Temperature dependent ultrasonic properties of aluminium nitride

Hexagonal wurtzite structured aluminium nitride has been characterized by the theoretical calculation of ultrasonic attenuation, ultrasonic velocity, higher order elastic constants, thermal relaxation time, acoustic coupling constants and other related parameters in temperature range 200-800 K for wave propagation along the unique axis of the crystal. Higher order elastic constants of AlN at different temperatures are calculated using Lennard-Jones potential for the determination of ultrasonic attenuation. A decrease in ultrasonic velocity with temperature has been predicted, which is caused by reduction in higher order elastic constants with temperature. The temperature dependent ultrasonic properties have been discussed in correlation with higher order elastic constants, thermal relaxation time, thermal conductivity, acoustic coupling constants and thermal energy density. Anomalous behaviour of the attenuation is found at 400 K. On the basis of attenuation, the ductility and performance of AlN have been studied.     

Ultrasonic wave propagation in IIIrd group nitrides

The ultrasonic attenuation in hexagonal structured (wurtzite) third group nitrides (GaN, AlN and InN) has been evaluated at 300 K for an ultrasonic wave propagating along the unique axis of the crystal. Higher order elastic constants of these materials are calculated using the Lennard-Jones potential for the determination of ultrasonic attenuation. The ultrasonic velocity, Debye average velocity, thermal relaxation time and acoustic coupling constant are evaluated along the z-axis of the crystal using the second order elastic constants and other related parameters. The contributions of the elastic constants, thermal conductivity, thermal energy density, ultrasonic velocity and acoustic coupling constant to the total attenuation are studied. On the basis of the ultrasonic attenuation, it can be concluded that the AlN is more ductile than either GaN or InN at 300 K. Orientation dependent characterization has been achieved by calculation of the orientation dependent ultrasonic velocity, Debye average velocity and thermal relaxation time for the materials.     

Measurement of the acoustic properties of a nerve-muscle preparation as a function of physiologic state

Nerve-muscle preparations of Sprague Dawley rats were exposed to low dosage ultrasound. The objectives were to measure the velocity of propagation and attenuation of ultrasonic energy in both the relaxed and contracted states. A tension-measuring system and associated ultrasonic instrumentation were designed to measure the tension developed in stimulated muscle and its corresponding acoustic parameters, ie the attenuation coefficient, (db cm^-1) and the velocity of propagation, c (ms^-1). Each test was performed at ultrasonic frequencies 3.1, 5.35, and 7.68 MHz and with the preparation maintained at 23 ± 0.5° C. Attenuation of ultrasonic energy was observed to increase by 10 ± 0.5% in the active state from its value in the relaxed state. The relation between the attenuation and the acoustic frequency was found to be approximately linear over the frequency range tested. The velocity of propagation in the active state did not change appreciably from its value in the relaxed state and was observed to be independent of the acoustic frequency in the range used.     

Acoustic properties of organic powders as ultrasonic contrast agents

The results of experiments on measuring attenuation and the effective acoustic nonlinear parameter of the second order are given for a suspension of cocoa-powder in water at different concentrations of the suspension. In the process of evaluating the value of the nonlinear parameter the attenuation in the suspension and generation of the second harmonic not only in the suspension but also in water are taken into account. The obtained results are evidence of the possibility of using a suspension of cocoa-powder in water as a technical substitute for ultrasonic contrast agents. The values of attenuation (up to 60 m⁻¹ at the concentration of 1 g of the powder per 1 l of water) and the nonlinear parameter (up to 120 m⁻¹ at the same concentration) mean that the suspension of cocoa-powder in water has smaller attenuation and the nonlinear parameter than ultrasonic contrast agents at the same concentration. However, these values for the suspension differ considerably from corresponding values for water or blood and, therefore, a suspension of cocoa-powder in water is a promising “substitute” for ultrasonic contrast agents in the case of technical testing of systems for nonlinear tomography of a blood flow, but cannot replace them in medical studies.     

非侵入式超声波浆料浓度测量技术研究*

设计了一种基于非侵入式超声波透射衰减法的浆料浓度测量系统,根据超声传播衰减原理,建立超声衰减值与浆料浓度之间的关系。实验中采用生物显微镜和激光粒度仪对颗粒标称粒径进行验证,采用中心频率为200 kHz的超声波换能器,利用一发一收模式对超声波在有机玻璃管内的浆料进行非侵入式测量并分析透射波信号,对体积百分浓度小于25%、不同粒径的石英砂浆料进行测量,通过拟合方法获得浆料温度、体积百分浓度与声衰减对应的关系,并据此构造浓度求解方程,通过现场实时在线测量并与取样结果进行对比验证方程的准确性,结果显示,本方法可有效测量浆料浓度。     

Visualization of human umbilical vein endothelial cells by acoustic microscopy

The morphology and acoustic properties of the human umbilical vein endothelial cells (HUVECs) were evaluated using a scanning acoustic microscope system. HUVECs were cultured for 4 days and exposed to the endotoxin for 4 h. The frequency of the scanning acoustic microscope was variable between 100 and 210 MHz. By changing the measuring frequency, ultrasonic amplitude and phase were measured and the quantitative value of attenuation was calculated. Before and after endotoxin stimuli, HUVECs were observed by scanning acoustic microscopy and the attenuation was measured. The acoustic images were successfully obtained to identify the outer shape of the HUVEC and the location of the nucleus in the cell. The attenuation of the nucleus is higher than that of the cytoplasm. The attenuation of the cytoplasm was increased and became inhomogeneous after endotoxin exposure. This finding would be related to the change of F-actin filaments, which is the main component of the cytoskeleton. Scanning acoustic microscopy is useful for assessing the cellular viscoelastic properties since it can detect both the morphological and acoustic changes without contacting the cellular surface.     

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).     

Resonance attenuation of ultrasonic waves in a superconductor with a moving vortex structure

Equations describing the interaction of ultrasonic waves with a moving vortex structure are derived. The addition to attenuation and the relative change in the velocity of longitudinal ultrasonic waves due to this interaction are calculated. It is found that when a longitudinal ultrasonic wave propagates along the direction of motion of the vortex structure and the velocity V of the structure is equal to half the velocity of the wave, then anomalous acoustic attenuation occurs and the contribution from the ultrasound-vortex interaction to the velocity of the ultrasonic wave vanishes. It is shown that if the vortex structure moves at a sufficiently high velocity, then (in contrast to the case of the structure at rest) a weakly damping collective mode propagating with velocity 2V arises in the structure. It is this mode that is responsible for anomalous attenuation of longitudinal ultrasonic waves.     

Frequency scanning ultrasonic pulse echo reflectometer

The frequency scanning ultrasonic pulse echo reflectometer (FSUPER) is a device which can be used to measure the ultrasonic velocity, attenuation coefficient and specific acoustic impedance of liquid samples as a function of frequency (0.3–6 MHz).     

Ultrasonic attenuation and velocity properties in rat liver as a function of fat concentration: a study at 100 MHz using a scanning laser acoustic microscope

This study examines the extent to which ultrasonic attenuation coefficients and velocity properties change between normal and fatty rat liver. The view of this problem is toward the application in clinical medicine in the future. Fatty livers were produced in rats by feeding them alcohol diets in liquid form. The animals were sacrificed and the fat concentration of the liver specimens determined. The fat concentration varied from 2.5% to 16.8% wet weight. The ultrasonic attenuation coefficient and velocity properties in 28 specimens were measured at 100 MHz with the scanning laser acoustic microscope (SLAM). Regression analysis was applied to the liver's ultrasonic propagation properties as a function of fat concentration. The results show that the attenuation coefficient increases at a rate of 1.08 dB/mm/% fat and the velocity decreases at a rate of 2.3 m/s/% fat as the fat concentration increases.     

Nonlinear elastodynamics in micro-inhomogeneous solids observed by head-wave based dynamic acoustoelastic testing

Dynamic acoustoelastic testing provides a more complete insight into the acoustic nonlinearity exhibited by micro-inhomogeneous media like granular and cracked materials. This method consists of measuring time of flight and energy modulations of pulsed ultrasonic waves induced by a low-frequency standing wave. Here pulsed ultrasonic head waves were employed to assess elastic and dissipative nonlinearities in a region near the surface of a solid. Synchronization of the ultrasound pulse sequence with the low-frequency excitation provided instantaneous variations in the elastic modulus and the attenuation as functions of the instantaneous low-frequency strain. Weak quadratic elastic nonlinearity and no dissipative nonlinearity were detected in duralumin. In limestone, distinction between tensile and compressive behaviors revealed an asymmetry in the acoustic nonlinearity and hysteresis in both the elastic modulus and the attenuation variations. Measured nonlinear acoustical parameters are in good agreement with values obtained by different techniques. Reversible acoustically induced conditioning modified the acoustic nonlinearity both quantitatively and qualitatively. It reduced tension-compression asymmetry, suggesting a nonequilibrium modification of the sources of acoustic nonlinearity. Additionally to the metrology of the acoustic nonlinearity, head wave based dynamic acoustoelastic testing may be a useful tool to monitor changes in the microstructure or the accumulation of damage in solids.     

经颅超声聚焦方法和声场特性研究

基于高分辨的CT数据建立了非均匀颅骨仿真模型,该模型引入了颅骨的声衰减系数,深入研究和分析了声波时间反转法和超声相控阵法在颅脑中的聚焦方法及效果。颅骨具有较强的声波衰减特性,使用时间反转聚焦时需要进行幅度补偿,对于0.7MHz的频率信号,幅度补偿后的时间反转聚焦声场主瓣宽度窄、旁瓣低,焦点处声场比无幅度补偿的时间反转法提高8.86dB,比超声相控阵聚焦法提高7.89dB,具有很好的空间聚焦精度和聚焦效率。研究了颅骨衰减系数、声场焦点位置、声波频率、换能器阵列位置和方位等参数对聚焦声场的影响,结果表明,幅度补偿时间反转法比相控阵法具有更低的旁瓣,且高频时的聚焦效果比相控阵好,相控阵聚焦对换能器阵列的位置和方位比较敏感,而时间反转经颅超声聚焦对声传播路径和入射角具有更高的鲁棒性。      

Anisotropic spectra of ultrasonic attenuation in aqueous dispersions with oriented carbon nanotubes

The method of acoustic spectroscopy is used for the first time for investigating the spectra of ultrasonic attenuation in the range of 3–100 MHz on oriented carbon nanotubes in the stabilized aqueous dispersion. The anisotropy of attenuation of ultrasound manifests itself in a significant distinction between the attenuation spectra for preferential perpendicular and parallel orientation of carbon nanotubes with respect to the wave-propagation direction. A qualitative agreement of the measured-spectra shape with that of the attenuation spectra calculated from the theoretical model is obtained.     

Information Content of Acoustic Attenuation Spectra

In recent years ultrasonic attenuation spectroscopy has gained much attention as a method for the characterisation of concentrated dispersions. Several publications have shown, that this method allows the accurate determination of particle size. In particular for submicron dispersions there is, however, some uncertainty to which degree the details of a size distribution can be resolved by acoustic attenuation measurements. Ideally the inversion of an attenuation spectrum into a size distribution would yield as much distribution parameters as sound frequencies. In practice, however, the measurement errors affect the inversion very strongly and may result in multiple solutions for the size distribution. The maximum number of distribution parameters, for which a unique solution exists, can be therefore regarded as the information content. For a given ultrasonic spectrometer and material system it is possible to quantify the information content. Such an information analysis has been conducted with selected material systems in the submicron range. The investigation shows that the information content of acoustic attenuation spectra with regard to particle size analysis in the submicron range is relatively low. On the other hand, the results imply that the number of frequencies can be reduced significantly without loss of information content or stability of inversion algorithms.     

Internal friction and Young’s modulus of a carbon matrix for biomorphic silicon carbide ceramics

The amplitude, temperature, and time dependences of the Young’s modulus and internal friction (ultrasonic attenuation) of a eucalyptus-based carbon biomatrix intended for preparing biomorphic silicon carbide ceramics were studied. Adsorption and desorption of molecules of the ambient medium (air) was shown to determine, to a considerable extent, the effective Young’s modulus and acoustic vibration decrement of a specimen. A doublet maximum in the temperature dependence of ultrasonic attenuation was observed at a temperature close to the sublimation temperature of solid CO₂. The microplastic properties of the material were estimated from acoustic measurement data.     

Acoustic properties of multiferroic BiFeO<Subscript>3</Subscript> over the temperature range 4.2–830 K

The longitudinal acoustic wave velocity and attenuation in BiFeO₃ ceramics have been measured by ultrasonic pulse-echo technique at a frequency of 10 MHz in the temperature range from 4.2 K to 830 K. The anomalies observed in the sound velocity and attenuation behavior versus temperature are attributed to the assumed relaxation in the temperature range 200–500 K and antiferromagnetic phase transition at higher temperatures. Order parameter fluctuations along with magnetostriction are discussed as the factors determining the acoustic wave velocity anomaly in the vicinity of the antiferromagnetic phase transition point.     

Propagation of ultrasonic waves in neptunium monochalcogenides

The ultrasonic attenuation and acoustic coupling constants due to phonon–phonon interaction and thermoelastic relaxation mechanisms have been studied for longitudinal and shear waves in B₁ structured neptunium monochalcogenides NpX (X: S, Se, Te) along 〈1 0 0〉 direction in the temperature range 100–300 K. The second and third order elastic constants (SOEC and TOEC) of the chosen monochalcogenides are also computed for the evaluation of ultrasonic parameters. The ultrasonic attenuation due to phonon–phonon interaction process is predominant over thermoelastic relaxation process in these materials. The ultrasonic attenuation in NpTe has been found lesser than other materials NpS, NpSe and GdY (Y: P, As, Sb and Bi). The semiconducting or semimetallic nature of neptunium monochalcogenides can be well understood with the study of thermal relaxation time. Total ultrasonic attenuation in these materials is found to be quadratic function of temperature. The nature of NpTe is very similar to semimetallic GdP. The mechanical and ultrasonic study indicates that NpTe is more reliable, perfect, flawless material.     

Determination of biomass concentration by measurement of ultrasonic attenuation

Experimental results are presented that demonstrate that ultrasonic attenuation can be used to estimate the biomass concentration of a biological suspension. The attenuation approach avoids the temperature sensitivity of established ultrasound velocity test methods, which have hindered their application in process environments. An empirical model is presented for the estimation of the biomass of Microcystis aeruginosa suspensions based on acoustic attenuation, temperature, and frequency, and is compared with experimental results.     

How diffraction limits ultrasonic screening in phononic plate composed of a periodic array of resonant slits

We explore experimentally the role played by diffraction in the phenomenon of acoustic shielding provided by a plate that is periodically perforated with subwavelength slits and immersed in water. We carried out ultrasonic transmission measurements for all directions of propagation in order to check the omnidirectionality of acoustic shielding. While a single slit acts as a Fabry–Perot resonator in the frequency range of interest, the coupling between adjacent slits provides an attenuation frequency band centered around the resonant frequency that is mostly independent of the angle of incidence. Beyond the incident angle of 45 degrees, however, we observe the appearance of scattered radiation that limits the attenuation of ultrasound. This spurious scattering is shown to arise from diffraction by the grating of slits.     

利用纳米力学方法对皮肤进行超声组织定征的研究

纳米力学方法适用于具有离散特性的材料.利用纳米力学方法,得出声波在多层皮肤组织中的波动方程.分别改变黑素瘤皮肤的泊松比、黑素瘤侵袭厚度(Breslow深度)、节间距离,计算多层皮肤结构模型对垂直入射纵声波的反射系数.同时计算了声速和声衰减系数随组织参数的变化.计算结果表明,可综合利用一定频率段内反射系数最小点数、声速和衰减系数的变化来表征正常皮肤和病变皮肤.