05 21 2007电滞回线测量.ppt

* * 鐵電材料與應用概論 (May 21, 2007) Measurement of Ferroelectric Hysteresis Loop 杜繼舜 電滯曲線量測 Pyroelectrics Pyroelectrics are the bridge between ferroelectrics and piezoelectrics. They possess a spontaneous polarization which is not necessarily switchable by an electric field. If their polarization is switchable, i.e. they are ferroelectric, then they are mainly used in situations in which ferroelectric properties are required. However, if they are not ferroelectric, then their properties as pyroelectrics are more useful. Comparison between pyroelectrics and ferroelectrics Introduction : The ferroelectric effect was first observed by Valasek in 1921, in the Rochelle salt. This has molecular formula KNaC4H4O6·4H2O. The effect was then not considered for some time, and it wasnt until a few decades ago that they came into great use. Nowadays, ferroelectric materials are used widely, mainly in memory applications. The electric field alters the energy profile, resulting in a ‘tilting’ of the potential well [see (c)] An increase in the electric field will result in a greater tilt, and lead to the dipole moments switching, leading to (d). (a) (b) (c) (d) Phase Transition of BaTiO3 Generation of Electric Polarization First order Second order reference capacitor Cr sample capacitor CS R1 R2 R3 Ferroelectric Hysteresis Loop and Switching Polarization There are three sections to this curve.1) Reversible domain wall motion.2) Linear growth of new domains.3) New domains reaching the limit of their growth. (001) PMNT30% (111) PMNT30% . [001] [010] [011] Measured at room temperature