Literatura académica sobre el tema "OH point defects"

Abstract. Research results of the past 6 decades on the incorporation of OH point defects in quartz are summarised and evaluated in terms of their application to natural samples and processes, and a link between experimental petrology, natural archives, and model calculations is made. A strong focus is put on recent studies on quartz as a rock-forming mineral, as a geochemical and/or petrological tracer, and as a tool for provenance analysis in sediments and sedimentary rocks. The most relevant defects for natural specimens are generated by coupled substitution involving mono- and trivalent cations, the most prominent being Li+, Al3+, and B3+. OH incorporation is rather a function of the availability of trace metals and water than of pressure and temperature, though temperature indirectly influences the incorporation by the solubility of trace metals in the fluid. Pressure has a negative influence on the formation of OH defects, so the most pure quartzes are probably formed in the deep crust close to the quartz/coesite transition. Natural quartz grains from the Earth's crust have on average 10 wt ppm (weight parts per million) water (5 wt ppm median), but grains with OH defect contents corresponding to up 250 wt ppm water have been discovered in sedimentary archives, matching the concentration of quartz from high-pressure experiments <4 kbar under water-saturated conditions in granitic systems. A rough division into three classes is suggested: (1) grains with pristine igneous and/or hydrothermal origin, (2) mildly thermally annealed grains, and (3) strongly dehydrated grains. While samples derived from the currently exposed Scandinavian Shield are dominated by the third class, considerable contributions of the first two classes are found in the younger rock systems in Central Europe. OH defect contents may be used to estimate mixing ratios for sediments with different sources, provided that a sufficiently large data set exists and that the different sources can be clearly distinguished by their OH inventory. Furthermore, metamorphic overprint leads to a higher degree of equilibration of OH defects between individual grains and may thus be used as a geothermometer. Finally, OH defect retention in quartz allows for estimating timescales of volcanic processes.

AbstractWe have performed studies of the structural and electronic properties of functionalized single wall silicon carbide nanotubes (SW-SiCNTs) with chirality (5,5). Our first principles studies are done using density functional theory. The exchange-correlation energies are modeled according to the Hamprecht-Cohen-Tozer-Handy functional in the generalized gradient approximation (HCTH-GGA) and the DNP basis function with double polarization is applied. To determine the most stable geometry, we have applied the minimum energy criterion considering several initial configurations of the hydroxyl (OH) functional group interacting with the single wall SiCNT. In particular, we tested different orientations of the OH adsorbed on the nanotube surface (oriented towards the Si or C atoms) and at the end of the nanotube. Results indicate that the most favorable geometry corresponds to OH adsorption (chemisorption) at the end of the nanotube. The polarity increases yielding better conditions for solubility and dispersion. The work function of the SW-SiCNT-OH is reduced, which in turn favors conditions for field emission properties (FEPs). Finally, the presence of silicon or carbon mono-vacancies in the functionalized nanotubes suggests this may be a good structure to fabricate semiconductor devices

The analysis of structural particularities of nominally pure and doped LiNbO3single crystals was performed by methods of Raman light scattering and infrared absorption. The concentration of OH groups, as well as the concentration of point defects is calculated from IR absorption spectra in the region of stretching vibrations of OH groups. A correlation has been determined in behavior of IR absorption spectra bands of strongly doped LiNbO3:Zn crystals with the behavior of the Raman spectra band that correspondsto stretching bridge vibrations of oxygen atoms in octahedra along the polar axis.

AbstractA defect structure and osseointegration capacity of sodium and chloride co-substituted hydroxyapatite (NaClAp) were newly studied. The NaClAp was prepared by reacting H3PO4 and Ca(OH)2 with NaNO3 and NH4Cl followed by sintering; pure hydroxyapatite (HAp) was synthesized as a control. After sintering, the co-substitution of Ca and OH with Na and Cl, respectively, produced charged point defects at Ca and PO4 sites. Also, OH molecules partially adopted a head-on structure. The calculated total system energy of NaClAp was higher, whereas the binding energies between each constituent elements and system were lower than those of HAp. These results suggest that NaClAp was less stable than HAp, due to the formation of various defects by co-substitution of Na and Cl. Indeed, NaClAp exhibited higher dissolution behavior in simulated body fluid (SBF) compared with HAp. Accordingly, this increased the capability to produce low crystalline hydroxyl carbonate apatite, likely due to the increasing degree of apatite supersaturation in SBF. Besides, the NaClAp granules showed noticeable improvements in osseointegration capacity four weeks after in vivo test compared with HAp. Collectively, these results imply that the defects made by multiple ion substitutions are useful to increase osseointegration capacity of hydroxyapatite.

Photoluminescence was studied in the volume of congruent and near-stoichiometric lithium niobate crystals obtained by different technologies. Luminescence intensity in near-stoichiometric crystals was found to be lower than that in a congruent crystal. We have shown that not only basic centers (caused by point defects NbLi, VLi) contribute to luminescence, but also complex defects caused by existence of OH-groups.

The photoluminescence intensity in lithium niobate crystals close to the stoichiometric composition being lower than in a congruent crystal was established. The increase of Li / Nb ratio leads to shifting the photoluminescence bands to the short-wavelength region of the spectrum and changing the fundamental absorption edge of the crystals under study. It was shown that, in addition to point defects in the cationic sublattice, complex defects due to the presence of OH groups and compensation defects (VLi / VNb) in the structure can also contribute to photoluminescence.

Исследованы ИК-спектры поглощения в области валентных колебаний OH - групп кристаллов ниобата лития стехиометрического и конгруэнтного составов. Рассчитана концентрация OH - групп, отношение Li / Nb, а также концентрация точечных дефектов Nb и V. Результаты расчетов совпадают с литературными данными и подтверждаются фазовой диаграммой ниобата лития. Показано, что увеличение количества свободных протонов (вносящих вклад в проводимость) может обуславливать более высокие электропроводность, скорость термической фиксации голограмм и снижение эффекта фоторефракции. The IR absorption spectra in the region of stretching vibrations of OH - groups of lithium niobate crystals of stoichiometric and congruent compositions are studied. The results of calculations of the OH - groups concentration, the Li / Nb ratio and the concentration of point defects Nb and V coincide with the literature data. The phase diagram of lithium niobate confirms these results. It is shown that an increase in the number of free protons that contribute to conductivity can lead to reducing the optical damage and higher electrical conductivity and the rate of thermal fixation of holograms.

Several methods are available to enhance the healing and regeneration of periodontal tissues after surgical therapy of intrabony defects. The main indications for the use of combined regenerative procedures are the extent and morphology of the osseous lesions. The six studies of the present dissertation focused on the clinical effect of different barrier techniques, bone substitutes, enamel matrix derivatives and a growth factors containing adjuvant used in various combinations on the healing of severe periodontal intrabony impairments. Synthetic, xenogenetic and autologous materials were used in these randomized clinical studies. Mechanical barriers (polytetrafluoroethylene and collagen membranes) for GTR, biological barriers/enamel matrix proteins (EMD), synthetic (β-TCP) and xenogeneic (NBM) bone grafts and autologous platelet-rich plasma (PRP) were combined in the test and control groups of the trials. The main clinical variable was the clinical attachment level (CAL) and the subsidiary one was the probing pocket depth (PPD), estimated at baseline and after one year. The summation of the results after the statistical analysis takes cognizance of the followings: a) Each of the eleven regenerative methods evaluated (ten combined procedures) leads to significant CAL gain and PPD decrease. b) Using β-TCP or NBM with EMD or with PRP + GTR and GTR’s, the difference between the parameters of the test and control groups were not statistically significant. c) It was confirmed in four studies that the addition of PRP to graft materials has not increased significantly the positive outcomes independent of the type of barrier or graft. d) Adding platelet-rich plasma to natural bone mineral, no benefit was observed from the point of view of the clinical variables. e) The polypeptide proteins of the platelet-rich plasma do not enhance the clinical regenerative effect of enamel matrix proteins. In conclusion, the option of the periodontal surgeons between these methods depends mainly on the defect morphology, the patient’s approach to the different types of materials, the medical concept of the physician, the technical possibilities and the clinical experience of the periodontist.

L'eau sous forme de défauts ponctuels dans la structure des pyroxènes (pxs) des xénolites du manteau est fréquemment utilisée pour tracer la teneur en eau du manteau lithosphérique. Cependant, on sait peu de choses sur le mécanisme qui permet aux xénolites de conserver les signatures hydrogène (H) profondes et si l'on peut éviter une réinitialisation complète ou partielle par réaction avec le magma hôte pendant le transport. En particulier, on ne sait pas : 1) comment la teneur en eau des xénolites est modifiée par le mode d'éruption et pendant le refroidissement de la lave en surface 2) comment les joints de grains (gb) peuvent affecter l'échange H du xénolite avec le magma. L'objectif de cette thèse est de mieux comprendre ces deux aspects de la préservation de la signature de l'eau dans les xénolites mantelliques.La première question est abordée à travers une étude des xénolites de péridotites de deux localités du Massif Central, Allègre et Ray Pic. Nous avons effectué environ 1000 analyses ponctuelles par FTIR et mesures de profil dans des cristaux ol, cpx et opx provenant de 16 xénolites collectés sur les deux localités. Les deux localités ont des structures de coulée de lave différentes, Allègre est un lac de lave figé et a une structure verticale tandis que Ray Pic est une coulée de lave typique allongée/horizontale. Les deux études montrent qu'il n'y a aucune preuve que le refroidissement et la solidification des coulées basaltiques affectent la teneur totale en eau des pxs dans les xénolites du manteau. Cependant, la comparaison des xénolites du dépôt pyroclastique avec ceux de la coulée de lave à Ray Pic montre que la concentration en eau est fortement affectée par le degré de dégazage du magma avant l'éruption. Par contre, des xénolites avec de pxs présentant différentes signatures spectrales, coexistent dans la même coulée de lave. La mise en place et le dégazage n'affectent donc pas les signatures spectrales, suggérant des signatures antérieurement peuvent être conservées.Pour la deuxième question, nous nous sommes concentrés sur l'étude du rôle des gb sur l'échange isotopique de H des xénolites avec leur environnement. Nous avons utilisé une approche combinant expérimentation et numérique. Nous présentons les résultats d'expériences d'échange hydrogène/deutérium réalisés dans monocristaux d'opx et des morceaux cubiques de taille centimétrique provenant d'un même xénolite. Les expériences ont été réalisées entre 600-900°C dans un gaz enrichi en deutérium (D), à pression ambiante. Nous avons comparé les profils de diffusion mesurés dans les monocristaux et les opx situés au bord des cubes avec les profils de diffusion dans les opx à l'intérieur des cubes. Les profils de diffusion ont été analysés grâce à un logiciel de modélisation numérique 2D (Idefick) original. Nos lois de diffusion H-D pour la diffusion intra-cristalline dans les opx mantelliques sont légèrement plus lentes mais comparables à celles proposées dans la littérature pour l'enstatite synthétique pure. Les profils OH-OD enregistrés dans les opx à l'intérieur des cubes ne sont que légèrement plus courts que ceux enregistrés dans les opx au bord des cubes. Ces résultats montrent que la diffusion isotopique de H dans les gb est suffisamment rapide pour équilibrer rapidement les cristaux d'opx à l'intérieur d’échantillons centimétriques. La diffusion aux gb impliquant l'échange H-D dans les xénolites est au moins 3 ordres de grandeur plus rapide que la diffusion intra-cristalline dans l'opx. C’est une première preuve que dans la nature, la signature δD des xénolithes est très probablement contrôlée par l'équilibre avec le magma hôte, même dans le cas de xénolithes de taille conséquentes. Ceci explique pourquoi les pxs de la plupart des xénolithes du manteau ont des signatures δD appauvries. Celles-ci reflètent plutôt un équilibre avec un magma dégazé qu'une signature originale du manteau
Water in the form of hydrous point defects in the crystal structure of pyroxenes (pxs) from mantle xenoliths is frequently used to trace the water content in the lithospheric mantle. However, little is known on the mechanism that allows xenoliths to preserve deep hydrogen (H) signatures and if we can avoid complete or partial reset by reaction with the host magma during transport. Especially, it is unknown: 1) how much water content of xenoliths is modified by the eruption mode (effusive versus explosive) and during lava emplacement 2) how grain boundaries (gb) can affect H exchange in the xenolith with the surrounding melt. The aim of this thesis is to provide better understanding on these two aspects of the preservation of water signature in mantle xenoliths. The first question is approached through an analytical study of peridotite xenoliths from two localities in the French Massif Central, Allègre and Ray Pic. We performed around 1000 FTIR point analysis and profile measurements in ol, cpx and opx crystals derived from 16 xenoliths collected on both localities. The two localities have different lava flow structures, Allègre is a frozen lava lake and has a vertical structure while Ray Pic basaltic lava flow is a typical elongated/horizontally one. In Allègre, we studied xenoliths from different heights in the 30 m lava body. In Ray Pic we sampled xenoliths along the 20 km lava flow and in a pyroclastic deposit at the volcanic edifice. Both studies show that there is no evidence that cooling and solidification of basaltic flows affect the total water content of pxs in mantle xenoliths. However, the comparison of the xenoliths from the pyroclastic deposit and the lava flow at Ray Pic shows that the water concentration is strongly affected by the degree of degassing of the magma prior the eruption. In addition, xenoliths with different spectral signatures of pxs coexist with the same lava flow suggesting that the emplacement and degassing does not affect spectral signatures, suggesting that they may preserve signatures acquired earlier.For the second question, we concentrated on the study of the role of gb on the H isotopic exchange of the xenoliths with its surrounding. We used a combine experimental and numerical approach. In our experimental approach, we present results from hydrogen-deuterium exchange experiments performed in cm-size cubic pieces of a natural xenolith, a spinel lherzolite aggregate. Experiments were performed between 600-900oC in a deuterium (D) enriched gas, at room pressure. We used single crystals of opx from the same xenolith, as sensors of the progress of the exchange within the polycrystalline aggregates. We compared diffusion profiles measured in single crystals and opx located at the edge of the cubes with diffusion profiles in opx inside the cubes, not directly in contact with the gas. Diffusion profiles were analysed through a 2D numerical modelling software (Idefick). Our H-D diffusion laws for intra-crystalline diffusion in mantle opx are slightly slower but comparable to the ones in literature for synthetic pure enstatite. OH-OD profiles recorded by FTIR in opx inside the cubes are only moderately shorter than the ones recorded in opx at the edge of the cubes (i.e. apparent diffusion inside the cubes are only moderately slower). These results indicate that the isotopic diffusion of H in gb is fast enough to equilibrate rapidly the opx crystals inside the cube xenoliths. It shows that grain boundary diffusion involving H-D exchange in xenoliths is at least 3 orders of magnitude faster than intra-crystalline diffusion in opx. This can be a first-evidence that in nature the δD signature of xenoliths is very likely controlled by the equilibrium with the host magma even in the case of xenoliths with large grain size. It provides explanation why pxs from most mantle xenoliths have depleted δD signatures. These rather reflect equilibrium with a degassed magma than an original mantle signature