Framboid Microcrystal Growth

Framboid microcrystals grow through surface reaction of S2(-II) or H2S with =FeS moieties at defect sites on the pyrite crystal surface. The surface energies of pyrite vary from the most stable cubic through octahedral to pyritohedral and dodecahedral surfaces. Microcrystals commonly develop as truncated octahedra as the supersaturation decreases during crystal growth in sedimentary environments, although cubic forms may be favored under hydrothermal conditions. Screw dislocation growth followed by surface nucleation growth are the normal growth modes in sediments, whereas surface nucleation growth is likely to dominate in hydrothermal systems. The rate of crystal growth of framboids is unknown but appears to be very fast and normally diffusion-limited. Linear approximations to the diffusion equations show that average 6 μ‎m diameter framboids form in five days in sediments, and formation times increase exponentially from a few hours for ca. 2 μ‎m framboids to three years for the largest 250 μ‎m framboids.

Reduction Mechanism of Fine Hematite Ore Particles in Suspension

In order to understand the pre-reduction behaviour of fine hematite particles in the HIsarna process, change of morphology, phase and crystallography during the reduction were investigated in the high temperature drop tube furnace. Polycrystalline magnetite shell formed within 200 ms during the reduction. The grain size of the magnetite is in the order of magnitude of 10 µm. Lath magnetite was observed in the partly reduced samples. The grain boundary of magnetite was reduced to molten FeO firstly, and then the particle turned to be a droplet. The Johnson-Mehl-Avrami-Kolmogorov model is proposed to describe the kinetics of the reduction process. Both bulk and surface nucleation occurred during the reduction, which leads to the effect of size on the reduction rate in the nucleation and growth process. As a result, the reduction rate constant of hematite particles increases with the increasing particle size until 85 µm. It then decreases with a reciprocal relationship of the particle size above 85 µm.

Toward a theory of the evolution of drop morphology and splintering by freezing

The drop freezing process is described by a phase-field model. Two cases are considered: when the freezing is triggered by central nucleation and when nucleation occurs on the drop surface. Depending on the environmental temperature and drop size, different morphological structures develop. Detailed dendritic growth was simulated at the first stage of drop freezing. Independent of the nucleation location, a decrease in temperature within the range from ~ −5 to −25°C led to an increase in the number of dendrites and a decrease in their width and the interdendritic space. At temperatures lower than about −25°C, a planar front developed following surface nucleation, while dendrites formed a granular-like structure with small interdendritic distances following bulk nucleation. An ice shell grew in at the same time (but slower) as dendrites following surface nucleation, while it started forming once the dendrites have reached the drop surface in the case of central nucleation. The formed ice morphology at the first freezing stage predefined the splintering probability. We assume that stresses needed to break the ice shell arose from freezing of the water in the interdendritic spaces. Under this assumption, the number of possible splinters/fragments was proportional to the number of dendrites, and the maximum rate of splintering/fragmentation occurred within a temperature range of about −10 °C to −20°C, in agreement with available laboratory and in situ measurements. At temperatures < −25°C, freezing did not lead to the formation of significant stresses, making splintering unlikely. The number of dendrites increased with drop size, causing a corresponding increase in the number of splinters. Examples of morphology that favors drop cracking are presented, and the duration of the freezing stages is evaluated. Sensitivity of the freezing process to the surface fluxes is discussed.

Growth Mechanism of Spherulitic Propylthiouracil-kaempferol Cocrystal: New Perspectives into Surface Nucleation

As an emerging technology that can simultaneously improve the powder performance and bioavailability of drugs, spherulitic cocrystallization has increasingly shown its unique advantages. In this study, we reported a new...