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handbook of modern sensors

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3.6 Piezoelectric Effect 73<br />

absorptive in the far-IR portion <strong>of</strong> the electromagnetic spectrum. The polymer melts<br />

at about 170 ◦ C. Its density is about 1780 kg/m 3 . PVDF is a mechanically durable<br />

and flexible material. In piezoelectric applications, it is usually drawn, uniaxially or<br />

biaxially, to several times its length. Elastic constants, (e.g., Young’s modulus) depend<br />

on this draw ratio. Thus, if the PVDF film was drawn at 140 ◦ C to the ratio <strong>of</strong> 4:1, the<br />

modulus value is 2.1 GPa, whereas for the draw ratio <strong>of</strong> 6.8:1, it was 4.1 GPa. The<br />

resistivity <strong>of</strong> the film also depends on the stretch ratio. For instance, at low stretch, it<br />

is about 6.3 × 10 15 cm, whereas for the stretch ratio 7:1 it is 2 × 10 16 cm.<br />

Polyvinylidene fluoride does not have a higher or even as high piezoelectric<br />

coefficient as other commonly used materials, like BaTiO 3 or PZT. However, it has a<br />

unique quality not to depolarize while being subjected to very high alternating electric<br />

fields. This means that even though the value <strong>of</strong> d 31 <strong>of</strong> PVDF is about 10% <strong>of</strong> PZT,<br />

the maximum strain observable in PVDF will be 10 times larger than in PZT because<br />

the maximum permissible field is 100 times greater for PVDF. The film exhibits good<br />

stability: When stored at 60 ◦ C, it loses its sensitivity by about 1–2% over 6 months.<br />

Comparative characteristics for various piezoelectric materials are given in Table<br />

A.8.Another advantage <strong>of</strong> piezo film over piezo ceramic is its low acoustic impedance,<br />

which is closer to that <strong>of</strong> water, human tissue, and other organic materials. For example,<br />

the acoustic impedance <strong>of</strong> piezo film is only 2.6 times that <strong>of</strong> water, whereas<br />

piezo ceramics are typically 11 times greater. A close impedance match permits more<br />

efficient transduction <strong>of</strong> acoustic signals in water and tissue.<br />

Some unique properties <strong>of</strong> the piezoelectric films are as follows 8 :<br />

• Wide frequency range: 0.001 Hz to 10 9 Hz<br />

• Vast dynamic range: 10 −8 –10 6 psi or µtorr to Mbar.<br />

• Low acoustic impedance: close match to water, human tissue, and adhesive systems<br />

• High elastic compliance<br />

• High voltage output: 10 times higher than piezo ceramics for the same force input<br />

• High dielectric strength: withstanding strong fields (75 V/µm), where most piezo<br />

ceramics depolarize<br />

• High mechanical strength and impact resistance: 10 9 –10 10 P modulus.<br />

• High stability: resisting moisture (

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