WO2015003849A1

WO2015003849A1 - Anisotropic rare earths-free plastic-bonded high-performance permanent magnet having nanocristalline structure, and method for production thereof

Info

Publication number: WO2015003849A1
Application number: PCT/EP2014/060786
Authority: WO
Inventors: Caroline Cassignol; Michael Krispin; Inga ZINS
Original assignee: Siemens Aktiengesellschaft
Priority date: 2013-07-12
Filing date: 2014-05-26
Publication date: 2015-01-15
Also published as: DE102013213644A1

Abstract

The invention relates to a method for producing a permanent magnet (PM), comprising the steps of mixing and coating (V) synthesized nanoparticles (1) with a plastics matrix (3), said step being carried out in the liquid phase, and orienting and shaping the dried matrix-coated nanoparticles (5) that are introduced into a mold and exposed to an external magnetic field (M). High degrees of filling are obtained.

Description

Patent application

Anisotropic rare earth plastic-bound

high-performance permanent magnet with nanocrystalline structure and process for its preparation

The invention relates to a method according to the main claim and a corresponding product.

Due to supply risks and high prices for rare earths, new rare earth-free solutions for the production of permanent magnets are being sought. Rare earths are used in particular for the production of permanent magnets. Conventional rare earth-free permanent magnet materials have an energy density which is too low for high-tech applications, for example using iron, cobalt, nickel or ferrites, or are too expensive from an economic point of view, for example FePt.

The permanent magnetic properties of magnetic materials are determined decisively by the microstructure or the microstructure in addition to the alloy composition. According to the micromagnetic theory as well as on the basis of experimental findings, it is known that high coercive field strengths can be achieved by a microstructural structure of single-domain, nanoscale structures. This enables the construction of a rare earth-free high-performance magnet made of nanoscale magnetic components. New nano-technological synthetic methods allow monocrystalline one-domain magnetic nanoparticles to be produced by combining shape and crystal anisotropy. In order to build up a macroscopic magnet, the magnetic nanoparticles must be embedded in organic or inorganic electrically insulating matrices in order to protect them against environmental influences and the resulting corrosion processes as well as to produce permanent magnets with corresponding mechanical, electrical and thermal properties. For example, a high electrical resistance advantageous. The resulting high-performance magnets can be used advantageously in high-efficiency drives and generators.

For the production of these magnetically and electrically optimized volume magnets a variety of criteria must be met.

Conventional permanent magnets are produced for example by means of a sintering technique (1) or by means of a plastic bond (2).

The conventional method of sintering technology enables production of anisotropic magnets by means of alignment of powder particles in the magnetic field before a pressing and sintering process. For the rare earth based magnets thus produced, the coercivity is limited due to the microcrystalline grain size, which is in the range of a few ym, and must be compensated by alloying very expensive and scarce heavy rare earth metals such as Dy or Tb. Due to the unfavorable temperature coefficient of the coercive field, this proportion must be additionally increased, the higher the working temperature. The heating of the magnet due to eddy current losses thus requires the use of a larger proportion of expensive heavy rare earth metals. As an alternative to this so-called sintered magnet, plastic-bonded magnets are conventionally also produced. For this purpose, several ten to several hundred micrometers magnetic particles based on rare earths embedded in a thermoset or thermoplastic matrix. In this case, a mixture which can also be called a compound is generated from the highest possible proportion of magnetic particles and the matrix. The mixture is then processed by injection molding, which is also called injection molding, which allows for a magnetic component of up to 60 vol%, or compression molding, which is called compression molding and allows up to 80% by volume of magnetic component, to form a volume magnet. Compared to the above described sintered magnets is the magnetic energy density of plastic-bonded magnets due to the dilution reduced by the polymer used.

For the production of nanocomposite formulations, which may also be referred to as a compound, by embedding nanoparticles in a matrix, conventionally no high fill levels are required. On the contrary, due to the difficult processing, it is traditionally attempted to maximize the effect at a minimum

To reach nanoparticle amount. For example, conventionally for carbon nanotubes or SiC> 2 ^_

Nanoparticles in an organic matrix reaches a filling level of up to 15 vol%. Therefore, use of such conventional standard methods is not expedient for nanoparticle-based magnets.

JP 2004296874 A discloses a hybrid rare earth bonded magnet for motors which suppresses mechanical damage between polycrystalline composite rare earth magnetic powder by a buffer action of rare earth magnetic powder having single domain particles for compression molding compound.

It is an object of the invention to produce highly effective permanent magnets with nanocrystalline structure in a simple manner reliably. In particular, magnetically and electrically optimized volume magnets are to be able to be produced, which fulfill in particular the following criteria: a high degree of filling, a homogeneous particle distribution with parallel alignment along the magnetic axis, a stationary binding of the magnetic particles after an alignment and a magnetic and electrical decoupling. In particular, a manufacturing process management should handle a large surface-to-volume ratio of nanoparticles. The object is achieved by a method according to the main claim and a corresponding product according to the independent claim.

According to a first aspect, a method for producing a permanent magnet is claimed with the following steps:

Synthesize rare earth-free ferromagnetic

anisotropic nanoparticles; by means of mixing in a liquid phase and coating of the synthesized nanoparticles with a plastic matrix; Orienting and shaping the dried matrix-coated nanoparticles introduced into an external magnetic field and into a mold.

According to a second aspect, a permanent magnet is claimed, which was produced by means of a method according to the main claim.

In particular, ferromagnetic means a very large permeability number and having a positive magnetic susceptibility and significantly enhancing a magnetic field.

Anisotropic means in particular a direction-dependent property, in particular magnetic property, having.

In particular, nanoparticles are single-phase and in particular have dimensions that are nanoscale and enforce a one-dimensional behavior.

The invention involves the construction of a rare earth permanent magnet whose magnetic properties, such as magnetization, coercive force and energy product, surpass those of conventional rare earth permanent magnets. The improvement in the magnetic properties of the rare earth free magnets proposed here allows replacement to be used conventionally rare earth based permanent magnets in electric motors and generators too. For this purpose, the magnet is made of nanoscale

Eindomänenteilchen, which can also be referred to as nanoparticles constructed. This magnetically optimized microstructure maximizes the coercive field to be achieved and also allows a large magnetization by means of a suitable choice of material.

The coating of the synthesized nanoparticles with a plastic matrix causes a very thin plastic layer, in particular in the nanometer range or smaller.

Further advantageous embodiments are claimed in conjunction with the subclaims.

According to an advantageous embodiment, prior to mixing, the synthesized nanoparticles may have been dispersed in a first solvent to a first dispersion and the matrix in a second solvent to a second dispersion each in a liquid phase.

According to a further advantageous embodiment, prior to mixing, the first solvent can be organic and polar or non-polar and the first dispersion stabilized by means of a polymer, oligomer or dendrimer, in particular a long-chain amine or a long-chain acid.

According to a further advantageous embodiment, the amine may be hexadecylamine and the acid may be palmitic acid or polyacrylic acid.

According to a further advantageous embodiment, the matrix may be organic or an organic-inorganic hybrid matrix. According to a further advantageous embodiment, the organic matrix may consist of epoxy resin and have a viscosity of about 100 to 300 mPa * s. According to a further advantageous embodiment, the second solvent may be compatible with epoxy resin, or in particular acetone, ethanol or chloroform.

According to a further advantageous embodiment, the matrix-coated nanoparticles can be dried by evaporating the liquid phase of the synthesized nanoparticles mixed with the matrix.

According to a further advantageous embodiment, the drying can be carried out by means of a rotary evaporator.

According to a further advantageous embodiment, the synthesizing of ferromagnetic anisotropic nanoparticles can not be industrialized. Anisotropy is particularly in terms of shape or crystal structure.

According to a further advantageous embodiment, the nanoparticles may have a core or a core / shell structure and optionally a protective cover. The shell can be soft magnetic. The thinnest possible coating, especially in the nanometer range, protects the nanoparticles against corrosion and oxidation. In addition, the shell reduces the agglomeration of the individual particles, which on the one hand reduces unfavorable contacts between the particles for the coercive field and, on the other hand, increases the anisotropy of the volume magnet to be achieved. The protective cover may for example consist of C and / or SiO 2. According to a further advantageous embodiment, after drying of the synthesized nanoparticles, they may be present in powder form. According to a further advantageous embodiment, the orientation and shaping can be performed simultaneously.

According to a further advantageous embodiment, during or after the molding, the matrix coating can cure and / or form crosslinked matrix envelopes.

According to a further advantageous embodiment, the curing and / or crosslinking can be activated, in particular thermally activated.

According to a further advantageous embodiment, the nanoparticles Co, Fe, Ni or Mn have. The nanoparticles can be synthesized wet-chemically, from the gas phase or by means of Millings.

According to a further advantageous embodiment, the core may consist of a soft magnetic and the shell of a hard magnetic material or vice versa.

According to a further advantageous embodiment, the protective layer may consist of carbon and be produced by means of storage of the nanoparticles for a period of a few hours and temperatures in the range of about 250 ° C to 350 ° C with an organic liquid.

According to a further advantageous embodiment, the protective layer can consist of silicon dioxide and be produced by means of hydrolysis and polycondensation of silane compounds in a polar solvent.

According to a further advantageous embodiment, permanent magnets according to the invention have been produced by one of the methods according to the invention.

The invention will be described in more detail by means of exemplary embodiments in conjunction with the figures. Show it: FIG. 1 shows a first embodiment of nanoscale magnetic components used according to the invention;

FIG. 2 shows a second embodiment of nano-scale magnetic components used according to the invention;

Figure 3 shows an embodiment of an inventive

Method;

FIG. 4 shows a further embodiment of a method according to the invention;

Figure 5 shows an embodiment of an inventive

Permanent magnets.

FIG. 1 shows an embodiment of nanoscale magnetic components 1 used according to the invention. As a result of a structural design as nanoscale one-domain particles having a combination of shape and crystal anisotropy, permanent magnet properties are favored according to the invention. For this reason, ferromagnetic anisotropic nanoparticles 1 are synthesized by means of suitable, for example, wet-chemical synthesis methods, which have a high magnetization and coercive field strength. These particles may be, for example, Co, Fe, Ni, Mn-based. Likewise, a core / shell structure is possible, wherein a core may consist of a soft magnetic material and a shell of a hard magnetic material. A reverse training is also possible. FIG. 1 shows a length L of nanoparticles <1000 nm, wherein a thickness D is smaller than the length L and the ratio L: D is approximately between 5: 1 to 100: 1. The arrow inside the magnetic module indicates a preferred magnetic direction.

FIG. 2 shows a further exemplary embodiment of nanoscale magnetic components or nanoparticles 1 used according to the invention. According to this advantageous embodiment, each nanoparticle is or will additionally be provided with a nanoparticle surrounded by nanoscale thin protective cover. The protective cover is shown as a strong border of a single magnetic module. A preferred magnetic direction again indicates an arrow in the magnetic component. As a first protection against environmental influences or as protection against corrosion, these nanoscale magnetic building blocks or nanoparticles 1 can be provided with a thin protective layer of, for example, carbon or silica. For this purpose, these nanoscale magnetic modules, for example, either by storage for several hours at high temperature, for example at temperatures between 250 ° C and 350 ° C, coated in an organic liquid with carbon or by hydrolysis and polycondensation of silane compounds in a polar solvent coated with SiC> 2. For example, silane bonds can be used

Aminopropylsilane (APS) or tetraethyl orthosilicate (TEOS). In addition to the protective function against environmental influences according to FIG. 2, an envelope according to FIG. 1 suppresses the formation of agglomerates by means of the reduction of the strength of a magnetic interaction. The formation of agglomerates has a negative influence on the magnetic properties to be achieved.

FIG. 3 shows an exemplary embodiment of a method according to the invention. According to the invention, it has been recognized for the production of volume magnets from protective nanoparticles that sintering methods conventionally used in rare earth-based magnets are not suitable because the nanoscale structure is destroyed due to the high thermal energy input. A possible

Solution is a further processing by embedding in a matrix at suitable temperatures, as shown for example below. According to the invention, the nanoparticles 1 are mixed with a matrix 3 in the liquid phase and coated. This mixing process is marked with the letter V. The mixture or coating V is dried and processed further. A stabilization of a dispersion of the magnetic nanoparticles 1 in an organic see non-polar or polar solvent can with

long-chain and especially multifunctional amines or corresponding acids, such as, for example, palmitic acid, polyacrylic acid (PAA) or hexadecylamine are achieved. This stable dispersion is then mixed with a solution of a very thin organic matrix 3, such as epoxy resin Epikote 05461 Momentive having a viscosity of 100 to 300 mPa * s, in a solvent such as acetone, ethanol or chloroform, or mixed with an organic-inorganic hybrid matrix such as the DS1000 manufactured by Hitachi. Volume fill levels of 50% to 70% can be achieved in this way. These matrices 3 are in particular 1-component, storage-stable at room temperature, have a glass transition temperature of over 90 ° C, good mechanical properties, for example, a flexural strength at 10 MPa, a long working time, which is also referred to as pot-life, at room temperature and a Curing temperature of below 400 to 450 ° C, which corresponds to an upper temperature resistance limit of the magnetic nanoparticles 1. The solvents are evaporated for example in a rotary evaporator. A powder of magnetic nanoparticles 5, which are produced with a thin reactive coating of the matrix 3, is obtained. The nanoscale magnetic particles 1 are now mixed with the matrix material 5 and now completely encased by a thin matrix layer 3. In this way, the degree of filling can be effectively increased because the degree of filling is inversely proportional to the layer thickness.

4 shows an exemplary embodiment of further method steps according to the invention. The powder of magnetic nanoparticles 1 obtained according to FIG. 3 with a thin reactive coating of the matrix 3 is transferred to a mold under an external magnetic field M for example, preferably oriented and pressed transversely to the pressing direction of a pressure P. In this case, pressures of a few MPa to a few GPa are built. At the same time for orienting and shaping Pressing or downstream, solidification or hardening of the matrix 3 is activated thermally or chemically. The result is bulk specimens with a high degree of filling of oriented, homogeneously distributed magnetic nanoparticles 1 in a matrix 3. The individual nanoscale magnetic bricks 1 have been aligned and compacted in the external magnetic field M, preferably transversely to the pressing direction, before the matrix cladding, for example thermally activated, be networked or softened.

5 shows an embodiment of a permanent magnet PM according to the invention. Fig. 5 shows a volume magnet, which is an anisotropic plastic-bonded magnet, which is composed of nanoscale magnetic modules 1 and has been prepared according to the inventive method. The liquid-phase-based method according to the invention for embedding magnetic nanoparticles 1 in a matrix 3 with subsequent densification and curing in the magnetic field leads to the greatest possible fill factor while at the same time having a homogeneous distribution and almost complete orientation in order to achieve the best possible magnetic properties. This is in contrast to conventional methods of embedding nanostructures that are optimized only for lower fill factors. Another advantage of embedding in a matrix 3 is the low processing temperature compared to conventional sintering processes. Thus, a magnetic view unfavorable particle growth is avoided. In addition, the method according to the invention makes it possible to produce close to final shape, which is also referred to as a near net shape. Due to the electrically insulating properties of the matrix material, the formation of eddy currents when used in the alternating magnetic field, which lead to an increase in temperature, is suppressed. The matrix coating of a permanent magnet according to the invention assumes three functions. 1. Connecting the individual nanomagnets or nanoparticles 1, to a volume magnet. 2. avoiding direct contact of the individual nanomagnets 1, that is, the magnetic insulation is formed and 3, electrical insulation to suppress eddy currents,

The invention relates to a method for producing a permanent magnet PM, comprising the steps by means of mixing and coating V of synthesized nanoparticles 1 with a plastic matrix 3 carried out in a liquid phase and orientation and shaping of the elements introduced into an external force field M and into a mold dried matrix-coated nanoparticles 5. There are high fill levels be acts

Claims

claims

1. A process for producing a permanent magnet (PM), comprising the steps of:

Synthesizing rare earth-free ferromagnetic anisotropic nanoparticles (1);

by means of mixing and coating (V) of the synthesized nanoparticles (1) with a plastic matrix (3) and producing a plastic matrix coating of the nanoparticles (1) in a liquid phase;

Orienting and shaping the dried matrix-coated nanoparticles (5) introduced into an external force field (M) and into a mold.

2. The method according to claim 1,

characterized in that

before mixing them, the synthesized nanoparticles (1) were dispersed in a first solvent to a first dispersion and the matrix (3) in a second solvent to a second dispersion each in a liquid phase.

3. The method according to claim 2,

characterized in that

before mixing, the first solvent is organic and polar or non-polar, and the first dispersion is stabilized by means of a polymer, oligomer or dendrimer, in particular a long chain amine or a long chain acid.

4. The method according to claim 3,

characterized in that

the amine is hexadecylamine and the acid is palmitic acid or polyacrylic acid.

5. The method according to any one of the preceding claims, characterized in that the matrix is organic or an organic-inorganic hybrid matrix.

6. The method according to claim 5,

characterized in that

the organic matrix consists of epoxy resin and has a viscosity of about 100 to 300 mPa * s.

7. The method of claim 2, 3, 4, 5 or 6,

characterized in that

the second solvent is acetone, ethanol or chloroform.

8. The method according to any one of the preceding claims, characterized by

Drying the matrix-coated nanoparticles by evaporating the liquid phase of the synthesized nanoparticles mixed with the matrix.

9. The method according to claim 8,

characterized in that

the drying is carried out by means of a rotary evaporator.

10. The method according to any one of the preceding claims, characterized by

industrial synthesizing of ferromagnetic anisotropic nanoparticles.

11. The method according to any one of the preceding claims, characterized in that

the nanoparticles have a core or a core shell structure and in particular a protective layer.

12. The method according to any one of the preceding claims, characterized in that

after drying of the synthesized nanoparticles, they are in powder form.

13. The method according to any one of the preceding claims, characterized in that

the orientation and shaping are performed simultaneously.

14. The method according to any one of the preceding claims, characterized in that

during or after molding, the matrix coating solidifies or cures.

15. The method according to claim 14,

characterized in that

the curing and / or crosslinking is activated, in particular thermally activated, is.

16. The method according to any one of the preceding claims, characterized in that

the nanoparticles Co, Fe, Ni or Mn have and / or are synthesized wet-chemically.

17. The method according to claim 11,

characterized in that

the core of a soft magnetic and the shell of a hard magnetic material or vice versa are formed.

18. The method according to claim 11,

characterized in that

the protective layer is made of carbon and produced by storage of the nanoparticles for a period of several hours and temperatures in the range of about 250 ° C to 350 ° C in an organic liquid.

19. The method according to claim 11,

characterized in that

the protective layer consists of silicon dioxide and was produced by hydrolysis and polycondensation of silane compounds in a polar solvent.

20. permanent magnet,

characterized in that

this was produced by means of a method according to one of the preceding claims.