Influence of the thickness of matching layers on narrow band transmitter ultrasonic airborne transducers with frequencies <100 kHz: Application of a genetic algorithm

The acoustic impedances of matching layers and their thicknesses are the most important and influential parameters in the performance of airborne ultrasonic transducers. In this paper, the optimum thicknesses of the matching layers of the narrow band transmitter ultrasonic transducer regarding transmission coefficient were determined by individual calculations using a genetic algorithm. The genetic algorithm was chosen because it is a powerful tool in the optimization domain. The results show that the permitted thickness variation is 0.0005% for one matching layer, and this value can be increased to 0.0031%, which corresponds to the permitted thickness variation for five matching layers. Approximately 55% enhancement in the transmission coefficient is theoretically possible, and 42% enhancement was observed experimentally when the genetic algorithm was applied to calculate the matching layer thicknesses in place of the quarter wavelength equation that is conventionally used for the determination of layer thickness. To verify our approach, the effect of the thickness variation on the transmission coefficient has been investigated experimentally for three, four and five matching layers. The experimental results displayed good agreement with the theoretical predictions.     

Modeling of functionally graded piezoelectric ultrasonic transducers

The application of functionally graded material (FGM) concept to piezoelectric transducers allows the design of composite transducers without interfaces, due to the continuous change of property values. Thus, large improvements can be achieved, as reduction of stress concentration, increasing of bonding strength, and bandwidth. This work proposes to design and to model FGM piezoelectric transducers and to compare their performance with non-FGM ones. Analytical and finite element (FE) modeling of FGM piezoelectric transducers radiating a plane pressure wave in fluid medium are developed and their results are compared. The ANSYS software is used for the FE modeling. The analytical model is based on FGM-equivalent acoustic transmission-line model, which is implemented using MATLAB software. Two cases are considered: (i) the transducer emits a pressure wave in water and it is composed of a graded piezoceramic disk, and backing and matching layers made of homogeneous materials; (ii) the transducer has no backing and matching layer; in this case, no external load is simulated. Time and frequency pressure responses are obtained through a transient analysis. The material properties are graded along thickness direction. Linear and exponential gradation functions are implemented to illustrate the influence of gradation on the transducer pressure response, electrical impedance, and resonance frequencies.     

Influence of the thickness of multilayer matching systems on the transfer function of ultrasonic airborne transducer

The effective ultrasonic energy radiation into the air of piezoelectric transducers requires using multilayer matching systems with accurately selected acoustic impedances and the thickness of particular layers. This problem is of particular importance in the case of ultrasonic transducers working at a frequency above 1 MHz. Because the possibilities of choosing material with required acoustic impedance are limited (the counted values cannot always be realised and applied in practice) it is necessary to correct the differences between theoretical values and the possibilities of practical application of given acoustic impedances. Such a correction can be done by manipulating other parameters of matching layers (e.g. by changing their thickness). The efficiency of the energy transmission from the piezoceramic transducer through different layers with different thickness enabling a compensation of non-ideal real values by changing their thickness was computer analysed. The result of this analysis is the conclusion that from the technological point of view a layer with defined thickness is easier and faster to produce than elaboration of a new material with required acoustic parameter.     

膜厚监控系统的光谱宽度对窄带滤光片性能的影响

讨论了膜厚监控系统的光谱宽度对波分复用窄带滤光片特性的影响,分析了监控过程中所出现的信号异常现象,其主要原因是控制光光谱宽度以及控制波长与滤光片中心波长不一致,所以控制光光谱分辨率必须小于单个法布里-珀罗滤光片最后2层膜折转点波长宽度的一半,即对100GHz的滤光片,监控系统的光谱宽度必须小于0．2nm。一旦产生中心波长偏离,就必定产生厚度控制误差。讨论了高折射率膜和低折射率膜的信号变化规律,发现当中心波长比监控波长长时,虽然信号变化规律正确,但判读到极值时的膜厚变薄。中心波长偏离越长,厚度将越薄。而当中心波长比监控波长短时,信号将出现反转。中心波长越短,反转量越大。最后指出了监控误差对滤光片Tmax和半峰全宽的影响。     

Variables in the use and design of acoustic emission transducers

Previously,¹1 we presented a theory to explain the resonant behaviour of a couplant/multilayer acoustic emission transducer system. Here we present experiments used to verify the applicability of the theory in describing the behaviour of such a system. An acoustic emission transducer/couplant system is typically five layers; but to obtain agreement between theory and experiment, it was necessary to extend the theory to six layers and carry out six-layer measurements.Agreement between theory and experiment is quite good and we believe that this theory provides a basis for a better understanding of the effect of couplant thickness on measured acoustic emission parameters. In the longer term, this will produce greater measurement precision and more effective interpretation of data.     

The analysis of adhesive bonds using electromagnetic acoustic transducers

The paper presented here outlines a technique for examining aerospace adhesive bonds using electromagnetic acoustic transducers (EMAT). The main restriction on the use of bonded structures is the lack of a reliable, applicable non-destructive test. Simple acoustic theory shows that a shear wave at normal incidence to an interface should be a more sensitive probe of interfacing coupling than a longitudinal wave. Conventional piezoelectric shear transducers require a very viscous couplant which makes scanning problematic. The EMAT described here consists of a pancake coil, and a permanent magnet behind the coil provides a static magnetic field normal to the surface of the sample and the plane of the coil. The EMATs used have the advantage of generating broadband radially polarized shear waves, while requiring no acoustic couplant. They are also comparable in size to typical piezoelectric transducers. The broadband nature of the transducer gives it a high spatial resolution in the direction of wave propagation. Experiments performed on plate-like samples have successfully detected deliberately constructed defects, while monitoring the adhesive thickness. Defects have been identified using a C-scan technique using a single EMAT in send-receive mode from either side of the bond.     

用铜和氯掺杂的CdS-CdSe双层光电导膜的性质

对掺Ｃｕ和Ｃｌ的ＣｄＳ－ＣｄＳｅ双怪光电导膜的暗电导和亮电导与掺杂浓度及Ｃｕ／Ｃｌ比的关系、响应时间和光谱响应进行了研究。试验发现：适当的Ｃｕ／Ｃｌ掺杂比可使暗电导降低而亮电导提高;掺杂薄膜的响应时间可达５－１０ｍｓ,而对掺杂一般为数百毫秒;随着ＣｄＳｅ浓度增加,光学吸收变大,光谱响应向长波移动 。     

Experimental and finite element modelling studies on single-layer and multi-layer 1-3 piezocomposite transducers

Finite element modelling (FEM) using ATILA code and experimental studies have been carried out on 1-3 piezocomposite transducers. FEM study was initially carried out on a piezocomposite infinite plate and then extended to transducers of finite size. The infinite-plate model results agree well with that of a simple analytical model and experiments. The acoustic performance of multi-layer finite-size piezocomposite transducers was also studied. Transducer stacks were fabricated with different number of layers. The transducer characteristics such as the electrical impedance, the transmitting voltage response (TVR) and the receiving sensitivity (RS) of the 1-3 piezocomposite transducers were evaluated as functions of frequency, ceramic volume fractions and the number of layers. TVR increases and RS decreases with increase in ceramic volume fractions. The model results are found to agree with the experimental data, especially when the number of layers is less.     

Optimising flat-walled multi-layered anechoic linings using evolutionary algorithms

Previous investigations indicated that a flat-walled, multi-layered anechoic lining system, with an overall thickness slightly less than a quarter of a wavelength, could be used to achieve a required cut-off frequency. However, the work proved to be tedious and time consuming because of the numerous trial-and-error measurements involved. On the other hand, the successful application of a method of calculating the overall acoustic impedance of multi-layered absorbing systems has indicated that the design of multi-layered absorbing systems can be carried out on a desktop computer. In the present work, a MATLAB genetic and evolutionary algorithm toolbox is implemented as the optimiser to aid and speed up the design process. The optimisation results indicate that a three-layered lining system can achieve results comparable with quality wedge-type anechoic linings with overall thickness slightly less than a sixth of a wavelength at the 100 Hz cut-off frequency.     

超薄原子蒸汽的透射与反射光谱

通过数值分析研究了超薄气体的透射与反射光谱特性与其厚度大小的树应关系。研究表明,透射光谱随薄膜厚度的增大而展宽,光谱呈现对称性,而在反射的情况下,这一对称性由薄膜厚度为1／4波长的奇数或偶数倍决定。     

Parylene anti-reflection coating of a quasi-optical hot-electron-bolometric mixer at terahertz frequencies

Parylene C was investigated as anti-reflection coating for silicon at terahertz frequencies. Measurements with a Fourier-transform spectrometer show that the transmittance of pure silicon can be improved by about 30% when applying a layer of Parylene C with a quarter wavelength optical thickness. The 10% bandwidth of this coating extends from 1.5 to 3 THz for a center frequency of 2.3–2.5 THz, where the transmittance is constant. Heterodyne measurements demonstrate that the noise temperature of a hot-electron-bolometric mixer can be reduced significantly by coating the silicon lens of the hybrid antenna with a quarter wavelength Parylene C layer. Compared to the same mixer with an uncoated lens the improvement is about 30% at a frequency of 2.5 THz.     

Acoustic impedance matching of medical ultrasound transducers

Ultrasonic transducers for pulse-echo systems are studied both theoretically and experimentally. For the theoretical calculations the Mason model for thickness-mode disc transducers with and without backing and matching layers is used. By building several of the theoretically investigated transducer configurations it is shown that theory and experiment agree well. Thus the properties of a transducer can be predicted to a good approximation before its experimental realization. To find transducers with good sensitivity and short pulses, the pulse shape and frequency response for the following classes of transducers were studied both theoretically and experimentally: transducers with backing only, transducers with heavy backing and front matching layers, and air-backed transducers with front matching layers.     

A dual-band flexible frequency selective surface with miniaturized elements and maximally flat (Butterworth) response

A dual-band flexible frequency selective surface （FSS） with miniaturized elements and maximally flat （Butterworth） response is presented in this paper. It is composed of three metallic layers, which are fabricated on thin flexible polyimide substrates and bonded together using thin bonding films. The overall thickness of the proposed structure is only about 0.3 mm, making it an attractive choice for conformal FSS applications. All the three layers can constitute a miniaturized- element FSS （MEFSS） and produce the first pass-band with miniaturization property, while the up and bottom layers can constitute a symmetric biplanar FSS and produce the second pass-band with maximally fiat （Butterworth） response. The two pass-bands are independent and there is a wide band spacing up to 30 GHz between them. The principles of operation, the simulated results by using the vector modal matching method, and the experimental values of the fabricated prototype are also presented and discussed.     

Determination of acoustic impedances of multi matching layers for narrowband ultrasonic airborne transducers at frequencies <2.5 MHz - Application of a genetic algorithm

The effective ultrasonic energy radiation into the air of piezoelectric transducers requires using multilayer matching systems with accurately selected acoustic impedances and the thickness of particular layers. One major problem of ultrasonic transducers, radiating acoustic energy into air, is to find the proper acoustic impedances of one or more matching layers. This work aims at developing an original solution to the acoustic impedance mismatch between transducer and air. If the acoustic impedance defences between transducer and air be more, then finding best matching layer(s) is harder. Therefore we consider PZT (lead zirconate titanate piezo electric) transducer and air that has huge acoustic impedance deference. The vibration source energy (PZT), which is used to generate the incident wave, consumes a part of the mechanical energy and converts it to an electrical one in theoretical calculation. After calculating matching layers, we consider the energy source as layer to design a transducer. However, this part of the mechanical energy will be neglected during the mathematical work. This approximation is correct only if the transducer is open-circuit. Since the possibilities of choosing material with required acoustic impedance are limited (the counted values cannot always be realized and applied in practice) it is necessary to correct the differences between theoretical values and the possibilities of practical application of given acoustic impedances. Such a correction can be done by manipulating other parameters of matching layers (e.g. by changing their thickness). The efficiency of the energy transmission from the piezoceramic transducer through different layers with different thickness and different attenuation enabling a compensation of non-ideal real values by changing their thickness was computer analyzed (base on genetic algorithm). Firstly, three theoretical solutions were investigated. Namely, Chebyshev, Desilets and Souquet theories. However, the obtained acoustic impedances do not necessarily correspond to a nowadays available material. Consequently, the values of the acoustic impedances are switched to the nearest values in a large material database. The switched values of the acoustic impedances do not generally give efficient transmission coefficients. Therefore, we proposed, in a second step, the use of a genetic algorithm (GA) to select the best acoustic impedances for matching layers from the material database for a narrow band ultrasonic transducer that work at frequency below the 2.5 MHz by considering attenuation. However this bank is rich, the results get better. So the accuracy of the propose method increase by using a lot of materials with exact data for acoustic impedance and their attenuation, especially in high frequency. This yields highly more efficient transmission coefficient. In fact by using increasing number of layer we can increase our chance to find the best sets of materials with valuable both in acoustic impedance and low attenuation. Precisely, the transmission coefficient is almost equal to unity for the all studied cases. Finally the effect of thickness on transmission coefficient is investigated for different layers. The results showed that the transmission coefficient for air media is a function of thickness and sensitive to it even for small variation in thickness. In fact, the sensitivity increases when the differences of acoustic impedances to be high (difference between PZT and air).     

Transparent conductive ITO/Ag/ITO multilayer electrodes deposited by sputtering at room temperature

ITO/Ag/ITO multilayers have been prepared onto conventional soda lime glass substrates by sputtering at room temperature. The optical and electrical characteristics of single layer and multilayer structures have been investigated as a function of the Ag and ITO film thicknesses. Transmittance and sheet resistance values are found mainly dependent on the Ag film thickness; whereas the wavelength range at which the maximum transmittance is achieved can be changed by adjusting the ITO films thickness. ITO/Ag/ITO electrodes with sheet resistance below 6 Ω/sq have been obtained for Ag film thickness above 10 nm and ITO layers thickness in the 30-50 nm range. These multilayers also show high transmittance in the visible spectral region, above 90% by discounting the glass substrate, with a maximum that is located at higher wavelengths for thicker ITO.     

Interference filters from porous silicon with laterally varying wavelength of reflection

Porous silicon reflection interference filters of Bragg type consists of up to 40 quarter wave layers with alternating high- and low-refraction index. The refraction index depends on the porosity of the silicon. The reflection wavelength can vary over a wide range and depends on the thickness and refraction index of the porous layers. A laterally continuously varying wavelength with linear profile of the filter can be achieved by manipulating the porosity and thickness of the silicon in the lateral direction. Our approach is to vary the Fermi level laterally by applying a potential parallel to the surface of the wafer. The slope of the Fermi level is easily controlled by the magnitude of the potential. The lateral current density and thus the porosity and thickness is related to the potential difference between the laterally varying Fermi level and the potential induced by the counter electrode. This relation is the well-known current–voltage characteristic of a Silicon hydrofluoric acid contact. The linearity of the etch profile across the wafer is demonstrated and the properties of preliminary reflection filters are shown.     

释压法混合吸声系统中多孔材料厚度优化研究

研究释压法主被动混合吸声结构吸声性能与多孔吸声材料参数的关系,利用释压法对材料厚度敏感的特性,选择合适的多孔材料厚度提高系统的低频吸声性能。提出一种释压法主被动混合双层吸声结构,将误差传声器置入两层吸声结构之间的空气层中,分别优化各层的厚度,在较宽频带内获得较好吸声。对流阻系数为15000NSm^-4的一类典型的玻璃棉数值仿真,发现选择优化厚度分别为2.8 cm和6 cm,中间空腔长为2 cm,可使有效吸声频带向下大幅扩展。实验结果验证了所提宽带吸声结构。     

Branching features of photonic bandgaps in Fibonacci dielectric heterostructures

We propose branching features of photonic band gaps in one-dimension Fibonacci dielectric heterostructures with arbitrary generation orders and thicknesses based on the gap map diagram, which is determined by the band edge formalism to avoid numerical instability. We find that the gap map diagram can be divided by the half-wave lines into several regions, each of which has similar gap pattern. The branching rules including the existence condition and the frequency range of each major gap as well as the number of all minor gaps in each region for systems with arbitrary thicknesses and generations are proposed. The number of major gaps in each region for the Fibonacci systems equals two rather than one for traditional binary layers. Moreover, the conditions of maximum gapwidth for the Fibonacci systems are not near the quarter wavelength, although the ones for traditional binary layers are at the quarter wavelength.     

Factors in the design of ultrasonic probes

This paper attempts to demonstrate the comprehensive way in which the Unit Impulse Response technique can be used to evaluate and calibrate thickness mode piezoelectric transducers and the ultrasonic probes made from these transducers. Two commonly used piezoelectric materials were compared in the form of batches of disc transducers at several different frequencies and diameters.At higher system gains spurious signals associated with transverse mode resonances seriously reduced the discrimination of the probe. These spurious effects were fully investigated and explained.The effects of backing; quarter-wavelength matching into water; and electrical tuning, were measured and explained.Other methods of measuring the transducer parameters based on frequency response analysis were considered.     

Study on measurement of dispersive characteristics of higher order mode Lamb waves

The dispersive characteristics of higher order mode Lamb waves (HOMLW) excited by interdigital transducers (IDT) are measured and analyzed, which are necessary for designing micro-sensor in ultrahigh frequency (UHF). A measurement system is set up, in which dispersive characteristics of HOMLW are obtained by the method of transform between frequency and time domains. The characteristics of amplitude-frequency and phase-frequency of Lamb wave are auto-measured by the system. By IFFT, the pulse response of the IDT device was obtained. Different modes were separated in time domain and dispersive curve of each mode is calculated by FFT. The best mode is chosen to design the micro-sensor in UHF. The phase velocity of HOMLW is greater than the surface wave (SAW) velocity and an oscillator in higher frequency can be made, so the absolute sensitivity of micro-sensor can be increased. In this paper, the dispersive characteristics of HOMLW excited by an IDT in a 127.86 degrees rotated Y-cut, X propagating lithium niobate plate is analyzed. An oscillator using a(13) mode is made, the phase velocity of which is measured about 19,652 m/s when h/lambda=0.94 (h=plate thickness, lambda=wavelength).