RU2118007C1 - Material for permanent magnets - Google Patents

Abstract

FIELD: electrical, electronic, and instrumentation engineering. SUBSTANCE: proposed material for permanent magnets is based on triple combination of iron, boron, and rare- earth metals incorporating at least one stable triple combination of Fe-B-rare-earth metal type (one or more rare-earth metals including yttrium) with tetragonal structure and nonmagnetic combinations incorporating amorphous compounds of oxide derivatives of rare-earth metals having following proportion of ingredients, mass percent: at least one stable triple combination of Fe-B-rare-earth metal type as base and amorphous compounds of rare-earth metals - 2.8-3.8. For example, triple combination may contain ingredients in following proportion, mass percent: neodymium - 2.5- 3.5; boron - 0.9-1.1; cobalt - 2.5-3.5; aluminum - 0.3-0.5; scandium - 0.01-0.05; iron - the rest. EFFECT: improved reproducibility of magnetic properties and facilitated production of material. 5 cl, 3 tbl

Description

 The invention relates to materials for permanent magnets, in particular to materials based on ternary compounds of the neodymium-iron-boron type containing at least one stable ternary compound of the type Fe-B-REM (rare-earth metals - one or more rare earth metals, including yttrium) with tetragonal structure, as well as non-magnetic compounds, and can be used for the production of permanent magnets for electrical engineering, electronics, instrumentation and other industries.

For the first time, Fe-B-REM permanent magnets based on a ternary compound were reported by M. Sagawa et al. (Japan Patent N59-46008, equivalent to EPO 0101552A and US Pat. No. 4,770,723, and publication in J. Appl. Phys., 55 (6) 1984, p. 2083). The proposed material was anisotropic sintered compositions containing 8-30 at.% REM. The highest level of magnetic properties was obtained for the composition ND ₁₅ Fe ₇₇ B ₈ [Nd (Fe _0.91 B _0.09 ) _5.67 ] :( BH) _m ≈35 MG • E and H _cj ≈10 kOe (Japanese patent 59-89401).

 However, this material was characterized by a low Curie point, and therefore, unsatisfactory thermal stability (patent EPO 0101552A). In the future, they tried to solve this problem in various ways, in particular by adding cobalt to the main phase, by doping with elements such as Al, Ti, Cr, Zr, Hf, Nd, Ta, Mo, Ge, Sb, Sn, Bi, Ni, Cu, and etc. (Japanese patent 59-89401), by replacing a part of neodymium with heavy rare-earth metals (Tb, Dy, Ho, etc.) (Japanese patent 60-32306, 60-34005). However, these solutions, leading to a positive effect in terms of increasing thermal and temporal stability, introduced additional disadvantages: a decrease in magnetic characteristics, an increase in the scarcity and cost of the material, complication of the technology for producing magnets, and a decrease in reproducibility of magnetic properties.

 The most effective of the proposals in this direction can be considered the joint doping of the Fe-B-REM material based on the ternary compound with cobalt and one of the elements forming together with Nd, Fe and Co a nonmagnetic grain-boundary phase, for example, Nd (Fe, Co, Al), in in the case of using Al (T. Mizoguchietal, Appl. Phys. Zet, 48, 1309, 1986) (EPO patent 0249973).

However, in these cases, the improvement of thermal stability was achieved by reducing the characteristics of the main magnetic phase R ₂ Fe ₁₄ B, where R is rare-earth metals. The smallest decrease in magnetic characteristics with the greatest increase in thermal stability was achieved as a result of complex alloying with Fe and B-REM base material with galium and tungsten (EPO patent 0258609).

The closest to the proposed material in technical essence and the achieved result is a material for permanent magnets based on the Fe-B-REM ternary compound (EPO patent 0389626), where a triad stable compound of the type R ₂ Fe ₁₄ B with a tetragonal structure is used as the basis, and as non-magnetic compounds non-magnetic derivatives of rare-earth metals and other elements, in particular fine powders of oxides, hydrides or intermetallic compounds in crystalline form with the following content, wt.%: at least one stable th ternary compound of the type Fe-B-REM - base powder REM crystalline compounds, in particular, crystalline compound oxide derivatives REM - 0.0005 - 3.0000.

This material has a high level of magnetic characteristics [Br up to 12.6 kG, H _cj up to 22 kOe, (VN) _max. up to 36.0 MGs • E], high corrosion resistance and thermal stability.

 However, this material is also characterized by low reproducibility of magnetic properties, for example, the production output of magnets with a magnetic energy of 35 MG · Oe does not exceed 35%. This is due, in particular, to the low technological suitability of the charge of this material - no more than 2-3 hours, which in mass production does not allow to stabilize the process.

 The technical result of the invention is to increase the reproducibility of magnetic properties and the manufacturability of obtaining material for permanent magnets based on the Fe-B-REM ternary compound.

 The result is achieved in that in the material for permanent magnets based on the Fe-B-REM ternary compound containing at least one stable Fe-B-REM type ternary compound and non-magnetic compounds, amorphous compounds of the REM oxide derivatives are used as non-magnetic compounds in the following content of components, wt.%: at least one stable ternary compound of the type Fe-B-REM — base, amorphous compounds of oxide derivatives of REM — 2.8–3.8.

Moreover, the content of rare-earth metals in a stable ternary compound of the Fe-B-REM type exceeds the content of rare-earth metals in amorphous compounds of oxide derivatives of rare-earth metals by 40 to 90 times, and the interface between amorphous compounds of oxide derivatives of rare-earth metals (phase I) and a stable compound of type Fe-B-REM (phase II) is 6000-12000 cm ² / cm ³ .

The proposed material differs from the known one (patent EPO 0389626) in that the oxide derivatives of rare-earth metals in an amorphous form are used as non-magnetic compounds with their contents from 2.8 to 3.8 wt.%, As well as a predetermined ratio of the content of rare-earth metals in stable ternary compounds of the type Fe-B-REM and oxide amorphous REM compounds (from 40: 1 to 90: 1) and a predetermined interface between amorphous compounds of oxide derivatives of REM and a stable ternary compound of the type Fe-B-REM (from 6000 to 12000 cm ² / cm ³ ) .

 The invention is based on the experimentally identified possibility of using the structural features of alloys of the Fe-RZM-B-O system to create the optimal, from the point of view of magnetization reversal, microstructure of the material for permanent magnets.

The proposed material is a grain of the main magnetic phase (tetragonal phase R ₂ Fe ₁₄ B), surrounded by a double layer. The inner (boundary) intergranular layer consists mainly of the ordered phase of Fe-B-REM enriched in REM, and the outer layer is composed of compounds of oxide derivatives of REM of an amorphous phase of the type REM-Fe-B-O. In this case, the surface layer of particles is morphologically associated with the surface layer, however, unlike it, it has a significant number of broken bonds, which leads to its high chemical activity. In the process of sintering the magnetic material of the Fe-B-REM-O system, such bonds are actively saturated first of all by REM and boron to an equilibrium state. As a result of the directed diffusion flow of these elements through the layers (surface and near-surface), equilibrium structures are formed: a non-magnetic amorphous REM-Fe-B-O phase in the surface amorphized layer and a Fe-B-REM phase enriched in REM in the surface layer. Moreover, the saturation of these REM layers also occurs due to the dissolution of inclusions of intergranular phases of REM-Fe-B enriched in REM and boron.

The dissolution of auxiliary phase precipitates located in tetrahedral gaps between the grains of the main magnetic phase R ₂ Fe ₁₄ B creates the prerequisites for increasing the packing density of the grains of the main magnetic phase R ₂ Fe ₁₄ B and, as a result, increases the magnetization of the material.

 On the other hand, the equilibrium double layer on each of the particles of the main magnetic phase in turn creates conditions for preventing the formation of nuclei of reverse magnetization and the displacement of domain walls through grain boundaries.

 This leads to increased coercive force of the material and, accordingly, high thermal stability.

 By the methods of high-resolution electron microscopy, the presence of such a structure in the inventive materials for permanent magnets was experimentally confirmed.

It was experimentally established that such a structure can be realized with an average grain size of the main magnetic phase R ₂ Fe ₁₄ B of the order of 5-10 μm, a thickness of intergranular layers enriched with rare-earth metals, in particular neodymium, of the rare-earth metals-Fe-B phases (fcc grain boundary phase) of the order 0.3-0.5 μm and the thickness of intergranular layers of the REM-Fe-B-O phase with an amorphous structure of the order of 0.2-0.3 μm.

With the indicated parameters of the structure of the proposed material, the interface between the phase of Fe-B-REM and amorphous compounds of oxide derivatives of REM should be from 6000 to 12000 cm ² / cm ³ .

With a value of less than 6000 cm ² / cm ^{3, the} proposed magnetic material acquires a coarse-grained structure with an average grain size of more than 10 μm, which leads to multi-domain grains and, as a result, to a deterioration in the texture and magnetization of the material. In addition, this may disrupt the continuity of the intergranular pinning layer, which helps to reduce the coercive force of permanent magnets from the proposed magnetic material.

A value of more than 12,000 cm ² / cm ^{3 is} fundamentally acceptable, however, as practice shows, it is difficult to implement, since it requires the creation of a very fine-grained structure (average grain size less than 3 microns). In addition, when the grain size of the main magnetic phase is less than 5 μm, their surface has excessive chemical activity to oxygen, which leads to the possibility of its partial penetration through the double pinning layer into the main magnetic phase R ₂ Fe _14B with a deterioration in its fundamental magnetic characteristics.

 A significant difference of the proposed material is the presence in it of compounds of oxide derivatives of rare-earth metals with an amorphous structure. Mostly these are compounds of oxide derivatives of neodymium and other light rare-earth metals, however, in small amounts, oxide derivatives of heavy rare-earth metals (dysprosium, terbium, gadolinium, holmium, and erbium) can also be present in the material when they are introduced into the main magnetic phase or sintering additive for purposeful changes in its basic magnetic characteristics (magnetization, coercive force, Curie point).

The content of the amorphous phase of the oxide derivatives of rare-earth metals is due to the need to create a thermodynamically stable phase of rare-earth metals-Fe-B-O at the interface between the grains of the main magnetic phase with tetragonal structure R ₂ Fe ₁₄ B, coated with a layer of the rare-earth phase enriched in rare-earth metals of the compound Fe-B-rare earth metals. It was experimentally established that in order to create a thermodynamically stable layer of the REM-FE-B-O phase, it is necessary that the content of the amorphous phase of compounds of the oxide derivatives of REM is 2.8-3.8 wt.%.

 When the content of the amorphous phase is less than 2.8 wt.%, The proposed material acquires a coarse-grained structure (grain size of the main magnetic phase is more than 10 μm), which leads to a deterioration in texture and a decrease in the magnetization of permanent magnets from the proposed material. However, in some cases, the content is less than 2.8 wt. % is permissible, in particular, in cases of obtaining permanent magnets with increased thermal stability with an average level of requirements for magnetic induction.

When the content of the amorphous phase is more than 3.8 wt.%, As a rule, a fine-grained structure is formed (the grain size of the main magnetic phase R ₂ Fe ₁₄ B is less than 5 μm), which leads to the instability of the permanent magnet production process and the unsatisfactory reproducibility of their magnetic properties.

 Another significant difference of the proposed material for permanent magnets from the known one is the controlled ratio of rare-earth metals in the elementary state and rare-earth metals in the oxidized state.

It was experimentally established that, taking into account the presence of rare-earth metals in the elementary state in the main magnetic phase R ₂ Fe ₁₄ B with a tetragonal structure and adjacent to the grain surface enriched rare-earth metals the main magnetic phase Fe-B-rare-earth metals, as well as the presence of rare-earth metals in the oxidized state, mainly in the intergranular layer of the Fe-B-REM-O phase with an amorphous structure, the content of rare-earth metals in the elementary state exceeds the content of rare-earth metals in the oxidized states by 40-90 times.

The boundary values of this interval are due to the fact that, at a value of less than 40, oxygen diffusion through the shell of the Fe-B-REM enriched REM phase into the main magnetic phase R ₂ Fe ₁₄ B is observed, which catastrophically worsens its magnetic characteristics, and at a value of more than 90 it is reached the equilibrium state of the amorphous phase Fe-В-РЗМ-О and the continuity of the intergranular oxide layer, which reduces the coercivity of the proposed material for permanent magnets.

 The combination of the above structural relationships is a prerequisite for achieving the desired technical result of the invention - improving the reproducibility of magnetic properties and manufacturability of the production of permanent magnets.

 The proposed material may contain neodymium as the main REM or its mixture with praseodymium, other light REMs as well as heavy REM groups Dy, Tb, Ho, Er, Yb, Ju, Tm as additional REM. In addition, it may include additional elements, for example, W, Nb, Ta, Mo, Cr, Mn, Ni, Co, Cu, Al, Hf, Ti, Si, Ga, Ge, Sn, SC, etc. Additional elements are introduced to give the material specified specific characteristics, in particular, increased heat resistance, corrosion resistance, mechanical strength, radiation resistance, etc.

 The proposed material is obtained by the traditional method of powder metallurgy: the preparation of the base alloy, its grinding, grinding with the introduction of additional elements at the stage of preparation of the charge, pressing the mixture in a magnetic field, sintering and heat treatment. However, in all operations, the composition of the initial alloy, additives, the ratio of the components of the charge and the treatment regimes are selected in such a way as to provide the necessary size of the interface between the phases Fe-B-REM and amorphous compounds of oxide derivatives of REM, the specified content of the amorphous phase of compounds of oxide derivatives of REM in elemental and oxidized states.

The invention is illustrated by the examples given in table 1-3. In these examples, a method for producing a material with predetermined structural relationships was used, including smelting a basic compound containing neodymium, iron, boron, dysprosium and cobalt, and an auxiliary compound containing neodymium, iron, boron, aluminum and scandium. The ratio of the base and auxiliary compounds in the mixture was chosen taking into account the receipt of the required total content of rare-earth metals. The ratio of dysprosium to neodymium in this case was from 88: 1000 to 122: 1000. The basic and auxiliary compounds were melted in an argon atmosphere in a high-frequency furnace in alundum crucibles. The mixture was crushed to a fineness of 300-500 microns in a jaw crusher, and then dosed and ground in a planetary mill in acetone to a fineness of 3-10 microns. The dried mixture was pressed in a magnetic field with a strength of 800-960 kA / m at a pressure of 3-5 T / cm ² into briquettes with a diameter of 13 mm and a height of 5 mm with an axial texture. Briquettes were sintered in a vacuum resistance furnace with program regulation during heating and soaking of the partial pressure of residual gases in the furnace. The change in vacuum during sintering ranged from 9 • 10 ^-2 to 1 • 10 ^-7 mm Hg. century, the samples were held at a temperature of 1100 ± 20 ^o C for 1-2 hours. Additionally, after sintering, the samples were heat treated at 550-650 ^o C for 15-20 minutes and cooled at a speed of 300-500 deg./min.

 Specific values of the content of the components of the material and technological indicators, providing the structural characteristics required according to this invention, as well as the parameters of the obtained magnets are given in table 1-3.

 The necessary structural characteristics in the proposed material for permanent magnets were obtained by varying the content of the components of the material, the dispersion of the main magnetic phase and sintering conditions. The structural characteristics of the material (Table 2) were calculated according to microscopic studies using Lorentz electron microscopy (Hitachi HU 650 transmission electron microscope), the oxygen content was analyzed on an O-mat 350 instrument (Strohlein), the composition of the material and the grain gap were determined spectroscopic method on a System 500 microanalyzer from Ortec.

 The magnetic characteristics of permanent magnets were measured in a closed magnetic circuit using ferromagnetic Hall transducers as sensors and in an open circuit according to the Helmholtz method. In all cases, preliminary magnetization was carried out in a pulsed solenoid with a magnetic field strength of 70 kOe. The accuracy of magnetic measurements is not lower than 5%.

 The dispersion of the powders was monitored microscopically using a Mini-Sem scanning electron microscope.

 In addition, in table. 3 shows the technological suitability of the powder (the manufacturability of the material), which was evaluated by holding the dry mixture in air until the decrease in one of the magnetic characteristics of the sintered magnetic material, exceeding the measurement accuracy (5%). To calculate the reproducibility of magnetic properties, the criterion of “rejection” (production) level is also introduced, as well as the criterion of “production yield” as a percentage of the number of magnets satisfying the rejection level to the total number of magnets of a given batch. Moreover, in the examples for known materials of the REM-Fe-B system, a lower rejection level is adopted for a number of parameters, since the values of these parameters taken for the proposed material are not achieved in them.

 As follows from table 1-3, the proposed material for permanent magnets (examples 1-5) is characterized by better reproducibility of magnetic properties (yield 60-90%) and higher adaptability (technological suitability of the powder mixture 48-96 h) than known materials, for the best of which (patent EPO 0389626), the yield is even by the lower rejection criterion no more than 35%, and manufacturability no more than 2-3 hours.

 The production of permanent magnets from the proposed material does not require a fundamental restructuring of existing technological industries, but comes down to only replacing certain types of technological equipment and providing the production facilities with structural control of the material and the charge.

 An additional advantage of the proposed material over the known ones is the stably high level of coercive force in magnetization (up to 25 kOe for 60-90% of magnets), which increases the thermal and thermal stability of magnets and significantly expands their field of application.

Claims (4)

1. Material for permanent magnets based on an iron - boron - rare earth metal triple compound containing at least one stable Fe - B - rare earth metal triple compound (rare earth metals - one or more rare earth metals, including yttrium) with a tetragonal structure and non-magnetic compounds, characterized the fact that as non-magnetic compounds it contains amorphous compounds of oxide derivatives of rare-earth metals in the following ratio of components, wt.%:
At least one stable ternary compound of the type Fe - B - REM base - amorphous compounds of oxide REM - 2.8 - 3.8
2. The material according to claim 1, characterized in that the content of rare-earth metals in a stable ternary compound of the type Fe - B - rare-earth metals exceeds the content of rare-earth metals in amorphous compounds of oxide derivatives of rare-earth metals by 40 to 90 times. 3. The material according to claim 1 or 2, characterized in that the interface between amorphous compounds of oxide derivatives of rare-earth metals and a stable ternary compound of the type Fe - B - rare earth metals is 6000 - 12000 cm ² / cm ³ .  4. The material according to claim 1, characterized in that it further comprises one or more elements selected from the group W, Nb, Ta, Mo, Cr, Mn, Ni, Co, Cu, Al, Hf, Ti, Si, Ga , Ge, Sn, Sc. 5. The material according to claim 4, characterized in that at least one ternary compound of the type Fe — B — REM contains neodymium and dysprosium as a rare-earth metal, and cobalt, aluminum and scandium as an additional element in the following ratio of components, wt. %:
Neodymium - 28 - 30
Dysprosium - 2.5 - 3.5
Boron - 0.9 - 1.1
Cobalt - 2.5 - 3.5
Aluminum - 0.3 - 0.5
Scandium - 0.01 - 0.05
Iron - The rest

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