Fabrication and modeling of piezoelectric transducers for High-Frequency medical imaging

International audienceWe have studied the processing of piezoelectric thick films using electrophoretic deposition (EPD) for high-frequency ultrasound applications. Lead-zirconium-titanate (PZT) particles synthetized by solid states synthesis were dispersed in ethanol using ammonium polyacrylate (PAA). The electrophoretic deposition of PZT particles was performed at a constant-current mode. PZT thick-films deposited at 1 mA for 60 seconds were sintered at 900oC for 2 hours in a PbO-controlled atmosphere. The scanning-electron microscopy (SEM) analysis shows that the thickness of PZT layer is uniform and that the pores are homogeneously distributed within the layer. The complex electrical impedance was measured and fitted by KLM scheme in order to deduce the dielectric, mechanical and piezoelectric parameters of the thick-films. The density and thickness of PZT thick films are used as inputs and the thickness coupling factor kt, dielectric constant at constant strain and resonant frequency are deduced. The results show that homogeneous PZT thick-film structures with tailored thickness and density prepared by EPD and sintering having a resonant frequency around 20 MHz can be used for noninvasive medical ultrasound imaging and diagnostics

Piezoelectric Thick-Film Structures for High-Frequency Applications Prepared by Electrophoretic Deposition

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Lead-Zirconate-Titanate Thick Films by Electrophoretic Deposition for High-Frequency Ultrasound Transducers

International audienceLead–zirconate–titanate (PZT) thick films for high‐frequency ultrasound applications were fabricated using an electrophoretic deposition (EPD) process. The PZT powder was synthesized from constituent oxides at close‐to‐room temperature by mechanochemical activation followed by calcination at 700°C. Homogeneous PZT thick films with a thickness of ~30 μm and a density of ~80% were produced from ethanol‐based colloidal suspensions containing PZT and PbO particles that were deposited using a constant‐current mode on a platinized alumina substrate and were sintered in the presence of a liquid phase at temperatures compatible with thick‐film technology. The dielectric, mechanical, and piezoelectric parameters of the PZT thick films with a porosity of ~20% were measured and a thickness‐coupling factor of 48% was obtained. Considering that a PZT thick film is a composite structure consisting of PZT and pores, there are several methods that can be applied to calculate the effective parameters where the composite is considered as a piezoelectric homogeneous material. For this study, a matrix method based on a generalization of 2‐2 connectivity was used to deduce the parameters of a completely dense configuration. Our results confirmed that the EPD process is suitable for the fabrication of efficient, high‐frequency transducers operating at frequencies above 40 MHz

Processing and Electromechanical Properties of Lead Zirconate Titanate Thick Films by Electrophoretic Deposition

International audienceElectrophoretic deposition (EPD) was used for the fabrication of piezoelectric [lead zirconate titanate (PZT)] thick films on alumina substrates. The EPD was performed in constant current mode from an ethanol based suspension consisting of PZT and PbO particles. The influence of addition of ethyl cellulose (EC) and sintering temperature on the thickness, density, homogeneity and functional response of PZT thick films is studied. Results show that the highest electromechanical performance is obtained for the PZT thick films sintered at 900 or 950°C, with a thickness coupling factor kt of 50%. The addition of EC influenced the thickness of the PZT thick films but had only minor effect on the porosity content for sintering temperatures over 900°C. Moreover, elastic constants of the thick films based on the suspension with EC were lower, which leads to lower acoustic impedance (15 MRa) while maintaining high (kt) value. In this last case, a better acoustic matching can be expected with propagation media such as water or biological tissues for ultrasound medical imaging applications

Electrophoretic deposition (EPD) process for lead zirconate titanate (PZT) thick films fabrication and high frequency medical imaging

International audienceHigh frequency ultrasonic devices operating over 20 MHz are required for medical diagnostics including skin and eye imaging. The main component in an ultrasonic transducer is a piezoelectric material such as PZT. It is usually surrounded by one or several matching layers, a focusing lens and a backing. Geometric and electromechanical properties of the piezoelectric layer strongly contribute to transducer performance. High-frequency operation requires piezoelectric materials with a thickness of few tens of micrometers that can be obtained by thick-film technologies. The use of a lens in high-frequency transducers implies a significant decrease of transducer sensitivity due to losses. So our interest is to use geometrical focusing of the acoustical beam. For this, the piezoelectric layer must be curved. Here, electrophoretic deposition (EPD) is used to deposit patterned piezoelectric structures on curved substrates. A stable suspension and an optimized deposition process have been developed to deliver homogeneous, crack-free deposits with uniform thickness. Deposited films were sintered, functionally and electromechanically characterized. Thickness coupling factors similar to those measured in bulk ceramics were observed. Finally, the piezoelectric structures were used to fabricate high-frequency ultrasonic transducers which were characterized and integrated in an echographic probe to obtain in-vivo skin images

High-frequency acoustic characterization of porous lead zirconate titanate for backing applications

International audienceAn 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

Effect of Pore Size and Porosity on Piezoelectric and Acoustic Properties of Pb(Zr0.53Ti0.47)O3 Ceramics

International audienceWe 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

Porous lead zirconate titanate ceramics with optimized acoustic properties for high-frequency ultrasonic transducer applications

International audienceAcoustic 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

Lead Zirconate Titanate Multi-Element Structure by Electrophoretic Deposition

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