High-frequency broadband transducers of ultrasonic oscillations

The amplitude-frequency responses (AFRs) of ultrasonic oscillation transducers based on piezoelectric lithium iodate crystals of hexagonal modification and on magnetic soft ferrite powders are experimentally studied. The possibility to control the efficiency of magnetostriction powder transducers (MPTs) by variation in the value and orientation of the external magnetic field is shown. Results of measurements of the transformation efficiency for ferrites of various chemical compositions are presented.     

Flux independent tests of neutrino oscillations

Zeitschrift für Physik C Particles and Fields - We discuss reactor and accelerator experiments which could provide further confirmation of neutrino oscillations. We address tests which are...     

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.     

Pulse compression techniques in ultrasonic non-destructive testing

The development of a pulse compression system applicable to ultrasonic flaw detection is described. The use of pulse compression technique permits a pulse-echo detection system to operate with long transmitted pulses for increased sensitivity, but without sacrificing resolution. The described system is economically implemented using an aluminium strip dispersive delay line. The operating system achieves a time-bandwidth product of 84 and sidelobe levels of —25 dB. Experimental evidence of the system's capabilities is presented.     

The efficiency of ultrasonic oscillations transfer into the load

The results of ultrasonic action to the substances have been presented. It is examined, the correlation between the electrical parameters of ultrasonic equipment and acoustic performances of the ultrasonic field in treating the medium, the efficiency of ultrasonic technological facility, and the peculiarities of oscillations introduced into the load under cavitation development. The correlation between the acoustic powers of oscillations securing the needed level of cavitation and desired technological effect, and the electrical parameters of the ultrasonic facility, first of all, the power, is established. The peculiarities of cavitation development in liquids with different physical-chemical properties (including the molten low-melting metals) have been studied, and the acoustic power of oscillations introduced into the load under input variation of electric power to the generator has been also estimated.     

Piezoelectric multilayer transducers for ultrasonic pulse compression

This paper describes piezoelectric multilayer transducers for application in ultrasonic pulse compression systems. The transducers are constructed in such a way as to produce binary pulse sequences, in particular Barker-coded sequences. This is achieved by the polarization pattern of the active layers. Pulse compression is effected without any electronic circuitry by using a receiver transducer with a pattern corresponding to that of the transmitter.     

In situ observation and analysis of ultrasonic capillary effect in molten aluminium

An in situ synchrotron radiographic study of a molten Al–10 wt% Cu alloy under the influence of an external ultrasonic field was carried out using the Diamond-Manchester Branchline pink X-ray imaging at the Diamond Light Source in UK. A bespoke test rig was used, consisting of an acoustic transducer with a titanium sonotrode coupled with a PID-controlled resistance furnace. An ultrasonic frequency of 30 kHz, with a peak to peak amplitude at 140 microns, was used, producing a pressure output of 16.9 MPa at the radiation surface of the 1-mm diameter sonotrode.This allowed quantification of not only the cavitation bubble formation and collapse, but there was also evidence of the previously hypothesised ultrasonic capillary effect (UCE), providing the first direct observations of this phenomenon in a molten metallic alloy. This was achieved by quantifying the re-filling of a pre-existing groove in the shape of a tube (which acted as a micro-capillary channel) formed by the oxide envelope of the liquid sample. Analytical solutions of the flow suggest that the filling process, which took place in very small timescales, was related to micro-jetting from the collapsing cavitation bubbles. In addition, a secondary mechanism of liquid penetration through the groove, which is related with the density distribution of the oxides inside the groove, and practically to the filtration of aluminium melt from oxides, was revealed. The observation of the almost instantaneous re-filling of a micro-capillary channel with the metallic melt supports the hypothesised sono-capillary effect in technologically important liquids other than water, like metallic alloys with substantially higher surface tension and density.     

超声阵列的多频脉冲压缩方法研究

脉冲压缩方法通过采用编码信号激励与脉冲压缩接收来提高超声检测的时间分辨率和信噪比。然而,受换能器带宽的限制,编码信号的时间带宽积往往有限,影响脉冲压缩效果。开展了基于超声阵列的多频脉冲压缩方法研究,超声阵列是由具有不同中心频率和带宽的多个阵元组成,从而使得整个阵列具有比单个换能器更宽的带宽。提出了压缩后叠加取包络的脉冲压缩方法,并研究了这种方法在提高时间分辨率和主副瓣比上的技术方案,对多频脉冲压缩的超声阵列进行了设计与优化,发现多频脉冲压缩方法能很大程度地提高时间分辨率,并能有效地抑制各阵元信号叠加产生的周期副瓣。理论和实验结果表明,这种压缩后叠加取包络的方法得到的时间分辨率在性能上受阵元带内不平整度的影响很小,能使阵元压缩包络中的主副瓣比得到明显改善。     

Microcomputer-based digital pulse compression system for ultrasonic NDT

A microcomputer-based digital pulse compression system suitable for ultrasonic non-destructive testing applications is described. By utilizing software for code generation as well as for correlation processing, this approach possesses a high degree of flexibility accommodating different types and lengths of transmission codes. The implemented system, which employs bi-phase modulation of the transmitter waveforms with m-sequences and Golay codes, provides significant signal-to-noise ratio enhancement with zero range sidelobes. Results from the operating system are in good agreement with theoretical predictions and confirm the feasibility of the method.     

Sputtered ZnO film on aluminium foils for flexible ultrasonic transducers

Nanocrystalline ZnO films with both C-axis vertical grown and inclined angled grown were sputter-deposited onto aluminium foils (50 μm thick) and characterised for using as flexible ultrasonic transducers. As-deposited C-axis grown ZnO films were annealed at different temperatures up to 600 °C to enhance film crystallinity and reduce film stress. The C-axis grown ZnO film on the Al foil were bonded onto steel plates, and the pulse-echo tests verified a good performance (with dominant longitudinal waves) of the ultrasonic transducers made from both as-deposited and post-annealed films. Inclined angled ZnO films on the Al foil glued onto steel plates generated mixed shear and longitudinal waves in the pulse-echo test.     

Acoustic analogue of electronic BLOCH oscillations and resonant Zener tunneling in ultrasonic superlattices

We demonstrate the existence of Bloch oscillations of acoustic fields in sound propagation through a superlattice of water cavities and layers of methyl methacrylate. To obtain the acoustic equivalent of a Wannier-Stark ladder, we employ a set of cavities with different thicknesses. Bloch oscillations are observed as time-resolved oscillations of transmission in a direct analogy to electronic Bloch oscillations in biased semiconductor superlattices. Moreover, for a particular gradient of cavity thicknesses, an overlap of two acoustic minibands occurs, which results in resonant Zener-like transmission enhancement.     

Brewster-angled chirped mirrors for broadband pulse compression without dispersion oscillations

We use Brewster-angled chirped mirrors for dispersion compensation of a noncollinear optical parametric amplifier. This novel mirror design virtually eliminates spurious surface reflections and resultant dispersion ripple. The absence of compression artifacts is demonstrated by the generation of clean 5.6 fs pulses, with what is believed to be an unprecedented low ripple-induced satellite content for a nonadaptive scheme. In addition, the 270 THz spectral coverage allows generation of widely tunable visible pulses of 8 to 15 fs duration.     

Thick aluminium nitride films deposited by room-temperature sputtering for ultrasonic applications

Materials in film form for electromechanical transduction have a number of potential applications in ultrasound. They are presently under investigation in flexural transducers for air-coupled ultrasound and underwater sonar operating at frequencies up to a few megahertz. At higher frequencies, they have the potential to be integrated with electronics for applications of ultrasound requiring high spatial resolution. However, a number of fabrication difficulties have arisen in studies of such films. These include the high temperatures required in many thick and thin film deposition processes, making them incompatible with other stages in transducer fabrication, and difficulties maintaining film quality when thin film--typically sub-1 microm--processes are extended to higher thicknesses. In this paper, we first outline a process which has allowed us to deposit aluminium nitride (AlN) films capable of electromechanical transduction at thicknesses up to more than 5 microm without substrate heating. As an ultrasonic transduction material, AlN has functional disadvantages, particularly a high acoustic velocity and weak electromechanical transduction. However, it also has a number of advantages relating to practicality of fabrication and functionality. These include the ability to be deposited on a variety of amorphous substrates, a very high Curie temperature, low permittivity, and low electrical and mechanical losses. Here, we present experimental results highlighting the transduction capabilities of AlN deposited on aluminium electrodes on glass and lithium niobate. We compare the results with those from standard simulation processes, highlighting the reasons for discrepancies and discussing the implications for incorporation of AlN into standard ultrasonic transducer design processes.     

Fourier thermal analysis of solidification kinetics in molten aluminium and in presence of ultrasonic field

In this paper a comparison between the Newtonian and Fourier method analysis available in thermal analysis techniques is reported. The experimental solidification of liquid aluminium under various conditions has been studied. Thermal analysis is a tool in elucidating the thermal events involved in liquid to solid transformation. The Newtonian method predicts a maximum in heat generation at the onset of solidification, while the Fourier method incorporates the effect of thermal gradient and predicts two heat generation peaks during the solidification process. This comparison between the Fourier and Newtonian method indicates that their predictions are appreciably different. In order to elucidate the effect of ultrasonic waves on solidification process, the measurements were carried out under similar conditions both with and without sonication. Our studies have shown that in presence of the ultrasonic field the kinetics of solidification is changed, leading namely to a decreased undercooling degree of the melt and thermal diffusivity.     

Friction and wear of a spherical indenter under influence of out-of-plane ultrasonic oscillations

This paper presents an experimental and theoretical investigation of friction and wear of a spherical indenter. With the pin-on-disc-tribometer the out-of-plane oscillations are applied to the sliding indenter. Oscillations lead to a decrease of the coefficient of friction, and this effect is also related to the sliding velocity and oscillation amplitude. During the sliding movement, the contact area of indenter increases due to the wear of material. This radius of the worn spherical cap is measured after each sliding period. It is found that the radius of the wear flat increases with sliding distance according to a power law with the power 1/4 and is independent of the sliding velocity. It further is practically insensitive to the presence of oscillations. A theoretical analysis and a numerical simulation based on the method of dimensionality reduction are carried out, both describing the experimental data very well.     

Determination of magnification and resonance length of samples used in ultrasonic fatigue tests

For ultrasonic fatigue tests various dumb-bell shaped specimens are used. For different sample shapes formulae are given, which allow a quick estimate of the resonance length and magnification factor. The calculations are based on the fact that the equation of motion can be solved explicitly when the shape of the stepped part, which, in longitudinal section, is usually part of a circle, is approximated by an exponential or catenoidal curve. The applicability of the presented method is discussed.     

Effect of ultrasonic oscillations on the fluidity of heavy oil products at low temperatures

We develop a laboratory setup to estimate the force of rotation of a metal branch pipe in a viscoelastic medium. We show that 2-min action of shearing ultrasonic oscillations (frequency, 32.5 kHz; specific power, no more than 0.008 W/cm²) reduces by 17% the static limit of fluidity brought to an initial temperature of ì-100 fuel oil cooled to −15°C in the wall layer of a rotating branch pipe. We obtain a linear regression dependence between the ratio of the threshold force of the onset of branch pipe motion to the consumption current of the ultrasonic transducer and the fuel temperature.     

An ultrasonic pulse compression system for non-destructive testing using maximal-length sequences

An ultrasonic pulse compression system is described. The system, which employs maximal-length sequences and possesses theoretically zero sidelobes, has been simply implemented using digital techniques. Experimental results from the working system demonstrate the capabilities of the system and confirm the feasibility of the approach. Because of its increased signal-to-noise ratio it can be advantageously applied to testing situations where highly absorbent media are encountered.     

Development of high-performance aluminium 6061/SiC nanocomposites by ultrasonic aided rheo-squeeze casting method

In the modern era, the need for new products with novel processing and multipurpose materials is increased. The current market requirements for engineering applications are lightweight, high strength and low-cost materials. This paper explores the novel development process of high-performance nano cermet material for aerospace applications. Herein, lightweight aluminium 6061 + 2% of SiC (40 nm) nano cermet was fabricated through the casting method. The effects of ultrasonication, double stir casting or rheocasting, and squeezing pressure on nano cermet fabrication were successfully investigated by comparing their physical, thermal and mechanical properties. Scanning electron microscopy was employed to analyse the morphology of the cermets, and the presence of reinforcements was verified through EDS. The reinforcement of SiC into Al 6061 improved density, hardness, and reduction in porosity and grain refinement. This study reveals a novel fabrication process of ultrasonic-aided rheo-squeeze casting technique which enhanced the mechanical properties of the cermets compared to Al 6061 alloy due to nanoparticles homogeneous distribution, nominal agglomeration and porosity.     

High accuracy non-contact ultrasonic thickness gauging of aluminium sheet using electromagnetic acoustic transducers.

Aluminium sheet thickness has been calculated from ultrasonic data obtained using a send-receive, radially polarised electromagnetic acoustic transducer (EMAT). Sheets in the thickness range between 0.1 and 0.5 mm have been measured using this non-contact approach at a stand-off of up to 1.5 mm. Normal incidence shear waves generated and detected in the sheet and the resultant waveforms have been processed using transit time measurements and Fourier analysis. Two broad band EMAT systems have been used to perform the measurements with centre frequencies of approximately 5 MHz and frequency content up to 10 and 20 MHz respectively. The most accurate measurements of thickness on thin sheets have been made using Fourier analysis and have yielded measurements accurate to within 0.2% (or 0.4 microm) for 280 microm thick aluminium sheets. Discrete shear wave echoes can be observed for sheets down to a thickness of 250 microm using the higher frequency EMAT system. However temporal measurements of these signals yield lower accuracy results when compared to the Fourier analysis method which is capable of sub-micron accuracy.