High-performance Piezoelectrics Research Articles

Practical high-performance lead-free piezoelectrics: structural flexibility beyond utilizing multiphase coexistence.

Due to growing concern for the environment and human health, searching for high-performance lead-free piezoceramics has been a hot topic of scientific and industrial research. Despite the significant progress achieved toward enhancing piezoelectricity, further efforts should be devoted to the synergistic improvement of piezoelectricity and its thermal stability. This study provides new insight into these topics. A new KNN-based lead-free ceramic material is presented, which features a large piezoelectric coefficient (d33) exceeding 500 pC/N and a high Curie temperature (Tc) of ∼200°C. The superior piezoelectric response strongly relies on the increased composition-induced structural flexibility due to lattice softening and decreased unit cell distortion. In contrast to piezoelectricity anomalies induced via polymorphic transition, this piezoelectricity enhancement is effective within a broad temperature range rather than a specific small range. In particular, a hierarchical domain architecture composed of nano-sized domains along the submicron domains was detected in this material system, which further contributes to the high piezoelectricity.

Understanding, Predicting, and Designing Ferroelectric Domain Structures and Switching Guided by the Phase-Field Method

Understanding mesoscale ferroelectric domain structures and their switching behavior under external fields is critical to applications of ferroelectrics. The phase-field method has been established as a powerful tool for probing, predicting, and designing the formation of domain structures under different electromechanical boundary conditions and their switching behavior under electric and/or mechanical stimuli. Here we review the basic framework of the phase-field model of ferroelectrics and its applications to simulating domain formation in bulk crystals, thin films, superlattices, and nanostructured ferroelectrics and to understanding macroscopic and local domain switching under electrical and/or mechanical fields. We discuss the possibility of utilizing the structure-property relationship learned from phase-field simulations to design high-performance relaxor piezoelectrics and electrically tunable thermal conductivity. The review ends with a summary of and an outlook on the potential new applications of the phase-field method of ferroelectrics.

Design development of (Ba1-xCax)(Ti1-ySny)O3 lead-free piezo ceramic by two manufacturing methods of CSS and SPS, promising for delamination damage detection

Delamination in laminated composite structures plays a major role in decreasing structural strength and stiffness, consequently downgrading system integrity and reliability. Therefore, the delamination detection by the method of sending and receiving propagated wave with piezoelectric sensors is very important. In order to detect the structural damage using piezo ceramics sensors with a high piezoelectric coefficient (d33), a lead-free piezo ceramic (B1-xCxT1-ySy)O3 or (Ba1-xCax)(Ti1-ySny)O3 was designed and manufactured by two methods of conventional solid-state reaction sintering (CSS) and spark plasma sintering (SPS). The electrical and electromechanical properties, X-ray diffraction analysis, thermal analysis (DSC) and scanning electron microscopy (SEM) were investigated on the samples and the samples properties were compared with different contents of Ca and Sn dopants. (Ba1-xCax)(Ti1-ySny)O3 (x = 0.05, 0.1 mole% and y = 0.02–0.12 mole %) ceramics with a high relative density over 97.02 (±0.2)% were prepared by CSS method at 1480 °C. A polymorphic phase transition from orthorhombic phase to tetragonal phase was confirmed in the composition range of x = 0.05, 0.1 and 0.02 ≤ y ≤ 0.12 by XRD analysis that was measured at room temperature. In CSS method a high piezoelectric coefficient (d33) up to 605 pC/N was obtained for x = 0.05 mole% and y = 0.09 mole%, which was higher than the other previously reported components of B1-xCxT1-ySy system. The corresponding planar electromechanical coupling factor (kp) and inversed piezoelectric coefficient (dS/dE) reached to 42.8% and 1203 pm/V, respectively. For the prepared SPS samples at 1330 °C and under the pressure of 55 MPa, the corresponding d33, kp, tanδ and dS/dE were 752 pC/N, 54.2%, 2.7% and 1248 pm/V, respectively. The average relative density of SPSed samples of the same composition reached to 99.50 (±0.2)%. It seems that designing and forming a suitable phase boundary starting from a tricritical triple point in the MPB zone that along with SPS manufacturing method could enhance and develop high-performance Pb-free piezoelectrics.

Enhanced electrical properties in donor–acceptor co-doped Ba(Ti0.92Sn0.08)O3 ceramics

In this work, lead-free piezoelectric ceramics Ba0.99Li0.005Al0.005(Ti0.92Sn0.08)O3-0.5%MnO2(BLATS-M) and Ba0.99Li0.005Nb0.005(Ti0.92Sn0.08)O3-0.5%MnO2(BLNTS-M) were synthesized by a solid-state reaction process. The influence of Li+–Al3+, Li+–Nb5+ ionic pairs on the microstructure, phase structure and electrical properties of Ba(Ti0.92Sn0.08)O3-0.5%MnO2(BTS-M) was systematically investigated. An orthorhombic (O) phase structure is obtained in all ceramics at room temperature. Both Li+–Al3+ and Li+–Nb5+ ionic pairs co-doping can enhance the piezoelectricity of BTS-M ceramic, while the Li+–Nb5+ doped BLNTS-M ceramic exhibits an optimum electrical properties: the piezoelectricity coefficient (d33) = 805 pC/N, the electromechanical coupling coefficient (kp) = 0.519, the relative permititivity (er) = 31900, the converse piezoelectric coefficient $$\left( {d_{{33}}^{*}} \right)$$ = 536 pm/V and the in situ quasi-static d33 with maximum $$\left( {d_{{33}}^{{\hbox{max} }}} \right)$$ = 1620 pC/N. The dielectric properties versus DC bias electric field experiments exhibit high dielectric tunability (ƞr = 97.78%) and figure of merit (FOM = 20.96) at 100 Hz and room temperature in BLNTS-M ceramic. The results suggest an effective approach to design high-performance piezoelectrics and dielectrics by appropriately selecting the types of donor–acceptor ionic pairs.

Understanding the mechanism of thermal-stable high-performance piezoelectricity

Lead zirconate titanate (PZT) ceramics have been widely used because of their large piezoelectric response and good temperature-stability in the vicinity of morphotropic phase boundary (MPB). However, the understanding on the mechanism of temperature-stable piezoelectricity in PZT is still blur and thus needs to explore properly since the urgently-needed Pb-free systems designed by the same MPB method commonly show poor temperature stability. In this paper, we investigate the property-microstructure relationship for the thermal-stable piezoelectricity through comparative studies of the phase diagram, in-situ piezoelectricity and microstructure evolution in both Pb-based and Pb-free systems. Our results show that the piezoelectric thermal-stability is closely related to the verticality of MPB and the doping elements. Tilted MPB shows worse thermal-stability as compared with that of the vertical MPB. While acceptor doped commercial PZTs show better thermal-stability when compared with that of the donor doped PZT. Moreover, our in-situ TEM reveals that it is the thermal-stable fine microstructure (i.e., no degeneration) that corresponds to the thermal-stable high-performance piezoelectricity. Our work provides an in-depth understanding of the relationship between the phase diagram, microstructure and thermal-stable piezoelectricity, which shall benefit the designing of new lead-free temperature-stable piezoelectric materials.

Atmospheric controlled processing enabling highly textured NKN with enhanced piezoelectric performance

High-performance lead-free piezoelectrics are of interest as alternate materials to confront potential legislative actions on lifting the exemption on lead-based piezoelectric materials. In this work, we demonstrated a highly textured commercial-grade Li- and Ta- modified (Na, K)NbO3 (NKN) lead-free piezoceramic that was sintered under low oxygen partial pressure (pO2) atmospheres using the solid state template grain growth (TGG) method. It was found that a moderate low pO2 condition not only facilitated the reactive template grain growth but also lowered the temperature required for such process. Highly textured NKN with a converse piezoelectric coefficient d33*≈680 pm V-1 was obtained, this remarkable piezoelectric property can be explained by the position of the polymorphic phase boundary of NKN. With considerations on both performance and cost, this work not only presented the potential for NKN to meet the requirement of commercial applications, but also provided an effective processing strategy to utilize low cost base metal inner electrodes in the future.

Temperature-driven phase transitions and enhanced piezoelectric responses in Ba(Ti0.92Sn0.08)O3 lead-free ceramic

Ferroelectric phases coexistence or transition is an important strategy on generating high piezoelectricity. Here, the temperature-induced phase structural evolution correlated with small signal piezoelectric response d33, bias-field piezoelectric activity dmax33(E), unipolar and bipolar strain piezoelectric outputs d*33 in Ba(Ti0.92Sn0.08)O3 (BTS0.08) ceramic was investigated in details. Temperature-driven successive phase transitions from rhombohedral (R) to orthorhombic (O), tetragonal (T), finally to cubic (C) phases took place around 14 °C, 38 °C and 61 °C, respectively. The highest d33 value of 675 pC/N is achieved in the T-C phase transition. However, the O-T phase boundary gives the highest dmax33 = 1170 p.m./V, bipolar d*33 = 822 pm/V and unipolar d*33 = 1318 pm/V. The temperature-driven phase transition exhibits large enhancements in piezoelectric property comparable to that of composition-induced phase boundary. These features suggest an effective method to design high-performance piezoelectrics by tailoring the types of phase boundary.

Evidence for existence of functional monoclinic phase in sodium niobate based solid solution by powder neutron diffraction

We have carried out systematic temperature-dependent neutron diffraction measurements in conjunction with dielectric spectroscopy from 6 to 300 K for sodium niobate based compounds (1-x) NaNbO3-xBaTiO3 (NNBTx). The dielectric constant is measured as a function of both temperature and frequency. It shows an anomaly at different temperatures in cooling and heating cycles and exhibits a large thermal hysteresis of ∼150 K for the composition x = 0.03. The dielectric constant is found to be dispersive in nature and suggests a relaxor ferroelectric behavior. In order to explore structural changes as a function of temperature, we analyzed the powder neutron diffraction data for the compositions x = 0.03 and 0.05. Drastic changes are observed in the powder profiles near 2θ ∼ 30.6°, 32.1°, and 34.6° in the diffraction pattern below 200 K during cooling and above 190 K in heating cycles, respectively. The disappearance of superlattice reflection and splitting in main perovskite peaks provide a signature for structural phase transition. We observed stabilization of a monoclinic phase (Cc) at low temperature. This monoclinic phase is believed to provide a flexible polarization rotation and considered to be directly linked to the high performance piezoelectricity in materials. The thermal hysteresis for composition x = 0.03 is larger than that for x = 0.05. This suggests that the addition of BaTiO3 to NaNbO3 suppresses the thermal hysteresis. It is also observed that the structural phase transition temperature decreases upon increasing the dopant concentration.

Unexpectedly high piezoelectricity of Sm-doped lead zirconate titanate in the Curie point region

Large piezoelectric coefficients in polycrystalline lead zirconate titanate (PZT) are traditionally achieved through compositional design using a combination of chemical substitution with a donor dopant and adjustment of the zirconium to titanium compositional ratio to meet the morphotropic phase boundary (MPB). In this work, a different route to large piezoelectricity is demonstrated. Results reveal unexpectedly high piezoelectric coefficients at elevated temperatures and compositions far from the MPB. At temperatures near the Curie point, doping with 2 at% Sm results in exceptionally large piezoelectric coefficients of up to 915 pm/V. This value is approximately twice those of other donor dopants (e.g., 477 pm/V for Nb and 435 pm/V for La). Structural changes during the phase transitions of Sm-doped PZT show a pseudo-cubic phase forming ≈50 °C below the Curie temperature. Possible origins of these effects are discussed and the high piezoelectricity is posited to be due to extrinsic effects. The enhancement of the mechanism at elevated temperatures is attributed to the coexistence of tetragonal and pseudo-cubic phases, which enables strain accommodation during electromechanical deformation and interphase boundary motion. This work provides insight into possible routes for designing high performance piezoelectrics which are alternatives to traditional methods relying on MPB compositions.

Ultrahigh Piezoelectric Properties in Textured (K,Na)NbO3 -Based Lead-Free Ceramics.

High-performance lead-free piezoelectric materials are in great demand for next-generation electronic devices to meet the requirement of environmentally sustainable society. Here, ultrahigh piezoelectric properties with piezoelectric coefficients (d33 ≈700 pC N-1 , d33 * ≈980 pm V-1 ) and planar electromechanical coupling factor (kp ≈76%) are achieved in highly textured (K,Na)NbO3 (KNN)-based ceramics. The excellent piezoelectric properties can be explained by the strong anisotropic feature, optimized engineered domain configuration in the textured ceramics, and facilitated polarization rotation induced by the intermediate phase. In addition, the nanodomain structures with decreased domain wall energy and increased domain wall mobility also contribute to the ultrahigh piezoelectric properties. This work not only demonstrates the tremendous potential of KNN-based ceramics to replace lead-based piezoelectrics but also provides a good strategy to design high-performance piezoelectrics by controlling appropriate phase and crystallographic orientation.

Effect of chemical pressure on competition and cooperation between polar and antiferrodistortive distortions in sodium niobate

We present results obtained from a combination of dielectric and x-ray diffraction measurements for compositional design of $(1\ensuremath{-}x)\phantom{\rule{0.28em}{0ex}}\mathrm{NaNb}{\mathrm{O}}_{3}\ensuremath{-}x\mathrm{BaTi}{\mathrm{O}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{NNBT}x)$, which can induce interferroelectric phase transitions. Anomalies are observed in dielectric measurements performed for various compositions at 300 K, as well as at different temperatures for NNBT03. We observed the appearance(disappearance) of the superlattice reflections along with change in the intensities of the main perovskite peaks in the powder x-ray diffraction data, which provide clear evidences for structural phase transitions with composition and temperature. We found that increasing the concentration of $\mathrm{BaTi}{\mathrm{O}}_{3}$ leads to the suppression of out-of-phase rotation of octahedra and an increment in tetragonality ($c/a$ ratio), which promotes the polar mode at room temperature. The temperature-dependent powder diffraction study shows that the ferroelectric rhombohedral phase of pure sodium niobate gets suppressed for the composition $x=0.03$, and the monoclinic phase $Cc$ gets stabilized at low temperature. The monoclinic phase is believed to provide for a flexible polarization rotation and is considered to be directly linked to the high-performance piezoelectricity in materials due to presence of more easy axes for spontaneous polarizations than the rhombohedral phase.

Ferroelectric materials for vibrational energy harvesting

Energy harvesting is an appealing technology that makes use of the ambient energy which is otherwise wasted. Piezoelectric materials directly convert the elastic energy to the electric energy, and thus have a great advantage in scavenging vibrational energy for simplicity in device structure with relatively high power density. This paper provides an overview on the research of piezoelectric materials in energy harvesting in recent decades, from basics of piezoelectricity and working principle of energy harvesting with piezoelectric materials, to the progress of development of high-performance piezoelectrics including ceramics, single crystals and polymers, then to experimental attempts on the device fabrication and optimization, finally to perspective applications of piezoelectric energy harvesting (PEH). The criteria for selection of materials for PEH applications are introduced. Not only the figure of merit but also maximum allowable stress of materials are taken into account in the evaluation of their potential in achieving high energy density and output power density. The influence of the device configuration on the performance is also acknowledged and discussed. The magnitude and distribution of induced stress in the piezoelectric unit upon excitation by the vibration source play an important role in determining the output power density and can be tuned via proper design of device configuration without changing its resonant frequency. Approaches to address the issue of frequency match accompanying with the resonant mode are illustrated with literature examples. Usage of PEH devices can be extended to a variety of vibration sources in everyday life as well as in nature. Some appealing applications of PEH, such as in implantable and wearable devices, are reviewed.

Effect of lanthanum doping on tetragonal-likeBiFeO3with mixed-phase domain structures

The recent discoveries of both chemical-driven and strain-driven morphotropic phase boundaries (MPBs) in $\mathrm{BiFe}{\mathrm{O}}_{3}$ (BFO) thin films have opened up new horizons in developing high-performance lead-free piezoelectrics. An attempt to bridge these two MPBs is made by doping La into highly strained BFO thin films with the coexistence of tetragonal-like and rhombohedral-like phases. The structural, morphological, and ferroelectric properties of such films are investigated. It is observed that La doping changes the energy landscape between the tetragonal-like and the rhombohedral-like polymorphs due to the chemical pressure imposed by the La substitution. Polar instability is found upon increasing La doping for the in-plane polarization component, which correlates with the vanishing of in-plane ferroelectric domain structures. The transition sequence of the in-plane ferroelectric polarization resembles that previously reported for the bulklike rhombohedral phase of BFO under continuous La doping, indicating the universality of the chemical-alloying effect on the ferroelectric order.

The missing boundary in the phase diagram of PbZr(1-x)TixO₃.

PbZr(1-x)Ti(x)O3 (PZT) is one of the most important and widely used piezoelectric materials. The study of its local and average structures is of fundamental importance in understanding the origin of its high-performance piezoelectricity. Pair distribution function analysis and Rietveld refinement have been carried out to study both the short- and long-range order in the Zr-rich rhombohedral region of the PZT phase diagram. The nature of the monoclinic phase across the Zr-rich and morphotropic phase boundary area of PZT is clarified. Evidence is found that long-range average rhombohedral and both long- and short-range monoclinic regions coexist at all compositions. In addition, a boundary between a monoclinic (M(A)) structure and another monoclinic (M(B)) structure has been found. The general advantage of a particular monoclinic distortion (M(A)) for high piezoactivity is discussed from a spatial structural model of susceptibility to stress and electric field, which is applicable across the wide field of perovskite materials science.

Thermotropic phase boundaries in classic ferroelectrics

High-performance piezoelectrics are lead-based solid solutions that exhibit a so-called morphotropic phase boundary, which separates two competing phases as a function of chemical composition; as a consequence, an intermediate low-symmetry phase with a strong piezoelectric effect arises. In search for environmentally sustainable lead-free alternatives that exhibit analogous characteristics, we use a network of competing domains to create similar conditions across thermal inter-ferroelectric transitions in simple, lead-free ferroelectrics such as BaTiO3 and KNbO3. Here we report the experimental observation of thermotropic phase boundaries in these classic ferroelectrics, through direct imaging of low-symmetry intermediate phases that exhibit large enhancements in the existing nonlinear optical and piezoelectric property coefficients. Furthermore, the symmetry lowering in these phases allows for new property coefficients that exceed all the existing coefficients in both parent phases. Discovering the thermotropic nature of thermal phase transitions in simple ferroelectrics thus presents unique opportunities for the design of 'green' high-performance materials.

Lead-free Piezoelectrics Based on Potassium–Sodium Niobate with Giant d33

High-performance lead-free piezoelectrics (d33 > 400 pC/N) based on 0.96(K0.5Na0.5)0.95Li0.05Nb1-xSbxO3-0.04BaZrO3 with the rhombohedral-tetragonal (R-T) phase boundary have been designed and prepared. The R-T phase boundary lies the composition range of 0.04 ≤ x ≤ 0.07, and the dielectric and piezoelectric properties of the ceramics with the compositions near the phase boundary are significantly enhanced. In addition, the ceramic with x = 0.07 has a giant d33 of ~425 pC/N, which is comparable to that (~416 pC/N) of textured KNN-based ceramics (Saito, Y.; Takao, H.; Tani, T.; Nonoyama, T.; Takatori, K.; Homma, T.; Nagaya, T.; Nakamura, M. Nature 2004, 432, 84). The underlying physical mechanisms for enhanced piezoelectric properties are addressed. We believe that the material system is the most promising lead-free piezoelectric candidates for the practical applications.

Enhanced electromechanical response of ferroelectrics due to charged domain walls

While commonly used piezoelectric materials contain lead, non-hazardous, high-performance piezoelectrics are yet to be discovered. Charged domain walls in ferroelectrics are considered inactive with regards to the piezoelectric response and, therefore, are largely ignored in this search. Here we demonstrate a mechanism that leads to a strong enhancement of the dielectric and piezoelectric properties in ferroelectrics with increasing density of charged domain walls. We show that an incomplete compensation of bound polarization charge at these walls creates a stable built-in depolarizing field across each domain leading to increased electromechanical response. Our model clarifies a long-standing unexplained effect of domain wall density on macroscopic properties of domain-engineered ferroelectrics. We show that non-toxic ferroelectrics like BaTiO3 with dense patterns of charged domain walls are expected to have strongly enhanced piezoelectric properties, thus suggesting a new route to high-performance, lead-free ferroelectrics.

Large piezoelectric effect in Pb-free Ba(Ti,Sn)O3-x(Ba,Ca)TiO3 ceramics

We designed a Pb-free pseudo-binary system, Ba(Sn0.12Ti0.88)O3-x(Ba0.7Ca0.3)O3 (BTS-xBCT), characterized by a phase boundary starting from a tricritical triple point of a paraelectric cubic phase, ferroelectric rhombohedral, and tetragonal phases. The optimal composition BTS-30BCT exhibits a high piezoelectric coefficient d33 ∼ 530 pC/N at room temperature. In view of the recent report of high piezoelectricity in another Pb-free system BZT-BCT (Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)), which possesses a similar tricritical triple point in the phase diagram, it seems that forming a suitable phase boundary starting from a tricritical triple point could be an effective way to develop high-performance Pb-free piezoelectrics.

Screening for high-performance piezoelectrics using high-throughput density functional theory

We present a large-scale density functional theory (DFT) investigation of the $\mathit{AB}{\mathrm{O}}_{3}$ chemical space in the perovskite crystal structure, with the aim of identifying those that are relevant for forming piezoelectric materials. Screening criteria on the DFT results are used to select 49 compositions, which can be seen as the fundamental building blocks from which to create alloys with potentially good piezoelectric performance. This screening finds all the alloy end points used in three well-known high-performance piezoelectrics. The energy differences between different structural distortions, deformation, coupling between the displacement of the $A$ and $B$ sites, spontaneous polarization, Born effective charges, and stability is analyzed in each composition. We discuss the features that cause the high piezoelectric performance of the well-known piezoelectric lead zirconate titanate (PZT), and investigate to what extent these features occur in other compositions. We demonstrate how our results can be useful in the design of isovalent alloys with high piezoelectric performance.

Structure and Piezoelectric Properties Near the Bismuth Scandium Oxide–Lead Zirconate–Lead Titanate Ternary Morphotropic Phase Boundary

Ternary phase diagram of BiScO3 (BS), PbZrO3 (PZ), and PbTiO3 (PT) was explored for identification of high‐performance piezoelectrics for actuator applications. The ternary morphotropic phase boundary (MPB) connecting the binary MPBs of BS–PT (45/65) and PZ–PT (52/48) was determined using X‐ray diffraction (XRD). High‐temperature XRD and dielectric measurements were used to determine the phase transformation temperatures. Curie temperature (TC) had a near linear dependence on composition, rate of which is determined for each component of the ternary independently. Specimens on the tetragonal side of the MPB exhibited lower high field resistivity and proper poling was not possible. Specimens on the rhombohedral side were superior with saturated hysteresis loops and piezoelectric coefficient (d33)>400 pm/V. Unlike phase transformation temperatures, the proximity to MPB was more dominant than the compositional effects in determining electrical and electromechanical properties, which maximized for the compositions closest to the MPB. Both weak‐ and high field properties are reported as a function of temperature.