10
1 Cavitation and Chemical Reactivity
very important and reactions must be thermostatically controlled. Although made of
hard materials, the emitting zone of the dipped probe undergoes deep damage with
time. Badly eroded parts should be changed or eventually sanded notably because tiny
metallic particles may be dispersed into the liquid medium and eventually interfere
on the course of the reaction (poisoning, catalytic side reactions, metallic inclusions
in frameworks, etc.).
1.7.3 ‘Cup-Horn’ Systems
This set-up is at midway between a horn and a cleaning bath system but remains of
more confidential use than the two previously described ones. It is comparable to a
high-intensity ultrasonic bath with irradiation from bottom to top with an inverted
probe placed in a glass- or metal-made vessel or ‘cup’. This system can be used in
both indirect and direct mode. The reaction vessel can be placed in the water filling the
ultrasonic reactor or chemicals can also be poured directly into the ultrasonic reactor.
This set-up enables much higher acoustic intensities than common ultrasonic baths
making necessary to use a double-jacketed reactor to enable thermostatic control of
the reaction medium.
For high-frequency systems, it is compulsory to adopt a ‘bottom to top’ irradiation mode in reason of the technological aspects of piezoceramic materials. Indirect
irradiation mode should be avoided since these systems are poorly energetic and the
presence of the walls of a glass reactor will enhance reflection of sound waves. However, in the case of aggressive liquid media, these fragile ceramics can be efficiently
protected by tightly glued glass windows bearing a multiple of n/4λ thickness (with
n 1–3), ensuring a sound ‘transparency’ of the glass window. Except for piezoceramics, the commercial availability of ‘ready to use’ high-frequency devices is very
scarce as compared to low-frequency set-ups. This is the consequence of finding in
literature almost solely high-frequency purpose-made irradiant systems.
1.8 Guidelines for Adequate Use of Ultrasonic Devices
Some rules of good use are hereafter given based on the own practical experience
of the authors with respect to important precepts in sonochemistry according to the
type of ultrasonic devices.
Precepts for bath systems:
• Fill the bath to the level preconized by the supplier.
• Never let an ultrasonic bath running out of liquid.
• Try to locate the most powerful cavitation zones (can be done by dipping a sheet
of alumina foil into the bath to map colonies of drilled spots on the foil).
• Always place the reaction vessel on the same X-Y-Z position for reproducibility.
1 Cavitation and Chemical Reactivity
very important and reactions must be thermostatically controlled. Although made of
hard materials, the emitting zone of the dipped probe undergoes deep damage with
time. Badly eroded parts should be changed or eventually sanded notably because tiny
metallic particles may be dispersed into the liquid medium and eventually interfere
on the course of the reaction (poisoning, catalytic side reactions, metallic inclusions
in frameworks, etc.).
1.7.3 ‘Cup-Horn’ Systems
This set-up is at midway between a horn and a cleaning bath system but remains of
more confidential use than the two previously described ones. It is comparable to a
high-intensity ultrasonic bath with irradiation from bottom to top with an inverted
probe placed in a glass- or metal-made vessel or ‘cup’. This system can be used in
both indirect and direct mode. The reaction vessel can be placed in the water filling the
ultrasonic reactor or chemicals can also be poured directly into the ultrasonic reactor.
This set-up enables much higher acoustic intensities than common ultrasonic baths
making necessary to use a double-jacketed reactor to enable thermostatic control of
the reaction medium.
For high-frequency systems, it is compulsory to adopt a ‘bottom to top’ irradiation mode in reason of the technological aspects of piezoceramic materials. Indirect
irradiation mode should be avoided since these systems are poorly energetic and the
presence of the walls of a glass reactor will enhance reflection of sound waves. However, in the case of aggressive liquid media, these fragile ceramics can be efficiently
protected by tightly glued glass windows bearing a multiple of n/4λ thickness (with
n 1–3), ensuring a sound ‘transparency’ of the glass window. Except for piezoceramics, the commercial availability of ‘ready to use’ high-frequency devices is very
scarce as compared to low-frequency set-ups. This is the consequence of finding in
literature almost solely high-frequency purpose-made irradiant systems.
1.8 Guidelines for Adequate Use of Ultrasonic Devices
Some rules of good use are hereafter given based on the own practical experience
of the authors with respect to important precepts in sonochemistry according to the
type of ultrasonic devices.
Precepts for bath systems:
• Fill the bath to the level preconized by the supplier.
• Never let an ultrasonic bath running out of liquid.
• Try to locate the most powerful cavitation zones (can be done by dipping a sheet
of alumina foil into the bath to map colonies of drilled spots on the foil).
• Always place the reaction vessel on the same X-Y-Z position for reproducibility.