Werkstoffe der technischen Keramik

3.4.2.8 Lead zirconate titanate

Currently the most important piezoelectric ceramic materials are based on mixed oxide crystal system consisting of lead zirconate and lead titanate known as lead zirconate titanate (PZT).

The specific properties of these ceramics, such as the high dielectric constant, are dependent on the molar ratio of lead zirconate to lead titanate as well as on substitution and doping with additional elements. A wide range of modifications can be implemented in this way, creating materials with highly varied specifications.

The piezoelectric effect

The piezoelectric effect links both electrical and mechanical properties.
Piezoelectricity refers to a linear electro-mechanical interplay between the mechanical and electrical states of a crystal.

The direct piezoelectric effect refers to an electrical charge, detectable as a voltage, being created in proportion to mechanical deformation of the crystal.

The reciprocal or inverse piezoelectric effect refers to a deformation that arises in proportion to an external electrical field created by the application of an electrical voltage.

Principles

The piezoelectricity of ferroelectric materials is a consequence of the existence of polar areas (domains) whose orientation changes as result of the polarisation, i.e. of the application of an electrical voltage. The polarisation is associated with a change in length, S.

Figure 23: Electric dipoles in a piezoelectric material before and after polarisation.

Lead zirconium titanate Pb(Zrx Ti(1-x)) O3 is processed in polycrystalline form. The two most common shaping methods are pressing and the tape casting process. The green body acquires its ceramic properties through firing.
The piezoceramic, however, only acquires its technically interesting piezoelectric properties through a polarisation process.

Figure 24:

Diagram of the domains in lead zirconate titanate before, during and after polarisation S = change in length during polarisation Sr = residual changing length after the polarisation process

Resonance modes in piezoelectric componen

All the components applied as sensors and actuators exploit the fundamental vibration modes.

Figure 25: Fundamental vibration modes in piezoceramic components

Structural forms

If the piezo-ceramic consists of one layer, we speak of single layer technology. If the piezo-ceramic component consists of a number of active piezo-ceramic layers, we speak of multi-layer technology. Nowadays it is usual for piezo-ceramic plates, strips, rings, domes, small tubes and a large number of special geometries to be manufactured.

Their compact form means that piezo-ceramic transducers take up little space, and use little energy when used as actuators. Multi-layer actuators are used when large movements are required that can also create high forces.
The individual layers are connected electrically in parallel so that the external voltage remains small.

Figure 26: Configuration and sectional diagram of a monolithic multi-layer piezo-ceramic actuator.

Another variant is represented by the bending transducer. This is created if, for instance, piezo-ceramic elements are glued to a carrier material. The piezo-ceramic material reacts with a change in its length when electrically stimulated. The result, similar to that of a bimetal strip, is a large change in the shape of the composite material, depending on the voltage and its polarity, associated with moderate forces.

Figure 27: Application of the operating voltage causes the piezo-ceramic to contract, causing the pair to bend

Actuators with a passive layer and a piezo-ceramic component are called monomorphic. A bimorphic bending transducer consists of two piezo-ceramic elements without an intermediate passive layer. A system consisting of two piezo-ceramic components together with a passive intermediate layer is referred to as trimorphic. A multimorphic bending transducer consists of a large number of piezo-ceramic components, and does not have passive intermediate layers.

Figure 28: Structural variants of piezo-ceramic bending converters with
P = polarisation direction and E = direction of the electrical field

Depending on the individual design of the bending transducer, movements of several millimetres, forces of up to a few newtons, and remarkably short movement times can be achieved. The bending transducer can therefore be used as a powerful and rapidly operating actuator.

Piezo-ceramics have won many applications in electronics, in the motor vehicle industry, in medical technology, in equipment and machine construction and consumer applications:
piezo-ceramic components are used as transducers in telecommunications, acoustics, hydro-acoustics, materials testing, ultrasonic processing, liquid atomisation, flow measurement, level measurement, distance measurement and medical technology. They are applied as actuators in micro-pumps, in optical systems, gas valves, low-pressure engineering, in inkjet printers, textile machines and in Braille modules (reading devices for the blind). Applied as sensors they react to force, pressure and acceleration, making it possible to monitor a wide range of processes.