Lead-free Piezoelectric System Research Articles

Role of BiScO3 in phase structure and electrical properties of potassium sodium niobate ternary materials

In this work, we mainly studied the effects of BiScO3 contents on the phase structure and electrical properties of lead-free (K,Na)NbO3-based ternary ceramics, and the material system of (0.97-x)K0.40Na0.60NbO3-xBiScO3 -0.03Bi0.5(Na0.6K0.3Li0.1)0.5HfO3 was prepared by the normal sintering method. The rhombohedral-tetragonal (R-T) phase boundary could be tuned in the ceramics by controlling BiScO3 contents. When the compositions approach the R-T phase boundary, the ceramics possess the enhancement of dielectric, ferroelectric and piezoelectric properties. The ceramics with x = 1.2% exhibited an excellent electrical behavior of d33 = 362 pC/N, kp∼0.42, Pr∼19.9 μC/cm2, εr∼1671, and tan δ∼0.02, which is assigned to the involvement of R-T phase boundary. Moreover, a high Curie temperature (TC) of ∼315 °C can be maintained even if the BiScO3 contents increase because of the addition of both BiScO3 and Li. As a result, the ceramics with x = 1.2% had both a large d33 and a high TC, showing that such a lead-free piezoelectric ternary system is suitable to the high-temperature applications.

Review and Perspectives of Aurivillius Structures as a Lead-Free Piezoelectric System

According to the EU-Directives 2002/95/EC, 2002/96/EC, lead-based piezoceramics must be substituted in the future with more environmentally friendly alternatives, only when a reliable alternative is found. This is why an increasing interest has grown in the research community to find lead free piezoelectric materials that fulfil the requirements for this substitution. Different families of compounds have been shown to be possible candidates for this use, such as bismuth and niobates based perovskites, pyrochlores, etc. However, a material with piezoelectric coefficients similar to those of PZT (lead zirconate titanate, Pb[ZrxTi1-x]O3) has not been yet found. Besides, each of these families has its specific characteristics in terms of remnant polarization, coercive field or application temperature. Thus, the choice of each material should be made according to the specific needs of the application. In this sense, Aurivillius-type structure materials (also known as Bismuth Layered Structure Ferroelectrics, BLSF) can take advantage of their specific properties for uses as Lead Free Piezoelectric systems. Some of them have a high Curie temperature, which make them good candidates to be used as high temperature piezoelectrics; high coercive fields, which facilitates their use as actuators; or a high switching fatigue resistance, which can be useful for future applications as Ferroelectric Random Access Memories (FERAM).

Probing local structure of the morphotropic phase boundary composition of Na0.5Bi0.5TiO3–BaTiO3 using rare-earth photoluminescence as a technique

(1-y)Na0.5Bi0.5TiO3-(y)BaTiO3 (NBT-BT) is one of the most investigated lead-free piezoelectric system in the recent past. Unlike the conventional piezoelectric alloys wherein the morphotropic phase boundary (MPB) compositions exhibiting maximum piezoelectric response, exhibit coexistence of ferroelectric phases with different symmetries on the global scale, the MPB composition (y = 0.06) of NBT-BT shows a cubic-like phase in the unpoled state. On poling it transforms to a non-cubic ferroelectric phase. Here we exploit the sensitivity of the photoluminescence (PL) property of doped rare-earth ions with regard to the local symmetry of the host crystal to investigate the local structure of the cubic-like phase of the MPB composition of NBT-BT. We performed a comparative study of the PL response by doping separately Er and Eu in very dilute concentration in the (i) MPB composition (y = 0.06) exhibiting a cubic-like phase, (ii) a sub-MPB composition (y = 0.03) exhibiting rhombohedral phase, and (iii) above the MPB composition (y = 0.10) exhibiting tetragonal phase. We found that both the Er and the Eu PL spectra of the MPB composition (y = 0.06) exhibits more number of Stark lines in its unpoled cubic-like phase as compared to that in the field-stabilized rhombohedral phase. The additional Stark lines in the cubic-like global phase are identified to be that of the tetragonal structure. Our study therefore confirms that what appears as a cubic-like phase on the global scale has intimately connected tetragonal and rhombohedral local structures. The success of our experiments suggests that rare-earth PL can be used as a simple yet powerful tool to investigate local structures in scenarios wherein the global structure need not comply with the local structure.

Microstructure and electrical properties of (1-x)K0.5Na0.5NbO3–xBi0.5Na0.5Zr0.85Sn0.15O3 lead-free ceramics

In this work, we simultaneously obtained a large d33 and high TC in a lead-free piezoelectric system of (1-x)K0.5Na0.5NbO3–xBi0.5Na0.5Zr0.85Sn0.15O3 [(1-x)KNN–xBNZS]. This lead-free piezoelectric system was synthesized by conventional solid-state method. We investigated the microstructure and electrical properties of this system. The rhombohedral-tetragonal (R-T) phase coexistence is demonstrated in the ceramics with 0.05 ≤ x ≤ 0.06. Owing to the R–T phase coexistence as well as the enhancement of ferroelectric and dielectric properties, the ceramics with x = 0.05 showed a large d33 of ∼350 pC/N together with a high TC of ∼315 °C, thereby illustrating that it is an effective way to obtain both large d33 and high TC in KNN-based ceramics. It is believed that such a ceramic system with large d33 and high TC is one of the promising candidates for piezoelectric devices.

Understanding the structure–property relationships of the ferroelectric to relaxor transition of the (1 − x)BaTiO3–(x)BiInO3 lead-free piezoelectric system

A structural and electromechanical investigation has been performed on (1 − x)BaTiO3–(x)BiInO3 in the region 0.03 ≤ x ≤ 0.12. A gradual structural phase transition has been observed where the structure changes from tetragonal (P4mm) and passes through two regions of coexisting phases: (1) P4mm + R3m in the range 0.03 ≤ x ≤ 0.075 and (2) $$ Pm\bar{3}m $$ + R3m for 0.10 ≤ x ≤ 0.12. The properties also transition from ferroelectric (x ≤ 0.03) to relaxor ferroelectric (x ≥ 0.05) as the dielectric permittivity maximum becomes temperature and frequency dependent. This transition was also confirmed via polarization-electric field measurements as well as strain-electric field measurements. At the critical composition of x = 0.065, a moderate strain of ~0.104% and an effective piezoelectric coefficient (d 33 * ) of 260 pm/V were observed. The original purpose of this study was to demonstrate the polarization extension mechanism as predicted in the literature, but due to the ferroelectric to relaxor transition, this mechanism was not found to be present in this system. However, this demonstrates that BaTiO3-based lead-free ceramics could be modified to obtain enhanced electromechanical properties for actuator applications.

Harnessing low frequency-based energy using a K0.5Na0.5NbO3 (KNN) pigmented piezoelectric paint system

A highly sensitive lead-free piezoelectric paint system for harvesting low-frequency vibration energy (<10 Hz) was developed.

Temperature and composition dependent phase transitions of lead-free piezoelectric (Bi0.5Na0.5)TiO3-BaTiO3 thin films.

Extensive studies on (Bi0.5Na0.5)TiO3-BaTiO3 piezoceramics have aroused growing interest in developing high-performance piezoelectric films based on this promising lead-free piezoelectric system. High quality (1 - x%) (Bi0.5Na0.5)TiO3-x% BaTiO3 (x = 0-15) thin films were synthesized on Pt(111)/Ti/SiO2/Si(100) substrates via a sol-gel method. The phase transitions as functions of composition and temperature were investigated by X-ray diffraction (XRD) and temperature-dependent Raman spectra analysis. A morphotropic phase boundary (MPB) located at 5 ≤ x ≤ 10 was found in the films, being identical to that in bulk materials. Additionally, complicated structural changes with temperature in a wide composition range were observed, which correspond to the evolution from a low-symmetry ferroelectric phase or a relaxor phase to high-symmetry relaxor phases. The ferroelectricity and piezoelectricity were also systematically studied with an emphasis on the influence of composition and phase structure. A phase diagram focusing on the above two kinds of phase transitions has been proposed in this work.

Structural transformations in morphotropic-phase-boundary composition of the lead-free piezoelectric systemBa(Ti0.8Zr0.2)O3−(Ba0.7Ca0.3)TiO3

The exceptionally large piezoelectric response of the morphotropic- phase-boundary (MPB) composition of the lead-free piezoelectric system (1 - x) Ba(Ti0.8Zr0.2)O-3 - x(Ba0.7Ca0.3) TiO3 has attracted great attention in recent years. Here in this paper we report a detailed investigation of the structural phase transformation behavior of the MPB composition (x = 0.50) driven by electric field, stress, and temperature. We show that the system exhibits metastable phases in a wide temperature range, and that the large piezoresponse at room temperature has a significant contribution from the increased fraction of the metastable phases induced by the poling field. Using a ``powder poling'' technique we also demonstrate the equivalence of stress and electric field with regard to the nature of the structural transformation. The fundamental significance of this interesting observation is discussed.

Phase Structures and Piezoelectric Properties of (K,Na,Li)(Nb,Sb)O3-(Bi,Ag)ZrO3 Lead-Free Ceramics

Samples in the pseudoternary lead-free piezoelectric ceramic system 0.94KNN-(0.06 − x)LiSbO3-x(Bi0.5Ag0.5)ZrO3 were prepared using a solid-state reaction technique and their phase transition behavior and electrical properties studied. Results showed that BAZ diffuses into KNN-LS to form a new solid solution, and induces a phase transition from tetragonal to rhombohedral phase with increase of x. At 0.02 ≤ x ≤ 0.03, coexistence of tetragonal and rhombohedral phases is observed, and enhanced piezoelectric properties are achieved in this composition range due to the polymorphic phase transition near room temperature. Doping with (Bi0.5Ag0.5)ZrO3 effectively promotes densification and further enhances the piezoelectric and dielectric properties of of the ceramics. Moreover, the ceramic with x = 0.025 possesses excellent electrical properties of kp = 42.3%, \({d_{33}^{*}}\) = 320 pm/V and d33 = 235 pC/N, tan δ = 0.039, and Tc = 326°C. This result indicates that 0.94KNN-0.035LS-0.025BAZ ceramic is a promising lead-free material for practical applications.

High piezoelectricity in (K,Na)(Nb,Sb)O3–(Bi,La,Na,Li)ZrO3 lead-free ceramics

A new lead-free piezoelectric system of (1 − x)(K0.48Na0.52+y )(Nb0.95Sb0.05)O3–x(Bi0.8La0.2)0.5(Na0.8Li0.2)0.5ZrO3 [(1 − x)KNa y NS–xBLNLZ] were prepared using the conventional solid-state sintering method. The effects of BLNLZ content and Na excess on the microstructure and electrical properties of the ceramics were investigated. It was found that an enhanced dielectric, ferroelectric, and piezoelectric behavior has been attained by successfully constructing the rhombohedral–tetragonal phase boundary in the ceramics with x = 0.04. The small Na excess can greatly improve ceramics properties due to the increase of abnormal grain growth caused by the formation of a small amount of liquid phase. As a result, the ceramic with x = 0.04 and y = 0.004 has optimum electrical properties of d 33 ~470 pC/N, k p ~50 %, 2P r ~32.3 μC/cm2, and 2E c ~14.2 kV/cm, together with a Curie temperature of ~210 °C. The giant d 33 of this ceramic system could be comparable with the most of the reported results about the alkali niobate-based lead-free piezoceramics. It was believed that such an excellent piezoelectricity of this material system will promote the development of KNN-based lead-free ceramics.

Phase evolution and correlation between tolerance factor and electromechanical properties in BNT-based ternary perovskite compounds with calculated end-member Bi(Me0.5Ti0.5)O3 (Me = Zn, Mg, Ni, Co).

In this work, the structure of end-member Bi(Me0.5Ti0.5)O3 (Me = Zn, Ni, Mg, Co) was calculated through a first-principles method and lead-free piezoelectric ternary systems (0.94 -x)(Bi0.5Na0.5)TiO3-0.06BaTiO3-xBi(Me0.5Ti0.5)O3 (Me = Zn, Ni, Mg, Co) (BNT-BT-Bi(Me0.5Ti0.5)O3) were designed to achieve a large strain response for actuator applications. Composition-driven phase transition characteristics and the resulting associated piezoelectric and electromechanical properties were systematically investigated, and schematic phase diagrams were constructed. XRD measurements, Raman spectra analysis and temperature-dependent polarization and strain hysteresis loops indicate that Bi(Me0.5Ti0.5)O3 substitution induces a phase transformation from a ferroelectric rhombohedral to an ergodic relaxor pseudo-cubic phase, accounting for the large strain response (>0.3%) with a high normalized strain Smax/Emax (≥550 pm V(-1)) at around the corresponding critical composition in the vicinity of room temperature. In addition, correlations between the tolerance factor t of the added end-member, the calculated tetragonality and the morphotropic phase boundary (MPB) composition were sought. In comparison to other reported BNT-based systems, there is a noticeable correlation between the MPB composition and the calculated tetragonality of the end-member Bi(Me0.5Ti0.5)O3, and the t value corresponding to the formation of the MPB composition is approximately 0.981 in the present ternary system with low tolerance factor end-members. As a result, we believe that the general correlations and design principles obtained from the present comprehensive research will be effective to predict the approximate MPB region quickly in BNT-based ceramics with an excellent actuating performance.

Ultrahigh strain response with fatigue-free behavior in (Bi0.5Na0.5)TiO3-based lead-free piezoelectric ceramics

In this letter, we report a lead-free piezoelectric ceramic system (Bi0.5Na0.5)1−xBaxTi0.98 (Fe0.5Sb0.5)0.02O3 which shows a surprisingly high field-induced nonlinear strain of 0.57% comparable to those obtained in Pb-based antiferroelectrics. The ultrahigh strain response of the composition stems from the composition proximity to the ferroelectric-nonpolar phase boundary, which leads to reversible transformation between a nonpolar phase and a polar ferroelectric phase under cyclic fields. In particular, this material is very attractive for its exceptionally good fatigue resistance (up to 106 cycles) and high temperature stability (25–100 °C) due to its stable nonpolar phase and lower defect density. These findings render the current material a great opportunity for novel applications in ultra-large stroke and nonlinear actuators demanding improved cycling and thermal reliabilities.

Structure evolution and enhanced piezoelectric properties of (K0.5Na0.5)NbO3–0.06LiTaO3–SrZrO3 lead-free ceramics

(K0.5Na0.5)NbO3–0.06LiTaO3–xSrZrO3 (KNN–LT–xST) lead-free piezoelectric ceramics with rhombohedral–tetragonal (R–T) phase boundary have been designed and prepared by the conventional solid-state reaction method, the phase transitional behavior and composition-dependent piezoelectric properties of KNN–LT–xST lead-free system were investigated. The R–T phase boundary is identified in the KNN–LT–xST ceramics with a composition of 0.04 ≤ x ≤ 0.06. The ceramics in the vicinity of the R–T phase coexistence zone exhibit a strong compositional dependence and significantly enhanced piezoelectric properties. A comprehensive performance of d33 (222–254 pC/N) and Tc (282–312 °C) could be obtained in the KNN–LT–xSZ ceramics with 0.04 ≤ x ≤ 0.06 by tailoring the SrZrO3 content. The ceramic with x = 0.045 shows the optimum electrical properties: d33 = 254 pC/N, kp = 28%, εr = 758, tanδ = 1.7%, Pr = 8.98 μC/cm2 and Tc = 300 °C. The existence of an R–T phase boundary and therefore the involvement of the more polarization states should be responsible for the enhanced piezoelectric properties of the KNN–LT–xSZ ceramics. The good piezoelectric properties together with a high cubic–tetragonal phase transition temperatures (Tc) make the KNN–LT–xSZ ceramics showing the promising lead-free piezoelectric material system for the practical applications.

The effect of BZT doping on phase formation, dielectric and ferroelectric properties of BNLT ceramics

In this work, the effect of BaZr0.05Ti0.95O3 (BZT) doping on phase, dielectric and ferroelectric properties of a lead-free piezoelectric ceramic system of Bi0.4871Na0.4871La0.0172TiO3 (BNLT) was studied. The different amounts of BZT were incorporated into BNLT to form (1−x)BNLT–xBZT ceramics (when x = 0, 0.06, 0.09 and 0.12 mol fraction). The rhombohedral phase was presented in the pure BNLT ceramic, which changed into the tetragonal phase with BZT incorporation. The lattice parameters and unit cell volume increased with increasing BZT content. Grain sizes tended to decrease with increasing BZT content. The dielectric constant of the BNLT ceramic was enhanced by BZT incorporation. The dielectric constant was maximized with the 0.09BZT incorporation. The temperature of the dielectric peak of BNLT ceramic decreased while the degree of diffuse phase transition increased with BZT substitution. The polarization–electric field (P–E) curve measured at temperature above 30 °C was found to be slimmer with increasing BZT content. The remnant polarization of the BZT-incorporated ceramics decreased with increasing temperature. The observed results were believed to be caused by the lattice distortion and the enhanced cation disorders in A and B-site lattices due to the incorporation of Ba2+ and Zr4+ ions into A and B-site lattices, respectively, of BNLT perovskite structure.

A new Na0.5Bi0.5TiO3 based lead-free piezoelectric system with calculated end-member Bi(Zn0.5Zr0.5)O3

This study shows a new lead-free piezoelectric system (1−x)Na0.5Bi0.5TiO3–xBi(Zn0.5Zr0.5)O3 [(1−x)NBT–xBZZ, x=0, 0.02, 0.03, 0.035, 0.04 and 0.05]. The structure of Bi(Zn0.5Zr0.5)O3 was calculated with first-principles method. A morphotropic phase boundary (MPB) near x=0.03 between rhombohedral and pseudo-cubic phases was found, and its improvement in electrical properties was demonstrated. In the MPB composition, the maximum polarization, remnant polarization, coercive field and piezoelectric coefficient are 46.4μC/cm2, 39.2μC/cm2, 4.8kV/mm and 95pC/N, respectively. The results not only show a new lead-free piezoelectric system but also suggest a new route to designing lead-free piezoelectric materials.

Hydrothermal Synthesis of Sodium Potassium Bismuth Titanates

(Bi0.5Na0.5)TiO3 (BNT) is an excellent candidate lead-free piezoelectric material system. This study prepares BNT ceramics using the hydrothermal method at 180°C with an NaOH concentration of 12 M. The effect of potassium content on the crystal structure of the ceramics is studied. A single phase of [Bi0.5(Na1−xKx)0.5]TiO3 ceramics is obtained using the process applied to obtain BNT. The phase structure transformed from rhombohedral to tetragonal at x = 0.3, as evidenced by the disappearance of the cubic phase. The surface morphology of the materials is affected by K substitution.

Electric field-induced giant strain and photoluminescence-enhancement effect in rare-earth modified lead-free piezoelectric ceramics.

In this work, an electric field-induced giant strain response and excellent photoluminescence-enhancement effect was obtained in a rare-earth ion modified lead-free piezoelectric system. Pr(3+)-modified 0.93(Bi0.5Na0.5)TiO3-0.07BaTiO3 ceramics were designed and fabricated by a conventional fabrication process. The ferroelectric, dielectric, piezoelectric, and photoluminescence performances were systematically studied, and a schematic phase diagram was constructed. It was found the Pr(3+) substitution induced a transition from ferroelectric a long-range order structure to a relaxor pseudocubic phase with short-range coherence structure. Around a critical composition of 0.8 mol % Pr(3+), a giant reversible strain of ∼0.43% with a normalized strain Smax/Emax of up to 770 pm/V was obtained at ∼5 kV/mm. Furthermore, the in situ electric field enhanced the photoluminescence intensity by ∼40% in the proposed system. These findings have great potential for actuator and multifunctional device applications, which may also open up a range of new applications.

Modification of both d33 and TC in a potassium-sodium niobate ternary system.

In this work, we simultaneously achieved a giant d33 and a high TC in a lead-free piezoelectric ternary system of (1-x-y)K0.48Na0.52NbO3-xBiFeO3-yBi0.5Na0.5ZrO3 {(1-x-y)KNN-xBF-yBNZ}. Owing to the rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence and the enhanced dielectric and ferroelectric properties, the ceramics with a composition of (x = 0.006, y = 0.04) show a giant d33 of ∼428 pC N(-1) together with a TC of ∼318 °C, thereby proving that the design of ternary systems is an effective way to achieve both high d33 and high TC in KNN-based materials. In addition, a good thermal stability for piezoelectricity was also observed in these ceramics (e.g., d33 > 390 pC N(-1), T ≤ 300 °C). This is the first time such a good comprehensive performance in potassium-sodium niobate materials has been obtained. As a result, we believe that this type of material system with both giant d33 and high TC is a promising candidate for high-temperature piezoelectric devices.

Large piezoelectric properties induced by doping ionic pairs in BaTiO3 ceramics

The high piezoelectric properties of piezoelectric ceramics are typically obtained by inducing a phase transition between two ferroelectric phases, using either the morphotropic phase boundary (MPB) or the polymorphic phase transition (PPT). Here we demonstrate that neither the MPB nor the PPT is necessary to achieve high piezoelectric properties. Our results show that the optimized distribution of Li+–Al3+ pairs parallel to the [001] direction, as found in our xLiAlSiO4/BaTiO3 (xLAS/BT) lead-free piezoelectric ceramic system prepared from ordinary raw materials by conventional solid-state reaction sintering, can generate a large piezoelectric constant (d33) of 378pC/N when x=7.5mol.%. The d33 value of the 7.5mol.% LAS/BT ceramic is more than three times that of the pure BT ceramic. The distortion connected to the proposed Li+–Al3+ pairs locally creates unit cells of less than tetragonal symmetry, and these low-symmetry cells can be responsible for the high piezoelectric response. The high-temperature stability testing reveals that these doped ceramics are usable at temperatures as high as 120°C. This piezoelectric mechanism coming from the doping ionic pairs provides a new method to achieve large piezoelectric properties in a wide range of ABO3-type perovskite systems.

A new Bi0.5Na0.5TiO3 based lead-free piezoelectric system with calculated end-member Bi(Zn0.5Hf0.5)O3

The phase structure, dielectric and piezoelectric properties of a new lead-free piezoelectric system (1 − x)Bi0.5Na0.5TiO3–xBi(Zn0.5Hf0.5)O3 [(1 − x)BNT–xBZH, x = 0, 0.01, 0.02, 0.03, and 0.04] were investigated. The structure of Bi(Zn0.5Hf0.5)O3 was calculated using first-principles method and (1 − x)BNT–xBZH ceramics were fabricated by conventional solid-state process. At room temperature, a morphotropic phase boundary (MPB) from rhombohedral to pseudocubic is identified near x = 0.02 by the analysis of X-ray diffraction patterns. The ceramics with MPB near room temperature exhibit excellent electrical properties: the Curie temperature, maximum polarization, remnant polarization, and coercive field are 340 °C, 56.3 μC/cm2, 43.5 μC/cm2, and 5.4 kV/mm, respectively, while the maximum positive bipolar strain and piezoelectric coefficient are 0.09% and 92 pC/N, respectively. In addition, a linear relationship between the MPB phase boundary composition and the calculated tetragonality of non-BNT end-member was demonstrated. Thus, this study not only shows a new BNT-based lead-free piezoelectric system but also suggest a new way to predict the composition at MPB a priori when designing new lead-free piezoelectric system.