Method for making ferrite magnets

Details Method for making ferrite magnets Method for making ferrite magnets

2002-07-10

2004-06-29

Koslow, C. Melissa (Department: 1755)

Compositions

Magnetic

Iron-oxygen compound containing

C252S062570

Reexamination Certificate

active

06755988

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the field of permanent magnets, and more particularly to magnets of the ferrite type comprising the magnetoplumbite type.
DESCRIPTION OF RELATED ART
The present invention relates to permanent magnets of the ferrite type based on the magnetoplumbite phase MFe
12
O
19
with M=Sr,Ba,etc . . . , in which the element M is partially substituted by an element R chosen from among the rare earths or bismuth, and in which the element Fe is partially substituted by at least one transition metal T.
Such magnets are already known for their high magnetic properties, as disclosed in the Japanese patent application J10-149910 or in the European patent application EP-0 905 718 or in the international patent application WO99/34379.
In these patent applications, lanthanum La is generally used as the element M, and cobalt Co as element R.
Manufacture of such magnets comprises the following stages:
a) formation of a mixture of raw materials either by wet process to form a dispersion, or by dry process to form granules,
b) calcination of the mixture up to 1250° C. to form a clinker, or fire clay, comprising the magnetoplumbite phase required, said mixture, under either dispersion form or granular form, being introduced into a calcination furnace.
c) wet grinding of the clinker until an aqueous dispersion of particles of around 1 &mgr;m is obtained, under the form of a paste with about 70% dry extract
d) the paste is concentrated and compressed under an orienting magnetic field of about 1 Tesla and under a pressure of 30 to 50 MPa in such a way as to obtain an anisotropic “green compact”, with about 87% dry extract.
e) after drying and elimination of the remaining water, sintering of the green compact,
f) final machining to obtain the magnet of the defined shape.
SUMMARY OF THE INVENTION
The major problem of magnets according to the state of the art described above is their very high cost. In fact, the substitution elements La and Co are very expensive, lanthanum being much more expensive than strontium, and cobalt oxide being about 200 times more expensive than iron oxide.
In any case, the aim of the invention is therefore to increase the quality/price ratio of present magnets, either by increasing their performance, or by lowering their price, or by playing on the two factors simultaneously.
The aim of the invention is a method making it possible to achieve this, together with the magnets obtained by this method.
DETAILED DESCRIPTION
According to the invention, in the manufacturing method for permanent magnets of the ferrite type comprising a magnetoplumbite phase, formula M
1−x
R
x
Fe
12−y
T
y
O
19
in which M=Ba, Sr, Ca, Pb, and R=Bi or elements of the rare earth group, T=Mn, Co, Ni, Zn, and with x and y comprised between 0.05 and 0.5:
a1) one forms in a mixing means, typically a mixer operating by batch, a pulverulent mixture MP of raw materials relative to the elements M, R, Fe and T, typically under the form of oxide, carbonate, hydroxide, etc., comprising water in a predetermined quantity, typically needed for the formation of granules in the subsequent stage,
a2) next one transforms, typically in a granulator operating continuously, said mixture of “green compact” pellets A, with possible addition of supplementary water,
b) one calcinates said “green compact” granules to form a clinker B, with a magnetoplumbite base of formula M
1−x
R
x
Fe
12−y
T
y
O
19
,
c) one carries out a wet grinding of said clinker, typically in an aqueous medium, to obtain a homogeneous dispersion C of fine non-agglomerated particles of average particle size lower than 1.2 &mgr;m,
d) one concentrates and compresses said particles under an orienting magnetic field to form an anisotropic “green compact” D which can be manipulated, to the pre-determined shape,
e) said anisotropic green compact is sintered to obtain a sintered element E,
f) possibly, a final dimensioning of said sintered element is carried out, typically by machining, to obtain the final magnet F, the method being characterised in that:
1) at the stage a1) of the method, said mixture is formed, by introducing in said mixing means, a dry mixture MS of powders corresponding to said raw materials relative to the elements M and Fe and a homogeneous fluid dispersion DF of said raw materials relative to the elements R and T typically in said pre-determined quantity of water,
2) at stage b) of the method, said green granules A are calcinated at a chosen temperature and for a chosen length of time,
in such a way as to obtain, at the end of stage b), a clinker B which is both homogeneous in chemical composition and in size, and in apparent density, comprised between 2.5 and 3.5 and which is easy to grind in stage c).
The method according to the invention is clearly different from the methods according to the present state of the art. In fact, the applicant has been able to compare the methods according to the state of the art and those according to the invention, all other things remaining equal. It was noted that the differences of method led to great differences of final magnetic performance of the magnets and/or manufacturing cost. Different hypotheses are put forward below and the facts observed are interpreted as follows:
A) Comparison of the Method According to the Invention with the State of the Art Wet Process
In this state of the art method, a dispersion of raw materials is formed in water and then the dispersion formed is introduced directly into the calcination furnace, typically a rotary furnace operating continuously and, at the exit from the furnace, the clinker B is recuperated.
In this method, the chemical composition of the dispersion to be calcinated is very homogeneous, and thus also that of the clinker B obtained; on the other hand, the sizes of the clinkers obtained at the end of calcination is very heterogeneous, and typically extends from 0.5 mm to 10 mm.
A clinker size is considered to be homogeneous when the spread T
M
−T
m
(difference between the maximum size and the minimum size) is lower than the average of these values (T
M
+T
m
)/2.
The applicant has observed that, during the wet grinding of the stage c), it was impossible to grind this clinker to obtain a dispersion C of particles of homogeneous particle size and centred around 1 &mgr;m, doubtless because the granules of small size are ground more quickly than the granules of large size, the ratio of sizes being typically between 1 to 20 and even more By using this method, the applicant has noted the formation of ultra-fine particles, namely of typical particle size lower than 0.3 &mgr;m. These particles are known to pose several disadvantages:
on the one hand, the concentration at the stage d), typically by filtration, is expensive because of the high level of losses (the finest particles being carried away with the water) and because of the length of time of filtration or concentration, taking into account the presence of ultra-fine particles obstructing the filtration media,
on the other hand, the ultra-fine particles cannot be oriented by the orienting field, because their magnetic couple is too low, which results in a final magnet with remanence Br which is too weak.
furthermore, the time and the energy required for grinding these ultra-fine particles are pure losses in cost.
B) Comparison of the Method According to the Invention and the Dry Process According to the State of the Art
In the method according to the state of the art, the first stage a1) comprises the formation of a mixture of powders MP comprising all the constituents of the final magnet together with the major part of the quantity of water needed for the formation of the granules of stage a2), and then this pulverulent mixture MP is transformed into green granules A at stage a2) . In order to achieve this, a granulator is typically used in which one introduces, continuously, on the one hand the MP mixture of powders and, on the other hand, the quantity of supplementary water needed for the formation of

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