0.16 Section 4.5. piezo-electricity and ferro-electricity.  (Page 2/2)

Paraelectricity occurs in crystal phases in which electric dipoles are unaligned (i.e. unordered domains that are electrically charged) and thus have the potential to align in an external electric field and strengthen it. In comparison to the ferroelectric phase, the domains are unordered and the internal field is weak.

The LiNbO3 crystal is ferroelectric below 1430 K, and above this temperature it turns to paraelectric phase. Other perovskites similarly exhibit paraelectricity at high temperatures. Paraelectricity may provide an alternative to the traditional heat pump. A current applied to a paraelectric material will cause it to cool down - which could be useful for refrigeration or for cooling computer chips.

As shown in Figure 4.7.b. below Curie Temperature it takes tetragonal symmetry. Each Unit Cell has built-in electric dipole which may be reversed by the application of electric field. These electric dipoles may be re-oriented in any desired direction by the application of appropriately directed electric field. This is analogous to Ferro-magnetism hence it is called Ferro-electricity.

In PZT, below Curie Temperature , there are domains of polarization known as WEISS DOMAINS as shown in Figure 4.8.a. Within WEISS DOMAIN, the dipoles are self-aligned hence WEISS DOMAINS has a net polarization measured by dipole moment per unit volume. But WEISS DOMAINS in PZT are randomly oriented as shown in the Figure 4.8.a.Overall polarization or Piezoelectric effect is zero. But this mass of minute crystallites in PZT containing randomly oriented WEISS DOMAINS can be induced to have net polarization by the application of strong electric field. This is called Electric Poling and has been illustrated in Figure 4.8.b.

As seen in Figure 4.8.b. after Electric Poling all Weiss Domains are forced to be oriented in a given direction by the application of Electric Field in the desired direction below Curie Temperature. The domains most nearly aligned with the applied electric field grow at the expnse of the other domains. Even after field is removed the alignment remains locked in the desired direction, giving PZT a remnant electric polarization and a permanent deformation making the material anisotropic. In anisotropic materials the material property depends on the direction of measurement. This is exactly as in Ferro-magnetic Materials. Just as we have B-H Hysteresis Loop in Ferro-Magnet we have Polarization-E Hysteresis Loop in Ferro-electric Materials as shown in Figure 4.9. D closely follows Polarization-E curve.

Remnent Polarization for soft PZT is P _r = 0.3 [(C-m)/m ³ ]= 0.3C/m ² =Dipole Moment per unit Volume.

In piezo-electric materials we have Mechanical Deformation versus Applied Electric Field curve. This also shows a hysteresis loop showing plasticity and plasticity loss.

In Table 4.5.1. the main electric parameters of standard Piezo-electric materials are given.

Table 4.5.1. Dielectric Constant and Q-Factor of Quartz, PZT 5A and PZT 4A.

Section 4.5.1.Applications of Piezo-electric and Ferro-electric Materials.

Quartz are used as high Q, high precision Mechanical Resonators and find wide applications as generating stable electric oscillations for Watches, Clock Waveforms in Computers and for generating Carrier Waves in Radio Broadcast Stations.

To date Quartz Crystal Oscillators (Xtal Oscillator) is the stab lest Frequency Generators. The Frequency of Xtal Oscillator does not drift with temperature , aging or with varying load. The Resonance Frequencies or Natural Frequencies are well defined by the physical dimensions of the crystal and oscillation occurs at natural frequencies. Quartz Xtal Oscillators are very small. It consists of a thin piece of Cut Quartz Wafer with two parallel surfaces metalized to make required electrical connections. The physical dimensions of the crystal are critical in faithfully producing a given frequency.

PZT are generally used as actuating systems in which they operate they operate below natural resonance frequencies and in which the ability to generate high forces and high spatial displacements is more important e.g. in high performance Ultra-sonic Transducers. PZT can be shaped in any fashion and it can be polarized in any direction.