Lead-free Piezoceramics Research Articles

Defect Engineering with Rational Dopants Modulation for High-Temperature Energy Harvesting in Lead-Free Piezoceramics

HighlightsThe solution limit of manganese ion in BiFeO3–BaTiO3 (BF–BT) was determined by combining multiple advanced characterization methods.The defect engineering associated with fine doping can realize the co-modulation of polarization configuration, iron oxidation state and domain orientation.The BF–BT–0.2Mn piezoelectric energy harvester shows excellent power generation capacity at 250 °C, which is an important breakthrough for lead-free piezoelectric devices.

Energy harvesting properties for potassium-sodium niobate piezoceramics through synergistic effect of phase structure and texturing engineering

In order to address the energy dilemma and meet environmental protection requirements, it is an urgent need to develop lead-free piezoelectric energy harvesters (PEHs). However, the low output power density has seriously hindered the application of lead-free piezoceramics as high efficiency energy harvesters. To solve above question, the combination of phase structure regulation and texturing technique in the K0.5Na0.5NbO3-xBi0.5Na0.5ZrO3-0.5%wtBiFeO3 (KNN-xBNZ) ceramics was adopted in this work. Because of both the Rhombohedral-Orthorhombic-Tetragonal (R-O-T) multiphases coexistence and texturing engineering, an ultrahigh piezoelectric activity quality factor d33×g33 ∼ 18324 ×10-15 m2·N-1 was achieved in the textured KNN-0.05BNZ (T-KNN-0.05BNZ) ceramics, which is about third times higher than that of random KNN-0.05BNZ (R-KNN-0.05BNZ) (d33×g33 ∼ 6672 ×10-15 m2·N-1) ones. A high output power of ∼ 0.32 mW was also obtained in the T-KNN-0.05BNZ ceramics by cantilever beam-based energy harvester devices under the resonance frequency of 75 Hz and matched RL of 1.2 MΩ, suggesting that the T-KNN-0.05BNZ ceramics have a great potential for application in the PEHs devices. This result also reveals that the combination of phase structure regulation and texturing technique is an effective way to achieved a high energy harvesting performances.

High Temperature-Insensitive Electrostrain Obtained in (K, Na)NbO3-Based Lead-Free Piezoceramics.

Over the last decades, notable progress is achieved in (K, Na)NbO3 (KNN)-based lead-free piezoceramics. However, more studies are conducted to increase its piezoelectric charge coefficient (d33). For actuator applications, piezoceramics need high electric-field induced strain under low electric fields while maintaining exceptional temperature stability across a wide temperature range. In this study, this work developes Li/Sb-codoped KNN (LKNNS) ceramics with high electrostrain by defect engineering and domain engineering. A remarkable strain of 0.43%, along with a giant d33* value of 2177 pm V-1, is attained in the LKNNS ceramic at 20 kV cm-1. The ceramic exhibits a minimal performance decrease of less than 15% over a temperature range from room temperature to 150 °C. The exceptional strain is attributed to the presence of A-site vacancy-oxygen vacancy ( ) defect dipoles and the increase in nano-domains. The hierarchical domain configuration and defect dipoles impede the switched domains from reverting to their original state as temperature increases, furthermore, the elongated dipole moments of caused by rising temperatures compensate for strain reduction results in exceptional temperature stability. This study provides a model for designing piezoelectric materials with exceptional overall performance under low electric fields and across a wide temperature range.

Ultrahigh thermal stability and piezoelectricity of lead-free KNN-based texture piezoceramics

The contradiction between high piezoelectricity and uniquely poor temperature stability generated by polymorphic phase boundary is a huge obstacle to high-performance (K, Na)NbO3 -based ceramics entering the application market as Pb-based substitutes. We possess the phase boundary by mimicking Pb(Zr, Ti)O3’s morphotropic phase boundary structure via the synergistic optimization of diffusion phase boundary and crystal orientation in 0.94(Na0.56K0.44)NbO3−0.03Bi0.5Na0.5ZrO3−0.03(Bi0.5K0.5)HfO3 textured ceramics. As a result, a prominent comprehensive performance is obtained, including giant d33 of 550 ± 30 pC/N and ultrahigh temperature stability (d33 change rate less than 1.2% within 25-150 °C), representing a significant breakthrough in lead-free piezoceramics, even surpassing the Pb-based piezoelectric ceramics. Within the same temperature range, the d33 change rate of the commercial Pb(Zr, Ti)O3−5 ceramics is only about 10%, and more importantly, its d33 (~ 350 pC/N) is much lower than that of the (K, Na)NbO3-based ceramics in this work. This study demonstrates a strategy for constructing the phase boundary with MPB feature, settling the problem of temperature instability in (K, Na)NbO3-based ceramics.

Enhancing piezoelectric coefficient and thermal stability in lead-free piezoceramics: insights at the atomic-scale

Given the highly temperature-sensitive nature of the polymorphic phase boundaries, attaining excellent piezoelectric coefficient with superior temperature stability in lead-free piezoceramics via direct compositional design remains a formidable challenge. We demonstrate the synergistic improvement of piezoelectric coefficient and thermal stability in lead-free piezoceramics via atomic-scale local ferroelectric structure design. Via modulation of the local Landau energy barrier at doping sites, we effectively mitigate fluctuations in piezoelectric d33. Our approach achieves an impressive d33 of ~430 pC/N with a minimal temperature fluctuation range (△d33 ~ 7%) across the room temperature to 100 °C in potassium sodium niobate ceramics. Further optimization through annealing extends this temperature up to 150 °C (△d33 ~ 8%) while maintaining a high d33 of ~380 pC/N, rivaling the performance of classic temperature stable lead zirconate titanate. This work establishes a framework for addressing the dilemma between high piezoelectric coefficient and inadequate temperature stability in lead-free piezoceramics.

Effect of alumina or zirconia particles on the performance of lead-free BCZT piezoceramics

The barium titanate derivative Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) offers high dielectric and piezoelectric performance but has poor mechanical properties. Ceramic composite materials combining BCZT with tougher Al2O3 or ZrO2 could be the solution to the inherent brittleness of electroceramics. This work focuses on BCZT with 1–5 vol.% addition of reinforcing phase dispersed in the microstructure. While hardness was improved in all composites, especially in ZrO2-reinforced BCZT, toughness was positively affected only by Al2O3. The chemical reactivity of BCZT during sintering resulted in the formation of new barium aluminate phases at the grain boundaries and possibly also within the grains, which affected the electrical properties of the composites. The addition of zirconia resulted in the formation of BaZrO3 or Ba(Zr,Ti)O3, which is a natural part of the BCZT solid solution and shifts the phase composition towards relaxor behaviour. ZrO2-reinforced composites exhibited higher permittivity but lower ferroelectric properties, accompanied by a substantial Curie temperature decrease.

Impact of Ca2+ and Zr4+ substitution on mechanical energy harvesting response of lead-free BaTiO3 piezoceramics with thermally stable storage density and efficiency

The Ba1-xCaxZryTi1-yO3 (BCZT; x = 0.18, y = 0.08; x = 0.15, y = 0.10; x = 0.12, y = 0.12; x = 0.09, y = 0.14; both x and y are in mol. %) piezoceramics were prepared by solid-state reaction method and investigated their energy storage and harvesting responses. The composition x = 0.15, y = 0.10 ( i.e. BCZT-2) reveals the morphotropic phase boundary (MPB) near room temperature with enhanced piezoelectric properties such as piezoelectric charge coefficient (d33) ∼ 380 pC/N, piezoelectric strain coefficient (d33*) ∼ 583.71 pm/V, piezoelectric voltage coefficient (g33) ∼ 14.09 ×10−3Vm/N, figure of merits (d33×g33) ∼ 5.35 ×10−12m2/N and the electromechanical coupling coefficient (kp) ∼ 0.291. The harvested peak-to-peak open circuit AC voltage is 30 V and the calculated current is in the range of 0.02–0.14 mA with varied load resistance and the maximum obtained power density is 13.38 μW/mm3 for BCZT-2 ceramics. The DC output signal of BCZT-2 ceramics successfully lit up the 'FCLNT' panel consisting of 53 red LEDs. At room temperature, BCZT ceramics have a recoverable energy storage density (Wrec) and storage efficiency (η) in the range between 506.46 and 257.49 mJ/cm3 and 50.53 – 64.93 % respectively. The composition x = 0.12, y = 0.12 (i.e. BCZT-3) revealed the highest energy storage efficiency (η) ∼ 64.93 % at room temperature. Nonetheless, the temperature-dependent (till its Curie temperature) maximum obtained Wrec of all BCZT ceramics is found in the range between 238.41 and 446.49 mJ/cm3 with an η of 66.98–79.67 %. Thus, compositionally tuned BCZT ceramics exhibit considerable piezoelectric properties, including energy storage and harvesting capabilities.

Fermi level limitation in Na1/2Bi1/2TiO3–BaTiO3 piezoceramics by electrochemical reduction of Bi

The (electro)chemical stability of undoped and Zn-doped 0.94Na1/2Bi1/2TiO3–0.06BaTiO3 lead-free piezoceramics (NBT–6BT) was studied. For this purpose, the Fermi level at the interface between NBT–6BT and Sn-doped In2O3 (ITO) electrode is varied by gradually reducing the ITO film either by annealing in vacuum or by applying a voltage across a Pt/NBT–6BT/ITO. The chemical and electronic changes are monitored in situ by x-ray photoelectron spectroscopy. The experiments reveal the formation of metallic Bi when the Fermi level is reaching a value of 2.23 ± 0.10 eV above the valence band maximum, while no reduction of Ti is observed. The electrochemical reduction of Bi constitutes an upper limit of the Fermi level at ≈1 eV below the conduction band minimum. High electron concentrations in the conduction band and a contribution of free electrons to the electrical conductivity of NBT–6BT can, therefore, be excluded. The reduction occurs for an ITO work function of 4.2–4.3 eV. As typical electrode materials such as Ag, Cu, Ni, or Pt have higher work functions, an electrochemical instability of the electrode interfaces in ceramic capacitors is not expected. Under the given experimental conditions (350 °C, electric fields <40 V/mm), no degradation of resistance and no enrichment of Na at the interface are observed.

A Comprehensive Review of Strategies toward Efficient Flexible Piezoelectric Polymer Composites Based on BaTiO3 for Next-Generation Energy Harvesting

The increasing need for wearable and portable electronics and the necessity to provide a continuous power supply to these electronics have shifted the focus of scientists toward harvesting energy from ambient sources. Harvesting energy from ambient sources, including solar, wind, and mechanical energies, is a solution to meet rising energy demands. Furthermore, adopting lightweight power source technologies is becoming more decisive in choosing renewable energy technologies to power novel electronic devices. In this regard, piezoelectric nanogenerators (PENGs) based on polymer composites that can convert discrete and low-frequency irregular mechanical energy from their surrounding environment into electricity have attracted keen attention and made considerable progress. This review highlights the latest advancements in this technology. First, the working mechanism of piezoelectricity and the different piezoelectric materials will be detailed. In particular, the focus will be on polymer composites filled with lead-free BaTiO3 piezoceramics to provide environmentally friendly technology. The next section will discuss the strategies adopted to enhance the performance of BaTiO3-based polymer composites. Finally, the potential applications of the developed PENGs will be presented, and the novel trends in the direction of the improvement of PENGs will be detailed.

Phase evolution and piezoelectric properties of SnO2 doped KNN based piezoceramics

The present research focuses on the electrical properties of lead-free piezoceramics (0.985-x)(K0.485Na0.485Li0.03)(Nb0.96Sb0.04)O3-0.015(Bi0.5Na0.5)ZrO3-xSnO2 (KNNLS-BNZ-xSnO2, x = 0.003, 0.006, 0.009, 0.012, and 0.015). This study examines the influence of varying SnO2 doping levels on the microstructure and piezoelectric properties of the ceramics. All the ceramics prepared demonstrate pure perovskite structure without any secondary phases. The Rietveld refinement analysis of XRD data reveals the existence of multiphase evolution around room temperature. The tetragonality (c/a) and cell volume of the ceramics tend to rise with an increase in Sn4+ content, potentially leading to improved ferroelectric characteristics. The optimum values obtained were Curie temperature (Tc) = 395 oC, remnant polarization (Pr) = 21.07 μC/cm2, piezoelectric coefficient (d33)=357 pC/N, piezoelectric voltage constant (g33) = 24 × 10−3 Vm/N, and figure of merit (FOMoff) = 10.34 pm2/N, corresponding to the ceramic doped with x = 0.012 SnO2. The findings indicate that an optimal amount of Sn4+ in the host composition KNNLS-BNZ improves piezoelectric properties by constructing an R-O-T phase boundary near room temperature. This study suggests that the ceramic with x = 0.012 SnO2 exhibits a favorably high Tc coupled with an exceptional energy harvesting capability, making it a suitable candidate for energy harvesting applications.

Adaptive ferroelectric states in KNN-based piezoceramics: Unveiling the mechanism of enhancing piezoelectric properties through multiple phase boundary engineering

Developing high-performance lead-free piezoceramics, such as (K,Na)NbO3 (KNN) material, is critical to achieving environmental sustainability in next-generation electromechanical devices. Although substantial advancements have been made in KNN-based ceramics, a general coherent framework is lacking that links microscopic structure to the enhancement of macroscopic properties. Our findings indicate that the enhanced performance of KNN-based ceramics in multiphase coexistence boundaries can be attributed to the formation of self-adjusting nanodomains in response to strong local structural heterogeneity. The self-adjusting behavior of domain structure is an outcome of minimizing the local stress generated by the lattice mismatch within KNN-based ceramics, which conforms to the thermodynamic principles that favor minimizing total free energy. Guided by this strategy, the present work achieves a significant improvement of electromechanical properties, including a piezoelectric coefficient (d33) of ∼585 pC N−1, an electromechanical coupling factor (kp) of ∼62 %, and a figure of merit (d33 × g33) of ∼12.78 ×10−12 m2 N−1. This study offers a new strategy for developing lead-free piezoceramics through dedicated design of novel phase boundaries.

Understanding the grain size dependence of functionalities in lead-free (Ba,Ca)(Zr,Ti)O3

Grain size effects in ferroelectric ceramics have long been exploited to tailor their functional properties. However, the underlying mechanism for the grain size effects is not yet fully understood. Here, we study the grain size dependence of domain wall activities in a lead-free piezoceramics system, (Ba,Ca)(Zr,Ti)O3, with grain sizes in the range of 4 − 22 μm. The dielectric permittivity is highest at intermediate grain sizes (∼12 μm) where there are moderate lattice distortion and the most active domain wall motion under low voltages. Despite the larger fraction of switched domains in the material with larger distortion (∼22 μm), as revealed by in situ electric-field synchrotron X-ray diffraction, time-resolved characterizations demonstrate an easier and faster domain wall dynamics in the sample with moderate lattice distortion. Our analysis and phase-field simulations show that the grain size dependence of the domain wall dynamics of polycrystalline ferroelectrics is dictated by the interaction between an external electric field and the grain size-related change in intergranular stress, and this interaction is most effective in stimulating the movement of non-180° domain wall at intermediate grain sizes during the initial phase of polarization reversal. Our results provide a new fundamental understanding to guide the future design of materials for improving functionalities.

Bi compensation and poling process to enhance piezoelectric properties of BiFeO3-BaTiO3 lead-free piezoceramics

BiFeO3-BaTiO3 (BF-BT) ceramics are an excellent choice for high-temperature lead-free piezoelectric ceramic due to their high Curie temperature and good piezoelectric properties, but the volatilization of Bi leads to the formation of A-site cationic vacancies and oxygen vacancies, culminating in heightened leakage currents and detrimental effects on piezoelectric properties. In this study, a chemical composition of 0.70Bi(1+x)Fe0·97Al0·03O3-0.30BaTiO3 ceramics, abbreviated as B1+xFA-BT, where 0 ≤ x ≤ 0.05, were synthesized, with special attention on the effects of Bi compensation and poling process on their insulation and piezoelectric properties. Due to the coexistence of R-PC phases and low leakage current, B1+xFA-BT ceramics exhibit excellent piezoelectric properties and high Curie temperature, among which the B1.02FA-BT ceramic boasts optimal overall properties: d33 = 196 pC N−1, TC = 480 °C, kp = 34%, θmax = 49.5° and Qm = 32, under optimal poling parameters Ep = 50 kV cm−1 and Tp = 100 °C. The poling process is directly linked to the reorientation of domains, the redistribution of positive/negative space charge and the aggregation or injection of holes, which is achieved by tuning the poling field and temperature in a ceramic with optimal integrated piezoelectric properties. The poling process of BF-BT ceramics is significantly influenced by their leakage characteristics, which can have a significant impact on their overall performance. Optimizing electrical poling conditions and minimizing leakage current is crucial to achieving favorable piezoelectric properties in BF-BT ceramics with R-PC coexisting structures.

Simultaneously enhanced electrical properties and high-power characteristics of (K,Na)NbO3 lead-free piezoceramics by hot-pressing

Piezoelectric materials enabling direct conversion between electrical and mechanical energies are ubiquitously implemented in high-power apparatuses, where both superior piezoelectricity and high-power performance are demanded. However, it remains a challenge to simultaneously achieve the two goals, which are hindered by their antagonistic relationship. Herein, hot-pressing is employed to address this dilemma, as exemplified in lead-free (K,Na)NbO3 (KNN) piezoceramics. In contrast to the samples prepared using normal sintering (NS–KNN), hot-pressed KNN (HP–KNN) polycrystals show higher density, lager piezoelectricity (d33∼129 pC/N), and more active domain wall movement. In particular, the normalized strain d33* of HP-KNN piezoceramics exhibits high thermal stability (variation of d33*<10 %) in a broad temperature range from ambient temperature up to 150 °C. Moreover, the HP-KNN samples show superior high-power characteristics with a maximum vibration speed of 1.1 m/s, which is almost 78 % larger than that of NS-KNN. This study not only demonstrates that the HP-KNN lead-free piezoceramics hold great potential for high-power implements, but also opens a new avenue for integrating antagonistic properties for the enhancement of the collective performance in functional materials.

Multiscale reconfiguration induced highly saturated poling in lead-free piezoceramics for giant energy conversion

The development of high-performance lead-free K0.5Na0.5NbO3-based piezoceramics for replacing commercial lead-containing counterparts is crucial for achieving environmentally sustainable society. Although the proposed new phase boundaries (NPB) can effectively improve the piezoelectricity of KNN-based ceramics, the difficulty of achieving saturated poling and the underlying multiscale structures resolution of their complex microstructures are urgent issues. Here, we employ a medium entropy strategy to design NPB and utilize texture engineering to induce crystal orientation. The developed K0.5Na0.5NbO3-based ceramics enjoys both prominent piezoelectric performance and satisfactory Curie temperature, thus exhibiting an ultrahigh energy harvesting performance as well as excellent transducer performance, which is highly competitive in both lead-free and lead-based piezoceramics. Comprehensive structural analysis have ascertained that the field-induced efficient multiscale polarization configurations irreversible transitions greatly encourages high saturated poling. This study demonstrates a strategy for designing high-performance piezoceramics and establishes a close correlation between the piezoelectricty and the underlying multiscale structures.

Investigation of piezoelectric properties in manganese doped alkaline niobate-based lead-free piezoceramics

In this work, an attempt is made for improving the piezoelectric properties of a lead-free (Na[Formula: see text]K[Formula: see text]Li[Formula: see text](Nb[Formula: see text]V[Formula: see text]O3 ceramic system by doping manganese in it. The Rietveld analysis of XRD micrographs of the ceramics indicates the samples crystallizing into 99.86% of orthorhombic phase and very small traces of tetragonal phase around room temperature. The Curie temperature ([Formula: see text]) is hardly affected by the incorporation of Mn[Formula: see text] into the system. At the manganese concentration of 0.02[Formula: see text]wt.%, the ceramic system attains the peak values in its density, dielectric constant at room temperature ([Formula: see text]), planar electromechanical coefficient ([Formula: see text], piezoelectric coefficient ([Formula: see text]), and remnant polarization ([Formula: see text]). The optimum piezoelectric properties of [Formula: see text]% and [Formula: see text] pC/N are observed for this composition. The study reveals that the modification of (Na[Formula: see text]K[Formula: see text]Li[Formula: see text]) (Nb[Formula: see text]V[Formula: see text]O3 with an appropriate quantity of Mn[Formula: see text] can produce the desired changes in the crystallographic properties and densification so as to eventually improve its piezoelectric properties.

Flash sintering of potassium-sodium niobate-based piezoceramics: Refining (micro)structure for properties enhancement

Low-consumption ceramics processing routes are expected to replace conventional ones due to environmental concerns. In this context, flash sintering is garnering interest because it allows dense ceramics to be obtained in just a few minutes and at relatively low temperatures. This is particularly interesting for sintering alkaline-based compounds due to the easy volatilization of these elements. In this work, current-controlled flash sintering is used to obtain potassium-sodium niobate (KNN)-based piezoceramics with refined microstructure and suitable stoichiometry, leading to improved functional properties. KNN-based materials are currently outstanding lead-free piezoceramics, with their properties highly sensitive to the proper construction of the polymorphic phase boundary, as in the case of the first promising composition (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3. Results of this work show that sintering parameters may determine the polymorphic behavior of this system, thereby evincing flash sintering allows polymorphic phase boundary to be fine-tuned. It is demonstrated that the convergence of microstructure refinement and compositional control holds the potential for enhancing properties through a proper electric current control during flash sintering.

Exceptional electrostrain with minimal hysteresis and superior temperature stability under low electric field in KNN-based lead-free piezoceramics

Exceptional electrostrain with minimal hysteresis and superior temperature stability under low electric field in KNN-based lead-free piezoceramics

Recent Developments in (K, Na)NbO3-Based Lead-Free Piezoceramics.

(K0.5Na0.5)NbO3 (KNN)-based ceramics have been extensively investigated as replacements for Pb(Zr, Ti)O3-based ceramics. KNN-based ceramics exhibit an orthorhombic structure at room temperature and a rhombohedral-orthorhombic (R-O) phase transition temperature (TR-O), orthorhombic-tetragonal (O-T) phase transition temperature (TO-T), and Curie temperature of -110, 190, and 420 °C, respectively. Forming KNN-based ceramics with a multistructure that can assist in domain rotation is one technique for enhancing their piezoelectric properties. This review investigates and introduces KNN-based ceramics with various multistructures. A reactive-templated grain growth method that aligns the grains of piezoceramics in a specific orientation is another approach for improving the piezoelectric properties of KNN-modified ceramics. The piezoelectric properties of the [001]-textured KNN-based ceramics are improved because their microstructures are similar to those of the [001]-oriented single crystals. The improvement in the piezoelectric properties after [001] texturing is largely influenced by the crystal structure of the textured ceramics. In this review, [001]-textured KNN-based ceramics with different crystal structures are investigated and systematically summarized.

Textured Potassium Sodium Niobate Lead-Free Ceramics with High d33 and Qm for Meeting High-Power Applications.

Potassium sodium niobate (KNN) lead-free piezoceramics have garnered significant attention for their environmentally friendly attributes, desired piezoelectric activity (d33), and high Curie temperature (Tc). However, the limited applicability of most KNN systems in high-power apparatus, including ultrasonic motors, transformers, and resonators, persists due to the inherent low mechanical quality factor (Qm). Herein, we proposed an innovative strategy for achieving high Qm accompanied by desirable d33 via synergistic chemical doping and texturing in KNN piezoceramics. Comprehensive electrical measurements along with quantitative structural characterization at multilength scales reveal that the excellent electromechanical properties (kp = 0.58, d33 ∼ 134 pC·N-1, Qm = 582, and Tc ∼ 415 °C) originate from the high <001> texturing degree, nanodomain, as well as acceptor hardening. Our findings provide an insight and guidance for achieving high-power performance in lead-free KNN-based piezoceramics, which were expected to be used in advanced transducer technology.