EP2869311B1

EP2869311B1 - Method of manufacturing fully dense Nd-Fe-B magnets with enhanced coercivity and gradient microstructure

Info

Publication number: EP2869311B1
Application number: EP13005137.8A
Authority: EP
Inventors: Paul John Mcguiness; Marko Soderznik; Kristina Zagar; Kocjan Andraz; Kobe Spomenka
Original assignee: Institut Jozef Stefan
Current assignee: Institut Jozef Stefan
Priority date: 2013-10-29
Filing date: 2013-10-29
Publication date: 2020-06-24
Anticipated expiration: 2033-10-29
Also published as: EP2869311A1

Description

Technical field of the invention

The present invention relates to a method of manufacturing fully dense Nd-Fe-B magnets.
It is well known that Nd-Fe-B magnets exhibit higher coercivity if the grain size is in the range of single-domain particles. In the case of Nd-Fe-B magnets, this is 300 nm or less. However, if heavy rare earths, such as Dy or Tb, are added to the tetragonal 2:14:1 crystal structure, the coercivity can be enhanced 2-3 times, from the theoretical standpoint. This is due to the increase in the anisotropy field of the material that contains some heavy rare earths and the knowledge that Dy₂Fe₁₄B and Tb₂Fe₁₄B have a 2-3 times higher value of the anisotropy field than Nd₂Fe₁₄B. US 2011/0057756 A1 describes a method of manufacturing rare earth composite magnets in which rare earth fluorides like DyF₃ are blended with rare earth magnet powders. The magnets are achieved from the mixture with methods such as sintering, spark plasma sintering, hot pressing and die upsetting and a combination thereof. A similar method is also known from US2012/0181476 A1 which discloses the use of a compacted mixture of finely milled melt-spun Nd-Fe-B particles and dysprosium-n-propoxide for the spark plasma sintering step.
It is an object of the present invention to provide a method of manufacturing fully dense Nd-Fe-B magnets with increased coercivity which requires only a reduced amount of heavy rare earth metals for increasing the coercivity.

Summary of the invention

The object is achieved with the method of claim 1. Advantageous embodiments of the method are subject matter of the dependent claims or can be derived from the subsequent description and preferred embodiment.
In the proposed method of manufacturing Nd-Fe-B magnets, Nd-Fe-B ribbons and a powder containing a heavy rare earth metal, in particular a fluoride, e.g. DyF₃ powder, are provided. The Nd-Fe-B ribbons are produced by the melt spinning technique. The Nd-Fe-B ribbons are mixed with the powder containing the heavy rare earth metal in a ratio selected to achieve a weight % of between 1 and 4 % of the heavy rare earth metal in the mixture. The powder can be added in various ways to the ribbons, including electrophoretic deposition (EPD). The mixture is pressed and spark plasma sintered (SPS) to a fully dense, nanocrystalline Nd-Fe-B magnet. The SPS process makes it possible to produce the fully dense magnet in short times of approx. 1-10 minutes. The advantage of the short densifying times is a limited grain-growth. After this SPS process the densified Nd-Fe-B magnet is annealed to allow the diffusion of the heavy rare earth metal. This annealing step is performed by heating the magnet to an elevated temperature without additional pressing, i.e., this step is different from a die upsetting process in which the geometry of the magnet is changed at elevated temperatures by pressing.
In a preferred embodiment the Nd-Fe-B ribbons are comminuted, e.g., crushed, to reduce their size to approximately 500 µm or lower in a maximum direction of the ribbons before mixing them with the powder containing the heavy rare earth metal. The powder containing the heavy rare earth metal is preferably provided with a particle size of between 1 and 30 µm.
The annealing is preferably performed for a time period of between 1 and 40 h and/or at a temperature of between 500 and 800 °C. An optimum result with respect to the increase in coercivity is achieved by annealing at a temperature of approximately 600 °C for a time period of approximately 20 h.
The proposed method combines the mixing of Nd-Fe-B ribbons and the heavy rare earth compound powder with the process of spark plasma sintering and the diffusion process. The formation process after the mixing is thus divided into two steps. The spark plasma sintering is the first step and the additional annealing is the second step. With the inventive combination of the above steps an increase in the coercivity H_ci up to 30 % can be achieved with only 1 to 4% added heavy rare earth metal. The method also results in a characteristic microstructure having a heavy-rare-earth gradient between the ribbon boundary and the centre of the Nd-Fe-B ribbons.

Brief description of the drawings

The proposed method is described in the following by way of example in connection with the accompanying figures. The figures show:

Fig. 1: an example of the heating regime in the SPS process;
Fig. 2: examples of the change in coercivity of the magnets during the annealing step for different weight proportions of the heavy rare earth metal; and
Fig. 3: a SEM image of a magnet showing an example of a microstructure achieved after the annealing step.

Detailed description of an embodiment

In the following an example of the proposed method is described in which Nd-Fe-B ribbons that were produced by the melt spinning technique are mixed with DyF₃ powder.
Rapidly quenched Nd-Fe-B ribbons without Dy were first crushed to reduce their size to approximately 500 µm in the maximum direction, i.e. in the direction of maximum extension of the ribbons. Such crushed ribbons were then mixed with the DyF₃ powder with a size from 1-30 µm. Different mixtures have been prepared in which the weight % of the Dy in the mixture ranged from 1-5 %. All the starting material was poured into a conductive graphite mould and pressed with 50 MPa. The hot compacting of the powder to the fully dense nanocrystalline Nd-Fe-B magnet was performed with a spark plasma sintering (SPS) device, also known as PECS (Pulsed Electric Current Sintering). This device makes it possible to produce the fully dense magnet, in the following also called SPS magnet, in short times of only 1-10 minutes. The advantage of the short processing times is limited grain-growth. The direct current that is passing through the mould and the material is as high as 1400 A, but the voltage used is only 4 V. The mould and the electrodes are placed in a vacuum of the order of 10 Pa. The heating regime in the SPS process is shown in Figure 1. As can be seen from this figure, the mixture is first heated for 3 minutes to a temperature of about 600°C, then further heated for 1 minute to a temperature of about 700°C and kept at this temperature for 1 further minute before being allowed to cool down.
With this spark plasma sintering as the first process step a coercivity of 2.1 T is achieved.
To allow the diffusion of the Dy from the surface of the ribbons towards the centre of the ribbons, the SPS magnet was further annealed at 600°C for 1-40 h in a furnace with an argon atmosphere. After annealing for 20h at 600° C an enhancement of coercivity of approximately 25 % (2.56 T) could be achieved in this second process step with a proportion of the Dy of 2.2 wt % in the mixture as can be seen from figure 2..This figure shows the change in coercivity during the annealing period for different weight proportions of the heavy rare earth metal in the mixture. The coercivity increases in this example when using weight proportions of the Dy between 1.45 and 3.68 wt%. If the weight proportion of the Dy in the mixture is too high, in this example 4.76 wt%, the coercivity decreases. The proposed method uses less heavy rare earth metals than conventional production methods and gives an equal or higher H_ci. With conventional methods a H_ci of 2.5 T may be achieved with a 3.7 wt % of Dy. With the proposed method in the above example a H_ci of 2.56 T is achieved with 2.2 wt. % of Dy.
The annealing process led to the microstructure shown in the SEM image of figure 3, where DyF₃ is concentrated at the surfaces of the ribbons. After spark plasma sintering the microstructure is mostly composed of Nd-Fe-B ribbons (grey phase) and a white phase consisting of fluorides and oxides. Due to the nature of the production of the Nd-Fe-B ribbons, they have a wheel side and a free side. Fast cooling rates are responsible for the formation of 50-100 nm Nd₂Fe₁₄B grains on the wheel side, while slightly lower cooling rates on the free side caused larger grains (app. 400 nm) which are marked with the arrows in fig. 3. Due to the annealing step a Dy-concentration gradient results from the edge of the ribbons towards the centre of the ribbons. The Dy-concentration in the outer part of the ribbons was found to be up to 6 % on the free side and 3 % on the wheel side, in both cases decreasing towards the centre of the ribbon.
While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, as long as they fall within the scope of the appended claims.
In the claims the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

A method of manufacturing Nd-Fe-B magnets comprising at least the steps of:
- providing Nd-Fe-B ribbons produced by melt spinning technique and a powder containing a heavy rare earth metal,

- mixing the Nd-Fe-B ribbons with the powder containing the heavy rare earth metal such that a mixture having between 1 and 4 weight % of the heavy rare earth metal in the mixture is achieved,

- pressing and spark plasma sintering the mixture to a fully dense nanocrystalline Nd-Fe-B magnet and

- subsequently annealing the Nd-Fe-B magnet to allow the diffusion of the heavy rare earth metal.
The method according to claim 1,
wherein Dy is used as said heavy rare earth metal.
The method according to claim 2,
wherein a DyF₃ powder is provided as said powder containing the heavy rare earth metal.
The method according to any one of claims 1 to 3,
wherein the Nd-Fe-B ribbons are comminuted to reduce their size to approximately 500 µm in a maximum direction of the ribbons before mixing them with said powder containing the heavy rare earth metal.
The method according to any one of claims 1 to 4,
wherein said powder containing the heavy rare earth metal is provided with a particle size of between 1 and 30 µm.
The method according to any one of claims 1 to 5,
wherein the annealing is performed for a time period of between 1 and 40 h.
The method according to any one of claims 1 to 6,
wherein the annealing is performed at a temperature of between 500 and 800°C.
The method according to any one of claims 1 to 6, wherein the annealing is performed at a temperature of 600 °C.