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Fraunhofer develops economical process for micro energy harvesting04.11.2014 - (idw) Fraunhofer-Institut für Elektronenstrahl- und Plasmatechnik FEP
The trend toward energy self-sufficient probes and ever smaller mobile electronics systems continues unabated. They are used, for example, to monitor the status of the engines on airplanes, or for medical implants. They gather the energy they need for this from their immediate environment - from vibrations, for instance. Fraunhofer researchers have developed a process for the economical production of piezoelectric materials. They will unveil a preliminary demonstration model at this years electronica trade show from November 11 to 14 in Munich (Hall A4, Booth 113).
When there is little space, or an exchange is complicated, then power supply for sensors via battery or cable is most often too circuitous. The best approach is to have the energy intake integrated and highly durable. One solution is offered by Energy Harvesting onsite power production for instance through solar cells, thermoelectric or piezoelectric materials.
Piezoelectric materials can convert mechanical vibrations into electric energy because the effect of mechanical force results in a charge separation. They can be applied in places where a defined but not necessarily constant state of vibration exists on industrial equipment, for example, or airplane engines, in car engines or even on the human body, where blood pressure, breathing or heartbeat are constantly creating momentum. Up to now, the piezoelectric material of choice has mainly been leadzirconium-titanium composites (PZT). Aluminum nitride (AlN) is another option. Compared to PZT, it possesses more favorable mechanical properties, is lead-free, more stable and biocompatible. Moreover, it is virtually no problem to integrate AlN layers into conventional manufacturing processes for microelectronics.
New process for manufacturing piezo coatings
Heres the dilemma: In order to integrate piezoelectric materials into increasingly smaller
electrical systems, they likewise have to be as small as possible on the one hand. On the
other hand, they need a certain volume in order to produce sufficient energy. So far it has
been impossible to produce the targeted coatings in a manner that is economically feasible
enough using the available methods to date. Deposition rates, homogeneity and coating
areas are too small. But now, scientists at Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP have developed a process by which they can precipitate highly homogeneous layers on diameters of up to 200 mm with simultaneously high deposition rates. Thus, the process is substantially more productive and profitable than previous processes.
The researchers deposited the layers by reactive magnetron sputtering of aluminum
targets in an argon-nitrogen atmosphere onto a silicon wafer. With this physical procedure,
atoms from solid bodies are discharged into the gas phase by bombarding the targets with highly energetic noble gas ions. They then deposit on the wafer as a layer. For this purpose, the Fraunhofer FEP scientists use the DRM 400, a double ring magnetron sputter source developed in-house that consists of two ring-shaped targets. Since the discharges of both targets overlap, it is possible to deposit the AlN layers homogeneously
onto a large coating surface with a piezo-coefficient d33 of up to 7 pC/N. The higher this
figure, the more strongly the material reacts. The typical values described in available
research literature for the piezo-coefficient d33 of AlN ranges between 5 to 7 pC/N. At the
same time, the mechanical stress of the layers can be flexibly modified to the relevant
field of application. These impact for example the adhesion strength of the coating, the
electromechanical coupling and the values of the energy produced.
Boosting energy yields even further
Working in collaboration with the Technical University of Dresden and Oulu University in
Finland, the Fraunhofer FEP researchers conducted tests on energy harvesting with AlN
coatings on silicon strips measuring 6×1cm². For demonstrations, they were able to reach
generated powers of several hundred W. According to project manager Stephan Barth,
this figure admittedly cannot be transferred to a practical application at a 1:1 ratio, since
the generated power depends on multiple factors: On the one hand, the design that
is to say the layer thickness, transducer geometry, volume, space and substrate materials
all have an impact; on the other hand, there is an effect from the vibrational behavior,
such as frequency, amplitude or ambient medium and one should also keep in mind the
necessity of the matching to the sensor electronics." Nonetheless, the AlN layers are a
practicable alternative for operating low-power sensors, as they are used in industrial
applications or with cardiac pacemakers.
In order to raise the power yield even higher, scientists are additionally using layers made
from aluminum-scandium-nitride, which they deposit by reactive co-sputtering. Compared
to pure AlN, these exhibit substantially higher piezo-coefficients with similar coating
rates. This means three to four times more power is produced through this. Another
future focus of the researchers work will be placed on optimization of the transducer
design for power production. The goal would be to downsize the entire construction, to
elevate capacities even further, and to better adapt resonance frequency to the respective
For further information
Stephan Barth | Phone +49 351 2586-379 | firstname.lastname@example.org
Organic Electronics, Electron Beam and Plasma Technology FEP| Phone +49 351 2586 452 | Annett.Arnold@fep.fraunhofer.de
Winterbergstraße 28 | 01277 Dresden | Germany | www.fep.fraunhofer.de
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