Lead-free Piezoelectric System Research Articles

Achieving Both Giant d33 and High TC in Patassium-Sodium Niobate Ternary System

Both giant d33 and high TC have been obtained in a lead-free piezoelectric ternary system (0.995 - x)K0.48Na0.52NbO3-0.005BiScO3-xBi0.5(Na0.7K0.2Li0.1)0.5ZrO3. Thanks to the rhombohedral-tetragonal phase coexistence and the enhanced dielectric and ferroelectric properties, the ceramic with a composition of x = 0.04 shows a giant d33 of ∼366 pC/N together with TC of ∼335 °C, thereby paving the way for achieving both high d33 and high TC in KNN-based materials. In addition, such a ceramic has a good thermal stability of d33 (e.g., d33 > 319 pC/N, T ≤ 300 °C) and an enhanced stability of ferroelectric properties against temperature. The domain-wall energy barrier of ∼0.15 eV is derived from the temperature dependence of the back-switching polarization.

Giant d33 in (K,Na)(Nb,Sb)O3-(Bi,Na,K, Li)ZrO3 based lead-free piezoelectrics with high Tc

In this work, a lead-free piezoelectric system based on (1−x)(K0.48Na0.52)(Nb0.95Sb0.05)O3-xBi0.5(Na0.7K0.2Li0.1)0.5ZrO3 [(1−x)KNNS-xBNKLZ] is developed, and a rhombohedral-tetragonal phase boundary is constructed in this system. The relationship between the phase boundary and the piezoelectric properties of the (1−x)KNNS-xBNKLZ ceramics is illuminated. The coexistence of a tetragonal phase and a rhombohedral phase is identified in the composition range of 0.03 < x < 0.05. For such composition, the ceramics show giant d33 of ∼380 pC/N, high Tc of ∼290 °C, and a good thermal-depolarization behavior (d33 > 300 pC/N) of ∼210 °C. We believe that the (1−x)KNNS-xBNKLZ system is very promising for lead-free piezoelectric applications.

Composition range and electrical properties of the morphotropic phase boundary in Bi0.5(Na0.80K0.20)0.5TiO3-(Ba0.7Sr0.3)TiO3 system

The lead-free piezoelectric ceramic binary system of (1-x)Bi0.5(Na0.80K0.20)0.5TiO3-x(Ba0.7Sr0.3)TiO3 or (1-x) BNKT-xBST (with x ranging from 0.05 to 0.15 mol fraction) near the morphotropic phase boundary (MPB) has been investigated. The ceramics were synthesized by a conventional mixed-oxide method and sintered at 1125°C for 2 h. All BNKT-BST samples had relative density higher than 98% of their theoretical values. X-ray diffraction patterns showed that all compositions had a pure perovskite structure and BST effectively diffused into the BNKT lattice during sintering to form a solid solution. Crystal structure changed from rhombohedral-rich phase to tetragonal-rich phase with increasing BST content. Because of such MPB-like behavior, the highest dielectric (Tc = 325°C, ɛr = 1827, tan δ = 0.0823) and piezoelectric performances (d33 = 225 pC/N) were obtained in the BNKT-0.11BST sample.

Micro fabrication of lead-free (K,Na)NbO 3 piezoelectric thin films by dry etching

Micro fabrication has been conducted for sodium potassium niobate [(K,Na)NbO 3 , KNN] thin films by dry etching and Pt/KNN/Pt unimorph micro cantilevers have been fabricated as a lead-free piezoelectric micro electro mechanical system (MEMS). KNN etching by Ar/C 4 F 8 plasma showed a high etching rate of about 60 nm/min and KNN/Cr selectivity of over 5. Tip displacement of the Pt/KNN/Pt micro cantilevers was measured and the frequency response and piezoelectric properties were evaluated. Young s modulus and piezoelectric coefficients d 31 of the KNN thin film were estimated to be 115 GPa and 99 to 219 pm/V, respectively. These results indicate that Ar/C 4 F 8 plasma etching does not degrade the piezoelectric properties of KNN thin films and enables one to fabricate various lead-free piezoelectric MEMS applications.

Domain evolution in lead-free thin film piezoelectric ceramics

Due to environmental and health concerns lead-free piezoelectric systems are currently being evaluated for use as replacements for lead-based ceramics. (1-x)Na0.5Bi0.5TiO3 – xBaTiO3 (NBT-xBT) is a promising alternative. In order to develop materials with improved performance, it is necessary to understand local structure effects on the piezoelectric response at the grain level. Using piezoresponse force microscopy (PFM), we have studied the domain evolution under locally applied electric fields. Using pulsed laser deposition thin films are deposited on Pt/Ti/SiO2/Si substrates at a temperature of 650 °C in a 150 mTorr O2 environment. In NBT-0.05BT films, domains are clearly visible after the application of a bias field and they relax after a period of time. After the films are exposed to ambient conditions for several weeks, they must be annealed before domains are again visible with PFM. Domain images after poling indicate that the polarization direction is rotating in the plane of the sample.

Giant piezoresponse and promising application of environmental friendly small-ion-doped ZnO

In recent years, with the growing concerns on environmental protection and human health, new materials, such as lead-free piezoelectric materials, have received increasing attention. So far, three types of lead-free piezoelectric systems have been widely researched, i.e., perovskites, bismuth layer-structured ferroelectrics, and tungsten-bronze type ferroelectrics. This article presents a new type of environmental friendly piezoelectric material with simple structure, the transition-metal(TM)-doped ZnO. Through substituting Zn2+ site with small size ion, we obtained a series of TM-doped ZnO with giant piezoresponse, such as Zn0.975V0.025O of 170 pC/N, Zn0.94Cr0.06O of 120 pC/N, Zn0.913Mn0.087O of 86 pC/N and Zn0.988Fe0.012O of 127 pC/N. The tremendous piezoresponses are ascribed to the introduction of switchable spontaneous polarization and high permittivity in TM-doped ZnO. The microscopic origin of giant piezoresponse is also discussed. Substitution of TM ion with small ionic size for Zn2+ results in the easier rotation of noncollinear TM-O1 bonds along the c axis under the applied field, which produces large piezoelectric displacement and corresponding piezoresponse enhancement. Furthermore, it proposes a general rule to guide the design of new wurtzite semiconductors with enhanced piezoresponses. That is, TM-dopant with ionic size smaller than Zn2+ substitutes for Zn2+ site will increase the piezoresponse of ZnO significantly. Finally, we discuss the improved performances of some TM-doped ZnO based piezoelectric devices.

Temperature-dependent electrical properties of B-site zinc substituted bismuth sodium titanate piezoceramics

Lead-free ceramics of B-site Zn substituted Bi0.5Na0.5TiO3, with the formula of (1−x)Bi0.5Na0.5TiO3–xBi(Zn0.5Ti0.5)O3 (BNT–BZT, x=0.01, 0.04, 0.07) have been prepared. These compositions have rhombohedral, rhombohedral–tetragonal morphotropic phase boundary, and tetragonal crystal structure, respectively. The polarization–electric field (P–E), bipolar and unipolar strain–electric field (S–E) curves of all the compositions are measured as the functions of temperature. The temperature-dependent values of the remnant polarization, maximum polarization, coercive field, bipolar and unipolar strain, as well as the shape change of P–E and S–E curves have been comparatively investigated and discussed. The observed different temperature-dependences of the above parameters are attributed to the fact that the introduction of BZN reduces the depolarization temperature and enhances relaxor characteristics, which is consistent with the temperature-dependent dielectric properties. These results are valuable in understanding and further designing noval BNT-based lead-free piezoelectric systems with high recoverable strain.

Dielectric and piezoelectric properties of Cu 2+-doped alkali Niobates

The effect of Cu 2+ addition (0.5–2 mol%) on microstructure and electromechanical properties of lead-free piezoelectric system of (K 0.44Na 0.52Li 0.04)(Ta 0.1Sb 0.06Nb 0.84)O 3 (KNN-LT-LS) was investigated through two processing methods; namely perovskite and mixed-oxide. The addition of Cu 2+ showed an increase in grain size and relative density of the undoped ceramics in both processing techniques. Introduction of Cu 2+ stabilized the orthorhombic phase at room temperature by shifting the tetragonal–orthorhomic phase transition to higher temperatures while did not show any major changes in T c . The polarization-field response of Cu 2+-doped ceramics showed a decline in both remnant polarization and coercive field, thus reducing the area of the hysteresis loop. This shrinkage in hysteresis loop was manifested through a large improvement in mechanical quality factor, nearly 4 times that of undoped ceramic. Within the studied range of Cu 2+ addition, the ceramic with 0.5 mol% of Cu 2+ prepared by mixed-oxide route represented a relatively desirable balance between the degradation of the electromechanical properties, improvement in temperature stability, and mechanical quality factor.

Dielectric and piezoelectric properties of Bi 0.5Na 0.5TiO 3–Bi 0.5K 0.5TiO 3–BiCrO 3 lead-free piezoelectric ceramics

A new ternary Bi-based lead-free piezoelectric ceramic system with perovskite structure (1 − x − y)Bi 0.5Na 0.5TiO 3– xBi 0.5K 0.5TiO 3– yBiCrO 3, was prepared by a conventional solid-state reaction method and the effect of amount of Bi 0.5K 0.5TiO 3 and BiCrO 3 on microstructure and electrical properties of the ceramics was investigated. The morphotropic phase boundary (MPB) of the system between rhombohedral and tetragonal locates in the range of x = 0.18–0.21, y = 0–0.02. And an obvious second phase was observed in the samples with y = 0.025 and x = 0.24. The addition of BiCrO 3 caused an insignificant promoted grain growth and Bi 0.5K 0.5TiO 3 suppressed the growth of grain. The dielectric constant ɛ r decreases and dielectric loss tan δ increases with the increase frequency. The optimum piezoelectric properties were obtained near the composition of the MPB. The piezoelectric constant d 33 and the electromechanical coupling factor k p show maximum values of d 33 = 168 pC/N and k p = 0.32 at x = 0.18, y = 0.015 and x = 0.18, y = 0.01, respectively.

Phase formation and electrical properties of BNLT–BZT lead-free piezoelectric ceramic system

Phase formation study in lead-free piezoelectric ceramics based on lanthanum doped bismuth sodium titanate (Bi0.4871Na0.4871La0.0172TiO3:BNLT) and zirconium doped barium titanate (BaZr0.05Ti0.95O3:BZT), has been carried out in the system of (1−x)BNLT–xBZT where x = 0.0–1.0, by two-step mixed oxide method. It was observed that the addition of BZT in the BNLT ceramics developed the dielectric and piezoelectric properties of the ceramics with the optimum piezoelectric constant (d33) and dielectric constant (εr) at room temperature of about 138 pC/N and 1651, respectively, from the 0.2 BNLT to 0.8 BZT ceramic sample. The Curie temperature (TC) of this ceramic was found at 295 °C which is 195 °C higher than that of pure BZT ceramics, promising the use of this ceramic in a higher range of temperature.

Morphotropic Phase Boundary Study of the BNT-BKT Lead-Free Piezoelectric Ceramics

A lead-free piezoelectric ceramic binary system based on bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT)-bismuth potassium titanate (Bi0.5K0.5)TiO3 (BKT) was synthesized by conventional mixed-oxide technique. The XRD analysis showed that the rhombohedral-tetragonal morphotropic phase boundary (MPB) of the Bi0.5 (Na1-xKx)0.5 TiO3 system was in the composition range of x = 0.16 ~ 0.20. In addition, the piezoelectric properties of this system were also investigated. It was indicated that the piezoelectric properties are better with the compositions near the rhombohedral phase within the MPB than the compositions near the tetragonal phase.

Lead-free piezoelectric ceramic transducer in the donor-doped K<sub>1/2</sub>Na<sub>1/2</sub>NbO<sub>3</sub> solid solution system

Lead-based piezoelectric ceramics are suitable materials for noninvasive applications of ultrasound in medicine. However, the embedded therapeutic and diagnostic procedures require the use of lead-free piezoelectric materials as active elements in transducers. With this goal in mind, we investigated the substitution of Ba(2+) cations in a lead-free piezoelectric system of K(1/2)Na(1/2)NbO(3)-LiTaO(3)-LiSbO(3) (KNN-LT-LS) with perovskite structure. The Ba(2+) was added to the system as an A-site dopant in the range of 0-2 mol% in increments of 0.5 mol%. The addition of Ba(2+) improved the piezoelectric charge coefficient, d(33), and longitudinal coupling coefficient, k(33). The composition with 1 mol% Ba2+ had 36% and 58% higher d(33) and k(33), respectively, than the undoped composition. It appeared that the addition of Ba(2+) induced "soft" characteristics in this lead-free piezoelectric system. This was verified by the increase of remnant polarization along with the decline of coercive field. The Ba(2+) behaved as a grain growth inhibitor and caused a drastic reduction in polarization level (approximately 60%) when the grain size became smaller than approximately 1.5 microm. Incorporation of Ba(2+) up to 1.5 mol% increased the bulk resistivity of the KNN-LT-LS system and then reduced it drastically at higher dopant concentrations. The electron-hole compensation model fit well with the results obtained in this study and verified the A-site substitution of donor-doped barium. KNN-LT-LS ceramics with 0 mol% and 1 mol% Ba(2+) were used to fabricate single-element ultrasonic transducers resonating at 5.5 MHz. The -6 dB fractional bandwidth and -20 dB pulse length of the probe made of doped ceramic were 50% and 1.68 micros, respectively. This indicated that this system could be considered as a candidate for invasive and/or embedded medical ultrasound applications.

Lead-free piezoelectric ceramics of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3–(Bi1/2Ag1/2)TiO3 system

The ternary lead-free piezoelectric ceramics system of 0.90[x(Bi1/2Na1/2)TiO3–(1 − x)(Bi1/2K1/2)TiO3]–0.10(Bi1/2Ag1/2)TiO3 (x = 0.77, 0.79, 0.81, 0.83, 0.85) were successfully synthesized by conventional ceramic sintering technique. The samples were studied by X-ray diffraction, dielectric, ferroelectric and piezoelectric measurements. The MPB composition of the system appears to be near BNKA-79 according to the results of the X-ray diffraction and ferroelectric properties. The sample with x = 0.79 showed the highest piezoelectric constant d 33 = 160 pC/N. The maximum electromechanical coupling factors, k p and k t, are 0.30 for BNKA-79 and 0.42 for BNKA-85, respectively.

Piezoelectric and Dielectric Properties of (Bi<sub>0.5</sub>Na<sub>0.5</sub>)TiO3- (Bi<sub>0.5</sub>K<sub>0.5</sub>)TiO<sub>3</sub>-BaTiO<sub>3</sub> Lead-Free Piezoelectric Ceramics

A lead-free piezoelectric ceramic ternary system based on (Bi0.5Na0.5)TiO3 (BNT)-(Bi0.5 K0.5)TiO3(BKT)-BaTiO3(BT) near the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases has been investigated. The samples were prepared by a conventional sintering technique. Piezoelectric properties, dielectric properties and microstructures of BNT-BKT-BT lead-free piezoelectric ceramic were studied. The X-ray diffraction (XRD) patterns show that the ceramics possess a single-phase perovskite structure. The measurements of piezoelectric properties reveal that this system has a relatively high piezoelectric performance: the piezoelectric constant d33 reached to 178pC/N and planar electromechanical coupling factor kp enhanced to 0.325. The BNT-BKT-BT ternary system ceramics could be well sintered at 11500C with a high-density ratio of more than 95% of the theoretical density.

Dielectric and Piezoelectric Properties of Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3-NaNbO3Lead-Free Ceramics

The ternary lead-free piezoelectric ceramics system of (1 − x) [0.88Na0.5Bi0.5TiO3-0.12K0.5Bi0.5TiO3] − xNaNbO3(x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) were synthesized by conventional solid state reaction method. The crystal structure, dielectric, piezoelectric properties and P-E hysteresis loops were investigated. The crystalline structure of all compositions is mono-perovskite phase ascertained by XRD, and the lattice constant was calculated from the XRD data. Temperature dependence of dielectric constant e r and dissipation factor tan δ measurement revealed that all compositions experienced two phase transitions: from ferroelectric to anti-ferroelectric and from anti-ferroelectric to paraelectric, and these two phase transitions have relaxor characteristics. Both transition temperatures Td and Tm are lowered due to introduction of NaNbO3. P-E hysteresis loops show that 0.88Na0.5Bi0.5TiO3-0.12K0.5Bi0.5TiO3 ceramics has the maximum Pr and E c corresponding to the maximum values of electromechanical coupling factor Kp and piezoelectric constant d33. The piezoelectric constant d33 and electromechanical coupling factor Kp decrease a little, while the dielectric constant e33 T /e0 improves much more when the concentration of NaNbO3 is 8 mol%.

Large Piezoelectric Constant and High Curie Temperature of Lead-Free Piezoelectric Ceramic Ternary System Based on Bismuth Sodium Titanate-Bismuth Potassium Titanate-Barium Titanate near the Morphotropic Phase Boundary

A lead-free piezoelectric ceramic ternary system based on bismuth sodium titanate, (Bi1/2Na1/2)TiO3 (BNT) - bismuth potassium titanate (Bi1/2K1/2)TiO3 (BKT) - barium titanate BaTiO3 (BT) near the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases has been investigated. In the case of a(Bi1/2Na1/2)TiO3–bBaTiO3–c(Bi1/2K1/2)TiO3 [BNBK(100a/100b/100c)] solid solution ceramics, the highest piezoelectric constant d33=191 pC/N, Curie temperature, Tc=301°C, electromechanical coupling factor, k33=0.56 and dielectric constant, ε33T/ε0=1141 are observed for the BNBK(85.2/2.8/12) composition which has a tetragonal phase near the MPB. The d33 value is the highest so far reported for all lead-free piezoelectric ceramics with Tc>300°C. The BNT-BKT-BT ternary ceramics system sintered at 1200°C for 2 h in air has a pure perovskite structure and a high density more than 95% of the theoretical density.