Synthesis and electrical properties of lead-free piezoelectric Bi0.5Na0.5TiO3 thin films prepared by Sol-Gel method

Lead-free Bi_0.5Na_0.5TiO₃ (BNT) piezoelectric thin films were deposited on Pt/TiO_x/SiO₂/Si substrates by Sol-Gel method. A dense and well crystallized thin film with a perovskite phase was obtained by annealing the film at 700 °C in a rapid thermal processing system. The relative dielectric constant and loss tangent at 12 kHz, of BNT thin film with 350 nm thickness, were 425 and 0.07, respectively. Ferroelectric hysteresis measurements indicated a remnant polarization value of 9 μC/cm² and a coercive field of 90 kV/cm. Piezoelectric measurements at the macroscopic level were also performed: a piezoelectric coefficient (d_33effmax) of 47 pm/V at E = 190 kV/cm was obtained. The piezoresponse force microscopy data confirmed that BNT thin films present ferroelectric and piezoelectric behavior at the nanoscale level.     

Microstructure and piezoelectric properties of CuO-doped 0.95(K0.5Na0.5)NbO3-0.05Li(Nb0.5Sb0.5)O3 lead-free ceramics

The effects of sintering temperature and the addition of CuO on the microstructure and piezoelectric properties of 0.95(K_0.5Na_0.5)NbO₃-0.05Li(Nb_0.5Sb_0.5)O₃ were investigated. The KNN-5LNS ceramics doped with CuO were well sintered even at 940 °C. A small amount of Cu²⁺ was incorporated into the KNN-5LNS matrix ceramics and XRD patterns suggested that the Cu²⁺ ion could enter the A or B site of the perovskite unit cell and replace the Nb⁵⁺ or Li⁺ simultaneously. The study also showed that the introduction of CuO effectively reduced the sintering temperature and improved the electrical properties of KNN-5LNS. The high piezoelectric properties of d₃₃ = 263 pC/N, k_p = 0.42, Q_m = 143 and tan δ = 0.024 were obtained from the 0.4 mol% CuO doped KNN-5LNS ceramics sintered at 980 °C for 2 h.     

Effect of Eu doping on structure and electrical properties of lead-free (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics

Eu-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (BNBT6-xEu, x=0.00–2.00 at%) lead-free piezoelectric ceramics have been synthesized by the solution combustion method. The effect of Eu doping concentration on the phase structure, microstructure and electrical properties of BNBT6 ceramics has been investigated. The XRD analysis confirms that the europium additive incorporates into the BNBT6 lattice and results in a phase transition from the coexistence of rhombohedral and tetragonal phases to a more symmetric pseudocubic phase. The SEM images indicate that the europium additive has little effect on the ceramic microstructure and the average grain size is about 2.0 μm. The electrical properties of BNBT6 ceramics can be improved by appropriate Eu doping. The 0.25 at% Eu doped BNBT6 ceramic presents excellent electrical properties: piezoelectric constant d₃₃=149 pC/N, remnant polarization P_r=40.27 μC/cm², coercive field E_c=2.95 kV/mm, dielectric constant ε_r=1658 and dissipation factor tan δ=0.0557 (10 kHz).     

Piezoelectric properties and microstructures of ZnO-doped Bi0.5Na0.5TiO3 ceramics

This article studies the microstructure and piezoelectric properties of a ceramic lead-free NBT under different amount of ZnO doping. X-ray diffraction shows that Zn²⁺ diffuses into the lattice of (Bi_0.5Na_0.5)TiO₃ to form a solid solution with a pure perovskite structure. By modifying the zinc oxide content, the sintering behavior of (Bi_0.5Na_0.5)TiO₃ ceramics was significantly improved and the grain size was increased. The piezoelectric coefficient d₃₃ for the 1.0 wt.% ZnO-doped (Bi_0.5Na_0.5)TiO₃ ceramics sintered at 1050 °C was found to be 95 pC/N, and the electromechanical coupling factor k_p = 0.13. However, the piezoelectric coefficient d₃₃ for the 0.5 wt.% ZnO-doped (Bi_0.5Na_0.5)TiO₃ ceramics sintered at 1140 °C was found to be 110 pC/N, and the electromechanical coupling factor k_p = 0.17.     

Composite microstructures and piezoelectric properties in tantalum substituted lead-free K0.5Na0.5Nb1-xTaxO3 ceramics

K_0.5Na_0.5Nb_1-xTa_xO₃ (KNNT) (with x?=?0.00, 0.05, 0.10, 0.20, 0.30, 0.50 and 1) ceramics are prepared by ball milling and two calcinations at 830?°C for 5?h. Subsequent sintering of centimeter size pellets, 1–2?mm thick, is studied using conventional and spark plasma sintering techniques with various conditions. X-Ray diffraction and Raman spectroscopy phase identification reveal orthorhombic to tetragonal phase transitions occurring at about x?=?0.50, associated to chemical disorder. Scanning electron microscope observations and associated energy dispersive X-ray spectroscopy analysis reveal some composite aspect of the ceramics. Substitution of niobium by tantalum, corresponding to x increase, decreases significantly the grain size but also the densification of the ceramics sintered by conventional sintering, while, enhancement of the piezoelectric properties is observed for both sintering techniques. Thanks to parameters optimization of the spark plasma sintering process, temperature-time-pressure, significant improvement of the relative density over 96%, is obtained for all the compositions sintered between 920 and 960?°C, under 50?MPa, for 5–10?min with heating rates of 100?°C/min. High relative permittivity (ε_r =?1027), piezoelectric charge coefficient (d₃₃ =?160 pC/N) and piezoelectric coupling factor (k_p =?46%) are obtained in spark plasma sintered K_0.5Na_0.5Nb_1-xTa_xO₃ composite ceramics, for x ranging between 0.10 and 0.30 and for some specific spark plasma sintering conditions. Thus, tantalum single element substitution on niobium site, combined with spark plasma sintering, is revealed to be a powerful combination for the optimization and the reliability of piezoelectric properties in KNN system.     

Hydrothermal synthesis and piezoelectric property of Ta-doping K0.5Na0.5NbO3 lead-free piezoelectric ceramic

Ta-doping K_0.5Na_0.5Nb_1−xTa_xO₃ (x = 0.1, 0.2, 0.3, 0.4) powder was synthesized by hydrothermal approach and its ceramics were prepared after sintering and polarizing treatment in this work. The K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics near morphotropic phase boundary (MPB), which exhibited optimum piezoelectric properties of d₃₃ = 210 pC/N and good electromechanical coupling factors of K_p = 0.3. The domain structure has been observed from TEM images which indicates that the K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics have good piezoelectric and ferroelectric properties for it is near the MPB.     

Phase transition behavior and electrical properties of lead-free (1 − x)(0.98K0.5Na0.5NbO3–0.02LiTaO3)–x(0.96Bi0.5Na0.5TiO3–0.04BaTiO3) piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(0.98K_0.5Na_0.5NbO₃–0.02LiTaO₃)–x(0.96Bi_0.5Na_0.5TiO₃–0.04BaTiO₃) (KNN–LT–BNT–BT) with x = 0–0.10 have been synthesized by a conventional sintering technique. All samples possess pure perovskite structure, showing room temperature symmetries of orthorhombic at x < 0.02, and tetragonal at 0.05 ≤ x ≤ 0.10. A coexistence of orthorhombic and tetragonal phases in the composition range of 0.02 ≤ x < 0.05 in this system is caused by the temperature of the polymorphic phase transition (PPT) decreasing to around room temperature but not the behavior of the morphotropic phase boundary (MPB). The samples near the coexistence region exhibit improved properties, which are as follows: piezoelectric constant d₃₃ = 155 pC/N, remnant polarization P_r = 24.2 μC/cm², and coercive electric field E_c = 2 kV/mm. The results indicate that although this kind of ceramics displays good properties, further study is needed to promote the stabilities of the ceramics in order to utilize them in varying temperature environments.     

Enhanced energy-storage properties of (1-x)(0.7Bi0.5Na0.5TiO3-0.3Bi0.2Sr0.7TiO3)-xNaNbO3 lead-free ceramics

A series of (1-x)(0.7Bi_0.5Na_0.5TiO₃-0.3Bi_0.2Sr_0.7TiO₃)-xNaNbO₃ (BNT-BST-100xNN) lead-free ceramics were fabricated using conventional solid-state reaction technique. The phase behavior, microstructure, dielectric, ac impedance and energy-storage properties of the sintered ceramics were systematically investigated. XRD patterns and surface SEM micrographs revealed the introduction of NaNbO₃ didn't change the perovskite structure of BNT-BST at low doping level. The NaNbO₃ doping gave rise to slimmer P-E loops and thus gained enhanced energy storage properties. Therefore, a maximum energy storage density of 1.03 J/cm³ was achieved at 85 kV/cm at x = 0.01 via increasing the dielectric breakdown strength (DBS). Temperature-dependent dielectric permittivity illustrated the enhanced relaxor characteristics, implying the long-rang ferroelectric order was further damaged due to the introduction of NaNbO₃. The results above indicate the sintered ternary ceramics can be a promising lead-free candidate for energy storage capacitors.     

Preparation and characterization of the lead-free piezoelectric ceramic of Bi0.5Na0.5TiO3 doped with CuO

This study investigates the effects of copper oxide (CuO) addition, calcining temperature, and sintering temperature on the microstructure and the electrical properties (such as dielectric constant and loss tangent) of lead-free piezoelectric ceramic of bismuth sodium titanate (Bi_0.5Na_0.5TiO₃), BNT, which was prepared using the mixed oxide method. Three kinds of starting powders (Bi₂O₃, Na₂CO₃ and TiO₂) were mixed and calcined. This calcined BNT powder and a certain weight percentage of CuO were mixed, calcined, and compressed into a green compact of BNT-CuO. This green compact of BNT-CuO was sintered to be a disk doped with CuO, and its characteristics were measured. In this study, the calcining temperature ranged from 700 to 1000 °C, the sintering temperature ranged from 950 to 1050 °C, and the weight percentages of CuO doping included 2, 4, 6, and 8 wt.%. The largest relative density of the BNT-CuO disk obtained in this study was 96.7% at the calcining temperature of 700 °C, the sintering temperature of 950 °C, and 4 wt.% of CuO addition. The corresponding dielectric constant and loss tangent were 494 and 0.181%, respectively. This study shows that adding CuO to the BNT not only improves the relative density and the dielectric constant of the BNT disk, but it also lowers the sintering temperature.     

Normal sintering of (K, Na)NbO3-based lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics have received more attention due to the environmental protection of the earth. (K, Na)NbO₃-based ceramics are one of the most promising candidates. Normal sintering of un-doped and Li/Ta co-doped (K, Na)NbO₃ ceramics was investigated to clarify the optimal sintering condition for densification, microstructure and electrical properties. It was found that density increased greatly within a narrow temperature range but turned to decrease when the sintering temperature slightly exceeded the optimal one. Piezoelectric properties also showed similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density. The grain growth and property change as a function of sintering temperature were discussed on basis of the formation of liquid-phase and the composition deviation caused by the volatilization of alkali components during sintering.     

Grain growth kinetics in Dy-doped Bi0.5Na0.5TiO3 ceramics

Dysprosium-doped bismuth sodium titanate ceramics were prepared using the conventional mixed-oxide method. The amount of dysprosium used was varied from 0 to 2 at.%. The mixed powders were calcined at 800 °C and checked for phase purity using X-ray diffraction technique. The calcined powders were then cold-pressed into pellets and sintered at 1050 °C for the time ranging from 2 to 48 h. The ceramics were checked for phases and microstructures using an X-ray diffractometer and a scanning electron microscope, respectively. The analysis showed that undoped BNT ceramics sintered at longer time exhibited a significant grain growth with non-uniform grain size distribution and shape. The Dy-doped BNT however showed a much more limited grain growth behavior, resulting in smaller grain size and more equiaxed grain shape. It was also found that all Dy-doped BNT ceramics sintered at 48 h possessed lower porosity than those sintered for shorter time.     

Sol-gel processing and nanoscale characterization of (Bi0.5Na0.5)TiO3-SrTiO3 lead-free piezoelectric thin films

(1-x)(Bi_0.5Na_0.5)TiO₃-xSrTiO₃ (BNT-xST) (0 ≤ x ≤ 0.4) thin films were fabricated using a sol-gel technique on Pt(111)/Ti/SiO₂/Si(100) substrates, which were investigated by piezoresponse force microscopy (PFM) and Raman spectroscopy. The composition-induced phase transition was analyzed by acquiring structural variations and the domain distribution on a local scale. Raman spectra showed phonon anomalies with peak broadening and shifting when increasing SrTiO₃ (ST) concentrations were used. Changes in the domain morphology with changes in the composition were observed, and grains smaller than 0.5 µm were observed at lower concentrations of x = 0–0.25, while larger grains appeared with increasing ST contents. The switching spectroscopy PFM (SS-PFM) results supported a ferroelectric (FE) to relaxor ferroelectric (RFE) phase transition at approximately x ≈ 0.3 by means of analyzing the parameters as a function of the composition including the piezoresponse parameters of hysteresis loops (D_max, D_rem) and amplitude butterfly loops (S_total, S_neg). Hence, these results demonstrated that the composition-driven FE to RFE phase transition behavior, which is consistent with the localized response behavior, is dependent on the ST content in bulk BNT-xST ceramics.     

Enhanced piezoelectric,electrocaloric and energy storage properties at high temperature in lead-free Bi0.5(Na1-xKx)0.5TiO3 ceramics

The piezoelectric, electrocaloric and energy storage properties were systemically investigated in lead-free Bi_0.5(Na_1-xK_x)_0.5TiO₃ ceramics from room temperature to high temperature region. These ceramics can be poled completely to obtain large piezoelectric coefficient (104–153 pC/N) at low electric field of ~30?kV/cm. The piezoelectric property shows good thermal stability due to high depolarization temperature (T_d). For BNKT₂₀, a large low electric field-induced strain of 0.36% is obtained at 120?°C under 50?kV/cm, the corresponding normalized strain coefficient is up to 720?pm/V, which is larger than other BNT-based ceramics at high temperature region. The electrocaloric properties of these ceramics are studied via indirect and direct methods. Large EC value (~1.08?K) in BNKT₂₀ ceramic is obtained at 50?kV/cm using indirect calculation. Above 100?°C, the dielectric energy storage density and efficiency of BNKT₂₀ is still up to ~0.85?J/cm³ and 0.75, respectively. The BNKT_x ceramics may become promising candidates in the fields of actuators, electrocaloric cooling and energy storage at high temperature region.     

The optimum conditions for preparing the lead-free piezoelectric ceramic of Bi0.5Na0.5TiO3 using the Taguchi method

Using the Taguchi method, this study analyzes the optimum conditions for preparing the disk of bismuth sodium titanate (Bi_0.5Na_0.5TiO₃), BNT, which is prepared using the mixed oxide method. The controllable factors used in this study consisted of the following: (1) the duration of milling; (2) the temperature of calcining; (3) the rotation speed of mill; and (4) the temperature of sintering. According to the optimum conditions, the confirmation experiment was carried out, and the relative density of the BNT disk, which was prepared without adding the additive, was up to 89.05%. The percentage contribution of each controllable factor was also determined. Most interestingly, the temperature of sintering is the most influential factor to press the BNT powder together tightly, and its value of percentage contribution is up to 94.94%.     

Microstructure and piezoelectric properties of thermal sprayed Bi0.5(Na0.70K0.20Li0.10)0.5TiO3 ceramic coatings

The microstructure of Bi_0.5(Na_0.70K_0.20Li_0.10)_0.5TiO₃ (BNKLT) coatings fabricated by thermal spray method was closely examined by TEM, revealing the coexistence of rhombohedral and tetragonal perovskite main phases, and very minor secondary phases, while all amorphous phase was crystallized after heat treatment. Obtaining coexisting rhombohedral and tetragonal perovskite phases after the thermal spray process involving the melting-recrystallization and heat treatment process resulted in piezoelectric ceramic coating with excellent electrical and electromechanical properties. The effective piezoelectric coefficient d₃₃ of the heat-treated BNKLT coating reached 86?pm/V with substrate clamping, measured over macroscale by laser scanning vibrometer.     

Effect of HfO2 content on the microstructure and piezoelectric properties of (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃–xHfO₂ [BNBT–xHfO₂] lead-free ceramics were prepared using the conventional solid-state reaction method. Effects of HfO₂ content on their microstructures and electrical properties were systematically studied. A pure perovskite phase was observed in all the ceramics with x=0–0.07 wt%. Adding optimum HfO₂ content can induce dense microstructures and improve their piezoelectric properties, and a high depolarization temperature was also obtained. The ceramics with x=0.03 wt% possess optimum electrical properties (i.e., d₃₃~168 pC/N, k_p~32.1%, Q_m~130, ε_r~715, tan δ~0.026, and T_d~106 °C, showing that HfO₂-modified BNBT ceramics are promising materials for piezoelectric applications.     

Enhancement in dielectric and ferroelectric properties of lead free Bi0.5(Na0.5K0.5)0.5TiO3 ceramics by Sb-doping

Lead free piezoelectric Bi_0.5(Na_0.5K_0.5)_0.5TiO₃ (pure and 1 wt.%, 2 wt.%, 4 wt.% Sb-doped) ceramics were synthesized away from its MPB. The crystalline nature of the BNKT ceramic was studied by XRD and SEM. Depolarization temperature (T_d) and transition temperature (T_c) were observed through phase transitions in dielectric studies which were found to increase after Sb-doping, thus increasing its usable temperature range. In the study of relaxation behavior, the activation energy for relaxation was found to be 0.33, 0.43, 0.57 and 0.56 eV for pure and Sb-doped samples, respectively. All samples were found to exhibit normal Curie-Weiss law above their T_c. Doping of Sb was found to restrain the diffused character of the pure sample. In P-E loop, Sb-doping was found to increase the ferroelectric properties.Pure and Sb-doped BNKT ceramics exhibited high values of piezoelectric charge coefficient (d₃₃) as 115, 121, 129 and 100 pC/N, respectively.     

CuO as a sintering additive for (Bi1/2Na1/2)TiO3-BaTiO3-(K0.5Na0.5)NbO3 lead-free piezoceramics

CuO as a sintering additive was utilized to explore a low-temperature sintering of 0.92(Bi_1/2Na_1/2)TiO₃-0.06BaTiO₃-0.02(K_0.5Na_0.5)NbO₃ lead-free piezoceramic which has shown a promise for actuator applications due to its large strain. The sintering temperature guaranteeing the relative density of greater than 98% is drastically decreased with CuO addition, and saturates at a temperature as low as ∼930 °C when the addition level exceeds ca. 1 mol.%. Two distinguished features induced by the addition of CuO were noted. Firstly, the initially existing two-phase mixture gradually evolves into a rhombohedral single phase with an extremely small non-cubic distortion. Secondly, a liquid phase induced by the addition of CuO causes an abnormal grain growth, which can be attributed to the grain boundary reentrant edge mechanism. Based on these two observations, it is concluded that the added CuO not only forms a liquid phase but also diffuses into the lattice. In the meantime, temperature dependent permittivity measurements both on unpoled and poled samples suggest that the phase stability of the system is greatly influenced by the addition of CuO. Polarization and strain hysteresis measurements relate the changes in the phase stability closely to the stabilization of ferroelectric order, as exemplified by a significant increase in both the remanent strain and polarization values. Electron paramagnetic resonance (EPR) spectroscopic analysis revealed that the stabilization of ferroelectric order originates from a significant amount of Cu²⁺ diffusing into the lattice on B-site. There, it acts as an acceptor and forms a defect dipole in association with a charge balancing oxygen vacancy.     

Good temperature stability of K0.5Na0.5NbO3 based lead-free ceramics and their applications in buzzers

(K_0.5−xLi_x)Na_0.5(Nb_1−ySb_y)O₃ (KLNNSx–y, x = 0–4 mol% and y = 0–8 mol%) lead-free piezoelectric ceramics were prepared by the conventional mixed oxide method. The denser microstructure and better electrical properties of the ceramics were obtained as compared to the pure K_0.5Na_0.5NbO₃ ceramic. The temperature stability of the electrical properties of the ceramics was also investigated. The experimental results show that the KLNNS2.5–5 ceramic exhibits good electrical properties (k_p  49%, k₃₁  30% and , tan δ  0.019), and possesses good temperature stability in the temperature range of −40 to 85 °C. The related mechanisms for improved electrical properties and temperature stability were also discussed. Moreover, buzzers based on the KLNNS2.5–5 ceramic have been fabricated and their characterization is presented. These results show that the KLNNS2.5–5 ceramic is a promising lead-free material for practical application in buzzers.