Chemistry Reference
In-Depth Information
Table 16.1
Some industrial uses of power ultrasound
0
10
10
2
10
3
10
4
10
5
10
6
10
7
Field
Application
Plastic welding
Fabrication of thermoplastic articles
Cleaning
Cleaning in aqueous media of engineering
items, medical instruments and jewellery
Human hearing
Conventional power
ultrasound
Extended range for
sonochemistry
Diagnostic ultrasound
16Hz - 18kHz
20kHz - 100kHz
Cutting
Accurate drilling and cutting of all forms of
material, from ceramics to frozen food
products
20kHz - 2MHz
Therapeutic
Destruction of cancerous tissue (high-
5MHz - 10MHz
medicine
intensity focused ultrasound, HIFU),
dissolution of blood clots, enhanced
chemotherapy
Fig. 16.1
Sound frequencies associated with sonochemistry.
Processing
Pigments and solid dispersion in liquid
media, crystallisation, filtration, drying,
degassing, defoaming, homogenisation,
emulsification, dissolution, deaggregation,
extraction
exceed the attractive forces of the molecules of the
liquid, and cavitation bubbles will form. It is the fate
of these cavities when they collapse in succeeding
compression cycles that generates the energy for
chemical and mechanical effects. The collapse is
thought to generate very high local temperatures
(around 5000°C) and pressures (in excess of 1000
atmospheres). Sonication of a liquid medium there-
fore can be thought of as generating high-energy
'hot spots' throughout the system.
Sonochemistry
Electrochemistry, environmental protection,
catalysis, benign synthesis
to diagnostic ultrasound (above 1 MHz), reaching a
limit defined as the frequency above which cavita-
tion is no longer possible (at about 3 MHz). This
will mean that a new and wider definition of sono-
chemistry will be required for the uses of
sound
in
chemistry.
In this chapter we will explore the range of appli-
cations of sonochemistry in clean technology and
waste minimisation and try to relate this to the
underlying principles.
Transducers
The first requirement for research in sonochemistry
is a source of ultrasound, and whatever type of com-
mercial instrument is used the energy will be gener-
ated via an ultrasonic transducer—a device by which
mechanical or electrical energy can be converted to
sound energy. There are three main types of ultra-
sonic transducer used in sonochemistry.
1.2 Power ultrasound
Ultrasound is defined as sound of a frequency
beyond that to which the human ear can respond
and generally is considered to lie between 20 kHz
and 500 MHz. The normal range of human hearing
is between 16 Hz (Hz = Hertz = cycle per second) and
about 18 kHz. For younger people 20 kHz is audible
but the frequency response limit reduces with age.
The full range of sound and its subdivisions is shown
in Fig. 16.1.
Power ultrasound enhances chemical reactivity
in a liquid medium through the generation and
destruction of cavitation bubbles. Like any sound
wave, ultrasound is propagated via a series of com-
pression and rarefaction waves induced in the mol-
ecules of the medium through which it passes. At
sufficiently high power the rarefaction cycle may
Liquid-driven transducers.
These are effectively
liquid whistles where a liquid is forced out of an
orifice and across a thin steel blade (Fig. 16.2). Cav-
itation is generated from two sources: the vibration
of the blade induced by liquid flow and a Venturi
effect as the liquid emerges as a jet. This style of
transducer is very effective in mixing and dispersion
but is not used generally for sonochemical reactions
requiring high energies.
Magnetostrictive transducers.
These are electro-
mechanical devices that use magnetostriction, an
effect found in some ferromagnetic materials, e.g.
nickel. Such materials reduce in size when placed in
