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Name: Steve
Status: Educator
Grade: N/A
Location: CA
Country: USA
Date: November 2005


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.



Replies:
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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