Lead-free Piezoelectric Thick Films Research Articles

Synthesis of a Stable and High-Concentration BaHfxTi1−xO3 Sol–Gel for High Electromechanical Performance of Bulk Ceramics

Lead-based materials are widely used in piezoceramics due to their high electromechanical properties. However, due to environmental protection and sustainable development, the use of the toxic element lead (Pb) in electronic devices is strictly restricted, therefore requiring the rapid development of piezoelectric-based devices with lead-free ceramics. In this context, a lead-free doped barium titanate was studied with a dual objective. First, a new sol–gel method to synthesize Hf4+-doped BaHfxTi1−xO (BHT) with x = 0.05, 0.075, and 0.10 is presented. Such BHT sols were prepared at high concentrations of up to 1 M. Dilution in ethylene glycol allowed parameters (viscosity, colloid sizes, etc.) to be controlled, which ensured a time-stable sol for several months at room temperature. Second, densified bulk ceramics with attrited powders were obtained from these sols and showed very good electromechanical properties, with a thickness coupling factor of kt = 47% (BaHf0.05Ti0.95O3 sintered at 1500 °C/6 h). These results are a first step that will allow the processing of lead-free piezoelectric thick films using a sol–gel composite method for vibrational energy harvesting applications.

Unleashing the Full Sustainable Potential of Thick Films of Lead-Free Potassium Sodium Niobate (K0.5Na0.5NbO3) by Aqueous Electrophoretic Deposition.

A current challenge for the fabrication of functional oxide-based devices is related with the need of environmental and sustainable materials and processes. By considering both lead-free ferroelectrics of potassium sodium niobate (K0.5Na0.5NbO3, KNN) and aqueous-based electrophoretic deposition here we demonstrate that an eco-friendly aqueous solution-based process can be used to produce KNN thick coatings with improved electromechanical performance. KNN thick films on platinum substrates with thickness varying between 10 and 15 μm have a dielectric permittivity of 495, dielectric losses of 0.08 at 1 MHz, and a piezoelectric coefficient d33 of ∼70 pC/N. At TC these films display a relative permittivity of 2166 and loss tangent of 0.11 at 1 MHz. A comparison of the physical properties between these films and their bulk ceramics counterparts demonstrates the impact of the aqueous-based electrophoretic deposition (EPD) technique for the preparation of lead-free ferroelectric thick films. This opens the door to the possible development of high-performance, lead-free piezoelectric thick films by a sustainable low-cost process, expanding the applicability of lead-free piezoelectrics.

Non-180° domain contributions in Bi0.5(Na0.82K0.18)0.5TiO3 lead-free piezoelectric thick films

Piezoelectric responses of Bi0.5(Na0.82K0.18)0.5TiO3 (NKBT) ceramic films with thickness of 10–100μm have been studied in a wide range of ac electric field and frequency. By the Rayleigh law fit, the typical irreversible Rayleigh coefficient, α, for piezoelectric response is 39.69±0.41×10−18m2V−2, which is much larger than that of the piezoelectric thin films, indicating significantly enhanced non-180° domain wall motions in NKBT thick films. The logarithmic frequency dependence of the piezoelectric response is associated with domain wall pinning in NKBT thick films, and the frequency dispersion increases with increasing ac field amplitude, which is attributed to the relaxation of non-180° domains.

Control of Paste Rheology and Piezoelectric Properties of Bi0.5(Na0.82K0.18)0.5TiO3 Lead-Free Piezoelectric Thick Films Deposited by Screen Printing

Bi0.5(Na0.82K0.18)0.5TiO3 (NKBT) lead-free thick films with a thickness of 10–100 μm have been deposited on a Pt-electroded alumina substrate using screen printing. The influences of the NKBT particle size, NKBT powder content, binder concentration, and dispersant on the rheological characteristics were systematically investigated. The resulting 60 μm thick films exhibit a dielectric constant of 582 (10 kHz), a dielectric loss of 2.8%, remanent polarization of 22.5 μC/cm2, a coercive field of 62 kV/cm, and a longitudinal effective piezoelectric coefficient, d33eff of 91 pm/V. The thick films are expected to be a new and promising candidate for lead-free piezoelectric microdevices especially the microactuator applications.

Nonlinear dielectric properties of (Bi0.5Na0.5)TiO3-based lead-free piezoelectric thick films

The effects of applied dc bias field, temperature, frequency, and doping types on the nonlinear dielectric behaviors of Bi0.5(Na0.82K0.18)0.5TiO3 piezoelectric films with thickness of 40 μm have been investigated. It is demonstrated that the dielectric permittivity versus ac field amplitude departed from linearity at high temperature, which is caused by a decrease of the coercive field with increasing temperature. The frequency dependence of the dielectric permittivity is related to domain wall pinning. The thicker piezoelectric films exhibit higher extrinsic contribution due to high domain wall mobility and less domain wall pinning, resulting from the increased grain size and film thickness.

Aerosol-deposited KNN–LSO lead-free piezoelectric thick film for high frequency transducer applications

In this report, we fabricated and characterized a high frequency transducer with lead-free piezoelectric thick film, 0.948(K 0.5Na 0.5)NbO 3–0.052LiSbO 3, formed on a platinized silicon substrate by aerosol-deposition. X-ray diffraction analysis and scanning electron microscopy revealed that the film had well-crystallized perovskite structure and was dense and crack-free microstructure. The film showed the dielectric constant of 766 at 1 kHz and the remnant polarization of 13.2 μC/cm 2. The film was used to fabricate a high frequency needle transducer with an aperture size of ∼200 μm. The center frequency of the transducer was 197 MHz and the −6 dB bandwidth was 50% which was comparable to the PZT-based transducers. A 6-μm tungsten wire phantom was imaged to assess the −6 dB axial and lateral resolution of the transducer, which were found to be 12 and 66 μm, respectively.

Low temperature preparation and electrical properties of sodium–potassium bismuth titanate lead-free piezoelectric thick films by screen printing

Sodium–potassium bismuth titanate (NKBT) thick films with thickness of 40 μm were prepared by screen printing. To improve the homogeneity, the sintering aids were added into the pastes as a chemical liquid-phase doping method. The results show that the addition of Bi–Li sintering aids was beneficial for both the reduction of the sintering temperature and the improvement of the electrical performance of the thick films. The thick films containing 5 wt.% Bi–Li sintering aids demonstrated optimal dielectric properties with the maximum dielectric constant of 725 and minimum dielectric loss of 2.5%. Moreover, the NKBT thick films containing 3 wt.% Bi–Li sintering aids sintered at 950 °C exhibited the remanent polarization of 19.6 μC/cm 2, room-temperature pyroelectric coefficient of 1.56 × 10 −4 C/(m 2 °C), figure of merit for specific detectivity of 0.48 × 10 −5 Pa −0.5, and effective longitudinal piezoelectric coefficient of 88 pm/V, which are comparable to that of the high-temperature sintered thick films without sintering aids.

Piezoelectric property in morphotropic phase boundary Bi0.5(Na0.82K0.18)0.5TiO3 lead free thick film deposited by screen printing

Piezoelectric Bi0.5(Na0.82K0.18)0.5TiO3 thick films were screen printed on Pt electroded alumina substrates. The resulting 38μm thick films sintered at 1100°C for 0.5h have maximum relative permittivity of 810, dielectric loss of 2.0% (at 10kHz), remanent polarization of 28.3μC∕cm2, coercive field of 62kV∕cm, and longitudinal effective piezoelectric coefficient d33eff of 109pm∕V. The temperature dependence of the dielectric constant shows two clear peaks that were associated with phase transitions. The performance of this lead free piezoelectric thick film could be comparable to the corresponding bulk ceramics.

Ferroelectric and piezoelectric properties of 0.948(K0.5Na0.5)NbO3–0.052LiSbO3 lead-free piezoelectric thick film by aerosol deposition

Lead-free, piezoelectric thick films of 0.948(K0.5Na0.5)NbO3–0.052LiSbO3 were fabricated by aerosol deposition and the effect of postannealing temperature on the dielectric, ferroelectric, and piezoelectric properties was investigated. The thickness of the films ranged from 5to17μm and highly dense films were obtained after deposition at room temperature. With increasing annealing temperature, the dielectric and ferroelectric properties were markedly increased. The maximum ferroelectric and dielectric properties of ε3T∕ε0=1012, Pr=15.5μC∕cm2, and d33=50pm∕V were obtained after annealing at 800°C for 1h. Above this temperature, the film exhibited rapid property degradation as it was decomposed due to Na2O evaporation.

Fabrication and ferroelectric properties of highly dense lead-free piezoelectric (K0.5Na0.5)NbO3 thick films by aerosol deposition

Lead-free piezoelectric thick films of (K0.5Na0.5)NbO3 were fabricated by aerosol-deposition method. The thickness of KNN film was 7.1μm and fully dense films were obtained. The dielectric constants ε3T∕ε0 of the as-deposited and annealed films at 1kHz were 116 and 545, respectively, which are higher than any previously reported values for lead-free piezoelectric thin/thick films, either without or with heat treatment. The ferroelectric properties were improved after annealing and the maximum values of Pr=8.1μC∕cm3 and Ec=100kV∕cm were achieved. These values are markedly superior to those of sintered KNN ceramic counterparts.