IE912618A1 - PROCESS FOR PRODUCING MAGNETIC MATERIAL BASED ON THE Sm-Fe-N¹MATERIAL SYSTEM - Google Patents

Abstract

The magnetic material of the Sm-Fe-N system contains a crystalline, magnetically hard phase having the Th2Zn17 crystal structure with N atoms incorporated in its crystal lattice. According to the invention, this magnetic material is to be prepared by first preparing a precursor containing the two-component Sm2Fe17 phase by mechanical alloying and heat treatment. The precursor is to have a predetermined microstructure. The precursor is then heated in a nitrogen-containing atmosphere, and in this process the magnetically hard phase is formed. The precursor with the predetermined microstructure can also be obtained by means of a quenching technique. <IMAGE>

Description

Siemens Aktiengesellschaft Process for producing magnetic material based on the Sm-Fe-N material system.

The invention relates to a process for producing magnetic material based on the Sm-Fe-N material system containing a crystalline, hard-magnetic phase with a Th2Zn17 crystal structure whose crystal lattice incorporates N atoms, by heating a starting product of the magnetic material to be produced containing a twocomponent Sm2Fe17 phase in a nitrogen-containing atmosphere.

Magnetic materials based on material systems which contain a rare earth metal and a transition metal and which are distinguished by high coercive field strengths Hcl and high energy products (B*H)max have been known for some years. The main representatives are Co-Sm as binary material system and Nd-Fe-B as ternary material system. Their hard-magnetic properties are due to inter20 metallic compounds having a high magnetic anisotropy and a high structure formation in the specific materials. These magnetic materials can be produced, for example, by sintering powders of the components of the corresponding material system (cf., for example, EP-A-0,134,304). In addition, it is also possible to produce corresponding magnetic materials by means of a so-called rapid solidification technique (cf., for example, EP-A-0,284,832).

In the case of the corresponding ternary magnetic materials, the Sm-Fe-Ti material system is also being discussed (cf. J.App.Phys., Vol. 64, No. 10, 1988, pages 5720 to 5722). Recently, the existence of Sm2Fe17Nx has also been known as a magnetic material. This material has the known Th2Zn17 crystal structure. To produce it, Sm2Fe17 is melted as starting material. The starting product thus obtained is then heated in an N2 or NH3 atmosphere, in which process the desired hard-magnetic phase is formed by incorporation of N atoms in the lattice structure of the starting product (cf. lecture by - 2 10 J.M.D.Coey during the conference Nato Advanced Study Institute on the Science and Technology of Nanostructured Magnetic Materials, 25.6 to 7.7.1990, Heraklion, Crete, GR) .

The object of the present invention is to develop this known process for producing a hard-magnetic material based on the Sm-Fe-N material system in a manner such that the formation of the desired hard-magnetic phase can be ensured in a comparatively simple manner and, at the same time, a material with high coercive field strength Hci of, in particular, over 5 kA/cm can be obtained.

This object is achieved, according to the invention, for a first process having the features mentioned at the outset in that a starting product containing the two-component Sm2Fe17 pha^v^H~Tirst produced by mechanically alloying corresponding starting powders and lonoi (having a microstructure corresponding to the hardmagnetic phase of the magnetic material^fie—produoodl by heat treatment, and in that said starting product is then converted into the hard-magnetic phase of the Sm-Fe-N material system in the nitrogen-containing atmosphere.

In this connection, the invention is based on the discovery that a material which has hard-magnetic properties can be obtained by means of the incorporation of N atoms inside the known Th2Zn17 crystal structure without change in the lattice type. At the same time, the starting product already having the microstructure of the final product to be produced can be advantageously formed by means of the process of mechanical alloying, which is known per se. Said starting product thus structured can then be nitrated comparatively simply and reproducibly at elevated temperature.

A further process for producing the magnetic material is characterised in that a startir^j^roduct containing the two-component Sm2Fe17 phaseY is first produced by rapid solidification of a molten master alloy of the composition Sn^Femo-j, where 10 χ < 20 (in atomic % in all cases) £having a microstructure corresponding to the hard-magnetic phase of the magnetic material^, and in - 3 that said starting product is then converted into the hard-magnetic phase of the Sm-Fe-N material system in the nitrogen-containing atmosphere.

Advantageous developments of the processes 5 according to the invention for producing the magnetic material emerge from the subclaims.

To explain the invention further, reference is made below to the diagrams of the drawing. In these, Figures 1 and 2 show X-ray diffraction curves and a magnetisation curve, respectively, of a magnetic material obtained by means of the mechanical alloying process according to the invention. Figures 3 and 4 show X-ray diffraction curves and the hysteresis loop, respectively, of a magnetic material for whose production a rapid solidification technique according to the invention was used.

The magnetic material having the required hardmagnetic properties can advantageously be formed by the processes outlined below as process A and process B, respectively: Process A In order to obtain the hard-magnetic material of the Sm-Fe-N material system, a starting product containing an Sm2Fe17 phase is first produced using a grinding process. For this purpose, the starting point is powders composed of or containing the components involved. Either elemental powders are used, or the elements involved may also be present in the form of alloys and/or compounds. The powdered starting components having predetermined, generally commonly available particle sizes are introduced into a suitable grinding apparatus such as is known in principle from mechanical alloying processes (cf., for example, Metallurgical Transactions, Vol. 5, Aug. 1974, pages 1929 to 1934). In this connection, the quantitative ratio of the individual components is determined by the predetermined resultant atomic concentration of the starting product to be prepared. Thus, an initial mass (in atomic %) should be provided which corresponds to the alloy Sm12 5Fe87 5 . The, in particular, elemental powders of the components involved are then subjected to the grinding process with the aid of hardened steel balls in a, for example, Arfilled steel container. The duration tm of the grinding process depends, in particular, on the grinding parameters. Important parameters are the ball diameter, the number of balls and the materials used in the grinding apparatus. The grinding rate and the ratio of steel balls to the amount of powder are also further parameters which determine the grinding duration necessary. In general, the grinding duration tn is between 1 and 100 hours. A grinding duration of two to three days is advantageous.

Optionally, the grinding process can also be undertaken at elevated temperature.

A two-phase grinding product composed of amorphous Sm-Fe and of finely divided a-Fe is then present at the end of the grinding process. The Sm-Fe phase may also possibly be at least partially crystalline. In this grinding product, the desired Th2Zn17 crystal structure having a predetermined grain size optimised in relation to the magnetic material to be produced still has to be established. The grain size (size of the crystallites in the grains) should under these circumstances be between 30 nm and 500 nm. For this purpose, a heat treatment is carried out under protective gas or vacuum at a temperature between 500°C and 1000°C, preferably between 650°C and 800°C. The duration of said heat treatment is between 1 minute and 10 hours, preferably between 10 minutes and 1 hour. Said heat treatment results, on the one hand, in a crystallisation of the Th2Zn17 phase and, on the other hand, in the establishment of the very fine microstructure which is indispensable for the magnetic hardening in this material. A corresponding starting product containing the soft magnetic Sm2Fe17 phase is then present at the end of the heat treatment.

In a further step, the powdered starting product - 5 thus obtained containing the Sm2Fe17 phase is then annealed in a nitrogen (N2) atmosphere. The temperature to be chosen for this purpose should be above 300’C and below 600°C. In general, a total duration of between 1 and 1000 hours, advantageously between 10 and 300 hours should be provided for this heat treatment. In this process, incorporation of nitrogen atoms in the crystal lattice takes place. This thus yields a compound Sm2Fe17Nx. In this connection it was found that this compound exists for nitrogen concentrations x for which 0 < x < 3.

It is furthermore remarkable from the process engineering point of view that the thermal stability of the compound Sm2Fe17Nx decreases markedly as the N content falls. This means that if the nitration is too rapid at, for example, 500°C, the Th2Zn17 phase may possibly decompose. Thus, for example, the decomposition temperature for x = 0.4 is about 100°C lower than for x = 2.94. For this reason, as high as possible x values (in the vicinity of x = 3) are to be regarded as advantageous.

Table 1 below shows the marked dependence of the decomposition temperature Td [in °C] on the nitrogen concentration x [in atoms per unit cell]. The measured values reported are approximate values above which a decomposition occurs (onset values): Table 1 x [N atoms per unit cell ] 0.5 1 1.5 2 2.5 Td [°C] 602 627 643 659 673 Owing to this dependence of the thermal stability of the Sm2Fe17Nx compound on the nitrogen concentration, it is particularly advantageous if the starting product nitriding process is carried out in two stages in relation to the temperature conditions, a temperature being chosen for the first stage which is, in particular, at least 50’ lower than for the second stage. An illustrated example of a corresponding two-stage nitration is specified below: - 6 First nitriding stage The nitriding is carried out at a temperature Tnl between 300"C and 400 °C for a duration tnl of between 10 and 1000 hours, the time specifically to be chosen depending on the grain size of the powder of the starting product to be nitrided. The N loading should take place at least up to a concentration of x = 1.5. Corresponding examples emerge from Table 2 below: Table 2 Grain size [pm] 10 10 5 5 Tnl [°C] 400 350 400 350 tnl 64 256 16 64 Second nitriding stage A further loading with nitrogen is carried out up to a maximum possible concentration of x < 3 at a temperature Tn2, which is higher than the temperature Tnl of the first nitriding stage. For example, for a grain size of pm, a temperature Tn2 of 500 °C is provided for a duration tn2 of 16 hours.

In this two-stage nitriding process, the thermal stability of the Sm2Fe17 nitride is advantageously increased in the first nitriding step to such an extent that the hard-magnetic phase cannot decompose at the higher temperature Tn2 necessary in the second nitriding step for complete nitriding.

The starting product nitriding takes place with expansion of the Th2Zn17 crystal structure without change in the lattice type and without change in the microstructure. This state of affairs is evident from the X-ray diffraction curves (X-ray diffraction patterns) which are shown in the diagram of Figure 1. In the diagram, the diffraction angle 2* Theta (0 in degrees) is plotted on the horizontal axis, while the associated intensity I (in arbitrary units of count rate per second) is plotted in the direction of the vertical axis. The curve marked Dl shown in the upper part of the diagram in this case shows the diffraction curve for the ground product of the composition Sm12 5Fea7 5 after mechanical alloying. The curve has the appearance typical for an amorphous state and additionally exhibits two broadened a-Fe reflections. The Th2Zn17 crystal structure of the starting product which establishes itself after a heat treatment at 700°C is evident from the central curve marked D2. This diffraction curve still contains residues of α-Fe. On the other hand, the lower diffraction curve designated D3 results for the Th2Zn17 crystal structure expanded by means of the incorporated N atoms at 500°C.

From a comparison of the two curves D2 and D3 it can readily be perceived that, in the case of the magnetic material according to the invention, the lattice type remains unchanged compared with the starting product.

The hysteresis curve of the crystalline hard20 magnetic final product composed of the Sm-Fe-N material system is shown by the diagram of Figure 2 as a curve designated m. In this diagram, the field strength H (in kA/cm) is plotted in the direction of the horizontal axis and the magnetisation J (in T) is plotted in the vertical axis direction. The hysteresis curve m exhibits a remanence Jr of about 0.71 T and a coercive field strength Hei of about 23.5 kA/cm.

The curves shown in the diagrams of Figures 1 and 2 are obtained if an annealing duration of 1/2 hour is provided for each of the individual annealing treatments.

Magnetic materials whose data found at 20°C are listed in the table below were also prepared with equivalent annealing times by the process A. The materials were nitrided at 500°C - 8 Table Material Coercive field Remanence Energy (initial mass strength Hci Jr density in atomic %) (kA/cm) (T) 5 (kJ/m3) 15.3 0.82 55 Smu 5Fe88 5+N 17.7 0.79 68 2Π*12.5^*®87.5+Ν 23.6 0.71 87 10 Sm13 5Fe86.5+N 24.0 0.69 73 The increase in the coercive field strength Hci and the decrease in the remanence Jr as the Sm component increases can be inferred from the table, while the energy density B*H passes through a maximum.

Process B As a departure from the process A which has been depicted, the magnetic material according to the invention can also be obtained equally as well via a starting product which has been prepared using a rapid solidifica20 tion technique. For this purpose, the starting components of the starting product having adequate purity first have to be melted in an Ar atmosphere to produce a master alloy. The proportions of the individual components are in this case so chosen that the master alloy has the composition SmxFe100-x, where x is between 10 and 20 (in atomic % in all cases). Pyrolytic BN or Al2O3 crucibles can be used for melting. In particular, melting in an electric arc furnace is also possible. The master alloy thus obtained from the starting components can then be converted by means of the known rapid solidification technique into a finely crystalline starting product. Advantageously, the so-called melt spinning can be provided for this purpose - a process which is generally known for producing amorphous metal alloys (cf., for example, Zeitschrift fur Metallkunde, Vol. 69, No. 4, 1978, pages 212 to 220). According to this, the master alloy is melted under protective gas such as, for example, Ar or under vacuum, for example, in a quartz or BN crucible by means of high frequency at a temperature between 1300°C and 1500eC, preferably between 1350°C and 1450°C and then extruded through a quartz nozzle having a nozzle diameter of, for example, 0.5 mm and at an extrusion pressure of, for example, 0.25 bar onto a rotating substrate such as, for example, onto a copper wheel or a copper roller. The wheel should at the same time rotate with a rotary speed such that a substrate velocity vs of between 5 m/s and 60 m/s, preferably between 10 m/s and 25 m/s is produced on the wheel circumference. Short strip-type pieces of the starting product which are comparatively brittle and contain Sm2Fe17 with the Th2Zn17 crystal structure as the main phase are thus obtained. In order to establish an optimum microstructure of the starting product one alternative is to provide a choice of suitable quenching parameters. Thus, for example, quenching from a melt heated to 1400°C with a substrate velocity v„ of between 15 m/s and 20 m/s is particularly beneficial. In addition, a thermal posttreatment, in particular of very rapidly quenched strips (at v, > 50 m/s) at temperatures between 500eC and 1000’C, preferably between 650*C and 800°C, can also be provided.

In that case the annealing duration is in general between minute and 10 hours, preferably between 10 minutes and 1 hour. During such a thermal post-treatment, the softmagnetic SmFe2 phase of the starting product is converted into the magnetically harder SmFe3 phase of an inter30 mediate product.

The starting or intermediate product thus to be obtained in the process B is advantageously also comminuted mechanically, preferably ground as finely as possible, before the final thermal treatment in a nitrogen35 containing atmosphere in order to achieve a reduction of the nitriding times necessary. In particular, the starting or intermediate product can be pulverised or crushed in a mortar to grain sizes below 40 pm. It is then subjected, in accordance with process A, to an - 10 10 annealing at temperatures between 300°C and 600°C for the purpose of magnetic hardening with the formation of the desired Sm2Fe17Nx phase.

The gradual formation of the hard-magnetic Sm2Fe17Nx phase by means of the various temperature treatments can be read off from the diagram of Figure 3. In this diagram, three X-ray diffraction curves of rapidly quenched Sm15Fe85 corresponding to Figure 1 are shown, and specifically a) after quenching from T = 1400°C at vs = 20 m/s (curve D4), b) after an additional annealing for 4 hours at 800’C in Ar (curve D5) and c) after a final annealing of 16 hours at 400eC in an N2 atmosphere (curve D6).

In this, peaks and also the α-Fe reflections to be assigned to the SmFe2 phase and the SmFe3 phase are specially marked in the diagram.

From Figure 4, a further hysteresis curve emerges which is obtained for a magnetic material of the composition Sm13Fe87Nx. This material was produced in accordance with the material which yields the curve D6 shown in Figure 3. For Figure 4, a representation corresponding to Figure 2 has been chosen. A maximum coercive field strength Hci of about 17.9 kOe, a remanence Jr of 0.63 T and an energy density (B*H)max of 44 kJ/m3 can be read off from the curve shown in Figure 4.

Claims (14)

1. Patent Claims

1. Process for producing magnetic material based on the Sm-Fe-N material system containing a crystalline, hard-magnetic phase with a Th 2 Zn 17 crystal structure whose 5 crystal lattice incorporates N atoms, by heating a starting product of the magnetic material to be produced containing a two-component Sm 2 Fe 17 phase in a nitrogencontaining atmosphere, characterised in that a starting { product containing the two-component Sm 2 Fe 17 phaseVIs 10 first produced by mechanically alloying corresponding starting powders and to nol (having Ffe-hel microstructure corresponding to the hard-magnetic phase of the magnetic material) fro produced/ by heat treatment, and in that said starting product is then converted into the hard-magnetic 15 phase of the Sm-Fe-N material system in the nitrogencontaining atmosphere.

2. Process according to Claim 1, characterised in that a two-phase grinding product composed at least partially of amorphous Sm-Fe and of α-Fe is produced by 20 means of mechanical alloying and said grinding product is then converted by means of the heat treatment into the crystalline starting product with the predetermined microstructure.

3. Process according to Claim 1 or 2, characterised 25 in that the heat treatment for producing the starting product is undertaken at a temperature between 500“C and 1000°C, preferably between 650°C and 800°C.

4. Process according to any of Claims 1 to 3, characterised in that the heat treatment for producing 30 the starting product is undertaken for a duration of between 1 minute and 10 hours, preferably between 10 minutes and 1 hour.

5. Process for producing magnetic material based on the Sm-Fe-N material system containing a crystalline, 35 hard-magnetic phase with Th 2 Zn 17 crystal structure whose crystal lattice incorporates N atoms, by heating a starting product of the magnetic material to be produced containing a two-component Sm 2 Fe 17 phase in a 90 Ρ 8552 DE nitrogen-containing atmosphere, characterised in that a starting product containing the two-component Sm 2 Fe 17 phaseY is u first produced by rapid solidification of a molten master alloy of the composition Sm I Fe 100 _ x , where 10 < x < 20 (in atomic % in all cases) ^having a microstructure corresponding to the hard-magnetic phase of the magnetic material), and in that said starting product is then converted into the hard-magnetic phase of the Sm-FeN material system in the nitrogen-containing atmosphere.

6. Process according to Claim 5, characterised in that the master alloy melted at a temperature between 1300°C and 1500 e C, preferably between 1350°C and 1450°C, is quenched by means of a melt spinning process by extruding the molten master alloy onto a rotating substrate having a circumferential velocity of between 5 m/s and 60m/s, preferably between 10 m/s and 25 m/s.

7. Process according to Claim 5 or 6, characterised in that the starting product is comminuted mechanically before the nitrogen treatment.

8. Process according to Claim 7, characterised in that grain sizes below 40 ^m are produced by means of the mechanical comminution.

9. Process according to any of Claims 5 to 8, characterised in that the starting product containing the Sm 2 Fe 17 phase is thermally post-treated at a temperature of between 500*C and 1000C, preferably between 650 e C and 800’C, under protective gas or vacuum.

10. Process according to any of Claims 1 to 9, characterised in that the formation of the hard-magnetic phase is undertaken in the nitrogen-containing atmosphere at a temperature between 300°C and 600 a C.

11. Process according to Claim 10, characterised by a two-stage process for forming the hard-magnetic phase, in which a temperature which is lower than the temperature to be chosen for the second stage is provided in the first stage.

12. Process according to Claim 11, characterised in that a temperature of between 300°C and 400°C is provided for the first stage.

13. Process according to any of Claims 10 to 12, characterised in that the heat treatment for forming the hard-magnetic phase is undertaken in the nitrogencontaining atmosphere for a total duration of between 1 5 and 1000 hours, preferably between 10 and 300 hours.

14. A process for producing magnetic material according to any preceding claim substantially as hereinbefore described.