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The synthesis of new layered rare-earth silicates K3[Y1-aEraSi3O8(OH)2] (AV-22 materials) has been reported. The photoluminescence properties of Y/Er-AV-22 and the material resulting from its thermal degradation, K3[Y1-aEraSi3O9] (Y/Er-AV-23), have been studied and compared. Both materials have a similar chemical makeup and structures sharing analogous building blocks, hence providing a unique opportunity for rationalising the evolution of the photoluminescence properties of lanthanide silicates across dimensionality.; http://www.sciencedirect.com/science/article/B6TWY-4NGKVJW-15/1/72c426adf5746bc81064c14dad0b54ff

As propriedades dosimétricas principais do jade e dos silicatos brasileiros da família do jade foram estudadas para aplicações em processos de radiação de doses altas. Jade é uma denominação comum a dois silicatos diferentes: jadeita, NaAl(Si2O6), e nefrita, Ca2(Mg, Fe)5(Si4O11)2 (OH)2, que pertencem respectivamente à subclasse dos piroxênios e anfibólios. As amostras de jade, estudadas neste trabalho, são provenientes da Áustria, Nova Zelândia, Estados Unidos e Brasil. Os silicatos brasileiros da família do jade estudados neste trabalho foram os anfibólios: tremolita,Ca2Mg5(Si4O11)2(OH)2 e actinolita, Ca2Fe5(Si4O11)2(OH)2; e os piroxênios: rodonita, MnSiO3 e diopsídio, CaMg(Si2O6). A composição mineralógica e química foi obtida pelas técnicas de análise por ativação com nêutrons e difração de raios X. As propriedades dosimétricas principais (curvas de emissão, curvas de calibração, dose mínima detectável, dependência angular e energética, entre outras) foram estudadas, utilizando as técnicas de termoluminescência, emissão exoeletrônica termicamente estimulada e ressonância paramagnética eletrônica. As amostras de jade-Teflon e as amostras de silicatos-Teflon apresentam pelo menos dois picos TL...

A forma isomórfica predominante nos biominerais (casca de ostra, coral, madrepérola e concha) estudados foi a aragonita. Contudo, o surgimento da fase calcita deu-se à temperatura de 500°C a uma taxa de aquecimento de 10°C/s para todas as amostras; e para a amostra de coral, à temperatura de 400°C, independente da taxa de aquecimento. O elemento mais abundante nas amostras de biominerais foi o Ca na forma de CaO e para os silicatos (tremolita, diopsídio e rodonita), o Si na forma de SiO. O elemento traço mais presente nas amostras de biominerais foi o Fe. A análise de ressonância paramagnética eletrônica mostrou as linhas de Mn2+ nas amostras de coral e madrepérola, antes da irradiação. Para amostras irradiadas, os defeitos encontrados foram os radicais CO2-, CO33-, CO3-, SO2-, SO3-, e num intervalo de g entre 2,0010 e 2,0062. Na análise por absorção óptica dos biominerais foram encontradas transições devido à presença de Mn nas amostras. Foi observado um pico termoluminescente (TL) em aproximadamente 140°C para os biominerais e em 180°C para os silicatos, cuja intensidade depende diretamente da dose. Para amostras expostas a diferentes tipos de radiações, o pico TL ocorre em temperaturas mais baixas. Para as curvas dose-resposta obtidas para esses materiais...

Esta tese tem como principal objectivo a síntese, a caracterização estrutural e o estudo das propriedades de fotoluminescência de novos silicatos, lamelares e microporosos de terras raras. As técnicas de caracterização utilizadas foram a difracção de raios-X de pós e de mono-cristal, a microscopia electrónica de varrimento, as análises térmica e elementar, e as espectroscopias de fotoluminescência, ressonância magnética nuclear, Raman e infravermelho. Os silicatos de terras raras cristalinos foram obtidos por síntese hidrotérmica em condições de temperatura e pressão moderadas. Os silicatos lamelares AV-22, K3[(RE)Si3O8(OH)2], RE3+=Y3+, Eu3+, Er3+ ,Tb3+, Gd3+ e Ce3+ são sistemas hóspede-hospedeiro convenientes para criar materiais multifuncionais com um amplo leque de propriedades. A estrutura dos materiais AV-22 foi determinada por difracção de raios-X de mono-cristal. Os materiais Tb- e Eu-AV-22 são emissores de luz visível (verde e vermelho, respectivamente), com eficiências comparáveis à dos padrões usados em lâmpadas comerciais, enquanto que Er-AV-22 é um emissor de infravermelho, à temperatura ambiente. A incorporação de Ce3+ e Tb3+ no mesmo silicato lamelar cria um efectivo canal de transferência da energia do primeiro para o segundo. Após calcinação a 650 ºC...

Copper containing silicates have been prepared by an acid-catalyzed sol-gel process. The materials were characterized by X-ray diffraction and fluorescence, EPR spectroscopy, elemental analysis, N2-physisorption, thermogravimetry, differential scanning calorimetry, temperature-programmed reduction, FTIR and UV/VIS spectroscopy. The silicates were shown to be efficient catalysts for the oxidation of cyclohexane with tert-butyl hydroperoxide as oxidant. Cyclohexanol and cyclohexanone were obtained as the main products. The metal was shown to be weakly bound to the silicate matrix and metal leaching was observed. Leaching was quantified by X-ray fluorescence and leaching tests showed that the catalytic activity is due to supported copper species. Leached copper showed no activity in the homogeneous phase.

Amorphous silicates having small particles with a large surface area were found to have high nonspecific adsorption capacity. Investigations with Escherichia coli T4 bacteriophage have shown adsorption capacity of up to 99.9% on different tested silicates. Optimal conditions for the adsorption of T4 bacteriophage on various commercially available silicates were determined. A continuous-flow adsorption of T4 bacteriophage on silicates appears to be a model system for the removal of viruses from potable water and sewage.

Micro- and nano- mesoporous silicate particles are considered potential drug delivery systems because of their ordered pore structures, large surface areas and the ease with which they can be chemically modified. However, few cytotoxicity or biocompatibility studies have been reported, especially when silicates are administered in the quantities necessary to deliver low-potency drugs. The biocompatibility of mesoporous silicates of particle sizes ~ 150 nm, ~ 800 nm and ~ 4 µm and pore sizes of 3 nm, 7 nm and 16 nm respectively are examined here. In vitro, mesoporous silicates showed a significant degree of toxicity at high concentrations with mesothelial cells. Following subcutaneous injection of silicates in rats, the amount of residual material decreased progressively over three months, with good biocompatibility on histology at all time points. In contrast, intra peritoneal and intra venous injections in mice resulted in death or euthanasia. No toxicity was seen with subcutaneous injection of the same particles in mice. Microscopic analysis of the lung tissue of the mice indicates that death may be due to thrombosis. Although local tissue reaction to mesoporous silicates was benign, they caused severe systemic toxicity. This toxicity could be mitigated by modification of the materials.

peer-reviewed; Mesoporous silicates (MPS) possess ordered pore structures with pore diameters sufficiently large to accommodate a range of enzymes. The successful immobilization of an enzyme on MPS usually requires modification of the surface of MPS to optimize the interactions between the support and the enzyme. Recent developments on the functionalization of MPS for the immobilization of enzymes are described in this review.; ACCEPTED; peer-reviewed

peer-reviewed; The objective of this research is to synthesise mesoporous silicates, incorporate metal complexes on to the surface of the silicates post synthesis and utilise these materials for the immobilisation of histidine-tagged enzymes to form stable, reusable biocatalysts. Studies in the synthesis of a range of mesoporous silicates and subsequent postsynthetic metal-cyclam functionalisation are presented. The generation of nickel-cyclam functionalised MCM-41/89, SBA-15 and MCF was successfully achieved in good yield. Post-synthetic methods for the metal-cyclam functionalisation of the mesoporous silicate mesocellular foam (MCF) are presented and compared. MCF functionalised using a one-pot functionalisation technique had a higher metal content than a step-wise functionalisation technique. Optimisation steps were required for the specific adsorption of histidine tagged proteins/enzymes onto metal functionalised mesoporous silicates. As a proof-of-concept system, the immobilisation of the histidine-tagged inhibitor protein Spi onto SBA-15- Ni-cyclam was investigated. Optimisation of the immobilisation using varying concentrations of NaCl and PEG400 demonstrated specific immobilisation. In the presence of 2 % PEG400...

This thesis investigates the nature and origins of minerals of a group known as layer (sheet) silicates or phyllosilicates. It consists of a collection of research papers grouped into 3 categories: modulated layer silicates, clay silicates and the weathering of rocks and minerals, including the formation of clay minerals. Central to the research has been the use of X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). All three research categories were carried out simultaneously.; Thesis (D.Sc.)--University of Adelaide, Dept. of Geology and Geophysics, 1999; Includes bibliographical references.; 1 v. :; Title page, contents and abstract only. The complete thesis in print form is available from the University Library.

Dissolution of silicate minerals is an important part of many geological processes taking place on Earth’s surface as well as in the deep crust. Weathering of rocks and soil formation, metasomatic rock alteration by hydrothermal fluids, and diagenetic transformation of sediments are controlled by dissolution reactions. Secondary pore space of natural reservoirs can be significantly altered by dissolution and the subsequent precipitation of clay minerals. As a consequence, the potential storage capacity of natural fuels, carbon dioxide, or radioactive waste, can be affected. In addition, the stability of rocks hosting potentially hazardous materials largely depends on their dissolution rates. The development of computational ab initio and Molecular Dynamics techniques drives the interest to study dissolution reactions at the molecular scale. At the same time, advanced microscopic techniques allow us to study dissolution process at the nm to micron scale and investigate spatio-temporal variations of surface reactivity. The combination of these methods has great potential for tackling fundamental questions of the mechanisms of mineral dissolution. My thesis work presents an integrated multiscale approach to studying the dissolution of silicates. The work consists of three main parts: experimental studies of the dissolving mineral surface at the micron scale...

Dissertação de mestrado integrado em Engenharia de Materiais; The present work introduces the transparent precursor as a new media for synthesis of tin silicates. Based on this approach a systematic work exploiting the potential for formation of various tin silicates and process optimization of the existing ones has been carried out. The conducted syntheses resulted in the creation of crystallization fields showing the dependence of the run product on initial molar composition. Within various synthesis compositions the typical structures of the sodium based silicate (AV-10) the potassium based tin silicate (AV-6) the sodium and potassium based tin silicate (AV-7) has been achieved. Another novelty is the fine control of the crystalline product by the time of agitation of the initial transparent precursor. This type of phase control has never been studied in tin silicates. The close structural and synthesis relation between the microporous phases AV-6 and AV-7 has been studied. The studies on the time of precursor agitation revealed that the transition between these two structures is time dependent and that AV-7 can be produced in absence of sodium ions. In the course of creation of the crystallization fields a new potassium tin silicate with layered structure was prepared. The structure was successfully indexed and the structural determination is in progress. Ion exchange properties of AV-6 and AV-7 were also explored. AV-6 was exchanged to copper and zinc forms and for the first time it has been revealed the ion exchange potential of AV-7. The phase was successfully converted into copper...

peer-reviewed; Enzymes often display high regio- and chemoselectivity and therefore are of great interest for industrial processes. However, in these processes, they can show low stability and are not reusable. Immobilisation can optimize stability and activity, providing controlled mass transport of substrate and enabling re-use of the biocatalyst. Immobilisation also allows for continuous processes. Mesoporous silicates provide a means of immobilizing proteins and enzymes in a stable environment while retaining physiological function. Mesoporous silicates possess large surface areas, highly ordered pore structures and a very narrow pore size distribution. Pore sizes range from 3-20nm making them ideal for protein encapsulation. One recurrent problem with physi-sorption onto mesoporous silicates is leaching of the enzyme. Covalent attachment can reduce leaching but can increase rigidity which could lead to a loss in activity. Methods of tailoring the properties of both the mesoporous silicate surface and the protein surface to improve adsorption and catalytic activity have been explored with a view to generate a stable biocatalyst. A biocatalyst has been generated through the tailored adsorption of a tagged enzyme onto a metal functionalized mesoporous silicate. By employing a standard method of protein purification...

The depletion of iron and sulphur into dust in the interstellar medium and the exact nature of interstellar amorphous silicate grains is still an open question. We study the incorporation of iron and sulphur into amorphous silicates of olivine- and pyroxene-type and their effects on the dust spectroscopy and thermal emission. We used the Maxwell-Garnett effective-medium theory to construct the optical constants for a mixture of silicates, metallic iron, and iron sulphide. We also studied the effects of iron and iron sulphide in aggregate grains. Iron sulphide inclusions within amorphous silicates that contain iron metal inclusions shows no strong differences in the optical properties of the grains. A mix of amorphous olivine- and pyroxene-type silicate broadens the silicate features. An amorphous carbon mantle with a thickness of 10 nm on the silicate grains leads to an increase in absorption on the short-wavelength side of the 10 $\mu$m silicate band. The assumption of amorphous olivine-type and pyroxene-type silicates and a 10 nm thick amorphous carbon mantle better matches the interstellar silicate band profiles. Including iron nano-particles leads to an increase in the mid-IR extinction, while up to 5 ppm of sulphur can be incorporated as Fe/FeS nano inclusions into silicate grains without leaving a significant trace of its presence.

We present evidence of a >10-sigma detection of the 10 micron silicate dust absorption feature in the spectrum of the gravitationally lensed quasar PKS 1830-211, produced by a foreground absorption system at redshift 0.886. We have examined more than 100 optical depth templates, derived from both observations of Galactic and extragalactic sources and laboratory measurements, in order to constrain the chemical structure of the silicate dust. We find that the best fit to the observed absorption profile is produced by laboratory crystalline olivine, with a corresponding peak optical depth of tau_10=0.27+/-0.05. The fit is slightly improved upon by including small contributions from additional materials such as silica, enstatite, or serpentine, which suggests that the dust composition may consist of a blend of crystalline silicates. Combining templates for amorphous and crystalline silicates, we find that the fraction of crystalline silicates needs to be at least 95%. Given the rarity of extragalactic sources with such a high degree of silicate crystallinity, we also explore the possibility that the observed spectral features are produced by amorphous silicates in combination with other molecular or atomic transitions, or by foreground source contamination. While we cannot rule out these latter possibilities...

Silicates are an important component of interstellar dust and the structure of these grains -- amorphous versus crystalline -- is sensitive to the local physical conditions. We have studied the infrared spectra of a sample of ultra-luminous infrared galaxies. Here, we report the discovery of weak, narrow absorption features at 11, 16, 19, 23, and 28 microns, characteristic of crystalline silicates, superimposed on the broad absorption bands at 10 and 18 microns due to amorphous silicates in a subset of this sample. These features betray the presence of forsterite (Mg_2SiO_4), the magnesium-rich end member of the olivines. Previously, crystalline silicates have only been observed in circumstellar environments. The derived fraction of forsterite to amorphous silicates is typically 0.1 in these ULIRGs. This is much larger than the upper limit for this ratio in the interstellar medium of the Milky Way, 0.01. These results suggest that the timescale for injection of crystalline silicates into the ISM is short in a merger-driven starburst environment (e.g., as compared to the total time to dissipate the gas), pointing towards massive stars as a prominent source of crystalline silicates. Furthermore, amorphization due to cosmic rays, which is thought to be of prime importance for the local ISM...

We have studied the dust along the line-of-sight towards the Galactic Center using Short Wavelength Spectrometer (SWS) data obtained with the Infrared Space Observatory (ISO). We focussed on the wavelength region from 8-13 micron which is dominated by the strong silicate absorption feature. Using the absorption profiles observed towards Galactic Center Sources (GCS) 3 and 4, which are C-rich Wolf-Rayet Stars, as reference objects, we are able to disentangle the interstellar silicate absorption and the silicate emission intrinsic to the source, toward Sgr A* and derive a very accurate profile for the intrinsic 9.7 micron band. The interstellar absorption band is smooth and featureless and is well reproduced using a mixture of 15.1% amorphous pyroxene and 84.9% of amorphous olivine by mass, all in spherical sub-micron-sized grains. There is no direct evidence for substructure due to interstellar crystalline silicates. We are able to determine an upper limit to the degree of crystallinity of silicates in the diffuse interstellar medium (ISM), and conclude that the crystalline fraction of the interstellar silicates is 0.2% (+/- 0.2%) by mass. This is much lower than the degree of crystallinity observed in silicates in the circumstellar environment of evolved stars...

We present a model using the evolution of the stellar population in a starburst galaxy to predict the crystallinity of the silicates in the interstellar medium of this galaxy. We take into account dust production in stellar ejecta, and amorphisation and destruction in the interstellar medium and find that a detectable amount of crystalline silicates may be formed, particularly at high star formation rates, and in case supernovae are efficient dust producers. We discuss the effect of dust destruction and amorphisation by supernovae, and the effect of a low dust-production efficiency by supernovae, and find that when taking this into account, crystallinity in the interstellar medium becomes hard to detect. Levels of 6.5-13% crystallinity in the interstellar medium of starburst galaxies have been observed and thus we conclude that not all these crystalline silicates can be of stellar origin, and an additional source of crystalline silicates associated with the Active Galactic Nucleus must be present.; Comment: 8 pages, 6 figures, accepted by MNRAS

[ES] El principal inconveniente para la utilización de los materiales compuestos C/SiC en aplicaciones estructurales de alta temperatura es la elevada velocidad de oxidación de la fibra de carbono por encima de 450°C (1). Por esta razón, es necesario el desarrollo de recubrimientos capaces de proteger de la oxidación a estos materiales durante periodos prolongados a temperaturas de hasta 1600°C. Habitualmente, los recubrimientos están formados por multicapas con características antioxidantes complementarias. El objetivo de este trabajo ha sido el desarrollo de un recubrimiento multicapa formado por una capa interior de SiC, una intermedia de silicatos de itrio: Y2Si2O7 y Y2SiO5, y una exterior de SiO2. En su procesamiento se han empleado diferentes técnicas: pulverizado con soluciones de policarbosilanos, inmersión en suspensiones coloidales y técnica solgel, respectivamente. Se ha analizado la influencia sobre la resistencia a la oxidación del material, y se ha realizado un estudio microestructural por MEB previo y posterior al ensayo de oxidación. Las presencia de fisuras en la capa interior de SiC origina una resistencia a la oxidación muy reducida a baja temperatura. Solamente cuando el substrato está protegido por la triple capa la reducción de la velocidad de oxidación es considerable y la pérdida de peso se reduce del 50%...

The structures of ternary and quaternary silicates are revisited on the basis of the Zintl–Klemm concept and the Pearson's generalised octet rule. The three-dimensional skeletons formed by the Si atoms can be interpreted as if the Si atoms were behaving as Zintl polyanions, adopting the structure of either main-group elements or Zintl polyanions showing the same connectivity. The O atoms are then located close to both, the hypothetical two-electron bonds and the lone pairs, giving rise to a tetrahedral coordination. In some silicates, the Si atoms clearly show an amphoteric character so that some Si atoms act as donors (bases) adopting an octahedral coordination, whereas others behave as acceptors (acid), adopting a tetrahedral coordination. Although the octahedral coordination seems to be favoured by the application of pressure, the results reported here indicate that the coordination sphere of silicon is not a function of the assumed ionic radius of the Si4+ cations but it depends on the nature of the other cations accompanying them in the structure.; The present work was supported by DGI of MEC (Spain) under project MAT2004-05867-C03-02.; Peer reviewed