Permanent Magnet Alloys Research Articles

Optimizing Composition in MnBi Permanent Magnet Alloys

MnBi is an attractive rare-earth-free permanent magnetic material due to its low materials cost, high magnetocrystalline anisotropy (1.6 × 106 J m-3), and good magnetization (81 emu g-1) at room temperature. Although the theoretical maximum energy product (BH)max of 20 MGOe is lower than that of NdFeB-based magnets, the low temperature phase (LTP) of MnBi has a positive temperature coefficient of coercivity, up to 200 °C, which makes it a potential candidate for high temperature applications such as permanent magnet motors. However, the oxygen sensitivity of the MnBi compound and the peritectic reaction between Mn and Bi make it difficult to synthesize into a material with high purity. This challenge is partly offset by adding excess Mn to the alloy, with composition close to Mn55Bi45 resulting in the highest saturation magnetization after common processing techniques such as arc melting, casting, melt spinning, and ball milling. Here we report a systematic process which reduces the amount of excessive Mn, while simultaneously providing a large saturation magnetization (MS) of 79 emu g-1 at 300 K in the annealed Mn52Bi48 ribbons. We also report excellent magnetic properties in the ball powders, resulting in 0.5-5 µm particles with MS of 75.5 emu g-1, coercivity Hci of 10.8 kOe, and (BH)max of 13 MGOe using 9 T applied field at 300 K. A secondary annealing treatment on various ball milled powders increased Hci by up to 21%, and also resulted in an increase in MS up to 78.8 emu g-1.

Effect of cooling rates on the microstructure and magnetic properties of MnAl permanent magnetic alloys

Abstract In this work, the microstructure, phase transformation and magnetic properties of MnAl permanent magnetic alloys prepared by the melt-spinning technique were systematically investigated by tuning the cooling rate. It was shown that the MnAl microstructure, phase transformation and magnetic properties can be tailored by the copper roller speed. With increasing of the roller speed, the phase transformation from the γ2, β phases to the e phase was observed. In addition, the variation of magnetic properties was attributed to the phase content and microstructure of the ferromagnetic τ phase in the MnAl alloys, being controlled by the cooling rate. As a result, the high-purity τ phase MnAl was obtained at a copper roller speed of 5 m/s, which exhibited a high magnetization of 118.2 emu/g. Therefore, this work can provide a method for the direct fabrication of the high-purity ferromagnetic τ phase MnAl alloy.

Reversible Changes of Coercive Force in Sm – Co – Cu – Fe – Zr Alloy for Permanent Magnets Under Cyclic Heat Treatment

Causes of reversible changes in the coercivity of alloys for permanent magnets as a result of cyclic heat treatment are analyzed. The magnetic properties and the metallographic and magnetic domain structures of alloy Sm(Co0.65Fe0.26Cu0.07Zr0.02)7 are studied as a function of the temperature of interruption of the process cooling. The own results and data of foreign authors are used for developing a scheme of the processes occurring in the alloy in order to explain the mechanism of the “damage – restoration” phenomenon in alloys of the Sm – Co – Cu – Fe – Zr system of type Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.

Microstructures and magnetic properties of cast alnico 8 permanent magnets under various heat treatment conditions

There is a close relationship between the magnetic properties of alnico permanent magnetic alloys and the nano-scaled spinodally decomposed structure which results from various heat treatment cycles. In this paper, different heat treatment cycles are employed to cast alnico 8 alloys with composition 32.3Fe-37.5Co-13.7Ni-6.2Al-5.8Ti-3.4Cu-0.2Zr-0.1S-0.8Nb. It is observed that magnetic field treatment at high temperature for specific time is the most effective method to obtain better microstructure and magnetic properties. The mosaic structures consisting of Fe-Co phase and Al-Ni rich phase are best separated during thermomagnetic treatment for 4–5 min and refined during low temperature treatments. The bias growth of the ferromagnetic phase does not develop in absence of magnetic field, and hence the phases are not refined and separated completely. This produces isotropic alnico alloys with low magnetic properties. However, continuous cooling of the alloys in magnetic field followed by isothermal treatment without magnetic field provides moderate magnetic properties. Treatment at high temperatures in field and without field for longer time leads to the coarsening of spinodal phases and poor magnetic properties. The optimum magnetic properties of Hcj = 1.7 kOe, Br = 8.0 kGs and (BH)max = 5.02 MGOe are attributed to the refined microstructure of the thermomagnetically treated alloys.

Electrodeposition of CoNiMnP Thick Films for Micromachined Magnetic Device Applications

CoNiMnP permanent magnetic alloys were electrodeposited on copper and brass substrates from a bath containing 26 g/L CoCl2�6H2O, 24 g/L NiCl2�6H2O, 3.6 g/L MnSO4, 4.6 g/L NaH2PO2�H2O as precursors and H3BO3, NaCl, saccharin, sodium lauryl sulphate and cerium(III) sulphate as additives. Influence of the presence of external magnetic field (2 T) was investigated using vibrating sample magnetometry and SEM/EDS microscopy.

Enhanced formation of 2:14:1 phase in La-based rare earth-iron-boron permanent magnetic alloys by Nd substitution

Aiming at promoting the formation of hard magnetic 2:14:1 phase in La-Fe-B alloys, the effects of Nd substitution on the melt-spun nanocrystalline (La1-xNdx)yFe94-yB6 alloys (x = 0–0.5; y = 12–16) have been investigated. No hard magnetic phase was formed in the (La1-xNdx)16Fe78B6 alloys with x < 0.2. A soft magnetic phase (La,Nd)2Fe23B3 with Curie temperature of 649 K was observed in the x = 0.2 alloy and it disappeared as x increased to 0.3. 30% and above Nd substitution can dramatically enhance the formation of 2:14:1 phase and the magnetic properties of La-based permanent magnets. Three different alloy systems (y = 16, 14 and 12) show similar behavior of magnetic properties dependences on Nd substitution. (La0.7Nd0.3)16Fe78B6 alloy with mean grain size 10–20 nm exhibits high remanent magnetization, maximum energy product and intrinsic coercivity up to 0.71 T, 10.0 MGOe and 562 kA/m, respectively, indicating it is possible to achieve excellent magnetic properties in La-based rare earth permanent magnets. The present work provides a good understanding on the fundamental magnetism of La-based magnets and the potential to develop low cost rare earth permanent magnets.

Relation between solidification microstructure and coercivity in MnAl permanent-magnet alloys

MnAl alloys are considered to be an important candidate to plug the gap between Nd-Fe-B magnets and ferrites, and their microstructure is closely related with the magnetic properties. In this work, the relation between solidification microstructure and the coercivity as well as the ε→τ solid-state transformation in MnAl permanent-magnet alloys are investigated. Different solidification microstructures are obtained in (Mn0·54Al0.46)98C2 alloys by various preparation methods. The solidification microstructure undergoes a significant refinement from single crystal to columnar grains, dendritic grains and finally to ultrafine columnar grains by increasing the solidification rate, and the width of the grains monotonously decreases from ∼3 mm for single crystal finally to ∼2 μm for the ultrafine columnar grains. Besides, the ε→τ transformation temperature and coercivity of τ-phase have significant correlation with the initial microstructure of ε-phase. The ε→τ solid-state transformation gradually shifts to lower temperatures due to the nucleation stimulation by higher density of grain boundaries with the refinement of solidification microstructure. Simultaneously, the coercivity is remarkably improved from 0.16 kOe in single crystal to 1.47 kOe in melt spun ribbons with ultrafine columnar grains because of the pinning effect of the increasing amount of grain boundaries.

BiMn: Synthesis, separation by centrifugation, and characterization

Several Mn-based compounds have been identified as promising materials to replace permanent magnetic alloys based on rare earth elements. Amongst them is the compound BiMn, in particular the low temperature modification α-BiMn (hP4, AsNi-type, B81), which has been shown to exhibit interesting magnetic properties. In the present study, it was attempted to synthesize phase-pure α-BiMn by annealing samples at 320 °C and centrifuging them at this temperature. In this way, it was possible to obtain a distinct phase separation with the intermetallic compound on top and the liquid Bi, which is always present after the high-temperature syntheses, at the bottom. After solidification, α-BiMn with a purity of up to 87%, though not phase-pure, could be recovered. Thermal analyses as well as the temperature dependence of the magnetization indicated a peritectic decomposition at 355 ± 2 °C whereas the phase re-formed on cooling at 341 ± 2 °C. High-temperature X-ray diffraction led to a complete oxidation of the sample even in a rather pure He atmosphere. Magnetic measurements showed an increase of the coercivity of α-BiMn with increasing temperature, in general agreement with earlier literature reports. The Curie temperature could not be determined experimentally as it is apparently higher than the temperature of the peritectic decomposition.

Coercivity enhancement and magnetization process in Mn55Bi45 alloys with refined particle size

The MnBi permanent magnetic alloys with high coercivity was prepared by a low-energy ball milling process. The coercivity of the MnBi alloys can be controlled by tuning the particle size and the size distribution of the MnBi alloys powders through the milling process. The size-dependent coercivity of the particles was expressed by a function of Hc = 106.17−0.56·log(D), where the particle size (D) was supposed to be lager than the single-domain size. With an optimized treatment, a room-temperature coercivity of ∼1.9 T was achieved in the MnBi powders. The magnetization curves and micromagnetic analyses indicated that the magnetization process of the MnBi alloys was governed by the coherent magnetization rotation. The hardening mechanism was corresponding to the nucleation process in the anisotropic MnBi powders with refined particle size.

Microstructure and magnetic properties of alnico permanent magnetic alloys with Zr-B additives

Alnico alloys are prepared with nominal composition of 31.4-xFe-7.0Al-36.0Co-4.0Cu-1.0Nb-14.0Ni-6.0Ti-0.6Zr-xB (x = 0.02, 0.04, 0.06, 0.08, in wt%) by arc melting and casting techniques and subsequent heat treatment. The alloys are characterized by X-ray diffraction method, optical microscope, scanning electron microscope and pulse field magnetometer by plotting magnetic hysteresis demagnetization curve. The results of HRSEM show at least two new phases at α-grain boundaries and triple junctions. These phases, when retained at low concentration, help in enhancing magnetic properties of alnico alloys by purifying spinodal phases and reducing the adverse effects of impurity elements. Two different heat treatment cycles are employed. In the first phase, the alloys are processed by using heat treatment cycles without magnetic field; and Hc of 1.35 kOe, Br of 4.87 kGs and (BH)max of 1.96 MGOe are obtained by furnace cooling below TC and subsequent tempering at 680 °C and 550 °C. In the second phase, the alloy with best magnetic properties is treated thermo-magnetically; and Hc of 1.68 kOe, Br of 7.1 kG and (BH)max of 4.45 MGOe are obtained.

СТРУКТУРА И СВОЙСТВА СПЛАВОВ ДЛЯ ПОСТОЯННЫХ МАГНИТОВ ЮНДК25БА, ПОЛУЧЕННЫХ МЕТОДОМ НАПРАВЛЕННОЙ КРИСТАЛЛИЗАЦИИ С ЖИДКОМЕТАЛЛИЧЕСКИМ ОХЛАДИТЕЛЕМ

СТРУКТУРА И СВОЙСТВА СПЛАВОВ ДЛЯ ПОСТОЯННЫХ МАГНИТОВ ЮНДК25БА, ПОЛУЧЕННЫХ МЕТОДОМ НАПРАВЛЕННОЙ КРИСТАЛЛИЗАЦИИ С ЖИДКОМЕТАЛЛИЧЕСКИМ ОХЛАДИТЕЛЕМ

Microstructural and magnetic property evolution with different heat-treatment conditions in an alnico alloy

Further property enhancement of alnico, an attractive near-term, non-rare-earth permanent magnet alloy system, primarily composed of Al, Ni, Co, and Fe, relies on improved morphology control and size refinement of its complex spinodally decomposed nanostructure that forms during heat-treatment. Using a combination of transmission electron microscopy and atom probe tomography techniques, this study evaluates the magnetic properties and microstructures of an isotropic 32.4Fe-38.1Co-12.9Ni-7.3Al-6.4Ti-3.0Cu (wt.%) alloy in terms of processing parameters such as annealing temperature, annealing time, application of an external magnetic field, as well as low-temperature “draw” annealing. Optimal spinodal morphology and spacing is formed within a narrow temperature and time range (∼ 840 °C and 10 min) during thermal-magnetic annealing (MA). The ideal morphology is a mosaic structure consisting of periodically arrayed ∼ 40 nm diameter (Fe-Co)-rich rods (α1 phase) embedded in an (Al-Ni)-rich (α2 phase) matrix. A Cu-enriched phase with a size of ∼ 3–5 nm is located at the corners of two adjacent {110} facets of the α1 phase. The MA process significantly increased remanence (Br) (∼ 40–70%) of the alloy due to biased elongation of the α1 phase along the 〈100〉 crystallographic direction, which is closest in orientation to the applied magnetic field. The optimum magnetic properties of the alloy with an intrinsic coercivity (Hcj) of 1845 Oe and a maximum energy product (BHmax) of 5.9 MGOe were attributed to the uniformity of the mosaic structure.

Self-organizing maps for pattern recognition in design of alloys

ABSTRACTA combined experimental–computational methodology for accelerated design of AlNiCo-type permanent magnetic alloys is presented with the objective of simultaneously extremizing several magnetic properties. Chemical concentrations of eight alloying elements were initially generated using a quasi-random number generator so as to achieve a uniform distribution in the design variable space. It was followed by manufacture and experimental evaluation of these alloys using an identical thermo-magnetic protocol. These experimental data were used to develop meta-models capable of directly relating the chemical composition with desired macroscopic properties of the alloys. These properties were simultaneously optimized to predict chemical compositions that result in improvement of properties. These data were further utilized to discover various correlations within the experimental dataset by using several concepts of artificial intelligence. In this work, an unsupervised neural network known as self-organizing maps was used to discover various patterns reported in the literature. These maps were also used to screen the composition of the next set of alloys to be manufactured and tested in the next iterative cycle. Several of these Pareto-optimized predictions out-performed the initial batch of alloys. This approach helps significantly reducing the time and the number of alloys needed in the alloy development process.

CaO-matrix processing of MnBi alloys for permanent magnets

The possibility to suppress agglomeration of MnBi alloy particles during milling and their unwanted sintering during subsequent annealing was explored by embedding the particles in CaO through co-milling. A 15 h annealing of the micron-sized MnBi particles embedded in the CaO matrix at 300 °C is not accompanied by sintering or growth of the particles while it significantly increases their coercivity – presumably by healing the milling-induced crystal defects. After separation from the CaO matrix, the annealed MnBi powder combines a calculated energy product of 10 MGOe with a room-temperature coercivity of 14.4 kOe. At the same time, the partial loss and degradation of the MnBi low-temperature phase during warm compaction of the powders makes the effect of the CaO-matrix annealing less pronounced in the case of fully dense magnets; the residue from the solvents employed for the removal of the CaO might have contributed to the decline of the properties. Still, a relatively high room-temperature coercivity of 8.5 kOe was obtained for the fuslly-dense MnBi magnet exhibiting an energy product of 5.3 MGOe.

The stability of newly developed (R,Zr)(Fe,Co)12−xTix alloys for permanent magnets

(R,Zr)(Fe,Co)12−xTix (R = Nd and Sm) alloys of ten different compositions were prepared by strip-casting. The elemental distribution of the sample particles after annealing was quite homogeneous. Existing phases in the prepared alloys were determined by X-ray diffraction and the occupation sites of the substituted elements (Ti, Co, and Zr) were determined using spherical aberration corrected scanning transmission electron microscopy. The ThMn12 structure is stable in Ti1.0 compounds for both R = Nd and Sm, but exhibited obvious precipitation of α-(Fe,Co) phases in low Ti content compounds with Ti0.5. Ti substitution into Fe(8i) sites is important for the stabilization of the ThMn12 structure. Furthermore, the stability of low Ti content compounds could be recovered by the substitution of Zr into R(2a) sites. Co was mainly in Fe(8j) and Fe(8f) sites, which contributed to the stability of the 1–12 structure by increasing the configuration entropy. Analysis based on three Fe hexagons and two local sub-structural units suggested that the stabilization of ThMn12 structure by Ti and Zr substitutions mainly originates by resolving local mismatches in interatomic distances in the structure.

(Invited) Ionic Liquid Enables Technologies for Winning Rare Earth Elements

Rare earth elements are amongst the most critical materials, they are crucial for a number of energy related technologies relying on the superior catalytic, optical or magnetic properties that can be achieved in their compounds. As catalysts they find application in fluidic catalytic cracking (FCC) catalysis, one of the most important processes in petrochemistry where they are used to convert the high-molecular weight hydrocarbon fraction of crude oil into more useful lower-weight hydrocarbons. Rare earth based phosphors are the key part for energy conversion in compact fluorescent lamps (CFLs) and light emitting diodes (LEDs) which are currently the most important energy efficient lighting devices. As phosphors rare earth based materials find also application in scintillators for radiation detection (as used by Homeland Security) or for medical imaging. As security makers they can be found in passports and banknotes. Currently an increased consumer demand for rare earth materials arises from the increased use of cell phones and tablets. The exceptional properties of rare earth elements are particularly used in alloys for permanent magnets. The properties of SmCo5 and Nd2Fe14B are unsuperseded. Because of their high field strength Nd2Fe14B magnets are used in hard drives, motors for hybrid cars or wind turbines. SmCo5 is less used as it not only has a weaker field strength, but is also more expensive because it contains two critical elements, samarium and cobalt. However, as the Curie temperature of SmCo5 is higher than that of Nd2Fe14B is used in applications where high field strengths at high temperatures are needed. However, the stable economic supply of rare earth elements is not guaranteed and, thus, the pproduction and use of rare earth compounds constitute an important strategic and dynamic segment of the electronic, chemical and energy industries. The presentation will discuss the importance and criticality of rare earth elements and will give insights into how ionic liquids can provide technology solutions for rare earth recycling, separation and production, most importantly, the electrodeposition from ionic liquids as an alternate, greener process to the currently used high temperature process.

Phase transformation kinetics and microstructural evolution of MnAl permanent magnet alloys

In order to provide a better understanding about the formation of ferromagnetic τ phase in MnAl Rare-Earth-free permanent magnet alloy, the phase transformation kinetics of τ phase has been analyzed for Mn55Al45 alloy. The local activation energy Ec(α) was introduced to explore the change of energy in the process of phases transformation and its value is closely related to the transformation volume fraction. The local activation energy is low (about 175 kJ/mol) in the process of nucleation. However, the threshold value of energy will increase slowly and approaches a constant value (about 188 kJ/mol) when the grain-boundary-nucleated τ phase steadily grows into the parent ε-phase grains. With the exception of anomalous stage, the local Avrami exponents fall in the range of 2.0–4.0, which implies that two- and three-dimensional nucleation and grain growth controlled by diffusion play a crucial role in the whole transformation process. Meanwhile, transmission electron microscopy (TEM) morphology and the results of transformation kinetics are accordance with each other, indicating that the variable local activation energies Ec(α) and local Avrami exponents n(α) are applicable and correct in describing the τ-phase transformation behavior.

Optimization of the magnetic properties of nanostructured Y-Co-Fe alloys for permanent magnets

The structural and magnetic properties of ball-milled Fe-doped Y Co5−xFex(0 ≤ x ≤ 0.5) were investigated. The magnetization increases with Fe-doping up to the solid solubility limit, x = 0.3 without destroying the crystal structure or degrading the coercivity. A special magnet array is designed using ring magnets for pressing the powders under magnetic field in order to achieve magnetic alignment. A dramatic increase in magnetization is observed for magnetically aligned Y Co4.8Fe0.2 pressed ingots.

Influence of mechanical milling on structure, particle size, morphology and magnetic properties of rare earth free permanent magnetic Zr2Co11 alloy

In an effort to explore finding rare earth free permanent magnet material for device applications, the Co based Zr2Co11 compound has been selected for investigation. A detailed study was carried out on the effect of mechanical milling on structure, particle size, morphology and magnetic properties. The alloy of nominal composition Zr18Co82 exhibited co-existence of minor Zr6Co23 phase along with main phase Zr2Co11. When subjected to long duration mechanical milling, the alloy has undergone structural disorder leading to partial amorphization. The particle size has decreased from 200 μm to 120 μm up on milling for 6 h in a planetary milling process. Further milling caused less influence on particle size reduction. However, it had an effect on particle size distribution. The size reduction is not very high due to ductile nature of Zr2Co11 phase and the morphology of the particles too showed coarse plate shape. The coercivity variation against particle size reduction is less than appreciable, given that the crystal structure being rhombohedral with significant magnetocrystalline anisotropy. The defects induced during milling and the presence of deleterious minor phases are considered the main cause for reduction in the coercivity.

Role of the Applied Magnetic Field on the Microstructural Evolution in Alnico 8 Alloys

Improving performance of permanent magnet (PM) alloys is a complex optimization of microstructure through control of chemistry and processing. Alnico alloys represent a class of chemically complex alloys whose functionality as a PM is dependent on the nano-structuring which develops during thermal heat treatments in an applied magnetic field. Initial phase separation during rapid quenching of a solid solution from 1523 K (1250 °C) results in a network structure consisting of interpenetrating continuous Fe-Co (α 1) and AlNi-based (α 2) phases. Annealing polycrystalline alnico with a fixed external magnetic field (H) at 1073 K (800 °C) causes isolation, coarsening, and faceting of the α 1 phase with increasing time. Also, the volume fraction and the Al, Ni, Ti, and Cu contents of the α 1 phase decreased, while the Fe and Co contents increased with increasing field annealing time. A Cu-rich phase forms along the adjacent corners of two α 1 facets in the α 2 phase during the phase separation and coarsening process. The phase distribution and micro-magnetic domain structure is closely related to the relative orientation between H and the principal axes of each grain. The α 1 rods are elongated in each grain along the 〈100〉 direction with projected H, where the preferentially extended length of the α 1 rods along the selected 〈100〉 is proportional to the magnitude of projected H.