DE69402551T2 - A RARE EARTH-COBALT TYPE OF FLUORINE-CONTAINING MAGNETIC POWDER AND DENSATED PERMANENT MAGNETS THEREOF AND THEIR PRODUCTION PROCESS - Google Patents

Description

The invention relates to a method for protecting magnetic powders and permanent magnets of the transition metal-rare earth metal type against oxidation and atmospheric corrosion by adding gaseous fluorine during the treatment of the powders. More specifically, it relates to powders and magnets of the transition metal-rare earth metal type in which the metal is essentially cobalt and the rare earth metal is essentially samarium and/or neodymium and/or praseodymium and/or cerium.

Patent FR 2 003 246 discloses a process for treating permanent magnet powders containing rare earth metals and transition metals in order to increase their coercive force; this process consists in treating the powders with an etching liquid based on an aqueous solution of hydrochloric acid, sulphuric acid or hydrofluoric acid.

In addition, the state of the art regarding these powders and magnets was described in detail some time ago, cf. K.J. Strnat, "Rare Earth-Cobalt Permanent Magnets" in "Ferromagnetic Materials", pp. 131-209, vol. 4, ed. E. P. Wohlfahrth and K.H. Buschow, North-Holland Amsterdam 1988.

Depending on the amount of rare earth metal contained, two main groups are distinguished:

- the group of molecular formula SmCo₅ (with 33.8 wt.% rare earth metal), in which only a small proportion of the cobalt is replaced by another transition metal, such as iron, However, depending on the desired technical or economic properties, the samarium may be partially or completely replaced by praseodymium or partially by other rare earth metals, of which neodymium, cerium, mischmetal and gadolinium are preferred.

- the group of the molecular formula Sm₂Co₁₇ (with 23.1 wt.% rare earth metal), in which the cobalt can also be replaced to a large extent by other metals such as iron, copper, vanadium, zirconium, niobium, titanium, manganese and chromium. By replacing Co with iron, the specific magnetization density can be increased in particular.

Materials in these two groups are widely used in electrical engineering because their high Curie temperature allows them to be used at high working temperatures. The most flexible method for their production is powder metallurgy, in which alloys are finely crushed and the grains are aligned in the presence of a magnetic field, pressed into the desired shape and sintered at high temperatures, followed by a thermal treatment for hardening and then machined according to the required specification.

The above-mentioned article highlights the serious problem (p. 158) of the stability of powders and magnets due to the special affinity of rare earth metals to atmospheric oxygen, especially in the presence of moisture. Usually, oxides, such as Sm₂O₃, and/or hydroxides are formed, which are non-magnetic compounds that can cause a reduction in the magnetization density and interfere with compaction; their formation Furthermore, samarium is consumed, which shifts the original stoichiometry and changes the formula towards a composition that may be unfavorable in its properties.

For example, according to the theoretical scheme

100 SmCo&sub5; + 21/4 O2 T 83 SmCo&sub5; + 5 Sm₂Co₁₇ + 7/2 Sm2O3

creates a fine dispersion of Sm₂Co₁₇ in the sintered material, which immediately leads to a breakdown of the coercivity (resistance to demagnetization).

Even if these products, when in the state of very fine powders (a few microns according to Fischer), are manufactured, stored and handled with the utmost care, protected from air and moisture, or shaped by compression, they must be thermally activated for densification by sintering using discontinuous or continuous furnaces under vacuum or inert gas.

For SmCo₅, a record was set by Narasimham (Sth International Workshop on Rare Earth-Cobalt Permanent Magnets and their Applications, June 1981, ed. University of Dayton, Ohio, USA, p. 629 ff.), but only in the laboratory, in that by working under maximum exclusion of air, an oxygen content of only 1000 ppm (by weight) oxygen was achieved, which nevertheless corresponded to a consumption of samarium of 0.6 wt.% and led to the formation of 0.7 wt.% Sm₂O₃. The author was able to achieve a magnetic flux density Br of 1060 mT and an energy density (BH) max of 220 kJ/m³ under good conditions of magnetic alignment.

Such a level of precaution cannot be achieved in technical processes and, for cost reasons, one must be satisfied with about 6000 ppm (by weight) of oxygen in the case of SmCo5; in particular, it is very difficult to remove the gaseous species contained in a molded article by pumping. Even the best magnets in this group do not achieve values higher than Br = 950 mT and (BH) max = 175 kJ/m3.

The same causes produce the same effects in the Sm₂Co₁₇ group, in which cobalt is replaced, albeit to a lesser extent, because the lower samarium content produces a lower affinity for oxygen. In the finished magnets, however, oxygen contents of around 4000 ppm (by weight) are found; it is assumed that small variations around this mean value in the squareness of the J (H) curve cause harmful changes, since the magnetic hardening of these alloys results from a multiphase structure in which the atoms preferentially segregate and which is difficult to manufacture.

After examining various means for controlling the uptake and release of oxygen and in particular after estimating the amounts definitively chemisorbed and temporarily physisorbed in the various process steps, the applicant concluded that it was necessary is to trap the active sites with an atom that is more electronegative than oxygen, making the oxygen more labile to removal. It has been shown that the introduction of fluorine could be effective, since a fluorine atom and an oxygen atom in the SmF₃ and Sm₂O₃ molecules block one-third of the samarium atoms and two-thirds of the samarium atoms, respectively.

The applicant has also found that this simple theoretical ratio can be greatly exceeded by a specific process consisting in introducing, during fine grinding when carried out in a gas jet mill, a mixture of nitrogen and fluorine gas into the inert gas stream (nitrogen, argon) commonly used in these grinders, for example. The weight ratios of the N₂ + F₂ mixture and the method of feeding it have been determined experimentally so as not to create any safety problems for the equipment or the environment. The fluorine gas content during grinding is generally less than 250 ppm (by volume).

It has been found that an optimum fluorine content in the powder or in the sintered magnet is found in a range of 600 to 3000 ppm (by weight), which is slightly different for the SmCo₂ and Sm₂Co₁₇ group; the preferred range is from 600 to 2000 ppm and can reach 600 to 1500 ppm for Sm₂Co₁₇. The fluorine content in the powders and sintered products was determined by a method which consists in placing the product to be analyzed in an acid solution, subjecting the fluorine to assisted distillation and and quantified at a specific electrode.

Fluorine gas is preferably used. Under other experimental conditions it can be replaced by other donor compounds such as hydrofluoric acid HF or nitrogen trifluoride NF3; however, the former is much more aggressive and difficult to control, the latter is much more expensive.

The following was found:

- below 600 ppm, there are not enough switched-off active sites when a large amount of fine powders are processed and the process is unstable;

- above 3000 ppm there is too much active fluorine, which forms samarium fluoride, thereby shifting the local metallurgical composition by depletion of samarium.

Finally, the applicant has found that these results can be somewhat improved if the homogenisation step in the mixer, which normally follows any comminution using gas jet mills in order to reduce segregation during comminution, is carried out under the same atmosphere of a mixture of nitrogen and fluorine.

Especially when stored in humid air, oxygen absorption is slower.

To clarify the invention, the applicant relies on the figures and the following examples.

Fig. 1 shows the known ranges of progressive oxygen uptake in the conventional production of SmCo₅ magnets.

The oxygen contents were determined using a commercially available LE-CO device equipped with a suitable furnace. The process steps are as follows: (1) starting materials; (2) coarse pre-crushing (50 µm); (3) fine crushing (5 µm); (4) intermediate storage; (5) after sintering.

Fig. 2 shows the results of the magnetic properties (magnetic flux density Br, coercive force HcB, magnetic reversal limiting field strength HK) and the density as a function of the rare earth metal content in the powder mixture and the sintering temperature (Δ 1115 ºC; X 1125 ºC; 0 1135 ºC) according to the state of the art.

Fig. 3 shows schematically an example of the technical implementation of the invention.

Fig. 4, in comparison with Fig. 2, illustrates the significant shift achieved according to the invention towards higher maxima, which lie at lower contents of rare earth metals, in particular below 36% for the compositions of the SmCo₅ type.

Fig. 5 illustrates a similar improvement achieved in the Sm₂Co₁₇ group, particularly with regard to the squareness of the curve (improvement of the remagnetization limit field strength at the limit of the strength HK), where HK corresponds to the demagnetization field through which the flux density Br is reduced to 90%. Curve (1) represents the state of the art, curve (2) is representative of the invention.

Example 1 (state of the art)

Two alloys were sourced from the market:

- the base alloy with 34.2% rare earth metal (analysis by the supplier),

- an additive resulting in an excess of rare earth metal of 41.2% (supplier's analysis).

These alloys were mechanically crushed to about 50 µm (sieve passage) and then mixed, whereby the values plotted on the abscissa of Fig. 2 were obtained. These mixtures were finely crushed (4 to 5 micrometers according to Fischer), pressed in a magnetic field, sintered in a vacuum at the temperatures indicated in Fig. 2, allowed to cool to 875 ºC for 4 hours and vigorously quenched as prescribed for the conventional treatment of this material (see K.J.Strnat).

For one of these compositions, the oxygen content was monitored in the various steps, resulting in the average values shown in Fig. 1. In the sintered product, the oxygen content, apart from measurement inaccuracies, is in the range of 6000 to 6500 ppm (by weight).

The physically interesting quantities are shown in Fig. 2. Although the alloys used were not of the best possible quality, which is provided by the suppliers from time to time, the The flux density, the remanence and the coercive force do not increase any further for an average content of 36.5% Sm and are somewhat sensitive to the sintering temperature; at this value, however, the magnetic reversal limit field strength HK, which guarantees thermal stability, has already fallen; all four values have fallen at 36%. It can thus be seen that the production, which is carried out at an average content of 37%, is critical, the cause being the unwanted absorption of 800 ppm oxygen (by weight).

The classic explanation in the literature is that the oxygen present in the material is labile and scavenges the samarium of the phases present in order to stabilize itself in refractory Sm₂O₃. There is no longer enough free residual samarium to support the grain-by-grain densification of the main phase, and the coalescence of the grains can even be disturbed by oxides at the grain boundaries.

Example 2 (according to the invention)

The same alloys were treated according to the claimed process. After coarse grinding, the mixtures were treated with continuous feeding in a gas jet mill where, according to the scheme in Fig. 3, the main nitrogen flow, which was passed through at 100 m³/h, was mixed with a small amount of nitrogen to which 10% fluorine was added as a calibrated addition. Some preliminary tests are necessary to ensure that the final product contains 600 to 2000 ppm (by weight) of fluorine. The sequence of operations is identical to that of Example 1; according to the results shown in Fig. 4, 1000 ppm of fluorine are present in the final product. It was surprisingly found that

- the final product had absorbed significantly less oxygen, about 3500 ppm,

- this causes a shift of the resulting curves to lower rare earth metal contents (maximum at 35 wt.%) and therefore a gain of 20 to 30 mT in terms of remanence

- and above all a significantly higher stability of the process with regard to the other variables is achieved.

One possible explanation is the hypothesis that the reactive sites of the samarium, which were exposed during crushing, were temporarily blocked by the binding of a fluorine atom. This then prevents the approach and physisorption of an oxygen molecule with two oxygen atoms, which could lead to the undesirable oxide Sm₂O₃. The simultaneous presence of samarium, fluorine and oxygen, on the other hand, promotes the formation of stable fluoride oxide SmOF under vacuum at high temperatures of around 900 to 1000 ºC, the formation of which preserves the mobility of the samarium during sintering.

Example 3 (according to the invention)

Before pressing, the finely ground products of Example 2 were homogenized by stirring under N₂ atmosphere containing 200 ppm fluorine. A slight No benefit was found in terms of oxygen content, as the level dropped to 3000 ppm. This is not enough to have a significant impact on the final properties. However, resistance to humid air was improved from a few hours to a few days.

Example 4 (according to the invention)

Of all the rare earth metals, praseodymium in PrCo₅ has the strongest magnetization (1200 mT compared to 1100 mT for SmCo₅). However, attempts to use this more abundant element have always failed, probably due to its higher oxidizability. Therefore, it is only used in partial substitution when the magnetization needs to be slightly increased, at the expense of greater technical instability.

As for Example 2, the preparation was repeated starting from a base alloy of SmCo₅ in which there was a 33% replacement by praseodymium. The same advantages were obtained as shown in Table 1 for the remanence (isostatic pressing). Table 1

These gains, which seem small when viewed in absolute terms, are very significant for the applications. The reliability and flexibility in manufacturing the products are significantly higher, combined with less critical thermal treatments, including quenching rates, especially when higher praseodymium contents are to be used.

Example 5 (according to the invention)

The process was also applied to alloys of the Sm₂Co₁₇ group. For this purpose, a composition with a high Fe content and a low Sm content was chosen in the hope of increasing the remanence.

Composition in wt.%:

Sm = 25, Fe = 22, Cu = 4.5, Zr = 2.4 and Co balance.

By the conventional method, the excellent properties of Fig. 5 with Br = 1160 mT, HcJ = 1070 kA/m, HcB =770 kA/m and (BH)max = 240 kJ/m³ can be achieved. However, this product is disadvantageous due to the lack of squareness of the curve, which leads to a value of HK of only 620 kA/m. Such a magnet contains about 4000 ppm oxygen.

Using the process according to the invention and a fluorine content of 700 ppm, it was possible to reduce the oxygen content to 2000 ppm and the samarium content to 24.7 wt.%. Two beneficial effects were achieved:

- Increase in remanence to 1180 - 1190 mT,

- better squareness at HK = 850 kA/m.

Claims (6)

1. Magnetic powders or permanent magnets of the group Sm-Co of the type SmCo₅ or Sm₂Co₁₇, characterized in that they contain 600 to 3000 ppm (by weight) of fluorine. 2. Magnetic powders or permanent magnets according to claim 1, characterized in that they contain 600 to 2000 ppm (by weight) fluorine. 3. Magnetic powders or permanent magnets according to claim 1 or 2 of the Sm₂Co₁₇ type, characterized in that they contain 600 to 1500 ppm (by weight) fluorine. 4. Magnetic powders and permanent magnets according to claim 1 or 2, which contain less than 36 wt.% of rare earth metals in the SmCo₅ type alloys. 5. Magnetic powders and permanent magnets according to claim 3, characterized in that they contain less than 25 wt.% rare earth metals. 6. Process for producing magnetic powders or permanent magnets according to one of claims 1 to 5, characterized in that a gas mixture containing less than 250 ppm (by volume) of gaseous fluorine is fed into the comminution device and/or during the step of homogenizing the powder.