Piezo2011 - Electroceramics for End-users VI
As the number of applications and market for piezoelectric materials and devices continues to grow the need for new materials, processing capabilities and device designs remains at the forefront in the development of piezoelectrics.Citation1 Reflecting this multidisciplinary aspect, Piezo2011 saw the presentation of work ranging from fundamental research of materials, through processing and on to applications of piezoelectrics in devices. The conference provided an opportunity for attendees to reflect on some of the changes that have occurred over the past two years since Piezo2009,Citation2 as well as look to the future to see where new trends are developing.
Materials development continues to be at the heart of much research with development of lead free alternatives to the ubiquitous lead zirconate titanate (PZT) family of materials. This has been on-going for a number of years and is predominantly driven by EC directives such as waste electrical and electronic equipment (WEEE) and restriction of hazardous substances (RoHS) with their focus on the reduction and elimination of heavy metals such as lead. This was well represented at Piezo 2011 with many talks on lead free including Dragan Damjanovic’s IEEE UFFC distinguished lecture presentation reviewing how the EC legislation relates to piezoelectric materials and journeying through some of the developments of lead free materials. While much of the work presented on piezoelectric materials explored lead free materials to replace existing lead containing materials, the work has evolved since the last conference in that it has now moved on to consider the processing routes by which the materials can be integrated into piezoelectric devices. As with the work done on lead containing materials over the last decade, the integration of lead free piezoelectrics with other materials is not without challengesCitation3 and requires innovative synthesisCitation4,Citation5 and integration solutions.Citation6 The lessons learned through the community’s work on lead containing ceramics – especially issues relating to interdiffision of mobile species and evaporation of volatile species – is as pertinent as ever, particularly with the prominence of volatile atomic species such as K and Na in the new generation of lead free materials.
Despite these challenges, the conference has shown that it is possible to achieve high levels of integration for both thin and thick films. Such approaches are predominantly focussed on reduced temperature and selection of appropriate barrier coatings.Citation3 Unusual heating profiles are also reflected in the work on processing of bulk materials to give rise to new microstructures. Here rapid thermal treatments through spark plasma sinteringCitation7,Citation8 have been used to overcome loss of volatile elements and to prevent excessive grain growth in difficult to sinter materials. These novel processing routes are of critical importance if the new generation of lead free materials are to find commercial success in piezoelectric products. Successfully integrating films or processing unusual ceramic compositions is but one step towards creating a successful device. Also key to this is the ability to shape the structures. This can be achieved via masking and etching technique, or by ink jet printing, but pad printingCitation9 also offers the exciting possibility of creating structured films on surfaces that are not flat. This is of particular interest in areas such as ultrasound where a non-flat transducer can be used to focus the ultrasonic waves to a point to achieve better imaging resolution.
The development of new piezoelectric devices featured strongly at the conference and is driven by the new processing capabilities and new materials that have been developed as well as the new strategic application areas that have come to prominence – perhaps the strongest of which is related to the increased global focus on energy and environmental issues. The application of piezoelectric materials to harvest energy, as well as sense the environment, has stimulated significant research in this area.
One of the driving forces behind the development of piezoelectric harvesters is minimising the need for batteries or wires for a range of sensor applications. At the heart of the harvester is the ability of piezoelectric materials to transform movement into electrical energy, but it is the shaping and arrangement of the piezoelectric material within a device that is crucial to maximising the power that can be extracted using this technology. In these applications thick films are often more effective than bulk materials as they can be integrated with the surrounding device architecture while at the same time providing a device that is sufficiently flexible to allow it to couple with the ambient vibrating environmentCitation10. By depositing PZT films onto a variety of substrates many of the speakers demonstrated effective harvesting capability. The effectiveness of a simple cantilever design can be enhanced through connecting multiple devices together, creating different shapes and constructing multilayer devices.Citation10 One of the continued challenges for such devices is that the maximum efficiencies are observed when they are operated at their resonant frequencies. Unfortunately in real life situations the most abundant vibrations are normally at low frequencies and spread across a wide frequency range. One route to solve this issue has been to convert such low frequencies into high frequency oscillations by means of a plucking action, much like the strumming of guitar strings or insertion of a playing card into the spokes of a child’s bike to create a ’motorbike’ effect. While films offer one possibility to increasing coupling with the environment, work is also underway to explore how nanoscale piezoelectric materials, such as nanopillars of PZT or ZnO, could potentially be excited directly to extract useful amounts of energy. In such approaches macroscopic motion is transformed to micro/nanoscopic motion via mechanically shaped surface such that individual nanorods are deformed. At the length scales involved in the nanoscale devices the ability to accurately model the behaviourCitation11 is critical for ensuring the optimum performance.
Using piezoelectric devices it has been shown that energy can be converted from mechanical vibrations, but often the voltages obtained are not as required. Here piezoelectric devices may also play a role as they have the ability increase or decrease the voltage. Such devices have been demonstrated for bulk materials,Citation12 some configurations may offer the potential for miniaturisation for inclusion in self-sustaining piezoelectric sensor nodes.
Professor Robert Dorey
Guest Editor
Cranfield University
References
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- Filoux E, Lou-Moeller R, Callé S, Lethiecq M, Levassort F: ‘Optimised properties of high frequency transducers based on curved piezoelectric thick films obtained by pad printing process’, Adv. Appl. Ceram, 2012, 112(2), 75–78.
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