EP3357074B1 - Process to manufacture a permanent magnet - Google Patents

Description

The invention relates to a method for producing a permanent magnet.

Permanent magnets, also known as permanent magnets, which have at least one element from the rare earth group, are characterized by a very high energy product, with neodymium-iron-boron magnets (NdFeB magnets) in particular having an energy product of over 385 kJ/m ³ or 48 MGOe enable new technical solutions for a variety of applications. In particular, significant reductions in the size of magnet systems or significantly higher magnetic energies are possible with the same size compared to conventional magnetic materials such as barium, ferrite or AlNiCo. However, producing such magnets requires complex processing steps, which also result in higher costs. One established way of producing such magnets involves melting an alloy of the magnetic material. Blocks of material cast from the melt are then broken and ground into a fine powder, pressed in a magnetic field and then sintered. However, only simple geometric shapes can be produced this way. To produce complex geometries, isostatically pressed and then sintered raw blocks in large dimensions are often used. Shaped magnets are cut from these raw blocks, which has to be done under water, for example, with a diamond saw because of the poor machinability of the magnetic material. Disc-shaped and/or ring-shaped magnets are also manufactured with diamond tools. Due to the machining process, there is considerable production waste, which in turn has a negative impact on costs. The same applies to diamond tools, which are expensive to procure and use.

The process of metal powder injection molding (MIM - Metal Injection Molding) is particularly known for the production of complex geometries, for example from steel or titanium materials. Here, a metal powder is processed with a typically organic binder material into an injection-mouldable mass, the so-called feedstock, and brought into shape by injection molding in an appropriate mold on injection molding machines. The binder is then removed, which is also referred to as debinding or debinding step. This is followed by a sintering step in which finished components that are true to the final shape or at least close to the final shape can be produced. A characteristic of the MIM process is the efficient use of raw materials, as it can be used with practically no production waste. Machining post-processing steps can at least be largely avoided. Because of the high raw material costs, the low availability of the starting materials and at the same time the very problematic machinability, complex-shaped permanent magnets based on elements from the rare earth group are generally suitable candidates for being manufactured using an MIM process. However, the problem arises that chemical elements from the group of rare earths and in particular the alloying element neodymium (Nd), which is very advantageous for powerful permanent magnets, have a very high reactivity with respect to oxygen (O ₂ ) and carbon (C). For a series-suitable and reproducible MIM process, it is important to use a binder material which has in particular organic compounds, in particular plastic compounds, which release carbon and oxygen during thermal debinding, i.e. during a burnout process to remove the binder material. This can lead to the formation of oxides or carbides of a rare earth metal, especially neodymium, which leads to a severe deterioration in the hard magnetic properties.

JP H09 283358 A shows a process in which oxidation of the rare earth metal is prevented by using an oxygen-free binder. Furthermore, carbide formation is prevented by carrying out the debinding with heating in a hydrogen atmosphere.

The invention is based on the object of creating a method for producing a permanent magnet in which the disadvantages mentioned do not occur.

The task is solved by creating a method with the steps of claim 1.

The object is achieved in particular by creating a method for producing a permanent magnet, the permanent magnet being a rare earth element, in particular neodymium, wherein a starting material is processed in a metal powder injection molding process (MIM process), wherein the starting material has a rare earth element, and wherein the rare earth element present in the starting material is converted into an inerted one before a debinding step of the metal powder injection molding process connection is converted. Because the rare earth element is converted into an inerted compound before the debinding step, it is in the Debinding step does not occur in its highly reactive form towards oxygen and carbon, but rather in a more inert form, so that the risk of formation of oxides or carbides of the rare earth element during the debinding step is significantly reduced, preferably completely avoided. In this way, a severe deterioration in the hard magnetic properties of the permanent magnet due to the debinding step can be prevented. Due to the suitable process control, this in turn enables the production of low-oxide and carbide or oxide and carbide-free permanent magnets with a high energy product by means of metal powder injection molding, so that this can for the first time be used in a large-scale manner for the production of such permanent magnets that have rare earth elements. With the help of the method, complex geometries for such magnets can be produced quickly, easily and cost-effectively, in particular with little material expenditure and preferably while avoiding production waste due to machining post-processing steps. Overall, the process enables efficient use of raw materials without the problem of a deterioration in the hard magnetic properties of the permanent magnets produced using the process.

A metal powder injection molding process, which is also briefly referred to as an MIM process, is understood to be a process in which a starting material is formed by injection molding, whereby a molded part is obtained, which is subsequently sintered. The starting material can be a metal powder, in particular a powder of a metal alloy, which is then processed in a separate step of the MIM process into an injection moldable material by mixing it with a binder material, this injection moldable material being referred to as feedstock, and wherein the step of mixing the metal powder with the preferably thermoplastic binder material is referred to as feedstock preparation. However, it is also possible for a previously processed feedstock to be used directly as the starting material in the MIM process, in which case the feedstock processing itself is not part of the process. The feedstock is injection molded in an injection molding machine, resulting in the molded part, which is also referred to as a green compact and which also has the metal powder and the binder material, i.e. the feedstock. The green compact is then subjected to at least one debinding step, in which the binder material is removed. In principle, catalytic debinding involves catalytic chemical conversion of the binder material into gaseous products that can be expelled, chemical debinding Dissolving the binder material in a typically liquid solvent, and/or thermal debinding with thermal conversion, in particular burning out, of the binder material is possible. A combination of these steps is also possible. In particular, it is possible to first chemically remove a first component of the binder material, also referred to as the primary binder component, in particular to dissolve it out of the green compact using a solvent, and then to thermally remove a second, secondary binder component, which is also referred to as the backbone binder. especially by burning out - to be removed. Through the at least one debinding step, the so-called brown compact is produced from the green compact, which only has the metal powder or - after a first debinding step - the metal powder and the secondary binder component. The brown compact is then sintered, with any remaining backbone binder being thermally removed. The sintering step ultimately results in the finished component manufactured using the MIM process. Instead of the terms “green body” and “brown body” the terms “green body” and “brown body” or “green part” and “brown part” are sometimes used.

With regard to the conversion of the rare earth element into the inerted compound, the starting material in the method proposed here can in particular be the metal powder before the feedstock preparation, the prepared feedstock, or the feedstock already in injection-molded form, i.e. the green body, or the green body after a first , in particular chemical debinding step, can be used. It is important that the rare earth element is converted into its inerted compound before a thermal debinding step, so that the formation of oxides and/or carbides is avoided or at least reduced during the thermal debinding step.

A rare earth element is understood to mean in particular a chemical element, also in the form of a compound and in particular an alloy with at least one other chemical element, which belongs to the rare earth metals. These are in particular the chemical elements of the third subgroup of the periodic table with the exception of actinium, as well as the lanthanides. The rare earth element is preferably selected from a group consisting of scandium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, yttrium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. The rare earth element is preferably selected from the so-called light rare earth elements, namely a group consisting of scandium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium and europium. Alternatively or additionally, the rare earth element is preferably selected from the group of so-called heavy rare earth elements, namely a group consisting of yttrium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. The rare earth element is preferably selected from a group consisting of neodymium, dysprosium, terbium and samarium. The rare earth element is preferably selected from a group consisting of neodymium, dysprosium and terbium. It is particularly preferred to use a starting material which has neodymium as the rare earth element.

It is possible that the starting material has more than one rare earth element. In particular, it is possible for the starting material to contain dysprosium and/or terbium as further rare earth element(s) in addition to neodymium. For the method proposed here, it is fundamentally sufficient if one of the rare earth elements contained in the starting material is converted into an inerted compound, in particular neodymium. However, it is additionally or alternatively possible for more than one rare earth element included in the starting material to be converted into an inerted compound. It is also possible for all rare earth elements included in the starting material to be converted into an inerted compound.

“Converting” into an inerted compound here is understood to mean, in particular, a chemical reaction of the rare earth element, in particular a chemical reaction of the rare earth element with at least one further element or at least one compound. The term “conversion” is not to be understood as limited to the formation of a covalent bond, but can include the formation of at least one covalent bond. Other bond types and/or chemical reaction types are also included in the term “conversion”. This particularly includes the formation of an ionic bond, a complex bond, and a metallic bond. The conversion can therefore in particular also include the formation of a metallic phase. The fact that the rare earth element is converted into an inerted compound means in particular that the rare earth element compound created by the conversion is less reactive than the rare earth element in its form before the conversion. The inerted compound is particularly unreactive towards the elements carbon and oxygen as the rare earth element in the form present before the conversion.

An embodiment of the method is described, which is characterized in that the inerted compound is converted back into the rare earth element after the debinding step. In particular, the inerted compound is preferably reconverted after the debinding step into the rare earth element in its form before the conversion into the inerted compound. This includes that the rare earth element may not have been in elemental form before being converted into the inerted compound, but in another form, in particular in the form of an alloy or metallic phase, whereby it then returns from the form of the inerted compound into this previously existing alloy or metallic phase or other form is converted back. As a result, after the debinding step, when there is no longer any risk of the formation of carbides or oxides that are harmful to the hard magnetic properties of the permanent magnet, the properties of the rare earth element, which are favorable for the permanent magnet, are recovered in the form that existed before the conversion into the inerted compound. This reconversion preferably takes place before sintering, in particular between the thermal debinding step and the sintering step of the MIM process.

An embodiment that is not part of the invention is characterized in that the conversion of the rare earth element into the inerted compound is carried out using a binder-free powder containing the rare earth element as the starting material. In this case, the conversion is carried out in particular before feedstock processing. In this case, the conversion is particularly preferably carried out simultaneously and/or together with a hydrogen decrepitation step, which is intended in particular to obtain a fine-grained powder for the MIM process. The conversion before feedstock processing has the advantage that shorter reaction times are possible due to the higher reactivity of the starting material without the binder material.

According to the invention, the conversion is carried out in at least two conversion steps, with the hydrogenation of the rare earth element being carried out in at least a first conversion step. The first conversion step can optionally be carried out before or after mixture formation. In at least a second conversion step, the hydrogenated rare earth element is completely and partially dehydrogenated. The second conversion step is carried out before the green compact is thermally debinded. In particular, in one embodiment it is provided that the conversion of the rare earth element into the inerted compound is carried out on a mixture of the powder containing the rare earth element and the binder material, that is in particular on the feedstock, as starting material. Although longer reaction times have to be accepted due to the lower reactivity of the metal powder bound in the binder material, there is the advantage of process separation of the feedstock preparation on the one hand and the conversion of the rare earth element on the other, so that the feedstock preparation is outsourced and in particular a fully prepared feedstock can be provided as starting material, on which the conversion step of the rare earth element is then carried out. This can result in particular economic advantages.

An embodiment of the method is also preferred, which is characterized in that at least the first conversion step is carried out in an atmosphere that contains hydrogen or consists of hydrogen. A hydrogenation of the rare earth element can be carried out, whereby it is chemically more stable in its hydrogenated form, i.e. as a compound with hydrogen, at least up to 500 ° C than in its dehydrogenated form, in particular more stable towards carbon and oxygen. The rare earth element is preferably converted into a hydrogenated form, i.e. into a hydrogen compound. The term hydrogen compound is not limited to a covalent compound. Rather, this can in particular be a hydrogen-containing, metallic phase. As part of the process, neodymium is particularly preferably converted into a neodymium hydride, in particular NdH ₃ .

In the context of the invention, the previously formed hydride of the rare earth element is reconverted as an inerted compound after the debinding step by dehydrogenation. Neodymium hydride, in particular NdH ₃ or NdH _2.7 , is particularly preferably converted back to neodymium (Nd).

The first conversion step is preferably carried out at a pressure of at most 2.5 bar, preferably at a pressure of at least 1.5 bar to at most 3.5 bar, preferably at a pressure of at least 1.5 bar to at most 3 bar, preferably at a pressure of at least 1.5 bar to at most 2.5 bar, preferably at least 1.5 bar to at most 2 bar, preferably at least 2 bar to at most 3 bar, preferably at least 2 bar to at most 2.5 bar, preferably from 2, 5 bar.

Alternatively or additionally, the first conversion step is carried out at a temperature of at least 15 °C to at most 200 °C, preferably at least 15 °C to at most 150 °C, preferably at least 100 °C to at most 150 °C, preferably 125 °C , preferably from at least 15 °C to at most 35 °C, preferably at least 15 °C to at most 30 °C, preferably at least 20 °C to at most 30 °C, preferably from 25 °C. It is therefore possible in particular to carry out the first conversion step at room temperature or at an elevated temperature, in particular up to 150 ° C.

Alternatively or additionally, the first conversion step is preferably carried out for a period of at least 5 minutes to a maximum of two hours, preferably from at least 30 minutes to a maximum of 90 minutes, preferably for a period of one hour. The specifically selected reaction time depends in particular on the amount of rare earth element to be converted and thus in particular on the amount of starting material to be treated.

The hydrogen-containing atmosphere preferably contains at least 200 mbar to at most 1 bar of pure hydrogen. In a preferred embodiment of the invention, the atmosphere consists of hydrogen in one of the aforementioned pressure ranges.

Highly pure gases with a low oxygen content are preferably used for the process, in particular hydrogen, which contains oxygen with a concentration - based on the volume - of at most 2 ppm (parts per million), that is, 2 ppmv (parts per million based on that Volume), preferably at most 1 ppmv, particularly preferably at most 0.5 ppmv. Particular preference is given to using hydrogen with a purity of at least 99.999% by volume (hydrogen 5.0), and very particularly preferably using hydrogen with a purity of 99.9999% by volume (hydrogen 6.0). The same applies to any protective gas that may be used, in particular argon.

According to the invention, at least a second conversion step is carried out. The conversion of the rare earth element into the inerted compound is therefore carried out in two steps. Particularly preferably, hydrogenation is carried out in the first conversion step, with partial dehydrogenation being carried out in the second conversion step, in particular to achieve an even more stable compound. In the second conversion step, neodymium hydride formed in the first conversion step is particularly preferred with the molecular formula NdH ₃ , the partially dehydrogenated neodymium hydride with the approximate molecular formula NdH _2.7 is produced by partial degassing, this partially dehydrogenated neodymium hydride being stable to carbon and oxygen up to about 500 ° C.

The at least one second conversion step is preferably carried out under vacuum, under an atmosphere containing or consisting of hydrogen, and/or under an atmosphere containing at least one inert gas or consisting of at least one inert gas.

A protective gas is understood to mean, in particular, a gas which is inert or at least unreactive towards the substances or mixtures of substances used to produce the permanent magnet. The protective gas is preferably selected from a group consisting of a noble gas, nitrogen, and carbon dioxide. The protective gas is particularly preferably selected from a group consisting of a noble gas and nitrogen. The protective gas is particularly preferably a noble gas. Argon is particularly preferably used as the protective gas.

Additionally or alternatively, the at least one second conversion step is preferably carried out at a pressure of at least 100 mbar to at most 1 bar, preferably at least 100 mbar to at most 700 mbar, preferably at least 200 mbar to at most 600 mbar. The second conversion step is particularly preferably carried out either under atmospheric pressure (1 bar) or in a pressure range of at least 200 mbar to at most 600 mbar.

Alternatively or additionally, the at least one second conversion step is preferably carried out at an increasing temperature, in particular by heating the compound resulting from the first conversion step. The temperature preferably increases from a starting temperature of 100 °C to a target temperature of 350 °C, preferably from a starting temperature of 150 °C to a target temperature of 300 °C.

Alternatively or additionally, the at least one second conversion step is preferably carried out for a period of at least 20 minutes to at most 120 minutes, preferably at least 30 minutes to at most 90 minutes, preferably at least 40 minutes to at most 80 minutes, preferably at least 50 minutes to at most 70 minutes, preferably 60 minutes. Within this period of time, a temperature ramp is preferably run, starting from one of the aforementioned start temperatures up to one of the aforementioned target temperatures. During the second conversion step, in particular partial degassing and, in particular, partial dehydrogenation of the in The compound produced in the first conversion step, resulting in an even more stable intertized compound that is even more inert to oxygen and carbon.

An embodiment of the method is preferred, which is characterized in that a binder material is used which has at least one secondary binder component which is completely decomposable below a temperature of 500 ° C or at most 600 ° C. This enables thermal removal of the binder material at a temperature at which the inerted compound is unreactive and, in particular, stable to carbon and oxygen. The debinding can therefore be carried out without the risk of a deterioration in the hard magnetic properties of the permanent magnet to be formed due to the formation of carbides or oxides, or of carbon- and/or oxygen-rich phases.

As part of the process, a binder material is preferably used which has a primary binder component and a secondary binder component. A primary binder component is understood to mean a component which can be a substance or a mixture of substances whose properties essentially contribute to the feedstock being flowable and processable by injection molding. This primary binder component is also referred to as the main binder. In contrast, the secondary binder component is in particular a substance or a mixture of substances which is intended to give the green compact - and possibly also the brown compact before sintering - mechanical stability by connecting the metallic particles to one another. This secondary binder component is also known as the backbone binder.

It is possible that both the primary and secondary binder components are thermally removed. In this case, the primary binder component can then also be completely decomposed, preferably below a temperature of 500 ° C or at most 600 ° C.

However, an embodiment of the process is preferred in which the primary binder component is chemically removable by dissolving in a solvent and is preferably removed, with the secondary binder component being thermally debinded. The solvent used for chemical debinding of the primary binder component is preferably an organic solvent, in particular heptane, cyclohexane, hexane, ethanol and/or acetone. In a preferred embodiment of the method is It is provided that no binder component is debinded catalytically, i.e. catalytic debinding is dispensed with.

A binder material is preferably used in which the primary binder component is selected from a group consisting of a paraffin wax, a carnauba wax, a polyolephine wax, polypropylene, polyethylene, polyethylene glycol, a water-soluble polymer, polymethyl methacrylate, and polyoxymethylene. The primary binder component can also have a plurality of these substances, in particular in any combination with one another, and/or comprise further, additional substances. However, it is also possible for the primary binder component to consist of one of the aforementioned substances.

As part of the process, a binder material is preferably used which has a secondary binder component which is selected from a group consisting of ethylene vinyl acetate, polyethylene, polypropylene, polystyrene or polystyrene, polyethylene glycol, polymethyl methacrylate, polyamide, polyoxymethylene, and polyvinyl butyral. The secondary binder component can also contain a plurality of the substances mentioned here, in particular in any combination with one another. In addition to the substances mentioned here, it can also have at least one further substance not mentioned here. However, it is also possible for the secondary binder component to consist of one of the substances mentioned here.

An embodiment of the method is preferred, which is characterized in that the - preferably second, thermal - debinding step takes place under vacuum, under an atmosphere that has at least one protective gas or consists of at least one protective gas, and / or under an atmosphere containing or consisting of hydrogen Hydrogen existing atmosphere is carried out.

Debinding under hydrogen, in particular under an atmosphere containing hydrogen or consisting of hydrogen, is particularly advantageous because, especially in combination with the conversion of the rare earth element into the inerted compound, it provides extremely efficient protection against the formation of carbides, oxides and / or of carbon and/or hydrogen containing phases of the rare earth element can be guaranteed.

The debinding step is additionally or alternatively preferably carried out at a pressure of at least 100 mbar to at most 1 bar, preferably at least 100 mbar to at most 700 mbar, preferably at least 200 mbar to at most 600 mbar.

Additionally or alternatively, the debinding step is preferably carried out at a temperature of at most 600 °C, preferably less than or at most 500 °C, preferably at least 100 °C to at most 500 °C, preferably at least 200 °C to at most 400 °C, particularly preferably carried out at 300 ° C. In particular, a distance of at least 200 ° C from the limit temperature of 500 ° C for the reactivity of the inerted compound ensures a sufficiently large process window for the complete and at the same time gentle decomposition of the binder material with regard to the hard magnetic properties of the resulting permanent magnet. In particular, in this way, a reaction with the rare earth hydride phase, which is stable to carbon and oxygen, can be prevented.

Additionally or alternatively, the debinding step is preferably carried out for a period of at least 80 minutes to a maximum of 170 minutes, preferably for a period of at least 100 minutes to a maximum of 150 minutes, preferably for a period of 125 minutes. The times mentioned here preferably ensure complete decomposition of the binder material. The specifically selected process time for the debinding step depends in particular on the amount of material to be decomposed, in particular on the size of the part to be debinded.

The term “debinding step” here is understood to mean, in particular, the thermal decomposition of binder material. This means in particular that this preferably refers to the thermal removal or burning out of the secondary binder component. It is therefore possible for the primary binder component to be removed, for example chemically, in a first debinding step, with the secondary binder component being thermally removed in a second debinding step, which is the debinding step relevant here. For the method proposed here, this second, thermal debinding step is particularly relevant as a debinding step, since in this debinding step carbon and oxygen compounds are formed, which are critical for the hard magnetic properties of the permanent magnet to be formed.

An embodiment of the process is preferred, which is characterized in that the reconversion of the inerted compound is carried out under vacuum and/or under an atmosphere which has at least one protective gas or consists of at least one protective gas. This means that unpleasant chemical reactions can be avoided during the reconversion.

Additionally or alternatively, the reconversion is preferably carried out at a pressure of at least 100 mbar to at most 1 bar, preferably at least 100 mbar to at most 700 mbar, preferably at least 200 mbar to at most 600 mbar.

Additionally or alternatively, the reconversion is preferably carried out at a temperature of at least 500 °C, preferably at a temperature of over 500 °C, preferably at a temperature of at least 500 °C to at most 900 °C.

Additionally or alternatively, the reconversion is preferably carried out for a period of at least 40 minutes to at most 100 minutes, preferably at least 50 minutes to at most 90 minutes, preferably at least 60 minutes to at most 80 minutes, preferably 70 minutes.

Additionally or alternatively, the reconversion of the inerted compound is preferably carried out in a sintering furnace. The term “sintering furnace” is understood in particular to mean a device which is designed to sinter the parts resulting from the previous steps of the MIM process. The thermal debinding step can also be carried out in the sintering furnace.

An embodiment of the method is preferred, which is characterized in that a sintering step is carried out in a vacuum and/or under an atmosphere which has at least one protective gas or consists of at least one protective gas. In this way, undesirable chemical reactions can be avoided during sintering.

Additionally or alternatively, the sintering step is preferably carried out at a pressure of at least 100 mbar to at most 1 bar, preferably at least 100 mbar to at most 700 mbar, preferably at least 200 mbar to at most 600 mbar.

Additionally or alternatively, the sintering step is preferably carried out at a temperature of at least 900 ° C to at most 1200 ° C, preferably at a temperature of at least 980 ° C to at most 1100 ° C.

Additionally or alternatively, the sintering step is preferably carried out for a period of at least 100 minutes to at most 160 minutes, preferably at least 110 minutes to at most 150 minutes, preferably at least 120 minutes to at most 140 minutes, preferably 130 minutes.

As part of the sintering step, a sintered body is produced. In particular, the sintered body is produced by sintering the brown compact or brown body.

An embodiment of the method is preferred, which is characterized in that cooling takes place after the sintering step, preferably under vacuum and/or under an atmosphere which has at least one protective gas or consists of at least one protective gas, with the cooling being followed by a heat treatment can connect. Such heat treatments are known in particular for conventionally sintered NdFeB magnets and serve to improve the hard magnetic properties of the permanent magnet. Such a heat treatment can also be advantageously carried out on a permanent magnet produced according to the method proposed here.

After the sintering step, preferably after cooling, and preferably before the heat treatment, at least one additional rare earth element or an additional amount of the at least one rare earth element is preferably introduced into the sintered body, particularly preferably via grain boundary diffusion, preferably by means of an electrochemical process. In this way, in particular, the coercive field strength of the permanent magnet can be increased even further. It is particularly preferred to introduce additional neodymium, dysprosium and/or terbium into the sintered body.

An embodiment of the method is also preferred, which is characterized in that a magnetic field is applied to the starting material. Alternatively or additionally, a magnetic field is applied to the sintered body. The magnetic field is particularly preferably applied to the starting material during injection molding. By applying the magnetic field, an alignment of the powder grains of the magnetic powder of the starting material can be achieved, resulting in good hard magnetic properties for the resulting permanent magnet can be ensured. The magnetic alignment is preferably carried out during the molding process, in particular when the starting material has been introduced into the injection mold or is about to be introduced, or alternatively only towards the end of the flow process during injection molding before the binder material has hardened.

A magnetic field, in particular a magnetic pulse, is preferably applied to the sintered body, whereby the permanent magnet receives its magnetization, or whereby the magnetization of the permanent magnet is increased.

Particularly preferably, a magnetic field is applied both to the starting material during injection molding and to the sintered body. This allows the magnetic particles to be aligned in the magnetic field during injection molding, with this alignment being retained during sintering. Typically, however, during sintering, the parallel alignment of electron spins of the magnetic material is lost, although the geometric alignment of the material is retained. By applying a magnetic field, in particular a magnetic pulse, to the sintered body, a parallel alignment of electron spins can in turn be generated in the sintered body and thus ultimately in the permanent magnet, so that it has a very high magnetization. The magnetic particles aligned by the magnetic field during injection molding are ultimately magnetized with respect to the electronic alignment by the magnetic field applied to the sintered body.

A magnetic pulse is preferably applied using a pulse magnetometer.

Finally, an embodiment of the method is preferred which is characterized in that a starting material is used which has NdFeB or consists of NdFeB. By using this starting material, permanent magnets based on NdFeB, in particular NdFeB magnets with a very high energy product, can advantageously be produced. The starting material preferably has NdFeB in powdered or granulated form, or it consists of NdFeB in powdered or granulated form.

In a preferred embodiment of the method, a starting material is used which is in powder form or in granulated form and which has a powder particle size of a maximum of 50 μm. The grain size is particularly preferably less than 10 μm, which makes an anisotropic magnetic alignment of the particles particularly easy to carry out during injection molding in the magnetic field. In particular, preferably has a as starting material NdFeB powder used or included in the starting material has a grain size of a maximum of 50 μm and preferably less than 10 μm.

It turns out that a hydride of the rare earth element that is stable up to 500 ° C can be created, in particular through hydrogenation of the starting material and through appropriate process control, whereby a reaction of the rare earth element with oxygen and carbon during the thermal decomposition of binder material is prevented, which allows the formation of a sintered structure with hard magnetic properties, which is not impaired by the formation of undesirable oxides, carbides, and/or phases containing oxygen and/or carbon.

Figur 1

eine schematische Darstellung einer Ausführungsform des Verfahrens in Form eines Flussdiagramms, und

Figur 2

eine schematische Darstellung eines Temperaturverlaufs während einer Ausführungsform des Verfahrens.

The invention is explained in more detail below with reference to the drawing. Show:

Figure 1

a schematic representation of an embodiment of the method in the form of a flowchart, and

Figure 2

a schematic representation of a temperature profile during an embodiment of the method.

Fig. 1 shows a schematic representation of an embodiment of the method for producing a permanent magnet in the form of a flow chart. Two alternative approaches to the process are shown in the upper part of the diagram.

A) shows a first alternative embodiment, in which a starting material in the form of an NdFeB powder is provided in a first step S1.

The neodymium comprised by the starting material is converted into an inerted compound in a second step S2, which is carried out in particular by hydrogen treatment at excess pressure of up to 2.5 bar, either at room temperature or at an elevated temperature of up to 150 ° C. A hydrogenation reaction of neodymium with hydrogen produces the neodymium hydride NdHa, which is significantly more stable than elementary neodymium. The hydrogenation to NdH ₃ is carried out in particular in a first conversion step, in particular for a period of at least 5 minutes to at most two hours, preferably at least 30 minutes to at most 90 minutes, preferably for one hour. In particular, this first conversion step can be done in conjunction with a Hydrogen decrepitation treatment of a molten NdFeB alloy can be carried out.

In a third step S3, the magnetic powder treated in the second step S2 is now mixed with a binder material and a feedstock is thus produced for further processing in the MIM process.

At b) an alternative embodiment of these first process steps is shown. In a first step, S1' is provided as a starting material in a powder mixture, namely a so-called feedstock, which has NdFeB powder and a binder material as a mixture.

This feedstock is treated in a second step S2` by hydrogen treatment at excess pressure of up to 2.5 bar, either at room temperature or elevated temperature, in particular up to 150 ° C, where neodymium is hydrogenated to the more stable neodymium hydride NdH <sub>3</sub> . This hydrogen treatment of a first conversion step also preferably takes place for a period of at least 5 minutes to a maximum of two hours, preferably from at least 30 minutes to a maximum of 90 minutes, preferably for a period of one hour. It should be noted that the time period selected for the alternative second step S2` should typically be longer than the time period selected for the second step S2 according to the first alternative, since the additional binder material present reduces the reactivity of the neodymium bound in the plastic and is therefore longer Requires reaction times. The feedstock treated in this way in the alternative second step S2` is then made available for the further MIM process.

The two alternative process routes described now lead to a common, further procedure, whereby in a fourth step S4 a green body is produced by injection molding from the feedstock. During injection molding, a magnetic field is preferably applied to the feedstock so that the individual magnetic grains in the injection molding material are aligned.

In a fifth step S5, the green compact is heated under vacuum, under a protective gas atmosphere or under a hydrogen atmosphere, at atmospheric pressure or at a reduced pressure of at least 200 mbar to a maximum of 600 mbar, at a temperature of at least 150 ° C to a maximum of 300 ° C Partial degassing of the NdH ₃ , whereby the opposite Carbon and oxygen form a stable compound with the approximate molecular formula NdH _2.7 at least up to approximately 500 °C. In particular, a temperature ramp can be run for this step, preferably from 150 ° C to 300 ° C, preferably over a period of 60 minutes. This is preferably followed by a holding time for the temperature at 300 ° C in order to ensure complete partial degassing of the neodymium hydride into its particularly stable form with the approximate molecular formula NdH _2.7 . This holding time is preferably 40 minutes.

In a sixth step S6, the green body is debinded. In general, debinding preferably takes place in two steps, with a primary binder component being chemically dissolved using a solvent, in particular heptane and/or acetone, and a secondary binder component, the so-called backbone binder, being thermally removed. It is possible for the first debinding step, namely the chemical debinding of the primary binder component, to be carried out before the fifth step S5, i.e. before the partial degassing of the neodymium hydride. In any case, the thermal debinding, i.e. the second debinding step for removing the secondary binder component, is preferably carried out after the fifth step S5, since this thermal debinding step is critical for the possible formation of carbides or oxides in the magnetic material. Therefore, before this thermal debinding step, this is preferably converted into its form, which is stable at least up to 500 ° C, with the approximate molecular formula NdH _2.7 . It is possible that both debinding steps, namely the first, chemical debinding step, and the second, thermal debinding step, are carried out after the fifth step S5. The decomposition of all remaining organic binder components, in particular the secondary binder component, preferably takes place below a limit temperature for the stability of NdH _2.7 of 500 ° C, particularly preferably at a lower temperature, in particular from at least 200 ° C to at most 400 ° C. The debinding is preferably carried out under a hydrogen atmosphere or under a protective gas atmosphere, in particular under an argon atmosphere, in particular either at atmospheric pressure, at reduced pressure, or also in a vacuum. The carbon and oxygen components that arise during debinding can be removed through process gas and/or vacuum without reacting with the NdH _2.7 phase, which is stable to them.

In a seventh step S7, the temperature is further increased - preferably in the sintering furnace - to over 500 ° C, preferably to over 600 ° C, and a corresponding holding time a dehydrogenation of the neodymium hydride NdH _2.7 to elementary neodymium. This dehydrogenation can take place in a vacuum or under protective gas, in particular under argon, at atmospheric pressure or at reduced pressure. Since there are no longer any carbon or oxygen residues in the sintering atmosphere, there can be no undesirable formation of neodymium carbides or neodymium oxides.

Sintering then takes place in an eighth step S8, with heating to a sintering temperature which is preferably between 980 ° C and 1100 ° C. With an appropriate holding time in a vacuum or under protective gas, in particular under argon, at atmospheric pressure or at reduced pressure, the elemental neodymium with the alloy elements iron and boron forms a structure which essentially consists of the phases Nd ₂ Fe ₁₄ , Nd. known from conventional sintered magnets _1+ε Fe ₄ B ₄ and an Nd-rich grain boundary phase.

In a ninth step S9, the sintered body formed by sintering is cooled.

In a tenth step S10, a usual heat treatment known from sintered NdFeB magnets optionally takes place. It is possible for a further post-treatment to take place before the heat treatment, in which in particular additional neodymium, additional dysprosium and/or terbium is introduced into the structure via grain boundary diffusion, for example via an electrochemical process, in particular to further increase the coercive field strength of the permanent magnet to increase.

Optionally, in an eleventh step S11, a magnetic field, in particular a magnetic pulse, is applied to the sintered body in order to magnetize it. The magnetic pulse is preferably applied using a pulse magnetometer.

Fig. 2 shows a schematic representation of a temperature curve in an embodiment of the method. Here a temperature T in °C is plotted against a process time t in minutes. The diagram starts within the process after the fourth step S4, i.e. after injection molding and thus the production of the green body. The temperature curve preferably begins at room temperature, particularly at 25 °C. First, heating takes place via a temperature ramp from room temperature to approximately 150 °C, during a heating time of approximately 80 minutes, with the partial degassing of the neodymium hydride NdH ₃ beginning at approximately 150 °C. This is followed by a steeper temperature ramp, over which a Heat up to around 300 °C for around 60 minutes. The partial degassing and formation of the more stable neodymium hydride with the approximate molecular formula NdH _2.7 is preferably completed during a holding time at approximately 300 ° C for approximately 40 minutes. If chemical debinding of the primary binder component has not already taken place before, in particular before the start of the temperature curve, this preferably takes place now. This is now followed by heating from approximately 300 °C to approximately 500 °C, with the heating ramp lasting approximately 170 minutes. The temperature is then maintained at approximately 500 °C for approximately 125 minutes. During the heating and holding time, thermal decomposition of the secondary binder component occurs, thus thermal debinding. This is followed by heating from 500 ° C to 600 ° C for a period of 115 minutes, and then holding at 600 ° C for 70 minutes, with the neodymium hydride being dehydrogenated to elementary neodymium during the ramp and in particular during the holding time .

The heat is then raised to a temperature of, for example, 1100 °C, namely during a ramp of 250 minutes, with the temperature being maintained at 1100 °C for 130 minutes for sintering. It is possible that the temperature is chosen to be slightly lower than 1100 °C, in particular between 980 °C and 1100 °C, as this is favorable for the magnetic properties of the resulting permanent magnet. After the sintering process has ended, cooling takes place along a non-linear cooling curve, which is determined in particular by the properties of the sintering furnace, with cooling preferably taking place down to room temperature.

Heat treatment can follow. In particular, this is preferably followed by the application of a magnetic field, in particular a magnetic pulse, preferably with a pulse magnetometer, to the sintered body formed during sintering.

Overall, it can be seen that the hydrogenation of the starting material and the neodymium hydride, which is stable at least up to 500 °C and has the approximate molecular formula NdH _2.7 , which is produced by appropriate process control, prevents the reaction of neodymium with oxygen and carbon during the thermal decomposition of the binder material in the MIM process, whereby it allows the formation of a sintered structure with hard magnetic properties, which consists primarily of the phases Nd ₂ Fe ₁₄ , Nd _1+ε Fe ₄ B ₄ and an Nd-rich grain boundary phase, and which is not affected by the formation of undesirable neodymium oxides or neodymium carbides.

Claims (10)

1. Method of manufacturing a permanent magnet comprising at least one rare earth element, wherein

   - a conversion of the at least one rare earth element into an inerted compound is carried out in at least two steps, wherein

   - in at least a first conversion step, hydrogenation of the rare earth element is carried out, wherein

   - in at least one second conversion step, the hydrogenated rare earth element is partially dehydrogenated completely into a hydride which is more stable to carbon and oxygen, wherein

   - a raw material comprising the at least one rare earth element is provided, wherein

   - the raw material is mixed with a binder material, wherein a mixture is formed, wherein

   - the at least one first conversion step is performed before or after formation of the mixture, wherein

   - the mixture is processed in a metal powder injection moulding process, wherein a green body is obtained, wherein

   - the at least one second conversion step is carried out prior to thermal debinding of the green body, wherein

   - thermal debinding of the green body is performed, wherein

   - reconversion of the inerted compound to the rare earth element is carried out by complete dehydrogenation after thermal debinding, wherein

   - the debinded green body is sintered.
2. Method according to claim 1, characterized in that the at least one first conversion step is carried out in an atmosphere comprising or consisting of hydrogen, preferably

   a) at a pressure of at most 2.5 bar, preferably of at least 1.5 bar to at most 3.5 bar, preferably of at least 1.5 bar to at most 3 bar, preferably of at least 1.5 bar to at most 2.5 bar, preferably of at least 1.5 bar to at most 2 bar, preferably of at least 2 bar to at most 3 bar, preferably of at least 2 bar to at most 2.5 bar, preferably of 2.5 bar, and/or

   b) at a temperature of at least 15 °C to at most 200 °C, preferably of at least 15 °C to at most 150 °C, preferably of at least 100 °C to at most 150 °C, preferably of 125 °C, preferably of at least 15 °C to at most 35 °C, preferably of at least 15 °C to at most 30 °C, preferably of at least 20 °C to at most 30 °C, preferably of 25 °C, and/or

   c) for a duration of at least 5 minutes to at most two hours, preferably of at least 30 minutes to at most 90 minutes, preferably of one hour.
3. Method according to any one of the preceding claims, characterized in that the at least one second conversion step is carried out

   a) under vacuum, under an atmosphere comprising or consisting of hydrogen, and/or under an atmosphere comprising or consisting of at least one shielding gas, and/or

   b) at a pressure of at least 100 mbar to at most 1 bar, preferably of at least 100 mbar to at most 700 mbar, preferably of at least 200 mbar to at most 600 mbar, and/or

   c) at an increasing temperature of 100 °C to 350 °C, preferably of 150 °C to 300 °C, and/or

   d) for a duration of at least 20 minutes to at most 120 minutes, preferably of at least 30 minutes to at most 90 minutes, preferably of at least 40 minutes to at most 80 minutes, preferably of at least 50 minutes to at most 70 minutes, preferably of 60 minutes.
4. Method according to one of the preceding claims, characterized in that the binder material comprises at least one secondary binder component which is completely decomposable below a temperature of 500 °C or at most at 600 °C.
5. Method according to any one of the preceding claims, characterized in that the debinding step is carried out

   a) under vacuum, under an atmosphere comprising or consisting of hydrogen, and/or under an atmosphere comprising or consisting of at least one shielding gas, and/or

   b) at a pressure of at least 100 mbar to at most 1 bar, preferably of at least 100 mbar to at most 700 mbar, preferably of at least 200 mbar to at most 600 mbar, and/or

   c) at a temperature of at most 600 °C, preferably of at most 500 °C, preferably of at least 100 °C to at most 500 °C, preferably of at least 200 °C to at most 400 °C, preferably at 300 °C, and/or

   d) for a duration of at least 80 minutes to at most 170 minutes, preferably of at least 100 minutes to at most 150 minutes, preferably of 125 minutes.
6. Method according to any one of the preceding claims, characterized in that the reconversion of the inerted compound is carried out

   a) under vacuum, and/or under an atmosphere comprising or consisting of at least one shielding gas, and/or

   b) at a pressure of at least 100 mbar to at most 1 bar, preferably of at least 100 mbar to at most 700 mbar, preferably of at least 200 mbar to at most 600 mbar, and/or

   c) at a temperature of at least 500 °C to at most 900 °C, and/or

   d) for a duration of at least 40 minutes to at most 100 minutes, preferably of at least 50 minutes to at most 90 minutes, preferably of at least 60 minutes to at most 80 minutes, preferably of 70 minutes, and/or

   e) in a sintering furnace.
7. Method according to any one of the preceding claims, characterized in that the sintering step is carried out

   a) under vacuum, and/or under an atmosphere comprising or consisting of at least one shielding gas, and/or

   b) at a pressure of at least 100 mbar to at most 1 bar, preferably of at least 100 mbar to at most 700 mbar, preferably of at least 200 mbar to at most 600 mbar, and/or

   c) at a temperature of at least 900 °C to at most 1200 °C, preferably of at least 980 °C to at most 1100 °C, and/or

   d) for a duration of at least 100 minutes to at most 160 minutes, preferably of at least 110 minutes to 150 minutes, preferably of 120 minutes to at most 140 minutes, preferably of 130 minutes,

   wherein a sinter body is produced.
8. Method according to any one of the preceding claims, characterized in that, after sintering, cooling and then heat treatment of the sinter body is carried out.
9. Method according to any one of the preceding claims, characterized in that a magnetic field is applied to the raw material and/or to the sinter body.
10. Method according to any one of the preceding claims, characterized in that the raw material comprises or consists of NdFeB.

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