Lead-free piezoelectric ceramics manufactured from tantalum-substituted potassium sodium niobate nanopowders

Nanopowders of lead-free (K_0.5Na_0.5)(Nb_0.9Ta_0.1)O₃ (KNNT) system were prepared by high-energy ball milling at different milling times keeping the milling speed fixed at 250 rpm. The particle size first decreases from 35 nm to 3 nm and then increases to 98 nm as the milling time increases in steps of 5 h from 10 h to 30 h. Without using any sintering aid, dense ceramics were formed by sintering the powders at 1050 °C for 1 h. With decreasing particle size of the starting nanopowders, the ceramics exhibit gradual increase in density from 93.1% to 95.8%, coercive field (E_c) from 10.9 kV/cm to 15.1 kV/cm, electromechanical coupling factor (k_p) from 35% to 48%, and piezoelectric charge constant (d₃₃) from 80 pC/N to 128 pC/N. The systematic changes observed in these parameters corroborate the observed increase in particle size as the milling time increases from 25 h to 30 h.     

Lead-free ceramics based on alkaline niobate tantalate antimonate with excellent dielectric and piezoelectric properties

This paper reports lead-free (Na_0.52K_0.48−x)(Nb_0.94−xSb_0.06)O₃-xLiTaO₃ compositions with significantly enhanced piezoelectric properties. The 6% Sb substituted Na_0.52K_0.48NbO₃ was modified by a small amount of LiTaO₃, leading to the formation of a morphotropic phase boundary between orthorhombic and tetragonal phases in the range of x = 0.035-0.04 where the materials show a strong compositional dependence of various electrical properties. Excellent properties of d₃₃ = 335 pC/N, k_p = 53%, , Q_m = 41 and T_c = 291 °C were obtained in the composition with x = 0.04, indicating that the ceramics studied are promising as a lead-free piezoelectric candidate.     

Lead-free piezoelectric thin films of Mn-doped NaNbO3-BaTiO3 fabricated by chemical solution deposition

Lead-free piezoelectric thin films of NaNbO₃-BaTiO₃ were fabricated on Pt/TiO_x/SiO₂/Si substrates by chemical solution deposition. Perovskite NaNbO₃-BaTiO₃ single-phase thin films with improved leakage-current and ferroelectric properties were prepared at 650 °C by doping with a small amount of Mn. The 1.0 and 3.0 mol% Mn-doped 0.95NaNbO₃-0.05BaTiO₃ thin films showed slim ferroelectric P-E hysteresis and field-induced strain loops at room temperature. The 1.0 and 3.0 mol% Mn-doped 0.95NaNbO₃-0.05BaTiO₃ films showed remanent polarization values of 6.3 and 6.2 μC/cm², and coercive field of 41 and 55 kV/cm, respectively. From the slope of the field-induced strain loop, the effective piezoelectric coefficient (d₃₃) was found to be 40-60 pm/V.     

Sol-gel processed barium titanate ceramics and thin films

Ferroelectric barium titanate (BaTiO3) ceramics and thin films have been prepared from barium acetate (Ba(CH3COO)2) and titanium (IV) isopropoxied (Ti((CH3)2CHO)4) precursors by a sol–gel technique. The as-grown powder and thin films were found to be amorphous, which crystallized to the tetragonal phase after annealing at 700°C in air for 1 h. Both the ceramics and thin films showed well-saturated polarization–field (P–E) hysteresis loops at room temperature. The value of the spontaneous polarization, PS, remnant polarization, Pr, and coercive field, Ec, of the ceramics and thin films determined from the P–E hysteresis loop were found to be 19.0 and 12.6; 14.0 and 3.2 G cm–2, and 30 and 53 kV cm–1, respectively. The coercive field of the film determined from the capacitance–voltage, C–V, characteristics is slightly lower than that determined from the P–E hysteresis loop (43 kV cm–1). The room-temperature dielectric constant, , of the ceramics and films was found to be 1135 and 370, respectively. Both the films and ceramics showed dielectric anomaly peaks at 125 °C, showing ferroelectric to paraelectric phase transition. © 1998 Kluwer Academic Publishers     

ZnO∶Li压电薄膜制备及其性能研究

ZnO薄膜的高阻特性在压电方面的应用极为重要.采用sol-gel法在Pt/Ti/SiO2/Si衬底上制备了c轴择优取向优良、电阻率高和化学计量比好的掺Li＋（Li/Zn摩尔比分别为0、0.05、0.10、0.15、0.20）ZnO压电薄膜.研究了退火温度、掺杂浓度对ZnO薄膜晶体质量和电学特性的影响.XRD结果表明,ZnO薄膜的c轴择优取向度受退火温度和掺杂浓度的强烈影响;I-V测试表明,掺Li^＋后薄膜的电阻率显著提高,当Li＋掺杂浓度为0.10（Li/Zn摩尔比）、退火温度为600℃时其电阻率达109Ω@cm;XPS分析结果表明,Li＋掺杂对ZnO薄膜中O1s和ZnL3M45M45的结合能以及Zn/O比都有一定的影响,掺杂后化学计量比更好.     

Micromachined piezoelectric force sensors based on PZT thin films

A micromachined lead zirconate titanate (PZT) force sensor for scanning force microscope (SFM) is conceptualized by its piezoelectricity. The fabrication procedure is interpreted, and mechanical characteristics of the micromachined PZT force sensors with various lengths are studied in this paper. A compact SFM is constructed by using the piezoelectric PZT sensor. A very clear image is taken by this SFM. The current study of the micromachined PZT force sensor can be considered as a breakthrough of design of SFM as well as a good example of integrated piezoelectric microdevices     

Lead-free piezoelectric BNKLT 1–3 composites

Bi_0.5(Na_0.725K_0.175Li_0.1)_0.5TiO₃ (abbreviated as BNKLT) is a soft-type piezoelectric ceramics with good piezoelectric properties and strong ferroelectricity at room temperature. The composites with different volume fractions of BNKLT ranging from 0.59 to 0.86 were fabricated by the dice-and-fill method. The composites have been characterized by the resonance techniques and it was found that the composites have good piezoelectric properties that agreed quite well with theoretical modeling. Those composites have potential to be transducer elements in various applications.     

Strain engineering of piezoelectric properties of strontium titanate thin films

The in-plane and out-of-plane piezoelectric properties of (001) strontium titanate (SrTiO₃, STO) epitaxial thin films on pseudo-cubic (001) substrates are computed as a function of in-plane misfit strain. A nonlinear thermodynamic model is employed, which takes into account the appropriate mechanical boundary conditions, the electromechanical coupling between the polarization and the in-plane lattice mismatch, and the self-strains of the ferroelastic and ferroelectric phase transformations. The piezoelectric behavior of epitaxial STO films is described in various strain-induced ferroelectric phase fields in a temperature range from ?50 to 50 °C. The calculations show that by carefully tailoring in-plane misfit strains in both tensile and compressive ranges, piezoelectric coefficients that are of the order of prototypical lead zirconate titanate and other lead-based piezoceramics can be realized. These results indicate that strain engineered STO films may be employed in a variety of sensor and actuator applications as well as surface acoustic wave devices and thin-film bulk acoustic resonators.     

Diffuse reflection of ceramics coated with dielectric thin films

We have studied the diffuse reflection properties of ceramics in the presence of dielectric thin films on the surface. A simple optical model was proposed in which interference effects in a thin film were considered for light scattered out of a ceramic in various directions. Measurements were performed on angle-resolved reflection spectra of a thin-film-coated alumina ceramic in the case of normal incidence. They showed that the presence of the thin film on the ceramic's surface modified the angular distributions of scattered radiation from that of a bare ceramic, which suggested a way to tailor the scattering properties of a diffuse reflector as needed.     

Advances in piezoelectric thin films for acoustic biosensors,acoustofluidics and lab-on-chip applications

Recently, piezoelectric thin films including zinc oxide (ZnO) and aluminium nitride (AlN) have found a broad range of lab-on-chip applications such as biosensing, particle/cell concentrating, sorting/patterning, pumping, mixing, nebulisation and jetting. Integrated acoustic wave sensing/microfluidic devices have been fabricated by depositing these piezoelectric films onto a number of substrates such as silicon, ceramics, diamond, quartz, glass, and more recently also polymer, metallic foils and bendable glass/silicon for making flexible devices. Such thin film acoustic wave devices have great potential for implementing integrated, disposable, or bendable/flexible lab-on-a-chip devices into various sensing and actuating applications. This paper discusses the recent development in engineering high performance piezoelectric thin films, and highlights the critical issues such as film deposition, MEMS processing techniques, control of deposition/processing parametres, film texture, doping, dispersion effects, film stress, multilayer design, electrode materials/designs and substrate selections. Finally, advances in using thin film devices for lab-on-chip applications are summarised and future development trends are identified.     

Miniaturized acceleration sensors with in-plane polarized piezoelectric thin films produced by micromachining

Miniaturized acceleration sensors employing piezoelectric thin films were fabricated through batch micromachining with silicon and silicon-on-insulator (SOI) wafers. The acceleration sensors comprised multiple suspension beams supporting a central seismic mass. Ferroelectric (Pb,La)(Zr,Ti) O(3) (PLZT) thin films were coated and in-plane polarized on the surfaces of the suspension beams for realizing electromechanical conversion through the piezoelectric effect. Interdigital electrodes were formed on the PLZT films and connected in parallel. Finite element analyses were conducted for the stress and strain distributions, providing guidance to the structural design, including optimizing electrode positioning for collecting the electrical output constructively. Uniformity of the beam thickness and sample consistency were significantly improved by using SOI wafers instead of silicon wafers. The measurement results showed that all the sensor samples had fundamental resonances of symmetric out-of-plane vibration mode at frequencies in the range of 8 to 35 kHz, depending on the sample dimensions. These sensors exhibited stable electrical outputs in response to acceleration input, achieving a high signal-to-noise ratio without any external amplifier or signal conditioning.     

Effects of outgassing on the reactive sputtering of piezoelectric AlN thin films

A series of study of the effects of outgassing on pulsed-DC reactive sputtering of highly (0002)-textured AlN thin film was conducted with systematically adjusted sputtering parameters like working pressure, atmosphere, and temperature. The film quality was evaluated by its rocking curve width and residual stress, both utilizing X-ray diffraction methods, as well as by SEM and XPS analyses. It is found that with lower outgassing all the above-mentioned sputtering parameters become less effective on both rocking curve width and residual stress. The rocking curve FWHM (Full Width at Half Maximum) measurements even exhibit apparently insensitive regions to the sputtering parameters, and accompanied threshold behaviors as well. XPS analysis reveals higher oxygen content while SEM observation shows thinner and slanter columnar structure in the AlN film when outgassing is higher upon sputtering.     

Measurement of longitudinal piezoelectric coefficient of thin films by a laser-scanning vibrometer

A laser scanning vibrometer (LSV) was used for the first time to measure the piezoelectric coefficient of ferroelectric thin films based on the converse piezoelectric effect. The significant advantages of the use of the LSV or this purpose were demonstrated. Several key points were discussed in order to achieve reliable and accurate results.     

Ferroelectric and dielectric properties of sol-gel processed barium titanate ceramics and thin films

Ferroelectric and dielectric properties of barium titanate (BaTiO3) bulk ceramics and thin films have been investigated. The bulk ceramics and thin film samples have been prepared from barium acetate [Ba(CH3COO)2] and titanium(IV)isopropoxide [Ti(CH3)2CHO)4] precursors by sol-gel technique. The as-grown bulk powder and thin films were found to be amorphous, which crystallized to tetragonal phase after annealing at 700°C in air for one hour. The values of the spontaneous polarization (Ps), remanant polarization (Pr) and coercive field (Ec) of the bulk ceramics were found to be 19.0, 12.6 C cm–2 and 30 kVcm–1 respectively. In the case of the film, the values of Ps, Pr and Ec were respectively found to be 14.0, 3.2 Ccm–2 and 53 kVcm–1. The capacitance-voltage (C-V) characteristics of the film also showed polarization hysteresis. The values of the dielectric constant () of the bulk ceramic and thin film at 1 kHz were found to be 1235 and 370 respectively. Both the films and ceramics showed dielectric anomaly peaks at 125°C, showing ferroelectric to paraelectric phase transition.     

Lead-free piezoelectric composites with high piezoelectric performance and high dielectric constant caused by percolation phenomenon

We report on lead-free piezoelectric composite with high dielectric constant (ε _r > 10⁵) and the d ₃₃ (above 70 pC/N) comparable to typical lead based piezoelectric composites, or even higher, reaching as high as 107 pC/N. We achieved this through the combination of the good piezoelectric properties of the alkaline niobate (KNN) based perovskite with the flexibility of polymer poly(vinylidene fluoride) (PVDF). The dielectric properties observed in the KNN–PVDF-based piezoelectric composite were well explained in terms of an interfacial percolation model.     

Review Nonlinearity in piezoelectric ceramics

The paper presents an overview of experimental evidence and present understanding of nonlinear dielectric, elastic and piezoelectric relationships in piezoelectric ceramics. This topic has gained an increasing recognition in recent years due to the use of such materials under extreme operating conditions, for example in electromechanical actuators and high power acoustic transducers. Linear behaviour is generally confined to relatively low levels of applied electric field and stress, under which the dielectric, elastic and piezoelectric relationships are described well by the standard piezoelectric constitutive equations. Nonlinear relationships are observed above certain threshold values of electric field strength and mechanical stress, giving rise to field and stress-dependent dielectric (), elastic (s) and piezoelectric (d) coefficients. Eventually, strong hysteresis and saturation become evident above the coercive field/stress due to ferroelectric/ferroelastic domain switching. The thermodynamic method provides one approach to describing nonlinear behaviour in the intermediate field region, prior to large scale domain switching, by extending the piezoelectric constitutive equations to include nonlinear terms. However, this method seems to fail in its prediction of the amplitude and phase of high frequency harmonic components in the field-induced polarisation and strain waveforms, which arise directly from the nonlinear dielectric and piezoelectric relationships. A better fit to experimental data is given by the empirical Rayleigh relations, which were first developed to describe nonlinear behaviour in soft magnetic materials. This approach also provides an indication of the origins of nonlinearity in piezoelectric ceramics, in terms of ferroelectric domain wall translation (at intermediate field/stress levels) and domain switching (at high field/stress levels). The analogy with magnetic behaviour is also reflected in the use of Preisach-type models, which have been successfully employed to describe the hysteretic path-dependent strain-field relationships in piezoelectric actuators. The relative merits and limitations of the different modelling methods are compared and possible areas of application are identified.An erratum to this article can be found at     

A model for longitudinal piezoelectric coefficient measurement of the aluminum nitride thin films

This paper proposes a new model for the longitudinal piezoelectric coefficient (LPC) measurement of the aluminum nitride (AlN) thin film on (100) Si substrate, the AlN thin film is fabricated by the direct-current magnetron sputtering and the piezoelectricity of the AlN thin film is measured by the piezoresponse force microscopy (PFM) in contact mode. In this model, the electric field distribution is taken into account, and the electrostriction displacement caused by the local field concentration is excluded from the measured displacement by the PFM. A LPC value of 4.22 ± 0.34 pm/V is obtained for the clamped AlN thin film by this model, and the deviation between this value and that measured under homogenous field condition is <5.7 %. Therefore, it is reasonable to apply our model to the piezoelectricity characterization of AlN thin films when using the PFM. Furthermore, piezoelectricity of other thin films could also be characterized using this model, which could simplify the measurement process.     

Fabrication of highly c-axis textured ZnO thin films piezoelectric transducers by RF sputtering

The influence of fabrication parameters on ZnO film properties has been analyzed through conducting several experiment processes to develop an appropriate deposition condition for obtaining highly c-axis textured films. A transducer with the structure of Al/ZnO/Al/Si was fabricated at low deposition rate and under a temperature of 380 °C in a mixture of gases Ar:O₂ = 1:3, and RF power of 178 W. Pt/Ti was employed as the bottom electrode of the transducer fabricated in a suitable substrate temperature, which starts increasing at 380 °C with an increment of 20 °C for each 2 h stage of the deposition. Highly c-axis textured ZnO films have been successfully deposited on Pt/Ti/SiO₂/Si substrate under feasible conditions, including RF power of 178 W, substrate temperature of 380 °C, deposition pressure of 1.3 Pa and Ar:O₂ gas flow ratio of 50%. These conditions have been proposed and confirmed through investigating the influences of the sputtering parameters, such as substrate temperature, RF power and Ar:O₂ gas flow ratio, on the properties of ZnO films.