Abstract. Recent observations suggest that the water-related defects associated with the so-called water weakening of single crystals of "wet" synthetic quartz are high-pressure clusters of molecular water. The microstructures which evolve in these crystals during both creep and constant strain-rate experiments and by heating alone were observed by TEM and show that the clusters act as highly efficient sources of the glissile dislocations which must be nucleated before plastic flow can be induced. These microstructural observations, together with simple microdynamical concepts based on the Orowan equation, are used to rationalize the creep behaviour and all the main features of the stressstrain curves observed in "wet" synthetic quartz crystals with a wide range of bulk water-contents, without postulating any direct influence of water on dislocation glide. It is proposed, therefore, that the relatively low yield stress of "wet" synthetic quartz is primarily due to the ease with which fresh glissile dislocations are nucleated, rather than to an enhanced glide of hydrolysed dislocations as is generally assumed in most models of water weakening.

Abstract. The uptake of water in quartz at 1.5 GPa total pressure, 1173 K and high water fugacity, over times up to 24 h, has been investigated using a newly developed assembly to prevent microcracking. It is found that the uptake is small, and below the detectability of the presently used technique of infrared spectroscopy and serial sectioning. This observation reflects either a low value for the diffusivity or the solubility or a combination of both, and is in agreement with the observations of Kronenberg et al. (1986) and Rovetta et al. (1986). It brings into question the interpretation of the early experiments on water weakening by Griggs and Blacic (1964) and the recent estimates of the solubility and diffusivity by Mackwell and Paterson (1985).Results of a combined T.E.M., light-scattering and infrared-spectroscopy investigation of 'wet' synthetic quartz before and after heating at 0.1, 300 and 1500 MPa total pressure and 1173 K, strongly suggest that the water in 'wet' quartz is mainly in the form of H20 in inclusions, consistent with the solubility being low, possibly less than 100 H/106Si. From these observations, water-containing inclusions appear to play a major role in the plasticity of quartz, while any role of water in solid solution remains to be clarified.

The dependence of the thermoelectric power and the electrical conductivity of chromium(III)oxide on the oxygen partial pressure was studied in the temperature range from 1000 to 1800 K. At low oxygen partial pressures n-type behavior is observed, while the material is p-type at high oxygen partial pressures. Both n-and p-type material can be obtained at temperatures as high as 1800 K, which means that up to this temperature chromium(III)oxide is not an intrinsic electronic conductor. The transition from n-to p-type is relatively fast, and proceeds at a considerable rate at temperatures above 1200 K. However a shift from p-to n-type does not take place below 1770 K. Two models for this phenomenon are presented. The present results provide evidence for a defect model for Cr20.~ with Cry as predominating point defects at low oxygen partial pressures and Vr at high oxygen partial pressures. Furthermore, it is concluded that the bandgap is larger than 3.6 eV. The transport properties of chromia scales during high temperature oxidation are discussed in view of these new data on the defect structure of chromium(III)oxide.Protective scales of chromium(III)oxide on high temperature alloys are of considerable technological importance. In order to be able to understand the mechanism of formation of these scales, information on the defect structure of the oxide is needed.Recent studies on the high temperature properties of chromium(III)oxide have demonstrated that, at low oxygen partial pressures, the material is an n-type electronic conductor. Chromium interstitials are thought to be the predominating point defects in the oxide under these conditions (1-5). At high oxygen partial pressures, p-type conductivity has been observed. Presumably chromium vacancies are the predominating point defects here (3)(4)(5)(6).In this study the character of the electronic charge carriers was determined by measuring the thermoelectric power and the electronic conductivity of sintered compacts of Cr203. The thermoelectric power of the sintered material was measured as a function of the oxygen partial pressure in the temperature range from 1400 to 1800 K.

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