Influence of nonparallelism of the surfaces of a piezoelectric layer on the frequency band of an electroacoustic piezoelectric transducer

The influence of nonparallelism of the surfaces of a piezoelectric transducer on its impedance and frequency band is analyzed theoretically. The model of a wedge-shaped piezoelectric layer taken for the analysis consists of a set of n plane-parallel elements arranged in succession along the length of the transducer with a gradual increase in the thickness of the piezoelectric layer. The analysis performed demonstrates the possibility of significantly expanding the band of working frequencies of a piezoelectric transducer when its electroacoustic conversion efficiency is reduced. In some cases, however, a decrease in conversion efficiency can be employed to optimize an acoustic device, for example, in creating a filter-type piezoelectric transducer, where electromagnetic energy is converted into acoustic energy successively from one cell of the filter to another. The proposed method for expanding the frequency band can be useful, for example, in creating high-frequency acousto-optic Bragg cells. Zh. Tekh. Fiz. 69, 72–73 (March 1999)     

声光器件的换能器带宽和镀层厚度的确定

根据压电换能器的Mason等效电路,就36°у切LN/PM声光器件和x切LN/TeO₂声光器件以及各种镀层材料和厚度计算了换能器损耗TL随规一化频率f/f₀的变化关系,由此可确定各镀层的厚度和器件的换能器带宽,还给出器件的电输入阻抗z_i在阻抗圆图上随f/f₀变化的轨迹,它可作为器件性能测试的理论依据。 关键词：     

基于2-2型压电复合材料的新型宽频带径向振动超声换能器

本文提出了一种基于2-2型压电复合材料的新型宽频带径向振动超声换能器,它主要由内金属圆环和外压电陶瓷复合材料圆环组成.首先利用Newnham串并联理论和均匀场理论推导了2-2型压电复合材料的等效参数;其次利用解析法得到了金属圆环和径向极化压电复合陶瓷圆环径向振动的机电等效电路;最后得到了换能器的六端机电等效电路,从而得到了换能器的频率方程.接着分析了换能器共振频率和反共振频率以及有效机电耦合系数与几何尺寸、两相体积占比的关系,采用仿真软件对新型换能器的径向振动进行了数值模拟.结果表明,利用解析法得到的共振频率和反共振频率与数值模拟结果吻合较好.此外,对换能器在水下的辐射声场进行了仿真研究,结果表明新型复合材料径向换能器相比传统纯陶瓷径向换能器,发射电压响应幅值更大,工作带宽提高接近一倍,声匹配更佳.     

压电陶瓷复合换能器固有频率的有限差分法*

为丰富换能器固有频率的研究理论，以及提供一种新的计算方法供工程人员选择，提出了计算其固有频率的有限差分法。以由径向极化的压电陶瓷圆管与金属预应力管沿径向复合而成的二元压电陶瓷复合换能器为例，建立并推导了其向振动的数学模型及其有限差分形式，给出了换能器径向振动的特征方程。利用MTALAB对计算实例的径向振动的固有频率进行编程计算，理论计算结果与已有实验结果符合很好，验证了有限差分法计算压电陶瓷复合换能器固有频率的可行性及准确性。通过仿真计算，给出了换能器径向振动固有频率与其结构尺寸的影响关系：换能器径向振动的固有频率随压电陶瓷圆管内径的增大而降低，随换能器壁厚比的增大而降低。该文所建立的换能器径向振动固有频率的有限差分法同样适用于结构形式相近的换能器及其他元器件。     

径向极化压电陶瓷长圆管复合超声换能器的径向振动

提出了一种圆管式径向复合压电陶瓷换能器,并对其径向振动特性进行了分析。该换能器由径向极化的压电陶瓷圆管以及金属外圆管组成。利用解析法得出了金属圆管以及具有任意壁厚的径向极化压电陶瓷圆管径向振动的机电等效电路。基于金属圆管与压电圆管的机械边界条件,得出了换能器的六端机电等效电路。在此基础上得出了换能器共振及反共振频率方程的解析表达式,给出了换能器的共振及反共振频率与其几何尺寸之间的依赖关系。利用数值方法对换能器的径向振动特性进行了模拟及仿真,并与解析结果进行了比较。最后,设计并加工了一些径向复合管式压电陶瓷换能器,利用精密阻抗分析仪对其共振及反共振频率进行了实验测试。研究结果表明,利用解析理论得出的换能器共振及反共振频率与数值模拟结果以及实验测试结果符合很好。      

Piezoelectric atomization of liquids with dynamic viscosities up to 175 cP at room temperature

Piezoelectric atomization has been applied in the field of respiratory medicine delivery and chemistry. However, the wider application of this technique is limited by the viscosity of the liquid. High-viscosity liquid atomization has great potential for applications in aerospace, medicine, solid-state batteries and engines, but the actual development of atomization is behind expectations. In this study, instead of the traditional model of single-dimensional vibration as a power supply, we propose a novel atomization mechanism that uses two coupled vibrations to induce micro-amplitude elliptical motion of the particles on the surface of the liquid carrier, which produces a similar effect as localized traveling waves to push the liquid forward and induce cavitation to achieve atomization. To achieve this, a flow tube internal cavitation atomizer (FTICA) consisting of a vibration source, a connecting block and a liquid carrier is designed. The prototype can atomize liquids with dynamic viscosities up to 175 cP at room temperature with a driving frequency of 507 kHz and a voltage of 85 V. The maximum atomization rate in the experiment is 56.35 mg/min, and the average atomized particle diameter is 10 µm. Vibration models for the three parts of the proposed FTICA are established, and the vibration characteristics and atomization mechanism of the prototype were verified using the vibration displacement measurement experiment and the spectroscopic experiment. This study offers new possibilities for transpulmonary inhalation therapy, engine fuel supply, solid-state battery processing and other areas where high-viscosity microparticle atomization is needed.     

Theoretical analysis of a quartz-enhanced photoacoustic spectroscopy sensor

Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors are based on a recent approach to photoacoustic detection which employs a quartz tuning fork as an acoustic transducer. These sensors enable detection of trace gases for air quality monitoring, industrial process control, and medical diagnostics. To detect a trace gas, modulated laser radiation is directed between the tines of a tuning fork. The optical energy absorbed by the gas results in a periodic thermal expansion which gives rise to a weak acoustic pressure wave. This pressure wave excites a resonant vibration of the tuning fork thereby generating an electrical signal via the piezoelectric effect. This paper describes a theoretical model of a QEPAS sensor. By deriving analytical solutions for the partial differential equations in the model, we obtain a formula for the piezoelectric current in terms of the optical, mechanical, and electrical parameters of the system. We use the model to calculate the optimal position of the laser beam with respect to the tuning fork and the phase of the piezoelectric current. We also show that a QEPAS transducer with a particular 32.8 kHz tuning fork is 2–3 times as sensitive as one with a 4.25 kHz tuning fork. These simulation results closely match experimental data.     

A Lotus shaped acoustofluidic mixer: High throughput homogenisation of liquids in 2 ms using hydrodynamically coupled resonators

This paper presents an acoustically actuated microfluidic mixer that uses an array of hydrodynamically coupled resonators to rapidly homogenise liquid solutions and synthesise nanoparticles. The system relies on 8 identical oscillating cantilevers that are equally spaced on the perimeter of a circular well, through which the liquid solutions are introduced. When an oscillatory electrical signal is applied to a piezoelectric transducer attached to the device, the cantilevers start resonating. Due to the close proximity between the cantilevers, their circular arrangement and the liquid medium in which they are immersed, the vibration of each cantilever affects the response of its neighbours. The streaming fields and shearing rates resulting from the oscillating structures were characterised. It was shown that the system can be used to effectively mix fluids at flow rates up to 1400 µl.min⁻¹ in time scales as low as 2 ms. The rapid mixing time is especially advantageous for nanoparticle synthesis, which is demonstrated by synthesising Poly lactide-co-glycolic acid (PLGA) nanoparticles with 52.2 nm size and PDI of 0.44.     

Dynamic modeling of thickness-mode piezoelectric transducer using the block diagram approach

This paper aims to provide an alternative method to determine the characteristics of a piezoelectric transducer from measurement. A block diagram approach is proposed to analyze the dynamic characteristics of a thickness-mode piezoelectric transducer at its resonance frequency. Based on the feedback loop framework, the input-output relations of the electromechanical interaction of the transducer are described in terms of linear block diagram models. Furthermore, the closed-loop relations from external force to vibration velocity and electric current from generated voltage are easily found by Mason’s rule to characterize the equivalent mechanical admittance and electrical impedance, respectively. An example of a Langevin transducer with 28.15 kHz resonance frequency is illustrated for dynamics analysis. The frequency responses of the piezoelectric transducer, resulting from a force and current input, are respectively measured to identify the system parameters of the feedback model. The experimental results demonstrate the effectiveness of the proposed method.     

Development of human IgE biosensor using Sezawa-mode SAW devices

This paper reports Sezawa-mode surface acoustic wave (SAW) devices with via-isolated cavity to construct the allergy biosensor. To fabricate Sezawa-mode SAW devices, the RF magnetron sputtering method for the growth of piezoelectric ZnO thin films are adopted and influences of the sputtering parameters are investigated. The optimal substrate temperature of 300 °C, RF power of 120 W and sputtering pressure of 2 Pa were used to deposit piezoelectric ZnO films with a smooth surface, uniform grain size and strongly c-axis-orientated crystallization. A back-etched SAW resonator is used in this study. The wet etching of (100)-oriented silicon wafers is used to form a back-side cavity which is critical to the formation of a hopper cavity for holding bio-analytes. The remaining membrane structure silicon thickness was 25 μm. In this report, the chrome (Cr, 12 nm)/gold (Au, 66 nm) layer was initially deposited onto the sensing area of SAW devices as the binding layer for biochemical sensor. The resonance frequency of the Sezawa-mode SAW device is 1.497 GHz. The maximum sensitivity of the Sezawa-mode is calculated to be 4.44 × 10⁶ cm²/g for human immunoglobulin-E (IgE) detection. The stability for human IgE detection is calculated to be 80% and the variation of the stability ±3% was obtained after several tests.     

压电换能器等效模型分析与阻抗匹配设计

声光可调滤光器(Acousto-optic tunable filter,AOTF)的衍射效率不仅与功率信号源的质量、声光晶体氧化碲(TeO_2)的切型有关,而且压电超声换能器的结构以及其阻抗匹配网络也对AOTF的衍射效率有很大影响。本文对X切型铌酸锂(LiNbO_3,LN)四层镀膜压电超声换能器的等效模型进行了分析,确定了声光介质存在时换能器的阻抗特性,利用射频微波仿真软件,设计了电感-电容复合匹配网络。将换能器压合在TeO_2上,用复色光作为光源进行了声光衍射实验,经仿真与实验验证表明,该匹配电路可以有效的改善压电换能器的阻抗特性,提高换能器带宽,提高能量传输效率,AOTF衍射效率最高可达92.67%。     

The use of acoustic radiation forces to position particles within fluid droplets

Handling of micrometer sizes particles, such as biological cells or coated beads, plays a relevant role in the field of life science. A number of devices have been presented in the last years, in which acoustic forces generated by coupling the vibration of a solid structure excited by a piezoelectric transducer to the particle suspension are used to collect particles in lines or position them in clumps on a grid. Following the trend of lab-on-a-chip devices, efforts have been made to shrink the size of such systems, aiming at less reagent consumption and shorter reaction times. The majority of these systems consist of closed fluid filled volumes, typically channels. Here the use of an open fluid volume, a droplet, is examined. By exciting resonances into the droplet positioned on a surface, particles can be gathered into a line, two parallel lines or, as the frequency of excitation is increased, into more complex patterns. Such a concentration process will have useful applications in improved detection sensitivity of low concentration particulate solutions.     

Study of the sandwiched piezoelectric ultrasonic torsional transducer

The sandwiched piezoelectric ultrasonic torsional transducer was studied. The transducer consists of front and back metal cylinders, and coaxially segmented, tangentially polarized, piezoelectric ceramic tubes. The torsional vibration of the tangentially polarized piezoelectric ceramic slender tube was studied first and its electromechanical equivalent circuit was derived. Based on the network theory and the electromechanical equivalent circuit, the torsional vibration of the piezoelectric ceramic cylinder, formed by stacking a number of identical short piezoelectric ceramic rings, was analysed and the electromechanical equivalent circuit of the piezoelectric ceramic stack in torsional vibration was developed. Finally, the sandwiched ultrasonic torsional transducer was studied and resonance frequency equations were derived which can be used to design and calculate the torsional transducers for different applications.     

Microfeeding with different ultrasonic nozzle designs

Microfeeding of dry powder excited by ultrasonic vibration makes use of relatively simple equipment and can be applied to solid freeforming and pharmaceutical dosing. The nozzle was a vertical glass capillary and four configurations for ultrasonic actuation were investigated: Type I had a piezoelectric transducer ring bonded to the base of a cylindrical water-containing vessel containing an axial nozzle; Type II had a piezoelectric transducer ring attached to the sidewall of the vessel; Type III used direct mechanical connection to the glass wall of the capillary to give nominally longitudinal vibration; and Type IV also used direct connection to the glass tube but arranged to give progressive wave vibration. The experimental results show that all four configurations realized powder microfeeding and dosing but the characteristics, in terms of minimum flow rate, dependence on voltage amplitude and uniformity of dose varied considerably. The discharge of particles was observed by a high-speed camera.     

Equivalent circuit of the non-uniform piezoelectric transducer

An equivalent circuit of the piezoelectric transducer with the step-by-step non-uniformity of piezoelectric properties is obtained by means of solving an inhomogenous wave equation by the method of Green's functions. Synthesis assumes that the transducer is governed by pure thickness vibrations. It is shown that the circuit is valid also for the transducer with piezoelectric properties continually varying from point to point. As an example of applying the equivalent circuit the maximum efficiency of electroacoustical transformation for a non-resonant transducer has been defined.     

Effects of driving frequency of longitudinal transducer on the vibration characteristics of a stepped plate

The flexural vibration characteristics of a stepped plate, driven at its center by different frequency of longitudinal transducer with a certain area are investigated. The variation in the nodal circle, fundamental frequency and displacement distribution of the stepped plate are calculated by using finite element method (FEM) under different driving frequencies. The results show that the fundamental frequency and nodal circle of the flexural-vibration stepped plate (FVSP) increase with an increase in the driving frequency of the longitudinal vibration ultrasonic transducer (LVUT), before the second-order flexural vibration occurs. When the driving frequency is f = 28 kHz, the displacement amplitude of the stepped plate can achieve the maximum, and the nodal circle radius of the stepped plate is 2.61 cm which fits evenly the edge of stepped profile. Meanwhile, the directivity and radiation efficiency of the FVSP would be greatly improved in a special driving frequency. The conclusions agree with the experimental ones and are significant for both design and applications of the stepped plate.     

Autoresonant control of nonlinear mode in ultrasonic transducer for machining applications

Experiments conducted in several countries have shown that the improvement of machining quality can be promoted through conversion of the cutting process into one involving controllable high-frequency vibration at the cutting zone. This is achieved through the generation and maintenance of ultrasonic vibration of the cutting tool to alter the fracture process of work-piece material cutting to one in which loading of the materials at the tool tip is incremental, repetitive and controlled. It was shown that excitation of the high-frequency vibro-impact mode of the tool-workpiece interaction is the most effective way of ultrasonic influence on the dynamic characteristics of machining. The exploitation of this nonlinear mode needs a new method of adaptive control for excitation and stabilisation of ultrasonic vibration known as autoresonance. An approach has been developed to design an autoresonant ultrasonic cutting unit as an oscillating system with an intelligent electronic feedback controlling self-excitation in the entire mechatronic system. The feedback produces the exciting force by means of transformation and amplification of the motion signal. This allows realisation for robust control of fine resonant tuning to bring the nonlinear high Q-factor systems into technological application. The autoresonant control provides the possibility of self-tuning and self-adaptation mechanisms for the system to keep the nonlinear resonant mode of oscillation under unpredictable variation of load, structure and parameters. This allows simple regulation of intensity of the process whilst keeping maximum efficiency at all times. An autoresonant system with supervisory computer control was developed, tested and used for the control of the piezoelectric transducer during ultrasonically assisted cutting. The system has been developed as combined analog-digital, where analog devices process the control signal, and parameters of the devices are controlled digitally by computer. The system was applied for advanced machining of aviation materials.     

DYNAMICAL ANALYSIS OF A CYLINDRICAL PIEZOELECTRIC TRANSDUCER

In the present paper, the vibration of a cylindrical piezoelectric transducer induced by applied voltage, which can be used as the stator transducer of a cylindrical micromotor, is studied based on shell theory. The transducer is modelled as a thin elastic cylinder. The properties of the vibration modes of the transducer, such as mode frequencies and amplitude ratios of the mode shapes, are determined following Galerkin method. The response of the transducer under the four electric sources with 90° phase difference is then obtained by the modal summation method. With the results, the performance of the transducer under the electric sources can be estimated. The present work provides a general and precise theoretical modelling on the dynamical movement of the transducer.     

Manipulation of micrometer sized particles within a micromachined fluidic device to form two-dimensional patterns using ultrasound

Ultrasonic manipulation, which uses acoustic radiation forces, is a contactless manipulation technique. It allows the simultaneous handling of single or numerous particles (e.g., copolymer beads, biological cells) suspended in a fluid, without the need for prior localization. Here it is reported on a method for two-dimensional arraying based on the superposition of two in-plane orthogonally oriented standing pressure waves. A device has been built and the experimental results have been compared with a qualitative analytical model. A single piezoelectric transducer is used to excite the structure to vibration, which consists of a square chamber etched in silicon sealed with a glass plate. A set of orthogonally aligned electrodes have been defined on one surface of the piezoelectric. This allows either a quasi-one-dimensional standing pressure field to be excited in one of two directions or if both electrodes are activated simultaneously a two-dimensional pressure field to be generated. Two different operational modes are presented: two signals identical in amplitude and frequency were used to trap particles in oval shaped clumps; two signals with slightly different frequencies to trap particles in circular clumps. The transition between the two operational modes is also investigated.     

阶梯圆环压电超声换能器径向振动性能的理论研究*

提出了一种阶梯圆环径向振动压电超声换能器,根据力电类比原理建立了阶梯圆环及阶梯圆环换能器的径向振动等效电路,推导了阶梯圆环的径向共振频率方程和位移放大系数,在此基础上进一步推导了换能器的径向共振和反共振频率方程。通过理论推导和有限元仿真模拟分析了阶梯圆环压电超声换能器的径向振动性能。结果表明,增大阶梯圆环中内外环的径向厚度之比K1或减小轴向厚度之比K2,阶梯圆环的一阶径向共振频率减小,二阶径向共振频率增大;在二阶径向共振模式下,K1、K2值在一定范围内阶梯圆环可实现由内向外的径向位移振幅放大;随着压电陶瓷圆环的内半径增大,阶梯圆环压电超声换能器的一阶、二阶径向共振和反共振频率减小,二阶径向共振下的有效机电转换系数趋于零;增大阶梯圆环内环的外半径,换能器的一阶径向共振和反共振频率减小,二阶径向共振和反共振频率先增大后减小,理论推导与仿真模拟结果符合良好。本文研究结果为阶梯圆环压电超声换能器的工程应用提供理论参考。