Decrease In Supersaturation Research Articles

Microstructure–hardness relationship in quenched and partitioned medium-carbon and high-carbon steels containing silicon

We describe here the effects of quenching and partitioning (Q&P) process on the evolution of microstructure and consequent changes in hardness in a set of medium-carbon and high-carbon steels containing varying percentage of chromium, manganese, and silicon, with the aim to advance our understanding of Q&P process. The study suggests that in medium-carbon steels, higher partitioning of carbon from martensite to retained austenite and stabilization of austenite occurs when martensite has a higher supersaturation of carbon after quenching, which is obtained at low quench temperature. Another important aspect that emerges from the study is that the transition ( ɛ) carbide decreases hardness and its formation is promoted in medium-carbon steels with higher silicon-content such that the precipitation occurs at lower temperature of 250 °C. However, in contrast to medium-carbon steels, the high chromium content in high-carbon steels has a negative impact on the Q&P process because of the formation of large cementite during the spheroidizing treatment that reduces the ability of austenite to be enriched with carbon. The decrease in hardness in high-carbon steels during partitioning is a cumulative effect of austenite stabilization, softening of martensite, and decrease in carbon supersaturation of martensite.

Pet Food Safety: Sodium in Pet Foods

Healthy dogs and cats appear to be able to adjust to differing amounts of sodium in their diet via the rennin-angiotensin-aldosterone mechanisms. There is no strong evidence that increased dietary sodium increases the risk of hypertension in dogs and cats, and the current recommendation for hypertensive animals is to avoid high dietary salt intake without making a specific effort to restrict it. The prevalence of salt sensitivity and its effect on blood pressure has not been determined for cats or dogs. The ideal amount of sodium in the diet of dogs and cats with cardiac deficiency has not been determined, as increasing may detrimentally increase the extracellular fluid volume, but decreasing it may detrimentally increase the activation of the rennin-angiotensin-aldosterone system. Increased dietary sodium increases urine output and may decrease the risk of forming calcium oxalate uroliths due to the decrease in relative supersaturation of solutes. However, caution should be used in increasing the sodium intake of patients with renal disease as increased dietary sodium may have a negative effect on the kidneys independent of any effect on blood pressure.

Mitigating the atmospheric CO2 increase and ocean acidification by adding limestone powder to upwelling regions

The feasibility of enhancing the absorption of CO2 from the atmosphere by adding calcium carbonate (CaCO3) powder to the ocean and of partially reversing the acidification of the ocean and the decrease in calcite supersaturation resulting from the absorption of anthropogenic CO2 is investigated. CaCO3 could be added to the surface layer in regions where the depth of the boundary between supersaturated and unsaturated water is relatively shallow (250–500 m) and where the upwelling velocity is large (30–300 m a−1). The CaCO3 would dissolve within a few 100 m depth below the saturation horizon, and the dissolution products would enter the mixed layer within a few years to decades, facilitating further absorption of CO2 from the atmosphere. This absorption of CO2 would largely offset the increase in mixed layer pH and carbonate supersaturation resulting from the upwelling of dissolved limestone powder. However, if done on a large scale, the reduction in atmospheric CO2 due to absorption of CO2 by the ocean would reduce the amount of CO2 that needs to be absorbed by the mixed layer, thereby allowing a larger net increase in pH and in supersaturation in the regions receiving CaCO3. At the same time, the reduction in atmospheric pCO2 would cause outgassing of CO2 from ocean regions not subject to addition of CaCO3, thereby increasing the pH and supersaturation in these regions as well. Geographically optimal application of 4 billion t of CaCO3 a−1 (0.48 Gt C a−1) could induce absorption of atmospheric CO2 at a rate of 600 Mt CO2 a−1 after 50 years, 900 Mt CO2 a−1 after 100 years, and 1050 Mt CO2 a−1 after 200 years.

Use of Sevelamer Hydrochloride as an Oxalate Binder

We tested the hypothesis that the cationic phosphate binder sevelamer hydrochloride could reduce hyperoxaluria and calcium oxalate supersaturation in patients with enteric hyperoxaluria by binding fatty acids, binding phosphate and rendering calcium free to bind oxalate, and/or directly binding oxalate. A secondary objective was to assess changes in the urinary excretion of other substances associated with nephrolithiasis. Ten patients with enteric hyperoxaluria were enrolled in a nonrandomized, open label trial of sevelamer hydrochloride (3,200 mg 3 times daily for 7 days). With treatment mean urinary oxalate decreased 17% (0.84 to 0.70 mmol per day) and the urinary oxalate-to-creatinine ratio decreased 11% (0.055 to 0.049 mmol/mmol, p not significant for both). Urinary calcium increased 25% (p not significant). Urinary citrate decreased 23% (p = 0.01) and urinary phosphorus decreased 44% (p = 0.0001). Mean supersaturation of calcium oxalate, brushite, hydroxyapatite, uric acid and sodium urate did not change significantly. However, the decrease in brushite supersaturation approached statistical significance (p = 0.07). Mean serum phosphorus was 3.6 mg/dl at baseline and 3.3 mg/dl with therapy (p not significant). Hypophosphatemia did not develop in any patients. One patient dropped out of study due to abdominal pain. Sevelamer hydrochloride dramatically decreased urinary phosphorus excretion with a lesser effect on urinary oxalate. Supersaturation of calcium oxalate did not decrease due to countervailing effects on other constituents including an increase in urinary calcium and a decrease in urinary citrate. Although sevelamer hydrochloride may not be an ideal agent for correcting hyperoxaluria, its potential to reduce calcium phosphate supersaturation merits further investigation.

Influence of vapor depletion on nucleation rate

During condensation in finite systems part of molecules is transformed from supersaturated mother phase to a new one and depletion of the mother phase occurs. Kinetic equations describing homogeneous nucleation process including decrease of supersaturation are solved numerically. It is shown that dependency of nucleation rate on nucleus size reaches some maximum, which decreases with time and moves to higher nucleus sizes. Nucleation rate is negative for undercritical size of nuclei.

Preparation, crystal structure and infrared spectroscopy of the new compound rubidium beryllium sulfate dihydrate, Rb 2Be(SO 4) 2·2H 2O

The solubility in the three-component system Rb 2SO 4–BeSO 4–H 2O at 25 °C was studied by the method of isothermal decrease of supersaturation. A new compound, Rb 2Be(SO 4) 2·2H 2O, is formed in a wide concentration range (from solutions containing 27.49 mass% beryllium sulfate and 20.16 mass% rubidium sulfate up to solutions containing 15.08 mass% beryllium sulfate and 39.07 mass% rubidium sulfate). Rb 2Be(SO 4) 2·2H 2O crystallizes in the monoclinic space group P2 1/ c ( a = 11.371(2) Å, b = 11.858(2) Å, c = 7.431(1) Å, β = 96.33(1), V = 996.0 Å 3, Z = 4, R1 = 0.039 for 2672 F o > 4 σ( F o) and 153 variables). The crystal structure is characterized by three-membered chain fragments, composed of a central BeO 2(H 2O) 2 polyhedron sharing corners with two SO 4 tetrahedra. These bent [Be(SO 4) 2(H 2O) 2] 2− units are linked by rubidium ions and hydrogen bonds to double layers and further to a three-dimensional framework structure. Rb 2Be(SO 4) 2·2H 2O is isotypic to the respective potassium sulfate and selenate compounds. The strengths of the hydrogen bonds in the title compound as deduced from the infrared wavenumbers of the uncoupled OD stretches of matrix-isolated HDO molecules (isotopically dilute sample) are discussed in terms of the O w⋯O hydrogen bond distances, the different hydrogen bond acceptor capabilities of the sulfate oxygen atoms and the strong Be OH 2 interactions (synergetic effect). The intramolecular O H bond lengths are derived from the ν OD versus r OH correlation curve [H.D. Lutz, C. Jung, J. Mol. Struct. 404 (1997) 63].

Crystallization of calcium sulfate dihydrate in the presence of some metal ions

Crystallization of calcium sulfate dihydrate (CaSO 4·2H 2O gypsum) in sodium chloride solutions in the presence of some metal ions, and over a range of relative super-saturation has been studied. The addition of metal ions, even at relatively low concentration (10 −6 mol l −1), markedly retard the rate of crystallization of gypsum. Retardation effect was enhanced with increase in the additives contents. Moreover, the effect was enhanced as the relative super-saturation decreases. Influence of mixed additives on the rate of crystallization (Cd 2++Arg, Cd 2++H 3PO 4 and Cd 2++PAA) has also been studied. Direct adsorption experiments of these metal ions on the surface of gypsum crystals have been made for comparison.

Nucleation, growth and ageing scenarios in closed systems II: Dynamics of a new phase formation

We simulate the dynamics of nucleation, growth and resorption of particles in an initially supersaturated closed medium, according to the approach worked out in the preceding paper, which applies to precipitation in solution, condensation in the vapor phase or crystallization from a melt. We find that the formation of the new phase obeys a generic scenario, involving mainly three stages, whatever the values of the parameters of the model: a phase of pure nucleation during which the supersaturation decreases, a plateau characterized by a quasi-equilibrium between nucleation and (positive) growth on the one hand, and resorption on the other hand, and an ageing stage ending with a single particle in thermodynamic equilibrium with its surrounding medium. During the plateau regime, very complex dynamical effects take place, which may be disentangled thanks to the full knowledge of the particle population obtained by our approach. We emphasize that the particles present in the system in the long time regime are not those formed at t = 0 , but rather during some (usually) short time interval(s) on the plateau. This point should be kept in mind when using crystal size distribution (CSD) to date events which have occurred in the past.

Potassium beryllium selenate dihydrate, K 2Be(SeO 4) 2·2H 2O: Preparation, structure and infrared spectroscopy

The solubility in the three-component system K 2SeO 4–BeSeO 4–H 2O at 25 °C is studied by the method of isothermal decrease of supersaturation. A new compound, K 2Be(SeO 4) 2·2H 2O, is formed in a wide concentration range (from solutions containing 39.40 mass% beryllium selenate and 8.50 mass% potassium selenate up to solutions containing 7.02 mass% beryllium selenate and 51.14 mass% potassium selenate). K 2Be(SeO 4) 2·2H 2O crystallizes in the monoclinic space group P2 1/ c ( a=11.503(2), b=11.846(1), c=7.491(1) Å, β=95.77(1)°, V=1015.5 Å 3, Z=4, R1=0.026 for 4220 F o>4 σ( F o) and 153 variables). The crystal structure is composed of [Be(SeO 4) 2(H 2O) 2] 2− groups, formed by a BeO 4 tetrahedron sharing two corners with SeO 4 tetrahedra. These units are linked via potassium cations and comparatively strong hydrogen bonds to double layers and further to a three-dimensional framework structure. K 2Be(SeO 4) 2·2H 2O is isotypic with the corresponding sulfate compound, K 2Be(SO 4) 2·2H 2O. The strengths of the hydrogen bonds in the title compound as deduced from the infrared wavenumbers of the uncoupled OD stretches of matrix-isolated HDO molecules (isotopically dilute sample) are discussed in terms of the O w⋯O hydrogen bond distances, the different hydrogen bond acceptor capabilities of the selenate oxygen atoms and the strong Be–OH 2 interactions (synergetic effect). The intramolecular O–H bond lengths are derived from the ν OD vs. r OH correlation curve [J. Mol. Struct. 404 (1997) 63].

The effect of magma flow on nucleation of gas bubbles in a volcanic conduit

We solve the dynamics of magma ascent and the kinetics of bubble nucleation and growth simultaneously, which allow us to predict bubble sizes and number densities under ascent conditions. As magma rises toward the surface, the pressure decreases and eventually becomes less than the solubility pressure. When the degree of supersaturation becomes great enough, bubbles nucleate. Nucleation will stop as the concentration of volatiles in the melt decreases due to growth of existing bubbles and hence the degree of supersaturation decreases. We show that a second nucleation event may occur just below the fragmentation level. Near that level, the degree of supersaturation continuously increases as the magma is rapidly decompressed. As a result, nucleation will not stop until fragmentation occurs. This second nucleation event should be taken into account when interpreting bubble size distribution measurements made on natural pumices. The bubbles of the second nucleation event have high internal gas pressures up to 2 MPa greater than the liquid pressure, suggesting that the second nucleation event may enhance fragmentation of magma. We apply the model to the calculation protocol defined at the “Volcanic eruption mechanism modeling workshop, Durham, 2002”. We found that as a result of disequilibrium degassing fragmentation occurs higher in the conduit than under equilibrium degassing.

Homogeneous nucleation of droplets from supersaturated vapor in a closed system.

Kinetic equations describing homogeneous nucleation kinetics within standard model are solved numerically under the condition of a constant number of molecules in the considered system. It has consequences to decrease the supersaturation of the supersaturated vapor during the process of the formation of small droplets of a new phase. The decrease of supersaturation occurs in a short time and reaches some value which remains unchanged for a relatively long time (quasistationary regime), especially at lower initial supersaturations. This time interval decreases with increasing value of the initial supersaturation. In the quasistationary regime the nucleation rate reaches its stationary value. At higher initial supersaturation, the rate of formation of nuclei goes to some maximum value corresponding to the stationary nucleation rate and then decreases with time due to the decrease of supersaturation.

Theory of the von Weimarn rules governing the average size of crystals precipitated from a supersaturated solution

More than 75 years ago, von Weimarn summarized his observations of the dependence of the average crystal size on the initial relative concentration supersaturation prevailing in a solution from which crystals were growing. Since then, his empirically derived “rules” have become part of the lore of crystal growth. The first of these rules asserts that the average crystal size measured at the end of a crystallization increases as the initial value of the relative supersaturation decreases. The second rule states that for a given crystallization time, the average crystal size passes through a maximum as a function of the initial relative supersaturation. Using a theory of nucleation and growth due to Buyevich and Mansurov (J. Crystal Growth 104 (1990) 861), we calculate the average crystal size as a function of the initial relative supersaturation. We confirm the von Weimarn rules for the case where the nucleation rate is proportional to the third power or higher of the relative supersaturation.

Supersaturation control in aragonite synthesis using sparingly soluble calcium sulfate as reactants

Sparingly soluble calcium salts were studied as reactants in the synthesis of needle-like precipitated calcium carbonate (PCC). The morphology and aspect ratio of the PCC particles were characterized with SEM. Polymorphs and crystal size were characterized using X-ray diffraction. The counterions of the sparingly soluble salts influenced the growth kinetics of PCC as well as the polymorphism and morphology of product particles. Either chrysanthemum-like or needle-like aragonite can be synthesized from calcium sulfate and sodium carbonate depending on the supersaturation and synthesis conditions. Low concentration and slow addition rate of sodium carbonate solution were favorable to the formation of aragonite. Addition of sodium sulfate to the reaction system (calcium chloride and sodium carbonate) promoted the formation of aragonite and decreased the crystal size of aragonite due to the decrease of supersaturation and adsorption of sulfate ion. Too much added sodium sulfate, however, did not further increase the aragonite fraction. An optimal temperature for the formation of aragonite was found to be ca. 60 °C. Slow dissolution kinetics of sparingly dissoluble salt also is very important for controlling PCC polymorphism and morphology.

Ostwald ripening with size-dependent rates: Similarity and power-law solutions

The dynamics of Ostwald ripening is treated by cluster distribution kinetics represented by a population balance equation that also describes growth or dissolution. Unlike simple crystal growth driven by supersaturation, the smaller, more soluble clusters in the distribution dissolve during ripening near equilibrium and vanish when they reach the critical nucleus size. Larger clusters accordingly grow as the supersaturation decreases. The long-time asymptotic result of the numerical solution of the scaled population balance equation is power-law decrease of cluster number and growth of average cluster mass, Cavg(θ). The cluster distribution approximates an exponential self-similar solution, and eventually narrows until but one large cluster remains, satisfying the mass balance. A previous theory is here extended to include mass-dependent rate coefficients for growth and dissolution that satisfy microscopic reversibility. The asymptotic power-law growth, Cavg∼θ1/(4/3-λ), is determined by the power λ on the mass for rate coefficients. The power is λ=1/3 for diffusion-controlled and λ=2/3 for surface-controlled processes. Experimentally observed ripening behavior is realized by an apt choice of λ for a given time range.

The effect of different diets on urine composition and the risk of calcium oxalate crystallisation in healthy subjects.

The aim of the study was to determine the impact of defined diet modifications on urine composition and the risk of calcium oxalate crystallisation. Ten healthy male volunteers consumed a self-selected diet (SD) for 14 days, and three different standard diets for a period of 5 days each. Whereas the western-type diet (WD) is representative of the usual dietary habits, the normal mixed diet (ND) and the ovo-lacto-vegetarian diet (VD) were calculated according to the requirements. The risk of calcium oxalate crystallisation, calculated as relative supersaturation (EQUIL2) from urine composition, was highest during ingestion of diets SD and WD. The intake of diet ND resulted in a significant decrease in relative supersaturation with calcium oxalate by 58% (p<0.05) compared with diet WD, due to a significant decline in urinary calcium and uric acid excretion and a significant increase in urinary volume, pH-value and citrate excretion. In spite of an increase in urinary pH, citrate and magnesium excretion and a decline in calcium excretion, no further significant decrease in the risk of calcium oxalate crystallisation was observed on diet VD, due to a significant increase in urinary oxalate by 30% (p<0.05) on average. The change of usual dietary habits for a normal mixed diet significantly reduces the risk of calcium oxalate crystallisation. With a vegetarian diet a similar decline in urinary supersaturation with calcium oxalate can be achieved compared to a normal mixed diet. Since urinary oxalate excretion increased significantly, a vegetarian diet without adequate intake of calcium may not be recommended to patients with mild hyperoxaluria.

PROPORTIONAL REDUCTION OF URINE SUPERSATURATION DURING NEPHROLITHIASIS TREATMENT

During metabolic stone therapy, urine supersaturation decreases in proportion to pretreatment levels. We gauge the quantitative contribution of regression to the mean for reducing urine supersaturation from high pretreatment to lower values during therapy. The 24-hour urine supersaturations for calcium oxalate, calcium phosphate and uric acid were measured on 2 pretreatment and at least 1 treatment 24-hour collection for each of the 2,667 patients in 2 networks and at a university based specialty clinic. Changes in supersaturation between the first and second pretreatment collections were an estimate of random change and compared to therapeutic changes. Supersaturations decreased between the first and second pretreatment collections, proportional to the supersaturation in the first collection. However, the magnitude of this effect was minor compared to therapeutic changes. Also, mean change between pretreatment collections was 0, whereas mean change with therapy was greater than 0 for all 3 supersaturations. Although regression to the mean can be detected, it cannot be responsible for the decrease in urine supersaturation with therapy or the fact that the decrease is proportional to pretreatment mean supersaturation. The mechanisms responsible for proportional reduction remain to be clarified.

SUPERSATURATION, INDUCTION PERIOD, AND METASTABLE ZONE WIDTH OF CaCl2—Na2C03—HCl SYSTEM

The supersaturation, induction period, and mctastablc zone width ofan aqueous supersaturated solution of CaC2—Na2C03—HCl system are studied in the present paper. The supersaturation is varied and predicted within a reasonable accuracy when a specified amount of HCl(aq) is added into the aqueous supersaturated CaC2—Na2C03—HCl solution. It is found that the decrease of solution supersaturation at low pH is caused mainly by the decrease of carbonate ions. Then, the induction periods are estimated by using the coagulation theory and they can be expressed as a function of supersaturation alone, which results from different values of reagent concentration and pH. Finally, the theoretical metastable zone widths based on arbitrary induction periods are obtained. Meanwhile, the induction periods are measured at various levels of reagent concentration and solution pH by applying the conductivity method. Accordingly, the metastable zone widths are determined experimentally and compared with the theoretical results. It is concluded that the present method is an useful tool for determining the metastable zone width of aqueous supersaturated CaC2—Na2C03—HCl solution.

Effect of carrier-gas pressure on barrier to nucleation: Monte Carlo simulation of water/nitrogen system

Carrier gases are used in most nucleation experiments for releasing the latent heat generated during vapor condensation. In the analysis of experimental data it is often assumed that the carrier gas is inert and would not participate in the nucleation process of the target gas. Several recent nucleation experiments show that the influence of carrier gases to nucleation rate is not negligible under certain conditions. To gain more insight into the carrier-gas effect, we carry out Monte Carlo simulation to compute the free energy of formation of water clusters in the presence of a nitrogen carrier gas. At fixed temperature (240 K) and chemical potential, it is found that the barrier height to nucleation increases with the carrier-gas pressure. This barrier enhancement is attributed to the increase of equilibrium vapor pressure of water in the presence of carrier gas, which results in a decrease of supersaturation. It is also found that the simulation results are consistent with the binary-nucleation theorem.

Growth of needle and plate shaped particles: theory for small supersaturations, maximum velocity hypothesis

A solution to the diffusion controlled growth of needle and plate shaped particles is presented as their shape approaches respectively a paraboloid of revolution or a parabolic cylinder, under small supersaturation values, when capillarity and interface kinetic effects are present. The solutions show that as supersaturation decreases, the growth rate and needle tip radius approach a common value regardless of interfacial kinetics effects as capillarity is the main factor that retards particle growth. Simple asymptotic expressions are thus obtained to predict the growth rate and tip radius at low supersaturations, assuming a maximum velocity hypothesis. These represent the circumstances during solid state precipitation reactions which lead to secondary hardening in steels.

The role of Mg2+ as an impurity in calcite growth.

Magnesium is a key determinant in CaCO3 mineralization; however, macroscopic observations have failed to provide a clear physical understanding of how magnesium modifies carbonate growth. Atomic force microscopy was used to resolve the mechanism of calcite inhibition by magnesium through molecular-scale determination of the thermodynamic and kinetic controls of magnesium on calcite formation. Comparison of directly measured step velocities to standard impurity models demonstrated that enhanced mineral solubility through magnesium incorporation inhibited calcite growth. Terrace width measurements on calcite growth spirals were consistent with a decrease in effective supersaturation due to magnesium incorporation. Ca(1-x)Mg(x)CO3 solubilities determined from microscopic observations of step dynamics can thus be linked to macroscopic measurements.