EP0499600B1 - Sintered permanent magnet or permanent magnetic material and process for production thereof - Google Patents

Abstract

The invention relates to a sintered permanent magnet (material) and a process for its production using a magnetic phase of the type Se2(FeCO)14B and at least one further sinter-active or particle-combining phase. In order to achieve a high saturation magnetisation, high coercive force and high energy product with good temperature stability and high Curie point of the permanent magnet (material), it is proposed according to the invention that the magnetic phase is formed of diffusion-moulded particles, which are decreased in their surface energy, having a diameter of at most 60 mu m, the magnetic phase has contents of Co and heavy rare earths (HRE) in a certain ratio to one another, the HRE concentration being inhomogeneous over the particle cross-section and the particle-binding phase having a higher activity of the HRE at the diffusion temperature compared to the magnetic phase.

Description

The invention relates to a sintered permanent magnet or material consisting essentially of a magnetic phase of the type SE₂ (FeCo) ₁₄B and at least one further sinter-active or grain-connecting phase which contains heavy rare earths and / or compounds.
Furthermore, the invention relates to a method for producing rare earth-containing permanent magnet (s) (- materials), wherein at least the magnetic phase of the type SE₂ (FeCo) ₁₄B forming or containing constituent is produced by melt metallurgy and then pulverized, whereupon the powder is pressed with additives containing heavy rare earths in the magnetic field and then sintered to form a magnetizable raw body and optionally heat-treated.

Permanent magnets or permanent magnet materials made essentially of an alloy of iron (Fe), boron (B) and rare earths (SE) in the sintering process are preferably used when high coercive force, high remanence and / or large energy product are required. The constituent which forms or contains the magnetic phase of the Se₂Fe₁₄B type is produced by melt metallurgy and pulverized, which powder, if appropriate mixed with additives, is pressed into a green compact in the magnetic field and this is sintered, the sintered body optionally being subjected to at least one further heat treatment.

EP-B1-0126802 discloses sintered permanent magnets of the Fe-BR type (R means at least one SE element including Y), in which Fe can be partially replaced by Co. The elements are homogeneously distributed in the magnetic phase due to the manufacturing process used, and a heat or aging treatment of the sintered body is said to improve the magnetic values. If Fe is partially replaced by Co, this increases the Curie point or the Curie temperature (T _c ) of the magnetic material, but its coercive force, as is known to the person skilled in the art, however decreases with increasing Co content, whereby the energy product can also be adversely affected.

In order to create permanent magnets with improved magnetic properties at room temperature, it is proposed according to EP-B1-101552 to use a Co-free alloy containing Fe-B-.R, which contains at least one stable compound of the ternary system Fe-BR, wherein R represents at least one rare earth element including yttrium. The main magnetic phase must be an intermetallic compound with a constant composition, which requires a homogeneous distribution of the alloy elements. In addition to the great expenditure on alloy technology in the production of the starting alloy and the strong scatter of the magnetic values of the sintered magnetic material, this shows a significant decrease in the magnetic characteristics with increasing temperature in the range from room temperature to 200 ° C, the Curie point already at about 300 ° C is reached.

Furthermore, from EP-A1-256006 a process for the production of sintered permanent magnets is known, in which stoichiometrically composed crystalline RE₂ (FeCo) ₁₄B material (RE means rare earths) is ground with another material, this other material during the heat treatment or in the sintering process forms a second non-magnetic liquid phase on the surface of the grains made of RE₂ (FeCo) ₁₄B. This is intended to ensure that the exact chemical composition can be set independently of the second phase, which can have special melting properties and / or compositions, with homogeneous distribution of all elements of the magnetic phase in the magnetic material. In this embodiment, however, there is a disadvantage in the high expenditure on alloy technology and the poor reproducibility of the magnetic material data.

From EP-A-395625 a sintered SE-Fe-B permanent material and a method for producing the same have become known, in which an element from the group of SSE and / or as an alloy additive at the grain boundaries a compound of an element from the group is optionally incorporated together with other grain boundary alloy additives.

The invention has for its object to eliminate the disadvantages of the known SE-containing magnets (materials) and their manufacturing processes and to create sintered permanent magnets that have high saturation magnetization, high coercive force and large energy product with good temperature stability and high Curie point. It is also the object of the invention to provide a new and improved manufacturing method for magnets, with which high magnetic characteristics can be achieved and their scatter can be reduced.

This object is achieved in a generic subject by the characterizing features of claim 1. Advantageous further developments are characterized in the subclaims. A method of the type mentioned is characterized according to the invention in claims 4 to 12.

With the permanent magnet or material according to the invention and due to the procedure according to the invention for producing the same, a number of advantages are achieved synergistically, disadvantageous interactions of individual measures being largely suppressed and the totality of the magnetic properties being significantly increased. The scientific basis and causes of these combination effects have not yet been fully clarified; however, these are essentially physico-chemical effects in connection with magnetokinetics.

According to the invention, grains of the magnetic phase are surface-smoothed or diffusion molded and have a diameter of at most 60 μm, but at least 3 μm. Grain surfaces designed in this way are energetically dependent, at least make it difficult to form and / or shift the domain walls, which generally improves the coercive force values. A corresponding grain size of the magnetic phase is of great importance because, as has been found, grain diameters of greater than 60 μm and less than 3 μm lead to a decrease in the coercive force or the magnetic induction to lead.

A special feature of the new permanent magnet (material) according to the invention is a partial replacement of iron (Fe) by cobalt (Co) in the magnetic phase formed with boron (B) and light rare earths (LSE) and heavy rare earths (SSE) , where the average SSE content is set at a certain value depending on the concentration value of Co. It is known that Co contents cause a slight increase in magnetization and an increase in the Curie point, but the coercive force or magnetic induction is reduced, which leads to a lower energy product (BH _max ) of the magnet and thus to a deterioration in all of the magnetic properties. These effects can be explained by the fact that Co atoms in the tetragonal crystal structure at room temperature cause a reorientation of the magnetic moments in the direction of the base plane and that the uni-axial magnetocrystalline anisotropy is deteriorated or the anisotropy field strength decreases. Completely surprisingly, it has been found that these disadvantages of a co-replacement can be eliminated or minimized if SSE is present in a specific concentration which is dependent on the Co content and the remaining rare earths (SE) - part of the hard magnetic phase is formed by LSE. This could be related to the fact that the magnetic moments of LSE, in particular the advantageously usable elements neodymium (Nd) and praseodymium (Pr), are aligned parallel to Fe or ferromagnetic and the SSE has an antiparallel direction to Fe or an antiferromagnetic direction of its have magnetic moments.

SSE Dyprosium (Dy) has been shown to be particularly effective and advantageous because, among other things, the anisotropy field strength increases sharply due to the antiferromagnetic coupling. However, it is important that the SSE content be at least 0.05 times the weight of Co, because lower concentrations cause a reduction in the coercive force. SSE contents higher than 0.2 times the weight of Co lead to a decrease in the saturation magnetization.

If, according to the invention, the local concentration of SSE atoms is inhomogeneous across the diameter of the grains, in particular increasing towards the surface-smoothed grain boundary, domain wall formation and / or domain wall displacement is further reduced, as a result of which the coercive force and the result of the energy product are further increased . An at least 3 times higher concentration of SSE atoms in a range of at most 1 µm at the grain boundary has proven to be particularly effective.

Another particularly important characteristic of the new permanent magnet according to the invention is a higher SSE content than the hard magnetic phase and / or a higher SSE activity at the diffusion temperature of the sinter-active or grain-connecting, essentially paramagnetic phase. Good magnetic values are preferably obtained if the SE concentrations of this grain-connecting phase are at least 25% and their SSE concentrations are at least 90% greater than those of the magnetic phase on average.

In the process according to the invention, the constituent which forms or contains the hard magnetic phase of the type SE₂ (FeCo) ₁₄B is produced by melting and casting an alloy containing in AT% 8 to 30 rare earths (SE), 2 to 28 B, remainder Fe and Co, if necessary, further alloying elements and impurities in which Co is adjusted with a concentration of 3 to 25 at.%, preferably 6 to 20 at.%, in particular 8 to 14 at.%, and the RE content consists of light rare earths ( LSE) and heavy rare earths (SSE) are formed, manufactured and ground into powders with a grain size of at least 3 μm, but, as is known per se, less than 60 μm, preferably less than 45 μm, in particular less than 30 μm. This powder contains one or more additive (s) containing rare earths (SE) with an SE concentration which is at least 25%, preferably at least 35%, in particular at least 80% greater than the powder grains and one at least 100%, preferably 150%, in particular 200%, greater SSE concentration introduced and distributed homogeneously. In order to bring the surface of the powder grains and the additives into good contact with one another, it is necessary for solid additives whose particle diameter is less than 5 μm or less than 15% of the Provide diameter of the powder grains and, if necessary, a further grinding process. The additives can also be introduced into the powder in liquid form, for example as SE compounds.

In the starting material of the powder, the SS content as a function of the Co content is adjusted in a range from 0.02 to 0.19 times the Co content by the melt metallurgical route. The SSE content of the additive is intended to be at least 100% greater than that of the powder.

A green compact is pressed from the material formed from powder with the additives, which is preferably sintered in a vacuum or, if appropriate, in a protective gas atmosphere at high temperature. The additives become at least partially liquid or pasty, essentially envelop the grains and act as a sinter-active or grain-binding agent which largely fills the edges and fissures in and between the grains. For this it is important that the sintering temperature is selected for a short time to such an extent that the sintering agent is given a sufficient degree of liquid to in particular fill or envelop the fissures and sharp-edged concave cavities of the grain surfaces.

After the sintering process, the sintered body is subjected to a diffusion treatment or diffusion annealing at a temperature below the sintering temperature at a temperature between 600 and 100 ° C. and for a period of 1 to 12 hours. The sinter-active or grain-connecting phase or mass has a sufficient degree of strength for shape stabilization. With a Diffusion treatment of the sintered body, which can be connected directly to the sintering, achieves surface structures and concentration profiles of atoms which are advantageous for the grains for the magnetic properties. The surfaces of the grains forming or containing the hard magnetic phase, which are made sharp-edged by the comminution process, are smoothed because the edges or tips represent energy irregularities and an increased atom diffusion takes place in these areas. A shaping of the grains or a largely directed atom diffusion brings about a reduction or minimization of their surface energy. Due to smoothed surfaces with reduced energy of the grains from the hard magnetic phase, based on the change in direction of the magnetic moments, a new formation of domain walls, which preferably occurs at the tips and edges, is effectively reduced and thus the coercive force of the magnets is increased. However, a certain grain size specified above and a sufficient filling, in particular the fissures and sharp-edged concave cavities of the grain surfaces with sinter-active mass or phase, are important.

Due to the set concentration difference of SSE atoms in the hard magnetic phase and the grain-connecting, largely paramagnetic phase, SSE atoms also penetrate into the magnetocrystalline phase during the diffusion treatment. Because in the case of elements diffused in, such as AL, for example, a rapid, essentially immediate, concentration equalization takes place, it was surprising that SSE atoms at the grain boundaries or in the region near the grain boundaries can be enriched to at least 3 times the content compared to the grain interior and one inhomogeneous concentration of SSE atoms can be formed in the grains. It is important to choose the diffusion treatment parameters in such a way that the strength of the area of the increased SSE concentration is set to at least 0.05 μm, but at most 1 μm. Smaller strengths only cause an insignificant further reduction in the formation of the domain wall and / or domain wall mobility, thus a slight increase in coercive force; greater strengths reduce the achievable saturation magnetization and reduce the energy product of the permanent magnet.

The invention is explained in more detail below with the aid of the accompanying tables 1 and 2, in which alloy contents and mean values of magnetic measurements of permanent magnet bodies are given.

Table 1 shows the magnetic values of reference magnets (materials) with different compositions. The respective starting material was produced by melt metallurgy and ground into powder. Under the influence of a magnetic field, the powder was pressed into a green body, which was sintered, heat-treated and magnetized. The composition and the measured magnetic values of the permanent magnet bodies (comparison magnets) are given under the designations A to F in Table 1.

The permanent magnets (materials) according to the invention are listed under numbers 1 to 14 in Table 2. The analytical determinations were carried out by transmission electron microscopy (TEM).
The SSE content in the magnetic phase on average was determined by averaging from point and area measurements over the grain cross section.

In the permanent magnets according to the invention, the Co content and the magnetic field strength or the magnetization are increased by the Co content and, as has been shown, the coercive force or induction is kept at high values as a result of the further measures, which synergistically brings about an increased energy product . With conventional magnets, largely without Co content, high coercive forces become low at low Curie temperatures and at high Co content high magnetization achieved at high Curie temperatures. However, the magnetic energy product is relatively low in both cases.

Claims (12)

1. Sintered permanent magnet or permanent magnet material consisting substantially of a magnetic phase of the type RE₂(FeCo)₁₄B and at least one further sinter-activated or grain-bonding phase which contains heavy rare earths, HRE, and/or compounds, characterised in that the magnetic phase is formed from grains with a smooth surface or shaped by diffusion with a minimum diameter of 3 µm and a maximum diameter, which is known per se, of 60 µm, preferably 45 µm, especially 30 µm, the magnetic phase or the grains formed from this phase having cobalt, Co, with a concentration of 3 to 25% by atom, preferably 6 to 20% by atom, especially 8 to 14% by atom, and rare earths, RE, with proportions of light rare earths, LRE, and heavy rare earths, HRE, wherein the average content of HRE is equal to 0.05 to 0.2, preferably 0.06 to 0.15, especially 0.1, multiplied by the concentration value of Co, and the local concentration of HRE atoms over the diameter of the grains, as is known per se, rises inhomogeneously in the direction of the grain boundary, preferably rising over-proportionally, and the sinter-activated or grain-bonding phase(s) which optionally contain(s) occlusions of metals and/or metal compounds has (have) relative to the magnetic phase or the grains a higher content of RE with a RE concentration value which is greater on average by at least 25%, preferably by at least 35%, especially by at least 80%, and a HRE concentration value which is greater by at least 90%, preferably 140%, especially at least 190%.
2. Permanent magnet or permanent magnet material according to Claim 1, characterised in that the concentration of HRE atoms on or in the region of the grain boundaries has a value which amounts to at least 3 times, preferably at least 4.5 times, especially at least 6 times the value in the interior of the grain.
3. Permanent magnet or permanent magnet material according to one of Claims 1 or 2, characterised in that the region with a higher concentration of HRE atoms on the grain boundaries has a thickness of 0.05 to 1 µm, preferably 0.09 to 0.9 µm, especially 0.2 to 0.4 µm.
4. Method of manufacturing permanent magnets or permanent magnet materials containing rare earths (RE), in which at least the component forming or containing the magnetic phase of the type RE₂(FeCo)₁₄B is produced by fusion metallurgy and then pulverised, whereupon the powder is compressed with additives containing heavy rare earths, HRE, in a magnetic field and then sintered to form a magnetisable blank and optionally heat-treated, characterised in that the component forming or containing the magnetic phase is produced by melting and casting an alloy containing in % by atom 8 to 30 rare earth, RE, 2 to 8 B, the rest Fe and Co, optionally further alloy elements and impurities, in which Co is set at a concentration of 3 to 25% by atom, preferably 6 to 20% by atom, especially 8 to 14% by atom, and the RE component is formed from light rare earths, LRE, and heavy rare earths, HRE, and crushed to powder with a grain size of at least 3 µm, but, as is known per se, smaller than 60 µm, preferably smaller than 45 µm, especially smaller than 30 µm, into which powder there is (are) introduced one or more additives containing rare earths, RE, with a RE concentration which, relative to the powder grains, is greater by at least 25%, preferably by at least 35%, especially by at least 80%, and a HRE concentration which is greater by at least 100%, preferably 150%, especially 200%, and homogeneously distributed, whereupon the mixture is compressed in the magnetic field to form a blank, the latter is sintered and the sintered body is subjected to diffusion treatment or diffusion annealing and subsequently optionally to one or more further heat treatment operation(s).
5. Method according to Claim 4, characterised in that the HRE proportion of the RE component of the alloy forming or containing the magnetic phase is set as a function of the Co concentration.
6. Method according to Claim 4 or 5, characterised in that in the alloy a HRE contont is set with a value in % by atom of 0.02 to 0.19, preferably 0.06 to 0.12, especially 0.08, multiplied by the concentration value of Co.
7. Method according to one of Claims 4 to 6, characterised in that the powdered additive or additives is or are introduced into the powder containing or forming the magnetic phase, with a grain size which is smaller than 15%, preferably smaller than 9%, especially smaller than 2%, of the grain size of the powder containing or forming the magnetic phase.
8. Method according to one of Claims 4 to 7, characterised in that an additive or additives in at least partially liquid form, especially as chemical compounds and especially as organometallic compounds which when heated form oxides and/or nitrides and/or carbides, is (are) introduced into the powder containing or forming the magnetic phase and homogeneously distributed.
9. Method according to one of Claims 4 to 8, characterised in that the sintered body produced by sintering of the blank is subjected to diffusion treatment or annealed in such a way that the grains forming or containing the magnetic phase are smoothed on the surface or shaped by diffusion and that HRE atoms from the sinter-activated or grain-bonding medium or phase containing the addition or additions and/or formed thereby are allowed to diffuse into the surface region of the grains forming or containing the magnetic phase.
10. Method according to one of Claims 4 to 9, characterised in that in the grains forming or containing the magnetic phase a concentration of HRE atoms is formed which is known per se, is inhomogeneous and in particular rises in the direction of the grain boundary.
11. Method according to one of Claims 4 to 10, characterised in that the diffusion treatment or diffusion annealing of the sintered body is carried out at a temperature of 500 to 1100°C, preferably 800 to 1050°C, especially 900 to 1000°C, which lies below the sintering temperature.
12. Method according to one of Claims 4 to 11, characterised in that the diffusion treatment or diffusion annealing is carried out for a period of 1 to 12 hours, preferably 2 to 8 hours, especially 3 to 5 hours, the period being prolonged in the event of low treatment temperatures.