A direct comparison of performance and acoustic properties of high intensity focused ultrasonic transducers utilizing lead-free (Sodium Bismuth Titanate - NBT) and lead-based (Lead Zirconate Titanate - PZT) piezoceramics is discussed. All transducers operate at 12 MHz at third harmonic frequency, having an outer diameter of 20 mm, a central hole of 5 mm in diameter and a radius of curvature of 15 mm. The electro-acoustic efficiency determined by a radiation force balance is evaluated in a range of input power levels up to 15 W. Schlieren tomography as well as hydrophone measurements are used for evaluation of the acoustic field distribution. It is found that the average electro-acoustic efficiency of NBT-based transducers is approximately 40% while it is around 80% in the PZT-based devices. NBT devices show significantly higher inhomogeneity of the acoustic field under schlieren tomography compared to PZT devices. From pressure measurements in the pre-focal plane, it was found that the inhomogeneity could be attributed to depoling of significant areas of the NBT piezo-component during the fabrication process. In conclusion, PZT-based devices performed significantly better than those using lead-free material. However, the NBT devices show promise for this application and their electro-acoustic efficiency as well as the uniformity of the acoustic field could be improved by employing a low-temperature fabrication process or repoling after processing.

While the consequences of humidity during solid-state processing of sodium potassium niobate-based lead-free piezoelectric powders are well established, the effect of humidity at later fabrication steps is less known. This study assesses the effect of humidity on the sintering and functional properties of 0.06LiNbO3-0.94(K0.5Na0.5)NbO3 (LKNN). Samples sintered in high-humidity air display a higher density, lower dielectric losses, and an increased mechanical quality factor. The observed properties persisted even after five months of storage with marginal reduction in the measured piezoelectric parameters. While the improvements shown with the high-humidity sintering method might be too small to justify investments in special atmosphere sintering, it more importantly indicates that no special equipment or atmosphere control is required to avoid adverse effects of humidity during sintering of sodium potassium niobate-based piezoceramics.

Research activities on lead-free piezoelectric materials have been ongoing for over 20 years. Generally, the applicability of the main material families is less universal than that of lead-based compositions such as lead zirconate titanate, but in some cases, the corresponding applications have already been identified. Due to the extensive research, it is now possible to manufacture demonstrators and prototypes for different applications and the authors propose in this article to take stock of these advances. For this, we have chosen to first recall briefly the main new material systems using a simplistic "soft" and "hard" classification for approaching the various resonant transducer applications. Medical imaging applications that represent one of the most important fields are presented in a second step together with other low-power transducers. Then, a variety of applications are merged under the heading of high-power transducers. In addition, we mention two points that are important to consider when manufacturing at a larger scale. For the design of transducers, complete datasets must be available, especially if modeling tools are used. Finally, the commercialization of these lead-free materials imposes essential secondary requirements in terms of availability, reproducibility, sample size, and so on.

This study delves into the crucial impact of humidity on the microstructure, phase composition, dielectric, ferroelectric, and piezoelectric properties of sintered 0.06LiNbO3–0.94(K0.5Na0.5)NbO3 lead-free piezoelectric ceramics during storage. By subjecting the samples to different humidity environments over six months, this research uncovers that storage conditions with 20% relative humidity or less maintain the samples unaltered. Moreover, the study reveals that exposure to higher humidity drastically decreases dielectric and piezoelectric performance in as little as two days due to adsorption of water. However, this research also demonstrates that even after six months in extremely humid conditions, the original performance can be restored with a simple and effective recovery procedure, indicating that the intrinsic performance after the adsorbed water is removed is mainly intact. This work therefore provides valuable insights into the shelf-life and possible resistance of piezoelectric ceramics to harsh environments.

Solid solutions of BiFeO3 and BaTiO3 are promising lead-free piezoelectric materials, especially around the morphotropic phase boundary at 0.67BiFeO3-0.33BaTiO3. Still, these materials are challenged by phase insta-bility and limited understanding of the processing-properties relationship. Here, we investigate mechanochem-ical activation and the use of BaTiO3 as seed particles for the 0.67BiFeO3-0.33BaTiO3 phase. Contrary to expectations from seeding in lead-based perovskites, the BaTiO3 seeds do not promote the 0.67BiFeO3- 0.33BaTiO3 perovskite phase neither during the mechanochemical activation nor the subsequent sintering, but cause an inhomogeneous structure with remnant BaTiO3. This results in ceramics with weaker low-field piezo-electric response than that of the unseeded route, but with higher field-induced strain, even up to 150 degrees C. Both routes produce ceramics of high density and without significant secondary phases visible by X-ray diffraction. This demonstrates the advantage of mechanochemical activation and the possibility to tailor the piezoelectric response of 0.67BiFeO3-0.33BaTiO3 through the processing route.

High-intensity focused ultrasound transducers operating at 4 MHz based on lead-free piezoceramics from the sodium bismuth titanate (NBT) family are described. First, the piezoelectric material (Pz12X) is evaluated from the standpoint of transducer design and its important characteristics, including temperature dependance of several parameters such as dielectric and mechanical coefficients. Then, the performance of six transducers of the same design is evaluated in terms of electro-acoustic efficiency and its dependency on the operating acoustic power level up to 30 W. Overall, the initial electro-acoustic efficiency of three independent transducers is approximately 50% at low acoustic power levels and slightly drops down to 42% as the input electric power reaches 10 W. This process is stable and fully reversible. Moreover, the stability of electro-acoustic efficiency over extended power burst cycling is studied using another two transducers up to 95 × 103 power bursts of 250-ms duration and acoustic power of 10 W. This protocol is beyond the typical clinical use of similar devices in practice. No significant changes in electro-acoustic performance are noted. Additionally, the input electric power and the output acoustic power, together with the temperature of the piezoelectric component, are evaluated simultaneously over the period of one power burst. It is found that the maximum operating temperature over a high-input electric power burst of 600 J is below 60°C, which defines the operational limit for such devices, as the de-poling temperature of the lead-free material is around 85°C. It is found that the lead-free material from the NBT family is also a promising alternative to lead-based PZT-type materials in high-power therapeutic ultrasound.

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 (Li 0.06 (K 0.52 Na 0.48 ) 0.94 )(Nb 0.71 Ta 0.29 )O 3 doped with 0.25 mol% Mn (KNNLTM) as textured ceramics. Templated grain growth from NaNbO 3 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 m 3 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.

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 crystal-lographic 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 Li-0.06(K0.52Na0.48)(0.94)Nb-0.71Ta0.29O3 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 polymethylmethacrylate (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 (k(t) = 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.

Structural Health Monitoring (SHM) is a general term for methods of detecting defects in structures before they become fatal. The typical areas of application cover aeronautics, civil engineering (buildings, bridges etc.) and recently also the automotive industry. One of the techniques used for SHM is based on the propagation of ultrasonic waves through a structure in order to assess its structural integrity. The common solution relies on the use of piezoelectric transducers to emit and receive ultrasonic waves. However, because of the high number of echoes and possible propagation modes within the structure, those applications require very extensive signal processing. In this work we present an approach involving piezo-composite transducers dedicated to SHM in combination with specific electronics that has been developed and successfully tested for reducing the complexity of the detection schemes. This has been achieved by selecting the mode and direction of the Lamb waves. The array transducer comprises a row of eight independently driven PZT elements integrated in a polymer matrix, effectively forming a 2-2 composite. Dielectric and piezoelectric characterisation is presented and the obtained results confirm an excellent uniformity and performance of the tested devices. Moreover, additional results of Finite Element Method modelling are given. Functional characterisation at the system level shows that it has been possible to successfully distinguish waves propagating at different velocities, confirming the concept of mode selectivity using piezoelectric arrays.

Ultrasonic-based SHM (Structural Health Monitoring) applications commonly rely on the use of piezo-electric patches to emit and receive ultrasonic waves. The objective is to study the propagation of the waves through a structure to assess its structural integrity. Because of the elevated number of echoes and possible modes of propagation of the waves within the structure, those applications suffer from a burden of signal processing. This paper presents a composite piezo-electric patch that was designed and successfully tested for reducing the complexity of the SHM detection schemes by selecting the mode and direction of the Lamb waves received. The piezo-composite is composed of a row of eight independent ceramic pillars separated with polymer, so it is a 1-D matrix of independent piezo-patches. Used with adequate electronics and signal processing, it was shown that it allowed selecting the direction and the mode of the Lamb waves.

FEM (Finite Element Method) modelling software such as COMSOL Multiphysics® can be a powerful tool for modelling the behavior and response of piezoelectric materials and devices [1]. Devices based on piezoelectric crystals are particularly well suited, because the polarization magnitude in crystals is predetermined and its orientation is defined by how it was cut with respect to the lattice structure. In the case of polycrystalline piezoelectric ceramic materials, the overall polarization vector in the material is not oriented in one single direction nor is constant all over the material. Therefore, a poling process, where a strong electrical field is applied across the material to align the dipole domains in one particular direction on a macroscopic level, should be performed. In this paper SAW [2] piezoelectric ceramic devices have been investigated. A diagram of a generic SAW sensor is shown in Figure 1, physical SAW sensors can be used to measure temperature, humidity, pressure, force, etc. [3]. Chemical/biological SAW sensors can also be used to detect oxygen, pathogens, etc. [4,5]. The initial SAW device has a polarization perpendicular to the substrate as shown in Figure 2. This means that the piezoelectric transduction relies mostly on d31, the charge coefficient in the transverse direction. In the case of Lead Zirconate Titanate, the most common piezoelectric ceramic, d31 is often less than half of the value of d33, the charge coefficient in the thickness direction [1]. Therefore a second SAW device is suggested, one that would gain higher sensitivity by using the d33 instead of the d31 mode of operation. This can be achieved by applying a high electric potential to one of the interdigitated electrodes while grounding the corresponding electrode, a polarization such as the one shown in Figure 3 can be achieved. This would not only enable the use of the higher charge coefficient for operation, but also would simplify the fabrication process of the device. The standard SAW device shown in Figure 2 and the interdigitated poled SAW device in Figure 3 have been modelled using the Piezoelectric Devices interface in COMSOL Multiphysics. In order to correctly model the interdigitated poled SAW device, the poling process has also been modelled by simulating the resulting electric field from the applied voltage on the electrodes and mapping the resulting magnitude according to the polarization magnitude correlated to the virgin hysteresis curve of the piezoelectric ceramic. The orientation of the polarization has been aligned in the direction of the simulated electric field. The module of the transfer function between the transmitting IDT (interdigitated transducer) and the receiving IDT provides the proper sensitivity comparison of the two SAW devices. It has been shown that COMSOL can model not only piezoelectric devices but also the poling process in the case of piezoelectric ceramic materials, which is of utmost interest for a piezoelectric ceramic producer such as Meggitt A/S. In this paper the focus has been mainly on SAW devices, but it can be applied to any piezoelectric ceramic based devices with non-trivial polarization.

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 crosstalk 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.

Screen- and pad-printed single-element ultrasonic transducers have been successfully commercialized over the recent years. Typically, PZT (Lead Zirconate Titanate) thick films are pad- or screen-printed on a curved ceramic substrate acting as integrated backing layer and providing mechanical pre-focus. The center frequency ranges between 8 MHz and 80 MHz. The devices are characterized by good sensitivity as well as high relative bandwidth. The main objective of the presented work has been to apply a similar technology to manufacture multi-element transducers enabling novel cost-effective fabrication of imaging arrays for medical applications. The thick film arrays have been integration-tested using a commercial ultrasound scanner (BK Ultrasound bk3000). The integration test revealed that the 32-element thick film transducers are compatible with a commercial scanner, have a frequency range of 7.5 MHz to 12 MHz, and a TX bandwidth of 70%. Moreover, the transducers support linear array beamforming as well as phased array beamforming. Here 32-element transducers are presented, however the technology can easily be extended to fabrication of transducers with 128 or more elements.

The use of porosity to modify the functional properties of piezoelectric ceramics is well known in the scientific literature as well as by the industry, and porous ceramic can be seen as a 2-phase composite. In the present work, examples are given of applications where controlled porosity is exploited in order to optimise the dielectric, piezoelectric and acoustic properties of the piezoceramics. For the optimisation efforts it is important to note that the thickness coupling coefficient kt will be maximised for some non-zero value of the porosity that could be above 20%. On the other hand, with a good approximation, the acoustic velocity decreases linearly with increasing porosity, which is obviously also the case for the density. Consequently, the acoustic impedance shows a rather strong decrease with porosity, and in practice a reduction of more than 50% may be obtained for an engineered porous ceramic. The significance of the acoustic impedance is associated with the transmission of acoustic signals through the interface between the piezoceramic and some medium of propagation, but when the porous ceramic is used as a substrate for a piezoceramic thick film, the attenuation may be equally important. In the case of open porosity it is possible to introduce a liquid into the pores, and examples of modifying the properties in this way are given.

High energy synchrotron XRD was employed to determine the lattice strain epsilon{111} and diffraction peak intensity ratio R{200} in tetragonal PZT ceramics, both in the virgin poled state and after a bipolar fatigue experiment. It was shown that the occurrence of microstructural damage during fatigue was accompanied by a reduction in the gradient of the epsilon{111}-cos(2) psi plot, indicating a reduction in the level of residual stress due to poling. In contrast, the fraction of oriented 90 degrees ferroelectric domains, quantified in terms of R{200}, was not affected significantly by fatigue. The change in residual stress due to fatigue is interpreted in terms of a change in the average elastic stiffness of the polycrystalline matrix due to the presence of inter-granular microcracks. (C) 2013 AIP Publishing LLC.

Recent progress in development of new functional materials that are flexible and can be processed at very low temperatures (below 100 °C) opens a new opportunity for applications, such as non-destructive evaluation (NDE), or structural health monitoring (SHM) by applying active materials directly on the structures made out of a variety of materials, e.g. metals (aluminium), plastics, and polymers, including CFRP (Carbon Fibre Reinforced Polymer). This paper presents sensor arrays based on a flexible piezoelectric material – PiezoPaintTM. The newly developed material exhibits relatively high sensitivity (d33 coefficient up to 45 pC/N), extremely low processing temperatures (< 120 °C), and high compliance in the cured state, enabling direct deposition of acoustic/vibration sensor arrays on structures to be monitored by means of screenor padprinting. The printed sensors have been applied for impact detection where four-element arrays and a fully integrated wiring system has been deposited directly on aluminium as well as CFRP plates. The presented results show very good performance in terms of sensitivity, flexibility of usage, and ultra-low weight, making PiezoPaintTM technology an attractive alternative for SHM particularly in aerospace applications.

Detection of high-order nonlinear components issued from microbubbles has emerged as a sensitive method for contrast agent imaging. Nevertheless, the detection of these high-frequency components, including the third, fourth, and fifth harmonics, remains challenging because of the lack of transducer sensitivity and bandwidth. In this context, we propose a new design of imaging transducer based on a simple fabrication process for high-frequency nonlinear imaging. The transducer is composed of two elements: the outer low-frequency (LF) element was centered at 4 MHz and used in transmit mode, whereas the inner high-frequency (HF) element centered at 14 MHz was used in receive mode. The center element was pad-printed using a lead zirconate titanate (PZT) paste. The outer element was molded using a commercial PZT, and curved porous unpoled PZT was used as backing. Each piezoelectric element was characterized to determine the electromechanical performance with thickness coupling factor around 45%. After the assembly of the two transducer elements, hydrophone measurements (electroacoustic responses and radiation patterns) were carried out and demonstrated a large bandwidth (70% at -3 dB) of the HF transducer. Finally, the transducer was evaluated for contrast agent imaging using contrast agent microbubbles. The results showed that harmonic components (up to the sixth harmonic) of the microbubbles were successfully detected. Moreover, images from a flow phantom were acquired and demonstrated the potential of the transducer for high-frequency nonlinear contrast imaging.