Dielectric and Piezoelectric Studies of Dysprosium Doped BZT-BCNT Perovskite Ceramic System for Sensors and Actuators Applications

Perovskite structured Dy doped BZT-BCNT system has been investigated with variation of Dy doping through the solid-state reaction method. X-ray diffraction (XRD) and microscopic studies conrm the results in the rhombohedral stoichiometry without any secondary phases, and uniform grain growth, respectively. Tolerance factor and electronegativity also conrm the presence of Dy doping in BZT-BCNT ceramic system with a stable rhombohedral crystallinity forming vacancy sites for oxygen ions. Optimum piezoelectric properties in non-Pb piezoelectric ceramic systems were obtained due to trivalent donor Dy doping at A-site and pentavalent donor Nb at B-site are possible members for actuator and energy harvesters. Dielectric and piezoelectric studies signifying the rhombohedral perovskite behavior of this lead-free ceramic system confess that the proposed investigation could yield further potential piezoelectric sensors and actuators applicative futuristic studies.

The dielectric and PE properties of the sintered ceramic powder are attributed to the volatile and precarious nature of the dopants, conjoining with the sintering conditions, microstructural, temperature dependent rhombohedral-tetragonal phase transition, surfactant effect on grain size, and composition ratios [20][21][22]. They are usually accounted for in terms of dielectric loss, tolerance factors, and electronegativity of the system. Enhanced PE coe cients are realized from the crystallinity and phase convergence nature of piezoceramic materials such as Barium Titanate (BaTiO 3 or BT), Barium Potassium Niobate (Ba 0.5 K 0.5 NbO 6 or BKN), Bismuth Potassium Titanate (Bi 0.5 K 0.5 TiO 3 or BiKT) and their modi ed compositions [23]. Notably, the electrical and mechanical energies of Barium-Zirconium-Titanate-Barium-Calcium-Titanate (BZT-BCT) is highly tunable [28][29][30][31] compared to the single phase BaTiO 3 owing to its binary-pseudo ferroelectric system [32]. The morphotropic phase boundary (MPB) separates the rhombohedral (Zr-rich near BZT site) and tetragonal (Ti-rich near BCT site) phases. In summary, the dopants of soft and hard ceramics in Lead-free systems play a vital role in evincing the PE properties leveraging electronic structure and high polarizability [33].
Soft [34] and hard [35] electroceramics are constituted by doping donor (at A-site) and acceptor (at B-site) cations of similar/(higher, and lower) ionic radii, respectively. The soft ceramics hold good for sensing and actuating applications [27], while the hard ones are principally used in high voltage, mechanical loads, and power applications [36]. The effect of modi ed composition and donor/acceptor rare-earth ions on the MPB of BZT-BCT has been explored to yield the optimal PE properties [37][38][39][40][41][42][43][44]. Nevertheless, a very few investigations were available on Nb-doped BZT-BCT [41,43,44] (i.e., BZT-BCNT) out of which, in particular, Dy 3+ dopant system is not available to the best of our knowledge.
In this study, Dy 3+ doped 0.48(BZT)-0.52(BCNT) series, has been synthesized by a solid-state reaction method. The stoichiometric composite structure effect of BZT-BCNT in the presence of donor-Dy at A-site over rhombohedral structure is evaluated. Furthermore, the stability of that unit cell corroborated by the calculated tolerance factor and oxygen vacancy formation is veri ed by the calculated electronegativity.
The effect of the presence of Dy at A-site, and Nb at B-site over microstructure and functional properties is characterized by the XRD, apparent density, dielectric, and piezoelectricity confessing the optimum PE properties of the proposed structure of non-Pb PE electroceramic system. This work prescience the capabilities of suggested material for speci ed prospective piezoelectric energy harvesters, multi-layer capacitors, and actuator applications.  (Table 1), and ball-milled these respective batch systems by using zirconia balls and ethanol in a polyethylene jar for 12 h. These ball-milled powders were ltered and dried at 100°C for 24 h, followed by calcination at 1150°C for 4 h. The set powders were ball-milled by using zirconia balls and ethanol in a polyethylene jar for 12 h to obtain agglomerate-free powders that were ltered and dried at 100°C for 24 h. The batch calcined powders were mixed with 5 wt% Polyvinyl Alcohol (PVA) binder, and compacted into pellets (with 12 mm in diameter and 2-3 mm in thickness by using a steel-die and conventional hydraulic press with a uniaxial pressure of 700-900 kg/cm 2 ), and sintered the respective green bodies at 1450°C for 3 h with a heating rate of 5°C/min, and after the sintering process, the samples were cooled to room temperature in the furnace. Dy doped BZT-BCNT powder ceramic systems were analyzed by using X-ray diffraction (XRD) with a Bruker Endeavor X-ray diffractometer model D4/MAX-Bat at a scanning rate of 0.02 °/min over a range of Bragg angles 2θ between 20 and 70°. The apparent densities of the respective sintered samples were measured through the Archimedes method. As sintered ceramic surfaces were analyzed with scanning electron microscopy (SEM; JEOL model JSM 840A). An air-dry silver paste was applied on the polished pellet surfaces to form the electrodes. The electrode samples were characterized for dielectric constant (ε), and dissipation factor (tan δ) using a 1260A Impedance Analyzer. The samples were electrically poled in a silicon oil bath at 100°C by applying a dc eld of 20 kV/cm. After aging for 1 day, the poled samples were characterized for piezoelectric properties by using a Berlincourt piezo-d-meter.

Results And Discussion
3.1 X-ray Diffraction Fig. 1(a) shows the powder XRD patterns of Dy doped 0.48BZT-0.52BCNT ceramics. The XRD peaks were indexed to a perovskite-type of rhombohedral structure, which shows a single rhombohedral phase as can be evidenced in the BZT-BCT phase diagram [47]. XRD patterns show sharp crystalline peaks that show the Dy diffusion in the BZT-BCNT lattice to form a solid-solution with the single-phase rhombohedral perovskite structure and no secondary phases are found that can be confessed by the tolerance factor in section 3.2. However, several reports con rm the rhombohedral-tetragonal phase transition with the variation in BZT:BCT ratio, and/or dopant modi cations at MPB [48][49][50]. To verify, we
Where t is the tolerance factor, r A is the ionic radii of the cations at A-site, r B is the ionic radii of cations at B-site, and r O is the ionic radii of the oxygen at respective sites in the perovskite unit cell. Tolerance factor values are tabulated in table 2. The tolerance factor is ranged in rhombohedral symmetry, and the rhomobhedrality is enhanced with Dy doping. It is well known that the trivalent ions could occupy B-site (r < 0.87 Å) or A-site (r > 0.94 Å), respectively. Dy 3+ (r = 1.083 Å) substitutes Ba 2+ (r = 1.61 Å), predominantly enhancing the rhombohedrality in the lattice. Furthermore, no secondary phases are evidenced from the XRD studies. Hence, it con rms the formation of stable rhombohedral (R3m) perovskite structure.  (Table 2) evaluates the oxygen vacancies and shows the tendency of the atomic bonding depending on the electron-ionization energy. It is well known that the lanthanides tend to replace Ba 2+ rather than Ti 4+ and make it a donor or acceptor doping depending on its ionic radii in the unit cell. Electronegativity is the difference between the sum of cations and anions that re ects the tendency of the bonded atom to attract shared electrons. Therefore, the greater the electronegativity, the greater the attraction of electrons, and the nature of bond is determined by the differences in the electronegativity. Electronegativity is caused as the chemical bonding between the atoms that are formed which could be positive or negative electrons that determines the bond type i.e., ionic, covalent, or metallic. If the electronegativity is below 0.5, then the bond is said to be a non-polar covalent. If the electronegativity is between 0.5 and 2.1, it is said to be a polar covalent. If the electronegativity is more than 2.1, it is said to be an ionic in nature. The electronegativity of Dy doped BZT-BCNT is in the range of 0.6354 to 0.6316, which attests the polar covalent bonding. The multiple cations in the BZT-BCNT rhombohedral perovskite ceramic system resulted in sharing of Dy 3+ and Ca 2+ ions in the place of Ba 2+ (A-sites) at BZT and BCNT, respectively, also, Nb 5+ replacing Ti 4+ at B-site of BCT resulted in a polar covalent bonding of the material, which can stabilize the piezoelectric (PE) perovskite phase [53,54].

Scanning Electron Microscopy (SEM)
The microstructure of Dy doped BZT-BCNT ceramics is presented in g. 2(a-c) for the 0.2, 0.8, 1 mol%. It is evident that, small concentration of pentavalent Nb enhances grain growth, while Dy doping supported the domain reorientation in aligning and densifying the coarse grains. The oxygen vacancies supported the charge balance in the BZT-BCNT ceramic system due to the sintering process that in turn enhanced the density. The sintering process (temperature and time) caused enhancement in grain growth with the increase in Dy content. The homogeneously dense grains attested by SEM in uenced the net polarization up to 0.8 mol% and thereafter attained saturation. Fig. 3 depicts the variation of density with Dy concentration for sintered ceramic samples. It is well known that the composition of the materials dictates the optimum sintering temperature [55,56]. Initially, the density intensi es with increase in Dy concentration up to 0.8 mol% and reduces thereafter. One of the prominent reasons for lowering the density is the incomplete removal of pore or trapped pore present in the grain. Further, the decrease in density for 1 mol% suggests the limit of addition of dopant to BZT-BCNT ceramics. Therefore, the density study results indicate that the sintering temperature of 1450 °C is optimum for 0.8 mol%. Fig. 4 shows the dielectric properties of Dy doped 0.48BZT-0.52BCNT ceramics characterized at 1 kHz. It is observed in Fig. 4(a) that the dielectric constant of the perovskite structured BZT-BCNT ceramic system increases gradually with temperature and reaches dielectric maximum at 98 0 C which is the Curie transition temperature (T c ), and with additional rise in temperature the dielectric constant declines for pure. It can be witnessed that for Dy doping (0.2 to 1.0 mol%) of BZT-BCNT exhibited a similar trend in this ceramic system. In addition to that, it is observed that maximum dielectric constant is achieved at a lower temperature for Dy doped BZT-BCNT systems up to 0.8 mol%, compared to the pure sample and thereafter the trend reverses. The curie transition temperature in the series has witnessed a similar trend. There are many factors that contribute to the high dielectric constant like, modi cations in the grain size and grain boundary, change in density, release of internal stress, and the defect and domain wall motion [57]. Fig. 4b shows the variation of dielectric loss of Dy doped BZT-BCNT ceramics with temperature. It is detected that at room temperature the dielectric loss decreases gradually with rising Dy doping up to 0.8 mol% (optimum) and thereafter increases at room temperature. Furthermore, it is seen that with the increase in temperature, the dielectric loss decreases and reaches minimum at ~ 80 0 C for all the Dy doping BZT-BCNT ceramic systems, which might be owing to the manifold cationic arrangements or saturation in the perovskite ceramic system.

Dielectric studies
In the rhombohedral symmetry, as Dy doping supported the space charge polarization due to the donor Dy 3+ dopant in BZT lattice and pentavalent Nb 5+ in the BCT lattice by balancing the charge and oxygen vacancies in the present system [58,59]. The polarization of a dielectric material is in uenced by the dipolar, electronic, ionic, and interfacial interactions in the unit cell. The incorporation of Dy in the BZT and Nb in BCT drastically in uenced the polarization in attaining optimum electrical properties that resulted in the increased dielectric constant. The dielectric constant (e RT =1342) at room temperature, dielectric maximum (e Tc =10667) at the Curie Temperature T c (81°C) is found to be optimum for 0.8 mol% at the frequency of 1 kHz. It was reported that there was a mixed trend of the dielectric loss and Curie temperature with Dy incorporation in BaTiO 3 [60], similarly, we con rm the same trend in this perovskite BZT-BCNT ceramic system. Fig. 5 shows the piezoelectric coe cient d 33 as a function of Dy concentration. It can be observed that with the increasing Dy concentration, the piezoelectric constant d 33 increases up to 0.8 mol% and attains a maximum value 368 pC/N and then decreases sharply with further Dy concentration. It is believed that the observed high piezoelectric properties should be ascribed to the phase, polarization due to multiple cations, dopants, enhanced grain size (possible net polarization from respective domains), dense ceramics [61]. Further, lattice distortion provoked by Dy doping content may be advantageous to ferroelectric domain reorientation during poling process have some bearing on the improvement of piezoelectric properties. However, with the increasing of Dy content to 1 mol%, Dy ions should occupy the A-site of Ba 2+ , which might cause deformation in the perovskite ABO 3 lattice to make a phase transition completed, resulting in a reduction of piezoelectric coe cient d 33 [9,61]. This shows that a proper addition of Dy induces a more signi cant piezoelectric activity in these perovskites, which can be chie y ascribed to nature of the dopant and its content, the densi cation and grain morphology of the ceramic system.

Conclusion
We report the perovskite structured Dy doped BZT-BCNT system resulting in the rhombohedral stoichiometry without any secondary phases being observed. Superior net polarization due to homogeneous dense coarse grains, which not only enhanced the electrical properties but also favoring the piezoelectric applicative investigation while exhibiting the rhombohedral phase of Dy doped BZT-BCNT ceramic system. Tolerance factor and electronegativity con rm the stable rhombohedral crystallinity in forming vacancy sites by oxygen ions and polar covalent bonding in the perovskite phase, respectively. Optimum dielectric and piezoelectric properties are observed at 0.8 mol% due to donor Dy 3+ doping at A-site and donor Nb 5+ at B-site in the non-Pb perovskite structured (BZT-BCNT) ceramic system. The obtained results could be helpful in developing sensors and actuators. Table 1 Stoichiometric compositions of the Dy doped BZT-BCNT ceramic system.