Non Contact Resonant Ultrasonic Spectroscopy

1. Introduction

2.  Applications   

             2.1. Silica aerogels

             2.2 Filtration membranes

             2.3 Mineral paper

             2.4 Ferroelectret films

             2.5 Vacuum packaged ham

              2.6 Plant leaves

              2.7 Light and Strong SiC Networks

1. Introduction

Resonant ultrasonic spectroscopy (RUS) is a well know technique to obtain the elastic constants of solid materials from the analysis of the resonant frequencies of different modes of vibration of samples having a well defined geometry (Migliori 1993, Migliori and Sarrao 1997, Zadler et al. 2004). Transducers used to excite and sense these resonances employ point contact with the samples to minimize any mechanical load that could shift the resonant frequency of any mode and so produce misleading results.

However, this approach is not possible for the case of very soft materials or for plate samples. In these cases, non-contact techiques to both excite and sense the resonances are required. A very good alternative is the use of air-coupled ultrasound.

In the case of plates, air-coupled ultrasound has been used to excite and sense thickness resonances with a similar purpose: to obtain elastic constants (some examples are: Hutchins and Wright 1994, Schindel and Hutchins 1995, Álvarez-Arenas 2003, 2010 and Álvarez-Arenas et al. 2002, 2009 and 2010). In this sense, this technique can be considered as non-contact RUS, though there are significant differences with conventional RUS as in this case the air-load is always considered.

2. Applications

2.1 Silica aerogels.

Silica aerogel are highly porous and brittle solids, that present unique acoustic and thermal properties. In this case, use of air-coupled ultrasound is very convenient as coupling fluid will destroy de material and the pressure required for dry coupling can break the samples. Some results were publishe in Applied Phys. Lett. (Álvarez-Arenas et al. 2002). In this work,the magnitude spectrum of the thickness resonances in the transmission coefficient at normal and oblique incidence was measured for some silica aerogel plates. Longitudinal and shear velocity and attenuation coefficient and their variation with the frequency were obtained.

Transmission coefficient, normal incidence, magnitude spectrum, aerogel plate 200 kg/m3, 3mm thick

Transmission coefficient, oblique incidence 19º, magnitude spectrum, aerogel plate 200 kg/m3, 3mm thick

2.2 Filtration membranes.

The results of the application of NC-RUS to the sutudy of filtrasion membranes hve been publised in J. Membr. Sci. (Alvarez-Arenas 2003a) and in IEEE Trans. Ultrason. Ferroelec. Freq. Control (Alvarez-Arenas 2003b). Filtration membranes are highly porous (open-pore porosity > 70%) and thin (thickness about 100 micron) polymer films. First thickness resonance normally appears between 0.1 and 2.0 MHz, so use of non-contact resonant ultrasonic spectroscopy is a convenient way to characterize these materials.

First thickness resonance of a Polyethersulfone membrane, pore size 0.1 micron, thickness 140 micron. NC-RUS provided: density: 370 kg/m3, ultrasound velocity: 660 m/s, ultrasound attenuation at resonance: 445 Np/m

Moreover, ultrasonic properties are closely related to other parameters normally used to characterize the membranes or as integrity test, like the flux resistivity and the bubble point.

Relationship between ultrasound velocity obtained from NC-RUS and bubble point.

 2.3 Mineral paper

2.4 Ferroelectret films

2.5 Vacuum packaged ham

2.6 Plant leaves

The first work about the use of NC-RUS to plant leaves was published in Appl. Phys. Lett in 2009 (Álvarez-Areans et al.)

Specially designed pair of air-coupled transducers mounted on an U-shaped holder to take NC-RUS measuremetns in Vitis vinifera leaves

2.7 Light and Strong SiC Networks

This special material was first produced by the Centre for Advanced Structural Ceramics of the Department of Materials of the Imperial College (London). NC-RUS were used to characterize plates of this material, different plates, cur along different directions, permited us to study different directions of propagation and determine the anisotropy of the material. Results were published in 2016 in Adv. Func. Mat. (Ferraro et al. 2016)

NC-RUS measurements of slabs of this material cut along different directions.

References

Álvarez-Arenas, T.E.G, Montero, F., Moner-Girona, M., Rodrı́guez, E., Roig, A., Molins, E., 2002. Viscoelasticity of silica aerogels at ultrasonic frequencies. Appl. Phys. Lett. 81, 1198.

Álvarez-Arenas, T.E.G., 2003a. Air-coupled ultrasonic spectroscopy for the study of membrane filters. J. Memb. Sci. 213, 195–207.

Alvarez-Arenas, T. E.G 2003b. A nondestructive integrity test for membrane filters based on air-coupled ultrasonic spectroscopy. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 50(6), 676–85.

Álvarez-Arenas, T.E.G, Sancho-Knapik, D., Peguero-Pina, J.J., Gil-Pelegrín, E., 2009. Noncontact and noninvasive study of plant leaves using air-coupled ultrasounds. Appl. Phys. Lett. 95, 193702.

Álvarez-Arenas, T. E.G. (2010). Simultaneous determination of the ultrasound velocity and the thickness of solid plates from the analysis of thickness resonances using air-coupled ultrasound. Ultrasonics, 50(2), 104–9.

Álvarez-Arenas, T.E.G., Calás, H., Cuello, J.E., Fernández, a. R., Muñoz, M., 2010. Noncontact ultrasonic spectroscopy applied to the study of polypropylene ferroelectrets. J. Appl. Phys. 108, 074110. Appl. Phys., 21, 89-93

Ferraro, C., García-Tuñón, E., G. Rocha, V., Barg, S., Fariñas, M. D., Álvarez-Arenas, T. E. G., Sáiz, E. (2016). Light and Strong SiC Networks. Advanced Functional Materials, 12.    DOI: 10.1002/adfm.201504051

Hutchins, D.A., Wright, W.M.D., 1994. Ultrasonic measurements in polymeric materials using air-coupled capacitance transducers. J. Acoust. Soc. Am. 96, 1634–1642.

Migliori, A. Sarrao, J.L. Visscher, W.M. Bell, T. M. Lei, M. Fisk, Z. and Leisure, R.G. 1993 Resonant ultrasound spectroscopic techniques for measurement of the elastic moduli of solids, Physica B, 183(1-2), 1–24.

Migliori, A, Sarrao, J.L. 1997, Resonant Ultrasound Spectroscopy, Wiley, New York, 1997 28.

Schindel, D.W., Hutchins, D.A., 1995. Through-thickness characterization of solids by wideband air-coupled ultrasound. Ultrasonics 33, 11–17. Soft Matter. 7, 7078.

Zadler, B. J. Le Rousseau, J. H. L. Scales, J. A. and Smith,  M. L. 2004 Resonant Ultrasound Spectroscopy: theory and application, Geophys. J. Int., 156, 154–169.