Microwave-assisted Al-substituted tobermorite synthesis
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The effect of microwave heating on the hydrothermal synthesis of Al-substituted tobermorite and the removal characteristics of resulting materials were examined and compared with the effect of conventional heating. The microwave heating reduced the crystallization time of Al-substituted tobermorite—i.e., Al-substituted tobermorite was synthesized within 80 min at around 140 °C—and produced smaller crystallites than the conventional heating. The minute crystallites were found to promote the removal characteristics for Cs+ ions in short reaction time.
Hydrothermal synthesis, solid-state synthesis, and sintering of ceramics by microwave heating are widely practiced. Zeolitic families, such as zeolite Y, ZSM-5, aluminophosphate-5, AlPO4-5, and so on, were synthesized by the microwave hydrothermal (MH) technique.1–6 The microwave heating was reported to reduce the crystallization time and improve the crystallinity of the final product, compared with the conventional heating. Moreover, the crystallization of ceramic oxides, hydroxylated phases, and metal powders was examined under MH conditions.7–12 This processing technique was reported to enhance the kinetics of reaction over the conventional hydrothermal (CH) process, form novel phases, and produce fine powders. Although most hydrated calcium silicates are synthesized by a low-temperature hydrothermal process below 250 °C, there are no references for the preparation by the MH technique. Tobermorite, Ca5Si6(OH)2O16 ⭈ 4H2O, which is a hydrated calcium silicate, has a layer-type structure13 and is able to be prepared using reactive Ca and Si starting materials under hydrothermal conditions below 180 °C.14–18 The exchangeable Na+ ions are incorporated into the interlayers of tobermorite substituted with the Al atoms for the Si atoms in order to maintain charge neutrality. Consequently, Al-substituted tobermorite was known to exhibit selectivity for cations such as Cs+ ions.16–18 In this paper, we report the comparison of the MH technique with the CH technique in the preparation of Al-substituted tobermorites and in the removal characteristics of the resulting materials for Cs+ ions. For the preparation of Al-substituted tobermorite, active calcium oxide, aluminium hydroxide, colloidal silica (40 mass%), and sodium hydroxide were used as starting materials. The starting materials with batch compositions of Al/(Al + Si) ⳱ 0.167 and Na/Al ⳱ 2.0 were added to de-ionized water, which was fully boiled, and were 850
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J. Mater. Res., Vol. 15, No. 4, Apr 2000 Downloaded: 15 Mar 2015
quickly stirred. The mixture was divided into two parts for loading into the MH and CH reactors. A Teflon-lined autoclave made with polyimide and a Teflon-lined steel autoclave were used in the MH and CH experiments, respectively. The MH synthesis was carried out in a household-type microwave range, and the CH synthesis was done in a preheated oven at 150 °C. The microwave range generates 600 W of microwave energy at a frequency of 2.45 GHz at full power. The temperatur
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