Piezoelectric materials

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Piezoelectric materials Polymers
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
range0.001 Hz to 109 Hz. Vast dynamic range
(10-8 to 106 psi or µ torr to Mbar). Low
acoustic impedanceclose match to water, human
tissue and adhesive systems. High elastic
compliance High voltage output10 times higher
than piezo ceramics for the same force input.
High dielectric strengthwithstanding strong
fields (75V/µm) where most piezo
ceramics depolarize. High mechanical strength
and impact resistance (1091010 Pascal
modulus). High stabilityresisting moisture
(lt0.02 moisture absorption), most chemicals,
oxidants, and intense ultraviolet and nuclear
radiation. Can be fabricated into unusual
designs. Can be glued with commercial
adhesives.
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Piezoelectric energy harvesting
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Piezoelectric energy harvesting Signal
conditioning

• The piezomaterial has an equivalent circuit of a
  capacitor in parallel to a resistor
• Lower resistor means that the charge will be
  decaying fast
• Common type of piezoelectric energy harvesting
  device are in the form of a microcantilever with
  integrated piezoresistive element
• Piezoresitive naoelements are also very
  attractive as the can provide very high energy
  density (why?)

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Piezoelectric materialsSemiconductors III-V
Nitrides
Ga-face
Three important parameters c0 unit cell
height a0 edge length of basal plane hexagon u0
c0 cation-anion bond length
All the nitrides have lower c/a ratio than ideal,
which is necessary for stability. This however,
causes spontaneous polarization
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Spontaneous polarization (polarization in
unstrained lattice)

• Condition for polarization to exist
• Bonds must be polar (atoms must be different)
• Crystals must lack inversion symmetry (different
  along 0001 and 000-1
• Similar atoms must be on the same plane (no
  polarization along a-plane where the atoms are
  mixed)
• Condition for spontaneous polarization
• The c/a ratio must differ from the ideal ratio of
  ?(8/3) 1.633

N. B. Spontaneous polarization for Ga-face
nitrides are all negative i.e. directed toward
the substrate
The spontaneous polarization of AlN is greatest
of all nitrides due to largest deviation from
ideal c/a ratio