Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
  • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
  • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
  • H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
  • H01F1/04—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys
  • H01F1/06—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys in the form of particles, e.g. powder
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S264/00—Plastic and nonmetallic article shaping or treating: processes
  • Y10S264/58—Processes of forming magnets

Definitions

• magnetically anisotropic particles can be aligned in a magnetic field or, taking advantage of the shape of the particles, by appropriate rolling, extrusion pressing etc., and can then be formed into a permanent magnet. This forming can be accomplished by compacting the magnetic anisotropic particles by application of high pressure and possibly also elevated temperature, or by sintering, or through bonding of the particles by means of an inorganic or an organic binder.
• inorganic binders one can use alloys having good ductility or low melting point which yield permanent magnets of suflicient mechanical strength.
• organic binders chemically hardening synthetic resins such as the epoxy resins can be used.
• a quality of special significance for the utility of a magnet is its energy product, which should be as high as possible.
• a high energy product is obtained when one uses an appropriate magnetically anisotropic material and sees to its that this material-which is usually available in powder formis present in the permanent magnet in the highest possible concentration, i.e., that it is packed to the highest possible density.
• the present invention has as its aim the creation of permanent magnets which have the highest possible packing density and thus a high energy product.
• the basic idea of the invention is to use magnetically anisotropic particles of different magnetic substances having different specific grain diameters and specific mixing ratios. Densely packed bodies made according to the proposed method may be bonded and further densifie'd by subsequent sintering, but the resulting magnets would have the above mentioned poor mechanical properties.
• the described technique permits the preparation of magnets avoiding sinter- 3,677,947 Patented July 18, 1972 ice ing which have nearly the same high densities as sintered magnets and therefore also good values of remanence and energy product.
• the invention is primarily concerned with the novel permanent magnet materials made from alloys of the rare earth metals with cobalt, which have a high crystal anisotropy and upon which is imparted a high coercive force by means of finely dispersing them or precipitation hardening them by means of alloying additions.
• the production method is also applicable, however, to other anisotropic magnet materials and mixtures of several such materials.
• the magnet according to this invention consisting of magnetically anisotropic and preferentially aligned and bonded particles, has these specific characteristics: It consists of magnetically anisotropic particles having different grain sizes, namely.
• the weight ratio of these fractions a:b:c is approximately 7:3:1.
• the condition that, e.g., the fraction (a) has to have an average grain size in the range between approximately 50 and 250 ,um. is not to be understood that the grain size distribution should cover the entire range from 50 to 250 ,uIIL, but rather that a fraction is used which is essentially uniform in grain size and whose average grain size lies within the specified range.
• the size of the particles of fraction (b), which in itself has again approximately uniform grain size, is determined in such a manner that it fills the voids between the particles of the coarsest fraction (a) in an optimal manner. This is the case when the average particle sizes of the two fractions are in a ratio lying between 1:7 and 1:4. The same considerations apply to the ratio of fraction (c) to fraction (b).
• the magnetically anisotropic particles of an average grain size in the range 50 to 250 ,um one uses preferentially rare earth-cobalt alloys which are precipitation hardened.
• Such alloys contain about 10 to 25 atomic percent of one, or a mixture of several, of the following elements: Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, in addition to to atomic percent cobalt and a third element which can cause the precipitation possibly even a combination of such third elements.
• Especially copper has become known as such a third element which can cause the precipitation.
• a portion of the cobalt may be replaced by one or both of the elements iron and manganese in order to increase the magnetic saturation.
• the rare earth-cobalt and copper are melted together, cooled and then heat treated. During this heat treatment, a finely dispersed second phase :forms in the interior of the grains. This second phase, which is probably nonmagnetic, impedes the motion of the magnetic domain walls (Bloch walls) and thus increases the coercive force.
• the fracture presumably takes place mostly along the grain boundaries of the magnetic matrix. Particles having a grain size in the range of approximately 50 to 250 ,um. already have a coercive force of 3,000 oersted or more and are thus useful for the production of permanent magnets.
• the precipitation hardening is not restricted to copper as the precipitation-causing element.
• Other precipitationhardening rare earth-cobalt-third element alloys may also be used.
• the essential feature is the presence of magnetic hardening through a phase (or phases) precipitated within the grain, or alternatively, through a fine subdivision of the magnetic rare earth-cobalt-alloy into regions of a size small compared with the particle size by means of a finely dispersed second phase.
• the magnetically anisotropic particles having an average grain size in the range of approximately 5 to 50 ,um. one can also use the previously described precipitation-hardened rare earth-cobalt alloys, or an RG alloy (R rare earth).
• SmCo for instance has an appropriately high coercive force. If one aspires to the highest possible energy product, one can for instance use PrCo or YC0 instead of SmCo While these alloys have a lower coercive force than SmCo for particle sizes between 5 and 50 m, they have a substantially higher saturation.
• the use of precipitation-hardened rare earth-cobalt alloys is attractive when economic considerations are in the foreground and whe the attainment of the highest possible energy product is not absolutely necessary. In this case, the low-priced precipitation-hardened -(cerium-mischmetal)- cobalt-third element alloys is especially advantageous.
• the average particle size of this fraction is to lie between 5 and 50 am it should be chosen within this range such that it is again approximately to A of the average particle size of the first fraction.
• the especially fine grain of the third fraction of 15 am. should, :for best results, again be to A of the grain size of the medium fraction.
• the alloys which offer themselves for this fraction are the well-known ones of rare earths with cobalt containing to 25 atomic percent of rare earth. These rare earth-cobalt alloys may also be precipitation hardened.
• another more conventional permanent magnet material of sufiicient coercive force in this particle size range can also be used. Examples for such a material are the barium and strontium ferrites.
• an alloy of very high saturation such as PrCo
• the production of permanent magnets according to this invention one proceeds as follows: the magnetically anisotropic particles having specified particle diameters are blended in the specified weight ratio.
• the weight ratio of the individual fractions may occasionally deviate from the specified ratio 7:3:1, since the packing density depends also on shape of the particles, and since the densities of the different fractions may be appreciably different if chemically dissimilar substances are used. It is best to determine the weight ratio of the individual fractons which yields optimum packing density for each combination by experiment. Mathematical studies have been made about the achievement of high packing density and its relation to grain size and fractional quantities. Results of such studies can, e.g., be found in the book Perspectives in Powder Metallurgy. Fundamentals, Methods and Applications, vol. 2, Plenum Press, especially in the chapters Mechanical Packing of Spherical Particles by R. K. McGeary and The Vibratory Packing of Powders by P. E. Evans and R. S. Millman.
• the orientation of the magnetically anisotropic particles may be achieved before and/ or during, and/or after the vibratory compacting.
• the final forming of the magnets is achieved in the well-known manner by application of pressure, possibly at elevated temperature, by sintering, or by means of binders.
• Such binders may be introduced either in liquid form or as a very fine powder whose average particle size is again best chosen in the range between ,6 and A of the third, the finest, magnetic fraction.
• This binding agent for instance ductile and/or lowmelting metals or alloys, or hardenable synthetic resinsfills the voids between the grains of the other two fractions.
• the magnetically anisotropic particles of rare earthcobalt alloys for use in the production of such permanent magnets may be improved by a special surface treatment. Examples for the latter are: soaking of the particles in a nickel bath, an acid treatment, and/or deposition of tin on the surface of the particles followed by a diffusion treatment. It is known that such measures increase the coercive force and provide a protection against ageing.
• the main objective of the invention is the preparation of magnets with an absolutely high energy product, which requires a high degree of orientation of the anisotropic particles in addition to a high density
• the procedures of the invention can nevertheless also be used to optimize isotropic permanent magnets.
• low production cost is a primary consideration, one can forego the particle orientation by magnetic or other meansas is done in the production of inexpensive ferrite magnets.
• the alloy is then ground with mortar and pestle, and powder fractions of the following ranges of particle size are prepared by sifting with the aid of a sieve shaker.
• the fractions are: 37 ,um., 37-53 ,uIIL, 53-74 ,am., 74- ,um., 105-149 ,um., and 149-250 1.111.
• a portion of the powder having 37 p.111. particle size was further ground for an additional 3 minutes under hexane in a laboratory vibratory mill (Spex Industries Mixer/Mill, Model 8000) and was then sifted through a copper screen with openings of 20 am first by shaking, then by brushing, so that two particle fractions of 20 37 m. and 20 m. are obtained.
• EXAMPLE 2 Production of particles of the precipitation hardened alloy MMCo Cu which are used in the subsequent Examples 9, 10, and 11 (MM is the symbol used here for the commercial cerium-rich mischmetal of the approximate composition 54.5 wt. percent Ce, 26% La, 13% Nd, 5% Pr, and ap proximately 1.5% other rare earths and other metals.)
• Example 1 Mischmetal, cobalt and copper were weighed out in proportions corresponding to the above formula as was done in Example 1.
• a charge weighing approximately 50 g. was then melted in an arc melting furnace with a water-cooled tungsten electrode under a protective gas mixture of 75% argon and 25% helium.
• the button-shaped ingot was turned over and remelted three times. After the current is turned off, the ingot cools very rapidly in the water-cooled copper mold.
• Five such ingots were prepared under the same conditions, were then broken into pieces weighing between .1 and 1 g., these were heat-treated in vacuum for 4 hours at 500 C., and were then powdered and classified by sifting as described in Example 1.
• a powder sample prepared as in Example 1 had a hysteresis loop which was slightly unsymmetric after magnetizing in a field of 22.4 koe. and showed a coercive force H 17,150 e. and a ratio of remanence-to-maximum induction of 0.92 after magnetization to the higher of the two maximum induction values.
• EXAMPLE 4 Production of the alloy powder of the nominal composition PrCo used in Example 11
• the metals praseodymium and cobalt were weighed out as in Example 1, except that an excess of approximately 2% Pr over the formula ratio was used in order to compensate the loss of Pr which is usually experienced in arc melting and so to avoid the presence of Pr Co in the ingot as a second phase.
• the alloy was prepared by are melting as described in Example 2 and was then homogenized at 1100 C. for 48 hours.
• EXAMPLE 6 Preparation of a magnet from SmCo Cu and SmCo
• Two particle fractions of the powder of Example 1 and one component of SmCo prepared according to Example 3 were intimately mixed in dry form by shaking them in the Spex Mixer/ Mill without grinding balls. The following particle sizes and quantities were used: (a) 7 g. of the fraction 149-250 m., (b) 3 g. of the fraction 37-53 ,um., both of SmCo Cu and (c) 1 g. SmCo powder of approximately 7 m. average particle size. About 5 g.
• the pressing mold used here permits the fabrication of prismatic briquets with the following dimensions: A inch (-6.3 mm.) in field direction, inch (-19 mm.) normal to the field and to the pressing direction.
• the short dimension, in the direction of the pressing force, depends on the amount of powder used and, for this magnet, was 6.1 mm. (All magnets described in the following had nearly the same dimensions.)
• Inserted into the pressing mold are pole pieces made from an ironcobalt alloy for the purpose of field enhancement, and these serve simultaneously as two walls of the pressing mold.
• the side walls and the pistons consist of a hardened copper-berryllium alloy.
• the pressing chamber can be evacuated.
• EXAMPLE 8 Preparation of a magnet with improved mechanical strength from SmCo Cu with a soft metal binder A magnet sample was prepared following essentially the procedures described in Example 6. As the finest powder fraction (c), however, two grams of a very finegrained tin powder were now used in the place of the one gram SmCo The tin powder had a relatively broad particle size distribution with an upper limit of about 10 ,um. and had been obtained by centrifugal separation of a fine-grained commercial tin powder of spherical particles. During the magnetic alignment of the particles before pressing, no liquid was added this time to increase the particle mobility.
• the pressing mold was tapped approximately 20 times with a hammer from each side While the aligning field was on.
• the pressed magnet sample had excellent mechanical strength and stable edges.
• EXAMPLE 10 Preparation of a magnet from MMCo Cu MMCo and barium ferrite
• EXAMPLE 11 Preparation of a magnet from SmCo Cu MMCo Cu and PrC0
• EXAMPLE 12 Preparation of a magnet from SmCo Cu with only one particle size fraction
• a magnet was made from SmCo Cu which contained only one magnetic fraction, and in addition only tin as a binder for mechanical cohesion. Seven grams powder of a fraction 37-53 ,am. was intimately mixed with 3 g. tin powder of 10 ,um. particle size and then magnetically aligned and compacted as described in Example 6. The sample had a density of 7.5 g./cm.
• EXAMPLE 13 Comparison of the magnets according to the invention with plastic bonded, pressed magnets made from Alnico alloys It is commercial practice to bond chips of magnet alloys of the types AlNi and AlNiCo into magnets by means of synthetic resins. Such magnets are less brittle, cheaper, but also magnetically inferior to cast magnets of the same alloys. The preparation of such composite magnets is described in the German Pat. No. 656,966. In view of the similarities in the manufacturing techniques for these magnets, which are traded under the names Tromalit and Domalit, and one of the composite magnets described in this invention, the properties of these two magnet types shall be compared. (Tromalit and Domalit are trade names for organic resin-bonded Alnico magnets.
• Tromalit comprises 22% of Ni, 12% of Al, remainder Fe and is marketed by the German firm M. Bermann, Benzberg- Wulfshof near Cologne, Germany. Domalit is marketed by the same company.)
• the two types of composite magnets have quite comparable remanence values. Because of the high values of the intrinsic coercive force, however, which is attributable to the extraordinarily high crystal anisotropy of the magnetic alloy component, the new magnets described here have much higher H and energy product values than the bonded Alnico powders.
• the magnet comprises compacted powders of magnetically anisotropic magnetic particles
• the improvement which comprises that the powders are composed of at least two particle fractions of diiferent average grain size, to wit (a) a first fraction having an average grain size in the range of about 50-250 m.
• the particles of said fraction (b) essentially consist of precipitation hardened rare earth-cobalt alloys containing about 10-25 atomic percent rare earths or of a RG0 alloy, wherein R stands for rare earths.
• the RG0 alloy is at least one of SmCo PrCo or YCo 6.
• the magnet contains said third fraction (c), the particles of said fraction (c) essentially consisting of rare earth-cobalt alloys containing about 10-25 atomic percent of rare earths or of a permanent magnet compound or alloy which exhibits sufiicient coercive force at particle sizes of 15 am.
• said permanent magnet compound or alloy is barium ferrite or strontium ferrite.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Hard Magnetic Materials (AREA)
• Powder Metallurgy (AREA)

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Cited By (31)

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• 1969
  • 1969-09-02 DE DE1944432A patent/DE1944432C3/de not_active Expired
• 1970
  • 1970-08-31 US US68119A patent/US3677947A/en not_active Expired - Lifetime
  • 1970-09-01 NL NL7012902.A patent/NL162502C/xx not_active IP Right Cessation
  • 1970-09-01 FR FR7031765A patent/FR2071664A5/fr not_active Expired
  • 1970-09-02 CH CH1310870A patent/CH516217A/de not_active IP Right Cessation
  • 1970-09-02 GB GB42084/70A patent/GB1299157A/en not_active Expired
  • 1970-09-02 JP JP45077018A patent/JPS4929046B1/ja active Pending

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