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Ceria based materials have attracted a great deal of interest particularly in area of UV shielding, oxide ion conductivity, solid state electrolyte for fuel cells, automotive exhaust catalysis, water gas shift (WGS) reaction catalysis and also in thermo-chemical water splitting cycles to generate hydrogen. Therefore great deal of efforts was devoted to synthesize nanocrystalline ceria and related materials with different shape and sizes. For example, hierarchically mesostructured doped CeO2 showed potential photvoltic response for solar cell applications. Substitution of lower valent metal ions (Ca2+, Gd3+, Tb3+, Sm3+) in CeO2 enhances oxide ion conductivity for solid oxide fuel cell applications. Eventhough ZrO2 is a nonreducible oxide, CeO2-ZrO2 solid solution has attracted a lot of attention in exhaust catalysis because it exhibited high oxygen storage capacity (OSC). Noble metal ion (M = Pt4+/2+, Au3+, Rh3+, Pd2+ and Ag+) substituted CeO2 (Ce1-xMxO2-δ and Ti1-xMxO2-δ, x = 0.01-0.03) prepared by solution combustion method have shown much higher three-way catalytic property compared same amount of noble metal impregnated to CeO2. Ionically substituted Pt and Au in CeO2 also showed high WGS activity. CeO2-MOx (M= Mn, Fe, Cu, Ni) mixed oxides have shown high activity for hydrogen generation by thermal splitting of water. In chapter 1, we have discussed recent developments on various synthesis strategies of ceria based materials for specific catalytic application. In this thesis, we have explored new route to synthesize Ce1-xMxO2-δ and Ti1-xMxO2-δ (M = transition metal, noble metal) nanocrystallites. Specifically we have addressed the effect of reducible metal ion substitution on the OSC of CeO2 for auto exhaust treatment, hydrogen generation and electro-chemical applications. Controlled synthesis of CeO2 and Ce1-xMxO2-δ (M = Zr, Ti, Y, Pr and Fe) nanocrystallites by hydrothermal method is presented in Chapter 2. The method is based on complexation of metal ion by diethylenetriamine (DETA) or melamine and the simultaneous hydrolysis of metal ion complexes in hydrothermal condition. Size of the crystallites can be controlled by varying the time and temperature of the reaction. 15% Fe3+ ion substituted CeO2 (Ce0.85Fe0.15O2-δ) nanocrystallites have shown higher oxygen storage capacity than Ce0.5Zr0.5O2 at lower temperature. A brief description of material characterization techniques such as powder X-ray diffraction (XRD) and Rietveld refinement of structure, high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) is presented. The home-built hydrogen uptake measurement system for OSC study and temperature programmed catalytic reaction system with a quadrupole mass spectrometer and an on-line gas-chromatograph for gas analysis is also described in this chapter. In chapter 3, hydrothermal synthesis of Ce1-xCrxO2+δ (0≤x≤1/3) nanocrystallites is presented. Up to 33% Cr ion substitution in CeO2 could be achieved only by the complexation of Ce(NH4)2(NO3)6 and CrO3 with DETA and simultaneous hydrolysis of the complexes in hydrothermal condition at 200 oC. Powder XRD, XPS and TEM studies confirm that the compound crystallizes in cubic fluorite structure where Ce exist in +4 oxidation state and Cr exist in 4+ and +6 (mixed valance) oxidation states in the ratio of 2: 1. Composition x = 0.33 (Ce2/3Cr1/3O2+δ) showed higher OSC (0.33 mol of [O]) than the maximum OSC observed for CeO2-ZrO2 solid solutions. Formation and higher OSC of Ce2/3Cr1/3O2+δ is attributed to interaction of Ce4+/3+ and Cr3+/4+/6+ redox couples in fluorite structure. The material shows oxygen evolution at ~400 oC in air and hence it is a true oxygen storage material. Oxygen evolution property of Ce0.67Cr0.33O2.11 and subsequent generation of hydrogen by thermal splitting of water is presented in chapter 4. Among the ceria based oxides, Ce0.67Cr0.33O2.11 being the only compound like UO2+δ to have excess oxygen possessing fluorite structure, it releases a large proportion of its lattice oxygen (0.167 M [O]/mole of compound) by heating the material under N2 flow at relatively low temperature (465 oC) directly and almost stoichiometric amount of H2 (0.152 M/Mol of compound) is generated at much lower temperature (65 oC) by thermosplitting of water. The reversible nature of oxygen release and intake of this material is attributed to its fluorite structure and internal coupling between the Ce4+/Ce3+ and Cr4+/6+/Cr3+ redox couples. In chapter 5, we present the hydrothermal synthesis and three-way catalytic activity of Ce1-xRuxO2-δ (0≤x≤0.1) nanocrystallites. Powder XRD, Rietveld refinement, TEM and XPS reveals that the compounds crystallized in fluorite structure where Ru exist in +4 state and Ce in mixed valent (+3, +4) state. Substitution of Ru4+ ion in CeO2 activated the lattice oxygen and Ce0.9Ru0.1O2-δ can reversibly releases 0.42[O]/mol of compound, which is higher than maximum OSC of 0.22 [O]/mol of compound observed for Ce0.50Zr0.50O2. Utilization of higher OSC of Ce1-xRuxO2-δ (x = 0.05 and 0.10) is also shown by low temperature CO oxidation with these catalysts, both in presence/absence of feed oxygen. Ru4+ ion act as active centre for reducing molecules (CO, hydrocarbon ‘HC’) and oxide ion vacancy acts as an active centre for O2 and NOx in this compound. Ce1-xRuxO2-δ not only act as a high oxygen storage material but it also shows high activity towards CO, hydrocarbon ‘HC’ oxidation and NO reduction by CO at low temperature with high N2 selectivity for 3-way catalysis. Study of water gas shift reaction over Ce0.95Ru0.05O2-δ catalyst is presented in chapter 6. The catalyst showed very high WGS activity in terms of high conversion rate (20.5 μmol.g-1.s-1 at 275 oC) and low activation energy (~50.6 kcal/mol). The reason for this seems to be high adsorption propensity of CO on Ru4+ ion and easy extraction of oxygen from lattice to form CO2. This step creates oxide ion vacancy in the catalyst lattice and H2O can adsorb on lattice sites oxygen vacancy and regenerate the lattice by releasing H2. Even in presence of externally fed CO2 and H2, complete conversion of CO to CO2 was observed with 100 % H2 selectivity with Ce0.95Ru0.05O2-δcatalyst in the temperature range of 305-385 oC and no trace of methane formation was observed in this temperature range. Catalyst does not deactivate in long duration on/off WGS reaction cycle because sintering of noble metal or active sites is avoided in this catalyst as Ru4+ ion is substituted in CeO2 lattice. Due to highly acidic nature of Ru4+ ion, surface carbonated formation is prohibited. In chapter 7, synthesis of Ce1-xFexO2-δ (0≤x≤0.45) and Ce0.65Fe0.33Pd0.02O2-δnanocrystallites is presented by sonochemical method. Powder XRD, XPS and TEM studies confirm that the compounds of ~4 nm sizes is crystallized in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of smaller Fe3+ ion in CeO2, lattice oxygen is activated and Ce0.67Fe0.33O1.835 reversibly releases 0.31[O] up to 600 oC which is higher or comparable to the maximum OSC observed for CeO2-ZrO2 based solid solutions. Due to internal interaction of Pd2+/0(0.89 V), Fe3+/2+ (0.77 V) with Ce4+/3+ (1.61 V) redox couples, Pd ion accelerates the electron transfer from Fe2+ to Ce4+ in Ce0.65Fe0.33Pd0.02O1.815, making it a high oxygen storage material as well as highly active catalyst for CO oxidation and WGS reaction. Activation energy for CO oxidation with O2 over Ce0.65Fe0.33Pd0.02O1.815 is found as low as 38 kJ/mol. CO conversion to CO2 is 100% H2 specific in WGS reaction with these catalysts. Conversion rate was found as high 27.2 μmol.g-1.s-1 and activation energy was found 46.4 kJ/mol for Ce0.65Fe0.33Pd0.02O1.815. Only 1-3% Pt, Pd ion can be substituted in CeO2 is by the solution combustion method. We show that even up to 10% of Pt and Pd ion can be substituted in CeO2 by sonication method. In chapter 8, we present the sonochemical synthesis redox property and methanol electro-oxidation activity of hierarchical Ce1-xMxO2-δ (M = Pt and Pd, 0≤x≤0.1) nanocrystallites. Powder XRD, TEM, SEM and XPS study confirms that hierarchical structure compound crystallize in fluorite structure. Pt exists in +4 state and Ce in mixed valent (+3, +4) state in Ce1-xPtxO2-δ and Pd exist in +2 state and Ce in mixed valent (+3, +4) state in Ce1-xPdxO2-δ. Substitution of Pt and Pd ion in CeO2 activated the lattice oxygen. Hydrogen absorption study show higher H/Pt ratio ~8.1 and H/Pd ratio ~4.2 in respective oxides. Reversible nature of higher oxygen storage capacity or higher H/P, H/Pd ratio is due to interaction of redox couples of Pt4+/2+(0.91V), Pt2+/0(1.18V), Pd2+/0(0.92V) and Ce4+/3+(1.61V). Due to participation of lattice oxygen, Ce0.95Pt0.05O1.95 and Ce0.95Pd0.05O1.90 have shown higher electro-oxidation of methanol compared to same moles of Pt in 5%Pt/C. In chapter 9, we present sonochemical synthesis of Ti1-xPtxO2 (0≤x≤0.1) nanocrystallites: a new high capacity anode material for rechargeable Li ion battery. Continuing our interest in synthesis of nanomaterials, we thought if we can extend the same sonochemical method to synthesize metal ion doped TiO2. Doping of TiO2 with a suitable metal ion where dopant redox potential couples with that of titanium (Ti4+) and act as catalyst for additional reduction of Ti4+ to Ti2+ (Ti4+ →Ti3+→Ti2+) is envisaged here to enhance lithium storage even higher than one Li/TiO2. 10 atom % Pt ion substituted TiO2, Ti0.9Pt0.1O2 nanocrystallites of ~4 nm size was synthesized by sonochemical method using diethylenetriamine (DETA) as complexing agent. Powder XRD, Rietveld refinement, TEM and XPS studies reveal that Ti0.9Pt0.1O2 nanocrystallites crystallize in anatase structure and both Ti and Pt are in 4+ oxidation state. Due to Pt4+ ion substitution in TiO2, reducibility of TiO2 was enhanced and Ti4+ was reduced up to Ti2+ state via coupling of Pt states (Pt4+/Pt2+/Pt0) with Ti states (Ti4+/Ti3+/Ti2+). Galvanostatic cycling of Ti0.9Pt0.1O2 against lithium showed very high capacity of 430 mAhg-1 or exchange of ~1.5Li/Ti0.9Pt0.1O2 which is much higher than the highest capacity of 305 mAhg-1 or insertion of ~0.9Li/TiO2 achieved for TiO2(B) nanowires. In chapter 10, we present the conclusions and critical review on the study of transition metal and noble metal ion substituted CeO2 and TiO2.

Properties of nanocrystals are extremely sensitive to their sizes when their sizes are smaller or of the order of the excitonic diameter due to the quantum confinement effect. The interest in this field has been concentrated basically in understanding the size-property relations of nanocrystals, for example, the pronounced variation in the bandgap of the material or the fluorescence emission properties, by tuning the sizes of the nanocrystals. Thus, the optical and electronic properties of semiconductor nanocrystals can be tailor-made to suit the needs of the specific application and hence is of immense importance. One of the major aspects necessary for the actual realization of the various applications is the ability to synthesize nanocrystals of the required size with a controlled size distribution. The growing demand to obtain such nanocrystals with the required size and controlled size distribution is met largely by the solution route synthesis of nanocrystals, that constitutes an important class of synthesis methods due to their ease of implementation and the high degree of flexibility. The main difficulty of this method is that the dependence of the average size and the size distribution of the generated particles on parameters of the reaction are not understood in detail and therefore, the optimal reaction conditions are arrived at essentially in an empirical and intuitive manner. From a fundamental point of view, understanding the growth kinetics of various nanocrystals can provide a deeper insight into the phenomena. The study of growth kinetics can be experimentally achieved by measuring the time evolution of diameter using several in-situ techniques like UV-absorption and small angle X-ray scattering. Having understood the mechanism of growth of nanocrystals, it is possible to obtain the required size of the nanocrystal using optimized synthesis conditions. The properties of these high quality nanocrystals can be further tuned by doping with a small percentage of magnetic ions. The optical and magnetic properties of these nanocrystals play an important role in the various technological applications. The first part of the thesis concentrates on the theoretical methods to study the electronic structure of semiconductor nanocrystals. The second part describes the studies performed on growth of various nanocrystal systems, both in the presence and absence of capping agents. The third part of the thesis describes the studies carried out on ZnO and doped ZnO nanocrystals, synthesized using optimal conditions that were obtained in the earlier part of the thesis. The thesis is divided into five chapters which are described below. Chapter 1 provides a brief overall perspective of various interesting properties of semiconductor nanocrystals, including various concepts relevant for the study of such systems. Chapter 2 describes experimental and theoretical methods used for the study of nanocrystals reported in this thesis. In Chapter 3 of this thesis, we report results of theoretical studies carried out on III-V and II-VI semiconductors using the tight-binding (TB) methodology. Chapter 4 presents the investigations on the growth kinetics of several nanocrystal systems. Chapter 5 presents experimental investigations carried out on undoped and various transition metal (TM) doped ZnO nanocrystals. In summary, we have performed electronic structure calculations on various nanocrystal systems, devised a novel method to obtain the size distribution from UV-absorption spectrum and studied the mechanism of growth in the presence and absence of capping agents in various II-VI semiconductors. Using the optimal conditions obtained from the growth studies, we prepare high quality ZnO nanocrystals of required size, both in free-standing and capped states and doped it with small percentages of various transition metals like Mn, Cu and Fe. We have then studied their optical and magnetic properties.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2014-12-01T12:15:19Z No. of bitstreams: 2 Dissertação - Mônica Pereira Soares - 2013.pdf: 2692345 bytes, checksum: 2640d3e6f5d3dcd342094a53e3cab858 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2014-12-04T14:19:24Z (GMT) No. of bitstreams: 2 Dissertação - Mônica Pereira Soares - 2013.pdf: 2692345 bytes, checksum: 2640d3e6f5d3dcd342094a53e3cab858 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2014-12-04T14:19:24Z (GMT). No. of bitstreams: 2 Dissertação - Mônica Pereira Soares - 2013.pdf: 2692345 bytes, checksum: 2640d3e6f5d3dcd342094a53e3cab858 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2013-12-16 / Nanocrystallitics of Zinc Oxide containing different amounts of Europium (Zn(1-X)EuxO – it being 0,0 ≤ x ≤ 0,035) were synthesized by a combustion reaction, with the aim of evaluating the effect of the concentration of Eu3+ in magnetic properties, after obtained. The chemical composition was determined by energy dispersive spectroscopy, X-ray (EDS). The structural characterization was performed by X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetrics Analysis (TGA), Spectroscopy Photoelectrons Excited by X-Rays (XPS) and Transmission Electron Microscopy (TEM). Crystallinestructure powders of ZnO in wurtzita were formed after the thermical treatment at 550°C about one hour, with average size crystallites of 20 nm. The X-Ray Diffraction results showed presence of diffraction peaks of well-defined of ZnO primary characteristic phase and traces of oxide Europium (Eu2O3) as secondary stage for samples doped with 3.5% of Eu3+. The analyzes showed the magnetic samples exhibit ferromagnetism at room temperature dependent content of ions Eu3+, but for the sample doped with 3.5% Europium the magnetization drops considerably, due to the formation of Oxide Europium (Eu2O3) non-magnetic. This fact suggests that the ferromagnetism of samples is associated with the Eu3+ ions, it is occupying the interstices of matrix semiconductor wafer. / Nanocristalitos de Óxido de Zinco (ZnO) contendo diferentes quantidades de Európio (Zn(1-X)EuxO - sendo 0,0 ≤ x ≤ 0,035) foram sintetizados por reação de combustão, com intuito de avaliar o efeito do teor de Eu3+ nas propriedades magnéticas dos pós obtidos. A composição química foi determinada por Espectroscopia de energia dispersiva de raios-X (EDS). A caracterização estrutural foi realizada por Difração de Raios-X (DRX), Espectroscopia de Infravermelho com Transformada de Fourier (FTIR), Análise Termogravimétrica (TG), Espectroscopia de Fotoelétrons Excitados por Raios-X (XPS) e Microscopia Eletrônica de Transmissão (MET). A caracterização magnética foi realizada em um Magnetômetro de Amostra Vibrante (VSM), com campos aplicados de -2000 Oe a +2000 Oe à temperatura ambiente. Pós cristalinos de ZnO na estrutura wurtzita foram formados após o tratamento térmico a 550°C por uma hora, com um tamanho médio de cristalitos de 36 nm. Os resultados de Difração de Raios-X mostram a presença de picos de difração característicos do ZnO e de óxido de európio (Eu2O3) como fase secundária para amostras dopadas com 3,5% de Eu3+. As análises magnéticas revelaram que as amostras apresentam ferromagnetismo à temperatura ambiente dependente do teor de íons Eu3+, porém na amostra dopada com 3,5% de Európio a magnetização cai consideravelmente, devido à formação do Óxido de Európio (Eu2O3) não magnético. Este fato sugere que o ferromagnetismo das amostras está associado aos íons Eu3+ que estão ocupando os interstícios da matriz semicondutora.

No description available.

Ceramics and polymer-ceramic composites associated with high dielectric constants are of both scientific and industrial interest as these could be used in devices such as capacitors, resonators and filters. High dielectric constant facilitates smaller capacitive components, thus offering the opportunity to miniaturize the electronic devices. Hence there is a continued interest on high dielectric constant materials over a wide range of temperatures. Recently, CaCu3Ti4O12 (CCTO) ceramic which has centro-symmetric body centered cubic structure has attracted considerable attention due to its large dielectric constant (ε ~104-105) which is nearly independent of frequency (upto 10 MHz) and low thermal coefficient of permittivity (TCK) over 100-600K temperature range. Apart from the high dielectric ceramics, high dielectric polymer-ceramic composites have also become promising materials for capacitor applications. By combining the advantages of high dielectric ceramics and low leakage behaviour of polymers, one can fabricate new hybrid materials with high dielectric constants, and high breakdown field to achieve high volume efficiency and energy storage density for capacitor applications. The CCTO polycrystalline powders were generally prepared by the conventional solid-solid reaction route with CaCO3, TiO2 and CuO as the starting materials. This method of preparation often requires high temperatures and longer durations. To overcome these difficulties, in the present investigations, an attempt has been made to synthesize CCTO by adopting microwave assisted heating technique and wet chemical synthesis routes. Also the CCTO crystallites (size varying from nano to micrometers) incorporated in the Polyvinyliden fluoride (PVDF) and Polyaniline (PANI) matrix and several composites with high dielectric constants were fabricated and investigated. Further, the high dielectric constant glasses in the system (100-x)TeO2-xCaCu3Ti4O12, (x=0.5 to 3) were fabricated by the conventional melt-quenching technique and their structural and dielectric properties were studied. The results obtained pertaining to these aforementioned investigations are classified as follows. Chapter 1 is intended to give basic information pertaining to the dielectrics and various mechanisms associated with high dielectric constants. Brief exposure to the high dielectric constant materials is also given. The structural aspects of CCTO, various synthetic routes adopted for the synthesis and the origin of the dielectric anomaly in CCTO are elaborated. In addition, basic information about the high dielectric polymer-ceramic composites and glasses are provided. In chapter 2 the various experimental techniques that were employed to synthesize and characterize the materials under investigation were discussed. Chapter 3 reports the synthesis and characterization of CaCu3Ti4O12, (CCTO) powders by microwave assisted heating at 2.45 GHz, 1.1kW. The processing and sintering were carried out at different temperatures for varied durations. The optimum calcination temperature using microwave heating was found to be 950oC for 20 minutes to obtain cubic CCTO powders. This is found to be fast and energy efficient as compared to that of the conventional methods. The structure, morphology and dielectric properties of the CCTO ceramic processed by microwave assisted heating were studied via X-ray diffraction, Scanning electron microscopy (SEM) and impedance analyser. These studies revealed that, the microwave sintered (MS) samples were less porous than that of the conventional ones. Relative density of about 95% was achieved for the MS pellets (1000oC/60min) while for the conventional sintered (CS) pellets (1100oC/2h) it was only 91%. The dielectric constants for the microwave sintered (1000oC/60min) ceramics were found to vary from 11000 to 6950 in the 100 Hz to 100 kHz frequency range. The presence of larger grains (6-10μm) in the MS samples contributed to the higher dielectric constants. Chapter 4 deals with the synthesis of complex oxalate precursor, CaCu3(TiO)4(C2O4)8 • 9H2O, by the wet chemical route. The various trials and the different reaction schemes involved for the preparation of complex oxalate precursor were highlighted. The oxalate precipitate thus obtained was characterized by the wet chemical analyses, X-ray diffraction, FTIR absorption and TG/DTA analyses. The complex oxalate precursor, CaCu3(TiO)4(C2O4 )8.9H2O was subjected to thermal oxidative decomposition and the products of thermal decomposition were investigated employing XRD,TGA, DTA and FTIR techniques. Nanocrystallites of CaCu3Ti4O12 with the size varying from 30-200 nm were obtained at a temperature as low as 680oC. The nanocrystallites of CaCu3Ti4O12 were characterized using Electron Spin Resonance (ESR) and optical reflectance techniques. The selected area electron diffraction (SAED) pattern with the zone axis [012] and spot pattern in electron diffraction (ED) indicate their single-crystalline nature. The optical reflectance and ESR spectra indicate that the Cu (II) coordination changes from distorted octahedra to nearly flattened tetrahedra (squashed) to square planar geometry with increasing heat treatment temperature. The powders derived from the oxalate precursor have excellent sinterability resulting in high density ceramics which exhibited giant dielectric constants upto 40,000 (1 kHz) at 25oC, accompanied by low dielectric loss < 0.07. The effect of calcium content on the dielectric properties of CaxCu3Ti4O12 (x=0.90, 0.97, 1.0, 1.1 and 1.15) derived from the oxalate route was described in Chapter 5. The structural, morphological and dielectric properties of the ceramics were studied using X-ray diffraction, Scanning Electron Microscope along with Energy Dispersive X-ray Analysis (EDX), and Impedance analyzer. The X-ray diffraction patterns obtained for the x= 0.97, 1.0 and 1.1 ceramics could be indexed to a body– centered cubic perovskite related structure associated with the space group Im3. The microstructural studies revealed that the grains are surrounded by exfoliated sheets of Cu-rich phase. The microstructure that is evolved for the Ca0.97 ceramic more or less resembles that of the Ca1.0 ceramic, but the density of such exfoliated sheets of cu-rich phase is lesser for the Ca0.97 ceramic and none for Ca1.1 ceramic. The sintered pellet (x=0.97) was ground and thinned to the required thickness (~ 20nm) and analyzed using Transmission Electron Microscopy (TEM). The current-voltage (I-V) characteristics of the ceramics exhibited non-linear behaviour. The dielectric properties of these suggest that the sample corresponding to the composition x=0.97, has a reduced dielectric loss while retaining its high dielectric constant. Chapter 6 illustrates the results concerning the fabrication and characterization of nanocrystal composites of Polyaniline (PANI) and CaCu3Ti4O12 (CCTO). These were prepared using a simple procedure involving in-situ polymerization of aniline in dil. HCl. The PANI and the PANI-CCTO composites were subjected to X-ray diffraction, Fourier Transform Infrared (FTIR), Thermo gravimetric, Scanning Electron Microscopic (SEM) and Transmission electron microscopic analyses. The FTIR spectra recorded for the composites was similar to that of pure PANI unlike in the case of X-ray diffraction wherein the characteristics of both PANI and CCTO were reflected. The TGA in essence indicated the composites to have better thermal stability than that of pure PANI. The composite corresponding to 50%CCTO-50%PANI exhibited higher dielectric constant (4.6x106 @100Hz). The presence of the nano crystallites of CCTO embedded in the nanofibers of PANI matrix was established by TEM. The AC conductivity increased slightly upto 2kHz as the CCTO content increased in the PANI which was attributed to the polarization of the charge carriers. The value of dielectric constant obtained was higher than that of the other PANI based composites reported in the literature. Chapter 7 deals with the fabrication and characterization of diphasic Poly(vinylidene fluoride) (PVDF)-CCTO composite. The CCTO crystallites (size varying from nano to micrometers) incorporated in the Polyvinylidene fluoride (PVDF) and composites with varying CCTO content were fabricated. The structural, morphological and dielectric properties of the composites were studied using X-ray diffraction, Thermal analysis, Scanning Electron Microscope (SEM), Transmission Electron Microscopic (TEM) and Impedance analyzer. The room temperature dielectric constant as high as 95 at 100Hz has been realized for the composite with 0.55 Vol.fraction of CCTO (micro sized crystallites), which has increased to about 190 at 150oC. Whereas, the PVDF/CCTO nanocrystal composite with 0.13Vol.fraction of CCTO has exhibited higher room temperature dielectric constant (90 at 100Hz). The PVDF/CCTO nanocrystal composite was further investigated for the breakdown strength and electric modulus. The breakdown strength plotted against the dielectric constant evidenced an inverse relationship of breakdown voltage with the dielectric constant. The relaxation processes associated with these composites were attributed to the interfacial polarization or Maxwell-Wagner-Sillars (MWS) effect. Various theoretical models were employed to rationalize the dielectric behavior of these composites. The fabrication and characterization details of optically clear colored glasses in the system (100-x)TeO2-xCaCu3Ti4O12, (x=0.5 to 3 mol%) are reported in Chapter 8. The color varies from olive green to brown as the CaCu3Ti4O12 (CCTO) content increased in TeO2 matrix. The X-ray powder diffraction and differential scanning calorimetric analyses that were carried out on the as-quenched samples confirmed their amorphous and glassy nature respectively. The optical transmittance of the glasses exhibited typical band-pass filter characteristics. The dielectric constant and loss in the 100 Hz-1MHz frequency range were monitored as a function of temperature (323K673K). The dielectric constant and the loss increased as the CCTO content increased in TeO2 at all the frequencies and temperatures under study. Further, the dielectric constant and the loss were found to be frequency independent in the 323-473 K temperature range. The value obtained for the loss at 1MHz was 0.0019 which was typical of low loss materials, and exhibited near constant loss (NCL) contribution to the ac conductivity in the 100Hz-1MHz frequency range. The electrical relaxation was rationalized using the electrical modulus formalism. These glasses are found to be more stable (a feature which may be of considerable interest) as substrates for high frequency circuit elements in conventional semiconductor industries. Thesis ends with summary and conclusions, though each chapter is provided with conclusions and complete list of references.