Effects Of Trivalent Cations Research Articles

Scandium diffusion in forsterite: concentration dependence, inter-site reactions and the effect of trivalent cations on Fe diffusion

Diffusion of scandium (Sc) in synthetic forsterite (Mg2SiO4) was studied at 1400 °C at atmospheric pressure, in air. The Sc source was a powder of forsterite and protoenstatite (Mg2Si2O6) in order to buffer the silica activity, doped with various Sc contents, from ∼9 wt. ppm Sc to ∼8.5 wt% Sc2O3. The powder with the highest Sc content also contained thortveitite (Sc2Si2O7), which additionally buffers the activity of Sc2O3. All experiments were conducted for the same duration (550 h), but the length of the Sc diffusion profile (i.e. the distance over which the concentration decreased from the rim to core/background value) varied from ∼200 to 800 μm, with higher Sc concentrations associated with longer diffusion profiles. The geometry of Sc diffusion profiles also changes systematically with the Sc concentration – profiles with low Sc (<10 wt. ppm at the interface) have stepped shapes, those with medium Sc contents (80–400 wt. ppm at the interface) have near-linear profiles, while experiments with high Sc at the interface (>500 wt. ppm) show profiles with a shape that is approximately, but not exactly, describable by error function forms. Small but variable amounts of Fe contamination resulted in Fe diffusion profiles with interface concentrations of 80 to 1100 wt. ppm. The diffusivity of Fe, at these trace concentrations, appears to be a function of the concentration of Sc, rather than its own concentration, with the increased Sc concentration associated with an increase in Fe diffusivity of at least an order of magnitude. Similar behaviour is observed for other contaminants, Ni, Co, Mn and Li, at trace concentrations. Additionally, Al, present in the original forsterite crystal at the level of a few 10s of wt. ppm developed concentration profiles with a stepped form, broadly showing out-diffusion. The point along the profiles at which the Al concentration changes dramatically, coincides approximately with the end of the Sc profiles. Models are tested in which Sc diffusion is assumed to be concentration-dependent. In these models, the concentration-dependence is defined using the energy of binding between Sc3+ and M-site vacancy, and the diffusivity of the cation-vacancy pair. Such models are generally successful, but cannot fully describe the behaviour observed in these experiments. Rather, the diffusive behaviour of Sc and Al is better described by a model in which Sc3+ substitutes for Mg2+, charge balanced by vacancies (the Sc4/3[vac]2/3SiO4 substitution), and diffuses relatively rapidly in a concentration-dependent regime. Interaction of Sc with Al (present in the starting material) is proposed to lead to the formation of an MgScAlO4 component, upon which the diffusivity of Sc is drastically reduced. The model is capable of describing the majority of Sc and Al profiles observed. One experiment was hydroxylated following the diffusion anneal, revealing a multitude of defects supporting the suggestion that this system is more complex than simple concentration-dependent diffusion. Whilst these experiments were conducted in a simple system with lower Fe concentrations and higher fO2 than those typical of natural olivine, an example from a previous study (Cr diffusion in San Carlos olivine) shows that such complex behaviour may also be found in systems closer to natural conditions. The addition of Cr (∼2000 wt. ppm at the diffusion interface) appears to cause a ∼ 1 order of magnitude increase in the diffusivity of Fe-Mg, Ca and Ni relative to the published values conventionally used for diffusion chronometry.

The Effect of Trivalent Cations on the Performance of Mg‐M‐CO3 Layered Double Hydroxides for High‐Temperature CO2 Capture

The effect of trivalent cations on the performance of Mg-M-CO(3) (M=Al, Fe, Ga, Mn) layered double hydroxides (LDHs) for high-temperature CO(2) capture is systematically investigated for the first time. We demonstrate that the M(3+) determines the structure evolution of LDH derivatives under thermal treatment, and finally influences the CO(2) capture capacity. Very different calcination temperatures are required for the different LDHs to obtain their maximum CO(2) capture capacities. To have a clear understanding of the reason behind these big differences the physicochemical properties, thermal stability, and memory effect of the LDHs were investigated. Both the thermal stability and the memory effect of LDHs are greatly influenced by the type of trivalent cation. The CO(2) capture capacities were also evaluated under various conditions. Another important finding of this work is that the quasi-amorphous phase obtained by thermal treatment at the lowest possible temperature gives the highest CO(2) capture capacity.

The Effect of Trivalent Cations on the Conduction Characteristics of PbF2

The conduction characteristics of containing large amounts of were investigated in the temperature range between −50° and 300°C. It was found that the inclusion of high concentrations of stabilizes the cubic structure of and the application of a high pressure and/or mechanical stress on the sample does not result in a conversion from the cubic form to the orthorhombic form at ambient temperatures. Furthermore, the conductivity of was substantially increased by the incorporation of. containing exhibited a room temperature conductivity of and an activation energy for conduction of in the temperature range of −50°–70°C. However, the conductivity of this solid decreased substantially after annealing at a temperature above 100°C. Furthermore, the activation energy for conduction was found to be higher after annealing. The rate at which the conductivity decreases and the activation energy increases appeared to depend upon the temperature and duration of the annealing process. Inasmuch as the conductivity of the annealed sample could be increased to some extent by prolonged grinding, and the presence of an orthorhombic phase together with the predominate cubic phase was noted in the x‐ray diffraction patterns of the annealed samples, we concluded that the decrease in conductivity by the annealing process may be attributed to (i) stress relief at the grain boundary or in the lattice upon annealing, (ii) partial structure conversion from the more conductive cubic form to the less conductive orthorhombic form upon annealing, and/or (iii) precipitation of or complexes from the supersaturated solid solution.

THE EFFECT OF pH ON THE WET STRENGTH OF SOIL CRUMRS

SummaryThe clay‐clay attractive forces were greater in crumbs from acid soils than crumbs from similar soils but Containing free CaCO3. This has two causes: (a) positive charges on the edge faces of the clay crystals; (b) exchangeable trivalent cations on the clay. Using the improved sodium‐saturation method these two causes were distinguished by measuring the reduction in the strength of the crumbs by (a) raising the pH of the percolating NaCl to 7.4 with para‐nitrophenol buffer, and (b) preliminary NaCl‐HCl leaching. The effect of trivalent cations was dominant in very acid soils where Al+++ can exist in the soil solution. The soil conditioner‘Flotal’strengthens soil crumbs, initially at least, by substituting Fe+++ ions on the exchange complex. No evidence was found for cementing by precipitated Al(OH)3, or Fe(OH)3.The same concentration of CaCl2 6 × 10−3 M was required to flocculate suspensions of moderately acid surface soils both at the natural pH of the soil and after buffering the suspensions to pH 7×4. However a Callite and an acid subsoil (no organic matter) flocculated at a CaCl2, concentration of 3 × 10−4 M at pH 5. Thus there appears to be sufficient organic matter present in normal cultivated acid soils to neutralize the positive charges on the edges of any clay particles dispersed from the crumbs.It is suggested that, although liming an acid soil decreases the attractive forces between clay particles within soil crumbs, the presence of free CaCO3, in the soil helps to maintain a sufficiently high concentration of electrolyte in the soil solution to reflocculate dispersed clay and so keep drainage waters clear.