O. Bou Matar

Tunable magnetoelastic phononic crystals

The feasibility of tuning the band structure of phononic crystals is demonstrated by employing magnetostrictive materials and applying an external magnetic field. Band structures are calculated with a plane wave expansion method that accounts for coupling between the elastic behavior and the magnetic field through the development of elastic, piezomagnetic, and magnetic permeability effective tensors. We show the contactless tunability of the absolute band gaps of a two-dimensional phononic crystal composed of an epoxy matrix and Terfenol-D inclusions. The tunable phononic crystal behaves like a transmission switch for elastic waves when the magnitude of an applied magnetic field crosses a threshold.

Band gap tunability of magneto-elastic phononic crystal

The possibility of control and tuning of the band structures of phononic crystals offered by the introduction of an active magnetoelastic material and the application of an external magnetic field is studied. Two means to obtain large elastic properties variations in magnetoelastic material are considered: Giant magnetostriction and spin reorientation transition effects. A plane wave expansion method is used to calculate the band structures. The magnetoelastic coupling is taken into account through the consideration of an equivalent piezomagnetic material model with elastic, piezomagnetic, and magnetic permeability tensors varying as a function of the amplitude and orientation of the applied magnetic field. Results of contactless tunability of the absolute bandgap are presented for a two-dimensional phononic crystal constituted of Terfenol-D square rod embedded in an epoxy matrix.

Band structures tunability of bulk 2D phononic crystals made of magneto-elastic materials

The feasibility of contactless tunability of the band structure of two-dimensional phononic crystals is demonstrated by employing magnetostrictive materials and applying an external magnetic field. The influence of the amplitude and of the orientation with respect to the inclusion axis of the applied magnetic field are studied in details. Applications to tunable selective frequency filters with switching functionnality and to reconfigurable wave-guides and demultiplexing devices are then discussed.

Multilayer magnetostrictive structure based surface acoustic wave devices

This study addresses the experimental and theoretical investigations of guided elastic waves propagation in piezo-magnetic multi-layered structure. The structure is composed of a 20×TbCo2(5nm)/FeCo(5nm) nanostructured multi-layer deposited between two Aluminum (Al) Inter-Digitals Transducers forming a surface acoustic wave delay line, on a Y-cut LiNbO3 substrate. We compare the calculated and measured phase velocity variation under the action of the external magnetic field orientation and magnitude. We find quantitative agreement between the measured and modeled phase velocity shift for all external magnetic field configurations (hard axis and easy axis) and for different shape modes of elastic waves at their first and third harmonic operation frequencies. The shear horizontal mode exhibits a maximum phase velocity shift close to 20% for a ratio close to 1 between magneto-elastic film thickness and wavelength.

Non contact measurement of vibration using airborne ultrasound

A non contact ultrasonic method for measuring the surface vibration of objects is studied. The instrument consists in a pair of 420 kHz air transducer. One is used to emit ultrasound toward the moving surface while the other receives the ultrasound reflected from the object under test. Two effects induce a phase modulation on the received signal. The first results from the variation of the round trip time interval /spl tau/ required for the wavefront to go from the emitter to the moving surface and back to the receiver. This is the Doppler effect directly proportional to the surface displacement. The second one results from the nonlinear parametric interactions of the ultrasonic beams (forward and backward) with the low frequency sound field emitted in the air by the vibrating surface. This latter phenomenon, which is a volume effect, is proportional to the low frequency field and then to the surface velocity. The relative contribution of Doppler and parametric effects are evaluated, and it is shown that both have to be taken into account when sensing broadband vibrations in the air.

Performances of the parametric acoustic vibrometer for vibration sensing

Ultrasonic vibration sensing is presently being developed as an alternative to optical techniques. It can be used in air or in liquid and is a highly sensitive method. A continuous ultrasonic beam is emitted towards the moving surface and is received after reflection from the object under test. Two different physical effects contribute to the phase modulation of the received signal: the Doppler effect and the nonlinear parametric interactions between the high frequency probing beams (forward and backward) and the low frequency sound field emitted by the vibrating surface. The following questions can then be addressed: What is the spatial resolution of a vibration measurement system based on the parametric effect? How does it compare to the resolution of the usual Doppler effect which is directly linked to the high frequency beam width? To answer that, the computation of the parametric field has been done by using a double decomposition: in Gaussian beams for the probing beam and in plane waves for the low frequency field.

An ultrasonic phase sensitive method for surface velocity measurements in a liquid

A highly sensitive ultrasonic method for measuring the surface velocity in a liquid is described. The method is based on sensitive detection of the phase of a high frequency continuous ultrasonic wave (probe beam) reflected from the moving surface. A simple analysis shows that the interaction, through the acoustic nonlinearity parameter B/A of the fluid, between the reflected carrier wave and the low frequency pressure wave transmitted by the moving surface in the liquid, produces a phase-shift of the carrier proportional to the surface velocity and to the time delay undergone by the probe beam. Results of experiments carried out in water with a 30-MHz focused transducer probe are in good agreement with the analysis. Surface velocity smaller than 0.1 mm/s, i.e. mechanical displacements smaller than 5 pm can be detected in a 5 MHz bandwidth. Lateral resolution of 0.4 mm has been achieved. Compared to optical techniques, this method has the advantages of compactness and low sensitivity to surface roughness.

Brillouin scattering-like effect and non-reciprocal propagation of elastic waves due to spatio-temporal modulation of electrical boundary conditions in piezoelectric media

The properties of a one-dimensional phononic crystal made of identical piezoelectric elements separated by thin metallic electrodes connected to the ground are studied theoretically for cases where the locations of the electrical connections change as a function of time with a specific speed. This spatio-temporal modulation of the electrical boundary conditions results in significant non-linear effects that are evidenced numerically. The interaction between an incident harmonic longitudinal wave and the time-dependent phononic crystal is shown to lead to frequency splitting analogous to Brillouin scattering. Moreover, the boundaries of the Bragg bandgaps are strongly affected, and for some specific modulation speed, one-way wave propagation can be achieved.

Acoustic nonlinear parameter measurement in solid with a contact phase modulation method

In this work, a new method to measure in contact the nonlinearity parameter /spl beta/ of solid plates is presented. This method is based on an idea developed by Barriere and Royer (2001) to measure the nonlinearity parameter in liquids. A high frequency (HF) tone-burst signal of 20 MHz is inserted in the material by a contact-transducer (with a suitable coupling). A low frequency (LF) pulse (2.5 MHz) is applied to the other face, in the opposite direction, such that the nonlinear interaction of the two waves takes place during the back propagation toward the HF transducer. This collinear interaction creates a phase modulation of the HF tone-burst which is directly proportional to the /spl beta/ coefficient and the particle velocity of the LF wave. To determine this particle velocity a self reciprocity calibration of the contact LF transducer is used, as proposed by Dace et al. (1991). A numeric phase demodulation has been performed, giving the beta coefficient of the sample. The proposed method is validated by nonlinearity parameter measurements in Fused Silica.

Application of multiple scattering theories to the evaluation of granular distributions based on phase velocity measurements

Using multiple scattering theories, the influence of granular distribution on acoustic phase velocity in particle-loaded liquids was studied. Theoretical results were compared to measurements in suspensions of acrylic spheres in ethylene glycol. It appears that the phase velocity versus frequency shows a minimum, the position and sharpness of which are related to the mean diameter of particles and to the standard deviation of the diameter distribution. Its amplitude is only influenced by the concentration. Therefore, it is shown that granulometric properties of suspensions can be determined by an ultrasonic method, through the behavior of phase velocity versus frequency.