Tomas Gomez Alvarez-Arenas

Achieving accurate measurements of inflammation levels in tissues or thickness changes in biological membranes (e.g., amniotic sac, parietal pleura) and thin biological walls (e.g., blood vessels) from outside the human body, is a promising research line in the medical area. It would provide a technical basis to study the options for early diagnosis of some serious diseases such as hypertension, atherosclerosis or tuberculosis. Nevertheless, achieving the aim of non-invasive measurement of those scarcely-accessible parameters on patient internal tissues, currently presents many difficulties. The use of high-frequency ultrasonic transducer systems appears to offer a possible solution. Previous studies using conventional ultrasonic imaging have shown this, but the spatial resolution was not sufficient so as to permit a thickness evaluation with clinical significance, which requires an accuracy of a few microns. In this paper a broadband ultrasonic technique, that was recently developed by the authors to address other non-invasive medical detection problems (by integrating a piezoelectric transducer into a spectral measuring system), is extended to our new objective; the aim is its application to the thickness measurement of OPEN ACCESS Sensors 2012, 12 15395 sub-millimeter membranes or layers made of materials similar to some biological tissues (phantoms). The modeling and design rules of such a transducer system are described, and various methods of estimating overtones location in the power spectral density (PSD) are quantitatively assessed with transducer signals acquired using piezoelectric systems and also generated from a multi-echo model. Their effects on the potential resolution of the proposed thickness measuring tool, and their capability to provide accuracies around the micron are studied in detail. Comparisons are made with typical tools for extracting spatial parameters in laminar samples from echo-waveforms acquired with ultrasonic transducers. Results of this advanced measurement spectral tool are found to improve the performance of typical cross-correlation methods and provide reliable and high-resolution estimations.

This work presents the design, construction and characterization of air-coupled piezoelectric transducers using 1–3 connectivity piezocomposite disks with a stack of matching layers being the outer one an active quarter wavelength layer made of polypropylene foam ferroelectret film. This kind of material has shown a stable piezoelectric response together with a very low acoustic impedance (<0.1 MRayl). These features make them a suitable candidate for the dual use or function proposed here: impedance matching layer and active material for air-coupled transduction. The transducer centre frequency is determined by the λ/4 resonance of the polypropylene foam ferroelectret film (0.35 MHz), then, the rest of the transducer components (piezocomposite disk and passive intermediate matching layers) are all tuned to this frequency. The transducer has been tested in several working modes including pulse-echo and pitch-catch as well as wide and narrow band excitation. The performance of the proposed novel transducer is compared with that of a conventional air-coupled transducers operating in a similar frequency range.

A novel non-contact and air-coupled ultrasonic technique to characterize ion-track membranes (ITM) is described and tested. It is based on the application of Fourier spectral analysis to ultrasound pulses transmitted through ITM, thus both magnitude and phase of the transmission coefficient (TC) are obtained. It takes advantage of the fact that ITM offer two rather decoupled paths to ultrasound propagation:

ABSTRACT In this letter, the possibility to use a technique based on the analysis of thickness resonances of air-surrounded aerogel plates at ultrasonic frequencies to obtain viscoelastic properties is investigated. These resonances were excited and sensed by airborne ultrasonic waves. Toward this purpose, specially designed air-coupled, high-sensitivity, and broadband piezoelectric transducers were used. Precise and simultaneous measurements of the velocity and attenuation of longitudinal and shear waves at different frequencies, as well as aerogel density, were obtained. It allowed us to afford a full characterization of the viscoelastic properties of these materials at ultrasonic frequencies. (C) 2002 American Institute of Physics.

Wide-band and air-coupled ultrasound pulses are propagated through slabs of open-cell reticulated solid foams and the transmission coefficient is measured. Under these conditions, fluid in the pores and solid skeleton are strongly decoupled; in addition, at frequencies well over that of the relaxation of the influence of the tortuosity, it is observed that energy loss approaches to a constant value: |γ|∞. Internal surface area (Si) and the length dimension parameter (Λ) of the foams are independently measured. Results reveal that |γ|∞ vs Λ or Si follows a power law very close to linear, suggesting a possible spatial normalization for |γ|∞.

A novel ultrasonic matching layer for improving coupling between piezoelectric transducers and an air load is presented and the results of a theoretical and experimental program of work are provided. A combination of a porous material that has very low acoustic impedance with a low-density rubber material forms the basis of the approach. These matching layers were first analyzed experimentally using scanning electron and optical microscopy to determine the microscopic structure. Air-coupled resonance measurements were then performed to reveal the acoustic parameters of the individual layers that were identified within this multilayered structure. These data were then incorporated into a conventional linear model, and this has been verified and used to study performance and produce designs. Close correlation between experiment and theory is demonstrated. The most efficient designs have been implemented in a pitch/catch air-coupled system, and an improvement in received signal amplitude of 30 dB was achieved when compared with the unmatched case.

ABSTRACT In this work, the feasibility of using non-contact ultrasonic techniques (air-coupled and scanning acoustic microscopy, SAM) for characterizing different dry-cured meat products was assessed. Air-coupled ultrasonic measurements were performed on vacuum packaged sliced dry-cured ham, and compared with contact measurements. The average ultrasonic velocity in dry-cured ham was 1846 ± 49 m/s and 1842 ± 42 m/s for air-coupled and contact measurements, respectively. The deviation (1% relative error) between both techniques was related to the influence of the heterogeneous structure and composition of dry-cured ham and the transducer focusing. The SAM was used to characterize dry-cured ham and chorizo samples. B-scan images for dry-cured ham and chorizo showed two dominant reflections from the sample, linked to reflections in the lean and fatty tissues. The results indicate that contact ultrasonic measurements could be replaced by the air-coupled technique, reducing the measuring time and the material handling. On the other hand, SAM technique allows the microscopic characterization of dry-cured meat products.

Noncontact ultrasonic spectroscopy (NCUS) is used to excite and sense thickness resonances in films of polypropylene ferroelectrets. From the comparison of these measurements with theoretical calculations it is possible to extract some material properties: film thickness and density, velocity, and attenuation of ultrasounds and variation in these two magnitudes with the frequency. Hence elastic compliance and acoustic impedance are worked out. Observed variation in the attenuation with the frequency exhibits classical viscoelastic behavior which can be used to investigate the underlaying physical mechanism. In addition, the influence of the metallization on the film response is studied. A modification of the NCUS method is proposed on the basis of the piezoelectric response of these films, which give rise to an alternative characterization method. Consistency of both methods is verified.

Air-coupled wideband ultrasonic piezoelectric transducers are used in the frequency range 0.3 to 1.3 MHz to excite and sense first-order thickness resonances in the leaves of four different tree species at different levels of hydration. The phase and magnitude spectra of these resonances are measured, and the inverse problem solved; that is, leaf thickness and density, ultrasound velocity, and the attenuation coefficient are obtained. The elastic constant in the thickness direction (c33) is then determined from density and velocity data. The paper focuses on the study of c33, which provides a unique, fast, and noninvasive ultrasonic method to determine leaf elasticity and leaf water content.

A novel experimental technique based on phase spectroscopy and through transmission of high-frequency airborne ultrasonic pulses is used to study rigid open cell foams. Phase velocity shows an anomalous relaxation like behavior which is attributed to a frequency variation of the apparent tortuosity. An explanation is proposed in terms of the relationship between the different length scales involved: microstructure and macroscopic behavior. The experimental technique together with the proposed apparent tortuosity scheme provides a novel and unique procedure to determine simultaneously tortuosity and characteristic length dimension and shape of the solid constituent of foams and porous materials in general.

A new theoretical frame is presented for the study of the elastic, dielectric and piezoelectric properties of two-phase piezoelectric composite materials of complex microstructure. Beginning from basic thermodynamic relations, new general constitutive relations for a two-phase composite have been obtained. These new constitutive equations consider in a general and novel way the dielectric and elastic interaction between the composite's components. This approach was successfully applied to the study of porous ceramics, and now its applicability to the study of piezocomposites will be analyzed. Some measurements of 3-3 composites' dielectric and mechanical properties are presented and compared with the results of porous piezoceramics. The existence and influence of these couplings is clearly observed

The characterization of piezoelectric materials is a well-known matter supported by several international standards -IEEE, -IEC. The assumptions normally accepted about the material properties in the standards are low dielectric and mechanical losses and material homogeneity. Some commercial piezoceramics have a porosity about 10% or even higher. The pore space is normally interconnected and filled with oils or polymers. Therefore the material is no more homogeneous and even the losses are not negligible. These materials, widely used to fabricate broad-band transducers, must be accurately characterized if good modelling work is to be undertaken. Some singular results when characterizing these materials following the standards are reported. In particular, the strong influence of the minor phase on the global properties of a porous ceramic is analyzed. The change of the main elastic, dielectric and piezoelectric parameters of some porous ceramics when different fillers are used is reviewed. Moreover, the dynamic behavior of the ceramic is also strongly affected by the properties of the minor phase. All these differences are justified in terms of a model which takes into account the different ways in which the two phases of the porous ceramic interact. The appearance of forbidden mechanical resonances are reported and analyzed in this paper. A characterization procedure based on the measurements of the complex coefficients is used together with the standard methods

This work presents a study of the properties of particulate composites. The whole range of particle volume fraction (0-1) and ideal 0-3, 3-3 and intermediate 0-3/3-3 connectivities are analysed. Two different approaches to produce a realistic model of the complex microstructure of the composites are considered. The first one is based on a random location of mono-dispersed particles in the matrix; while the second incorporates a size distribution of the particles based on experimental measurements. Different particle shapes are also considered. A commercial finite element package was used to study the propagation of acoustic plane waves through the composite materials. Due to the complexity of the problem, and as a first step, a two-dimensional model was adopted. The results obtained for the velocity of sound propagation from the finite element technique are compared with those from other theoretical approaches and with experimental data. The study validates the use of this technique to model acoustic wave propagation in 0-3/3-3 connectivity composites. In addition, the finite element calculations, along with the detailed description of the microstructure of the composite, provide valuable information about the micromechanics of the sample and the influence of the microstructure on macroscopic properties.

"The performance of air-coupled piezoelectric transducers (0.1-2.0 MHz) is mainly determined by the lack of materials to produce impedance matching layers (ML). Hence, most of these transducers operate under non-ideal matching conditions and the properties of the outer ML become critical. Using outer ML with acoustic impedance (Z) in the range 0.02-0.1MRayl and attenuation coefficient, α <1000 Np/m @ 1 MHz, two-way Insertion Loss figures up to -25 dB have been reported, with 6dB bandwidths about 20% and 60% for ceramic and composite transducers, respectively.1, 2 Further bandwidth increments were tried by using PMN-PT single crystal composites but only a moderate improvement was achieved. This was mainly attributed to the limitations of this matching scheme.3 The introduction of a new range of ML (0.1MRayl < Z < 1.9 MRayl and α < 300 Np/m @ 1 MHz ) is described. These ML present significant technological advantages over previous solutions and offer the possibility to produce a more efficient and flexible matching by either increasing the number of ML, or optimizing the tuning of their thicknesses and impedances. Use of this novel strategy along with single crystal composites appears promising to overtake the limitations of previous matching schemes and get the full potential of single crystal composites.
Response of air-coupled transducers with different matching schemes is calculated and obtained results are compared. Three different piezoelectric elements were considered: bulk PZ27, and two 1-3 composites: PZT5A ceramic (random fibers, 65%) and PMN-PT single crystal (dice&fill, 50%). 1, 2 Results show that for bulk ceramics and ceramic piezocomposites the use of this novel matching strategy is able to provide results similar to those obtained before with more conventional matching schemes; however, when used together with single crystal piezocomposites a remarkable increment of the bandwidth is obtained, while sensitivities are kept at similar levels. Finally, air-coupled transducers prototypes in monolithic and phased array configurations for the best theoretical configurations were built, characterized, tested and compared with the purpose to illustrate the real possibilities of this approach.
The level of transducer performance here presented is essential for applications that require high sensitivity and wide-band. This includes applications in NDT, materials characterization, surface metrology, etc, but new possibilities can be opened up in fields like fully non-invasive characterization of biological tissues, contact-less human machine interfaces or gesture recognition devices, short range and secure wireless information transfer, wireless airborne power transmission, detection of objects and image formation using air-coupled phased arrays. Finally, some of these ideas can also be exported to the design of transducers in other fields whenever the ideal matching solution is not attainable due to the lack of proper materials."

A novel technology in the paper industry makes possible to produce paper by using a mineral powder and a polymer instead of cellulose fibers. This new product is called mineral paper, it presents some potential environmental advantages compared with conventional paper, while it exhibit a similar appearance and properties. The purpose of this work is to determine the possibilities of an air-coupled ultrasonic technique using wide band signals and spectral analysis to study this kind of materials. As no direct contact nor coupling fluids between the paper and the transducers is required, this technique is specially well suited to this problem. It also offers good perspectives for the development of a on-line quality control system. A through transmission technique (0.15–2.3 MHz) is employed and Fourier analysis is performed to obtain both magnitude and phase spectra of the transmission coefficient. Properties in the thickness direction as well as in the paper plane has been determined by the excitation and analysis of thickness and plate resonances at several incident angles and different directions within the paper plane. Different paper grades (from 140 to 480 g/m2) have been studied. Very high attenuation coefficients and very low propagation velocities (and hence elastic constant) have been obtained for most cases, this can be explained by considering the large porosity of this material (up to 50%) and the microstructure: a mixture of solid grains with a resin with a relatively large fraction of air-filled pores. Measurements show that unlike conventional cellulose machine made paper this material is transversely isotropic (isotropic in the paper plane) and that the degree of anisotropy (when in-plane directions are compared with the thickness direction) largely depends on the level of resin impregnation.

Shear waves are investigated in leaves of two plant species using air-coupled ultrasound. Magnitude and phase spectra of the transmission coefficient around the first two orders of the thickness resonances (normal and oblique incidence) have been measured. A bilayer acoustic model for plant leaves (comprising the palisade parenchyma and the spongy mesophyll) is proposed to extract, from measured spectra, properties of these tissues like: velocity and attenuation of longitudinal and shear waves and hence Young modulus, rigidity modulus, and Poisson's ratio. Elastic moduli values are typical of cellular solids and both, shear and longitudinal waves exhibit classical viscoelastic losses. Influence of leaf water content is also analyzed.

This work presents the design, construction and characterization of air-coupled
piezoelectric transducers using 1–3 connectivity piezocomposite disks with a stack of
matching layers being the outer one an active quarter wavelength layer made of
polypropylene foam ferroelectret film. This kind of material has shown a stable
piezoelectric response together with a very low acoustic impedance (<0.1 MRayl). These
features make them a suitable candidate for the dual use or function proposed here:
impedance matching layer and active material for air-coupled transduction. The transducer
centre frequency is determined by the λ/4 resonance of the polypropylene foam
ferroelectret film (0.35 MHz), then, the rest of the transducer components (piezocomposite
disk and passive intermediate matching layers) are all tuned to this frequency. The
transducer has been tested in several working modes including pulse-echo and pitch-catch
as well as wide and narrow band excitation. The performance of the proposed novel
transducer is compared with that of a conventional air-coupled transducers operating in a
similar frequency range.

This work presents an investigation carried out to apply a broadband ultrasonic spectroscopy technique to the study of membrane filters. The technique is based on the analysis of the amplitude spectra of broadband airborne ultrasonic pulses transmitted through filter membranes. In particular, analysis of the through thickness resonances is used. Density of the membrane and velocity and attenuation of sound waves are obtained. These magnitudes are correlated to other properties of the membrane like porosity, pore size, water flow and bubble point. Observed relations suggest that this technique can be used as a filter integrity test and as a non-invasive characterization procedure.

The directional freezing of microfiber suspensions is used to assemble highly porous (porosities ranging between 92{\%}and 98{\%}) SiC networks. These networks exhibit a unique hierarchical architecture in which thin layers with honeycomb-like structure and internal strut length in the order of 1–10 µm in size are aligned with an interlayer spacing ranging between 15 and 50 µm. The resulting structures exhibit strengths (up to 3MPa) and stiffness (up to 0.3 GPa) that are higher than aerogels of similar density and comparable to other ceramic microlattices fabricated by vapor deposition. Furthermore, this wet processing technique allows the fabrication of large-size samples that are stable at high temperature, with acoustic impedance that can be manipulated over one order of magnitude (0.03–0.3MRayl), electrically conductive and with very low thermal conductivity. The approach can be extended to other ceramic materials and opens new opportunities for the fabrication of ultralight structures with unique mechanical and functional properties in practical dimensions.