Metallic nickel colloids in plastically deformed nickel-doped MgO crystals

Abstract

Heat treatment of nickel-doped MgO crystals in a reducing atmosphere produces metallic nickel colloids. The presence of these colloids is characterized by two very broad optical absorption bands centring at about 2·2 and 5·0eV. The threshold temperature for the nickel colloid formation in chemically polished (dislocation-free) crystals was determined to be 1440·30K. A lower threshold temperature (subthreshold) for colloid production in mechanically deformed crystals was found to be about 1320K. Optical and transmission electron microscopy showed that regions of high dislocation density were decorated with the colloids. These results indicate that dislocations and/or their debris serve as nucleating sites for metallic colloids. These colloids are stable even at 1300 K in oxygen. The simple method of producing metallic colloids and the high thermal stability of the colloids, as demonstrated in this study, lend themselves to many useful applications such as solar absorbers.