Fabrication, modelling and use of porous ceramics for ultrasonic transducer applications

Journal of Electroceramics (Impact Factor: 1.42). 08/2007; 19(1):127-139. DOI: 10.1007/s10832-007-9117-3

ABSTRACT Porous ceramics are of interest for ultrasonic transducer applications. Porosity allows to decrease acoustical impedance,
thus improving transfer of acoustical energy to water or biological tissues. For underwater applications, the dhgh figure of merit can also be improved as compared to dense materials. In the case of high frequency transducers, namely for
high resolution medical imaging, thick film technology can be used. The active films are generally porous and this porosity
must be controlled. An unpoled porous PZT substrate is also shown to be an interesting solution since it can be used in a
screen-printing process and as a backing for the transducer. This paper describes the fabrication process to obtain such materials,
presents microstructure analysis as well as functional properties of materials. Modelling is also performed and results are
compared to measurements. Finally, transducer issues are addressed through modelling and design of several configurations.
The key parameters are identified and their effect on transducer performance is discussed. A comparison with dense materials
is performed and results are discussed to highlight in which cases porous piezoceramics can improve transducer performance,
and improvements are quantified.

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    ABSTRACT: Bi0.5(Na0.82K0.18)0.5TiO3(NKBT) lead-free ferroelectric thick films with various porosities have been produced by screen printing. Their microstructures, pyroelectric and piezoelectric properties were investigated with variation of porosity. The results show that the relative dielectric constant of the resulting 90 μm NKBT thick films with 19% and 32% porosity was down to 161 and 56, respectively. The pyroelectric voltage figure of merit (Fv) and detectivity figure of merit (FD) of NKBT thick films were increased from 10.2 × 10−13 to 19.7 × 10−13 Cm/J and 1.1 × 10−5 to 3.8 × 10−5 Pa−0.5. Moreover, the hydrostatic voltage constant and hydrostatic figure of merit of the NKBT thick films with 32% porosity reached 81 × 10−3 V/mPa and 8200 × 10−15 Pa−1 respectively. The decreasing of relative dielectric constant, volume specific heat, and piezoelectric coefficient with increasing porosity was responsible for the improved pyroelectric and piezoelectric figures of merit of the NKBT thick films. Furthermore, screen printing with the addition of controlled fraction of organic vehicle in pastes has been proved to be an alternative method for fabrication of porous pyroelectric and piezoelectric thick films. Our work demonstrates that introduction of pores in ferroelectric thick films creates a matrix void composite resulting in high figures of merit for pyroelectric and piezoelectric applications.
    Journal of the American Ceramic Society 03/2010; 93(7):1957 - 1964. · 2.43 Impact Factor
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    ABSTRACT: The rapid development of the electronics industry has created the need for highperformance, high-reliability, miniaturised electronic components integrated into various electronic devices. Additional requirements, such as the desired size and weight, low cost, low power consumption, and portability, should be considered to make the devices user friendly and widely accessible. Attempts to miniaturise discrete elements have generally failed due to the difficulty in handling and assembly. A lot of waste material and high costs are also involved. In this approach, the ceramic parts are manufactured as a bulk ceramic, followed by a reduction in size by cutting, polishing, etc., to specified dimensions. The final step is the assembling of a thin layer of ceramic with the other components. This topdown approach imposes limits on the minimum dimensions of the manufactured parts. It constrains the geometry of the parts to simple shapes, like discs, plates, rings, cylinders, etc.
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    ABSTRACT: In previous work, screen-printing technology was used to elaborate an integrated piezoelectric structure on a porous substrate [1]. The substrate was chosen to withstand the film sintering temperature which was lowered at 800degC thanks to the addition of PGO to a PZT composition. The acoustical impedance of this substrate is very close to that of the deposited piezoelectric thick film, making it an adequate backing to deliver a short pulse-echo response, but the resonance frequency is lowered. Intermediate functional layers were necessary between the substrate and the piezoelectric layer. The transducer based on a porous PZT structure including a dense barrier layer and a gold rear electrode is used as a reference device. Here, another set of materials is considered as possible candidate to fulfill both functions of substrate and backing: porous alumina associated with a dense alumina barrier layer and platinum rear electrode. Since the thicknesses of these layers is not negligible compared to the wavelength of the first thickness mode of the piezoelectric layer, they have a relatively strong influence on the electro- acoustic response. The input acoustic impedance of this stack must be controlled very precisely in order to make it an adequate backing The damping of the backing is optimized to deliver a relatively short pulse-echo response, without excessive lowering of the resonance frequency. The piezoelectric thick film on alumina substrate has similar properties to the one on porous PZT. A mean thickness around 30 micrometers and a thickness coupling factor around 40% were obtained in both cases. As a result of the damping, the structure resonates at 40 MHz, the anti-resonance of the piezoelectric thick film alone being around 65 MHz. Pulse-echo measurements allows comparison of the performance of the two devices in terms of sensitivity, axial resolution and bandwidth. The results are compared and discussed, showing that the sensitivity/bandwidth trade-offs of - the two transducers are significantly different.
    Proceedings of the IEEE Ultrasonics Symposium 01/2007;