Fine annealing

AH intrinsic germanium metal sold is specified to be N-type with a resistivity of at least 40 H-cm at 25°C or 50 H-cm at 20°C. Germanium metal prepared for use in infrared optics is usuaHy specified to be N-type with a resistivity of 4-40 Hem, to be stress-free and fine annealed, and to have certain minimum transmission (or maximum absorption) characteristics in the 3—5 or 8—12 pm wavelength ranges. Either polycrystaHine or single-crystal material is specified. 

There are, of course, applications for which processing steps have to be performed in the temperature interval between 130 and 320 °C and for which a final fine annealing from 320 °C is not possible e.g., when coatings are not stable at this temperature. For such applications, which at the same time require very precise optical figures, a new Zerodur variant, Zerodur M , has been developed [4.31]. Its composition is free of MgO and does not show any hysteresis effect between 320 °C and room temperature within the accuracy of current measurement techniques (see Fig. 4.10) [4.32,33]. The expansion characteristic of Zerodur M is comparable to that of Zerodur , (see Fig. 4.11), thus also guaranteeing very stable optical figures in precision optical applications. 

Second fine annealing according to the rate cooling process, a certain temperature distribution is established in the blank until the solidification temperature is reached. It determines the permanent stress (cr) after annealing at room temperature ... 

During machining the stress symmetry is disturbed, and it must subsequently be re-established by fine annealing lasting about a inonth. 

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