Formation of core–shell and hollow nanospheres through the nanoscale melt-solidificationeffect in the Sm–Fe(Ta)–N system

Abstract

Sm–Fe–Ta–N–O nanospheres were synthesized by pulsed-laser deposition from aSm13.8Fe82.2Ta4.0 target in a nitrogen atmosphere. Three structurally and compositionallydistinct types were identified: amorphous, core–shell and hollow nanospheres.Amorphous spheres were compositionally homogeneous and completely oxidized.The core–shell spheres were composed of an iron-rich crystalline core with upto 10 at.% interstitially incorporated nitrogen, surrounded by an amorphousand oxidized shell. The hollow spheres were characterized by voids filled withN2 gas.It was found that the formation of either amorphous or complex nanospheres is defined by an initialFe/Sm ratio within the molten droplet. The formation of hollow spheres is believed to be relatedto the general affinity of liquid metals for gas intake. During rapid solidification thedissolved gas in the melt is trapped within the surrounding solid rim, preventing theoutwards diffusion of gas. As long as the amount of gas atoms in the melt is keptbelow its solubility limits it can be completely interstitially incorporated into thesolid, thus forming crystalline Fe(N)-rich cores. If the melt contains more than anequilibrium amount of nitrogen it is possible that the gas recombines to formN2 molecules, which are condensed inside the spheres.