Finite-element modeling of lead magnesium niobate electrostrictive materials: dynamic analysis

A finite-element model is proposed for the time-domain analysis of electrostrictive materials. Hom's material model, developed for lead magnesium niobate (PMN) ceramics, is used. It includes the quadratic dependence of strain with polarization, the saturation of polarization, assumes constant temperature, and excludes hysteresis. The theoretical formulation is justified by the principle of virtual works. The numerical model is obtained after discretization in space and time. The validation is performed by comparing numerical results with semianalytical results for an electrostrictive spherical shell subjected to a step in voltage or in charge. From these results, a method to compute the coupling coefficient of electrostrictive materials, based on Ikeda's definition, is proposed and applied to a bar with parallel electric field.     

Ultrasonic transducers using electron-irradiated vinylidene fluoride-trifluoroethylene copolymers

Single-element, planar transducers have been fabricated using electron-irradiated poly(vinylidene fluoride-trifluoroethylene) 80/20 mol% copolymers with different electron dosage. Electrical field-induced strain response of copolymer film with 100 Mrad dosage has been studied at 5 kHz and the electrostrictive coefficient was calculated. The transmitting response of the air-backing and epoxy-backing transducers was evaluated with the application of high DC bias voltages. Clear ultrasonic amplitudes and high frequency spectrum (>20 MHz) were observed when driven from a standard ultrasonic voltage source through a decoupling circuit. It has also showed that larger generation of ultrasonic waves will be induced under high DC bias field, which is due to the increase of induced d(33) piezoelectric coefficient. Two different polar bias voltages, positive and negative, were applied to the transducers and inverse waveforms were received, which was coincident with the theoretical analysis of the strain response of electrostrictive film.     

Antiferroelectric phase transitions ceramics low frequency projector

In order to discuss the nonlinear electromechanical properties of the La-modified lead zirconate titanate stannate antiferroelectric ceramics PbLa（Zr,Sn,Ti）O₃（PLZST）,the approximate linearized electrostrictive equations exactly the same form as the classical linear piezoelectric equations were derived from the second-order terms of the generalized piezoelectric equations.An analytic approach of solving the electroacoustic transduction problems of the nonlinear antiferroelectric phase transitions ceramics transducers was presented.The nonlinear properties of the PLZST can be linearly equivalent with a maintained bias based on the approximate linearized electrostrictive equations for analyzing the electroacoustic transduction problems of the transducers.An innovative antiferroelectric ceramics low frequency classⅣflextensional transducer with resonance frequency 1.1 kHz based on the approach was developed which utilizes the PLZST as active elements.The lake authentications indicated that the use of PLZST for transducers yielded presently a 3 dB increase in TVR and a 9 dB increase in SL over its PZT-4 counterpart,and the approach was valid.     

Giant axial electrostrictive deformation in carbon nanotubes

An exceptionally large axial electrostrictive deformation is demonstrated in single walled carbon nanotubes using Hartree-Fock and density functional quantum mechanics simulations. Armchair and zigzag open-ended tubes and capped tubes are studied and in all of them the external electric field induced axial strains can be greater than 10% for a field strength within 1 V/A. The corresponding volumetric and gravimetric work capacities are predicted to be three and six orders higher than those of the best known ferroelectric, electrostrictive, magnetostrictive materials and elastomers, respectively.     

Stress analysis of electrostrictive material with an elliptic defect

It is shown that the constitutive equation and electric body force used to discuss the stress analysis of electrostrictive material in some previous literature are not appropriate. This paper presents the corrected stress solution for the infinite plane with an insulated elliptic hole under an applied electrical field. The numerical result obtained for the PMN material constants show that the stress near the end of the narrow elliptic hole is the tensile stress.     

Investigations on ferroelectric PMN-PT and PZN-PT single crystals ability for power or resonant actuators

Ferroelectric single crystals of PZN-PT and PMN-PT exhibit outstanding properties: high charge coefficient (dij), high coupling factor (kij) and high strain levels under DC fields. Besides, their mechanical quality factor is believed to be low. Their usefulness for non-resonant or large bandwidth transducers has therefore been previously investigated. However, few studies have been devoted to the dielectric and mechanical losses of single crystals and to their stability under high levels of excitations (electric fields, temperature and mechanical stress). A knowledge and understanding of such performances is needed to determine whether single crystals are suitable materials for power or resonant transducers. In this work, losses and non-linearity versus external excitations are investigated. Dielectric losses and mechanical losses are measured versus electric field for different compositions, orientations. The evolution of d33 and epsilonT33 are obtained versus electric field and temperature for the longitudinal mode. Strain and hysteresis versus sweep mode (up and down) are measured near the resonance frequency using a laser Doppler vibrometer.     

High nonlinearities in Langevin transducer: a comprehensive model

The design and simulation of power transducers are difficult since piezoelectric, dielectric and elastic properties of ferroelectric materials differ from linear behavior when driven at large levels. This paper is devoted to modeling of a resonant power transducer at a high level of dynamic mechanical stress. The power transducer is subjected to a sine electrical field E of varying frequency which was considered as the excitation of the transducer.The mechanical equation of the piezoelectric element is written using electrostriction. The dielectric part is written as a nonlinear function of an equivalent electric field including stress influence (scaling relationship between electric field and mechanical stress). Using various simulations, we show then that typical resonance nonlinearities are obtained, such as jump phenomenon of transducer speed amplitude and phase, resonance peak that become asymmetric, and diminution of mechanical quality factor. As a consequence, we state that those typical nonlinearities are only due to dielectric nonlinearities, in good correlation with typical ferroelectric behavior. Moreover, this demonstrates the usefulness of scaling relationships in ferroelectrics, which explain static depoling under stress and butterfly strain hysteresis loop. The same scaling law gives here several nonlinearities for resonant transducers as well.     

Advantage analysis of PMN-PT material for free-flooded ring transducers

The acoustic radiation characteristics of free-flooded ring transducers made of PZT4 and PMN-PT materials are calculated and compared.First,the theoretical formulae for free-flooded ring transducers are studied.The resonant frequencies of a transducer made of PZT4 and PMN-PT materials are calculated.Then,the transmitting voltage responses of the free-flooded ring transducers are calculated using the finite element method.Finally,the acoustic radiation characteristics of the free-flooded ring transducers are calculated using the boundary element method.The calculated results show that the resonant frequencies of the free-flooded ring transducer made of PMN-PT are greatly reduced compared with those made of PZT4 with the same size.The transmitting voltage response of the transducer made of PMN-PT is much higher than that of the transducer made of PZT4.The calculated 3-dB beamwidth of the acoustic radiated far-field directivity of the free-flooded ring transducer made of PZT4 at the resonant frequency 1900 Hz is 63.6 and that of the transducer made of PMN-PT at the resonant frequency 1000 Hz is 64.6.The comparison results show that the free-flooded ring transducer made of PMN-PT material has many advantages over that made of PZT4.The PMN-PT is a promising material for improving the performance of free-flooded ring transducers.     

Electric field effects in RUS measurements

Much of the power of the Resonant Ultrasound Spectroscopy (RUS) technique is the ability to make mechanical resonance measurements while the environment of the sample is changed. Temperature and magnetic field are important examples. Due to the common use of piezoelectric transducers near the sample, applied electric fields introduce complications, but many materials have technologically interesting responses to applied static and RF electric fields. Non-contact optical, buffered, or shielded transducers permit the application of charge and externally applied electric fields while making RUS measurements. For conducting samples, in vacuum, charging produces a small negative pressure in the volume of the material - a state rarely explored. At very high charges we influence the electron density near the surface so the propagation of surface waves and their resonances may give us a handle on the relationship of electron density to bond strength and elasticity. Our preliminary results indicate a charge sign dependent effect, but we are studying a number of possible other effects induced by charging. In dielectric materials, external electric fields influence the strain response, particularly in ferroelectrics. Experiments to study this connection at phase transformations are planned. The fact that many geological samples contain single crystal quartz suggests a possible use of the piezoelectric response to drive vibrations using applied RF fields. In polycrystals, averaging of strains in randomly oriented crystals implies using the “statistical residual” strain as the drive. The ability to excite vibrations in quartzite polycrystals and arenites is explored. We present results of experimental and theoretical approaches to electric field effects using RUS methods.     

Dual field nonlinear dielectric spectroscopy in a glass forming EPON 828 epoxy resin

Results of the dual field nonlinear dielectric spectroscopy (NDS) studies in supercooled glass forming epoxy resin EPON 828 are presented. For the NDS, changes of dielectric permittivity induced by DC (rectangular) or AC (sine-wave) pulses of a strong electric field were probed by a weak radio frequency electric field. A clear stretched exponential (x < 1) decay after switching off the DC pulse and a single exponential decay (x = 1) after switching off the AC pulse were found. The same results are presented for preliminary studies in superpressed low molecular glass former di-isobutyl phthalate. This observation may be considered as an argument for the heterogeneous picture of supercooled glass forming materials. The temperature dependences of the stationary responses related to DC and AC strong electric field excitations are also shown. The sensitivity of the applied set up made it possible to detect NDS outputs even for electric fields E(strong) < 10 kV cm(-1), qualitatively weaker than in similar 'nonlinear, dielectric' experimental studies on glass forming materials carried out so far.     

Fine grains ceramics of PIN-PT, PIN-PMN-PT and PMN-PT systems: drift of the dielectric constant under high electric field

Lead-based ferroelectric ceramics with (1-x)Pb(B1 B2)O3-xPbTiO3 formula have emerged as a group of promising materials for various applications like ultrasonic sonars or medical imaging transducers. (1-x)PMN-xPT, (1-x)PIN-xPT and ternary solutions xPIN-yPMN-zPT ceramics are synthesised using the solid state reaction method. Our objective is to achieve higher structural transition temperatures than those of PMN-PT ceramics with as good dielectric, piezoelectric and electromechanical properties. Ceramics capacitance and loss tangent are measured when the ac field of measurement increases up to E=500 V/mm. Behaviours of these materials under ac field are related to their coercive field and Curie temperature.     

Large electrostrictive strain in lead-free Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–BaTiO<Subscript>3</Subscript>–KNbO<Subscript>3</Subscript> ceramics

In the present work, (1−x)(0.935Bi_0.5Na_0.5TiO₃–0.065BaTiO₃)–xKNbO₃ (BNT–BT–KN, BNT–BT–100xKN) ceramics with x ranging from 0 to 0.1 were prepared by the conventional ceramic fabrication process. A large electrostrictive coefficient of ∼10⁻² m⁴ C⁻² is obtained with the composition x ranging from 0.02 to 0.1, which is close to the well-known electrostrictive material Pb(Mg_1/3Nb_2/3)O₃. Under an electric field of 4 kV/mm, the electrostrictive strain can reach as high as 0.08%. Besides, the electric field induced strain behavior indicates a temperature independent behavior within the temperature range of 20 to 150°C. The large electrostrictive strain is suggested to be ascribed to the formation of non-polar (NP) phase developed by the KNbO₃ substitution, and the high electrostrictive coefficient of BNT–BT–KN ceramics makes them great candidates to be applied in the new solid-state actuators.     

碳纳米管--一种新的巨电致变形纳米智能材料

纳米机电系统的发展要求智能材料具有大的应变和功率密度，以获得优良的能量转换能力．文章介绍了作者的最新发现：在电场的作用下，碳纳米管能产生巨大电致变形；而且与传统智能材料如铁电体等相比，其体积功率密度和质量功率密度预计可分别提高3个和4个量级，是一种极具潜力的新型智能材料．     

A new method for the absolute measurement of piezoelectric coefficients on thin polymer films

A new quasistatic method to measure piezoelectric coefficients on thin polymer films is presented. This method is based on a combined experimental/analytical approach, where small polymer samples (6 mm x 3 mm x 110 microm) are encapsulated in a soft silicone rubber and an electric field is applied across their thickness (3-direction). Strains are measured optically along three perpendicular directions using a laser Doppler vibrometer, and the experimental measurements are used in a Rayleigh-Ritz energy minimization procedure implemented symbolically in MATHCAD, which yields the absolute piezoelectric coefficients d(3ii). These measured coefficients are material properties of the polymer and do not depend on the specific boundary conditions of the problem. The validity of the method is established using the ATILA finite element code. Experimental values of d(311), d(322), and d(333) obtained for polyvinylidene fluoride (PVDF) at room temperature, in the frequency range 500-2000 Hz, are presented and compared with existing data; excellent agreement is found. The extension of the method to the determination of electrostrictive coefficients on soft polyurethane materials is introduced.     

Development of a technique for measuring static electricity distribution using focused ultrasound waves and an induced electric field

A novel method is proposed for non-contact measurement of static electricity distribution on a surface using focused ultrasound to excite movement of the charge. The focused ultrasound is generated by controlling individually the phases of 285 airborne ultrasound transducers, and it was demonstrated local excitation could be measured. An electric field is induced by local excitation of a charged object. The electric field intensity and phase are related to the surface potential and electrical polarity of the object, respectively. It is possible to measure static electricity distribution over an entire object surface by scanning the position of the focused ultrasound.     

The application of high permittivity piezoelectric ceramics to 2D array transducers for medical imaging

Two-dimensional (2D) array transducers have become of great interest in the last few years, in view of real-time volumetric ultrasonic imaging. The electrical matching between the high electrical impedance of elements and the standard cables and electronics is one of the key issues in 2D array design. The use of high-permittivity ceramics such as PNNZT either in bulk configuration or in 1-3 piezocomposites decreases the electrical impedance. In this paper, bulk samples of PNNZT and PZT ceramics are characterised, and results are compared. 2D array elements are then manufactured and their electrical impedances measured. Theoretical predictions of homogenisation models for 1-3 piezocomposites allow the simulation of the electroacoustic behaviour of 2D array elements. Results for both piezocomposite and bulk materials can be obtained. Calculations of the input impedance, the sensitivity and the bandwidth of the different configurations are compared and discussed. These results demonstrate the advantages of the PNNZT compositions over standard PZT.     

Analysis of temperature effect in dielectric response of electrostrictive polymers for pseudo-pyroelectric operations

Thanks to their flexibility and easy processability, electroactive polymers have gained a lot of attention over the last decades. More specifically, dielectric electrostrictive polymers have been demonstrated to provide interesting ways for mechanical actuation and energy harvesting or for electrocaloric applications. This Letter aims at presenting an additional application potential of such materials, showing their ability in terms of converting thermal energy into electrical energy. More particularly, it is shown that such materials, once polarized through the application of a bias electric field, allow a polarization variation with the temperature, yielding the so-called pseudo-pyroelectric effect. Theoretical analysis, supported by experiments, therefore demonstrates that such a material can exhibit pseudo-pyroelectric activity that can be tuned with the applied electric field.     

Characterization of multilayered piezoelectric ceramics for high power transducers

In some circumstances, large vibrational displacements at ultrasonic frequency must be generated using a low voltage drive. This result cannot be obtained with monolithic PZT ceramics which require voltages larger than 1000 V to produce displacements of the micrometer order at resonance. The use of multilayered hard lead zirconate titanate ceramics as transduction material in resonant devices is experimentally investigated for Langevin-type transducers. Large amplitudes are obtained under low drive (5 microm under 10 V). Material constant (compliance, losses) variations under large dynamic stress are, at least, one order of magnitude larger than for monolithic ceramics. Depolarization is found to be a critical issue when the transducer is driven continuously. It is demonstrated that this problem can be solved by polishing the interfaces between different parts of the device and applying an electrical DC bias to the transducer.     

Ultrasound generation with high power and coil only EMAT concepts

Electro-magnetic acoustic transducers (EMATs) are intended as non-contact and non-destructive ultrasound transducers for metallic material. The transmitted intensities from EMATS are modest, particularly at notable lift off distances. Some time ago a concept for a “coil only EMAT” was presented, without static magnetic field. In this contribution, such compact “coil only EMATs” with effective areas of 1–5 cm² were driven to excessive power levels at MHz frequencies, using pulsed power technologies. RF induction currents of 10 kA and tens of Megawatts are applied. With increasing power the electroacoustic conversion efficiency also increases. The total effect is of second order or quadratic, therefore non-linear and progressive, and yields strong ultrasound signals up to kW/cm² at MHz frequencies in the metal. Even at considerable lift off distances (cm) the ultrasound can be readily detected. Test materials are aluminum, ferromagnetic steel and stainless steel (non-ferromagnetic). Thereby, most metal types are represented. The technique is compared experimentally with other non-contact methods: laser pulse induced ultrasound and spark induced ultrasound, both damaging to the test object’s surface. At small lift off distances, the intensity from this EMAT concept clearly outperforms the laser pulses or heavy spark impacts.     

Fabrication and comparison of PMN-PT single crystal, PZT and PZT-based 1-3 composite ultrasonic transducers for NDE applications

This paper describes fabrication and comparison of PMN-PT single crystal, PZT, and PZT-based 1-3 composite ultrasonic transducers for NDE applications. As a front matching layer between test material (Austenite stainless steel, SUS316) and piezoelectric materials, alumina ceramics was selected. The appropriate acoustic impedance of the backing materials for each transducer was determined based on the results of KLM model simulation. Prototype ultrasonic transducers with the center frequencies of approximately 2.25 and 5 MHz for contact measurement were fabricated and compared to each other. The PMN-PT single crystal ultrasonic transducer shows considerably improved performance in sensitivity over the PZT and PZT-based 1-3 composite ultrasonic transducers.