
3.9984
|
3.9984
|
89.5362
|
16.3(R3m:R)
|
9.48
|
1.03
|
|
|
3.9687
|
3.9687
|
4.0114
|
90
|
83.7(P4mm)
|
|
|
1.25
|
R-T
|
3.9883
|
3.9883
|
3.9883
|
89.7915
|
25.9(R3m:R)
|
9.94
|
1.09
|
|
|
3.9740
|
3.9740
|
4.0126
|
90
|
74.1(P4mm)
|
|
|
1.5
|
R-T
|
3.9824
|
3.9824
|
3.9824
|
89.9918
|
53.3(R3m:R)
|
9.20
|
1.01
|
|
|
3.9706
|
3.9706
|
4.0092
|
90
|
46.7(P4mm)
|
|
|
According to the εr–T curve of KNN-0.03BNKLZ ceramic ring sample (Fig. 3 (a)), there was a sharp dielectric peak representing a typical ferroelectric-paraelectric transition (i.e., Tc) and the Tc determined from the corresponding dielectric peak position was about 369 °C. The chemical modification with BS induced a gradual decline of Tc with a peak broadening associated with the transition from normal phase to diffuse ferroelectric phase. The loss of Tc was minor (ΔTc ≈ 46 °C) in the studied BS concentrations: the lowest Tc was measured to be 323 °C at y = 0.015. Indeed, the decline of Tc induced by doping with the BNKLZ and BS perovskite compounds was not significant, as compared to the case of the well-known Sb5+ ion doping [26].
For a pure KNN ceramic, there are two intrinsic polymorphic phase transitions, R-O transition at –123 oC (TR-O) and the O-T transition at 200 oC (TO-T). The presence of a phase boundary at each phase transition temperature is typically characterized by slight dielectric anomalies (Fig. 3 (b)). In the absence of BS doping, the KNN-0.03BNKLZ ceramic had the shift of TR-O and TO-T values to about –83 oC and 147 oC, respectively, owing to the effect of Bi3+ and Zr4+ ions that are known to play the roles in decreasing TO-T and increasing TR-O, respectively [27, 28]. When the BS was added, both the dielectric peaks gradually approach the room-temperature region. Finally, the peaks converge to create a broad bulge peak centered at/near room temperature (y = 0.01). Then, the bulge peak becomes weaker, as the BS was added further. The appearance of a dielectric bulge peak is associated with the R-T structure caused by the extinction of O phase. The weakened bulge peak relates to the enrichment of R phase relative to T phase. As a result, the BS doing induces the transition from a triphasic R-O-T to a diphasic R-T structure, varying the fraction of each polar phase. The room-temperature phase structure predicted from such εr–T behaviors is well supported by the above XRD and Rietveld refinement results.
It should be noted that the observed phase evolution significantly affects the magnitude of room-temperature εr (Fig. 3 (c)). The relatively low εr values between 467 − 473 were obtained in the R-O-T region containing a considerable amount of O phase (at less than y = 0.0025). Then, the εr value increased with the reduction of O phase. The peak value of about 1630 was obtained at y = 0.01125 that corresponded to the T-rich R-T structure. However, the εr continuously decreased, as the y increased further with the enrichment of R phase in the R-T structure .
The results indicate that the R-O-T phase boundary, if it has a fair proportion of O phase, can result in the decreased εr at room temperature. This is believed to be due to the formation of a relatively flat εr–T region near room temperature between two dielectric anomalies. On the contrary, the formation of R-T phase boundary, characterized by the bulged dielectric peak formed by merging of two dielectric anomalies (TR-O and TO-T), increases the room-temperature εr value. The suppression of the bulge peak, associated with the enrichment of R phase in the R-T structure, can also lead to the reduction of the room-temperature εr. Therefore, it is found that the magnitude of room-temperature εr can be greatly changed with the type of phase boundary structure, which originates from an intrinsic characteristic of a KNN material system that possesses two temperature-dependent polymorphic phase transitions, i.e., R-O and O-T.
Figure 4 (a) shows the dependence of room-temperature piezoelectric properties on the BS concentration (y). The trend of d33 as a function of BS concentration is found to be roughly similar to but different in detail from that of εr. That is, the d33 shows a continuous increase up to y = 0.01, above which a dramatic decrease follows. It should be noted that this behavior was different from that observed in εr (Fig. 3 (c)). Namely, the εr slightly increases until y = 0.00625, after maintaining nearly constant levels up to y = 0.0025. Then, it shows a large increase up to y = 0.01125, after which it decreases.
Notably, this difference in increasing and decreasing rates of εr and d33 leads to substantial changes in the magnitude of their ratio, eventually giving a wide range of g33 values from 8.0 to 46.9 × 10-3 V·m/N. The maximum g33 (46.9 × 10-3 V·m/N) was observed for the sample with y = 0.0025 thanks to the best combination of d33 (208±1.2 pC/N) and εr (502±3.7). In fact, this g33 value was significantly higher than those found in well-known polycrystalline ferroelectrics including commercial PZT, PMN-PT, and BaTiO3 ceramics (12.6 − 28 × 10−3 V·m/N) (Fig. 4 (b)). It was also comparable to those of textured PMN-PT and PMN-PZT [9, 17] or porous PZT ceramics [14]. We also ascribe a much lower g33 value (23.8±0.2 × 10−3 V·m/N) obtained from the sample with y = 0.01 to the high-εr effect (1571±2.5), in spite of the best d33 property (330.7±2.3 pC/N).
As shown in Fig. 4 (c), the behavior of εr and d33 as a function of BS concentration can be characterized by two regions, i.e., Regions A and B, based on their change rates with respect to the value at y = 0. Region A corresponds to the samples with y = 0 − 0.00625, characterized by a lower increase rate of εr compared to that of d33. For the samples with y = 0.0075 − 0.015, however, the trend is reversed and the change (increase or decrease) rate of εr is higher than that of d33, as indicated by Region B. Hence, the d33-to-εr ratios in Region A should be higher than those in Region B, which yields high magnitudes of g33. The peak g33 value was obtained at the highest d33-to-εr ratio (y = 0.0025), while the lowest d33-to-εr ratio (y = 0.015) led to the lowest g33 (indicated by red arrows in Fig. 4 (c)). From this observation, some valuable information can be drawn that the d33-to-εr ratio needs to be increased in such a way that a larger increase in d33 goes with an increase in εr as small as possible in order to increase g33. Generally, it is well known that the variation in magnitude of d33 and εr is similar and they increase or decrease simultaneously upon modification with dopants or processing technique, leading to minor changes in the magnitude of their ratio [16, 22]. Contrary to one’s expectation, our results demonstrate that the independent control of d33 and εr for a polycrystalline KNN ceramic system is possible by means of manipulation of the phase boundary structure to finally yield a greater level of the d33-to-εr ratio.
The transduction coefficient (d × g) was also determined (Fig. S4) because it is an important parameter required to generate higher power in energy harvesting or generator applications [29]. In this work, high levels of transduction coefficient (9,601 − 10,163 × 10−15 m2/N) comparable to that of PZT-5A (9,699 × 10−15 m2/N) were obtained in the R-O-T region (y = 0.0025 − 0.00625). For the sample with the highest g33 (y = 0.0025), the transduction coefficient was 9,765 × 10−15 m2/N and, on the other hand, the sample (y = 0.01) with the highest d33 and R-T structure had its much lower value (only 7,871 × 10−15 m2/N).
The microstructure was characterized for the corresponding sintered ceramic rings (Fig. 5). The bimodal grain structure, characterized by large grains (defined as more than 20 µm) imbedded into the smaller matrix grains (defined as less than 2 µm) [30], was typically observed until y = 0.0125 (Fig. S5). Upon the evolution of microstructure, the large grains became dominant with the increase in y up to y = 0.01, accompanied by the reduction of the small-grained area. With the further increase in y, the size of the large grains dramatically decreased owing to the effect of the solubility limits of low-melting-point Bi3+ ion or its oxides in KNN lattice. The observed structural evolution was typical of many KNN-based ceramics doped with Bi-containing perovskite oxides [30–33]. Regarding the d33 behavior shown in Fig. 4 (a), the well-established grain size effect should have a strong contribution. Namely, as the increased grain boundaries generally constrain the domain wall motion, the decreased grain sizes induced the decline of d33. This is possibly combined with the involved phase boundary structure. It has been reported that a T-rich phase boundary structure in a KNN system is favored for piezoelectricity owing to the more positive role of the irreversible tetragonal-electric induced phase transition compared to the rhombohedral- or orthorhombic-electric induced phase transition [34–36]. Hence, the T-rich R-T phase boundary structure as well as the large-grained structure is considered to be responsible for the enhanced d33 observed at near y = 0.01.
Finally, to investigate the performance of the modified-KNN ceramics in practical device applications, two KNN compositions were introduced to fabricate the prototype accelerometer sensors: they were y = 0.0025 and y = 0.01 with the highest g33 and d33 values, respectively. Most of the dielectric and electromechanical properties measured for three types of ring-shaped samples including PZT-5A are presented in Table S1. Their sensing performances were investigated and compared through the vibration test. Figs. 6 (a) and (b) shows the constituent components and piezoceramic rings fabricated according to the proposed design [25] and the compression-mode prototype accelerometer sensor assembled using them, respectively. The vibration test results obtained from the prepared sensors are shown in Fig. 7. The continuous vibration was applied under cyclic loading with sine-wave modes and the output voltage signals were observed. It can be seen that the peak-to-peak amplitudes of time-dependent output voltages as a function of gravitational acceleration g (= 9.8 m/s2) were different for three sensors (Fig. 7 (a)), indicating different sensing capabilities. The generated output voltage determined at a constant frequency of 159 Hz increased linearly with the applied acceleration (Fig. 7 (b)). In the acceleration range of 1 − 10 g, the values of Pearson’s correlation coefficient R, as a measure of the linear association between two variables, were almost “unity”, demonstrating a perfect linear response to the applied acceleration and thereby excellent reliability of the fabricated sensor prototypes. For the sensor using the modified-KNN (y = 0.0025) with the highest g33, the slope, representing a voltage sensitivity Sv, was determined to be 183 mV/g. Notably, it was significantly higher than that using commercial PZT (142 mV/g) with a 29% increase, showing a clear potential to replace the PZT-based sensors. On the contrary, the Sv for the sensor using the KNN ceramic with y = 0.01 was much lower (119 mV/g), even if this composition had the highest d33.
The voltage sensitivity Sv of a piezoelectric sensor is linearly associated with g33, while its charge sensitivity Sq is related to d33 [6, 7]. By considering only the piezoceramic and seismic mass, those relations are simplified by the following equations:
Here, Cs, Cc, n, ms, g, t and A are the capacitances of sensor and cable, number of piezoceramic layer, weight of seismic mass, acceleration, thickness and area of piezoceramic, respectively, as listed in Table 2. Following this theoretical relation, the test results revealed a strong correlation between the Sv and g33, as shown in Fig. 7 (c): a larger magnitude of g33 leads to a higher level of Sv. Meanwhile, it can be seen that there is a deviation from the direct proportional relationship between the Sv and g33. The observed deviation may relate to the effect of another material factor besides g33, that is, the energy conversion efficiency η. Since the η is only dependent on the electromechanical coupling factor kp and mechanical quality factor Qm, high efficiency for piezoelectric conversion devices usually requires large kp and Qm values [37]. When the η was determined based on the impedance analysis, we obtained different values of η for three samples, as presented in Table 2. The higher η of about 85.9% for PZT-5A compared to those for modified-KNN ceramics (64.1% and 75.8% for y = 0.0025 and y = 0.01, respectively) is considered to be responsible for the Sv higher than predicted by a linear correlation in Eq. (2). The slight deviation from a linear relationship between the Sv and g33 for two KNN compositions might also be explained by such a different-η effect.
Table 2
Properties of modified (m)-KNN and PZT piezoceramic rings and accelerometer sensor prototypes built using them (seismic mass ms = 21.5 g, cable capacitance Cc = 15 pF)
Material
|
Piezoceramic ring
|
Cs (pF)
|
Sv (mV/g)
|
A (mm2)
|
t (mm)
|
d33 (pC/N)
|
εr (-)
|
g33 (10-3V·mN-1)
|
η
|
m-KNN (y = 0.0025)
|
334.86
|
2.55
|
208.2
|
502
|
46.9
|
64.1±0.8
|
311
|
183
|
m-KNN (y = 0.01)
|
306.90
|
2.55
|
330.7
|
1571
|
23.8
|
75.8±1.1
|
838
|
119
|
PZT-5A
|
303.26
|
2.55
|
371.6
|
1607
|
26.1
|
85.9±0.3
|
804
|
142
|