Aerosol Fe Nanoparticles with the Passivating Oxide Shell

Structure and Magnetic Properties of Aerosol Nanoparticles of Fe and Its Alloys

“…Hysteresis loops allow one to define saturation magnetization (M S ) ( Figure 29) and coercivity H C (Figure 30). A magnitude M S at a bulk alloy is approximately in 2 times above that at nanoparticles with a low Cr content, but as the latter grows, the magnitudes of M S approach, becoming equal in an alloy with composition near Fe 20 Cr 80 . On the contrary, the value H C at particles close to that for Fe 20 Cr 80 foil sharply increases when diminishing Cr content.…”

“…To completely avoid a self-ignition of the deposits upon exposure to air, the aerosol particles were kept in a preparation chamber with the volume of 0.12 m 3 for 12 h in Ar atmosphere at the same pressure. After this procedure the particles were covered with a thin passivating oxide shell allowing further handling of the powder in ambient atmosphere [20]. For example, iron particles in diameter of about 50 nm were aged in air for 20 years without any noticeable change of the oxide content.…”

“…The chamber allowed one to get 9 X-ray patterns of 6 mm in width for one run. A sample temperature was evaluated by displacement of reverse Pt Xray lines with accuracy of up to [15][16][17][18][19][20] • C. In recent works structural characterization of powder samples under study was performed by analyzing XRD patterns recorded with a Siemens D5000 powder diffractometer with Cu K α incident radiation (λ = 1.5406Å) and a graphite 4 International Journal of Inorganic Chemistry monochromator in a θ-2θ Bragg-Brentano geometry. The XRD data were analysed by Rietveld refinements using the MAUD program [25].…”

“…[18,20]. The kinks are seen on the temperature dependencies of H C and M R for sample II, which correspond to Verwey transition in the oxide shell of Fe particles.…”

“…It can be explained by supposition that at high-temperature heating the outward layer of the primary amorphous shell is crystallized in spinel, whereas the metallic core continues to slowly oxidize even in vacuum of 2.7·10 −3 Pa. A newly forming oxide appears to be amorphous and superparamagnetic owing to the low thickness. It is known [18,20] that the SP doublet is lacking in the spectra of pure Fe nanoparticles though an amorphous oxide is likely to constitute no less than 30% of the particle volume. However, the additive of some metals to iron leads to occurrence of oxide doublet in Mössbauer spectra.…”