Low-expansion glass ceramics

Neoceram, Low-Expansion Glass-Ceramic," Nippon Electric Glass Co. Ltd., 12 (1995). "Neoceram, brochure, Nippon Electric Glass, 1998. 

Pannhorst W, "Low-Expansion Glass-Ceramics—Review of the Glass-Ceramic Ceran and Zerodur and Their Application" pp. 267—76 in Ceramic Transactions, Vol. 30, Nucleation and Crystallization in Liquids and Glasses. Edited by M.C. Weinberg. American Ceramic Society, Westerville, OH, 1993. 

The development of the low-expansion glass ceramics is a commercially very successful part of a much broader effort to understand nucleation and crystal growth phenomena, on the one hand, and to develop products based on the glass ceramic approach, on the other hand. These fields have, therefore, been the topics of many conferences and the accompanying proceeding volumes [1.16-19] as well as of several books [1.20-23]. 

The incentives for new developments in the area of low-expansion glass ceramics are rather low. It seems that the basic understanding of these materials has been achieved so that ideas for further improvements mainly address optimizations of the production processes and of existing products. These ideas are well-kept secrets inside each company and are not communicated to the scientific community. 

In recent years LAS glass ceramics received some new attention by the observation that keatite s.s. [1.38] and h-quartz s.s. [1.39] containing glass ceramics can be formed by the photonucleation process. But it still has to be shown that low-expansion glass ceramics can be obtained via the photonucleation process. 

Because of the unique property achievable with low-expansion glass ceramics this material class attracted most attention at the beginning of the development of glass ceramics but of course many researchers tried to apply the basic ideas of the formation of glass ceramics to other composition fields, and often they were very successful. Nowadays there is no principal reason why the glass ceramic approach could not work in other composition fields, although the specific details have to be worked out for each field separately. 

Low-expansion glass ceramics are widely used as precision parts (see Chap. 4), cooktop panels, stove windows, and cookware. In this section we will focus on the development of transparent glass ceramics used as stove windows (colourless) and cooktop panels (tinted) glass ceramics for cookware are also briefly discussed within this section. 

Depending on the application of low-expansion glass ceramics - we are now focussing on tinted cooktop panels and colourless stove windows - further physical and chemical properties are required from both the production and the user side. To lower production costs, fast and effective processing is required, for example, tank melting, on-line hot forming of panels, as well as fast nucleation rates and crystal growing are absolutely necessary. Moreover, the application of decoration, which, for example, indicates the hot zones of a final cooktop panel, should be performed at the same time as ceramization takes place, without any influence on the production quality. 

Thus, commercial low-expansion glass ceramics contain at least about eight oxides, each one having a particular function. Table 3.2 shows the typical composition range and the major role of the constituents of low-expansion glass ceramics. 

Low-expansion glass ceramics, however, show an excellent thermal shock resistance and a high thermal load capacity. For that reason they are used as table and oven ware, oven plates, or fire resistant glasses. With all these applications, low-expansion glass ceramics are exposed to relatively high temperatures. In most of these cases, compressive stresses due to thermal toughening would be reduced. 

The low compressive stresses obtainable by thermal toughening and the reduction of prestressing in daily use are the reasons why low-expansion glass ceramics are usually not prestressed by thermal toughening. 

Low-expansion glass ceramics have a crystalline as well as a vitreous phase. In this vitreous phase normal ion exchange can take place. But for the chemical strengthening of low-expansion glass ceramics at transformation temperatures above 700 °C, salt melts of potassium nitrate cannot be used, because the salt melts of nitrates decompose at temperatures above 500 °C and produce the toxic NO . Other potassium salts such as sulphates and chlorides and their eutectics have melting temperatures above 700 °C. 

Surface Strengthening of Low-Expansion Glass Ceramics 99 Used time... 

As base glasses for strengthening by surface crystallization, primarily low-expansion glass ceramics are eligible because after ceramization they exhibit a lower thermal expansion than does the base glass. The difference in thermal expansion is between 4 and 5 x 10 units. 

Radiation Programme. The high-quality properties of the low-expansion glass ceramics may be severely attacked by ionizing radiation. Electrons and pro-... 

W. Pannhorst, R. Haug, E. Rodek, K. Stetter Hysteresis effects in low expansion glass-ceramics , J. of Non-Cryst. Sohds 131-133, 488-491 (1991)... 

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