Degree Of Solution Supersaturation Research Articles

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.

In Situ Observation on the Dynamic Process of Evaporation and Crystallization of Sodium Nitrate Droplets on a ZnSe Substrate by FTIR-ATR

Sodium nitrate is a main component of aging sea salt aerosol, and its phase behavior has been studied repeatedly with wide ranges observed in the efflorescence relative humidity (RH) in particular. Studies of the efflorescence dynamics of NaNO3 droplets deposited on a ZnSe substrate are reported, using an in situ Fourier transform infrared attenuated total reflection (FTIR-ATR) technique. The time-dependence of the infrared spectra of NaNO3 aerosols accompanying step changes in RH have been measured with high signal-to-noise ratio. From the IR difference spectra recorded, changes of the time-dependent absorption peak area of the O-H stretching band (ν-OH, ∼3400 cm(-1)) and the nitrate out-of-plane bending band (ν2-NO3(-), ∼836 cm(-1)) are obtained. From these measurements, changes in the IR signatures can be attributed to crystalline and solution phase nitrate ions, allowing the volume fraction of the solution droplets that have crystallized to be determined. Then, using these clear signatures of the volume fraction of droplets that have yet to crystallize, the homogeneous and heterogeneous nucleation kinetics can be studied from conventional measurements using a steady decline in RH. The nucleation rate measurements confirm that the rate of crystallization in sodium nitrate droplets is considerably less than in ammonium sulfate droplets at any particular degree of solute supersaturation, explaining the wide range of efflorescence RHs observed for sodium nitrate in previous studies. We demonstrate that studying nucleation kinetics using the FTIR-ATR approach has many advantages over brightfield imaging studies on smaller numbers of larger droplets or measurements made on single levitated particles.

Regularities in the impurity element content of minerals during hydrothermal ore formation

Many goldfields host scores of discovered gold ore occurrences with unclear prospects because of their insufficient geologic exploration. Since exploration is expensive, the occurrences should be first appraised by low-cost methods. Studying the typomorphic features of minerals from the goldfields of the Upper Yana–Kolyma region, we have established a relationship between the contents of isomorphic impurity elements (IIE) and the degree of solution supersaturation (DSS). Preliminary studies show that, based on this relationship, criteria can be developed for revealing “concealed” mineralogical–geochemical zonation and predicting mineralization to depth. The determining factor for the behavior of IIE, depending on the DSS, is their distribution coefficient ( K) with respect to the host element. If K > 1, the content of these elements decreases with increasing DSS during ore deposition; if K < 1, it increases. When a solution contains IIE with K > 1 and K < 1, they behave according to their distribution coefficients. In other words, when the DSS increases during ore deposition, IIE with K > 1 will be typical of early mineral segregations, and IIE with K < 1, of late ones. If the crystallization front migrates, this must give rise to a vertical zonation in the content of both groups of elements. The content of elements with K > 1 will decrease upsection, and that of elements with K < 1 will increase (“concealed” mineralogical–geochemical zonation). In connection with this subject, the paper reports data using native gold as an example. The behavior of Ag, Sb, Hg, Cu, Ni, and Co is discussed depending on the depth of deposit formation from the paleosurface (deep-seated → shallow); in the vertical section of individual deposits; depending on mineralization stages; in the cross-section of individual crystals with a zonal structure. The behavior of Ag, Sb, and Hg has been shown to be opposite to that of Cu, Ni, and Co in all these aspects. With respect to Au, the first group of elements has K < 1, and the second one, K > 1. Copper and antimony are of particular interest. In the vertical section of individual deposits, the Cu content of gold increases with depth and the Sb content decreases. The Cu/Sb ratio can be used to estimate the erosional truncation level of deposits and thus predict their extension to depth. Examples of well-explored deposits are given.

Microstructural Stability of Gas-Atomized Al-Based Nanophase Composites

The microstructure evolution and grain growth behavior, of gas-atomized Al 88 Ni 9 Ce 2 Fe 1 nanocomposite materials, were studied by means of the X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. The results showed that the microstructure of as-atomized powders are affected by the degree of solute supersaturation in matrix, the microstrain contained, the size, distribution and volume fraction of nanophase precipitated, and these are essentially sensitive to the powder particle sizes. Moreover, there appeared to be a two-stage phase transformation during thermal crystallization, i.e., the precipitation of α-Al and the growth of Al 3 (Ni,Fe) nanophase in preference to Al 11 Ce 3 nanophase, respectively. Finally, the grain growth kinetics of α-Al at temperatures of 250 to 300°C, revealed that the volume-diffusion grain growth mechanism with an activation energy of 1.3 eV is controlling in the nanophase materials.

MECHANISM OF CRYSTALLIZATION IN AGITATED SOLUTIONS

A new concept of molecular cluster dynamics in turbulent flows is used to develop a physical model and mechanism of homogeneous and heterogeneous nucleation in solutions. The proposed physical model considers the motion of molecular clusters in centrifugal fields of free eddies induced in turbulent flows. The clusters are moved to an eddy boundary region where they are concentrated and aggregated to form critical nuclei. It is shown theoretically that a critical fluid velocity exists, below which nuclei are not formed, and that its value depends upon the degree of solution supersaturation. The nucleation theory takes into account the hydrodynamic parameters of the process and permits calculation of the number of nuclei formed. Satisfactory agreement was obtained between calculated values and published experimental data.

Crystal growth of calcium apatites from dilute solutions

The dependence of the kinetics of crystal growth of hydroxyapatite, HA, on concentration of seeds and degree of solution supersaturation is examined. Empirical relations are obtained between the initial rates of precipitation and the two aforementioned variables. The results clearly indicate that the precipitating phase, under the selected experimental conditions, is HA. The driving force for precipitation is the supersaturation with respect to this calcium phosphate. The data obtained from kinetic studies are consistent with the BCF theory of crystal growth which relates the mean linear rate of growth to the degree of supersaturation. This observation is also valid for systems in which the precipitating phases are fluoridated hydroxyapatites, Ca 5F x (OH) 1− x (PO 4) 3, FHA. In this case, supersaturation is calculated with respect to the specific FHA precipitated. The degree of fluoridation, x, appears to be determined by the activity of hydrofluoric acid in solution which remains fairly constant during precipitation but varies for each initial fluoride concentration used. Studies are reported on the kinetics of crystal growth of HA in the presence of biological inhibitors. Two proline-rich proteins and a peptide, statherin, of salivary origin are powerful inhibitors of HA crystal growth. It appears that their mode of action is related to their adsorption onto the surface of apatite seeds. The results suggest that the adsorption sites are the same as the sites where crystal growth takes place.

The influence of fluoride on the seeded growth of apatite from stable supersaturated solutions at pH 7.4.

Previous studies have shown that fluoride affected the formation of apatite in spontaneous precipitations from unstable calcium phosphate solutions partly through its effect on the stability of such nonapatitic precursors as octacalcium phosphate, Ca8H2(PO4)6.5 H2O. The present study shows that fluoride had a comparable effect on the seeded growth of apatite from stable solutions when the degree of solution supersaturation favored the transient development of octacalcium phosphate precursors on the seed surface.