The Formation of Barite and Celestite through the Replacement of Gypsum

Barite (BaSO4) and celestite (SrSO4) are the end-members of a nearly ideal solid solution. Most of the exploitable deposits of celestite occur associated with evaporitic sediments which consist of gypsum (CaSO4·2H2O) or anhydrite (CaSO4). Barite, despite having a broader geological distribution is rarely present in these deposits. In this work, we present an experimental study of the interaction between gypsum crystals and aqueous solutions that bear Sr or Ba. This interaction leads to the development of dissolution-crystallization reactions that result in the pseudomorphic replacement of the gypsum crystals by aggregates of celestite or barite, respectively. The monitoring of both replacement reactions shows that they take place at very different rates. Millimeter-sized gypsum crystals in contact with a 0.5 M SrCl2 solution are completely replaced by celestite aggregates in less than 1 day. In contrast, only a thin barite rim replaces gypsum after seven days of interaction of the latter with a 0.5 M BaCl2 solution. We interpret that this marked difference in the kinetics of the two replacement reactions relates the different orientational relationship that exists between the crystals of the two replacing phases and the gypsum substrate. This influence is further modulated by the specific crystal habit of each secondary phase. Thus, the formation of a thin oriented layer of platy barite crystals effectively armors the gypsum surface and prevents its interaction with the Ba-bearing solution, thereby strongly hindering the progress of the replacement reaction. In contrast, the random orientation of celestite crystals with respect to gypsum guarantees that a significant volume of porosity contained in the celestite layer is interconnected, facilitating the continuous communication between the gypsum surface and the fluid phase and guaranteeing the progress of the gypsum-by-celestite replacement.

Формирование бикристаллических пленок ZnO на ромбоэдрической плоскости сапфира при высоких скоростях роста

A route for the formation of a ZnO bicrystal film using the features of the magnetron sputtering method and the orienting action of the rhombohedral sapphire plane is proposed. It is shown that, when two deposition modes with with growth rates of ~ 2 and ~ 16 nm/s are used consistently, a bicrystalline ZnO film with a bottom (110)-oriented sublayer and a top (002)-oriented one is formed. Recrystallization annealing of the film at 1000°C for 10 h does not affect the top (002)-oriented layer and leads to relaxation of stresses in the bottom (110)-oriented layer.

Enhanced performance of piezoelectric composite nanogenerator based on gradient porous PZT ceramic structure for energy harvesting

Oriented layer and interconnected transverse bridges between layers in the progressive lamellar region effectively improved the electromechanical conversion efficiency.

Optical Properties of Laterally Oriented Self-Assembled Monolayers of Silver Nanoplatelets on Cationic Polymers

Silver plasmonic nanoplatelets functionalized with sodium mercaptoethanesulfonate were electrostatically deposited onto thin films of cationic co-polymers to form a dense planar-oriented layer. We observed a linear dependence of the optical density of as-prepared films versus the amount of tertiary amino groups in the polymeric film. Our novel method allows direct examination of the optical properties and morphology of silver nanoplatelets in the same polymeric film.

Ferroelectric liquid crystals with sub-wavelength helix nanostructure pitch for the generation of axial-symmetric vortex light fields

Experimental results of the formation of axially symmetric vortex light fields with a frequency of up to 2 kilohertz are presented. The results were obtained by using a 12-sector spiral phase plate (SPP) based on a planar-oriented layer of a 50 μm-thick ferroelectric liquid crystal (FLC), having the spiral nanostructure pitch substantially less than 100 nm.

Silicon (001) Heteroepitaxy on 3C-SiC(001)/Si(001) Seed

We report for the first time the successful heteroepitaxial growth of Si(100) oriented layer on top of a 3C-SiC(001) seed. By using a post-growth modification of the 3C-SiC surface (pulse insertion of precursors during cooling), it led to a change in Si nucleation, favoring squared (100) islands instead of elongated (110) ones. Without this surface modification step, the Si layers grown on 3C-SiC were always polycrystalline with a mixture of (110) and (100) orientations. Using such Si(100) layer grown on top of 3C-SiC(100), a (100) oriented 3C-SiC single crystalline layer was successfully grown on top, fabricating thus for the first time a fully (100) oriented multilayer heterostructure made of Si(substrate)/SiC/Si/SiC.