Enhanced transduction coefficient and thermal stability of 0.75BiFeO3-0.25BaTiO3 ceramics for high temperature piezoelectric energy harvesters applications

Owing to the coexistence of high depolarization temperature of 500 °C and good piezoelectric properties, 0.75BiFeO₃-0.25BaTiO₃-0.5mol%MnO₂ (BFBTMn) ceramics show potential energy harvesting applications at elevated temperature. Chemical modifications are usually difficult to improve transduction coefficient (d₃₃×g₃₃) due to the opposite directions of d₃₃ and g₃₃. Herein, the BFBTMn ceramic powders with different average grain sizes of 500 (S₁), 700 (S₂) and 800 (S₃) nm were mixed together for ceramics preparation. This method promotes the grain size, relative density and rhombohedral phase volume fraction of BFBTMn ceramics, leading to the much improved piezoelectric transduction coefficient and its thermal stability. The transduction coefficient of ceramics with optimized weight ratio (1S₁:2S₂:1S₃) reaches up to 4644 × 10^?15 m²/N, about 51% higher than those without mixed powder and at least 60% higher than those reported in the literatures, while its degradation from room temperature to 500 °C is about 15%, much smaller than that (30%) of ceramics without mixed powders. These investigations pave a new way to tailor the transduction coefficient and its thermal stability of BF-BT piezoelectric ceramics for high temperature energy harvesting devices.     

Influence of Grain and Grain‐Boundary Resistances on Dielectric Properties of KNbO3 Under Small DC Bias Field

Lead‐free piezoelectric potassium niobate (KNbO₃) system was synthesized by conventional solid‐state ceramic route. Rietveld analysis of X‐ray diffraction data of this system revealed that the sample crystallized in pure orthorhombic perovskite phase at room temperature. SEM micrograph of this system depicted presence of grains having diffuse brick structure with an average grain size of 500 nm. Dielectric properties of KNbO₃ ceramic were investigated under different DC bias voltage in a broad frequency (from 20 Hz to 1 MHz) and temperature (from 200°C to 500°C) ranges in its three crystalline phases. The dielectric constant was found to increase with increasing bias field in all three phases. The loss tangent of this system was found to increase first, and then it becomes constant with increasing bias field. These properties have been explained in terms of variation of grain and grain‐boundary resistances with bias field.     

Grain growth anomaly and piezoelectric properties of liquid phase sintered high TC 0.36BiScO3 - 0.64PbTiO3 ceramics with sillenite Bi12PbO19

The sintering behavior of 0.36BiScO₃-0.64PbTiO₃ (0.36BS-0.64 PT) ceramics was studied to investigate the effect of grain growth by the sillenite Bi₁₂PbO₁₉ (BP) phase on their piezoelectric properties for application in high-temperature piezoelectric devices. The BP phase formed during calcination at temperatures <750 °C led to a grain growth anomaly of the 0.36BS-0.64 PT ceramics sintered at 1000 °C. This phase assisted the grain growth of the 0.36BS-0.64 PT ceramics by liquid phase sintering. In particular, the 0.36BS-0.64 PT ceramic calcined at 700 °C exhibited excellent piezoelectric properties with a d₃₃ of 531 pC/N, g₃₃ of 41×10⁻³ Vm/N, k_p of 61.8%, and Q_m of 16. In addition, the 0.36BS-0.64 PT ceramics exhibited ferroelectric relaxor-like characteristics with an extremely large relaxation coefficient (γ) of 1.94 along with high maximum dielectric permittivity temperature (426 °C).     

Ultra-high-temperature piezoelectric responses and ultra-high thermal stability of piezoelectricity in ceramic PbZr0.54Ti0.46O3

To date, most piezoceramics with a high piezoelectric coefficient (d₃₃ > 500 pC/N) and a high Curie temperature (T_C around 400°C) are BiScO₃-PbTiO₃-based (BS-PT-based) systems, containing the rare-earth element Sc, whose high cost hinders mass production. We investigated the effect of Nd-doping on the morphotropic phase boundary and synthesized low-cost Nd-doped PbZr_0.54Ti_0.46O₃ (PZT) piezoceramics, achieving high piezoelectric performance. At room temperature, the piezoelectric coefficient d₃₃ reached 550 pC/N with a T_C = 375°C and this changed by only 3.6% over a broad temperature range (30–260°C). The d₃₃ value reached an ultra-high value of 941 pC/N at 345°C, which is higher than that of a BS-PT-based ceramic (810 pC/N at 350°C). The developed PZT ceramic material has a superior electrostrictive strain of 0.45% at 40 kV/cm, and a room temperature piezoelectric coefficient d₃₃* of 1312 pm/V at 20 kV/cm. Our research provides a new paradigm for designing piezoceramics that can be used over a wide temperature range.     

Compositional inhomogeneity and segregation in (K0.5Na0.5)NbO3 ceramics

In this report, the effects of the calcination temperature of (K_0.5Na_0.5)NbO₃ (KNN) powder on the sintering and piezoelectric properties of KNN ceramics have been investigated. KNN powders are synthesized via the solid-state approach. Scanning electron microscopy and X-ray diffraction characterizations indicate that the incomplete reaction at 700 °C and 750 °C calcination results in the compositional inhomogeneity of the K-rich and Na-rich phases while the orthorhombic single phase is obtained after calcination at 900 °C. During the sintering, the presence of the liquid K-rich phase due to the lower melting point has a significant impact on the densification, the abnormal grain growth and the deteriorated piezoelectric properties. From the standpoint of piezoelectric properties, the optimal calcination temperature obtained for KNN ceramics calcined at this temperature is determined to be 800 °C, with piezoelectric constant d₃₃=128.3 pC/N, planar electromechanical coupling coefficient k_p=32.2%, mechanical quality factor Q_m=88, and dielectric loss tan δ=2.1%.     

Low-temperature-sintered Bi0.5Na0.5TiO3-based lead-free ferroelectric ceramics with good piezoelectric properties

Li₂CO₃ has been used as a sintering aid for fabricating lead-free ferroelectric ceramic 0.93(Bi_0.5Na_0.5TiO₃)-0.07BaTiO₃. A small amount (0.5 wt%) of it can effectively lower the sintering temperature of the ceramic from 1200 °C to 980 °C. Unlike other low temperature-sintered ferroelectric ceramics, the ceramic retains its good dielectric and piezoelectric properties, giving a high dielectric constant (1570), low dielectric loss (4.8%) and large piezoelectric coefficient (180 pC/N). The “depolarization” temperature is also increased to 100 °C and the thermal stability of piezoelectricity is improved. Our results reveal that oxygen vacancies generated from the diffusion of the sintering aid into the lattices are crucial for realizing the low temperature sintering. Owing to the low sintering temperature and good dielectric and piezoelectric properties, the ceramics, especially of multilayered structure, should have great potential for practical applications.     

Improved ferroelectric,piezoelectric, and dielectric properties in pure KNN translucent ceramics by optimizing the normal sintering method

In this study, it is reported that various properties can be selectively derived in a pure (K_0.5Na_0.5)NbO₃, KNN ceramics through optimizing the sintering temperature by the conventional sintering method. High piezoelectric, ferroelectric, and dielectric properties such as d₃₃ = 127 pC/N, P_r = 31 μC/cm², and ε_r = 767 are obtained at the sintering temperature of 1100 °C. On the contrary, the specimen sintered at 1130 °C does not show high piezoelectric and ferroelectric properties, but it is translucent with a transmittance of 22% and 57% at the wavelength of 800 and 1600 nm respectively and shows a very high dielectric constant ε_r of 881. The origin of the high piezoelectric constant owes to large remanent polarization and dielectric constant, and dense microstructure with uniform distribution of large grains with the conjunction of relatively large crystal anisotropy. On the other hand, dense microstructure with almost no porosity, highly compacted grain boundaries, uniform distribution of grains, and relatively low crystalline anisotropy are responsible for the translucency and large dielectric constant of the ceramic specimens. This study demonstrates that the lead-free KNN ceramic has the potential to show multiple noteworthy properties such as piezoelectric, ferroelectric, dielectric, and transparent properties. This work provides a pure KNN ceramic simultaneously with high piezoelectric and transparent characteristics prepared only by using the conventional sintering method at a moderate sintering temperature for the first time in the literature.     

Ba5Li2W3O15: A New Li‐Containing Perovskite‐Type Microwave Ceramic with High Q and Low τf

A new Li‐containing microwave ceramic Ba₅Li₂W₃O₁₅ with hexagonal perovskite structure was prepared through a solid‐state ceramic route. Small amount of scheelite BaWO₄ appeared as a second phase during sintering. The Ba₅Li₂W₃O₁₅ could be well densified at 1120°C and exhibits good microwave dielectric properties with permittivity (ε_r) of 25.4, high Q × f value about 39 000 GHz, and low temperature coefficient of resonate frequency (τ_f) of 10 ppm/°C. The addition of BaCu(B₂O₅) can effectively lower the sintering temperature from 1120°C to 900°C and does not induce degradation of the microwave dielectric properties. These results indicate that the Ba₅Li₂W₃O₁₅ ceramic might be a promising candidate in microwave dielectric resonators.     

Piezoelectricity,thermal stability,and fatigue resistance in Nb and Ta-doped Bi4Ti3O12 high-temperature piezoceramics

The effect of Nb/Ta donor doping on the piezoelectricity, thermal stability, and fatigue resistance of bismuth titanate Bi₄Ti₃O₁₂ (BIT) ceramics was investigated in relation to their structural and oxygen vacancy-related electrical properties. As the Nb/Ta doping amount increased, the activation energy of oxygen vacancy conduction increased, indicating a reduction in the concentration of oxygen vacancies. The improved electrical insulating properties of the Nb/Ta-doped Bi₄Ti₃O₁₂ ceramics (BTNT) with fewer oxygen vacancies, contributed to their effective poling and strong piezoelectricity. Outstanding piezoelectric performance with high piezoelectric constant (39 pC/N) and Curie temperature (690 °C) could be achieved in the 0.005 mol Nb/Ta-doped BTNT ceramic with high density and anisotropic grain growth. The BTNT ceramics exhibited superior thermal aging stability and fatigue resistance compared to the BIT ceramic, suggesting that the reduction of oxygen vacancy defects plays a decisive role in enhancing elevated-temperature-induced and electric-field-induced degradation stabilities.     

Grain size effects and structure origin in high-performance BaTiO3-based piezoceramics with large grains

Grain size shows significant influence on electrical properties of piezoceramics. However, there are few works to investigate the grain size effects in high-performance and large-grain piezoceramics and uncover the structure origin. In this work, large grain size from ~53 to ~92 µm was achieved in a high-performance BaTiO₃ (BT)-based ceramic via tuning sintering conditions. With grain size increasing, the ceramics exhibit same multiphase coexistence state, similar phase transition temperature, upward T_C dielectric peaks and reduced diffuseness degree. Because of the larger and more complex non-180° domains within bigger grains, the improvement in remnant polarization (P_r), coercive field and negative strain were observed in bigger-grain ceramics. The elevated P_r finally leads to the piezoelectric coefficient d₃₃ increasing from 500 to 650 pC/N. However, too large grains may cause the reduced strain due to the high remnant strain in first cycle. Therefore, big grain size is conducive to achieve high piezoelectricity while moderate grain size can facilitate strain response in high-performance ceramics with large grains, which is quite different with pure BT ceramic. This work presents insights into the grain size effects and affords guides to further optimize electrical properties in high-performance piezoceramics with large grains.     

High‐temperature piezoelectric properties of 0‐3 type CaBi4Ti4O15:x wt%BiFeO3 composites

The 0‐3 type CaBi₄Ti₄O₁₅:30 wt%BiFeO₃ composite shows much better high‐temperature piezoelectric properties than the single‐phase CaBi₄Ti₄O₁₅ or BiFeO₃ ceramics. The composite with 0‐3 type connectivity exhibits a high density of 7.01 g/cm³, a saturated polarization of 21.5 μC/cm² and an enhanced piezoelectric d₃₃ of 25 pC/N. After the poled composite was annealed at 600°C, its d₃₃ is 21 pC/N at room temperature. Resistance of the composite decreases slowly from 10⁹ ohm at 20°C to ~10⁵ ohm at 500°C. Furthermore, the poled composite shows strong radial and thickness dielectric resonances at 20°C‐500°C.     

Structure,infrared spectra and microwave dielectric properties of the novel Eu2TiO5 ceramics

The microwave dielectric properties of Eu₂TiO₅ ceramic prepared by a conventional solid-state method were investigated for the first time. An orthorhombic structure with Pnam space group was obtained from x-ray diffraction. On the basis of P-V-L chemical bond theory and refined lattice parameters, the bond parameters of bond ionicity, lattice energy, bond energy, and coefficient of thermal expansion of Eu₂TiO₅ were computed. The relationship between chemical bond characteristics and microwave dielectric properties was discussed. Besides, far-infrared reflective spectra indicated the absorption of structural phonon oscillation might be the main contribution to polarization for Eu₂TiO₅ ceramic. Eu₂TiO₅ ceramic sintered at 1300°C for 6 hours possessed excellent microwave dielectric properties of ε_r ~ 14.4 ± 0.2, Q × f ~ 21 000 ± 500 GHz, and τ_f  ~ −10 ± 2 ppm/°C.     

Processing of 0.55(Ba0.9Ca0.1)TiO3-0.45Ba(Sn0.2Ti0.8)O3 lead-free ceramics with high piezoelectricity

We report a large piezoelectric constant (d₃₃), 720 pC/N and converse piezoelectric constant (d₃₃^*), 2215 pm/V for 0.55(Ba_0.9Ca_0.1)TiO₃-0.45Ba(Sn_0.2Ti_0.8)O₃ ceramics; the biggest value achieved for lead-free piezoceramics so far. The ceramic powders were calcined between 1050°C-1350°C and sintered at 1480°C. The best properties were obtained at a calcination temperature (CT) of 1350°C. The fitting combination of processing and microstructural parameters for example, initial powder particle size >2 µm, ceramics density ~95%, and grain size ~40 µm led to a formation of orthorhombic-tetragonal-pseudo-cubic (O-T-PC) mixed phase boundary near room temperature, supported by Raman spectra, pointed to the extremely high piezoelectric activity. These conditions significantly increase piezoelectric constants, together with high relative permittivity (ε_r) >5000 and a low loss tangent (tan δ) of 0.029. In addition, the d₃₃ value stabilizes in the range of 400-500 pC/N for all samples calcined between 1050°C and 1250°C. The results entail that the (Ba,Ca)(Sn,Ti)O₃ ceramics are strong contenders to be a substitute for lead-based materials for room temperature applications.     

Low sintering temperature for lead-free BiFeO3-BaTiO3 ceramics with high piezoelectric performance

Through modification of the heat-treatment process using a higher heating rate and a lower binder burnout temperature, the piezoelectric performance of water-quenched 0.67Bi_1.05FeO₃-0.33BaTiO₃ (BF33BT) lead-free piezoelectric ceramics was improved. The observed physical properties of BF33BT ceramics were very sensitive to the process temperatures. The sintering temperature (T_S) was changed within a narrow temperature range, and its effects were investigated. The largest rhombohedral distortion (90°-α_R = 0.14°) and tetragonality (c_T/a_T = 1.022) were observed for the ceramic sintered at 980°C, and its Curie temperature was 476°C. This ceramic showed good piezoelectric properties and large grains; the piezoelectric sensor charge coefficient (d₃₃) was 352 pC/N, and the piezoelectric actuator charge coefficient () was 270 pm/V. The high piezoelectric performance and low T_S of BF33BT ceramics indicate their potential as new low-cost eco-friendly lead-free piezoceramics.     

Large Piezoresponse and Ferroelectric Properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–Bi(Mg0.5Ti0.5)O3 Thin Films Prepared by Chemical Solution Deposition

Bulk ceramic 72.5 mol%(Bi_0.5Na_0.5)TiO₃–22.5 mol%(Bi_0.5K_0.5)TiO₃–5 mol%Bi(Mg_0.5Ti_0.5)O₃ (BNT–BKT–BMgT) has previously been reported to show a large high‐field piezoelectric coefficient (d₃₃* = 570 pm/V). In this work, the same composition was synthesized in thin film embodiments on platinized silicon substrates via chemical solution deposition. Overdoping of volatile cations in the precursor solutions was necessary to achieve phase‐pure perovskite. An annealing temperature of 700°C resulted in good ferroelectric properties (P_max = 52 μC/cm² and P_r = 12 μC/cm²). Quantitative compositional analysis of films annealed at 650°C and 700°C indicated that near ideal atomic ratios were achieved. Compositional fluctuations observed through the film thickness were in good agreement with the existence of voids formed between successive spin‐cast layers, as observed with electron microscopy. Bipolar and unipolar strain measurements were performed via double laser beam interferometry and a high effective piezoelectric coefficient (d_33,f) of approximately 75 pm/V was obtained.     

Improved Dielectric and Nonlinear Electrical Properties of Fine‐Grained CaCu3Ti4O12 Ceramics Prepared by a Glycine‐Nitrate Process

Pure CaCu₃Ti₄O₁₂ was successfully prepared by a glycine‐nitrate process using a relatively low calcination temperature and short reaction time of 760°C for 4 h. Fine‐grained CaCu₃Ti₄O₁₂ ceramics with dense microstructure and small grain size were obtained after sintering for 1 h. The nonlinear coefficient of a fine‐grained CaCu₃Ti₄O₁₂ ceramic calculated in the range 1–10 mA/cm² was found to be very high of ~16.39 with high breakdown electric field strength of 1.46 × 10⁴ V/cm. This fine‐grained CaCu₃Ti₄O₁₂ ceramic also exhibited a very low loss tangent of 0.017 at 20°C with temperature stability over the range ?55°C to 85°C. The grain growth rate of the CaCu₃Ti₄O₁₂ ceramics was found to be very fast after increasing the sintering time from 1.5 to 3 h, leading to formation of a coarse‐grained CaCu₃Ti₄O₁₂ ceramic with grain size of about 100–200 μm. The dielectric permittivity of this coarse‐grained ceramic was found to be as high as 1.03 × 10⁵ with a low loss tangent of 0.054.     

Temperature-stable Na0.5Bi0.5TiO3-based ceramics with favorable low-temperature dielectric and energy storage property

In this work, (1 − x)(0.94Na_0.5Bi_0.5TiO₃–0.06BaTiO₃)–xKTaO₃ (x = 0–0.30) ceramics are developed for dielectric capacitor applications. The introduction of KTaO₃ from x = 0 to 0.30 increases the tolerance factor t from 0.984 to 1.005 and causes the decrease of ferroelectric rhombohedral phase in the ceramics. Besides, a gradual structural change toward a higher symmetry can be detected, accompanied by the obvious domain refinement. In the aspect of electrical property, the strengthened dielectric relaxation leads to the greatly enhanced thermal stability of dielectric response. The decline in T_s from 98 to −96°C causes a significant widening of the low-temperature region with temperature-stable dielectric constant ε_r and low dielectric loss tan δ. The x = 0.30 ceramic shows a high ε_r (25°C) of 1094 with the temperature coefficient of capacitance ≤±15% over −70 to 200°C, which exceeds the X9R standard. Meanwhile, tan δ is less than 0.02 in a wide temperature range of −35 to 300°C. In addition, the ultrafine grain size of 290 nm, large bandgap of 3.22 eV, and high resistance of the x = 0.30 ceramic contribute to its electrical breakdown strength. A linear-like P–E loop with the large discharged energy density W_D ∼ 3.50 J/cm³ and high energy efficiency η ∼ 90.1% is obtained under 28 kV/mm at room temperature. The thermal stability of the energy storage performance is also satisfactory with the variation of W_D less than 15% over −40 to 200°C, and the η is higher than 85%.     

Microwave Dielectric Properties of Scheelite Structured PbMoO4 Ceramic with Ultralow Sintering Temperature

In this work, a low‐firing microwave dielectric ceramic PbMoO₄ with tetragonal structure was prepared via a solid‐state reaction method. The sintering temperature ranges from 570°C to 670°C. Ceramic samples with relative densities above 97% were obtained when sintering temperature was around 600°C. The best microwave dielectric properties were obtained in the ceramic sintered at 650°C for 2 h with a permittivity ~26.7, a Q × f value about 42 830 GHz (at 6.2 GHz) and a temperature coefficient value of 6.2 ppm/°C. From the X‐ray diffraction, backscattered electron imaging results of the cofired sample with 30 wt% silver and aluminum additive, the PbMoO₄ ceramic was found not to react with Ag and Al at 630°C. The microwave dielectric properties and low sintering temperature of PbMoO₄ ceramic make it a candidate for low‐temperature cofired ceramic applications.     

Low-temperature synthesis of bismuth titanate by modified citrate amorphous method

Bismuth titanate is a lead-free piezoelectric ceramic with outstanding properties that strictly depend on the composition and microstructure. However, bismuth-based materials are difficult to synthesize due to bismuth volatilisation that causes secondary phases and stoichiometry deviations. In this work, we propose a low-temperature chemical route, i.e. a modified amorphous citrate method, that allows a reduction of thermal treatment temperature, when compared with solid-state or other chemical routes, to obtain single-phase bismuth titanate samples. Single-phase powders with particle size under 300 nm are produced by calcination at 700 °C, and prepared into homogeneous dense pellets (density above 95%), with only isolated pores. The pellets show two distinctive features in the electrical behaviours directly associated with their mica-like microstructure: planar oriented boundaries are responsible for oxygen conduction, while the bulk is dominated by electronic conductivity. The samples show a high dielectric constant, around 200 at room temperature, while maintaining a low loss factor. The pellets also achieved a maximum polarisation of 5.85 μC/cm² and an inverse piezoelectric coefficient of 7.4 pm/V. The dielectric and piezoelectric properties obtained are comparable or superior to the state-of-the-art.