Preparation of c-axis-oriented Bi4Ti3O12 thick films by templated grain growth

Highly c-axis-oriented Bi₄Ti₃O₁₂ thick films were successfully fabricated by templated grain growth. The effects of template particles and sintering conditions on grain orientation in thick films were investigated. SEM micrographs and X-ray diffraction (XRD) patterns exhibited that thick films were c-axis-oriented. The degree of grain orientation (Lotgering factor, f) increases with increasing sintering temperature and soaking time. Highly c-axis-oriented thick film (orientation degree of ∼ 0.98) is obtained with the use of only 5 wt.% template particles by sintering at 1000 °C for 2 h. This film exhibits a better temperature-independent dielectric constant and a lower dielectric loss.     

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.     

Enhanced grain growth and dielectric properties in aerosol deposited BaTiO3

Piezoelectric ceramics are envisioned as cell stimulating materials for in-vivo, load-bearing applications. To compensate for their brittle nature developing ceramic films on medically accredited metals is a promising approach. However, high temperature consolidation is often required to achieve highly dense ceramics with suitable functional properties, which can compromise the metal substrate integrity. With aerosol deposition highly dense thick films can be produced at room temperature. Still, an annealing step is required to enhance the functional properties of piezoelectric ceramics. Thermal annealing of dense, aerosol deposited BaTiO₃ thick films on 304SUS stainless steel gave a clear enhancement of the dielectric properties. An increase in saturation polarization and the adoption of ferroelectric hysteresis at 750 °C coincided with a significant reduction in mechanical properties. The simultaneous appearance of grain growth and diffusion of chromium from the substrate at 750 °C suggests that chromium acts as a sintering aid.     

Composition gradient (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 film with improved dielectric,piezoelectric and ferroelectric temperature stability

Lead-free (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BZT-BCT) possesses comparable piezoelectric constant with lead zirconate titanate (PZT), but its poor temperature electric performances stability and low Curie temperature limit its application. Here we designed composition graded BZT-BCT films with improved temperature stability of piezoelectric, ferroelectric, and dielectric performances over a wide temperature range, and the d₃₃ reaches 21 pm/V with hysteresis loop even at 180 °C, which is far above the Curie temperature of BZT-BCT ceramic and BZT-0.5BCT film. The excellent temperature stability is ascribed to the lattice distortion and strain gradient in the grains caused by ions diffusion, and could suppress phase transition. This work could bring forward a feasible design for dielectric/piezoelectric/ferroelectric devices operating in harsh temperature environment.     

Tetragonal BF-xPT-0.1BZT ceramics with high Curie temperature and large piezoelectric constant

Obtaining both high Curie temperature and large piezoelectric constant simultaneously is of great significance for the application of actuators and sensors. In this research, the piezoelectric ceramics of (0.9-x)BiFeO₃-xPbTiO₃-0.1Ba(Zr_0.5Ti_0.5)O₃ (BF-xPT-0.1BZT, x = 0.29, 0.30, 0.31 and 0.32) were fabricated by the traditional solid-state reaction method. BF-xPT-0.1BZT ceramics exhibit the tetragonal perovskite structure without detectable second phases, and the tetragonality c/a ratio and the tolerance factor of ceramics are about 1.03 and 0.98, respectively. SEM images reveal that ceramics are well densified with the average grain size of 5–12 μm. The Curie temperature T_C of BF-xPT-0.1BZT ceramics is about 490–509 °C, slightly changing with the variation of PT content. The excellent comprehensive dielectric and piezoelectric properties are achieved for BF-0.31PT-0.1BZT ceramics with dielectric constant ε_r (1 kHz), tanδ, piezoelectric constant d₃₃ and T_C of 1001, 0.008, 230 pC/N and 509 °C, respectively. The significant enhancement of piezoelectric constant in the x = 0.31 sample is attributed to the enlargement of grain size. Moreover, the d₃₃ and the planar coupling coefficient k_p remain stable in the elevated temperature range of 200–400 °C, and the fluctuations are only 2% /℃ and 0.007% /℃, respectively, both of which are superior to that of BS-PT based piezoelectric ceramics. Our results indicate that BF-0.31PT-0.1BZT ceramics with high T_C, large d₃₃ and good thermal stability possess great potential for high temperature piezoelectric applications.     

Interdependence of piezoelectric coefficient and film thickness in LiTaO3 cantilevers

Electromechanical energy demands on homogenous thick films of piezoceramics with sufficiently large piezoelectric constant and reproducible performance. Single-phase LiTaO₃ films deposited by sol-gel processing have been fabricated as cantilevers to investigate the interdependence of dielectric and piezoelectric properties as a function of film thickness. Phase pure LiTaO₃ films with varying thickness in the range of 2.07-4.37 µm on stainless steel substrates were obtained after calcination of samples at 650°C. The relative permittivity of optimized spin-coated films peaked at 479.73 (1 kHz), whereas the piezoelectric coefficient (d₃₃ mode) determined by piezoresponse force microscopy was in the range of 21-24 pm/V. The effect of poling was studied through the butterfly and phase curves. A figure of merit (FOM) up to 3.29 (10⁻¹⁸ m²/V²) was determined for cantilever devices, which were able to generate a peak-to-peak voltage of 0.046-0.15 V using a 1 MΩ resistor as an impedance load at a fixed acceleration of 1.5 m/s². While the power density was in the range of ~4-20 × 10⁻⁹ W/m³, which increased with the increasing film thickness. The leakage current density decreased in the range of 4 × 10⁻⁵-6 × 10⁻⁷ A/m² in the same direction. As both ferroelectric and piezoelectric properties of LiTaO₃ films are dependent on film thickness, an optimal energy conversion efficiency was obtained for a thickness of ~3 µm. Furthermore, these devices were tested up to a temperature of 150°C for voltage generation. Given the need for lead-free piezoelectric materials for environmental applications, these LiTaO₃ cantilevers are very promising for vibrational energy harvester (VEH) applications especially due to their cost effectiveness, small size, stability at higher temperatures, and repeatable properties, which makes them suitable for MEMS devices for industrial applications.     

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).     

Microstructure-related piezoelectric properties of a ZnO film grown on a Si substrate

We investigated the effect of grain size on the piezoelectric properties of ZnO using films of different grain sizes and a fixed thickness of 800 nm deposited on a Si substrate by pulsed laser ablation in the temperature range of 300–700 °C. All of the deposited films have a crystal structure with a c-axis orientation. The grain size of the grown films, characterized by transmission electron microscopy (TEM), increases with the deposition temperature. In contrast, their piezoelectric efficiency (PE, d₃₃), characterized by piezoelectric force microscopy (PFM), was found to initially increase with the deposition temperature up to 500 °C, after which it decreased with further increases in temperature. The maximum PE value is observed for the film deposited at 500 °C with a grain size of approximately 60 nm. The peculiar PE behavior observed was theoretically explained by a competition between the contribution of the c-axis orientation favoring a larger d₃₃ value due to the enhanced static asymmetry and the strong grain size effect that influences the piezoelectric polarization as a result of domain motion.     

Effects of thickness on structures and electrical properties of K0.5Na0.5NbO3 thick films derived from polyvinylpyrrolidone-modified chemical solution

Lead-free ferroelectric K_0.5Na_0.5NbO₃ (KNN) films with different thicknesses were prepared by polyvinylpyrrolidone (PVP)-modified chemical solution deposition (CSD) method. The KNN films with thickness up to 4.9 μm were obtained by repeating deposition-heating process. All KNN thick films exhibit single perovskite phase and stronger (1 1 0) peak when annealed at 650 °C. The variation of dielectric constant with thickness indicates that there exists a critical thickness for the dielectric constant in the KNN films which should lie in 1.3–2.5 μm. The similar trend is observed for the ferroelectric and piezoelectric properties of KNN films. Both the remnant polarization P_r and the piezoelectric coefficient d₃₃ of KNN thick films increase with the film thickness and become saturated after the critical thickness.     

Effects of Sm-substitution on dielectric,ferroelectric, and piezoelectric properties of 0.36(Bi1-xSmx)ScO3-0.64PbTiO3 ceramics

Dielectric, ferroelectric, and piezoelectric properties of 0.36(Bi_1-xSm_x)ScO₃-0.64PbTiO₃ (BSPT-xSm) ceramics were investigated to assess effects of Sm-substitution on 0.36BiScO₃-0.64PbTiO₃ for high temperature piezoelectric device application. Optimal sintering was achieved at 1200°C when the BSPT-xSm ceramics were fully densified and crystallized with a perovskite structure without any secondary phase. The substitution of Bi³⁺ with Sm resulted in degradation of rhombohedral side in BSPT-xSm ceramics having morphotropic phase boundary. In addition, variations of grain size and ferroelectric behavior after Sm-substitution were insignificant. However, dielectric constant (ε^T₃₃/ε₀) was significantly enhanced with an increasing of amount of Sm to 5%. Although a slight decrease of relative density in case of x exceeding 3% led to deterioration of piezoelectric values of d₃₃, k_p, and d₃₃*, the BSPT-3%Sm ceramic exhibited excellent values of d₃₃ of 628 pC/N, k_p of 62.4%, and d₃₃* of 718 pm/V at 4.5 kV/mm, along with a high ferroelectric transition temperature of 421°C. The highly increased diffusion coefficient of 1.909 also implies that the Sm-substitution contributed to relaxor-like ferroelectric behavior of BSPT ceramics.     

Pyroelectric,ferroelectric, piezoelectric and dielectric properties of Na0.5Bi0.5TiO3 ceramic prepared by sol-gel method

Sodium bismuth titanate (BNT) nanopowder of molar composition 50/50 (Na_0.5Bi_0.5TiO₃) was prepared by a sol-gel processing method. The structure and microstructure of the precursor gel as well as the ferroelectric, pyroelectric, dielectric and piezoelectric properties of the BNT were studied. BNT crystallized in the rhombohedra perovskites structure Na_0.5Bi_0.5TiO₃ was obtained from the precursor gel by heating at 700 °C for 2 h in air. The BNT ceramic at 1100 °C sintering temperature present high crystallinity, good dielectric properties at 1 kHz (ε′=885, tan δ=0.03, T_c=370 °C), piezoelectric properties (k₃₃=0.39, c₃₃=105 GPa, e₃₃=12.6 C/m², d₃₃=120 pC/N), high remnant polarization (P_r=47 μC/cm²) and pyroelectric coefficient ($p$=707 μC/m² K) and low coercive field (E_c=55 kV/cm). Hence, the BNT prepared by sol-gel method could be used for silicon based memory device application where a low synthesis temperature is a key requirement.     

Fabrication and characterization of intergrown Bi4Ti3O12-based thin films using a metal-organic precursor solution

Ferroelectric intergrowth-structured Bi₄Ti₃O₁₂-based thin films have been fabricated by chemical solution deposition. Bi₄Ti₃O₁₂–SrBi₄Ti₄O₁₅ (BiT–SBTi) and SrBi₂Nb₂O₉–Bi₄Ti₃O₁₂ (SBN–BiT) precursor films crystallized in the desired intergrown BiT–SBTi and SBN–BiT structures on Pt/TiO_x/SiO₂/Si substrates by optimizing the processing conditions. Synthesized BiT–SBTi and SBN–BiT thin films exhibited ferroelectric P–E hysteresis loops. The BiT–SBTi thin films crystallized at 750 °C showed a 2P_r value approximately 20 μC/cm². Although a little smaller P_r value was observed for the SBN–BiT thin films, the squareness of a P–E hysteresis loop was superior to that of BiT–SBTi thin films. Also, the SBN–BiT thin films had a smoother surface morphology compared with BiT–SBTi thin films.     

Enhanced piezoelectric properties of Mn-modified Bi5Ti3FeO15 for high-temperature applications

Bi₅Ti₃FeO₁₅ (BTF) has recently attracted considerable interest as a typical multiferroic oxide, wherein ferroelectric and magnetic orders coexist. The ferroelectric order of BTF implies its piezoelectricity, because a ferroelectric must be a piezoelectric. However, no extensive studies have been carried out on the piezoelectric properties of BTF. Considering its high ferroelectric-paraelectric phase transition temperature (T_c ~ 761°C), it is necessary to analyze the piezoelectricity and thermal stabilities of BTF, a promising high-temperature piezoelectric material. In this study, lightly manganese-modified BTF polycrystalline oxides are fabricated by substituting manganese ions into Fe³⁺ sites via the conventional solid-state reaction method. X-ray diffraction and Raman spectroscopy analyses reveal that the resultant manganese-modified BTF has an Aurivillius-type structure with m = 4, and that the substitutions of Fe by Mn lead to a distortion of BO₆. The temperature-dependent dielectric properties and direct-current (DC) resistivity measurements indicate that the Mn ions can significantly reduce the dielectric loss tanδ and increase the DC resistivity. The piezoelectricity of BTF is confirmed by piezoelectric constant d₃₃ measurements; it exhibits a piezoelectric constant d₃₃ of 7 pC/N. Remarkably, BTF with 4 mol% of Mn (BTF-4Mn) exhibits a large d₃₃ of 23 pC/N, three times that of unmodified BTF, whereas the Curie temperature T_c is almost unchanged, ~765°C. The increased piezoelectric performance can be attributed to the crystal lattice distortion, decreased dielectric loss tanδ, and increased DC resistivity. Additionally, BTF-4Mn exhibits good thermal stabilities of the electromechanical coupling characteristics, which demonstrates that manganese-modified BTF oxides are promising materials for the use in high-temperature piezoelectric sensors.     

Electrical properties of high density PZT and PMN–PT/PZT thick films produced using ComFi technology

To achieve high actuation forces in piezoelectric film actuators and transducers it is desirable to have relatively thick films. Sol-gel derived films are often limited in the maximum thickness that is obtainable due to the increased probability of cracking and delamination during processing. Composite film (ComFi) technology combines conventional sol-gel processing with ceramic powder processing to enable thick (>2 μm) ferroelectric films to be deposited onto silicon substrates at temperatures as low as 710 °C. Ten micrometre thick films have been fabricated using three different piezoelectric powders [hard doped PZT, soft doped PZT and PMN–PT(85–15)]. The resultant films have high densities with relative permittivities of 800, 900 and 1800, respectively. The d₃₃ piezoelectric coefficients were found to be lower than corresponding values for the bulk material. This has been attributed to a combination of small grain size and the clamping effects of the rigid substrate. Hysteresis loop measurements show that greater fields are required to achieve a similar degree of polarisation to that of the bulk material. This indicates that the presence of the substrate also affects the ability to pole the material so further reducing the observed piezoelectric coefficient.     

Mixed grains and orientation-dependent piezoelectricity of polycrystalline Nd-substituted Bi4Ti3O12 thin films

Polycrystalline Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin films were prepared on Pt/Ta/glass substrates by a pulsed laser deposition method. X-ray diffraction measurements revealed that the BNT thin films were preferentially oriented along the (117) direction although they possessed a polycrystalline structure. Good ferroelectric properties of the BNT thin film were observed with a remnant polarization of 13 μC/cm² (2 P_r ~26 μC/cm²). The fatigue resistance test exhibited that the ferroelectric polarization of the BNT thin film degraded significantly after around 10⁹ switching cycles, which can be attributed to its crystal structure. We investigated the surface morphology and ferroelectric domain structure by atomic force microscopy (AFM) and piezoresponse force microscopy (PFM), respectively. Interestingly, mixed grains consisting of long and circular shapes were observed on the BNT film surface, which corresponded to a- and c-axes orientations of crystal growth, respectively. The PFM study revealed that the piezoelectric coefficient (d₃₃) of the long grains was much larger than that of the circular grains.     

Microstructure,dielectric, piezoelectric,and ferroelectric properties of fine-grained 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics

For preparing fine-grained 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ lead-free ferroelectric ceramics, the precursor powders were synthesized via sol-gel method and calcined at various temperatures. The precursor powders calcined at 520 °C, 550 °C, and 600 °C exhibit mean grain sizes of 30 ± 4 nm, 54 ± 3 nm, and 78 ± 5 nm, respectively. By optimizing the synthesis parameters, the fine-grained ceramics with high relative densities (>97%) and mean grain size around 100 nm were prepared. The ferroelectric, dielectric, and piezoelectric behavior were investigated. The ceramics prepared using the different precursor powders show different piezoelectric, ferroelectric, and dielectric behavior. The ceramic calcined at 550 °C and sintered at 900 °C exhibits the breakdown strength higher than 85 kV/cm, which exhibits the maximum polarization of 38.4 ± 0.3 μC/cm², remanent polarization of 20.6 ± 0.4 μC/cm².     

Highly (00l)-textured BiFeO3 thick films integrated on stainless steel foils with an optimized piezoelectric performance

Piezoelectric BiFeO₃ films were successfully prepared on 304 stainless steel (SS) foils via RF-magnetron sputtering at 450 °C. By adopting a LaNiO₃/Pt/Ti tri-layer bottom electrode, a predominant (100)-orientation and a better crystalline morphology were achieved in the BiFeO₃ film, resulting in better ferroelectric and dielectric properties targeted for piezoelectric applications than those directly grown on SS or LaNiO₃/SS. These properties include a large remnant polarization (P_r~68 μC/cm²), a large coercive field (E_c~241 kV/cm) and a sizable self-bias (E_bi~ 158 kV/cm), as well as a low dielectric constant and a reduced dielectric loss (ε_r≤200 and tan δ ≤ 0.026 in 1 kHz-1 MHz). Based on this optimal BiFeO₃-SS heterostructure, large converse and direct transverse piezoelectric coefficients |e_31,f| ~ 1.7 C/m² and 1.07 C/m² were achieved in prototype piezoelectric actuator and energy harvester structures, respectively. These results suggest that BiFeO₃-SS heterostructures have a great potential for applications in lead-free, metal-based piezoelectric micro-electro- mechanical systems (piezo-MEMS).     

Stress Modulation and Ferroelectric Properties of Nanograined PbTiO3 Thick Films on the Different Substrates Fabricated by Aerosol Deposition

Nanograined PbTiO₃ (PT) thick films were deposited on Si, yttria‐stabilized zirconia (YSZ), and Ni substrates using an aerosol deposition (AD) method at room temperature. The AD PT thick films on each different substrate were annealed at 500°C and 700°C for 1 h to increase the crystallinity. The stresses in the PT film were modulated by controlling the difference in the coefficient of thermal expansion (CTE) between the films and substrates during the thermal annealing process. The morphology of the AD PT films was examined from the polycrystalline dense structure (thickness ~8 μm), and the changes in the crystallographic phase, in‐plane stresses, and ferroelectric properties in annealed films were investigated. In‐plane stress analysis showed that the PT films annealed at 500°C and 700°C on each substrate exhibited compressive stress. Owing to the effects of compressive stress in the PT film, the film showed less tetragonality (c/a ratio) and enhanced ferroelectric behaviors. The change in the polarization–electric field (P–E) hysteresis loop of the PT films was explained by the stress induced from CTE mismatch between the films and substrates.     

Ferroelectric and piezoelectric properties of Ba0.85Ca0.15Ti0.90Zr0.10O3 films in 200 nm thickness range

Lead-free piezoelectric Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCZT) thin films were fabricated on Si/SiO₂/TiO₂/Pt (100) substrates following chemical solution deposition technique. Microstructure of the nano-sized BCZT particles crystallized in the thin film was thoroughly characterized. Ferroelectric, dielectric and piezoelectric properties of the films were investigated in detail. The BCZT films annealed at 800°C temperature exhibited high remanent polarization of 25 ± 1 μC/cm², energy density of 17 J/cm³, dielectric constant of 1550 ± 50 and dielectric tunability of 50%. Converse piezoelectric coefficients (d₃₃) obtained from piezo-response force microscopy (PFM) measurements on BCZT grains of different grain size (20-100 nm) distributed on the BCZT 700 film varied widely from 90 to 230 pm/V. The same for BCZT 800 measured on different grain size (30-130 nm) varied from 120 to 295 pm/V. These BCZT thin films with high dielectric, ferroelectric, and piezoelectric properties might be good alternative to the PZT films for thin film piezoelectric device applications.     

Dielectric and piezoelectric properties of low temperature synthesized iso-valent modified BT ceramics

Ba_(1?x)Y_xTiO₃ (where Y = Ca, Mg and Sr, x = 0.02, 0.04, 0.06 and 0.08) ferroelectric ceramic samples were synthesized in single perovskite phase by modified solid state reaction (MSSR) route. For single perovskite phase formation and dense grain morphology, 900 °C and 1300 °C were optimized as calcination and sintering temperatures. With the increase of substitutions% of Ca in BCT ceramic samples, the position of T_c increases slightly, whereas with the increase of Mg and Sr substitution% in BMT and BST systems, the position of T_c decreased but remains above RT. Decreased processing temperature with low temperature coefficient of capacitance made BCT ceramic samples useful for dielectric applications. Symmetric nature of the S–E loops indicated the increase of piezoelectric nature with the increase of Ca substitution% in BCT system.