No description available.

No description available.

A series of closely related lithium aryl thiolates with pyridine as a donor ligand have been prepared. X-ray structure determination (Dr. W. Clegg) has shown that [PKSLi.(NC(_5)H(_5))] crystallises as an infinite chain polymer. Insertion of a methylene group between the phenyl ring and sulphur as in (PhCH(_2)SLi .NC(_5)H(_5). ] (_oo) produced an infinitely folded ladder polymer with Li-S rungs, while introduction of an 0-methyl group produces the monomeric complex [o-MeC(_6)H(_4)SLi .(NC(_5)H(_5))(_3)].The unusual symmetrical complex [Li(_14) (SCH(_2)Ph)(_12) S(TMEDA.)(_6)] has been prepared, in which a central sulphur atom is surrounded by a distorted cube of lithium atoms, each edge of which is bridged by a benzyl thiolate group such that the twelve S atoms form a cubo- octahedron: a further six Li atoms form a larger outer octahedron. When TMEDA is present in excess it appears that both PhSLi and PhCH(_2)SLi form dimeric compounds with a central Li(_2)S(_2) ring but structure determination of these complexes is incomplete.[PhOOSLi.TMEDA](_2)has been prepared and crystallises as a centrosymmetric dimer containing a chair shaped central eight membered (OOSLi)(_2) ring with the Li atoms out of the (COS)(-2), molecular plane. Lithiation of the related acids PhOOOH and PhCSSH has been carried out in the presence of TMEDA but crystal growth has so far been unsuccessful. Ab initio m.o. calculations on related model compounds are included. A preliminary study has been carried out on the lithiation of the thio-oxime (Ph(_2) C-NSH). Results were encouraging but detailed investigation is needed. The reaction of S(_4) N(_4) with BI(_3) produces, not the expected adduct, S(_4)N(_4) .BI(_3) , but an intractable polymer, empirical formula, S(_3)N(_3)BI. Other adducts of S(_4).N(_4) and (PhCN_2) S(_2))(_2), were prepared but were unsuitable for further reactions. The reaction of lithium borohydride and sulphur in THF to produce sulphurated lithium borohydride has been modified to give a controllable reaction. Further reaction with TMEDA gives clear orange crystals of a complex the composition of which is still unknown.

In the field of high performance primary battery systems lithium anoded cells have been shown to have an excellent performance and extremely good shelf life. The major drawback with the lithium technology is one of safety, whereby abuse conditions (such as charging of the cell) lead to an unstable system with the very real possibility of a cell explosion. For a commercially available cell consideration of safety issues would preclude the marketing of a high performance lithium cell for general use, rather, it will be reserved for specialist e.g. Military use where the personnel having contact with the power source can be trained in its safe operation. The work described in this thesis is concerned with the development of a high performance battery system utilising calcium as the anode material. Calcium has received attention as an anode material for a high performance battery system because it removes many of the safety problems associated with lithium. The major disadvantages of calcium have been addressed namely the shelf life and discharge performance. The electrochemical techniques of cyclic voltammetry and a.c. impedance have been used in conjunction with physical methods such as scanning electron microscopy to define the mode of operation of these cells.

Aluminium alloys suffer from localised attack termed exfoliation corrosion which is intergranular in nature where the attack proceeds around the elongated grains present in the microstructure. The formation of a more voluminous corrosion product than the original material results in the lifting of grains giving the characteristic of exfoliation corrosion - lamellar attack. This results in surface degredation in the form of pitting, flaking or blistering. The precipitates present within the microstructure and the grain shape can alter how susceptible the material is to exfoliation corrosion. The distribution of precipitates present can be altered by heat treatment which affects the exfoliation corrosion susceptibility. A study on the effects of heat treatment and grain shape on exfoliation corrosion has been completed for two AlCuMg alloys (2014 and 2024) and two aluminium - lithium alloys (8090 and 2091). A MASTMAASIS salt - spray cabinet was used to determine the exfoliation corrosion susceptibility of all the alloys. A four - point bend test was developed to determine the depth of penetration and compared to depths measured metallographically. The results for the 2XXX series alloys suggest that as the grain aspect ratio increases the exfoliation susceptibility increases. Although the aspect ratio did not differ greatly between each alloy, the different phases present in the two alloys seem to contribute more markedly to the susceptibility. Heat treatment in the 2024 alloy suggested that the peak - aged condition gave a higher susceptibility to corrosion than the under - aged and over - aged conditions. Results indicated that the heat treatment has a greater effect on exfoliation corrosion than the grain aspect ratio, showing that the grain boundary microstructure is an important factor in the exfoliation process. The four - point bend test tended to underestimate the depth of attack by virtue of measuring the whole area of the specimen whereas metallographic measurements only took into account the attacked areas. End grain testing suggested that the attack can not only proceed longitudinally but also "short - circuit" down the transverse path of the elongated grains in the 2XXX series alloys. For the aluminium - lithium alloys the over - aged condition showed more resistance to attack than the under - aged and peak - aged conditions but the under - aged was more resistant to attack for the 2091 alloy. Stress corrosion cracking showed trends similar to exfoliation corrosion.

Le processus de vieillissement des accumulateurs lithium-ion est un facteur déterminant lors de leur utilisation pour des applications spatiales. Les couches de passivation présentes à la surface des électrodes, composées de différents sels et polymères, ont une grande influence sur les performances des batteries, en particulier en termes de cyclage et de durée de vie. La compréhension des phénomènes de dissolution et de précipitation des espèces présentes aux électrodes est donc essentielle pour appréhender le vieillissement des batteries. Dans un premier temps, les propriétés de solubilisation et de transport de ces espèces dans l’électrolyte ont été étudiées afin d’analyser leur comportement en solution. Les investigations ont ensuite porté sur l’influence de ces composés sur le cyclage des accumulateurs lithium-ion. Les résultats obtenus ont permis d’identifier les constituants des couches de passivation ayant un impact critique sur le fonctionnement des batteries, en vue d’optimiser les performances en vieillissement des systèmes existants. / The ageing process of lithium-ion batteries is a key issue for their use in space applications. The passivation layers deposited on the electrode surface are composed of various salts and of polymers, and hence, have a great impact on battery performances or on cycle life. The understanding of dissolution and precipitation phenomena of species deposited at the electrode surface is therefore essential to enlighten the ageing process in batteries. At first, solubilization and transport properties of organic or mineral lithium salts in the electrolyte were determined. Using solubility data, investigations were then carried out to study the effect of these compounds on cycling properties. It has been shown that if some lithium salts have a critical impact on the battery efficiency, others do not have any detrimental effect on battery capacity or faradic yield. As a conclusion, the dissolution with time of compounds belonging to the solid electrolyte interphase is an important parameter to take into account when studying battery ageing.

Ferroelectrics are dielectric materials possessing a switchable spontaneous polarization, which have attracted a growing interest for a broad variety of applications such as ferroelectric lithography, artificial photosynthesis, random and dynamic access memories (FeRAMs and DRAM), but also for the fabrication of devices for nonlinear optics, etc. All the aforementioned applications rely on the control of the ferroelectric domains arrangement, or the charge distribution and transport. In this regard, the main prerequisite is the engineering of the spontaneous polarization, obtained by reversing its orientation or locally inhibiting it. In the latter case, the interface created by the spatial discontinuity of the spontaneous polarization generates local charge accumulation, which can be used to extend the capabilities of ferroelectric materials. This thesis shows how engineering the spontaneous polarization in lithium niobate (LN) by means of proton exchange (PE), a temperature-activated ion exchange process, can be used to develop novel approaches for ferroelectric domain structuring, as well as fabrication of self-assembled nanostructures and control of ionic/electronic transport in this crystal. In particular, it is shown how the electrostatic charge at PE:LN junctions lying below the crystal surface can effectively counteract lateral domain broadening, which in standard electric field poling hampers the fabrication of ferroelectric gratings for Quasi-Phase Matching with periods shorter than 10 μm. By using such an approach, ferroelectric gratings with periods as small as ~ 8 μm are fabricated and characterized for efficient nonlinear optical applications. The viability of the approach for the fabrication of denser gratings is also investigated.  The charge distribution at PE:LN junctions lying on the crystal surface is modelled and used to drive the deposition of self-assembled nanowires by means of silver photoreduction. Such a novel approach for PE lithography is characterized for different experimental conditions. The results highlight a marked influence of the orientation of the spontaneous polarization, the deposition times, as well as the reactants concentrations and the doping of the substrate with MgO. Based on the fact that proton exchange locally reduces the spontaneous polarization, a quick and non-destructive method for imaging PE regions in lithium niobate with nanoscale resolution is also developed by using Piezoresponse Force Microscopy. Moreover the relative reduction of the piezoelectric d33 coefficient associated to PE is estimated in lithium niobate substrates with and without MgO-doping. Finally, by using advanced Scanning Probe Microscopy techniques, the features of charge transport in PE regions are further investigated with nanoscale resolution. A strong unipolar response is found and interpreted in light of ionic-electronic motion coupling due to the interplay of interstitial protons in the PE regions, nanoscale electrochemical reactions at the tip-surface interface, and rectifying metal-PE junctions. / <p>QC 20150325</p>

No description available.

Le niobate de lithium LiNbO3 (LN) est un excellent matériau pour diverses applications en raison de ses propriétés photoréfractives (PR). Un des enjeux de recherche de ce matériau consiste à réaliser des guides optiques PR performants pour l'optique intégrée. L'objectif de la thèse consiste à déterminer les conditions optimales de réalisation de la diffusion du fer (Fe) dans le LN, par le contrôle de sa microstructure et l'estimation de ses propriétés PR par une seule et même technique, la spectroscopie Raman.Plusieurs échantillons LN:Fe ont été étudiés lors de ce travail. Il s'agit entre autres de contrôler le profil de diffusion du fer, et d'estimer l'influence du traitement oxydant ou réducteur sur les différents cristaux.En effet j'ai pu montrer que la microstructure était modifiée non seulement par l'introduction d'un dopant comme Fe, mais aussi par les divers traitements. J'ai aussi mis en évidence pour la première fois le mécanisme de l'incorporation des ions Fe dans des structures de LN obtenues par diffusion.Par ailleurs, j'ai proposé une nouvelle approche des règles d'activité Raman, qui ainsi peuvent prendre en compte des non linéarités optiques du second ordre, ce qui est en général négligé dans la littérature. J'ai ainsi établi les configurations Raman dans lesquelles, soit les intensités de raies sont amplifiées, soit de nouvelles raies sont activées par un processus non linéaire. Ces prédictions ont été confirmées par des résultats expérimentaux obtenus sur plusieurs échantillons. Enfin, j'ai pu proposer une nouvelle méthode d'estimation de l'efficacité PR permettant de comparer utilement divers échantillons selon leur dopage ou traitement. / Lithium niobate LiNbO3 (LN) is an excellent material for various applications in particular thanks to its photorefractive (PR) properties. One of the research goals for this material consists in performing efficient PR optical waveguides for integrated optics.The objective of the thesis is to determine the optimum performing conditions for the iron (Fe) diffusion in LN, by controling its microstructure and estimating its PR properties with only one technique, Raman spectroscopy.Several LN:Fe samples have been studied within this work. The aim is, among others, to control the iron diffusion profile, and to estimate the influence of the oxidizing or reducing treatment on the different crystals.Indeed I was able to show that the microstructure has been affected not only by the introduction of a dopant as Fe, but also by the various treatments. I showed for the first time the mecanism of Fe ions incorporation in LN structures obtained by diffusion.Otherwise, I proposed a new approach of the Raman activity rules, in the way they can take into account optical nonlinearities of the second order, which is generally neglected in litterature. Thus I have established the Raman configurations where, or the intensities of the lines are enhanced, or new lines are activated by a nonlinear process. These predictions have been confirmed by experimental results obtained on many samples. At last, I was able to propose a new method for the estimation of the PR efficiency, allowing to compare usefully several samples according to their doping or treatments.

In this study it was proposed to investigate the use of 3D metal oxides (specifically titanium oxides) as potential electrode materials for lithium ion batteries. Three different approaches were taken: mesoporous materials to increase the surface area and improve the capacity; nanocrystalline materials to increase the surface area and to investigate any changes that may occur using nanocrystals; and microporous materials that are more open, allowing rapid diffusion of lithium and higher capacities. Of the three categories of materials studies, mesoporous TiO2 was the least promising with low reversible capacities (20 mAh??g-1) due to densification resulting in a loss of surface area. In nanocrystalline rutile an irreversible phase change occurred upon initial intercalation, however after this intercalation occurred reversibly in a single phase mechanism giving capacities of 100 mAh??g-1. A trend in intercalation potential was observed with crystallite size that was related to the ability of the structure to relax and accept lithium. Doping of rutile yielded no real improvement. Brookite gave only low capacities from a single phase intercalation mechanism. TiO2 films produced by a novel electrochemical technique showed that while amorphous films give greater capacities, more crystalline (anatase) films give greater reversibility. Overall, microporous titanosilicates showed the most promise with sitinakite giving a reversible capacity of 80 mAh??g-1 after twenty cycles or double this when dried. The intercalation was found to occur by two steps that generate large changes in crystallite size explaining the capacity fade witnessed. While doping did not improve the performance, cation exchange has proven beneficial. The remaining titanosilicates did not perform as well as sitinakite, however a trend was observed in the intercalation potentials with the wavenumber of the Ti-O Raman stretch. This was due to the covalent nature of the bonding. Upon reduction an electron is added to the bond meaning the energy of the bond determines intercalation potential. Overall, most promise was shown by the microporous titanosilicates. The capacities of sitinakite after drying, are comparable to those of the "zero strain" material Li4Ti5O12. Investigation of the titanosilicates and their ion-exchanged derivatives is a promising path for new lithium-ion battery electrode materials.