High Electromechanical Coupling Coefficient Research Articles

Optical properties of lead- free ferroelectric potassium sodium niobate (KxNa1-xNbO3) thin films

Abstract Potassium Sodium Niobate (KxNa1-xNbO3 or KNN) is the most attractive lead- free ferroelectric material used in sensor and transducer applications due to its promising piezoelectric properties and high electromechanical coupling coefficient. Besides that, KNN also has good non-linear optical properties and high transparency in the visible range. In the present work, high quality KxNa1-xNbO3 (x=0.35; KNN) thin film has been synthesized on quartz substrate by means of pulsed laser deposition (PLD) process under optimized processing conditions and later post- annealed for better crystallinity. X-Ray Diffraction (XRD) pattern and Raman scattering spectroscopy confirm the highly crystalline nature of KNN thin film. The optical properties of annealed KNN thin film are interrogated from the UV- Visible transmittance spectra. The band-gap of the KNN thin film in optical region is calculated to be equal to 3.75 eV.

Observation of Novel Multifunctionalities in Monolayer CdO

AbstractMultiferroicity, especially ferroelectric, usually fades away at the atomic limit of materials. Here, based on multiscale ab initio many‐body calculations employing first‐principles density functional theory, density functional perturbation theory, the self‐consistent quasiparticle Green function and screened Coulomb method, and the Bethe–Salpeter equation, intrinsic spontaneous polarization and ferroelectricity in monolayer CdO are observed. Using the data, a rather high electromechanical coupling coefficient of ≈6.26% is predicted. The results further reveal a direct optical bandgap of ≈3.16 eV. This direct excitation is dominated by strongly bound excitons with a binding energy of ≈1.09 eV. The predicted multifunctionalities of monolayer CdO are crucial for emerging applications in optoelectromechanical nanodevices and provide insights into its novel properties.

High performance 33.7 GHz surface acoustic wave nanotransducers based on AlScN/diamond/Si layered structures

Surface acoustic wave (SAW) devices are essential devices for communication and sensing, but usually have an operation frequency limit well below 20 GHz due to the constraints of material properties and fabrication capability. By using an AlScN/diamond layered structure with a high electromechanical coupling coefficient K2 and our proposed two-step exposure electron beam lithography (EBL) process for ultra-fine patterns, we have fabricated SAW devices with resonant frequency up to 33.7 GHz in the Ka-band, the highest one ever reported for SAW devices electrically excited by interdigital transducers (IDTs). Combined with finite element analysis, we identified that series resonances are fundamental and high order Rayleigh modes, and K2 are in the range of 1.21%–2.32%, 200% higher compared to those of traditional AlN/diamond-based SAW devices. The high order modes become stronger and dominant, particularly suitable for the development of ultrahigh frequency SAW devices and applications. In addition, the proposed EBL process showed its superb capability to make ultra-fine IDTs down to the nano-scale with excellent smooth edges and uniform patterns, suitable for ultrahigh frequency SAW development.

Numerical Investigation of Mechanically and Electrically Switching SSHI in Highly Coupled Piezoelectric Vibration Energy Harvester

In aiming to increase output power for piezoelectric vibration energy harvesters, a self-powered synchronized switch harvesting on inductor (SSHI) using an electrical or mechanical switch has considerable attention. However, the advantages and disadvantages of the two switching technique for the self-powered SSHIs remains unclear. In addition, for a harvester with a high electromechanical coupling coefficient k, the piezoelectric damping force, which enhances by the SSHI’s voltage increase, is likely to reduce the harvester’s displacement and thus lower the output power. We developed simulation technique, and numerically investigated the performance for the electrical switch SSHI (ESS) and for the mechanical switch SSHI (MSS) harvester, considering the feedback of the piezoelectric damping force. The numerical investigation revealed that, for the ESS, the piezoelectric damping force reduces the displacement every switching on at the maximum/minimum displacement, and thus lowers the output power. In contrast, the MSS, in which the switch turns on only when the displacement exceeds the gap distance, achieved a higher output power, and exhibited intriguing phenomena that the output power continues to increase, whereas the displacement is held constant. Therefore, for a harvester with high k, the MSS can outweigh the ESS.

Rapid and simple measurement of hypersonic wave velocity by Brillouin scattering with induced phonons

Brillouin scattering is a non-contact method to measure wave velocities in the GHz range. One problem of the Brillouin scattering technique is weak light scattering from thermal phonons, which results in the long measurement time and necessity of a complex tandem Fabry-Perot interferometer. To overcome this problem, we have proposed techniques to make use of induced strong coherent phonons from a high frequency transducer. In this study, we have tried to induce strong longitudinal coherent phonons by a ScAlN film transducer (composition of the film : Sc0.41Al0.59N), which has a high electromechanical coupling coefficient. The transducer was fabricated on the quartz sample and composed of the ScAlN film grown by an RF magnetron sputtering and electrodes. The transducer was deposited on one side of the sample. Due to the induced phonons, the scattered light became much stronger than those of thermal phonons and could be observed by a simple confocal Fabry-Perot interferometer. The measured frequency shift of the Brillouin scattering peak was equal to the excitation frequency of the ScAlN transducer (883 MHz). This technique enables easy, rapid and simple measurement of wave velocity in the GHz range, which can be applied for the 2D velocity imaging of the sample.

High frequency transducer for vessel imaging based on lead-free Mn-doped (K0.44Na0.56)NbO3 single crystal

We report a high frequency ultrasonic transducer up to 50 MHz for vessel imaging based on a lead-free (K0.44Na0.56)NbO3–0.5 mol. % Mn (Mn-KNN) single crystal, which has a high electromechanical coupling coefficient kt of 0.64 and a large thickness frequency constant Nt of 3210 kHz · mm. The Krimholtz, Leedom, and Mattaei (KLM) equivalent circuit model was utilized to simulate and optimize the pulse-echo response combined with PiezoCAD software. Theoretically, a high -6 dB bandwidth of 74.94% was obtained at a center frequency of 50.47 MHz and optimized matching conditions. The experimental results showed a center frequency of 51.8 MHz with a −6 dB bandwidth of 70.2%. The excellent global performance makes this lead-free single-crystal transducer quite potential in an environmentally friendly vessel imaging transducer.

Effects of LiNbO3-doping on properties of (Na0.535K0.48)NbO3 piezoelectric ceramics with high electromechanical coupling coefficient for application in surface acoustic wave devices

Abstract In this study, lead-free LiNbO3-doped (Na0.535K0.48)NbO3 (NKN) ceramics (NKLxN, x = 0–0.09) were prepared using a conventional mixed oxide method to investigate the effects of LiNbO3 additives on the microstructure and electrical properties. The addition of LiNbO3 reduced the optimal sintering temperature from 1100 to 950 ℃ and increased the bulk relative density to 99.61% (g/cm3) at x = 0.05 through the formation of a homogeneous microstructure. The electrical properties remained highly stable, even when the ambient temperature was increased from room temperature to 140 ℃. NKLxN ceramics sintered at 950 ℃ with x = 0.05 exhibited the following excellent piezoelectric properties: kp：50% (34%); kt：53% (40%); Qm：168 (112); d33：150 pC/N (80 pC/N); and tan δ ：4% (8%). These values far exceed those of pure NKN ceramics sintered at 1100 ℃. Surface acoustic wave (SAW) devices fabricated using the proposed NKLxN (x = 0.05) ceramics presented high phase velocity of 3210 m/s, k2 of 6.7%, and TCF of approximately −282 ppm/℃. Our results clearly demonstrate the effectiveness of adding LiNbO3 to prevent deliquescence and thereby enhance resistance to the effects of humidity. The resulting devices are highly sensitive to changes in temperature, making them ideally suited to electromechanical transducers as well as SAW temperature sensors.

Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 Piezoelectric Single-Crystal Rectangular Beams: Mode-Coupling Effect and Its Application to Ultrasonic Array Transducers

Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT) piezoelectric single-crystal rectangular beams with the PIN:PMN:PT ratio of 0.33:0.35:0.32 are prepared, and their mode-coupling effect is investigated both theoretically and experimentally for ultrasonic array transducer applications. The PIN–PMN–PT rectangular beams become a tall-narrow beam and a short-wide plate, and so exhibiting an uncoupled height-extensional (beam) mode and an uncoupled thickness-extensional (plate) mode, at a width-to-height ratio G (= L / H) of 6.0, respectively. With G varying in the range of 1.6 to 3.1, the beam mode not only couples strongly with the width (lateral) mode, but also coexists obviously with the plate mode, giving high electromechanical coupling coefficients k_33^' and k_t of ~0.75 and ~0.50, respectively. With the guide of the mode-coupling results, a multifrequency ultrasonic array transducer having three distinct operational frequencies of 1.52, 2.60, and 6.01 MHz, corresponding to the coupled/coexistent beam mode, lateral mode, and plate mode, respectively, is developed using a mode-coupled rectangular beam of G = 1.6. Two different single-frequency ultrasonic array transducers, fabricated using two different uncoupled rectangular beams, one operating in uncoupled beam mode with G = 0.6 at 2.24 MHz and one working in uncoupled plate mode with G = 10.0 at 5.75 MHz, are also developed for comparison.

Lithographically defined aluminum nitride cross-sectional Lamé mode resonators

This paper reports on aluminum nitride (AlN) cross-sectional Lamé mode resonators (CLMRs) showing high electromechanical coupling coefficient () in excess of 4% in a lithographically defined 307 MHz frequency range around 920 MHz. In addition, we report the performance of a CLMR showing a figure of merit (FoM, defined as the product of quality factor, Q, and ) in excess of 85. To the best of the authors’ knowledge, such FoM value is the highest reported for AlN resonators using interdigitated metal electrodes (IDTs).

Creating adjoining polymorphic phase transition (PPT) regions in ferroelectric (Ba,Sr)(Zr,Ti)O3 ceramics

In this work, we report the polymorphic phase transitions(PPT) in ferroelectric Ba0.95Sr0.05ZrxTi(1-x)O3 (BSZT, x=0.01–0.10) ceramics synthesized by using a solid-state reaction method. The doping elements and composition ratios were selected to create adjoining PPT phase boundaries near room temperature, hence to achieve a broadened peak of piezoelectric performance with respect to composition. The temperature-composition phase diagram was constructed and the effects of PPT on the electromechanical and ferroelectric properties of the ceramics were investigated. It was revealed that the two adjacent PPT regions at room temperature showed different characteristics in property enhancement. However, due to the proximity of the phase boundaries, Ba0.95Sr0.05ZrxTi(1-x)O3 ceramics in a fairly broad range of compositions (0.02≤x≤0.07) showed excellent piezoelectric properties, including a large piezoelectric constant (312 pC/N≤d33≤365 pC/N) and a high electromechanical coupling coefficient kp (0.42≤kp≤0.49).

Magnetoelectric coupling studies on (x) (0.5BZT-0.5BCT) – (100-x) NiFe2O4 [x=90−70 wt%] particulate composite

In this article, the effect of composition on magnetoelectric properties of lead-free (x) [0.5BZT – 0.5BCT] – (100-x) NiFe2O4 (x=90, 80, 70wt%) particulate composites synthesized by sol-gel method was studied. Presence of ferroelectric and ferromagnetic phases in the composites was confirmed using x-ray diffraction technique. The microstructure of the composites displayed uniform grains distribution and dispersion of NFO phase in BCZT matrix. The co-existence of ferroelectric and magnetic orderings at room temperature was confirmed by measuring the P-E and M-H loops respectively, of the particulate composites. Frequency dependence of admittance measurements exhibited high electro-mechanical coupling coefficient (kp) in the range of 0.41 to 0.37. The magneto-electric (ME) properties of the composites were studied both in longitudinal (αE,33) and transverse (αE,31) modes at low frequency of 1kHz under external bias field (Hdc). High strain mediated ME coupling responses, αE,31=13.3mV/cmOe, 8mV/cmOe and 4.3mV/cmOe were recorded for samples with x=80, 90, and 70wt% BCZT respectively.

Study on tangentially polarized composite cylindrical piezoelectric transducer with high electro-mechanical coupling coefficient

In this work, we proposed an effective way to fabricate a tangentially polarized composite cylindrical transducer with high electro-mechanical coupling coefficient (EMCC) by radially connecting an inner tangential polarized piezoelectric tube with an outer metal ring. The resonance frequency and EMCC of the proposed transducer are calculated according to frequency equations, which are obtained from the equivalent circuit of the transducer, and the results indicate that EMCC of the tangentially polarized cylindrical transducer is much higher than that of the cylindrical transducer polarized in radial direction. Furthermore, the Finite Element Method (FEM) model of the transducer is established and used for numerical simulation of the vibration models and the optimum configuration parameters. On the basis of those theoretical results, serials of prototype transducers are manufactured with an inner tangential polarized piezoelectric tube connecting with different outer metal cylindrical shells. The admittance characteristics of the fabricated transducers measured by Impedance Analyzer clearly demonstrate that the resonance frequencies of the transducers are in good agreement with those of simulation results, and the effective EMCC of transducers varies with the material of metal cylindrical shell, in which aluminum metal shell possesses the highest EMCC.

A novel langasite crystal microbalance instrumentation for UV sensing application

In this paper, we present a novel low-power UV sensor instrumentation based on ZnO thin-film and langasite crystal microbalance (LCM) composite resonator. The design of this sensor utilizes the exceptional transient response characteristics of thickness shear mode langasite crystal and UV sensitivity of annealed ZnO thin film. Our comparative transient analysis of langasite and quartz crystals shows that langasite crystal has high stability and fast relaxation times, and requires a fraction of excitation power in comparison to that of quartz. Upon investigating the equivalent circuit components, we discovered that high value of motional capacitance of LCM resulted in these contrasting transient responses in LCM and QCM. This high motional capacitance can be further attributed to the high electromechanical coupling coefficient (K2) of piezoelectric langasite crystal. Motivated by these observed characteristics, we proposed a novel sensor instrumentation that is simple yet effective in simultaneous measurements of the frequency, dissipation factor, and the amplitude of oscillation of a LCM. Moreover, our measurements allow for direct monitoring of any change in piezoelectricity of ZnO thin film. Our new measurement approach uses a one-shot pulse instead of conventional sinusoidal waveform to drive the crystal, which allows for simple instrumentation and requires very low input power. The transient response of the crystal shows that the crystal oscillates at its fundamental frequency and the amplitude of oscillation decays exponentially. From the recorded decay curve, the frequency of the freely oscillating crystal, and other important crystal parameters were calculated. We have utilized this mechanism to demonstrate highly sensitive ZnO thin film and LCM based UV sensor. Highest sensitivity of 35.8ppm per μW/cm2 was observed for the LCM coated with 400nm ZnO film. Additionally, the influence of UV on the piezoelectric output of ZnO thin film was also characterized by monitoring the DC offset of the output oscillations. Our findings in this work opens scope for developing low-power acoustic sensors with significant implications towards self-powering sensors.

Fiber-Optic Current Sensor Head Based on the Giant Magnetostrictive Materials

A current sensing method based on giant magnetostrictive material (GMM) and fiber Bragg grating (FBG) was proposed, which not only combines the advantages of FBG such as wavelength encoding, compatibility with fiber and distributed measurement system realization, but also the advantages of GMM as large flexible coefficient, high electromechanical coupling coefficient and short response time. The minimal residual method was put forward to effectively identify the Jiles-Atherton model parameters of ferro-magnetic material. The parameters of the giant magnetostrictive ( Terfenol-D bar ) and the Jiles-Atherton model was established and optimized by using the Mathematica software. Then the optical fiber current test system with ferromagnetic loop was set up to investigate the current sensing properties of the sensor. The relationship between current excitation and GMM-FBG reflected wavelength was obtained, which provides basic support for further current sensing experiment research.

Dielectric and piezoelectric properties of lead-free (K0.44Na0.46)NbO3-0.5%MnO2 single crystals grown by the TSSG method

Lead-free (K0.44Na0.46)NbO3-0.5%MnO2 (KNN-0.5%MnO2) single crystals with dimensions of Ф30×10mm were successfully grown by a top seeded solution growth technique (TSSG). The X-ray diffraction pattern has shown that the as-grown crystals have an orthorhombic perovskite structure. The orthorhombic-tetragonal (TO-T) and tetragonal-cubic phase transition temperature (the Curie temperature TC) of the single crystal were found at 184°C and 412°C, respectively. The KNN-0.5%MnO2 single crystals exhibited high piezoelectric constants d33 and dielectric permittivity εr, being 261pC/N and 275. Well saturated P-E hysteresis loop with remnant polarization Pr=22.06µC/cm2 and coercive field Ec=17.93kV/cm was obtained at a maximum electric field of 3kV/mm. A high strain (0.24%) and electromechanical coupling coefficient Kt (65.9%) were obtained along the (001) orientation. These excellent results indicated that the KNN-0.5%MnO2 single crystals could be used as high quality lead-free material.

Application of TiO2 thin film with nanorods to surface acoustic wave type ultraviolet photo detection

TiO2 thin films with nanorods grown on 128° Y−cut LiNbO3 and 90° rotated 42°45′ ST−cut quartz were used to fabricate surface acoustic wave ultraviolet photodetectors. TiO2 thin film was deposited by radio−frequency magnetron sputtering and TiO2 nanorods were then synthesized on the thin film via the hydrothermal method. 128° Y−cut LiNbO3 is a Rayleigh wave substrate with a high electromechanical coupling coefficient, whereas 90° rotated 42°45′ ST−cut quartz is a surface skimming bulk wave substrate with a high wave velocity. The effects of substrate characteristics and TiO2 nanorod morphology on the ultraviolet sensitivity of the surface acoustic wave photodetectors were investigated. The variations of insertion loss, phase, resistance, and capacitance under ultraviolet illumination were examined. The performance of the TiO2 thin film with nanorods deposited on 128° Y−cut LiNbO3 is much greater than that of the film deposited on 90° rotated 42°45′ ST−cut quartz, which can be attributed to the former’s high electromechanical coupling coefficient.

Influence of magnetic flux density and cooling rate on orientation behavior of Tb0.27Dy0.73Fe1.95 alloy during solidification process

The rare-earth giant magnetostrictive material Tb0.27Dy0.73Fe1.95 is one of the most important functional magnetic materials. Their superior properties include high saturation magnetostrictive coefficient at room temperature, high electromechanical coupling coefficients, high output power, fast response, high energy density, and non-contact drive. Thus, they can be used to build sensors, precision machinery, magnetomechanical transducers, and adaptive vibration-control systems. In this material, the magnetic phase (Tb, Dy)Fe2 has a typical MgCu2-type cubic Laves phase structure and exhibits different magnetostrictive properties along different crystal orientations. The 111 direction of this phase is the easy magnetization axis, along which the linear magnetostriction is higher than other directions. Thus, researchers have focused on preparing (Tb, Dy)Fe2 with a crystallographic orientation along or close to the 111 direction. Generally, the directional solidification method is used to prepare the Tb0.27Dy0.73Fe1.95 alloy. However, a crystal orientated along the 110 or 112 direction is always obtained and both of these directions require a high external magnetic field for improved magnetostrictive performance. The 111 preferred growth orientation can be acquired using seed crystal technology. However, the relatively low growth velocity can cause the appearance of the linear (Tb, Dy)Fe3 phase which induces a high brittleness of the material. Therefore, new methods to prepare Tb0.27Dy0.73Fe1.95 products with high 111 orientation at higher growth velocity are required. In this paper, we solidify the Tb0.27Dy0.73Fe1.95 alloys under various high magnetic field and cooling rate conditions. We study the effects of the magnetic flux density and cooling rate on the crystal orientation of the (Tb, Dy)Fe2 phase and the magnetization behavior of the alloys. It is found that after field-treated solidification, a high 111 orientation of (Tb, Dy)Fe2 along the magnetic field direction can be produced. As a consequence, the magnetostriction without applying stress remarkably increases. By increasing the magnetic flux density applied during the solidification of the Tb0.27Dy0.73Fe1.95 alloys, the 111 orientation of (Tb, Dy)Fe2 could be obtained at higher cooling rates. Ranging from 4 T to 10 T, with increasing cooling rate the magnetic flux density, at which the 111 or 110 orientation of (Tb, Dy)Fe2 occurs, increases or decreases, respectively. The saturated magnetization of the alloys increases with increasing cooling rate. The application of the magnetic fields does not affect the saturated magnetization.

Lead-free 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 nanowires for energy harvesting.

Lead-free piezoelectric nanowires (NWs) show strong potential in sensing and energy harvesting applications due to their flexibility and ability to convert mechanical energy to electric energy. Currently, most lead-free piezoelectric NWs are produced through low yield synthesis methods and result in low electromechanical coupling, which limit their efficiency as energy harvesters. In order to alleviate these issues, a scalable method is developed to synthesize perovskite type 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) NWs with high piezoelectric coupling coefficient. The piezoelectric coupling coefficient of the BZT-BCT NWs is measured by a refined piezoresponse force microscopy (PFM) testing method and shows the highest reported coupling coefficient for lead-free piezoelectric nanowires of 90 ± 5 pm V(-1). Flexible nanocomposites utilizing dispersed BZT-BCT NWs are fabricated to demonstrate an energy harvesting application with an open circuit voltage of up to 6.25 V and a power density of up to 2.25 μW cm(-3). The high electromechanical coupling coefficient and high power density demonstrated with these lead-free NWs produced via a scalable synthesis method shows the potential for high performance NW-based devices.

Functional Piezocrystal Characterisation under Varying Conditions.

Piezocrystals, especially the relaxor-based ferroelectric crystals, have been subject to intense investigation and development within the past three decades, motivated by the performance advantages offered by their ultrahigh piezoelectric coefficients and higher electromechanical coupling coefficients than piezoceramics. Structural anisotropy of piezocrystals also provides opportunities for devices to operate in novel vibration modes, such as the d36 face shear mode, with domain engineering and special crystal cuts. These piezocrystal characteristics contribute to their potential usage in a wide range of low- and high-power ultrasound applications. In such applications, conventional piezoelectric materials are presently subject to varying mechanical stress/pressure, temperature and electric field conditions. However, as observed previously, piezocrystal properties are significantly affected by a single such condition or a combination of conditions. Laboratory characterisation of the piezocrystal properties under these conditions is therefore essential to fully understand these materials and to allow electroacoustic transducer design in realistic scenarios. This will help to establish the extent to which these high performance piezocrystals can replace conventional piezoceramics in demanding applications. However, such characterisation requires specific experimental arrangements, examples of which are reported here, along with relevant results. The measurements include high frequency-resolution impedance spectroscopy with the piezocrystal material under mechanical stress 0–60 MPa, temperature 20–200 °C, high electric AC drive and DC bias. A laser Doppler vibrometer and infrared thermal camera are also integrated into the measurement system for vibration mode shape scanning and thermal conditioning with high AC drive. Three generations of piezocrystal have been tested: (I) binary, PMN-PT; (II) ternary, PIN-PMN-PT; and (III) doped ternary, Mn:PIN-PMN-PT. Utilising resonant mode analysis, variations in elastic, dielectric and piezoelectric constants and coupling coefficients have been analysed, and tests with thermal conditioning have been carried out to assess the stability of the piezocrystals under high power conditions.

Vibration Modes of Radially Polarized Thin Cylindrical Piezoceramic Rings

Radially polarized cylindrical piezoceramic rings are studied experimentally. It is shown that the inphase resonance is characterized by very high amplitudes and electromechanical coupling coefficient. The distribution of internal stresses along the height differs from cosinusoidal in the middle part of the ring. The edge resonance is characterized by high stresses near the ends and is very weak in the middle part. The amplitude–frequency and admittance–frequency responses are plotted and discussed