Design,production and testing of PMN-PT electrostrictive transducers

Lead magnesium niobate ceramics (PMN) are promising materials for application in the field of high power transducers. The advantage of PMN materials are the large strains generated under moderate electric field and the low hysteresis. The electrostrictive effect is non-linear, the corresponding physical constants depend on temperature and frequency and a DC electrical bias is required. These difficulties must be considered at the design stage. A finite element model has been developed and validated in the ATILA code for non-linear static and time-domain analyses. These numerical modelings are used to design and test two Langevin-type electrostrictive transducers. The first transducer is made of PMN-PT-La (90-10-1%) ceramics (TRS Ceramics), the second one of ESCI ceramics (Morgan Matroc). For given static mechanical prestresses, resonance frequencies and effective coupling coefficients are measured at different DC electric fields and temperatures.     

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.     

Magnetization and magnetic excitations in HoAl2

The magnetization of HoAl₂ single crystals parallel and perpendicular to the applied field for three crystallographic directions has been measured between 4.2 and 300 K in fields up to 8 T. Further the spin wave excitations were investigated by inelastic neutron scattering at 5.2 K along <100>, <110> and <111> directions in reciprocal space. The results are interpreted consistently in terms of a cubic crystal field using a simple molecular field to calculate the magnetization and an isotropic Heisenberg exchange interaction to obtain the magnetic excitations.     

弛豫铁电单晶的研究进展—压电效应的起源研究

由于具有优异的压电性能,弛豫铁电单晶自上世纪90年代问世以来即成为了铁电压电领域研究的热点材料,并被认为是研发下一代高性能换能器、传感器等器件的重要压电材料。弛豫铁电单晶不但压电常数可达2500 pC/N,约为软性Pb(Zr,Ti)O3(PZT)陶瓷的5倍,而且其电致应变滞后也远小于软性PZT陶瓷。因此,弛豫铁电单晶高压电性能的产生机理一直是铁电压电领域的研究热点。本文主要介绍了弛豫铁电单晶材料在近些年的发展,从本征压电效应(晶格压电畸变)的角度归纳总结了弛豫铁电单晶高压电效应的产生机理,着重探讨了弛豫铁电单晶的重要特点—剪切压电效应。在本征效应的基础上,本文对弛豫铁电单晶压电效应与晶体组分、切向以及温度的关系进行了分析。需要指出的是,目前基于本征角度对弛豫铁电单晶高压电效应的分析仍处于定性的阶段,因而还不能完全排除一些可能导致弛豫铁电单晶高压电效应的非本征物理机制。     

Trap ionization in phosphors in a strong electric field

Temperature dependences of the intensity of Gudden-Pohl flashes in zinc sulfide crystals in various electric fields were measured in order to study trap ionization by electric fields. The decrease in the flash intensity from trap levels of a given depth in a certain temperature range is due to a decrease in the probability for the liberation of an electron from a trap as the temperature decreases. The temperature dependences of the flash intensity found in various electric fields are compared with the theoretical temperature dependence of the probability for electron liberation from a trap. The agreement between experimental and theoretical curves indicates that electrons are liberated from the deeper levels through ionization by an electric field involving many phonons.Translated from Izvestiya VUZ. Fizika, No. 10, pp. 53–58, October, 1969.     

Thermo-mechanical stress effect on 1-3 piezocomposite power transducer performance

This paper deals with the emission performance of 1-3 piezoelectric composite power transducers made with a hard PZT (Navy III) and epoxy resins with a high glass-rubber transition temperature. Following the "dice and fill" technique, various composite transducers with 30 and 50% PZT volume fractions were fabricated with an air backing and no front matching layer with resonance operating frequencies around 500 kHz. The transducers were first evaluated under isothermal conditions, with a low emission duty cycle. Efficiencies as high as 95% were monitored as a function of the instantaneous input power up to a 60 W/cm(2) density. The effect of the polymer matrix mechanical losses and the fabrication conditions is then discussed. For the transducer thermal stability, the case of long duty cycle or continuous emission was considered in a second evaluation. In this case the transducer working temperature and axial radiated pressure were monitored as functions of the input power density up to 40 W/cm(2). It is shown that the transducer efficiency and working temperature were strongly dependent on the type of resin used but also on the PZT material, even for hard PZT compositions. A composite transducer configuration with strongly improved thermal stability was investigated demonstrating a working temperature higher than 90 degrees C and an extended power range (30-40 W/cm(2)). The composite thermal breakdown mechanism was analyzed and the effect of the curing-induced thermo-mechanical stresses on the PZT mechanical losses was considered in relation to the composite working temperature. Measurements of the composite mechanical losses versus the temperature were obtained and related to the variation of the PZT mechanical losses with the stresses due to the composite transducer temperature change. It is found that the thermally induced stress can strongly influence the PZT ceramic mechanical losses and that it can be the reason for a thermal breakdown taking place at a temperature much lower than the epoxy resin transition.     

聚酰亚胺电导率随温度和电场强度的变化规律

介质深层充电对航天器安全运行构成了重大威胁.以聚酰亚胺为代表的此类聚合物绝缘介质的电导率受温度影响显著,又因为充电过程中局部产生强电场(10~7V/m量级),因此,其电导率模型需要综合考虑温度和强电场的影响,这对介质深层充电的仿真评估意义重大.已有的两类模型,不是低温区间不适用,就是没有充分考虑强电场的影响.基于跳跃电导理论,本文分析对比了现有电导率模型,提出了适用于较宽温度范围且合理考虑强电场增强效应的电导率新模型,并采用某型聚酰亚胺电导率测试数据做出验证.此外,为了提高新模型在强电场下的低温适用范围,尝试对强电场因子中的温度做变换,取得了满意的效果.参数敏感度分析表明新模型在电导率拟合与外推方面具有参数少、适用性强的优势.     

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.     

Field-induced piezoelectric response in Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

The electric field induced strain, piezoelectric and electromechanical response in relaxor based single crystal piezoelectrics (1−x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃(PMNT100x) were investigated as a function of dc bias and temperature along the crystallographic [001] orientation. With increasing temperature and dc bias, monoclinic and rhombohedral phases were found to coexist in the crystals, exhibiting large strain hysteresis, while above a threshold temperature and dc bias, the induced monoclinic and tetragonal phases were found to show very low strain hysteresis. The electric field level for the monoclinic to tetragonal phase transformation was found to exponentially decay with temperature. The mechanical quality factor Q for the tetragonal phase exhibits higher values when compared to rhombohedral and monoclinic phases, displaying improved domain stability. The domain stability of tetragonal crystals as a function of sample thickness was also studied, it was found that a higher dc bias was needed for thinner tetragonal crystals in order to stabilize the single domain configuration and reduce the strain hysteresis.     

Dielectric properties of betaine phosphite films

Polycrystalline films of betaine phosphite are grown on single-crystal quartz and lithium niobate substrates atop interdigitated transducers. It is revealed using a light-polarizing microscope in the reflection mode that the films are made up of large (up to 1 mm in size) single-crystal blocks. The temperature dependences of the film capacitance measured at frequencies from 120 Hz to 1 MHz pass through a strong maximum at the ferroelectric phase transition point. The effect of a bias on the permittivity of the films is investigated. It is shown that the differences in the temperature dependences of the permittivity of single crystals and films originate from the influence of dynamic strains in the substrate on the film and from the presence of an effective depolarizing field. Original Russian Text ? E.V. Balashova, B.B. Krichevtsov, V.V. Lemanov, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 525–532.     

Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.     

Electret states and the phase transition in a surface layer of the TlGaSe2 ferroelectric semiconductor

The electret polarization is investigated in the TlGaSe₂ ferroelectric semiconductor. It is proved for the first time that stable internal electric fields associated with residual electret polarization are induced in crystals of the TlGaSe₂ ferroelectric semiconductor at temperatures T < 200 K. It is experimentally established that the peak of the pyroelectric current measured in the vicinity of the phase transition to the ferroelectric polar phase depends substantially on the temperature at which the external electric field is switched off when the TlGaSe₂ ferroelectric crystal under investigation is preliminarily cooled from room temperature. The results obtained are discussed in the framework of a model according to which internal electret fields are induced by charges localized at different levels in the bulk and on the surface of the TlGaSe₂ ferroelectric crystal. These fields drastically change at temperatures in a narrow range near 135 K. The inference is made that a phase transition occurs in the surface layer of the TlGaSe₂ crystal at a temperature close to ～135 K.     

Molecular theoretic study of Freedericksz transition. Symmetry breaking of oblique axial order

Freedericksz transition, which is usually analyzed by an elastic theory, is studied on the basis of statistical mechanical ground, where nematics with positive dielectric anisotropy in homogeneous anchoring cell is exposed to an electric field in the direction of wall normal. In low temperature region, an oblique axial symmetry breaking occurs, which is nothing but the Freedericksz transition. In high temperature and high field region, biaxial nematic phase with principal axis parallel to the field direction at interior area of the system is proved to appear. A phase diagram on the field versus temperature plane is obtained and compared with the one at a bulk with common biaxial symmetry, where both of electric and magnetic fields are applied in directions perpendicular to each other. In the latter, no symmetry breaking occurs, in contrast with the former case above-mentioned, and the reason why this difference occurs is elucidated.     

Variation of the soft-mode frequency in ferroelectric semiconductors in a static electric field

The action of a static electric field on the soft-mode frequency in narrow-gap ferroelectric semiconductors, as a result of the contribution of the electron subsystem, is investigated within the framework of the vibronic mechanism. It is shown that the frequency of the soft mode varies directly as the square of the electric field strength under the action of the field, and the soft-mode frequency is determined in the vibronic model by the electron distribution function, whose parameters also vary under the action of the electric field. The effect is stronger for crystals with a one-dimensional electron spectrum than for those with an isotropic three-dimensional spectrum; according to estimates for ferroelectric semiconductors of the type Pb_1–xGe_xTe, a frequency variation of the order of the soft-mode frequency itself is attained in fields of the order of tens of volts per centimeter.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 33–37, January, 1988.     

利用压电力显微镜研究PMN-30%PT单晶体中铁电畴的结构及其演变

本文总结了我们近年米利用压电力显微镜(PFM)研究PMN-30%PT单晶体中铁电畴的结构及其演变的结果.选择PMN-30%PT品体是因为该组分在超声传感器等应用方面具有最大的潜力.铁电畴的观察是基于反压电现象;具体来讲就是当交变电场通过原子力显微镜探针加到晶体表面时,会引起品体表面的起伏振荡,而锁相放大器可以解出该振荡信号的振幅和相位角;其中振幅衬度反映了压电系数d33的大小,而相位衬度则反映了铁电畴的极化方向.文中介绍了平面内以及垂直平面的PFM成像技术,并演示了影响畴的图像的一些因素,其中包括静电倚效应,表层效应和机械抛光的影响.本文还利用有限无模型对PFM成像原理进行了模拟分析.着重研究了晶体中铁电畴的尺寸分布,畴与晶体取向,时间和温度的相关性,以及畴的演变过程.     

Thermal behavior of acousto-optic devices: effects of ultrasound absorption and transducer losses

In the present paper we analyze the electric and acoustic losses in acousto-optic devices, especially in their ultrasonic transducers and the related thermal effects. We include electric and acoustic losses into the classical electric equivalent model of the transducer, to explain the characteristics of the measured electric and thermal behavior. Measured temperature distributions on the acousto-optic crystal faces serve visualization of the conversion efficiency of the radio-frequency input to bulk acoustic waves. We show that the pronounced acoustic frequency dependence of the temperature distribution is in correlation with the frequency dependent losses in the transducer and in the bulk. We also demonstrate experimentally the effectiveness of our active and passive heat removing and temperature stabilization methods.     

Effect of carbon substitution on low magnetic field AC losses in MgB2 single crystals

The DC magnetization and AC magnetic susceptibilities were measured for MgB₂ single crystals, unsubstituted and carbon substituted with the composition of Mg(B_0.94C_0.06)₂. AC magnetic losses were derived from the AC susceptibility data as a function of the AC amplitude and the DC bias magnetic field. From the DC magnetization loops critical current densities were derived as a function of temperature and DC field. Results show that the substitution with carbon decreases critical current densities at low external magnetic fields, in contrast to the well known effect of an increase of the critical current densities at higher magnetic fields.     

利用压电力显微镜研究PMN-30%PT单晶体中铁电畴的结构及其演变(英文)

本文总结了我们近年来利用压电力显微镜(PFM)研究PMN 30%PT单晶体中铁电畴的结构及其演变的结果。选择PMN-30%PT晶体是因为该组分在超声传感器等应用方面具有最大的潜力。铁电畴的观察是基于反压电现象;具体来讲就是当交变电场通过原子力显微镜探针加到晶体表面时,会引起晶体表面的起伏振荡,而锁相放大器可以解出该振荡信号的振幅和相位角;其中振幅衬度反映了压电系数d_(33)的大小,而相位衬度则反映了铁电畴的极化方向。文中介绍了平面内以及垂直平面的PFM成像技术,并演示了影响畴的图像的一些因素,其中包括静电荷效应,表层效应和机械抛光的影响。本文还利用有限元模型对PFM成像原理进行了模拟分析。着重研究了晶体中铁电畴的尺寸分布,畴与晶体取向,时间和温度的相关性,以及畴的演变过程。     

Non-ohmic properties and electric breakdown of quasi one-dimensional organic conductors

The dependence of the d.c. conductivity of single crystals of TTF-TCNQ on the electric field strength has been investigated. At 4.2 K drastic deviations from Ohm's law are observed. At an electric field strength of 300–600 V/cm, dependent on the room temperature conductivity, a reversible breakdown occurs connected with a rise of the conductivity by about three orders of magnitude. Similar effects have been found in single crystals of (TTF)₇J₅ and (TTF)J₂ in the temperature range around 100 K. Possible mechanisms responsible for this large conductivity changes are discussed.     

Tomonaga-Luttinger liquid in a quasi-one-dimensional S = 1 antiferromagnet observed by specific heat measurements

Specific-heat experiments on single crystals of the S = 1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9H24N4)(NO2)ClO2 (NTENP) have been performed in magnetic fields applied both parallel and perpendicular to the spin chains. We have found for the parallel field configuration that the magnetic specific heat (C(mag)) is proportional to temperature (T) above a critical field H(c), at which the energy gap vanishes, in a temperature region above that of the long-range ordered state. The ratio C(mag)/T increases as the magnetic field approaches H(c) from above. The data are in good quantitative agreement with the prediction of the c= 1 conformal field theory in conjunction with the velocity of the excitations calculated by a numerical diagonalization, providing conclusive evidence for a Tomonaga-Luttinger liquid.