User Manual

Page 2
New copolymers of PVDF, developed over the last few years, have expanded the applications of
piezoelectric polymer sensors. These copolymers permit use at higher temperatures (135bC) and
offer desirable new sensor shapes, like cylinders and hemispheres. Thickness extremes are possible
with copolymer that cannot be readily attained with PVDF. These include ultrathin (200 Å) spin-cast
coatings that enable new sensor-on-silicon applications, and cylinders with wall thicknesses in excess
of 1200µm for sonar. Piezo cable is also produced using copolymer.
PIEZOELECTRIC FILM PROPERTIES
Piezo film is a flexible, lightweight, tough engineering plastic available in a wide variety of
thicknesses and large areas. Its properties as a transducer include:
• Wide frequency range—0.001 Hz to 10
9
Hz.
• Vast dynamic range (10
-8
to 10
6
psi or µ torr to Mbar).
• Low acoustic impedance—close match to water, human tissue and adhesive systems.
• High elastic compliance
• High voltage output—10 times higher than piezo ceramics for the same force input.
• High dielectric strength—withstanding strong fields (75V/µm) where most piezo ceramics
depolarize.
• High mechanical strength and impact resistance (10
9
—10
10
Pascal modulus).
• High stability—resisting moisture (<0.02% moisture absorption), most chemicals, oxidants, and
intense ultraviolet and nuclear radiation.
• Can be fabricated into unusual designs.
• Can be glued with commercial adhesives.
One major advantage of piezo film over piezo ceramic is its low acoustic impedance which is closer
to that of water, human tissue and other organic materials. For example, the acoustic impedance
(Z
O
= ρ υ) of piezo film is only 2.6 times that of water, whereas piezo ceramics are typically 11 times
greater. A close impedance match permits more efficient transduction of acoustic signals in water
and tissue.
Piezo film does have some limitations for certain applications. It makes a relatively weak
electromechanical transmitter when compared to ceramics, particularly at resonance and in low
frequency applications. The copolymer film has maximum operating/storage temperatures as high
as 135
o
C, while PVDF is not recommended for use or storage above 100 bC. Also, if the electrodes
on the film are exposed, the sensor can be sensitive to electromagnetic radiation. Good shielding
techniques are available for high EMI/RFI environments.
Table 1 lists typical properties of piezo film. Table 2 provides a comparison of the piezoelectric
properties of PVDF polymer and two popular piezoelectric ceramic materials.
Piezo film has low density and excellent sensitivity, and is mechanically tough. The compliance of
piezo film is 10 times greater than the compliance of ceramics. When extruded into thin film,
piezoelectric polymers can be directly attached to a structure without disturbing its mechanical
motion. Piezo film is well suited to strain sensing applications requiring very wide bandwidth and
high sensitivity. As an actuator, the polymer's low acoustic impedance permits the efficient transfer
of a broadband of energy into air and other gases.