Large Electromechanical Coupling Coefficient Research Articles

Large piezoelectric strain observed in sol–gel derived BZT–BCT ceramics

High performance lead (Pb)-free piezoelectric ceramics with excellent piezoelectric properties is in great demand for sensor and actuator applications. Barium zirconate titanate–barium calcium titanate [xBZT–(1 − x)BCT] (x = 0.5) is one such lead free system, which exhibits high piezoelectric properties similar to lead zirconate titanate (PZT). In this study we report the synthesis and characterization of this lead free [xBZT–(1 − x)BCT] (x = 0.5) via wet chemical sol–gel method. Calcination of the BZT–BCT precursor only at 1000 °C (against 1300 °C reported in the literature) for 4 h resulted in formation of single phase nanoparticles (<50 nm) as confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. Highly dense and homogenous microstructure with 95% of the theoretical density was obtained by solid-state sintering of the green pellets at 1550 °C. Remanent polarization (Pr) of 11.55 μC/cm2 and relative permittivity of 20,020 at the Curie temperature of 95 °C were obtained. Electrically poled BZT–BCT ceramics samples exhibited high piezoelectric charge coefficients, d33 ∼ 530 pC/N, d33* ∼ 942 pm/V, large electromechanical coupling coefficient kp ∼ 0.45 and a large strain of 0.15%, which are comparable to those of lead based piezoelectric ceramics. The excellent piezoelectric properties of this sol–gel derived BZT–BCT system has been analyzed and correlated to its structure in this report.

Direct measurement of piezoelectric shear coefficient

Piezoelectric materials exhibit electromechanical coupling which has led to their widespread application for sensors, actuators, and energy harvesters. These materials possess anisotropic behavior with the shear coefficient, and have the largest electromechanical coupling coefficient. However, the shear mode is difficult to measure with existing techniques and thus has not been fully capitalized upon in recent devices. Better understanding of the full shear response with respect to the driving electric field would significantly help the design of optimized piezoelectric shear devices. Here, a simple and low cost direct measurement method based on digital image correlation is developed to characterize the shear response of piezoelectric materials and its nonlinear behavior as a function of external field. The piezoelectric shear coefficient (d15) of a commercial shear plate actuator is investigated in both bipolar and unipolar electric fields. Two different nonlinearities and hysteresis behaviors of the actuators were observed, and the relation between the driving field amplitude and the corresponding d15 coefficient is determined. Moreover, the measured transverse displacement of the plate actuator in simple shear condition is validated through a laser interferometry technique.

Surface acoustic wave devices on AlN/3C–SiC/Si multilayer structures

Surface acoustic wave (SAW) propagation characteristics in a multilayer structure including a piezoelectric aluminum nitride (AlN) thin film and an epitaxial cubic silicon carbide (3C–SiC) layer on a silicon (Si) substrate are investigated by theoretical calculation in this work. Alternating current (ac) reactive magnetron sputtering was used to deposit highly c-axis-oriented AlN thin films, showing the full width at half maximum (FWHM) of the rocking curve of 1.36° on epitaxial 3C–SiC layers on Si substrates. In addition, conventional two-port SAW devices were fabricated on the AlN/3C–SiC/Si multilayer structure and SAW propagation properties in the multilayer structure were experimentally investigated. The surface wave in the AlN/3C–SiC/Si multilayer structure exhibits a phase velocity of 5528 m s−1 and an electromechanical coupling coefficient of 0.42%. The results demonstrate the potential of AlN thin films grown on epitaxial 3C–SiC layers to create layered SAW devices with higher phase velocities and larger electromechanical coupling coefficients than SAW devices on an AlN/Si multilayer structure. Moreover, the FWHM values of rocking curves of the AlN thin film and 3C–SiC layer remained constant after annealing for 500 h at 540 °C in air atmosphere. Accordingly, the layered SAW devices based on AlN thin films and 3C–SiC layers are applicable to timing and sensing applications in harsh environments.

Finite Element Modeling of Ring-Shaped Piezoelectric Transformer

In this work, we developed a finite element package to investigate the dependence of electromechanical coupling coefficient on ring-shaped piezoelectric transformer parameters, and proposed how to design the transformer in achieving optimum efficiency using the ceramic PZT-5H as an application. The ring transformer firstly used full driven electrode to observe piezoelectric vibration characteristic with varying the ring dimensional (structural) parameters. Then, with the optimized ring parameters, electrode layout was designed in accordance to the distribution of electrical potential within the ring transformer, where both alternating and non-alternating electrode layouts were considered. From the calculation, results showed that the piezoelectric vibration characteristic strongly depends on the ring-shaped transformer dimension. Specifically, with increasing piezoelectric ceramic thickness and the width between inner- and outer-ring-radius, the resonance frequencies of the transformer decrease. On the other hand, in terms of the electrode layouts, the alternating electrode was a better choice in obtaining desired resonance frequency mode, as it provides larger electromechanical coupling coefficient than the non-alternating electrode. These results are in agreement with previous experimental investigation, where applicable.

Realizing Deep-Submicron Gap Spacing for CMOS-MEMS Resonators

Integrated complementary metal-oxide-semiconductor (CMOS)-microelectro mechanical systems (MEMS) beam-array resonators that take advantage of the pull-in effect to surmount limitations of standard CMOS foundry processes have been demonstrated to attain electrode-to-resonator gap spacing at a deep-submicrometer range. Such deep-submicrometer gaps lead to much larger electromechanical coupling coefficient and smaller motional impedance as compared with conventional CMOS-MEMS technologies. In addition, a unique frequency tuning capability by modulating the mechanical boundary conditions (“BC”) of the resonators has also been proposed. Furthermore, the mechanically coupled array approach used in this paper not only provides five times smaller motional impedance compared with that of a single resonator but offers superior power handling capability due to higher stiffness of the arrayed resonator. In such an array design, high velocity coupling for beam-array resonators effects a single resonance behavior without spurious modes. With the increase of an applied dc-bias which simultaneously serves for functions of pull-in and resonator operation, the upward frequency shift of resonance caused by boundary condition change offers opposite tuning mechanism against the well-known effect of electrical stiffness. As a result, frequency variation induced by the BC-modulation and electrical stiffness would yield a frequency-insensitive region under a certain dc-bias voltage. Furthermore, the use of metal/oxide composite materials for resonator structures in this paper substantially alleviates residual stress often seen in the CMOS layers as compared to mere-metal resonators.

Calculation of thermostable directions and the effect of external electric field on the propagation of Lamb and SH waves in a langasite-crystal plate

The effect of dc electric field E on the characteristics, propagation conditions, and temperature delay coefficients of Lamb and SH waves in a piezoelectric langasite plate is considered based on the theory of acoustic-wave propagation in piezoelectric crystals exposed to such a field. The cuts characterized by a thermal stability and large electromechanical coupling coefficient are determined.

Modified single crystals for high-power underwater projectors

Underwater electroacoustic projectors using single crystals based on the lead magnesium niobate-lead titanate (PMNT) composition were investigated. The large electromechanical coupling coefficient (k(33) > 0.90) and piezoelectric coefficient (d(33) > 1500 pC/N) of PMNT have been demonstrated to improve transducer bandwidth and source level relative to conventional piezoelectric ceramics. The low mechanical quality factor (Q(M) < 200) and low temperature stability (T(RT) < 95°C) of PMNT, however, limit its utility in high-power, high-duty-cycle applications. Use of modified single crystals was shown to result in transducers which exhibit up to 5 dB improvement in source level over PMNT when operated at resonance. Compared with a PZT4 transducer, these modified crystals offer similar source level and power handling capability at resonance, but the available bandwidth is doubled and a 6 dB improvement in maximum source level is achieved when driven off resonance.

Wideband tunable Love wave filter using electrostatically actuated MEMS variable capacitors integrated on lithium niobate

A tunable bandpass filters with low insertion loss, sharp cut-off characteristics, wide bandwidth and wide tunability is strongly expected for future reconfigurable and cognitive wireless systems. In this study, a wideband tunable filter with the center frequency of 1.08GHz was fabricated by directly integrating Love wave resonators and electrostatically actuated MEMS variable capacitors on a 15° Y LiNbO3 wafer. The Love wave resonators have an extremely large electromechanical coupling coefficient (k2∼30%), providing wide bandwidth and wide tuning range. The 3dB bandwidth was tuned from 146MHz to 130MHz by applying 15V to one of the MEMS variable capacitors.

Surface acoustic wave device properties of (B, Al)N films on 128° Y– X LiNbO 3 substrate

A c-axis orientated aluminium nitride (AlN) film on a 128° Y– X lithium niobate (LiNbO 3) surface acoustic wave (SAW) device which exhibit a large electromechanical coupling coefficient ( k 2) and a high SAW velocity property, is needed for future communication applications. In this study, a c-axis orientated (B, Al)N film (with 2.6 at.% boron) was deposited on a 128° Y– X LiNbO 3 substrate by a co-sputtering system to further boost SAW device properties. The XRD and TEM results show that the (B, Al)N films show highly aligned columns with the c-axis perpendicular to the substrate. The hardness and Young's modulus of (B, Al)N film on 128° Y– X LiNbO 3 substrates are at least 17% and 7% larger than AlN films, respectively. From the SAW device measurement, the operation frequency characteristic of (B, Al)N film on 128° Y– X LiNbO 3 is higher than pure AlN on it. The SAW velocity also increases as (B, Al)N film thickness increases (at fixed IDT wavelength). Furthermore, the k 2 of (B, Al)N on the IDT/128° Y– X LiNbO 3 SAW device shows a higher value than AlN on it.

Rayleigh and shear horizontal surface acoustic properties of (100) ZnO films on silicon

(100) ZnO films on silicon will excite Rayleigh SAWs along the z-axis and shear horizontal (SH) SAWs along the y-axis. Rayleigh SAW modes of (100) ZnO films on silicon have smaller film thickness ratios, higher phase velocities, and larger electromechanical coupling coefficient (K(2)) than the those of (002) ZnO films on silicon. This is special for Rayleigh mode 1, where the K(2) curve shows a maximum value of 3.37% at h/lambda = 0.28 and the velocity is 5258 m/s. This is special for SH mode 0, where the K(2) curve shows a maximum value of 3.3% at h/lambda = 0.28 and the velocity is 3242 m/s.

Rayleigh surface acoustic wave modes of (100) ZnO films on (111) diamond

(100) zinc oxide (ZnO) films were combined with (111) diamond to be a composite high velocity surface acoustic wave (SAW) substrate. Rayleigh SAW modes of (100) ZnO films on (111) diamond were theoretically analyzed. Those Rayleigh SAW modes have smaller films thickness ratios (h/λ), higher phase velocities, and larger electromechanical coupling coefficients (K2) than the ones of (002) ZnO films on (111) diamond. The research results provide a predictable and theoretical basis for further application on the high velocity SAW devices.

Piezoelectric Energy Harvesting using Single Crystal Pb(Mg1/3Nb2/3)O 3-xPbTiO3 (PMN-PT) Device

We present the analytical and experimental studies of a piezoelectric energy harvesting device that can be used as a standalone energy source for powering microsensors and electronics. Single crystal piezoelectric Pb(Mg1/3 Nb2/3)O3-xPbTiO3 (PMN-PT) with large electromechanical coupling coefficient was employed for device design and fabrication in this study. An analytical model for estimating the power-harvesting performance was derived for a cantilever-mounted aluminum alloy plate with a PMN-PT device bonded near the clamped end and a proof mass at the other end. Considering the plate was subjected to both a steady-state sinusoidal vibration and a pulse impact excitation, static, and dynamic analyses were performed for device structure to achieve efficient energy harvesting. In the static analysis, the effect of geometrical dimension of piezoelectric device on the energy harvesting performance has been discussed. In the dynamic analysis, transient response of the device subjected to a pulse impact excitation was studied using a single degree of freedom system model. The resonance characteristics and the power generation capability of the device were discussed. Based on the analytical results, a cantilever device was fabricated and evaluated with sinusoidal excitation and impact excitation. The results are applicable for the structural design of piezoelectric energy harvesting devices.

Rayleigh surface acoustic wave modes of interdigital transducer/(100) AlN/(111) diamond

In this research, Rayleigh surface acoustic wave (SAW) modes of interdigital transducer (IDT)/(100) AlN/(111) diamond were theoretically analyzed and exhibited some excellent acoustic properties. Those Rayleigh SAW modes have smaller film thickness ratios (h/λ), higher phase velocities, and larger electromechanical coupling coefficients (K2) than the ones of IDT/(002) AlN/(111) diamond. Especially for mode 1, the phase velocity is 10 474 m/s, the K2 is 2.31%, and the film thickness ratio (h/λ) is 0.3. The research results exhibit that IDT/(100) AlN/(111) diamond has some excellent properties and provides a predictable and theoretical basis for further application in high velocity SAW devices.

Surface acoustic wave element

A surface acoustic wave element according to the present invention has a structure including a hard layer containing a composition component essentially consisting of at least one of diamond and a diamond-like carbon film [2], a piezoelectric layer formed on the hard layer [4], a silicon dioxide (SiO₂) [5] layer formed on the piezoelectric layer, and electrodes combined with the piezoelectric layer to perform electro-mechanical conversion. The surface acoustic wave element having the above structure according to the present invention has a larger electro-mechanical coupling coefficient and a higher surface acoustic wave propagation velocity than those of a conventional surface acoustic wave element having no silicon dioxide layer, thereby obtaining a surface acoustic wave element operated in a high-frequency range. In particular, the electro-mechanical coupling element is increased. The silicon dioxide (SiO₂) layer is an electric insulator and rarely reacts with moisture or an acid. The silicon dioxide layer can protect the piezoelectric layer and the electrodes from an external environment, thereby providing a surface acoustic wave element having good high-frequency characteristics and a high resistance to environment.

Film bulk acoustic resonator

A film bulk acoustic resonator comprises a substrate (12) of a single silicon crystal, a base film (13) formed on the substrate (12) and composed of a dielectric film mainly containing silicon oxide, and a piezoelectric multilayer structure (14) formed on the base film (13). A vibratory section (21) composed of a part of the base film (13) and a part of the piezoelectric multilayer structure (14). The piezoelectric multilayer structure (14) includes a lower electrode (15), a piezoelectric film (16), and an upper electrode (17) in this order from below. The substrate (12) has a via hole in the region corresponding to the vibratory section (21). The via hole forms a space for allowing vibration of the vibratory section (21). The piezoelectric film (16) is an aluminum nitride thin film containing 0.2 to 3.0 atom% of alkaline earth metal and/or a rare earth metal. Thus, the film bulk acoustic resonator has a large electromechanical coupling coefficient, an excellent acoustic quality factor (Q), an excellent frequency-temperature characteristic, high characteristics, and a high performance.

Surface acoustic wave device and electronic device using the same

A reliable SAW device has excellent reflection and a small size, which is achieved by reducing the number of fingers defining reflectors, such that losses due to a large electromechanical coupling coefficient are small and the film thickness of electrodes has much less effect on frequencies of the device. In the SAW device having pluralities of first fingers and second fingers, disposed on a quartz substrate, constituting an IDT for exciting SH waves and reflectors for reflecting the SH waves, respectively, the first and second fingers made mainly from Al are disposed on the ST-cut 90° X-propagation quartz substrate with the Euler angles (0°, θ, 90°±2°), wherein the angle θ is within the range of about 110° to about 150°, and a normalized film thickness (H/λ) of the fingers is within in the range of about 0.025 to about 0.135.

The Strain Response of Lead Zinc Niobate-Lead Titanate Piezocrystals: Phase Transitions and Non-Linear Effects

Single crystals of Pb(Zn 1/3 Nb 2/3 )O 3 -PbTiO 3 (PZN-PT) produce high levels of strains with large electromechanical coupling coefficients and they exhibit high dielectric permittivity. These properties make PZN-PT single crystals promising materials for transducer and actuator applications. The large strains in these materials are partly due to electric-field-induced rhombohedral–tetragonal phase transitions that are temperature dependent. We have studied the phase transitions in ⟨001⟩ oriented Pb(Zn 1/3 Nb 2/3 )O 3 -xPbTiO 3 single crystals for x = 4.5% and x = 8% by measuring the piezoelectric strains of the crystals as a function of applied electric field and temperature. We have used laser Doppler interferometry to make direct measurements of the strain responses of the two types of ⟨001⟩ and ⟨111⟩ oriented PZN-PT single crystals in the rhombohedral phase as a function of AC electric fields and DC bias fields. When the applied electric field exceeds a threshold value, the dielectric and piezoelectric constants become a function of the applied field, and hysteresis loops are observed. Our results show that a positive DC bias stabilizes the domain configuration in the crystals and increases the range of AC voltages over which the response is linear. The threshold field, at the onset of non-linearity, is found to have a linear relation with DC bias field in the field range investigated. Very large shear strains have been observed for crystals poled in the pseudocubic ⟨111⟩ direction with d 15 values in the neighbourhood of ∼5500 pC/N for PZN-4.5%PT and ∼7500 pC/N for PZN-8%PT.

Analysis of SAW properties in ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ structures

Piezoelectric thin films on high acoustic velocity nonpiezoelectric substrates, such as ZnO, AlN, or GaN deposited on diamond or sapphire substrates, are attractive for high frequency and low-loss surface acoustic wave devices. In this work, ZnO films are deposited on AlxGa1-xN/c-Al2O3 (0 < or = chi < or = 1) substrates using the radio frequency (RF) sputtering technique. In comparison with a single AlxGa1-xN layer deposited on c-Al2O3 with the same total film thickness, a ZnO/AlxGa1-xN/c-Al2O3 multilayer structure provides several advantages, including higher order wave modes with higher velocity and larger electromechanical coupling coefficient (K2). The surface acoustic wave (SAW) velocities and coupling coefficients of the ZnO/AlxGa1-xN/c-Al2O3 structure are tailored as a function of the Al mole percentage in AlxGa1-xN films, and as a function of the ZnO (h1) to AlxGa1-xN (h2) thickness ratio. It is found that a wide thickness-frequency product (hf) region in which coupling is close to its maximum value, K(2)max, can be obtained. The K(2)max of the second order wave mode (h1 = h2) is estimated to be 4.3% for ZnO/GaN/c-Al2O3, and 3.8% for ZnO/AlN/c-Al2O3. The bandwidth of second and third order wave modes, in which the coupling coefficient is within +/- 0.3% of K(2)max, is calculated to be 820 hf for ZnO/GaN/c-Al2O3, and 3620 hf for ZnO/AlN/c-Al2O3. Thus, the hf region in which the coupling coefficient is close to the maximum value broadens with increasing Al content, while K(2)max decreases slightly. When the thickness ratio of AlN to ZnO increases, the K(2)max and hf bandwidth of the second and third higher wave modes increases. The SAW test devices are fabricated and tested. The theoretical and experimental results of velocity dispersion in the ZnO/AlxGa1-xN/c-Al2O3 structures are found to be well matched.

Fabrication of KNbO3 Epitaxial Thin Films on Sapphire Substrates by Pulsed Laser Deposition

KNbO3 epitaxial thin films were fabricated on sapphire r-plane substrates with an MgO buffer layer by pulsed laser deposition. X-ray diffraction measurements revealed that both MgO and KNbO3 were epitaxially grown on the Al2O3 (1-102) substrate with an orientation relationship as KNbO3 (100)pc/MgO (100)/Al2O3 (1-102) (out-of-plane) and KNbO3 [010]pc//MgO [010]//Al2O3 [11-20] (in-plane), although both MgO and KNbO3 were tilted 6–9° with respect to the substrate surface. Raman scattering measurements revealed that the KNbO3 film was composed of the orthorhombic ferroelectric phase, and that the (010)ortho (Y) domain, in which the polarization axis was parallel to the substrate surface, was dominant. The present study enables the production of KNbO3 epitaxial thin-film wafers, which correspond to Y-cut KNbO3 single crystals with a large electromechanical coupling coefficient (k2=0.53), for surface acoustic wave propagation.

Temperature Coefficient of SAW Device on SiO2/Proton Exchanged LiNbO3 Substrate

Proton exchange (PE) is an attractive method for the fabrication of optical and acoustic waveguides in lithium niobate (LiNbO3). LiNbO3 substrate has large electromechanical coupling coefficient (K2) and moderate SAW velocity, but its temperature coefficient of frequency (TCF) is as large as -73 ppm/°C. Silicon dioxide (SiO2) is a well-known non-piezoelectric material having the positive TCF for SAW. The SiO2 thin films are deposited on the PE-LiNbO3 substrates to improve the TCF of the SAW devices. The results show that the temperature stability of SAW on SiO2/PE-LiNbO3 substrate is improved to be about -55 ppm/°C.