Abstract

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When Ca(HCO3)2 solution is put at 80 °C, ACC initially formed grow into calcite, aragonite and vaterite particles. The trunk of snow-shaped particles are self-assembled from hexagonal vaterite particles by one or more steps, followed by the filling in the pores of snow-shaped particles by small crystals.

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Vaterite is a metastable phase of CaCO3 and was prepared mechanochemically for the first time with the assistance of sodium hexametaphosphate (SHMP). First, CaCO3 was prepared without SHMP and was characterized using X‐ray diffraction (XRD) to study the effect of milling times and speeds on the polymorphs of product. The results indicate that the reaction is complete at 60 minutes producing only calcite. Additionally, amorphous CaCO3 (ACC) was obtained at a milling speed of 300 rpm, while calcite was obtained at 600 and 1000 rpm. Then, the effect of SHMP concentration on the fraction of vaterite was investigated, and the vaterite fraction increased with increasing SHMP amount. Subsequently, the effect of milling speed in the presence of 0.8 g of SHMP was studied, and the vaterite fraction increased with decreasing milling speed. Finally, gentler manual milling was employed, and the effect of the amount of added SHMP on vaterite formation was evaluated. The results confirmed that vaterite increased with increasing amounts of SHMP, and that vaterite formed more readily via manual milling than via mechanical milling. Observations with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that calcite and vaterite particles formed by mechanical milling were irregular agglomerates composed of primary nanoparticles while calcite particles formed by manual milling were irregular microparticles. Moreover, vaterite readily aggregated into spherical particles as the amount of SHMP increased. To investigate the reaction process and mechanism, the ethanol‐washed product was characterized using XRD, SEM/EDS and TEM/SAED. The results demonstrate that ACC and calcite were concomitant during the milling process, and ACC transformed into vaterite during subsequent water washing.

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Atmospheric pressure micro-discharges in helium gas with a mixture of 0.5% water vapor between two pin electrodes are generated with nanosecond overvoltage pulses. The temporal and spatial characteristics of the discharges are investigated by means of time-resolved imaging and optical emission spectroscopy with respect to the discharge morphology, gas temperature, electron density, and excited species. The evolution of micro-discharges is captured by intensified CCD camera and electrical properties. The gas temperature is diagnosed by a two-temperature fit to the ro-vibrational OH(A2Σ+–X2Π, 0–0) emission band and is found to remain low at 425 K during the discharge pulses. The profile of electron density performed by the Stark broadening of H α 656.1-nm and He I 667.8-nm lines is uniform across the discharge gap at the initial of discharge and reaches as high as 1023 m−3. The excited species of He, OH, and H show different spatio-temporal behaviors from each other by the measurement of their emission intensities, which are discussed qualitatively in regard of their plasma kinetics.

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