Rare earth permanent magnets and their preparation

Abstract

A sintered magnet body (RaT1bMcBd) coated with a powder mixture of an intermetallic compound R1iM1j, R1xT2yM1z, R1iM1jHk), alloy (M1dM2e)or metal (M1) powder and a rare earth (R2) oxide is diffusion treated. The R2 oxide is partially reduced during the diffusion treatment, so a significant amount of R2 can be introduced near interfaces of primary phase grains within the magnet through the passages in the form of grain boundaries. The coercive force is increased while minimizing a decline of remanence.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application Nos. 2011-102787 and 2011-102789 filed in Japan on May 2, 2011 and May 2, 2011, respectively, the entire contents of which are hereby incorporated by reference.TECHNICAL FIELD[0002]This invention relates to an R—Fe—B permanent magnet having an enhanced coercive force with a minimal decline of remanence, and a method for preparing the same by coating a sintered magnet body with a mixture of an intermetallic compound, alloy or metal powder and a rare earth oxide and heat treating the coated body for diffusion.BACKGROUND ART[0003]By virtue of excellent magnetic properties, Nd—Fe—B permanent magnets find an ever increasing range of application. The recent challenge to the environmental problem has expanded the application range of these magnets from household electric appliances to industrial equipment, electric automobiles and wind power generators....

AI Technical Summary

Benefits of technology

[0064]When a mixture of an oxide containing a rare earth element such as Dy or Tb and an intermetallic compound or metal powder is used for coating, the oxide is partially reduced during subsequent heat treatment. Thus a significant amount of the rare earth element such as Dy or Tb can be introduced near interfaces of primary phase grains within the magnet through the passages in the form of grain boundaries, as compared with the method of effecting heat treatment after coating with a rare earth inorganic compound powder such as fluoride or oxide. As a consequence, the coercive force of the magnet is increased while minimizing a decline of remanence. Additionally, the process is improved in productivity over the prior art processes. The R—Fe—B sintered magnet exhibits excellent magnetic performance, an increased coercive force, and a minimal decline of remanence, despite a minimal amount of Tb or Dy used.

Problems solved by technology

The recent challenge to the environmental problem has expanded the application range of these magnets from household electric appliances to industrial equipment, electric automobiles and wind power generators.

When Dy or Tb is added in an ordinary way, however, a loss of remanence is unavoidable because Dy or Tb substitution occurs not only near the interface of the primary phase, but even in the interior of the primary phase.

Another problem arises in that amounts of expensive Tb and Dy must be used.

The results are still not fully satisfactory.

However, the processes utilizing evaporation or sputtering have many problems associated with units and steps when practiced on a mass scale and suffer from poor productivity.

However, since Dy or Tb diffuses through substitution reaction between the powder and the magnet component, it is difficult to introduce a substantial amount of Dy or Tb into the magnet.

The method is advantageous for introducing a substantial amount of Dy or Tb into the magnet, but less productive because the calcium or calcium hydride powder needs careful handling.

The method of coating with only metal alloy has the drawback that it is difficult to coat the metal alloy onto the magnet surface in a large and uniform coating weight.

However, this method requires many steps as compared with the method of coating a mother alloy magnet with a dispersion of a powder mixture of an intermetallic compound and a rare earth oxide in a solvent.

The method is time consuming and is not industrially useful.