Chapter I

The unquenching thrust among the scient~sts to impmve tlic pn\pcrt\es of

available materials lead to the development of new materials with astounding physicul

properties. One such area in which researchers are t ~ i n g to find out new lnugnctlc

materials with unbelievable physical propertles hetter than knitcs to have flowahility.

unlque thermal propeny like h a t transfer abll~ty to work etfwtively, swlunt, dampers

etc. In addition, in later part of 20Ih century. the new nano pm~clcs w e into being to

utilize the above unbelievable physical properties. Nanomugnetism and maynetlc

behaviour of nano part~cle are net+ areas where pcwple wsntd to exploit their advantaps.

Hence the process of understanding the magnetic hchavior of nanopan~cles needs to hegiti

w~th the fundamentals of magnctlsm. These foundat~ons will cnablc to havr (in 111-depth

understanding and appreciat~on of the uniquc behavior of magnetic nanopartlclcs and

their deviations from respective hulk properties.

1.1 Origin of magnetism

The force of magnetism, experienced at a distance, has always fascinated

mankind. Magnetism originates from the mov~ment of elwtric charges. All materiuls

exhibit some kind of magnetic responses. However most of these eN'bts are very small.

The electrons In an atom govern magnet~c propertles of matter in two difircnt ways The

first is the electron acting as a splnning charged sphere, where the spin rcs~mblcs the

magnetic field of a tiny bar magnet. The spin is a quantum mechanical propdy wh~ch

can be oriented in one of two directions, the upward ( f ) direction or the downward (1 ) direction. The second one is the effect of the electron circulating around the nucleus of

the atom, which resembles a current loop. The flow of charge in a circular current loop

produces magnetic lines of force known as a dipole [ I ]

The magnetic moment of a material is the measure of the strength of the dipole.

An electron in an atom has a magnetic moment due to its spin only or due to both spin

and orbital motion. An orbiting electron responds to an applied magnetic field, which is

gowned by the spin configuration of the atoms of the material. The orbital motions of

e l e m n s create atomic current loops, which generate the magnetic field. Therefore, all

Chapter l

materials inherently possess mapellc fields generated by the o h ~ t a l mcltlons of the~r

electrons

1.1.1 Magnetism in materials

Materials are classified by the~r response to an externally applied magnctlu ficld.

Descriptions of orientations of the magnetic naments in a material enable us to identify

different fonns of magnetism observed in naturc (Flg.l.1). Fivc hns~c types of magnetism

can be described as diamagnetism, paramagnetlsni, f'erromug~lct~sm, ant~tiem~magnetism

and fernmagnetism.

Figure 1.1 Different types of magnetic behavior 121

In the presence of an externally applied magnetic field the atomic current loops

created by the orbital motion of electrons tend to oppose the applied field. The materials

which display this type of weak repulsion to a magnetic ficld known as diamagnetic.

However, diamagnetism is very weak and therefore any other form of magnetic behavior

that a material may possess usually over-powers the effects of the current loop. In terms

of eiectronic configurations of materials, diamagnetism is observed in materials with

Chapter 1 -~

filled electronic sub-shells where the mapettc moments u e pii~rat and overall cancel

each other. Dlarnagnaic matenals have a negative susceptibility ( x < 0 ) and we&ly

repel the applied magnetic field. The eti'mts of these atomic currcnl Imps un. suppn3sd

if the material displays a net magnetlc moment or has long-range ordering of magti~qic

moments.

All the other types of magnetic behavior.ohserved in muterials tve at least partially

attributed to unpaired electrons in atomic shells, offen 111 the 3d or 4t'shells ofeach atonl.

Materials whose atomic magnetic moments are uncoupled dtsplay paranlagnetlsm

Therefore, paramagnetic materials have no long-range order. A gcilcrulizd descript~c~n of'

paramagnetism is when each atom carries a magnetlc moment, which pafl~ully al~gns with

the direction of applied magnetic field and hence enhances the mngnetlc tlux dens~ty.

This field acts independently on each atomic dipole. Therefore, there IS no long-range

order. There is a small positive magnetic susceptibil~ry ( X 2 0 ) (31. Materials, wh~ch

possess ferromagnetism, have aligned atomic magnetic moments of 'qual magnitude and

their crystalline structure allows for direct coupling interactions between the moments.

which may strongly enhance the flux dens~ty. Further, the aligned moments In

ferromagnetic materials can confer a spontantnus magnetlzatlon In the absence of an

applled magnettc field. Materials, which retain their permanent magnetization in the

absence of an applied field, arc known as hard magnets. The hard magnetic malcnals

retain a wnsiderable amount of magnetic energy; even when the magnetlc lield is

removed. These matenals may be used to form permanent magnets. Materials, which are

easy to magnetise and demagnetize, are called soft magnetic materials. The soft magnetic

materials retain a small amount of magnetic energy even after the magnetic field is

removed. These materials are also called as temporary magnets.

Materials having atomic magnetic moments of equal magnitude arranged in a

antiparallel fashion display antifemmagnetism. The exchange interaction coupia the

moments such that they are a n t i p d k l leaving a zero net magnetization [4]. Above the

NCI temperature (TN) thermal energy is sufficient to cause the equal and oppositely

aligned atomic moments to randomly fluctuate leading to a disappearance of their long-

range order. In this state, the material exhibits paramagnetic behavior. Ferrirnlypletism is

similar to antifemmagnetism where two different sub-lattices exist with

Chapter I -

antiferromagnetic exchange interactions. However, the malyl~tudcs of the two d i l T m t

types of moments, arranged antiparallel, not being cqual in magnitude lead to a net non

zero magnetization.

1.1.2. Theory of ferromagnetism and ferrimagnetisrn

A complete discussion of the theory of femagnet ism is beyond the smpc of this

introduction. However, the fundamentals are provided to addrcss the magnctizotion

processes in ferrimagnetic nanoparticles and thelr dlspers~ons. We~ss tint developed the

molecular theory of ferromaqetism in early 1Q00s, uh~cl i gave o sen11-quantltatlve

description. The theory is based on the assumption that each atornlc dlpolc IS subjected to

a local field that is proportional to the mubmetization summ~vi over all thc other dip)lcs in

the material [1,4,5]. Later, the development of quantum mechanics led lo the conccpt of

exchange interactions between two atoms which is a spin-dependent intcract~ons. The

spin exchange interaction is derived From the Paul1 exclus~on princ~ple, whcrc antiparallel

spln arrangements in an atomic shell arc forbidden [ h ] . The exchange Interaction is

essentially different from coulomhic cnerglcs applicable to direrent spln configurations

in the sample [j]. Above a certain temperature, known as the C'uric tcmpcraturc ('I'( ). the

aliqmcnt of the moments in ferromagnetic and Serrimagnetic matenals is lost due to

thermal tnergy and the material displays paramagnetic hchavior.

Ferrimagnetism is similar to antiferrornagnetism sincc the spln arrangemcnts arc

antiparailel, however, the sub-lattice magnetlc moments are of unequal magnitude, which

produces a net magnetization in an applied field. The net magnetization observed in

fenimagnetic materials is typically lower than that of ferromagnetic matenals primarily

because of the antiparallel spin arrangement in the former. Despitc the differences in

alignments of magnetic moment the theories that describe ferromagnetism can he applied

to fenimagnetism with modifications to account for the existence of the sub-lattice

interactions and antiparallel orientations of spins.

The ferrimagnetic crystalline structure is comprised of two different magnetic ions

occupying two kinds of lattice sites, tetrahedral 'A' site and octahedral 'B' site [1,5]. For

spinel crystalline structures, such as magnetite, surprisingly all exchange interactions

Chapter I - --

(AA, AB and BB) favor antlparallel allpment. The fernmapet~c behavior IS due to thc

strong AB lnteractlon. [ I ]

1.1.3. Ferrites

Femtes are one class of the magnetic niatcnals. They ure also calld fi.nimugnrtic

materials. These are complex compounds of vanous metals and ox~des, which exhibit thc

phenomenon of fermmagnetism. They are non-metals, hut possess strong ferromagnetic

properties such as domain structure, hysteresis remwence and coew~vity etc.. sinlilar to

those of metallic ferromagnets.

1.1.4. Structure and Magnetic Properties of Spinel Ferritcs

The magnetic part~cles possess a spinel structure. This structure consists ofa cuhic

close packing of oxygen ions with the metallic ions occupying the tetrahedral ' A ' s~ te and

octahedral 'B' interstitial sites [7]. The structure of a cuhic splnel ferntc IS shown In

Fig. 1.2.

Fig.1.2. Structure of vpinel lerrlte

The unit cell contains eight formula units ABzO,, with eight tetrahedral 'A ' sites,

16 octahedral 'B' sites and 32 oxygen atoms. The 32 large oxygen ions form a cubic

closed packed (fcc) m y with two kinds of interstitial sites mupied by the metallic ions.

Chapter I --

One IS called tetrahedral 'A' srte because catron IS located at the center ol a tetrahedron

with comers occupled by the oxysen Ions. The= are 64 tetrahedral sites surrounded by 4

oxygen Ions The other 'B' site IS called octahedral slte. because the oxygen ions around

catlon occupy the comers of an octahedron There are 32 octahedral s~tes surrounded by 6

oxygen Ions [l.7-IS]

The general formula for representing the spin4 stmctum is given as

where Me is a d~valent metal ions such as Fe" , ~ o " . NI". MI,". %II" ctc.. und

'x ' is the degree of invers~on. The octahedral 'B' posltlons arc usually shown hy squure

brackets.

According to distribution of cations the ferrospinels arc of following types.

x= l leads to normal spinel

x =O leads to inversc spinel

O< x< I mixed spinel

I ) In normal spinel structure, all Me2' ions occupy A-sites. The structural formula of

such ferrites is Me2'[~e2''] 0 4 ' . This type of distribution exists In the ~ i n c fkrritc: Zn!'[ Fe3' Fe"l 0 2.

4 .

2) In inverse spinel structure, all ~ c " are In 'B' plsltlons and Fe" ions are equally

distributed between 'A' and 'B' sites. The structural formula of these ferrites IS

Fe"[Me2' Fe"'~04''. Magnetite (FelOd). Nickel fenita (NiFc204) and Cobalt ferrites

(CoFe204) have inverse spinel structure.

3) In mixed spinel structure, the cations Me2' and Fe" occupy both ' A ' and 'B'

positions. The structural formula of such ferrites is (~e~',1., ,~e'*. ) [ ~ e ' * & Fe"2.J 04'.

where parameter 'x' indicates the degree of inversion. CoFe20, represent this typc of

structure. If it an invmion degree of x 4 . 2 and its structural formula is:

(CO~'~.~F~"O.~)[CO~*O~F~"I s]04'..

Chapter I - -

Neel explained the magnetic propaty of ferrites. He postulated that the magnetic

moments of fenites are a sum of magnetic moments of individual sublatt~ces. In the

fcrrospinels, the magnetic structure depends upon the types of the magnetic ions resld~ng

on the sublattice 'A ' consisting of cattons in tetrahedral positions and sublatt~ce '6' with

cations in octahedral psitions and the rclat~vc strengh of the inter (JAll) and the intrii

(JAA and JBE) sub-lattice interactions. Since In spinel. all the thrtv excllarigc intmctions

viz: JAA, JBB, JAB are negative, there exists a compatit~on hetwcc~i intra and inter suh-

lattice interactions and generally. JAB >> Jtls '> JAA The spontaneous niagnet~zation 111

spinel ferrites is then the diflercnce between the net magnetlc moment of 'B' s ~ t c and 'A '

site ions. This interaction leads to coniplcte or partla1 (u~iconipcnsutd)

antiferromagnetism (ferrimabmetism) [XI

The possibility of changing continuously the ccincentratlon o f non-nlagnctlc on

like Zn, Cd, Mg. Ca etc, in the d~fferent sub-lattices makes the krntes very Itltercstlng

material to study the appearance of the various magnetic structures like ferrimugnetlc

order, superparamagnetic, spin glass depcndlng on the rclat~ve s ~ t c prefcrcncc oftlie ions

present. These non -magnetic ions may enter e~ther one o f the suh-latticcs or hoth. 'I'hc

diamagnetic substitution in ' A ' sub lattice represent the most clear cut case, in wh~ch the

Zn substituted ferrites hcing a well known example, bwausc o f J A A - - ~ . thc 'AD'

interactions are effect~vely weakened.

If the number of substituted ions In ferromagnet~c materials is not too high (up to

30 to 40%) then the overall ferromagnetic arrangements cannot bc destroyed even at m o

Kelvin. But some loosely bound spins may become locally canted or disordered at higher

temperature Tq,. For larger substitution the 'AB' interaction may become comparable to

or even weaker than the 'BB' interactions and which may lead to a non-wllinear spin

structure.

1.2. Ferrofluid

F m f l u i d &e a colloidal suspension of nano sized magnetic particles in a cania

liquid. The cania liquid can be an organic solvent, water, or a variety of oil bases. The

magnetic particles are coated with a d q m m t to prevent agglomeration of particles.

Chapter I

These suspensions are stable and maintain their properties at extreme temperatures and

over a long period of time.

1.2.1. Ferrofluid: components

Ferrofluids are new class of magnetic materials,ahich combine both fluid and

magnetic properties comparable to those of a solid magnet. This rare combination of

fluidity and the capability of interacting with a magnetic field are achieved in ferrofluids

because of their composition [16]. Ferrofluids constitute of single domain particles

suitably dispersed in a camer liquid (two phases) with the help of a surfactant (three-

components). In short, ferrofluid is single domain two phase and three component system.

The two phases of the system are the solid magnetic phase (dispersed or discontinuous),

which is colloidally dispersed in a magnetically passive liquid phase.

The three components required to prepare ferrofluids are

1. Magnetic particles

2. Canier liquid

3. Surfactant (dispersant)

1.2.2. Magnetic Particles

The nature of the particle is ferromagnetic e.g., Fe, Co, Ni or ferrimagnetic like

magnetite, maghemite or mixed femtes. Scholten reported that, though a large number of

magnetic materials exist; only a few are candidates for making magnetic fluid [17].

Ferromagnetic materials have high saturation magnetization, but when the size is reduced

to the nano range, surface oxidation becomes more pronounced. Hence ferrimagnetic

materials have low saturation magnetization when compared to ferromagnetic materials.

In addition, fenimagnets, i.e., most of the ferrites are stable in atmosphere. Moreover,

adsorption of surfactant over the particle surface should also be possible. Hence, mostly

ferrites are preferred forthe preparation of ferrofluids.

The magnetic particles to be dispersed should be of single domain, size of which

normally ranges from 30 to 100 A' [IS]. The size of the magnetic particles should be

C h a p t e r 1

small enough to be suitably dispersed in a canier liquid, but not too small, since at sizes

less than 10-20 A' their magnetic properties tend to disappear [18]. Therefore, it is

advantageous to develop the ability to prepare particles having controlled size and size

distribution still retaining uniformity among other important properties such as

composition, qstallinity and magnetization.

1.2.3. Car r i e r liquid

Camer liquid plays a dominant role in deciding the suitability of magnetic fluid

for different applications, as it governs the overall physical properties of a magnetic fluid.

Camer liquid should be so chosen as to cater to the need of specific application.

Commonly used carrier liquids are water, hydrocarbon, ester, mineral and vacuum oil,

silicon oil, fluorocarbon etc. Properties of carrier liquids depend on the ultimate use. In

general, the camer-liquid must have following characteristics. It should be magnetically

passive and chemically stable. Its vapor pressure, viscosity and other physical properties

must be chosen according to the type of application. It should be compatible with the

surfactant.

Low evaporation rate, low viscosity and chemical inertness are desirable

properties for the carrier liquid. Scholten has discussed the evaporation and solidification

problems along with the advantages and disadvantages of using different camer liquids,

the availability of dispersant etc [17]. The canier liquid should bind itself with the free

chain of the surfactant. Finally, the carrier liquid should be compatible with the

environment (construction materials) with which the magnetic fluid comes in contact.

1.2.4. Surfactant o r Dispersant

The requirements meant for a surfactant are rather specific. It should have an

anchor group that has a high affinity for the particle surface, as well as a flexible tail of

proper length, which is soluble in the canier liquid. The surfactant therefore has to be

tailored to the particlesurface as well as to the carrier liquid [17]. The surfactant should

also be chemically stable in the environment in which it is to be used. Surfactant must

prevent particles from aggregating.

Chapter I

For sterically stabilized ferrofluids, steric repulsion between particles is usually

achieved by coating the particle surface with long chain monomolecular layer of an

appropriate surfactant, which accounts for the stability of a magnetic fluid. A molecule of

a simple surfactant consists of a linear chain of non-polar hydrocarbon atoms at one end

and an anchor polar group of atoms at the other end. The anchor group is called polar

head. The chain is called non-polar tail. The surfactant is so chosen that its molecules rn

interact with the magnetic particles, through a bond of hnctional group, to form a tightly

bonded monomolecular layer around the particles. The long-chain fragment of a

surfactant molecule must resemble that of the carrier liquid, so that the thermal motion is

not inhibited.

The choice of a surfactant for each particular application presents a rather dificult

problem, and is usually selected experimentally. Scholten has given the choices of

appropriate surfactant for different carrier liquids with their structures [19]. Usually

unsaturated fatty acids having chain length between Cg - CIX are used as surfactants. This

is because for shorter chain length acid, formation of clump occurs while for saturated

fatty acids the complete dispersion of magnetic particles is not achieved. The best

stabilizers available namely the polymeric ones, generally take up too much space

because of the long chain length. This will reduce the number concentration of the

magnetic particles and hence the magnetic property of the fluid. It has been reported that

CI2 - CIS acids can stabilize water based magnetic fluid despite their slight insolubility in

water [20]. Oleic acid (CIR) is the most commonly used dispersing agent for hydrocarbon-

based (with relatively low molecular weight) magnetic fluid. Oleic acid is used as the

surfactant for the present work and hence study on mode of adsorption of surfactant (oleic

acid) becomes mandatory.

1.2.5. Mode of adsorption of surfactant

For preparing ferrofluids having heptane (less viscous and volatile hydrocarbon)

as the camer liquid, ole!c acid is used as surfactant. Oleic acid contains a polar head and

non-polar tail.

Chapter I

Oleic acid:

P d u Non.plutril head r l r 1

CHdCH2)rCH = C H ( C H z ) , C - O H

Polar head - Carboxylic group Tail - Non polar end ( hydrocarbon chain)

The polar head reacts with the surface bound OH ions and get adsorbed on the

particle surface and non-polar chain provides a hydrophobic sheath to individual particles

[21]. Excepting the anchor part (polar head chemically adsorbed on the particle surface),

the adsorbed molecules (tail) perform thermal movements. When a second particle

approaches closely, the chains have to bend aside and their motion is restricted severely.

Thus when magnetic particles coated with a layer or shell of surfactant, long chain

molecules come close together. A repulsive force (referred to as a steric force) arises

thereby preventing aggregation of particles avoiding sedimentation.

Theoretical calculations can give the required amount of surfactant. However the

suitable amount of a surfactant can be decided only through experiments. The stabilizer

must not only prevent particles from agglomeration but also provide the formation of a

colloidal solution in canier liquid. For clear understanding, the appearance of the particles

coated with surfactant is given in Fig.l.3. The effective thickness of the solvated sheath

is therefore very important in order to determine the stability of magnetic fluid against the

inter-particle attractive forces like van der Waals and magnetic attractive forces. Hence it

will be appropriate to study the stability requirements.

Chapter 1

Fig.l.3 Appearance of the surfactant coated particles

1.3. Stability conditions for ferrofluids

The practical applications of ferrofluid are decided considering the stability of the

fluid and the vapor pressure of the canier liquid at the operating temperature [17]. In

order to ensure the suitability of the magnetic fluid for the particular application, the

stability of fluid is an important parameter. A magnetic fluid is called a stable one if no

ageing or phase separation occurs after a long exposure to a gradient magnetic field [2].

In order to prevent the phase separation particle size should be reduced such that the

Brownian motion keeps them in suspension. This is the most important factor but not the

only requirement. A magnetic fluid becomes stable when several inter particle attractive

as well as repulsive forces are balanced. The energy of the thermal motion in the fluid

must exceed that of attractive forces between the particles.

In a magnetic fluid the inter particle forces encountered are magnetic attraction,

van der Waals adraction, steric repulsion or electric repulsion.

The various energy terms involved are

Thermal energy = k~ T

Magnetic energy = po MHV

Gravitational energy = ApVg L

where,

ke is the Boltzmann constant,l.38 x 1v2' ~ . k "

Chapter I

T is the absolute temperature in Kelvin

p, is the permeability of free space and has the value 4n x 10" Him

V = n d3 1 6 in m' Volume of a spherical particle of diameter 'd'

L is the elevation in the gravitational field.

M is the magnetization

H is the applied field

1.3.1. Stability in a magnetic field gradient

The particles are attracted to the higher-intensity regions of a magnetic

field, while thermal motion counteracts the field force and provides statistical motions

that allow the particle to sample all portions of the fluid volume [2]. The magnetic energy

(b MHV) represents the reversible work in removing a magnetized particle from a point

in the fluid, where the field is H to a point in the fluid that is outside the field.

when some part of the fluid volume is located in a field free region, then stability

against segregation is favored by a high ratio of the thermal energy to the magnetic

energy

Thermal energy = k B T Magnetic energy p,MHV

Rearranging and substituting for the volume of a sphere gives an expression for

maximum particle size

Considering Fe304 particles subjected to the magnetic gradient field, it can be

shown that the size is d98.1 x lw9m or 8.1 nrn. From the calculation, it can be

seen that the particle size cannot be much larger than the range of a colloid lOnm to

remain stable against segregation in a field gradient [2]

C h a p t e r I

The concentration of particles in regions of more intciise magnetic field is Illnitd.

Steric hindrance puts an upper limit on the particle number concentration. Although

concentration gradient can be established in situation like this, when the field is removed

particles of a well-stabilized ferrofluid spontaneously redistribute throughout the fluid

volume over a period of time.

1.3.2. Stability against Gravitational Field

Gravity constantly pulls an individual particle downward in the container

while thermal agitation tends to keep the particle disperscd throughout the fluid matrix.

This is similar to a magnetic force providing unidirectional forcc. Thc rclativc influencc

of gravity to magnetism is descrihcd hy the ratio

gravitational energy A p g l = -- magnetic energy poM H

The ratio is approximately equal to 0.047. Thus b~avity is less of a threat to

the segregation than in a magnetic lield [2].

1.3.3. Stability against magnetic agglomeration

A typical ferrofluid contains particles of the order of 10~'im' and collisions

between particles are frequent. Hence if the particles adhere together agglomeration will

be rapid. Each particle is permanently magnetized, so more energy to separate a pair of

particles of diameter 'd' is needed when the particles are aligned.

m, .m, = m2

(m,.rXm,.r)= m'r"

is the dipole -dipole pair energy

Chapter 1 --- --

where L = 2Sld. with S surface to surface separation d~stancc. Whcn the panlclcs are in

contact,

Again thermal agitation is available to disrupt the agglomerates, with the

effectiveness of the disruption governed by the ratio

thermal energy 24k ,T dipole - dipole contact energy = m]

Accordingly for particles to escape aglomeration, this ratio must bc b~cater than

unity,

Hence particle size is given by

From t h ~ s equation for magnetic particles at room temperature the diameter of the

particle should be less than or equal to 9.8nm.

1.3.4. Net interaction curve

The algebraic sum of attraction and repulsion energies gives the net potential

energy curve. Fig.l.4. shows the net potential energy curve [2]. It is seen that the net

potential energ)! curve presents an energy barrier of more than 20ksT in order to prevent

the agglomeration. The sum of attraction energy is equated to repulsion energy

A value of ' 6 ' , thickness of stabilizing layer is calculated (6=2nm for D=lOnm) which

gives a reasonable energy banier say 25ke.T.

Chapter I

Fig.l.4. Potential energy versus surface-to-surface separation of sterically stabilized

lOnm size particle

1.3.5. Dispersant Structural Guidelines

The polar adsorbing group can be carboxyl, sulfosuccinate, phosphate, phosphoric

acid etc. The polar reactive group may be located at the head of the dispersant or

distributed over the length of a polymer. These coating take up a good deal of volume and

limit the concentration of magnetic solid in the colloidal magnetic fluids as given by

where 9, is the volume fraction of magnetic solid and

cp, the volume fraction of coated particle.

For 6 = 2 m and d=lOm, q, = 0.74 and (p,= 0.27.

The increase in particle packing leads to a rapid increase of colloidal viscosity.

Hence fluids with solid fraction of less than 20 vol% alone can provide flowability.

C h a p t e r I

1.4. Superparamagnetism: Fine particle a n d magnetic properties of ferrofluids

Ferrofluids usually exhibit superparamagnetic behavior [22]. The typical particle

size in a ferrofluid should be about 10 nm, which is smaller than a size of magnetic

domain [2]. Because of the nano-size, magnetic fluidJ contain only single domain

particles. Therefore they have a permanent magnetic moment proportional to their

volume. Hence each particle is permanently magnetized [18]. The particles experience

appreciable thermal translation that maintains them in suspension against the force of

gravity, while in the absence of field, thermal re-orientation of particles results in zero

remanence and coercivity, i.e. ferrofluids are magnetically soft. The magnetic moments of

ferrofluids are much larger than the magnetic moment of a paramagnet and it is for this

reason, ferrofluids are called as supcrparamagnetic [2.23].

If the ferrofluid is to be superparamagnetic, the dipole moment of each particle

should be free enough to rotate. Each single domain particle in the camer bears magnetic

moment 'm' that can freely orient itself in the direction of an external magnetic field 'H'.

Thermal motion tends to disorient parallel alignment of magnetic moments to 'H'. The

effect of these two forces determines the resulting value of magnetization 'M' (magnetic

moment per volume M=mlV) 1241

1.4.1. Magnetic relaxation observed in ferrofluids

Usually two modes of rotation are possible in ferrofluids. The two different

processes help in understanding how the magnetization of these fluids obeys the changes

in external field. If the magnetic moment of the particle is fixed with respect to its crystal

structure, the relaxation will take place by the physical rotation of the particles. Such

particles are called magnetically hard, and the relaxation of magnetically hard particles

will take place by rotation of whole particle. This way of rotation of the particle is called

Brownian rotation and the relaxation processis called as Brownian relaxation [24]. The

Brownian Relaxation time is given by,

Chapter I

where 'Vh' denotes the hydrodynamic volume of the particles (including the surfactant

layer), '11' is the dynamic viscosity of the liquid.

On the other hand, if the magnetic moment is rotating inside the particle relative

to the crystal structure, the rotation is called Neel rotation and the relaxation is called

Neel relaxation. This kind of relaxation of particles, magetically weak, can take place if

the thermal energy is high enough to overcome the energy barrier provided by the

crystallographic anisotropy of the magnetic material. The Neel relaxation time is given by

the expression

where 'K' is the anisotrdpy constant of the particles and 'El' is the Lannour frequency of

the magnetization vector in the anisotropy field of the particle [24].

The Neel relaxation depends on the volume of the magnetic core of the particle,

while the Brownian relaxation is influenced by its hydrodynamic size. In the case of

mono-dispersed ferrofluid the relaxation of magnetization will follow the process of

shorter relaxation time. For smaller particles 'TN' will be smaller than 're' and the

relaxation will take place by the rotation of the moment inside the particle.

Near the critical diameter (transition from Nee1 to Brownian relaxation) the

relaxation takes by a mixture of both given by effective relaxation time.

In the case of ferrofluids some of the particles relax by Nee1 process and some

other by Brownian process due to poly dispersive nature.

Assuming that ferrofluid is a system of non-interacting spherical particles having

thermally agitated magnetic dipoles, the well-known relation deduced by Langevin for

paramagnetic system can be used to describe the dependence of the magnetization of the

fluid 'M' on the strength of the magnetic field 'H'.

Chapter 1

The Langevin argument ' a ' represents the ratio of magnetic interaction energy of

the particle mH to the thermal energy 'ksT' [2].

1.5. Ferrofluid: Historical background

In the early part of the 20Ih century, almost all the efforts of scientists were put

into dealing with the diverse phenomena of the solid matter. The conceptual paradigm of

the solid matter is the picture of a dense periodic lattice and basis of atoms. This

paradigm has been spectacularly successful in describing the properties of the solid matter

and continued to underlie much of the ongoing work [ I , 8-10]. In the mean time, the other

significant progress in science and technology was, the sprouting of the nanoscale science

and technology [25,26]. Nanoscience and technology emphasize materials of small

dimensions, typically IOOnm, which connect the macroscopic and the microscopic areas

of research. Strict control of the chemical and structural perfection of the samples is the

essence of the nanoscale technology. These novel techniques capable of fabricating

nanoscale building blocks have opened up a revolutionary method of exploring material

properties and device characteristics, and the work is expanding rapidly worldwide.

Therefore, nanoscience is a very broad and interdisciplinary area of research. Hence this

brings tremendous common interest of research to chemists, physicists, engineers and

biologists. Generally, the worldwide research in nanoscience is carried out as

nanoparticles, nanostructured materials and nano devices. Among these categories, the

work on the dispersion of nanoparticles in liquids leads to a hybrid of colloidal science

and nanoscience.

There have been many attempts in the past decade to create platform for

technologies to have a colloidally stable ferrofluid, which combine both fluidic and

magnetic properties. New technological demands have motivated the preparation of

ferrofluids. They are stable colloidal suspensions of magnetic particles in a liquid

medium. They respond to magnetic field and thus have found their way into a variety of

applications such as sealing rotating shafts and delivery of drugs in a human being. Early

Chapter 1

in 1779, Gowan Knight tried to disperse iron particles in water, but the mixture did not

have a long-term stability [27]. Following him, Bitter prepared a colloidal solution of

magnetite, which was stable under gravity but became unstable in the presence of

magnetic fields [28]. Elmore, Craik and Griffiths [29-311 produced fine dispersions of

magnetite (Bitter colloid), which was used to detect the boundaries of magnetic domains

in bulk ferromagnetic materials. But as the parti~les~sisize was too large to prevent

sedimentation it did not have the quality of ferrofluid. In 1960's, a particular class of

materials appeared named ferrofluid, which was also called magnetic fluid 1321. A major

breakthrough in developing magnetic fluids was achieved by Papell [32]. In 1965, Papell,

under a grant by NASA, first developed successfully a method for making the magnetic

colloid in kerosene by wet ginding with steel balls in presence of a surfactant (Oleic

acid). Then he synthesized magnetic fluid which can be mixed with rocket fuels so that

fuels can be pumped under zero gravity condition by means of external magnetic tields.

This motivated Papell to develop an additive to missile fuel such that the fuel could be

kept by magnetic means from entraining vapour during pumping [32].

In 1968, Papell was the first to report the fonation of colloid magnetic

nanoparticles dispersions with oleic acid [33]. In the mid of 1960's, Neuringer and

Rosensweig performed a substantial amount of the pioneering work in developing

physic0 chemical aspects of ferrofluid. They were interested in preparing ferrofluids and

used them for magneto caloric energy conversion devices [34-361. Gable and Kerr

employed chemical synthesis, which has a remarkable solubility in water [37]. All these

ferrofluids use a surface adsorbed layer of long-chain surfactant, polymer or protein

molecule to provide a short- range repulsion preventing particles from sticking to each

other.

In the case of magnetite, particle can be prepared from the co-precipitation of

hydroxides from an aqueous solution of ~ e ~ ' and ~ e " in the mole ratio of approximately

1:2 using a base [38]. Khalafalla and Reimer prepared magnetic fluids using the above

mentioned particle by peptization techniques [38]. In 1978, Scholten gave a review of the

chemical and physical problems associated with the preparation of h o f l u i d , which listed

the advantages and disadvantages, associated with different camer liquids 139). Massart

Chapter I

developed ferrofluids in aqueous media that are stabilized by the electric double layer

mechanism [40].

Massart proposed a convenient chemical method of synthesizing ionic ferrofluids.

The fine particles of femte were precipitated and then dispersed in water using an

appropriate particle surface treatment [40]. This methoiwas widely used to synthesize

several ferrite particles and to obtain aqueous and non aqueous ferrofluids [41,42]

Massart, in 1981, studied aqueous colloidal suspensions using surfactants to stabilize the

system known as sterically stabilized ferrofluids. But suspension can also be stabilized by

the presence of a charge on the particles (electrostatically), which are known as charge

stabilized ferrofluid or ionic ferrofluids [40].

Hoon et al. have reported the preparation of colloidal solution of nickel particles

by the irradiation of nickel carbonyl with ultrav~olet light and by the reduction of

nickelocene [43]. Co-precipitation process is an extremely versatile method of producing

femte particles. Particles of different size and magnetic properties may be prepared by

simply controlling the experimental conditions. Massart, Cabuil and his co-workers

showed that the particles of the desired size can be prepared by controlling the mole

fraction ratio of ~ e " : ~ e ~ ' , their concentration, and that of the alkali medium[41,44,45].

Davies et al. studied the effect of precipitation temperature on particle size, the

effect of heating the precipitate in the alkaline medium, and the effect of addition of

surfactant to the reaction mixture [46]. Nakatani and Furubayashi et al. prepared iron

nitride particles by two methods, first by plasma chemical vapour deposition (CVD)

reaction of iron pentacarbonyl with nitrogen gas and second one, by heating a solution of

iron pentacarbonyl in kerosene containing a surfactant in which a constant stream of

ammonia was passed. It might be also possible to prepare alloy particle of the transition

metals by the thermolysis of mixed-metal organometallic compounds in the presence of

surfactants [47,48].

Fujita et al. reported that ferrites have relatively low Nee1 temperature (-100°C)

and thus high thenno magnetic coefficient at these temperatures, wuld be used to study

thennomagnetic convection [49]. Nakatsuka prepared temperature sensitive fluids for

Chapter 1

heat transfer applications [SO]. Misra et al. have studied the preparation of ferrotluid by

reverse micelle technique [S 1-53].

Currently, various magnetic fluids were prepared by different techniques using

various solvents for different technological applications. These magnetic fluids are also

known as ferrofluid.

1.5.1. Advancement in Co-Zn substituted ferrites for preparation of ferrofluids

The physical properties of Co -Zn nanoparticles are of current interest due to their

size-dependent behavior observed in the nanometer length scale, high crystallinity and

temperature sensitive application. Several groups of workers reported thc production of

cobalt zinc particles. Thomas et al. reported preparation of a very narrow size particle

ideal for the production of stable colloids [54]. Hess and Parker did detailed investigation

in the presence of different copolymers of acrylonitril-styrene in the reaction solution

showed that greater the concentration of polar groups along the polymer chain, the

smaller the size of the particles formed [5S]. Smith reported that the reaction is polymer-

catalyzed, which results in the formation of a polymer-carbonyl complex which then

decomposes to form elemental cobalt [56]. Anantharaman et al. has studied acicular

gamma fenic oxide containing cobalt by using oxalate precursors method [57]. Papirer et

al. showed that the decomposition could also be catalyzed by the presence of surfactants

[58]. Charles and Well reported that by selecting an appropriate surfactant, the size of the

particle can be controlled effectively [59]. They found that the saturation magnetization

of the particles to be consistently lower (75%) than that of the bulk value of 17900 Gauss

(l.79T) [S9]. Measurements of the magnetic anisotropy of cobalt particles have been

undertaken by several groups of workers [59, 601. Lattice constant for Co-Zn femtes, has

been reported by Jin-Ho Lee et al. [61]. Yeong 11 Kim et al. have synthesized CoFe204

magnetic nanoparticles by temperature controlled co-precipitation method [62]. Shi-Youg

Zhan et al, have reported the surface modification by surfactant adsorption (Oleic acid) on

Fe304 and CoFezOl [63]. Wagener and Blums have also reported the preparation of

stable colloidal based oil [64]. Massart has prepared the ionic based manganese and

cobalt ferrites [65]. Dey and Ghose has reported the preparation of the Coo.zZna.sFe204

fine particles by chemical co-precipitation method followed by sintering [66]. Jeyadevan

Chapter 1

has successfUlly prepared nearly monodispersed single-domain cobalt femte particles,

which could be used for the high-density recording media [67]. Morais et al, reported the

possibility of controlling the size of nanoparticla using different stirring speeds 1681.

Temperature sensitive magnetic fluid having Coo 3Zm 7Fe204 particles was used for the

study of thennal convection [49]. Fannin et al. showed the preparation of diester based

ferrofluid with Coo7Zm 1Fe204 nanoparticles [69]. Arulmlylgan et al, have synthesized

Col.,Zn,Fe204 nanoparticles with x varying from 0.1 to 0.5. [70]. Gul and Maqsood have

studied the influence of Zn-Zr ions on physical and magnetic properties of co-

precipitated cobalt ferrite nanoparticles [71]. Park et al. has studied size-controlled

magnetic nanoparticles with lecithin for biomedical applications [72]. Abu Bakar reported

liquid-liquid phase-transfer synthesis of magnetite nanocrystals affording enhanced size

reduction[73]. Structural, magnetic and electrical properties of Col-,Zn,Fe204

synthesized by co-precipitation method has been studied by Gul 1741. Giovanni

Baldi et al. has reported the control of the particle size and surface state and their effects

on magnetic properties of cobalt femte nanoparticles [75]. Upadhyay and his co-workers

have reported the effect of size and synthesis route on the magnetic properties of

chemically prepared nanosize ZnFe204 1761. Yimin Xuan, Qiang Li and Gang Yang have

reported on the synthesis and magnetic properties of Mn-Zn ferrite nanoparticles [77].

1.6. Ferrofluids: application

In the recent years there has been an increased interest in the application of

ferrofluids. Magnetic fluids are an interesting group of liquids because they have liquid

properties and act like ferromagnetic material. The applications of ferrofluids are often

based on their controllability by an external magnetic force in addition to their fluid

properties. Thus, a number of different applications of ferrofluids in various fields have

been investigated.

The magnetic control of ferrofluids forced strong efforts in the design of

applications using the influence of a magnetic fluid. Berkovsky et al. have used to

Position the fluid inside a technical device, which leads to several applications [78]. Some

of them reached commercial importance and are widely used in day-to-day life. For

example, a loud speaker consists mainly of a membrane connected with the voice coil

Chapter I

being located in the magnetic field of a permanent magnet system. For high power

loudspeakers or speakers with a reduced size as they are used in car Hi-Fi systems, the

ohmic heat produced in the voice coil leads to a critical limitation of the maximum power

of the speaker. Placing a ferrofluid in the magnetic field around the voice foil increases

thermal conductivity in this region and enables thus an increased heat transfer to the

speaker structure. This increases the cooling p ~ s s i b i l i t i e s ~ d connected with an increased

cooling, there by, the maximum power of the system can be increased [79,80]. Besides

this for thermal application, Raj et al. reported the application of ferrofluid in D'Arsonval

galvanometer, stepper motors and ferrofluid sensors [81]. John Philip et al. showed that

the ferrofluid could be used as a new optical technique for the detection of defects in

ferromagnetic materials 182,831. Ferrofluids are also used in mechanical systems like

bearings as frictionless sealing [78]. Charles in his invited paper reported the use of

ferrofluid as magnetic ink [84]. Childs et al. explained use of magnetic fluid to grind the

surface [85]. Recently, ferrofluids have also been used in biomedical field. Biological

application using maghemite ferrofluids: magnetic cell sorting and magnetocytolysis were

reported by Roger et al. [86]. Especially, treatment of cancer by magnetic hyperthermia

[87,88], magnetic drug targeting 1891 eye surgery [90] are gaining much attentilatior. in

recent days.

1.7. State of problem and motivation

From the literature survey, it is clear that synthesis and characterization of

transformer oil based ferrofluid has not been studied extensively for different types of

ferrofluid. This thesis projects to address the problem of preparation of transformer oil

based ferrofluid, which could be a possible candidate for solving the heat transfer and

thermo magnetic problems.

1.7.1. Ferrofluids for heat transfer enhancement

Heat transfer in a fluid having a temperature gradient is accompanied by natural

convection caused by hon-uniform density distribution (Archimedes force or Buoyant

force). In the case of ordinary fluids the more dense layers tend to move downwards

displacing the less dense layers. But thennoconvective motion does not develop, if denser

layers are at the bottom. Similarly in the magnetic fluids, the layers with higher

Chapter i

magnetization tend to move in the direction of magnetic field gradient, provided the

layers with less magnetization are in this direction. But if the magnetization increases in

the direction of field gradient, magnetically induced convection for thermal dissipation

will not be possible [16]. The driving force and conditions (both temperature and

magnetic field should oppose each other) for thermomagnetic convection are depicted in

Fig.l.5.

Fig.l.5 Conditions for thermomagnetic convection

When a magnetic fluid, whose magnetization strongly depends on temperature

(temperature sensitive magnetic fluid), is heated in a non-uniform magnetic field and the

gradient of magnetic tield intensity coincides with the temperature gradient, a

destabilizing magnetic body force appears in the fluid. 'This force can drive a convective

flow that is purely controlled by the strength and direction of the applied magnetic

field 1911.

If a volume element of the fluid with temperature 'T' and Magnetization 'M' is

displaced adiabatically to a position 'TI' and 'MI' in the direction of the temperature

gradient, it will find itself in a region of high temperature, T P T and lower magnetization

MI<M. This is because the direction of magnetization gradient is opposite to that of the

temperature gradient (shown in the Fig.l.5). The volume element will be surrounded by

hotter fluid and 'MI' will be less than 'M' (as a rule, aMlaT<O). Therefore the volume

element will have higher magnetization than the surrounding and will thus experience a

force in the direction of initial displacement [24,92].

Chapter I

Thus, if the vectors 'VH' and 'VT' are parallel, any displacement of the fluid

element from the equilibrium position will result in a force tending to the displacement of

the fluid from the equilibrium position and promote the development of convective

motion in the fluid. Magnetically induced convection for thermal dissipation can be

achieved only in the case of magnetic fluids whose mamizat ion changes considerably

in a temperature gradient, provided the magnetic field intensity is maximum at the high

temperature region (as seen in Fig. 1 .S.). Normal magnetic fluids cannot serve this purpose

ind requires explicitly temperature sensitive magnetic fluids.

1.7.2. Temperature sensitive magnetic fluid

For different technological applications, one has to synthesize magnetic fluids

with varying physical properties. Thermomagnetic energy conversion using ferrofluids

proposed during the early stage of research has not yet been fully realized because of the

need for improved material properties [2]. In order to use ferrofluids for heat transfer

application or energy conversion devices, it is necessary to use a fluid with a large

pyomagnetic coefficient (dM1aT)~ i.e., fluid with a high saturation magnetization and a

low Curie temperature [93].

The particles, which are commonly used for the preparation of magnetic fluids,

are magnetite (FelOh), maghemite (yFezO1) substituted femtes etc., [79,94-1201. These

particles have high saturation magnetization and Curie temperature higher than the

boiling point of the carrier liquid. Hence in order to prepare a temperature sensitive

ferrofluid, particles having Curie temperature lower than the boiling point of the carrier

liquid and relatively high magnetization are required. This is because the change in

magnetization with respect to temperature is large near the Curie temperature region. It is

well known that for the inverse spinels ( ~ e ~ ~ ) * [ ~ e ~ ~ ~ e ~ + ] ~ 0 4 with Me2+ = Fez+, co2*,

~ i " and CU*' have a C.urie temperature (Tc) between 728 K and 858 K. Due to strong

'AB' and 'BB' interactions compared to 'AA' interactions the Curie temperature

depends on the respective exchange integrals 'J' as

Chapter I

where 'ke' is the Boltzmann constant, 'n,,' is the n u m a of nearest exchange-coupled

neighbors and 1 S lis the absolute spin value [121]. For Fel04, the change in

magnetization with temperature was found to be large between 500-700 K.

Hence Fe304 cannot be utilized practically for energy conversion or thermal

dissipation purpose, as the boiling point of most of the camer liquid is lower than

473 K [121].

In the case of MnFe204, 80% of the ~ n ~ ' ions occupy the .A' site while the

remaining 20% occupies the 'B' site, t h ~ s results in the reduction of the exchange

interaction between 'MnA' and 'Fee' by a factor of 2 compared to 'FeA-Fee' interactions.

This will reduce the Curie temperature of the bulk MnFezOd, which is 585 K. But the

temperature range, which can be practically utilized for application, is very small

(between 373-573 K). Now if the Mn ion from the 'A' site is replaced with a non-

magnetic ion such as Zn2', the Curie temperature will be further reduced. The saturation

magnetization may increase provided the cation distribution is not altered [93]. Zinc

substituted mixed fenites are attractive because Zn2' substitution alters their magnetic

parameters over a wide range.

Preparation and properties of temperature sensitive magnetic fluid having

MqZnl.,Fe204 fine particles, with Me=Mn2+, x=0.5 and Me=~e", ~ ~ 0 . 5 and 0.7 are

reported [121-1271. Upadhyay et al, have reported Gd substituted Mn-Zn ferrite for the

preparation of temperature sensitive magnetic fluid [128]. Zins et al. have reported new

aqueous magnetic fluid.having Nil.,ZnxFezOd particles with x varying b m 0.1 5 to 0.66

[129]. Temperature sensitive ferrofluids having different Zn2+ substitution in mixed

fenites ( M @ , , c ~ . ~ Z n o ~ F q o ~ : Me=h4nZ', Fez', co2', cu2* and M ~ Z ' , Ni0.2Cao.1Zno.6Fe204

C h a p t e r I

and Me0 ,Zna,Fez01: Me = ~ e " , co2', ~ i ~ ' and CU") have heen used for the study of

thermal convection [49].

1.8. Scope of the work

The aim of the thesis is to prepare ferrofluids W i n g Co-Zn substituted ferrite

nano-particles which can be used for heat transfer enhancement making use of the

magnetically induced convection and to study the effect of zinc substitution on the

particle size and magnetic properties of fine particles. Literature survey on Co-Zn ferrite

nano-particles reveals that the preparation conditions and properties of Co-Zn femtes

have not yet been studied exhaustively. They almost do not touch the problem of

synthesis and characterization of these ferrofluids. Therefore, a detailed systematic study

on the preparation of Co-Zn fine particles, the effect of zinc substitution on the magnetic

properties of fine particles, study on thermomagnetic co-efficient and the magnetic

properties of the fluids becomes mandatory.

The thesis entitled "Synthesis and characterization of Co-Zn ferrite

nanoparticles used for ferrofluid preparation" consists of seven chapters and the

contents of each chapter are as follows:

The first chapter provides a review of the important developments in both

preparation and application aspects of ferrofluid. It includes a brief introduction to

ferrofluid dealing with the preparation aspects, size restriction imposed by the stability

requirements, magnetic materials used and their magnetic properties. Purpose and task of

the thesis with justification for the samples chosen and ferrofluid for heat dissipation

using the magnetically induced convection are discussed.

The second chapter deals with the theoretical concepts of the various analytical

tools employed for the characterization of Co-Zn substituted fenite nanoparticles with x

v%ng from 0 to 1.0 are discussed.

Chapter 1

The third chapter of the thesis elaborates the experimental techniques used for

the characterization of fine particles and ferrofluid. The following experimental

techniques have been used.

X- ray difiaction was used for structural analysis and estimation of particle size.

Powder-X software was used for indexing and r e f in i e of cell parameters. Average

crystallite size for each composition was calculated using the Debye-Scherrer formula.

The full-width at half maximum values was estimated for the indexed peaks after

deconvoluting. For this peakfit software was used.

Room temperature magnetic measurements with a maximum magnetic field of

1194.15kAim were carried out using a Lakeshore vibrating sample magnetometer (VSM)

(model 7404). The parameters like specific saturation magnetization (M,), coercive force

(H,) and remanence (MJ were evaluated for Col.,Zn,Fe204 nanoparticles.

The magnetic parameters of transformer oil based Co1.,Zn,FezO4 ferrofluid with x

varying from 0 to 0.7 were canied out using pulse field technique and the specific

saturation magnetization of the fluid samples were estimated.

Estimation of co2' ,2n2* and ~ e " in the final product was carried out using ICP

Analyst 5.2 Ultima 2 (JY Jobin Yvon Horiba). The particles were dissolved using

concentrated HCl and carefully diluted so that the dilution was well within the linear limit

depending on the sensitivity for the estimation of the respective cation.

FTIR spectra were recorded for dry samples (uncoated and coated with oleic acid)

of Col.,Zn,Fe20r with x varying from 0 to 1.0 with an ABB BOMEM 104 FTIR (range

400-4000m-') spectrometer. The dry samples were mixed with the KBr matrix, and

spectra were measured according to transmittance method. The spectra were resolved

with a resolution of 4cm". For spinel structure an attempt has been made to identify some

Me-0 vibrations as well as vibrations to indicate the presence of the water adsorbed on

the particle surface. To understand the adsorption mechanism of the oleic acid on the

surface of the cobalt nanoparticles, FTIR measurements were also carried out on pure

Chapter I

oleic acid, transformer oil and for transformer oil based Co1.,Zn,Fe204 ferrofluid with x

varying from 0 to 0.7.

EPR measurements were canied out using JEOL JES-TE 100 spectrometer having

X-band frequencies (9GHz). EPR spectra were recorded at room temperature (RT) at

300K and at liquid nitrogen temperature (LNT) at 77K fonhe dry samples (uncoated and

coated with oleic acid) of Col.,Zn,Fe~04 with x varying from 0 to 1.0. The powder

samples containing in 3mm diameter quartz tubes were cooled to 77K using liquid

llitrogen dewar insert loaded in the cavity. The cooling was done at different values of the

applied magnetic fields. The spectrum was the first derivative microwave absorption with

respect to f i e l d ( d ~ / d ~ ) . For each sample, the value of peak-to-peak line-width (AH,,)

was computed as the difference between the extreme values 'Hi' and 'H2' of the magnetic

field (the maximum and minimum of the resonance curves, respectively). The resonant

magnetic field (H,) was computed as (Hl+H2)/2. Similarly the EPR spectrum was also

recorded at room temperature (RT) at 300K and at liquid nitrogen temperature (LNT)

77K for the transformer oil based Col.,Zn,Fe204 ferrofluid with x varying from 0 to 0.7.

The value of peak-to-peak line-width (AH,) and the resonant magnetic field (H,) was

computed.

Simultaneous differential thermal analysis and thermogravimetry (DTA-TG) were

carried out on the dried precipitate by using a thermal analyzer (NETZSCH STA 409).

Samples were heated from room temperature to 1273K at a heating rate of S0C/min. The

associated water content was estimated by TG analysis by monitoring the weight of the

sample when heated to a maximum of 1273K (rate S°C/min.) in a nitrogen atmosphere

(NETZSCH STA 409). The water content was estimated from the weight difference

measured from room temperature to 1273K.

The fourth chapter describes the synthesis and characterization of Co-Zn ferrite

nano particles. Details about the preparation technique used f w the precipitation of fine

particles, ferrofluid preparation, are described here. The preparation technique includes

the effects of pH, initial molar concentration, and temperature. Ultra fine particles of

COI.,ZII,F~O~ with x varying from 0 to 1.0 were prepared by co-precipitating aqueous

Chapter I

solutions of CoC12,ZnCl2 and FeCll mixtures respectively in alkaline medium. The mixed

solution of CoC12, ZnC12 and FeCI, in their respective stoichiometry (100 ml. of 0.5 M

CoC12, 100 ml. of 0.5 M ZnCI2 and I00 ml. of 2 M FeCI, in the case of Coo ~Znci 5Fe204

and similarly for the other values of x) was prepared and kept at 60 '~ . This mixture was

added to the boiling solution of NaOH (0.63 M dissolved in 1200 ml. of distilled water)

within 10 seconds under constant stirring. 4

Nano ferrites were formed by conversion of metal salts into hydroxides, which

took place immediately, followed by transformation of hydroxides into femtes. The

solutions were maintained at 95'C for one hour. This duration was sufficient for the

transformation of hydroxides into spinel ferrite (dehydration and atomic rearrangement

involved in the conversion of intermediate hydroxide phase into ferrite).

In addition the results obtained from the XRD, VSM. FTIR, EPR and DTA-TG

(STA) are also discussed. The amount of zinc concentration used in partial substitution

with Co has a definite impact in the ferrite formation and its properties.

The XRD pattern has indicated the formation of ferrite nano particles for Co-Zn

ferrite. All the peaks in the diffraction pattern were indexed and the refinement of the

lattice parameter was done using Powder-X software. The average clystallite size for each

composition calculated using the Debye-Scherrer formula was found to decrease with the

increase in the zinc concentration. The lattice constant increased with the increase in zinc

substitution.

The percentage of zinc affected the associated water content. The water content

varied from 14.78% to 22.52% in the case Co-Zn ferrite. The loss of water content in the

sample was found to be maximum around 300 to 700K. This indicated the presence water

molecules chemically adsorbed to the magnetic particle surface (associated water

content). Further from 700 to 1273K the loss of water content was small.

The formation of fcnites was in accordance with their initial stoichiometry. The

ratio of ( ~ e ~ * ) / ( ~ e ' + ) initially taken was 0.5 and the ratio obtained form the final product

varied form 0.507 to 0.484 in the case of Co-Zn ferrite. Co-Zn fenites did not deviate

Chapter I

(within the allowed experimental errors including estimation of water content, dilution

etc.,) from their initial stoichiometry and matched well with the initial degree of

substitution. It is interesting to note that the initial and final Zn concentration do not

deviate and the preparation condition completely favors the formation of femtes allowing

us to study the effect of Zn substitution on the properties of the femtes. 9

The DTA curves for Col.,Zn,Fe20~ with x varying from 0 to 1.0 show an

endothermic peak around 350K, which confirms the presence of water, content in the

Col.,ZnxFe20~ samples. The broad hump at around 800 to 1200K may correspond to

complete crystallization in the cubic spinel phase (formation of femte).

Chapter five gives the magnetic properties of the fine particles. The room

temperature magnetization and the temperature dependence of magnetization of the

powder samples are estimated and discussed.

The magnetic parameters like specific magnetization (Ms), remanence (M,) and

coercivity (Hc) of the prepared powder samples (Co-Zn substituted femtes) measured at

room temperature in a maximum field of 1194.15kAlm showed a strong dependence on

the Zn concentration. The change in magnetic properties such as Ms, Hc and Curie

temperature was due to the influence of the cationic stoichiometry and their occupancy in

the specific sites.

For particles, the variation pattern of specific saturation magnetization (M,) as a

function of Zn content shows an increase for small substitutions, reaches a maximum

value of 46.55 Am2/kg at 1 194.1SkA/m for x=0.1 and then decreases. The particles do not

show any saturation for x=0.9 and 1.0. even at 1194.1SkAim and it almost behaves linear.

The remanence (M,) and coercivity (H,) for Col.,ZnXFezO4 with x varying from 0 to 1.0

decrease with the increase in zinc substitution.

Temperature dependent magnetization was measured at 8 O W m for

Co1,Zn,Fe~0~ with x varying from 0 to 0.7. The Curie temperature decreases with the

addition of zinc. This is due to the replacement of more non-magnetic ion (zn2+) instead

of co2+ in the A site. The Curie temperature for the samples xM.7 cannot be obtained

Chapter 1

because of the low magnetic volume force when dispersed in a camer liquid, particles

with x > 0.7 are not considered. Thermomagnetic coefficient k~ ( k ~ = -AMIAT) is

calculated from the first derivative of the temperature dependent magnetization curve for

each concentration.

m The practical applications of ferrofluid are decided considering the stability of the

fluid and the vapor pressure of the camer liquid at the operating temperature. Ferrofluid

can be operated at a temperature of around 423K (150°C) without losing much of the

stability. Ferrofluid having the maximum value of thermomagnetic coefficient around this

temperature range can be utilized for energy conversion applications. Simultaneous

reduction in the Curie temperature and higher value of thermomagnetic coefficient shown

by Co-Zn ferrites will enable us to prepare temperature sensitive ferrofluid for practical

applications

Preparation and properties of ferrofluid having Co-Zn fine particles are given in

sixth chapter. Procedure for preparing low viscous transformer oil based ferrofluid with

the role of pH, mode of adsorption while coating of the surfactant is discussed. In

addition preparation of temperature sensitive ferrofluid, based on the value of

thermomagnetic coefficient of the prepared particles is also given. Such temperature

sensitive ferrofluid can be used for energy conversion application.

The pH of the solution was reduced to ~ 1 0 . 5 for effective coating of the

surfactant. Oleic acid (ClaH3402) was used as the surfactant and coating of surfactant was

carried out at a temperature of about 80°C. To coagulate the oleic acid coated particles,

dilute HCI was added. After decantation, the coated particles were dispersed in

transformer oil and centrifuged.

The results obtained from the magnetization of the fluid samples carried out by

pulse field technique to. a maximum field of lkOe at room temperature are discussed.

FTIR measurements were canied out on pure oleic acid, transformer oil and for

transformer oil based Col.,Zn,Fez04 ferrofluid with x varying from 0 to 0.7. EPR

spectrum was also recorded at room temperature (RT) at 300K and at liquid nitrogen

temperature (LNT) 77K for the transformer oil based Col.,ZnxFe2O4 f m f l u i d with x

Chapter I

varying from 0 to 0.7 and the value of peak-to-peak line-width (AH,) and the resonant

magnetic field (H,) were computed. The results are discussed.

SANS measurements were carried out on transformer oil based Col.,Zn,Fe~O~

ferrofluid with x varying from 0 to 0.7. The particle s i z w a s calculated using the core

shell program. The variation of core particle size with different Zn substitution has been

given.

In the case of magnetically responsive fluids, the viscosity of the fluid depends not

only on the carrier fluid, but also on the number concentration and temperature. In

addition, the presence of an external magnetic field includes a change in the viscosity of

ferrofluid. By the application of a sufficiently large field, the orientation of the particles

can be fixed, thereby altering the viscosity. It is attempted to study the variation of

viscosity of a ferrofluid in the presence of an external magnetic field. Viscosity studies

have been carried out using a Brookfield LV DV-111 ultra programmable rheometer.

Modification has been made in the spindle to make it non-magnetic. Instead of stainless

steel spindle, aluminum spindle (non-magnetic) has been utilized and variation in

viscosity has been studied.

The salient features of the experimental results with the summary and conclusion

are given in the seventh chapter.