ultrasound cavitation cleaning wilson m.phil uws 1999
TRANSCRIPT
Ultrasound, Cavitation and Cleaning
Benjamin Paul Wilson B.Sc. Hon’s (Wales)
A thesis submitted in fulfilment of the requirements of the degree of Master of Philosophy in the University of Wales
Department of Materials Engineering
University of Wales, Swansea September 1997
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Aim of the Thesis This thesis describes work initially aimed at elucidating the interaction of
power ultrasound and electrolytic current in the removal of oxide scale from metal
(steel) surfaces. The descaling of metals is currently carried out using an acid
‘pickling’ process and electrolytic descaling in neutral salt would be an
‘environmentally friendly’ alternative. As such the work has three parts:
I. A systematic study of ultrasound propagation and ultrasonically induced
cavitation in aqueous solution proximal to an ultrasound transducer. This
work was undertaken by using the sonogenerated chemiluminescence (SCL)
of luminol to produce images of ultrasonically generated cavitation fields.
The background to ultrasound and cavitation in aqueous systems is
introduced in sections 1.1 and 1.4. the phenomenon of luminol SCL is
introduced in section 3.4
II. A systematic study of the influence of ultrasound intensity and electrolytic
current density on the rate of oxide scale removal from a steel surface. The
theory of electrochemical rate processes is introduced in section 1.5. The
nature and causation of oxide heat scales is introduced briefly in section 1.3.
III. A study of the influence of ultrasound on the rate of cathodic hydrogen
evolution and cathodic oxygen reduction at a titanium “sonotrode” (i.e. an
electrode that is also an ultrasonic transducer.) This last piece of work was
undertaken in order to determine how, and to what extent, ultrasound might
affect the rate of interfacial electron transfer processes occurring at a metal
solution interface.
The original intention was to produce a body of knowledge, which would be
of use in the design of equipment for the rapid, environmentally friendly, descaling
of metal surfaces. However, in the course of the above work information came to
light, which may be of more general significance. For this reason an effort was
made to investigate the propagation of ultrasound in cavitation fields and the
operation of a "sonotrode” at as fundamental level as time allowed.
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Acknowledgements
I would like to thank Dr. Neil McMurray for his supervision and for being a
source of inspiration, encouragement and astonishment (in varying degrees)
throughout the duration of this project. Thanks must also go to Duncan MacDonald
for allowing me the opportunity to participate in this research and all at Maysonic
Ultrasonics Ltd. for providing technical assistance throughout the year.
Personal thanks to Sue, Fiona, Jafar, Ahmed, Siva and Dave for all the help,
drama, comedy, and for all those hours you’ve had to endure the less than pleasant
hullabaloo that has been emanating from my little “box of tricks” in the corner of
the lab. Thanks once again to Justin for putting up with lack of computer literacy
your assistance is always much appreciated! Finally a big “diolch yn fawr” for my
good friend Sarah, for all the hours of fun and entertainment that we have both
experienced during the course of our respective write ups.
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Ultrasound, Cavitation and Cleaning
Aim of the Thesis....................................................................................... iii
Acknowledgements ................................................................................... iv
Chapter 1 Introduction...............................................................................2
1.1 Ultrasound and Ultrasonic Mechanisms .................................................................................... 2 1.1.1 Background ......................................................................................................................... 2 1.1.2 History of Ultrasound.......................................................................................................... 2
1.2 Applications of Ultrasound ........................................................................................................ 5 1.2.1 High Frequency (or Diagnostic) Ultrasound ....................................................................... 5 1.2.2 Power or (Low Frequency) Ultrasound............................................................................... 5 1.2.3 Ultrasonic Welding ............................................................................................................. 5 1.2.4 Biological Uses of Ultrasound ............................................................................................ 6 1.2.5 Ultrasound in Medical Applications ................................................................................... 6 1.2.6 Engineering and Ultrasound, ............................................................................................... 7 1.2.7 Dentistry.............................................................................................................................. 8 1.2.8 Ultrasonic Cleaning............................................................................................................. 8
1.3 Heat Scale Formation and Composition..................................................................................... 9 1.3.1 Introduction......................................................................................................................... 9 1.3.2 The Classical Three Layer Scale Formation ....................................................................... 9
1.4 Ultrasonic Effects on Aqueous Media ..................................................................................... 11 1.4.1 Introduction....................................................................................................................... 11 1.4.2 Acoustic Cavitation and Streaming................................................................................... 13 1.4.3 Factors Affecting Cavitation ............................................................................................. 16
1.5 Electrochemistry ...................................................................................................................... 20 1.5.1 Electrochemical Reactions ................................................................................................ 20 1.5.2 The Electrical Double Layer Hypothesis .......................................................................... 27
1.6 Ultrasound in Electrochemistry................................................................................................ 29 1.6.1 Introduction....................................................................................................................... 29 1.6.2 Mass Transport in Electrochemistry ................................................................................. 31 1.6.3 Mass Transport Boundary Layer (Nernst Diffusion Layer) .............................................. 31
Chapter 2 Experimental Set-up and Protocol ........................................34
2.1 Materials .................................................................................................................................. 35 2.1.1 Chemicals.......................................................................................................................... 35
2.2 Methods ................................................................................................................................... 36 2.2.1 Ultrasound Probe............................................................................................................... 36 2.2.2 Photomultiplier Tube ........................................................................................................ 36 2.2.3 Low Light Camera ............................................................................................................ 36 2.2.4 Galvanostat ....................................................................................................................... 36 2.2.5 Function Generator ........................................................................................................... 38 2.2.6 Potentiostat........................................................................................................................ 38
2.3 Calibration of the Ultrasound Probe ........................................................................................ 38 2.3.1 Calorimetry ....................................................................................................................... 38 2.3.2 Equipment Utilised in the Calibration of the Ultrasound Probe........................................ 39 2.3.3 Method .............................................................................................................................. 39 2.3.4 Results............................................................................................................................... 39
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2.4 Luminol Sonochemilumunescence Experiments ..................................................................... 42 2.4.1 Experimental Set-up.......................................................................................................... 42
2.5 Wire Cleaning Experiments ..................................................................................................... 43 2.5.1 Wire Samples .................................................................................................................... 43 2.5.2 Sample Preparation ........................................................................................................... 43 2.5.3 Electrolyte Preparation...................................................................................................... 47 2.5.4 Electrolytic Current........................................................................................................... 47 2.5.5 Current Density ................................................................................................................. 47
2.6 Sonotrode Experiments ............................................................................................................ 48
Chapter 3 The Visualisation of Ultrasonically Induced Cavitation with
Luminol ..................................................................................................50
3.1.1 Introduction....................................................................................................................... 51 3.2 Materials .................................................................................................................................. 51
3.2.1 Chemicals.......................................................................................................................... 51 3.2.2 Experimental Equipment................................................................................................... 51
3.3 Experimental Details................................................................................................................ 52 3.3.1 Kinetic Measurements....................................................................................................... 52 3.3.2 Image Capture and Analysis ............................................................................................. 53
3.4 Results and Discussion............................................................................................................. 53 3.4.1 Kinetic Investigations ....................................................................................................... 53 3.4.2 Mechanism: sonochemical generation of OH• and O2
•- .................................................... 58 3.4.3 SCL Image Analysis ......................................................................................................... 64 3.4.4 Acoustic Attenuation in Cavitating Water. ....................................................................... 73 3.4.5 Conclusions....................................................................................................................... 75
Chapter 4 Determination of the Effect of Ultrasound Intensity and
Proximity on Wire Cleaning Kinetics ......................................................76
4.1 Introduction.............................................................................................................................. 77 4.2 Experimental Details................................................................................................................ 77
4.2.1 Samples ............................................................................................................................. 77 4.2.2 Method .............................................................................................................................. 77 4.2.3 Ultrasonic Configuration................................................................................................... 78 4.2.4 Measurement of Surface Cleaning. ................................................................................... 78
4.3 Results and Discussion............................................................................................................. 81 4.3.1 Influence of Ultrasound Power and Transducer Surface Distance. ................................... 81 4.3.2 Ultrasound Shadowing. ..................................................................................................... 84
Chapter 5 Hydrogen Evolution at the Titanium Sonotrode...................89
5.1 Introduction.............................................................................................................................. 90 5.2 Experimental Details................................................................................................................ 91
5.2.1 Methodology ..................................................................................................................... 91 5.3 Results and Discussion............................................................................................................. 92
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Appendices ...............................................................................................98
Paper: ‘Hydrogen evolution and oxygen reduction at a titanium
sonotrode’ Chem. Commun. (1998) ........................................................99
References ..............................................................................................100
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Chapter 1 Introduction
2
1.1 Ultrasound and Ultrasonic Mechanisms
1.1.1 Background
Sound comprises of a series of waves transmitted through a medium - solid,
liquid or gas – which possesses elastic properties allowing the periodic
displacement of molecules from the mean position. Sound waves are ‘longitudinal’
waves and comprise of a series of alternating compressions and rarefractions, along
the axis of propagation, as shown in Figure 1.1. In effect, the movement/vibration
of the sound source is communicated to a layer of molecules of the medium, which
in turn transmits the motion to an adjoining layer before returning to an
approximately unperturbed position. This is contrast to electromagnetic waves (e.g.
light, radio waves, x-rays)1 which are transverse waves oscillating at right angles to
the direction of propagation. The pitch of sound depends solely on its frequency i.e.
the higher the pitch the higher the frequency. For humans, the range at which sound
waves are audible is around 16 to 17,000 Hz (vibratory waves/cycles per second).
Ultrasound is a term that is used to describe sound waves that possess a frequency
in excess of that discernible by the human ear (>17kHz). The upper level of the
ultrasound is fairly indistinct but is usually taken to be in the region of 5 MHz for
gases and 500 MHz for liquids and solids.
1.1.2 History of Ultrasound
The beginnings of modern day ultrasound technology can be traced back as
far as the 1800’s with the discovery of ultrasonic generation and detection
techniques. In 1847 Joule 2 revealed the phenomenon of Magnestriction which
involves the modification of a magnetic material’s dimensions whilst exposed to a
magnetic field. The Curie Brothers followed this, in 1880 3,4 with their observation
of the Piezoelectric Effect and it’s inverse. Piezoelectricity involves the production
of electrical charge on certain crystalline surfaces when they are put under tension
or pressure and is a method of ultrasound detection. The inverse of this effect uses
alternated potential, which, when applied to the crystal, will be converted from
electrical energy to mechanical (sound) energy – akin to a loudspeaker. If the
alternating potential is of a high enough frequency then ultrasound is generated.
Galton 5 discovered ultrasound’s first practical use in 1883. His specially
designed whistle ‘transducer’ had a resonance cavity which was adjustable and
3
allowed sound of a known frequency to be generated. The so-called ‘Silent Dog
Whistle’ was originally used to investigate the audible range for humans and
animals. As its name suggests it is still in use today for certain applications.
Commercial exploitation of ultrasound didn’t occur until 1917,when
Langevin developed an ultrasonic echo sounding technique for water depth
estimation. The impetus for the invention came out of a competition to detect
icebergs in the open sea and thus prevent another Titanic disaster. This early ‘Echo-
sounder’ method utilised a simple pulse of ultrasound from the ship’s keel to the sea
bottom that reflected it back to a detector. From Equation 1 for seawater:
waterin sound of velocity time21 Travelled Distance ××=
Equation 1
The depth could be gauged and any foreign object between the bottom of the sea
and the ship appeared as an echo in advance of that from the bottom. This system
was the forerunner of what is known as SONAR (SOund, Navigation And Ranging)
today.5,6
Sir John Thornycroft and Sidney Barnaby were the first to observe the
phenomenon of cavitation 7 in 1895. Problems with HMS Daring (a newly built
destroyer) led to discovery that the propeller blades were incorrectly aligned
causing the water structure to be torn apart by the mechanical action and the
induction of cavitation bubbles. The periodic rarefractions produced by intense
‘Power’ ultrasound fields in water can also produce cavitation 5. Since the 1940’s,
an increased understanding of ultrasound and its associated cavitation phenomena
has led to significant developments in the application of power ultrasound to
chemical processes – Sonochemistry. Modifications to chemical reactions by
ultrasound are caused by the occurrence of cavitation.
4
Rarefaction
Axis ofpropagation
Compression
Wavelength
Figure 1.1: An illustration of the periodic compressions and rarefractions present in a sound wave along the axis of propagation.
Frequencies
(Hz)0 101 102 103 104 105 106 107
Bat navigation signals(70 kHz)
Dolphin whistle(120 kHz)
Grasshopper(7 kHz)
Bumble bee(150 Hz)
Range of Human Hearing (16 Hz – 17 kHz) High Frequency Ultrasound (1MHz – 10 MHz)
Low Frequency Ultrasound (20 kHz – 100 kHz)
Figure 1.2: A diagram of frequency range.
5
1.2 Applications of Ultrasound
Practically, ultrasound can be divided into two main areas, High Frequency
or Diagnostic ultrasound and Low Frequency or Power ultrasound.
1.2.1 High Frequency (or Diagnostic) Ultrasound
This utilises ultrasound within the 2-10 MHz range and the effects of the
medium on the wave (attenuation). Typical uses of high frequency ultrasound
include non-destructive materials testing, SONAR and medical scanning of
foetuses.
1.2.2 Power or (Low Frequency) Ultrasound
The second area is known, most commonly, as Power Ultrasound and
involves the region between 20 and 100 kHz in frequency. These are high-energy
waves and are used for a variety of applications that include the welding of plastics,
cleaning and modifications to chemical reactivity. (See Figure 1.2)
1.2.3 Ultrasonic Welding
Ultrasonic welding is one of the major uses of power ultrasound in industry
today. It has the advantages of not involving long heating and cooling cycles in
comparison to more traditional methods. Welding of plastics in this manner also
provides a weld with a high joint strength that is equivalent to between 90-98% of
the normal material strength.8 Application of such technology to is not just limited
to plastics, it has also been used weld snap on lids to paint tins and weld aluminium,
which is extremely difficult by more conventional means due to it’s hard oxide
layer. Welding metal 5 ultrasonically, by producing lateral vibratory movement,
leads to the oxide layer being broke up and adsorbed into the metal surround the
weld. As with the plastic, this forms a high strength weld by preventing the
formation of brittle inter-metallic compounds. Such flexibility also allows for the
precision welding of delicate components e.g. musical instruments instead of using
the more common hard soldering method.
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1.2.4 Biological Uses of Ultrasound
The main use of ultrasound within the field of biology is for the disruption
of cell walls to release their contents (particularly genetic material) for further
examination. Outer cell wall collapse is achieved using a 20 kHz probe to produce
cavitation. The cavitation acts in ‘machine gun’ fashion, with bubbles driven at very
high speed from the tip of the probe into the cell wall, eventually leading to
penetration and subsequent disruption. This method reduces the cells to their
components with limited denaturation of the cellular material, e.g. macromolecular
protein and nucleic acids, if appropriate measures to limit the bulk heating effects
caused by cavitational collapse are used, i.e. keeping the sample cool during
sonication.
1.2.5 Ultrasound in Medical Applications
(i) Therapeutic applications
Power ultrasound found its first medical use as an alternative to massage in
the 1930’s. The mechanical movement of the tissues by the waves of ultrasound
was found to mimic the rubbing movements of a masseur and as a consequence
improve circulation and muscle physiology. Today, the ultrasound can be applied
direct to the skin of the afflicted area, caused by e.g. sporting injuries, strains etc.,
by the use of flat, earthed, quartz crystals and is a common treatment in the
physiotherapist’s armoury.
Another, more recent, use of power ultrasound has been the removal of
kidney stones. By using the mechanical effect of the ultrasound the stone can be
annihilated and excreted via the patient’s urine, without the need for invasive
techniques. Ultrasonic baths have also been used for sterilisation of medical
instruments e.g. scalpels, forceps etc. (see section 1.2.8)
(ii) Diagnostic Uses
This utilises high frequency (diagnostic) ultrasound to provide a non-
invasive technique for human body scanning. It finds particular in foetal imaging9
and continuous/non-hazardous visualisation of surgical instruments during in-vitro
procedures. The basis for what are termed ‘Percussion Techniques’ stem from an
18th century method of tapping on the skin of a patient and listening to the resonant
note produced from either the either air or fluid areas of the body. Diagnosis is then
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based on the pitch and quality of the resonant note. Modern day ‘percussion’
involves high frequency pulses of acoustic ultrasound (3-10 MHz) of short duration.
Backscattering of the pulse from the boundaries in the tissues leads to a series of
echoes, which can be detected at the skin.
1.2.6 Engineering and Ultrasound
(i) Ultrasonic Machining (USM)
With the increasing use of hard, brittle materials e.g. carbides, stainless
steel, glass, ceramics, an alternative to the more usual methods of machining had to
be developed.10 Ultrasonic machining usually involves a 20 kHz cutting tool in an
abrasive slurry (containing alumina, silicon or boron carbide) and is shaped so as to
produce the required profile in the workpiece. An alternative is to use ultrasound to
augment traditional machining methods, which gives an increased efficiency.
(ii) Abrasive Jet Machining (AJM)
Abrasive Jet Machining is similar to conventional sandblasting with the
exception that is more controllable and uses a finer abrasive. Such advantages mean
it can be used to clean, deburr and cut a variety of hard and brittle materials – mica,
germanium, glass, ceramics 5.
(iii) Drilling and Cutting
Use of ultrasound by the aerospace industry includes the use of
ultrasonicated tungsten carbide cutting blades for chiselling complex carbon fibre
shapes prior to sealing with epoxy resin. This allows continuous production of a
quality which would be unattainable by more established methods. Ultrasound is
also used to drill carbide turbine blades in aero-engines due to their fragile nature.
(iv) Metal Tube Drawing
Application of ultrasound to the cold drawing of metal tubing has led to
significant improvements. Ultrasonication of the die with 20 kHz vibrations leads to
a reduction in draw pressure and draw times, and gives a better finish to the final
product. Similar results have been found for the ultrasonic drawing of wire.
8
1.2.7 Dentistry
As with engineering applications (see section 1.2.6) ultrasound is used in the
dentist’s surgery for a whole host of tasks like cleaning, descaling, polishing,
drilling and root canal treatments. Cleaning involves an instrument resonating at 25
kHz that ultrasonicates a spray of sodium carbonate, water and air. Projection of
this spray on to a tooth surface gives a cleaning/polishing action, which is much
gentler for tooth enamel than the previous use of pumice stone grinding. The use of
a diamond-tipped drill attachment and alumina slurry allows for the drilling of tooth
enamel. The same instrument, with the addition of a file, can be used for root canal
treatment that is self-cleaning due to effect of the ultrasonic vibrations transmitted
through the irrigation fluid.
1.2.8 Ultrasonic Cleaning
Since the 1950’s power ultrasound has been utilised for surface cleaning,
particularly in industry, in areas as diverse as the electronics, optics and medicines.
The usefulness of ultrasound for cleaning depends greatly on the nature of the
material to be cleaned. Sound absorbing materials like rubber suffer from mediocre
cleaning quality, whereas the method is highly effective for sound-refracting items
made of glass, metal and plastic. Known in some quarters as ‘Brushless
Scrubbing’,11 ultrasonic cleaning achieves its effects via cavitation bubbles which
slowly erode the insoluble surface contaminants. A majority of the bubbles formed
by cavitation are transient in nature, but a significant number can remain in the
solution on a semi-permanent basis and oscillate for multiple acoustic cycles.12
Hence, hard surface contaminant removal can be achieved by the combination of
the transient cavitations, which crack the contaminant layers, and the stable bubbles
which can lift surface contamination by forming in the cracks between the coating
and the surface. Another phenomenon, which occurs during ultrasonication of a
liquid medium is ‘microstreaming’, whereby liquid is rapidly convected away from
the ultrasound transducer surface and through the bulk medium. This convection
further enhances the process of cleaning by accelerating the dissolution of
contaminants and constantly supplying a fresh solution to the surface of the item to
be cleaned.
For over forty years the advantages of ultrasonic cleaning have been
exploited in a whole host of areas. The technique allows significant time saving and
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more importantly allows unusually formed articles with indents and holes to be
cleaned with similar efficiencies. As recently as 1980, Geckle13 stated that
“…ultrasonic cleaning proves faster than any conventional cleaning method for the
removal of soil and contaminants.” C.T. Walker and R. Walker14 provided further
evidence for this when they demonstrated that, ultrasonication of metal items in an
alkali solution led to a reduction in cleaning times by up to a factor of 1500 when
compared to that achieved in a stirred solution.
1.3 Heat Scale Formation and Composition
1.3.1 Introduction
While the oxygenation of steel in air is kinetically slow at room temperature,
under conditions of intense heat, the increase in rate is such that oxidation products
rapidly accumulate on the metal. When, after a relative short period of time, this
surface contamination has grown into a hard and thick oxide layer, it is referred to
as ‘heat scale’. While consisting predominately of an iron oxide matrix, a heat scale
may contain some of the alloying elements in the steel as silicone and chromium,
which also have a great affinity for oxygen.15 Consequently, determining the scale
composition of highly alloyed steels can become an extremely complex problem.
The following section will limit itself to a general outline of the heat scale
formation of low carbon steels, which, in essence, mechanistically follow the high
temperature oxidation of pure iron.
1.3.2 The Classical Three Layer Scale Formation
The oxygen partial pressure (PO2) and temperature of the surrounding
atmosphere both strongly influence the type of oxide formed on exposed iron
surfaces. For example, a classical three layer scale, such as the one shown in Figure
1.3 is formed when oxygen when the oxygen partial pressure of the furnace gas is
high (such as for air) and the heating temperature is above 570°C.16 Under such
conditions, a compact adherent scale is formed consisting of three distinct layers,
each representing a different oxide phase. Their sequential appearance in the scale
is dictated by the relative availability of oxygen. At the oxide/gas boundary where
oxygen is abundant, a layer of hematite (Fe2O3) develops, as it is
thermodynamically the most stable oxide of iron at high PO2. In the middle region
10
Steel
FeO
Fe3O4
Fe2O3
Figure 1.3: Classical three layer scale (not to scale) consisting of würsite (FeO), magnetite (Fe3O4) and hematite (Fe2O3).
11
of the scale where the oxygen availability is moderate, the formation of magnetite
(Fe3O4) dominates. Finally, at the metal/oxide interface, a layer of würsite since it is
the most stable iron oxide at low PO2. This innermost layer is normally represented
as the stoichiometric compound FeO but in reality, is a grossly non-stoichiometric
phase. This is due to its high iron deficiency and, in consequence, is more
accurately described as Fe1-yO, where y is a measure of the concentration of the
vacancies,17 typically 0.95.18 Diffraction studies have shown that the complex
clusters of crystals, which often exist in würsite19,20 comprise of vacant octahedral
sites (‘normal’ iron vacancies) as well as tetrahedral sites occupied by iron ions (i.e.
interstitial iron ions).19 the chemical formulae of magnetite and hematite, Fe3O4 and
Fe2O3 respectively, represent their ideal composition but both tend to be non-
stoichiometric. However, such deviations are significantly smaller (particularly for
hematite) than is the case for FeO. Würsite is known to exhibit the rock salt crystal
structure, magnetite exhibits a mixed Fe2+ and Fe3+ spinel structure, and hematite is
known to have a corundum structure.21 The source of such heat scales on the
surface of wires comes from the various stages of manufacture that are used to
produce the final product, which include wire drawing, annealing, patenting,
hardening and tempering.
1.4 Ultrasonic Effects on Aqueous Media
1.4.1 Introduction
In general, most cleaning applications involve the use of power ultrasound
(within the region of 20-100 kHz, see section 1.2.2) within an aqueous medium.
The sound energy transmitted into the solution at these lower frequencies is
significantly greater than is the case for high frequency ultrasound – giving rise to
the rapid formation of cavitation bubbles, which are responsible for the ‘scrubbing’
action of the ultrasound.22
Application of the longitudinal ultrasonic waves (see section 1.1.1) to a
solution leads to the creation of physical compressions and rarefractions along the
axis of propagation. (See Figure 1.1) The speed of this propagation has been
reported to be as much as 1505ms-1 in degassed water with an equilibrium
temperature of 31°C23. The passage of sound waves through the liquid causes the
propagation of periodic pressure and velocity variations as the water molecules
oscillate around equilibrium positions. These variations are clearly illustrated by
12
Figure 1.4: An illustration of the phase differences between sound wave pressure and velocity. (Reproduced from Ref.24).
Figure 1.5: Illustration of the standing wave pattern set up due to interference between incident and reflected sound waves. (Reproduced from Ref.25)
VELOCITY
PRESSURE
13
(Figure 1.4) and show the relative velocity and pressure changes in successive
liquid layers. Both velocity and pressure variations occur along the axis of wave
propagation, but they are out of phase by 90° with the pressure lagging behind
velocity. When these propagating (travelling) sound waves reach an interface –
liquid/air or liquid/solid – they are reflected. This reflection leads to the formation
of a ‘Standing Wave Pattern’ caused by the interference between the incident and
reflected waves (see Figure 1.5). Areas where pressure excersion is at a minimum
are termed ‘Pressure Nodes’ and can be observed at three key areas:
(I) Transducer/liquid interface
(II) The half wavelength point (1/2λ)
(III) The liquid/air boundary.
Conversely areas of maximum pressure (‘Pressure Antinodes’) occur at the
1/4λ and 3/4λ points. For travelling waves velocity nodes are out of phase by 90°
(1/4λ) with pressure nodes in a standing wave pattern.
1.4.2 Acoustic Cavitation and Streaming
“Cavitation occurs when bubbles – cavities – filled with gas or vapour
develop within the body of a liquid.”26 A good example of this phenomenon is the
effervescence of supersaturated gas containing liquids. As previously mentioned (in
section 1.1.2), cavitation was first alluded to by Sir John Thornycroft during
investigations into HMS Daring’s propeller corrosion. This was followed by
Langevin’s 27 observation of acoustic cavitation in liquids due to ultrasound. By
1936, the first observations of the presence of cavitation in degassed liquids (at
room temperature and pressure) were reported by Söllner.28 The late 1930’s saw the
interest in cavitation increase as numerous researchers including Harvey 16, Boyce 16 and Kornfeld/Suvorov 29 began to investigate various facets of the phenomenon
and increase understanding of the mechanisms of its inception.
(i) Acoustic cavitation
The formation of cavities in response to an alternating acoustic pressure is
known as ‘Acoustic Cavitation’. This can be generalised to include all observable
14
activity i.e. bubble formation, motion and lifetime caused by the acoustic field.
Henceforth ‘cavitation’ will be taken to mean acoustic cavitation.
Applying ultrasound of a high intensity to aqueous media leads to the
formation of alternating cycles of compressions and rarefractions (see section
1.4.1). Passage of these sonic-waves through the liquid establishes a sinusoidal
wave of varying acoustic pressure (PA):
fTπ2 sin P P Aa =
Equation 2
Where PA is oscillating acoustic pressure amplitude, ƒ Frequency, T time.
This Applied Acoustic Pressure is superimposed on the pre-existing, ambient
Hydrostatic Pressure (PH).
Pressure reaches a maximum during the positive half cycle when the
molecules in the solution reach a maximum compression. Therefore maximum
liquid pressure (PL) is defined as:
aHL P P P +=
Equation 3
Conversely the negative half cycle occurs when the molecules reach their
most spaced, or rarefied state, leading to the minimum liquid pressure:
aHL P- P P =
Equation 4
During the negative half-cycle the liquid pressure (PL) can actually become
negative i.e. when Pa > PH. Should –PL exceed the intermolecular binding forces
within the fluid the molecules are torn apart and cavitation is induced.
An ‘ideal’ cavity is one that is purely a void and collapses almost
instantaneously during the following compression cycle. This collapse releases huge
15
quantities of energy that can generate temperatures as high as 5000°K, pressures in
the region of 1000 atmospheres 30, weak light emission or sonoluminesence 31
arising from the collapse of cavities often accompanies sonication of water at 25°C.
Such violent implosions are known as ‘Transient Cavitations’32 and are generally
formed by lower frequency ultrasound with intensities in excess of 10Wcm-2.19
The existence of transient cavitation bubbles is short-lived (usually less than
one cycle) and typically involves expansion to a radius that is two times greater than
the initial radius of the nucleus, before ending in the violent collapse. This
implosion is considered to be so extreme as there is so little time from initiation to
collapse for gas or vapour to diffuse into the bubble and cushion the impact of the
imploding liquid surfaces.
Transient cavitation is not the only type of cavitation that can be observed.
‘Stable Cavitations’ last for many acoustic cycles and oscillate in a non-linear
fashion around an equilibrium size. It is thought that such micro-bubbles owe their
stable nature to the presence of gas and/or vapour within the bubble preventing the
implosion. Thus the lifetime of cavitation bubbles is dependent on whether there is
sufficient time for gas and vapour diffusion into the cavity during the rarefraction
period. If there is, this allows for the formation of stable cavitation, if there isn’t the
bubble is mainly a void (or highly rarefied solvent vapour) tending to favour a more
transient nature to the cavitation.
(ii) Acoustic Streaming
In addition to the production of cavitation, application of ultrasound to
liquid media creates a continuous displacement of particles around their equilibrium
positions in what is termed ‘Acoustic Streaming’.
This displacement is also sinusoidal in nature (like the variation in acoustic
pressure) and is given by:
Tsin2 YY A fπ=
Equation 5
Where Y is particle displacement,
YA particle displacement amplitude,
ƒ Frequency,
16
T time.
Production of a continuous movement of the particles within the medium
produces the effect of displacing loose contaminants from a contaminated surface
and constantly supplying fresh solution to the surface during ultrasonic cleaning.
(See section 1.2.8)
1.4.3 Factors Affecting Cavitation
A number of experimental parameters can influence the extent and type of
cavitation formed within a liquid medium by the application of ultrasound. This is
of prime importance as the production of cavities is key to the ability of solution to
provide ultrasonic cleaning via the scrubbing action.
(i) Effect of Temperature
Increasing temperature causes a reduction in the intensity of ultrasound
required for the formation of cavitation within an ultrasonicated medium. This is
probably due to the lowering of the surface tension and/or viscosity of the liquid,
either of which would lead to a reduction in the cohesive forces within the fluid and
the energy required to tear it apart. However, near a liquid’s boiling point, the high
temperature causes a significant increase of vapour pressure. So, whilst high
temperature favours the nucleation of cavities the presence of liquid vapour within
the bubble leads to a cushioning of the implosion during the compression cycle.
Hence stable cavitation is favoured and the impact of ultrasound is reduced, as the
shock wave released on cavity implosion becomes less intense.
The effect of temperature on cavitation in tap water is clearly illustrated in
Figure 1.6. Increasing temperature to 55°C leads to an analogous increase in the
intensity of cavitation. Further increases in temperature towards water’s boiling
point sees a reduction in cavitation intensity observed due to the increase in vapour
pressure and it’s associated cushioning effect. Also noteworthy is the hysteresis that
occurs when the water is heated to boiling and then cooled back down to the 50°C -
60°C optimum. The upper curve in Figure 1.6 shows that the intensity of cavitation
is increased when compared to the initial optimum, probably due to the degassing of
solution caused by the boiling of solution.
17
Figure 1.6: The effect of temperature on cavitation in tap water and its hysteresis effect (Reproduced from Ref. 33)
Figure 1.7: Effect of ultrasonic frequency on the cavitation threshold (Reproduced from Ref.34)
18
(ii) Dissolved Gas and Particulate Matter A majority of liquids are heterogeneous in nature and contain a certain
amount of dissolved gas or gas bubbles. These gas bubbles can have a positive
effect on the frequency of cavitation when contained in an ultrasonicated liquid as
they can act as nuclei for the growth of cavitation bubbles. (See Figure 1.7)
Unfortunately if there is a large amount of dissolved gas present it can also have a
detrimental effect on the cleaning effects of cavitation as the cushioning of shock
wave intensity provided by such relatively large quantities of gas far outweighs the
increase in cavitation frequency.
An analogy often used to explain the cushioning effect of dissolved gas is
that of the fizzy drink bottle. When the bottle is sealed it is done so under pressure,
so as to maintain the drink’s fizziness by super saturating it with gas. On opening
the bottle’s pressure is suddenly reduced, leading to an immediate release of
dissolved gas that floats to the surface and diffuses into the surrounding
atmosphere. A similar thing happens as the pressure is quickly reduced during the
rarefraction period of an ultrasound cycle. Here the reduction in pressure causes
dissolved gas to be drawn towards and into a cavitation bubble. In the succeeding
compression cycles this gas softens, and can even prevent, cavity collapse.
Prevention of collapse inevitably leads to an increase in gas attracted into the
bubble on subsequent rarefractions causing it to grow, until it floats to the surface
and discharges into the atmosphere.
Particulate matter can also lower the cavitation threshold by acting as
sources of trapped vapour gas nuclei.
(iii) Ultrasound Intensity (Irradiation Power)
Increasing the intensity intensifies the vibration amplitude. As a
consequence, collapse pressure rises causing faster and more violent transient
cavitation implosion. However there is a limit to the intensity of irradiation that can
be applied to a system due to practical and engineering considerations.
At sufficiently high acoustic intensities a ‘Decoupling Phenomenon’ occurs
leading to a loss of power being transferred into the medium from the source. This
decoupling is due to the source of ultrasound being unable to remain in contact with
the liquid medium for the complete cycle.
19
Another consequence of high power ultrasound on a liquid medium is the
increase in the number of cavitations per unit volume. A large concentration of
cavities can cause a significant amount of conglomeration leading to the formation
of larger, more stable bubbles 26. Such bubble clusters also have the effect of
dampening the sound energy as it passes through the solution causing a decrease in
ultrasound impact and it’s application to surface cleaning for example.
From the point of view of transducer design, an increase in irradiation
intensity requires a greater dimensional change in the transducer material. Such
changes can increase the strain on these components causing them to break and
hence reduce the equipment’s lifetime. Compromise is therefore necessary to give
both optimum performance and longevity.
(iv) Ultrasound Frequency
To produce gas or vapour filled voids by completely rupturing a liquid
requires a finite time. High frequency sound waves can suffer problems as the time
needed to create a bubble is longer than the time available during the rarefraction
cycle. For example at 20 kHz the rarefraction cycle lasts 25µs (=½ƒ) with
maximum negative pressure at 12.5µs, whereas at 20 MHz the rarefraction cycle is
0.025µs.35 Consequently as frequency is increased, cavitation bubble formation
becomes more and more difficult. This problem can be overcome by increasing
ultrasound intensity, which increases acoustic pressure amplitudes and in turn gives
a more powerful rarefraction cycle than can overcome the liquid’s intermolecular
forces. Figure 1.7 demonstrates this quite clearly, illustrating the variation of
threshold intensity with ultrasonic frequency. As can be seen on the plot, there is a
significant rise in the ultrasound intensity required to produce cavitation above
~100 kHz. Operating transducers at this and higher frequencies with sufficient
intensities is extremely difficult. With this in mind, the typical range used for a
power ultrasound application e.g. cleaning, is normally between 20-50 kHz
(anything lower maybe audible which can produce discomfort in the ear of the
user.)
20
1.5 Electrochemistry
1.5.1 Electrochemical Reactions
Electrochemical reactions are concerned with the reduction and oxidation
(so called 'REDOX') processes that occur heterogeneously at a surface which is
electrically conducting i.e. an electrode. Reduction processes or 'Cathodic
Reactions' are so called because they occur at the cathode and have the general
formula:
Red Ox ne n- →+ +
Equation 6
Where Red and Ox are the reduced and oxidised forms of the ‘Redox Couple’. Such
cathodic reduction reactions include metal deposition from an ionic solution 36
(s)-
)( n M ne M →++
aq
Equation 7
And the cathodic evolution of hydrogen gas:
(g) 2-
)( H 2e 2H →++aq
Equation 8
The opposites of such reactions are the oxidation processes or 'Anodic
Reactions’ that occur at the anode. The general form for these anodic oxidation
reactions is:
-n ne Ox Red +→ +
Equation 9
Commonly encountered anodic reactions include the solvation of metal ions:
21
-(aq)
n (s) ne MM +→ +
Equation 10
E.g. -(aq)
2 (s) 2e Fe Fe ++ +
Equation 11
And anodic evolution of oxygen from water:
- 22 4e O 4H O2H ++→ +
Equation 12
It is worthy of note that both the anodic and cathodic processes can occur at
the same electrode surface simultaneously. Each redox couple has its own, unique
electrode potential (Eeq) where there is no net oxidation or reduction occurring i.e.
the couple is in equilibrium (N.B. Cathodic and anodic rates are non-zero here, a
finite exchange velocity is associated with the process of equilibrium.) When such
conditions are reached the value of Eeq can be determined by the Nernst Equation37:
[ ][ ]RedOxlnE E eq
nFRTφ=
Equation 13
Where Eǿ is the standard electrode potential of a couple
R is the universal gas constant
F is the Faraday constant
N is the number of electrons exchanged
T is the temperature
[Ox] concentration of the oxidised form of the couple
[Red] concentration of the reduced form of the couple
22
Application of an external current produces an ‘overpotential’, i.e. causes
the electrode potential to be displaced from Eeq resulting in the net oxidation or
reduction of the redox couple. When this occurs electrical currents are related to
observed chemical rates by Faraday’s law, i.e.:
rate) (anodic nF iA =
Equation 14
(Anodic currents are positive by convention)
rate) (cathodic nF- i C =
Equation 15
(Cathodic currents are negative by convention)
And
CA i i i +=
Equation 16
Where iA is the anodic current component
iC is the cathodic current component
i the external circuit current
The rates (and therefore current contributions) of electrochemical reactions
depend, exponentially on the potential of the electrode and are expressed in the
following form:
For an anodic reaction38
)E-(E )nF-(1 exp i i eqoA
=
RTα
Equation 17
23
Figure 1.8: Anodic and cathodic Tafel Plots (Reproduced from Ref.39)
Pote
ntia
l
Log Current
Tafel Region(Potential Controlled)
Diffusion Limited(Diffusion Controlled)
Figure 1.9: Illustration of the Tafel plot diffusion limited current plateau
24
A cathodic reaction
) E- (E RT
nF- exp i- i eqoC
= α
Equation 18
where io is the exchange current of the redox equilibrium
(A measurement of the exchange velocity)
∝ is the 'Asymmetry Factor' of the redox reaction
When the applied overpotential | E – Eeq| is large (>75mV), then reverse
currents become insignificant i.e. either iA or iC predominates within i leading to
straight line plot of E verses Log i. These plots are known as 'Tafel Plots' and can be
observed experimentally. (Figure 1.9) The current increases exponentially with
potential until it eventually becomes limited by diffusion processes, leading to a
plateau on the Tafel plot (Figure 1.8.)
Another factor that can have a bearing on the appearance of the Tafel plot
for anodic metal dissolution is metal surface passivation. To simplify the
understanding of the metal redox system, the most likely anodic reactions for a
given metal are plotted verses pH on what is termed a 'Pourbaix Diagram'. (See
Figure 1.10.)
Such a diagram for a metal indicates the following zones:
(i) Corrosion (Active)
This is where anodic metal dissolution occurs, E > Eǿ M/Mn+ leading to the
production of water soluble species (ions) and corrosion of the metal (where Eǿ is
the standard electrode potential for the metal.)
(ii) Immunity
Here E < Eǿ M/Mn+ and anodic metal dissolution is impossible,
thermodynamically.
25
Figure 1.10: Pourbaix diagram showing the equilibrium potential-pH for the FE-H2O system, illustrating the areas of immunity, corrosion and passivation. m = the equilibrium of H2O/O2 at Po2 = 1, I = H2O/H2 equilibrium at pH 2. (Reproduced
from Ref.38)
26
Log i
E
EF
Epp
Ec
Transpassive Region
Passive Region
Active or Tafel Region
Figure 1.11: Diagram to illustrate the active-passive regions of a Tafel plot
27
(iii) Passivation
E > Eǿ M/Mn+ and although dissolution is thermodynamically possible the
reaction leads to water insoluble products (usually and an oxide or hydroxide
species.) Deposition of such a non-conductive substance prevents further
dissolution by blocking the electrode surface resulting in a kinetic inhibition.
When the metal surface becomes passive the Tafel plot displays a rapid
reduction in current. (See Figure 1.11) The current increases with electrode
potential up to the 'Passivation Potential' (Epp). Once the electrode attains the
'Breakdown Potential' (EC) the passive film decays and current again begins to
increase. Such an electrode is termed 'Transpassive' 40
Reduction of the electrode potential sees a similar current-potential
relationship with the exception that the passive-active transition is observed at the
'Flade Potential' (Ef). The difference between the Flade Potential and the
Passivation Potential (Ef < Epp) is probably due to potential drops that occur as the
pre-passive film forms at the active regions upper end and local pH changes
associated with anodic current.40
The shape of a Tafel plot for a metal and it's associated values of EC, Epp and
Ef depends on the sort of electrolyte used. Some electrolyte anions may cause the
complexation of the oxidised metal species produced by the anodic reaction, leading
to them becoming soluble and hence hindering the formation of a passive film at the
metal surface. Likewise these anions can breakdown a pre-formed passive film.
This property is termed 'Aggressiveness' i.e. the ability of an anion to complex and
solubilise anodic reaction products. Hence an aggressive anion is one that readily
forms soluble complexes and reduces a metal passivity. In contrast a non-aggressive
anion is one that either doesn't form a complex with the products of the anodic
reaction or reacts to form insoluble complexes, causing the metal surface to remain
or become passive. Halide ions are an example of a highly aggressive anion,
sulphate to is also termed aggressive though is quite mild when compared to
chloride.
1.5.2 The Electrical Double Layer Hypothesis
The interface between an electrode and electrolyte is analogous to that of a
circuit, (Figure 1.12) with a 'Charge Transfer' resistance (associated with the
28
RctCdl
Figure 1.12: The electrode/electrolyte interface is analogous to an electrical circuit in which Rct
is a ‘charge transfer’ resistance and Cdl is the double layer capacitance.
Current
Potential
Time
Figure 1.13: Depiction of the electrode potential–current lag caused by double layer capacitance charging
29
electrochemical reaction) Rct and a double layer capacitance Cdl.. Values of Rct are
potential dependent due to the exponential relationship of the electrochemical
reaction to the electrode potential (see section 1.5.1.) Cdl values are also known to
vary with potential.41 However this does not prevent a simple equivalent circuit
(shown in Figure 1.12) from being used to explain, qualitatively, pulsed or transient
electrochemical current phenomenon.
Application of pulsed or stepwise current to an electrode causes a lag
between electrode potential and the transient current due to double layer
capacitance charging 88 – illustrated by Figure 1.13.
Also impedance (Z) of the double layer varies with f :
dlfCπ21Z =
Equation 19
Whereas values of Rct are frequency independent.
Hence low frequencies of AC current produce electrochemical reactions as
they tend to pass 'Faradically' through Rct, alternatively high frequency AC
produces no electrochemical reaction as it passes through Cdl 'Non-Faradically'.
1.6 Ultrasound in Electrochemistry
1.6.1 Introduction
Sonoelectrochemistry 5, 42 involves the coupling of electrochemical systems
and power ultrasound together to produce new processes caused by the influence of
ultrasound on reaction kinetics. This phenomenon can be compared to other
synergistic approaches in which two independent sources of “activation” energy are
joined together. Such techniques usually result in new methodology for the study of
each activation source separately and the detection of new reactions caused by so-
called ‘Dual Activation’43. Ultrasound may merely initiate processes by activation
but also by influences mass transport.
Direct exposure of homogenous and heterogeneous chemical reactions to
intense ultrasound has a profound effect on reactivity 11,44. Ultrasound has also been
30
usefully applied to organic synthesises that involve electrochemistry,45,46
electroanalytical problems, degradation/mineralisation of toxic materials and
wastewater treatment 47. The application of ultrasound, particularly in electroplating
has led to significant improvements in terms of higher density/quality metal surface
film build-up. Similarly, ultrasound has been shown to produce better quality
conducting polymer films when these are generated by electrodepostition. 48
Recent studies involving ultrasound in conjunction with analytical
electrochemical techniques found that the application of ultrasound to be most
useful under certain conditions. Experiments made by Bard 49 utilised High Speed
Coulometry with ultrasound induced mass transport and Dewald and Peterson 50
found that using pulsed ultrasound led to a new form of Hydrodynamic Modulation
Voltametry. Compton 51 has been particularly active in this field having been
involved in a number of studies including determining the effect of ultrasound on
current/voltage characteristics 52 and developing a form of Anodic Stripping
Voltametry with an added ultrasonic component 53. Brett et al 54 have also shown
the usefulness of in-situ ultrasonicated anodic stripping voltametry and the cleaning
effect provided by ultrasound during voltametric nucleic acid detection.
Studying the effects of ultrasound on an electrochemical system is not
without its difficulties. Ultrasound is known to influence a large number of physical
and chemical processes 55 and its application to an electrode surface must be
carefully controlled. In an ideal situation the sound intensity should be known and
be able to consistently applied.
The effect of ultrasound in electrochemistry can be divided into two areas:
(i) Homogeneous Effects
Such effects can be induced by cavitation, which is coupled to the pressure
changes within the solution (see section 1.4.2.) The collapse of cavitation bubbles,
and the associated high transient pressures and temperatures 56, 57 may cause
homolytic cleavage of chemical bonds in compounds present within, or close to the
bubble. Such cleavage gives rise to the formation of free radicals e.g. hydroxyl
radicals 58, halogen radicals 59 and others 60 depending on the solution composition.
Confirmation of such radical formation has been obtained from radical
trapping/spectroscopic detection experiments.61
31
If a solvent medium contains dissolved macromolecules, ultrasound can
mechanically rupture bonds within these macromolecules by the inducing strong
shear forces in solution. Such effects have resulted in the application of ultrasound
to limit molecular weight, and hence control material properties during
polymerisation processes.
(ii) Heterogeneous Effects
The most dramatic effects caused by ultrasound on heterogeneous processes
are the significant change in mass transport at phase boundaries and solid surface
erosion.62 Erosion is caused by asymmetric cavitation near the solid/solution
interface 63 and is more prominent on materials such as lead and copper materials,
which have relatively low hardness.
Mass transport plays a substantial role in electrochemical as well general
heterogeneous reactions. A great number of heterogeneous/electrochemical
processes are controlled by mass transport and the variation in transported material
to and from interfaces can cause a change in the reaction pathway involved. 64
Electrochemical mass transport depends on the mass transport boundary layer
thickness, δ, (see below) and control of this thickness can be used to vary the nature
of the chemical process.
1.6.2 Mass Transport in Electrochemistry
Transfer of charge at an electrode surface is accompanied by ion transport
within the electrolyte. The transport of uncharged species is dependent on
convection and diffusion. However, charged species are influenced by the effect of
electric fields and for such species charge migration also contributions to mass
transport. Electrochemical processes often owe their limitation to the rate at which
reactant is transported to the surface of the electrode. This limited mass transport
rate can be increased in a variety of ways including an increase in solution
temperature, reactant concentration or fluid agitation.65
1.6.3 Mass Transport Boundary Layer (Nernst Diffusion Layer)
The complex nature of the mass transport boundary layer means that
simplifications are required. One of these simplifications is to assume that there is
32
Con
cent
ratio
n
Distance from electrode0
Csurface
Cbulk
δ
Extrapolation of initialconcentration
Actual concentration
Figure 1.14: Nernst diffusion layer model. The solid line represents the actual concentration profile and the dashed line from Csurface is the extrapolation of the
initial slope
33
the presence of a laminar sub-layer close to the electrode surface that possesses a
linear concentration gradient. This is equivalent to the assumption that near the
electrode mass transport occurs exclusively by diffusion across a constant diffusion
barrier. The concentration gradient of the electroactive species the boundary layer
(known as the ‘Nernst Diffusion Layer’ see figure sad) determines the flux density
and hence current at the electrode surface. The thickness of the Nernst layer (δ) can
be used as a measure of resistance to mass transport. For a given electrode δ is
hydrodynamically determined i.e. the greater the fluid agitation, the thinner the
boundary layer and easier the resultant mass transport.
Nernst diffusion layer treatment of a typical electrochemical system gives an
equation for mass transport of:
δ)c - (c nFD
i surfacebulklim =
Equation 20
Where ilim is the limiting current
n is the number of transferred electrons
F is the Faraday Constant
D is the diffusion co-efficient
C is the concentration
δ is the diffusion layer thickness
The limiting current may be increased by the use of ultrasound because this
thins the diffusion layer thickness by increasing the agitation within the fluid.
Hence there is an enhancement of mass transport due to the influence of ultrasound.
This increase in mass transport has been observed in a number of experiments
carried out by both Compton et al 66 and Walton and co-workers.67
34
Chapter 2 Experimental Set-up and Protocol
2.1 Materials
2.1.1 Chemicals
All the chemicals utilised in this work, listed below, were of
ANLAR purity (unless otherwise stated) and all solutions made up using doubly
distilled water.
Supplier Chemicals Used
Fisher Chemicals UK Sodium Hydroxide (NaOH)
Sodium sulphate (Na2SO4)
Sodium chloride (NaCl)
Hydrochloric acid (HCl)
Sulphuric acid (H2SO4)
Di-sodium Hydrogen orthophosphate
(Na2HPO3)
Aldrich Hydrogen peroxide (H2O2)
3-Aminophtalhydrazine
(Luminol)
Diaminoethanetetra-acetic acid EDTA
(CH2N[CH2COOH]2)2
BOC, UK Argon (Ar)
Albright and Wilson, UK Ltd Phosphoric acid (H3PO4)
36
2.2 Methods
2.2.1 Ultrasound Probe
The ultrasound source in all experiments was a Branson Sonifier 250
variable power 20 kHz ultrasound generator with piezo electric transducer horn.
The transducer horn was used in combination with a variety of 10mm diameter
cylindrical titanium tips including a standard flat tip and a tip ended in a 450 wedge
section, both supplied by Branson Ultrasonics Ltd, UK.
2.2.2 Photomultiplier Tube
All non-spatially-resolved light intensity measurements were performed
using an Electron Tubes Ltd, model QL30F photomultiplier working into a type A1
transconductance amplifier. Light was focussed onto the pmt photocathode using a
3cm-diameter compound glass lens with a 10cm focal length.
2.2.3 Low Light Camera
Luminescence imaging and spatially resolved light intensity measurements
were carried out using a Merlin Low Light camera with fibre optic interface (LTC
216F40E) and type (CCU 2025) control unit supplied by Custom Cameras. The
camera was fitted with a Ziess 55-mm f 2 lens. All images were captured with
integration over either 16 or 64 frames (at a sample rate 25 frames per second) to
improve the signal to noise ratio and produce clearer images. Greyscale images
were digitised in the form of a 320 x 240 resolution matrix of 8-bit pixels using a
Xciplite software.
2.2.4 Galvanostat
The galvanostat apparatus comprised of a Wenking Instrument LB75
Potentiostat operated in galvanostatic mode by passing current through a standard
1Ω resistor as shown Figure 2.6. This apparatus was used for all experiments, which
involved electrolytic scale loosening.
a b
Figure 2.1: A illustration showing the configurations of the ultrasound transducer horn tips a) the standard flat and b) the 45° angled wedge.
38
2.2.5 Function Generator
A Thandar TH501 model, 5MHz Function Generator, RS Components
Limited was used in conjunction with the galvanostat (described above) to produce
current pulse waveforms.
2.2.6 Potentiostat
The potentiostat used for all potentiostatic and potentiodynamic experiments
was a Solartron Instruments electrochemical measurement unit (Model SI 1280)
under computer control, utilising Omega Pro DC software.
2.3 Calibration of the Ultrasound Probe
2.3.1 Calorimetry
Before any of the experimental work could be carried out a calibration of the
ultrasonic probe was undertaken to determine the ultrasound output power
associated with each of the defined generator intensity settings. This was achieved
by the use of calorimetry68. A rise in temperature produced by the addition of any
given amount of energy (in this case ultrasound) to a body is determined by its heat
capacity. Thus when the body in question is a volume of water the input of energy
from the ultrasound probe can be calculated from:
tT c m (W)Power
∆∆×=
Equation 21
Where m is the mass of water (in grams)
c is the specific heat capacity (water = 4.2 JK-1)
∆T is the change in temperature (°C)
∆t is the time taken for the observed temperature
rise (in seconds)
39
2.3.2 Equipment Utilised in the Calibration of the Ultrasound Probe
Figure 2.2 illustrates schematically the experimental set-up used for the
calibration of the ultrasound probe’s power output.
2.3.3 Method
A 100ml of distilled water was measured out and placed in the polystyrene
cup within the insulated beaker. The solution was then allowed to equilibrate for 5
to 10 minutes until a constant temperature reading was achieved.
After equilibration, the water was then sonicated at generator intensity one,
with readings every 10 seconds until an appreciable temperature rise was observed
(usually a few degrees Celsius). This was then repeated for the individual generator
intensities, the results of, which can be observed in Table 2-1 and graphs Figure 2.3
2.3.4 Results
Intensity Energy Input - m x
c x ∆T (Joules)
Time - ∆S
(seconds)
Power Output
(Watts)
1 5 x (4.2 x 100) =
2100
404 5.2
2 8 x (4.2 x 100) =
3360
228.78 14.68
3 10 x (4.2 x 100) =
4200
148.7 28.24
4 8 x (4.2 x 100) =
3360
78.78 42.65
5 20 x (4.2 x 100) =
8400
148.95 56.39
6 10 x (4.2 x 100) =
4200
59.35 70.8
Table 2-1: Power Outputs for the respective ultrasound generator settings
B
A
C
ED
Figure 2.2: Diagram of the apparatus used for calibrating the ultrasound probe power output. A = 20 kHz ultrasound generator, B = ultrasound
probe, C = insulated beaker, D = polystyrene reaction vessel, E = electronic thermometer
20
25
30
35
40
45
50
0 50 100 150 200 250 300 350 400 450 500
Time (seconds)
Tem
pera
ture
Figure 2.3: Calibration curves for ultrasound generator power output levels: 1, 2, 3, 4, 5, 6.
2.4 Luminol Sonochemilumunescence Experiments
2.4.1 Experimental Set-up
The apparatus was set-up as illustrated below. The ultrasound was
introduced into the buffered solution of EDTA, luminol and peroxide (e) via the
ultrasonic transducer horn (a) at varying power levels. The temperature of the
solution was maintained at 50°C by the constant circulation of thermostated water
(f) around the cell (e). For the capture of non-spatially resolved SCL data the
apparatus was used in conjunction with (d) the photomultiplier tube, focussed on
the area directly beneath the tip of the ultrasound transducer horn with a 10cm focal
length, compound glass lens. For the capture spatially resolved data a low light
(CCD) camera was employed.
a
e
b
d
g
c
f
f
Figure 2.4: Schematic diagram of the experimental set-up (a) ultrasound transducer; (b) thermostated cell; (c) optically flat window; (d) photon multiplier tube/low light camera; (e) buffered solution of EDTA, luminol and peroxide; (f)
thermostatic fluid; (g) lightproof box.
43
2.5 Wire Cleaning Experiments
2.5.1 Wire Samples
Wire supplied by Garphyttan Wire Limited was used throughout all the
experiments. This wire consisted of steel with small amounts of carbon (0.50%),
silicon (1.40%), chromium (0.70%) and manganese (0.70%). As a result of the
manufacturing process there was an oxide layer of approximately 15 µm
(determined by microscopic analysis) covering the surface of the metal.
2.5.2 Sample Preparation
Test samples for electrolytic/ultrasonic cleaning consisted of 15cm lengths of
wire complete with a watertight tip of epoxy resin. A coating of insulating tape was
utilised to provide a predetermined surface area for electrolysis (See Figure 2.5).
Steel wire
Exposed area
Waterproofepoxy resin tip
Insulating tape
Figure 2.5: An illustration of the wire sample preparation.
44
B
A
C
To workingelectrode
Frompotentiostat
output
To potentiostat reference inputs
Figure 2.6: Set-up of the 1Ω resistor to provide galvanic output from the
potentiostat. A = ± 1V square waveform, B = 1 Ω resistor, C = ± 1A square
waveform.
45
Figure 2.7 illustrates the equipment used for the ultrasonic used for the
ultrasonic probe distance/cleaning experiments. In light of earlier research 69,70 all
cleaning experiments employed 2 minutes of square wave current with a 95%
anodic duty cycle, frequency 1 Hz and 1A of current provided by the
galvanostat/potentiostat (C) via the function generator (B) to soften the surface
scale to ensure that all observed descaling was entirely due to ultrasound intensity
alone. After the electrolytic pre-treatment the wire was then subjected to 10 second
bursts of ultrasound (the minimum time required by the generator to produce a
stable ultrasound field) for a maximum of 120 seconds in total. After application of
ultrasound the surface was examined and the percentage of the remaining scale
visually assessed. The experiments were repeated for a number of probe to wire
distances and at a number of ultrasound output powers.
C
E
B
F
D
A
G
Figure 2.7: Diagrammatic illustration of the experimental set-up for the probe distance/cleaning tests. (A = 20 kHz generator, B = function generator, C = potentiostat/galvanostat, D = ultrasound probe, E = wire sample, F = graphite counter electrode, G = inert plastic support
47
2.5.3 Electrolyte Preparation
All experiments utilised a 10% w/w solution of sodium sulphate at pH 7. A
standard volume of solution (1400ml) contained in a beaker - thermostated at 50°C
by a stainless steel coil connected to a Grants Model L14 waterbath - was allowed
to reach a thermal equilibrium before any pH adjustments were made. Adjustments
were carried out by the addition of small quantities of sulphuric acid or sodium
hydroxide. Measurement of the pH was made using a Jenway (Model 3071) pH
Meter that was calibrated prior to each use with buffers of pH 4 and pH 10 at 20°C.
The cleaning solution was then subjected to 45 minutes sonication in an ultrasound
bath to ensure that total degassing of the liquid and optimum ultrasound efficiency
for the descaling process.
2.5.4 Electrolytic Current
Pulsed anodic current is generated by connection of the galvanostat to a
function generator (see Figure 2.6) which allowed the anodic duty cycle, current
waveform and frequency to be set.
Anodic duty cycle is defined as the percentage time during which the sample
is anodically polarised during the total current time period:
100 period totalperiod anodic Cycle Duty Anodic ×=
Equation 22
2.5.5 Current Density
Current density on the wire samples was kept constant at 1Acm-2 by keeping
the current passing between the working and the counter electrode at 1A and the
exposed area on the prepared wire sample constant at 1cm2, this can be calculated
from:
area exposedcurrent density Current =
Equation 23
48
2.6 Sonotrode Experiments
Experiments were carried out to determine the current voltage
characteristics (Polarisation Curves) of hydrogen evolution and oxygen reduction at
the tip of the titanium sonoprobe, with and without ultrasonic stimulation. The
experimental set-up is illustrated in Figure 2.8 and measurements were recorded
with a Solartron Instruments electrochemical measurement unit (Model SI 1280)
under computer control, utilising Omega Pro DC software. The reference electrode
used for these experiments was a saturated calomel electrode (SCE) which has a
stable potential of 0.268 Volts when compared to the standard hydrogen electrode
(SHE). For experiments that involved deaerated conditions the electrolyte solution
was purged with pre-purified argon (supplied by BOC, UK) for a period of thirty
minutes to ensure total removal of the air present.
Cathodic polarisation curves were obtained by using a Linear Sweep
Voltametry (LSV) technique 71. LSV involves the potential of the electrode of
interest (working electrode) being varied relative to a fixed reference (reference
electrode potential) at a constant rate. The flow of current between the working
electrode and the electrode solution is recorded as a function of the working
electrodes potential.
For all experiments to determine the cathodic polarisation characteristics of
the tip of the titanium sonoprobe, the LSV potential sweep commenced at –2.5 V
and was swept at a rate of 0.014 Vs-1 to 0 V verses SCE. To ensure that all
voltammograms would be reproducible, the potential of the sonoprobe tip was
cycled from –2.5 V to 0 V at a rate of 0.014 Vs-1 without ultrasonic stimulation, and
before the collection of data, until a stable pattern was observed (typically < three
cycles.)
Electrolyte
Perspex
Reference electrode
Magnetic stirrer
Ar inlet fordegassing
Graphite counter electrode
INSULATEDSONIC HORN
Ti tip
Transducer
Electrochemical measuring unit
To sonic horncontrol unit
Computer
Figure 2.8: Schematic Diagram of sonotrode and electrochemical cell arrangement (Reproduced from Ref. 72
Chapter 3 The Visualisation of Ultrasonically Induced Cavitation with Luminol
51
3.1.1 Introduction
Alkaline solutions of luminol have been known for quite sometime to emit
light when exposed to a source of power ultrasound. The observed light intensity is
significantly more (orders of magnitude) intense than the visible sonoluminescence
caused by the direct sonication of aerated water 63,73,74,75,76 and is thought to be due
to an oxidative chemiluminescent process involving OH• generated by ultrasound.
This method of sonochemically inducing the chemiluminescence of luminol has
been used in a number of studies to investigate the cavitation mechanics 63,76. The
spatial distribution of cavitation in solution has also been determined by using this
set-up in conjunction with either a scanning fibre optic probe 75 or by
photographically capturing images 74,75. As for the kinetics and mechanisms
involved in the sonochemiluminescence of luminol not a great deal is known.
Previous studies of both luminol chemiluminescence 77,78 and electrogenerated
chemiluminescence 79,80 in aqueous solution have demonstrated that pH and
hydrogen peroxide concentration have a dynamic effect on the emitted light
intensity.
Investigation of sonoluminescence involving luminol with additional H2O2
is affected by divalent transition metal cations 81,82,83,84 that catalyse a non-
sonochemical/background chemiluminescence. Solution contamination by transition
metals is introduced through reagent impurities or via leachate from equipment,
which is nigh on impossible to avoid totally. The solution to the problem is to add a
chelating agent, like EDTA, which is able to complex and thus prevent the catalytic
actions of the divalent transition metal cations 81,84.
3.2 Materials
3.2.1 Chemicals
All chemicals used in this work (detailed below) were of ANLAR grade and
all solutions created with doubly distilled water.
3.2.2 Experimental Equipment
The experimental set-up used is illustrated in Figure 2.4. Image analysis and
iso-luminescence contour plot generation was performed using “Surfer”
cartography software obtained from Golden Software Ltd. In all cases where
digitised images were subject to quantitative analysis care was taken that the image
52
bitmap contained no areas of saturation i.e. that all the 8 bit pixel values fell
between zero and 255. Care was also taken that the camera’s depth of field was
adequate to keep the luminescent feature in sharp focus. The object distance was
typically 40cm.
3.3 Experimental Details
3.3.1 Kinetic Measurements
(i) Effect of pH and H2O2 concentration
Experiments to determine the effect of H2O2 concentration on the intensity
of luminol sonoluminescence over a range of solution pH (pH 7-13) at an
ultrasound power of 60W.
A luminol stock solution containing 10-3M of luminol, aqueous phosphate
buffer Na2HPO4 (0.1M) and 5 x 10-6M EDTA was prepared using doubly distilled
water. A 50ml aliquot of this stock solution was taken, heated to 50°C and the pH
adjusted by addition of either 0.3M aqueous NaOH or 0.1M aqueous H3PO4.
Variation in concentration of H2O2 was achieved by additions of known volumes of
0.02M aqueous H2O2 prepared by volumetric dilution of a 30% stock. All H2O2
additions came from prepared solutions that were less than 12 hours old to ensure
no decay of the peroxide concentration.
The aliquot was then placed in the reaction cell, allowed to thermally
equilibrate for 5 minutes before sonication by the ultrasound probe. Results were
plotted as relative intensities verses hydrogen peroxide concentration. (See Figure
3.1)
(ii) Effect of Ultrasound on Sonochemical Luminescent Intensity.
50ml of the luminol stock solution (10-3M luminol, 0.1M Na2HPO4, 5 x 10-6
EDTA) was adjusted to pH 12 with 0.3M aqueous sodium hydroxide solution. This
was then placed in the glass reaction cell and allowed to equilibrate to 50°C before
addition of 10-4M H2O2. The solution was then sonicated at each of the calibrated
ultrasound generator intensities (see section 2.3) and the results plotted as a graph of
power input (W) verses the relative intensity of sonoluminescence. (Figure 3.2)
53
(iii) Effect of EDTA
A solution containing 10-3M luminol, 0.1M Na2HPO4 and 10-4M H2O2 was
prepared. A 50ml aliquot of this solution was adjusted to pH 12 by using 0.3M
aqueous sodium hydroxide, placed in the glass reaction vessel and allowed to
equilibrate to 50°C. Before sonication, a silent or “background” reading was taken
from the photomultiplier tube with another reading recorded during sonication.
The experiment was then repeated with the addition of 5 x 10-7M EDTA and
the results recorded by the photomultiplier tube. EDTA was then added in a
stepwise fashion up to a maximum of 5 x 10-5M.Results were recorded prior to
(background/silent) and during sonication. These results were plotted as relative
‘background’ (unsonicated) or sonicated intensities against concentration of EDTA
and are illustrated in Figure 3.3.
3.3.2 Image Capture and Analysis
The apparatus was used in conjunction with a solution comprising of
luminol (10-3M), H2O2 (10-4M), Na2HPO4 (0.1M) and EDTA (0.02M) adjusted to
pH 12 as previously described, with sodium hydroxide. Experiments were initially
carried out on the flat, 10mm diameter probe tip. Sonication of the luminol solution
was carried out at the calibrated ultrasound generator intensities, two, four and six
(see 2.3) with an individual image being captured for each separate intensity by
CCD video camera. These images were stored on PC via the video capture unit as
greyscale images, digitised in the form of a 320 x 240 resolution matrix of 8-bit
pixels using a Xciplite software prior to further analysis using “Surfer” cartography
software. The experiments were then repeated with a wedge shaped tip (also 10 mm
in diameter).
3.4 Results and Discussion
3.4.1 Kinetic Investigations
The Influence of H2O2 concentration ([H2O2]) on spatially unresolved SCL
intensity was studied over a range of solution pH at constant temperature and using
constant ultrasound power. Figure 3.1 shows the relative intensity of SCL emission
(ISCL) as a function of [H2O2] obtained from a solution containing luminol and
54
EDTA at pH values between pH 7 and pH 13. It may be seen from Figure 3.1 that
any given value of [H2O2] ISCL increases monotonically with solution pH up to pH
12. Above pH 12 the trend becomes reversed and ISCL is decreased at pH 13. It may
also be seen from Figure 3.1 that at pH ≤ 10 increasing [H2O2] has no significant
effect on ISCL. At pH > 10 however, ISCL increases monotonically with [H2O2] for
[H2O2] < 10-4M. The effect is most pronounced at pH 12 where the value of ISCL
approximately doubles as [H2O2] increases from 10-6M to [H2O2] = 10-4M. At
[H2O2] > 10-4M the trend is reversed and ISCL decreases with increasing H2O2
concentration.
The dependence of ISCL on ultrasound power was investigated at pH 12 using
a solution containing luminol, EDTA and 10-4M H2O2. Figure 3.2 shows the
substantially linear relationship (linear correlation coefficient = 0.9998) relationship
observed between ISCL and ultrasonic output power under these conditions. The
observed dependencies of ISCL mechanisms shall be discussed in the subsequent
sections.
The effect of EDTA in suppressing the background (silent)
chemiluminescence of aqueous luminol / H2O2 is illustrated in Figure 3.3. Figure
3.3 shows the relative intensity of light emitted by a solution containing luminol and
H2O2 at pH 12 as a function of EDTA concentration, under both sonicated and
silent conditions. It may be seem from Figure 3.3 that the addition of EDTA has
little effect on the ISCL, which decreases by < 5% as the EDTA concentration
increases from zero to 5 x 10-5M. However it may also be seen from Figure 3.3 that
the intensity of chemiluminescence under silent conditions decreases monotonically
with increasing EDTA concentration and is reduced by approximately 95% at an
EDTA concentration of 5 x 10-5M.
Aqueous solutions of divalent transition metal cations such as Cu2+, Fe2+,
Co2+ and Mn2+ are well known to catalyse the chemiluminescence of luminol in the
presence of dissolved O2 and/or H2O2.81,82,83 In the presence of H2O2, significant
increases in emitted light intensity may be produced by even trace concentrations of
transition metal cation.83 The exact mechanism of this transition metal catalysis is
not known but the production of HO• through a Fenton type reaction has been
suggested as an important step81 i.e.
10-6 10-5 10-4 10-3
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
I SCL,
arbi
trary
uni
ts
[H2O2] , M
Figure 3.1: The effect of H2O2 concentration on spatially unresolved ISCL at different values of solution pH. pH 7, pH 8, pH 9, pH 10, pH 11, pH 12, pH 13 (10-3M luminol, 10-4M EDTA, Temperature 50ºC ultrasound power 70W).
0 20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
light
inte
nsity
, arb
itrar
y un
its
Ultrasound power, Watts
Figure 3.2: Relationship between ultrasound power output power and spatially unresolved ISCL (10-3M luminol, 10-4M H2O2, 10-4M EDTA, Temperature 50ºC, pH 12).
0 1 2 3 4 5 6
0.0
0.2
0.4
0.6
0.8
1.0
1.2
light
inte
nsity
, arb
itrar
y un
its
[EDTA], M x 10-5
Figure 3.3: The effect of EDTA concentration on spatially unresolved intensity of luminol chemiluminescence under: sonicated and silent conditions (10-3M luminol, 10-4M H2O2, pH 12, Temperature 50ºC).
58
HO HO MO H M -322
2 •++ ++→+
Reaction 1 It is also known that the complexation of divalent transitional metal cations by
chelating agents such as EDTA greatly diminishes their ability to catalyse luminol
chemiluminescence.81,83 On the basis of the above it is proposed that the levels of
background light emission observed in the absence of added EDTA result from the
luminol chemiluminescence catalysed by traces of transition metal cation present in
the reagents and/or water used in the experiments. It is further proposed that the
observed suppression of background chemiluminescence by EDTA result from the
complexation and deactivation of the trace metal cations. The finding that ISCL is not
significantly reduced at EDTA concentrations up to 5 x 10-5 M implies that EDTA
acts specifically to inhibit the background chemiluminescence. This suggests that
the EDTA cannot be acting as a reductive “quencher” for HO• or O2• - radicals (vidi
infra) and so tends to confirm the hypothesis that is the properties of EDTA as a
chelating agent that are important here.
3.4.2 Mechanism: sonochemical generation of OH•••• and O2•••• -
The propagation of ultrasound waves in aqueous solution leads to cyclic
pressure variations, which cause the nucleation, growth and collapse of microscopic
cavitation bubbles filled with gas and/or vapour 85,86 (see section 1.4.2.)
Furthermore, it has been shown that the extremely high local temperatures and
pressures may be generated during the collapse or implosion of such bubbles.85,86
Consequently, it is generally accepted that it is within the cavitation bubble, or the
layer of solution immediately contacting the cavitation bubble, that the
sonochemical effects of molecular activation and dissociation take place.85,86,87, 88
In air saturated water the principal sonochemical dissociation processes
involve the homolytic cleavage of H2O and dissolved O2.89,90,91
•• +→ HO H OH 2
Reaction 2
O O O2 +→
Reaction 3
59
Such cleavage products may then recombine or participate in further reactions:
22O H HO HO →+ ••
Reaction 4 •• +→+ HO HO O H O 2
Reaction 5 •• →+ 22 HO O H
Reaction 6
22222 O O H HO HO +→+ ••
Reaction 7
Both hydroxyl radicals (HO•) and hydrogen atoms (H•) have been detected in ESR
spin trapping experiments when water containing permanent gases in solution is
subjected to ultrasound.61,89,92 The production of hydroperoxyl radical (HO2•) has
been presumed from the involvement of this species in specific sonochemical
reactions.93, 94, 95 Furthermore, the sonochemical generation of H2O2 is well
documented, and reactions 4-7 are reported to be significant routes of H2O2
formation91,92,96
It should be noted that HO2• is itself a weak acid with a pKa = 4.8 97, which
causes it to immediately dissociate at pH’s that are neutral or alkali in nature to
form the superoxide radical (O2• -).
-
22 O H HO •+• +↔
Reaction 8
H2O2 is also a weak acid with a pKa = 10-11.65, and dissociates to give the
hydroperoxyl anion (HO2-). The HO• radical has a redox potential of 2.8V 98 and is
readily capable of oxidising both H2O2 and HO2-. Hence when significant
60
concentrations of H2O2 are present in the sonicated solution the following reactions
become of critical importance
+•• +→+ O H O O H OH 3
-2
k22
8
Reaction 9
O H O HO OH 2-
2k
29- +→+ ••
Reaction 10
k8 and k9 are second order rate constants and have been determined to have
the values of k8 = 3.7 x 107 M-1s-1 and k9 = 6.7 x 109 M-1s-1 respectively.99 Thus
leads the rate of O2• - production through reactions 9 and 10 is predicted to increase
with pH as a result of H2O2 dissociation.
There have been a number of mechanistic pathways proposed that contribute
to the oxidative chemiluminescence of luminol and the inter-relationship of such
pathways is far from straight forward. 77,99,100 The principal mechanism that is
proposed for sonoluminescence for the experimental conditions utilised is
illustrated by Scheme 1 and is similar to that proposed for gamma-ray radiolysis
induced luminol chemiluminescence.77 Luminol is known to be a weak dibasic acid
with first and second pKa values of 6.3 101 and approximately 13.80 It may therefore
be understood that over the experimental pH range the predominant luminol species
will be the luminol monoanion (I). Step (i) in Scheme 1 shows the oxidation of the
luminol monoanion (I) to produce the diazaquinone radical anion (II). Step (ii) is
the reaction of the diazaquinone radical anion (II) with the superoxide radical, O2• -
to form the hydroperoxide addition product (III). (III) is a weak acid, pKa = 10.4,99
and it is only the monoanion form of (III) which decomposes through step (iii) to
give the excited state of the aminophthalate monoanion (IV).77,99 The neutral form
of (III) decomposes via a dark reaction (iv) to give the starting material (I) and O2.
Step (iii) is thought to proceed via a concerted mechanism involving an unstable
endoperoxide intermediate99 and the aminophthalate product (IV) relaxes to the
ground state with emission of light at 430nm.
NH2
N
N
O
NH2
N
N
O
O O
NH2
N
N
O
OOHONH2
O
O
O
OH
*
OH
+ N2
-HpKa10.4
O2
(ii)
(iii)
(iv)-O2 + H+
NH2
N
N
(I) (III)
(IV)
O
(II)OOH
HO
(i)
Scheme 1: Reaction pathways of luminol sonogenerated chemiluminescence (SCL)
62
If we consider SCL occurring in the absence of added H2O2, conditions
approximated by the lowest H2O2 concentration data in Figure 3.1, O2• - will be
produced directly through the dissociation of HO2• produced by reaction 6. Some
H2O2 will be generated internally through reactions 4 and 7 and this may be
oxidised to O2• - through reactions 9 and 10. In addition O2
• - is produced through the
slow reaction of (II) with O2 through reaction 11 below. Thus sufficient O2• - will be
present for light to be emitted through Scheme 1. The monotonic increase in ISCL
with pH seen in Figure 3.1 at pH ≤ 12 may be explained by the progressive
dissociation of the hydroperoxide (III).99 The decrease in ISCL at pH > 12 is
consistent with the known decrease in quantum yield of aminophthalate at high
pH.99 The finding that increasing H2O2 concentration only increases ISCL at pH ≥ 10
may be explained if we assume the rate of reaction 10 is fast enough to significantly
increase the steady state concentration of O2• - whereas the rate of reaction 9 is not.77
This assumption would seem reasonable given the relative values of k9 and k10.
Regarding the ISCL maxima observed in the pH 12 and pH 13 data shown in
Figure 3.1, it may be understood that reaction 10 and step (i) of Scheme 1 are in
direct competition for sonochemically generated HO• . Furthermore, it has been
shown that this competition leads to maximum light emission occurring when the
steady state concentrations of (II) and O2• - are equal.77 Thus the observed reduction
in ISCL values at H2O2 concentrations > 10-4M may be ascribed to the depletion of
HO• through reaction 10 suppressing reaction step (i) and leading to a condition
where steady state concentration of (II) < concentration of O2• -. The second order
rate constant for step (i) of Scheme 1 has been reported as 8.7 x 109 M-1s-1,102 i.e.
similar to the value of k10 and close to the diffusion limit. This would suggest that at
pH ≥ 12 maximum light emission should occur when the concentration of luminol
and hydrogen peroxide are approximately equal, provided (II) and O2• - are
consumed at a similar rate. The finding that ISCL is maximal at luminol:H2O2
concentration ratio of ~10:1 suggests that (II) is actually consumed more rapidly
than O2• -. This would seem probable given the additional reactions in which (II) is
known to be involved (vidi infra).
63
In addition to involvement in step (ii) of Scheme 1 (kii ≅ 10-7M-1 s-1)77 (II)
has been shown to react slowly (forward rate constant kf (O2 + II) ~ 550 M-1s-1)
with molecular oxygen through the formal equilibrium
nediazaquino O O -22 ++↔+ • III
Reaction 11
to give a neutral diazaquinone product.100 (II) is also known to undergo rapid self-
recombination and quantitative dismutation (k12 = 5 x 108 M-1s-1)99,103 to give (I)
and the same neutral diazaquinone.
-
2 HO nediazaquino I O H II II ++→++
Reaction 12 The neutral diazaquinone product my either be destroyed through hydrolysis (k
(OH- + diazaquinone) ≅ 108 M-1s-1)99 or react with HO2- to produce the
hydroperoxide adduct (III) (k (HO2- + diazaquinone) ≅ 108 M-1s-1)99. The small
value of kf for reaction 11 implies that the formal equilibrium is never actually
attained. In comparison with (II) O2• - is relatively stable with respect to dismutation
to O2 and H2O2 over the range of experimental pH used here.104 Furthermore, the
steady state concentration of O2• - may be augmented through reaction 11 and the
reaction of neutral diazaquinone with HO2-.
One consequence of the competition between reaction 10 and step (i) of
Scheme 1 is that ISCL is primarily determined by the ratio of luminol and H2O2
concentrations. The absolute concentrations of luminol and H2O2 are immaterial
provided they are greater than the steady state concentration of HO• .77 Under
conditions of alkaline pH and constant H2O2: luminol concentration ratio it has been
shown that the integrated intensity of light emission is linearly dependent on γ-ray
pulse radiolytic dose, and hence HO• yield.77 Thus the linear dependence of ISCL on
ultrasound output power shown in Figure 3.2 is consistent with rate of
sonochemical HO• generation being directly proportional to the ultrasound power
entering the solution. It should be noted that the non-linear, and even inverse
relationships, have been reported between ISCL and ultrasound at high transducer
64
power levels.73,74 However, such relationships have not been observed under the
stated experimental conditions.
3.4.3 SCL Image Analysis
Figure 3.4 and Figure 3.5 show images of SCL in the region of a plane and
wedge ended 1cm-diameter cylindrical titanium sonoprobe tips, respectively. These
images were each obtained by the integration of 64 video frames, captured at 25-
frame sec-1. For both figures (3.4 and 3.5) a primary lobe or “plume” of
luminescence can be seen extending into solution such that the luminescent plume
is co-axial ultrasound transducer horn. In addition, the wedge shaped sonoprobe tip
displays secondary lobes of luminescence that are centred on the base angles of the
wedge section joins the main body of the tip. The localisation of SCL activity in
Figure 3.4 is similar to that reported previously in the case of a plane ended 20kHz-
transducer horn.76 Furthermore, the SCL images obtained using the planar
sonoprobe tip were uninfluenced by the volume of the sonicated solution, or the
dimensions of the sonication cell, provided that these were such that resonance and
a standing wave pattern did not arise.
It is difficult to extract quantitative information from the images shown in
Figure 3.4 and 3.5 ‘by eye’. However the digital nature of the images permit a full
analysis. Figure 3.6 and Figure 3.7 show the iso-luminance contour plots of
spatially resolved ISCL data calculated from the pixel values associated with Figure
3.4 and 3.5 respectively. Iso-luminance contour lines were constructed using the
‘inverse distance squared’ method of weighted interpolation and are spaced at
intervals corresponding to 10% of maximum light intensity. It may be seen from
Figure 3.6 that in the case of the plane tip, maximum ISCL values are located at, or
very near, the transducer-solution interface and decay rapidly with distance from
that interface. However, Figure 3.6 shows that, in the case of the wedge tip, an area
of maximum ISCL values occur approximately 0.25mm from the tip vertex and that
this area is elongated coaxially with the principal axis of the transducer horn.
Images similar to, of SCL emission at the plane ended sonoprobe tip were
obtained over a range of transducer power levels. These images were analysed with
the intention of determining the extent to which spatial distribution of SCL activity
was influenced by ultrasound intensity. In all cases, ISCL was found to decay
exponentially with perpendicular distance (d) from the transducer surface.
65
Figure 3.4: Image of luminol SCL activity, proximal to the standard, flat; 10mm diameter titanium sonoprobe tip. (10-3M luminol, 10–4M H2O2, 0.02M EDTA,
Temperature 50ºC power output - 30W).
66
Figure 3.5: Image of luminol SCL activity, proximal to the wedge shaped, 10mm diameter, titanium sonoprobe tip. (10-3M luminol, 10–4M H2O2, 0.02M EDTA,
Temperature 50ºC power output - 30W).
67
A-A’
B-B’
Figure 3.6: Iso-luminance contour plot of SCL activity proximal to the plane ended, 10mm diameter, titanium sonoprobe tip. (Contours spaced at 10 percent light
intensity intervals).
68
Figure 3.7: Iso-luminance contour plot of sono-chemiluminescent activity for the wedge-ended 10mm diameter, titanium sonoprobe tip. (Contours spaced at 10
percent light intensity intervals).
69
However the half-length of this exponential decay was found to decrease
significantly with increasing transducer output power. Figure 3.8 shows a semi-
logarithmic plot of normalised ISCL values a line coaxial with the principal axis of
the ultrasound horn, as indicated by the line A-A’ in Figure 3.6. The data in Figure
3.8 correspond to at least three half-lengths of ISCL-d decay. The solid lines shown
in Figure 3.8 were constructed by least squares linear regression and exhibit
gradients of 4.3 ± 0.1 cm-1 and 10.3 ± 0.4cm-1 for transducer output powers of 5W
and 70 W respectively. Both data sets exhibit a linear correlation coefficient > 0.98.
By contrast the radial distribution of ISCL in the luminescent plume
generated by the plane ended sonoprobe tip was substantially independent of
ultrasound output power. Figure 3.8 shows the distribution of normalised ISCL
(image pixel values) along a line normal to principal axis ox the ultrasound
transducer horn, as indicated by the line B-B’ in Figure 3.6 for ultrasound powers
5W and 70W. It may be seen from Figure 3.9 that neither the shape nor the absolute
widths of the radial ISCL distribution are changed significantly by the change in
ultrasound output power. Furthermore, the diameter of the sonoluminescent plume
did not change rapidly with perpendicular distance, d, from the transducer surface,
i.e. the plume was not strongly divergent. In no case did the width-at-half-maximum
value of the radially resolved ISCL distribution increase by more than 10% in going
from d = 2.5mm to d = 5mm.
In the previous section it has been proposed that ISCL is proportional to the
rate of sonochemical HO• generation. However, the extent to which Figure 3.4 to
Figure 3.7 can be interpreted, quantitatively, as maps of cavitational intensity is
unclear. One unknown quantity is the extent to which light scattering from
cavitation bubbles 105 contributes to the SCL images. It has been previously stated,
in the case of luminol SCL stimulated by a titanium tipped 20kHz-ultrasound horn,
that “Luminescence is located on the surface of the of the titanium horn”.76 For this
statement to be literally true the appearance of a the luminescent plumes observed
in Figure 3.4 and Figure 3.6 could be explained by the surface of the titanium tip
acting like a plane mirror and reflecting a beam of light out into solution. Scattering
of light would then cause the beam to be visible. That this is not the case is made
evident by Figure 3.5 and Figure 3.7, where the direction of the main plume of
luminescence remains co-axial with the long axis of the transducer horn (possibly
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7-3
-2
-1
0
Ln (n
orm
alis
ed I SC
L)
distance from surface, cm
Figure 3.8: Normalised ISCL as a function of perpendicular distance d from the plane ended sonoprobe tip i.e. along the line A-A’ shown in Figure 3.6 at various ultrasound power values. (10-3M luminol, 10–4M H2O2, Temperature 50ºC, and ultrasound power: 5W, 70W.)
-0.5 0.0 0.5-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
norm
alis
ed I SC
L
distance from centre axis, cm
Figure 3.9: Normalised radial ISCL distribution proximal to the plane ended sonoprobe tip i.e. along the line B-B’ shown in Figure 3.6 at various ultrasound power values. (10-3M luminol, 10–4M H2O2, Temperature 50ºC, and ultrasound power: 5W, 70W.)
73
due to the constructive interference of waves propagating from the tip surfaces)
even though the plane surfaces of the wedge ended sonoprobe tip lie at
approximately 45º to that axis. Thus, we can conclude that, whilst there is probably
a contribution from scattering, the light intensity distribution shown in Figure 3.4 to
Figure 3.7 derive principally from the spatial distribution of SCL activity.
The next difficulty in interpreting images shown in Figure 3.4 (and its ilk) is
the fact that they representations, in two dimensions, of a three dimensional
phenomenon. The light contributing to a single point on the image will therefore
derive from a volume of luminescent solution. If the object distance is sufficiently
great that the light rays entering the camera lens are effectively parallel, and the
absorption and scattering of light in solution is negligible, the observed light
intensity will be linearly dependant on the optical path length (L) through the
luminescent volume along the axis of observation. Given that the spatially resolved
ISCL data in Figure 3.4 and Figure 3.5 derive from an approximately cylindrical
plume of SCL activity the radially resolved ISCL profiles shown in Figure 3.9 are
expected to derive substantially from radial variation of L. Conversely, As the
diameter of the luminescent plume changes very little over the first few millimetres
from the transducer surface, i.e. L is approximately independent of d, the axially
resolved ISCL data shown in Figure 3.8 are expected to correspond closely to the
axial distribution of sonochemical activity. Thus, the exponential decay in ISCL
shown in Figure 3.8 reflects an exponential fall off in the rate of sonochemical HO•
generation with perpendicular distance, d, from the transducer surface.
3.4.4 Acoustic Attenuation in Cavitating Water.
Given the preceding arguments, the axially resolved ISCL – d data shown in
Figure 3.8 may be used to characterise the propagation of ultrasound travelling
waves in the volume of cavitating solution proximal to the transducer surface. In
order to facilitate the necessary analysis it is assumed that the axially resolved ISCL
values are proportional to the local acoustic intensity (I, Wcm-2). The assumption
that, microscopically, ISCL ∝ I is not unreasonable given the linear macroscopic
relationship between spatially unresolved ISCL values and transducer output power
shown in Figure 3.2. Obviously, this relationship will only apply, macroscopically
or microscopically, above the cavitation threshold.
74
When a plane acoustic wave propagates through a homogenous medium the
intensity of the wave decreases with the distance from the radiation source due to
the absorption of acoustic energy and its conversion into heat. Absorption results
from: viscous effects, thermal conduction and chemical relaxation processes
occurring within the medium.106,107 The acoustic intensity, I, at some distance, d,
from a source of intensity I0 is given by:
d)(-2 exp I I 0 α=
Equation 24
Where α is the acoustic absorption coefficient.106,107 For acoustic intensities below
the cavitation threshold below the cavitation threshold the value of α depends
predictably on the mechanical and thermodynamic properties of the medium and
increases with the square of the acoustic frequency, f,106,107 For water the quantity
α/f2 has a measured value of 21 x 10-17 cm-1 s2 over a wide range of frequencies,108
implying α = 8.6 x 10-8 cm-1 at 20 kHz. However, bubbles, such as those produced
through cavitation, are known to be effective absorbers and scatterers of acoustic
energy109,110,111,112,113. Sound absorption occurs through the damping bubble
oscillations by: viscous, re-radiative and thermal conduction mechanisms,109 and the
absorption cross-section of a bubble near its resonant frequency may be 1000 times
its geometrical cross section 110. For these reasons the value of α is predicted to
increase significantly in the presence of cavitation104,113,114 and will depend on the
number concentration and size distribution of cavitation bubbles. However, under
these conditions the ultrasound wave will be subject to multiple scattering from
cavitation bubbles and any experimental value of α would be more properly
regarded as an “attenuation” coefficient containing both absorption and scattering
contributions.
Equation 24 is immediately consistent with the exponential form of the
axially resolved ISCL – d data shown in Figure 3.8, given the assumption that I ∝
ISCL as argued above. This being the case, the gradients of the lines shown in Figure
3.8 correspond to the α values of 4.3 ± 0.1 cm-1 and 10.3 ± 0.4 cm-1 at I0 values of
6.4 Wcm-2 and 89 Wcm-2 respectively. The finding that, in the presence of acoustic
cavitation, α values in water may increase by > 8 orders of magnitude at the
75
cavitation producing frequency implies that, when present, cavitation is the
predominant mechanism of acoustic energy absorption. It also helps to explain the
“shielding” effect,113 significantly reducing acoustic intensities elsewhere in a
sonicated aqueous solution. The observed increase in α with I0 suggests that either
the number concentration of cavitation bubbles increases with acoustic intensity, or
there is an increase in their individual adsorption cross-sections, or both. This
finding also tends to support the notion that an enhancement of acoustic absorption
through cavitation may contribute to the phenomenon of ‘decoupling’34 whereby the
efficiency of energy transfer from the ultrasound transducer a liquid medium
decreases progressively with increasing I0 at high I0 values.
3.4.5 Conclusions
The sonogenerated chemiluminescence (SCL) of aqueous luminol is
strongly influenced by pH and by the concentration of H2O2. In the presence of 10-4
M H2O2 the intensity of SCL is linearly proportional to ultrasound transducer output
power. EDTA (10-4M) reduces the background (silent) chemiluminescence of
luminol/ H2O2 solutions by >95% whilst minimally affecting the intensity of SCL.
These findings are consistent with SCL light emissions following the decomposition
of a hydroperoxide adduct formed through the reaction of luminol mono-anion with
sonogenerated HO• and O2• -. Spatially resolved light intensity information derived
from digitally captured SCL video images may be analysed to provide quantitative
data on the spatial distribution of sonochemical activity in solution, provided
variations in optical pathlength are taken into account. SCL intensity (ISCL) decays
exponentially with perpendicular distance (d) from a planar ultrasound transducer-
solution interface and that the decay half-length decreases with increasing
transducer output power. Acoustic attenuation coefficients (α) in cavitating solution
may be estimated non-invasively using ISCL-d data by assuming a linear
macroscopic relationship between ISCL and transducer input power. The α values
thus obtained increase with transducer power and may be >108 times greater than α
values for homogenous water
76
Chapter 4 Determination of the Effect of Ultrasound Intensity and Proximity on Wire Cleaning Kinetics
77
4.1 Introduction
Previous studies115,116 have shown that oxide heat scale may be removed from
the surface of steel wires by a combination of electrolysis and ultrasonication in
neutral (or near neutral) electrolyte. It is thought that electrolysis serves to disrupt
the bonds, which hold the scale to the metal surface. Ultrasound then acts to break
up and remove the loosened scale.115,116 No synergistic interaction has been found
between the electrolytic current and ultrasound. Best results are obtained when
these two methodologies are applied separately, with electrolysis preceding
ultrasonication.
The work to be described was aimed at investigating the influence of
ultrasound transducer power and transducer-wire distance in determining the rate of
removal of an electrolytically loosened heat scale. A further aim was to relate the
rate of scale removed to the intensity of cavitation proximal to the scaled surface. It
is generally assumed that it is cavitation and the impingement of microjets
generated by the collapsing cavitation bubbles, which are responsible for the surface
cleaning effects of ultrasound. (See section 1.2.8). However, the difficulties in
quantifying cavitational activity in a spacial resolved manner make this hypothesis
hard to test. Here the relationships demonstrated in the preceding chapter between
ultrasound transducer power, distance and cavitational driven sonochemical activity
are correlated with scale removal rates.
4.2 Experimental Details
4.2.1 Samples
All the work was carried out on a wire used in piston ring manufacture
(described in section 2.5.1). The wire, consisting of a high carbon, silicon
manganese steel, was covered with a heat scale layer of approximately 15 µm.
Preparation of the steel wire samples followed the protocol outlined in section 2.5.2
to obtain a current density of 1 Acm-2 during all electrolytic treatments.
4.2.2 Method
The experimental set-up for the kinetics of wire descaling experiments is
shown in Figure 2.7. All experiments featured a constant, 95% anodic duty cycle of
1 Hz frequency with square wave characteristics. All electrolytic baths consisted of
78
a 10% w/w aqueous solution of sodium chloride adjusted to pH 7 and thermostated
at 50 °C by a stainless steel coil connected to a waterbath. Ultrasound was applied
at a variety of intensities and wire to probe distances as outlined below.
4.2.3 Ultrasonic Configuration
Variation of Intensity and Probe Distance Tip to Wire Distance
The distance of the wire samples from the tip of the ultrasound probe was
varied between 5-25mm in 5mm steps by the use of an adjustable inert plastic
support stand. After the initial electrolytic pre-treatment the wire was then exposed
to a 10-second burst of ultrasound from the probe (this was the shortest time that
produced a stable ultrasound field). After visually inspecting the surface of the wire
to determine the progress of the cleaning, the cycle of ultrasound/inspection was
repeated until either the sample was 100% scale free or a consistent percentage of
scale remained.
The experiments were repeated for the various probe to wire distances at
calibrated ultrasound intensities 1, 2, 4 and 6. (See section 2.3.4)
4.2.4 Measurement of Surface Cleaning.
The prepared wire sample and graphite counter electrode were immersed in
the 10% sodium sulphate solution and connected to the galvanostat in preparation
for electrolysis (See Figure 2.7.) For all the wire samples used the wire was
prepared in such a way that the surface current density was equivalent to 1Acm-2
(see Figure 2.4.) The subsequent cleaning was achieved by an initial two minutes
electrolytic treatment of the wire followed by 10-second bursts of ultrasound as
described in previous section. Experiments were repeated three times in total for
each of the different conditions employed from which an average cleaning rate was
determined.
Cleaning progress was monitored by withdrawal of the wire sample from the
sodium sulphate solution after each 10-second exposure to the ultrasound. An
assessment of the remaining heat scale soiling was determined by the viewing of the
upper surface (hemi-cylinder see Figure 4.4) of the wire through a millimetre grid.
Estimated results are presented as percentage of remaining soil verses total
sonication times.
79
(a) 5.2W
0
20
40
60
80
100
0 20 40 60 80 100 120 140
Cleaning Time (seconds)
Perc
enta
ge s
cale
rem
aini
ng
(b) 14.68W
0
20
40
60
80
100
0 20 40 60 80 100 120 140
Cleaning time (seconds)
Perc
enta
ge s
cale
rem
aini
ng
80
(c) 42.65W
0
20
40
60
80
100
0 20 40 60 80 100 120 140
Cleaning time (seconds)
Perc
enta
ge s
cale
rem
aini
ng
(d) 70.8W
0
20
40
60
80
100
0 20 40 60 80 100 120 140
Cleaning Time (seconds)
Perc
enta
ge s
cale
rem
aini
ng
Figure 4.1: The variation in the percentage of scale removed with distance from the ultrasound probe tip at various power outputs of (a) 5.2W, (b) 14.68W, (c) 42.65W,
(d) 70.8W ( -5mm, -10mm, -15mm, -20mm, -25mm.)
81
NB These DO NOT reflect total experimentation time, ONLY the
accumulative of the times that the wire was exposed to ultrasound
4.3 Results and Discussion
4.3.1 Influence of Ultrasound Power and Transducer Surface Distance.
Figure 4.1 shows the fraction of the exposed metal surface which remains
covered with scale as a function of time for various wire-probe distances at probe
ultrasound powers of a) 5.2W b) 14.6W c) 45W and d) 70.8W. It maybe seen from
Figure 4.1 that for each probe output power the rate of scale removal increases
markedly with decreasing probe-wire distances. It may also be seen from Figure 4.1
that at any given probe-wire distance the rate of scale removal increases with
increasing probe output power. It should be noted that the minimum experimental
periods of sonication was 10 seconds and at higher probe output powers and lower
probe-wire distances the surface is completely de-scaled after 10 seconds of
sonication. This means descaling times indicated in Figure 4.1 under these
conditions must be regarded as minimum estimates of the true descaling time
Figure 4.2 shows the fraction of the exposed wire surface, which has been
de-scaled after 120 seconds of ultrasonication as a function of transducer output
power at various transducer-wire distances. The data in Figure 4.2 is replotted from
Figure 4.1 and represents a crude estimation of scale removal rates over the 120
seconds experimentation period. Figure 4.2 serves to illustrate in a more condensed
fashion how the rate of scale removal increases with increasing transducer output
power and increases with decreasing transducer-wire distance. Again with the
caveat that the fastest rates of scale removal are underestimated due to the
experimental procedure used. A more refined estimate of scale removal rate was
made by constructing tangents to the scale area verses time plots shown in Figure
4.1 at time zero. It may be appreciated that the gradient of such a tangent represents
an estimate of the initial rate of scale removal (Ri).
Figure 4.3 shows a semi-logarithmic plot of Ri as a function of transducer-
wire distance for various values of transducer output power. It may be seen from
Figure 4.3 that for Ri values < 10 the plots of lnRi verses transducer wire distance
are approximately linear. The slopes of the linear portions of the lnRi-distance plots
were all approximately equivalent at 3.5cm-1 ± 0.1 cm-1, which correlates quite
readily with the calculated α value of 4.3 ± 0.1 cm-1 at lower ultrasound intensities.
0 10 20 30 40 50 60 70
0.0
0.2
0.4
0.6
0.8
1.0
Frac
tiona
l Are
a C
over
age
(Ac/
A)
Power (Watts)
Figure 4.2: Graph illustrating the changes in the area of the wire surface covered with scale as a fraction of the total surface area (Ac/A) with applied ultrasound density for distances of 5mm; 10mm; 15mm; 20mm; + 25mm; from the surface of the sonoprobe.
0.0 0.5 1.0 1.5 2.0 2.5-6
-4
-2
0
2
lnR
i
Distance from probe tip(cm)
Figure 4.3: Graph to show the changes in initial cleaning rate (ln Ri) with distance from the sonoprobe tip for various ultrasound power densities: = 5.2W. = 14.68W; = 42.65W;= 70.8W.
84
Figure 4.3 shows clearly that the maximum measured value of Ri is 10 and all the
experimental plots converge to this limit as Ri increases. Once again the limiting
value of Ri almost certainly arises from the minimum experimental period of 10
seconds and the error that this introduces when de-scaling is rapid.
The finding that (non-limiting) Ri values vary with exponentially with
transducer-wire distance is directly consistent with the exponential relationship
between cavitationally driven sonochemical activity and distance from the
transducer surface shown in the previous chapter. Furthermore, the slope of the ln
Ri-distance plots are of a similar magnitude to the cavitation field acoustic
coefficients (α) values of 4.3 ± 0.1 cm-1 and 10.3 ± 0.4 cm-1 determined by the
analysis of luminol SCL image data. These findings tend to support the hypothesis
that it is cavitation, and its related properties, which are responsible for the
ultrasonic removal of electrolytically loosened oxide scale from the wire surface. It
further implies an approximately linear dependence of de-scaling rate on ultrasound
intensity.
4.3.2 Ultrasound Shadowing.
It remains to explain the observation that only the half of the wire surface
proximal to the ultrasound transducer becomes significantly de-scaled over the 120
seconds experimental period (see Figure 4.4). It may be appreciated from the
preceding section that the observed exponential decrease in scale removal rate with
transducer-surface distance might be considered as a contributing factor. That is to
say the distal side of the wire is more distant than the ultrasound transducer and is
therefore more slowly de-scaled. However, the experimental ln RI-distance scores
are insufficient to explain the marked dissimilarity in observed descaling rates
An alternative explanation is that the wire-solution interface scatters
ultrasonic energy such that ultrasound intensity is significantly attenuated in
solution lying behind the wire. That is to say that the wire casts an ultrasound
shadow thus reducing the intensity of the cavitational activity near the distal portion
of the wire surface. To test this hypothesis a luminol SCL imaging experiment was
carried out, as described in the preceding chapter. In this experiment a prepared
wire sample (See section 2.5.2) was placed 5mm from the transducer tip and
sonicated. The image was captured and analysed as before.
85
A)
UltrasoundProbe
Wire Oxide
UltrasoundProbe
Wire Hemicylinder ofcleaned wire
B)
Figure 4.4: The hemi-cylindrical cleaning of the oxide scale of wire: A) diagrammatic illustration and B) the appearance of the wire surface after combined
electrolytic-ultrasonic de-scaling.
86
Figure 4.5: Image of luminol SCL activity proximal to the plane ended, 10mm diameter, titanium sonoprobe tip during wire de-scaling. (10-3M luminol, 10–4M
H2O2, 0.02M EDTA, Temperature 50ºC power output – 30W.)
87
Figure 4.6: False colour iso-luminance contour plot of SCL activity proximal to the plane ended, 10mm diameter, titanium sonoprobe tip during wire cleaning.
88
Both Figure 4.5 and Figure 4.6 clearly show a reduction in luminol SCL in
the region of solution immediately behind the wire. The SCL intensity at points A
and B in Figure 4.6 was 26 and 12 pixel values respectively. Assuming the linear
relationship between ultrasound intensity and SCL light intensity argued in the
preceding chapter these values suggest that the ultrasound intensity immediately
behind the wire is less than half the ultrasound intensity found immediately in front
of the wire. This difference offers a possible explanation for the observed difference
in the cleaning rate between the two sides of the wire’s surface.
89
Chapter 5 Hydrogen Evolution at the Titanium Sonotrode.
90
5.1 Introduction
The cleaning of metal surfaces electrolytically involves, by definition, some
type of electrolytic cell. This usually consists of two electrodes, (I) the working
electrode, which for metal cleaning purposes is usually the item to be cleaned and
(II) the counter electrode, which is traditionally an inert substance that allows the
passage of current (e.g. graphite). Both of these electrodes are immersed in an ionic
conductor like an aqueous salt solution, which completes the ‘electronic circuitry’
and allows the passage of electric current between the electrodes.
Effects caused by Power Ultrasound (15-40 kHz) cavitation on electrode
processes is known to fall into five distinct modes:
(i) Mass transport enhancement (see section 1.6.2) caused by increased
turbulence and microstreaming 67,85,117,118 ,
(ii) Continuous electrode surface activation 119,
(iii) Radical, ion or other high energy intermediate formation 117,
(iv) Product desorption 120 and
(v) The increase in heterogeneous electron transfer within the chemical
processes 121.
Traditionally ultrasonic baths and probes have been utilised to indirectly
stimulate electrodes 122. More recent research carried out by Compton, Ecklund et al 123 as well as Reisse, Francois and co-workers 124 has, however, indicated that a far
greater rate enhancement can be achieved by using the tip of an ultrasound
transducer horn as an electrode. Direct stimulation of these so-called ‘Sonotrodes’
causes the observed increase in observed reaction rate.
For this study experiments were carried out to investigate the beneficial
effects that the direct application of ultrasound would have, if any on hydrogen
evolution (Reaction One) and oxygen reduction (Reaction Two) in aqueous
electrolyte. This was achieved by substituting the working electrode for a
‘sonotrode’ – in this case the tip of a titanium ultrasound horn.
91
(g) 2-
(aq) H 2e 2H →++ (1)
(aq) --
2(aq)2 4HO 4e O2H O →++ (2)
Both of these reactions play an important part in both cathodic acid pickling,
AC based electrolytic de-scaling and metallic corrosion in aqueous systems. (The
presence of cavitation phenomena causing increased corrosion rates being of
notable importance 125.) Each of the reactions outlined above involve complex,
multistep mechanisms that incorporate the presence of chemisorbed intermediates 126,127 and whose reaction could be affected by any over the previously stated modes
of action (i) – (v). The present chapter aims to determine to what extent intense
ultrasound cavitation influences such processes.
5.2 Experimental Details
5.2.1 Methodology
(i) Aerated
The experimental set-up employed is illustrated in Figure 2.8. The
electrolytic cell bath consisted of a 10% w/w solution of sodium sulphate (Na2SO4)
adjusted to pH 7 with 0.1M NaOH. The power level used throughout all
experiments was 26Wcm-2, which was determined by calorimetry 68 (see section
2.3.1). Temperature control was provided by a Grants Y14 waterbath, which
circulated thermostated water through a stainless steel cooling coil 123 and
maintained electrolyte temperature constant (to within +/- 1ºC). A Solatron 1280
potentiostat controlled by computer was used to carry out all voltametric
measurements at a temperature of 30ºC. The collection of data for the linear sweep
voltammograms involved a sweeping rate of 0.014Vs-1 of the sonotrode potential
between –2.5V to 0V verses SCE (Standard Calomel Electrode). To ensure that the
resultant voltammogram was reproducible the potential of the sonotrode was cycled
from –2.5 to 0V at a rate of 0.014V-1 without sonication, prior to data collection,
until a stable pattern was observed (typically < three cycles).
92
(ii) Deaerated
For experiments that involved deaerated conditions, the electrolyte was
purged for 30 minutes with pre-purified argon (supplied by BOC) prior to
measurements being carried out.
5.3 Results and Discussion
The results of the investigations are displayed as Tafel plots. Figure 5.1
shows the voltametric responses under deaerated conditions and Figure 5.2 the
effect of aerated electrolyte. Evolution of hydrogen was clearly observable at
potentials <-1.6V. Both Figure 5.1 and Figure 5.2 show that the distinctive
appearance of the Tafel plot is more or less unchanged for sonicated and silent
conditions, the only difference being that the values of current density increase in
the presence of ultrasound, for all potentials.
The reproducibility, reversibility and magnitude of the observed sono-
electrochemical effect are emphasised by Figure 5.3 and Figure 5.4. For deaerated
and aerated conditions respectively these figures (5.3 and 5.4) demonstrate how the
sonotrode’s current responses are time dependent/transient when short pulses of
sonication are imposed on it at constant potential. The result of such bursts of
ultrasound in deaerated electrolyte at a potential of –1.3V can be found in Figure
5.3 whereas Figure 5.4 displays the response of the sonotrode in aerated electrolyte
at a constant potential of –1.25V. For both conditions, the changes in current
density caused by application of ultrasound are seen to be instantaneous and
reversible. Such results mean that bulk heating effects that can be caused by
ultrasound are not responsible for producing the rapid increases in current density.
Titanium sonotrodes rapidly form a TiO2/TiO3 layer (which acts like an n-Type
semiconductor) when exposed to air. This layer has been found to limit
electrochemical reactions –especially anodic- at titanium sonotrodes 123 but the
results in Figure 5.1 to Figure 5.4 show little evidence of this. The figures
demonstrate that the titanium sonotrode is more than acceptable cathode for
reaction (1) and (2) at the stated potentials and that sonication leads to a substantial
enhancement of reaction rate.
-6 -5 -4 -3 -2 -1 0-3
-2
-1
0
Pote
ntia
l (V)
Figure 5.1 Tafel plot of titanium sonotrode voltametric response under silent (---) and sonicated (___) conditions in deaerated neutral 10% wt Na2SO4 solution
Log I (A cm-2)
-6 -5 -4 -3 -2 -1 0-3
-2
-1
0
Pote
ntia
l (V
)
Figure 5.2: Tafel plot of titanium sonotrode voltametric response under silent (---) and sonicated (___) conditions in aerated neutral 10% wt Na2SO4 solution
Log I (A cm-2)
150 200 250
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
Time (s)
Figure 5.3: Transient (time dependent) current response of the titanium sonotrode in neutral 0.7M aqueous sodium sulphate at 30ºC (Deaerated, 5 second ultrasound pulses.
Cur
rent
/ m
A cm
-2)
50 150
-4.0
-3.0
-2.0
-1.0
0.0
Time (s)
Cur
rent
/ m
A cm
-2
100
Figure 5.4: Transient (time dependent) current response of the titanium sonotrode in neutral 0.7M aqueous sodium sulphate at 30ºC (Aerated, 1 second ultrasound pulses
97
For the reaction under deaerated conditions (Figure 5.1) it can be assumed
that reaction (1) is entirely responsible for the observed cathodic currents. The Tafel
plot under silent conditions is more or less linear between –1.25 and –1.6V with a
Tafel slope of 190mV decade-1. This corresponds to reaction (1) being activation
controlled. When the Tafel plot enters the –1.7V region it curves away suggesting
that reaction becomes current limited. On the application of ultrasound the Tafel
plot becomes anodically shifted by a value in the region of 500mV and there is also
an observed change in the linear region’s Tafel slope, which increases by about
80mV decade-1 to 270mV decade-1. A change in Tafel slope indicates that reaction
(1) has undergone a change in mechanism or rate determining step caused by the
ultrasound. The enhancement of current density at constant potential by sonication
has the greatest magnitude within the linear, activation controlled region of the
Tafel plot and show an increase approximate to a factor of five. A less dramatic
effect is seen in the current limited portion of the plot where there is a factor of two
increase observed. As the greatest effect of ultrasound is seen in the activation-
controlled region of reaction (1) it would be sensible to conclude that ultrasound
interacts through modes (ii) - (v) with the electrochemical process.
In the case of aerated conditions, the silent Tafel plot (Figure 5.2) possesses
an unmistakable current plateau that spans the –0.9 to –1.4V region. This is caused
by the O2 in reaction (2) on the sonotrode surface becoming mass transport limited 128. Therefore it can be assumed that at potentials cathodic of –1.4V, reaction (2)
predominates whereas at potentials anodic of –1.4V a combination of reaction (1)
and (2) leads to the cathodic currents seen. The increases in current density at
constant potential caused by sonication are most profound (greater that one order of
magnitude) in the region anodic of –1.4V. The presence of the diffusion limited
current plateau also becomes obscured in the sonicated Tafel plot. Between –0.9
and -1.4V the current density must be due to mode (i) and suggests that the Nernst
diffusion layer (see section 1.6.3) has been reduced tenfold by the sonication. At
potentials anodic of –0.9V where reaction (2) is not limited by mass transport,
modes (ii) – (v) may play a role but mode (i) is still likely to make significant
additions to the current increase caused by the sonication.
98
Appendices
99
Paper: ‘Hydrogen evolution and oxygen reduction at a titanium sonotrode’ Chem. Commun. (1998)
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101
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