CHAPTER 1 AN OVERVIEW ON ELECTROSPINNING PROCESS...
Transcript of CHAPTER 1 AN OVERVIEW ON ELECTROSPINNING PROCESS...
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CHAPTER 1
AN OVERVIEW ON ELECTROSPINNING PROCESS,
VARIATION OF ELECTROSPINNING PARAMETERS AND
CHARACTERIZATION TECHNIQUES
1.1 INTRODUCTION
Electrospinning is a versatile process by which electrostatically
driven polymer jet is coated on a collector with nanofibers of uniform
diameter less than 100 nm and length of several kilometers (Pinto et al 2004).
These nanofibers are one dimensional in nature which resembles the
nanotubes, nanopillars, nanowires, nanorods, etc. Nanofibers are flexible by
nature and can be used to connect two terminals at different orientations in
electronic circuits such as MEMS and NEMS (Chandra S. Sharma et al
2011). Quite a lot of research work has been carried out in this area to utilize
the properties of these nanofibers to be applied for various potential
applications. For effective utilization of these fibers, it is noteworthy to study
the topography, morphology, structure, atomic arrangement, physical and
chemical properties of these nanofibers.
There are numerous methods have been followed to produce the
nanofibers such as bubble electrospinning (Ruirui Yang et al 2009, Yong Liu
and Ji-Huan He 2007), melt spinning (Young-Pyo Jeon and Christopher
2009, Xiaoyan Yuan et al 2001, Jason Lyons et al 2004), wet spinning,
drawing (Ondarcuhu and Joachim 1998), template synthesis (Feng et al 2002,
Martin 1996), phase separation (Ma and Zhang 1999), controlled synthesis
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(Hailiang Wang and Limin Qi 2008), self-assembly (Liu et al 1999,
Whitesides and Grzybowski 2002), co-electrospinning process
(Zaicheng Sun et al 2003) , upward needleless electrospinning (Yarin and
Zussman 2004),electrospinning with dual collection rings (Paul D. Dalton
et al 2005), melt coaxial electrospinning (Jesse T. McCann et al 2006),
multi-jet electrospinning (Bin Ding et al 2004,Wac aw Tomaszewski and
Marek Szadkowski 2005), multilayering and mixing electrospinning (Satoru
Kidoaki et al 2005 ), high frequency electrospinning (Zheng-Wen Fu et al
2005), radiation grafting techniques(Robinette and Palmese 2005), two pole
air gap electrospinning (Balendu S. Jha 2011), nanofiber seeding
(Xinyu Zhang et al 2004) and layer by layer spinning
(Heidi L. Schreuder-Gibson and Phil Gibson 2005). From these methods
dense mesh of polymer fibers can be prepared.
Nanofibers are studied extensively due to their small diameter, one
dimensional nature, small pore size, very high porosity, high surface area per
unit mass (Ioannis S. Chronakis et al 2006), large surface area to volume
ratio, flexibility in surface functionalities, superior mechanical performance,
high aspect ratio, high axial strength combined with extreme flexibility, low
basis weight and cost effectiveness (Zheng-Ming Huang et al 2003).
Electrospinning process can control the deposition of the polymer
nanofibers. Controlling the electrospinning process has resulted in nanorods,
flexible fibers, porous fibers, hollow fibers, etc. These nanofibers could be
aligned to construct unique functional nanostructures such as nanotubes and
nanowires. A characteristic feature of the electrospinning process is the
extremely rapid formation of the nanofiber structure in a short time of
milliseconds. It has a large material elongation rate in the order of 1000/s, and
a cross-sectional area reduction in the order of 105 to 106, that have been
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discovered to influence on the orientation of the structural elements within the
fiber (Ioannis S. Chronakis et al 2006).
Electrohydrodynamic (EHD) concepts were applied to synthesize
fibers by the electrospinning process was explained in the innovative work of
Lord Rayleigh (1882). Electrospinning process was first patented by Formhals
(1934) and he got several patents in the successive years. In the early 1990s,
Reneker’s group replenished the electrospinning field of research (Doshi and
Reneker 1995, Reneker and Chun 1996).
Materials in fiber form are unique and they are stronger than bulk
materials. As fiber diameter decreases, it has been well established in glass
fiber science that the strength of the fiber increases exponentially, due to the
reduction of the probability of including flaws. As the diameter of matter gets
even smaller as in the case of nanotubes, the strain energy per atom increases
exponentially, contributing to the enormous strength of over 30 GPa for
carbon nanotube. Mechanical strength of an individual nanofiber is also
expected to be enhanced with decreasing diameters (Kwon et al 2005).
Polymer nanofibers possess very large length to diameter aspect ratios (in the
order of 1010) and very large surface to volume ratios (in the order
of 107). These nanofiber materials are in a form that maximizes surface area
for light collection and minimizes the volume while being mechanically very
flexible which is preferable for space deployment (Zhang et al 2005). Higher
electrical conductivity is always desired to have high capacitance and high
power density in supercapacitors (Dalton et al 2003).
Several polymers such as polymethacrylate, polyamides,
polystyrene, polyester, polyacrylonitrile, polyolefine, polyurethanes, poly
(vinyl alcohol), poly vinyl acetate, cellulose acetate, polyethylene oxide,
polycaprolactone, poly(3-caprolactone), polyvinyl pyrrolidone and silk/PEO
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blend have been electrospun into fibers in the nanoscale by several
researchers in the past (Audrey Frenot and Ioannis S. Chronakis 2003).
Electrically conducting polymers, optically active fibers, porous
and catalytic fibers and carbon nanofibers were also synthesized. Biological
polymers like DNA, proteins, polypeptides were also electrospun. These
fibers have a surface area to mass ratio of 100 m2 / g approximately for a
diameter of 100 nm (Ioannis S. Chronakis 2005). The electrostatic charges
applied to the fibers are influenced by the electrical properties of these fibers
and retained or dissipated by them.
Electrospun mesoporous metal oxide fibers have been studied
(Minedys Mac ´as et al 2005). Nataraj et al (2011) have reviewed the
polyacrylonitrile-based nanofibers. Electrical and morphological properties of
conductive polypyrrole nanofibers prepared by electrospinning were studied
by Ioannis S. Chronakis et al (2006). Charge consequences in electrospun
polyacrylonitrile (PAN) nanofibers have been studied by Veli E. Kalayci
et al (2005).
Critical length of straight jet in electrospinning was studied by
Ji-Huan He et al (2005). Feng (2003) investigated the stretching of a straight
electrically charged viscoelastic jet. A mathematical model of electrospinning
was proposed and a variational approach to nonlinear problems was applied
by Ji-Huan He and Hong-Mei Liu (2005). The use of AC potentials in
electrospraying and electrospinning processes were studied by Royal Kessick
et al (2004).
Electrospun polyethylene terephthalate (PET) nanofibers were
synthesized and their surface engineering has been studied and used as new
material for blood vessel engineering (Zuwei Ma et al 2005). Morphology,
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structure and electrochemical properties of single phase electrospun vanadium
pentoxide nanofibers were studied and used as lithium ion batteries
(Yan L. Cheah et al 2011). Electrospun nano-vanadium pentoxide is used
effectively as cathode material (Chunmei Ban 2009). PBI-based carbon
nanofiber web prepared by electrospinning has been used as supercapacitor
electrodes (Chan Kim et al 2004). Electrospun polymer nanofibers are used
as single light emitters (Nikodem Tomczak et al 2006) due to the effect of
local confinement on radiative decay. Electrospun activated carbon nanofibers
were used as electrode in supercapacitors after electrochemical
characterization (Chan Kim 2005). Electrospun nanofibrous membranes
coated quartz crystal microbalance are used as gas sensor for NH3 detection
(Bin Ding et al 2004). Electrospun silk-BMP-2 scaffolds are used in bone
tissue engineering (Chunmei Li et al 2006). Electrospun PVdF-based fibrous
polymer electrolytes are used as lithium ion polymer batteries
(Jeong Rae Kim et al 2004).
Supercapacitor performances of activated carbon fiber webs
prepared by electrospinning of PMDA-ODA poly(amic acid) solutions were
studied (Chan Kim et al 2004a). Conductive polypyrrole nanofibers were
electrospun and their electrical and morphological properties were
investigated (Ioannis S. Chronakis et al 2006). Transport properties of
electrospun nylon 6 nonwoven mats were explored by Young Jun Ryu et al
(2003). Anisotropic electrical conductivity of MWCNT/PAN nanofiber paper
was studied by Eun Ju Ra et al (2005). Rheological properties of sisal
fiber/poly (butylene succinate) composites were determined by Yan-Hong
Feng et al (2011). Transport and vibrational properties of
poly(3,4-ethylenedioxythiophene) nanofibers have been studied by
Duvail et al(2002). Self assembly and correlated properties of electrospun
carbon nanofibers were explored by Rutledge et al (2006). Tensile testing of a
single ultrafine polymeric fiber was performed by Tan et al (2005).
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Electrostatic fabrication of ultrafine polyaniline / polyethylene oxide blend
ultrafine conducting fibers was done by Ian D. Norris et al (2000).
Mesomechanics for fiber reinforced composites with nanofiber reinforced
matrix was studied by Christos C. Chamis (2009).
1.2 ELECTROSPINNING PROCESS AND VARIATION OF
PARAMETERS
The schematic diagram of the electrospinning set-up is shown in
Figure 1.1. A high voltage electrostatic field is applied to a polymer solution
kept in a spinneret. The spinneret contains a cylindrical vessel with a needle at
the tip. The positive of the D.C. high voltage source is connected to the needle
and negative of it is connected to a grounded collector. Collector can be a
metal plate like aluminium foil, copper foil, 32OlA foil, etc. The spinneret
was placed downwards to have gravitation pull and the collector was placed
down facing up. In some cases, the spinneret was kept in an inclined position
to have good flow of the sol. Recently, a motorized pump is used which
ejects the sol with uniform flow rate and the deposition is made to be uniform.
Figure 1.1 Schematic diagram of the electrospinning set-up
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Figure 1.2 Electrospinning set-up with stationary collector
The collector can be a stationary or rotating one. Stationary
collector is used to deposit thin layer of nanofibers and shown in Figure 1.2.
It shows the electrospinning set-up kept inside an electrical shielding
arrangement. This prevents electrical shock as well as the electrospinning
process could not be disturbed by the surroundings. A rotating grounded
collector is controlled by a motor as shown in Figure 1.3. The speed of
rotation and the direction can be controlled by a switch. The rotating cylinder
is covered with the aluminium foil. Thin continuous nanofibers with greater
length can be deposited by using this method.
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Figure 1.3 Electrospinning set-up with rotating collector
Electrospinning process is a fluid dynamics associated problem. To
prepare nanofibers with superior properties, the clear understanding of how a
sol present in a spinneret on the application of electrostatic forces is formed
into a thin fiber is inevitable. The electrospinning process is assumed to be
composed of the following three stages (Fong and Reneker 2001),
(i) Initiation of the jet and the extension of the jet along a straight
line
(ii) The growth of whipping instability and further elongation of
the jet accompanied by jet branching / splitting
(iii) Solidification of the jet into nanofibers
1.2.1 Initiation of the Jet
Taylor showed that in a viscous liquid, fine threads are created due
to the maximum instability of the liquid surface induced by the electrical
forces (Yarin et al 2001). He also showed that a viscous liquid exists in
equilibrium in an electric field when it has a conical form with a semi-vertical
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angle of = 49.3°(Taylor 1969).This means that the fluid jet is developed
only when the semi vertical cone angle = 49.3° . This cone is named after
Taylor as Taylor cone and shown in Figure 1.4.
Figure 1.4 Taylor cone formation and ejection of the jet
The strength of the necessary electrostatic field is an important
information in the electrospinning process. The critical voltage (Vc) at which
the maximum instability occurs in kilovolt was calculated by Taylor (1969).
RR
LIn
L
HVc 117.05.1
242
22 (1.1)
where L is the length of the capillary tube in meter, R is the radius of the tube
in meter, is the surface tension of the fluid in dyne per cm and H is the
distance between the electrodes in meter.
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Henricks et al (1964) calculated the minimum spraying potential of
the suspended, hemispherical, conducting drop in air as
rV 20300 (1.2)
where r is the jet radius and is the surface tension of the fluid
1.2.2 Thinning of the Jet
Fluid instabilities occur during this stage. When an electrically
accelerated jet fluid moves and thins along its trajectory, the radial charge
repulsion results in splitting the jet into multiple small jets through a process
called ‘splaying’. If the number of jets splitted is more, it would reduce the
fiber diameter.
Jet splitting is caused by whipping instability in which bending and
stretching of the jet takes place. Instability is caused due to the applied
electric field and the electric field strength is directly proportional to the
instability level. Rayleigh instability occurs when the applied field is lowest.
Due to higher applied field values, bending or whipping instability occur. The
splitting of the primary jet is also caused due to bending instability.
Three instabilities were proposed by Hohman et al (2001a, 2001b),
namely the classical Rayleigh (capillary) hydrodynamic instability, the
azimuthal /varicose and the whipping/bending instabilities induced by the
electric field on a liquid of finite conductivity. The bending instability, in
particular, is strong for highly conducting fluids. It tends to dominate when
the interfacial charge density and the jet radius are large. At high fields and
flow rates, the whipping instability controls the behavior of the jet and leads
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to bending and stretching of the jet. The whipping jet thins dramatically,
while traveling the short distance between the electrodes. Thinning of the
fluid jet in the whipping regime is a consequence of normal stresses
originating from the bending of the centerline of the jet. During
electrospinning the electrodriven jet undergoes a set of instabilities. The
whipping instability causes the jet to bend and stretch, ultimately producing
superfine fibers. The resulting morphology and physical properties of the
fiber depend on the fluid material properties. Bending instability in
electrospinning nanofibers were studied by Yarin et al (2001a).
Electrically driven fluid jet driven by electrostatic forces is found to
be unstable during its trajectory before reaching the collector screen.
Whenever the viscosity of the polymer solution increased, the spinning drop
changed its shape approximately from hemispherical to conical. Baumgarten
(1971) arrived at an expression to determine the radius of a spherical drop of
the jet as
K
mr 03 4
(1.3)
where is permittivity of the fluid (in coulomb/volt-cm), m0 is the mass flow
rate (g/sec), K is a dimensionless quantity related to the electric currents, is
electric conductivity in (amp/volt.cm) and is density (g/cm3).
1.2.3 Solidification of the Jet
Yarin et al (2001) explained the decrease and variation of the fluid
jet due to the evaporation and solidification. They found that the
cross-sectional radius of the dry fiber was 31031.1 x times of that of the initial
jet. Solidification process is important because it decides the morphology and
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topography of the fibers. It also decides the continuous formation, porosity
and uniform cross section of the fibers.
1.2.4 Allometric Scaling Relationship Between Current and Voltage
The charged jet can be considered as a one-dimensional flow.
Conservation of mass given by Ji-Huan He and Yu-Qin Wan (2004) is
Qr 2 (1.4)
where Q is mass flow rate, r is the radius of the jet, is velocity and is
liquid density.
The current passing through the jet is composed of two parts : the
Ohmic bulk conduction current (Ic) and surface convection current (Is).
Current I = Ic + Is. (1.5)
The resistance of the ohmic conductor is
2~1
~ rA
RC (1.6)
where CR is resistance, r is radius of the conductor and A is cross sectional
area.
Ohmic conduction current
21 ~~~ rAcRI C (1.7)
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which corresponds to kErIC2 , where k is the dimensionless
conductivity of the fluid.
rI S 2 , where is the surface density of the charge and is
velocity.
In electrospinning process, the current (I) is the sum of ohmic bulk
conduction current and the surface convection current. Conservation of
charges gives
EkrrI 22 (1.8)
Ohm’s law is valid for metallic conductors and invalid for non-metallic
conductors.
R ~A
1 ~ 2r (1.9)
where is a parameter relating to conductivity of polymer solution. When
=1, it becomes a metal-like conductor, =1/2, free ions or electrons do not
exist in the bulk. Generally, for electrospinning process, takes values
between2
1 and 1.
The current in the electrospinning process is proportional to the
cube of the applied voltage.
3EI (1.10)
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1.2.5 Variation of Electrospinning Parameters
Electrospinning has been used to synthesize various nanostructures
for several decades, extensive work has been carried out by various
researchers to study the fiber morphology due to the modification of various
parameters. Effect of concentration, applied voltage and calcining temperature
were investigated (Jeerapong Watthanaarun et al 2005). Effect of viscosity,
concentration, conductivity, temperature and electric field were studied by
Demir et al (2002). Effect of solvents on electro-spinnability of polystyrene
solutions were studied by Teeradech Jarusuwannapoom et al (2005).Polymer
concentration, feed rate, molecular weight, conductivity and voltage were
studied by Tan et al (2005a). Effect of viscosity and conductivity were studied
by Lei Li and You-Lo Hsieh (2005). Effect of applied voltage, working
distance and concentration on PAN fibers were investigated by Gu et al
(2005). Effect of molecular weight, applied field and feed rate were studied
by Jason Lyons et al (2004).Effect of applied voltage and working distance
have been studied (Lin-Jer Chen et al 2011).Effect of solution concentration
was studied by Hualin Wang et al (2011).Effect of voltage and working
distance were investigated by Sajeev et al (2008).Effect of voltage, working
distance and feed rate were studied by Rouhollah Jalili et al (2005). The
change of bead morphology formed on electrospun polystyrene fibers were
experimented by Lee et al (2003). Experimental investigation of the
governing parameters in the electrospinning of polymer solutions was studied
by Theron et al (2004). Alignment of electrospun nanofibers in a large area
were improved by using an insulating tube on the collector designed by Ying
Yang et al (2007). Vibration Technology was applied in the electrospinning of
nanofibers by Ji-Huan He et al (2004a). Electrospinning processing
parameters and geometric properties were studied by Sachiko Sukigara et al
(2003).
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The morphology and characteristics of fibers can be modified by
the variation of different parameters. These parameters are broadly classified
as
1. Parameters of the sol derived from the sol-gel solution. They
are
(a) Solution concentration
(b) Viscosity of the solution
(c) Conductivity
(d) Surface tension
(e) Solution temperature
(f) Dielectric constant
(g) Molecular weight
2. External Factors
(a) Applied voltage
(b) Distance between the spinneret and collecting screen
(working distance)
(c) Size of the needle opening
(d) Calcining temperature
1.2.5.1 Solution concentration
Solution concentration plays a major role in the deposition of fibers
(Park et al 2004, Mit-uppatham et al 2004). The increase in solution
concentration was experimented by adding Poly (vinyl alcohol) to deionised
water at 10%, 20%, 30%, 40% and 50% (wt. percentages) and the
corresponding fiber diameter was measured. Increase in concentration
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increased the fiber diameter as shown in the Figure 1.5. This has been
reported earlier by Deitzel et al (2001a, 2001b, 2002). According to
Demir et al (2002 ) power law relationship,
Fiber diameter d C3 (1.11)
where C is the solution concentration.
Figure 1.5 Fiber diameter vs solution concentration
1.2.5.2 Viscosity of the solution
When viscosity was very low, droplets were formed. At very high
viscosity values, the fibers were not formed. At intermediate values, for an
increase in viscosity there was an increase in diameter of the fibers. Viscosity
increase is caused due to the increase in polymer concentration and the
increase in diameter is studied by Baumgarten (1971) as d 0.5.
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1.2.5.3 Conductivity of the solution
Electrical conductivity of the polymer solution also plays a role in
forming the fibers. When the concentration of the polymer increases, more
molecules can be ionized and higher conductivity can be attained. Fiber
diameter increases with respect to increase in conductivity (Demir et al 2002,
Tan et al 2005, Lei Li 2005). The conductivity in the solution tends to reduce
the fiber sizes (Fong et al 1999).
1.2.5.4 Surface tension
At higher surface tensions, the reductions of surface areas are
increasingly favored due to the increased free energy of the system forcing a
breakup of the polymer jets into spheres. Decrease in surface tension will
result in lower electric field required for jet initiation. Higher surface tension
drives beaded fibers (Yao et al 2003).
1.2.5.5 Solution temperature
Polyurethane nanofibers were electrospun by Demir et al (2002)
and they recognized that the fiber diameters obtained from the polymer
solution at a high temperature of 343 K were much more uniform than those
at room temperature. It should be noted that the viscosity of the polyurethane
solution with the same concentration at some higher temperature was
significantly lower than that at room temperature. Therefore, uniformity in the
diameter of fibers was obtained only at high temperature of the polymer
solution. This could be due to the decrease in viscosity at high temperatures.
In PVA solution, the conductivity was measured at various temperatures.
Conductivity increased due to increase in temperature.
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1.2.5.6 Dielectric constant
Dielectric constant of the solvent controls the average diameter of
the fibers (Won Keun Son et al 2005). The large is the dielectric constant of
the solvent, the electrospun fibers formed are finer. Charges have much
greater effect to a polar solvent than to a non-polar solvent. Therefore, a
solvent with high dielectric constant has a higher net charge density in ejected
jet. Higher viscosity and higher net charge density favor the fiber formation
without beads (Son et al 2004, Lee et al 2003a).
1.2.5.7 Applied voltage
In electrospinning process, applied voltage decides the formation of
the fibers and improves the perfection of the fibers. Diameter of fibers
increased due to the increase in applied voltage. This may be due to the
ejection of more solution at high applied voltages. Density of fiber mesh
increased with increase in applied voltage (Gu et al 2005, Rouhollah Jalili
et al 2005).
1.2.5.8 Distance between spinneret and collector screen (working
distance)
Distance between the spinneret and collector should have a
specified value for optimized formation of fibers. As the distance (H)
increases the amount of fiber deposition decreases. When the distance
decreases, more beads were formed (Sajeev et al 2008, Rouhollah Jalili et al
2005).
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1.2.5.9 Size of the needle opening
Size of the needle opening of the spinneret could control the
formation of fibers. Smaller needle opening allows less solution as it is
observed in 24 gauge needle and increase in size of the opening to 22 gauge
produced large diameter fibers. Above 25 gauge of the opening, the gel was
obstructed and no fibers were formed because of reduction in the needle
opening. 20 gauge needle allowed more solution and produced more number
of beads. This has been experimented by Minedys Macias et al (2005) and
similar results have been obtained.
1.2.5.10 Molecular weight
The molecular weight of PVA and the solution concentration have
an important effect on the structure of the electrospun polymer. At each
molecular weight, there is a minimum concentration required to stabilize the
fibrous structure and maximum concentration where the solution cannot be
electrospun. The molecular weight of the polymer may have a noteworthy
effect on the electrical conductivity, rheological properties , dielectric strength
and on surface tension of the solution (Koski et al 2004). Mark–Houwink
relationship for PVA in water was given by Tacx et al (2000) in terms of
viscosity and molecular weight as
= 6.51x10-4 Mw0.628 (1.12)
1.2.5.11 Calcining temperature
In the poly vinyl pyrrolidone (PVP) / titanium (IV) oxide fibers,
after calcinations the calcined fibers appeared to be more distorted and the
surface appeared to be more rough, even though calcination did not affect
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much the fibrous nature of the fibers. Apparently, the higher the calcination
temperature, the greater the shrinkage and the diameter decreased
considerably. Higher calcination temperature is considered to exhibit better
thermal stability, which is desirable for catalytic applications (Jeerapong
Watthanaarun et al 2005).
1.2.6 Sol-Gel Process
Sol-gel is a useful self assembly process for nanomaterial synthesis.
Important characteristic of a solution is that it has to be clear. Molecules in
the nanometer scale are dispersed and move around randomly and clear
transparent solution can be obtained. A colloid that is suspended in a liquid is
called a sol. A suspension that keeps its shape is called a gel. Thus sol-gels
are suspensions of colloids in liquids that keep their shapes.
In the sol-gel process, it involves the evolution of networks through
the formation of a colloidal suspension called as sol and gelation of the sol to
form a network in a continuous liquid phase known as gel. The precursors for
synthesizing these colloids normally consist of ions of a metal but sometimes
other elements surrounded by various reactive species, called as ligands.
Metal alkoxides and alkoxysilanes are most popular because they react readily
with water. Because of the fact that water and alkoxides are immiscible, a
mutual solvent such as an alcohol is used.
Sol-gel process involves the following four stages,
1. Hydrolysis
2. Condensation and Polymerization of monomers to form
particles
3. Growth of particles and
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4. Agglomeration of particles followed by formation of networks
that extend throughout the liquid medium resulting in
thickening, which forms a gel.
The properties of a particular sol-gel network are related to a
number of factors that affect the rate of hydrolysis and condensation
reactions, such as pH value, temperature and time of reaction, reagent
concentrations, nature and concentration of catalyst, aging temperature, time
and drying.
Advantages
It is a low temperature process.
Nanomaterial can be obtained in any form like plate, disc, powder,
thin film, etc.
It can be attached to other materials easily.
It can be polished to fine optical quality.
1.2.7 Polymer Selection
Polyvinyl alcohol (PVA) was carefully selected as the polymer in
our research work. It was selected due to the following reasons. Polyvinyl
alcohol is a non-ionic synthetic polymer that is soluble in water. It has a large
number of hydroxyl groups which allows it to react with many types of
functional groups (Surawut Chuangchote and Pitt Supaphol 2006). These
reasons makes it suitable to be used as a biocompatible material (Bin Ding et
al 2002). Poly (vinyl alcohol) is a semi-crystalline, hydrophilic polymer with
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good chemical and thermal stability and non-toxic. It can be processed easily
and has high water permeability. PVA is a water-soluble polymer that readily
reacts with different cross-linking agents to form a gel. PVA solutions can
form physical gels from various types of solvents (Youliang Hong et al 2006).
These properties have led to the use of PVA in a wide range of applications in
medical, cosmetic, food, pharmaceutical and packaging industries.
PVA has been widely used in various fields, ranging from
thickening agent to solution-spun fiber. Viscosity plays an important role in
the industrial applications of PVA (Tong Lin et al 2006). There are several
factors that influence the rheological behavior of aqueous PVA solutions such
as molecular weight, temperature and degree of hydrolysis (Koski et al 2004).
1.2.8 Nanofibers Synthesized in Our Laboratory
The photographs of the obtained nanofiber mat synthesized in our
laboratory by electrospinning the sol obtained from the sol-gel process are
given in the following figures.
Figure 1.6 PVA nanofiber mat prepared by electrospinning technique
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Figure 1.7 ZnO/CuO nanofiber mat grown on glass plate with metallic
wire electrode fixed on it
1.3 CHARACTERIZATION TECHNIQUES
Physical and chemical properties of materials can be assessed from
the different characterization techniques. Nanomaterials possess fascinating
properties which could be sensible from studying the chemical composition,
structure and defects. In our present work, the nanocomposites were subjected
to various experimental techniques and the background of those experimental
techniques is explained in this chapter.
1.3.1 Optical Absorption Studies
UV-Visible spectrometry is the most preferred methodology among
all other methods due to its less cost, accuracy, less time consumption,
versatility, speed and simplicity.
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UV Spectroscopy
Study of the interaction of light and matter is called as
spectroscopy. Measurement of the absorption of light by molecules or ions in
the gaseous, vapour or dissolved state is termed as spectrometry. Ultraviolet
and visible spectrophotometers have been used in the analytical
instrumentation laboratories for more than four decades. In the wavelength
range of 190 nm to 780 nm, absorption of the electromagnetic radiation is
made by the excitation of the bonding and non-bonding electrons of the ions
or molecules. The absorption spectrum is the plot between wavelength and
absorbance.
According to Beer-Lambert’s law,
absorbance A = ect (1.13)
where ‘e’ is the molar absorbance or absorption coefficient, ‘c’ is the
concentration of the compound in the solution and ‘t’ is the thickness of the
cell.
From the absorption spectrum, the wavelength at which maximum
absorption takes place gives the information about the structure of the ion or
molecule due to the fact that the absorption is directly proportional to the
amount of the material absorbing the light.
Instrumentation
The following components are present in the ultraviolet and visible
spectrophotometer.
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1. Radiation source
2. Wavelength selectors
3. Sample holders
4. Radiation detectors and
5. Signal processing and read-out devices
In our work, VARIAN CARY 5E spectrophotometer was used to
obtain the UV spectrum. A continuous UV spectrum in the region of
180-780 nm is produced by a hydrogen or deuterium discharge lamp
operating at a low pressure of 2 to 3 torr with quartz. Cells and cuvettes used
to hold the sample and the solvent are made from quartz or fused silica
because of their transparent nature to UV region. Prism or diffraction grating
are used as monochromators which may lead to severe energy loss and filter
photometers are preferred to avoid this. In the past, photovoltaic cells and
photomultiplier tubes were used. Photodiodes are used in the recent time.
Modern spectrometers are computerized and the detector output is digitalized
and stored. Scanning of samples yield absorbance values and the spectrum is
displayed on the screen. The photograph of the UV spectrophotometer is
given in Figure 1.8.
The optical absorption coefficient ( ) varies with the excitation
light energy h (Toyoda et al 1985) and is given by the expression,
( )ngh A h E (1.14)
near the band gap, where A is the constant independent of photon energy, hv
is the photon energy and Eg is the direct allowed energy gap. ( )2 is related
to hv linearly. For allowed direct transition, the constant n = 1/2. From the
26
Figure 1.8 Photograph of the VARIAN CARY 5E - spectrophotometer
plot of ( )2 versus h shown in the Figure 3.11, band gap energy Eg is
evaluated by extrapolating the linear fitted regions to ( )2 = 0.
1.3.2 Fourier Transform Infrared Spectroscopy (FTIR)
Infrared spectroscopy has been widely used in the instrumentation
laboratory for more than eighty years. An IR spectrum provides the
fingerprint of the sample with absorption peaks which correspond to the
vibrational frequencies between the bonds of constituent atoms of a material.
Infrared spectrum of any compound is unique because the combination of
atoms in each material is different. This helps the identification of each and
every material. Amount of material present produces different sizes of peaks
in the spectrum. Recent advancements in software helps a lot in quantitative
analysis. All infrared instruments are based on the principle of dispersion.
These instruments separate the individual frequencies of energy emitted from
27
the infrared source with the help of grating or prism. Grating is preferred over
the prism because it is a superior dispersive element than a prism. The
detector measures the energy at each frequency from the sample. The output
would be plotted in the form of a graph in terms of intensity vs frequency.
Instrumentation
The following components are present in a FTIR spectrometer.
They are
1. Radiation source
2. Interferometer and
3. Detector
FTIR is time domain spectroscopy, in which the radiant power
changes with time. The modulation of the high frequency signal is formed by
Michelson’s interferometer. Basic components in a FTIR spectrometer are
moving mirror, fixed mirror and a beam splitter. Radiations from the IR
source are collimated by the mirror and the resultant beam is divided at the
beam splitter. Half of the beam is passing towards the fixed mirror and other
half of the beam is reflected to the moving mirrors, which are placed at right
angles to each other. The two beams interfere constructively or destructively
after reflection depending on the path difference when the movable mirror is
moved with a constant velocity the intensity of the emerging radiation is
modulated in a regular sinusoidal manner. The modulated frequency after
passing through the sample compartment is focused on to the detector. The
detector signal is sampled at precise intervals during mirror scan. The
resulting signal from the detector is called as interferogram and the spectrum
is reconstructed using Fourier Transformation. The transform is carried out by
a computer and the spectrum is plotted on a paper.
28
FTIR is used to identify types of chemical bonding in a molecule
by producing an IR absorption spectrum. It has several advantages such as
non destructive testing (NDT) , very fast and higher optical throughput.
Figure 1.9 Photograph of the FTIR spectrometer
The Perkin Elmer spectrumone FT-IR instrument consists of Nernst
glower as source, an interferometer chamber consisting of KBr beam splitters
followed by a sample chamber and detector. It operates in the entire range of
4000 to 450 cm-1. This spectrometer functions under purged conditions. Solid
specimen samples are dispersed in KBr or polyethylene pellets based on the
region of interest. The resolution of this instrument is 4.0 cm-1. Several
functionalities like signal averaging, signal enhancement, baseline correction
and other spectral manipulations are possible in this instrument. The
photograph of the Perkin-Elmer Spectrumone FTIR spectrometer is shown in
Figure 1.9. FTIR spectrum is taken by making KBr pellets by applying a
pressure of 370 MPa using a hydraulic press.
29
1.3.3 Powder X-Ray Diffraction Studies
In the characterization of crystalline materials, powder X-ray
diffraction technique is widely applied due to its non-destructive testing
nature. Using this instrument, crystal structure, phase identification,
quantitative analysis, extraction of three dimensional microstructural
properties and determination of structure imperfections are studied. Crystal
structure of a crystalline material can provide information on the unit cell
dimensions.
Principle
Bragg’s law is the fundamental of X-ray diffraction. According to
Bragg’s law, a beam of monochromatic X-rays incident on a crystal creates
each atom to act as a source of scattering radiation. Certain planes in a crystal
possess rich number of atoms. The scattering of X-rays is assumed to be
reflections from these planes. At certain glancing angles, the reflections from
these set of parallel planes are in phase with each other, and they reinforce
each other to produce maximum intensity. For the other angles, the reflections
from different planes are out of phase, and they reinforce and produce either
zero or less intensity. From Bragg’s law,
nd sin2 (1.15)
where n is order of diffraction, d is the interplanar distance, is the
wavelength of the X-rays used and is the glancing angle. If the wavelength
of X-rays is known, the angle of diffraction and interplanar distance could be
determined. A set of interplanar distance values thus obtained from a single
compound will represent a set of planes which could be used to determine the
crystal structure.
30
Instrumentation
The set-up has a cathode ray tube acting as a source. Electrons are
produced by heating a filament and by applying a voltage the electrons are
accelerated to collide with the target material. Whenever the electrons have
sufficient energy to remove inner shell electrons in the target material,
characteristic X-ray spectra are produced. These spectra consist of
components like K and K . K consists of 1K and 2K . 1K has a
slightly shorter wavelength and twice the intensity as that of 2K . The
specific wavelengths emitted are characteristic of the target material (Cu, Fe,
Mo, Cr). Filters or crystal monochromators are used to produce
monochromatic X-rays used in diffraction. 1K and 2K are sufficiently
close in wavelength such that a weighted average of these two is used. These
X-rays are collimated and directed onto the sample. Sample placed on the
sample holder is rotated for different positions and the detector is also rotated
to measure the scattered radiation. When the X-rays fall on the sample at the
Bragg’s angle, constructive interference occurs and a peak in intensity is
obtained. Detector records and processes the X-ray signal and converts the
signal to a count rate and gives the output to a printer or computer monitor.
In our present work, Rigaku D/max-A diffractometer is used for
structural analysis. It is capable of producing beam of monochromatic X-rays
from a CuK radiation source of wavelength 1.5406 Å. In this method, the
nanofiber mesh is carefully collected and ground into fine powder which
possesses thousands of grains with random orientations. Scan is performed
from 10 to 70 , and diffraction has occurred through the material associated
with different atomic spacing. The photograph of powder X-ray
diffractometer is given in Figure 1.10.
31
An electronic detector is used to detect the diffracted X-rays. It is
rotated from 10 to 70 . For each angle, it sends the detected signal to a
computer. A graph is plotted between measured X-ray intensity and 2 .
Crystal structure is determined from diffraction peaks corresponding to ‘d’
spacing using Bragg’s law.
Figure 1.10 Photograph of the X-ray powder diffractometer
1.3.4 Non-Linear Optical Test (NLO)
Non linear optics plays a major role in photonics, image processing
and information devices (Ushasree et al 1999). Kurtz and Perry proposed a
powder Second Harmonic Generation (SHG) testing method for
comprehensive analysis of the second-order non linear optical effects.
32
Overview on non-linear optics (NLO)
When an electric field (E) is applied to a dielectric material, dipole
moments are created. The dipole moments created per unit volume is called as
dielectric polarization P(t) at time ‘t’. The value of P(t) depends on the
applied field and can be expressed as
tExtP 1 (1.16)
where 1x is the polarizability of the material
The non-linear optic phenomenon occurs at relatively high field
values. When the applied field value increases as in lasers, the polarization
behavior of the medium would not be linear as in equation 1.16. In such
cases, the equation can be modified as
tExtExtExtP 33221 (1.17)
where 2x and 3x are the second and third order susceptibility coefficients
of the medium.
The above equation can be written as
lkjijkl
kjijk
jij tEtEtEtP 32 (1.18)
where ij = Polarizability
ijk = First hyperpolarizability (second order effects)
ijkl = Second hyperpolarizability (third order effects)
and i, j, k and l correspond to the molecular co-ordinates.
33
In a three wave mixing process, the second order term is important
because it is a non-zero value in non-centrosymmetric materials.
1 21 2. . *i t i tE t E e E e C (1.19)
where C* is the complete conjugate.
E1 and E2 are the incident beams and the second order term can be read as
2 2 1 1 2 21 20 1 2 *i m m tn nP t x n E E e C (1.20)
Second harmonic generation (SHG)
Let us consider a plane wave of amplitude E travelling in a non
linear medium in the direction of its k vector. The polarization produced at the
second harmonic frequency
20 ,,'222 EdP ffe (1.21)
where 22 xd ffe
The wave equation at 2 is
Zkiffe eEd
cn
i
Z
E 2
2
..2
(1.22)
where 2 2k k k , at low conversion efficieny 2E E .
34
For an interaction length of l, the amplitude E remains
constant.
Applying the boundary condition
00,2 zE we get
2
2 0
2 , .l
i k ziE z l E e
n C
= 2 2
2
.2
. .
2
i k lk l
l Sini
E ek ln C
(1.23)
Optical intensity I is given by
2
0
02
EE
nI , (1.24)
Therefore,
2 2 22
2 32 0
2 . 22 ,
2
e ff
k lSind l
I l Ik ln n C
(1.25)
For the phase matched condition 0k , the intensity is maximized. If phase
matching is not performed, the polarization at 2 goes in and is out of phase
with the generated wave 2E and the conversion oscillates as2
lkSin ,
coherence length can be expressed ask
lc .
35
The process of transformation of light with a frequency into light
with double frequency is referred as Second Harmonic Generation. If p1
and
p2
are the momenta of the absorbed photons, and p is the momentum of the
emitted one. The process is spontaneous process and involves three-photon
transitions i.e., two photons with a energy of h per each photon are absorbed
spontaneously to emit a photon with an energy of 2 h .
Energy of the photons will give
+ h = 2h (1.26)
Momenta of the photons will produce
p1+ p
2= p (1.27)
The transformation of the light wave with frequency into two new
light waves with frequencies 1
and 2is termed as parametric generation. This
also represents three-photon process in which one photon with energy h is
absorbed and two photons, one with energy h1
and the other
with h2are emitted.
Instrumentation
Light wavelength of 1064 nm emitted by a Nd-YAG laser is
splitted into two rays. Reflected ray passes on to the trigger and given as
reference to the CRO which gives the fluctuation in the input pulse.
Transmitted beam passes towards the sample holder. After reaching the
sample, the beam from the sample is focused by the mirror onto the
36
photomultiplier tube. Narrow band pass filter filters the fundamental laser
radiation and allows only the second harmonic signal. The size and shape of
the particle affects the strength of second harmonic signal. Figure 1.11 gives
the schematic diagram of the Kurtz and Perry powder technique.
Figure 1.11 Schematic diagram of Kurtz- Perry set-up for SHG
measurement
Chemla and Zyss (1987) carried out SHG test on organic and
inorganic NLO materials for device fabrications. Powder SHG test is an
efficient tool to assess the non-linearity of NLO materials. Kurtz and Perry
(1968) introduced a method for the analysis of second order non-linear effect
using powder samples. Kurtz studied the NLO properties of quite a number
of compounds for various technological applications.
Light of wavelength 1064 nm, with 35 ps pulse-width emitted from
a Q-Switched Nd-YAG laser produces a spot of radius 1 mm. It is used to
illuminate the sample powder of dimension in nanometers placed between
two glass plates, using copper spaces of 0.4 mm thickness.
37
1.3.5 Atomic Force Microscopy (AFM)
The interactions between a sharp probe and a sample placed on the
platform are utilized for imaging. Atomic Force Microscopy (AFM) produces
a topographical image by moving a sharp tip about 2 m long held at the apex
of a cantilever, across a surface. The extension of the piezoelectric crystal is
responsible for the movement of the tip across the surface. Whenever the tip
scans the surface, the force between the tip and the surface causes the
cantilever to bend. An optical lever measures the deflection of the cantilever.
This optical lever consists of a laser beam reflected from the gold coated back
of the cantilever onto a positional sensitive diode. The photodiode is capable
of measuring changes in the position of incident laser beam up to 1 nm, which
produces sub-nanometer resolution. Figure 1.12 shows the schematic diagram
of Atomic Force Microscope.
Figure 1.12 Schematic diagram of Atomic Force Microscope
38
The microfabricated tips of Si or Si3N4 are used. The spring
constant of the tip is of the order of 1 N/m and smallest vertical displacement
observable is 0.5 nm. Cantilever thickness is 0.8 m and it is vibrated at a
resonant frequency of 140 kHz.
It is important to place the surface of the cantilever at a particular
distance. Weak attractive van der Waals forces are created between the
surface and the cantilever atoms when the cantilever approaches the surface.
When the cantilever moves near the surface these forces become repulsive,
since the cantilever attempts to displace the atoms on the surface. Both types
of forces displace the cantilever and are used to study the surface topography
and other properties. The photograph of the AFM is given in Figure 1.13.
Figure 1.13 Photograph of the Atomic Force Microscope
An AFM can be operated in two modes namely contact and
non-contact mode. Repulsive forces are measured when the cantilever moves
towards the sample in the contact mode. These repulsive forces are the
interactions between the atoms on the material’s surface and the atoms in the
tip of the cantilever. As the tip is in contact, it can damage the atoms in the
surface of the material. To overcome this, in the ‘tapping mode’, the
39
cantilever vibrates so that the tip contacts the surface intermittently. This
reduces the lateral forces incident on the sample and avoids possible surface
damage to the sample and tip contamination.
In non-contact mode the cantilever is oscillated with a small
amplitude about 5 to 10 nm from the surface of the sample and the attractive
van der Waals forces are measured. As the weak attractive forces are
measured in the non contact mode, the lateral resolution is less than that
achieved in the contact mode. In our work, AFM images were captured to
study the morphology of the nanofibers by using AFM (NanoSurf Easy
Scan2, Switzerland). It has maximum XY- Scan range of 70 µm and
maximum Z-range of 14 µm. The AFM has Z resolution of 0.21 nm and
XY resolution of 1.1 nm.
1.3.6 Scanning Electron Microscopy (SEM)
Scanning Electron Microscope was developed in the year 1942.
SEM has become more popular than TEM because it can produce images of
greater clarity with three-dimensional quality and little sample is only needed.
SEM has a large depth of the field, and most part of the image is magnified. It
can produce a magnification of up to 1,00,000 times than the normal size.
In Rutherford elastic scattering, the path of the electron is deflected
when it encounters a nucleus in the sample. Some of these electrons are
completely backscattered, and emerges again from the surface of the sample.
Since the atomic number of the nucleus decides the scattering angle, the
primary electrons reaching the detector can be used to produce images with
the information about atomic composition and topology. The electrons present
at a short distance from the surface of the sample are able to come out and are
observed by the detector. The images obtained from these secondary particles
40
contain large information. This enables to obtain topographical images with
higher resolution. Figure 1.14 shows the schematic diagram of Scanning
Electron Microscope.
Instrumentation
Important components in a SEM are
1. Electron gun
2. Electron lenses
3. Scanning coils
4. Electron detector
Figure.1.14 Schematic diagram of Scanning Electron Microscope
41
Beam of electrons emitted from the electron gun in the SEM,
passes through a condenser lens and made into a fine beam. An objective lens
focuses the electron beam onto the sample. A set of coils is present in the
objective lens applied with varying voltages. These coils generate an
electromagnetic field on the beam of electrons and redirect the electrons to
scan the sample in a controlled manner called as raster. As the electrons from
the electron beam incident on the sample, a series of interactions deflect
secondary particles to a detector. The detector converts the signal into voltage
and amplifies it. This voltage is then applied to a Cathode Ray Oscilloscope
and converted into an image. Depending on the topography of the sample,
based on the angle in which the electrons bounce off from the sample, the
secondary particles are emitted and the intensity of the image is varied. The
photograph of a scanning electron microscope is given in Figure 1.15.
Figure 1.15 Photograph of Scanning Electron Microscope
42
The major drawback of a SEM is that the biological samples that
give off vapour could not be analyzed by a SEM. The vapour would interact
with the electrons. The schematic diagram of a scanning electron microscope
is given in Figure.1.14. In our work, to observe the surface topography,
morphology and cross-section of the nanofibers, scanning electron
microscopy images were captured using a JEOL GSM-5900 scanning electron
microscope. The SEM will give best results with conductive samples. Two
coaters from Quorum Technologies are available to coat non-conductive
samples. The Polaron High Resolution SC7640 Gold Coater is used for ideal
imaging while the Polaron CC7650 Carbon Coater is the perfect instrument
used for elemental analysis.The Scanning Electron Microscope provides
magnification of up to 80,000 times.
1.3.7 EDAX or EDS Spectroscopy (Energy Dispersive Analysis of
X-rays)
EDAX used in conjunction with a SEM and it can perform an
elemental analysis on the microscopic sections of the material or
contaminants that may be present in the sample.
In addition to viewing the three dimensional images by a
microscope, it is often necessary to find out the different elements present in
the sample. This is achieved by using the built-in spectrometer known as
Energy Dispersive X-ray Spectrometer. This analytical technique uses X-rays
which are emitted by the sample when bombarded by the electron beam to
find out the elemental composition of the sample. Whenever the sample is
bombarded by the electron beam of the SEM, electrons are ejected by the
atoms from the surface of the sample.
43
The vacancy of the electron created by the ejection, is occupied by
an electron from the higher shell and to balance the energy difference between
the two electrons an X-ray is emitted. The EDAX X-ray detector measures
the number of X-rays emitted with respect to their energy. The emitted X-ray
energy is characteristic of the element from which the X-ray is emitted. A
spectrum is plotted between the energy and relative counts of the detected
X-rays. This is utilized to determine elements present in the sample
qualitatively and quantitatively.
EDAX is advantageous in detecting low atomic number elements
like oxygen and carbon, which are present everywhere in our environment.
Elements like H, He, Li, Be and isotopes cannot be tested. It is also
advantageous because of its cost-effectiveness and fast collection of the
spectra. The main drawbacks are that it has poor resolution and spectral
artifacts. In our present work, JEOL JSM 5900 Scanning Electron Microscope
along with Oxford INCA Energy EDS System is employed. The EDS system
allows point by point elemental analysis.
1.3.8 Vibrating Sample Magnetometer (VSM)
Nanomagnetism is mainly the study of magnetically ordered
materials when they are restricted in one dimension. Comparing the magnetic
properties of materials in the nanoscale with bulk state brings out interesting
facts. Reduction of dimensions to one dimensional or zero dimensional
structures makes the complication due to additional effects such as
demagnetization, dipole interaction and change in magnetic anisotropy.
Understanding of these effects and measurement of them are the main
objectives of nanomagnetism.
44
Size and Shape dependent magnetic properties
Moving charges create magnetism. Elementary particles like
electrons have an intrinsic magnetic moment (spin), which determines the
quantum state. The orbital and spin motion of the electrons are the origin for
the magnetic properties of materials. The contribution due to the nuclear
magnetic moment is very small and negligible. The nanometer-sized metal
particles’ electronic structure is mainly dependent on size and it influences on
the magnetic behavior also.
All materials respond to the magnetic field by attracted towards the
magnetic pole in ferromagnetic and paramagnetic materials and repelled from
the magnetic pole in diamagnetic materials. Application of a magnetic field to
a material can cause magnetization (M) of the material. Magnetization can be
measured by Super conducting Quantum Interference Devices (SQUIDs) and
Vibrating Sample Magnetometer (VSM). A ferromagnetic material exhibits
hysteresis behavior due to a cycle of magnetization. At larger field values, the
magnetization reaches a saturation called as Saturation Magnetisation (Ms).
Whenever the applied field is reduced to zero, there is residual magnetization
in the material called as retentivity (Mr). A reverse-magnetic field is applied
to remove the magnetization in the material and called as coercivity (Hc).
In bulk ferromagnetic materials, the material is divided into several
domains separated by domain walls of thickness of nanometer. All the
moments in each domain are oriented in the same direction and the orientation
of successive domains is completely different. When the particle size
decreases below a critical value, the formation of domain walls become
energetically unfavorable, and can support only a single domain. This critical
size depends on the material and is usually in the order of tens of nanometers,
45
varying from ~ 14 nm for Fe upto ~ 170 nm for 32 OFe . Magnetic particles
in the nanometer scale are usually in single domain.
Magnetic susceptibility is the ratio between Intensity of
magnetization and applied field. Susceptibility of a material also depends on
the direction in which it is measured. This is called as anisotropy. When
magnetic anisotropy exists, the total magnetization of a system will prefer to
lie along a special direction, called the easy axis of magnetization. For single
domain particles, the energy associated with this alignment is called as
anisotropy. Energy Ea can be written in the simplest uniaxial approximation
form as
2SinkVEa (1.28)
where k is anisotropy constant, V is the volume of the particle and is the
angle between the moment and the easy axis.
The coercivity of a magnetic particle strongly depends on its size.
In a single domain particle, the change of direction of magnetization can
occur only by coherent rotation of spins, which produces higher coercivity of
single domain particles compared to multi-domain particles. Further decrease
in particle size, reduces coercivity due to the increasing role of thermal
fluctuations.
VSM operates based on the Faraday’s law of Induction. It explains
the change in magnetic field caused by an electric field. If the electric field is
measured accurately then it can provide the information about the change in
magnetic field. Thus, a VSM can be effectively used to measure the magnetic
behavior of magnetic materials.
46
The operation of a VSM is such that the sample to be studied is
kept in a constant magnetic field. If the sample placed possesses magnetic
property then the constant magnetic field will magnetize the sample by
arranging the magnetic domains or the individual magnetic spins along the
field. Larger magnetization would be created if the constant field applied is
strong. A magnetic field is created around the sample due to the magnetic
dipole moment of the sample. This is called as the magnetic stray field. When
the sample is moved up and down, the magnetic stray field varies as a
function of time and a set of pick-up coils can sense them.
According to the Faraday’s law of induction, the alternating
magnetic field will create an electric field in the pick-up coils. The current
produced would be proportional to the magnetization of the sample. The
induced current is large if the magnetization is large.
Figure 1.16 Photograph of Vibrating Sample Magnetometer
47
Instrumentation
The VSM instrument has the following parts.
1. Water cooled electromagnetic and power supply
2. Sample holder and vibration exciter
3. Sensor coils
4. Amplifier
5. Lock in amplifier
6. Computer interface
Amplification of the induction current is performed by
transimpedance amplifier and lock-in amplifier. Based on the magnetization
of the sample, its dependence on the strength of the constant magnetic field is
suitably explained by controlling and monitoring software.
The processes involved in VSM measurements are explained here.
Initially the strength of the constant magnetic field is set. Then the sample
begins to vibrate. The magnetic moment of the sample is calculated from the
signal received from the probe. The strength of the constant magnetic field is
changed to a new value and magnetization of the sample is changed and
noted. The constant magnetic field varies over a range and plot of
magnetization (M) and magnetic field strength (H) is generated. In our work,
M-H hysteresis loops were obtained from Lakeshore VSM 7410 magnetometer.
The Vibrating Sample Magnetometer can give plots of M vs H at Room
Temperature and M vs T at constant H. ZFC/FC measurements can also be
included. It can also give plots of M vs H at constant temperature (T). The
temp range can be selected with regular intervals in the Low Temperature
range of 20-300 K and High Temperature range of 300-1270 K. The
photograph of Vibrating Sample Magnetometer is given in Figure 1.16.
48
1.3.9 Dielectric Characterization
Impedance analysis is the basic means of evaluating electronic
components and materials. Depending on the dielectric and insulation
properties, every material has a unique set of electrical characteristics.
Measurement of these properties with precision can provide valuable
information to ensure the intended application and maintain a proper
manufacturing process.
Dielectric constant measurement is one of the most popular
methods of evaluating the solid samples, such as electric insulators and
polymers due to its simplicity. It helps in evaluating the electrical properties
and physical properties like the structure of elements.
Dielectric constant of solids which can be shaped into a disc can be
obtained using the following equation.
mFA
Ctr /
0 (1.29)
Relative permittivity
02
20 xdx
Ct
A
Ctr (1.30)
where r is dielectric constant
0 is dielectric constant of vacuum
t thickness of test device
A area of the disc
49
C Capacitance of the disc
d diameter of the disc
Whenever the strength of the electric field through solid changes,
the polarization lags behind the electric field. This is known as ‘dielectric
after effect’ and can be derived +as a function of time as
tet .
1(1.31)
where is dielectric relaxation time and t is time
Dielectric constant can be written in complex form as *= ’- j ”
* is complex dielectric constant
’ is real part of dielectric constant
” is imaginary part of dielectric constant
tan is dissipation factor
D is dissipation factor of test device
We can obtain ’and ” by using , using the Debye equation
220 1
1' rrr (1.32)
220 11
" rr (1.33)
where 0r is the dielectric constant when frequency ~ 0
where r is the dielectric constant when frequency ~ .
50
Dielectric constant is defined as the ratio of the capacitance of the
material to the capacitance of the air. This value for dry air is 1.00053, which
is very close to vacuum. For a material to be used for insulating purposes, the
dielectric constant can be as low as possible. If it has to be used for storage of
electrical charge, it would be better to have as high as possible. More charge
is stored when a dielectric is present than if no dielectric is present.
Dissipation factor is defined as the ratio of the resistance of the
insulating material to its capacitative reactance at a specified frequency. This
measures the loss and it is always higher than zero, but usually less than the
dielectric constant. In our work, we have used HIOKI 3532-50 LCR
HITESTER meter for dielectric measurements. It operates between a wide
range of frequencies from 42 Hz to 5 MHz. It has high resolution and high
basic accuracy of ±0.08%. It has the fastest measurement time of 5 ms.
Several parameters that could be measured are absolute value of impedance,
absolute value of admittance, equivalent series inductance, equivalent parallel
inductance, equivalent series capacitance, equivalent parallel capacitance,
equivalent resistance, equivalent series resistance, equivalent parallel
resistance, quality factor, dissipation factor, equivalent reactance, equivalent
parallel conductance , equivalent parallel susceptance, impedance phase
(radian), impedance phase (degree), admittance phase (radian) and admittance
phase (degree). Figure 1.17 gives the photograph of HIOKI 3532-50 LCR
HITESTER meter.
51
Figure 1.17 Photograph of HIOKI 3532-50 LCR HITESTER meter
1.3.10 Overview on Ferroelectric Property
Ferroelectric materials exhibit electric dipole moment even in the
absence of the electric field called as spontaneous electric dipole moment.
They exhibit hysteresis loop when the electric displacement is plotted against
the applied field. The positive and negative charges in these materials do not
coincide in the ferroelectric state as in polar molecules. This ferroelectric
behavior disappears above a certain temperature called as the Curie
Temperature (Tc) and becomes paraelectric. A ferroelectric material can be
pyroelectric if the spontaneous moment of polarization varies with
temperature.
There are two types of ferroelectric phase transitions namely order-
disorder and displacive. Ferroelectricity was discovered in 1920 in Rochelle
salt by Valasek.
Ferroelectric crystals possess several domains in which the
moments are oriented in the same direction and the orientation in the nearby
domains would be in different directions. Changes in the dipole moment can
be brought by changing the temperature or applying an electric field.
52
In polycrystalline materials like ceramics, each ceramic grain will
not have properties like that of a single crystal. Presence of the grain
boundaries and the crystallography axes of the grains are randomly oriented,
the macroscopic properties of the ceramic will in general differ to large extent
from those of a single crystal.
In ferroelectric materials, dielectric constant changes with
temperature according to Curie-Weiss law,
Cr TT
CB' (1.34)
where B and C are constants independent of temperature. C and TC are called
as Curie-Weiss constant and Curie temperature. The second term is very large
compared to the first term. Therefore, we can ignore B and we can rewrite the
equation as
Cr TT
C' (1.35)
Frequency dependent dielectric constant can be derived from the
Debye’s equation as
22'
1
0 rrrr (1.36)
r is the dielectric constant measured at high frequency and equals the
square of the optical index of refraction.
We can rewrite the above equation in another form by taking2nr ,
53
2' 2
2 2
0
1r
r
nn (1.37)
Square of optical index of refraction 2n for ferroelectric materials
is negligible with respect to 'r and 0r ,
22'
10r
r (1.38)
Frequency dependent dielectric loss from the Debye’s equation is
given by
2222"
1.0
1.0 rrrr (1.39)
Substituting from equation 1.38 in 1.39, we get
.'''rr (1.40)
and by substituting form 1.35 in 1.40, we get
TcTCr ." (1.41)
ThenC
TcTr ." (1.42)
Therefore when TcT , then 0 and equation 1.38 can be written as
0'max rr .
54
From this conclusion, the dielectric peak at the phase transition
temperature in ferroelectric materials can be considered as 0'r in the
Debye’s equation for the dielectric materials.
But 'maxr is a function of frequency
ie 'max
' 0 rr (low frequency) 'maxr (high frequency)
At the phase transition temperature Tc , equation (1.42) cannot be
used for the calculation of relaxation time. The model described by equation
(1.42) can be modulated into another simplified model and taken into
consideration that the variation of temperature, when TcT then c ,
where c is relatively the smallest value for the relaxation time in the
ferroelectric material and corresponding to the highest value of dielectric
constant. From the value of c and equation (1.38), for low and high
frequencies
max'max 2 2
0
1
rr
L c
L (1.43)
22
max'max
1
0
cH
rr H (1.44)
From these equations, we can get
55
max22
max 1
R
R
LHc (1.45)
whereH
LR
r
r'max
'max
max (1.46)
Substituting the value of c in equation 1.44, the value of 0maxr
can be obtained. Debye’s relaxation time can be found out as a function of
temperature by substituting the value of 0maxr instead of 0r in
equation 1.38.
can be calculated for all temperatures, except at the Curie
Temperature as
21
max1 10
Tr
r (1.47)
The values of from the previous equation are corrected for the
whole range of temperatures both for ferroelectric and paraelectric phases
except at T = Tc and = c. Relaxation frequency (Fr) can be found out using
the formula.
1rF (1.48)
1.4 SCOPE OF THE THESIS
The thesis objectives are mainly focused on synthesis and
characterization of one dimensional composite nanofibers. Sol-gel method
was used to synthesize the sol of Poly (Vinyl alcohol) and metal acetate
56
composite. The sol is electrospun to obtain the nanofibers. These nanofibers
were calcined at a higher temperature and fibers in the crystalline phase were
obtained. By varying the parameters of the sol, the diameter of the obtained
fibers can be controlled.
In chapter 2, synthesis and characterization of ZnO / CuO
composite nanofibers are presented. The effect of applied voltage on the
morphology of the fibers was studied. The electrospun nanofibers were
characterized by using FTIR, UV, SEM, AFM, powder XRD, EDAX,
dielectric and Kurtz powder techniques. These fibers exhibited a higher SHG
efficiency of 11.1 times compared with KDP.
Chapter 3 deals with the synthesis and characterization of BaO /
MnO composite nanofibers. Morphology of these nanofibers were studied by
varying the size of the needle opening. The synthesized fibers were
characterized by using UV, SEM, AFM, powder XRD, EDAX, dielectric and
VSM techniques. These fibers exhibited room temperature ferromagnetic
behavior. At low temperatures, the ferromagnetic behavior was masked by
paramagnetic property.
In chapter 4, ZnO / BaO composite nanofiber synthesis and
characterization are presented. The effect on morphology of these fibers was
studied with respect to working distance (distance between the electrodes).
Synthesized fibers were characterized by using UV, SEM, AFM, EDAX,
dielectric, powder XRD and Kurtz powder techniques. These fibers exhibited
ferroelectric to paraelectric phase transition at 323 K.
Chapter 5 deals with the synthesis and characterization of ZnO /
CaO composite nanofibers. After synthesis, these nanofibers were
characterized by using FTIR, SEM, AFM, EDAX , powder XRD, UV,
57
dielectric and Kurtz - powder techniques. These nanofibers had a high
dielectric constant of 2814 at 50 Hz at room temperature. ZnO/Ca0.5O and
Zn0.5O/CaO composite nanofibers were also synthesized by electrospinning
and characterized by UV, dielectric and Kurtz- powder techniques. Second
harmonic generation efficiency of ZnO/Ca0.5O and Zn0.5O/CaO
nanocomposites were found to be 1.63 and 2.94 times than that of KDP. The
A.C.conductivity of ZnO/Ca0.5O, Zn0.5O/CaO and ZnO/CaO nanocomposite
materials were also investigated.
Summary of the work based on the present research investigation
and suggestions for the future work to be carried out on the basis of the results
obtained are presented in chapter 6.