Estudo da formação do AlNbO4 no sistema (x Al2O3 + 1-x Nb2O5) por técnicas de difração de Raios X e microscopia eletrônica em função da composição e temperatura de tratamento térmico

Neste trabalho, a formação da fase AlNbO4 no sistema (x Al2O3 + 1-x Nb2O5) foi estudada por técnicas de difração de raios X e microscopia eletrônica. 0; 0,15; 0,5; 2 e 4% m/m de Nb2O5 foram incorporados em Al2O3-α e sinterizados a 1400, 1500 e 1600°C durante 60 min a uma taxa de aquecimento de 2,5°C.min-1. Os sinterizados foram analisados por difração de raios X, tendo o ajuste dos picos de difração e as fases formadas quantificadas pelo Método de Rietveld. Os materiais produzidos também foram analisados por microscopia eletrônica de varredura tendo a composição química de pontos específicos da amostra analisada por espectroscopia de raios X por dispersão de energia. Mapas de raios X característicos e perfis de composição química microestrutural também foram analisados por espectroscopia por dispersão de comprimento de onda através de microssonda eletrônica. Detalhes da microestrutura foram escolhidos para uma investigação mais aprofundada e removidos da microestrutura original por meio de feixe de íons localizados e analisados por microscopia eletrônica de transmissão de alta resolução. Os resultados mostraram que uma fase de origem não identificada se formou a partir da adição de 0,15% m/m de Nb2O5 em Al2O3-α e que a fase estável do sistema, AlNbO4 foi verificada por difração de raios X a partir de 0,5% m/m de Nb2O5 Também se verificou que todo o Nb2O5 foi consumido durante a sinterização das amostras. Os ajustes dos picos de difração pelo Método de Rietveld permitiram concluir que não existem distorções significativas na rede cristalina da Al2O3-α e que nenhum vestígio de solubilidade do Nb na Al2O3-α foi observado. O AlNbO4 pôde ser quantificado pelo Método de Rietveld apenas para as amostras contendo 2 e 4% m/m de Nb2O5, excluindo-se do refinamento os picos da fase de origem não identificada. As análises por microscopia eletrônica de varredura mostram que com o aumento do teor de Nb2O5 incorporado há um aumento de fase intergranular no sinterizado. Mostra, também, que o Nb2O5 atua como agente de sinterização da Al2O3-α favorecendo o aumento de seus grãos e de sua densificação. A análise por microscopia eletrônica de transmissão de alta resolução mostrou que a região de interesse e a região de interface possuem orientações cristalográficas distintas. O padrão de difração de elétrons desta região mostrou que a mesma possui estrutura monocristalina e está associada à fase AlNbO4. Mostrou, ainda, que esta região monocristalina está em uma matriz com orientação distinta e que nesta orientação puderam ser verificadas distâncias interplanares correspondentes às distâncias interplanares dos picos de origem não identificada.In this work, the AlNbO4 phase formation in the system (x Al2O3 + 1-x Nb2O5) was studied by X-ray diffraction and electron microscopy techniques. 0; 0.15; 0.5; 2 and 4 Nb2O5 wt% were added in α-Al2O3 and sintered at 1400, 1500 and 1600°C for 60 min using a heating rate of 2.5°C.min-1. The sintered samples were analyzed by X-ray diffraction, with the adjustment of the diffraction peaks and the formed phases quantified by Rietveld Method. The samples were also analyzed by scanning electron microscopy and the chemical composition of specific points of the sample analyzed by energy dispersive X-ray spectroscopy. Characteristic X-ray maps and microstructural chemical composition profiles were analyzed by wavelength dispersion spectroscopy through an electron microprobe. Details of the microstructure were chosen for further investigation and removed from the original microstructure by focused ion beam system and analyzed by high resolution transmission electron microscopy. The results showed that an unidentified phase was formed when 0.15 Nb2O5 wt% were added in α-Al2O3 and that the stable phase of the system, AlNbO4 was verified by X-ray diffraction after the addition of 0.5 Nb2O5 wt% It was also found that all Nb2O5 was consumed during the sintering of the samples. Adjustments of the diffraction peaks by Rietveld Method allowed to conclude that no significant distortions in the α-Al2O3 crystalline lattice were observed and that no trace of Nb solubility in α-Al2O3 was verified. AlNbO4 could be quantified by Rietveld Method only for samples containing 2 and 4 Nb2O5 wt%, after excluding the peaks of the unidentified phase. The scanning electron microscopy analysis shows that the increase of the incorporated Nb2O5 content increases the intergranular phase in the sintered samples. It also shows that Nb2O5 acts as a sintering agent for α-Al2O3 favoring the increase of its grains and its densification. High resolution transmission electron microscopy showed that the interest region and the interface region have different crystallographic orientations. The electron diffraction pattern of this region showed a monocrystalline structure and it was associated to the AlNbO4 phase. It also showed that the monocrystalline region is in a different orientation matrix and in this orientation, interplanar distances corresponding to the interplanar distances of the unidentified peaks could be verified

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Experimental evidence of pressure effects on spinel dissolution and peridotite serpentinization kinetics under shallow hydrothermal conditions

Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs, as well as fluid and element transfer between the Earth’s mantle and crust. However, fluid-rock element exchange and serpentinization kinetics under shallow hydrothermal conditions is still largely unconstrained. Here we present two constant temperature (230 °C) time-series of natural peridotite (77.5% olivine; 13.7% enstatite; 6.8% diopside; 2% spinel) serpentinization experiments: at 13.4 MPa; and 20.7 MPa. Al-enriched lizardite was the main secondary mineral in all runs after olivine (olv) and orthopyroxene (opx) serpentinization (without any detectable brucite, talc or magnetite), while primary spinel and diopside partially dissolved during the experiments. Initial serpentinization stages comprises intrinsically coupled reactions between olivine and enstatite, as Al and Si are progressively transferred from orthopyroxene-derived to olivine-derived serpentine, while the opposite is true for Mg and Fe, with homogenization of serpentines compositions after 40 days. The Ni/Cr ratios of serpentines, however, remain diagnostic of the respective primary mineral. Estimated average serpentine content indicates fast serpentinization rates of 0.55 wt.%·day−1 (0.26 mmol·day−1) and 0.26 wt.%·day−1 (0.13 mmol·day−1) at 13.4 and 20.7 MPa, respectively. Approximately 2x faster serpentinization kinetics at lower pressure is likely linked to enhanced spinel dissolution leading to one order of magnitude higher available Al, which accelerates olivine serpentinization while delays orthopyroxene dissolution. Additionally, time-dependent increase in solid products masses suggests rock volume expands linearly 0.37% ± 0.01% per serpentine wt.% independently of pressure. Mass balance constrains suggests olv:opx react at ∼5:2 and ∼3:2 M ratios, resulting in Si-deficient and Si-saturated serpentines at the end of the low-pressure series (13.4 MPa) and high-pressure series (20.7 MPa), respectively. Elevated starting peridotite olv:opx ratio (7.94:1) therefore indicates orthopyroxene serpentinization is ∼3.3x and ∼5.4x faster than olivine at 13.4 MPa and 20.7 MPa, respectively. This contradicts previous assumptions that olivine should dissolve faster than orthopyroxene at experimental conditions. Finally, serpentinization-derived fluids develop pH > 10 and become enriched in H2, CH4, Ca2+ and Si within 6 weeks. Aqueous silica concentrations are highest after 5 days (265.75 and 194.79 µmol/kg) and progressively decrease, reaching 13.84 and 91.54 µmol/kg at 13.4 and 20.7 MPa after 40 days, respectively. These concentrations are very similar to the low-silica (M6) and high-silica (Beehive) endmembers of the Lost City Hydrothermal Field (LCHF). Beyond fluid characteristics, serpentinization products and conditions analogous to the LCHF suggest similar mechanisms between our experiments and natural processes. Our results demonstrate constant temperature serpentinization of a common protolith leads to distinct serpentine and fluid compositions at different pressures. Although additional data is necessary, recent studies and our experiments suggest peridotite serpentinization rates at 230 °C rapidly decrease with increasing pressures at least up to 35 MPa. Whether pressure directly influences olivine and orthopyroxene serpentinization kinetics or indirectly controls reaction rates due to spinel dissolution under hydrothermal conditions deserves further investigation

The influence of different concentrations of a natural clay material as active principle in cosmetic formulations

The clay minerals have characteristics and properties that allow their applicability in the cosmetic area, being incorporated into formulations as an active principle. Therefore, the aim of this work is characterizing and evaluating the influence of a clay from Miracema do Norte, Tocantins, Brazil, named Clay V, on the physicochemical characteristics of a hydrophilic gel of Aristoflex, a copolymer of the neutralized acryloyldimethyltaurate and vinylpyrrolidone sulphonic acid. Clay V was decontaminated and characterized through microbiological evaluation, crystalline phases present by X-Ray Diffraction (XRD), chemical composition by X-Ray Fluorescence (XRF), thermogravimetric analysis (TGA) and granulometric distribution by sieving. Gels were prepared using Clay V with particles in the range ≤ 180 μm in three different concentrations: 1%, 3% and 5% (w/w) and a standard gel without clay. The formulations were evaluated according to organoleptic characteristics, pH, viscosity, spreadability and the centrifugation test. The microbiological evaluation showed that the clay sample is in compliance with the parameters established by the legislation. The following mineral phases were identified by XRD: kaolinite, illite, vermiculite and quartz, mainly composed of silica and alumina according to XRF. Thermal analysis showed that the clay has two thermal decomposition reactions, the largest being 547.6°C. The granulometric analysis identified that the largest fraction (63.22%) was of particles with sizes greater than 710 μm. The organoleptic characteristics presented by the formulations were suitable, with characteristic gel odor, homogeneous appearance, soft and refreshing texture and staining based on the concentration of Clay V used. The obtained pH values were within the range between 5.5 and 6.5, and it was verified that the gel has high spreadability, distributing evenly on the skin. The values obtained for viscosity showed that the formulations are non-Newtonian fluids with pseudoplastic behavior. The centrifugation test showed that the formulations are stable, with no phase separation. The results obtained with the tests showed that the natural clay V material is beneficial in cosmetic products and can be used for incorporation in cosmetic gel formulations of Aristoflex type