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Chapter I
Introduction
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Chapter 1
INTRODUCTION
INTRODUCTION:
During the past decade and a half the production and use of Nano materials
has established a foot hold. The term nanotechnology is employed to describe the
creation and exploitation of materials with structural features in between those of
atoms and bulk materials, with at least one dimension in the nanometer range (1nm =
10-9
m).The Scientist and Technocrats have well realized that the use of Nano sized
materials have not only helped in the production of compact and smaller machines
and equipment’s, but has also relaxed the strain on the fast depleting of the limited
resources. It has been further recognized that the ultrafine particles have properties
which are different form their counterpart.
Magnetic nanoparticles are of great interest in recent years due to their
extensive use in the technological and chemical applications. Among these, spinal
ferrites have attracted considerable attention due to their useful electrical and
magnetic properties and applications in several important technological fields.
About for half of the century ferrites have been established as new category of
magnetic materials. Research and development continue to take place in many new
theories, synthesis methodologies; characterization and analysis techniques are
currently under development in the field of ferrites to be used in ever widening range
of applications. Generally the term ferrite is referred to all magnetic oxides containing
iron as a major component. They have a general chemical formula MFe2O4 [M = any
divalent cation (Zn, Cu, Ni, Co, Mg, Fe etc.) [Ramesh and Spaldin 2007].Ferrites are
considered as advance materials for their crucial role as pace setters and the role they
found in pushing the development of civilization at a great pace. [Santos, Costa et al.
2009]Spinal ferrites are considered as important catalysts for a number of industrial
processes such as in ammonia synthesis, Fisher-Tropsch, dehydrogenation of
butylene [Li, Wang et al. 2014; Rennard and Kehl 1971] and decomposition of
alcohols and H2O2[Lahiri and Sengupta 1991].
Nanocomposites include multiphase solid materials wherein one of the phases
has a dimension of less than 100 nm. The mechanical, electrical, optical,
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electrochemical, and catalytic properties of the Nano composite will differ markedly
from that of the component materials. Other kinds of Nano particulates may result in
enhanced optical properties, dielectric properties or mechanical properties such as
stiffness and strength.
In the recent years, cumulative consideration has been paid in the area of
Nanocomposites magnet [Asti, Solzi et al. 2004; Erokhin, Berkov et al. 2012]as it
delivers an integrated system comprising of components whose properties are
harmonizing to each other [Roy and Kumar 2013]. One such dynamic field of
research is the exchange spring magnet [Uzdin, Vega et al. 2012; Kneller and Hawig
1991; Shield, Zhou et al. 2006; Zhou, Skomski et al. 2005; Suess, Schrefl et al.
2005], where high saturation magnetization of the soft and the high magnetic
anisotropy of the hard magnetic phases are exchange coupled in the Nano metric
scale.
One of the fascinating properties of ferrites is the possibility to prepare
different compositions and thereby alter the magnetic properties. One of the
challenges is to improve the magnetic properties of soft ferrites such as saturation
magnetization, magnetic hysteresis, demagnetizing force and anisotropic energy.
Researchers are trying to produce hard and soft ferrites by using simple methods. In
view this, many studies have focused on new systems, such as
CoFe2O4/ZnFe2O4[Masala, Hoffman et al. 2006], earth-iron-boron [Maeda, Sugimoto
et al. 2004] and Fe/Z-type ferrite [Liu, Itoh et al. 2006].The results suggest that
coupling exchange exists between the nanoparticles and the interaction significantly
influences magnetization and coercivity of the composite powders. [Masala, Hoffman
et al. 2006], they reported that exchange interaction between hard and soft magnetic
phases improve the microwave absorption and magnetic properties of Nano
composites.
Recently, due to development of electronic technology, the trends of
miniaturization and excellent electromagnetic properties are the utmost requirements
of materials to be used for different purpose and these have been and are being
fulfilled by the materials called Composites [Grössinger et al. 2008; Goldman,
Gardner, Moss et al. 1966]. For few years extensive research has been carried out on
Multiferroic (MF) composite materials [Ma, Hu et al. 2011; Ramesh and Spaldin
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2007] which have been under the focus of researchers due to their potential
applications in electronics technology (as magnetic–electric sensors in radio-
electronics, optoelectronics, microwave electronics and transducers). In MF materials,
magnetic and electric orders coexist simultaneously and the coupling between spin
and charge degrees of freedom gives rise to a wide range of magneto electric
phenomenon [Eerenstein, Mathur et al. 2006; Fitchorov, Chen et al. 2011]. The
control of polarization by applying magnetic fields or the magnetization by applying
electric fields, which is known as the magneto electric (ME) effect, appears in the
materials when the electric polarization and magnetic orders are coupled to each other
[Verma and Negi 2010; Verma and Kotnala 2011].
The ME effect can also be given as direct ME effect which is characterized as
magnetic-field-induced polarization and electric-field-induced magnetization, respectively
[Chu, Martin et al. 2008] .The different types of single-phase Multiferroic such as BiFeO3
[Chu, Martin et al. 2008], TbMn2O5 [Hur et al. 2004], BaTiO3-CoFe2O4 [Agarwal et al.
2012], 0.62Pb (Mg1/3Nb2/3)O3-0.38PbTiO3, Ni47.4Mn32.1Ga20.5/PZT [Wang et al. 2010] etc.
are investigated in literature. Mostly these MF systems are extensively studied and they
are the focus of current research because of the advancement in every field. To overcome
the scarcity of single-phase Multiferroic, one approach is to enhance the specific
characteristics by doping or the other is the development of new Multiferroic materials
such as ferroelectric- ferromagnetic. However the composite of ferrite such as NiFe2O4,
NiZnFe2O4 and CoFe2O4 etc. with Perovskite such as BaTiO3, PbTiO3 and CaTiO3 is of
technological importance. Because these ferrites based composites are results in
Multiferroic properties of higher magnetization in spintronics devices. Also the electric
behaviour of ferrites is highly usable in high frequency based devices.
1.1 Ferrite:
Based on the magnetic properties of high or low coercivity ferrites are
classified as soft and hard ferrites. Ferrites can be classified according to crystal
structure—that is, cubic vs. hexagonal ferrite or magnetic behaviour; that is, soft vs.
hard ferrite. Soft magnetic materials exhibit magnetism only when they are exposed to
a magnetic field, while hard magnetic materials retain magnetism when they are
removed from a magnetic field. Soft ferrites are easy to magnetize and demagnetize.
Hard ferrites are hard to magnetize and demagnetize. Hard magnetic materials are
commonly used for permanent magnetic applications [Srivastava and Yadav 2012].
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More commonly it can be seen that magnetic heads of a tape deck are made up of
magnetically soft material, and the tape is made of magnetically hard material.
Because of low price and very good chemical stability ferrites are included in the
most important magnetic materials which cannot be easily replaced. One of the
factors used to express the properties of a magnetic material is coercive force. Ferrite
materials are broadly divided into those that do not have coercive force and those that
have high coercive force. Both soft and hard ferrite can store powerful magnetic
energy internally and play key roles in a wide range of electronic circuits and
electronic devices. There are many metallic ferromagnetic materials with strong
magnetic force, but ferrite is a type of ceramic, which means it has high electrical
resistance and maintains its excellent properties even when used with high-frequency
signals [Ohashi et al. 1993].
1.1.1 Soft ferrites:
Ferrites that are used in transformer or electromagnetic cores contain nickel,
zinc, or manganese compounds. They have a low coercivity and are called soft
ferrites. Due to their comparatively low losses at high frequencies, they are
extensively used in the cores of switched-mod power supply (SMPS) and radio-
frequency (RF) transformers and inductors [Srivastava and Yadav 2012].
Noteworthy is the recent rapid increase in the production of soft ferrites used
in transformers for switching regulators. Soft ferrites, compared with magnetic
metals, have such advantages as high electric resistivity, excellent magnetic properties
in the high frequency region, and superior corrosion resistance, but also such
disadvantages as low saturation magnetic flux density, low Curie point, and inferior
mechanical properties. The application of soft ferrites may be divided into two main
fields, one is the field where high permeability and low power loss are required as
represented by Mn-Zn and Ni-Zn ferrites with less than 300 MHZ, while the other is
the microwave region of 300 MHZ or higher where magnetic resonance is involved.
MN-Zn ferrites are used in a frequency region of several megahertz or less as
transformers for SWRs, flyback transformers and communication coils. Ni-Zn ferrites
on the other hand, are used in such applications such as rotary transformers at
frequencies higher than for Mn-Zn ferrites, and as intermediate-frequency
transformers and coils[ Ohashi et al. 1993].The soft ferrites are particularly important
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since they are relatively inert and their properties can be tailored by chemical
manipulations. The magnetic characteristics of ferrites are strongly affected when the
particle size approaches the critical diameter below which each particle is a single
magnetic domain. The quantum size effects of the large surface area of these
nanometer ferrite particles dramatically changes some of the magnetic properties &
inhibit quantum tunnelling of magnetization .They have boosted up new electronic
technology and are widely used in electromagnetic cores of transformers, switching
circuits in computers and for motors and generators. Ferrites of Ni, Zn, Li, Mn, and
Cu as individual or in mixed compositions do have less value of coercivity causing
low hysteresis loss at high frequency, so are the best material for new technology.
Magnetic soft materials have low coercivity and also low value of remanent magnetic
induction Mr.
1.1.2 Hard ferrites:
In contrast, permanent ferrite magnets (or hard ferrites) which have a high
remanence after magnetization are composed of iron and barium or strontium oxides.
In a magnetically saturated state, they conduct magnetic flux well and have a high
magnetic permeability. This enables these so-called ceramic magnets to store stronger
magnetic fields than iron itself. Hard ferrites have a hexagonal structure and can be
classified as M-, W-, X-, Y-, and Z-type ferrites.
It is an integral property associated with material like Barium, Strontium
ferrite having a characteristic feature of having high value of retentivity and
coercivity. They retain magnetization even when magnetic field is taken off, so after
considered as permanent magnet. Hard ferrite magnets have a wide variety of
applications: Speakers magnets, DC Motors, Sweepers, Magnetic separators for
ferrous materials, Automotive Sensors, MRI’s, and Reed Switching, Hall Effect
devices, Refrigerators and Arts and Crafts as well as many other novel applications.
Magnetic hard ferrites have wide hysteresis loop and coercivity Hc > 2.5 kA/m. They
also express high value of remanent magnetic induction Mr and high value of
maximum energy product (BH) max. These ferrites with hexagonal structure and
strong magneto-crystalline anisotropy are suitable for producing of permanent
magnets.
1.2 Origin of thesis:
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The particles or grains in Nano size play a vital role in the improvement of
properties of Multiferroic materials as compared to bulk i.e. low leakage current and
show dielectric response up to higher frequency region apart from magnetic
properties. With the increasing demand of miniaturized/smaller, faster electronic
devices, sensitive detectors for biomedical and environmental applications, it has
become a necessity to synthesize materials in Nano range < 100 nm.
1.2.1 Importance of soft ferrites:
Nickel ferrite is an important member of the spinal family and it is found to be
the most versatile technological material switched for high frequency application due
to its high resistivity [Albuquerque et al. 2001].Spinal ferrites are good dielectric
materials and they have wide applications ranging from microwave frequency to radio
frequency. Nickel zinc ferrite nanoparticles have potential technological importance
in different applications such as storage media, biomedical fields, and high
performance microwave devices because of their high resistivity, high Curie
temperature, chemical stability, and good soft magnetic properties even at high
frequencies [Tsay et al. 2000; Harris 2012].
Generally, the high resistivity ferrite is possible by having very small size
nanoparticles, responsible for higher frequency dependent of dielectric properties
reasonably involving superparamagnetism which lower its magnetization. Therefore,
the large surface to volume ratio of ferrite nanostructures (Nanorods, nanowires etc.)
exhibits unique properties such as spin canting, surface anisotropy, high resistivity
etc. which may recover the required limitation of dielectric and magnetic properties.
Therefore, Nano size, high purity and uniform distribution of particles are
essential to get enhancement in tailoring various properties including ferromagnetic as
well as electrical properties of ferrites with low preparation cost and small device size.
Recently one of the challenges is to improve the magnetic properties of soft ferrites
such as saturation magnetization, magnetic hysteresis, demagnetizing force and
anisotropic energy.
For soft ferrite synthesis in the thesis work,, among various methods a simple
chemical combustion route and hydrothermal route are employed to synthesize
various compositions of pure and zinc substituted nickel ferrite nanoparticles using
Poly-ethylene glycol as the reducing and chelating agent, which neither requires
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sophisticated instrument nor high sintering temperature and ethanol water mixture as
solvent cum surfactant in an autoclave.
1.2.2 Importance of Hard Ferrites:
Researchers are trying to produce hard and soft ferrite using simple methods.
In view this, many studies have focused on new systems, such as
CoFe2O4/ZnFe2O4[Masala et al. 2006], earth-iron-boron [Maeda et al. 2004] and
Fe/Z-type ferrite [Liu et al. 2006].The results suggest that coupling exchange exists
between the nanoparticles and the interaction significantly influences magnetization
and coercivity of the composite powders. Masala et al. [2006] reported that exchange
interaction between hard and soft magnetic phases improve the microwave absorption
and magnetic properties of Nano composites.
[Shen et al. 2012] investigated the magnetic properties of SrFe12O19/Ni0.5Zn0.5
Fe2O4 Nano composite in their research. They have pointed to this thread that the
Nano composite magnets combining a high saturation magnetization of the soft phase
and high coercivity of the hard phase will be recognized as the next generation of
permanents.
Recently, the demand for increasing information density and signal-to-noise
ratio and allowing writeability, for e.g. exchange - coupled composite media,
composite granular continuous media and percolated media [Verma and Kotnala
2011a], a composite of soft/hard ferrite layer proposes excellent properties. It is based
on direct exchange coupling across grain boundaries which makes an intimate mixture
of Nano size grains behave differently from a pure superposition of the grains
individual magnetic properties. The coupling could permit that the grain size of the
soft phase should not largely exceed the exchange length of the hard magnetic phase.
Otherwise, a domain wall can form in the anisotropic phase at sufficient distance from
the hard phase, and the so-initiated magnetization process will easily reverse the
whole magnet. To reduce bit size in magnetic recording require higher uniaxial
anisotropy, exchange coupling was proposed to achieve moderate coercivity and thus
write fields while maintaining the stability against thermal demagnetization at room
temperature [Eckert et al. 1996].
1.2.3 Multiferroic (ferrite /ferroelectric) composites:
Ferrites based composites have two advantages such as ferrite as well as
Multiferroic properties which are used in spintronics and high frequency electronic
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devices. To enhance ferrite properties there is need to combine some hard ferrite
(CoFe2O4, SrFe2O4 etc.) with soft ferrite (NiFe2O4, ZnFe2O4, MnFe2O4 etc.).
Therefore it is important to study best quality of ferrites for improved electrical and
magnetic based applications and their composite with Perovskites for fabricating MF
materials.
Recent work on nanomaterial’s has revealed ME behaviour in
NiFe2O4/BaTiO3 systems, NiFe2O4/PZT, NiCoFe2O4/Ba(0.8)Pb(0.2)TiO3 [Kadam et al.
2003; Kothale et al. 2003] and NiFe2O4/Ba(0.8)Sr(0.2)TiO3 etc.BaTiO3 [Wang et al.
2008]which shows these systems have high permittivity, low dielectric loss and high
tenability whereas NiFe2O4 and NiZnFe2O4 are known for their chemical stability,
high resistivity and excellent electromagnetic properties [Costa et al.
2010].Composites of these materials and individually they need to be researched for
further improvement and their possible applicability in different fields.
Interest in nanoparticle materials permanently increases because of the
significant influence of large surface/volume ratio of nanoparticles on their physical
properties, compared to their bulk counterpart [Vetrone et al. 2004].Ferrite Nano
crystals are also of interest in various applications, such as inter-body drug delivery
[Li et al. 2007; Sun et al. 1995], bio separation, and magnetic refrigeration systems
[Chen and Zhang 1998], in particular due to their specific properties, such as
superparamagnetism. In addition, among ferrospinels zinc ferrites are used in gas
sensing [Niu et al. 2004; Ikenaga et al. 2004], catalytic application [Toledo-Antonio
et al. 2002], photo catalyst [Qiu et al. 2004; Fan et al. 2009],and absorbent materials
[Kobayashi et al. 2002].
Doping ferrite Nano crystals with various metals, such as chromium, copper,
manganese, and zinc are usually used to improve some of their electric or magnetic
properties [Gubbala et al. 2004; Saafan et al. 2010; Singhal and Chandra 2007].For
example, Zn/Ni ferrites have applications as soft magnetic materials with high
frequency (due to high electrical resistivity and low eddy-current loss [Tsay et al.
2000Along that line, (Cu, Zn)/Ni ferrites offer a further improvement as softer
magnetic materials [Aphesteguy et al. 2009].
Transition metal oxide Nanoparticles represent a broad class of materials that
have been investigated extensively due to their interesting catalytic, electronic, and
10
magnetic properties relative to those of the bulk counterparts, and the wide scope of
their potential applications[Raghasudha et al. 2013; Farhadi et al. 2013].Among these
materials, ferrites have attracted immense attention of the scientific community
because of their novel properties and technological applications especially when the
size of the particles approaches to nanometer scale [Chander et al. 2004].As magnetic
materials, Nano-sized ferrites cannot be replaced by any other magnetic material
because they are relatively inexpensive, stable, and have a wide range of
technological applications [Costa et al. 2010].
The spinel ferrites have remarkable magnetic and electrical properties. Among
them, CoFe2O4 is interesting because of its perfect chemical properties, thermal
stability, high electrical resistivity, and excellent magnetic properties [Múzquiz-
Ramos et al. 2010]. Nano crystalline CoFe2O4 with such properties have potential
applications in high frequency devices, memory cores, recording media, and in
biomedical field [Pervaiz and Gul 2012].
The first practical soft ferrite application was in inductors used in LC filters in
frequency division multiplex equipment. The combination of high resistivity and good
magnetic properties made these ferrites an excellent core material for these filters
operating over the 50-450 kHz frequency range. For four decades ferrite components
have been used in an ever widening range of applications and in steadily increasing
quantities.
From a technology development point of view, as global trend for higher
efficiency and miniaturization of the electronic devices, the application requirement
for soft ferrite is getting more and more demanding and challenging. For power
ferrite, it requires lower loss, higher saturation flux density, higher frequency and a
wider temperature range. For high permeability ferrite, wider temperature stability
and frequency stability, higher insert loss, higher impedance and lower THD (Total
Harmonic Distortion) are general requirements.
Soft ferrites are widely used in electronic devices as magnetic cores for high
frequency applications. The advantages of ferrites for these applications are higher
electronic resistivity as opposed to metals, high machinability, and ease of the
pressing, chemical stability and lower cost. Various performance characteristics of
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ferrites are necessary for varied applications. However, basically high permeability,
high saturation magnetization, high Curie temperature, and low loss are expected.
1.3 Ferrite and Multiferroics nanostructures:
1.3.1 Nanoparticles:
Nano scaled materials are of scientific and technological interest due to their
unique optical, electric, and magnetic properties. Porous solids have higher surface
area, pore volume and tunable pore size compared to nonporous materials. These
properties make the materials interesting in different fields including catalysis,
sorption, separation, drug delivery, sensors, photonics and Nano-devices. As a
colouring and coating material, the iron oxides such as magnetite, hematite and
goethite are commonly used as pigments for black, red, brown and yellow colours
respectively. In general, particle size from 2 to 10 nm increases transparency 3-10
time when compared to the bulk form. These are strong absorbers of ultraviolet
radiation [Sreeram et al. 2006] and mostly used in automotive paints, wood finishes,
construction paints, industrial coatings, plastic, nylon, rubber and print ink. The
excellent weather fastness, UV absorption properties, high transparency and colour
strength makes them to enrich the colours, increase colour shades when combined
with organic pigments and dyes.
Many toxic cations (Co, Zn, Pb, Cd, Cs, U, Sr etc.) and anions like AsO43−
,
CrO42−
, PO43−
, CO32−
etc. are removed by using various phases of iron oxide
[Benjamin and Leckie 1981; Todorović et al. 1992; Ding et al. 2000; Zhou et al.
2001; Luengo et al. 2006; Mohapatra and Anand 2010].Use of iron oxide
nanoparticles is thus becoming very attractive in the area of adsorption or recovery of
metal ions from industrial wastes or natural water streams.
Nanoparticles of magnetic oxides, including most representative ferrites, have
been studied for many years for their application as magnetic refrigeration
[McMichael et al. 1992] photo anode for possible photo-electrochemical cells [Prosini
et al. 2002]. The Nano size of magnetic particle with large surface area change some
of the magnetic properties and exhibit superparamagnetic phenomena and quantum
tunnelling of magnetization which offer a high potential for several biomedical
applications [Reimer and Weissleder 1996; Bonnemain 1998; Pankhurst et al.
2003].Their super paramagnetic property, together with other intrinsic properties,
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such as low cytotoxicity, colloidal stability, and bioactive molecule conjugation
capability, makes such Nano magnets ideal in both in-vitro and in-vivo biomedical
applications .In case of mitigation of anions/cations from aqueous solutions, iron
oxides in Nano form will have higher number of active sites for adsorption, thereby
reducing the amount required per litre of solution. The adsorption process involves
surface hydroxyl group interaction with adsorbents. Nano iron oxides exhibit very
different magnetic properties which can be used for soft ferrites and biomedical
applications including drug delivery and magnetic resonance imaging. Down to the
Nano scale, superparamagnetic iron oxide nanoparticles can only be magnetized in the
presence of an external magnetic field, which makes them capable of forming stable
colloids in a physio-biological medium. The Nano particles usually have much larger
surface area due to their smaller size and can reduce the volume required to achieve
same effect when used as a catalyst. Considering numerous applications of iron
oxides in various emerging fields, tremendous efforts on synthesis of Nano-dispersed
particles are continuing. The biggest challenge in this field is to economically produce
iron oxide Nano particles of desired characteristics for specific application in large
scale. There has been a lot of progress in understanding the basic science of Nano iron
oxides but evaluation of economic viability for commercial application needs much
more attention.
The adsorption properties of the iron oxide is due to combination of both
surface complexation by inner or outer sphere bonding with adsorbate and ion
exchange by Vander Wall forces. Again the small size of Nano particle also gives a
high surface area-to-volume ratio, which facilitates interaction with several kinds of
chemical species, both gaseous and aqueous [Hiemstra et al. 2004].At the Nano scale
these materials are potentially highly efficient for binding metal ions. By tailoring the
composition of the metal oxides, one can induce selective adsorption of different
metal ions.
When a magnetic field is applied, the particles acquire a certain magnetization
but, because of the high thermal energy, the long range order is lost when the field is
removed, and the particles have no remanent magnetization [Uheida et al. 2006]. This
makes magnetic nanoparticles excellent candidates for combining metal binding and
selective adsorption properties with ease of phase separation.
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Iron oxides have relatively high surface area and surface charge, therefore,
often regulate free metal and organic matter concentrations in soil or water through
adsorption reactions.
Iron oxide-based materials have been found to be good candidates as cheap
and efficient catalysts, especially in environmental catalysis. [Miyata et al. 1978]
studied the catalytic activity of several iron oxides and oxide hydroxides of various
particle sizes for the reduction of 4-nitrotoluene using hydrazine hydrate as reducing
agent, and found β-FeOOH was the most effective catalyst. Iron oxide (usually mixed
with other metal oxides) in particular, has been shown to be a very active (although
unstable) catalyst for the oxygen evolution process as well as other related processes,
such as water splitting, chlorine evolution, the oxidation of organic molecules, the
oxygen reduction process and for the hydrogen peroxide decomposition.
1.3.2 Nanorods:
In nanotechnology, Nanorods are morphology of Nano scale objects. Each of
their dimension ranges from 1-100 nm. Nanorods may be synthesized from metals or
semiconducting materials with ratios (length divided by width) are 3:5.One-
dimensional (1D) nanostructures represent a group of Nano-materials with highly
anisotropic morphologies and have received much attention since the discovery of
carbon nanotubes (CNTs) in 1991. [Iijima 1991] Controlled nucleation and growth in
a particular crystallographic direction is considered the basis for the formation of a 1D
nanostructure. However, the growth mechanisms by which anisotropic development
occurs can differ depending on the Nano-material and its method of production.
On the basis of experiments showing large variation in magnetic behaviour due
to size or morphology differences in nanoparticles as well as a few recent studies on
ferrite Nanorods and nanowires, 1D nanostructured ferrites are expected to exhibit
many properties unlike those of particles of the same phase. In contrast to spherical
nanoparticles, Nanorods with their inherent one-dimensional (1- D) shape anisotropy
may exhibit unique magnetic behaviour which is significantly different from that of
the bulk material. Few investigations of the magnetic properties of hematite Nanorods
have been reported.
For 1-D Nanorods with a high aspect ratio, the shape anisotropy may play an
important role in dictating the magnetic properties. The magnetic domain structures,
14
which determine the magnetic properties of the materials, have been reported to be
affected by the 1-D nanostructure. Ad atoms and Nano clusters adsorbed at the
Nanorods surfaces may also have an impact on the magnetic properties. Further, the
impurities, defects, and internal stresses of the Nanorods that are likely to strongly
depend on the preparation methods could also be important factors influencing the
magnetic properties.
Recently, some of the researchers have prepared nanostructured materials by
different synthesis methods. [Bousquet-Berthelin et al. 2008] have reported NiFe2O4
nanoparticles with elementary particle size close to 4–5 nm by flash microwave
synthesis and investigated their possible applications as cathode materials for lithium-
ion battery. [Kavas et al. 2009] prepared NiFe2O4 nanoparticles by surfactant assisted
hydrothermal process and their structural and magnetic properties were investigated in
detail.
The micron sized rod-like particles of nonstoichiometric Co and Ni ferrites
were synthesized by aging co-precipitated Fe(OH)2 and (NiOH)2 at 90 oC in the
presence of an external magnetic field and mechanism for the formation of rod-like
particles was investigated by the time-dependent observation of growing Ni ferrite
rods [Vereda et al. 2008].[ Wang et al. 2008]synthesized MFe2O4 (M = Co, Ni),
ribbons with Nano porous structure which were prepared by electro spinning
combined with sol–gel technology. [Liu et al. 2009] reported NiFe2O4 nanoparticles
and Nanorods synthesized by a facile hydrothermal treatment of Ni(DS)2, FeCl3 and
NaOH aqueous solution. [Zhang et al. 2005] synthesized Nanorods by polyethylene
glycol assisted route and investigated their structural and magnetic properties.
The exciting discovery of the fullerenes was followed closely by the discovery
of nanotubes of carbon. Nanotubes show tremendous promise as building blocks for
new materials. Because of their topology, nanotubes have no dangling bonds, and so
despite being very small, they do not exhibit ―surface effects.‖ As a consequence,
individual nanotubes exhibit nearly ideal electrical, optical, and mechanical
properties. Nanorods are also under extensive development and investigation.
Nanorods have wide applications; they find their applications in dye solar cells, for
oligonucleotide detection, applied electric field, for applied humidity sensitive.
1.3.3 Nanowires:
15
A nanowire is a wire of dimension of the order of nanometer (10-9
meters). At
these scales, quantum mechanical effects are important-hence such wires are also
known as ―quantum wires‖. The nanowires could be used, in near future, as
components of nanotechnology to create electrical circuits out of compounds that are
capable of being formed into extremely small circuits. Some early experiments have
shown that they can be used to build the next generation of computing devices. To
create active electronic elements, the first key step was to chemically dope a
semiconductor nanowire. This has already been done to individual nanowires to create
p-type and n-type semiconductors. Nanowires are not observed in nature and must be
produced in a laboratory.
These nanowires can be suspended, deposited or synthesized from the
elements. Nanowires show peculiar properties due to their size. Unlike carbon
nanotubes, whose motion of electrons can fall under the regime of ballistic transport,
nanowires conductivity is strongly influenced by edge effects. The edge effects come
from atoms that lay at the nanowire surface and are not fully bonded to neighbouring
atoms like the atoms that lay at the surface and are not fully bonded to neighbouring
atoms like the atoms within the bulk of the nanowires. The unbounded atoms are
often a source of defects within the nanowire, and may cause the nanowire to conduct
electricity more poorly than the bulk material. As a nanowire shrinks in size, the
surface atoms becomes more numerous compared to the atoms within the nanowire,
and edge effects become more important. Recent research has shown that the high
aspect ratio of magnetic nanowires can produce a larger magnetic moment than that
observed in particles of the same volume, providing significant benefits in numerous
applications. In fields such as local drug delivery, improved magnetic properties
would allow the ability to deliver drugs more quickly and accurately, enabling
treatment of smaller areas with lower dosages and decreased side effects. Novel
ferrite properties could likewise benefit current communications, defence, memory
storage, and energy technologies, among others.
The synthesis of transition metal doped BaTiO3 nanostructures are of great
importance [Kaur et al. 2012] and has attracted much attention due to its novel shape
and size dependent properties. For instance, the ferroelectric Curie temperature of the
zero dimensional BaTiO3 nanoparticles decreases progressively with particles size
[Verma et al. 2012].On the other hand one dimensional BaTiO3 nanowires still retain
16
their ferroelectric properties, and non-volatile polarization domains with dimensions
can be induced in the nanowires. This further open ups the possibility of fabricating
BaTiO3 nanowire-based non-volatile memory devices and for many more applications
[Yun et al. 2002; Mao et al. 2003].The polarization within individual ferroelectric
domains of the nanowire generally orient along the wire axis [Morber et al. 2006].
1.4 Different parameters responsible for fabrication of nanostructures:
It is well known that increasing or decreasing the concentration of the
chemical reactants will eventually influence the resultant products. The properties of
ferrites and their composite materials are sensitive to the grain size and also strongly
influenced by the distribution of metallic ions among crystallographic crystal lattice
sites. These in turn are sensitive to the method used to prepare those materials
[Mouallem-Bahout et al. 2005].As a whole there are Several factors are important in
the preparation of nanostructures, such as the nature of the cations, their ratio, and the
nature of the anions, pH, temperature, aging, fuel, solvent, surfactant and the
preparation method. Commercial applications of all these different nanostructures
required a high degree of control over the processing as well as the structure and
composition of the resulting materials. This requires a much better understanding of
the underlying mechanisms, chemistry and physics behind the processes occurring
during synthesis. As much more is dependent on synthesis methodology, so it should
be versatile, simple and rapid process which should allow effective synthesis of a
variety of Nano size materials. The development of this knowledge is currently still in
its infancy and clearly much more work needs to be done in this area in the near
future.
1.4.1 Surfactants:
In synthetic techniques fundamental goal is to produce atoms in solution
which quickly (rather spontaneously) formulate into nanoparticles and then to control
their size/shape by utilizing surfactants. Shape controlled Nano crystals possess well-
defined surfaces and morphologies because their nucleation and growth are controlled
at the atomic level. The recent years have seen tremendous progress in the preparation
of nanostructured materials using surfactant. Surfactant: The name "surfactant" refers
to molecules that are surface active, usually in aqueous solutions [Kanel et al.
2006].They are typically soluble in both organic solvents and water. There are
17
hundreds of compounds that can be used as surfactants and are usually classified by
their ionic behaviour in solutions; anionic, cationic, non-ionic or amphoteric
(zwiterionic). Each surfactant class has its own specific properties.
A huge variety of different organic and inorganic compounds can be prepared
into a wide range of different nanostructures. A systematic study of commonly used
surfactants cationic or anionic and different pH values lead to surface induced
reactions that are responsible for fast decomposition of the raw material being used
for preparation and chemical reduction due to active groups [Kaczmarek and Ninham
1997]. Nanoparticles grow with increasing the temperature, while surfactant prevents
the particle growth under the same condition. Commonly used surfactants include,
PVA, PVP, PEG and CTAB etc. An ideal surfactant should have qualities like it
should be cheap, easily available and utmost requisite property it should act as solvent
and fuel. The properties of ferrite nanoparticles can be altered by controlling their
size, which can provide an advantage in formulating new composite materials with
optimized properties for various applications.
Thus, to control the growth of the spinel ferrite nanoparticles and their
corresponding Nanocomposites, organic stabilizers (polymers), e.g., polyvinyl alcohol
(PVA), polyethylene oxide (PEO), polymethacrylic acid (PMAA), and Poly vinyl
pyrrolidone (PVP), are added during the synthesis. New methods of synthesizing
Nano scale materials have also shown that in addition to size, a nanostructure’s shape
can also profoundly affect its physical properties. Non spherical architectures such as
one-dimensional (1D) wires and rods and shapes have demonstrated an enthralling
diversity of properties [Alarifi et al. 2009; Kavas et al. 2009].
1.4.2 Role of pH and temperature:
It has been found that both temperature and base play a very important role in
the formation of well-defined confined nanostructures. In general role of base is to act
as a mineralizer. High calcination temperature (above 450°C) is usually required to
form a regular crystal structure but it is also observed that above 700 oC crystal size
growth takes place. The properties of ferrite do change from expected ones with a
small change in sintering temperature. Temperature is a crucial factor and its role
starts from homogenous mixing of raw materials to till the formation of product. The
effect of the pH is crucial because hydroxide ions (OH−) are strongly related to the
18
reactions that produce nanoparticles. The pH of solution is one of the main factors on
which the final composition of the product depends, which can be varied to get the
desired final product.
It is well known that the morphology of the precipitation/solution synthesized
metal-oxides strongly depend on the amount of H+ or OH
- ions in the sol that
effectively determines the polymerization of the metal–oxygen bonds. Precursor
solution pH variation affects the hydrolysis and condensation behaviour of the
solution during gel formation, and hence influences the morphology. The pH of
solution appears to be critical parameter for the phase formation, particles size and
morphology of the structure during preparation method. The pH is particularly
important when some impurities are present in the growth medium because it
influences, for example, the formation of either zwitterions (ions having both positive
and negative charges) or complex ions. The presence of these various species during
the Nano crystal growth modifies the growth of certain crystal faces. The changes in
shape are due to the differences between the growths rates of the various
crystallographic faces.
1.4.3 Stoichiometric ratio of the compound, doping:
It is well known that increasing or decreasing the concentration of the
chemical reactants will eventually influence the resultant products. Substitutions in
simple, inverse and mixed ferrites and their composites have received a great deal of
attention over the past few years. The substitution of various magnetic and
nonmagnetic ions at different sub lattices in ferrites materials has provided interesting
magnetic structures and electrical properties.
The spinel ferrites and composites are very attractive among them, as
substitution/doping allow a variety of magnetic, electrical and structural disorders,
and also surface chemistry is altered which in turn introduces numerous novel
properties in them. As these substitutions have different sitting preferences for the two
sites (A and B) in the spinel structure and can change many properties as an effect of
modified cation distribution in the material. This may be due to the fact that in spinel
ferrite, the intra-sublattice interactions (A-A and B-B) are weaker than the inter-
sublattice interactions (A-B) as a result of the unsatisfied bonds in antiferromagnetic
phase.
19
The presence of unsatisfied bonds results in increase in magnetic dilutions,
which generate competition between the various exchange interactions. These
exchange interactions result in a variety of magnetic structures. Scientist are still
improving the property of different preparation technique and also doping with
impurities. Any change in surface chemistry may directly affect the gas sensing
properties of one-dimensional (1D) nanostructures which is governed by the
distribution of anions and cations in the structure. For example, the sensitivity of
magnetic mixed oxide-based sensors can be boosted by various doping schemes and a
number of different dopants such as Pd, Sn, Ti, Zn etc. have been used [Kanai et al.
1992; Neri et al. 2006; Reddy et al. 2002; Vasiliev and Polykarpov 1992; Gurlo et al.
1997; Korotcenkov et al. 2007; Tiemann 2007].
1.4.4 Synthesis routes:
Nano-scale science and engineering is likely to produce the strategic
technology breakthroughs of tomorrow. Our ability to work at molecular level- atom
by atom- to create something new, something we can manufacture from the ―bottom
up‖ and ―bottom down‖ opens huge vistas for many of us. The continuous study &
enhancement of nanofabrication techniques is a crucial activity in Nano-science
/technology. By choosing a method that leads to a reduction of the particle size, the
magnetic properties such as coercive field, Curie temperature, saturation
magnetization and increase in absorption coefficients may change significantly in
comparison with those of the bulk material.
In case of nanostructures, it is possible to control the crystallinity and
stoichiometry during the growth process which allows for the manipulation of the
crucial parameters that control their properties. There are several methods for
preparation of nanomaterial’s Viz. Chemical vapour deposition, Physical vapour
deposition, Sputtering, Hydrothermal, Co-precipitation, chemical combustion method
etc. [ Dube and Darshane 1993; Upadhyay et al. 2004; Shannon and Prewitt 1970].
Every method has its own impact on the properties of Nano materials which depend
upon various parameters. Among these we have selected/chosen Chemical
combustion and hydrothermal method for the preparation because of their individual
and distinctive features. Chemical combustion method produces nanoparticles with
much ease and comfort, simple calculation, homogenous and un-agglomerated
20
powder, inexpensive raw material. On the other hand hydrothermal method is a low
temperature synthesis routes resulting in the fabrication of different nanostructures (in
the form of Nanorods, Nano particles, Nano wires, etc.) also provides ease in
optimization of process parameters and restricts size to remain in between 1-20 nm
which further enhances various properties of synthesized material.
Details of processing steps
(A) Processing steps used for preparation of materials by chemical combustion
method
1. Analysis, purification of raw materials.
2. Checking feasibility of Stoichiometric ratios of constituent of different
powders.
3. Processing of nanoparticles by chemical combustion method using PEG.
4. Crystallization and annealing of powdered samples, their washing and
purification.
5. Pellet formation by pressing crystallized powder by using PVA as binder.
(B) Processing steps used for synthesis of materials by hydrothermal method
1. Analysis, purification of raw materials.
2. Checking solubility of stoichiometric ratios of different raw materials.
3. Processing of materials by hydrothermal treatment in an autoclave.
4. Crystallization and annealing of powdered samples, also their washing and
purification.
5. Pellet formation by pressing crystallized powder by using PVA as binder.
1.5 Application of Ferrites and Multiferroics:
Due to advancement in all aspects of life the development in electronic
technology is directly coupled with the advances made in materials science. Within
the broad class of materials available today, functional materials provide exclusive
opportunity for developing novel components and devices as their physical and
chemical properties are sensitive to changes occurring in the environment such as
temperature, pressure, electric field and magnetic field. Ferromagnetic and
ferroelectric materials are presently utilized in a wide range of systems.
21
1.5.1 Ferrite:
Ferrite materials have wide range of applications. Among oxide compounds,
spinel ferrites emerge as subjects of intense research activity, mainly due to the
magnetic, electrical, chemical and other properties exhibited by this class of materials.
[Srivastava and Yadav 2012]Recently, ferrite materials have received extensive
applications in humidity sensors, gas sensors, catalysts, pigments, and anticorrosive
agents [Sun et al. 1995; Chakrabarti et al. 2005; Hotta et al. 1991; Gardner et al.
1966; Darshane et al. 2008; Jing 2006].Filter inductors, Antenna core, Flyback
Transformers, Magnetic Amplifiers, Magnetic memories and switches, IF
transformers and tuned inductor, Ceramic magnet as medical treatment.
Besides well-known applications related to data storage, new fields utilizing
magnetic Nano sized particles are emerging particularly in the biomedical
technologies development. Ferrites also play a significant role in thermochemical
hydrogen production from water-splitting cycles [Padella et al. 2005; Varsano et al.
2011]. Ferrites in Nano-scale have exhibited great potential for their applications as
catalytic materials, wastewater treatment adsorbents, pigments, flocculants, coatings,
gas sensors, ion exchangers, magnetic recording devices, magnetic data storage
devices, toners and inks for xerography, magnetic resonance imaging, bio separation
and medicine [Mohapatra and Anand 2010]. These nanostructures find applications as
catalysts, sorbents, pigments, flocculants, coatings, gas sensors, ion exchangers and
for lubrication [Lim et al. 2001; Sharrock and Bodnar 1985; Sestier et al. 1998; Choo
and Kang 2003].
Magnetic Nano-composites have potential applications in areas such as
magnetic recording, magnetic data storage devices, toners and inks for xerography,
and magnetic resonance imaging, wastewater treatment, bio separation, and medicine
[Raj and Moskowitz 1990; Pieters et al. 1991; Ziolo et al. 1992; Šafařík 1995; Häfeli
1997; Schütt et al. 1997; Denizli and Say 2001]. Ni-Zn substituted mixed ferrites have
properties like low coercivity, high resistivity values and little eddy current losses
which makes them excellent core materials for power transformers in electrical and
telecommunication properties [Costa et al. 2003].Magnetic nanoparticles get heated
on subjection to alternating magnetic fields, this can be utilized in destroying tumour
cells.
22
1.5.2 Multiferroic:
Recently, ferroelectric–ferromagnetic composite materials that exhibit a large
ME effect at above room temperature and at low bias have attracted noteworthy
attention from the scientific community due to their capability of efficient energy
transfer between electric energy and magnetic energy as shown by the rising number
of publications in the last few years and their subsequent applications for their
significant usages such as, magnetic–electric sensors in radio-electronics [Petrov et al.
2007], oscillators, phase shifters, memory devices, transducers and also as compact
electrical filters for suppressing electromagnetic interference (EMI) and so onto the
next generation multifunctional devices that can be electrically written and
magnetically read [Kang et al. 2009].
The ME (Magneto electric) composites can be used as magnetic probe for
detecting ac or dc fields. Sensitive magnetic sensors can be obtained using the ME
composites with high ME coefficients. Due to novelty of the ME effect, these
composites may find applications in memory devices, a memory device so produced
will be accompanied with the combination of best functionalities of FeRAMs and
MRAMs (ferroelectric write and magnetic read operations) would efficiently improve
the writing speed and reduce the energy consumption. Moreover, device
miniaturization can further lead to reduced energy consumption and higher speeds
[Bichurin et al. 2012; Eerenstein et al. 2006].
1.6 Objective and present work:
The concept for the present plan of research work was aimed to undertake a
systematic study on synthesising parameters and possibility of multifunctional
properties i.e. structural, microstructural, magnetic, electric and dielectric properties
in nanostructured materials. Nanomaterial’s are of prodigious scientific interest as
they are an actual bridge between bulk materials and atomic or molecular structures.
The properties of bulk materials are only reliant on their chemical composition.
However at the Nano-scale, the properties of materials are not only determined by
chemical compositions, but also by sizes and shapes.
The survey of recent literature studies have showed that there still remains
scope of research for the production of Ni1-xZnxFe2O4/SrFe2O4 and
Ni1-xZnxFe2O4/BaTiO3 system in both bulk and Nano forms with low cost, efficient,
23
ease of processing and with desired properties and structure. Ni1-xZnxFe2O4, Ni1-
xZnxFe2O4/SrFe2O4, NiFe2O4/BaTiO3 and Ni1-xZnxFe2O4/BaTiO3 system by primarily
employing Chemical combustion method and then subjecting selected samples from
the chemical combustion method to be prepared by hydrothermal method. These
methods are cheap, simple and provide free choice of the composition of components.
So we have planned to involve both methods due to their unique features to
investigate changes in properties by applying these methods.
Further, these results can be useful to a large extent in giving new dimensions
to the emerging technologies. Because of exclusive physical, chemical properties and
numerous applications, a lot of work has been done in the field of ferrite
Nanomaterial’s. However, there are still challenges ahead that we intend to address
effectively in the thesis.
In view of various research challenges in the Ferrite materials, the objectives
of the present study were structured as.
1) To chemically synthesize the Nanomaterial’s of pure and
doped with Zn and composite materials with , and the
enhancement in ferrite properties of with hard ferrite
. All the compositions of these ferrite and Multiferroic
composite have been prepared by two methods: Chemical Combustion
and Hydrothermal.
2) For combustion PEG (Poly Ethylene Glycol) is used as an efficient
fuel and solvent and urea is used to create an overall redox system.
3) In hydrothermal synthesis, have been used as
a basic medium for pH adjustment.
4) Effects of particle size on the various properties of ferrite nanoparticles
and Multiferroic nanoparticles have been investigated.
5) Comparative analyses of preparation methodology on properties of
resulting materials have been investigated.
24
6) Effect of Zn concentration on the structural, Microstructural, dielectric
and magnetic properties of system have been
thoroughly investigated.
7) Effects of preparation of composite of and with
and have been investigated.
1.7 Outline of the thesis:
Chapter 1 gives an insight of the brief introduction of ferrites; composites of
hard and soft ferrites along with Multiferroic have been taken up. It also includes their
applications, role of different parameters in their synthesis part along with objectives
of the research work. In the Second chapter attempts have been made to
systematically classify the available information .This chapter incorporates
information to assist in understanding the aim and objective of the investigation ,
literature survey of Ferrite nanostructured materials their structure, history; general
methods of synthesis of nanomaterial and techniques used to analyse their various
parameters are explained. Third chapter enunciates with the detailed insight of the
material and experimental methods used to prepare samples where in quantity of
material, reactions involved and methodology is discussed. Fourth and Fifth chapter
comprises of results and discussions obtained from prepared samples. In chapter sixth
a summarized conclusion of all previous chapters is given. A complete list of
references has been given towards the end of the thesis. Finally a concise list of
publications based on research work has been presented at the end.
1.8 Future scope:
With the arrival of nanotechnology, an incredible rush in research on
miniaturization and high efficiency electronic devices is on growth. These materials
suit these demands and are considered to shape the future of advanced technology. In
recent years, nanotechnology has not only opened new vistas for the preparation of
various novel Nano size oxides and composites, but also prospered in continuous
synthesis methods of Nano powders and development of various supported catalysts
25
and coatings. As a result, conditions are developed for breakthroughs in these areas
over the next several years.
The author is interested in the study of the magnetic properties of the Nano
crystalline soft magnetic material, and the changes in the magnetic, electric, dielectric
and catalytic and other properties of Nano crystalline spinel ferrites and their
composites. The future challenges of nanoparticles and their applications are abundant
only a few of them are explored, for instance the preparation of ferrite samples with
low eddy current losses and a useful frequency of the order of gigahertz is a
challenging one. Also in the pursuit of high density recording the challenge is to have
unidirectional permanent magnetism. The magnetic relaxation which tends to destroy
magnetization has to be successfully overcome by novel methods of preparation of
samples. These nanostructured materials having large surface to volume ratio would
act as efficient catalyst for heavy and toxic metal separation from waste water and
also in degradation of coloured effluents from dye and related industries. Their
efficiency in various organic reactions is needed to be explored in the form of
extension of this work.
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