Search

In this work, the structure and microstructure of Nd:KY3F10 nanoparticles was probed using X-ray synchrotron diffraction analysis. Rietveld refinement was applied to obtain cell parameters, atomic positions and atomic displacement factors to be compared with the ones found in literature. X-ray line profile methods were applied to determine mean crystallite size and crystallite size distribution. Thermoluminescent (TL) emission curves were measured for different radiation doses, from 0.10 kGy up to 10.0 kGy. Dose-response curves were obtained by area integration beneath the peaks from TL. The reproducibility of the results in this work has shown that this material can be considered a good dosimetric material.

There are many commercially important multiphase materials which contain amorphous phases, such as cement pastes, porcelains, glass-ceramics or pharmaceutical compounds. However, the analysis of amorphous phase(s) within cement matrices that contain high amounts of crystalline phase(s) is a challenge. Here, we report measurements of total scattering data quantitatively analyzed by Pair Distribution Function (PDF) minimization and Rietveld methodology to determine phase compositions including both amorphous and nanocrystalline phase contents in cement-related samples. Furthermore, laboratory techniques were used to complement the sample characterization. In addition to five reference materials, the main phase of calcium aluminate cements, CaAl2O4 hydrated at 50 °C to yield crystalline hydrogarnet, Ca3Al2(OH)12, (43wt%) and nanocrystalline aluminum hydroxide gel, Al(OH)3·0.1H2O (50wt%) was also investigated. The PDF analyses revealed that the hydroxide gel has a gibbsite local structurewith an average particle size close to 5nm. PDF and Rietveld quantitative phase analysis results fully agree. This work has been supported by Spanish MINECO through BIA2014-57658-C2-1-R and BIA2014-57658-C2-2-R, which is cofunded by FEDER, research grants. R.U.I. acknowledges funding of CNPQ, grant no. 206983/2014-0/SWE. We also thank CELLS-ALBA (Barcelona, Spain) for providing synchrotron beamtime at BL04-MSPDand Dr. Oriol Vallcorba for his assistance during the synchrotron experiment. 2018-07-01

Publicación versión impresa: diciembre de 2008, In-situ monitoring of the changes that take place in the phase composition of cement pastes during accelerated carbonation (100% CO2) for different binders, has been carried out, by taking Neutron Diffraction patterns in parallel with the carbonation experiments. The variation of the intensity of chosen reflections for each phase along the experiment has been used to monitor concentration changes and has supplied data, in real time, for fractional conversion of different phases (Portlandite, Ettringite and CSH gel) of the hydrated cement pastes. Fitting of these results has allowed to make a qualitative approach to the kinetics of the carbonation of the different phases and extracting conclusions on the microstructural changes that takes place during the carbonation of cement pastes., The experiments reported here were done thanks to the beamtime (Exp 5-25-50) granted by the Institute Max von Laue- Paul Langevin (ILL). The authors are especially grateful to the staff of D20. The authors also acknowledge the funding provided by the Spanish MMA through the project n° 005/2006/2-3.1.

The transformation b-a in Mg-substituted Ca3(PO4)2 was studied. The results obtained showed that, contrary to common belief, there is, in the system Mg3(PO4)2–Ca3(PO4)2, a binary phase field where b1a-Ca3(PO4)2 solid solutions coexist. This binary field lies between the single-phase fields of b- and a-Ca3(PO4)2 solid solution in the Ca3 (PO4)2-rich zone of the mentioned system. In the light of the results and the Palatnik–Landau’s Contact Rule of Phase Regions, a corrected phase equilibrium diagram has been proposed. The practical implications of these findings with regard to the synthesis of pure a- and b- Mg-substituted Ca3(PO4)2 powders and to the sintering of related bioceramics with improved mechanical properties are pointed out

Intermetallic compounds in the Al–Ni system were prepared by Self-propagating high-temperature synthesis (SHS). The front high speed in the range of 1–100 mm/s – typical of these processes made necessary the use a very intense synchrotron source (ESRF) coupled to a very fast acquisition system. Finely divided powders of Ni and Al were intimately mixed in a 1:1 mole ratio, and then compressed to form compact cylindrical of the right density to assure a steady combustion front. Diffraction patterns were acquired every 100 ms. As the reaction proceeded, patterns from the preheated, reaction front, post-heated and cooling zones were sampled. The crystal phases appearing were identified. Similarly, reaction mechanisms and nucleation kinetics were established. Also, high speed pyrometers were used to obtain combustion front temperature profiles and its propagation was video recorded. Finally, SEM images were taken to characterize the microstructure of end products.

The hydration behaviour of Ca3Al2O6 ,C a 12Al14O33 and CaAl2O4 with added amorphous silica at 40, 65 and 901C has been studied for periods ranging from 1 to 31 days. In hydrated samples crystalline phases like katoite (Ca3Al2(SiO4)3� x(OH)4x) and gibbsite, Al(OH)3, were identified, likewise amorphous phases like Al(OH)x, calcium silicate hydrates, C–S–H, and calcium aluminosilicate hydrates, C–S–A–H, were identified. The stoichiometry of Ca3Al2(SiO4)3� x(OH)4x (0p3� xp0.334), which was the main crystalline product, was established by Rietveld refinement of X-ray and neutron diffraction data and by transmission electron microscopy.

TiC–WC composites have been obtained in situ by self-propagating high-temperature synthesis (SHS) from a mixture of compacted powders of elemental titanium, tungsten and graphite. The Rietveld method has proved to be a useful tool to quantify the different phases in the reaction and calculate the cell parameters of the solid solution found in the products. The reaction has also been followed in real time by X-ray diffraction at the European Synchrotron Radiation Facility (ESRF ID-11 Materials Science Beamline). The mechanism of the reaction is discussed in terms of the diffusion of liquid titanium to yield titanium carbide with a solid solution of tungsten. The microstructures of the materials obtained by this method are presented.

Composites of TiC and TiB2 were prepared by self-propagating high-temperature synthesis (SHS). Two routes were attempted; from the elements and from a mixture of anatase, boron oxide, graphite and magnesium. The reactions were monitored in situ by synchrotron X-ray diffraction ( lambda = 0. 26102 Angstrom). The powder mixtures were compacted as cylindrical pellets and upon ignition diffraction patterns were collected every 65 ms with a CCD camera. TiC was the first phase to form, followed by TiB2. The reactions take place in time scales of 0.1 s. The temperature profile for the first route was established from the peak position and the known thermal expansion coefficients. The microstructure of the final products was different: particles of 10 mum for the first and submicron for the second. The viability of the second route to produce ceramic powders in a cheaper way was confirmed. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. (Less)

The reaction process to obtain CaZrO3/MgO specimens in the ZrO2–CaO–MgO system from two mixtures of natural and synthetic raw materials were analyzed by neutron thermodiffractometry; differential thermal analysis–thermogravimetry (DTA-TG) also was used. The results from the different techniques provided a complete description of the decomposition and reaction process in the samples and proved that high-temperature neutron diffraction is a powerful tool for analyzing in situ reactions up to 1250°C in both mixtures. In the present work, the evolution of the crystalline phases, the mechanism and temperature of formation of CaZrO3, and the activation energy were properly established in terms of the neutron thermodiffraction study.

Composition and microstructure changes of cement pastes when heating until 620 °C and cooling afterwards have been monitored on site by neutron diffraction. The parameters involved in the study have been the heating ramp, the state of the sample (in block or ground) and the type of cement. The residual state of the samples has also been studied by mercury intrusion porosimetry (MIP) and thermogravimetry analysis (TGA). As a result, it has been possible to monitor the major features of the experiments, i.e., the phases existence domains and their growing and decaying for portlandite, ettringite, calcite, lime, larnite, and hydrated calcium silicate (CSH gel), as well as the loss of free and bounded water during the experiments. In addition, the different residual phases in the matrix depending on the experimental set up have been established, which might have a significant influence in the durability of a concrete after a fire.

The reaction mechanism that takes place in ZrSiO4–Mg Ca(CO3)2 mixtures was studied in air up to 13001C by collecting neutron diffraction patterns during the heating ramp. Neutron diffraction intensities were used to monitor and establish the mechanism of reaction that occurs in successive stages. (a) MgCa(CO3)2 decomposition yielding MgO and CaCO3; (b) CaCO3 decomposition; (c) reactions between CaO, MgO, and ZrSiO4 involving the formation of phases such as: tetragonalZrO2, a-Ca2SiO4, and Ca3MgSi2O8, some of them acting as transitory phases; and (d) formation of CaZrO3. The results obtained by this technique agree with data obtained by differential thermal analysis and thermogravimetry. The final product has a porous structure, due to the release of CO2, with a very narrow pore size distribution (E1 lm). This open-porosity can be controlled by tailoring the reaction sintering process. # 2002 Elsevier Science (USA)

Partially deuterated Ca3Al2(SiO4)y(OH)12−4y–Al(OH)3 mixtures, prepared by hydration of Ca3Al2O6 (C3A), Ca12Al14O33 (C12A7) and CaAl2O4 (CA) phases in the presence of silica fume, have been characterized by 29Si and 27Al magic-angle spinning–nuclear magnetic resonance (MAS–NMR) spectroscopies. NMR spectroscopy was used to characterize anhydrous and fully hydrated samples. In hydrated compounds, Ca3Al2(OH)12 and Al(OH)3 phases were detected. From the quantitative analysis of 27Al NMR signals, the Al(OH)3/Ca3Al2(OH)12 ratio was deduced. The incorporation of Si into the katoite structure, Ca3Al2(SiO4)3−x(OH)4x, was followed by 27Al and 29Si NMR spectroscopies. Si/OH ratios were determined from the quantitative analysis of 27Al MAS–NMR components associated with Al(OH)6 and Al(OSi)(OH)5 environments. The 29Si NMR spectroscopy was also used to quantify the unreacted silica and amorphous calcium aluminosilicate hydrates formed, C–S–H and C–A–S–H for short. From 29Si NMR spectra, the amount of Si incorporated into different phases was estimated. Si and Al concentrations, deduced by NMR, transmission electron microscopy, energy dispersive spectrometry, and Rietveld analysis of both X-ray and neutron data, indicate that only a part of available Si is incorporated in katoite structures., This research was partially supported by MCYT under the project MAT-2006-12749-C02-01 and MAT-2004-04923-CO2-01. Neutron diffraction data were acquired at the SINQ (experiment number no II/025-18, Swiss Spallation Neutron Source-Paul Scherer Institute Villigen, Suisse). We thank Dr. D. Sheptiakov for the experimental assistance in collecting the ND data.

International audience; The ability of gypsum, a common sulfate mineral, to host arsenic atoms in its crystalline structure, is demonstrated through experimental structural studies of the solid solutions formed upon synthetic coprecipitation of gypsum (CaSO4 ·2H2O) and arsenic. Neutron and X-ray diffraction methods show an enlargement of the gypsum unit cell proportional to the concentration of arsenic in the solids and to the pH solution value. The substitution of sulfate ions (SO42-) by arsenate ions is shown to be more likely under alkaline conditions, where the HAsO42- species predominates. A theoretical Density Functional Theory model of the arsenicdoped gypsum structure reproduces the experimental volume expansion. Extended X-ray Absorption Fine Structure (EXAFS) measurements of the local structure around the arsenic atom in the coprecipitated solids confirm solid state substitution and allow some refinement of the local structure, corroborating the theoretical structure found in the simulations. The charge redistribution within the structure upon substitutions of either the protonated or the unprotonated arsenate species studied by means of Mulliken Population Analyses demonstrates an increase in the covalency in the interaction between Ca2+ and AsO4 3-, whereas the interaction between Ca2+ and HAsO4 2- remains predominantly ionic.

Hydrogarnet (Ca12Al2(OH)12) and katoite of composition Ca3Al2(SiO4)0.176(OH)11.3 were obtained by hydration of tricalcium aluminium oxide, and for katoite synthesis, by addition of amorphous silica. The thermal dehydration was monitored in situ, by neutron thermodiffractometry, from room temperature to 700 °C at atmospheric pressure and at a heating rate of 2 °C/min. On heating, powder neutron diffraction patterns were collected every 300 s. Cell parameters were fitted by the Pawley method. Hydrogarnet decomposed to yield Ca12Al14O32(OH)·mH2O and Ca(OH)2 that eventually transformed to CaO. For katoite, phases of formula Ca12Al14−ySiyO32(O,OH)1+y/2 related to mayenite were formed. Complementary annealing experiments, for 10 h, at higher temperatures and subsequent quenching lead to the formation of mayenite and traces of Ca3SiO5. Thermogravimetric curves were in agreement with the thermodiffractometry experiments. Also, neutron diffraction data allowed to measure thermal expansion coefficients, at atmospheric pressure, between 25 and 250 °C for hydrogarnet and katoite: 1.89 × 10−5 ± 0.09 and 1.63 × 10−5 ± 0.07 °C−1, respectively., This research was supported by MCYT under the projects MAT-2003-08331-CO2-01 and MAT2006-12749-C02-01-02. We thank ILL for beam time granted (experiment number 5-25-83) and Dr. J. Campo for his assistance in collecting ND data. We also thank to Dr. D. Sheptyakov for experimental assistance in collecting the neutron diffraction data at SINQ (experiment number II/02 S-18).

Here we report on two structural studies performed on As-doped gypsum (CaSO4 2H2O) and calcite (CaCO3), using neutron (D20-ILL) and x-ray (ID11-ESRF) diffraction data and EXAFS (BM8-ESRF). The aim of this study is to determine whether As gets into the bulk of gypsum and calcite structures or is simply adsorbed on the surface. Different mechanisms of substitution are used as hypotheses. The combined Rietveld analysis of neutron and x-ray diffraction data shows an expansion of the unit cell volume proportional to the As concentration within the samples. DFT-based simulations confirm the increase of the unit cell volume proportional to the amount of carbonate or sulphate groups substituted. Interpolation of the experimental Rietveld data allows us to distinguish As substituted within the structure from that adsorbed on the surface of both minerals., Comment: Proceedings of the Eighth International Conference on Neutron Scattering

Sorption of As(III) by calcite was investigated as a function of As(III) concentration, time and pH. The sorption isotherm, i.e. the log As(III) vs. log [As(OH)3 degrees / Assat] plot is S-shaped and has been modelled on an extended version of the surface precipitation model. At low concentrations, As(OH)3 degrees is adsorbed by complexation to surface Ca surface sites, as previously described by the X-ray standing wave technique. The inflexion point of the isotherm, where As(OH)3 degrees is limited by the amount of surface sites (ST), yields 6 sites nm-2 in good agreement with crystallographic data. Beyond this value, the amount of sorbed arsenic increases linearly with solution concentration, up to the saturation of arsenic with respect to the precipitation of CaHAsO3(s). The solid solutions formed in this concentration range were examined by X-ray and neutron diffraction. The doped calcite lattice parameters increase with arsenic content while c/a ratio remains constant. Our results made on bulk calcite on the atomic displacement of As atoms along [0001] direction extend those published by Cheng et al., (1999) on calcite surface. This study provides a molecular-level explanation for why As(III) is trapped by calcite in industrial treatments., Comment: 9 pages

10 pages, 12 figures, 3 tables., Even though the electroosmotic flux through hardened cementitious materials during laboratory realkalisation trials had been previously noticed, it has never been in-situ monitored, analysing at the same time the establishment of the electroosmotic flux and the microstructure changes in the surroundings of the rebar. In this paper, two series of cement pastes, cast with CEM I and CEM I substituted in a 35% by fly ash, previously carbonated at 100% CO2, were submitted to realkalisation treatments followed on line by simultaneous acquisition of neutron diffraction data. As a result, it has been possible to confirm the electroosmosis as the driving force of carbonates towards the rebar and to determine the range of pH in the anolyte in which most of the relevant electroosmotic phenomena takes place. On the other hand, the behaviour of the main crystalline phases involved in the process has been monitored during the treatment, with the precipitation of portlandite as main result., The authors also acknowledge the funding provided by the Spanish MMA through the project n- 055/2004/3.

The influence of an external DC voltage in an aluminous cement paste has been studied by in situ neutron diffraction, by monitoring the evolution of CaAl2O4 and Ca3Al2(OH)12 at different distances from the electrodes. The microstructure of the whole aluminous paste becomes dramatically altered. Both phases have been more attacked in zones closer to the electrodes. Major changes took place in the zone next to the positive electrode, in which the anhydrous phase has disappeared completely and the Ca3Al2(OH)12, has decreased significantly. In the cathodic zone, a decrease is also noticed due to the aluminium amphoteric character. The noticeable background diffraction increase in the area close to the positive electrode has been attributed to the formation of an amorphous alumina-like-gel that incorporated water to its composition.

An external dc voltage was applied to cured cement pastes to simulate their natural degradation over time; i.e. migration of ions and dissolution of solid phases. The presence of crystalline phases before and after applying a voltage was quantified by powder diffraction using synchrotron X-ray radiation. The results were refined by the Rietveld method. The specimens were also analysed by mercury intrusion porosimetry. Results indicated that a portion of the crystalline phases dissolved and later precipitated as amorphous phases in smaller pores. This accelerated treatment led to the formation of a larger proportion of amorphous phases in the specimens studied.

Dolomite powder from Coin (Spain) was heated in air at a constant rate of 2°C/min to 1000°C, while neutron diffraction patterns were collected every 150 s. Rietveld refinement was applied and raw intensity data were used to monitor decomposition. The full process happened in two stages: dolomite decomposition to give calcite and periclase, and calcite breakup. The first stage activation energy was 47 kcal·mol−1 from fitting to a contracting sphere model. The dolomite mean thermal expansion coefficients were (6.7 ± 0.4) × 10−6 and (2.7 ± 0.2) × 10−5 K−1 along the a and c axes, respectively. Changes in the Ca–O and Mg–O bond distances were also measured.