Lead-free Piezoelectric Films Research Articles

Achieving High Piezoelectric Performance across a Wide Composition Range in Tetragonal (Bi,Na)TiO3-BaTiO3 Films for Micro-electromechanical Systems.

Tetragonal (1-x)(Bi,Na)TiO3-xBaTiO3 films exhibit enhanced piezoelectric properties due to domain switching over a wide composition range. These properties were observed over a significantly wider composition range than the morphotropic phase boundary (MPB), which typically has a limited composition range of 1-2%. The polarization axis was found to be along the in-plane direction for the tetragonal composition range x = 0.06-1.0, attributed to the tensile thermal strain from the substrate during cooling after the film formation. A "two-step increase" in remanent polarization against an applied maximum electric field was observed at the high-field region due to the domain switching, and a very high piezoelectric response (effective d33 value, denoted as d33,f) over 220 pm/V was achieved for a wide composition range of x = 0.2-0.5 with high tetragonality, exceeding previously reported values for bulk ceramics. Moreover, a transverse piezoelectric coefficient, e31,f, of 19 C/m2 measured using a cantilever structure was obtained for a composition range of at least 10 atom % (for both x = 0.2 and 0.3). This value is the highest reported for Pb-free piezoelectric thin films and is comparable to the best data for Pb-based thin films. Reversible domain switching eliminates the need for conventional MPB compositions, allowing an improvement in the piezoelectric properties over a wider composition range. This strategy could provide a guideline for the development of environmentally acceptable lead-free piezoelectric films with composition-insensitive piezoelectric performance to replace Pb-based materials with MPB composition, such as PZT.

Ferroelectric and piezoelectric properties of lead-free Li0.06(K0.5Na0.5)0.94NbO3 thin films

Lead-free ([Formula: see text][Formula: see text])NbO3 (KNN) and [Formula: see text]([Formula: see text][Formula: see text])[Formula: see text]NbO3 (LKNN) thin films were fabricated by a sol-gel method. The effects of Li substitution on crystal structure, microstructure and electrical properties of KNN film were systematically studied. Li doping can enhance the ferroelectric and piezoelectric properties of KNN film. Compared with pure KNN film, the LKNN film possesses larger remanent polarization ([Formula: see text] [Formula: see text] 9.3 [Formula: see text]C/[Formula: see text]) and saturated polarization ([Formula: see text] [Formula: see text] 41.2 [Formula: see text]C/[Formula: see text]) and lower leakage current density ([Formula: see text]A/[Formula: see text] at 200 kV/cm). Meanwhile, a typical butterfly shaped piezoelectric response curve is obtained in the LKNN film with a high piezoelectric coefficient ([Formula: see text] [Formula: see text] 105 pm/V). Excellent fatigue resistance ([Formula: see text][Formula: see text] switching cycles) and aging resistance ([Formula: see text] 180 days) demonstrate the long-term working stability of LKNN film. These findings indicate that KNN-based lead-free piezoelectric films have a broad application prospect in microelectromechanical systems (MEMS).

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

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

Stable self-polarization in lead-free Bi(Fe0.93Mn0.05Ti0.02)O3 thick films

The BiFeO3-based film is one of the most promising candidates for lead-free piezoelectric film devices. In this work, the 1 [Formula: see text]m-thick Bi([Formula: see text][Formula: see text][Formula: see text])O3 (BFMT) films are grown on the ITO/glass substrate using a sol-gel method combined with spin-coating and layer-by-layer annealing technique. These films display a large saturated polarization of 95 [Formula: see text]C/cm2, and a remanent polarization of 70 [Formula: see text]C/cm2. Especially, the films are self-poled caused by an internal bias field, giving rise to asymmetric polarization-electric field ([Formula: see text]) loops with a positive shift along the [Formula: see text]-axis. A stable self-polarization state is maintained during the applied electric field increasing to 1500 kV/cm and then decreasing back. The weak dependence of [Formula: see text] loops on frequency (1–50 kHz) and temperature (25–125[Formula: see text]C) indicate that the internal bias field can be stable within a certain frequency and temperature range. These results demonstrate that the self-polarized BFMT thick films can be integrated into devices without any poling process, with promising applications in micro-electro-mechanical systems.

KNN lead-free piezoelectric films grown by sputtering

Potassium sodium niobate [(K,Na)NbO3, KNN] films are promising lead-free piezoelectric materials for microelectromechanical systems (MEMS) devices. We previously developed technologies for forming high-quality KNN films by sputtering, which showed excellent piezoelectric properties comparable to lead zirconate titanate (PZT) films. In the present study, we addressed several challenges with the aim of introducing KNN films into commercialized MEMS devices. First, we optimized the dielectric and piezoelectric properties to realize a suitable performance for actuator and sensor devices. The sensor-type KNN films had a low dielectric constant (248) and a self-poling function, which are greatly beneficial for sensor device performance and the fabrication process. The actuator-type KNN films had a very high e31 value of −13.5 C/m2, which is almost comparable to the top level of PZT films. Next, we found that the DC stress lifetime of KNN films strongly depended on the material used for the adhesion layer of the top electrode. When we used a Pt/RuO2 top electrode, the actuator-type KNN films had a long lifetime (>130 000 s, 300 kV/cm and 200 °C), which is long enough to be used for commercialized MEMS devices. Furthermore, we realized 8-in. KNN wafers with good thickness uniformity across the wafer (±3.5%) by introducing a mass-production-ready sputtering tool: the EB-2500 produced by Canon Anelva. In the near future, these promising results will open the way to replace PZT with KNN in the piezoMEMS industry.

Manipulation of defects to achieve fast domain switching and enhance the piezoelectric properties of thin films

High-performance lead-free piezoelectric films play an important role in micro-electromechanical systems (MEMS), and the inevitable defects can affect the performance of the films. In particular, the coupling between defects and domain wall motion can be regarded as a general strategy for regulating the piezoelectric properties of thin films. Here, based on the existing vacancy defects in 0.8Bi0.5Na0.5TiO3-0.2(Sr0.7Bi0.2□0.1)TiO3 system, we control the balance of A-site defects through the introduction of rare earth La ions, release the rapid switching of domain structures and enhance the piezoelectric response of the thin film. The maximum inverse piezoelectric coefficient can reach 425.9 pm V−1. Under the in-situ electric field, a fast response of the domain structure can be observed, which is closely related to the reduction of defect concentration and releasing the pinning effect of domain walls. Also, phase coexistence was observed by TEM, which is attributed to the optimization of MPB by defect regulation. It is hoped that this process of balancing defects can provide a certain reference for the improvement of piezoelectric properties of thin films.

Orientation dependent intrinsic and extrinsic contributions to the piezoelectric response in lead-free (Na0.5K0.5)NbO3 based films

The intrinsic and extrinsic contributions to the piezoelectric response are deeply dependent on the crystallographic orientation in the thin films. Herein, (100), (110) and (111)-oriented (Na0.5K0.5)NbO3 (KNN) based lead-free piezoelectric films are synthesized by a sol-gel method. The dielectric, ferroelectric and piezoelectric properties of the KNN-based films at the rhombohedral-orthorhombic (R-O) phase boundary are investigated with the focus on the influence of the film orientation. The maximum piezoelectric response is obtained in the (100)-oriented film, resulting from the extrinsic contribution stimulated by the external electric field, although the (110) and (111)-oriented films have the higher dielectric constant and remnant polarization. There are more varied ways of domain switching and domain configuration under the external electric field in (100)-oriented film, leading to the higher extrinsic contribution to the piezoelectric response. The study of the orientation effect on the piezoelectric response can shed light on the exploration of optimal piezoelectricity by tailoring the preferred orientation in KNN-based lead-free films.

Morphotropic Relaxor Boundary Construction Highly Boosts the Piezoelectric Properties of Bi-Based Lead-Free Thin Films.

To achieve large electrostrain and low hysteresis, we further optimized a morphotropic phase boundary (MPB) by modulating its local polar symmetries. The construction of a morphotropic relaxor boundary (MRB) in thin films can be achieved by suitable introduction of Bi(Fe0.95Mn0.03Ti0.02)O3 into (Bi0.5Na0.5)TiO3-SrTiO3 to form a solid solution. The designed thin film achieves surprising piezoelectric properties with an inverse piezoelectric coefficient of 179.7 pm V-1 and negligible hysteresis. The composition of two relaxors with different local polar symmetries (tetragonal nanoregions and rhombohedral nanoregions), namely, an MRB, and the coexistence of multiscale domain structures can greatly weaken the anisotropy of polarization, degrade the energy barrier, attenuate the discontinuity of polarization, and achieve a large electrostrain and low hysteresis. The domain dynamics of the PNRs under the action of an external excitation field are analyzed to clarify the enhancement mechanism. This construction of MRBs is feasible for producing lead-free piezoelectric films with high-voltage electrical properties and low hysteresis, and various experimental design and theoretical references are provided.

New lead-free piezoelectric thin film fabricated using metal-oxide nanosheets at low temperature

A new lead-free piezoelectric film consisting of Sr2NaNb4O13− (SNNO−) and TiNbO5− (TNO−) nanosheets was fabricated via electrophoresis. SNNO and TNO films display paraelectric polarization versus electric field (P-E) loops. However, a new film composed of a mix of SNNO− and TNO− (S/T) nanosheets displayed a ferroelectric P-E hysteresis loop with large maximum polarization (18.7 μC/cm2), remnant polarization (7.7 μC/cm2), and a coercive electric field (86 kV/cm). The interfaces formed between the SNNO and TNO layers induced ferroelectric properties in the S/T film through the occurrence of polar distortion and octahedral tilting in the film. Ferroelectric properties were also observed in piezoelectric force microscopy images of the S/T film, which showed 90° domains after the removal of the applied electric field. The dielectric constant of the S/T film was 70, which is higher than those of SNNO and TNO films, indicating that the S/T film is a ferroelectric material. The piezoelectric strain constant of the S/T film was 156 p.m./V and promising insulating properties were observed therein. The growth temperature of the S/T film was low (300 °C), suggesting that the S/T film can be used for flexible electronic devices.

Investigation of Material Constants of CaTiO₃ Doped (K,Na)NbO₃ Film by MEMS-Based Test Elements.

A CaTiO3-doped (K,Na)NbO3 (KNN-CT) film is a lead-free piezoelectric film that is expected to substitute Pb(Zr,Ti)O3 (PZT) film in piezoelectric micro electro mechanical systems (MEMS). However, the full set of the material constants (elastic constants, piezoelectric constants and dielectric constants) of the KNN-CT film have not been reported yet. In this study, all the material constants of a sputter-deposited blanket KNN-CT film were investigated by the resonance responses of MEMS-based piezoelectric resonators and the phase velocities of leaky Lamb waves on a self-suspended membrane. The phase velocities measured by a line-focus-beam ultrasonic material characterization (LFB-UMC) system at different frequencies were fitted with theoretical ones, which were calculated from the material constants, including fitting parameters. A genetic algorithm was used to find the best-fitting parameters. All the material constants were then calculated. Although some problems arising from the film quality and the nature of deliquescence are observed, all the material constants were obtained exhibiting accuracy within 16 m/s in the phase velocity of leaky Lamb wave.

Preparation of 0.055BaZrO3–0.935(K0.45Na0.5Li0.05)NbO3–0.01(Bi0.5Na0.5)TiO3 piezoelectric thin films from precursor solution by Pechini method

Lead-free piezoelectric thin films of 0.055BaZrO3–0.935(K[Formula: see text]Na[Formula: see text]Li[Formula: see text])NbO3–0.01(Bi[Formula: see text]Na[Formula: see text])TiO3 (BZ–KNLN–BNT) were fabricated on (100)-LaNiO3/SiO2/Si substrates. The Pechini (polymeric precursor) method was carried out to prepare the complicated multi-component precursor solution. This method was suited for controlling the complex components accurately. The films crystallized at 700[Formula: see text]C showed a (100)pc-oriented perovskite structure, whose grain size was about 200[Formula: see text]nm and thickness was about 700[Formula: see text]nm. The Curie temperature [Formula: see text]c of the films was 292[Formula: see text]C that was near to that of the bulk ceramics. The preparation method used in this work provided a possibility for the application of multi-component lead-free piezoelectric films.

Large transverse piezoelectric properties of lead-free Bi0.5(Na0.82K0.18)0.5TiO3 films

The ferroelectric and piezoelectric properties of Bi0.5(Na1-xKx)0.5TiO3 (x = 0, 0.18 and 0.25) films deposited on Pt(111)/TiO2/SiO2/Si(100) substrates using a chemical solution deposition technique were examined as part of an ongoing study into the development of lead-free piezoelectric films. The effective transverse piezoelectric coefficient (e∗31,f) was examined to assess the potential applications of these piezoelectric films for device applications using the Bi0.5(Na0.82K0.18)0.5TiO3 (BNKT18) cantilever, which is the morphotropic phase boundary composition in the Bi0.5(Na1-xKx)0.5TiO3 system. The BNKT18 cantilever showed good linearity of piezoelectric displacement with low hysteresis under an applied field, and exhibited a high effective transverse piezoelectric coefficient (e∗31,f) of ∼5.15 C/m2 and a figure of merit ((e∗31,f)2/ε0εr) of 3.8 GPa, which are comparable to the preferred oriented lead-free piezoelectric and lead zirconate titanate thin films. This suggests that the BNKT18 film is a potential candidate for lead-free piezoelectric film devices.

Mask-Less Direct-Writing Deposition of Lead-Free Piezoelectric Films for Microsystems

Piezoelectric films fabricated with lead-free piezoelectric inks by means of a mask-less Direct-Writing (DW) technique are presented. A lead-free piezoelectric material, K0.5Na0.5NbO3-δ (KNN), has been produced via Solid State Reaction (SSR) and Molten Citrate (MC) route in order to improve the microstructural properties of powders. The slurry composition and rheology have been optimized in order to obtain inks with physical properties compatible with the DW technique and layers with enhanced mechanical and piezoelectric properties. Experimental results obtained with pellets and films deposited on alumina substrates demonstrate the piezoelectric properties of the fabricated devices. The developed direct-writing technique will be implemented for the realization of precise patterns of piezoelectric sensors or actuators inside microsystems, without the requirement of manufacture expensive masks.

Sol–gel synthesis of 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 lead-free piezoelectric films: effect of pyrolysis temperature on phase evolution and electrical properties

Sol–gel processing was applied to fabrication of 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 (BNT–BT6) lead-free ferroelectric thin films on Pt(111)/Ti/SiO2/Si(100) substrates, whose phase structure and electrical properties were found to be greatly affected by the pyrolysis temperature. It was found that a large amount of pyrochlore phase formed in the films if pyrolysis was conducted around 350 °C; pyrolyzing at lower and higher temperatures in the range from 300 to 450 °C could avoid its formation even though other conditions are the same. The reasons are revealed by the DSC and TG experiments conducted in inert gas and air, and a phenomenological model was proposed with a focus on the thermal evolution of carbonaceous in pyrolysis and crystallization process. High-quality BNT–BT6 films with a relatively low surface roughness and a dense cross section were obtained by conducting the pyrolysis at the optimized temperature (400 °C), which also show the best ferroelectric and piezoelectric properties. The effects of pyrolysis temperatures on BNT–BT6 films deposited by the sol–gel technique were investigated. A phenomenological model was built to clarify why phase structures and electrical properties of BNT–BT6 lead-free thin films are greatly affected by pyrolysis temperature.

Preparation and Ferroelectric Properties of K0.5Na0.5NbO3 Thin Films Derived from Non-alcohol Niobium Salt Sol-gel Process

Lead-free K0.5Na0.5NbO3 (KNN) thin films were prepared on Pt (111) /Ti/SiO2/Si (100) substrates by a polyvinylpyrrolidone (PVP)-modified sol-gel method without any metal alkoxides. The effects of PVP on the crystallization, surface morphology and electrical properties were investigated. It was found that the introduction of PVP into the sol could reduce the annealing temperature, enhance the surface quality and improve the KNN film of (100) oriented growth. Compared with the pure non-PVP-modified KNN thin films, the dielectric and ferroelectric properties of the KNN films were significantly enhanced by adding PVP into the sol. In particular, the PVP-modified KNN films which were annealed at 550°C exhibited relatively saturated polarization-electric field (P-E) hysteresis loops with high remnant polarization Pr of 22 μC/cm2, dielectric constant of 280 and dielectric loss of 0.09, respectively, indicating promising lead-free piezoelectric film candidate for future applications.

Electrical properties of lead-free KNN films on SRO/STO by RF magnetron sputtering

Abstract Highly (001) oriented (K,Na)NbO 3 lead-free piezoelectric films were fabricated on SrRuO 3 buffered SrTiO 3 (001) single crystal substrates by RF magnetron sputtering. The films showed a stable frequency dependence of dielectric permittivity and loss tangent in the range of 1 kHz–1 MHz. A fairly high tunability of 69.7% (@400 kV/cm) was obtained in the films. Well saturated P – E hysteresis loops were obtained at E =250 kV/cm, the remnant polarization P r was 8 μC/cm 2 , and coercive field E c was only 40 kV/cm. These films showed a fatigue-free behavior up to 10 10 switching cycles. The decreased coercive field, improved dielectric and fatigue properties may be related to SRO bottom electrodes, which can efficiently suppress the “dead layer” near the ferroelectric-electrode interface. The leakage current density of the films was only 3.48×10 −6 A/cm 2 at the electric field of 250 kV/cm, which is even lower than that of Mn-doped KNN films. In addition, the piezoelectric constant d 33 was estimated to be 36 pm/V.

Research on Lead-free Piezoelectric Films for MEMS Applications

Research on Lead-free Piezoelectric Films for MEMS Applications

Friction and wear performance of the 0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 piezoelectric film

As a lead-free piezoelectric material with potential application, 0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-0.5BCT) ceramics, which has a morphotropic phase boundary composition, deserves much attention due to its excellent ferroelectric and piezoelectric properties. BZT-0.5BCT lead-free piezoelectric film has been synthesized on a Si (100) substrate by Sol-Gel process. The topography of the film measured using an atomic force microscope and a scanning electron microscope shows that the surface of the prepared film is smooth, and the grain is in the shape of hemisphere with a diameter of 80-100 nm. The film is 1.7 μm in thickness, with pores inside. Friction experiments show that the friction between the tip and the piezoelectric film is much larger than that between the tip and the SiO2 substrate, because of the existence of electrostatic force between the film and the silicon tip. However, the friction coefficients obtained are approximately equal. Nano-scratch experiments show that the BZT-0.5BCT film has a high normal carrying capacity, but a poor tangential wear resistance. The average elastic modulus of the film is 23.64 GPa ± 5 GPa, and its hardness is 2.7-4 GPa, both being slightly lower than those of the bulk value in PZT ceramics.

Fabrication of (K,Na)NbO3 lead-free piezoelectric films deposited by sputtering

Fabrication of (K,Na)NbO₃ lead-free piezoelectric films deposited by sputtering

Improvement of Piezoelectric Properties of (K,Na)NbO3 Films Deposited by Sputtering

(K,Na)NbO3 (KNN) films with high transverse piezoelectric coefficients were successfully deposited onto Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structures with preferential <001 > orientation. To improve their piezoelectric properties, we investigated the effects of annealing after the deposition and the Na/(K+ Na) ratio of the films. Annealing in air at 750 °C led to a decrease in the residual strain in the KNN crystal and the disappearance of openings at the grain boundary, thereby improving the transverse piezoelectric coefficient and leakage current properties. We also investigated the transverse piezoelectric coefficient and dielectric constant as a function of the Na/(K+ Na) ratio; both had maximum values at a ratio of approximately 0.55. For the KNN films, e31* ranged between -10.0 and -14.4 C/m2; thus, it was superior to previously reported values for lead-free piezoelectric films and was comparable to the best commercially available Pb(Zr,Ti)O3 films.