The spinel structure of LiMn2O4 powders react with the 1 mole Li2CO3 and 4 mole MnO2 powders by solid-state reaction at 800 oC, and then sintered at 1300 oC to become ceramics specimen. There are accompany phase transformation and non-stoichiometric composition during the cooling process. In X-ray diffraction analysis, the sintered specimen contains principal Li-deficiency Li1-XMn2O4 composition and minor of second phases LiMnO2 and Mn3O4. Lattice parameters also distorted by John-Teller effect. In electron microscopy observation, there are lamellae grains and defects in the specimen, such as twins, dislocations and stacking faults. In TEM analysis, tetragonal-LiMn2O4 structure has lamellae domains, and reflection twinning. However, this study for cubic-LiMn2O4 structure found that edge dislocations with Burger vector of 1/2<110> slip on {110} plane, and mixed(45o) dislocation with Burger vector of 1/2<100> slip on {100} plane.

Abstract A vast majority of the studies devoted to Lithium manganese oxide deals with their electrochemical characteristics in lithium batteries. The main project of this study is to realize the structure evolution upon electrochemical cycling. The phase transformations under the charge and discharge testing are an interesting project. Nitrate or oxide precursor calcined at 800¢XC can produce single phase stoichiometric LiMn2O4. The hypo-stoichiometric compositions (xLi2O¡Ñ4MnO, x < 1) synthesized by Li-poor situation contain LiMn2O4 and Mn2O3. The hyper- stoichiometric compositions (xLi2O¡Ñ4MnO, x > 1) synthesized by Li-rich situation contain non-stoichiometric spinel LixMn2O4 (such as Li4Mn5O12 or Li2Mn4O9) and Li2MnO3. The lattice parameter of LiMn2O4 increases slightly with increase of the lithium content at x < 1 (0.823 ~ 0.824 nm), but decreases sharply for x = 1.0 ~ 1.8 (0.824 to 0.817 nm). Differential thermal analysis showed at temperature higher than 935&#x00BA;C, rocksalt phase (with tetragonal symmetry), Mn3O4 will be produced. Above 1045&#x00BA;C, the crystallite phases contain cubic LiMn2O3 spinel, tetragonal Mn3O4 and orthorhombic symmetry LiMnO2. After high temperature annealing (> 935&#x00BA;C), the residual phase is lithium-deficient structure, Mn3O4. Apparent facets with {111}, {011}, and {001} (and {113}) planes are usually observed. The LiMn2O4 crystallite appears to be a truncated cubo-octahedron. The lowest surface energy gsv for LiMn2O4 spinel is located at the {111} planes. Lamellae domain and twinned structure are usually observed in LiMn2O4 particles. The occurrence of domain boundary and twin plane are {111} mostly. Forbidden reflections {200}, {420} in the initial powder and 1/2{311} and 1/3{422} superlattice reflections occurred after charging and discharging test reveal LiMn2O4 structure is a violation of space group. [311]/[111] peak ratio in the XRD traces is increase after electrochemical cycling. Fraction of inverse phase increased upon electrochemical cycling. The results for structure evolution under charging and discharging test can be divided into two parts for reversible and irreversible. First, unit cell of cubic spinel contracted upon charging and returned to original after discharging. The lattice constant varies back and forth between 0.824 nm to 0.814 nm for cycle between 3.3 and 4.3 V. LiMn2O4 transits to Li4Mn5O12 and l-MnO2 after fully charging to 4.3 V, which then recovers to cubic spinel LixMnyO4 after discharging to 3.3 V. The structure variations in the cycle of changing and discharging are LiMn2O4 ¡V (Li4Mn5O12 + l-MnO2) ¡V LixMnyO4. And metastable circular or rectangle LiMn2O4 particles observed in the surface can be extracted and inserted Li+ ion upon charging and discharging test. This process is reversible. Second, (1) tetragonal, rhombohedral and triclinic distorted within cubic spinel particles; (2) nanoscale regions of highly disordered lattices observed; (3) amorphous film observed in the powder particle surface; (4) crystalline phase Mn2O3 increased; (5) structure collapse inside the particle and the domain boundary; (6) inverse spinel structure. The structure of LixMn2O4 had distorted upon electrochemical cycling. These results are irreversible.

Ce mémoire décrit la synthèse, la caractérisation – notamment thermogravimétrique et structurale – et les propriétés d'intercalation électrochimique du lithium de plusieurs types d'oxydes de manganèse. On décrit tout d'abord la préparation d'une 'ramsdellite synthétique', à faible taux de défauts structuraux de type rutile. Les oxydes de manganèse lamellaires (phyllomanganates) ont donné lieu à une nouvelle voie de synthèse du phyllomanganate de lithium par une succession de réactions topotactiques (échanges d'ions) à basse température. Sa stabilité thermique et ses propriétés d'intercalation sont examinées en comparaison avec celles du composé de sodium. La majeure partie de ce mémoire est consacrée aux spinelles Li1+xMn2–xO4, qui sont des matériaux d'électrode positive prometteurs pour les accumulateurs au lithium. Ce travail montre la faisabilité de synthèses à basse température à partir de béta-MnO2 (procédé breveté), et l'existence d'une corrélation entre température de synthèse et composition de la phase spinelle. L'intercalation du lithium est étudiée en électrolyte solide et liquide pour plusieurs compositions. L'emploi d'une cellule électrochimique in situ dans un diffractomètre de rayons X a permis de mettre en évidence le caractère biphasé de l'intercalation, même pour des spinelles de Li:Mn = 0.69. Les performances électrochimiques de spinelles substituées au magnésium et à l'aluminium sont également examinées. L'étude thermogravimétrique des spinelles Li–Mn–O a permis de mettre en évidence des réactions réversibles avec dégagement d'oxygène. Des affinements structuraux à partir de diagrammes de diffraction neutronique mettent en évidence des réactions différentes en fonction de la température d'équilibre, avec apparition de lacunes d'oxygène dans un échantillon trempé à 925°C. Enfin, un nouveau composé appelé "phase m", de formule Li0.25MnO2, a été obtenu à 150°C. Sa caractérisation structurale aux rayons X et par diffraction électronique montre qu'il s'agit d'une phase nouvelle monoclinique avec une sous-structure pseudo-hexagonale proéminente.

This thesis is concerned with kinetic characterisation of theinsertion compound LiMn2O4, which is used as positive electrodematerial in rechargeable lithium batteries. Three different typesof electrode configurations have been investigated, namely singleparticles, thin films and composite electrodes. Differentelectrochemical techniques, i.e. linear sweep voltammetry (LSV),electrochemical impedance spectroscopy (EIS), potential step, andgalvanostatic experiments were applied under various experimentalconditions. The majority of the experimental data were analysedby relevant mathematical models used for describing the reactionsteps of insertion compounds. It was concluded that a model based on interfacialcharge-transfer, solid-phase diffusion and an external iR-dropcould be fairly well fitted to LSV data measured on a singleelectrode system over a narrow range of sweep rates. However, itwas also found that the fitted parameter values vary greatly withthe characteristic length and the sweep rate. This indicates thatthe physical description used is too simple for explaining theelectrochemical responses measured over a large range of chargeand discharge rates. EIS was found to be a well-suited technique for separatingtime constants for different physical processes in the insertionand extraction reaction. It was demonstrated that the impedanceresponse is strongly dependent on the current collector used.According to the literature, reasonable values of theexchange-current density and solid-phase diffusion coefficientwere determined for various states-of-discharge, temperatures andelectrolyte compositions. Experiments were carried out in bothliquid and gel electrolytes. A method which improves thedistinction between the time constants related to thematerial’s intrinsic properties and possible porous effectsis presented. The method was applied to composite electrodes.This method utilises, in addition to the impedance responsemeasured in front of the electrode, also the impedance measuredat the backside of the electrode. Finally, the kinetics of a composite electrode was alsoinvestigated by in situ X-ray diffraction (in situ XRD) incombination with galvanostatic and potentiostatic experiments. Noevidence of lithium concentration gradients could be observedfrom XRD data, even at the highest rate applied (i.e. ~6C), thusexcluding solid-phase diffusion and also phase-boundary movement,as described by Fick’s law, as the ratelimiting step. <b>Key words:</b>linear sweep voltammetry, electrochemicalimpedance spectroscopy, potential step, in situ X-raydiffraction, microelectrodes, electrode kinetics, LiMn2O4cathode, rechargeable lithium batteries

<p>This thesis is concerned with kinetic characterisation of theinsertion compound LiMn2O4, which is used as positive electrodematerial in rechargeable lithium batteries. Three different typesof electrode configurations have been investigated, namely singleparticles, thin films and composite electrodes. Differentelectrochemical techniques, i.e. linear sweep voltammetry (LSV),electrochemical impedance spectroscopy (EIS), potential step, andgalvanostatic experiments were applied under various experimentalconditions. The majority of the experimental data were analysedby relevant mathematical models used for describing the reactionsteps of insertion compounds.</p><p>It was concluded that a model based on interfacialcharge-transfer, solid-phase diffusion and an external iR-dropcould be fairly well fitted to LSV data measured on a singleelectrode system over a narrow range of sweep rates. However, itwas also found that the fitted parameter values vary greatly withthe characteristic length and the sweep rate. This indicates thatthe physical description used is too simple for explaining theelectrochemical responses measured over a large range of chargeand discharge rates.</p><p>EIS was found to be a well-suited technique for separatingtime constants for different physical processes in the insertionand extraction reaction. It was demonstrated that the impedanceresponse is strongly dependent on the current collector used.According to the literature, reasonable values of theexchange-current density and solid-phase diffusion coefficientwere determined for various states-of-discharge, temperatures andelectrolyte compositions. Experiments were carried out in bothliquid and gel electrolytes. A method which improves thedistinction between the time constants related to thematerial’s intrinsic properties and possible porous effectsis presented. The method was applied to composite electrodes.This method utilises, in addition to the impedance responsemeasured in front of the electrode, also the impedance measuredat the backside of the electrode.</p><p>Finally, the kinetics of a composite electrode was alsoinvestigated by in situ X-ray diffraction (in situ XRD) incombination with galvanostatic and potentiostatic experiments. Noevidence of lithium concentration gradients could be observedfrom XRD data, even at the highest rate applied (i.e. ~6C), thusexcluding solid-phase diffusion and also phase-boundary movement,as described by Fick’s law, as the ratelimiting step.</p><p><b>Key words:</b>linear sweep voltammetry, electrochemicalimpedance spectroscopy, potential step, in situ X-raydiffraction, microelectrodes, electrode kinetics, LiMn2O4cathode, rechargeable lithium batteries</p>

著作権、出典、利用制限の表示を出版社より求められている。 / 京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第19824号 / エネ博第330号 / 新制||エネ||66(附属図書館) / 32860 / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)准教授 高井 茂臣, 教授 萩原 理加, 教授 佐川 尚 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

Philosophiae Doctor - PhD / Aqueous ion-intercalation batteries (AIB’s) have the potential to provide both high power for hybrid-electric transport, and low cost bulk energy storage for electric grid supply. However, a major setback to AIB development is the instability of suitable ionintercalation anode material in aqueous electrolyte. To counter this problem, the use of activated carbon (AC) (a supercapacitor anode) paired against the low cost ionintercalation cathode spinel LiMn2O4 (LMO) provides a stable alternative. This thesis comprises two novel areas of investigation concerning: (1) the development of the AC/LMO cell for high power applications, and (2) the introduction of PbSO4 as a high capacity alternative anode material paired against LMO for low cost bulk energy storage. The study on AC/LMO explores the electrode combination’s practical specific energy and power capability at high P/E (power to energy ratio) of 50:1 suitable for hybrid electric vehicle batteries. To study the relationship between electrode material loading density, active material performance, and current collector mass contribution, a specially designed cell was constructed for galvanic cycling of different thicknesses of electrode. Between a loading density range of 25 – 100 mg, ~50 mg of total active material between two 1 cm2 current collectors produced the highest 50:1 P/E ratio values of 4 Wh/kg and 200 W/kg, constituting a 4-fold reduction of the active material values of thin films at 50:1 P/E. The cycling potentials of the individual electrodes revealed that doublings of electrode film loading density increased the LMO electrode’s polarization and voltage drop to similar levels as doublings in applied current density. However, by increasing the charging voltage from 1.8 V to 2.2 V, 6 Wh/kg and 300 W/kg was obtainable with minimal loss of energy efficiency. Finally a large-format cell of a calculated 3 Ah capacity at 50:1 P/E was constructed and tested. The cell produced ~60% of the anticipated capacity due to a suspected high level of resistance in the electrode contact points. The overall conclusion to the study was that AC/LMO holds promise for high power applications, and that future use of higher rate capability forms of LMO offers a promising avenue for further research. v The second part of this thesis presents the development of a novel cell chemistry, PbSO4/LMO, that has yet to be reported elsewhere in existing literature. The cell uses aqueous pH 7, 1 M, Li2SO4 electrolyte, and forms an electrode coupling where the PbSO4 anode charge/discharge is analogous to that in Pb-acid batteries. The average discharge voltage of the cell was 1.4 V and formed a flat charge/discharge plateau. The use of a low cost carbon coating method to encapsulate PbSO4 microparticles had a marked improvement on cell performance, and compared to uncoated PbSO4 improved both rate capability and specific capacity of the material. The active materials of the carbon-coated PbSO4/LMO cell produced a specific energy 51.1 Wh/kg, which, if a 65% yield is possible for a practical cell format, equals 38.4 Wh/kg, which is 15 Wh/kg higher than AC/LMO bulk storage cells at 23 Wh/kg, but lower than Pb-acid batteries at ~25-50 Wh/kg. Interestingly, the specific capacity of PbSO4 was 76 mAh/g compared to 100 mAh/g in Pb-acid cells. The predicted cost of the cell, providing a 65% value of the active material specific energy for a practical cell can be realized, is on par with Pb-acid battery technology and, importantly, uses 2.3 × less Pb/kWh. The cycling stability achieved thus far is promising, but will require testing over comparable cycle life periods to commercial batteries, which could be anywhere between 5 – 15 years.

A Comprehensive review of literature on electrode materials for lithium-ion batteries is provided in Chapter 1 of the thesis. Chapter 2 deals with the studies on porous, sub-micrometer size LiNi1/3Co1/3O2 as a positive electrode material for Li-ion cells synthesized by inverse microemulsion route and polymer template route. The electromechanical characterization studies show that carbon-coated LiNi1/3Co1/3O2 samples exhibit improved rate capability and cycling performance. Furthermore, it is anticipated that porous LiNi1/3Co1/3O2 could be useful for high rates of charge-discharge cycling. Synthesis of sub-micrometer size, porous particles of LiNi1/3Co1/3O2 using a tri-block copolymer as a soft template is carried out. LiNi1/3Co1/3O2 sample prepared at 900ºC exhibits a high rate capability and stable capacity retention of cycling. The electrochemical performance of LiNi1/3Co1/3O2 prepared in the absence of the polymer template is inferior to that of the sample prepared in the presence of the polymer template. Chapter 4 involves the synthesis of sub-micrometer size particles of LiMn2O4 in quaternary microemulsion medium. The electrochemical characterization studies provide discharge capacity values of about 100 mAh g-1 at C/5 rate and there is moderate decrease in capacity by increasing the rate of charge-discharge cycling. Studies also include charge-discharge cycling as well as ac impedance studies in temperature range from -10 to 40º C. Chapter 5 reports the synthesis of nano-plate LiFePO4 by polyol route starting from two reactants, namely, FePO42H2O and LiOH.2H2O. The electrodes fabricated out of nano-plate of LiFePO4 exhibit a high electrochemical activity. A stable capacity of about 155 mAh g-1 is measured at 0.2 C over 50 charge-discharge cycles. Mesoporous LiFePO4/C composite with two sizes of pores is prepared for the first time via solution-based polymer template technique. The precursor of LiFePO4/C composite is heated at different temperatures in the range from 600 to 800ºC to study the effect of crystalllinity, porosity and morphology on the electrochemical performance. The compound obtained at 700ºC exhibits a high rate capability and stable capacity retention on cycling with pore size distribution around 4 and 46nm. In Chapter 6, the electrochemical characterization of LiMn2O4 in an aqueous solution of 5 M LiNO3 is reported. A typical cell employing LiMn2O4 as the positive electrode and V2O5 as the negative electrode was assembled and the characterized by charge-discharge cycling in 5 M LiNO3 aqueous electrolyte. Furthermore, it is shown that Li+-ion in LiMn2O4 can be replaced by other divalent ions resulting in the formation of MMn2O4 (M = Ca, Mg, Ba and Sr) in aqueous M(NO3)2 electrolytes by subjecting LiMn2O4 electrodes to cyclic voltametry. Cyclic voltammetry and chronopotentiometry studies suggest that MMn2O4 can undergo reversible redox reaction by intercalation/deintercalation of M2+-ions in aqueous M(NO3)2 electrolytes.

Philosophiae Doctor - PhD (Chemistry) / Lithium ion capacitors are new and promising class of energy storage devices formed from a combination of lithium-ion battery electrode materials with those of supercapacitors. They exhibit better electrochemical properties in terms of energy and power densities than the above mentioned storage systems. In this work, lithium manganese oxide spinel (LiMn2O4; LMO) and lithium manganese phosphate (LiMnPO4; LMP) as well as their respective nickel-doped graphenised derivatives (G-LMNO and G-LMNP) were synthesized and each cathode material used to fabricate lithium ion capacitors in an electrochemical assembly that utilised activated carbon (AC) as the negative electrode and lithium sulphate electrolyte in a two-electrode system. The synthetic protocol for the preparation of the materials followed a simple solvothermal route with subsequent calcination at 500 - 800 ?C. The morphological, structural and electrochemical properties of the as prepared materials were thoroughly investigated through various characterisation techniques involving High resolution scanning electron microscopy (HRSEM), High resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), Small-angle X-ray scattering (SAXS), Electrochemical impedance spectroscopy (EIS), Cyclic voltammetry (CV) and Galvanostatic charge/discharge.

Rechargeable Li-ion batteries and supercapacitors are the most promising electrochemical energy storage devices in terms of energy density and power density, respectively. Recently, nanostructured materials have gained enormous interest in the field of energy technology as they have special properties compared to the bulk. Commercially available Li-ion batteries, which are the most advanced among the rechargeable batteries, utilize microcrystalline transition metal oxides as cathode materials which act as lithium insertion hosts. To explore better electrochemical performance the use of nanomaterials instead of conventional materials would be an excellent alternative. High Li-ion insertion at high discharge rates causes slow Li+ transport which in turn results in concentration polarization of lithium ions within the electrode material, causing a drop in cell voltage. This eventually, leads in termination of the discharge process before realizing the maximum capacity of the electrode material being used. This problem can be addressed by decreasing the average particle size which leads to an increase in surface area of the electrode material. Nanostructured materials, because of their high surface area and large surface to volume ratio, to some extent can overcome the problem of slow diffusion of ions. Supercapacitors are electrical energy storage devices which can deliver large energy in a short time. A supercapacitor can be used as an auxiliary energy device along with a primary source such as a battery or a fuel cell to achieve power enhancement in short pulse applications. Active materials for supercapacitors are classified into three categories: (i) carbonaceous materials, (ii) conducting polymers and (iii) metal oxides. Among the materials studied over the years, metal oxides have been considered as attractive electrode materials for supercapacitors due to the following merits: variable oxidation state, good chemical and electrochemical stability, ease of preparation and handling. The performance of supercapacitors can be enhanced by moving from bulk to nanostructured materials. The theme of the thesis is to explore novel routes to synthesize nanostructured materials for Li-ion batteries and supercapacitors, and to investigate their physical and electrochemical characteristics. Chapter I is an introduction of various types of electrochemical energy systems such as battery, fuel cell and supercapacitor. A brief review is made on electrode materials for Li-ion batteries and supercapacitors, and nanostructured materials. Chapter II deals with the study of nanostrip orthorhombic V2O5 synthesized by a two-step procedure, with the formation of a vanadyl ethylene glycolate precursor and post-calcination treatment. The precursor and the final product are characterized for phase and composition by powder X-ray diffraction (XRD), infrared (IR) spectroscopy, thermal analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The morphological changes are investigated using field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HRTEM). It is found that the individual strips have the following dimensions, length: 1.3 μm, width: 332 nm and thickness: 45 nm. The electrochemical lithium intercalation and de-intercalation of nanostrip V2O5 is investigated by cyclic voltammetry (CV), galvanostatic charge-discharge cycling, galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy. Chapter III describes the synthesis of nanoparticels of LiMn2O4 by microwave assisted hydrothermal method. The phase and purity of spinel LiMn2O4 are confirmed by powder XRD analysis. The morphological studies are carried out using FE-SEM and HRTEM. The electrochemical performance of spinel LiMn2O4 is studied by using CV and galvanostatic charge-discharge cycling. The initial discharge capacity is found to be about 89 mAh g-1 at a current density of 21 mA g-1 with reasonably good cyclability. Chapter IV deals with synthesis of MoO2 nanoparticles through ethylene glycol medium and its electrochemical characterization. XRD data confirms the formation MoO2 on monoclinic phase, space group P21/c. Polygon shape of MoO2 is observed in HRTEM. MoO2 facilitates reversible insertion-extraction of Li+ ions between 0.25 to 3.0 V vs. Li/Li+. CV and galvanostatic charge-discharge cycling are conducted on this anode material to complement the electrochemical data. Chapter V reports the synthesis of nanostructured MnO2 at ambient conditions by reduction of potassium permanganate with aniline. Physical characterization is carried out to identify the phase and morphology. The as prepared MnO2 is amorphous and it contains particles of 5 to 10 nm in diameter. On annealing at a temperature > 400 °C, the amorphous MnO2 attains crystalline α-phase with a concomitant change in morphology. A gradual conversion of nanoparticles to nanorods (length 500-750 nm and diameter 50-100 nm) is evident from SEM and TEM studies. High resolution TEM images suggest that nanoparticles and nanorods grow in different crystallographic planes. The electrochemical lithium intercalation and de-intercalation of nanorods was performed by (CV) and galvanostatic charge-discharge cycling. The initial discharge capacity of nanorod α-MnO2 is found to be about 197 mAh g-1 at a current density of 13.0 mA g-1. Capacitance behavior of amorphous MnO2 is studied by CV and galvanostatic charge-discharge cycling in a potential range from -0.2 to 1.0 V vs. SCE in 0.1 M sodium sulphate solution. The effect of annealing on specific capacitance is also investigated. Specific capacitance of about 250 F g-1 is obtained for as prepared MnO2 at a current density of 0.5 mA cm-2 (0.8 A g-1). Chapter VI pertains to electrochemical supercapacitor studies on nanostructured MnO2 synthesized by polyol method. Although X-ray diffraction (XRD) pattern of the as synthesized nano-MnO2 shows poor crystallinity, it is found that it is locally arranged in δ-MnO2 type layered structure composed of edge-shared network of MnO6 octahedra by Mn K-edge X-ray Absorption Near Edge Structure (XANES) measurement. Annealed MnO2 shows high crystalline tunneled based α-MnO2 as confirmed by powder XRD pattern and XANES. As synthesized MnO2 exhibits good cyclability as an electrode material for supercapacitor. In Chapter VII, capacitance behavior of nanostrip V2O5, TiO2 coated V2O5 and nanocomposites of PEDOT/V2O5 are presented. Structural and morphological studies are carried out by powder XRD, IR, TGA, SEM and TEM. Cyclic voltammogram of pristine V2O5 shows the regular rectangular shape indicating the ideal capacitance behavior in aqueous 0.1 M K2SO4. The SC value of pristine V2O5 is found to be about 100 F g-1. Nanostrip V2O5 is modified with TiO2 using titanium isobutoxide to enhance the capacitance retention upon cycling. Only 48 % of the initial capacitance remains in the case of pristine V2O5 after 100 cycles, while TiO2 coated V2O5 exhibits better cyclability with capacitance of 70 % of the initial capacitance. The capacitance retention is attributed to the presence of TiO2 on the surface of V2O5 which prevents the vanadium dissolution into the electrolyte. Microwave assisted hydrothermally synthesized PEDOT/V2O5 nanocomposites are utilized as capacitor materials. The initial SC of PEDOT/V2O5 (237 F g-1) is higher than that of either pristine V2O5 or PEDOT. The enhanced electrochemical performance is attributed to synergic effect and an enhanced bi-dimensionality. Details of the above studies are described in the thesis with a conclusion at the end of each Chapter.