1

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

22

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.