Lithium aluminosilicate system

A range of glass-ceramics, each having different crystalline phases, has been used as the matrix material. The most widely used matrices are based on the lithium aluminosilicate (LAS) system. The constituents of typical glass-ceramic matrices are given in Table 3.12. 

Influence of Ethanol. Three different amorphous aluminosilicate solids of Si/Al ratios 1.33, 1.48 and 4.28 were synthesized by mixing sodium silicate and aluminate solutions of various concentrations. These solids were extensively ion-exchanged with LiCl and NaCl solutions. The lithium and sodium containing solids (2g) were then mixed with 50 mL of 1JJ LiOH and NaOH, respectively. The hydroxide solutions contained 0%, 10%, 25%, 50% and 75% ethanol (volume by volume). These samples were then heated to 90-95 C, and formation of zeolites was monitored by powder diffraction. In one experiment, the lithium aluminosilicate solid was reacted in the NaOH system. 

Crystallization sequences have been determined for over 400 compositional points in the Li,Na-aluminosilicate system at 100°C and reaction times of up to 14 days. A total of 9 zeolitic species has been observed thereby. Crystallization times and the nature of the first appearing solid phase depend largely on the lithium and sodium concentrations. The same crystallization sequences occur within certain fields, and these sequences are nowhere reversed. A simple scheme illustrating the relative stabilities of the solid phases in the system is presented. 

Recent work in Versailles and Santa Barbara has led to the synthesis of several nanoporous nickel(II) phosphates. A zeolitic nickel(II) phosphate, VSB-1 (Versailles/Santa Barbara-1), was prepared under simple hydrothermal conditions [22] and has a unidimensional pore system delineated by 24 NiO and PO4 poly-hedra with a free diameter of approximately 0.9 nm (Figure 18.7). It becomes microporous on calcination in air at 350 °C, yielding BET surface areas up to 160 m g and is stable in air to approximately 500 °C. The surface area appears low compared with aluminosilicate zeolites, but the density of VSB-1 is twice that of a zeolite and the channel walls are particularly thick. VSB-1 can be prepared in both ammonium and potassium forms, and exhibits ion-exchange properties that lead, for example, to the formation of the lithium and sodium derivatives. Other cations (e.g. Mn, Fe, Co, and Zn) can be substituted for Ni in VSB-1, up to a level as high as 30 atomic%. The parent compound shows canted antiferromagnetic order at Tn = 10.5 K with 6 = —71 K on doping with Fe, Tn increases to 20 K and 6 decreases to —108 K. 

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