Physical properties of oxides containing metal precipitates

Abstract

The optical and mechanical properties of MgO single crystals containing coherent metal precipitates or colloids have been investigated. The optical absorption spectra of these materials were characterized by broad optical bands centred near 2·2 and 5·0eV. By changing the size distribution of the precipitates, it was possible to obtain optical spectra of these materials suitable for solar selective absorber applications. Dislocations and sub-boundaries were found to provide nucleation sites for the formation of precipitates. The presence of these extended defects decreased the minimum temperature at which precipitates can be formed. The precipitates increased yield stress by a factor of about 4, and resulted in an increase in the room-temperature ductility of about 50°. Microstructures of deformed specimens revealed anchoring or pinning of mobile dislocations by precipitates as the probable cause for the increase in yield stress. The precipitates also enhanced ductility by providing dislocation sources, including punching out of dislocation loops. The coherent precipitates were found to transform into incoherent ones on deformation or laser irradiation. In situ fracture studies have shown the role of precipitates in preventing the propagation of cracks. By providing dislocation sources, the precipitates inhibit the propagation of cracks.