Precipitate Stoichiometry Research Articles

Kinetics and mechanisms of cyanobacterially induced precipitation of magnesium silicate.

The biomineralization of CO2 , in the form of carbonate minerals, is considered as one of the efficient solutions of atmospheric CO2 removal, allowing stable and sustainable storage of this greenhouse gas. Cyanobacteria are among the most powerful microorganisms capable of precipitating carbonate minerals, both in the present and in the past. In the modern environments, high Si concentration during geoengineering biomineralization could occur due to dissolution of Mg-bearing primary silicates such as olivine. However, most of experimental studies aimed to understand the formation of these carbonates were performed in Si-poor solutions. Thus, experimental characterizations of the nature, rate, and stoichiometry of precipitated minerals in Si-rich solutions in the presence of bacteria are lacking. The present study attempted to reproduce, in controlled laboratory experiments, the processes of biomineralization in a carbonate- and Mg-bearing medium having high Si concentrations (2-4 mM, which is below the saturation with respect to amorphous silica). These experiments have been carried out in the presence of three contrasting cyanobacteria: Synechococcus sp., Chroococcidiopsis sp. and Aphanothece clathrata in order to characterize the rate of formation, stoichiometry and mineralogical nature of precipitates. The results demonstrated significant role of cyanobacteria in the precipitation of carbonate and silicate minerals by increasing the pH of the medium during photosynthesis. Magnesium precipitation rates measured between 50 and 150 h of reaction time ranged from 0.05 to 0.5 mmol h-1 gdry1 and decreased (Synechococcus sp. and Chroococcidiopsis sp.) or increased (A.clathrata) with an increase in the Si:Mg ratio in solution. The abiotic instantaneous rates of Mg and Si removal from alkaline solutions were similar to those in the presence of cyanobacteria at the same pH value suggesting that photosynthetically induced pH rise was the main factor of mineral formation. The transmission electron microscopy (TEM) and spectroscopic observations and associated analyses identified an amorphous magnesium silicate together with hydrous Mg carbonates (hydromagnesite). The formation of carbonate solid phase at high Mg: Si ratios indicated the potential for the removal of inorganic carbon at pH > 10. The difference in the degree of C removal between different species was primarily linked to different degree of pH rise during photosynthesis. Taken together, the results obtained in this study allowed an efficient reproduction of combined magnesium hydroxo-carbonates and hydrous silicates precipitation under cyanobacterial activity, suitable for geoengineering of biologically controlled CO2 sequestration in Si-Mg-carbonate-bearing solutions.

Tuning Solution Chemistry for Compositional Control of Multicomponent Lithium-Ion Battery Materials

A common route to synthesize lithium-ion battery active materials in the literature is to precipitate transition metals with a desired stoichiometry and then perform a calcination to produce the final active material. While this approach has produced many high performance materials, depending on the details of the solution conditions used during precipitation the stoichiometry of the precipitate can deviate from the stoichiometry of the feed. This stoichiometry deviation is largely ignored; however, a few groups have attempted to address this discrepancy with solution equilibrium models to rationally approach compositional control in the final active material. In this talk, we will present results that show the extent of the deviation of the precipitate stoichiometry from the feed stoichiometry across a number of solution conditions and compare experimental results to equilibrium calculations for one co-precipitation process. In addition, we will show initial results for the rate of precipitation of different transition metals in solution and the potential impact on the resulting final material.

3D modelling of β′′ in Al–Mg–Si: Towards an atomistic level ab initio based examination of a full precipitate enclosed in a host lattice

We extend a first principles based hierarchical multi-scale model scheme for describing a fully coherent precipitate in a host lattice to 3D simulations. As our test system, the needle-shaped main hardening Al–Mg–Si alloy precipitate β′′ is chosen. We show that computational costs do not impose practical limits on the modelling: the scheme can probe the full interface energy for physically sized and well isolated precipitates. Examining a series of energetically competitive bulk β′′ configurations, we highlight a series of results: (i) the scatter in the structural parameters for different β′′ configurations clearly exceeds experimental uncertainties also when interaction with the host lattice is taken into account. (ii) Structural and compositional β′′/Al interfaces generally coincide. This implies that precipitate stoichiometry is retained only for the two β′′ configurations with the lowest formation energy (compositions Mg5Al2Si4, Mg4Al3Si4). (iii) β′′–Mg4Al3Si4 emerges as a minimum energy configuration for large precipitates. Finally, (iv) more complete modelling, with precipitates surrounded by Al in all three dimensions, is expected to highlight a non-negligible influence of the precipitate misfit along the main growth (needle) direction.

Comparative quantification and statistical analysis of η′ and η precipitates in aluminum alloy AA7075-T651 by TEM and AFM

Quantification of nanometric precipitates in metallic alloys has been traditionally performed using transmission electron microscopy, which is nominally a low throughput technique. This work presents a comparative study of quantification of η′ and η precipitates in aluminum alloy AA7075-T651 using transmission electron microscopy (TEM) and non-contact atomic force microscopy (AFM). AFM quantification was compared with 2-D stereological results reported elsewhere. Also, a method was developed, using specialized software, to characterize nanometric size precipitates observed in dark-field TEM micrographs. Statistical analysis of the quantification results from both measurement techniques supports the use of AFM for precipitate characterization. Once the precipitate stoichiometry has been determined by appropriate analytical techniques like TEM, as it is the case for η′ and η in AA7075-T651, the relative ease with which specimens are prepared for AFM analysis could be advantageous in product and process development, and quality control, where a large number of samples are expected for analysis on a regular basis.

Ab initio interface configuration determination for β″ in Al–Mg–Si: Beyond the constraint of a preserved precipitate stoichiometry

The precipitate-host lattice interface configuration stabilities for the β″ phase in the Al–Mg–Si alloy system have been examined using density functional theory. Usually, the supercell based calculations underlying such studies assume a preserved precipitate stoichiometry. Relaxing this assumption in the present work, we highlight significant results that may easily be overlooked when the stoichiometry constraint is operative. Our main findings are the following: (i) we reject the often proposed Mg5Si6 composition for β″. (ii) We stress the possibility of significant Mg–Al compositional disorder for the more likely β″-Mg5Al2Si4. Finally, (iii) we propose the existence of more than one locally stable interface configuration for the coherent β″/Al interfaces. It is argued that quantifying the solute atom diffusivity near the interface as well as within β″ is fundamental to addressing the importance of points (ii and iii).

Determination of silicon oxide precipitate stoichiometry using global and local techniques

The effects of high temperature annealing in an oxidizing atmosphere on defect evolution have been studied using global and local techniques. Modeling the evolution of silicon oxide precipitates requires knowledge of the precipitate stoichiometry. In this study we followed precipitate size evolution during the annealing process. The stoichiometry was determined using two different methods: first, results were obtained using Laser Scattering Tomography (LST) technique to determine the size and density of precipitates and compared with model predictions, considering both SiO and SiO2 cases. Then, the precipitate composition and stoichiometry were measured directly, using Electron Dispersive X-ray Spectroscopy (EDX) in an Transmission Electron Microscope (TEM). It is shown that the most probable stoichiometry is SiO and both methods provide similar results.

Oxidation behaviour of zirconium alloys and their precipitates – A mechanistic study

The precipitate oxidation behaviour of binary zirconium alloys containing 1wt.% Fe, Ni, Cr or 0.6wt.% Nb was characterised in TEM on FIB prepared transverse sections of the oxide and reported in previous studies [1,2]. In the present study the following alloys: Zr1%Cu, Zr0.5%Cu0.5%Mo and pure Zr are analysed to add to the available information. In all cases, the observed precipitate oxidation behaviour in the oxide close to the metal-oxide interface could be described either with delayed oxidation with respect to the matrix or simultaneous oxidation as the surrounding zirconium matrix. Attempt was made to explain these observations, with different parameters such as precipitate size and structure, composition and thermodynamic properties. It was concluded that the thermodynamics with the new approach presented could explain most precisely their behaviour, considering the precipitate stoichiometry and the free energy of oxidation of the constituting elements.The surface topography of the oxidised materials, as well as the microstructure of the oxide presenting microcracks have been examined. A systematic presence of microcracks above the precipitates exhibiting delayed oxidation has been found; the height of these crack calculated using the Pilling–Bedworth ratios of different phases present, can explain their origin. The protrusions at the surface in the case of materials containing large precipitates can be unambiguously correlated to the presence of these latter, and the height can be correlated to the Pilling–Bedworth ratios of the phases present as well as the diffusion of the alloying elements to the surface and their subsequent oxidation. This latter behaviour was much more considerable in the case of Fe and Cu with Fe showing systematically diffusion to the outer surface.

Oxalate-precursor processing for high quality BaZrO3

BaZrO3 is by far the most inert crucible material that has been used for melt processing of high quality single crystal YBCO superconductors. To overcome the processing difficulties of existing solid-state methods, solution processing methods are increasingly important in powder synthesis. This study investigates several methods of producing oxalate precursors for subsequent thermal decomposition to BaZrO3 with a view to producing high quality BaZrO3 ceramics. The most favourable system used barium acetate, ammonium oxalate and zirconium oxychloride, which unlike other previously reported oxalate processes allowed near stoichiometric precipitation without requiring a large excess of Ba reagents, elevated precipitation temperatures or slow addition of reagents. Precise control over precipitate stoichiometry was achieved by variation of the solution Ba:[Zr+Hf] mole ratio without requiring accurate control over oxalate addition. XRF, XRD, N2 BET adsorption, DTA/TGA and TEM analysis showed this process to be capable of producing BaZrO3 powders suitable for ceramics applications. The phase purity, particle size and surface areas of BaZrO3 powders produced by calcination of these precursors can be adjusted by variation of stoichiometry and calcination temperature. Crucibles formed from oxalate precursors have been able to contain Y2O3-BaCuO2-CuO melts for up to seven days.

Formation and Release of Cobalt(II) Sorption and Precipitation Products in Aging Kaolinite–Water Slurries

The uptake and release behavior of cobalt(II) was studied over thousands of hours in CO2-free aqueous suspensions of kaolinite under three pairs of total cobalt concentration (CoT) and near-neutral pH (7.5–7.8) conditions. Dissolved cobalt, aluminum, and silicon concentrations were monitored by ICPMS, and cobalt-containing products were identified by EXAFS spectroscopy. In each uptake experiment, cobalt sorbed to kaolinite as a mixture of surface-adsorbed monomers or polymers and hydrotalcite-like precipitates of the approximate composition CoxAl(OH)2x+2(An−)1/n, where 2≤x≤4 and An− is nitrate or silicate anion. Precipitate stoichiometry varied with experimental conditions, with the highest Co:Al ratio in the high CoT/high pH experiment. Cobalt surface adsorption occurred within seconds, whereas precipitation was slower and continued for the duration of the experiments. Consequently, the proportion of precipitate in the sorbed mixture increased with time in all experiments. The most rapid precipitation occurred in the high CoT/high pH experiment, where solutions were most supersaturated with respect to cobalt hydrotalcite. Precipitates incorporated some previously adsorbed cobalt, as well as cobalt from solution. Cobalt release from the solid phase was effected by lowering solution pH to 7.0. Release experiments initiated after shorter sorption times returned a larger fraction of cobalt to solution than those initiated after longer sorption times, for a fixed duration of release. In other words, sorption product stability increased with sorption time. Specifically, under the conditions of the release experiments, the hydrotalcite-like precipitates are more stable than smaller adsorbates, and precipitates that formed over longer time periods are more stable than those that formed rapidly. The latter result suggests that precipitates ripened or modified their structure or composition to become more stable over the course of the several-thousand-hour sorption experiments. Precipitates that formed over hundreds of hours or longer did not dissolve over thousands of hours at the lower pH.

Studies on polyfunctional O-ligands. Solubility and thermal stability of 1,2,4,5-benzenetetracarboxylate complexes with alkali and alkaline earth metal ions

A solubility and thermoanalytical study of alkali and alkaline earth solid complexes of pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid, H 4L) is reported. The insoluble Na + species is Na 2LH 2. K +, Rb + and Cs + form MLH 3. The stoichiometry of precipitation of H 4L with Ca 2+, Ba 2+ and Sr 2+ depends on pH: at pH < 4, MLH 2 is formed; in the range 4 < pH < 4.5, we found a mixture of MLH 2 and M 2L; and at pH > 4.5, only M 2L is formed. The p K s0 of all the species were determined. The insoluble compounds were also characterized by thermogravimetric analysis.

The growth of nonstoichiometric apatite from aqueous solution at 37°C: I. Methology and growth at pH 7.4

The apatites found in vivo are invariably nonstoichiometric. It has been shown that such phases may also be formed by spontaneous precipitation at lower temperatures and pH. When carbonate is excluded, the stoichiometry of thse phases can be closely approximated by the formula Ca 10−u H u( PO 4) 6( OH) 2−u , in which 0 ⩽ u ⩽ 2. A modification of the constant solution composition method has been used to assess the stoichiometry of phases growth at 37°C in solutions, undersaturated with respect to octacalcium phosphate, of ionic strength 0.100 mole liter −1 (potassium nitrate), pH 7.40, and with total molar concentrations T Ca and T P of 4.0 × 10 −4 and 2.4 × 10 −4 mole liter −1, respectively. The titrants used to replenish these solutions during crystallization experiments contained calcium and phosphate concentrations calculated for precipitated phases having nonstoichiometric coefficients, u, between 0.00 and 1.60. When the titrant stoichiometry did not match that of the precipitating phase, systematic variations in the analyzed solution concentrations of calcium and phosphate were used to determine the composition of the grown phase. In verification experiments with corrected titrant concentrations, the constancy of the analyzed calcium and phosphate concentrations indicated that the solid phase stoichiometry was consistent with a value, u = 0.57. The use of different apatite seed under the same experimental conditions has confirmed this precipitate stoichiometry.

Phase transformations during aging of a nitrogen-strengthened austenitic stainless steel

An analytical electron microscopy study was undertaken in order to characterize intergranular and matrix precipitation accompanying intermediate temperature aging in NITRONIC 50, a nitrogen-strengthened austenitic stainless steel. Extensive precipitation on most grain boundaries had occurred after aging for 24 hours at 675 °C. The primary intergranular phase at that time was Cr-rich M23C6, and energy dispersive spectra taken on grain boundary segments between these carbides indicated Cr-depletion and Fe- and Ni-enhancement relative to the matrix. After aging for 336 and 1008 hours at 675 °C, M6C (eta-carbide) precipitates were also present on grain boundaries. These precipitates were distinguished from M23C6 on the basis of their lattice parameters and chemistries, with M6C containing less Cr and Fe, and more Ni, Mo, and Si than M23C6. The differences in chemistry were clarified by a statistical treatment of the spectra. The statistical analysis also showed that precipitates with a range of chemistries between M23C6 and M6C coexisted with these phases on the grain boundaries. Associated with this shift in precipitate stoichiometry was an increase in the average concentration of Cr and a decrease in the average concentration of Ni at the grain boundaries. Intergranular sigma phase was also observed after times 24 hours at 675 °C, with sigma precipitating on grain boundaries containing carbides. Intragranular precipitates observed to be stable up to 1008 hours at 675 °C included Z-phase, a complex nitride which had formed during solution annealing; M7C3 carbides, which nucleated at Z-phase/austenite interfaces; M23C6 carbides, which precipitated on incoherent twin boundaries; and Cr-rich MN precipitates, which nucleated on dislocations.

Precipitate dissolution by high energy collision cascades

The evolution of the microstructure during irradiation is now widely recognized due to: (1) radiation altered kinetic phenomena; (2) collisional processes by energetic cascades. In this paper, we investigate the specific nature of the collisional interaction between energetic cascades and precipitates. A new Monte Carlo-based computer program, TRIPOS, has been developed for the TRansport of Ions in POlyatomic Solids. The computer code utilizes standard nuclear and electronic energy loss formulas, and compares well with experimental data on particle reflection, penetration and sputtering. One of the unique features of the code is its applicability to problems involving multispecie media in multilayers of 3-dimensional configurations. The interaction of neutron-initiated high energy collision cascades is demonstrated to result in the partial dissolution of precipitates. However, the maximum precipitate size that may be completely destroyed by a high energy collision cascade is only a small fraction of the cascade size. Matrix atom implantation inside precipitates as well as preferential sputtering of light atoms from the surface of precipitates into the matrix is demonstrated to lead to changes in precipitate stoichiometry.

Stoichiometry of precipitation under conditions of double diffusion

The mechanism of natural precipitation processes has been studied using as a model system the one-dimensional double diffusion of the precipitating components in a gel. It was found, theoretically and experimentally, that precipitation under conditions of double diffusion occurs when the equivalent concentrations of both precipitating components are equal to a “critical” concentration. Under the experimental conditions used here the conventional solubility law does not apply. A general relation is deduced which enables the estimation of the relative error in the determination of the critical concentration for precipitation. Theoretical treatments of other authors are discussed.

Determination of the stoichiometry of precipitates by the method of continuous variations

Determination of the stoichiometry of precipitates by the method of continuous variations