Chemistry Reference
In-Depth Information
homogenised
mixture out
clamping
and electrical
contact bolt
cavitation
zones
adjustable channel
for liquid flow
back mass
stainless
steel
block
heterogeneous
mixture
pumped in
piezoceramic
discs
electrical
contacts
thin metal
blade
front mass
Fig. 16.2
Schematic diagram of a liquid whistle.
face for attachment
(normally by epoxy resin)
metal plate
for attachment
normally by solder
copper coil
windings
Fig. 16.4
Schematic diagram of a piezoelectric sandwich
transducer.
copper coil
windings
Table 16.2
Methods of introducing ultrasound into a system
metal core
• Immerse reactor in a tank of sonicated liquid
(e.g. flask dipped into a cleaning bath)
• Immerse an ultrasonic source directly into the reaction
medium
(e.g. probe placed in a reaction vessel)
• Use a reactor constructed with ultrasonically vibrating walls
(e.g. a tube operating through radial vibrations)
Fig. 16.3
Schematic diagram of a magnetostrictive transducer.
a magnetic field and return to normal dimensions
when the field is removed. This type of transducer is
constructed using laminated nickel as the core of a
solenoid (Fig. 16.3). There are two major disadvan-
tages of this type of transducer: the useful frequency
range is restricted to below 100 kHz and the system
is only about 60% electrically efficient, with consid-
erable energy loss through heating. As a result of the
latter problem, all magnetostrictive transducers must
be cooled. The major advantages are that the system
is of extremely robust construction and produces
very large driving forces.
ments are combined so that their overall mechanical
motion is additive (Fig. 16.4). Piezoelectric trans-
ducers are better than 95% electrically efficient and
can operate over the whole ultrasonic range.
1.3 Apparatus available for sonochemistry
Piezoelectric transducers.
These are the most
common devices employed for the generation of
ultrasound and they utilise ceramics containing
piezoelectric materials such as barium titanate or
lead metaniobate. The piezoceramic element com-
monly used in ultrasonic cleaners and for probe
systems is produced in the form of a disc with a
central hole. Ceramic transducers are potentially
brittle and so it is normal practise to clamp them
between metal blocks. This serves both to protect the
delicate crystalline material and to prevent it from
overheating by acting as a heat sink. Usually two ele-
There are essentially three methods for the intro-
duction of ultrasound into a reacting system (Table
16.2), and of those listed only the first two have
received any extensive use in the chemical labora-
tory. The majority of these systems rely upon the
piezoelectric transducer as a source of power ultra-
sound and all three suffer from the disadvantage
that optimum performance is obtained at a fixed fre-
quency that depends upon the particular transducer
employed. For most commercial probe systems
this frequency is 20 kHz, and for baths it is around
40 kHz.
