S. Berliner, III's berliner-ultrasonics.org Sonochemistry Page keywords = sonochemistry sonoluminescence sonocatalysis ultrasonic ultrasound cavitat cavitate cavitating cavitation sonotrode acoustic sonic sound wave ultra liquid processing Ultrasonic Industry Association UIA bubble shock wave immersi vapor react accelerat pollut abat toxi waste treat beneficiat remediat particl

Updated:   03 Feb 2012, 17:15  ET
[original AT&T Worldnet Website begun 30 May 1996.]
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URL:  http://berliner-ultrasonics.org/usonochm.html

S. Berliner, III
Consultant in Ultrasonic Processing
"changing materials with high-intensity sound"

[consultation is on a fee basis]

Technical and Historical Writer, Oral Historian
Popularizer of Science and Technology
Rail, Auto, Air, Ordnance, and Model Enthusiast
Light-weight Linguist, Lay Minister, and Putative Philosopher


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[Please note that I am an independent consultant, NOT a manufacturer;
I WAS Director of Technical Services for Heat Systems-Ultrasonics
(now Misonix) for many years.
The Misonix SONICATOR line of ultrasonic liquid processors is now
manufactured by Qsonica, LLC, q.v.]



S. Berliner, III's

Sonochemistry Page

ULTRASONICS

INDEX to ULTRASONICS

PLEASE NOTE:  If some internal links refuse to work,
please click on Back and scroll down.

[Please also note the alternative spelling of American usage "homogenize"
vis-ŕ-vis the British usage "homogenise", etc.]

note-rt.gif - it was my original intention to use the main "home" or "index" page of this site for this purpose but the coverage of ultrasonics has become too extensive and complex even for me to follow; thus there is now a more-detailed index, which is in two forms.  The first part is a straight-forward index of the ultrasonics pages, brought forward from the site index page and amplified.  The second part is a linked alphabetical index to all or most of the terms used herein.

On the main Ultrasonics Page:

    Applications List.

    Keywords (Applications) Index.

    Probe-type Ultrasonic Processing Equipment.

SOME HIGHLIGHTS OF OTHER PAGES:

    more-detailed index;

index of the ultrasonics pages, and
linked alphabetical index

    AL-1C - "CONDENSED GUIDE TO ULTRASONIC PROCESSING"
      (A Layperson's Explanation of a Complex Letterhead)

    AL-1P - "A POPULARIZED GUIDE TO ULTRASONIC PROCESSING".
      (A Non-Technical Explanation of a Complicated Letterhead)

    AL-1V - "A POPULARIZED GUIDE TO ULTRASONIC CAVITATION"
      (A Non-Technical Explanation of "Cold Boiling"

    Power vs.Intensity.

    AL-4 - AMPLITUDE MEASUREMENT.

    Call for Contributions for Book.

    More on Cavitation.

    AL-2 - "ULTRASONICS AND FINE PARTICLES -
      BENEFICIATION OF SLURRIES AND FINE-PARTICLE SUSPENSIONS
        [CERAMICS, COAL & ORES, COATINGS, COLUMN PACKINGS,
            SINTERING, SLIPS].     AM-1 - "ULTRASONIC STERILIZATION and DISINFECTION".

    UM-1 - "ULTRASONICS, HEARING, and HEALTH"

    Keywords (Applications) Index - moved to this main page.

    Quick Links for Ultrasonic Probe Manufacturers

    ULTRASONICS GLOSSARY {in process}.

    ULTRASONICS BIBLIOGRAPHY
      Ultrasonic Bibliography Page 1 - Reference Books on Acoustics,
          Vibration, and Sound.
      Ultrasonic Bibliography Page 2 - Sonochemistry.
      Ultrasonic Bibliography Page 3 - Selected Articles.

Other pages are shown on the Ultrasonic Index page.


You are invited to visit the ULTRASONIC INDUSTRY ASSOCIATION home page.


CALL FOR CONTRIBUTIONS:  I am writing a book on "High-Intensity Ultrasonic Technology and Applications" (intended for Marcel Dekker's "Mechanical Engineering Series", edited by Profs. Lynn L. Faulkner and S. Bradford Menkes).  This book will focus on the practical application of power (high intensity) ultrasonics, the use of ultrasonic energy to change materials.  Contributions are welcome.


THE CAVITATION BUBBLE

Larry Crum's Cavitation Bubble

[image from University of Washington, Applied Physics Laboratory (Lawrence Crum, Ph.D.)
- bubble diameter approximately 1mm]


ULTRASONICS

[Please note that over the many years since this site was begun (30 May 1996), I have abandoned the formal academic usage of
"the author" and "your Webmaster" in favor of the more informal first person singular "I" and "me".]

I defined "ULTRASONICS" as the application of sound at extremely high intensity and high frequency (normally above human hearing, 20kHz - 20,000 cycles per second - and above) to change materials.  The term "MEGASONICS" is now being used to describe frequencies of 1,000,000Hz (1,000kHz) and above.

There are other types of "ULTRASOUND", especially those used for Imaging and Sonar, Characterization of Materials and NDE (Non-Destructive Evaluation), pest-control (supposedly), and so forth; these do NOT change materials and are not covered herein.  This series of pages is concerned only with changing materials with ultrasonics.

Such change can clean, homogenize, and accelerate both physical and chemical reactions, among many other things.

That is a key phrase worthy of repetition:

CAVITATION ACCELERATES BOTH PHYSICAL AND CHEMICAL REACTIONS.


SONOCHEMISTRY

[Please note that the following is largely excerpted from the draft text on Sonochemistry by my friend and former colleague, the late Dr. Daniel R. Raichel (1935-2006), for a book on which we were working on "High-Intensity Ultrasonic Technology and Applications" (intended for Marcel Dekker's "Mechanical Engineering Series", edited by Profs. Lynn L. Faulkner and S. Bradford Menkes).  This book will focus on the practical application of power (high intensity) ultrasonics, the use of ultrasonic energy to change materials.  Contributions are welcome.]

Sonochemistry is the application of ultrasonics to influence chemical processes.  It has only relatively recently come into prominence as a field of research, but its history reaches back to the close of the nineteenth century.  In the course of testing newly developed high-speed torpedo boats in 1894, the Englishmen Sir John Isaac Thornycroft (1843-1928) and Sydney W. Barnaby (c.1860-1925) observed severe vibrations from the ship’s propeller and the resultant rapid erosion of the propeller itself.  They noted that large bubbles (or cavities) formed on the rotating propeller and hypothesized that the formation and collapse of these bubbles caused the rapid deterioration of the propeller.  They were able to mitigate this problem of cavitation by increasing the propeller size and reducing the rotational speed of the propeller.  But when ship speeds increased, cavitation became a serious concern, and the Royal Navy commissioned John William Strutt, 3rd Baron Rayleigh (1842-1919) to investigate the problem.  Lord Rayleigh established that the erosive effects were due to enormous turbulence, heat, and pressure gradients produced when cavitation bubbles imploded on the propeller surface.  [He also mentioned in his report that the process of cavitation is responsible for teakettle noise!]

The cavitation phenomenon occurs in liquids not only as the result of turbulent flow but also as the result of high-intensity ultrasonic irradiation.  Cavitation not only causes propeller erosion; it also gives rise to chemical results from ultrasonic irradiation.

Alfred L. Loomis (1887-1975) observed the chemical effects of ultrasonics in 1927, but the field of sonochemistry lay dormant as an area of research for more than a half century.  The advent of economical and reliable ultrasonic generators in the 1980’s led to the rebirth of sonochemistry.  It is now realized that the chemical effects of ultrasonics are rather diverse.  Ultrasonics can provide accelerations of both stoichiometric and catalytic chemical reactions.  In some applications, ultrasonic irradiation has increased reactivities by more than a millionfold.  The chemical effects of ultrasonics fall into three categories:

(a) homogenous sonochemistry of liquids,

(b) heterogeneous sonochemistry of liquid-liquid or liquid-solid systems, and

(c) sonocatalysis (something of an overlap of the first two categories).

Cavitation can occur only in liquids, so chemical reactions, as a rule, do not occur during ultrasonic irradiation of solids or solid-gas systems.

Cavitation Growth

The process of cavitation, i.e. generation of bubbles in the liquid, and the associated collapse of these bubbles, gives rise to immense highly localized temperature and pressure gradients.  Ultrasonics may fall in the frequency range of low-frequency radio waves, but it consists of the mechanical effect of compression and expansion of matter that serves as a sound transmission medium, not the phenomenon of electromagnetic transmission of energy.  Even at high frequencies, ultrasonics has wavelengths in the order of 10cm to 10-3cm.  These lengths are not in the order of molecular dimensions; and because of this dimensional mismatch, the chemical effects of ultrasonics can not occur from a direct interaction of sound with molecular species.  Yet ultrasonic irradiation of liquids does produce a surfeit of high energy chemical reactions, owing to the fact that ultrasonics causes other physical conditions in liquids that provide the impetus for chemical reactions.  The most important of these is cavitation --- the creation, growth, and implosive collapse of bubbles in a liquid.  The dynamics of cavity growth and collapse depends strongly on the local environment.  The collapse of cavities in a homogeneous liquid differs very considerably from the corresponding phenomenon near a liquid-solid interface.

When an ultrasonic signal passes through a liquid, the expansion cycles exert negative pressure on the liquid, causing molecules to pull away from one another.  When the signal is sufficiently strong, the expansion cycle will generate cavities in the liquid.  This occurs because the negative pressure exceeds the local tensile strength of the liquid.  Tensile strength is the maximum stress that a material can withstand from a stretching load without rupture, and in the case of liquids, this stress varies according to the type and purity of the liquid itself.  Cavitation usually occurs as a nucleated process at preexisting weak points in the liquid, such as gas-filled crevices in suspended particulate matter or evanescent microbubbles from earlier cavitation processes.  Cavitation can readily be commenced at moderate negative pressures owing to sufficient contamination by small particles usually present in most liquids.

Upon formation, small gas bubbles irradiated with ultrasonics will continue to absorb energy from the impact of the sound and increase in size.  The growth of the cavities depends on the intensity of the irradiation.  If the cavity expansion is sufficiently quick during the expansion half of the wave cycle, there will be almost no time to recompress during the compression phase of the acoustic cycle.

At lesser acoustic intensities, cavitation growth may also occur through a slower process called rectified diffusion.  In this case, a cavity will oscillate in size over many expansion and compression cycles. In the course of such oscillations, the amount of gas or vapor that diffuses in or out of the cavity depends on the surface area (which is slightly larger during expansion than during compression).  Cavity growth during each expansion is thus slightly greater than shrinkage that occurs during compression.  Therefore, the cavities will grow, on the average, over many acoustic cycles.  The growing cavity may eventually reach a critical size where it can more efficiently absorb the energy from ultrasonic irradiation.  This critical size, also termed the resonant size, depends on the liquid and the sound frequency.  A typical value is roughly 170 micrometers (µm) at 20 kHz. At this stage the cavity can grow rapidly during a single cycle of sound.

There is, however, a limit to how large a cavity can grow, either at low or high intensities.  Once the cavity has reached its maximum, it will no longer absorb energy as efficiently.  Without the energy input, the cavity can no longer be sustained.  The surrounding liquid rushes in and the cavity implodes, thus giving rise to an environment for chemical reactions.

Hot Spots in Cavitation

Compression of a gas results in heat.  When cavities implode in a liquid, their resulting compression generates highly localized ‘hot spots’.  These hot spots result from the fact that the implosions occur so quickly that heat has too little time to dissipate into the surrounding colder liquid.  Such hot spots constitute the source of homogeneous sonochemistry. A typical hot spot has a temperature in the neighborhood of 5000ºC (9000ºF), a pressure of roughly 1000 atmospheres, a lifetime somewhat less than a microsecond, and heating and cooling rates above 109ºC per second.  The maximum transient temperature is comparable to that of the surface of the sun, the maximum pressure is approximately the pressure at the bottom of the ocean, and the cooling rate exceeds that of a red-hot iron bar plunged into water by a millionfold.  The process of cavitation can serve as a powerful means of concentrating the diffuse energy of sound into a chemically useful format.

The task of measuring the temperatures reached in cavitating bubbles constitutes a difficult experimental problem.  Direct measurement is obviously precluded by the extremely transient nature of the cavitation process.  Chemical reactions per se can be utilized to determine reaction conditions.  The effective temperature of a system can be found through the use of competing unimolecular reactions whose rate dependencies on temperature have already been measured.  This methodology of “comparative-rate chemical thermometry” was applied by Suslick, Hammerton, and Cline at the University of Illinois to determine the effective temperature during cavity collapse.  According to Suslick, “for a series or organometallic reactions, the relative sonochemical rates were measured in combination with the known temperature behavior of these reactions, the conditions present during cavity collapse could then be determined.  The effective temperature of these hot spots was 5,200°K.”  In the context of the highly transient nature of the temperature profile, the comparative rate data represent only a composite temperature.

Not only chemistry occurs in a liquid subjected to ultrasonics irradiation, light or sonoluminescence is also produced.  Sonoluminescence provides an alternative means of measuring the temperature of high-energy species produced during cavitation.  Flint and Suslick analyzed high-resolution sonoluminescence spectra to measure the effective cavitation temperature of the emitting species to be at 5100ºK. This is in very close agreement with the determination made by comparative rate thermometry of sonochemical reactions.

Liquid-Solid Systems

In the presence of a nearby surface, the dynamics of a cavity collapse changes greatly.  In pure liquids, the cavity tends to remain spherical during collapse, owing to the uniformity of the surroundings.  Near a solid boundary, however, the cavity collapse becomes very asymmetric and produces high-speed jets of liquid.  The expanded bubble’s potential energy converts into kinetic energy of a liquid jet that moves through the bubble’s interior and penetrates the opposite bubble wall.  Lauterborn measured liquid jets impinging into the surface with velocities of 400 km/h (see frontispiece, above, per Crum).  This process is capable of inflicting severe damage at the point of impact and producing newly exposed, highly reactive surfaces.  This accounts for the corrosion and erosion of metals observed in ship propellers, turbines, and pumps where cavitation presents an ever-present threat to the integrity of the machinery.

The distortions of the bubbles during collapse depend on a surface several times larger than the resonant size of the bubble.  The presence of powders or small particulates does not induce jet formation.  But in the situation of liquid-powder slurries, the shock waves produced by homogeneous cavitation are capable of generating high-velocity interparticle collisions.  The combined effect of turbulent flow and shock waves generated by intense ultrasonics can drive the metal particles together at very high speeds that are sufficient to cause melting at the collision points.  These interparticle collisions can result in remarkable changes in surface texture, composition, and reactivity.

Doktycz and Suslick used metal powders to find the effective maximum temperatures and speeds achieved during interparticle collisions.  Chromium, molybdenum, and tungsten powders of a few micrometers in particulate size were irradiated in decane at 20kHz and 50W/cm².  Agglomeration and welding of particles were observed for chromium and molybdenum but not for tungsten (which has the highest melting point of the three metals).  On the basis of the melting points of these three metals, it was concluded that the effective transient temperature reached at the points of impact during interparticle collisions is approximately 3000ºC.  With the volume of the melting region at the impact point taken into consideration, the amount of energy generated during collision was estimated.  On this basis, it was estimated that the velocity of impact was in the order of 1800km/h, which corresponds to half of the speed of sound in liquids.  It should be realized here that the conditions occurring during interparticle collisions do not directly correlate to the temperatures resulting from cavitational collapse of bubbles.

Homogeneous Liquids

In sonochemistry, high-intensity probes capable of producing 10 to 500W/cm² beams constitute the most reliable and effective tools in laboratory-scale work.  A laboratory set-up should feature a simple control system for the ambient temperature and atmosphere.  Lower acoustic intensities can be applied to liquid-solid heterogeneous systems on account of the reduced liquid tensile strength at the liquid-solid interface.  For these types of reactions a commercially available, relatively cheap ultrasonic cleaning bath, such as that used by jewelers, will often suffice, but the low intensity (usually 1 W/cm²) can impose a limitation.  For large-scale purposes involving more intensive irradiation, there are available on the market flow reactors in modular units that are as powerful as 20kW.

The chemical effects of ultrasonics on aqueous solutions have been investigated over a long time.  The primary products of the irradiation are molecular hydrogen H2 and hydrogen peroxide H2O2.  Intermediary products yielded at high energy levels may include the superoxide HO2, atomic hydrogen H·, hydroxyl OH·, and solvated electrons e-.  At the National Institutes of Health, Riesz and his collaborators applied electron paramagnetic resonance with chemical spin traps to definitely prove the production of H· and OH·.  The sonoanalysis of water produces both strong reductants and oxidants, thereby rendering the process capable of causing secondary oxidation and reduction reactions.

As for ultrasonic irradiation of organic liquids, little work has been done to date.  But Suslick and his co-workers determined that under the effect of ultrasonic irradiation nearly all organic liquids will generate free radicals (uncharged, reactive intermediates featuring an unpaired electron), as long as the total vapor pressure is sufficiently low to permit effective bubble collapse. The sonolysis of simple hydrocarbons generates the same types of products associated with very high temperature pyrolysis.  Many of the products, e.g. H2, methane (CH4), and the smaller 1-alkenes, stem from a well-known radical chain mechanism.  Copious amounts of acetylene C2H2 are also yielded in the process.

In the realm of sonochemistry, solutes dissolved in organic liquids remain largely uncharted territory, with the exception of metal carbonyl compounds.  Schubert, Goodale, and Suslick reported in 1981 the first sonochemical investigation of discrete organometallic complexes and described the effects of ultrasonics on metal carbonyls.  Further, more intensive studies of these systems led to increased understanding of the sonochemistry of these substances.  Unusual reactivity patterns during ultrasonic irradiation have been observed, most notably novel metal cluster formation and the initiation of homogeneous catalysis at low ambient temperatures, with than greater than105-fold rate increase in the rate of production.

Polymers and Biomaterials

Polymers (or ‘plastics’) are giant molecules formed by the coupling of small molecules, called mers.  The effect of ultrasonics on plastics has been well studied over the past three decades, particularly the controlled cleavage (fragmentation) of polymers in solutions irradiated with ultrasonics.  Polymer degradation yields chains of smaller lengths with relatively uniform molecular weight distributions with cleavage occurring principally in the middle of the polymer chain.  The sonochemical cleavage process, which can be described as a mechanical rupture of the polymeric chains, occurs under the impact of shock waves or solvent flow generated by cavitation during the ultrasonic irradiation of liquids.

Ultrasonic fragmentation of polymers have been applied by Price at the University of Bath in England to synthesize block copolymers of different kinds.  Block copolymers can be described as long chain polymers with two different but linked parts.  A polymer of one kind can be linked to a different polymer to yield a combination of desired physical properties that are individually represented by separate species.

A relatively new application of ultrasonics that is making rapid strides is the synthesis of biomaterials.  It is true that the chemical effects of ultrasonics in aqueous materials have been studied over the past few decades, but the advent of aqueous sonochemistry for biomaterial synthesis is only very recent.  Protein microencapsulation is a very interesting application.  Microencapsulation entails the enclosing of materials in capsules that are a few micrometers in size and has various applications, such of drug delivery systems and as medical diagnostic agents.  In non-biomedical applications, encapsulation can be used with dyes, flavors, and fragrances.

As an example of medical use, high-intensity ultrasonics is used to make aqueous suspensions of long-lived proteinaceous microspheres filled with air or with water-insoluble liquids.  Emulsification itself does not suffice to produce these long-lived microspheres —  chemical reactions requiring oxygen are essential in forming them.  The sonolysis of water yields hydrogen atoms that react with oxygen to produce superoxide, Suslick and Grinstaff determined that the proteinaceous microspheres are held together by disulfide bonds between protein residues and the superoxide acts as a cross-linking agent.

Sonoluminescence

Not long after the discovery of sonochemical reactions, Frenzel and Schultes in 1934 were the first to report sonoluminescence (emission of light due to sound) in water.  Sonoluminescence can be attributed to acoustic cavitation, just as with sonochemistry.  Only recently has some significant work has been reported on sonoluminescence in non-aqueous (i.e., containing no water) liquids. In any event, the emission of light is caused by the high-temperature formation of reactive chemical species in electronically excited states.  The emitted light from these excitation states gives a spectroscopic probe of the cavitation event.

Flint and Suslick obtained high-resolution sonoluminescence spectra for hydrocarbons and silicone oil.  The observed emission is produced by excited state diatomic carbon, which is identical to the transitions that are responsible for the blue color of natural gas used in a kitchen stove.

In 1990, Gallan and Crum discovered that sonoluminescence in a single, oscillating gas bubble can be observed.  With the aid of extremely sophisticated instruments Putterman at UCLA was able to examine these bubbles with a time resolution in picoseconds.  Gaitain, Crum, and Putterman used light scattering techniques to measure the radius time curve of the luminescing bubble and to correlate the optical emissions with a specific phase of the sound field.  It was found that the light emissions occurred during cavity collapse, as was expected.  A surprising discovery was made that the duration of the sonoluminescence emissions is less than 100 picoseconds (ps), approximately one millionth of the time duration of the acoustic cycle applied.  This extremely brief emission seems to originate from the formation of shock waves inside the collapsing bubble during the earlier stages of compression.

Heterogeneous Sonochemistry

Heterogeneous sonochemistry essentially deals with the reactions of solids with liquids under the influence of ultrasonic irradiation.  This field originated from the early work of Pierre Renaud conducted in France during the 1950’s and from the later breakthroughs of J.-L. Luche of the University of Grenoble, France.  The application of high-intensity ultrasonics to increase the reactivity of metals as stoichiometric agents has gained significance as a synthetic technique for many heterogeneous organic and organometallic reactions, particularly those involving reactive metals, such as magnesium, lithium, and zinc.

The use of heterogeneous sonochemistry grew apace during the last decade, and the general effects thereof apply to reactive inorganic salts as well.  Enhancements of more than ten-fold have been achieved in the reactivity rates, the yields are substantially increased, and by-products are avoided.

Ultrasonics can be used at room temperature and atmospheric pressure to generate heterogeneous reactions that normally occur at highly elevated conditions in the order of hundreds of atmospheres and hundreds of degrees.  Johnson and Suslick obtained good results with the use of ultrasonics to effect some of the most difficult reactions known for transition metals, e.g. the attack of carbon monoxide on the normally very unreactive metals such as vanadium, tantalum, molybdenum and tungsten.

ultrasonics can also be used in the process of intercalation.  Intercalation is the adsorption of organic or inorganic compounds as guest molecule between the atomic sheets of layered solid hosts, such as graphite or molybdenum sulfide. The usefulness of intercalation lies in the fact that it can be used to allow a systematic change of optical, electronic, and catalytic properties. Modification of such materials have many technological applications, e.g. lithium batteries, hydrodesulfurization catalysts, and solid lubricants. Without ultrasonics, the kinetics of intercalation is inexorably slow, and syntheses generally necessitate high temperatures and very long reaction times.  M. L. H. Green at Oxford University, Suslick and their respective students established that high-intensity ultrasonics increases dramatically the intercalation rates of a wide range of compounds (among them amines, metallocenes, and metal sulfur clusters) into a variety of layered solids such as ZrS2, V2O5, TaS2, MoS3, and MoO3.  Scanning electronic microscopy of layered solids, combined with chemical kinetics theory, revealed that the origin of the observed reaction rate enhancements arises from particle fragmentation that dramatically increases surface areas, and to a minor degree from surface damage. High intensity ultrasonics can rapidly form uniform dispersions of micrometer-sized powers of brittle materials, so it is used in a wide range of liquid-solid reactions.

Amorphous Metals

Amorphous metals can be prepared by the use of heterogeneous sonochemistry.  When molten metal alloy is cooled rapidly enough, it freezes into a solid before it has a chance to crystallize.  This results in an amorphous alloy that lacks crystalline order and may possess unique electronic, magnetic, and corrosion resistance properties.  Producing amorphous metals is a difficult task because extremely rapid cooling of molten metals is required to prevent crystallization.  Cooling rates in the order of one million degrees K per second are required.  Plunging a rod of red-hot steel into cold water produces cooling at only about 2500ºK per second.  In the last few years, the use of ultrasonics to synthesize amorphous metallic powders by using sonochemical means to effect decomposition of colatile organometallics was reported by Suslick, Chloe, Cichowlas, and Grinstaff.  This major discovery opens up new vistas for ultrasonics for the low temperature synthesis of uncommon alloy phases.  For example, the sonolysis of iron pentacarbonyl produces nearly pure amorphous iron, which was found through materials characterization techniques to have a definite lack of crystallinity.  Sonochemically synthesized amorphous powers have potential technological applications.  Amorphous iron powder, for example, is an active catalyst for several important reactions, including the synthesis of liquid fuel from CO and H2, both of which can be readily produced from coal.  Amorphous iron is found through magnetic measurements to be an extremely soft ferromagnet, i.e. it is a material that very quickly ‘forgets’ its magnetization once the magnetic field has been turned off.  This type of ferromagnet would obviously make terrible permanent magnets, but it would be superb for making magnetic shielding, electrical transformer cores, and magnetic recording heads.

Sonocatalysis

The purpose of a catalyst is to speed up the rate of a chemical reaction without being consumed itself.  Catalysts therefore perform major roles in speeding up reactions in both laboratory and industrial applications.  Catalysts generally fall into two categories: if the catalyst is a molecular or ionic species dissolved in a liquid then the system is ‘homogeneous”; if the catalyst is a solid, with the reactants either in a percolating liquid or gas, then it is “heterogeneous”.  It is often a challenging problem to activate the catalyst or to keep it active.

Heterogeneous catalysts are more widely used in industry than homogeneous systems. In the petroleum industry nearly all of its processes are based on a series of catalytic transformations.  Heterogeneous catalysts often require the use of rare and expensive metals such as platinum and rhodium, both of which are considerably more expensive than gold.

Ultrasonics has the potential of activating less reactive (and less costly) metals.  Some early investigations in the Soviet literate indicated increases in turnover rates were observed, but rarely exceeding ten-fold.  These relatively modest increases may have been due to increased effective surface areas.  Greater amounts of acceleration have more recently been reported, including hydrogenations (catalytic reactions of hydrogen with unsaturated organic compounds) by nickel, palladium, or platinum.  Casadonte and Suslick found that hydrogenation of alkenes by nickel powder increases enormously (105-fold) under ultrasonic irradiation.  The surface morphology was observed to have been affected.  Ultrasonic irradiation smoothens (on a microscopic scale) the initially crystalline surface and brings about agglomeration of small particles.  Both of these effects are possibly attributable to interparticle collisions caused by cavitation-generated shock waves.  Moreover, Auger electron spectroscopy indicates that a considerable decrease in the thickness of the oxide coat has occurred after ultrasonic irradiation.  The removal of this layer probably causes the great increase observed in catalytic activity.

[Please note that the foregoing is largely excerpted from the draft text on Sonochemistry by my friend and former colleague, the late Dr. Daniel R. Raichel (1935-2006), for a book on which we were working on "High-Intensity Ultrasonic Technology and Applications" (intended for Marcel Dekker's "Mechanical Engineering Series", edited by Profs. Lynn L. Faulkner and S. Bradford Menkes).  This book will focus on the practical application of power (high intensity) ultrasonics, the use of ultrasonic energy to change materials.  Contributions are welcome.]


Those persons interested in SONOCHEMISTRY might wish to look at
Prof. Kenneth S. Suslick's and Shiga University's Sonochemistry pages.


The author gratefully acknowledges inclusion of these pages
in INTUTE: Science, Engineering and Technology
[formerly EEVL - the Enhanced and Evaluated Virtual Library
The Internet Guide for Engineering, Mathematics and Computing
(previously the Edinburgh Engineering Virtual Library)
a service of the Heriot-Watt University funded by the JISC.]



LEGACY

  What happens to all this when I DIE or (heaven forfend!) lose interest?  See LEGACY.

COPYRIGHT NOTICE

See Copyright Notice on Berliner-Utrasonics home page.



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