Piezo Electric Shoes
Date: November 2005
I would like to explain to a classroom how the piezo
crystals light up the led in the kids shoes. Also I would like to know
if anyone has developed a piezo/LED that is a single unit, and how small
could they make this. For example could it be made as a film or in paint
so that contact would light the area of contact. Thank you so much for
this great service.
Crystalline piezoelectric materials derive their properties from the fact
that there is charge separation (although evenly distributed so that the
crystal is neutral) within the crystal lattice structure. When stress is
applied onto the material, the sample will be strained (by a few
nanometers), the crystal lattice deforms, and there is a charge asymmetry.
Such asymmetry results in a voltage. If you think of voltage as a
potential difference, then by stressing/straining the crystal, you are
developing a changing potential difference between a symmetric and
asymmetric charge distribution within the crystal.
In piezoelectric polymers, since the sample is highly compliant, easily
deformed, and does not contain charges, the voltage is produced from a
change in the dipole concentration. As the sample is squeezed, the dipole
density changes. This change in dipole density may result simply from the
change in the number of dipoles per unit volume or from a partial change
in the crystal lattice structure as the sample is stressed. Since the
strain in polymer samples are much higher than in crystals, this dipole
density change can be very large. If the stress is like a wave (the
amplitude changes), the strain and deformation go in cycles - this is
equivalent to a changing magnetic field that induces current.
I do not know of any material in which the light-emitting function and
piezoelectricity come from one unit.
Greg (Roberto Gregorius)
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Update: June 2012