EP0707739A1

EP0707739A1 - Process for the preparation of cobalt/rare earth type magnetic powders containing fluorine and corresponding densified permanent magnets

Info

Publication number: EP0707739A1
Application number: EP94921696A
Authority: EP
Inventors: Armand Gabriel; Henri Lemaire; Fernand Vial
Original assignee: Ugimag SA
Current assignee: Ugimag SA
Priority date: 1993-07-08
Filing date: 1994-07-07
Publication date: 1996-04-24
Anticipated expiration: 2014-07-07
Also published as: FR2707192A1; DE69402551T2; DE69402551D1; WO1995002252A1; FR2707192B1; EP0707739B1

Abstract

This invention pertains to the addition of fluorine to magnetic powders and to sintered permanent magnets of the Sm - Co family in order to enhance their atmospheric corrosion and oxydation resistance. These powders and magnets contain from 600 to 3000 ppm of fluorine, which is introduced in gaseous form during grinding, and/or homogenizing of said powders following grinding.

Description

PROCESS FOR THE PREPARATION OF FLUORINE-CONTAINING RARE EARTH COBALT MAGNETIC POWDERS AND CORRESPONDING DENSIFIED PERMANENT MAGNETS

This invention relates to a method of protecting magnetic powders and permanent magnets of the transition metal-rare earth metal type against oxidation and atmospheric corrosion by the introduction of gaseous fluorine during the treatment of powders. It applies more particularly to powders and magnets of the transition metal - rare earth family, where the metal is essentially cobalt, and the rare earth essentially of samarium and / or neodymium and / or praseodymium and / or cerium. .

The state of the art on these powders and magnets has recently been fully described by K.J. STRNAT "Rare Earth-Cobalt permanent magnets" in "Ferromagnetic Materials", p. 131 to p. 209 Vol. 4, Publisher E.P. WOHLFARTH and K.H. BUSCHOW, North-Holland Amsterdam 1988.

There are essentially two families depending on the amount of rare earth contained:

- the family of molecular formula Sm Co_. (containing 33.8% by weight of

5 rare earth) tolerating little substitution of cobalt by another transition metal such as iron, but where the samarium can be replaced in part or in whole by praseodymium, in part by other rare earths whose preferred are neodymium, cerium, misch-metal, gadolinium depending on the technical or economic property sought.

- the family of molecular formula Sm Co „(containing 23.1% by weight of rare earth) where, in addition, cobalt can be more widely substituted by other metals such as iron, copper, vanadium, zirconium , niobium, titanium, manganese, chromium. The substitution of Co by iron makes it possible in particular to increase the specific magnetization density.

These two families are commonly used by the electrotechnical industry because their high Curie temperature allows their use at high operating temperatures. The most flexible practice for their preparation is Metallurgy αes Powders where we finely grind alloys, we align the grains in the presence of a magnetic field, we compress to the required shape, we sinter at high temperature then applying a hardening heat treatment, we factory according to required specifications.

The review cited above highlights a serious problem (p. 158) of the stability of powders and magnets due to the particular affinity of rare earth for oxygen in the air, especially in the presence of moisture. The natural evolution is towards the formation of the oxide, for example Sm 0 and / or of the hydroxide which are non-magnetic compounds causing a reduction in the density of magnetization and being able to interfere with densification; moreover, their training consumes samarium, therefore shifts the original stoichiometry to make the formula evolve towards a composition which may be unfavorable for the properties.

For example, a theoretical scheme like:

100 Sm Co_. + 21 0 -> 83 Sm Co, _ + 5 Sm_ Co. „+ 7 Sm., 0_- b -. b __ 1 / - __ d 4 2 generates in the sintered material a fine dispersion of Sm Co which immediately leads to a collapse of the coercivity (resistance to demagnetization).

The greatest precautions are therefore taken during the development of these products, when they are in the form of very fine powders (a few microns in Fischer measurement), to produce them, store them, and handle them under cover. of air and humidity, or when, shaped in the press, they must be thermally activated to densify by sintering, in the use of discontinuous or continuous ovens placed under vacuum or under inert gas.

For Sm Co_, the record was obtained, and only in the laboratory,

3 by Narasimham (5th International orkshop on Rare Earth-Cobalt Permanent Magnets and their Applications, June 1981, Ed. University of Dayton, Ohio, USA, p.629 et seq.) Which, proceeding as far as possible air, managed to have only 1000 ppm by weight of oxygen, which nevertheless consumes 0.6% by weight of samarium and generates 0.7% by weight of Sm ₂ 0 ₃ - ^a P ^u obtain, under good conditions of magnetic alignment, a density of immission Br = 1060 mT, a specific energy of (BH) max = 220 kJ / m3.

Industrial techniques cannot reach such a level of precaution, and must be content, for reasonable costs, to be maintained for Sm Co-. around 6000 ppm by weight of oxygen; in particular, b it is very difficult to pump out the gaseous species contained in a tablet. Also the best magnets in this family

3 do not exceed Br = 950 mT and (BH) = 175 kJ / m. max

The same causes produce the same effects in the Sm Co „family where cobalt is substituted, to a lesser degree, however, because the lower samarium content results in a lower affinity for oxygen. However, there are on finished magnets contents close to 4000 ppm by weight of oxygen; it is suspected that small variations around this average cause harmful variations on the rectangularity of the curve J (H), because the magnetic hardening of these alloys results from a polyphase structure where the atoms preferentially segregate and whose obtaining is delicate.

After having examined various means for controlling the intakes and departures of oxygen, and in particular after having estimated the quantities chemi-sorbed definitively and physi-sorbed temporarily in the various stages of the process, the applicant concluded that it was necessary to seek to trap active sites by an atom more electronegative than oxygen making it more labile for its evacuation. It turned out that an addition of fluorine could be effective, because an atom of fluorine or oxygen respectively blocks a third of Samarium atoms or two thirds of samarium atoms in the molecules Sm F_ and Sm 0 „ .

In addition, the Applicant has found that this simple theoretical ratio could be largely exceeded by a particular method consisting in introducing during a fine grinding step, when it is carried out in a gas jet mill, a mixture of nitrogen and fluorine gas in the inert gas stream (nitrogen, argon) for example usually used in these mills. The weight proportions of the N + F mixture and its method of introduction have been determined experimentally so as not to generate safety problems for equipment and the environment. The fluorine content of the grinding gas is generally less than 250 ppm (by volume).

It has been found that an optimum lies in a range from 600 ppm to 3000 ppm by weight of fluorine in the powder or retained in the sintered magnet, this optimum being slightly different for the Sm Co family and the Sm Co family. „; the preferred range is from 600 to 2000 ppm and can reach 600-1500 ppm for Sm Co., - ,. The fluorine contents on powders and on sintered products were determined according to the method consisting in placing the product to be analyzed in acid solution, distilling the fluorine assisted and assaying it by specific electrode.

Fluorine gas is preferably used. With other experimental conditions, it can be replaced by other donors such as hydrofluoric acid HF or nitrogen trifluoride NF; but the former is much more aggressive and difficult to control, the latter is much more expensive. It was noted that: - below 600 ppm, there are not enough neutralized active sites when handling large quantities of fine powders, the process is unstable; - above 3000 ppm, there is too much active fluorine, which transforms into samarium fluoride, - thus displacing the local metallurgical composition by depletion in samarium.

Finally, the Applicant has found that the results could still be slightly improved if the blending homogenization step, which normally follows any grinding in a gas jet mill for the purpose of reducing grinding segregation, was carried out under the same atmosphere of nitrogen plus fluorine mixture.

In particular, storage in humid air shows a slower evolution of oxygen recovery.

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

Figure 1 shows the known ranges of oxygen uptake in the classic manufacture of Sm Co_ magnets.

D

The oxygen contents are established on the LECO commercial apparatus equipped with the appropriate furnace. The process steps are as follows: (1) raw materials; (2) coarse pre-grinding (50 Uni); (3) fine grinding (5 ”m); (4) intermediate storage; (5) after sintering.

FIG. 2 gives the results of the magnetic characteristics (remanence Br, induction coercivity H, limiting stiffness field Hk) and the density p as a function of the rare earth content of the powder mixture and of the sintering temperature (A 1115 ° C; x: 1125 ° C; o: 1135 ° C) according to the prior art.

Figure 3 shows schematically an example of technical embodiment of the invention.

FIG. 4 illustrates, by comparison with FIG. 2, the significant slip obtained according to the invention towards higher optima lying towards lower rare earth contents, in particular below 36% for the compositions of Sm Co type, ..

3

FIG. 5 illustrates a similar gain obtained in the Sm Co family, in particular on the rectangularity of the curve (gain in field of stiffness limit Hk), Hk corresponding to the demagnetizing field reducing the induction to 90% of Br. The curve (1) is representative of the prior art, the curve (2) is representative of the invention.

Example 1 (according to the prior art)

Two alloys have been supplied from outside to constitute:

- the base at 34.2% rare earth (supplier analysis).

- the additive bringing the excess of rare earth to 41.2% (supplier analysis).

These alloys were mechanically ground to around 50 µm (passing sieve) and then mixed to provide the contents plotted on the abscissa of Figure 2. These mixtures were finely ground (4 to 5 Hm Fischer), compressed under magnetic field, frits under vacuum at the temperatures indicated in Figure 2, then returned to 875 ° C for 4 hours and vigorously quenched, as prescribed by conventional treatment of this material (see KJ STRNAT).

One of the compositions was followed in oxygen content at the various stages, to give the average values of FIG. 1. On sintered product, with measurement uncertainties, we place ourselves in the current level of 6000 to 6500 ppm by weight of oxygen.

The interesting physical quantities are shown in FIG. 2. Even though the alloys used are not of the maximum quality that the suppliers deliver from time to time, we find the usual behavior of an extremely critical situation. Density, remanence and coercivity of induction peak for an average content of 36.5% of Sm and are a little sensitive to the sintering temperature; on the other hand, at this content, the limit stiffness field H, which guarantees stability

K thermal, is already collapsed. And the four sizes have collapsed to 36%. We thus realize the criticality of the manufacturing, which will be aligned on an average rate of 37%, but which is at the mercy of an untimely taking of 800 ppm by weight of oxygen.

The classic explanation from the literature is that the oxygen present in the material is labile and captures the samarium of the phases present to stabilize in refractory Sm 0 „. There is then no longer sufficient residual free samarium to assist the grain-to-grain densification of the main phase, said coalescence of the grains being able to even be hampered by intergranular oxides.

Example 2 (according to the invention).

The same alloys were treated according to the claimed procedure. After the coarse grinding, the mixtures were treated in continuous feed in a nitrogen gas jet mill where, according to the diagram in Figure 3, a small amount of d was introduced into the main nitrogen delivered at 100 m3 / h nitrogen loaded with 10% fluorine by means of a calibrated nozzle. Some preliminary experiments are necessary so that the final product contains from 600 to 2000 ppm by weight of fluorine. The continuation of the operations is identical to that of Example 1, and in the results presented in FIG. 4, there was a level of 1000 ppm of fluorine in the finished product. It was surprising to find that: - the final product had captured much less oxygen, around 3500 ppm.

- this resulted in a shift in the response functions towards low rare earth contents (optimum at 35% by weight), with a consequent gain of 20 to 30 mT on the afterglow.

- and above all, a much greater stability of the process with regard to the other quantities.

An explanatory hypothesis can be advanced in the temporary blocking of active samarium sites exposed to grinding by the fixation of a fluorine atom. This then prevents the approach and physi-sorbtion of a molecule with two oxygen, which could evolve towards the harmful oxide Sm 0 „. On the contrary, the simultaneous presence of samarium, fluorine and oxygen would favor, under vacuum and high temperature around 900-1000 ° C, the evolution towards a stable oxyfluoride Sm 0 F, whose formation retains mobility in the samarium in the sintering operation.

Example 3 (according to the invention)

Before being compressed, the finely ground products of Example 2 were homogenized by stirring under an N atmosphere containing 200 ppm of fluorine. A slight gain on the oxygen content was noted, since its content fell to 3000 ppm. This is not enough to have a significant effect on the final properties. However, the wet air resistance could be increased from a few hours to a few days.

Example 4 (according to the invention)

Of all the rare earths, praseodymium in Pr Co .. has the most

5 strong magnetization (1200 mT) against 1100 mT for Sm Co ... But the b attempts to use this more abundant element have always failed, probably in connection with increased oxidizability. So we only use it as a partial substitution when we want to slightly increase the magnetization, at the cost of increased technical instability.

As for Example 2, the preparation was repeated from an alloy basic Sm Co_. 33% substituted with praseodymium. The same advantages have been obtained, as Table I illustrates for the remanence (in isostatic compression).

TABLE 1

These modest gains in absolute terms are very important for applications. Much more important are the reliability and flexibility provided in the manufacture of products, with less critical heat treatments, including the quenching speeds, especially when one wants to use higher praseodymium contents.

Example 5 (according to the invention) The method was also applied to alloys of the S Co family. For this, we chose a composition rich in Fe and poor in Sm with the hope of increasing the afterglow.

For composition by weight%

Sm = 25 Fe = 22 Cu = 4.5 Zr 2.4 Co = balance

The usual process makes it possible to obtain the excellent performances of FIG. 5, with Br = 1160 mT, H 1070 kA / m, H _, = 770 kA / m, (BH) cJ cB max 240 kJ / m3. But this product is handicapped by the lack of rectangularity of the curve, resulting in an H of only 620 kA / m. Such a magnet contains approximately 4000 ppm of oxygen. With the process according to the invention, and at a level of 700 ppm of fluorine, it it was possible to reduce the oxygen content to 2000 ppm and to lower the samarium to 24.7% by weight. Two beneficial effects have been obtained:

- the rise in remanence to 1180 - 1190 mT - the best rectangularity at H = 850 kA / m

Claims

1. Magnetic powder or permanent magnet from the Sm Co Sm family

Co_ or Sm_ Co, _, characterized in that it (it) contains between 600 and b 2 17

3000 ppm of fluorine.

2. Magnetic powder or permanent magnet according to claim 1 characterized in that it (it) contains between 600 and 2000 ppm of fluorine.

3. Magnetic powder or permanent magnet according to claim 1 or 2 aappppaarrtteennaanntt àà llaa ffaammiillllee ddeess SSmm CCoo tcaractérisé in that it (it] contains between 600 and 1500 ppm of fluorine.

4. Magnetic powder and permanent magnet according to one of claims 1 to 2 containing less than 36% (by weight) of rare earths in alloys type Sm Co_ .. b

5. Magnetic powder and permanent magnet according to claim 3 characterized in that it (it) contains less than 25% of rare earths.

6. Method for obtaining magnetic powder or permanent magnet according to one of claims 1 to 5 characterized in that a mixture of gases containing less is introduced into the mill and / or in the powder homogenization step 250 ppm (by volume) of gaseous fluorine.