Ask A Scientist Chemistry Archive

(Created prior to 1993)

Question: I need specific info on the piezoelectric effect.  I want to know 
details on alpha and b forms of the polymer used.  Why they work.
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I would suggest that you try the physics list for a different 
slant on this, but generally speaking, it is a reversible electromechanical 
interaction which occurs in materials which have certain symmetry properties.  
All compounds which exhibit this effect lack a center of symmetry on the 
molecular scale.  What this means is that if you moved all of the atoms in a 
way that changed the signs of their coordinates (x --) -x, y --) -y, z--) -z), 
the geometry would be different.  An example of a shape which has a center of 
symmetry is a circle (in two dimensions) or the corners of a cube (in three 
dimensions).  For complex reasons, this lack of symmetry makes it so that 
external forces (electrical or mechanical) produce an electrical polarization 
in the crystal, which changes the dielectric constant of the crystal and 
stiffens it a bit.  Since this is reversible, you can set up a feedback loop 
between the crystal and the external force, causing the two to resonate 
(oscillate at the same frequency).  This is the basis of quartz watches; the 
resonance controls the clock frequency.  I am not sure what you mean about the 
"alpha and b forms of the polymer used."  There are zillions of compounds 
which exhibit this effect!
Topper
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