Elimination of boron species via solvent extraction with 2,2,4-trimethyl-1,3-pentandiaol was engaged in a flow-type device into which the aqueous solution dissolving diatomaceous earth, a candidate of the resource for high-purity silicon material. The flow-type device has a 2-stage channel with 1.0 mm width, 0.10 mm depth, and 25 mm length each, and the specific interfacial area is comparable to a typical microchannel device. Residual boron content in the aqueous solution decreased to less than 0.1 ppm after the solvent extraction under the restriction of short contact duration shorter than 100 ms. In addition, the analysis with density functional theory indicated that the number of neighbor carbon on the central carbon is one of the keys for improvement of reactivity of extractant. Hence, it is suggested that the flow-type microchannel device with solvent extraction with alkyldiol extractant can be utilized as an attractive approach toward the production of high-purity silica as a source for solar-grade silicon.
Purification of silica solution to chemically remove impurities is a novel approach for preparing solar-grade Si. Complete elimination of boron is necessary, as it significantly affects the semiconductor properties of Si if included. To build an efficient reactor for boron extraction, the mechanism of extraction reaction based on molecular behavior should be well understood. Here, we investigated the liquid−liquid extraction of boron (boric-acid) using 2,2,4-trimethyl-1,3-pentanediol as an extractant, which takes place at the liquid−liquid interface experimentally and theoretically. Raman spectroscopy for the microflow reactor provided the concentration of boric acid after extraction, whereas density functional theory calculation showed the reaction energy profiles of the underlying chemical reactions. Calculation results suggested that H 2 O molecules from the aqueous phase promote extraction by enabling the formation of [BOH-HOH-HOC], a sufficiently stable, nonsteric structure, which stabilizes the transition state and facilitates boric acid esterification. Otherwise, this reaction cannot take place in standard conditions. Raman spectroscopy applied to the extraction process in a microflow reactor supported this conclusion experimentally. These results suggest that the extraction reaction at the liquid−liquid interface is mass transfer-limited. This can help the design of effective reactors to eliminate boron impurity from silica solution.
We have attempted to develop a novel process to produce solar-grade silicon (SOG) from diatomaceous earth using wet chemical processes, which have potential advantage to conventional process with long-term, high temperature reactions [1]. The key step of the process is purification of silica solution by solvent extraction with micro-channel device to eliminate light-element impurities such as boron [2], and in order to achieve higher purity, its reaction mechanism should be elucidated in detail. In the present work, we investigate the behavior of reactant species in the channel, using computational and spectroscopic approaches. In our previous study, 2,2,4-trimethyl-1,3-pentanediol (TMPD) was proposed as an efficient extractant for boron (boric acid) [3]. Boric acid is extracted in the form of ester with TMPD at the interface between aqueous solution containing silica and organic solvent with TMPD, whose reaction equation is shown in Fig. 1. Figure 2 shows schematic view of the micro-channel device, which has two channels to provide liquid-liquid interface for the reaction. In order to investigate the reaction mechanism, the following three issues should be considered; (i) local area of the reaction system should be measured precisely, (ii) mass transfer, including fluidic flow, should be simplified in the measured reaction system, (iii) reactant behavior during the extraction should be understood at molecular level. To satisfy these conditions, we used micro analysis system with simulation technique. For the local area measurement, Raman spectroscopy, which is capable to provide chemical information within less than 1μm range, was utilized. For the flow state control in the system, micro-channel device has great advantage, since laminar flow state is predominant in the device. For molecular level understanding of the reaction, density functional theory (DFT) calculation was performed. Rate constant of the reaction in Fig. 1 was estimated by transition state theoretically estimated from DFT. Using this rate constant, finite element method (FEM) was performed to calculate concentration profile of reactants within the channel, comparing to the result of the spectroscopic study. From the result of transition state optimization by DFT, the ester formation was considered to be enhanced by water molecule. Comparing the concentration profile obtained from FEM and that from Raman spectroscopy, the measurement system in this study was expected to have capability to analyze the detailed reaction mechanism of the process. Acknowledgements This work performed as part of the CREST Program, JST, Japan References [1] J. Komadina, T. Akiyoshi, Y. Ishibashi, Y. Fukunaka, T. Homma, Electrochim. Acta, 100, 236-241 (2013). [2] N. Matsuo, Y. Matsui, Y. Fukunaka, T. Homma, J. Electrochem. Soc., 161(5), E93-E96 (2014). [3] N. Matsuo, T. Ishihara, T. Oyanagi, K. Nakajima, M. Kunimoto, Y. Fukunaka, T. Homma, ECS Trans., submitted. Figure 1
The improvement in the quality of ZnO:Al films grown on glass substrates was investigated using a sputtered buffer layer inserted between a glass substrate and a ZnO:Al CVD film. ZnO:Al layers were grown at 773 K using dimethylzinc (DMZn), trimethylaluminum (TMAl), and high-temperature H2O generated by a catalytic reaction as zinc, aluminum, and oxygen sources, respectively. The electron mobility increased by approximately 20 cm2 V−1 s−1 with the use of a buffer layer with a thickness of approximately 40 nm. In addition, the optical transmittance in the wavelength range of 380–600 nm increased with the insertion of the buffer layer. For the growth of ZnO films on a sputtered buffer layer, the average surface roughness was reduced, and as a result the fluctuation in crystal orientation along the c-axis became smaller than that of the film grown directly on the glass substrate by the proposed CVD. This resulted in improvements in the optical transmittance and electron mobility of the ZnO:Al films.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.