Abstract
In this work we report on the fabrication process for the development of a flexible piezopolymeric transducer for health monitoring applications, based on lead-free, piezoelectric zinc oxide (ZnO) thin films. All the selected materials are compatible with the space environment and were deposited by the RF magnetron sputtering technique at room temperature, in view of preserving the total flexibility of the structures, which is an important requirement to guarantee coupling with cylindrical fuel tanks whose integrity we want to monitor. The overall transducer architecture was made of a c-axis-oriented ZnO thin film coupled to a pair of flexible Polyimide foils coated with gold (Au) electrodes. The fabrication process started with the deposition of the bottom electrode on Polyimide foils. The ZnO thin film and the top electrode were then deposited onto the Au/Polyimide substrates. Both the electrodes and ZnO layer were properly patterned by wet-chemical etching and optical lithography. The assembly of the final structure was then obtained by gluing the upper and lower Polyimide foils with an epoxy resin capable of guaranteeing low outgassing levels, as well as adequate thermal and electrical insulation of the transducers. The piezoelectric behavior of the prototypes was confirmed and evaluated by measuring the mechanical displacement induced from the application of an external voltage.
Highlights
In parallel to the development of strong approaches to face reliability problems, two solutions are normally proposed: redundancy, with an obvious drawback for space exploration purposes in terms of weight and complexity, and fault detection/recovery
Sputtering was selected as the synthesis technique for growing all the involved materials, while a couple of gold (Au) and zinc oxide (ZnO) were selected as the electrode material and the piezoceramic, respectively
Suitable deposition conditions were selected for the sputter deposition of high-quality Au electrodes and PE ZnO thin films on flexible Polyimide substrates
Summary
In parallel to the development of strong approaches to face reliability problems, two solutions are normally proposed: redundancy, with an obvious drawback for space exploration purposes in terms of weight and complexity, and fault detection/recovery. It is a non-biocompatible, toxic material which requires high deposition temperatures or post-deposition thermal treatments (generally higher than 500 ̋C) in order to get the right crystalline structure and avoid the formation of the pyrochlore phase which suppresses piezo/ferroelectricity [21,22] Another important issue of PZT is its superior brittleness [23], representing a strong limitation in view of the development of mechanical flexible transducers conformable to structural surfaces. AlN is suitable for Micro Electro-Mechanical Systems (MEMS) application, with Complementary Metal-Oxide Semiconductor (CMOS)-compatible processes applied even on flexible substrates, such as Polyimide [36] Despite both these materials showing several advantages and similar properties, AlN is characterized by a lower d33 value. The fabricated PPT prototypes were characterized by measuring their mechanical displacement under the application of an electric voltage, which resulted in the PE behavior of the fabricated transducers, and provided an estimation of the ZnO PE coefficient d33
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