Damir Kralj

The influence of gel properties on the kinetics of crystallization and particulate properties of MFI-type zeolities. I. The influence of time and temperature of gel ageing on the particulate properties of silicalite-1 microcrystals

Abstract The influence of time and temperature of ageing of the amorphous gel precursor (2.5Na 2 O-8TPABr-60SiO 2 -800H 2 O) on the particulate properties (particle size distribution, particle shape, number of particles) of silicalite-1 synthesized at 170°C were studied by different experimental methods such as X-ray diffractometry (XRD), differential thermogravimetric analysis (TGA), particle size analysis, FT infra-red spectroscopy and scanning-electron microscopy (SEM). In spite of negligible changes in the crystal morphology, ageing of the gel considerably affects the number and size of silicalite-1 crystals in the crystalline end products. The observed effects were discussed in terms of the formation of specific ordered subunits between silicate species and TPA + ions (germ nuclei) and their role in the nucleation and crystal growth of silicalite-1 crystals.

Effects of magnesium and temperature control on aragonite crystal aggregation and morphology

In this study, the influence of Mg2+ on the aggregation and morphology of precipitated aragonite crystals is investigated at different temperatures. Different from some investigations described in the literature, aragonite precipitates in chemical systems in which Mg2+ acts specifically as crystal habit modifiers and not as polymorphic selectors. The results show that at increased Mg2+ concentration and temperature the aragonite crystals are less aggregated and that the amount of crystals sharing {110} faces and having larger extension of {001} faces increases. These outcomes may be relevant in biological, geochemical and technological contexts.

How similar are amorphous calcium carbonate and calcium phosphate? A comparative study of amorphous phase formation conditions

Amorphous calcium carbonate (ACC) and calcium phosphate (ACP) increasingly attract attention as initial solid phases in vertebrate and invertebrate hard tissue formation, as well as in materials science as a possible new synthetic route for advanced materials preparation. Although much is known about these two amorphous phases and similarities in the mechanisms of their formation are recognized, no attempt has been made to investigate their formation under defined and comparable initial experimental conditions viz supersaturation, constituent ions ratio, ionic strength and presence of relevant inorganic additives. In this paper, the formation of ACC and ACP in three model precipitation systems of increased chemical complexity were investigated: (a) systems containing constituent ions, (b) systems containing additional co-ions, and (c) systems with higher ionic strength and addition of Mg2+. The results have shown that ACP is more stable and was formed at lower relative supersaturations in comparison to ACC. The precipitation domain of both phases expanded with increasing complexity of precipitation systems, with the ACP precipitation domains always being larger than that of ACC. In addition to stability, the presence of inorganic ions, especially Mg2+, influences the composition of both amorphous phases. The obtained results indicate that general similarity between ACC and ACP exists, but it could also be concluded that similar chemical environment in which they form not necessary lead to similar structural properties.