The 4th Annual Energy Harvesting Workshop organised on 28-29 January by Virginia Tech in Blacksburg Virginia was largely academic and biased toward piezoelectrics which are moving up to rival photovoltaics in potential energy harvesting applications. About 110 people from many countries attended. Some presentations concerned thin films (defined as below one micrometer) and thick films (defined as 1-100 micrometers).
Technical rather than cost advantages over bulk materials were the primary concern of the researchers presenting. Professor Dong-Soo Park of Korea Institute of Material Science in Korea described aerosol deposition, something successfully commercialised by Optomec in the West for printed electronics. His interest was very different, involving thick PZT films by the above definition that can meet performance requirements intermediate between those of thin film and bulk piezoelectrics. He demonstrated superior performance over bulk piezoelectrics - such as higher density than conventionally sintered bulk PZT - in a miniature tube motor and in micro-mirror actuations. Micro-mirrors are used in miniature projectors in mobile phones for example. His tube motor had loss of only 0.008 vs 0.15 for bulk and it had a better quality factor as well.
Lead free piezoelectrics are sought given environmental laws increasingly banning lead even when tightly bound chemically. Prof Park, Fraunhofer Institute for Ceramic Technologies (Fraunhofer IKTS), Dresden and many others are looking at this although, curiously, barium containing ceramics are often being considered as alternatives despite them being close to lead in the Periodic Table and having similar environmental challenges. Prof Park has looked at the piezoelectric "NKN" (Na0.5 K0.5 NbO3) with oxide additives (usually ZnO) and conductive "LNO" LiNiO4 electrodes deposited by aerosol where 350 ohm cm 10-4 can be optimally achieved with ten micrometer film but only after 700ºC annealing.
Structural Health Monitoring
He was followed by several speakers that described their mathematical modelling of various forms of piezoelectric harvesting. Thomas Roedig of Fraunhofer IKTS in Dresden expressed the opinion that the primary potential markets for piezoelectric energy harvesting are low power electronics and wireless sensor nodes.
There are 15 of them working on piezoelectrics in the Institute's team of 150, the rest being dedicated to such things as fuel cells, which can also involve thin films. The "Silicon Saxony" area is about to put Euros 15 million into structural health monitoring. For example, fiber reinforced composite structures increasingly used in aircraft must be continuously monitored.
For monitoring everything from bridges to wind power generators, trains, pipelines and buildings, self powered, ultrasound wireless sensors are required, he said. If the harvesting piezo is thin, you get high current and, if thick, you get high voltage. In his electromechanical modelling, he considered a few volts - which translate into 250 micrometer layers - are optimal.