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The lengths and the thermal expansion values of the glass ceramic Zerodur show a reversible dependence on the thermal history within two temperature intervals. Typical effects associated with this dependence are, for example, isothermal length changes within and permanent length changes below the temperature intervals. It is assumed that relaxation causes the observed effects. The phenomena in the upper temperature range from 1300C to 300 0 C are related to the MgO content. The reversibility of the relaxation effects allows adjustment for lengths and thermal expansion values by appropriate cooling processes.

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