Work advances on thin-film piezoelectric power sources

The only way to really make use of piezoelectric materials on a large scale is to produce them in the form of thin films that can be used in a wide variety of settings.

So says Dr. Madhu Bhaskaran of RMIT University in Austrialia. She and three other researchers recently assessed how piezoelectric thin-films at the nanoscale generate energy, for the first time precisely measuring the amount of electrical voltage and current they produce.

Writing in the journal Advanced Functional Materials, Bhaskaran says the group synthesized PSZT composed of (Pb 0.92 Sr 0.08 ) (Zr 0.65 Ti 0.35 )O₃, a composition that has been demonstrated to produce the highest piezoelectric response for a thin film on a silicon substrate reported to date. A 200-nm platinum coating with a 20-nm titanium dioxide adhesion layer formed a conductive bottom electrode metallization and PSZT thin films were created with thicknesses of 700 nm and 1,400 nm.

A scanning electron microscope view showing nanoislands of PSZT. A close-up of one island at right shows its grain structure.

Researchers checked nanoscale electromechanical properties of the piezoelectric thin films using a device called a Hysitron Triboindenter, which uses a tip made with boron-doped diamond to touch the films. The tip-contact resistance varies as a function of the penetration depth, as the interaction area increases. Researchers used the device to gauge open-circuit voltage and short-circuit current generation during nanoindentation. Nanoindentation took place with maximum loads ranging from 0.1 to 10 mN.  Bhaskaran says the mechanical response is purely elastic for loads of 2.5 mN and below, with contributions from plastic deformation apparent at higher loads. There is a “kink” in the loading curve at around 3 mN which Bhaskaran thinks is an indication of plastic deformation.

Researchers say the voltage the film produces is approximately linear with applied force and sustained when the applied force was held constant. This is interesting because while piezoelectric voltage generation is expected only for changes in force, the sustained voltage researchers observed can be attributed to charge storage due to the extremely high dielectric constant of PSZT. The generated voltage as a result of these capacitive properties gradually decays.

Here are the kinds of currents you can expect to get out of PSZT nanoislands.

Multiple force cycles generated the same voltage with minimal hysteresis, researchers say. In the case of multiple loading/unloading cycles, the PSZT films respond elastically to the indentation following the first cycle. Researchers figure that voltage generation is due only to elastic deformation.

As researchers expected, peaks in current arose at abrupt changes in the applied force. The polarity
of current also varies, often with negative current produced during film compression (application of force) and positive current produced during removal of force.

All in all, researchers measured voltage outputs of up to ∼ 40 mV and current transients up to ∼ 200 pA. They say these compare well to published results for nanowire generators of ∼ 30 mV and ∼ 100 pA which have been shown to be suitable for powering nanosensors. The effective power generated is ∼ 250 μW/mm 2 at 5.0 mN force. The researchers also figure the independence of voltage generation and enhanced current generation they observed indicates potential for capitalizing on current nanofabrication advances to potentially increase current density.

RMIT put out a press release on this work. You can see it here: http://www.rmit.edu.au/browse;ID=ft6u8i5edsko