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

Top-Down Processing of Nanopowder

We report the processing of NaNbO3 nanopowder by combining the solid-state synthesis and subsequent milling in the agitator bead mill. The effect of different rotation speeds of the agitator shaft on the comminution process was followed by laser granulometry. The morphology and specific surface area of the powders were investigated by scanning electron microscopy and the N2 adsorption method, respectively. With the optimized milling parameters, we obtained NaNbO3 nanoparticles with an average size of 25 nm and a narrow particle size distribution. The result is comparable to other processing techniques, such as solution-based chemical routes or mechanochemical synthesis; however, the presented method does not require any complicated processing and it can be easily upscaled to yield large quantities of the NaNbO3 nanopowder. Furthermore, the compaction behaviour of the obtained nanopowder was investigated, and a compaction-response diagram was constructed revealing good compactability of the powder. The green compacts, isostatically pressed at 740 MPa, had a relative density of 70% and a narrow pore size distribution with an average pore radius of 4 nm

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