EP0468903B1 - Method for obtaining powdered magnetic material of the rare earth-transition metal-boron type for corrosion resistant magnets - Google Patents

Abstract

The invention relates to a method of obtaining friable and relatively inert TR Fe B type magnetic materials in divided form which lead to magnets having improved corrosion resistance. This method involves treating the material in an atmosphere containing (or capable of containing) hydrogen under the following conditions of absolute pressure (P) and of temperature (T DEG C.): if P</=Pa, 250<T<550; and if P>Pa, 250+100 log (P/Pa)<T<250+100 log (P/Pa) log base 10, Pa being atmospheric pressure. The invention is used for obtaining sintered TR Fe B magnets having improved corrosion resistance.

Description

The invention relates to a method for obtaining, in divided form, magnetic materials of the TR Fe B type which are brittle and relatively inert with respect to air and which lead to magnets with improved corrosion resistance.

By magnetic materials type TR Fe B is meant materials essentially consisting of a magnetic tetragonal phase T1, analogous to TR2 Fe14 B, TR designating one (or more) rare earth (s), including Yttrium, Iron and Boron can be partially substituted, as is known, by other elements such as cobalt, with or without the addition of metals such as aluminum, copper, gallium, or refractory metals. See EP-A-101552, EP-A-106948, EP-A-344542, and French patent applications Nos 89-16731 and 89-16732.

Preferably, most of the rare earth is constituted by Neodymium, which can be partly replaced by praseodymium and dysprosium.

The magnets of this family, particularly the sintered magnets, have to date, the most efficient magnetic properties in particular with regard to the residual induction (Br), the intrinsic coercivity (H _cJ ) and the specific energy [( BH) _max ].

However, the constituent materials of these magnets have a drawback which is that of their high sensitivity to corrosion, in particular in a humid atmosphere, both in the solid and divided state. Partial substitutions of cobalt for iron have been made to reduce this sensitivity, they have given insufficient results.

The classic method of manufacturing magnets of this type consists in obtaining a fine powder, possibly compressing it under magnetic field and sintering it before various finishes and final magnetization.

• élaboration par fusion de l'alliage qui est d'abord concassé (morceaux de l'ordre de quelques cm3), prébroyé jusqu'à une taille de 5/10 mm environ (mécaniquement ou par décrépitation à l'hydrogène) et broyé finement dans un broyeur à jet de gaz (jet mill), ou par attrition dans un milieu humide, jusqu'à une taille inférieure à 50 µm et de préférence 20 µm.
• réduction par le calcium des oxydes, en présence de poudres métalliques, la taille maximale des granulés formés par les particules d'alliage ainsi obtenus étant de l'ordre de 300 µm, les autres étapes du procédé restant les mêmes.

Powders are generally obtained by two routes:

• elaboration by fusion of the alloy which is first crushed (pieces of the order of a few cm3), pre-ground to a size of approximately 5/10 mm (mechanically or by decrepitation with hydrogen) and finely ground in a gas mill (jet mill), or by attrition in a wet environment, up to a size less than 50 μm and preferably 20 μm.
• reduction by oxides of calcium, in the presence of metal powders, the maximum size of the granules formed by the alloy particles thus obtained being of the order of 300 μm, the other steps of the process remaining the same.

By hydrogen decrepitation is meant a process for dividing an alloy consisting in subjecting an alloy in pieces to a hydrogen atmosphere under temperature and pressure conditions dependent on the alloy and allowing the conversion at least partially. into a hydride and then subjecting it to different conditions of temperature and pressure such that the hydride decomposes. This cycling often leads to a noisy fragmentation of the alloy, which is said to "decrepitate". In principle, it is generally described in GB 1 313 272 and GB 1 554 384 for binary combinations of a rare earth and a transition metal, mainly cobalt, combinations for which this process has not provided 'major advantages over conventional grinding methods, and therefore has not received notable industrial applications. The same method was applied in FR 2,566,758 to obtain fine and reactive powders by passing through new hydrides TR₂ Fe₁₄ BH _y by hydriding, preferably at room temperature and under a hydrogen pressure at least equal to 20 bars. , then by partial dehydriding by heating them above 150 ° C at ambient pressure, or by total dehydriding by heating them to at least 400 ° C under primary vacuum.

This method was then repeated in EP A-0280372, where a neutral gas such as argon or nitrogen is added to hydrogen, to reduce the risk of explosion. If the dehydriding conditions are described there as in FR 2 566 758 (start of dehydration of Nd₂ Fe₁₄ BH _y towards 150-260 ° C, the rest of the hydrogen going around 350-650 ° C), the hydriding conditions are more vague, by limiting themselves to staying below 300 ° C to avoid a risk of decomposition of l 'alloy ("disproportionation" in English) with the formation of highly divided iron. In this application, the powder is formed therein as a permanent permanent magnet by compression, when it is in the hydrated state since it is said to be less reactive towards oxygen in dry air. Dehydriding takes place in the sintering furnace, which, for industrial loads, represents high quantities of gas to be evacuated by sustained pumping.

Although the grinding, compression and sintering operations can be carried out under protective atmospheres, the powders partially oxidize during their transformation before densification (sintering) by reaction with the residual O2 and / or H2O contents of said atmospheres. This oxidation is particularly strong when the developed surface of the material is large, for example in the stages of pre-grinding, grinding, storage, compression of the powders, and during the rise in sintering temperature. As the Applicant has observed for itself, the method of decrepitation with hydrogen does not make it possible, in the art described above, to resolve these drawbacks.

• cette réaction consomme la TR, diminuant ainsi la fraction de phase intermétallique riche en TR active.
• la présence d'oxydes (ou d'hydroxydes) entraîne des difficultés au frittage (densification moindre)
• elle diminue les propriétés magnétiques de l'aimant final, en particulier la rémanence Br, l'énergie spécifique (BH)_max et peut accroître considérablement sa sensibilité à la corrosion atmosphérique.
• elle augmente le coût du produit fini: nécessité d'accroître le taux initial de TR de l'alliage et d'utiliser des équipements protégés complexes.

This oxidation, which essentially affects the rare earth (s) (TR) contained in the material, results in the following drawbacks:

• this reaction consumes the TR, thus reducing the fraction of intermetallic phase rich in active TR.
• the presence of oxides (or hydroxides) causes difficulties in sintering (less densification)
• it decreases the magnetic properties of the final magnet, in particular the remanence Br, the specific energy (BH) _max and can considerably increase its sensitivity to atmospheric corrosion.
• it increases the cost of the finished product: need to increase the initial rate of TR of the alloy and to use protected equipment complex.

For all these reasons, the applicant has sought a method considerably reducing the reactivity of these materials with respect to atmospheres, in particular those containing oxygen and / or water vapor, and leading to sintered magnets increased resistance to corrosion.

It found that other conditions than those described previously made it possible to prepare, by hydrogen treatment, friable materials, which can be used after grinding for the manufacture of permanent magnets, relatively passive with respect to atmospheric air at ambient temperature, therefore easier to handle during the various stages of the process, requiring only reduced degassing treatments in the sintering oven, and above all leading to magnets remarkably resistant to corrosion.

The process according to the invention consists in treating the material (crushed ingot or granules resulting from reduction of oxides) in a reactor where hydrogen is introduced under specific conditions of temperature (T) and pressure (P) defined below, at least in a final phase.

• Si P = < Patm. on doit avoir 250 < T°C < 550
• Si P > Patm. on doit avoir :
  250 + 100 log (P/Patm.) < T°C < 550 + 100 log (P/Patm.) ; (log base 10).

"Patm." Designating the normal atmospheric pressure (1 bar, or 0.1 MPa.

• If P = <Patm. we must have 250 <T ° C <550
• If P> Patm. we must have :
  250 + 100 log (P / Patm.) <T ° C <550 + 100 log (P / Patm.); (base 10 log).

Preferably, and to better control the reaction kinetics, the temperature T is chosen between 350 ° C and 550 ° C and in particular between 350 and 500 ° C if P <Patm. and the conditions 350 + 100 log P / Patm. <T <550 + 100 log (P / Patm.) and in particular 350 + 100 log (P / Patm.) <T <500 + 100 log (P / Patm.) if P> Patm.
More preferably, the temperature is kept above 400 ° C.

It has in fact been curiously observed that the exothermicity of the reaction is lower the higher the initial temperature, which constitutes a factor of safety in use and longevity of the apparatus.

Furthermore, for the reaction kinetics to be sufficient, it is preferable to operate with a pressure P greater than or equal to 50.6 kPa (0.5 atm.); in addition, for reasons of safety and simplicity of construction of the treatment enclosure, in particular with regard to its sealing, it is preferable to operate below 101.3 kPa (1 atm.).

By hydrogen pressure P is meant its absolute pressure in the case of a gas atmosphere alone, or its partial pressure in the case of a mixture of gases containing hydrogen or of a body providing nascent hydrogen such as ammonia NH₃. By temperature T to which H₂ is introduced, is meant the minimum temperature to which the product is brought by a heat source, independently of the heating which may result from the exothermic hydriding reaction; the actual temperature of the material is that reached by it during its transformation.
The duration of treatment depends on the operating conditions used; the reaction is considered to be complete when the hydrogen pressure and the temperature have become constant.

The reactor containing the product is then brought back to the usual temperature, pressure and atmosphere conditions.

It is remarkable to note that under conditions outside the field claimed above, the hydrogen treatment leads to materials extremely sensitive to oxidation, as shown by certain examples given below.

It is possible that the greater sensitivity of the powders prepared by the methods already described in the prior art, of decrepitation by hydrogen, is to be linked to the effective formation of the stable hydride associated with the magnetic phase TR₂ Fe₁₄ BHy ( 0 <y <5), the subsequent decomposition of which must generate many active sites vis-à-vis the environment.

This decomposition can moreover, under certain temperature and pressure conditions lead to the destruction of the magnetic phase TR₂ Fe₁₄ B ("disproportionation") with formation of very divided α - Fe, Fe ,B, TR₂ Fe₁₇ and TR. The plaintiff found, under the conditions that it has explored, that this disproportionation does not occur and it attributes it to the absence of formation of the stable hydride of the magnetic phase, which would absorb and transmit hydrogen by simple solid diffusion, without creation, or weakly, active sites.

It is known that rare earth hydrides are not strict defined compounds, but the stoichiometry of which can vary within wide limits. Thus, it is known that these hydrides, of formula TR Hx have a value of x which can vary continuously from 1.8 to 3.

ou de α -Fe ou d'un hydrure plus hydrogéné tel que NdH₃. Le matériau à l'issue du traitement à l'hydrogène est essentiellement constitué de 3 phases principales : $$\text{TR₂ Fe₁₄ B,appelée T1,}$$

$$\text{"TR H₂",}$$

et une phase riche en bore déjà décrite dans l'art antérieur. On attribue à la formation de cet hydrure riche en terre-rare l'apparition d'une friabilité appréciable des produits hydrogénés stables et passifs, et ceci sans création de la phase hydrurée de T1. Cependant, cette friabilité ne constitue pas un désavantage pour la santé du comprimé lors de la montée en température vers le frittage, car cette phase est minoritaire en volume en face de T₁.Continuing her research, the Applicant has however noted that during the hydriding according to the invention, there essentially forms a TR hydride of formula TR Hx with x between 1.8 and 2.45 - designated here by "TRH2" - to the exclusion of all others; in particular, the formation of a hydride of the standard formula has not been detected under the conditions of the invention $$\text{TR₂ Fe₁₄ B Hy}$$

or α -Fe or a more hydrogenated hydride such as NdH₃. The material at the end of the hydrogen treatment essentially consists of 3 main phases: $$\text{TR₂ Fe₁₄ B, called T1,}$$

$$\text{"TR H₂",}$$

and a boron-rich phase already described in the prior art. The formation of this rare earth-rich hydride is attributed to the appearance of appreciable friability of the stable and passive hydrogenated products, without creating the hydrated phase of T1. However, this friability does not constitute a disadvantage for the health of the tablet during the rise in temperature towards the sintering, since this phase is a minority in volume opposite T₁.

Conversely, outside the claimed domain, the Applicant has found that the hydrogen treatment also leads to brittle materials but comprising significant amounts of the hydride of T1, the hydride NdH₃, or of α -Fe. These materials did not make it possible to obtain magnets resistant to corrosion, see the examples outside the invention.

The invention will be better understood using the following examples:
Tests have been carried out on materials obtained by fusion, having the following composition (in at%), which is not limiting, which contains a low TR content in order to obtain the highest remanences. They made it possible to test the passivity of the materials obtained under different conditions according to the invention and outside the invention and the quality of corrosion resistance of the final magnets. The method described in this invention has been successfully applied to other TR or B compositions, or comprising substitutions and / or additions described in the prior art (see EP-A-101552, EP-A-106558, EP-A-344542), or alternatively to granules coming from the so-called diffusion reduction process.

The friability was measured by the particle size spectrum (% by weight passing through the sieve, without external constraint) of the material obtained after the hydriding treatment.

The nature of the phases present in the hydrated material was determined by X-ray diffraction.

The magnetic characteristics - B _r and H _cJ - were determined on sintered magnets, prepared according to the process recalled in the introduction, and without extreme precautions for handling atmospheres.

The oxygen content of the magnets obtained is situated according to their composition in the most desirable range for the particular use thereof. It is known that the prior art recommends either relatively high oxygen contents in order to improve the resistance to corrosion, this is the case of US Pat. No. 4,588,439; or on the contrary very low rates, as in the patent. EP 0.197.712, if it is desired to achieve high magnetic properties (Br, (BH)).

The corrosion resistance of sintered magnets has been estimated by their service life in autoclave at 115 ° C, under 0.175 MPa at 100% relative humidity. In all cases, the magnets were coated before testing under identical conditions, with an epoxy resin after a surface preparation (phosphating). The resistance of the coating was estimated by visual examination (blisters) and by the cross-cutting test.

The results are reported in Tables 1 to 8 (following).

Examples 1, 6 and 7 relate to the prior art, or to conditions outside the invention, the other tests (Examples 2 to 5 and 8) relate to the invention.

EXAMPLE 1 HYDRATION AT 25 ° C UNDER P = 0.1 MPa H₂ (outside the invention)

EXAMPLE 2 HYDRATION AT 300 ° C UNDER P = 0.1 MPa OF H₂ (invention)

EXAMPLE 3 HYDRATION AT 400 ° C UNDER P = 0.1 MPa OF H₂ (invention)

EXAMPLE 4 HYDRATION AT 400 ° C UNDER P = 0.01 MPa OF H₂ (invention)

EXAMPLE 5 HYDRATION AT 400 ° C UNDER P = 0.001 MPa H₂ (invention)

EXAMPLE 6 HYDRATION AT 550 ° C UNDER P = 0.1 MPa OF H₂ (outside the invention)

EXAMPLE 7 HYDRATION AT 250 ° C UNDER P = 100 BAR (10 MPa) OF H₂ (outside the invention)

EXAMPLE 8 HYDRATION AT 700 ° C AT P = 100 BAR (10 MPa) OF H₂ (invention)

Example 1 shows that under conditions close to those of the prior art (25 ° C. at around 0.1 MPa of H₂), and for the composition exemplified a duration of 4 days is the maximum that the magnet coated in autoclave, before blistering occurs, sign of corrosion.

Example 2 shows that hydriding at 300 ° C under conditions representative of the invention leads to a considerably increased autoclave service life (+ 100%) compared to Example 1, which may be related to an improved compactness.

A similar result is obtained by hydriding at 400 ° C under 0.1 MPa of H₂ (example 3), under 0.01 MPa of H₂ (example 4), or else under 10⁻³ MPa of H₂ (example 5 ).

Example 6 shows that at 550 ° C, there is no longer any embrittlement. Mechanical pre-grinding is then necessary. Densification becomes difficult; the service lives in the autoclave are extremely short, as are the magnetic properties, no doubt due to the presence of numerous open porosities.

At 250 ° C under 100 bar (10MPa) - example 7- and in an identical manner to example 1, there is easy corrosion.

At 700 ° C. (example 8), the magnetic properties as well as the corrosion resistance are optimal, similar to those of example 2.

• moindre consommation d'H₂ puisque la phase riche en terre-rare, qui occupe quelques % de la structure, est hydrurée à son niveau le plus bas
• faible désorption de l'H₂ en cours de frittage, ce qui évite l'apparition de défauts tels que soufflures ou fissurations, et permet l'obtention de pièces de volume unitaire important
• facilité de broyage des matériaux passivés
• absence de formation d'une phase ferromagnétique Fe α par suite de la réaction de "disproportionation", décrite dans l'art antérieur
• moindre consommation de TR
• sécurité améliorée par le volume réduit d'H₂ à mettre en oeuvre.

In addition to the high passivity of the materials obtained and the improved resistance to corrosion of the magnets which are prepared with them, the method according to the invention provides the following economic and technical advantages:

• lower consumption of H₂ since the phase rich in rare earth, which occupies a few% of the structure, is hydrated at its lowest level
• low desorption of H₂ during sintering, which avoids the appearance of defects such as blowing or cracking, and allows obtaining parts of large unit volume
• easy grinding of passivated materials
• absence of formation of a ferromagnetic Fe α phase as a result of the "disproportionation" reaction, described in the prior art
• lower RT consumption
• safety improved by the reduced volume of H₂ to be used.

Claims (6)

1. Method of obtaining a friable and relatively inert Fe TR B type magnetic material in divided form containing at least the T1, Nd₂ Fe₁₄ B phase and a rare earth hydride, TR H_x wherein x is between 1.8 and 2.45, allowing highly corrosion resistant sintered permanent magnets to be formed in a hydrogen-containing atmosphere, characterised in that the conditions of absolute pressure (P) and temperature T (°C) are as follows:
      if P ≦ Patm., 250 < T < 550°C
      if P > Patm., 250+100 log (P/Patm.) < T < 500+100 log (P/Patm.)
   in which formulae Patm. denotes atmospheric pressure and log the logarithm of base 10.
2. Method according to claim 1, characterised in that:
      if P ≦ Patm., 350 < T < 550°C
      if P > Patm., 350+100 log (P/Patm.) < t <550+100 log (P/Patm.)
3. Method according to one of claims 1 or 2, characterised in that:
      if P ≦ Patm., 350 < T < 500°C
      if P > Patm., 350+100 log (P/Patm.) < T < 500+100 log (P/Patm)
4. Method according to one of claims 1 to 3, characterised in that the temperature is > 400°C.
5. Method according to one of claims 1 to 4, characterised in that the pressure P is higher than 50.6 kPa (0.5 atmosphere).
6. Method according to claim 5, characterised in that the pressure P is lower than 101.3 kPa (1 atmosphere).