Applications of Piezoelectric Materials | Electrical Engineering

The following points highlight the top nine applications and uses of piezoelectric materials in electrical engineering. The applications are: 1. Crystal Oscillators 2. Transducer 3. Delay Lines 4. Medical Ultrasound Applications 5. Gas Igniters 6. Displacement Transducers 7. Accelerometers 8. Piezoelectric Transformers 9. Impact Printer Head.

Application # 1. Crystal Oscillators:

A piezoelectric crystal will vibrate naturally in several mechanical modes, the frequencies of vibration being dependent on the dimensions of the specimen and the elastic constants of the material (f = 1/2l √Y/√ρ). If the crystal is placed between electrode and an alternating voltage at one of the resonant frequencies is applied, the amplitude of oscillation will build up at this frequency.

The stability of the oscillatory system will be controlled by the constancy of the elastic constants of the crystal. Especially cut quartz disks are generally used for this purpose, the stability being particularly high since the coefficient of thermal expansion of quartz is very low. The use of quartz crystal oscillators in electronic circuitry is already very great.

Application # 2. Transducer:

Another application of piezoelectric oscillators lies in the conversion of mechanical pulses into electrical ones, and vice versa. The crystal is here used as a transducer. Acoustic pulses are used in underwater search and other applications. In almost all such cases the acoustic pulses are produced by piezoelectric transducers shock excited by electric fields.

In a phonograph cartridge, as the stylus traverses the grooves on a record, a pressure variation is imposed on a piezoelectric material located in the cartridge, which is then transformed into an electric signal, and amplified before going to the speaker.

Application # 3. Delay Lines:

The piezoelectric materials can be used as delay lines. If an electrical signal is converted into an acoustic one at one end of a quartz rod, the signal will pass along the rod as an acoustic wave. It will travel in the quartz with the appropriate sound velocity. On reaching the end of the rod, the acoustic wave may be picked off as an electrical signal. The initial electrical signal has been delayed, a requirement often found in communication devices.

Application # 4. Medical Ultrasound Applications:

Piezoelectric material can be used for both active and passive transducer applications. In the passive mode the transducer acts as a sound receiver i.e. there is conversion of sound energy into an electrical signal. The converse piezoelectric effect permits a transducer to act as an active sound transmitter.

In the pulse echo mode, the transducer is used, to perform both the active and passive functions at the same time. A sound wave is propagated into the medium and a faint echo is received back after a small time gap due to the acoustic impedance mismatch between the interface materials. This principle is used in transducers for ultrasonic medical imaging applications.

The ultrasound imaging method is superior as the diagnosis can be done without the need of cutting as in surgery. When compared to X-Ray imaging, it is safer at the low power levels used in normal scans. It is also helpful in distinguishing between soft tissues which is not possible with X-Rays. However, X-Ray and ultrasonic imaging can be said to complementary to each other as the X-Rays can be used to image the bone which is not possible in biomedical imaging.

The principle of ultrasonic imaging is based on the pulse echo mode of operation. The transducer is excited by an electrical signal which in turn produces a vibrational pulse in the medium of propagation; part of the energy is reflected back towards the transducer.

This reflected echo produced a voltage signal which is used to generate the image of the internal organs and tissues in the body. The acoustic impedance difference between one tissues to another is small so the vibrational pulse penetrates to larger depths and gives a good imaging capability.

Application # 5. Gas Igniters:

Gas igniters consists of two oppositely poled ceramic cylinders attached end to end in order to double the charge available for the spark. The compressive force has to be applied quickly to avoid the leakage of charge across the surfaces of the piezoelectric ceramic.

The generation of the spark takes place in two stages. The application of a compressive force ‘F’ on the poled ceramic (under open circuit conditions) leads to a decrease in the length by σL^D. The potential energy developed across the ends must be higher than the breakdown voltage of the gap, for sparking to occur. When the spark gap breakdown occurs the second stage of energy generation starts. The electric discharge across the gap results in a change to a lower level.

The compliance of the material increases and allows further compression of the ceramic by (σL^E – σ L^D) where σ L^E is the displacement that would have occurred if the force ‘F’ had been applied under short circuit conditions. The combination of the strains from the open and short circuit conditions produce more energy that can be dissipated in the spark. Usually PZT ceramic disks are used for this application.

Application # 6. Displacement Transducers:

When force is applied to a long piezoelectric cantilever beam, one side is in tension while the other side will be in compression. No electrical output can be obtained from this homogenous body by bending. Bimorphs made with two halves of separate beams with an electrode in between as well as on the top and bottom surfaces is shown in Fig. 6.15.

If the beams are poled in the opposite direction then on the application of force ‘F’ the voltage generated on the outer electrodes will be additive (Fig. 6.15(a)). If the beams are poled in the same direction, the additive output can be obtained by connecting the outer electrodes and the centre electrode as shown in Fig. 6.15(b).

Conversely the application of a voltage to the bimorph causes it to bend. As shown in Fig. 6.16, the application of an electric field causes one half of the cantilever beam to expand and the other half to contract. If they are joined in the form of a bimorph it leads to a net displacement on the application of an electric field.

Application # 7. Accelerometers:

An accelerometer is device which gives an electrical output proportional to the acceleration. The transducer is a piezoelectric cylinder which is poled along its axis but has its poling electrodes removed and the sensing electrodes applied to its inner and outer surfaces.

The cylinder is joined to the fixed central pole on the inside and a cylindrical mass on the outside. When an axial acceleration takes place the cylinder is subjected to a shear force between the outer mass and inner pole. Any motion in the radial direction does not give any output as the piezo-coefficient for PZT ceramic accelerometer is zero. So the device is highly directional.

Application # 8. Piezoelectric Transformers:

Low voltage to high voltage transformation can be done by using a piezoelectric plate. Fig. 6.18 shows a flat plate having electrodes on half of its larger face and on an edge. The region between the larger face electrodes and the edge electrodes are poled separately.

A length mode resonance is excited by applying a low AC voltage source between the larger face electrodes. The step up voltage ratio would be proportional to the ratio of the input to output capacitance and the efficiency of the device. This principle has been used for making EHT transformers for miniature television receivers.

Application # 9. Impact Printer Head:

Dot matrix impact printers driven by multilayer piezoelectric ceramic actuators have been successfully produced on a large commercial scale. The printing pin element consists of a piezo-actuator, a stroke amplifier operated on the lever principal and a printing wire. When a pulse with a peak voltage of 150 V is applied to the piezoelectric actuator, the printing wire moves by about 40 μm, making the tip of the wire to hit the paper through the ink ribbon.

The advantages of the dot matrix printer over the conventional electromagnetic drive printers includes a higher printing speed (double the speed because of the presence of 24 printing pin elements) and half the power consumption as compared to the latter printer.