Residual glass phase

TRANSFORMED -8l3N4 GRAINS WITH RESIDUAL GLASS PHASE AT 20 C... 

FIGURE 13. HRTEM image reveals the existence of residual glass phase at grain boundary and triple junction area. (W p—Si3N4 whisker) (Reprinted with permission ofThe American Ceramic Society, www.ceramics.org. 

Glass ceramics are polycrystalline solids formed from suitable parent glasses by a controlled heat treatment, the so-called ceramization. The glass ceramics consist of varying crystalline phases and a residual glass phase, depending on the base glass composition. 

Compared with technical glasses the electrical resistivity of Zerodur is rather poor. The temperature for a resistivity of 10 Qcm is 178 °C. The dielectric constant e and the loss factor tan 5 at 1 MHz are 7.4 and 0.015, respectively. These data are not surprising. /3-eucryptite, a crystalline material with the special h-quartz s.s. composition Li20 AI2O3 2Si02, is known to be a good one-dimensional ion conductor [4.21], and due to the structure of h-quartz also other solid solution compositions are expected to behave similarly. So, neither the residual glass phase nor the crystalline phase are expected to block ionic conductivity effectively. 

During the development of Zerodur for the MPI telescope mirror blank the question was raised whether Zerodur shows any effect of thermal ageing, which is common to many glasses. Petzoldt [4.23] performed some investigations but did not observe any effect. He concluded that one reason for that could be the relatively small amount of 30 vol% of the residual glass phase. But when Zerodur was tested for other applications, a sensitivity of the length of Zerodur to thermal history was observed. 

The surface roughness at the bottom of a structure is significantly higher than at the side walls, which is caused by the etching attack on the crystals and the reduced chemical attack on the residual glass phase which results in the formation of a porous layer of the residual glassy phase. An example is shown in Fig. 9.11 (right). 

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