Chemistry Reference
In-Depth Information
Piezoelectric element
Electrical matching
and tuning
Focal zone
Backing
δ
d
fp
Fo c u s
point
Electrodes
Front plate
x
fzb
x
fze
Near field
Far field
Fig. 3.2
Scheme of the transducer and the pressure field (x
fzb
and x
fze
, focal zone begin
and end; d
fp
, diameter of the beam at the focus point) with the divergence half angle
δ
.
3
.
1
.
3
.
7
Cape Peninsula University of Technology, CapeTown, South Africa
This research group applied UVP-PD mainly for the measurements in
highly concentrated mineral suspensions of kaolin and bentonite (Kotze
et al.
, 2008b).
3.2
ULTRASOUND TRANSDUCERS
The transducer is a converter between the electrical and ultrasonic signal
and works in both directions. It consists, as also illustrated in Fig. 3.2,
of the piezoelectric element which is usually round and has a diameter
of a few millimetres, a front plate made of epoxy which has usually
a thickness of λ
2(λ is wavelength), backing material which has a
damping function to avoid reverberations, the housing and the electrical
connection. The main design parameters of a transducer are the central
frequency
f
0
and the diameter of the piezoelectric element
D
, also known
as active diameter, which is usually smaller than the visible diameter of
the front plate. These two parameters are important for the pressure field
(Fig. 3.2) generated by the transducer, which consists of the near field,
focal zone and the far field. The length of the near field
l
N
is equal to
D
2
f
0
/
/
4
c
,where
c
is the sound velocity. Although it is possible to make
measurements inside the near field, it is not advisable as the pressure
field is irregular and has a not very well defined diameter. Another
parameter to be considered is the half-divergence angle
δ
=
arcsin(0.51
λ
D
). Thus, a large active element has the advantages of an increased
sensitivity and a reduced beam divergence but the disadvantage of a
long near-field length. The central frequency influences the near-field
length, the beam divergence and the attenuation respectively. For most
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