The clathrates feature large cages of silicon, germanium, or tin, with guest atoms in the cage centers. The group IV clathrates are interesting because of their thermoelectric efficiency, and their glasslike thermal conductivity at low temperatures. Clathrates show a variety of properties, and the motion of cage center atoms is not well understood. In Sr8Ga16Ge30, we found that the slow atomic motion in the order 10-5 s is present in this system, which is much slower than what would be expected for standard atomic dynamics. NMR studies of Sr8Ga16Ge30 showed that Knight shift and T1 results are consistent with low density metallic behavior. The lineshapes exhibit changes consistent with motional narrowing at low temperatures, and this indicates unusually slow hopping rates. To further investigate this behavior, we made a series of measurements using the Carr-Purcell-Meiboom-Gill NMR sequence. Fitting the results to a hopping model yielded an activation energy of 4.6 K. We can understand all of our observations in terms of non-resonant atomic tunneling between asymmetric sites within the cages, in the presence of disorder. For Ba8Ga16Ge30, the relaxation behavior (T1) deviates from the Korringa relation, and the Knight shift and linewidth change with temperature. Those results could be explained by carrier freezout, and the development of a dilute set of magnetic moments due to these localized carriers. For Ba8Ga16Ge30 samples made from Ga flux, we observed different T1 and Knight shift behavior as compared to n type material. This is due to the differences in carrier type among these different samples. The p type sample has a smaller Knight shift and a slower relaxation rate than n type samples made with the stoichiometric ratio, which is consistent with a change in orbital symmetry between the conduction and valence bands. WDS study for Ba8Al10Ge36 showed the existence of vacancies in the Al-deficient samples, which results in some degree of ordering of Al occupation on the framework sites. In Al NMR measurements on Ba8AlxGe40-x with x = 12 to 16, we found that T1 of all Al samples follows the Korringa relation. The broadening of the single NMR central peak of Ba8Al16Ge30 is due to the inhomogeneous Knight shifts for occupation of different framework sites. For Ba8Al12Ge34 and Ba8Al13Ge33, we observed two peaks, and NMR results show that they are from distinct Al sites, while for each peak, the inhomogeneous broadening is much smaller. The difference in lineshapes we attributed to the existence of vacancies which we detected in the Al-deficient materials, and we assign one of the two Al peaks to Al adjacent to a vacancy.

Reproduction de : Thèse doctorat : Physique : Lyon 1 : 2003. / Titre provenant de l'écran titre. 289 Réf. bibliogr.

Philosophiae Doctor - PhD / Organic noncyclic compounds were used in the development of immunosensor for rapid fish species detection. Flourene derivatives show unique chemical and physical properties because they contain a rigid planar biphenyl unit, and the facile substitution at C9 position of the flourene can improve the solubility and processability of materials containing flourene without significantly increasing of steric interactions in the compounds backbone. 9-(4- methoxyphenyl)-9H-xanthen-9-ol is bulky, rigid and has an hydroxyl moiety that may act as a hydrogen – bond donor, as well as a pyranyl oxygen which is a potential hydrogen –bond acceptor. We have successfully synthesised 9,9’-(ethyne1,2-diyl)bis(flouren-9-ol) by reflux method and 9-(4-methoxyphenyl)-9H-xanthen-9-ol through stirring at room temperature. The products were characterised using spectroscopic methods and were found to be both UV/Vis active (λmax = 400 nm flourene derivative and λmax = 337 nm xanthene derivative ) and fluorescent (440nm and 467nm flourene derivative and 344 and 380 xanthene derivative). These compounds were drop coated onto commercial glassy carbon electrode (GCE) to produce thin films. Scan rate dependent cyclic voltammetry (CV) confirmed the electrodynamics of the thin films to be consistent with monolayer diffusion (De = 1.37x10-21 cm2/s flourene derivative and De = 9.79x10-21 cm2/s xanthene derivative). Surface concentration was estimated to be 1.55x10-13 mol cm-2 flourene derivative and 2.00x10-13 mol cm-2. These compounds were used for the inclusion of parvalbumin antibodies immobilised onto clathrate platform by incubation and were evaluated as immunosensors for fish species identification. The antibody/antigen binding event was evaluated using UV/Vis spectroscopy, electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM). The immunosensor response to parvalbumin in real samples of snoek (an indigenous fish species), tuna, fish paste, eyeshadow, lipstick, omega 3&6 and Scott's emulsion was evaluated. The sensitivity as calculated from EIS for each immunosensor was found to be 5.36x104 flourene derivative immunosensor and 4,11x104 xanthene derivative immunosensor and the detection limit of 1.50 pg/ml flourene derivative immunosensor and 2.42 pg/ml xanthene derivative immunosensor. The antibody/antigen binding was monitored as decrease in charge transfer resistance and increase in capacitance by EIS. The interfacial kinetics of the immunosensors were modelled as equivalent electrical circuit based on EIS data. The UV/Vis spectroscopy was used to confirm the binding of the antibody/antigen in solution by monitoring the intensity of the absorption peak.

Various porous media were tested with biosurfactant solutions (rhamnolipid or Emulsan) at concentrations ranging from 0 ppm to 1000 ppm. The biosurfactant solutions in the presence of porous media often showed substantial gas hydrate catalyzation, localization on selected surfaces, and/or specific hydrate form (massive, stratified, dispersed.) At 1000-ppm concentrations of rhamnolipid, all porous media surfaces exhibited the same hydrate formation rate increase of 187% over the control. The curves generated for rhamnolipid or Emulsan concentration versus peak hydrate formation rate resembled certain classical adsorption curves. Bentonite and aragonite showed hydrate catalyzation properties with or without biosurfactants. The preference for hydrate formation on porous media surfaces (no surfactant) was: Bentonite/nontronite > aragonite/stainless steel > Ottawa sand/kaolinite. Porous media/biosurfactant concentration combinations play marked roles in the types of gas hydrates formed: massive, dendritic, or needle-like. The research helps to explain the vast occurrence of gas hydrates in ocean sediments.

Gas hydrates are molecular host-guest mixtures where guest gas species are encapsulated in host water networks. They play an important role in gas storage in aqueous environments at relatively low pressures, and their stabilities are determined by weak interactions of the guest species with their respective host water frameworks. Thus, the size and the amount of the guest species vary, depending on the size of the empty space provided by the host water structures. The systems studied here are noble gas (He, Ne, Ar) and diatomic (H2) hydrates. Because of the similarity of the guests' sizes between the noble gases and the di-atomic gases, the noble gas hydrates act as simple models for the di-atomic gas hydrates. For example, He, Ne and H2 have approximately the same size. Density functional theory calculations are used to obtain the ground state formation enthalpies of each gas hydrate, as a function of host network, guest stoichiometry, and pressure. Dispersion effects are investigated by comparing various dispersion corrections in the exchange-correlation functionals (semi-local PBE, semi-empirical D2 pair correction, and non-local density functionals i.e. vdW-DF family). Results show that the predicted stability ranges of various phases agree qualitatively, although having quantitative difference, irrespective of the methods of the dispersion corrections in the exchange-correlation functionals. Additionally, it is shown in gas-water dimer interaction calculations that all DFT dispersion-corrected functionals overbind significantly than the interaction acquired by the coupled-cluster calculations, at the CCSD(T) level, which is commonly accepted to provide the most accurate estimation of the actual interaction energy. This could lead to an overestimation of the stability of the hydrate mixtures. Further study in the gas-water cluster indicates that less overbinding effect is found in the cluster than in the dimer. This implies that the overbinding energy caused by DFT might become less pronounce in the solid phase. Graph invariant topology and a program based on a graph theory are used to assign protons based on the 'ice rule' to fulfill the incomplete experimental structural data such as unknown/unclear positions of protons in the host water lattices. These methods help constructing host water networks for computational calculations. Several configurations of the host water structures are tested. Those configurations having lowest enthalpies are used as the host water networks in this research. Furthermore, the enthalpic spread between the configurations having the highest and the lowest enthalpy in the pure water ice network is very small (about 10 meV per water molecule). Nevertheless, it is still unclear to conclude that this protonic effect is also trivial in the gas-water compound. Therefore, this study also calculates the enthalpies of the gas-water mixtures having various proton configurations in the host water networks. Results indicate that very small enthalpic distributions among the proton configurations are found in the compounds as well. Furthermore, the enthalpic spread is almost constant as pressure increases. This suggests there is no pressure effect in the enthalpy gap amoung the proton distributions in both pure water ice and the gas-water compounds. Predicted stable phases for the noble gas compound systems are based on four host water networks, namely, ice Ih, II and Ic, and the novel host water network S!. The He-water system adopts ice Ih, II and Ic network upon increasing pressure. In the Ne-water system, a phase sequence of Sx/ice-Ih, II and Ic with a competitive hydrate phase in the S! host network at very low pressure is found. This is similar to the phase evolution of the H2-water system. For the Ar-water mixture, only a partially occupied hydrate in the Sx host network is found stable. This Sx phase becomes metastable if taking the traditional clathrates (sI and sII) into account. This result agrees very well with the experiment suggesting only two-third filling is found the large guest gases i.e. CO2. For the diatomic guest gas compound systems, the traditional clathrate structure (sII) that found to be existed experimentally in the H2-H2O system is also included in this study together with those four host water networks. Predicted phase stability sequence as elevated pressure is as follows: Sx, ice-Ih, II and Ic. This computationally prediction agrees very well with experiment. Results in this work suggest that the compound based on the traditional clathrate structure II (sII) host water framework is found to be metastable with respect to the decomposition constituents - in this case, they are pure water ice and the S!. The metastability of the hydrogen hydrates based on the sII structure might due to zero-point motions or other dynamic/entropic mechanisms uncovered in this research. Dynamic studies concerning the transition states of the hydrogen guest molecules in three competitive phases at very low pressure (less than 10 kbar), based on Sx, ice-Ih, and ice-II host water network, are considered. The energy barriers required by the hydrogen guest molecules in those three host frameworks are calculated by using Nudged Elastic Band (NEB) method. Results suggest that the hydrogen molecules are more mobile in the Sx than the other two host structures significantly. In the S! host water network, the energy barrier is about 25 meV/hydrogen molecule. This energy is about the room temperature suggesting that the hydrogen guest molecules are easily mobile in the Sx host water network if there is an empty site adjacent to them.

Reproduction de : Thèse de doctorat : Physique et nanophysique : Toulouse 3 : 2007. / Titre provenant de l'écran-titre. Bibliogr. à la fin des chapitres.

Reproduction de : Thèse de doctorat : Physique et nanophysique. Chimie théorique, physique et recherche de la structure des matériaux : Toulouse 3 : 2007. Reproduction de : Thèse de doctorat : Physique et nanophysique. Chimie théorique, physique et recherche de la structure des matériaux : Université Eötvös Loránd de Budapest, Hongrie : 2007. / Thèse soutenue en co-tutelle à l'Université Paul Sabatier-Toulouse III et à l'Université Eötvös Loránd de Budapest, Hongrie. Titre provenant de l'écran-titre. Bibliogr. à la fin des chapitres.

Gas hydrates are known to form plugs in pipelines. Hydrate plug dissociation times can be predicted using the CSMPlug program. At high methane mole fractions of a methane + ethane mixture the predictions agree with experiments for the relative dissociation times of structure I (sI) and structure II (sII) plugs. At intermediate methane mole fractions the predictions disagree with experiment. Enthalpies of dissociation were measured and predicted with the Clapeyron equation. The enthalpies of dissociation for the methane + ethane hydrates were found to vary significantly with pressure, the composition, and the structure of hydrate. The prediction and experimental would likely agree if this variation in the enthalpy of dissociation was taken in to account. In doing the plug dissociation studies at high methane mole fraction a discontinuity was observed in the gas evolution rate and X-ray diffraction indicated the possibility of the presence of both sI and sII hydrate structures. A detailed analysis by step-wise modelling utilising the hydrate prediction package CSMGem showed that preferential enclathration could occur. This conclusion was supported by experiment. Salts such as tetraisopentylammonium fluoride form semi-clathrate hydrates with melting points higher than 30 ℃ and vacant cavities that can store cages such as methane and hydrogen. The stability of this semi-clathrate hydrate with methane was studied and the dissociation phase boundary was found to be at temperatures of about (25 to 30) K higher than that of methane hydrate at the same pressure.

Increasing efforts have been put into research about thermoelectric materials for the last few decades, especially recently, faced with the crucial demand for new energy and energy savings. Among the potential candidates for new generation thermoelectric materials are the intermetallic clathrates. Clathrates are cage-structured materials with guest atoms enclosed. Previous studies have shown lower thermal conductivities compared with many other bulk compounds, and it is believed that guest atom vibration modes are the reason for such thermal behaviors. Several models, including the Einstein oscillator and soft potential models, have been used to explain the guest motion. However the characterization of the anharmonic oscillating motion can be a challenge. In this work, Nuclear Magnetic Resonance (NMR), heat capacity and transport measurements have been used to study several clathrate systems, especially the well- known type-I Ba8Ga16Sn30, which has been reported to have one of the lowest thermal conductivities for bulk compounds. In this material the strong anharmonic rattling behavior was investigated and analyzed according to a double well potential model, yielding good agreement with the experimental results. Furthermore, the resistivity and heat capacity results were studied and analyzed according to the influence of the anharmonic contribution. This offered a way to connect the NMR, transport and heat capacity properties, providing an advantageous way to study strongly anharmonic systems. In further work, several related intermetallic materials were examined for their structure, motion and NMR properties. Dynamical and electrical behaviors were investigated by studying the magnetic and quadrupole NMR spin-lattice relaxation. Type-VIII Ba8Ga16Sn30 exhibits an enhanced dynamics-related term at low temperature, but no rattling response as observed for the type-I structure. Type-I Ba8In16Ge30 was compared with the type-I Ba8Ga16Sn30 because their cage structures are similar. No strong anharmonic contribution was found in the NMR T1 behavior of Ba8In16Ge30, however the T2 showed behavior characteristic of atomic motion. In all cases, the magnetic relaxation was used to characterize the electron structures, and n- type Ba8Ga16Ge30 exhibited a spin-lattice relaxation behavior which is characteristic of impurity band structures near the Fermi surface. Also, a series of Ba8CuxGe46-x clathrates were investigated and showed much more insulating like behavior. In related work, the layered BaGa4 and BaGa3Sn have shown interesting NMR spin-spin relaxation behavior that indicates atomic fluctuations. This is similar to the situation found in type-I Ba8In16Ge30. The influence of atomic motion on the NMR and also the atomic structures of these alloys is further discussed in this work.

Cette thèse porte sur l’étude de la synthèse et des données thermodynamiques des clathrates intermétalliques du système Na-Si, qui sont des matériaux pertinents pour les applications thermoélectriques et qui sont les précurseurs pour de nouvelles formes de Si prometteuses pour des applications photovoltaïques et photoniques. Dans ce travail la formation de clathrates Na-Si a été étudiée in situ et ex situ sous haute pression et haute température. Un nombre de nouvelles formes de silicium a été observé en chauffant des clathrates sous vide. Sous pression inférieure à 3 GPa les clathrates ne se forment pas par l’interaction des éléments. A la place, le siliciure Na4Si4 se forme et reste stable jusqu’à la fusion. Au-dessus de 3 GPa, Na4Si4 formé au début du chauffage, réagit avec l’excès de Si et donne Na30Si136 (structure II) et ensuite, si Na30Si136 réagit avec l’excès de Si pour former Na8Si46 (structure I) au dessous de 7 GPa et le clathrate NaSi6 au-dessus de 7 GPa. Des expériences ont été réalisées en utilisant des cellules standard Paris-Edimbourg (enclumes opposées) jusqu'à 6 GPa et la presse multi-enclumes pour les pressions au-dessus de 6 GPa. La forte sensibilité des produits de cristallisation à la concentration en sodium a été observée. Un schéma de transformations de clathrate pour les pressions entre 0 à 10 GPa et températures entre 300 et 1500 K a été proposé. Le diagramme de phase (concentration – température sous 4 GPa) qui représente les transformations observées et les domaines de stabilité a également été proposé et suggère l’existence de trois équilibres eutectiques : entre Si et structure I, entre structures I et II, et entre structure II et Na4Si4. Pour comprendre la thermodynamique et les interactions dans le système Na-Si, on a également effectué la modélisation du diagramme de phase à pression ambiante, ce que nous a permis de trouver les caractéristiques thermodynamiques de fusion et les équations des liquidus observés. Pour prendre en compte l’impact de la pression sur la thermodynamique, les équations d’état p-V-T de Si à structure diamant, structure I et la forme haute pression de structure II ont été utilisés. / The objective of this thesis is to study the synthesis and thermodynamics of intermetallic clathrates of the Na-Si system, which are the important materials for thermoelectric applications and are the promising precursors for new forms of Si for the photovoltaic and photonic applications. In this work the formation of the Na-Si clathrates have been studied in situ and ex situ at high pressure and high temperature. A number of new forms of silicon have been observed while heating the clathrates under vacuum. Under pressures below 3 GPa the clathrates does not form by the interaction of elements. Instead, the silicide Na4Si4 forms and remains stable up to melting. Above 3 GPa, Na4Si4 formed in the beginning of the heating, react with the excess of Si and gives Na30Si136 (structure II) and after that, if Na30Si136 react with the excess of Si, to form Na8Si46 (structure I) below 7 GPa and the clathrate NaSi6 above 7 GPa. The experiments have been performed by using the standard Paris-Edinburg cell (opposite anvils) up to 6 GPa and multianvil press for pressures above 6 GPa. The strong sensibility of the crystallization product to the concentration of sodium has been observed. The diagram of transformations in clathrates for pressures between 0 and 10 GPa and temperatures between 300 and 1500 K has been proposed. The phase diagram (concentration – temparature at 4 GPa) that represents the observed transformations and stability domains has been also proposed and suggests the existence of three eutetic equilibria: between Si and structure I, between structures I and II, and between structure II and Na4Si4. To understand the thermodynamics and interactions in the system Na-Si, the modelisation of phase diagram at ambient pressure has also been performed that allowed us to establish the thermodynamic characteristics of melting and the equations of observed liquidi. To take into account the impact of pressure on the thermodynamics, the p-V-T equations of state of diamond Si, structure I and high-pressure form of structure II have been used.