EP0499600A1 - 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 (material) according to the preamble of claim 1.

The invention further relates to a method for producing permanent magnets (materials) according to the preamble of claim 6.

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 component which forms or contains the magnetic phase of the SE₂Fe₁₄B type is melt-metallurgically produced and pulverized, which powder, optionally mixed with additives, is pressed into a green compact in the magnetic field and sintered, the sintered body optionally being subjected to at least one further heat treatment.

EP-B1-0126802 discloses sintered permanent magnets of the Fe-B-R type (R means at least one SE element including Y), in which Fe can be partially replaced by Co. The elements are in due to the manufacturing process used the magnetic phase is homogeneously distributed 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, the coercive force of which, as is known to the person skilled in the art, however, decreases with increasing Co content, which also results in the energy product can be adversely affected.

In order to create permanent magnets with improved magnetic properties at room temperature, it is proposed according to EP-B1-0101552 to use a Co-free alloy containing Fe-BR, which contains at least one stable compound of the ternary system Fe-BR, where R is at least means a 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-A1-0265006 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 forms a second non-magnetic liquid phase on the surface of the grains of RE (FeCo) B during the sintering process. 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. However, in this form of performance Disadvantages in the large expenditure on alloy technology and the poor reproducibility of the magnetic material data.

The invention has for its object to eliminate the disadvantages of the known RE-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 an 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 6 to 18.

A number of advantages are achieved synergistically in the permanent magnet (material) according to the invention and due to the procedure according to the invention for producing the same, 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 their surface energy is reduced or minimized by diffusion molding in sinter-active or grain-connecting phases and have a diameter of at most 60 μm, but at least 3 μm. With grain surfaces designed in this way, energetically, a domain wall formation and / or shift at least made difficult, which generally leads to an improvement in 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 drop in the coercive force or the magnetic induction.

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

If, according to the invention, the local concentration of SSE atoms is also inhomogeneous across the cross-section of the grains, in particular increasing in the direction of 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 activity of the SSE at the diffusion temperature of the sinter-active or grain-connecting, essentially paramagnetic phase. Good magnetic values are preferably obtained if the SE concentration of this grain-connecting phase is greater by at least 25% and its SSE concentration by at least 90% than that of the magnetic phase on average.

In the inventive method, the hard magnetic phase of the type SE₂ (FeCo) ₁₄ B forming or containing component by melting and casting an alloy with 8 to 30 at.% SE, consisting of LSE and SSE, 2 to 28 at.% B , 3 to 25 at.% Co, remainder iron and optionally further alloying elements and impurities produced and to powder with a grain size crushed between 60 µm and 3 µm. Additives containing at least one SSE element, preferably to an extent of 5 to 15% by weight, are introduced into this powder 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 to provide their particle diameter below 5 .mu.m or less than 15% of the diameter of the powder grains and, if appropriate, 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 and 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-connecting agent which largely fills the edges and fissures in and between the grains. For this purpose, it is important that the sintering temperature is selected so high for a short time 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 with a temperature between 600 and 1100 ° C. and 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 directly connected 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 energetic 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, a new formation of domain walls, which preferably occurs at the tips and edges, is effectively reduced in relation to the change in direction of the magnetic moments, 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 difference in concentration 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 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 thickness of the area of the increased SSE concentration is set to at least 0.05 μm, but at most 1 μm. Smaller strengths cause only an insignificant further reduction in the formation of domain walls 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 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 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 (20)

Sintered permanent magnet (material) essentially consisting of a magnetic phase of the type SE₂ (Fe, Co) ₁₄B and at least one further sinter-active or grain-connecting phase, characterized in that the magnetic phase of surface-smoothed or in their surface energy reduced or minimized diffusion-molded grains with a diameter of at most 60 μm, preferably at most 45 μm, in particular 3 to 30 μm, the magnetic phase or the grains Co formed from this phase with a concentration of 3 to 25 at%, preferably 6 up to 20 at.%, in particular 8 to 14 at.% and rare earths (SE) with proportions of light rare earths (LSE) and heavy rare earths (SSE), the average SSE content being 0, 05 to 0.2, preferably 0.06 to 0.15, in particular about 0.1, multiplied by the concentration value of Co and the local concentration of SSE atoms Across the cross-section of the grains is inhomogeneous, in particular in the area near the grain boundary or in the direction of the grain boundary, preferably increasing disproportionately, and the sinter-active or grain-bonded phase (s), which may contain metal and / or Compounds contain (s), have a higher SSE content and / or a higher activity of the SSE at the diffusion temperature than the magnetic phase or the grains. Permanent magnet (material) according to claim 1, characterized in that the SSE content of the magnetic phase is at least partially formed by SSE atoms diffused into the grains becomes. Permanent magnet (material) according to claim 1 or 2, characterized in that the concentration of SSE atoms at or in the region of the grain boundaries has a value which is at least 3 times, preferably at least 4.5 times, in particular is at least 6 times the value inside the grain. Permanent magnet (material) according to one of Claims 1 to 3, characterized in that the region with a high concentration of SSE atoms at the grain boundaries has a thickness of 0.05 to 1 µm, preferably 0.09 to 0.9 µm, in particular from 0.2 to 0.4 µm. Permanent magnet (material) according to one of Claims 1 to 4, characterized in that the sinter-active or grain-connecting phase (s), optionally having deposits, has an average of at least 25%, preferably at least, compared to the magnetic phase 35%, in particular by at least 80%, higher SE concentration value and an SSE concentration value higher by at least 90%, preferably 140%, in particular at least 190%. Process for the production of rare earth (SE) containing permanent magnet (s) (- material (s)), wherein at least the magnetic phase of type SE₂ (FeCo) ₁₄B forming or containing constituent is produced by melt metallurgy and then pulverized, after which the powder with additives is pressed in the magnetic field and then sintered to form a magnetizable raw body and optionally heat-treated, characterized in that the constituent which forms or contains the magnetic phase is produced by melting and casting an alloy containing 8 to 30 rare earths (SE) in at.%, 2 to 28 B, balance Fe and Co, optionally further alloying elements and impurities, in which Co with a concentration of 3 to 25 at.%, Preferably 6 to 20 at.%, In particular 8 to 14 at.%, Adjusted and the SE portion is made up of light rare earths (LSE) and heavy rare earths (SSE) Powder with a grain size of less than 60 microns, preferably less than 45 microns, in particular 3 to 30 microns, is ground, in which powder one or more heavy rare earth additives (SSE) are introduced and homogeneously distributed, whereupon the mixture in the magnetic field pressed into a green compact, this is sintered and the sintered body is subjected to a diffusion treatment or annealing and subsequently optionally one or more further heat treatment (s). A method according to claim 6, characterized in that the SSE portion of the SE portion of the alloy is set as a function of the Co concentration. A method according to claim 6 or 7, characterized in that an SSE content in the alloy with a value in at% of 0.02 to 0.19, preferably 0.06 to 0.12, in particular about 0.08 , multiplied by the concentration value of Co is set. Method according to one of claims 6 to 8, characterized in that in the powder containing or forming the magnetic phase, a powdery additive or powdery additives with a grain size of less than 5 µm, preferably less than 1 µm, in particular less than 0.5 µm , introduced and distributed homogeneously. Method according to one of claims 6 to 9, characterized in that the powdered additive (s) with a grain size which is less than 15%, preferably less than 9%, in particular less than 2%, of the grain size of the the powder containing or forming the magnetic phase is introduced into it. Method according to one of claims 6 to 8, characterized in that in the powder containing or forming the magnetic phase, an additive (additives) in at least partially liquid form, preferably chemically, in particular organometallic compounds which, when heated, contain oxides and / or nitrides and / or form carbides, is introduced and distributed homogeneously. Method according to one of claims 6 to 11, characterized in that the homogeneous distribution of the additive (s) to the powder is carried out by a grinding process. Method according to one of claims 6 to 12, characterized in that as additive (s) substance (s) with a compared to the powder grains by at least 25%, preferably at least 35%, in particular at least 80%, higher SE concentration and one at least 100%, preferably 150%, in particular 200%, higher SSE concentration is (are) used. Method according to one of claims 6 to 13, characterized in that the sintered body produced by sintering the green compact is diffusion-treated or annealed and that SSE atoms from and / or containing the additive (s) (n) formed sinter-active or grain-preventing agent or phase is allowed to diffuse into the surface area of the grains forming or containing the magnetic phase. Method according to one of Claims 6 to 14, characterized in that an inhomogeneous, in particular a concentration of SSE atoms which increases in the direction of the grain boundary, is formed in the grains forming or containing the magnetic phase. Method according to one of claims 6 to 15, characterized in that the diffusion treatment or annealing of the sintered body is carried out at a temperature below the sintering temperature of 600 to 1100 ° C, preferably 800 to 1050 ° C, in particular 900 to 1000 ° C becomes. Process according to one of Claims 6 to 15, characterized in that the diffusion treatment or annealing is carried out with a time period of 1 to 12 hours, preferably 2 to 8 hours, in particular 3 to 5 hours, the duration of the treatment temperatures being low is extended. Method according to one of claims 6 to 17, characterized in that the sintered body produced by sintering the green compact is diffusion-treated or annealed and that the grains forming or containing the magnetic phase are surface-smoothed or their surface energy is reduced or minimized by diffusion molding. Use of a method according to one of claims 6 to 18 for the enrichment of SSE atoms in the region of the grain boundaries of grains containing or forming the magnetic phase. Use of a method according to one of claims 6 to 18, for producing a sintered permanent magnet (material) according to claims 1 to 5 with a high energy product and high Curie temperature.

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