Preparation of lithium aluminate monolith of controlled porosity by a sol-gel method

Abstract

Lithium aluminate (LiA102) has received much attention due to its promising use as a tritium breeding material in fusion reactors [1, 2], and as an electrolyte matrix for the molten carbonate fuel cell (MCFC) [3, 4]. LiA102 powder can be prepared by a solid-state reaction between 7-A1203 and a lithium compound such as carbonate and hydroxide, or a sol-gel method starting from metal alkoxides [11]. The sol-gel processing of LiA102 produces a powder with a better controlled ratio of lithium to aluminum due to the low processing temperature [5]. Various combinations of alkoxides of lithium and aluminum were tested to prepare LiA102 powder by Turner et al. [7]. They found that amorphous LiA102 was directly transformed to v-LiA102 at temperatures as low as 550 °C, when lithium methoxide and aluminium n-butoxide were hydrolyzed at 25 °C. Most of the research on the sol-gel route to LiA102 has involved preparation with powders; preparation methods using monolithic porous LiA102 were not seriously examined. In this letter, we propose a direct route to the preparation of pore-controlled monolithic LiA102 by a sol-gel method. Crack formation over the drying period has limited accessibility to this monolithic route [7, 8]. There have been several suggestions to avoid cracks during drying, such as DCCA (drying control chemical additive) addition [9], supercritical drying, and freeze drying [10]. The DCCA addition method has advantages over other methods. First, no costly additional equipment, such as high pressure equipment or freeze drier, is needed. Secondly, porosity can be controlled by varying the amount of DCCA or other additives such as carbon black or active carbon. Currently, fonnamide, dimethylformamide (DMF) [7, 8], glycerol, and oxalic acid [8] are widely used as a DCCA. Regarding the role of a DCCA, it is reported that the DCCA makes pore size larger and reduces capillary stress [7]. Specific roles of the DCCA, however, vary depending on the types of DCCA, and are not yet fully understood. The procedure for preparing monolithic gel is summarized in Fig. 1. Lithium isopropoxide (LiOC3H7) and aluminium isopropoxide (AI[OC3 H713), 0.05 moles of each, were mixed with 100 ml of isopropanol in a three-neck flask. This mixture was then stirred with a magnetic stirrer at 80 °C. After 2 h of stirring, yellowish brown precipitates were obtained at the bottom of the flask. These precipitates were mixed with 200 ml of ethanol, refluxing under nitrogen gas at 80 °C for 2 h. The 2012 I-'soUtrhp uomx'de ]