Size effect on the crystal phase of cobalt fine particles

Abstract

We have synthesized Co fine particles with the average diameter (D) of less than 500 Å by sputtering Co in a somewhat high inert-gas pressure. It has been found that there is a close relationship between the particle size and the crystal phase; that is, pure fcc ͑␤͒ phase for Dр200 Å, a mixture of hcp ͑␣͒ and ␤ phases for D ϳ300 Å, and ␣ phase with inclusion of a very small amount of ␤ phase for Dу400 Å. Precise structural characterizations have revealed that the ␤ particles are multiply twinned icosahedrons and the ␣ particles are perfect single crystals with external shape of a Wulff polyhedron. In order to explain the size effect on the crystal phase of Co fine particles, we have performed theoretical calculations for total free energies of an ␣ single crystal, a ␤ single crystal, and a multiply twinned ␤ icosahedron. The present calculations well explain the size dependence of the crystal phase of the Co fine particles, and have revealed that the stabilization of ␤ phase, confirmed by previous studies, is the intrinsic effect caused by the small dimensionality of fine particles. Moreover, the phase transformations that occurred in annealing experiments can also be explained by the theory. ͓S0163-1829͑97͒05145-X͔

Size effects in NiTi from density functional theory calculations

“…The authors find either partially or fully suppressed transformation at sizes less than 25 nm and indicate that for spheres of diameter less than 16 nm, the martensite transformation is completely suppressed. In case of FeNi-B alloys, the authors show that the martensite transformation is not completely suppressed but the critical transformation temperature is reduced to as low as 4.2 K. Similar trends have been observed in fine Cobalt powders, 11 polycrystalline samples of low-alloy steels, 12 Au-Cd, 13 and Fe-Ni systems. 14 Size also affects the mechanical response of shape memory materials.…”

C59oNMR study of the allotropic phase transformation in small ferromagnetic cobalt particles

“…The errors quoted refer to the errors deduced from the reproducibility of the fits from data that has undergone processing using the procedure described in the text, starting from the different sets of raw data. der are much smaller than that of bulk Co materials, 5 and ͑2͒ differences in the heat treatments: the measurement of hcp/ fcc ratio in bulk Co in the reference was done in situ using XRD methods, whereas the measurements in Co powder were carried out after cooling from a treatment temperature allowing some Co to recover back to hcp phase ͑fcc heating cooling hcp͒. For the same reason, a small hcp/fcc ratio of ϳ0.9 is found for CC above 550°C.…”

“…4 From XRD, the phase-transition temperature was found to be at 417°C, with considerable hysteresis between 390-430°C. 1 However, Kitakami et al 5 have reported that there is a close relationship between the Co crystallite size and the crystal phase. In general at room temperature, hcp is the dominant phase for Co crystallites with diameter ͑d͒ Ͼ 40 nm, fcc phase for d Յ 20 nm, and a mixture of hcp and fcc phases for d in the range of 20-40 nm.…”

Recent Progresses in Understanding of Co-Based Fischer–Tropsch Catalysis by Means of Transient Kinetic Studies and Theoretical Analysis

“…Large efforts have been devoted to the studies of adsorption of intermediates on hcp Co (0001), which is the thermodynamically most stable surface of the industrial Co catalysts at FTS condition [110][111][112][113]. The chemisorption energies of the most stable configurations for the related surface species on Co (0001) are summarized in Table 1.…”