Lithium aluminosilicate

Fig. 4.52. SIMS and IBSCA depth profiles of the altered layer region of a lithium aluminosilicate (LAS) glass ceramic (conditions SkeVAr" ).

Most structural PMCs consist of a relatively soft matrix, such as a thermosetting plastic of polyester, phenolic, or epoxy, sometimes referred to as resin-matrix composites. Some typical polymers used as matrices in PMCs are listed in Table 1.28. The list of metals used in MMCs is much shorter. Aluminum, magnesium, titanium, and iron- and nickel-based alloys are the most common (see Table 1.29). These metals are typically utilized due to their combination of low density and good mechanical properties. Matrix materials for CMCs generally fall into fonr categories glass ceramics like lithium aluminosilicate oxide ceramics like aluminnm oxide (alnmina) and mullite nitride ceramics such as silicon nitride and carbide ceramics such as silicon carbide. 

Figure 5.116 Temperature dependence of bending strength for SiC-reinforced lithium aluminosilicate (LAS) CMC. Reprinted, by permission, from R. W. Davidge and J. J. R. Davies, in Mechanical Testing of Engineering Ceramics at High Temperatures, B. F. Dyson, R. D. Lohr, and R. Morrell, eds., p. 264. Copyright 1989 by Elsevier Science Publishers, Ltd.

Rubidium occurs in lepidolite (lithium aluminosilicate, in amount up to 1% Rb). in certain mineral waters and rare minerals. Rubidium salts may be recovered from the mother liquor upon crystallization of (1) lithium salts, (2) potassium salts. Rubidium metal is obtained by electrolysis of the fused chloride out of contact with air. 

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. 

Table 3.72 Typical compositions of lithium aluminosilicate (LAS) glass-ceramics74...

Kagawa, Y., Kurosawa, N., Kishi, T. (1993), Thermal shock resistance of SiC fibre-reinforced borosilicate glass and lithium aluminosilicate matrix composites , J. Mater. Sci., 28, 735-741. 

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. 

Lithium carbonate is utilized as a starting material for the manufacture of all other lithium compounds and in large quantities in the manufacture of aluminum by melt electrolysis (ca. 25% of the total lithium consumption). Lithium carbonate is also used as a flux in the glass, enamel and ceramic industries, which accounts for a further ca. 25% of lithium consumption. Glasses with high lithium content (on the basis of lithium aluminosilicate) are as a result of their low thermal expansion coefficients virtually fireproof. In psychiatry high purity lithium carbonate is utilized for the treatment of manic-depressive complaints. 

Figure 6.26. A. O MQMAS NMR spectra of albite glass, showing the data presented in both direct double FT and sheared format, with the observed and simulated one-dimensional slices below. From Dirken et al. (1997), by permission of the copyright owner. B. Isotropic projections of the triple-quantum MAS spectrum of sodium aluminosilicate and lithium aluminosilicate glasses. From Lee and Stebbins (2000), by permission of Elsevier Science.

Although not explicitly stated, the discussion so far is only strictly true for isotropic, e.g., cubic, polycrystalline materials. Crystals that are noncubic and consequently are anisotropic in their thermal expansion coefficients behave quite differently. In some cases, a crystal can actually shrink in one direction as it expands in another. When a polycrystal is made up of such crystals, the average thermal expansion can be very small, indeed. Cordierite and lithium-aluminosilicate (LAS) (see Fig. 4.4) are good examples of this class of materials. As discussed in greater detail in Chap. 13, this anisotropy in thermal expansion, which has been exploited to fabricate very low-a materials, can result in the buildup of large thermal residual stresses that can be quite detrimental to the strength and integrity of ceramic parts. 

Within the category of silicate powders there are two different varieties (1) glass and/or glass-ceramics and (2) crystalline. The first variety includes various compositions of glasses and glass-ceramics (e.g., lithium aluminosilicate and magnesium aluminosilicate), and mullite and zircon are typical examples of silicates that belong to the latter variety. 

BMAS - barium-magnesium-aluminosilicate glass ceramic LAS - lithium-aluminosilicate glass ceramic)... 

Varshneya AK, Spinelli IM. High-strength, large-case-depth chemically strengthened lithium aluminosilicate glass. American Ceramic Society Bulletin, 2009 88(5) 27-32. 

Commercial lithium aluminosilicate glass-ceramics provide excellent examples of such behavior. The initial glass used for production of transparent cookware, for example, has a thermal expansion coefficient of 4 ppm K , Tg 730 °C, and T = 760 °C. After processing, the thermal expansion coefficient is == 0.5 ppm K and Tg and T can no longer be detected on an expansion curve below 1000 °C. Heat treatment results in the formation of a lithium aluminosilicate crystal which has a very low thermal expansion coefficient. Removal of lithium from the residual glassy phase also decreases the thermal expansion coefficient of that phase, while simultaneously increasing the transformation and softening temperatures. 

C. E. Lord, in Crystallization and Properties of Lithium Aluminosilicate Glass-Ceramics, M.S. Thesis, Alfred University, 1995. 

Lithium-aluminosilicate glass ceramics Supports for telescope mirrors... 

Glass-ceramics, such as the lithium-aluminosilicates (LAS), can be designed to have very low, in fact near zero, as over a wide temperature range making them particularly suitable as supports for telescope mirrors. Tele-... 

Shelby, J. E. (1978) Viscosity and Thermal Expansion of Lithium Aluminosilicate,... 

Lapp, J. S. and Shelby, J. E. (1987) The Mixed Alkali Effect in Lithium Aluminosilicate Glasses, J. Non-Cryst. Solids, 95-96, 889-896. 

---