Search

International audience; 1-3 piezocomposites are first choice materials for integration in ultrasonic transducers due to their high electromechanical performance, particularly, in their thickness mode. The determination of a complete set of effective electroelastic parameters through a homogenization scheme is of primary importance for their consideration as homogeneous. This allows for the simplification of the transducer design using numerical methods. The method proposed is based on acoustic wave propagation through an infinite piezocomposite, which is considered to be homogeneous material. Christoffel tensor components for the 2 mm symmetry were expressed to deduce slowness curves in several planes. Simultaneously, slowness curves of a numerical phantom were obtained using a finite element method (FEM). Dispersive curves were initially calculated in the corresponding heterogeneous structure. The subsequent identification of the effective parameters was based on a fitting process between the two sets of slowness curves. Then, homogenized coefficients were compared with reference results from a numerical method based on a fast Fourier transform for heterogeneous periodic piezoelectric materials in the quasi-static regime. A relative error of less than 2% for a very large majority of effective coefficients was obtained. As the aim of this paper is to implement an experimental procedure based on the proposed homogenization scheme to determine the effective parameters of the material in operating conditions, it is shown that simplifications to the procedure can be performed and a careful selection of only seven slowness directions is sufficient to obtain the complete database for a piezocomposite containing square-shaped fibers. Finally, further considerations to adapt the present work to a 1-3 piezocomposite with a fixed thickness are also presented.

For transducer design, it is essential to know the acoustic properties of the materials in their operating conditions. At frequencies over 15 MHz, standard methods are not well adapted because layers are very thin and backings have very high attenuation. In this article, we report on an original method for measuring the acoustic properties in the 15–25 MHz frequency range, corresponding to typical skin-imaging applications, using a backing/piezoelectric multilayer structure. Onto a porous Pb(Zr0.53Ti0.47O3 (PZT) substrate, a piezoelectric PZT-based layer with a thickness of $\sim 20~\mu \text{m}$ was deposited and directly used to excite an acoustic signal into water. Herein, the measured signal corresponds to the wave that is first reflected on a target in water, then propagates back to the multilayer structure, and is transmitted through the thick film and further to the rear face of the porous backing, where it is again reflected and returns to the piezoelectric thick film, thus avoiding overlap with the electrical excitation signal. Two types of PZT backings with similar porosity of ~20% and spherical pores with size of 1.5 and 10 $\mu \text{m}$ were processed. The ultrasound group velocities were measured at ~3500 m/s for both samples. The acoustic attenuation of the backings with pore size of 1.5 and 10 $\mu \text{m}$ were 12 and 33 dB/mm, respectively, measured at 19 MHz. This advanced measuring technique demonstrated potential for the simple measurements of acoustic properties of backing at high frequencies in operating conditions. Importantly, this method also enables rapid determination of the minimum required thickness of the backing to act as a semi-infinite medium, for high-frequency transducer applications.

International audience

International audience; BaTiO3-based crystal fibres with mm-sized grains were grown by the micro-pulling down technique from the BaTiO3–CaTiO3–BaZrO3 solid solution with pulling velocities of about 6, 9 and 15 mm h−1. The natural growth direction was identified as (001)pc. For the pulling velocities of about 15 mm h−1 and 9 mm h−1, effective partition coefficients have been calculated from Castaing micro-probe measurements, and gave, respectively, 1.3 and 2 for Zr, and 0.95 and 0.9 for Ca. Laser-induced breakdown spectroscopy measurements reveal a strong inhomogeneity and variations of Zr contents while Ca contents show an opposite variation trend with a more steady distribution. Coexistence of two crystallized perovskite solid solutions is suggested. Most efficient polycrystals with mm-sized grains and 0.5 mol% Zr and 11 mol% Ca as average contents exhibit Curie temperatures higher than 113 °C, electromechanical coupling factors kt up to 41% and piezoelectric charge coefficients d33 up to 242 pC N−1 at room temperature. These values are similar to piezoelectric coefficients reported in the literature for oriented flux-grown single crystals with close compositions. Both chemical and physical results obtained in the BCTZ system make the μ-PD technique a promising way to improve the piezoelectric response of lead-free solid solution-based single crystals.

We have investigated the processing of lead-zirconate-titanate-based thick films by inkjet printing Pb(Zr0.53Ti0.47)0.98Nb0.02O3 with a 6 mol% excess of PbO nanosized powder dispersed in water. Different waveforms were employed to determine the optimum size and shape of the drops. A uniform, defect-free pattern with dimensions of 4 mm × 4 mm can be printed using 20 V and a drop spacing of 20 μm. The inkjet-printed films were heated to 400 °C to remove the organics and subsequently sintered at 750 and 850 °C. The correlations between the density, grain size and electromechanical properties of the thick films and bulk ceramics are qualitatively discussed. A thickness coupling factor of 46% was obtained for a 15-μm-thick film sintered at low temperature of 850 °C, which is comparable to the value of the bulk ceramic with an identical nominal chemical composition. Our results are important for the economic and environmental-benign printing of piezoelectric materials applicable in variety of electronic devices, such as sensors, actuators, transformers, piezoelectric energy harvesters and transducers.

The miniaturization of electronic devices and power systems requires the fabrication of functional components in the form of micrometer-sized thick films. A major challenge is the integration of functional ceramics with metals, which are considered incompatible with high-temperature ceramic processing. To overcome the integration barrier, an aerosol deposition (AD) spray-coating method based on room temperature deposition can be used. By employing the AD method, we were able to deposit relaxor-ferroelectric 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 ceramic thick films on low-cost stainless-steel substrates. The as-deposited films were dense, with ∼97% of the theoretical density. Moreover, the post-deposition annealing at 500 °C did not result in any microstructural changes. Compared to the as-deposited films, the annealed films exhibit improved energy storage and electromechanical properties. The annealed thick films achieve a recoverable energy density of 15.1 J⋅cm−3 at an electric field of 1350 kV⋅cm−1 and an electric-field cycling stability of 5 million cycles. A piezoelectric response was detected through the entire film thickness by piezoelectric force microscopy. Macroscopic displacement measurements revealed a maximum relative strain of 0.38% at 1000 kV⋅cm−1, corresponding to inverse effective piezoelectric coefficient of ∼40 pm⋅V−1. In this study, we overcame the integration challenges and demonstrated the multifunctionalization of future ceramic-metal structures, as the deposited thick films on stainless steel exhibit energy storage capability and piezoelectric properties.

In this work, the effects of thermal annealing at 500 °C on aerosol-deposited 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 thick films on stainless-steel substrates are investigated using two complementary methods at high and low applied external electric fields. The first one is the positive up negative down method, which allows us to obtain information about the switching and non-switching contributions to the polarization. It shows that the as-deposited film is ferroelectric before annealing, since it has a switching contribution to the polarization. After annealing, both the switching and non-switching contributions to polarization increased by a factor of 1.6 and 2.33, respectively, indicating a stronger ferroelectric behavior. The second method is based on impedance spectroscopy coupled with Rayleigh analysis. The results show that post-deposition thermal annealing increases the reversible domain wall contribution to the dielectric permittivity by a factor of 11 while keeping the threshold field similar. This indicates that, after annealing, domain wall density is larger while domain wall mobility remains similar. These two complementary characterization methods show that annealing increases the ferroelectric behavior of the thick film by increasing the domain wall density, and its influence is visible both on polarization vs electric field loop and dielectric permittivity.

This numerical study compares the acoustic performance of three focalization principles (using a concave piezoelectric element, a lens, or a Fresnel Zone Plate) for a high-frequency (50 MHz) PVDF-based single-element transducer. A pseudo-spectral time-domain method was used to simulate the ultrasound wave propagation in water and to deduce acoustic properties in the focal zone. The focal distance was investigated between 10-20 mm with a step of 1 mm. The external diameter was identical for all configurations and fixed at 6 mm. The results showed a similar behavior between the concave shape and acoustic lens in terms of spatial resolution. However, the comparison showed a significant loss in sensitivity (−22 dB) for the FZP technique mainly due to the large bandwidth at −6 dB (27 MHz) of the electric excitation signal. But, with low bandwidth excitation signal, the simulation showed better performance in the focal zone with FZP transducer and a sensitivity difference with the two others configurations in the range 4.5-8.5 dB.

International audience; Highly textured cm-sized (Ba1−xCax)TiO3 (x = 0.05) polycrystals at off-eutectic composition were grown by the micro-pulling down freezing technique from a BaTiO3–CaTiO3 system at a high pulling velocity of about 48 mm h−1. Textured polycrystals exhibiting submillimetre-sized dendrites along the (110)pc growth direction and geometrical grains along the (100)pc direction were revealed by a complementary analysis performed by Laue diffraction and EBSD techniques. An average Ca content of about x = 0.0435 was measured throughout the whole volume of the boule and led to an effective partition coefficient of Ca of about 0.87. LIBS measurements highlighted the local segregation of Ca and major impurity contents within dendrites, grains and at the grain boundaries. Dielectric and piezoelectric measurements performed on oriented polycrystalline textured samples led to Curie temperatures up to 116 °C and d33 up to 214 pC N−1, in very good agreement with literature values. Both chemical and physical results obtained in the BCT system make the μ-PD technique a promising and alternative way to improve the piezoelectric response of lead-free solid solution-based crystals through their texturing along preferential crystallographic directions.

In the field of high-frequency ultrasound imaging ( ${\geq} 20$ MHz), tools for characterizing the performance of imaging systems are lacking. Indeed, commercial phantoms are often inadequate for this frequency range. The development of homemade phantoms on the laboratory scale is often required but is hindered by the difficulty in making very small structures that must be distributed with high accuracy in 3-D space. We propose investigating the use of 3-D photopolymer printing to create resolution and calibration phantoms designed for high-frequency ultrasound imaging. The quality and importance of these phantoms are discussed from the point of view of ultrasound parameters and imaging. First, the compressional wave group velocity, acoustic impedance, and attenuation of six photopolymerized materials were measured using temporal and spectral methods in a substitution experimental setup. Measurements were performed on printed samples using a broadband-focused single-element transducer covering a large frequency range (15–55 MHz). Two 3-D phantoms incorporating different shapes and dimensions were designed and printed. Finally, 3-D acoustic images were obtained using either a mechanically driven single-element transducer or a high-frequency commercial imaging system. Three-dimensional printing enabled us to generate phantoms suitable for high-frequency imaging with complex geometry inclusions and with a surrounding material having acoustic properties close to those of human skin. The calculated SNR between the inclusion and surrounding media is approximately 50 dB. In conclusion, 3-D printing is a useful tool for directly, easily, and rapidly manufacturing ultrasound phantoms for ultrasound imaging system assessments and computational calibration or validation.

Piezoelectrics are key materials for energy conversion, for example in ultrasound transducers and energy harvesters. This work presents the synthesis and characterization of the lead-free piezoelectric composition (Li0.06(K0.52Na0.48)0.94)(Nb0.71Ta0.29)O3 doped with 0.25 mol% Mn (KNNLTM) as textured ceramics. Templated grain growth from NaNbO3 platelet templates aligned by tape casting was used to introduce texture, and after sintering for 14 h at 1100 °C this produced up to 84% (100)pc grain orientation. After high temperature poling, the textured samples exhibit reasonable piezoelectric response with d 33 values up to 171 pC N−1, and k t values of 0.35, which is 71% of the response obtained in a single crystal of the same composition. The low relative dielectric permittivity of the textured and high temperature-poled KNNLTM (ϵ 33 T/ϵ 0 down to 182) resulted in record-high piezoelectric voltage constants (g 33 up to 101 mV m N−1), higher than previously reported for lead-free piezoelectric ceramics, as well as very high figure of merit (d 33 g 33 up to 16 × 10−12 m3 J−1) for non-resonant energy harvesting in compression. These numbers make the textured KNNLTM materials of this work highly promising for use in thickness mode, non-resonant piezoelectric energy harvesters.

A new model for piezoelectric textured ceramics was developed that considers the presence of porosity, which can appear during heat treatment (ceramic sintering). In the long wavelength approximation, a matrix method, which has already been applied to piezoelectric composites, was extended to textured ceramics for three phases [porosity (air), piezoelectric single-crystal (related to the texturation degree), and ceramic] to calculate the effective electroelastic modulus. This method was first compared and validated with finite-element calculations. A computation was applied to two systems with lead-based (PMN-PT) and lead-free (KNN) compositions. The results showed that the introduction of porosity in the whole material promotes electromechanical performance, particularly the electromechanical coupling factor $k_{t}$ , while limiting the degree of texturation. As an example, for the chosen PMN-PT system, an equivalent $k_{t}$ factor of 60% can be obtained with 1% porosity and an 85% single-crystal volume fraction or with 16% porosity and a 40% single-crystal volume fraction. According to the database used, this tradeoff is different. With the chosen lead-free composition, the degree of texture is less important than in the lead-based composition. Consequently, the porosity content is of primary importance for significantly improving the electromechanical coupling factor $k_{t}$ .

We investigated the innovative processing of poly(vinylidene fluoride-trifluoroethylene) P(VDFx-TrFE1-x) (x = 83 mol. %) by inkjet printing to deliver uniform and thickness-controlled layers on silicon substrates. Here, we provide detailed processing steps and optimize film deposition conditions. The thickness coupling factor for a P(VDF-TrFE) film around 11 μm thick was 22%, demonstrating good electromechanical performance after poling. These multilayer structures were specifically for high-frequency, single-element ultrasonic transducer applications. The measurements of electro-acoustic responses were in water. The maximal frequency was centered at 33.2 MHz and had a fine axial resolution at 22 μm, corresponding to a fractional bandwidth at −6 dB of 100%. In the context of technological evolutions aimed at miniaturized devices and integrated electronics, these results allow for the consideration of complex structures such as multi-element transducers for high-frequency imaging applications.

International audience; Lead–zirconate–titanate PbZr(1−x)TixO3 (PZT), materials for piezoelectric applications are the dominant piezoceramics since their properties are high. However PZTs need to be substituted by safer materials due to health and environmental problems of lead. In this work, the crystal structure modifications of BaTiO3 induced by cobalt doping and niobium/cobalt co-doping were investigated in the range 0.1–25 at%. Low Co and Co/Nb doping level significantly improve piezoelectric properties of BaTiO3. The piezoelectric constant, obtained for BaTiO3 doped with 0.075 at% of Cobalt (d33=200 pC/N), is twice of that measured for pure BaTiO3. At high doping concentration, the structure of BaTi1−x CoxO3 and BaTi1−x CoxNbxO3 change from tetragonal to hexagonal while BaTi1−x CoxNb2xO3 remains tetragonal and gradually turns into cubic. It suggests that the ionic charge defect is balanced by oxygen vacancies formation when Ti4+ are substituted by Co2+/Co3+.

International audience; We studied the effect of porosity and pore morphology on the functional properties of Pb(Zr0.53Ti0.47)O3 (PZT) ceramics for application in high frequency ultrasound transducers. By sintering a powder mixture of PZT and polymethylmetacrylate spherical particles (1.5 and 10 μm) at 1080°C, we prepared ceramics with ∼30% porosity with interconnected micrometer sized pores and with predominantly ∼8 μm spherical pores. The acoustic impedance was ∼15 MRa for both samples, which was lower than for the dense PZT. The attenuation coefficient α (at 2.25 MHz) was higher for ceramics with ∼8 μm pores (0.96 dB mm− 1 MHz− 1), in comparison to the ceramic with smaller pores (0.56 dB mm− 1 MHz− 1). The high α value enables the miniaturisation of the transducer, which is crucial for medical imaging probes. The dielectric and piezoelectric coefficients, polarisation, and strain response decreased with increased porosity and decreased pore/grain size. We suggest a possible role of pore/grain size on the switching behaviour.

Dense barium titanate (BaTiO

Vibrational energy harvesting is a very promising way to replace batteries in many types of application like industry, infrastructure, aeronautic or healthcare. Piezoelectric based energy harvesting are commonly employed to scavenge mechanical energy from environmental vibrations in which resonance frequencies are mostly below 200 Hz. Piezoelectric materials that are generally based on lead zirconate titanate (PZT)are increasingly prohibited in many areas in the world for their toxicity to human health and to the environment. A serious alternative to PZT is the sodium potassium niobate (KNN)which presents appropriate properties like its Curie temperature, its coupling coefficient and its density. In this paper we propose to integrate a KNNSr (strontium doped sodium potassium niobate)ceramic in a bimorph structure in a cantilever beam configuration to scavenge vibrational energy in the frequency range of 25 Hz (for medical application). A thinned bulk approach is chosen for the fabrication of the bimorph in order to maintain the good properties of the bulk ceramic. Vibrational measurements demonstrate a power density of 13μW/G2/ cm 3and a resonance frequency of 24 Hz which is highly compatible with external vibrations at low frequencies.

Nowadays, piezoceramics are commonly used in ultrasonic transducers (underwater applications, medical imaging, NDT …). Most of them are PZT-based materials (Pb(Zr, Ti)O3), which causes environmental issues due to the presence of lead. To comply with international regulations on the use of lead, numerous studies are conducted on lead-free piezoceramics (K0.5Nao.5Nb03, BaTiO 3 ) which can offer similar electromechanical performance to lead - based materials. Fabrication of lead-free textured ceramics enables the production of large area of piezoelectric materials with good electromechanical capabilities. Textured ceramics can be simply modeled as a 3-phases composite: one for the single-crystal that increases with the texturation degree, another one for the ceramic matrix and a last one for porosity. The amount of porosity depends on the sintering conditions. In the long wavelenght approximation, this material can be considered as homogeneous and characterized by effective properties. In this context, few studies were carried out for this homogeneization step and, in this paper, a homogeneization scheme is developped, based on the generalization of series and parallel connections. For the modeling, a unit cell is defined and used to determine the effective electroelastic moduli. Then, a validation step for this model is performed using FEM calculation. Results show that a high texturation is necessary and that the presence of porosity is not always damaging to optimize the targeted performance. Finally, theoretical properties of two single-element transducers based on textured piezoceramics are determined and compared to those based on PZT.

Piezoelectric materials are essential for the conversion between mechanical and electrical energy, for example in ultrasound imaging and vibrational energy harvesting. Here, we are making and exploring the effects of a new design: co-sintered multilayers with texture (grains of a preferential crystallographic direction). The motivation is the combination of increased piezoelectric response in certain crystallographic directions; multilayer structures where thick films rather than bulk materials can allow higher frequency operation and large area; and co-sintering to avoid detrimental effects from gluing layers together. Samples of the lead-free piezoelectric material Li0.06(K0.52Na0.48)0.94Nb0.71Ta0.29O3 with 0.25 mol% Mn (KNNLTM) were made by tape casting and co-sintering. NaNbO3 platelets with (100) orientation which were used as templates to introduce texture, and polymethyl methacrylate (PMMA) was used as a pore forming agent for making porous substrates. The electrical impedances of the co-sintered samples were recorded and analyzed by equivalent electrical circuit modelling. A texture up to 85% in the [100] crystallographic direction was obtained. The samples displayed ferro- and piezoelectricity, with a maximum thickness coupling coefficient (kt = 0.18) between mechanical and electrical energy in the most textured sample. This demonstrates that the introduction of texture in multilayered, co-sintered piezoelectrics shows promise for improving devices for ultrasound imaging or energy harvesting.

International audience; We demonstrate numerically and experimentally the opening of a locally resonant bandgap in an active phononic crystal (PC) made of a homogeneous piezoelectric plate covered by a 1D periodic array of thin electrodes connected to inductive shunts. The application of periodic electrical boundary conditions (EBCs) enables an at will tailoring of the dispersion properties of the PC plate, thus leading to a control of the dispersion of the propagating guided elastic waves in the plate. Depending on the nature of the EBCs, several bandgaps open up, the most important being a Hybridization Bandgap (HBG) in the subwavelength regime. The PC behaves as a locally resonant metamaterial. The HBG originates from the interaction of propagating elastic waves (Lamb modes) with an electrical resonant mode whose dispersion can be effectively described through an equivalent transmission line model.

Nowadays, bimorph piezoelectric cantilevers are commonly used in ambient vibrational piezoelectric energy harvesting. They are constituted of two thin layers of piezoelectric material separated by ...

Due to their enhanced properties with respect to conventional piezoelectric ceramics, 1–3 piezocomposite materials are predominant in high performance transducer applications. The determination of their effective electroelastic modulus has been the subject of numerous investigations. Simple modeling of the thickness mode is well documented in literature, however precise simple modeling of the full effective modulus is less abundant. A coupled model is proposed to predict the complete electroelastic moduli of 1–3 piezocomposites. The main aim of this model is to deliver accurate effective elastic, dielectric and piezoelectric parameters while taking into account the internal geometry of the composite. Various piezoelectric pillar shapes and spatial distribution were tested to assess the effect on the effective parameters.

Barium titanate (BaTiO3) is a well-known ferroelectric material which was intensively studied in the early 50's. Indeed, BaTiO3 has been rapidly substituted by lead zirconate titanate (PZT) materials, which exhibit higher piezoelectric properties. However, PZT materials contain lead and their increasing success is associated to health and environmental problems. Therefore tremendous efforts have been devoted to the development of competitive lead-free counterparts and BaTiO3 compositions comeback on center stage as promising candidates. In this work co-doped barium titanate ceramics were fabricated and characterized. Particular attention was paid to the delivery of a complete accurate set of elastic, dielectric, and piezoelectric constants. These databases are relatively rare in literature but are essential for numerical simulations of new devices integrating these piezoelectric materials.

International audience; An original method to measure the acoustic properties of porous Pb(Zr0.53Ti0.47)O3 (PZT) backings at high-frequencies (HF, i.e. between 15 and 25 MHz), was developed. To this end, a piezoelectric PZT-based thick film was deposited onto the porous ceramic backing and directly used to generate an acoustic signal in water. Here, the considered signal is the one that was generated in water, reflected on a target, back toward the multilayer structure, transmitted through the piezoelectric thick film and then to the rear face of the porous backing and finally back to the active film. This method avoids the overlap of the electrical excitation and the measured signal. In this study, two types of porous PZT backings with 1.5 and 10 micrometers pore sizes (volume fraction around 20%) were characterized. For both samples, group velocities were around 3500 m/s and for the 10 micrometers pores size sample, the measured acoustic attenuation coefficient was 1.7 dB/mm/MHz in the frequency range of the transducer bandwidth. Finally, this method is found to be an easy way to determine, in operating conditions, the minimum required thickness of the porous PZT to be used as a backing (in order to be considered as a semi-infinite medium) for HF transducer applications.

Propagation of guided Lamb waves in a piezoelectric phononic plate, on which metallic electrodes are laid on periodically, is studied. Electric boundary conditions (EBCs) applied on the electrodes induce some changes in the effective elastic properties of the plate. More particularly, the case of inductive impedance charges connected on the PC is investigated. This configuration enables the creation of low-frequency gap in the sub-wavelength regime.

International audience; Acoustic properties of porous PZT ceramics intended to be used as backing in high-frequency transducer applications are investigated using a novel method where an electroded piezoelectric thick film is deposited on the backing under test. Two backings with pore sizes 1.5 μm and 10 μm were obtained by sintering a mixture of ceramic powder and an organic template, their porosity was evaluated by scanning electron microscopy at 15%, leading to a density around 6.5 g/cm3. The electroacoustic impulse responses of these devices were measured considering the backing as a propagation medium, the initial thickness of which was chosen small enough to allow back-wall echoes to be detected and large enough to be able to separate the signals in time domain. Then the thickness of the backing was reduced (from around 2 mm to less than 1 mm) and the measurements were repeated. Acoustic properties were then deduced: attenuation coefficients reaching 4 dB/mm/MHz and group velocities around 3400 m/s were obtained, leading to an acoustic impedance around 22 MRa. Such combination of high attenuation and moderate acoustical impedance make these materials an interesting solution for high-resolution ultrasonic imaging transducers.

The standard fabrication method for 1–3 piezocomposites for ultrasound transducers is the “dice and fill” method (DFM) in which lateral periodicity is introduced. This contributes to the appearance of spurious modes that can drastically affect the performance of the device if they appear near its thickness mode frequency, thus limiting the effective frequency range. A new 1–3 piezocomposite fabricated with a super-cell structure [1–3 super cell (13SC)] was designed in order to overcome these limitations. It consists of the merging of several periodic cells with 47% PZT volume fraction and epoxy resin as the matrix. Two lateral periodicities in one direction are defined as well as two different kerfs. The chosen cell shape is composed of five nonaligned square section rods ( $1 \times 1$ mm $^{{\mathrm {2}}}$ ). For comparison of performance, two regular 1–3 piezocomposites (the same materials and equivalent periodicities) were fabricated by DFM. Electroacoustic responses in water were measured for the three composites being considered as transducers. Successive regular thinnings (from 2.8 to 1.1 mm) were carried out for each sample to increase the operating frequency (from around 0.4 to 1.3 MHz) and study the evolution of the characteristics (bandwidth and sensitivity). The experimental results confirmed the behavior of those obtained with numerical simulations, showing that the 13SC composite can be used in this entire frequency range, unlike regular composites.

This paper deals with advanced characterization approaches of thin piezoelectric materials enabling accurate modelling of low frequency (

Multi-element transducers enabling novel cost-effective fabrication of imaging arrays for medical applications have been presented earlier. Due to favorable low lateral coupling of the printed PZT, the elements can be defined by the top electrode pattern, leading to a kerf-less design with low cross-talk between the elements. The linear arrays have proved to be compatible with a commercial ultrasonic scanner and to support linear array beamforming as well as phased array beamforming. The main objective of the presented work is to investigate the performance of the devices at the transducer level by extensive measurements of the prototype structures. The arrays have been characterized at the transducer level by several different measurement techniques. Firstly, electrical impedance measurements on several elements in air and liquid have been conducted. The arrays have also been characterized by a pulse-echo system. The measured sensitivity is around −60 dB, the fractional bandwidth is close to 60%, while the center frequency is about 12 MHz over the whole array. Finally, laser interferometry measurements have been conducted. The in-depth characterization of the array structure have given insight into the performance parameters for the array based on PZT thick film and the obtained information will be used to optimize the key parameters for the next generation of arrays based on piezoelectric thick film.

International audience; A Phononic Crystal (PC), constituted of a homogeneous piezoelectric plate covered by a 1Dperiodic arrangement of thin metallic electrodes on both surfaces, is studied. The application ofElectric Boundary Conditions (EBCs) on the electrodes enables the propagation control of theultrasonic guided waves in the PC. The band structure is investigated for different EBCs: theelectrodes are either at a floating potential or they are alternately short-circuited and at a floatingpotential. In the latter case, a Bragg gap appears for the fundamental S$_0$ guided Lamb mode. Theseresults are verified experimentally and compared to finite element calculations. A physical interpretationis also given, relying on the symmetry of the electric potential fields associated with theseguided modes

International audience; Pad-printed thick-film transducers have been shown to be an interesting alternative to lapped bulk piezoceramics, because the film is deposited with the required thickness, size, and geometry, thus avoiding any subsequent machining to achieve geometrical focusing. Their electromechanical properties are close to those of bulk ceramics with similar composition despite having a higher porosity. In this paper, pad-printed high-frequency transducers based on a low-loss piezoceramic composition are designed and fabricated. High-porosity ceramic cylinders with a spherical top surface are used as the backing substrate. The transducers are characterized in view of imaging applications and their imaging capabilities are evaluated with phantoms containing spherical inclusions and in different biological tissues. In addition, the transducers are evaluated for their capability to produce high-acoustic intensities at frequencies around 20 MHz. High-intensity measurements, obtained with a calibrated hydrophone, show that transducer performance is promising for applications that would require the same device to be used for imaging and for therapy. Nevertheless, the transducer design can be improved, and simulation studies are performed to find a better compromise between low-power and high-power performance. The size, geometry, and constitutive materials of optimized configurations are proposed and their feasibility is discussed.

The transient analysis of piezoelectric transducers is often performed using finite-element or finite-difference time-domain methods, which efficiently calculate the vibration of the structure but whose numerical dispersion prevents the modeling of waves propagating over large distances. A second analytical or numerical simulation is therefore often required to calculate the pressure field in the propagating medium (typically water) to deduce many important characteristics of the transducer, such as spatial resolutions and side lobe levels. This is why a hybrid algorithm was developed, combining finite- difference and pseudo-spectral methods in the case of 2-D configurations to simulate accurately both the generation of acoustic waves by the piezoelectric transducer and their propagation in the surrounding media using a single model. The algorithm was redefined in this study to take all three dimensions into account and to model single-element transducers, which usually present axisymmetrical geometry. This method was validated through comparison of its results with those of finite-element software, and was used to simulate the behavior of planar and lens-focused transducers. A high-frequency (30 MHz) transducer based on a screen-printed piezoelectric thick film was fabricated and characterized. The numerical results of the hybrid algorithm were found to be in good agreement with the experimental measurements of displacements at the surface of the transducer and of pressure radiated in water in front of the transducer.

Recent fundamental results concerning the ultimate performance of electromagnetic absorbers were adapted and extrapolated to the field of sound waves. It was possible to deduce some appropriate figures of merit indicating whether a particular structure was close to the best possible matching properties. These figures of merit had simple expressions and were easy to compute in practical cases. Numerical examples illustrated that conventional state-of-the-art matching structures had an overall efficiency of approximately 50% of the fundamental limit. However, if the bandwidth at -6 dB was retained as a benchmark, the achieved bandwidth would be, at most, 12% of the fundamental limit associated with the same mass for the matching structure. Consequently, both encouragement for future improvements and accurate estimates of the surface mass required to obtain certain desired broadband properties could be provided. The results presented here can be used to investigate the broadband sound absorption and to benchmark passive and active noise control systems.

The fabrication of piezoelectric transducers for high resolution medical imaging applications requires a backing material to damp the piezoelectric resonance, resulting in a shorter time response, i.e. improved resolution, but lower sensitivity. Thus, the choice of such a substrate must be made according to its acoustical properties, namely the ratio of acoustic impedance of the backing material and the piezoelectric layer. Moreover, this backing material must have a relatively high attenuation, and provide good surface properties such as a low roughness and diffusion potential. Finally, the substrate material must have a good mechanical behavior at high temperature (around 900°C for the sintering of the piezoelectric thick film). In this context a review of available materials is first given, and the corresponding list is found to be quite limited. Several PZT/PGO piezoelectric thick films deposited by screen-printing have been fabricated on dense alumina [1] and have delivered good electromechanical per...

In this work, piezoelectric PbZr0.52Ti0.48O3 (PZT) thick films were deposited onto silicon and alumina substrates using sol-gel composite slurry. Thick films prepared with rough and attrited powders are compared. A time stable polymeric PZT sol was first synthesised as the binder. It was then used as a precursor for PZT powder fabrication. Powder attrition ball milling was optimised to reduce the particles size to 200 nm. Composite slurries were obtained by mixing PZT sol with attrited or rough powders and thick films were deposited by dip-coating. The film thickness, roughness and microstructure were studied as a function of powder content, particle size, sol concentration and slurry ageing. Electromechanical measurements were performed. By strictly controlling each step of the thick film fabrication, high coupling factor (thickness mode, kt) can be obtained using both types of powder. Nevertheless, with attrited powder, better and more reproducible effective coupling coefficients in thickness mode (kt up to 46%) were reported on silicon and alumina substrates.

In this study, high performance piezoelectric thick films were prepared by depositing only a few sol–gel infiltrated composite layers on Si/SiO 2 /Ti/Pt wafers. Lead zirconate titanate (PZT) powder, binder and infiltration sol were synthesised from the same-laboratory made time-stable polymeric PZT sol. Adding coarse powder (50 wt%) to the sol precursor, crack-free coatings were prepared by dip coating in the 30–50 μm thickness range in only 3–6 layers. The results showed that good performance can be achieved by controlling sol concentration, powder charge, composite sol ageing, number of infiltrations and heat treatments. The initial results showed that only a limited number of infiltrations are needed in the final stack to obtain high piezoelectric performance. For instance, a 40 μm thick film was prepared with 6 composite layers with a 0.7 M PZT sol charged with 50 wt% PZT powder after 2 months’ ageing. Poling was performed for 5 min under 15 V/μm in an oil bath at 170 °C after 5 infiltrations and final annealing at 700 °C for 10 min in RTA. Despite substrate clamping, this coating reached an effective thickness coupling factor up of to 39% at a resonance frequency of 30 MHz. This result was obtained with a non-doped PZT powder.

A successful fabrication process of textured ceramics by homo-template grain growth (HTGG) is described. Two samples fabricated with this technique were characterised. Functional properties obtained are competitive with for example a thickness coupling factor over 50%. Different characteristics were compared with those obtained by a unit cell model previously developed for piezocomposites and extended to the case of two piezoelectric phases. Behaviours of several connectivities (0–3, 3–0 and 3–3) are calculated and results show that the spatial arrangement does not have a great influence on effective parameters. A transducer based on a textured ceramic sample has been fabricated and characterised. The measured performance shows the efficiency of these new piezoelectric materials. Finally, electroacoustic responses of identical single element transducer configurations have been simulated. They show that a sensitivity increase of 3dB can be obtained at equivalent relative bandwidth of 40% with textured ceramic sample in comparison with PMN-PT ceramic used as a matrix in the textured ceramic.

A lens-focused single-element transducer designed for high-resolution medical imaging requires a high ratio of radius of curvature to source radius. Therefore, classical models neglecting the radial contribution may not be accurate. The objective of this study is to evaluate the contribution of radial displacement to the pressure response of the transducer, both in terms of focal spot and pulse response characteristics. To achieve this objective, two finite element method calculations (FEM) were performed (commercial atila® software), namely those for free and clamped radial displacements. A propagation code adapted to an axisymmetric transducer geometry was implemented to compare the radiated fields, and FEM results for the transducer surface were used as inputs to obtain the radiated fields. Subsequently, the differences between the results of the two calculations results were determined. However, it was demonstrated that the radial displacements slightly affect the propagated field and can therefore be neglected in realistic transducer designs. Moreover, the effects of lens acoustic properties were studied for realistic configurations in terms of resolution and sensitivity to obtain an optimal ultrasound image quality.