Piezoelectric Effect(PE)The piezoelectric effect(PE) is the tendency of certain materials to accumulate electric charge in response to an applied mechanical stress. The mechanical stresses cause a deflection in the materials lattice displacing ions which create a potential of electric charge.The most known PE is in crystal like quartz which is used in clocks and watches as an oscillator which increases the accuracy the timekeeping as seen in the picture to the right. The effect is reversible, so by applying a electric charge to a piezoelectric material a mechanical stress caused by electrostatic forces can be created. This reverse effect is used to great effect in noise generation equipment especially in ultra-sound applications.
When studying the PE, the piezoelectricity is often refered to in constants. The physical constants relate to both the direction of the applied mechanical or electric force and the directions perpendicular to the applied force. Consequently, each constant generally has two subscripts that indicate the directions of the two related quantities, such as stress and strain for elasticity. The orientation coinciding with the subscript number can be seen to the right. The most common constant used is the carge constant which represents the polarization generated per unit of mechanical stress applied to a piezoelectric material. So a charge consant D33 is the induced polarization in direction 3 per unit stress applied in direction 1. Other constants used are the voltage constant, g, which is the electric field generated by a piezoelectric material per unit of mechanical stress and dielectric constant, ε, which is the dielectric displacement per unit electric field. |
Many materials in the human body have been found to have a PE such as bone, the enamel in teeth,tendons and even DNA. The PE in the human body has been subject of great interest due to its probable significance in biological systems. A lot research has been carried out to see how the body might use the PE to its advantage. It is thought that the PE is used for bone and tissue in regeneration but it is unknown whether this is the case. It is understood that the PE originates from the collagen fibrils in the bone and tendons and is created by the shearing of these fibrils. The PE therefore is directly dependent on the orientation of the mechanical stress in relation to the fibril orientation.This can be seen in the femur where the charge distribution along the extent of the bone (pcoul/cm2). The positive and negative charge is due to the bone either being in compression or tension. In compression the PE creates a negative voltage and in compression a negative. |