Expansion hysteresis upon thermal cycling of Zerodur

Applications requiring ultralow thermal expansivity make use of Zerodur or ULE (type 7971) materials. There are cases in which ULE cannot be considered. Thus, laser gyros cannot tolerate its helium permeability. For these reasons, observations regarding a small instability in Zerodur caused concern. It had been found that, upon thermal cycling, Zerodur dimensions did not return precisely to their former equilibrium values. In another study, it was observed that in addition to this effect there are, near 250 and 450 K, surprisingly large dimensional excursions occurring between thermal equilibria. The manufacturer of Zerodur advised that these instabilities would be reduced if temperature changes were kept always less than the annealing rate. The present investigation is concerned with the results of rapid thermal cycling between 100 and 340 K, and experiments in which the temperature was increased to 480 K. Hysteresis effects near 250 and 450 K could be observed in Zerodur, while such effects were absent in ULE or fused silica.