Problems of Sintering Metallic Ultrafine Powders

Abstract

It seems that ultrafine media (UFM), i.e., the macroscopic ensembles of small particles (SP) with size of about 10 nm, have been systematically studied only for the last twenty years. The results obtained for these objects testify to the fact that their physical properties differ essentially, sometimes qualitatively, from the properties of the same materials in the bulk state I. This distinction is caused by the dominant role of the surface which noticeably contributes to the net balance of the system free energy at these sizes. This contribution becomes comparable with that of the volume, the reconstruction of the surface atomic structure which initiates the drastic change in the atomic structure type of a whole SP, the rebuilding of elementary excitation spectra in SPs. In addition, the typical lengths of most of the transfer processes (electroconductivity, thermoconductivity, galvanomagnetic effects, etc.) become comparable with the particle sizes in this particular range of dimensions, which brings to life interesting „size“ anomalies in transport processes. First of all, these processes are structure sensitive, while the defect types and their densities are specific for UFM. Dislocations were observed in UFM neither by the electron microscopy, nor by the positron annihilation method. It is of no surprise, since the sizes of the dislocation loops for typical metals are one order-of-magnitude larger than the particle size. In contrast, SPs are liable to twinning and reveal different stacking faults. This fact is important, since most of the processes analogous to recrystallization and sintering develop in the foreground of the initial defect structure present in the system. In particular, this implies that the primary recrystallization is not typical of UFMs.