Water milling and gas passivation method for production of corrosion resistant Nd-Fe-B-N/C powder and magnets

Abstract

Nd-Fe-B powder produced by conventional methods is pyrophoric, and exhibits poor corrosion resistance. Magnets made from powder are also susceptible to corrosion. Conventional methods of production are complicated, potentially hazardous, and relatively expensive. A novel, low cost, less hazardous method of producing powder and magnets with high corrosion resistance and Curie point is discussed. Nd-Fe-B alloys are milled in water, vacuum dried, and passivated at a suitable temperature in a nitrogen or carbon dioxide medium. During passivation, a protective layer, comprised of nitride and/or carbide phases, is formed in the surface region of the powder particles. This powder is not pyrophoric, and may be stored in a laboratory air environment for an extended period of time with no loss in magnetic properties. Compacted and sintered magnets produced from this powder are also highly corrosion resistant, and exhibit a higher Curie point compared to conventionally produced magnets. A description of the novel technology, and a discussion of the properties of Nd-Fe-B-C/N powder and magnets is given.

Passivation agents and conditions for Mo2C and W2C: Effect on catalytic activity for toluene hydrogenation

“…However, the adsorption is partially reversible and the subsequent exposure of the surface to 16% O 2 at 40 °C results in a rapid weight gain, indicating that the samples are not passivated (Figure 3b). This result is in conflict with reports in the literature that H 2 O and CO 2 can passivate Mo 2 C at room temperature, and that CO 2 can passivate mixed nitrides or carbides at temperatures below 400 °C [22,23]. TP reaction experiments (Figure 4a show that significant weight gain and reaction with the surface of the carbide begin at about 580 °C with the production of CO and presumably oxidation of the carbide surface, Figure 4b.…”

“…The conclusions were based on IR spectra of adsorbed carbon monoxide, which was used as a probe molecule to identify changes to the molybdenum oxidation state and the number of surface sites that could be regenerated after passivation. Bogatin et al, who investigated the magnetic properties of neodymium-iron-boron nitrides and carbides, reported passivating their powdered carbides with CO 2 or N 2 at temperatures between 125 and 300 °C [23]. The passivation resulted in surface neodymium carbides and nitrides that stabilized the material although neodymium oxides were observed after storage.…”

Permanent magnets based on R-Fe-B and R-Fe-C alloys

“…For example in Nd 29 Dy 4 Fe bal Co 5 B 1.15 (wt%) without and with Nb, i H c values increase rapidly with increasing O 2 content up to 0.4 wt% and these start to decrease with a further increase in the oxygen-figure 35 (Kim and Camp 1995). Bogatin et al (1991) after milling the Nd-Fe-B alloys in water and vacuum drying, passivated the powders at a suitable temperature in a nitrogen or carbon dioxide medium. During passivation, a protective layer, composed of nitride and/or carbide phase is formed in the surface region of the powder particles.…”

“…In order to avoid the oxidation, soda-silica glass coatings by the sol-gel method of the powders for plastic-bonded magnets were performed (Cavallotti et al 1992). Bogatin et al (1991) used a method in which Nd-Fe-B alloys were passivated in a nitrogen or carbon dioxide medium. During passivation, a protective layer, comprised of nitride and/or carbide phases, is formed on the surface region of the powder particles.…”