Supersaturation Of Solution Research Articles

Effect of heat treatment on the microstructure and anisotropy in mechanical properties of A357 alloy produced by selective laser melting

A357 alloy was manufactured by selective laser melting (SLM) and subjected to different heat treatments, by considering the stress relief step as critical, to establish the microstructure-mechanical property relationship and model the yield strength. For the as-SLM processed condition, the imposed fast cooling rate refines and limits the amount of Fe-rich intermetallic and eutectic Mg2Si phases, and enables a higher level of supersaturation of solutes. The latter leads to significant yield strength contributions from solid solution strengthening and natural ageing. The inter-cellular Si network within the columnar grains remains unchanged but small Si-rich particles appear within cells after direct ageing. The Si network breaks up during stress relieving, leading to high ductility. The lower ductility for the vertically-built samples under as-SLM processed, directly aged, and stress relieved conditions is due mainly to the finer Si networks brittle nature. Porosity resulting from trapped hydrogen gas enlarges significantly with solutionisation, deteriorating the ductility. Grain growth is not obviously observed with heat treatment. Si particles coarsen preferentially at grain boundaries initially, however, they distribute more evenly by coarsening within the grains as well during prolonged solutionisation. Anisotropy in both yield strength and ductility subsequently disappears when a more homogeneous microstructure is obtained.

Elucidating the Effect of Water-To-Cement Ratio on the Hydration Mechanisms of Cement.

The hydration of cement is often modeled as a phase boundary nucleation and growth (pBNG) process. Classical pBNG models, based on the use of isotropic and constant growth rate of the main hydrate, that is, calcium–silicate–hydrate (C–S–H), are unable to explain the lack of any significant effect of the water-to-cement (w/c) ratio on the hydration kinetics of cement. This paper presents a modified form of the pBNG model, in which the anisotropic growth of C–S–H is allowed to vary in relation to the nonlinear evolution of its supersaturation in solution. Results show that once the supercritical C–S–H nuclei form, their growth remains confined within a region in proximity to the cement particles. This is hypothesized to be a manifestation of the sedimentation of cement particles, which imposes a space constraint for C–S–H growth. In pastes wherein the sedimentation of cement particles is disrupted, the hydration kinetics are no longer unresponsive to changes in w/c. Unlike C–S–H, the ions in solution are not confined, and hence, the supersaturation-dependent growth rate of C–S–H diminishes monotonically with increasing w/c. Overall, the outcomes of this work highlight important aspects that need to be considered in employing pBNG models for simulating hydration of cement-based systems.

Improved volume stability of mortar bars exposed to magnesium sulfate in the presence of bicarbonate ions

The effect of the simultaneous occurrence of sulfate and bicarbonate ions in magnesium containing solutions was investigated for a CEM I and CEM III/B mortar under laboratory conditions. The presence of bicarbonate ions, which is common in natural waters, prevents the typical expansion and surface erosion of CEM I mortars and the surface deterioration of CEM III/B mortars. The chemical effect of the bicarbonate ions is twofold. The precipitation of CaCO3 lowers the availability of CaO for the formation of additional sulfate bearing phases like ettringite and gypsum. The second effect is the reduction of supersaturation of the pore solution with respect to ettringite. The absence of deterioration is linked to the formation of only small amounts of ettringite and to the absence of gypsum in the affected zones. Thermodynamic modeling suggests that relatively small bicarbonate concentrations suffice to prevent damage, contribute to the scarcity of damage under natural exposure conditions and to the diversity of results under laboratory exposure conditions.

Potassium Citrate is Better in Reducing Salt and Increasing Urine pH than Oral Intake of Lemonade: A Cross-Over Study.

BackgroundUrine solute supersaturation leads to the formation of urinary tract caliceal stones. Many parameters can be involved in the supersaturation of solutes in urine, such as pH. Uric acid has pKa ≤5.5, and it is solubilized at pH ≥5.5. The objective of the study was to evaluate the effects of potassium citrate and lemonade supplementation in pediatric patients with urolithiasis.Material/MethodsA total of 126 children who had lower ureteral stones calculi and fragments with severe colic pain participated in this cross-over study. Children drank lemonade (2 mEq/kg/day citrate) in 3 divided doses for 5 days. After a 15-day washout period, children drank 2 mEq/kg/day of potassium citrate in 3 divided doses for 5 days. On the sixth of the day of individual intervention, a 24-h urine sample was collected and evaluated for pH, urine volume, citrate level, uric acid level, magnesium, phosphorus, potassium, and sodium. Urinary parameters for 1-day urine collection measurements after each supplementation were compared with baseline using the Mann-Whitney test following Tukey post hoc test at 95% confidence level.ResultsPotassium citrate supplementation resulted in reduction of sodium concentration (p=0.0337; q=3.76) and increased pH of urine (p=0.0118; q=4.389). However, urine volume, citrate level, and uric acid level, as well as elemental magnesium, phosphorus, and potassium, remained unchanged after 5 days of supplementation with potassium citrate or lemonade.ConclusionsPotassium citrate supplementation is an effective therapy for preventing pediatric urolithiasis, with acceptable adverse effects.

A Cannibalistic Approach to Grand Canonical Crystal Growth.

Canonical molecular dynamics simulations of crystal growth from solution suffer from severe finite-size effects. As the crystal grows, the solute molecules are drawn from the solution to the crystal, leading to a continuous drop in the solution concentration. This is in contrast to experiments in which the crystal grows at an approximately constant supersaturation of a bulk solution. Recently, Perego et al. [ J. Chem. Phys. 2015, 142, 144113] showed that in a periodic setup in which the crystal is represented as a slab, the concentration in the vicinity of the two surfaces can be kept constant while the molecules are drawn from a part of the solution that acts as a molecular reservoir. This method is quite effective in studying crystallization under controlled supersaturation conditions. However, once the reservoir is depleted, the constant supersaturation conditions cannot be maintained. We propose a variant of this method to tackle this depletion problem by simultaneously dissolving one side of the crystal while letting the other side grow. A continuous supply of particles to the solution due to the crystal dissolution maintains a steady solution concentration and avoids reservoir depletion. In this way, a constant supersaturation condition can be maintained for as long as necessary. We have applied this method to study the growth and dissolution of urea crystal from water solution under constant supersaturation and undersaturation conditions, respectively. The computed growth and dissolution rates are in good agreement with those obtained in previous studies.

Biopolymer Control on Calcite Precipitation

The influence of biopolymers, alginate (Alg) and polyaspartate (pAsp), on the kinetics of calcite precipitation was studied using the constant composition method for supersaturation states ranging from 2.4 to 4.5, at pH = 8.5. Biopolymer presence changes the mechanism of calcite precipitation and its inhibition depending on the system history. In a system without polymers, calcite precipitates by spiral growth. Introducing polymers during growth inhibits step movement by pinning at a number of kink sites along step edges. Polymer adsorption induces growth by two-dimensional nucleation. Both polymers inhibit calcite growth. Inhibition is stronger at higher concentration and lower solution supersaturation. The interfacial free energy, a key parameter in the control of nucleation and growth, was estimated from the analysis of the precipitation rates, as well as data obtained from vapor adsorption, which are quite identical for calcite with either alginate or polyaspartate adsorbed. This is confirmed by electrokinetic measurements, which show similar ζ- potential values for calcite with each of the polymers. Increasing polymer concentration and adsorption time led to a progressive decrease of the effective interface free energy, which could explain the much lower supersaturation needed for the onset of surface nucleation.

Nominal vs. actual supersaturation of solutions

Following the formalism of the Classical Nucleation Theory beyond the dilute solution approximation, this paper considers a difference between the actual solute supersaturation (given by the present-to-saturated solute activity ratio) and the nominal supersaturation (given by the present-to-saturated solute concentration ratio) due to formation of subcritical transient solute clusters, called heterophase fluctuations. Based on their distribution function, we introduce an algebraic equation of supersaturation that couples the nominal supersaturation of a binary metastable solution with its actual supersaturation and a function of the specific interface energy and temperature. The applicability of this approach is validated by comparison to simulation data [(Clouet et al., Phys. Rev. B 69, 064109 (2004)] on nucleation of Al3Zr and Al3Sc in model binary Al alloys.

Nanotransducers on printed circuit boards by rational design of high-density, long, thin and untapered ZnO nanowires

Nanotransducers can offer crucial advantages in comparison with conventional sensors and actuators. However, interfacing and packaging nanostructures into complete electronic systems is very complex. Here we describe a wet chemical method for cointegrating arrays of ZnO nanowires into systems on printed circuit boards (PCBs). First, we deposit on the PCB a MnOOH layer for reproducibly increasing the nanowires density. Afterwards, we numerically demonstrate that the ligand ethylenediamine, at the isoelectric point of the ZnO nanowires tips, can effectively control, at very low concentrations, both zinc speciation and supersaturation in the nutrient solution. Accordingly, we combine potassium chloride and ethylenediamine, produced in situ from a safer precursor (ethanolamine), for concurrently thinning the nanowires (top width around 60nm) and stabilizing their top faces. Our synergic approach enables the solution growth of ZnO nanowires which are untapered and have high densities (> 8/μm2) and record length (> 15µm) and aspect ratio (> 200) for plastic substrates which may not withstand high temperatures. These characteristics permit to simultaneously add the top electrode, package the nanowires, improve their mechanical robustness and connect to the electronic interface by a simple flip chip adhesive bonding procedure. As proofs of concept we describe flexible PCB heaters (electrical to thermal energy conversion) with power densities comparable with state of the art flexible heaters and a wearable piezotronic heartbeat detector comprising both ZnO nanowires (mechanical to electric energy conversion) and electronics on a single flexible PCB. Our results open the way to the cointegration of nanotransducers and electronics on conventional PCBs.

Analysis of age hardening precipitates of Al-Zn-Mg-Cu alloys in a wide range of quenching rates using small angle X-ray scattering

The volume fraction of age hardening precipitates upon peak ageing after different quenching rates has been studied for five Al-Zn-Mg-Cu alloys using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The rate of change in volume fraction of the η′ age hardening precipitates as a function of cooling rate was used to evaluate changes in the degree of solute supersaturation, and hence the quench sensitivity of the Al-Zn-Mg-Cu alloys. The results demonstrate that the amount of η′ age hardening precipitates can be changed by varying the thermomechanical processing history or the alloy compositions. It was found that the volume fraction of age hardening η′ precipitates in Al-Zn-Mg-Cu alloys increases with increasing cooling rates and increasing Mg content as well as that the size of η′ precipitates is related to the Cu/Mg ratio.

Mechanisms of Modulation of Calcium Phosphate Pathological Mineralization by Mobile and Immobile Small-Molecule Inhibitors.

Potential pathways for inhibiting crystal growth are via either disrupting local microenvironments surrounding crystal-solution interfaces or physically blocking solute molecule attachment. However, the actual mode of inhibition may be more complicated due to the characteristic time scale for the inhibitor adsorption and relaxation to a well-bound state at crystal surfaces. Here we demonstrate the role of citrate (CA) and hydroxycitrate (HCA) in brushite (DCPD, CaHPO4·2H2O) crystallization over a broad range of both inhibitor concentrations and supersaturations by in situ atomic force microscopy (AFM). We observed that both inhibitors exhibit two distinct actions: control of surface crystallization by the decrease of step density at high supersaturations and the decrease of the [1̅00]Cc step velocity at high inhibitor concentration and low supersaturation. The switching of the two distinct modes depends on the terrace lifetime, and the slow kinetics along the [1̅00]Cc step direction provides specific sites for the newly formed dislocations. Molecular modeling shows the strong HCA-crystal interaction by molecular recognition, explaining the AFM observations for the formation of new steps and surface dissolution along the [101]Cc direction due to the introduction of strong localized strain in the crystal lattice. These direct observations highlight the importance of the inhibitor coverage on mineral surfaces, as well as the solution supersaturation in predicting the inhibition efficacy, and reveal an improved understanding of inhibition of calcium phosphate biomineralization, with clinical implications for the full therapeutic potential of small-molecule inhibitors for kidney stone disease.

Stress–Corrosion Cracking of Surface-Engineered Alloys in a Simulated Boiling-Water Reactor Environment

The potential of surface engineering by concentrated interstitial solute (SECIS) for improved resistance to stress–corrosion cracking (SCC) in a simulated boiling-water reactor (BWR) environment was studied for an austenitic stainless steel (AISI-316L) and a Ni-base superalloy (IN-718) via slow-strain-rate tests. For these tests, tensile rods of both alloys were carburized at a low temperature, which generates a concentrated solution (supersaturation: 105) of interstitially dissolved carbon within a ≈25 μm deep zone below the surface. In BWR-NWC (normal water composition), SECIS AISI-316L exhibits high susceptibility to SCC with a reduction in elongation and a fracture surface consistent with transgranular SCC (TGSCC). We found that the crack-tip strain rate plays an important role for crack initiation and propagation. To understand the role of concentrated interstitial carbon on the failure mode, we also studied specimens treated by ultrasonic nanocrystal surface modification (UNSM), a carbon-free method for surface hardening. We conclude that the carbon-rich zone enables nucleation of sharp cracks, which normally do not form in AISI-316L. Crack propagation in the underlying carbon-free alloy core and, ultimately, TGSCC failure, depends on the environment (solution and potential), as demonstrated by tests in BWR-HWC (hydrogen water composition). The stress–strain curve of SECIS IN-718, in contrast, is similar to that of non-treated material. In this case, the fracture surface was consistent with ductile failure.

A Novel Dual-Step Nucleation Pathway in Crystalline Solids under Neutron Irradiation

Innovations in nanostructuring of inorganic crystalline solids are often limited by prerequisite critical nucleation energy and solute supersaturation for formation of a phase. This research provides direct evidence supporting the viability of an unconventional irradiation-induced nanostructuring process, via transmission electron microscopy, that circumvents these preconditions. Using polymorphic silicon carbide (SiC) as a prototype, a surprising two-step nucleation route is demonstrated through which nanoscale distribution of the second phase is achieved by reaction of solutes with neutron irradiation-induced precursors. In the first step, nanoscale α–SiC precipitates in a β–SiC matrix unexpectedly nucleate heterogeneously at structural defects. This occurs at significantly lower temperatures compared with the usual β→α transition temperature. Subsequently, α–SiC precipitate acts as a surrogate template for its structural and compositional transition into a fission product precipitate, palladium silicide. These discoveries provide a modern view of irradiation engineering in polymorphic ceramics for advanced applications.

Interface engineered surface morphology evolution of Au@Pd core-shell nanorods.

Engineering the interfacial structure of bimetallic nanocrystals is an effective method to improve their electrocatalytic performances. Here, we design a facile strategy for controlling the surface morphology evolution of Au@Pd core-shell nanorods by adjusting the solution supersaturation. The Pd shell of the as-prepared Au@Pd bimetallic nanorods can be modulated from a (111) facet-exposed island to a (100) facet-exposed conformal shell. The conformal shell structure exhibited enhanced catalytic performance toward the ethanol oxidation reaction, while the core-island structure possessed better catalytic stability. This work provides a facile method for interfacial engineering of bimetallic nanocrystals with desired morphology and properties.

Formation of calcareous nodules in loess–paleosol sequences: Reviews of existing models with a proposed new “per evapotranspiration model”

Loess is a product of aeolian deposition during Quaternary glaciation cycles. Loess–paleosol sequences are rich in calcareous nodules (CNs). In the literature, two models are widely cited for the formation of CNs, namely “per descendum” and “per ascendum”. However, there has been no direct testing or monitoring to support either of these contradictory models. This paper reviews a large number of multidisciplinary literature to evaluate the consistency, reliability and rationality of these two models. Three main conclusions are drawn: (1) the causative factors (variation of pH value along loess–paleosol sequence, decrease of CO2 partial pressure, and reduction of solvent water) that are used to support the per descendum model do not completely explain the supersaturation of infiltration solution with CaCO3, thereby making this model questionable; (2) the per ascendum model explains the formation of CNs along narrow horizons through upward evaporation; however, it fails to produce sporadic distributions and irregular shapes of nodules on loess slope faces and the frequent appearance of nodules around plant roots. In order to reconcile these deficiencies, we conducted an extensive field survey in various parts of Shanxi province. Based on this new set of observations, it was concluded that the “per ascendum” model can be extended to explain all occurrences of CNs. This extended model is called “per evapotranspiration”.

Low biodegradability of dissolved organic matter and trace metals from subarctic waters

The heterotrophic mineralization of dissolved organic matter (DOM) controls the CO2 flux from the inland waters to the atmosphere, especially in the boreal waters, although the mechanisms of this process and the fate of trace metals associated with DOM remain poorly understood. We studied the interaction of culturable aquatic (Pseudomonas saponiphila) and soil (Pseudomonas aureofaciens) Gammaproteobacteria with seven different organic substrates collected in subarctic settings. These included peat leachate, pine crown throughfall, fen, humic lake, stream, river, and oligotrophic lake with variable dissolved organic carbon (DOC) concentrations (from 4 to 60mgL−1). The highest removal of DOC over 4days of reaction was observed in the presence of P. aureofaciens (33±5%, 43±3% and 53±7% of the initial amount in fen water, humic lake and stream, respectively). P. saponiphila degraded only 5% of DOC in fen water but did not affect all other substrates. Trace elements (TE) were essentially controlled by short-term (0−1h) adsorption on the surface of cells. Regardless of the nature of organic substrate and the identity of bacteria, the degree of adsorption ranged from 20 to 60% for iron (Fe3+), 15 to 55% for aluminum (Al), 10 to 60% for manganese (Mn), 10 to 70% for nickel (Ni), 20 to 70% for copper (Cu), 10 to 60% for yttrium (Y), 30 to 80% for rare earth elements (REE), and 15 to 50% for uranium (UVI). Rapid adsorption of organic and organo-mineral colloids on bacterial cell surfaces is novel and potentially important process, which deserves special investigation. The long-term removal of dissolved Fe and Al was generally consistent with solution supersaturation degree with respect to Fe and Al hydroxides, calculated by visual Minteq model. Overall, the biomass-normalized biodegradability of various allochthonous substrates by culturable bacteria is much lower than that of boreal DOM by natural microbial consortia.

Microstructure Formation and Resistivity Change in CuCr during Rapid Solidification

The formation of the surface-near microstructure after a current interruption of CuCr contact materials in a vacuum interrupter is characterized by a fast heating and subsequently rapid solidification process. In the present article, we reveal and analyse the formation of two distinct microstructural regions that result from the heat, which is generated and dissipated during interruption. In the topmost region, local and global texture, as well as the resulting microstructure, indicate that both Cu and Cr were melted during rapid heating and solidification whereas in the region underneath, only Cu was melted and elongated Cu-grains solidified with the <001>-direction perpendicularly aligned to the surface. By analysing the lattice parameter of the Cu solid solution, a supersaturation of the solid solution with about 2.25 at % Cr was found independent if Cu was melted solely or together with the Cr. The according reduction of electrical conductivity in the topmost region subsequent to current interruption and the resulting heat distribution are discussed based on these experimental results.

Precipitation analysis of as-cast HK40 steel after isothermal aging

As-cast HK40 steel was aged at 700, 800, or 900°C for times as long as 2000 h. Microstructural characterization showed that the primary M7C3 carbide network contained a substantial content of manganese, in agreement with the microsegregation of manganese calculated by Thermo-Calc using the Scheil–Gulliver module. The dissolution of primary carbides caused the solute supersaturation of austenite and subsequent precipitation of fine M23C6 carbides in the austenite matrix for aged specimens. During prolonged aging, the carbide size increased with increasing time because of the coarsening process. A time–temperature–precipitation diagram for M23C6 carbides was calculated using the Thermo-Calc PRISMA software; this diagram showed good agreement with the experimental growth kinetics of precipitation. The fine carbide precipitation caused an increase in hardness; however, the coarsening process of carbides promoted a decrease in hardness. Nanoindentation tests of the austenite matrix indicated an increase in ductility with increasing aging time.

Dynamics of ensemble of gas bubbles with account of the Laplace pressure on the nucleation stage at degassing in a gas-liqiud mixture

A new comprehensive analysis of growth dynamics of an ensemble of gas bubbles on the nucleation stage at degassing in a gas-supersaturated liquid solution is presented. The Laplace pressure in supercritical bubbles is strictly taken into account in the analysis based on the mean-field approach to nucleation kinetics. The whole distribution of supercritical bubbles in sizes, gas supersaturation in the liquid solution at any time at the nucleation stage as well as the total duration of the nucleation stage have been described analytically. The scaled universal forms independent of specific parameters of the gas and solvent components have been found for the maximal bubble radius and the reduction of mean gas supersaturation as functions of time. It has been shown that the Laplace pressure in the supercritical bubbles considerably affects the bubble distribution at the beginning of nucleation stage but becomes less important to the end of the nucleation stage.

Iron and silicon isotope behaviour accompanying weathering in Icelandic soils, and the implications for iron export from peatlands

Incipient warming of peatlands at high latitudes is expected to modify soil drainage and hence the redox conditions, which has implications for Fe export from soils. This study uses Fe isotopes to assess the processes controlling Fe export in a range of Icelandic soils including peat soils derived from the same parent basalt, where Fe isotope variations principally reflect differences in weathering and drainage. In poorly weathered, well-drained soils (non-peat soils), the limited Fe isotope fractionation in soil solutions relative to the bulk soil (Δ57Fesolution-soil=−0.11±0.12‰) is attributed to proton-promoted mineral dissolution. In the more weathered poorly drained soils (peat soils), the soil solutions are usually lighter than the bulk soil (Δ57Fesolution-soil=−0.41±0.32‰), which indicates that Fe has been mobilised by reductive mineral dissolution and/or ligand-controlled dissolution. The results highlight the presence of Fe-organic complexes in solution in anoxic conditions. An additional constraint on soil weathering is provided by Si isotopes. The Si isotope composition of the soil solutions relative to the soil (Δ30Sisolution-soil=0.92±0.26‰) generally reflects the incorporation of light Si isotopes in secondary aluminosilicates. Under anoxic conditions in peat soils, the largest Si isotope fractionation in soil solutions relative to the bulk soil is observed (Δ30Sisolution-soil=1.63±0.40‰) and attributed to the cumulative contribution of secondary clay minerals and amorphous silica precipitation. Si supersaturation in solution with respect to amorphous silica is reached upon freezing when Al availability to form aluminosilicates is limited by the affinity of Al for metal–organic complexes. Therefore, the precipitation of amorphous silica in peat soils indirectly supports the formation of metal–organic complexes in poorly drained soils. These observations highlight that in a scenario of decreasing soil drainage with warming high latitude peatlands, Fe export from soils as Fe-organic complexes will increase, which in turn has implications for Fe transport in rivers, and ultimately the delivery of Fe to the oceans.

Nonisothermal nucleation in a solid solution of CuCl in glass: Concentration effect upon heating of the solid solution

The influence of the heating rate of a solid solution of CuCl in glass on the size distribution of the produced CuCl nanoparticles is studied. The distribution curves of CuCl nanocrystals are determined by the method of exciton-thermal analysis. It is established that the concentration of CuCl nanoparticles increases by ten times upon slowing the sample heating process from 2 to 60 min, while the mean radius of particles decreases almost twice. The concentration of CuCl nanoparticles passes through a maximum in the process of heating the sample. The numerical simulation of the nucleation upon slow heating of a solid solution showed that the formation of the concentration maximum of the new phase clusters is determined by a rapid increase in the critical radius owing to an increase in temperature and decrease in the solution supersaturation. As a result, the formation of new phase nuclei ceases at a certain temperature, and a part of the previously formed clusters dissolves.