conversion of electrical pulses to mechanical vibrations and the
conversion of returned mechanical vibrations back into electrical
energy is the basis for ultrasonic testing. The active element
is the heart of the transducer as it converts the electrical energy
to acoustic energy, and vice versa. The active element is basically
a piece of polarized material (i.e. some parts of the molecule are
positively charged, while other parts of the molecule are negatively
charged) with electrodes attached to two of its opposite faces.
When an electric field is applied across the material, the polarized
molecules will align themselves with the electric field, resulting
in induced dipoles within the molecular or crystal structure of
the material. This alignment of molecules will cause the material
to change dimensions. This phenomenon is known as electrostriction.
In addition, a permanently-polarized material such as quartz (SiO2)
or barium titanate (BaTiO3) will produce an electric field when
the material changes dimensions as a result of an imposed mechanical
force. This phenomenon is known as the piezoelectric effect. Additional
information on why certain materials produce this effect can be
found in the linked presentation material, which was produced
by the Valpey Fisher Corporation.
Piezoelectric Effect (PPT,
Elements (PPT, 178kb)
active element of most acoustic transducers used today is a piezoelectric
ceramic, which can be cut in various ways to produce different
wave modes. A large piezoelectric ceramic element can be seen
in the image of a sectioned low frequency transducer. Preceding
the advent of piezoelectric ceramics in the early 1950's, piezoelectric
crystals made from quartz crystals and magnetostrictive
materials were primarily used. The active element is still sometimes
referred to as the crystal by old timers in the NDT field. When
piezoelectric ceramics were introduced, they soon became the dominant
material for transducers due to their good piezoelectric properties
and their ease of manufacture into a variety of shapes and sizes.
They also operate at low voltage and are usable up to about 300oC.
The first piezoceramic in general use was barium titanate, and
that was followed during the 1960's by lead zirconate titanate
compositions, which are now the most commonly employed ceramic
for making transducers. New materials such as piezo-polymers and
composites are also being used in some applications.
The thickness of the active element is determined by the desired
frequency of the transducer. A thin wafer element vibrates with
a wavelength that is twice its thickness. Therefore, piezoelectric
crystals are cut to a thickness that is 1/2 the desired radiated
wavelength. The higher the frequency of the transducer, the thinner
the active element. The primary reason that high frequency contact
transducers are not produced is because the element is very thin
and too fragile.