Ask A Scientist Materials Science Archive

name         Steve
status       educator
location     CA

Question -   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.
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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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