Near-Net-Shape Forming of Celsian Ceramics from Ion-Exchanged Zeolite Precursors
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ABSTRACT A technique has been developed for the synthesis and near-net-shape forming of celsian glass-ceramics. This technique involves ion exchange of zeolite precursor powders, heat treatment of the powders to temperatures between 800-900*C to form a glassy phase during which the samples are formed under low stresses (_3% volume change. Hexacelsian, however, is the first product that forms using techniques which have been used for the synthesis of celsian, including crystallization from a melt, solid state reactions, hydrothenrnal treatments, and sol-gel processes. Hexacelsian persists over the entire temperature range and prolonged high temperature treatments (1500°C to 1800°C for periods of >20 hours) and the addition of mineralizers have been necessary to obtain monoclinic celsian 3 -6 . Such processing conditions render these techniques undesirable for efficiently producing large quantities of monoclinic celsian. In this paper, a technique is described for the preparation and near-net-shape forming of monoclinic celsian ceramics using heat treatments of less than an hour at about 1000°C. Zeolites are used as starting precursors which are ion exchanged and decomposed to form aluminosilicate based ceramics. The general formula for zeolites is M2/n A1203 xSiO2 yH20 where M is a cation, and n and x are usually greater than two, and y varies from 0 to 10 and the structure consists of sodalite cages of Al, Si, and 0, inside which water molecules and alkali metal cations are trapped. The cages are linked through a network of channels resulting in an open framework structure and porosity such that cation exchange can take place through the open channels with relative ease. The ion exchange process is simple and allows reasonable control over the stoichiometry of the resulting product. The use of zeolites for synthesis of aluminosilicate ceramics was first studied by Subramanian and Corbin of Dupont 7 -10. Using this technique they were able to produce hexacelsian upon heat treatment to 1000*C for 3 hours. In our study the M cations are replaced for barium and lithium, the ion-exchanged zeolites are seeded with monoclinic celsian and heat treated to form celsian at about 1000°C. 493 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL PROCEDURE Synthetic zeolite 13X* (Na86A186Si106O384 H20) was utilized in this study. The zeolite powders were first ion-exchanged several times in an aqueous solution of ammonium nitrate. The ammonium exhanged zeolites were next ion-exchanged in an aqueous solution of either barium nitrate or barium plus lithium nitrate. Several ion-exchanges were conducted to ensure complete exchange of the cations for sodium ionsqiFinely ground monoclinic celsian seed particles (1.3 wt.%) were mixed with the barium and lithium exchanged zeolites prior to heat treatment. Seed particles were obtained from barium exchanged powders that had been fired to 1550*C for >20 hours. The cation exchanged powders were either free-sintered in air or hot-pressed under very low stresses
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