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Efficient fabrication of nanoporous si and Si/Ge enabled by a heat scavenger in magnesiothermic reactions.

Luo W, Wang X, Meyers C, Wannenmacher N, Sirisaksoontorn W, Lerner MM, Ji X - Sci Rep (2013)

Bottom Line: Magnesiothermic reduction can directly convert SiO2 into Si nanostructures.Despite intense efforts, efficient fabrication of highly nanoporous silicon by Mg still remains a significant challenge due to the exothermic reaction nature.Our methodology is potentially competitive for a practical production of nanoporous Si-based materials.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, USA.

ABSTRACT
Magnesiothermic reduction can directly convert SiO2 into Si nanostructures. Despite intense efforts, efficient fabrication of highly nanoporous silicon by Mg still remains a significant challenge due to the exothermic reaction nature. By employing table salt (NaCl) as a heat scavenger for the magnesiothermic reduction, we demonstrate an effective route to convert diatom (SiO2) and SiO2/GeO2 into nanoporous Si and Si/Ge composite, respectively. Fusion of NaCl during the reaction consumes a large amount of heat that otherwise collapses the nano-porosity of products and agglomerates silicon domains into large crystals. Our methodology is potentially competitive for a practical production of nanoporous Si-based materials.

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Investigation of decomposition of alkaline carbonates during a MRR without or with NaCl heat scavenger, indicating a much lower reaction temperature with the heat scavenger.XRD patterns of (a) the product collected after heating the mixture of diatom, Mg powder, and BaCO3 at 650°C for 2.5 h under Ar and (b) the product collected after heating the mixture of diatom, Mg powder, NaCl, and SrCO3 at 650°C for 2.5 h under Ar and the removal of NaCl.
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f6: Investigation of decomposition of alkaline carbonates during a MRR without or with NaCl heat scavenger, indicating a much lower reaction temperature with the heat scavenger.XRD patterns of (a) the product collected after heating the mixture of diatom, Mg powder, and BaCO3 at 650°C for 2.5 h under Ar and (b) the product collected after heating the mixture of diatom, Mg powder, NaCl, and SrCO3 at 650°C for 2.5 h under Ar and the removal of NaCl.

Mentions: We propose the following process for the MRR with the NaCl heat scavenger. When the furnace temperature rises close to the melting point of Mg at 650°C, the exothermic MRR is initiated. Without an efficient dissipation, heat released by the reaction accumulates, and the reaction temperature continues to rise. At 801°C, NaCl in the reaction mixture begins to melt, which consumes the heat generated by the MRR and retards further temperature rising. For one gram of silica, we used 10 grams of NaCl heat scavenger. It takes approximately 1.4 kJ to heat up the reaction mixture from 650°C to 801°C. With good thermal conductivity of molten NaCl (0.8 W/m·K)39, heat from the MRR can be readily conducted to surrounding NaCl crystals and consumed by NaCl fusion. The fusion of NaCl (10 g) removes heat of 5.0 kJ. At 801°C, in fact, Mg vaporization heat should be partially provided by the MRR as well (Mg: ΔHvap = 6.1 kJ/g). Totally, the heat consumption by the reactant mixture is between 6.4 kJ to 12.5 kJ, comparable to the heat release from the MRR (9.8 kJ/gsilica). We can expect that the reaction temperature may be slightly higher than 801°C. In order to probe the real reaction temperatures, we used alkaline carbonates as temperature indicators that were well mixed with the reactants (Supplementary Methods). After a MRR without NaCl, the absence of BaCO3 peaks in the XRD pattern of the product indicates a reaction temperature above 1300°C, the decomposition temperature of BaCO3 (Fig. 6a), consistent to the reported reaction temperature31. In sharp contrast, with NaCl heat scavenger, SrCO3, with the decomposition temperature of 1100°C, survives a MRR in the product, confirmed by its strong XRD peaks and a lack of XRD peaks from other Sr containing compounds than SrCO3 (Fig. 6b). However, XRD peaks of CaCO3 disappeared after a MRR even with NaCl as the heat scavenger (see XRD results in Supplementary Fig. S10). Even with 20 g NaCl that is theoretically sufficient to absorb all the heat, CaCO3 still decomposes. This implies that the method could be limited by the mixing degree between NaCl and diatom. We are currently working on porous silica nanofibers as precursors in order to increase the mixing extent with NaCl heat scavenger. The results reveal that the reaction temperature with NaCl should be between 840°C and 1100°C. This is the first time that the real reaction temperature in a magnesiothermic reaction can be controlled by a heat scavenger and “measured” with a temperature indicator.


Efficient fabrication of nanoporous si and Si/Ge enabled by a heat scavenger in magnesiothermic reactions.

Luo W, Wang X, Meyers C, Wannenmacher N, Sirisaksoontorn W, Lerner MM, Ji X - Sci Rep (2013)

Investigation of decomposition of alkaline carbonates during a MRR without or with NaCl heat scavenger, indicating a much lower reaction temperature with the heat scavenger.XRD patterns of (a) the product collected after heating the mixture of diatom, Mg powder, and BaCO3 at 650°C for 2.5 h under Ar and (b) the product collected after heating the mixture of diatom, Mg powder, NaCl, and SrCO3 at 650°C for 2.5 h under Ar and the removal of NaCl.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3713525&req=5

f6: Investigation of decomposition of alkaline carbonates during a MRR without or with NaCl heat scavenger, indicating a much lower reaction temperature with the heat scavenger.XRD patterns of (a) the product collected after heating the mixture of diatom, Mg powder, and BaCO3 at 650°C for 2.5 h under Ar and (b) the product collected after heating the mixture of diatom, Mg powder, NaCl, and SrCO3 at 650°C for 2.5 h under Ar and the removal of NaCl.
Mentions: We propose the following process for the MRR with the NaCl heat scavenger. When the furnace temperature rises close to the melting point of Mg at 650°C, the exothermic MRR is initiated. Without an efficient dissipation, heat released by the reaction accumulates, and the reaction temperature continues to rise. At 801°C, NaCl in the reaction mixture begins to melt, which consumes the heat generated by the MRR and retards further temperature rising. For one gram of silica, we used 10 grams of NaCl heat scavenger. It takes approximately 1.4 kJ to heat up the reaction mixture from 650°C to 801°C. With good thermal conductivity of molten NaCl (0.8 W/m·K)39, heat from the MRR can be readily conducted to surrounding NaCl crystals and consumed by NaCl fusion. The fusion of NaCl (10 g) removes heat of 5.0 kJ. At 801°C, in fact, Mg vaporization heat should be partially provided by the MRR as well (Mg: ΔHvap = 6.1 kJ/g). Totally, the heat consumption by the reactant mixture is between 6.4 kJ to 12.5 kJ, comparable to the heat release from the MRR (9.8 kJ/gsilica). We can expect that the reaction temperature may be slightly higher than 801°C. In order to probe the real reaction temperatures, we used alkaline carbonates as temperature indicators that were well mixed with the reactants (Supplementary Methods). After a MRR without NaCl, the absence of BaCO3 peaks in the XRD pattern of the product indicates a reaction temperature above 1300°C, the decomposition temperature of BaCO3 (Fig. 6a), consistent to the reported reaction temperature31. In sharp contrast, with NaCl heat scavenger, SrCO3, with the decomposition temperature of 1100°C, survives a MRR in the product, confirmed by its strong XRD peaks and a lack of XRD peaks from other Sr containing compounds than SrCO3 (Fig. 6b). However, XRD peaks of CaCO3 disappeared after a MRR even with NaCl as the heat scavenger (see XRD results in Supplementary Fig. S10). Even with 20 g NaCl that is theoretically sufficient to absorb all the heat, CaCO3 still decomposes. This implies that the method could be limited by the mixing degree between NaCl and diatom. We are currently working on porous silica nanofibers as precursors in order to increase the mixing extent with NaCl heat scavenger. The results reveal that the reaction temperature with NaCl should be between 840°C and 1100°C. This is the first time that the real reaction temperature in a magnesiothermic reaction can be controlled by a heat scavenger and “measured” with a temperature indicator.

Bottom Line: Magnesiothermic reduction can directly convert SiO2 into Si nanostructures.Despite intense efforts, efficient fabrication of highly nanoporous silicon by Mg still remains a significant challenge due to the exothermic reaction nature.Our methodology is potentially competitive for a practical production of nanoporous Si-based materials.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, USA.

ABSTRACT
Magnesiothermic reduction can directly convert SiO2 into Si nanostructures. Despite intense efforts, efficient fabrication of highly nanoporous silicon by Mg still remains a significant challenge due to the exothermic reaction nature. By employing table salt (NaCl) as a heat scavenger for the magnesiothermic reduction, we demonstrate an effective route to convert diatom (SiO2) and SiO2/GeO2 into nanoporous Si and Si/Ge composite, respectively. Fusion of NaCl during the reaction consumes a large amount of heat that otherwise collapses the nano-porosity of products and agglomerates silicon domains into large crystals. Our methodology is potentially competitive for a practical production of nanoporous Si-based materials.

Show MeSH
Related in: MedlinePlus