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PRACTICE
AND APPLICATION* *Based on a paper
presented at the Sonochemistry Symposium, Annual Chemical Congress, held at Intoduction With chemists turning more and
more to ultrasound as a source of energy for the acceleration or modification
of chemical reactivity, so it becomes increasingly important that they
understand some of the ultrasonic engineering principles which underpin the
whole topic of sonochemistry. This article is intended to provide an
introduction to the principles of generation of ultrasound for the
non-specialist. It is to be hoped that a chemist, armed with this
information, will be in a much better position to decide on the type of
equipment most appropriate for the intended laboratory application. The majority of chemists who
have an interest in sonochemistry will naturally have acquired some
familiarity with the types of ultrasonic equipment used for their work.
However, with the increased usage of ultrasound in chemistry, as evidenced by
the large number of sonochemical papers currently being published in the
literature, more chemists are seeking information on the choice of equipment
available and those already in the field may well be looking for more refined
instrumentation. As part of any refinement one would expect a move towards
some means of monitoring acoustic performance. The purpose of this article is
to explore some of the principles behind the generation of power ultrasound.
In doing so it is hoped that a practising chemist may gain not only a better
knowledge of these fundamentals but also an appreciation of some of the
parameters which ought to be monitored when applying acoustic energy in
fundamental sonochemical studies. There are two basic methods of
applying acoustic power to liquid loads (see Figure 1).
One is a low intensity system;
essentially an ultrasonic cleaning bath in which a liquid filled tank has
multiple transducers bonded around the base and walls. The other is a single
transducer, which couples energy into a chemical reaction by means of a horn
or velocity transformer. The latter will be referred to as a probe system.
Each transducer arrangement is powered by a generator or power amplifier. One of the basic parameters in
ultrasonic engineering is power density which can be defined as the
electrical power into the transducer divided by the horn radiating surface
area. The low intensity (bath) system uses a power density at the transducer
face of the order 1-2 W cm-2 for a modern piezoelectric transducer. It is
normal, therefore, to employ a number of transducers to put high powers into
liquids contained in such tanks. For the probe system we can achieve a much
greater maximum power density at the radiating face of the horn. This can be
of the order of several hundred W cm-2. In both of these cases the working
frequencies are of the order 20-40 kHz. Both types of systems have merit but
for research purposes the probe system seems to offer the following
advantages: 1. It is possible to
achieve much greater vibrational amplitudes; Probe system Construction of the transducer Transducers used in modern power
ultrasonic systems are almost without exception based upon the pre-stressed
piezoelectric design. In this construction, a number of piezoelectric
elements - normally two or four - are bolted between a pair of metal end masses.
The piezo elements would be a pre-polarized lead titanate zirconate
composition, which exhibit high activity coupled with both low loss and
ageing characteristics. They are ideally suited to form the basis of an
efficient and rugged transducer. If we consider a length of
polarized piezoelectric rod and drive this with an alternating Voltage, at a
frequency corresponding to its resonant length, then this dimension will
change in sympathy with the applied voltage. Such a rod would have a length
of the order 70 mm at a frequency of 20 kHz. Its power handling capacity
would be low since these ceramics have poor thermal capacity and low tensile
strength. To overcome these inherent weaknesses a number of thin elements are
clamped between two acoustically low loss metal end masses; either titanium
or aluminium would be used for this purpose. The assembly would be designed
so that the overall length was one half-wave at the required frequency of
operation. Figure 2 illustrates a typical
transducer construction. Two piezo elements are positioned near to the point
of maximum stress in a half-wave resonant assembly. Because the elements are
pre-polarized they can be so arranged that they are mechanically aiding but
electrically opposing. This feature enables both end masses to be at an earth
potential. The assembly is clamped together by means of a high tensile bolt,
which ensures the ceramics are in compression at maximum transducer
displacement. Transducers constructed in this
way can have potential efficiencies of 98% and will handle power transfers of
the order For a detailed analysis of the
pre-stressed piezoelectric transducer, the reader is directed to References 1
and 2.
Horns or velocity transformers The vibrating motion generated
by the transducer is normally too low for practical use and so it is
necessary to magnify or amplify this motion. This is the function of the horn
which, like the transducer, is a resonant element in the compression mode.
Normally, these are half a wavelength long, although, should the distance
between the transducer and the sample being treated need to be increased,
they can be designed in multiples of half wavelengths. This can also be
achieved by screwing one horn into the other thereby building up the overall
length. The most popular horn designs
are shown in Figure 3. (a) Linear taper: Simple to make
but its potential magnification is limited to a factor of approximately
four-fold. (b) Exponential taper: This
design offers higher magnification factors than the linear taper. Its shape
makes it more difficult to manufacture but its length coupled with a small
diameter at the working end makes this design particularly suited to micro applications. (c) Stepped: For this design the
magnification factor is given by the ratio of the end areas. The potential
magnification is limited only by the dynamic tensile strength of the horn
material. This is a useful design and easy to manufacture. Gains of up to
16-fold are easily achieved in a practical horn.
Magnification and stress
distribution are shown for the three types (Figure 4). Note the stress
discontinuity in the case of the stepped horn. Great care must be taken when
machining these since any marks in the nodal region will create 'stress
raisers' causing metal fatigue in high magnification horns.
When choosing a material for
acoustic horns, then, we look for the following characteristics: 1. high dynamic fatigue
strength; In order of preference,
suitable materials which fit the above are: 1. titanium alloy; Titanium alloys are way ahead of
the other two in each of the four required characteristics. Aluminium alloys
are too soft for the irradiation of liquids and, compared with titanium
alloys, losses in aluminium bronze and stainless steel will result in end
amplitudes reduced by factors of 0.75 and 0.5 respectively, assuming a given
power going into the transducer. This is because the latter two materials are
acoustically lossier and this will show up as heat, ie the horn will become
hot and transfer heat to the reaction - an undesirable side effect. Monitoring of acoustic input In the industrial field,
equipment is designed to undertake a particular application and vibrational
levels are invariably predetermined. Nevertheless, these systems do contain
some rudimentary monitoring. A meter indicating relative power supplied to
the transducer is common but this does not normally indicate actual watts.
One or two manufacturers do offer, in addition, a measurement of transducer
amplitude. Industrially, this is quite adequate since either will indicate
the consistency of machine performance. However, they are performing an
industrial application and the final arbiter of this performance is the test
or quality engineer whose responsibility it is to obtain a consistent final
product. In the research situation the
end result is not necessarily known and therefore the performance of
equipment cannot be judged by results. It would seem sensible therefore that
a number of acoustic parameters should be monitored which would ensure: 1.
That the day to day acoustic conditions are consistent; There are two basic parameters
which should be known, the operational frequency and the acoustic power in
the treated sample. Frequency is not normally critical within 5-10% and so
the nominal frequency of the system quoted by the manufacturer will probably
suffice. Acoustic power is more difficult to measure because of the power by
demand characteristics of most ultrasonic systems. This means that the rated
power of the generator cannot be used as an indication of acoustic power,
since the power transferred will depend upon: (a) how heavily the transducer
is loaded (this is a function of horn magnification); and (b) the area of the
horn immersed in the treatment sample. There are three possible
approaches to the determination of acoustic power: Calorometric method: This is
simply the calculation of power input by measuring the rate of temperature
rise in the system, taking into account its thermal capacity. It is a rather
cumbersome method and to be used properly it should be undertaken each time a
sample is treated in case there are system variations. Thus, it is not really
a practical method. Measurement of vibrational
amplitude: This is the direct measurement of the amplitude at the working
face of the horn, and will give a parameter that is at least proportional to the
acoustic power [Equation (1)].
It does have the advantage that
it can be continuously monitored but it cannot really be considered as an
absolute method since r and c in a cavitating medium cannot easily be
determined. Amplitude measurement does offer
a very sensitive measurement of acoustic change. It changes as the horn is
immersed further into the treatment sample, ie as it becomes more loaded.
Additionally, it will give warning of any changes in acoustic transmission
caused by: 1. dirty interfaces
between transducer and horn or horn and tip; A combination of the
calorimetric method and measurement of vibrational amplitude might well be
the most useful method of power monitoring and therefore control. In any
event, by measuring amplitude we do have an indication of the acoustic power output
rather than the electrical power into the transducer. Measurement of the real electrical power to the transducer. This can be converted to the
acoustic power if the overall acoustic transfer efficiency is known. The use
of a wattmeter to measure electrical power to the transducer can, in certain
circumstances, lead to a measurement of the true acoustic power transmitted
to the sonicated sample. This can certainly be true of probe systems used for
sonochemical treatment. If the system is driven in a controlled manner then
we can derive the transmitted acoustic power from the unloaded and loaded
electrical powers. The acoustic power transmitted
by a transducer/horn arrangement driven in such a manner is shown in Figure
5.
In these examples the treatment
sample was tap water (80 cm-2) at room temperature. A number of different
horns were screwed in torn to the same transducer and the results are as
shown. (If the gain/area product ratio were made the same for each case then
the transmitted acoustic power would be similar). For the examples chosen,
the energy density has been varied by selecting appropriate probe end areas.
It has to be pointed out, however, that some limitations do exist, eg if the
gain/area product is made too large then the electrical control system will
be unable to handle it and the system will either stall out or the horn will
exceed its dynamic fatigue limits and fracture. In summary, we have seen that a
measure of either or both amplitude and electrical power can offer a valuable
means of checking the acoustic performance of the systems and of monitoring
the transmitted acoustic power. Methods of measuring amplitude Directly by microscope: We can measure amplitude just by looking at the end
of a free transducer with a microscope. A metallurgical microscope with a Continuous monitoring: Having the ability to measure amplitude with a
microscope is clearly impractible during a sonochemical experiment. A method
is required which provides continuous monitoring with a display. There are
two possible approaches; electro-mechanical and purely electrical. 1.
Electro-mechanical: The alternating stress in a resonant element is at a
maximum in the centre (see Figure 2). If a strain gauge is bonded to the
centre of such an element then the output from this will be proportional to
the displacement or amplitude of vibration. This output signal can be
rectified and displayed for example on a meter. The meter can then be
calibrated by the use of a microscope (see the 'Directly by microscope'
section above). It is also possible to derive a purely electrical signal that
is proportional to transducer dis-placement and this possibly offers a more
elegant solution since it eliminates the use of the strain gauge which can be
a somewhat fragile element. Using this method it should only
be necessary to calibrate the meter once since any sub-sequent change in
transducer amplitude due to loading will be accompanied by a proportional
change in strain in the transducer/acoustic system. The meter will thus
follow any induced changes of amplitude which occur either as a result of
power input or load variation. 2. Purely
electrical: Electrical methods of measurement can be contained within the
ultrasonic generator. Essentially this is a power amplifier which converts
energy at the mains frequency to energy at a chosen ultrasonic frequency. Because
of the very narrow operating frequency band of the transducer, it is
essential that the amplifier tracks any changes in resonant frequency of the
system. This can be done by sensing with electrical means the transducer
motion in a similar manner as that just examined. The same electrical signal used
to display amplitude can be fed back into the amplifier and this will enable
the power generated to follow any frequency changes in the
transducer/acoustic system. This is very important because the resonant frequency
of the transducer decreases as it becomes warm and lengthens. Changes in the
treatment sample can also affect the frequency. Both of these effects would
be sufficient to shift the system off resonance with an accompanying
performance loss were it not for the automatic tracking, normally referred to
as automatic frequency control (AFC). Another desirable feature of
this method is that it can be used to limit the transducer amplitude and thus
ensure that it does not damage either itself or the coupled resonant elements
due to overstress. Cavitation phenomena There is little doubt that
cavitation does influence the chemical acceleration process and so an
appreciation of the mechanism is necessary. The subject has been and
continues to be covered by many workers and there is much detailed published
work. Some important references considering much of the detail are given but
the following gives a brief general explanation. Cavitation may occur when
applying high intensity ultrasound to liquids. In generating cavitation a
sinusoidal pressure is superimposed on the steady ambient pressure. Two aspects of cavitation that
may be of interest are shown in Figures 6 and 7. Figure 6 illustrates the
frequency dependence of the intensity required to produce cavitation. The
example given is for degassed water at room temperature. It will be noted
that the intensity required to produce vaporous cavitation above the
frequency of » 100 kHz rises rapidly.
Figure 7 illustrates the effect
of temperature on cavitation and its associated hysteresis effect. This
example is for tap water and the increase in intensity as the temperature is
increased can be observed before it falls away at the boiling point. When the
temperature is allowed to fall an increase in intensity is found in the
region of 50-60ºC. This is quite a significant effect and appears to occur in
all liquids. Health and safety aspects Cavitation effects, produced
during the sonication of a liquid sample, results in the generation of a wide
spectrum of noise. This noise is radiated into the atmosphere and
consideration must therefore be given to the health and safety aspects
associated with the use of ultrasound. The frequency spectrum generated
by a single source frequency ƒ0 is shown in Figure 8. Note the strong
subharmonic which gives a very audible level even for a 20 kHz system.
Permitted exposure limits do vary from country to country but are normally
found to have a maximum of between 85 and 90 dBA over a 8 h period. Shorter
exposure times permit higher pressure levels. If, for example, the exposure
time is halved then the pressure limit may be doubled. This means that for a
4 h exposure, then the limit would be increased by 3 dBA, a 2 h exposure by 3
dBA and so on, up to a maximum of 120 dBA.
There are two ways of guarding
against radiated noise: either acoustic ear muffs or an acoustic screen
around the apparatus. The first solution would be acceptable for one isolated
worker with short exposure times, although muffs can become uncomfortable if
worn for long periods particularly in a warm atmosphere. The second solution
would take the form of a box lined with a proprietary sound absorbing
materials. The transducer and treatment sample would be housed within the
box. A 6mm thick Perspex door would permit observation of the sonication
process. A well fitted door is essential however since noise, like water,
will escape through any gaps. Noise levels in excess of 100
dBA would not be uncommon at a distance of 1 m when processing small samples
at 20 kHz without acoustic screening. It is certainly possible to reduce
these levels to 75 dBA by a well designed screening box. Large scale applications In a production situation the
volumes treated will be very much larger than those considered in the
laboratory. Almost certainly the type of process will govern the choice of
transducer energy density required and it could well be that some processes
would be suited to a low intensity sonication, whereas others may need the
higher intensity of the probe system. In the case of low intensity
treatment, the reacting liquids could be flowed in a controlled manner
through an ultrasonic tank (see Figure 9) and out over a weir to the next
process.
A number of such sonically
activated stages could be connected in line. The tanks would be constructed
in an appropriate grade of stainless steel or if plastic tanks are used then
the transducer could be bonded onto a stainless or titanium plate and bolted
with a gasket into the tank. Alternatively a scaled submersible transducer
assembly could be employed. If high intensity treatment is
needed it is possible to couple a probe transducer into a flow pipe by means
of a 'T' section. A number of such transducers could be employed in this
manner (see Figure 10). The actual number and position in the process line
would need to have been determined during the process development phase.
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(material
taken from Internet)
