Sm2Co17-type Magnets Research Articles

Correlation between ordered solid solution and cellular structure of Sm2Co17 type magnets with high iron content

The ordering transformation of solid solution and the insufficient 1:5H boundary phase in the final magnets challenge the realization of high performance in Sm2Co17 magnets with high iron content. The investigation correlates the ordering degree of solid solutions to the cellular structure of the final magnets with the composition of Sm(CobalFexCu0.06Zr0.02)7.8 in which x ranges from 0.30 to 0.36. The ordering tendency of solid solution increases with increasing iron content. Rietveld structure refinement demonstrates that increasing iron content reduces the occupancy of Sm at Co-Co dumbbell sites in 2:17R, as well as Zr solubility on Sm 6c sites, leading to higher ordering degree and less Z planar phase formation separately. Antiphase boundaries (APBs) arising from the different 2:17R variants with the same or different directions were frequently observed in solid solutions, which facilitate the nucleation and formation of 1:5H phase during aging treatment. With increasing Fe content, the width of nanotwins increases and thus the APB density decreases in the solid solution. Furthermore, overlapping zones appear in solid solutions with higher Fe content, which will retain and enlarge as the twins grow during aging treatment, inhibiting the formation of 1:5H boundary phase. In addition, the density of Z phase declines and more defective platelet phases occur in x = 0.36 magnets, which depress the element diffusion and re-distribution during slow cooling. The above correlation between solid solution and cellular structure well explain the structure origin for the deterioration of magnetic performance with higher Fe content, which may provide guidance for further performance improvement in Sm2Co17 type magnets with higher iron content.

Nanoscale short-range ordering induced cellular structure and microchemistry evolution in Sm2Co17-type magnets

Sm2Co17-type permanent magnets have important application value due to their advanced magnetic properties, which can be ascribed to their unique cellular structure and element segregation in various phases developed after isothermal aging treatment. In this study, the evolution of the microstructure combined with the microchemistry of Sm2Co17-type magnets from solid solution precursor to the cellular structure is investigated, and a new insight into the formation of the cellular structure is provided. We report for the first time that a large number of strip-like nanoscale short-range ordered micro-domains are distributed in the disordered matrix of the solution precursor, which serve as the nucleation centers in the formation of the embryos of the cellular structure. Based on atomic scale structural analyses, a new mechanism concerning the ordering transformation from short-range 2:17R microtwins to long-range 2:17R phase is proposed, involving collective glide of three partial dislocations on successive basal planes with the net Burgers vector to be zero. Furthermore, microchemistry analysis reveals that the segregation of Cu and Fe in the early stage of isothermal aging is accelerated substantially during the ordering transformation, indicating that the ordering transformation can act as a new driving force for element segregation in Sm2Co17-type magnets. These findings provide new considerations for the development of high performance Sm2Co17-type magnets.

Magnetic properties and microstructures of high heat-resistance Sm-Co magnets with high Fe and low Zr content

The magnetic properties and microstructures of 2-17 type Sm-Co magnets with high Fe and low Zr content were investigated. The developed magnet achieved maximum energy product, [BH]m of 34.5 MGOe, intrinsic coercivity, Hcj of 21.3 kOe and squareness of 73.3% at 25 °C. Temperature coefficients of remanent magnetic flux density, Br and Hcj were 0.034%/K and 0.28%/K respectively, which values were almost as same as the conventional Sm-Co magnets. Moreover, the developed magnets had high magnetization orientation. For XRD, it was found that Zr was preferentially substituted by Co-Co pair, this made interaction between Co and Co stronger, so that heat resistance was maintained. Magnetic domain structures were observed with a Kerr effect microscope, and then it was observed that the developed magnet had strong pinning force. In the microstructures, the developed magnet had 200∼500 nm cell size with Fe and Cu separated clearly. This led to large gap of domain wall energy which produces strong pinning force. Because the developed magnet had high magnetization orientation and large gap of domain wall energy, we achieved high magnetic properties and high heat resistance on the developed magnet.

Optimization of both coercivity and knee-point magnetic field of Sm2Co17-type magnets via solid solution process

It is confirmed that phase homogenization is very important for improving the magnetic properties of 2:17-type Sm–Co sintered magnets. In this work, the influence of solid solution process on microstructure and magnetic properties of the Sm(CobalFe0.233Cu0.073Zr0.024)7.6 sintered magnets was systematically studied. With the solid-solution treating duration (ts) increasing from 0 to 4 h, intrinsic coercivity (Hcj) increases from 12.83 to 36.54 kOe, magnetic field at knee-point (Hknee) increases from 2.76 to 19.14 kOe, and the maximum energy product increases from 19.79 to 29.48 MGOe. The electron probe microanalyzer results reveal that there mainly exist gray and dark regions besides “white” rare earth-rich phase, and the content of Sm, Fe and Cu elements for the two kinds of regions changes a lot for the specimens. Furthermore, with ts increasing up to 4 h, the elements content deviation between the gray and dark regions becomes small gradually from 3.94 at% to 0.27 at%, 7.66 at% to 0.21 at% and 7.27 at% to 0.16 at% for Sm, Fe and Cu elements, respectively. Moreover, transmission electron microscopy results show that the distribution of cell size is much more concentrated for aged specimens when ts is 4 h. It is also found that the Cu concentration at cell boundaries for the 4 h solid-solution treatment case shows relatively higher values and greater concentration gradient (1.94 at%/nm). It is verified that sufficient solution treatment duration is prerequisite to form these homogeneous microstructural features, which are the key points for obtaining both high Hcj and Hknee.

Zirconium content induced mitigation of mechanical anisotropy in 2:17 type SmCo magnets

The mitigation of mechanical anisotropy is observed in 2:17 type SmCo magnets by adjusting the Zr content. This behavior is supposed to be closely related to the density of lamellar phase, the density of which is enhanced obviously with increasing Zr content. The other reasons which could cause the reduction of the mechanical anisotropy is discussed from the Zr-rich impurity phase to the atom substitutions and crystal lattice distortion. The observation of crack in nanoscale that clearly forms angles of 60° and 90° with respect to the lamellar phase, indicates that the probable cleavage planes are crystal faces (101¯1) and (101¯0). The results of investigation can deepen the understanding of mechanical anisotropy and cleavage fracture in the SmCo magnets.

Grain boundary modification induced magnetization reversal process and giant coercivity enhancement in 2:17 type SmCo magnets

Grain boundary modification induced giant coercivity enhancement from 945 kA/m to 1904 kA/m was achieved in Sm25Co48Fe20Cu5Zr2 magnets by doping 0.4 wt% CuO. Different magnetization reversal behaviors were clearly observed between un-doped and doped magnets. Compared with un-doped magnets, Cu enrichment along grain boundary promoting regular cellular structure was obviously found in doped magnet. The in-situ Kerr observation indicated that complete cellular structure could strengthen the pinning field of grain boundary regions and the propagations of magnetization reversals into interiors of grains were restrained by the modified grain boundaries, which resulted in the remarkable coercivity increment.

2:17 type SmCo magnets with low temperature coefficients of remanence and coercivity

Spin-reorientation-transition cell boundary phases are used to improve the temperature stability of coercivity in 2:17 type SmCo magnets. A magnet with both low temperature coefficients of remanence (α) and coercivity (β) is developed. In comparison with traditional 2:17 type SmCo magnets with low temperature coefficient of remanence, this magnet exhibits comparable (BH)max and α, meanwhile the β(20–100 °C) is improved by more than two orders of magnitude. The comparative study of reversible magnetization/irreversible magnetizations indicates that the flux losses of this magnet are barely affected by temperature, which is an remarkable improvement in developing temperature-stable magnet.

Mechanism of phase transformation in 2:17 type SmCo magnets investigated by phase stabilization

Two kinds of lamella phases were observed in the phase transformations during aging of Sm(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets, which formed before and after the disappearance of microtwins respectively. Microtwins appeared with the precipitation of disordered 2:17R phases and disappeared with 2:17R phase ordering. Dy was introduced to investigate these transformations by stabilizing the 2:17H phases. A simplified atomic model was given to describe the formation of microtwins and a reasonable assumption was proposed as well to clarify the mechanism of these transformations.

ChemInform Abstract: Recent Progress in High Temperature Permanent Magnetic Materials

AbstractReview: focus on SmCo‐based magnets; 52 refs.

Stoner-Wohlfarth Model for Nanocrystalline Anisotropic Sm<sub>2</sub>Co<sub>17</sub> Magnets

The Stoner-Wohlfarth model can be used for predicting hysteresis curves of either isotropic or anisotropic nanocrystalline Sm2Co17 type magnets. For isotropic magnets, with Mr/Ms=0.5, the Stoner-Wohlfarth model predicts coercive force of 48% of the anisotropy field. Here, Mr is remanence and Ms is magnetization of saturation. Two texture distributions were compared: Pearson VII and cosn. The calculations indicate that increasing the alignment degree of the grains, the coercivity increases. However, this increase of the coercive force is small. It is found that a very well aligned magnet, with Mr/Ms ratio of 0.96, presents coercivity only ~20% higher than that of the isotropic magnet.

Recent progress in high temperature permanent magnetic materials

Permanent magnetic materials capable of operating at high temperature up to 500 °C have wide potential applications in fields such as aeronautics, space, and electronic cars. SmCo alloys are candidates for high temperature applications, since they have large magnetocrystalline anisotropy field (6–30 T), high Curie temperature (720–920 °C), and large energy product (>200 kJ·m−3) at room temperature. However, the highest service temperature of commercial 2:17 type SmCo magnets is only 300 °C, and many efforts have been devoted to develop novel high temperature permanent magnets. This review focuses on the development of three kinds of SmCo based magnets: 2:17 type SmCo magnets, nanocrystalline SmCo magnets, and nanocomposite SmCo magnets. The oxidation protection, including alloying and surface modification, of high temperature permanent magnets is discussed as well.

High-temperature oxidation resistance and magnetic properties of Si-doped Sm2Co17-type magnets at 500 °C

The high-temperature oxidation resistance and magnetic properties of Si-doped Sm2Co17-type magnets at 500°C were systematically investigated. The Sm(Co0.76, Fe0.1, Cu0.1, Zr0.04)7Six (x=0–0.6) magnets were prepared by the conventional powder metallurgical technique. It is found that the addition of silicon in the Sm2Co17-type magnet can remarkably improve its oxidation resistance. Moreover, a small amount of silicon addition can also increase its high-temperature intrinsic coercivity. A maximum intrinsic coercivity of 6.7kOe at 500°C was obtained for the Sm2Co17-type magnet with Si content x=0.4, whose high-temperature maximum energy product loss was about 2.5 times smaller than pure Sm2Co17-type magnet after oxidation at 500°C for 100h, indicating the enhanced oxidation resistance. Its corresponding Curie temperature and saturation magnetization are about 723.9°C and 7.4kG, respectively.

Phase transformation in nanocrystalline Sm2Co17permanent magnet material

Single-phase ultrafine nanocrystalline Sm2Co17, a candidate material for permanent magnets, was prepared by a novel simple route based on a home-built `oxygen-free'in situfabrication system. The as-prepared nanocrystalline Sm2Co17has the stable hexagonal Th2Ni17-type (2:17 H) crystal structure at room temperature, which is distinctly different from the conventional polycrystalline Sm2Co17having the rhombohedral Th2Zn17-type (2:17 R) crystal structure at room temperature. Phase transformations in the nanocrystalline Sm2Co17alloy were investigated systematically by a series of annealing experiments. It was found that, along with the nanograin growth in the heat-treatment process, the crystal structure of the room-temperature phase transforms from 2:17 H to 2:17 R. The magnetization behavior of Sm2Co17alloys in different structural states was characterized. The relationship between the magnetic performance and the structure characteristics was analyzed. Understanding the phase transformation of Sm2Co17nanocrystals facilitates the development of nanocrystalline Sm2Co17-type magnets for high-temperature applications.

Radial cracks and fracture mechanism of radially oriented ring 2:17 type SmCo magnets

Radially oriented ring 2:17 type SmCo magnets have different microstructure in the radial direction (easy magnetization) and axial direction (hard magnetization). The structure of the cross-section in radial direction is close-packed atomic plane, which shows cellular microstructure. The microstructure of the cross-section in axial direction consists of a mixture of rhombic microstructure and parallel lamella phases. So the magnets have obvious anisotropy of thermal expansion in different directions. The difference of the thermal expansion coefficients reaches the maximum value at 830–860 °C, which leads to radial cracks during quenching. The magnets have high brittlement because there are fewer slip systems in crystal structure. The fracture is brittle cleavage fracture.

Magnetic Microstructures of 2:17 Type Sm(Co, Fe, Cu, Zr) z Magnets Detected by Magnetic Force Microscopy

The magnetic microstructures of 2:17 type Sm(Co, Fe, Cu, Zr) z magnets were detected by magnetic force microscopy. Comparing the microstructures of the specimens coated with and without Ta thin film before and after heat-treatment, it is found that: (a) as a protection layer, Ta coating layer about 20 nm thick can effectively restrain Sm volatilization under high temperature; (b) the stress built in the 2:17 type Sm-Co magnets during specimen preparation only affects some local parts of the domain structures; (c) the magnetic microstructures vary largely for specimens heat-treated at high temperature without Ta film coating due to Sm volatilization. In addition, by comparing with high coercivity Fe-Pt point tips, it is found that the Co-Cr thin-film tips are not suitable for detecting the magnetic microstructures of strong permanent magnets.

Design of Permanent Magnet Systems Using Finite Element Analysis

Rare earth permanent magnets have a wide range of magnetic properties to meet the requirements of an extensive variety of applications. Sintered Sm 2Co 17-type magnets have the best thermal stability with high magnetic performance at temperatures up to 550 °C. Sintered NdFeB magnets have the highest maximum energy product, ( BH) max, but are limited to applications with relatively low operating temperatures. Bonded magnets offer some design flexibility at the expense of magnetic properties. In view of these complexities, it is very important to understand the critical factors when designing the magnetic circuit. Using design examples based on finite element analysis (FEA), we will discuss magnetic materials selection, magnetic circuit design principles and design trade-offs for various applications.

Microstructures and magnetic properties of spark plasma sintered Fe–Cr–Co type and Sm2Co17 type magnets

The differences in microstructures and magnetic properties are studied between Fe–Cr–Co type magnets and Sm2(Co,Fe,Cu,Zr)17 type magnets due to the processing variables, such as starting powders and sintering temperatures of spark plasma sintering (SPS). The grain growth can be suppressed by SPS. Furthermore, SPS process makes it possible to obtain fully dense magnets in a short periods at low sintering temperature compared with the conventional process.

Evolution of microstructure, microchemistry and coercivity in 2.17 type Sm-Co magnets with heat treatment

A systematic study has been undertaken to understand the evolution of microstructure, microchemistry, and coercivity of sintered Sm(Co/sub bal/Cu/sub 0.06/Fe/sub 0.015/Zr/sub 0.027/)/sub 6.4/ magnets with heat treatment using magnetometry, transmission electron microscopy, Lorentz microscopy, and nanoprobe chemical analysis. In general, the homogenized and quenched Sm(Co/sub bal/Cu/sub 0.06/Fe/sub 0.015/Zr/sub 0.027/)/sub 6.4/ magnets have a featureless microstructure with the 2:17 hexagonal structure. During isothermal aging at 700-850/spl deg/C, the 1:5 nuclei precipitate and then coalesce and start forming the cellular structure with 2:17 rhombohedral cells surrounded by 1:5 hexagonal cell boundaries. Uniform cellular and lamellar structures are formed after 2 hours of isothermal aging, and both the cell size and density of lamella phase slightly increase with longer aging. Nanoprobe chemical analysis shows that the Cu content in 1:5 cell boundaries increases during the slow cooling to lower temperatures, reaching a maximum value around 500/spl deg/C, which is consistent with the development of coercivity. Also the Cu content in the triple cell boundary junctions is twice as much as the amount at the regular cell boundaries regardless of cell size and boundary width. Lorentz microscopy indicates that the triple cell boundary junctions may play a major role in domain wall pinning.

Optimisation of 2/17 permanent magnets using the quinary Sm–Co–Cu–Fe–Zr phase diagram

The combination of high magnetisation and high Curie temperature of the Sm2Co17 type magnets makes them attractive for a number of applications requiring high-energy products and long term stability for continuous operating up to 300°C. In order to optimise the elaboration process, it was necessary to investigate phase equilibria in the Sm–Co–Zr–Cu–Fe system. The presented part of the work concerns two regions of interest near 1150°C and 850°C. A specific method is proposed to represent the diagram where a two dimension projection of the different domains is coupled with a three dimensional one. This representation appears as a tool to understand the metallurgical process and to obtain better reproducibility of the alloys magnetic properties. Then an optimisation could be made. Magnetic behaviour of alloys is discussed.

The ternary system Sm [sbnd]Co [sbnd]Zr isoplethic section at Co = 89 at.%

A knowledge of solid-liquid equilibria is necessary to optimise sintering temperature in industrial Sm 2Co 17-type magnets. A single-phase state is reported as precursor of the cellular microstructure of the Sm 2Co 1, permanent magnets. The formation of this phase at high temperature seems for a large part to be governed by Zr addition. In order to understand the behaviour of 2:17-type magnets, phase relationships have been investigated in the Co-rich field of the Sm Co Zr system. The 1150°C isothermal section has been reinvestigated. A two-phase region was observed involving hexagonal and rhombohedral Sm 2Co 12. An isoplethic section has been drawn for temperatures between 1150°C and the melting points of the alloy with Co - 89 at.%, (Sm + Zr) = 11 at.%. Two four-phase equilibria have been observed. A tentative polythermal representation of the Sm Co Zr system in the Co-rich field has been drawn.