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Inexpensive, high-perforMance, lead-free piezoelectric crystals and their applications in transducers for ultrasonic MEdical DIAgnostic and industrial Tools and Equipments

Final Report Summary - IMMEDIATE (Inexpensive, high-performance, lead-free piezoelectric crystals and their applications in transducers for ultrasonic medical diagnostic and industrial tools ...)

Concerned with toxicity of lead (Pb) contained in most high performance piezoelectric materials, recent legislations in EU and other regions (notably Japan) require replacement of lead based materials with non-toxic alternatives, whenever this is possible, i.e. when alternatives can assure the same functionality as existing products. This is particularly important for medical devices employing piezoelectric materials: thousands of lives are saved and illnesses cured using ultrasonic imaging and therapy based on piezoelectric devices. The alternative materials thus must meet or surpass the performance of the existing lead-based products.

Recent engineering and scientific advances in Japan have suggested that materials based on potassium sodium niobate may be comparable in performance with classical lead zirconate titanate ceramics, presently most widely used piezoelectric material. This project represents a European effort to seek alternative lead-free solutions for piezoelectric materials used in medical and industrial devices and a broad spectrum of other applications.

One success was the development of monodomain and domain engineered KNbO3 based piezoelectric elements. This activity involved development of crystal growth conditions, crystal poling into monodomain and domain engineered state and preparation of KNbO3 (KN) elements for transducers (e.g. cutting, thinning and polishing). Special cuts of KN crystals exhibit low dielectric permittivity (clamped permittivity below < 30) and high thickness coupling coefficients (69 %) that are particularly well suited for high frequency single element transducers. Such piezoelectric elements are presently not commercially available. One reason for this is difficulty in growth and preparation of KN crystals and the other is the high cost of monodomain crystals which were until now prepared only for optical applications.

Doppler probes for transcranial measurement of cerebral blood flow velocity were successfully constructed using KN crystals. The main technical issue is the low dielectric permittivity of KN making the electrical impedance of the probes too high at low frequencies. MTB has shown that the impedance mismatch can be compensated electrically. When compensation was implemented, the performance of probes was superior to those using lead-based transducers. An obstacle in commercialization of probes could be the high price of large crystals (16 mm) needed for 2 MHz probes. However, the price issue is being considered by FEE and significant decrease in fabrication cost of crystals has been already achieved during the project. While KN-based probes represent a technically viable lead-free alternative to classical PZT based devices, a more convenient solution for low frequency applications would be lead-free materials with a higher permittivity and the same high coupling coefficient as KN. As planned in the DoW, this problem was addressed by developing ceramics and crystals with modified compositions. These activities will continue within future activities of FEE, Ferroperm, JSI and LC-EPFL.

SSCG method was chosen as one of the possibilities to achieve cost reduction in production of KN based crystals. Prior to work in IMMEDIATE, to our best knowledge, no attempts were made in growth of KN based materials by SSCG technique. Growth conditions (such as temperature, pressure, time), substrate choice and matrix used for the growth were investigated and developed. Crystals sufficiently large for property characterisation (up to 4 mm) were grown for the first time. Because of the low crystal growth rate the size of the crystals was, however, too small for prototype construction. Thus, the cost reduction efforts were concentrated on developing TSSG domain engineered crystals that could be used in prototypes during the project duration. From the scientific and research point of view the demonstrated possibility of growth of KN based crystals is significant. It is shown that this technique is useful for exploratory growth of single crystals of KN based materials that are difficult to grow with standard methods.

In agreement with the DoW, it was demonstrated that examined systems could potentially be used in ceramic form as a replacement for PZT. Moreover, prototypes were fabricated for all intended applications and tested. In particular, it was demonstrated that machinability of ceramics is excellent allowing production of small elements (tens of microns size) for composite transducers. Up-scaling of production from laboratory to industrial level was also accomplished although not optimised before completion of the project.

Considering that many of the results are trade secrets of end-users, the consortium is not yet ready to publish project results on CORDIS public web page. Selected results of the project haves been presented at conferences (8 international conferences) and scientific publications (6 papers). The dissemination of the activities will continue at international conferences and in scientific publications after the project is completed (1 PhD thesis work is carried out at EPFL).

The project web page (see http://lc.epfl.ch/lc/electro/Projects/ImmediatePublic/Home.htm online) was created and maintained by the coordinator. This page was used for external dissemination of information about project activities and for internal distribution of information to project partners.

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