Dinâmica da umidade no preenchimento de meso- e nanoporos em nanosilicatos sintéticos

Dissertação (mestrado)—Universidade de Brasília, Instituto de Física, 2012.A expansão das partículas de argilas esmectitas causam mudanças nas distâncias interplanares (espaçamento-d) como função da temperatura e da umidade relativa. Diferentemente do Sódio Fluorohectorita que possui os estados de hidratação de zero, uma e duas camadas de água, a amostra de Lítio Fluorohectorita, os estados de hidratação estáveis são os de zero, uma, uma e meia, duas e três camadas de água, com saltos discretos no espaçamento-d ocorrendo durante as transições entre esses estados. Mantendo a temperatura fixa e variando a umidade relativa do ambiente, somos capazes de reproduzir essas mudanças no espaçamento dentro dos estados de hidratação. A confiabilidade e a reprodutividade do controle dessa umidade permite-nos usar as distâncias interplanares como uma medida da umidade que circunda localmente as partículas de argila. Queremos usar essas observações para estudar o transporte de umidade ao longo da amostra. Impondo um gradiente de umidade em uma amostra com temperatura controlada quase unidimensional, e usando difração de raios X para coletar as distâncias interplanares, nós poderemos extrair padrões de umidade ao longo da amostra. A evolução temporal desses padrões descreve o transporte de água nos mesoporos dentro da argila. _______________________________________________________________________________________ ABSTRACTThe swelling of layered smectite clay particles consists of a change in the interlayer repetition distance (d-spacing) as a function of temperature and humidity. In this work, a ne scan of the relative humidity under room temperature was done for the synthetic clay Lithium Fluorohectorite. This sample has hydrodynamically stable hydration states with zero, one, one and a half, two and three intercalated monolayers of water which are described in a similar work for the Sodium Fluorohectorite, with discrete jumps in d-spacing at the transitions between the hydration states. These changes are monotonous as a function of relative humidity, and one order of magnitude smaller than the shift in d-spacing that is typical for the transition between two hydration states. The reproducibility and reliability of this relative humidity controlled d-shift enables us to use the interlayer repetition distance d as a measure of the local humidity surrounding the clay particles. We provide an example of application of this observation: imposing a humidity gradient over a quasi-one-dimensional temperature-controlled sample, and using x-ray diffraction to record the d − spacing, we are able to extract pro files of the relative humidity along the sample length. Their time evolution describes the transport of water through the mesoporous space inside the clay

Vapor transport in a smectite clay: from normal to anomalous diffusion

International audienceSmectite clays are widely found on the Earth surface. They possess a connected mesoporous space in the micrometer range, and nanopores inside the mineral grains. The grains are stacks of individual 1 nm-thick clay particles (the layers) with the ability to swell by incorporating H2O molecules (or other molecules such as CO2) in-between the layers, depending on the ambiant temperature and on the relative humidity (RH) present in the mesoporous space surrounding the grain. Imposing a gradient of RH along a temperature- controlled dry sample of smectite clay, we investigate the diffusive transport of water molecules in vapor phase through the material. As water molecules diffuse through the mesoporous space, (i) some of them intercalate into the nanopores, (ii) causing the grains to swell and the separation of clay grains into particles of smaller thickness. From (ii) results a change in the geometry of the mesoporous space, with a decrease in the mesoporous volume available for vapor diffusion. These two effects (i and ii) render the transport process potentially anomalous. We monitor it using space- and time-resolved X-ray diffraction at a synchrotron source. Indeed, water absorption into the nano-layered grains changes the interlayer repetition distance (d-spacing) of the stacks, which is seen in the diffraction data. A separate calibration experiment allows mapping this monotonous evolution of d as a function of the RH. By measuring d in space and time in the transport experiments we thus record the time evolution of RH profiles along the direction of the initial RH gradient. To model the data we consider a 1D effective diffusion process described by a fractional time diffusion equation with a diffusion coefficient that depends on the RH. It is possible to rescale all RH profiles onto a single master curve as a function of (x/t)γ/2, where γ is the exponent characteristic of the fractional derivative. We observe that when the clay sample is prepared with sodium cations intercalated in the nanopores, vapor transport is normal (γ=2), while if the interlayer cation is lithium the transport is strongly subdiffusive. This is explained by the different dynamics of cation intercalation in these two clays. In both cases we also obtain the dependence of the effective diffusion coefficient on relative humidity

Microstructural Characterization of Spheroidal Graphite Irons: A Study of the Effect of Preconditioning Treatment

The effect of preconditioning treatments on the control and improvement of spheroidal graphite iron (SGI) microstructure was evaluated. In the melt, 0.15% of Zr-(Ca, Al) FeSi preconditioner was added into different conditions. Four samples were produced for this investigation: (1) in the first melt, there was no addition of a preconditioner for comparative purposes; (2) in the second melt, the preconditioner was added at the cold charge; (3) in the third melt, the preconditioner was added before the last cold charge; and (4) in the fourth melt, the preconditioner was added at tapping from the furnace. Microstructural characterization was conducted to understand the effect of the treatment on the SGI. Optical microscopy results show that preconditioning treatment increases graphite’s nodule density, ferrite content, and nodularity. Scanning electron microscopy (SEM), energy dispersive energy (EDS), and electron backscatter diffraction (EBSD) analysis were used to identify the types of microparticles present in the graphite nodules. Some complex microparticles were identified as AlMg2.5Si2.5N6, MgS, and CaS. The microstructural characteristics of the matrix, such as grain size, crystallographic orientation, and misorientation, were also evaluated by the EBSD. The addition of the preconditioning at tapping results in a higher ferrite fraction, smaller grain size, misorientation, and hardness values. This work suggests that the different preconditioning practice has a crucial effect on the microstructural characteristics of the SGI. This knowledge is vital, allowing the microstructure tailoring to enhance the mechanical properties of SGI to obtain the best performance of these materials

Water vapor diffusive transport in a smectite clay: Cationic control of normal versus anomalous diffusion

International audienceThe transport of chemical species in porous media is ubiquitous in subsurface processes, including contaminanttransport, soil drying and soil remediation. We study vapor transport in a multiscale porosity material, a smectiteclay, in which water molecules travel in mesopores and macropores between the clay grains but can also intercalateinside the nanoporous grains, making them swell. The intercalation dynamics is known to be controlled by thetype of cation that is present in the nanopores; in this case exchanging the cations from Na+ to Li+ acceleratesthe dynamics. By inferring spatial profiles of mesoporous humidity from a space-resolved measurement of grainswelling, and analyzing them with a fractional diffusion equation, we show that exchanging the cations changesmesoporous transport from Fickian to markedly subdiffusive. This results both from modifying the exchange dynamicsbetween the mesoporous and nanoporous phases, and from the feedback of transport on the medium’spermeability due to grain swelling. An important practical implication is a large difference in the time needed forvapor to permeate a given length of the clay depending on the type of intercalated cation

Intercalation and Retention of Carbon Dioxide in a Smectite Clay promoted by Interlayer Cations

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Water Vapor Diffusive Transport in a Smectite Clay: Cationic Control of Normal vs. Anomalous Diffusion

International audienceThe transport of chemical species in porous media is ubiquitous in subsurface processes, including contaminant transport, soil drying, and soil remediation. We study vapor transport in a multiscale porosity material, a smectite clay, in which water molecules travel in mesopores and macropores between the clay grains but can also intercalate inside the nanoporous grains, making them swell. The intercalation dynamics is known to be controlled by the type of cation that is present in the nanopores; in this case exchanging the cations from Na+ to Li+ accelerates the dynamics. By inferring spatial profiles of mesoporous humidity from a space-resolved measurement of grain swelling, and analyzing them with a fractional diffusion equation, we show that exchanging the cations changes mesoporous transport from Fickian to markedly subdiffusive. This results both from modifying the exchange dynamics between the mesoporous and nanoporous phases, and from the feedback of transport on the medium's permeability due to grain swelling. An important practical implication is a large difference in the time needed for vapor to permeate a given length of the clay depending on the type of intercalated cation

Análise da Reflexão 001 da Níquel-Fluorohectorita por Difração de Raios X Síncrotron

In the present work X-ray diffraction was used in order to observe the stable hydration states of the clay nickel-fluorohectorite. The diffraction patterns were measured as function of relative humidity and temperature. The Bragg peak positions were used to calculate the basal spacing of the clay at each temperature (the spacing along the stacking direction). The results showed that the water intercalation process is more dependent on the relative humidity than on the temperature. The crystallite size of the clay increases as the relative humidity rises. This study exposes the necessity for more experiments at higher temperature range in order to reach other hydration states

Domain Orientation and Molecular Ordering in Thin Films of Small Molecule Donor for Organic Photovoltaics

The relationship between the nanomorphology and the charge transport characteristics in solution-processed organic thin films has been the focus of extensive research, as it is key to the further advancement of organic photovoltaics. Understanding the nanostructured domains and their dependence on the applied thermal annealing conditions is one crucial challenge. Here, the conjugated small-molecule donor p-DTS(FBTTh2)2 is used as a model system to demonstrate how isothermal annealing alone, without addition of any processable additives during solution casting, leads to a high ordering of domain structures. Using scanning transmission X-ray microspectroscopy (STXM), the domain structures are probed at the resonance energy of 284.7 eV corresponding to the C1s to Pi* transition. The STXM data analysis reveals the in-plane orientation of the conjugated backbone comprising the heterocyclic aromatic compounds dithienosilole (DTS), bithiophene (BT), and fluorobenzothiadiazole (FBT). The analysis further exhibits the presence of ordered domains accompanied by the disordered boundaries in thin films where the increase in the domain sizes and the high degree of molecular order (anisotropy) is associated to the relatively slow crystallization dynamics of p-DTS(FBTTh2)2 molecules during isothermal annealing at 90 °C. The analysis of the out-of-plane component of the directed resonance intensity shows deviations in the local out-of-plane tilt angles to the lower values, for the most ordered regions in the thin film, suggesting the more out-of-plane orientation of Pi* resonance. These results manifest a promising role of STXM technique in enhancing the knowledge of domain structures in small molecule organic semiconductors

EXAFS and XRD studies in synthetic Ni-Fluorohectorite

In the present work the synthetic clay mineral fluorohectorite was studied by means of an extended X-ray absorption fine structure (EXAFS) in a powder sample with the intention to observe the number of neighboring atoms to the Ni interlayer cation. In addition X-ray diffraction (XRD) was performed in order to follow the hydration states of Ni-fluorohectorite in terms of basal-spacing measurements. The sample conditions were the same for both types of experiments. The EXAFS results show that Ni2 + forms a brucite-like structure in the form of Ni(OH)2, and that this structure coexists with the clay mineral particles. This shows that the Ni atom observed by means of our EXAFS measurements is predominantly the Ni which composes the brucite-like structure and not the interlayer Ni2 + cation. In order to confirm the formation of the brucite-like structure, the EXAFS data from Ni-fluorohectorite were compared to Ni-salt water solutions at various pH