Ferroelectret

From Wikipedia, the free encyclopedia

1 Definition
2 See also
3 References
4 External links

[edit] Definition

Ferroelectrets are thin films of polymer foams, exhibiting piezoelectric and pyroelectric properties after electrical charging. Ferroelectret foams usually consist of a cellular polymer structure filled with air. Polymer-air composites are elastically soft due to their high air content as well as due to the size and shape of the polymer walls. Their elastically soft composite structure is one essential key for the working principle of ferroelectrets, besides the permanent trapping of electric charges inside the polymer voids. The elastic properties allow large deformations of the electrically charged voids. However, the composite structure can also possibly limit the stability and consequently the range of applications [1].

]

How it works

The most common effect related to ferroelectret is the piezoelectricity, but in this materials the effect occurs in different way as those in ferroelectric polymers. A negative stress in the 3-direction mainly decreases the distance between the molecular chains, due to the relatively weak van der Waals and electrostatic interactions between chains in comparison to the strong covalent bonds within the chain. The thickness decrease thus results in a increase of the dipole density and thus in an increase of the charges on the electrodes, yielding a positive d33 coefficient dipole-density piezoelectricity. In cellular polymers (ferroelectrets), negative stress in the 3-direction also decreases the thickness of the sample. The thickness decrease occurs dominantly in the voided region, the macroscopic dipole moment decreases and so does the electrode charge, giving a negative d33 (intrinsic (quasi)-piezoelectricity) [2].

Image:Ferroelectret.jpg

[edit] See also

R. Gerhard-Multhaupt, Less Can Be More - Holes in Polymers lead to a new paradigm of piezoelectric materials for electret transducers, IEEE Transactions on Dielectric and Electrical Insulation, Vol. 9, No. 5, 2002.

S. Bauer, R. Gerhard-Multhaupt, and G. M. Sessler, Ferroelectrets: Soft Electroactive Foams for Transducers, Physics Today Vol. 57, pages 37-43, February 2004.

M. Wegener and S. Bauer, Microstorms in Cellular Polymers: A route to soft piezoelectric transducer materials with engineered macroscopic dipoles, ChemPhysChem, Vol. 6, pages 1014-1025, 2005.

[edit] References

[1] - Mellinger, Axel; Wegener, Michael; Wirges, Werner; Mallepally, Rajendar; Gerhard-Multhaupt, Reimund,”Thermal and Temporal Stability of Ferroelectret Films Made from Cellular Polypropylene/Air Composites”, Ferroelectrics, Volume 331, Number 1, pp. 189-199(11), 2006.

[2] - Michael Lindner, Heinz Hoislbauer, Reinhard Schwödiauer, Simona Bauer-Gogonea and Siegfried Bauer, “Charged Cellular Polymerswith ‘‘Ferroelectretic’’Behavior”, IEEE Transactions on Dielectrics and Electrical Insulation Volume 11, Number 2, pp. 255-263; April 2004.

[3] Joachim Hillenbrand and Gerhard M. Sessler, “High-sensitivity piezoelectric microphones based on stacked cellular polymer films (L)“, J. Acoust. Soc. Am. 116 (6), December 2004.