1.7 Generation of Ultrasonic Waves and Subsequent Equipment
9
Noteworthy, piezoelectric ceramics do resonate at some specific frequencies
depending on their initial shape (thickness, diameter, etc.). The ceramic is then incorporated in different set-ups, called transducers to enable ultrasonic waves emissions.
These latter can be then mounted on several types of equipment with or without contact (direct or indirect modes) between the ultrasonic emitting zone and the reaction
medium. Many types of ultrasonic apparatuses are now commercially available from
the simplest to the most complex architecture and for small to big volumes. However,
availability of high-frequency devices remains still nowadays anecdotic as compared
to low-frequency ones. This comes probably from a set of reasons as compared to
low-frequency devices. Typically, the thinness of high-frequency ceramics renders
them more fragile and more delicate to shape. The poor conversion ratio of electrical
energy and the quasi-absence of physical effects of ultrasound make high-frequency
ultrasound hardly adequate for heterogeneous chemical systems.
1.7.1 Ultrasonic Bath
This apparatus is the most available, cheapest and the most known ultrasonic device,
historically designed for cleaning and analytical grade solvent degassing. Transducers are usually glued at the bottom and/or on the sides of a water-containing tank. The
reaction vessel is generally plunged into the tank enabling only an indirect irradiation
mode. Most of the pioneering works in the 80s/90s have been accomplished with
this equipment at the dawn of sonochemistry field. However, it exhibits also some
important drawbacks owing to its extremely basic conception. First, this conception
prevents the formation of a homogeneous acoustic field highlighting the importance
of the position of the reaction vessel. Then, chemical effects cannot be expected at
both such a low ultrasonic frequency and amplitude. Mechanical effects predominate making this system adequate for heterogeneous chemical systems for several
purposes among extraction, analyses or organic synthetic chemistry.
1.7.2 Ultrasonic Horn System
It is the most powerful available ultrasonic device. Contrary to the ultrasonic bath, a
horn (or probe) enables a direct irradiation mode with the emitting tip plunging into
the reaction medium (such a system is inherited from biological cell disruptors). A
commonly designed ‘ultrasonic probe’ is in fact constituted of four main parts with
a generator, a piezoelectric transducer, an amplifying part (or booster) and the horn
itself. The booster and horn parts must be made of very hard materials not only to
stand with relatively high mechanical stresses but also to resist to cavitation erosion.
Thus, they are usually made of titanium alloys or stainless steel. Ultrasonic probes
are all low-frequency ultrasonic devices from 20 to 100 kHz. Since high-intensity
ultrasonic waves are emitted directly into the reaction vessel, the thermal release is
9
Noteworthy, piezoelectric ceramics do resonate at some specific frequencies
depending on their initial shape (thickness, diameter, etc.). The ceramic is then incorporated in different set-ups, called transducers to enable ultrasonic waves emissions.
These latter can be then mounted on several types of equipment with or without contact (direct or indirect modes) between the ultrasonic emitting zone and the reaction
medium. Many types of ultrasonic apparatuses are now commercially available from
the simplest to the most complex architecture and for small to big volumes. However,
availability of high-frequency devices remains still nowadays anecdotic as compared
to low-frequency ones. This comes probably from a set of reasons as compared to
low-frequency devices. Typically, the thinness of high-frequency ceramics renders
them more fragile and more delicate to shape. The poor conversion ratio of electrical
energy and the quasi-absence of physical effects of ultrasound make high-frequency
ultrasound hardly adequate for heterogeneous chemical systems.
1.7.1 Ultrasonic Bath
This apparatus is the most available, cheapest and the most known ultrasonic device,
historically designed for cleaning and analytical grade solvent degassing. Transducers are usually glued at the bottom and/or on the sides of a water-containing tank. The
reaction vessel is generally plunged into the tank enabling only an indirect irradiation
mode. Most of the pioneering works in the 80s/90s have been accomplished with
this equipment at the dawn of sonochemistry field. However, it exhibits also some
important drawbacks owing to its extremely basic conception. First, this conception
prevents the formation of a homogeneous acoustic field highlighting the importance
of the position of the reaction vessel. Then, chemical effects cannot be expected at
both such a low ultrasonic frequency and amplitude. Mechanical effects predominate making this system adequate for heterogeneous chemical systems for several
purposes among extraction, analyses or organic synthetic chemistry.
1.7.2 Ultrasonic Horn System
It is the most powerful available ultrasonic device. Contrary to the ultrasonic bath, a
horn (or probe) enables a direct irradiation mode with the emitting tip plunging into
the reaction medium (such a system is inherited from biological cell disruptors). A
commonly designed ‘ultrasonic probe’ is in fact constituted of four main parts with
a generator, a piezoelectric transducer, an amplifying part (or booster) and the horn
itself. The booster and horn parts must be made of very hard materials not only to
stand with relatively high mechanical stresses but also to resist to cavitation erosion.
Thus, they are usually made of titanium alloys or stainless steel. Ultrasonic probes
are all low-frequency ultrasonic devices from 20 to 100 kHz. Since high-intensity
ultrasonic waves are emitted directly into the reaction vessel, the thermal release is