Method of obtaining a magnetic material of the rare earth/transition metals/boron type in divided form for corrosion-resistant magnets

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.degree. C.): if P.ltoreq.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.

The invention relates to a method of obtaining rare earth (RE) Fe B type 
magnetic materials in divided form which are friable and relatively inert 
toward air and lead to magnets having improved corrosion resistance. 
The term RE Fe B type magnetic materials covers materials essentially 
consisting of a T1 tetragonal magnetic phase similar to RE.sub.2 Fe.sub.14 
B, wherein RE designates one (or more) rare earth(s), including yttrium, 
wherein the iron and the boron can be partially substituted, as known, by 
other elements such as cobalt with or without addition of metals such as 
aluminium, copper, gallium etc. or refractory metals. See EP-A-101552, 
EP-A-106558, EP-A-344542 and French patent applications nos. 89-16731 and 
89-16732. 
The rare earth preferably consists mainly of neodymium which can be 
partially substituted by praseodymium and dysprosium. 
The magnets in this family, in particular sintered magnets, nowadays have 
the most high-powered magnetic properties, in particular with regard to 
residual induction (Br), intrinsic coercivity (H.sub.cJ) and specific 
energy [(BH).sub.max ]. 
However, the materials constituting these magnets have a disadvantage which 
is their high sensitivity to corrosion, particularly in a damp atmosphere, 
in both the bulk and divided state. The iron has been partially 
substituted by cobalt to reduce this sensitivity, but this yielded 
inadequate results. 
The conventional method of producing magnets of this type involves 
obtaining a fine powder, possibly compressing it in a magnetic field and 
sintering it prior to various finishing treatments and final 
magnetisation. 
The powders are generally obtained in two ways: 
preparation by melting of the alloy which is firstly ground (lumps of 
approximately a few cm3), precrushed to a size of about 5/10 mm 
(mechanically or by hydrogen crackling) and finally crushed in a jet mill 
or by attrition in a moist medium to a size smaller than 50 .mu.m and 
preferably 20 .mu.m. 
reduction by calcium of the oxides in the presence of metallic powders, the 
maximum size of the granules formed by the particles of alloy thus 
obtained being approximately 300 .mu.m, the other stages of the process 
remaining the same. 
The term hydrogen crackling refers to a process for dividing an alloy 
involving subjecting a lump-form alloy to a hydrogen atmosphere under 
temperature and pressure conditions which depend on the alloy and allow at 
least partial conversion into a hydride, then subjecting it to different 
temperature and pressure conditions such that the hydride decomposes. This 
cycle frequently leads to noisy fragmentation of the alloy which is called 
"decrepitation". The principle thereof is described fairly generally in GB 
1 313 272 and GB 1 554 384 for binary combinations of a rare earth and a 
transition metal, mainly cobalt, this process not having produced major 
advantages over conventional crushing methods and not therefore having 
received significant industrial application for these combinations. The 
same method has been applied in FR 2 566 758 for obtaining fine reactive 
powders by passing through new RE.sub.2 Fe.sub.14 BH.sub.y hydrides by 
formation of hydrides, preferably at ambient temperature and under a 
hydrogen pressure at least equal to 20 bars, then by partial dehydridation 
by heating them above 150.degree. C. at ambient pressure or by total 
dehydridation by heating them to at least 400.degree. C. under low vacuum. 
This method has then been dealt with in EP A-0280372, where an inert gas 
such as argon or nitrogen is added to the hydrogen to reduce the risks of 
explosion. Although the dehydridation conditions are described therein as 
in FR 2 566 758 (beginning of dehydridation of Nd.sub.2 Fe.sub.14 BH.sub.y 
at about 150.degree. to 260.degree. C., the remainder of the hydrogen 
issuing at about 350 to 650.degree. C.), the conditions for formation of 
hydrides are vaguer, involving a restriction to below 300.degree. C. to 
avoid a risk of disproportionation of the alloy with the formation of 
finely divided iron. In French application, the powder is formed into a 
permanent magnet while untreated, by compression, when it is in the 
hydrided state because it is said to be less reactive toward the oxygen in 
dry air. Dehydridation is carried out in the sintering furnace, so large 
quantities of gas have to be discharged by sustained pumping when 
industrial charges are used. 
Although the crushing, compression and sintering operations can be carried 
out in a protective atmosphere, the powders oxidize in part during their 
transformations prior to densification (sintering) by reaction with the 
residual O2 and/or H2O contents of said atmospheres. This oxidation is 
particularly pronounced when the developed surface area of the material is 
large, for example in the precrushing, crushing, storage and powder 
compression stages and during the rise in the sintering temperature. As 
the Applicants have themselves found, these disadvantages are not overcome 
by the hydrogen crackling method in the art described above. 
This oxidation, which essentially affects the rare earth or earths (RE) 
contained in the material, is accompanied by the following disadvantages: 
this reaction consumes the RE, thus reducing the fraction of intermetallic 
phase which is rich in active TR. 
the presence of oxides (or hydroxides) leads to difficulties during 
sintering (less densification) 
it reduces the magnetic properties of the final magnet, in particular the 
residual magnetism Br, the specific energy (BH).sub.max and can 
considerably increase its sensitivity to atmospheric corrosion. 
it increases the cost of the finished product: need to increase the initial 
RE content of the alloy and to use complex protected equipment. 
For all these reasons, the Applicants have sought a method which will 
considerably reduce the reactivity of these materials toward atmospheres, 
in particular those containing oxygen and/or steam, and will lead to 
increased corrosion-resistance in the sintered magnets. 
They have found that conditions other than those formerly described allowed 
the preparation, by hydrogen treatment, of friable materials which could 
be used after crushing for the production of permanent magnets which are 
relatively passive toward atmospheric air at ambient temperature, 
therefore easier to handle during the various stages of the process, and 
require only reduced degasification treatments in the sintering furnace 
and, in particular, lead to magnets which are particularly resistant to 
corrosion. 
The process according to the invention involves treating the material 
(ground ingot or granulates issuing from reduction of oxides) in a reactor 
where the hydrogen is introduced under the particular conditions defined 
below of temperature (T) and pressure (P), at least in a final phase. 
"Pa" designates normal atmospheric pressure (.perspectiveto.1 bar, that is 
0.1 MPa). 
If P=&lt;Pa, 250&lt;T.degree. C.&lt;550 should apply 
If P&gt;Pa, 250+100 log (P/Pa)&lt;T.degree. l C.&lt;550+100 log (P/Pa); (log base 
10) should apply. 
In a preferential manner and to improve control of the reaction kinetics, 
the temperature T is selected between 350.degree. C. and 550.degree. C. 
and, in particular, between 350.degree. and 500.degree. C. if P&lt;Pa and the 
conditions 350+100 log(P/Pa)&lt;T&lt;550+log (P/Pa) and in particular 350+100 
log (P/Pa&lt;T&lt;500+100 log (P/Pa) if P&gt;Pa. 
Again in a preferred manner, the temperature is kept above 400.degree. C. 
It has in fact curiously been found that the higher the starting 
temperature, weaker the exothermicity of the reaction, and this 
constitutes a safety factor with regard to use and the longevity of the 
apparatus. 
Furthermore, for the reaction kinetics to suffice, it is preferable to work 
with a pressure P which is higher than or equal to 0.5 atmosphere; 
moreover, with regard to safety and to the simplicity of construction of 
the treatment chamber, in particular with regard to its impermeability, it 
is preferable to work under less than 1 atmosphere. 
The term hydrogen pressure P denotes its absolute pressure in the case of a 
gas atmosphere only or its partial pressure in the case of a mixture of 
gases containing hydrogen or a body providing nascent hydrogen such as 
ammonia NH.sub.3. The term temperature T at which H.sub.2 is introduced 
means the minimum temperature to which the product is brought by a source 
of heat, independently of the heating possibly resulting from the 
exothermic hydride-forming reaction; the actual temperature of the 
material is that attained by the material during its transformation. The 
duration of treatment depends on the operating conditions employed; it is 
considered that the reaction is completed when the hydrogen pressure and 
the temperature have become constant. 
The reactor containing the product is then brought to the usual 
temperature, pressure and atmosphere conditions. 
It is worthy of note that under conditions external to the range claimed 
above, the hydrogen treatment leads to materials which are extremely 
sensitive to oxidation, as demonstrated by certain examples which are 
given below. 
It is possible that the higher sensitivity of the powders prepared by the 
methods already described in the prior art of hydrogen crackling is linked 
to the effective formation of the stable hydride combined with the 
magnetic phase RE.sub.2 Fe.sub.14 BH.sub.y (0&lt;y&lt;5), of which the 
subsequent decomposition should generate many sites which are active 
toward the environment. 
Under certain temperature and pressure conditions, this decomposition can 
also lead to the destruction of the magnetic phase RE.sub.2 Fe.sub.14 B 
(disproportionation) with formation of finely divided .alpha. - Fe, 
Fe.sub.2 B, RE.sub.2 Fe.sub.17 and TR. The Applicants have found that, 
under the conditions which they have investigated, this disproportionation 
does not occur and they attribute it to the absence of formation of the 
stable hydride of the magnetic phase which would absorb and transmit the 
hydrogen by mere solid diffusion without creation or with weak creation of 
active sites. 
It is known that the hydrides of rare earths are not strict defined 
compounds but that the stoichiometry thereof can vary within wide 
proportions. Thus, it is known that these hydrides of formula RE H.sub.x 
have a value x which can vary continuously from 1.8 to 3. 
Continuing their research, the Applicants have however found that during 
hydride formation according to the invention, a TR hydride of formula RE 
H.sub.x, with x between 1.8 and 2.45--designated here by "REH.sub.2 "--is 
essentially formed to the exclusion of all others; in particular, the 
formation of a hydride of RE.sub.2 Fe.sub.14 B Hy type formula or of 
.alpha. -Fe or of a more highly hydrogenated hydride such as NdH.sub.3 has 
not been detected under the conditions of the invention. The material 
issuing from the hydrogen treatment consists essentially of three main 
phases: RE.sub.2 Fe.sub.14 B, known as T1, "RE H.sub.2 ", and a boron-rich 
phase already described in the prior art. The appearance of appreciable 
friability of the stable and passive hydrogenated products is attributed 
to the formation of this hydride which is rich in rare earth, without 
creation of the hydrided phase of T1. However, this friability does not 
constitute a disadvantage for the well-being of the compact during the 
rise in temperature toward sintering because this phase is in the minority 
in volume vis-a-vis T.sub.1. 
On the contrary, outside the disclosed range, the Applicants have found 
that the hydrogen treatment also leads to materials which are friable but 
contain large quantities of T1 hydride, NdH.sub.3 hydride or .alpha. - Fe. 
These materials did not allow highly corrosion resistant magnets to be 
obtained, see the examples outside the invention. 
The invention will be understood better by means of the following examples: 
Tests have been carried out on materials obtained by melting, having the 
following composition (in at %) which is non-limiting and has a small 
content of RE in order to obtain the highest residual magnetism. They 
allowed the passivity of the materials obtained under various conditions 
according to the invention and outside the invention and the corrosion 
resistance quality of the final magnets to be tested. The process 
described in this invention has been successfully applied to other 
compositions with TR or with B or containing the substitutions and/or 
additions described in the prior art (see EP-A-101552, EP-A-106558, 
EP-A-344542), or again to granulates originating from the so-called 
diffusion reduction process. 
______________________________________ 
Nd Dy B Al Fe 
______________________________________ 
C1 13.5 1.5 8 0.75 remainder 
______________________________________ 
The friability was measured by the grain size spectrum (% by weight passing 
through the sieve without external stress) of the material obtained after 
the hydride-forming treatment. 
The nature of the phases present in the material subjected to hydride 
formation was determined by X-ray diffraction. 
The magnetic characteristics--B.sub.r and H.sub.cJ --were determined on the 
sintered magnets prepared by the process recalled in the introduction and 
without extreme precautions for the handling atmospheres. 
The oxygen content of the magnets obtained lies, as a function of their 
composition, in the range which is most desirable for the particular use 
thereof. It is known that the prior art recommends either relatively high 
oxygen contents in order to improve the corrosion resistance, as is the 
case in U.S. Pat. No. 4,588,439; or, on the other hand, very low contents, 
as in the patent EP 0.197.712, if high magnetic properties (Br, (BH)max) 
are to be obtained. 
The corrosion resistance of the sintered magnets has been estimated by 
their life in an autoclave at 115.degree. C. under 0.175 MPa at 100% 
relative humidity. In all cases, the magnets were coated before testing 
under identical conditions by an epoxy resin after a surface preparation 
(phosphatation). The content of the coating has been estimated by visual 
examination (blisters) and by the cross-cutting test.

The results are compiled in Tables 1 to 8 (as follows). 
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 
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Formation of hydrides at 25.degree. C. under P = 0.1 MPa of H.sub.2 
(outside invention) 
Composition C1 
______________________________________ 
Grain size % (by weight) 
0-100 .mu.m 1.6 
100-500 .mu.m 8.5 
500-1000 .mu.m 89.9 
1000 and greater 0 
Main phases present 
(NdDy)2 Fe14 BH3 
(NdDy) H3 
Nd Fe4 B4 
Density (g/cm3) 7.4 
Residual magnetism (T) 
1.14 
Coercivity (kA/m) 
1480 
Life in autoclave (days) 
4 
______________________________________ 
EXAMPLE 2 
______________________________________ 
Formation of hydrides at 300.degree. C. under P = 0.1 MPa of H.sub.2 
(invention) 
Composition C1 
______________________________________ 
Grain size % (by weight) 
0-100 .mu.m 1.0 
100-500 .mu.m 11.3 
500-1000 .mu.m 87.7 
1000 and greater 0 
Main phases present 
(Nd,Dy)2 Fe14 B 
"(Nd,Dy) H2" 
Nd Fe4 B4 
Density (g/cm3) 7.5 
Residual magnetism (T) 
1.16 
Coercivity (kA/m) 
1616 
Life in autoclave (days) 
9 
______________________________________ 
EXAMPLE 3 
______________________________________ 
Formation of hydrides at 400.degree. C. under P = 0.1 MPa of H.sub.2 
(invention) 
Composition C1 
______________________________________ 
Grain size % (by weight) 
0-100 .mu.m 1.2 
100-500 .mu.m 11.1 
500-1000 .mu.m 87.7 
1000 and greater 0 
Main phases present 
(Nd,Dy)2 Fe14 B 
"(Nd,Dy) H2" 
Nd Fe4 B4 
Density (g/cm3) 7.5 
Residual magnetism (T) 
1.16 
Coercivity (kA/m) 
1608 
Life in autoclave (days) 
8 
______________________________________ 
EXAMPLE 4 
______________________________________ 
Formation of hydrides at 400.degree. C. under P = 0.01 MPa of H.sub.2 
(invention) 
Composition C1 
______________________________________ 
Grain size % (by weight) 
0-100 .mu.m 1.0 
100-500 .mu.m 12.2 
500-1000 .mu.m 86.8 
1000 and greater 0 
Main phases present 
(Nd,Dy)2 Fe14 B 
"(Nd,Dy) H2" 
Nd Fe4 B4 
Density (g/cm3) 7.5 
Residual magnetism (T) 
1.16 
Coercivity (kA/m) 
1600 
Life in autoclave (days) 
9 
______________________________________ 
EXAMPLE 5 
______________________________________ 
Formation of hydrides at 400.degree. C. under P = 0.001 MPa of H.sub.2 
(invention) 
Composition C1 
______________________________________ 
Grain size % (by weight) 
0-100 .mu.m 0.8 
100-500 .mu.m 9.1 
500-1000 .mu.m 90.1 
1000 and greater 0 
Main phases present 
(Nd,Dy)2 Fe14 B 
"(Nd,Dy) H2" 
Nd Fe4 B4 
Density (g/cm3) 7.5 
Residual magnetism (T) 
1.16 
Coercivity (kA/m) 
1600 
Life in autoclave (days) 
8 
______________________________________ 
EXAMPLE 6 
______________________________________ 
Formation of hydrides at 550.degree. C. under P = 0.1 MPa of H.sub.2 
(outside invention) 
Composition C1 
______________________________________ 
Grain size % (by weight) 
0-100 .mu.m 0 
100-500 .mu.m 0 
500-1000 .mu.m 30.2 
1000 and greater 69.8 
Main phases present 
(Nd,Dy)2 Fe14 B 
"(Nd,Dy) H2" 
Nd Fe4 B4 
Density (g/cm3) 7.1 
Residual magnetism (T) 
0.82 
Coercivity (kA/m) 
320 
Life in autoclave (days) 
1 
______________________________________ 
EXAMPLE 7 
______________________________________ 
Formation of hydrides at 250.degree. C. under P = 100 bar (10MPa) 
of H.sub.2 (outside invention) 
Composition C1 
______________________________________ 
Grain size % (by weight) 
0 &lt; % &lt; 100 .mu.m 
1.2 
100 &lt; % &lt; 500 .mu.m 
10.0 
500 &lt; % &lt; 1000 .mu.m 
88.8 
1000 &lt; % 0 
Main phases present 
(Nd,Dy)2 Fe14 BH3 
(Nd,Dy) H2.9 
Nd Fe4 B4 
Density (g/cm3) 7.3 
Residual magnetism (T) 
1.13 
Coercivity (kA/m) 
1380 
Life in autoclave (days) 
4 
______________________________________ 
EXAMPLE 8 
______________________________________ 
Formation of hydrides at 700.degree. C. under P = 100 bar (10MPa) 
of H.sub.2 (invention) 
Composition C1 
______________________________________ 
Grain size % (by weight) 
0 &lt; % &lt; 100 .mu.m 
2.2 
100 &lt; % &lt; 500 .mu.m 
12.3 
500 &lt; % &lt; 1000 .mu.m 
85.5 
1000 &lt; % 0 
Main phases present 
(Nd,Dy)2 Fe14 B 
"(Nd,Dy) H2" 
Nd Fe4 B4 
Density (g/cm3) 7.5 
Residual magnetism (T) 
1.16 
Coercivity (kA/m) 
1650 
Life in autoclave (days) 
9 
______________________________________ 
Example 1 shows that under conditions close to those of the prior art 
(25.degree. C. at about 0.1 MPa of H.sub.2) and for the exemplified 
composition, a duration of 4 days is the maximum which the coated magnet 
can withstand in the autoclave before blistering which is a sign of 
corrosion. 
Example 2 shows that hydride formation at 300.degree. C. under conditions 
which are representative of the invention leads to a life in an autoclave 
which is considerably increased (+100%) over Example 1, which is perhaps 
linked to improved compactness. 
A similar result is obtained by hydride formation at 400.degree. C. under 
0.1 MPa of H.sub.2 (Example 3), under 0.01 MPa of H.sub.2 (Example 4), or 
under 10.sup.-3 MPa of H.sub.2 (Example 5). 
Example 6 shows that at 550.degree. C. there is no more embrittlement. 
Mechanical precrushing is therefore necessary. Densification becomes 
difficult; the lives in an autoclave are extremely reduced as well as the 
magnetic properties, undoubtedly owing to the presence of numerous open 
pores. 
At 250.degree. C. under 100 bar (10 MPa)--Example 7--and identically to 
Example 1, easy corrosion is found. 
At 700.degree. C. (Example 8), the magnetic properties as well as the 
corrosion resistance are at an optimum, similar to those in Example 2. 
Apart from the high passivity of the materials obtained and the improved 
corrosion resistance of the magnets prepared with them, the process 
according to the invention provides the following economic and technical 
advantages: 
less consumption of H.sub.2 since the phase which is rich in rare earth and 
occupies several per cent of the structure is hydrided at its lowest level 
slight desorption of the H.sub.2 during sintering which prevents the 
appearance of defects such as blow holes or cracks and allows parts having 
a large unit volume to be obtained 
facility of crushing of the passivated materials 
absence of formation of a ferromagnetic phase Fe .alpha. owing to the 
disproportionation reaction described in the prior art 
lower consumption of RE 
safety improved by the reduced volume of H.sub.2 to be used.