The piezoelectric effect

Chapter 3 The piezoelectric effect



CONTENTS


The piezoelectric effect 13

Piezoelectric materials 13

The piezoelectric crystal as a transmitter of sound 14

The piezoelectric crystal as a receiver of sound 15

Production of a pulsed sound wave 16

Resonance 16

Factors affecting wavelength/frequency 16

Important properties of piezoelectric materials 16




LEARNING OBJECTIVES



1. Explain what is meant by the piezoelectric effect.

2. Describe how piezoelectric crystals are used for the production and detection of sound.

3. Explain what is meant by the resonant frequency of a piezoelectric crystal.

4. Describe the factors which determine the output frequency of a piezoelectric crystal.


THE PIEZOELECTRIC EFFECT


This is the ability of a material to generate an electrical charge in response to applied pressure. When a piece of piezoelectric material is compressed a potential difference is generated across opposite faces – the one side becomes positive, the other negative (Fig. 3.1). Conversely, if an electric field is applied across the crystal it changes its shape (Fig. 3.2).


image

Fig. 3.1 Compression of a piezoelectric crystal results in a potential difference across opposite faces


image

Fig. 3.2 When an electric field is applied to a piezoelectric crystal it changes its shape



PIEZOELECTRIC MATERIALS


This property is exhibited by certain crystalline materials such as quartz, which is a naturally occurring crystal, and lead zirconate titanate, which is a man-made ceramic.


Piezoelectric materials are crystalline materials composed of dipolar molecules, which are positive at one end and negative at the other (Fig. 3.3). Normally these dipolar molecules have a random arrangement within the material and they are unable to align themselves with an applied electric field (Fig. 3.4). However, if the material is heated above the Curie temperature in the presence of an electric field, the molecules will align themselves with that field. If the material is then allowed to cool below the Curie temperature, while the electric field is maintained, the molecules will remain aligned to the electric field and maintain this position even after the field is removed (see Fig. 3.5). This piezoelectric crystal can now be used in an ultrasound transducer to transmit and to detect sound. Opposite faces of the crystal have conducting plates attached to their surfaces. These surfaces are at right angles to the previously applied electric field (Fig. 3.6).


image

Fig. 3.3 A dipolar molecule having positive and negative regions

Related posts:

Default ThumbnailMultiple choice questions and answers The ultrasound beam Ultrasound interactions and attenuation Transducers Resolution Quality assurance and performance testing

Stay updated, free articles. Join our Telegram channel
Tags: ,
Mar 10, 2016 | Posted by in ULTRASONOGRAPHY | Comments Off on The piezoelectric effect

Full access? Get Clinical Tree
Get Clinical Tree app for offline access