Spodumene Solid Solution Glass-Ceramics

The structure of P-spodumene is described in Section 1.2.1 in connection with the structurally related thermal properties. The solid solution P-spodumene ranges between n = 4 and n = 10 for Li20 Al203 Si02. The effect of the increasing Si02 content in the solid solution on the thermal expansion is shown in Section 1.3.1. 

As in the case of P-quartz, the Li ion can also be replaced by Mg in P-spodumene, a stuffed derivative of keatite. The degree of substitution, however, is lower than that of P-quartz. 

The transformation of P-quartz to P-spodumene is possible between 900 C and 1000°C. The irreversible procedure always entails the enlargement of the crystallites. As a result, the optical properties are often affected and the transmissibility decreases. Furthermore, if T1O2 is used as a nucleating agent, crystalline Ti02 forms as rutile. This crystal phase is known to demonstrate a high refractive index, which gives the material an opaque appearance. 

The microstructure of a typical P-spodumene solid solution glass-ceramic in the form of Corning Ware is presented in Fig. 2-6. The width of the main crystal phase of P-spodumene solid solution measures 1-2 pm. 

The compositions of commercial P-spodumene solid solution glass-ceramics are shown in Table 2-8. The composition of Corning Ware differs from that of Cercor in that it contains a number of additions that Cercor does not contain. 

---

Commercial B-Spodumene Solid Solution Glass-Ceramics (wt%)... 

Beall (1992) attributes a great deal of importance to the glass matrix representing the cellular membrane in the solid-state reactions of P-quartz solid solution and P-spodumene solid solution glass-ceramics, the tenacious residual... 

Other P-spodumene solid solution glass-ceramics were developed under the trademarks Cer-Vit (Owens-Illinois) and Hercuvit (PPG) in the United States and in the United Kingdom. The latter was used to produce cooking surfaces (Pincus 1971). 

In Japan in 1962, P-spodumene solid-solution glass-ceramics were also produced on the basis of developments achieved by Tashiro and Wada (1963). The products... 

Properties of B-Spodumene Solid-Solution Glass-Ceramic Neoceram N-11 and the B-Quartz Solid-Solution Glass-Ceramics Neoceram N-0, Ceran , and Robax ... 

Also in Japan in 1962, a white nontransparent / -spodumene solid-solution glass ceramic was produced. Nippon Electric Glass manufactured cooktop panels imder the trademark Neoceram N-11. With a coefficient of thermal expansion of approximately 1.3 x 10 they were used for induction... 

Figure 1-37 TEM image showing crystaiiized Si02-Al2 37 " 2 2 glass, heat treated to 1000 C for 45 min. p-spodumene solid solution was formed in the glass-ceramic (bar = 1 pm).

In addition to Neoceram N-0, Nippon Electric Glass Co., Ltd., also produces another glass-ceramic that demonstrates zero expansion. Neoceram N-11 is a white, nontransparent glass-ceramic that contains P-spodumene solid solution primary crystals measuring approximately 1 pm. This glass-ceramic is used for induction cooker top plates, Kitchenware, (Section 4.2.1) or for optical components, such as the coupler housings of optical fibers (Neoceram 1992, 1995). 

In many cases, more than one crystalline phase forms. Often, phases that form initially transform into another phase as heat treatment progresses. For example, crystalline phases of /5-quartz solid solutions (e.g., (S-eucryptite), which are not stable at high temperatures, convert to more stable phases (e.g., 6-spodumene, cristobalite, sapphirine, or lithium disilicate). The stable high temperature phase may have different properties— in this case, higher thermal expansion. Therefore, glass ceramics with the same chemical composition may have very different properties, depending on the heat treatment. 

Findings from the theories of nucleation have also contributed to the optimization of these comprehensive experimental investigations. As a result, glass-ceramics with improved properties have been produced, specifically glass-ceramics with specific solid-solution limits like j3-spodumene and stuffed P-quartz or with stoichiometric compositions like cordierite and lithium disilicate. Initial results for the successfU application of the theory of nucleation to multicomponent glass-ceramics with a nonstoichiometric composition were achieved with mica and anosovite glass-ceramics. 

This type of cellular membrane microstructure can be developed in glass-ceramics that exhibit a very low coefficient of thermal expansion because of the precipitation of p-quartz solid-solution or P-spodumene crystals (Beall 1992). The glass matrix, which is very thin and surrounds the crystals, plays the part of the cellular membrane. As a result, the crystals are separated from each other. The membrane accounts for approximately 10 vol% of the micro-structure and can, for example, act as a diffusion barrier between the crystals. 

---