Synthesis and thermal properties of ultrafine powders of iron group metals

From the viewpoint of commercial production, the synthesis of ultrafine powders (UFPs) of iron group metals has been studied experimentally in detail by using activated plasma evaporation of the metals in a hydrogen and argon gas mixture. Different experimental parameters, including hydrogen content, are current and gas pressure, were investigated for their effects on the evaporation rate of the metals. It should be noted that Co UFP, unlike Fe UFP and Ni UFP, consists of two phases: alpha-Co (h.c.p.) and beta-Co (f.c.c.). The fraction of the former phase in Co UFP ranges from 10% to 25% as the average grain size decreases from 24 to 14 nm. The thermal stability of metal UFPs upon heating in a stream of helium can be understood by powder densification via intergrain sliding and grain growth. Besides this, phase transitions from ferromagnetism to paramagnetism were clearly observed in the differential thermal analysis (DTA) curves of Fe UFP (at about 755 degrees C) and Ni UFP (at about 355 degrees C), indicating that there is a decrease of several degrees in the Curie point with respect to their bulk counterparts. In addition, a structural transition of alpha-Fe to gamma-Fe occurred at 901-908 degrees C, a little lower than the bulk value (912 degrees C). The oxidation behavior of metal UFPs was attributed to oxidation reactions of metal grains, followed by grain growth and sintering of powder. Moreover, Co UFP exhibits one endothermic peak at 882-930 degrees C which was identified as due to the thermal decomposition of cobalt oxide formed in the oxidation reaction at low temperatures.