Lead-free Piezoelectric System Research Articles

(1-x)(Na₀.₅Bi₀.₅)TiO₃-xCaTiO₃ ceramics: Investigating structural and microstructural features for enhanced dielectric properties

(1-x)(Na₀.₅Bi₀.₅)TiO₃-xCaTiO₃ Lead-free piezoelectric systems, positioned near the morphotropic phase boundary, were synthesized for varying compositions (x = 0.0, 0.05, 0.10, 0.15, and 0.20) using the solid-state reaction route. This study delves into the comprehensive investigation of the compositional effects on phase, structure, and electrical characteristics. Specifically, a morphotropic phase boundary (MPB) involving rhombohedral (R3c) and orthorhombic (Pnma) structures was seen in a (1-x)NBT-xCT crystal structure close to the composition of x = 0.10. Information on the pure phase formation and grain size of the intended composite system has been obtained using Rietveld refinement of the X-ray diffraction (XRD) diagram as well as scanning electron microscopy (SEM). The impact of the CT phase on the NBT lattice was investigated through an analysis of the charge density distribution. Using Williamson-Hall plots from XRD data, the average particle diameter was estimated to be between 131.87 nm and 136.54 nm. The relative permittivity increases with the addition of Ca2+, according to dielectric measurements. All ceramics exhibit a diffuse phase transition near (Tm) with a diffusivity range of 1.5–1.8, and a downward shift in depolarization temperature (Td). At the morphotropic phase boundary (MPB), excellent dielectric properties were observed at x = 0.10, which are attributed to the presence of both rhombohedral and orthorhombic structures as well as an appropriate particle size. The conduction process at different temperatures is thermally activated, as determined by the frequency-dependent ac conductivity.

A dispersed polycrystalline phase boundary constructed in CaZrO3 modified KNN based ceramics with both excellent piezoelectric properties and thermal stability

In this paper, the ceramics with composition of (0.98-x)(K0.5Na0.5)(Nb0.96Sb0.04)O3-0.02(Bi0.5Na0.5)(Zr0.8Ti0.2)O3-xCaZrO3 (abbreviated as (0.98-x)KNNS-0.02BNZT-xCZ, x = 0, 0.01, 0.015, 0.02, 0.025, 0.03) were prepared by a traditional solid-state reaction method. The effect of the additional amount of CaZrO3 on the phase structure, microstructure, dispersion index, domain structure and piezoelectric properties of ceramics was systematically studied. Finally, the piezoelectric properties and thermal stability of ceramics could be controlled by adding different amounts of CaZrO3. The addition of CaZrO3 transferred the phase structure of the ceramics from orthogonal-tetragonal (O-T) coexistence phase to rhombohedral-orthogonal (R–O) coexistence phase, which could be demonstrated by XRD test, temperature-dependent Raman spectra and εr‐T plot analysis. And when x = 0.02, the ceramics possessed the best piezoelectric and dielectric properties (d33 = 253 pC/N, εr = 1185, tanδ = 0.044). Such excellent electrical properties could be originated from the heterogeneous domain structure and small-size nano-domains of the ceramics. Moreover, with the increase of CaZrO3 doping amount, the dispersion index of ceramics gradually increased from 1.404 to 1.871, which showed more obvious dispersion phase transition characteristics and improved the thermal stability of ceramics. Particularly, when x = 0.02, after annealing at a high temperature of 220 °C (close to its Curie temperature), the d33 tested at room temperature remained above 85% of that without annealing. The results indicated that (0.98-x)KNNS-0.02BNZT-xCZ ceramic was a promising lead-free piezoelectric ceramic system.

Design and Development of a Low-Power Wireless MEMS Lead-Free Piezoelectric Accelerometer System

In this paper, a complete 3-D integrated sensing system intended for practical applications was presented on a PCB board, starting with lead-free piezoelectric films deposited using an RF sputtering system, a MEMS structure design/simulation, device fabrication, and a signal processing chip design including charge-to-voltage transformation, signal amplification, 60-Hz noise rejection, and low-power RF wireless transmission chip for remote monitoring. Both the signal processing chips and the RF wireless transmitter chips were manufactured using the TSMC CMOS process. The piezoelectric coefficient d33 was measured as 27.21pm/V of the proposed MLZO (ZnO: Li doped Mg) films. To meet the frequency specifications of industrial applications, the ring-structure devices with a 7730-Hz resonance frequency were designed using ANSYS simulation software. There was only a 0.39% deviation as compared to the simulation results with the actual experimental data. The measurement results of the proposed system showed that the acceleration detection range was approximately 137 mV/g(p-p). The voltage-controlled oscillator adopted in the wireless transmitter chip had a four-leaf clover structure inductor with two symmetrical axes resulting in better suppression of magnetic field radiation. Finally, this system was applied to industrial spindles, and the normal and abnormal operation information of spindles was successfully and wirelessly monitored.

Enhanced ferroelectric, piezoelectric, and dielectric properties in potassium sodium niobate lead-free piezoelectric ceramics by constructing of multiple phase boundaries

In this work, we report excellent ferroelectric, piezoelectric, and dielectric properties in 0.95(K0.5−x/2Na0.5−x/2Lix)NbO3-0.04BaZrO3-0.01BiFeO3, ((KN)1−xLxN-BZ-BF) lead-free piezoelectric ceramics system by constructing a coexistence of orthorhombic-tetragonal phase boundary near the room temperature, and increasing of grain size as well through the conventional sintering process. The addition of Li+ ions, in ((KN)1−xLxN-BZ-BF) composition, increased the grain size, and shifted the orthorhombic-tetragonal phase transition to near room temperature, and shifted the tetragonal-cubic phase transition toward higher temperatures, since both large electrical properties, and moderate Curie temperature was achieved simultaneously. The sample with x = 0.03 showed large average grain size of (9 µm), and possess coexisting of orthorhombic-tetragonal phases, since the sample showed excellent physical properties, such as d33 = 330 pC/N, kp = 0.5, Pr = 25 µC/cm2, and εr = 1740. The origin of these decent combinations of physical properties owes to the coexistence of orthorhombic-tetragonal phase transition to near room temperature, large grain size, and hybridization between Bi 6p, and O 2p as well.

Enhanced electromechanical response in (K, Na)NbO3-based ferroelectrics by phase boundary and nonstoichiometry engineering

Compositional fluctuation of the K and Na elements during the preparation process becomes a major issue for (K,Na)NbO3 (KNN) lead-free ceramics to maintain relatively stable electrical properties. The raw material Bi2O3 is more sensitive to high temperature. To improve electrical properties, it is more necessary to compensate the volatile Bi. In this work, the 0.96K0.48Na0.52Nb0.96Sb0.04O3-0.04Bi0.5*(1+x)Na0.5ZrO3 (x = −0.05, −0.02, 0.00, 0.02, 0.05, and 0.08) ceramics were selected as basis material. The excess Bi can increase the phase fraction of the T phase and decrease oxygen vacancy content, which effectively enhance the extrinsic contribution of the piezoelectric response. The electromechanical properties were optimized by excess Bi, and the optimal properties (d33 = 522 pC/N, kp = 58.9%, Tc = 235 °C) were obtained in the ceramic with x = 0.05. The present work provides new guidelines for enhancing the piezoelectric performance of KNN ceramics by a simple method, which can be applied to other lead-free piezoelectric ceramic systems.

Bipolar cycling effects in BiFeO3–BaTiO3 piezoelectric ceramics

BiFeO3–BaTiO3 (BF-BT) lead-free piezoelectric system has been paid much attention due to good piezoelectric properties and high Curie temperature. Poling is a process to align ferroelectric domains and increase the piezoelectric coefficients. During the poling process, unipolar direct current (dc) electric fields were applied conventionally, but recently bipolar alternating current (ac) cycling was reported to improve piezoelectric properties in rhombohedral structure piezoelectric materials. We investigated the effects of dc-poling and ac-cycling in BF-BT ceramics. The d33 increased from 210 pC/N with dc-poling to 240 pC/N with ac-cycling in the morphotropic phase boundary region of BF-BT with domain engineering. This improvement of piezoelectric properties with ac-cycling was consistent with the structural evolution related to ferroelectric domains.

Effects of Ba(Ti0.90Sn0.10)O3 on Electrical Properties of Bi0.5Na0.5TiO3-0.06BaTiO3 Lead-Free Piezoelectric Ceramic System

Lead-free piezoelectric ceramics with the composition of (1-x)[Bi0.5Na0.5TiO3-0.06BaTiO3]-xBa(Ti0.90Sn0.10)O3 or (1-x)[BNT-0.06BT]-xBTS ternary system (where x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05 mol fraction) have been synthesized by a conventional mixed-oxide method. The optimum sintering condition for all samples was found to be 1125 °C/2 h with a heating/cooling rate of 5 °C/min. The relative densities of all samples were ∼98% with the accompanying linear shrinkage of ∼18%. The XRD patterns of all samples showed no detected impurity phase. It was found that the composition of x = 0.03 exhibited optimum electrical properties (εr = 1895, tan δ = 0.0355 and d33 = 255 pC/N at room temperature), suggesting that this composition had potential to be a promising lead-free piezoelectric candidate compound for dielectric and piezoelectric applications.

Phase transition and piezoelectric property of (Ag,K)NbO3 ceramics

AgNbO3 is an antiferroelectric (AFE) material with double hysteresis loop. Both the antiferroelectricity and ferroelectricity can be enhanced by doping. Herein, the ferroelectricity of AgNbO3 ceramics was enhanced via K-doping and the phase diagram of the (Ag1-xKx)NbO3 ceramics was upgraded. In details, (Ag1-xKx)NbO3 ceramics are ferrielectric (FIE) M1 phase as x = 5.00–5.50 mol% and ferroelectric (FE) O phase as x = 5.75–6.00 mol% before poling, and FE O phase as x = 5.00–6.00 mol% after poling at room temperature. With increasing temperature, (Ag1-xKx)NbO3 ceramics show the phase evolutions from FIE M1, AFE M2 to paraelectric (PE) T phase at x = 5.00–5.50 mol% and from FE O, FE T to PE T phase at x = 5.75–6.00 mol% before poling, and from FE O, FE T to PE T phase at x = 5.00–6.00 mol% after poling. High d33 values of 180 pC/N and 285 pC/N are obtained at the FE O-FE T and FE T-PE T phase boundaries. This work sheds light on a novel and promising lead-free piezoelectric system.

Nano-Interfacial Layer Effects on Dielectric and Piezoelectric Characteristics in Chemical Solution-Derived (K,Na)NbO3 Thin Films

Since non-piezoelectric interfacial layers even at the nanoscale significantly affect the performance of lead-free piezoelectric thin films, the quantitative characterization of property changes of thin films due to interfacial layers is of great importance and should be precisely undertaken for piezoelectric microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) devices. In contrast to widely accepted concepts for interfacial layer thickness estimation based on the existing series capacitor model, we find that the interfacial layer thickness at the top and the bottom interfaces is clearly different in chemical solution deposition (CSD)-derived (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 (KNMN) thin films. Interestingly, the thickness of the bottom interface increases linearly with increasing the thin film thickness, while the thickness of the top interface is constant regardless of the thin film thickness. In this work, nano-interfacial layer effects of CSD-derived KNMN thin films are theoretically and experimentally addressed in a combinatorial way using a modified series capacitor model. The gained information is used to envisage the origins and the mechanisms of non-piezoelectric interfacial layers and associated dielectric and ferroelectric properties of KNMN thin films. Our research connects macroscopic properties with the microscopic origins and is greatly facilitated by separating intrinsic and extrinsic contributions to phenomenological behaviors, as well as engineering interface-related properties of the films. We believe these studies to be crucial for the further development and applications of KNN-based lead-free piezoelectric devices, and will also open the door to future studies on other lead-free piezoelectric material systems for practical MEMS and NEMS applications.

Understanding the structure–property relationships of lead-free piezoelectric systems

Understanding the structure–property relationships of lead-free piezoelectric systems

Depoling phenomena inNa0.5Bi0.5TiO3−BaTiO3: A structural perspective

The structural complexities of the lead-free piezoelectric system $(1\text{\ensuremath{-}}x){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_{3}\text{\ensuremath{-}}x{\mathrm{BaTiO}}_{3}$ (NBT-BT) continues to pose challenge regarding understanding the mechanisms underlying several interesting phenomena. Issues like (i) whether thermal depoling across compositions is triggered by a structural transformation event or not, (ii) what causes the average Cc structure to partially transform to $R3c$ at $x\ensuremath{\sim}0.03$ in unpoled specimens, (iii) what makes complete depoling of the compositions $0.03\ensuremath{\le}x\ensuremath{\le}0.05$ occur in a considerably small temperature interval as compared to those for $x<0.03$, (iv) what makes the $R3c$-$P4bm$ transition temperature $({T}_{2})$ abruptly become smaller than the depolarization temperature $({T}_{d})$ at $x=0.06$, etc., have remain unresolved. Here, we offer structural insights on these issues by carrying out a detailed investigation using a set of complementary tools involving temperature-dependent x-ray powder diffraction, neutron powder diffraction, dielectric, ferroelectric, piezoelectric, and thermally induced depoling current measurements. We show that onset of thermal depoling in NBT $(x=0)$ well below its depolarization temperature is caused by abrupt reduction of intrinsic polarization in the ferroelectric $R3c$ phase, triggered by the appearance of the $P4bm$ phase. Our study suggests that partial conversion of the Cc average structure to $R3c$ in unpoled NBT-BT at $x\ensuremath{\sim}0.03$ (more precisely in the range $0.03\ensuremath{\le}x\ensuremath{\le}0.05$) is catalyzed by the appearance of $P4bm$ phase. The overlap of ${T}_{d}$ and ${T}_{2}$ for this composition range is correlated with the collapse of the tetragonality of the $P4bm$ phase and significantly reduced kinetic barrier associated with the $R3c\ensuremath{\rightarrow}P4bm$ transformation. We show that the abrupt crossover between ${T}_{d}$ and ${T}_{2}$ at $x=0.06$ is due to takeover of the thermal depoling process by an emergent tetragonal $(P4mm)$-like ferroelectric distortion. We present updated phase diagrams of poled and unpoled specimens which highlight all the subtle details needed to explain the temperature-dependent properties of this complex piezoelectric alloy system.

Nanointerfacial Layer Effect on Dielectric and Piezoelectric Responses in Chemical Solution-Derived Lead-Free Alkaline Niobate-Based Thin Films.

Since nonpiezoelectric interfacial layers even at the nanoscale significantly affect the performance of lead-free piezoelectric thin films, the quantitative characterization of property changes of thin films due to interfacial layers is of great importance and should be precisely undertaken for piezoelectric microelectromechanical system (MEMS) and nanoelectromechanical system (NEMS) devices. In contrast to widely accepted concepts for interfacial layer thickness estimation based on the existing series capacitor model, we find that the interfacial layer thickness at the top and the bottom interfaces is clearly different in chemical solution deposition (CSD)-derived (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 (KNMN) thin films. Interestingly, the thickness of the bottom interface increases linearly with increasing thin-film thickness, while the thickness of the top interface is constant regardless of the thin-film thickness. In this work, nanointerfacial layer effects of CSD-derived KNMN thin films are theoretically and experimentally addressed in a combinatorial way using a modified series capacitor model. The obtained information is used to envisage the origins and the mechanisms of nonpiezoelectric interfacial layers and associated dielectric and ferroelectric properties of KNMN thin films. Our research connects macroscopic properties with microscopic origins and is greatly facilitated by separating intrinsic and extrinsic contributions to phenomenological behaviors, as well as engineering interface-related properties of the films. We believe these studies to be crucial for the further development and applications of KNN-based lead-free piezoelectric devices, which also open the door to future studies on other lead-free piezoelectric material systems for practical MEMS and NEMS applications.

The origin of chemical inhomogeneity in lead-free potassium sodium niobate ceramic: Competitive chemical reaction during solid-state synthesis

Excellent electromechanical properties have been reported in lead-free potassium sodium niobate (KNN)-based piezoelectric ceramics over the past 15 years. However, poor reproducibility of the electromechanical properties of KNN has been a major barrier to industrial development, due to a lack of full understanding of some of the processing aspects. Chemical inhomogeneity is one of the most critical challenges in processing, as the properties of KNN-based ceramics are strongly composition-dependent. In the present study, in-situ temperature-dependent X-ray diffraction and thermogravimetric analysis were employed to investigate the chemical reactions that occur during solid-state synthesis. Chemical homogeneity was found to be sensitive to the competition among reactants during synthesis. The phenomenon can reasonably explain the chemical inhomogeneity in KNN and possibly other lead-free piezoelectric ceramic systems prepared via solid-state synthesis.

Abrupt change in domain switching behavior within tetragonal phase regime of (x)Na1/2Bi1/2TiO3-(1 − x)K1/2Bi1/2TiO3

We show that the lead-free piezoelectric system (x)Na1/2Bi1/2TiO3-(1 − x)K1/2Bi1/2TiO3 [xNBT-(1 − x)KBT] exhibits a boundary within its tetragonal phase regime at 0.58 < x < 0.60 across which the system exhibits an abrupt increase in reversible domain switching and high-field electrostrain for x > 0.58. We demonstrate that the abrupt changes, mimicking a composition-driven phase transition like scenario, are due to a sudden increase in the structural disorder for x > 0.58.

Coexistence of excellent piezoelectric performance and high Curie temperature in KNN-based lead-free piezoelectric ceramics

A new KNN-based lead-free piezoelectric material system of (1-x)K0.4Na0.6Nb0.96Sb0.04O3-x(Bi0.45Sm0.05)Na0.5ZrO3 (abbreviated as KNNS-xBSNZ, 0.00 ≤ x ≤ 0.06) is designed to obtain high performance to meet the requirement of next generation electronic devices. The relevant electric properties such as piezoelectricity, strain, and temperature stability are analyzed in detail. An excellent comprehensive property (d33∼508 pC/N, TC∼268 °C) is obtained in ceramic with x = 0.04 through the construction of rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary. In addition, a good temperature stability of strain for KNNS-0.04BSNZ ceramic is also obtained within the range from room temperature to 175 °C (Suni100°C/SuniRT∼97.81%, Suni150°C/SuniRT∼84.67%), which is comparatively superior to some reported KNN-based ceramics and lead-based ceramics. Therefore, we believe that this new material system would be a promising lead-free candidate for the practical applications.

Understanding the Nature of Temperature Stability in Potassium Sodium Niobate Based Ceramics from Structure Evolution under External Field.

Preferable temperature stability of ferroelectric and strain properties can be achieved in potassium sodium niobate (KNN)-based ceramics compared to that of other lead-free piezoelectric material systems, giving it greater potential for use in electronic devices once the temperature reliability issue related to the piezoelectric constant (d33) can be further resolved. Through exploration of the nature of the temperature stability from the evolution of a crystal structure under external fields, it has been determined that the electric field has no effect on the temperature stability of d33, resulting in almost the same trend of changes in d33 with temperature under both in situ and ex situ measurements, which is totally unlike the close relationship between the electric field and the strain temperature stability. With analysis of multiple factors including intrinsic phase coexistence, electric-field-induced domain texturing, lattice distortion, and thermally induced domain detexturing, the intrinsic contribution of phase evolution plays a dominant role in the temperature stability of d33 because of the mutual counteraction of domain texturing, lattice distortion, and thermal domain detexturing. Our research provides a systematic approach to understanding the nature of the d33 temperature stability in KNN-based ceramics and proposes some favorable suggestions on the future design of materials with good temperature reliability.

Enhanced mechanical energy harvesting capability in sodium bismuth titanate based lead-free piezoelectric

The demand for high-performance lead-free piezoelectric materials towards mechanical energy harvesting has been driven by the continuous requirements of self-powered applications such as wireless sensor network systems and biomedical devices, etc. Herein, manganese-doped sodium bismuth titanate based lead-free ternary piezoelectric system 0.92(Na0.5Bi0.5)TiO3-0.04(K0.5Bi0.5)TiO3-0.04BaTiO3 (NBT-KBT-BT) was developed and the critical role of manganese on tailoring the microstructure and electrical properties was investigated. The highest piezoelectric coefficient (d33) of 140 pC/N with figure of merit (d33 × g33) of 1790 × 10−15 m2/N has been achieved. Both high open-circuit voltage of ∼12.2 V and power density of ∼0.37 μW/mm3 were obtained based on cantilever-type energy harvester. This work provides a promising paradigm for the development of high-performance mechanical energy harvesting devices by tailoring the composition of lead-free piezoelectric ceramics.

Influence of porous configuration on dielectric and piezoelectric properties of KNN–BKT lead-free ceramic

Porous piezoelectric ceramics are the far and wide known graded functional materials. Of late, lead-free porous piezoelectric systems have increased enormous consideration, because of the factors inclusive of environmental friendly, light weight, low acoustic impedance and versatility. Porous lead-free (1-x) (K, Na) NbO3. x (Bi, K) TiO3 (KNN–BKT) solid solution, was synthesized by solid state method. The porogen utilized as part of this work was ammonium oxalate monohydrate in 10, 20, 30 and 40 wt% to get diverse porosity levels in the lead-free specimen. Primary evaluation of the porous specimen has been done with respect to its phase by XRD and surface microstructure using SEM. The specimens were poled and their piezoelectric coefficients were assessed at varied temperatures. Dielectric properties as a function of temperature and frequency are also reported for the specimens.

Role of Y3+ ion substitution in the enhanced electrical properties of Ba(0.9−x)YxCa0.1Zr0.07Ti0.93O3 lead-free piezoceramics

Ba(0.9−x)YxCa0.1Zr0.07Ti0.93O3 lead free piezoelectric ceramics were synthesized for x = 0–0.035 in the steps of 0.005. The lead free piezoceramics are gaining importance as there is an urgent demand for replacing the highly toxic PZT family piezoceramics. Ba0.9Ca0.1Zr0.07Ti0.93O3 (BCZT) is one such system. It shows some high electrical properties but suffers with a low Curie temperature, which restricts its usage in low temperature range only. In the present study, the Curie temperature has been improved by 9 °C. A polymorphic phase transition (PPT) has also been observed around x = 0.015, consisting of orthorhombic and tetragonal phases, which provides the polarization vector more number of favorable directions. As a result the remnant polarization (Pr), piezoelectric charge coefficient (d33) and electromechanical coupling coefficient (kp) attain their maximum values of 5.21 μC/cm2, 200 pC/N and 24.78% respectively for x = 0.015. This increase in transition temperature and other electrical properties makes BCZT a potential candidate for a lead free piezoelectric system.

Structural evolution and electrical properties of lead-free (1-x) (K0.4Na0.6)Nb0.96Sb0.04O3-xBa0.1(Bi0.5Na0.5)0.9ZrO3 ceramics

New lead-free piezoelectric ceramic system based on (1-x) (K0.4Na0.6) Nb0.96Sb0.04O3-xBa0.1(Bi0.5Na0.5)0.9ZrO3 was designed and fabricated by using conventional ceramic processing techniques. An investigation was carried out with a focus on the effects of Ba0.1(Bi0.5Na0.5)0.9ZrO3 content on the phase structure, dielectric and piezoelectric properties of the ceramics. It was found that adding Ba0.1(Bi0.5Na0.5)0.9ZrO3 could move the rhombohedral-orthorhombic phase transition temperature upwards and simultaneously shift the orthorhombic-tetragonal phase transition temperature downwards, and thereby resulted in the formation of a rhombohedral-tetragonal phase boundary with 0.045 ≤ x ≤ 0.05. The piezoelectric properties were obviously enhanced for the ceramics with compositions near the rhombohedral-tetragonal phase boundary. Furthermore, an analysis was as well conducted on the main characteristics of the ferroelectric-paraelectric phase transition of the ceramics. Our experimental results suggest that the Ba0.1(Bi0.5Na0.5)0.9ZrO3 is an effective additive for the construction of phase boundaries in (K,Na)NbO3-based ceramics, therefore benefiting the improvement of piezoelectric activity.