.

Indian Journal of Engineering & Materials Sciences : " ,ii." '..',

Vol. 5, December 1998, pp. 372-389,

Ferrofluid technology -An overview

K Raj & AF Chorney

Ferrofluidics Corporation, Nashua, NH 03061, USA

Received 6 February 1998; accepted 17 November 1998

Researchers and product designers have been intrigued by ferrofluids ever since the discovery ofthis fascinating magnetically responsive liquid. The purpose of this article is to provide an overview ofthe progress that has been made to date in the development of ferrofluids and affiliated applications.Actual examples of magnetic fluid based devices performing in commercial environments have beenpresented. The technology has been growing steadily. New innovations and solutions to challenging ..

problems promise further scientific advancements and commercial opportunities.

Although the dream and challenge of producing a developed. The aim of this paper is to review theliquid that possesses strong magnetic properties significant advances in the field of magnetic liquidhad been in the scientific comm4nity for a long technology and to present challenges for futuretimel; the first successful colloid was not realized innovations. Important properties of variousuntil the early 1960s by a researcher, Papell2 at commercial ferrofluids have been discussed. TheNASA. This ferrofluid was kerosene based, had a operational principles of a number of magnetichigh evaporation rate and was not suitable for fluid devices along with field examples is expectedmany industrial applications. After a few years of to illuminate the unique aspects of this space ageNASA fund~d magnetic colloid re'f'earch at A VCO, technology. ,Ferrofluidics corporation was founded by the twovisionaries, Rosensweig and Moskowitz, to Magnetic Fluidscommercialize the technology. At that time, Magnetic particles, large or small, are utilized inalthough the scientific disciplines were well many practical devices and are of great industrialestablished, the technology was unproven in the significance (Fig. 1). Ferrofluids occupy the lowestindustrial market and lacked rigorous evaluation. limit of magnetism, with a particle size of aboutToday, after about 30 years of existence, 0.01 Ilm and share this regime with aquaticferrofluids are a global industry with estimated magnetotactic bacteria4. It has been speculated thatannual revenues of $150 million. Because of their as the particle becomes smaller in size the magneticunique features, products based on magnetic fluids moment becomes vanishingly small. In particleshave become a corner stone of many high tech with diameters of 0.002 J.1lll only feeble magnetism ...industries. Ferrofluid is a subject of research at is observed 5. The particles suitable for magnetic '

various leading institutes throughout the world. It is ink are roughly 0.1 Ilm and for toners 0.3 Ilm. Thea material that has enticed v~rious scientific particles used in magnetic recording media are

enq~irie~ a.nd o,ffers ch~llenging fundame~tal close to 0.5 Ilm. In production of permanent ferritestudles.1n dispersion ~hemls~ a~d s~all particle magnets,." the particle size employed is IJ.1lll. Themagnetism. For practlc~l ap~lIcatlons It has. been particles are made of ferrimagnetic orsought after and investigated. by diverse ferromagnetic materials such as Fe 0engineering disciplines such as mechanical, CrO Y-Fe 0 or MFe 0 (M= Ba Sr or ~b

)4,

I . I h . I b. d. I I h " I 2, 2 3, 12 19 , "e ectrlca , ~ emlca, tome Ica, e ectrome~ a~lca Except for the case of ferrofluids, the particles areand acoustical. The total number of publications multidomain. Furthermore particles in a ferrofl "dand ~~tent~' as of 1997 adding to the pr~vious are incorporated in a li~uid medium possess~~g ..score In this fiel.d are 5800 and 2?70 respectlvel~., colloidal features, whereas in other cases either

To meet vaned market requirements, a wide they form a suspension or are imbedded in a sol"drange of ferrofluids and devices have been matrix. I

, .:',~}C

RAJ & CHORNEY: FERROFLUID TECHNOLOGY 373

APPLICATIONS OF MAGNETIC PARTICLES

15 D = 45 Ao10 FWHM = 23 Ao

E~ IIi..~ 0.3 0,5 .~N tin 0.1 0.1 J.; f .

.! ~ ~ j ! ~uSa "0

~ 0.01 J/.} ..~ ~ c'0 .f fa..

0-001PrVllers Copiers -yDi,k. Speak",s MClowave CI~"s

Man.,;cTSeals g 'PO' ""'Ior. 8f.k~ D = 60 AO

Dampers FWHM = 31 AO

Fig. I -lndustaes developed around various sizes of

magnetic particles.

~

*;JI;; * FWH~ == ~~1 ~:

iJ;c .~ *~ 0 50 100 150 200

~ * ~ Fig. 3-Log n:~~s~~E:::::(~;ParriCleS ofvarioils

I l' M1~%n:Jic ","""..rsurfN . I diameters obtained with magnetization measurements.MagnetIc ' ..r 0100 AO -.; Surfactant Carri.r

5-04(10000

c

Fig. 2-Model ora ferrofluid. ~ 1000"D-..If) 100..

The model of a ferrofluid is a three component :gmaterial with one homogeneous phase (Fig. 2). The i! 1 1 10 100

particles that have typically been used in Ferrorfiuid Mogn~imtlon,Gouss

commercial ferrofluids are the ferrimagnetic Fe304. h .. I F Fig. 4 -Particle separation as a function of saturation, Wit an Inverse spIne structure. or reasons of t. t. f fi fl .d (C R .

R E~ magne lza Ion 0 a erro UI ourtesy. osenswelg -

colloidal stability the average particle diameter IS personal communication).

about 10 nm (0.01 J.Lm) with a distribution that may

extend from 50-150 nm. The average particle is

composed of approximately 6200 molecules of value for various particle diameters .is shown in

Fe304. Ferrofluids based on small particles tend to Table 1. A plot of particle-particle separation in the

have a narrow size distribution compared with colloid is depicted in Fig. 4 (Rosenweig R E,

large particles, Fig. 3. The curie temperature of a personal communication). For example, in a

ferrofluid is the same6 as for Fe304, i.e. 585°C, ferrofluid composed of 10 nm particles and having

however, the maximum temperature at which the the magnetization value of 100 G, the particle

colloid can be used is dictated by the selection of separation is roughly 220 nm.

...the surfactant and the carrier liquid and is about Although a number of ferromagnetic elements,

200 °C. The calculated number of particles per unit alloys and compounds have been cited in the

volume of ferrofluid as\a function of magnetization literature7.8 such as the Fe, Co, Ni; Fe-Co, Fe-Ni-)

374 INDIAN J. ENG. MATER. SCI., DECEMBER 1998

Table 1 -Particle density versus saturation magnetization

MagnetiZhtion 11 nm 10 nm 9 nm 8 nm 7 nmvalue, G

1 2.5xl014 3.4xlW4 4.7xl014 6.7xl014 10xl0145 1.3xl01s 1.7xl01s 2.4xlWs 3.4xl01s 5.0xl01s

25 6.3xl01s 8.5xl01s 1.2xl016 1.7xl016 2.5xlW650 1.3xlW6 1.7xl016 2.4xl016 3.9xl016 5.0xl016

100 2.5xl016 3.4xl016 4.7xl016 6.7xl016 1.0xl017200 5.1xl016 6.8xl016 9.4xl016 1.3xl017 2.Oxl017300 7.6xlW6 1.0xl017 1.4xl017 2.Oxl017 2.5xl017400 1.0xl017 1.4xl017 1.9xl017 2.7xl017 4.0xl0~

Co Fe-N; Sm-Co, Nd-Fe-B etc. which may be Table 2-Liquid carriers used in synthesis offerrofluidsconsidered, as candidates for colloidal particles in Inorganic solvent: Aqueousthe preparation of magnetic fluids, either the Organic solvents: Heptane, Xylene, Toluene, MEK Achemistry or the long-term stability of these Oils: Esters. .materials have not yet been proven compared with H.ydrocarbons : MineraI and SynthetIc

d h . d fi . S I f h I.. bl Sllahydrocarbons .Fe304 an t e mlxe emtes. evera 0 tee Igl e Perfluoropolyethersmaterials possess large magnetic moments and thus Polyphenylethersare attractive to advance the technology into the Glycolsrealm of high magnetization ferrofluids. Silicones

Th . I b . d . h . I Vegetable: Sunflower, Cannola, Castore partlC es may e vlewe as sp enca

nanomagnets dispersed randomly in a continuous biodegradibility and toxicity are of concern,phase. In order to achieve a stable suspension, vegetable oils offer the best choice; however, theythese magnets must be kept apart and prevented suffer from poor thermal stability and their pourfrom agglomeration under the attractive magnetic points are high. Silicones have the highest viscosity ~and Van der Waals forces. The role of a surface- index of all the carriers, exhibit low volatility andactive agent in a ferrofluid is to mail1tain a certain can be used over a wide operating temperatureminimum distance9, namely 1-2 nm, between the range. However, the magnetization values ofparticles and at the same time invoke further silicones are limited to about 100 G. Aqueousstabilization through steric repulsion. The solvated based ferrofluids are environmental friendly with astabilizing molecules have a polar head group, high volatility and a restricted temperature range ofwhich anchors, firmly to the surface of the particle 10-80°C.and a tail, which resembles the solvent. Different The first magnetic fluid was based on a lowsurfactants are required to disperse particles in molecular weight mineral oil 2. Early commercialdifferent carriers. Stabilization of colloidal Fe304 ferrofluids used mostly ester and water as carriers,with ionic charges has also been achieved1o. The followed by the developments of fluorocarbon and -',particles are kept in a colloidal state against the synthetic hydrocarbon products. Then came thegravitational settling by the Brownian movement. polyphenyl ethers, glycols, halocarbons, andThe selection of the third component, i.e., the vegetable oils. Most recently, a stable siliconecarrier, depends on the application. Oil based based ferrofluid has been synthesized. A completeferrofluids are used for their low volatility and long list of industrial magnetic colloids is shown inlife. Ester based ferrofluids can operate over a wide Table 2. Each category except for the aqueoustemperature range, have a high viscosity index and medium represents a chemical family of carriersare more oxidative stable than the hydrocarbons. offering a choice of parameters as viscosity,On the other hand, hydrocarbons possess a greater operating temperature, volatility and environmentalhydrolytic stability than esters. Mineral oils are not compatibility to suit the particular requirements ofas thermally stable as the synthetic hydrocarbons. an application. Including the option of various "For harsh chemical environments, perfluoropoly- magnetization values in each family, this representsethers are the best liquids. Under nuclear radiation, more than 100 different ferrofluids that arepolyphenyl ethers are most durable. Where available for engineering designs.

RAJ & CHORNEY: FERROFLUID TECHNOLOGY 375

COLLOID STABILITY~Q9nrlic Firld GrQ~irnt= 7.2 X 10"o./cm Table 3 -Achievable parameters in commercial ferrofluids

100", S t t...;: 80'1. a ura Ion magnetIzation, 25°C 1100 G-~ , External field for 90% Saturation 1600 Oe~60t, 1 " 1 .u ' 0" mtla magnetIcPermeability 200G 25°C 2 5..~O'l .., ' .~ 20'; P~rom~gnetic coefficient, 1000 Oe, 20°C 3.5xI0-2 emu/gK

, VISCOSity, 27°C 10000 cp0 '" Viscosity Index 300

2 3 4 5 6 Th I d ..S .erma con UCtIVlty, 25°C 215 mw/mK

UCC~SSIV~ G~nar at ' E .~ Ions vaporatlon rate, 175°C 2.7xI0-9gicm2sFig. 5 -Improvements made in colloidal stability of Upper Temperature 250°Cferrofluids. Lower Temperature -80°C

Vapour Pressure, 25°C -10-11 TorrTHERMAL STABILTY 175°C Particle. size ..55 A

0.3 2.1 2 CoefficIent of Fnctlon 0.05'"' 250 9 cm Electrical Resistivity, 25°C 35x1090-cm

255 O. I . C200 Ie ectnc on stant, I QOG, 25°C 2.5

.c 150 145~

E 100 100 magnetic fluids. More than 200,000 of these sealsj: 50 have been manufactured since commencement. The

0 seals have the distinguished record of providing1 2 3 4 5 6 years of maintenance free service in a wide

Succ~ssiv~ G~n~rations commercial sector such as the semiconductor,Fig. 6-lmprovements made in thennal stability of t / aerospace, laser, process and nuclear industriesl2.

es er A II th . I..hydrocarbon based ferrofluids. mong a e competIng sea mg technologIes, this

is the only product which has a unique combination.The chemistry of ferrofluids has steadily of st.andard perf~rmance ~aramete.rs: bi-directionalImproved over the years. The growth was fueled by rotation, hermetic behavior, static and dynamic?Iarket. opportunities requiring technological sealing, non-rubbing, pressure range: 10-8 torr to 5Innovations. The physical properties that have been bars, temperature range: -10-1 OO°C, shaftthe focus of continual refinement are colloidal diameter: 5-100 mm, DN (shaft diameter instability, thermal stability, volatility, temperature mmxspeed in rpm)=200,000, 100% torquerange, durability under harsh environments transmission and very low power consumption.compatibility with materials and merit ratio (th; However, state-of-art seals have been built toratio of viscosity and saturation magnetization operate at; ultrahigh vacuum: 10-9 torr, positive,,:ithout impacting any other physical parameters). pressure: 15 bars, shaft diameter: I m, upperFig. 5 shows progressive improvements in colloid temperature: 200°C, DN=500,000, but these values

.l stability and Fig. 6 in thermal stability. Table 3 is a do not necessarily represent the limit of thesummary of the maximum physical parameters that technology. The seal is predominantly a gas sealhave been achieved in ferrofluids. It must be noted but it can withstand occasional liquid splashes.that no single ferrofluid can achieve all these Bec~use of its zero leakage capability, the sealspecifications. When one parameter is optimized pe~its con.tamination free processing of silIconthe others are compromised to various degrees. For ChiPS, contaInment of nuclear radiation, fabricationexample, an 1100 G ferrofluid will be very viscous of optical fibers in clean environments and growthand can be used only in a limited temperature of def~ct free. semi~onductor single crystals, andrange. Furthermore, only certain carriers can be thus It provides Invaluable service to these

employed for this purpose. Similarly, a very low indu.stries.pour point ferrofluid will in general have a high Fig. 7 shows the basic design of the seal. An~ volatility associated with it. axially polarized ring magnet is the source of

magnetic flux, which is channeled and focused intoVacuum rotary ~eal an air gap by a set of pole pieces attached to ~ach

Introduced m 1969-70, vacuum rotary seals were face of the magnet. A magnetically permeable shaftthe very first industrial product I I based on supported by two precision bearings completes the

INDIAN J. ENG. MATER. SCI., DECEMBER 1998 _~76

PERMANENT electromechanical subsystems providing combinedMAGNET linear, rotary and oscillatory motions, liquid

cooling and electrical power under vacuum

STATIC conditions. The examples of these subsystems are:D- RING SIDE crucible lift assembly for crystal growth, platenVACWM SIDE INGBALL BEARII«; actuation in control vapor deposition machines and

substrate manipulation in RF sputtering units. The

seal integrated subassemblies eliminate the

problems of incorrect installation of ferrofluid seal

in the equipment, space and interface requirements

for the seal and simplification of the procedure of

electromechanical design.-'

SemiconduGtor processing subsystem-Magneticfluid seals are utilized in all phases of '"'

VACUUM ATMOSPHERE semiconductor processing, from the very first step(PROCESS) f d . fh . h d II .. 1. 0 pro uctlon 0 Ig gra e monocrysta me Sl Icon

Fig. 7 -Elements of a multistage magnetic fluid rotary seal. material to the last step of fabrication of integrated

devices. Several seals are employed in the machine

RF SPUTTERING SUBSVTEM that converts chunks of polY.crystalline silicon into

SPUTTERING defect free, dimensionally controlled monocrystal-

T~ ATER COOLED line silicon ingotsl3. Each ingot is then sliced intoMOVABLE ~~ AfERPLATTENSHUTTER -wafers, about 0.5 mm thick, using a diamond saw.

WAFER The wafers, which may range in diameter from 100

to 200 mm, go through several processing steps~~~~~~~TE such as photoresist, chemical vapor deposition, .

plasma etching, ion milling, eptitaxial growth and

sputtering depending upon the specifications of theRF POWER -end product. In these processes the surface of the

silicon wafer is modified to include SiO2 layers, p

WATER IN and n type impurities, metallization and electricalWATER OUT .

elements. Because of the small feature sIze ofFig. 8 -Schematic of an RF s~u~erin.g ~ubsyst~m w~th a circuits the process environments must be clean.

hallow shaft ferro~uid seal pr~vldmg liquid coolIng, lInear Fig. 8 is a schematic of a RF sputtering

displacement rotation and electncal power under vacuum. .. h fi fl .d I b bl' equIpment WIt a erro UI sea su assem y.

Sputtering is a method of depositing a thin film of

magnetic circuit. A series of stages formed either material on a desired surface. An energetic beam of ,

on the shaft or pole pieces create alternate regions ions, initiated by a RF power, bombards a target

of intense and weak magnetic fields. When the seal (substance to be sputtered), and releases

is charged with a ferrofluid distinct liquid O-rings microscopic amounts of the target material which

are formed occupying the gaps under the stages is deposited on the substrate. The process consists

(regions of strong magnetic field) with intervening of evacuating the chamber and then back filling it

air annular spaces (regions of weak magnetic field). with argon gas. The ferrofluid feedthru is a conduit

Depending upon the mechanical design and to various services that are needed for the process

ferrofluid properties each ring can hold a practical such as water cooling, RF power and rotation. The

pressure differential of 15-30 kPa. However, with shaft of the seal is 38 mm in diameter and is

recent advances in the technology, each stage can hollow. It is connected to a planten carrying wafers

withstand up to 50 kPa without burst. The total to be processed. The planten is cooled with water ~,

pressure capacity of the seal is the sum of the to minimize temperature rise of wafers resulting

pressure holding strength of each stage. from the deposition of the target material. The

From the simple seal component, the technology water is carried from a rotary union into a

in the last few years has evolved into complex concentric pipe and then to the planten with one

~- RAJ & CHORNEY: FERROFLUID TECHNOLOGY 377

SIX DEGREES OF FREEDOM I Movable Elements IFERROFLUID FACE SEAL

IIIIIII

II1III1

-Diaphragm

Voice Coil

MOVABLE PLATE

-Spider

6 DDf'ACTUAT

I Fixed Elemen1s1...

Fig. 9 -Six degrees Qf freedom magnetic liquid seal used in Baske\

X-ray lithography.Pole Piec~

, Ferrofluidpipe serving as an inlet and the-other as the return Front Plote

path for water. The outer surface of the outside netpipe is silveJ plated to conduct RF (- 13.5 Mz, 1 k Plate

kW) with minimum losses. The planten is indexed

to various angular positions for sputtering of Fig. 10- Schematic diagram of a dynamic loudspeakerdifferent materials on to the wafers. The bellows identifying fixed and moveable parts..attached

to the seal provide linear displacement to

the planten to facilitate loading and unloading of the wafers from an adjacent load-lock. In addition Into the s~al. Helium IS emplo~ed as It does not

to the main seal there are also other ferrofluid seals absorb.or1~pede the X-rays as air would and at the

in the machine which are used to control the s~e tl~e I~ cools the wafer and tJte ~ask. The seal

position of various shutters interposed between the ?eslgn ~s .dlfferent from the conventional struct.ure

wafer and the target, The shutters start and m that It IS a face seal rat.her than a rotary device.

terminate the process .The purpose of the seal IS to allow a very small

S .-1 iffr .-1 I A ~ fl . d I movement of the mask relative to the wafJr with

IX uegrees 0 eeuom sea -lerro UI seat .

I f h I.. fi I.. dth X very little mechanical resistance. The seal positionre ams a co umn 0 e lum m a me me-wIl.

th h h . t I . htl ..IS controlled by stepper motors operatIng m theray I ograp y mac me a a s Ig y posItive ..-'

d b d . I d .. d ' f ~ mlcrostep mode. The motors are not capable ofpressure an can e ISp ace m SIX I lerent .... t t . I d ' t . II overcomIng the friction of an elastomer type seal or

orlen a Ions name y X, y, z- Irec lons as we as.t h d t t. (F . 9) Th I d . t an edge welded metal bellows. Larger motors canpi c , yaw an ro a Ion Ig. .e sea lame er .. 180 d th d . I 0 5 solve this problem but at the expense of fine

IS mm an e ISp acements are z=+ .mm I .. d . h. I..2 2 .-, reso utlon require m t IS app Icatlon,

x=:t mm,_y=:t mm. It IS attached to a movable

plate (one seal per face) which also carries a mask. Moving coil loudspeaker

At one end of the column is an X-ray source and at

the other a silicon wafer. The wafer is mounted a This application of magnetic fluids followed the

few microns away from the mask to minimize the sealing devices in the chronology of commercial

diffraction of X-rays. Lithography is a process of events, The use of ferrofluids in moving coil,"

transferring an image from a mask to a wafer and is loudspeakers may be traced back to 1973. It is

one of the steps in the manufacturing of integrated estimated that about 350 million speakers have

circuits. The section of the wafer to be modified by been built with ferrofluids to date. Various

the X-ray beam is positioned in line with the mask. components of a loudspeaker are shown in Fig. 10,

A complete alignment between the mask and the A strong magnetic field is produced in an air gap

wafer is achieved with the degrees of freedom built by forming a magnetic circuit composed typically

378 INDIAN J. ENG. MATER. SCI., DECEMBER 1998

of an axially polarized ceramic magnet and However, today the fluid applications are moremagnetically permeable steel elements: the front wide spread, they are successfully used not only inplate, black plate and pole piece. However, some tweeters but also in woofers, midranges,recent speaker designs also include powerful Nd- compression drivers, alarm type speakers,Fe-B magnets with polarization in ~he radial intercoms and under water acoustical systems. Thedirection. Inside the gap is suspended a bobbin or speaker industry now takes advantage of severalvoice coil with one or more layers of copper or other attributes of ferrofluids such as thealuminum windings. The majority of speakers simplification of cross-over networks, sealing,contain a single magnetic gap and a cylindrical corrosion protection and improved productioncoil. There are also commercial speakers with dual yields which previously lacked diligence. The largemagnetic gaps and a flat voice coil14.Common to demand for ferrofluids in loudspeakers and relatedall speaker constructions is the formation of a applications mainly created the need for themagnetic air gap in which a coil can vibrate freely development of a new generation of ferrofluidswhen excited with an audio signal in the frequency with properties tailored to solve more difficult ...

range of 20-20,000 Hz. The coil is connected to a problems. Some of the more unusual speakerdust cap, spider and diaphragm. The diaphragm applications are described below:oscillates with the coil and produces compression Spiderless woofer -FeiTofluids have traditionallyand rarefaction of surrounding air, thus generating been used for the centering of voice coils insound waves. The ferrofluid resides in the gap and tweeters I 5. Ferrofluid has also been employed to

is retained by the prevailing magnetic field. It is a align a flat voice coil of a line source speakerl4.freely suspended column of liquid enveloping both However, more recently ferrofluid was designed tosides of the coil and cannot be easily dislodged by provide a support mechanism for the coil in theimpact or g-forces of the moving mass. The coil most demanding application, i.e., a line of 4 to 8now travels in a liquid medium rather than air; and inch woofers. These drivers reproduce lowthis feature brings about a vast improvement in frequency sonic information by moving a large coil ~

speaker performance. The thermal conductivity of and heavy diaphragm. The forces acting on theferrofluid is about four times that of air and the spring- mass system are quite large. A corrugatedviscosity of ferrofluid, which may be adjusted, is disk element made of a flexible fabric called theseveral orders of magnitude higher than air. The 'spider' aligns the coil in the gap so that it vibratesfluid forms a bridge between the moving coil and in a truly axial direction without any rotation,the stationary metal structure. The structure serves rocking or uneven motion. Such a coiltwo functions: (i) it acts as a heat sink for the coil displacement is necessary to ensure low distortionwhich becomes very hot during operation and (ii) it of sound. The retention of the ferrofluid iri theseestablishes a velocity gradient across the film of drivers posed a challenge during operation as airferrofluid introducing a viscous damping term into pressure is created in various cavities causingthe force equation of the moving spring- mass ferrofluid to spew out of the gap. Furthermore, i

system. The presence of ferrofluid controls the ferrofluid magnetostatic forces needed to be highmechanical damping coefficient (Qm) of the enough to counter the mechanical forces. A highvibrating system. The Qm-factor is proportional to magnetization and low viscosity ferrofluid wasthe viscosity of the colloid. With a proper choice of found suitable for this application. The fluidviscosity, the resonance peak can be critically provided the centering force for the coil and thusdamped leading to a smooth frequency response the requirement for the disk element was alleviated.curve of the driver, a requisite for a hi-fi speaker The woofers are vented through the pole piece andsystem. The fluid protects the coil from bum out rear plate to relieve excess pressure enablingunder high power levels. The fluid is known to ferrofluid to stay in the gap. The magnetic fluxincrease the transient power handling capability of density in the gap is more than 10,000 Gauss, the 'fa. driver by as much as a factor of four. The coil excursion about 6 mm, the power rating in themagnetic fluids in the beginning were range of 150 Wand the operating frequency 20 Hz-predominantly used in tweeters, which reproduce 2 kHz. Both the cone and bobbin are made oftreble sound and are known to be highly aluminum. The spun aluminum diaphragm remainsunderdamped and susceptible to thermal failures. unaffected under water, Fig. 11, and its rigidity

RAJ& CHORNEY: FERRqFLUID TECHNOLOGY 379

oor

Fig. 12 -Outline of a tactile sound transducer.

along with the coil. The system can be operatedcontinuously at 100 W RMS and up to 200 Wtransient. A high power level is desired to create

'-- the maximum sensation. However, at these powerlevels the voice coil may be overheated especiallyat instantaneous peaks and may bum out. Theferrofluid in the gap prevents thermal failures andextends the service life of the transducer. The gapflux density is in excess of 1.4 T to create a hightransduction force. The fluid possesses both high

Fig. 11 -Spiderless woofer with ferrofluid submerged in colloidal and thermal stabilities to ensure long termwater and tested over a year. reliable performance of the tactile audio system.(Courtesy, Kieltyka W, New England Audio Resource, Maine) The device is used in home theaters, flight

simulators, amusement parks, schools for the deaf,, etc.

ensures an absence of a high frequency peak, found Alarm type speaker -The speaker is installedtypically in paper cones. Its large metallic conical as a warning device on heavy transportation andsurface area provides additional heat transfer for construction vehicles to alert people of thethe coil. These speakers are the preferred choice at impending danger of the moving vehicle. TheDisney World and theme parks and are either driver is subjected to harsh environmentalsubmerged in water or experience splashing of conditions such as extreme temperatures andwater. A fabric spider cannot function in these humidity. In this application the speaker is operatedenvironments and the speaker will fail at the resonance peak to ensure the maximumimmediately. Ferrofluid is the only choice to align sound pressure level (SPL) for the alarm. Thethe coil and as an additional advantage improve the broader and higher the peak, the better the device.

F-' heat transfer. The selected fluid is compatible with Thus the requirements for the ferrofluid arewater. opposite to the conventional speakers. The

Tactile sound transducer -Tactile sound is the sPe;cification for the SPL is 107-103 dB, which isexperience of low frequency vibrations, 20-800 Hz, achieved with a tight air gap having a highpassing through the whole body and not just magnetic flux density. Ferrofluid eliminated thelistening to the bass music through the ear. This problem of voice coil rubbing created by the"flying saucer" shaped transducer, about 200 mm narrow radial gap and improved production yields.in diameter and 50 mm high, is directly fastened to The resonance peak was slightly (-2 dB) damped-a physical structure such as a chair, bed or floor in the presence of ferrofluid but the speaker wasthrough a threaded fitting, (Fig. 12) and transmits still within the specification. The enhanced heatsonic energy into the object rather than into the air. transfer allowed an increased current flow into the~ The magnetic circuit is similar in design to a coil thereby restoring some of the loss inconventional speaker and is rigidly connected to efficiency. The fluid caused the peak to broadenthe bottom half of the shell. The voice coil is over a wider band and improved the alarmattached to the top half of the shell, which vibrates frequency selection.

INDIAN J. ENG. MATER. SCI., DECEMBER 1998 '.

much larger in size than the flying height of the

disk and can easily destabilize the head and cause it

MAGNETICh Th . f h . HEAD to cras. e rotation 0 t e disk creates a small

ICALLY negative air pressure, which P romotes mi g ration ofSILVER P~INT I DUCTIVECONDUCTIVE NETIC FLUID bearing g rease P articles towards the disk cavi tyGLUE .

The ferrofluid exclusion seal prevents this flow ofETIC SEAL . I Add .. IIpartlc es. ItIona y, electrostatic charges are

! generated on the moving disk. If the potential

difference between the disk and the head is allowed

to exceed the dielectric breakdown of air (0.8

kV/mm), a spark may be produced resulting in

IC) system failure. The accumulation of static charges

also constitutes a flow of low level electrical ""'

Fig. 13 -Computer disk drive spindle equipped with a current through the bearings. This may cause a loss

electrically conducting magnetic fluid seal. of accuracy, pitting of the bearings and a shortened

bearing life. An electrically conducting ferrofluid,

ISO FERROFLUID STEPPER in addition to providing sealing, also protects the

spindle from static electrical discharge' .

Shaftless seal designs and new ferrofluids were

developed in tandem around 1979 specifically for

sealing magnetic disk drive spindles. The current

mechanical sealing technology that existed was not

performing satisfactorily and a reliable sealing

method was sought by the computer industry. The )

disk cavity needed to be hermetically sealed from

external and internal sources of contaminants to

avoid hard (head crash) or soft (information loss)

errors. The most challenging problem was to seal

Figure 14. Cross section of a permanent magnet stepper motor. the rotating shaft so that the bearing grease vapors

and any other particulate matter produced

internally did not escape to the disk area.

Exclusion seal for hard disk drive Conventional elastomer type seals were not

A magnetic disk drive spindle equipped with a adequate as they self-generated particles due to

ferrofluid exclusion seal is shown in Fig. 13. The friction. A ferrofluid exclusion seal is a low

shaft of the spindle is supported by two precision pressure, thin profile seal designed to protect the ~

ball bearings. One end of the shaft is attached to a critical elements of a rotating system from

motor (for rotation of the spindle) and the other is deleterious environments. The torque and power

fixed to a hub (for placement of memory disks). A requirements of the seal are minimal which makes

magnetic pick- up head stores or retrieves the technology attractive. Although these seals are

information from the rotating disk. In a floppy disk also utilized successfully in textile, robotics and

drive, the head rides on the surface of the disk in an marine industriesl2, the largest application has been

intimate contact, but in a hard drive the head in the computer industry. Presently, about four

initially rests on the surface of the disk but as soon million of these seals are manufactured per month

as the disk begins to rotate, it is aerodynamically to serve the needs of the computer industry. The

lifted at a height of about 0.2 J.1m above the surface first exclusion seal application was in a 14" disk

of the disk. The latter technology, called the drive spindle, followed by 10" and 8" drives used'

Winchester drive, enables one to achieve a high in main frame computers. The shaft diameter for

magnetic storage density. The operating the 14" drive was 46 mm and smaller for the

environments for the head and the disk must be others. The speed of rotation for all these spindles

kept clean. Even a small dust particle (typical was 3600 rpm with the sealing pressure

dimension: 24 J.1ffi) or human hair (-100 J.lm) is requirement of 5" H2O.

RAJ & CHORNEY: FERROFLUID TECHNOLOGY 381

..AUTOMATED STA6E LIGHTINGTable 4 -Recovery of minerals from concentrate of a placergold mine using magnetic fluid separator

MMER

(Courtesy: Ralphael Smoltin, Magnetic TechnologiesforRecovery of Minerals Ltd, Israel)

First Step: Magnetic separation in a week magnetic field: 5

0.26-0.3 Tesla

Magnetite 35.5%Magnetic wastes 60.8%Chromite 3% ~ LAMPZircon 0.2% tc:::DCinnabar 0.5%Loss of 2old after this process 0.4%

,j.

Second Step: Magnetic separation in a strong field: 2.3 Tesla F. 15 F fl .d t tta h d t d.Th"- Ig. -erro UI s epper ace 0 a Immer. e

Chromite 77% magnetic fluid improves step accuracy.Zircon 8%Ilmenite 2.6%Cinnabar 3.6% pressure capacity:> 5 H2O, temperature range: -20Hematite 8% to 80°C, life: 5 years, shock: 6-150 g. The seals areand the rest of 0.9% consisted of garnet of Fe(OHh bl d . t t. h.

t t II .I assem e ID au oma IC mac IDes 0 mee argep aglOC ase .. d b..Loss of 2&ld after this process 0.3% productIon requIrements an cost 0 ~ectlves.

,j.Third Step: Separation with kerosene based magnetic fluid, Ferrofluid filled stepper

density+0.9 g/cc, Field gradient -0.1 T/cm b d .b d ..A stepper motor may e escn e as a precIsIon

Chromite 31 % incremental actuator based on the interaction,Zircon 23.2% b .

fi ld d"Cassiterite 0.2% etween a permanent magnetic Ie an anCinnabar 42% electromagnetic field in an air gap. Ferrofluids arePyrite 3.0% used in permanent magnet or hybrid types ofEpidot ..0.6~0 steppers which possess a magnetic field in the gapLoss of 2old m thiS sta2e 0.2"/0 even when the motor is nonoperating, and not in

General recove~ of gold 99.1% variable reluctance motors where the field isproduced only when the windings are energized.The shaft of the device moves in discrete angular

With the advent of personal and lap top positions in response to a sequence of excitation.computers the seal volume surged dramatically. pulses. Being of digital nature and driven by logicThe spindle sizes shrunk successively to 51/4", circuits, these motors can easily interface with

'-' 31/2", 21/2" and 1.8" due to space constraints and computers. The applications of steppers areso did the dimensions of the ferrofluid exclusion numerous; some examples are: plotters, scanners,seals. Seals with an axial length as low as 0.75 mm silicon wafer transport, machine tool, mail sortinghave been produced but the most common size is '1 and robots. The angular displacement and themm. The sealing technology has been one of the direction of the shaft rotation depends upon thefew innovations which has successfully kept up number of stator and rotor teeth and the phasewith the changing trends of the spindle designs and relationship between the coils. A cross-section of aof the computer industry. The new generation of PM stepper is shown in Fig. 14. The rotor is madedisk drive spindles are both fixed and rotating shaft of a nonmagnetic steel shaft. Mounted on the shafttypes with operating speeds in the range of 3600- are a cylindrical magnet and two soft iron yokes7500 rpm. The seals, which are the scaled down affixed coaxially on either side of the magnet. This

"' version of the large two pole seals, are furbished is the construction of a single stack rotor. A

with ferrofluid after they are installed on the multiple stack rotor (a design used for boosting thespindle, and can be either a single or double pole motor output tQrque) consists of the basic magnetfill. The operating parameters of the seal are; and two yokes unit repeated several times along the

382 INDIAN J. ENG. MATER. SCI., DECEMBER 1998

length of the shaft. Such a structure will invariably problem!? The maximum stepper motor housinguse a higher amount of ferrofluid than the single temperature is about 130°C. Being in sealedstack. Each yoke is composed of a number of teeth environments, the ferrofluid evaporative losses arealong its circumference; The teeth on the second minimal. The fluid life is generally not an issue.yoke are displaced by half a pole pitch from the However, the capillary migration of magnetic fluidfirst one but are of exactly the same shape and through the laminations must be prevented bynumber. For a 1.8° angular displacement there are proper adhesive bonding of the individual sheets.50 teeth on each yoke, and the adjacent teeth on The advantages of using ferrofluids in PM stepperseither yoke are spaced apart by 7.2°. The stator of a are: noise reduction, decrease in settling time,1.8° stepper is comprised of 8 electrically increased torque output at resonance, co"rrosionenergized laminated poles with each pole equipped protection and heat transfer. These benefits arewith 5 teeth spaced the same as on the rotor. The realized by the viscous shear effect of the ferrofluidteeth of adjacent poles are 1/4 of a tooth pitch retained by the magnetic field between the statordisplaced from each other. When the stator is and rotor teeth. Sporadic commercial activities

.A

excited with four phase windings, the shaft relatmg to ferrofluid steppers have existed sinceexecutes 200 steps in one revolution. Each step is a 1985, but it was not until 1992 that more consistentresult of the ,nagnetic force of attraction acting to market acceptance was realized. It is estimated thatalign the rotor teeth with the stator teeth of there are close to 200,000 steppers operating at thisopposite polarity. When a phase is energized, the time in the field. A most recent application iselectromagnetic flux thus generated passes through described below:the pole into the ~ir gap, then into the yoke, Automated stage lighting-In the entertainmentthro.ugh. the m~gnet, mto the yoke? then to the stator industry, automated stage lighting is quitelaml~at!ons, mto the magnetically ~e~eable commonly employed. Originally conceived tohousmg and bac~ throug~ the stator lammatlon to eliminate the need for a lighting technician to climbthe pole of o.pposlte polarity. ..into the rafters to refocus lights and change their

.Several dlffere?t types of ferroflulds are used m colored lenses, automated lighting has also gained '1steppers dependmg upon the nature of the wide popularity because of its ability to create

special effects. The automated function isVERTICAL PUMP controlled by steppers which interface with a

computer. In fact the whole show can beMOTOR programmed in advance. A schematic of the

lighting arrangement is shown in Fig. 15. A size 17MA~EE;~C L stepper is used in a mechanism designef to move a

lens in front of a high intensity bulb. The motorVAPO was deficient in positional accuracy, and because

of vibrations it was also noisy. A mechanicalLIQUID damper was used to correct this problem but "1

""'-' ""'" ., "'"" without avail. The fluid filled motor not only'"'" ---~ -::::::::::- -::: -=-- ~ substantially increased the positional accuracy, but

also reduced the noise. Additionally, the lensbecame quite hot due to its close proximity to the

SHAFTALIGBUSHING bulb. The heat flowed along the shaft of the motor

and resulted in an unacceptably high rotortemperature. By bridging the gap b~tween the rotorand stator, ferrofluid provided a path for heat flowinto a larger sink, lowering the operatingtemperature of the rotor by 30°C.

~

Environmental sealsFig. 16 -A vertical sump pump with a magnetic fluid seal Preservation and protection of the environmentproviding containment of hazardous gases. has become a major concern in the world. The

g

RAJ & CHORNEY: FERROFLUID TECHNOLOGY 383

environmental seal based on magnetic fluid is the pressure inside a process chamber. A stirrer may be~atest product designed to contain the hazardous employed to agitate a liquid mixture in a reactor. Avolatile compounds within the process chamber pump may serve to remove liquid from a sump orand prevent them from polluting the atmosphere18. evacuate a process vessel. The seal performs a dualThe seal at the same time does not allow function, i.e., containment of hazardous gases inatmospheric contaminants to leak into the process the enclosure and protection of processthereby affecting quality and yield of finished environments from outside contamination.product. These seals are used either as 'stand- Blower seal- A ferrofluid emission controlalone' or in tandem with a mechanical seal. For seal was installed in a nickel ore refinementmost applications, the pressure capacity of the seal system. The seal is attached to a blower whichneed not be more than 34 kPa and only in few forces poisonous' CO over nickel ore, thusinstances may the sealing requirement exceed 101 producing Ni(CO)4. The mixture is then carriedkPa. In the process generally both liquid and over to another part of the system where undervapour phases are present and a consideration of higher temperature Ni(CO)4 decomposes into pure

~ these environments and location of seal dictates the Nickel. Four seals are used per system with the

seal design. If a liquid is to contact the magnetic operating speed of about 1100 rpm. The sealsfluid seal, a mechanical seal is desired to avoid the hermetically retain the toxic fumes within themixing of liquid with ferrofluid. A mechanical seal system, and at the same time do not allow the oilis also necessary if the operating pressures are used for lubrication of bearings and gears to enterexcessively high. However, mechanical seals such the process. Lip seals are used to prevent mixing ofas labyrinths, packings and bushings inherently lubricating oil with ferrofluid.leak and with the passage of time as the wear Pump seal-Fig. 16 shows the schematic ofincreases the emission of harmful process vapours application of the sealing technology to a verticalincreases. Magnetic fluid seals, on the other hand, sump pump. These types of pumps are used forare nonwearing and hermetic in nature and thus in evacuation of difficult to access industrial wastes,

( combination with a mechanical seal they ensure a corrosive and noncorrosive liquids. The pumpcomplete retention of toxic organics or inorganics vtumn is attached to a motor which drives anIn cases where the seal is exposed to only vapours impeller. Except for the motor, most of thewhich may be present on top of the liquid, the assembly is submerged in the process liquid. The'stand alone' version works fine. If the gases are oottom of the column is filled with liquid and thechemically active, a purge of inert gas such as top contains the vapour phase. The pressure isnitrogen or argon in front of the seal may be close to vapour pressure, however, a seal with anecessary to extend its life. The seal may be sufficient high pressure capacity i.e. 34 'kPa wasrefurbished with the ferrofluid through a fill port used. The shaft rotated at a speed of 3500 rpm, andwhile mounted and operating on the equipment, if its diameter was 28 mm. The gases consisted ofso desired, providing multiple years of replacement toluene and methyl ethyl ketone at a temperature of

-free operation. The interstage regions may also be about 90°C., accessed to monitor the leakage of any corrosive Mixer seal- In a low speed mixer application,

gas as the seal approaches its terminal life to Fig. 17, the vapours produced as a result ofprevent catastrophic impact on environments and to chemical reaction were chlorofluorocarbons andenhance planned maintenance. These seals are hydrochlorofluorocarbons in a tank containingtypically not liquid cooled and may operate in the liquid diphenylmethane diisocynate prepolymer.temperature range of 60-125°C. The operating The maximum pressure of the vapours was 510speeds are from 1500 to 6000 rpm, and the shaft kPa, but the seal was designed with a sufficientdiameters may range from 10 to 100 mm. To meet safety factor to sus~ain 1100 kPa. To protect theenvironmental and life specifications, fluorocarbon magnetic fluid seal from an occasional liquidbased ferrofluids are ideal for these applications. splash, a guard seal was incorporated into the

~ The seal is used as a positive barrier around assembly. The atmosphere side of the seal was als()rotating shafts of industrial fans, blowers, agitators, equipped with a guard seal to protect it from gearmixers and pumps. The fan and other similar oil and dust. The temperature of the gaseousdevices move air or gas under vacuum or positive environments was less than 40°C. The rotary speed

384 INDIAN J. ENG. MATER. SCI., DECEMBER 1998

MIXER Two types of particle separators utt'lizingmagnetic fluid technology have been developed.

One is a static unit, known as magnetogravimetric

or sink- float method2O.21, in which the magnetic

MAGNETIC LIQUID fluid is station ary in the ta p ered P ole P ieces of anSEAL.

electromagnet, Fig. 18. The fluid is subjected to a

VAPO magnetic field gradient. The apparent specific

LIQUID gravity of the fluid is proportional to the field

gradient. Mixed nonferrous materials are fed into

the middle of the fluid reservoir with a conveyor

MIxER BLA belt. Lighter materials float to the top, and the

heavier materials sink to the bottom of the

ferrofluid pool. Conveyor belts, one for the top part

and the other for the bottom, scoop these materials

Fig. 17 -A mixer fitted with an environmental seal based on and transfer them into separate buckets. The -..0

ferrofluid. apparent density of the fluid can be controlled by

the current in the electromagnet; thus materials of a

SINK FLOAT SEPARATION wide range of densities can be separated by this

method. Kerosene based ferrofluids are typically

used for this application because of their high

colloid stability in large field gradients and low

viscosity. They are relatively inexpensive, easily

diluted, and can be prepared with a high

magnetization value. These separators have

predominantly been used in recovery of precious

metals and stones.Sink-float equipment using a hydrocarbon based 1

Electromagnet ferrofluid has recently been marketed. With this

system, materials in the density range of 0.85-22

Fig. 18-Schematic of a static material separator using g/mL can be separated with the resolution of 0.1

magnetic fluid. g/mL. The technique can be effectively used in the

grain size of 0.03-16 mm, and feed rates of up to

of the shaft was 10-50 rpm. The seal operated 30 kg(h. T?e re~overy of .go.ld in placer and ore

successfully in these environments. deposits With thiS process IS In the range of 98.6-

99.5%. An example of mineral separation in a

Material separation placer gold mine is shown in Table 4. The gold

Various methods exist for concentration and sands and rocks are always mixed in with weak and

separation of ores19 and a number of them utilize strong paramagnetic compounds. After the -~

gravimetric principles and rely upon the density of conventional gravity beneficiation process, the gold

the processing fluid. Heavy liquids such as concentrate is subjected to three stage separation.

methylene bromide and acetylene tetrabromide The first two steps remove the magnetic impurities;

possessing densities of up to 5 g/cc are some times the trailing is then passed through the sink~float

employed in centrifuges for this purpose. However, apparatus for final recovery of gold.

these liquids are toxic, threaten personnel safety The second system is known as Magstream(R).

and pose environmental hazard. A ferrofluid, on The magnetic fluid along with the particles to be

the other hand, can have an apparent density as separated rotate in a vertical duct placed in a

high as 20 g/cc and it can be controlled in precise magnetic field (Fig. 19). A septum at the lower end

steps with a magnetic field gradient. Water based of the duct divides the mixed mineral slurry into of

ferrofluids, generally recommended for this two fractions. Each particle in the separator is

application, are nontoxic, inexpensive, can be subjected to four radial forces22: the outward

easily diluted and reclaimed from the wet particles. centrifugal force, inward buoyancy force resulting

RAJ & CHORNEY: FERROFLUID TECHNOLOGY 385

from the outward centrifugal force on the MAGNETOHYDRODYNAMIC SEPARATIONsurrounding ferrofluid, inward levitation force bythe magnetic field gradient and .outward magneticattractive force on weakly paramagnetic particles.The centrifugal and the levitation are the dominantforces and the other two forces are relatively weak. STATIONARYThe magnetic attractive force term is zero for ~~I~~ 1/ ;o~o \.. 1 Th . fu 1 d FERROFLUIO oto +

\nonmagnetic partlC es. e centrl ga an I +0+0+0

levitation forces can be independently varied on a ! i\~: Iparticle by changing the speed of rotation and I ~oi+ I. fild ... 1 I 0+0+ I magnetic Ie Intensity or more convement y ~r:;~~ 1 +0'" ..I

magnetization of the fluid. The net force varies : ~:o"+ :linearl y with the radius and is diretted either loot I

0 ~ I

inward or outward depending upon the selection of : go~~ I~ speed, field intensity, magnetization of ferrofluid \ 'loAt: Iand the density difference between the fluid and the M ',- ~.1 + /

particle. By properly balancing these forces a HFAVY IttBI. LIGHTd . d .fi . 1.. b h. d F:RACTION IllER ERACTIONeslre speci IC gravity sp It pOInt may e ac leve

whereby the slurry is divided into two fractions; Fig. 19 -Material separation achieved with a rotating columnone with the 'density below the chosen split point offerrofluid.

and the other above that point. The lighter R-actionis collected at the inside and the heavier at the Magnetization versus Densit y foroutside of the septum. Each of these fractions MSC W 11

which indeed may be a mixture of several different E 1'~.~minerals can be further processed in the separator E. fi I M .. 1 f 01 1.15Into mer cuts. n agstream, umt partlC es 0 ;;:

f d.ffi d . 1 d .. 1 ..1.1I erent enslty va ues are riven opposite y .~ 1005

independent of their shape, size and position in the ~ 1duct. Precise separations in a wide density range, 0 50 100 150 200i.e., 1.5 to over 21 g/cc can be achieved with Magnetization ,Gaussparticle size less than 250 ~m. A controlled mineralrecovery experiment was performed using the Fig. 20 -= Ca1ibrati~n (magnetization versus density) curve forcommercial Magstream(R) unit model 50 and water water based ferrofluld MSG WIt.

based ferrofluid MSG WIt. The calibration curve,(Fig. 20) provides a straight line relationship the minerals having SG greater than 3.1. Thisbetween the density and magnetization of the procedure produced two fractions namely PCD (H)ferrofluid. Ten glacier sediment specimens (already at the outside and PCD (L) at the inside of theexamined for absence of indicator minerals) septum. Both fractions were dried and weighed.ranging in weight from 137 to 270 g but of The fraction PCD (H) contained all of the indicatordifferent origin and contents were spiked with ten minerals. These results are shown in Table 5. Thespecimens of the known indicators: ~¥rope (PY)- quality of separation was determined in two ways:

SG: 3.51, chrome diopside (CD)-SG: 3.3 and (i) the weight of PCD (H) should be as low asilmenite (IL)-SG :4.72. The concentrated MSG possible so that when the indicators are sortedWI I was diluted with water to obtain a density manually with a microscope, the time requiredvalue of 1.05 g/cc with the corresponding would be minimum ensuring cost savings, and (ii)magnetization of 37 G. The split point was there should be 10 of each of the indicators in theexperimentally determined to be at 3.1 SG. The PCD (H) fraction. If this number is less than 10,r- speed of rotation, 370 rpm, was calculated from the this means that some of the indicators are presentknown values of ferrofluid parameters and mineral in the PCD (L) fraction and the separation was lessdensity. Each of the ten samples were processed at than perfect. The results show varying weights andthe feed rate of IS g/cc only once to separate all of the number of indicators recovered. The variation

386 INDIAN J. ENG. MATER. SCI., DECEMBER 1998

-Table 5 -Results of mineral separation using Magstream(!) unit

(CrJurtesy.. Bahjal Koshaba, Canamera Geological Ltd., Canada)

Ferrofluid MSG WI IMagnetization 167.4 GDensity 1.23 g/mLFeed Rate 15 g/mL

Sample Sample SG RPM PCD PCD PY CD IL# weight, g ferrofluid (L) (H)

370A 215.5 1.05 370 113.58 101.92 08\10 09\10 06\10B 136.96 1.05 370 75.44 61.52 09\10 10\10 10\10C 235.51 1.05 370 201.44 34.07 10\10 10\10 06\10D 233.38 1.05 370 177.78 55.6 08\10 07\10 05\10E 231.05 1.05 370 172.12 58.93 10\10 10\10 07\10F 198.94 1.05 370 162.12 36.82 07\10 07\10 07\07 '""

G 198.32 1.05 370 167.94 30.38 10\10 08\10 07\10H 198.77 1.05 370 176.84 21.93 09\10 08\10 07\10---

FERROFLUID SOLENOID VALVE mechanical stop or butt affixed near the center

prevents the slug from being ejected out of thebore. A spring attached to the plunger keeps itaway from the center when the coil carries noc\XTent. Thus a reciprocating motion of the plungeris produced as the coil is excited with a series ofelectFical pulses. This back and forth movementcan ~ used to actuate a valve, which in turn may lcontrol a flow of liquid or gas. In miniaturesolenoid valves this displacement is of the order of1 mm. A magnetic field is created in the gapbetween the plunger and the butt. The higher the

Fig. 21 -Components of a solenoid valve and location 01 value of the magnetic field (or the current in theferrofluid. .

1) h th fi h I A hCOt, t e greater e orce on t e p unger. s t eslug approaches the butt, the force increases rapidly

was attributed to the differences in the sediments due to a decrease in the reluctance of the magneticand not to any inadequacies in the fundamental flux path. The plunger strikes the butt with theprinciples of the separation process. Subsequent maximum force; creating noise, vibrations andexperiments have shown that the weight of PCD chattering. Random noise of solenoid valves poses ,(H) could be as low as 20 g with 100% recovery of serious restrictions in their utility in machinesthe indicators. If desired it is possible to further which must perform quietly. For example, inseparate IL (SG 4.72) from the other two minerals medical applications such as dialysis equipment,at the split point of SG 4.1. IL will be collected at blood chemistry instruments, blood pressurethe outside of septum and PY and CD at the inside. monitors and ventilators/respirators it is necessary

that the valyes be quiet for patient's comfort.Quiet solenoid valve The presence of ferrofluid in a solenoid valve

A solenoid consists of a hollow bobbin with a may lead to several advantages such as noisewound coil and a plunger or magnetic slug which is reduction, lubrication of moving parts, corrosionfree to move inside the bore (Fig. 21). When the resistance, and increased force on plunger and heatcoil is energized with an electrical pulse, a dissipation. The fluid fills all of the gaps inside the .,magnetic field is produced in and around the core. solenoid. A small external magnet retains the fluidAs a result of this field the plunger is magnetically when the valve is not. functioning.attracted towards the center of the coil. A A miniature solenoid valve was filled with a

RAJ & CHORNEY: FERROFLUID TECHNOLOGY 387

SOL ENOIO VALVE and the butt through the ferrofluid seal thatprovides the noise control.

Conditions 2Hz5 5 50 0 Nois~ l~\'t' I

T' Transformers .I. 5 .Responst" ,1m e(msec) The use of magnetic liquids in transformers, FIg.

;~ '35 23, is a relatively new area. The ferrofluids for t~is

30 application are typically based on petroleu~ o~l.s,2 5 which are liquids of choice for heat conductIon m20 transformers. Large transformers such as1 5 distribution, traction and power transformers are1 0 5-0 cooled with dielectric oils2 .The heat generated by

8 ::l.:':.~":. ~-_. the current passing through the primary andNo Ferrofluid With Ft"rrofl uid secondary windings as well as the heat generated

by the eddy currents in the laminated core of theFig. 22 -Comparison of solenoid noise level and response transformer must be conducted away to avoid antime with and withoutterrofluid. Conditions: 2 Hz, I eV. excessive temperature build- up. The temperature

degrades the insulation, increases power losses andreduces the op~rating life of the transformer. It is

high viscosity fluorocarbon based ferrofluid. The suggested that for every 4 °C rise in temperature thevalve operated at 12 VDC with a frequency of2 or insulation life is reduced 24by 30%. The core is20 Hz. The- noise level of the valve was measured cooled by the natural convective flow of the oil.after it reached the steady state temperature of When transformer oil is converted into a ferrofluid,45°C with a B&K acoustical meter in an anechoic the heat transfer is significantly improved. There ischamber. Without the ferrofluid, the noise level an additional force that comes into play by virtuewas 50 dBA and with ferrofluid it decreased to 32 of the fact that the liquid now has magneticdBA almost indistinguishable ~om t.he backg~ound properties. This force, called the magnetic bodynoise; Fig. 22. There was a SlIght Increase m the force, is considerably higher than the buoyancyresponse time of the valve; it increa~ed from 5.0.!0 force. The origin of this force is the chan~e in5.5 ms in the presence of ferrofluld. The pu."-m magnetization of ferrofluid with temperature 5. Atcurrent was about 60 mA, less than the specIfied elevated temperatures the magnetization ofvalue of 65 mA. This was believed to be cfue to an ferrofluid is lower than at room temperatu~e. Theincrease in the magnetic permeability of the air gap ferrofluid next to the windings is not as stronglyin the presence of ferrofluid and the resulting attracted in comparison to the fluid awa~from theincrease in acceleration of the plunger.. The val~e windings in the cooler regions of the transformer.was contin~ously operated for a penod of SIX The time varying magnetic fields, already present inmonths at 2 Hz and at a pressure of 50 kPa thus the tr'ansformer due to the flow of electric currents,accumulating over 15 million cycles with out any cause the ferrofluid to circulate, thus providing theproblems. Separately, a life test was performed on heat transfer which is predominantly radial ina different valve operating at 20 Hz. The valve direction. The winding temperature may beactuated for 20 million cycles at which time the test decreased by as much as 10°C with the fluid.was stopped; its electrical and seal parameters were ...measured and then the device was disassembled for In addition to heat transfer and elImInatIon ofinspection of wear. The solenoid met all the hot spots, ferrofluids for. transformer app!ic~tionsoperating specifications and no wear was. observed. mus~ also ~osse~s electrIcal p~ramet~rs. s~mIlar. to

A model is proposed for the dampIng of the the InsulatIng otis such as hIgh resIstIvIty, hIghsolenoid valve with ferrofluid. It is believed that breakdown voltage, negligible charging tendency,this is not viscous damping since this type of low dielectric constant and low power factor.

"" damping is affected by the temperature, which was Whereas much work has been done to show that

found not to be the case. The damping mechanism ferrofluids can be used for cooling26, theis understood to be of dash pot type. It is the ~ploration of their electrical behavior has justmovement of entrapped air between the plunger begun.

388 INDIAN J. ENG. MATER. SCI., DECEMBER 1998.

T ANK several demanding research problems such as the

electrically conducting ferrofluids, high saturation

magnetization fluids, ultrastable colloids,

nonwetting liquids, ferrofluid composites and-"""'"'""'"' , e ~"" &--, extreme temperature fluids. These new capabilities& ::~d-" ~""'"'o.-" :: ::::~.;:-'~ once proven for commercial environments will lead

~ ::: -~ e.-- e-- ~ ~..-:::: to a new wave of engineering solutions and high

:::: :;:;::- performance products with significant impact on

the market place. In regard to the devices, the

challenge is found in the development of linear

seals, liquid sealing, ink jet printers, high DN seals,

compact and cost effective film bearings and ultra

high vacuum seals (~ 10-11 Torr). To this list may

be added the conceptual applications described by ...

Rosensweig27 namely the magnetoresistance

devices, optical displays, and hydroacoustic

projector, and those visualized by Zahn [Zahn M,

personal communication] in the far distant horizon

such as devices based on traveling magnetic fields,

P~h optical fibers coated with ferrofluids, optical

shutters and heat pumps.

References1 Elmore W C, Phys Rev, 54 (4) (1938) 309,1092.

Fig. 23 -Schematic diagram of a power transformer filled 2 Papell S S, US Pat. 3,215,572, 1965.

with a ferrofluid. 3 Bhatnagar S P & Rosensweig R E, J Magn Magn Mater,149 (1955) 198. ~

..4 Guell 0 C, Brenner RB, Frankel R B & Hartman H, JOther Applications Theo Bioi, 135 (1988) 525.

In addition to the visibly successful and 5 Yaacob 11, Nunes A C & Bose A, J Coil In[ Sci, 171commercially promising products just discussed, (19?5) 73. ..there is a class of small but unique applications of 6 Cralk.D J & Teb.ble, FerromagnetIsm and ferromagnetIc

0 12 .0 domaIns (John Wiley & Sons Inc., New York), 1965.ferroflulds which lie between the two extremes. 7 Bozorth R M, Ferromagnetism (D. Van Nostrand

These areas have been proven technologically but Company, Inc., Princeton, NJ), 1964.the level of business activity has been low. 8 Wohlfarth E P, in Ferromagnetic Materials: A handbook

F errofluids particles can be used as metal on the properties of magnetically ordered materials, editedtal t £" wth f b fib h o h . tu by Wohlfarth E P (North-Holland Publishing C~mpany,

ca ys s lor gro. 0 coar ?n I ers ': IC m rn Amsterdam), 1980, 1.

are employed m fabrIcation of high strength 9 Rosensweig R E, Ferrohydrodynamics (Cambridgecomposite materials, for observation of magnetic University Press, Cambridge), 1985. '1

domains or magnetized regions in memory disks, 10 M~sart R, in Magn~tic fluids and applications handbook,tapes and pipe weldments and for diagnostic and edited by Berkovsk.l B & Bashtovoy V (Begell House,

0 . d o 0 h b o d . 1 fi Id M .Inc., New York), 1996,24.clmlcal stu les m t e tome Ica Ie. agnetlc 11 Rosensweig R E, US Pat. 3,620,584 (to Ferrofluidics

liquids are utilized for quality control of complex Corp.), 1971.internal mechanical structures such as turbine 12 Raj K, in Magnetic fluids and applications handbook,blades, in precision film bearings, in a variety of edited by Berkovski B & Bashtovoy V (Begell House,

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Islon 0 e 0 or th ,/" h d I 26 I 31e pages OJ t e au 10 engIneerIng socIety, vo .-vo .The challenges for the technology are far from (1978-1983), (Audio Engineering Society, New York),

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