Modern engineering systems, such as computers, high-performance engines and nuclear reactors, require new and better materials than have previously been available. Improved materials and processes will play an ever increasing role in efforts to improve energy efficiency, promote environmental protection, develop an information infrastructure and provide modern and reliable transportation and civil infrastructure systems. The natural resources from which these and other materials are created are less available today than in the past, creating a need for research concerned with the selection and design of materials. In certain industries, the need for energy-efficiency, energy conversion systems and materials with the ability to withstand hostile chemical and/or thermal environments has sparked a growing interest in processing and properties of ceramic materials. In addition, the continuing trend towards miniaturization in the electronics industry imposes constantly growing demand on semiconductor properties and requires the development of new and innovative processing techniques.
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While ceramics have traditionally been admired for their mechanical and thermal stability, their unique electrical, optical and magnetic properties have become of increasing importance in many key technologies including communications, energy conversion and storage, electronics and automation. Such materials are now classified under Electroceramics as distinguished from other functional ceramics such as advanced structural ceramics. Historically, developments in the various subclasses of Electroceramics have paralleled the growth of new technologies. Advances in materials research, therefore, enable progress across a broad range of scientific disciplines and technological areas, with dramatic impact on society.
The extensive research devoted to the materials of solids during the last three decades has led to great advances in the understanding of the properties of solids in general. Recently one area of very active research has been in the mixed oxide ceramic systems. These have been of particular interest because of their ease of fabrication, flexibility and the fact that a wide range of properties can be obtained by substitution of one ion for another. One of the most extensively studied groups of compounds is the oxygen octahedral type ferroelectrics.
The word Nanotechnology simply nano in its form, means a billionth (1 x 10-9) of a meter length of measurement. It is the study of control of matter at atomic and molecular scale level and it deals with structures of the size100nm and/or below. Reflecting many of the nanoscience possibilities, Richard P. Feynman emphasized in his talk on "There is Plenty of Room at the Bottom" in the year 1959. In his talk, Feynman thought that, what might be on the molecular scale, and challenged the people of technology "to find ways of manipulating and controlling things on a small scale". Inspired by the visualization of Feynman, today, nanoscience is defined as the study of material manipulation and control at the molecular scale, that is, a spatial scale of the order of a few hundred angstroms, less than one-thousandth of the width of a human hair.
Now, in the present days, nanotechnology is used in almost all fields such as medicine, electronics, energy production etc, ranging from novel extensions of conventional device physics, to complex new designs based on molecular self-assembly, to developing new materials and devices etc on nanoscale measurement.
Nanoscale materials are those materials which fall in between atoms / molecules and condensed matter. These materials have one or more dimensions in the range of 1 to 100 nanometers. The size of a nanoparticle usually is less than 100 nm. The reason that size matters is that the properties of materials can have some unexpected differences from their behavior in larger bulk forms that makes for new application opportunities. The two reasons for this change in behavior are an increased relative surface area (producing increased chemical reactivity) and the increasing dominance of quantum effects. The physicochemical properties of nanomaterials depend on their size, shape, composition, etc. The interesting properties that change with size and shape are chemical reactivity, melting point, optical and magnetic properties, specific heat, etc .
Incremental nanomaterials are materials that have improved properties at the nanoscale, typically as a result of their greatly increased surface area, but do not typically take advantage of the quantum effects. Nanoparticles are already seeing application, taking advantage primarily of the high surface area of these fine powders. Nanoceramic powders, the most commercially important of which are simple metal oxides, constitute almost 90% of the total market. For example, nano-sized zinc oxide particles are in use in sunscreen.
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Evolutionary nanotechnology takes advantage of the changes that can occur in materials at the nanoscale related both to increased chemical reactivity and the increasing importance of quantum effects. Examples include nanoscale sensors that exploit the large area of nanotubes and semiconductor nanostructures such as quantum dots and quantum wells.
1.2.2 Basic Structures of Nanomaterials
Nanoparticles - ultrafine solid particles on a nanoscale including nanopowders and nanocrystals;
Nanotubes - hallow nanoscale particles including nanotubes, nanohorns and nanocapsules;
Nanostructured materials and coatings - materials made of structural elements (clusters, crystals, molecules) with dimensions in the nano-range and which may form films or be free standing; and
Nanocomposites - mixtures of components at least one of which has nanoscale dimensions.
1.2.3 Applications of Nanomaterials
Clay nanoparticles in packaging materials, where reduced porosity leads to less gas entering (e.g. less gas such as oxygen that spoils foods);
Rolled graphite nanotubes used in coatings on car bumpers that better hold their shape in a crash;
Carbon nanotubes which are sources of filed-emitted electrons and create enhanced phosphorescence e.g. in "jumbotron" lamps used at many athletic stadiums;
Nanoparticles of zinc oxide in sunscreens, more efficient at absorbing UV than more traditional white titanium dioxide lotions and leaving the lotion smooth and transparent;
Textiles which are dirt and crease resistant due to naocoatings;
Nanoparticles used as antiseptics, for abrasives and in paints;
Nanocoatings for spectacle glasses (making them scratchproof and crack resistant);
Nanocoatings for tiles to reduce slipping;
Electrochromic of self-cleaing nanofilm coatings on windows, which in sunshine breaks down dirt and helps the water falling on it to carry the dirt away;
Nanofilms with non-stick properties used as anti-graffiti coatings for walls;
Ceramic coatings for solar cells to improve scratch and erosion resistances;
Nanoceramics for more durable and better medical prosthetics.
Nanoparticles research is currently an area of intense scientific research, due to a wide variety of potential applications in biomedical, optical, and electronic fields. Processing, properties and cost issues are pushing down the particle sizes of powders used in a variety of industries. Fine, ultrafine and nanostructured powders are now critical to advancement of numerous applications.
1.3 PEROVSKITE FERROELECTRICS
Many oxide structures are based on close packing of cations and anions. A somewhat different structure occurs where large cations are present, which can form a close packed structure along with the oxygen ions. Calcium titanate (CaTiO3) was the first compound to be classified as a perovskite, by Russian geologist Perovsky. The most commonly studied ferroelectrics have the cubic perovskite structure (in paraelectric phase) with chemical formula ABO3. As conveniently drawn (Fig. 1.1), A- site cations occupy the corners of a cube, while B-site cations sit on the body center. Three oxygen atoms per unit cell rest on the faces. The lattice constant of these perovskite is always close to 4a (where 'a' is the lattice constant) due to the rigidity of the oxygen octahedral network and the well-defined oxygen ionic radii of 1.35 Å.
The A- and B-site cations are 12- and 6-coordinated respectively, and the A-site cation is normally larger than the B-site cation. The oxygen anion has a coordination number of 6. Assuming that all the ions are hard spheres, the lattice parameter a of the cubic perovskite is given by
as shown in Fig. 1.2 (a) and (b). Here, , and are the ionic radii of the 12-coordinated A-site cation, 6-coordinated B-site cation, and 6-coordinated oxygen anion, respectively. The stability of the perovskite structure also can be described geometrically as the ratio of Eq. (1.2) to Eq. (1.3), defined as the tolerance factor 't' by
It is advantageous that the A- and B-site cations are in contact with oxygen anions for an ABO3 compound to form a stable perovskite structure. That is, the perovskite structure is more stable if t ï‚» 1.0.
A practical advantage of the perovskite structure is that many different cations can be substituted on both A and B site without drastically changing the overall structure. Complete solid solution is easily formed between many cations, often across the entire range of composition. Simple solid solution series are basically of two types: substitutional solid solutions and interstitial solid solutions. In substitutional solid solutions the atom or ion that is being introduced directly replaces an atom or ion of the same charge in the parent structure while in case of interstitial solid solutions the introduced species occupies a site that is normally empty in the crystal structure. Even though two cations are compatible in solution, their behavior can radically be different when apart from each other. Thus it is possible to alter the properties of the materials such as Curie temperature, piezoelectric coefficient etc. with a small substitution of given cation.
Fig: 1.1 Cubic ABO3 Perovskite Structure and Geometrical Consideration for Lattice Parameter 'a'
CARBON NANOTUBES (CNTs)
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In the year 1991 Iijima , discovered Carbon Nanotubes (CNTs) accidentally during his experiment on synthesis of fullerenes by arc-discharge method. Later the CNTs have been drawn most scientists and research scholars into great interest, both from fundamental view point and for future applications. He invented first the Multiwalled Carbon Nanotubes (MWCNTs)  and then derives from there the Single Walled carbon Nanotubes (SWCNTs). The MWCNTs consist of 2 to 30 concentric graphitic layers, diameter of which range from 10 to 50nm and length of more than 10Âµm. on the other hand, SWCNTs have much thinner from 0.34 to 1.4nm. The most noticeable features of these structures are their mechanical, electronic, electrical, chemical and optical characteristics, which open a means to future applications. On single wall nanotubes, these properties can also be measured. For commercial application, large quantities of purified nanotubes are needed. Moreover, these structures are atomically specific significance that each carbon atom is still three-fold coordinated without any dangling bonds. CNTs have been used to build prototype nanodevices in many laboratories. These nanodevices consist of field-effect transistors, metallic wires, electromechanical sensors and displays. They prospectively form the basis of future all-carbon electronics.
An ideal nanotube can be consideration of as a hexagonal network of atoms of the carbon, which has been rolled up to build a seamless cylinder. Just a nanometre across, the cylinder can be tens of microns long, and each end is "capped" with half of a fullerene molecule. Single-wall nanotubes can be consideration of as the fundamental cylindrical structure, and these form the building blocks of both multi-wall nanotubes and the ordered arrays of single-wall nanotubes called ropes. Many theoretical studies have predicted the properties of single-wall nanotubes. Primary synthesis methods to prepare nanotubes include methods of laser ablation, arc discharge, gas phase catalytic growth from carbon and carbon sources and chemical vapour deposition. Considering the application of carbon nanotubes as reinforcements in composites which requires production of large amount of carbon nanotubes economically, gas phase techniques like chemical vapour deposition (CVD) tenders the greatest potential for optimization of nanotube production.
MOTIVATION (Seeking for new materials)
For all the amazing advances that have been made in semiconductor technology, most importantly in miniaturization and processor speed, some goals remain elusive. Among these goals is realizing the "ideal" nonvolatile memory - memory that retain information even when the power goes. As modern portable electronic devices such as mobile phones and note book computers become more and more popular, there is a confirmed increase in the demand for non-volatile memories. Semiconductor memories such as dynamic random access memories (DRAMs) and static random access memories (SRAMs) currently dominate the market. However, the main problem with these memories is that they are volatile.
Since the first discovery of ferroelectricity in Rochelle salt many other materials with crystal structures of perovskite, pyrochlore and tungsten bronze have been found and studied for the applications in memory devices. Ferroelectric Random Access Memories (FeRAM) are most promising alternatives, which are non-volatile and have the added benefits of greater radiation hardness and higher speed.
These devices use the switchable spontaneous polarization arising due to the positional bistability of constituent ions and thereby storing the information in the form of charge. Since the response time of the ion displacement is of the order of nano second or less, non-volatile random access memory can be realized using ferroelectric capacitors in which two binary states of "0" and "1" are represented by the direction of spontaneous polarization. Their non-volatility is because the polarization remains in the same state after the voltage is removed, and their radiation hardness allows devices containing these memories to be used in harsh environments, such as outer space. One of the problems with ferroelectric memories is their tendency to loose data after a certain number of read/write cycles. This phenomenon is called fatigue. At the moment the fatigue resistance of FRAMs is not sufficient for them to replace completely semiconductor memories, but this can be improved with further optimization of both composition and microstructure.
Organization of the thesis
To obtain high quality materials (such as ferroelectrics, CNTs etc) for a specific use, it is necessary to understand various phenomena concerning structural and electrical properties of the materials. For this purpose it is desired to make available as many experimental data as possible by using various techniques. Hence the aim of the research described in this thesis is focused on the synthesis and characterizations of BiFeO3, xCrFe2O4-(1-x)BiFeO3 nanoceramics and Carbon Nanotubes (CNTs). The effect of various-dopant-induced changes in structural, dielectric, ac impedance, ferroelectric hysteresis, mechanism of the dielectric peak broadening and frequency dispersion have been addressed. The basic idea is to develop a low processing temperature and enhance the dielectric and ferroelectric properties of these ceramics.
The following are the main objectives of the present work.
To study the processing conditions and synthesize pure/doped, homogeneous and reactive BiFeO3, xCrFe2O4-(1-x) BiFeO3 nanoceramics and Carbon Nanotubes at a low processing temperature based on the understanding of their transformation kinetics.
To investigate the decomposition and phase formation behavior of the material by TGA, DTA, FTIR and XRD for better understanding of the structural phase transition.
To study the electrical and morphological characteristics of the synthesized materials in order to understand the fundamental science, this will lead to further understanding of the link between microstructure and ferroelectric properties.
To study the mechanical properties such as bending, buckling and torsion effects under different conditions for Single and Multi walled Carbon Nanotubes using Finite Element Analysis simulation technique.
The brief description and the contents of this thesis (chapter-wise) are given below:
Chapter-1 describes the brief introduction of the ferroelectrics and fundamental aspects of ferroelectricity with a specific mention of bismuth ferrite oxide (BFO: a Perovskite ferroelectric); chromium ferrite oxide (CrFe2O4) belonging to the aurivillius family of Perovskite ferroelectrics, their structure and importance and Carbon Nanotubes (CNTs). In Chapter-2 a brief literature survey has been presented related to bismuth ferrite oxide (BFO), chromium ferrite oxide (CFO) perovskite structures, and Carbon Nanotubes. In order to get better understanding of structure-property relations of solids, various experimental techniques are normally used. Hence the basic principles and working of various instruments used in the present work are given in Chapter-3.
Chapter-4 discusses Polycrystalline samples of BiFeO3 were synthesized at low temperature using sol-gel technique and studied the characteristics under XRD, SEM, DTA analysis etc. A part of this chapter has been published as a research paper entitled "Effect of Sintering Temperature on Structural and Electrical Properties of BiFeO3 Multiferroic Nanoceramics" in Indian Journal of Engineering Materials and sciences - , 2010
Chapter-5 presents experimental study on synthesis and characterizations of xCrFe2 O4-(1-x) BiFeO3 Multiferroic Nanocomposites, with x = 0.0, 0.1, 0.2, 0.3 and 0.4. A part of this chapter has been published as a research paper entitled "Structural, Magnetic and Dielectric Properties of xCrFe2 O4-(1-x)BiFeO3 Multiferroic Nanocomposites" in Journal of Physica B - Condensed Matter, 405, 2010, pp 1325-1331.
Chapter-6 contains a detailed study of synthesis and characterizations of Carbon Nanotubes (CNTs). All the experimental results are supported on theoretical basis from literature. A part of this chapter has been published in the form of a research paper entitled "Synthesis and Characterization of Carbon Nanotubes" in International Journal of Technology Spectrum - JNTU-H, (accepted, 2009). Chapter 7 presents successful modeling and analysis of Single Walled Carbon Nanotubes (SWCNTs) and studied bending and buckling deformations under static and dynamic loading conditions. A part of this chapter has been published in the form of a research paper entitled "Simulation Studies on Dynamic response of single walled carbon Nanotubes under bending - buckling loads in FEA Approach" in the International Journal of Nanotechnology and Applications - Vol: 2, No.3, (2009), , pp.49-60 published by Research India Publications - New Delhi.
In Chapter 8, deals with simulation studies on multiwalled carbon nanotubes using Finite Element Analysis. For analysis purpose, 13, 14 and 15 walled tubes are considered for bending, buckling and torsional loads. The results were compared with previous work in the literature. Finally, Chapter 9 gives the conclusions and recommendations for future research. At the end of this thesis copies of the published research papers are enclosed.