W.F. Miao

MECHANOCHEMICAL SYNTHESIS OF ULTRAFINE FE POWDER

Ultrafine Fe powders were synthesized by mechanochemical solid‐state reduction of FeCl3 by Na and subsequent removal of the reaction by‐products. X‐ray diffraction, Mossbauer spectroscopy, and transmission electron microscopy measurements showed that Fe particles with a relatively uniform particle size ∼10 nm were formed during the mechanical milling process. The powders exhibited coercivity values of 350–550 Oe, consistent with that expected for separated nanosized particles. This study demonstrates that mechanochemical processing has significant potential for the synthesis of ultrafine metal powders in an economic and efficient way.

High-coercivity ferrite magnets prepared by mechanical alloying

Abstract Nanocrystalline hexaferrite (BaFe 12 O 19 or SrFe 12 O 19 ) and mixed Fe,Co-ferrite ((Fe x Co 1− x )Fe 2 O 4 with x =0–1) materials have been prepared by mechanical alloying and subsequent annealing. High coercivities were obtained in these nanocrystalline materials, 6–7 kOe for hexaferrite and ∼3 kOe for Co-ferrite. Hexaferrite powders prepared by mechanical alloying have been used as the starting material for high-coercivity bonded magnets. Hot-pressed anisotropic hexaferrite magnets have been produced with high values of coercivity and remanence. High magnetic performance was also achieved in some mixed Fe,Co-ferrites after magnetic annealing.

Hexaferrite magnetic materials prepared by mechanical alloying

Abstract The structure and properties of hexaferrites in the form of MFe 12 O 19 with M = Ba, Sr and Pb prepared by mechanical alloying and heat treatment have been studied. Coercivities of 6–7 kOe were measured for Ba- and Sr-hexaferrite powders. The high values of coercivities have been associated with small particle sizes (∼ 0.1 μm) resulting from the mechanical alloying and subsequent heat treatment. High-coercivity anisotropic samples have been synthesized using hot-pressing, with remanences of 70–75% of the saturation magnetisation being obtained.

High coercivity Ba hexaferrite prepared by mechanical alloying

Abstract Ba-hexaferrite has been prepared by mechanical alloying and subsequent heat treatment. Powders were studied by X-ray diffraction, transmission electron microscopy, Mossbauer spectroscopy and magnetic measurements. High coercivities of up to 6 kOe were measured for these fine-particle materials.

High magnetic performance in mechanically alloyed Co‐substituted Fe3O4

Nanocrystalline Co0.5Fe2.5O4 has been prepared by mechanical alloying and subsequent vacuum annealing. Significant increase of coercivity was observed after a postannealing at low temperatures in air. Magnetic annealing resulted in anisotropic sample with a coercivity of 2.8 kOe and a remanence of 81% of the saturation magnetization, providing a theoretical maximum energy product of 4 MG Oe.

Fe3O4Fe magnetic composite synthesized by mechanical alloying

High-energy ball milling (mechanical alloying) has been widely used for the production of amorphous or nanocrystalline materials. Recently, mechanical alloying has been introduced as a method for the preparation of magnetic materials. Ceramic/metal nanocomposites have also received increased attention because of their unique mechanical, electrical and magnetic properties. As is well known, ceramic magnetic materials are not only economic permanent magnets, but are also widely used soft magnets for high frequency applications because of their excellent electrical properties. However, the magnetic properties of ceramic materials (such as the maximum energy product for permanent magnets and the permeability for soft magnets) are far below those obtained in metallic compounds because of their low saturation magnetizations, e.g., the magnetization of {alpha}-Fe s more than twice that of Fe{sub 3}O{sub 4}. In this paper the authors report the results of a study of the structure and magnetic properties of nano-composites of Fe/Fe{sub 3}O{sub 4} synthesized by mechanical alloying and heat treatment.

Mechanical alloying of iron–hematite powders

Abstract The structure and magnetic properties of mechanically alloyed and heat treated x Fe·(1− x )Fe 2 O 3 powders have been investigated. As-milled powders had a nanocrystalline structure with a particle size of 5–10 nm and consisted of Fe 2 O 3 and Fe 3 O 4 for x ≤0.2, Fe 3 O 4 and FeO for x =0.2–0.5 and FeO and Fe for x ≥5.0. Nanocrystalline metastable FeO decomposed into nanocrystalline Fe 3 O 4 and Fe after annealing at 250–400°C and reformed again to submicron FeO after annealing at temperatures above 550°C. Nanocomposites of Fe 3 O 4 /Fe obtained by decomposition after annealing at ∼300°C exhibited high values of magnetisation and coercivity.

Structural evolution of Fe + Fe2O3 during mechanical milling

Abstract The evolution of structure during the mechanical milling of the Fe + Fe 2 O 3 mixture has been studied. Magnetite, Fe 3 O 4 , was initially formed after milling for short times. Nearly single-phase wustite, FeO, was found after milling for 30 h or longer. The kinetics of the phase transformations was studied by X-ray diffraction, Mossbauer spectroscopy and magnetic measurement. Nanocrystalline wustite formed during mechanical milling, decomposed into nanocrystalline Fe 3 O 4 and Fe after annealing at 200–500°C. Annealing at higher temperatures resulted in reformation of the wustite phase.

Magnetic behaviour of mechanically milled (Nd1-xDyx)8Fe88B4

Abstract The structure and magnetic properties of mechanically milled (Nd 1− x Dy x ) 8 Fe 88 B 4 ( x = 0–1) have been investigated. It was found that suitably annealed samples consisted of a nanocrystalline mixture of magnetically soft α-Fe and hard (Nd,Dy) 2 Fe 14 B. Optimally annealed samples exhibited remanence enhancement and single-phase magnetic behaviour, due to exchange interactions between hard and soft phases. Whilst the remanence decreased with increasing Dy concentration, the coercivity increased. Moreover, as the Dy concentration increased, the magnetic behaviour of the two-phase Nd(Dy)FeB alloys became more sensitive to heat treatment conditions.

Remanence-enhanced Nd8Fe87M1B4 (M=Fe, V, Si, Ga, Cr) Alloys

Abstract The effect of additions of V, Si, Ga or Cr on the structure and magnetic properties of mechanically milled Nd 8 Fe 88 B 4 has been investigated. Mechanical milling followed by suitable heat-treatment resulted in the formation of nanocomposites of Nd 2 Fe 14 B and α-Fe. For all compositions, optimally annealed samples exhibited remanence enhancement and single-phase magnetic behaviour, due to exchange interactions between Nd 2 Fe 14 B and α-Fe. However, no significant improvements in either remanence or coercivity were observed as a result of the additions.