Piezoelectric crystals have multifunctional applications in the electronic industry for various devices such as sensors, transducers, actuators, resonators, filters, microphones etc.  . These devices work based on the fundamental piezoelectric properties, i.e., polarize electrically when subjected to a mechanical stress and conversely, induce mechanical deformation (change in the shape of crystal) under an applied electric field  . As of now, lead (Pb) based systems are exhibiting high piezoelectric constants due to the presence of Morphotropic Phase Boundary (MPB), where the two phases co-exist. However, its toxicity to humans and environment, has motivated to look for lead-free piezoelectric systems. Besides the advantage of MPB, a high Curie temperature (TC > 250˚C) is an essential factor for exhibiting stable piezoelectric characteristics over a wide temperature range  .
Among such high temperature piezoelectric systems, Na0.5Bi0.5TiO3 (NBT) and K0.5Na0.5NbO3 (KNN) are believed to replace lead-based systems with high “TC”  . Smolenskii et al.  found NBT system with high piezoelectric properties compared to that of lead-based systems. Similar to PZT and PMN-PT, NBT  bismuth (Bi3+) based compounds possess ion off centering driven by chemically active 6s2 lone pair on Bi3+ ion, which is responsible for large structural distortions leading to greater polarizability. Nevertheless, due to the high coercive field (EC), NBT requires very high field to change the domain orientation while poling which hinders the use of NBT in the conventional device  .
The electrical and structural properties of NBT system can be modified by the substitution of larger K+ ion in the Na-site which results in lattice distortion and can enable the formation of MPB with co-existence of rhombohedral and tetragonal phases  . In turn, the K+ ion acts as a pinning site for long-range lattice diffusion thereby resulting in reduced grain size and increase in the domain mobility. This facilitates in achieving the low “EC” and enhanced ferro/piezo electric properties for NKBT ceramics  . A brief comparison of recently reported structural, ferroelectric and piezoelectric characteristics of both lead-based and lead-free      composites are given in Table 1. Compared to lead-based systems, the strain values are low in the case of lead-free systems due to their higher “EC” and hence require higher electric fields to induce the polarizability during the poling process. Hence, the objective of the present work is to enhance the strain and converse piezoelectric properties with reduced grain size by preparing dense Na0.4K0.1Bi0.5TiO3 (NKBT) ceramics.
2. Materials and Methods
2.1. Sample Preparation
Na0.4Bi0.5TiO3-K0.1Bi0.5TiO3 solid solution was prepared by a conventional solid-state reaction method. High purity precursors (>99% from Sigma Aldrich, USA) used to prepare NKBT ceramics are Sodium carbonate (Na2CO3), Potassium carbonate (K2CO3), Bismuth oxide (Bi2O3) and Titanium oxide (TiO2). Stoichiometric amounts of carbonates and oxides were dried at 100˚C for 4 h to remove any adsorbed moisture. Initially, the mixture of powders was ground in a mortar with acetone as a medium for 6 h for proper homogeneity and then
Table 1. Recently reported structural, ferroelectric and piezoelectric properties of lead-based and lead-free systems.
PLZT: (Pb0.93La0.07)(Zr0.60Ti0.40)O3PLZT (7/60/40). PMN-PT: (1 − x)Pb(Mg0.33Nb0.67)O3 − xPbTiO3 (x = 10) PMN-10PT. KNN: K0.5Na0.5NbO3. LNN-BT: (100 − x)Li0.12Na0.88NbO3-xBaTiO3 (x = 12.5) LNN-12.5BT. NBT: Na0.5Bi0.5TiO3. NKBT: Na0.4K0.1Bi0.5TiO3.
solid state reacted at 800˚C for 2 h to remove the volatile materials. The resultant powder was pulverized and X-ray powder diffraction (XRD) analyses were performed using a PANalytical X’PERT PRO MRD. To attain uniform particle size, calcined powders were ball milled in ethanol using Zirconia balls at 100 rpm for 12 h. 1 wt% Polyvinyl Alcohol (PVA) was added as a binder and uniaxially pressed into pellets (10 mm diameter with 1 mm thick) under a pressure of 145 MPa. Green pellets (density ~62% - 64%) were sintered at 1150˚C for 4 h at a slow heating rate of 3˚C/min in a high temperature programmable furnace. The measured volume shrinkage of the sintered NKBT pellets was around 30% - 35%. At 1150˚C, the bulk density of the NKBT ceramic was in the range of 5.75 - 5.86 g/cm3, which is~96% - 98% of the theoretical density (5.96 g/cm3), measured in accordance with Archimedes principle using water as the immersion fluid.
The phase content and crystallinity of the sintered pellets are measured using PANalytical X’PERT PRO MRD with Cu Kα (λ = 1.5417 Å) radiation at room temperature in the 2θ range from 20˚ to 80˚ with a step size of 0.0167˚. The Rietveld analysis of the X-ray diffraction pattern was carried out for the MPB composition using FULLPROF software package. The microstructures of the polished samples are characterized using field emission scanning electron microscopy (FE-SEM) (JSM-7600). For proper grain boundary determination, the sintered NKBT pellets were thermally etched at 1100˚C for 10 min (50˚C lower than the sintering temperature). The average grain size was determined using Image J software. In order to measure dielectric and ferroelectric measurements, samples were electroded using silver paste on both sides of the sintered NKBT pellets, dried and fired at 100˚C for 2 h. An Alpha-A high-resolution impedance analyzer (Novocontrol GmbH, Germany) was used for dielectric measurements. The electric field induced polarization (P-E) hysteresis loop, piezoelectric strain-electric field (S-E) curves and current-electric field (I-E) measurements were conducted using a standard FE loop tester (aixACCT TF Analyzer 2000) at 1 Hz connected with a miniature plane-mirror interferometer and the accessory laser interferometric vibrometer (SIOS-SP-S 120). For bipolar strain measurements, the maximum applied electric field was 5.5 MV/m at 1 Hz.
3. Results and Discussion
Figure 1(a) shows the room temperature X-ray diffraction patterns of calcined (800˚C/2 h) and sintered (1150˚C/4 h) NKBT ceramics. All the diffraction peaks exhibited pure perovskite structure without any secondary phases within the measured 2θ range of 20˚ to 80˚, indicating the formation of complete solid solution in the studied composition. A visual inspection of the peaks represented in Figure 1(b) imply that the splitting of (111)R and (200)T at 2θ angles around 40˚ and 46˚ indicates that K-substituted NKBT have co-existence of rhombohedral (R) and tetragonal (T) phases. The unit cell parameters of NKBT composition was determined by Rietveld refinement as shown in Figure 2. Since, the chosen composition is expected to have both the phases, the parameters of the R3c space group with hexagonal lattice parameters  and tetragonal P4bm space group  were simultaneously used for Rietveld refining to obtain the minimum error in fitting of the lattice parameters. The obtained Rietveld refined values of NKBT are found to be consistent with the previously reported values  . The refined perovskite lattice parameters, discrepancy factor (Rwp), expected values (Rp) and percentage of both R- and T-phases along with goodness of fit index (χ2) of NKBT are given in Table 2.
Figures 3(a)-(e) show the microstructure of as-sintered and thermally etched fractured surface of NKBT ceramics at different magnifications. A relatively high dense (96% - 98%) and uniform morphology was seen from the microstructure with uniform grain size distribution ranging from 1 - 1.2 μm. The measured
Figure 1. (a) X-ray diffraction patterns of calcined and sintered NKBT ceramics (b) enlarged view of (111) and (200) reflections representing fitted rhombohedral and tetragonal splitting.
Figure 2. Rietveld fitted X-ray diffraction pattern of NKBT using R3c (hexagonal) and P4bm (tetragonal) space group.
Figure 3. FESEM micrographs of (a) as-sintered, (b)-(d) thermally etched (1100˚C for 10 min) and (e) fractured surface of NKBT sintered at 1150˚C for 4 h.
grain size of NKBT is much smaller when compared to NBT (7.8 ± 3.2 μm) system  . Therefore, even a smaller substitution of K+ in the NKBT solid solutions
Table 2. Rietveld structural properties of NKBT ceramic sintered at 1150˚C for 4 h.
χ2 = 2.79, Rp = 2.80 and Rwp = 3.47.
inhibits the grain growth. Absence of phase segregation at grain boundaries in the microstructure confirms the complete solubility of K+ in the NBT lattice. From Figure 3(d), it is clearly observed that stripes pattern exist inside the each grain which are the characteristics of domain configuration in ferroelectric ceramics. G. Arlt et al.  ascribed that twinning occurs in all ferroelectric ceramics at a temperature below the structural phase transition in order to minimize the elastic energy. In turn, the fractured surface (Figure 3(e)) confirms the compact structure of fine grained NKBT ceramics with no traces of porosity leading to enhancement in ferroelectric and piezoelectric properties as discussed in the below sections.
Electric field dependent polarization (P-E) hysteresis loop and their corresponding switching current (I-E) of NKBT ceramics are shown in Figure 4(a). The observed well saturated loop depict typical ferroelectric behavior. In the present study, the measured “EC” value of 30 kV/cm which is lowest compared to the reported literature   and “Pr” value of 37 μC/cm2 is comparable to those prepared by conventional solid-state sintering method  . The reduced “EC” could be attributed to the smaller grain size due to the substitution of K+ which in turn reduces the domain width and enables the easy mobility of ferroelectric domains  . In addition, effective poling can be achieved since low electric field is sufficient to cause switching of domains. I-E hysteresis loop exhibits two typical current peaks at the threshold electric field of +EF and −EF at the Ist and IIIrd quadrant. These loading and unloading cyclic electric fields represent intrinsic ferroelectric character, indicating the domain switching behavior in NKBT ceramics.
Figure 4(b) represents bipolar strain curve of NKBT ceramics measured for a maximum electric field of 5.5 MV/m at 1 Hz. The bi-polar strain curve exhibits symmetric “butterfly” curve with a maximum strain of 0.3%. Moreover, in the present study, the negative strain (Sneg) was absent when compared to reported literature  which is an additional feature of NKBT making more suitable for actuator applications. A high converse piezoelectric coefficient ( ) of 554 pm/V is calculated from the ratio of maximum unipolar strain to maximum electric field (Sunipolar/Emax). Rao et al. has reported a maximum strain value of 0.12%, where the grain size decreased and porosity will be increased upon K-substitution in NBT lattice  . A similar study was carried on electric field
Figure 4. (a) and (b) shows polarization hysteresis loops, current (I) and corresponding piezoelectric response (strain) vs. electric field (E) properties of NKBT ceramics.
dependence of strain curves by Liu et al.  reports a maximum strain of 0.15 % ( = 250 pm/V) in NKBT system poled at RT (5 kV/mm for 15 min). The large strain in the NKBT ceramics could possibly be attributed to: extrinsic contribution from movement and switching of non-180˚ domain walls driven by the external electric field in addition to pure piezoelectric response and intrinsic contribution due to change in the shape of the crystal lattice  .
Temperature dependent dielectric constant at different frequencies of NKBT ceramic is shown in Figure 5. The temperature of maximum permittivity (εmax, Tmax) values increases with increasing frequency. However, the opposite trend is noticed for the measured dielectric constant (εr) values. The εr-T plot shows two broad dielectric peaks in the measured temperature range of 50˚C - 500˚C. The first hump between 90˚C - 120˚C exhibits dielectric maxima at ~100˚C, which represents relaxor behavior and corresponds to polymorphic phase transition (PPT) from R- to T-phases. The second broad peak signifies T to C (cubic) transition at ~330˚C. The transition temperature (TC) was found to be increasing with increasing frequency. The possible reason for this kind of diffusive nature could be attributed to the weakening of long-range ferroelectric order, higher internal stresses and space charge effect   . The measured values of “εr” are comparable with the reported NKBT ceramics   .
The effect of reduced grain size on piezoelectric properties in K-substituted NBT system has been studied. A remarkable increase in strain (0.3%) and reduced coercivity (EC = 30 kV/cm) in Na0.4Bi0.5TiO3-K0.1Bi0.5TiO3 system could be attributed to highly dense and uniform microstructure. Interestingly, enhanced converse piezoelectric coefficient, of 554 pm/V, which is higher than the reported literature, could be a potential candidate for actuator applications. Further study includes optimizing the poling conditions (poling field, temperature and time) to enhance piezoelectric charge and voltage coefficients as it offers more understanding of domain switching mechanisms in NKBT. For an actuator, in addition to piezoelectric properties, precise mechanical control is also
Figure 5. Temperature dependent dielectric constant (εr) of NKBT measured at different frequencies.
equally important for device fabrication and the above studies are currently underway.
The authors gratefully acknowledge the SAIF and IRCC IIT Bombay for providing FE-SEM and Broadband dielectric spectrometer facilities.