piezoelectric ceramics 压电陶瓷 1讲解材料.ppt

PIEZOELECTRIC CERAMICS outline Definitions of terms and symbols Measurement techniques Piezoelectricity is the ability of certain crystalline materials to develop an electric charge proportional to a mechanical stress. + + + + + + + + - - - - - - - - - - - F F - + material - + DEFINITIONS OF TERMS AND SYMBOLS vacuum - + A given voltage Q1 Q2 Q1/Q2=K or the relative dielectric constant T T is the constant or zero the “free” dielectric constant KT S is the constant or zero the “clamped” dielectric constant KS S several hundred to several thousand under 20 under 5 K piezoelectric ceramics inorganic solids Organic materials alternating voltages the loss the complex permittivity DEFINITIONS OF TERMS AND SYMBOLS The direct piezoelectric effect D: dielectric displacement The converse effect For both effects, the piezoelectric constant S T D E d d g e g h e h The equations of state relating the electric and elastic variables : The direct piezoelectric effect The converse effect 6 4 3 1 2 5 i gives the “electrical” direction j gives the component of mechanical deformation or stress DEFINITIONS OF TERMS AND SYMBOLS For the symmetry of ceramics (∞mm) The electromechanical coupling factor k DEFINITIONS OF TERMS AND SYMBOLS 0.9 0.5-0.7 0.4 0.1 k Rochelle salt (at 24℃) PZT ceramic BaTiO3 ceramic quartz DEFINITIONS OF TERMS AND SYMBOLS + + + + + + + + - - - - - - - - - - - This dipole effect is called pyroelectricity The external charge due to the net internal dipole in the crystal is the polarization uniformly heat Change in magnitude of the dipole with temperature MEASUREMENT TECHNIQUES Dielectric Measurements Low field measurements: the low field dielectric constant and the loss High field measurements: high driving fields increase the effective dielectric constant and also cause increased loss Piezoelectric Measurements Resonance method Static methods The mechanical quality factor Q and the piezoelectric constant d *