Piezoelectricity Definition: A property of certain materials, which expand or contract in an electrical field or a property of certain materials which generates an electrical charge when pressure is applied.
PVDF: the polymer we work with exhibits both of these properties.
PVDF, Poly(vinylidene fluoride), beta phase
PVDF is chains of CH2CF2 and is a semi crystalline polymer. It is produced in large thin clear sheets which are then stretched and poled to give it the piezoelectric properties. The stretch direction is the direction along the sheet in which most of the carbon chains run (C’s connected by lines in the figure above) and is visible to the naked eye when the sheet is held up to the light. The poled direction "p" is either to the top or bottom of the sheet. The hydrogen atoms (H’s, above), which have a net positive charge and the fluorine atoms (F’s, above), which have a net negative charge end up on opposite sides of the sheet. This creates a pole direction (indicated by the small p, above) and is either directed to the top or bottom of the sheet.The PVDF we currently use is 32 micrometers thick and comes in long rolls 16 inches wide from Measurement Specialties, Inc. The dielectric constant is around 10.
When an electric field (E) is applied across the sheets they either contract in thickness and expand along the stretch direction or expand in thickness and contract along the stretch direction depending on which way the field is applied. This is due to the physical nature of the positive hydrogen atoms attracting to the negative side of the electric field and repelling from the positive side of the electric field. The negative fluorine atoms attract to the positive side of the electric field and repelling from the negative side of the electric field. Effects of the two electric field directions on a sheet of PVDF can be seen below.
Above the electric field is in the opposite direction of the poled direction and the sheet is stretched length. Below the electric field is in the same direction of the poled direction and the sheet is contracted in length.
Updated: March 14, 2013 14:24