Recent progress in high temperature permanent magnetic materials

Permanent magnetic materials capable of operating at high temperature up to 500℃ 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℃）, 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℃, 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.

A linear elastic Ni_(50)Mn_(25)Ga_9Cu_(16) martensitic alloy

The linear elasticity was studied in a martens- tic alloy NisoMn25Ga9Cu16. A 0.4 % linear elastic strain is btained in the polycrystalline sample under compressive stress of 745 MPa. The elastic modulus is 186 GPa. The obtained linear elastic strain and elastic modulus are much higher than that of ternary Ni-Mn-Ga martensitic alloys.~.bstract The linear elasticity was studied in a martens- tic alloy NisoMn25Ga9Cu16. A 0.4 % linear elastic strain is ~btained in the polycrystalline sample under compressive stress of 745 MPa. The elastic modulus is 186 GPa. The obtained linear elastic strain and elastic modulus are much higher than that of ternary Ni-Mn-Ga martensitic alloys.

Phase transition of Ni_(55-x)Co_(x)Mn_(20)Ga_(25)(8.5≤x≤11.0)alloys with different compositions and magnetic fields

Studies on Co-doped Ni–Mn–Ga ferromagnetic shape memory alloys(FSMAs)have been quite active topics in recent years.Unlike previous reports where the amount of Co doping was generally less than 8 at%,in this work Ni_(55-x)Co_(x)Mn_(20)Ga_(25)(8.5≤x≤11.0)alloys were studied with high Co doping.Unusual effects of both composition and magnetic field on phase transitions were observed.With the increase in substitution of Co for Ni,the magnetic transition temperatures increase gradually but the martensitic transformation temperature decreases quite sharply.In particular,the average decrease in the martensitic transformation temperatures is up to 100 K which is nearly twice that in the case of Co content of less than 8 at%.Further,under an applied magnetic field ranging from 0.03 to 0.60 T,abnormal stabilization of a martensite phase with lower magnetization was monitored.Magnetic entropy change of 1.6 J·kg^(−1)·K^(−1)was induced at the martensitic transformation of Ni46.5Co8.5Mn20Ga25 alloy by an applied field of 1 T.The magnetic contributions,including the magnetocrystalline anisotropy and the Zeeman energy,to the thermodynamics of the martensitic transformation are considered to understand the observed unusual phenomena.This work results in new insights into the understanding of Co-doped Ni–Mn-based ferromagnetic shape memory alloys.

Internal friction behavior of Ni_(50.5)Mni_(25)Ga_(24.5)alloy with cellular microstructure

Ni_(50.5)Mn_(25)Ga_(24.5)alloy with cellular microstructure was fabricated.The effect of the cellular microstructure on the internal friction(IF)behavior associated with martensitic transformation(MT),premartensitic transformation(PMT)and twin boundary motion(TBM)was investigated.Compared with conventional NiMn-Ga alloys,the temperature span of the IF_(MT)peak is significantly expanded,the IF_(PMT)is weakened,and no IF_(TBM)is detected.Both IF_(MT)and IF_(PMT)in the cellular microstructure exhibit unique frequency dispersion feature which has never been observed in conventional Ni-Mn-Ga alloys.The effect of point defects associated with the cellular microstructure is considered to understand the observed results.

Pseudoelasticity and elastocaloric effect of Fe_(75.5)Ga_(24.5) single crystal

<100>-,<112>-and <149>-oriented single crystals of Fe_(75.5)Ga_(24.5) alloy were prepared by optical float zone melting method.The pseudoelasticity behavior and elastocaloric effect of the single crystals were investigated,as well as the associate microstructures.D0_(3) phase structure was realized by solution treatment at 800 0C and annealed at 600℃ for 10 h.The compressive deformation behavior exhibits significant dependence on the crystalline directions.Excellent pseudoelasticity with recoverable strain up to 5% is obtained by compression along <149>direction.The pseudoelasticity disappears after five deformation cycles.Adiabatic temperature change is simultaneously detected during the pseudoelasticity,especially during loading process.

Correlation of microstructure and magnetic properties in Sm(Co_(bal)Fe_(0.1)Cu_(0.1)Zr_(0.033))_(6.93) magnets solution-treated at different temperatures

The correlation of microstructure and magnetic properties in Sm(Co_(bal)Fe_(0.1)Cu_(0.1)Zr_(0.033))_(6.93) magnets solution-treated at different temperatures was systematically investigated. It is found that the magnets solution-treated at 1219℃ possess a single 1:7 H phase, exhibiting the homogeneous cellular structure during further aging treatment, leading to the optimum magnetic properties. However, for the magnets solution-treated at 1211 and 1223℃,2:17 H or 1:5 H secondary phase will also form besides 1:7 H main phase, which cannot transform into cellular structure,thus deteriorating the magnetic properties greatly. The irreversible magnetization investigations with recoil loops also propose a non-uniform pinning in the magnets induced by the secondary precipitates. At proper solution temperature, Zr is supposed to occupy the Fe-Fe dumbbell sites in the form of Zr-vacancy pairs, leading to the minimum c/a ratio and thus stabilizing the 1:7 H phase. Finally,Sm(Co_(bal)Fe_(0.1)Cu_(0.1)Zr_(0.033))_(6.93) magnets with the maximum energy product and intrinsic coercivity at 550℃ up to 60.73 kJ·m^(-3) and 553.88 kA·m^(-1) were prepared by powder metallurgy method.

Microstructure and magnetostrictive properties of Tb doped Fe–Ga bulk alloys prepared by melt quenching

The microstructure and magnetostrictive properties were investigated in the Tb doped Fe83Ga17-xTbx（x = 0.05, 0.10, 0.20, 0.30, 0.40, 0.50） bulk rods prepared by melt rapidly quenching. The partial solid solubility of Tb in the Fe-Ga matrix was preliminary detected by the lattice parameters and SEM observation. The matrix keeps A2 structure and the second phase appears surround the grain boundary as x C 0.1. h100 i preferred orientation is also observed for x = 0.1 sample along the axis of the quenched rod. The saturation magnetostriction first increases and maximum value reaches at x = 0.1, and then decreases with Tb addition increasing. The initial increase of the magnetostriction should be associated with the partial solution of Tb in the matrix, the maximum value at x = 0.1 should be attributed to the h100 i preferred orientation, and the decrease of the magnetostriction is correlated with the appearance of the second phase along the grain boundary.

Temperature stability of SmCo(2:17) magnets modified by Ni-Cr two-layer coating

The oxidation resistance behavior of SmCo(2:17)-type high-temperature magnets modified with Ni-Cr two-layer coating was studied. The study depicts the mass gain kinetics and magnetic properties of uncoated and NiCr-modified magnets oxidized at high temperature(500 ℃) in air for 200 h. The oxidation test results illustrate that the mass gain of uncoated magnet is6.95 mg·cm^(-2) which is more than that(0.08 mg·cm^(-2)) of coated magnet after 200 h. For the magnetic properties concerned, there is a great loss for uncoated magnet, while for coated magnet, magnetic properties do not change much. The study of uncoated magnet through X-ray diffraction(XRD) and electron probe microanalysis(EPMA) shows that the invasion of oxygen at high temperature leads to the loss of magnetic properties by changing the microstructure of magnet.

Synthesis of SmCo_5 nanoparticles with small size and high performance by hydrogenation technique

SmConanoparticles(NPs) have promising applications in high-density magnetic storage and magnetic nanocomposites. In this work, A novel method to yield SmCoparticles with small size and high coercivity was reported. Firstly, SmO-Co NPs with size of 6-15 nm were fabricated by a solvothermal route. Then the agglomerated SmCoparticles were obtained by thermal reduction of the precursor, which show high coercivity of2.0 T at room temperature. At last, the as-synthesized SmCoparticles were further hydrogenated under high hydrogen pressure of 4 MPa at room temperature, where hydrogen absorption process could form small-sized SmCoHparticles due to their lattice expansion and hydrogen desorption process could convert SmCoHNPs into SmCoNPs. The prepared SmCoNPs after hydrogenation, showing well distribution, have a small size of5-20 nm and room temperature coercivity of 1.22 T.

Chemical synthesis of SmCo5/Co magnetic nanocomposites

A three-step chemical synthesis of SmCo5/Co nanocomposites was developed. Firstly, the Co-Sm(OH)3- Ca(OH)2 precursors were prepared by co-precipitation. Secondly, SmCo5 particles were obtained by reductive annealing of the precursors. At last, the SmCo5/Co nanocoinposites were achieved by chemical deposition based on SmCo5 particles. The SmCo5/Co nanocomposites contain hard magnetic phase of SmCo5 with about 100 nm in size and soft magnetic phase of Co wilh about 8 nm in size, exhibiting in dependent two-phase structure without alloying. Compared to that of single-phase SmCo5 particles, the saturation magnetization of SmCo5/Co nano composites is increased by 27.5%. The synthesis provides a new route to fabricate SmCo-based nanocomposites.

Hot corrosion of surface-modified Sm_(2)Co_(17) high-temperature magnet with Ni and Ni/Cr bilayer coatings in 75 wt% Na_(2)SO_(4)–NaCl mixture

Hot corrosion behavior of Sm_(2)(Co, Fe, Cu,Zr)17-type high-temperature magnetic alloy without and with a protective coating of Ni and Ni/Cr bilayer at 500 ℃ in a corrosive mixture of 75 wt% Na_(2)SO_(4)–NaCl for 100 h was reported in this paper. The obtained results of timedependent weight change established the parabolic growth of oxidized surface of bare magnets(8.87 mg·cm^(-2)),which caused a rapid loss of magnetic properties measured both at room and high temperatures(500 ℃). X-ray diffraction(XRD) analysis revealed the growth of oxides for bare magnet and Cr–oxides(Cr_(2) O_(3)), top layer of Ni/Cr bilayer coating, but in the case of single coating, Ni–sulfides formation indicated penetration of sulfur, which is further verified by electron probe microanalysis with energy dispersive spectroscopy(EPMA/EDS) study.Results showed that bilayer-coated samples(Ni/Cr) performed better than Ni-coated and bare samples.

Low remanence temperature coefficient Sm1-xErx(Co,Fe,Cu,Zr)z magnets operating up to 400℃

Er-doped Sm1-xErx(CobalFe0.15Cu0.08Zr0.03)7.8(x=0,0.1,0.2,0.3)magnets with a low remanence temperature coefficient were prepared by powder metallurgy method.The influence of Er content on the remanence and microstructure was investigated.X-ray diffractometer(XRD)analysis showed that the magnets with different Er contents consist of 2:17 R phase and 1:5 H phase.Scanning electron microscopy(SEM)analysis showed that the composition of the matrix is consistent with stoichiometric composition and no obvious precipitated phase appears.With the increase in doped Er amount,the temperature stability of Sm1-xErx(CobalFe0.15Cu0.08Zr0.03)7.8(x=0,0.1,0.2,0.3)is getting better.When x is up to 0.3,the magnets with a low remanence temperature coefficient are obtained and the remanence descends tardily from 0.86 to 0.80 T as the temperature rises from room temperature to 400℃.These results indicate that Er substitution for Sm in SmCobased permanent magnets together with optimal composition and proper heat treatment could achieve a desired magnetic performance combined with high thermal stability.

Phase transition and mechanical properties of Ni_(30)Cu_(20)Mn_(37+x)Ga_(13-x)(x=0–4.5) alloys

Ni30Cu20Mn37＋xGa13-x（x = 0–4.5） alloys were studied with the phase transformation and mechanical properties. With the increase of Mn content, the martensitic transformation temperatures increase and the Curie temperature decreases. Simultaneously, the room temperature microstructure evolves from single phase of austenite to dual phases containing martensite and precipitation. Both the ductility and the strength of the polycrystalline alloys are significantly improved by the precipitation. Coupled magnetostructural transition from weak magnetic martensite to ferromagnetic austenite is obtained in both single-phase and ductile dual-phase alloys.

Chemical synthesis and characterization of SmCo_(5)/Co magnetic nanocomposite particles

Magnetic nanocomposite material has been widely focused for the potential to become the next generation of magnetic material.In this paper,two kinds of chemical coating methods were used to prepare SmCo_(5)/Co nanocomposite particles which were further characterized and compared.The two methods were carried out by using different materials and at different temperatures.In Method I,oleylamine(OAm),oleic acid and Ca(acac)2 were used and the reaction was carried out at the temperature of 300℃.In MethodⅡ,anhydrous isopropanol,polyvinylpyrrolidone(PVP),N_(2)H_(4)·H_(2)O and CoCl_(2)·6 H_(2)O were used and the reaction temperature was~55℃.It was found that by using the two methods,the growth and the crystal structure of the Co nanoparticles(NPs)are different.In Method I,epitaxial growth on the surface of SmCo_(5) NPs was observed and the Co NPs were in a facecentered close packing crystal structure.While in Method II,the coated Co NPs were self-nucleated with a crystal structure of hexagonal close packing.Using MethodⅡwith the addition of surfactant,anisotropic nanocomposite particles were achieved with an enhanced saturated magnetization of 84.2 A·m^(2)·kg^(-1).And the coercivity change of the NPs illustrates that a nonmagnetic interlayer between the hard and soft magnetic phase is beneficial to maintain the coercivity.

Influence of cooling rate on magneto-structural transition and magnetocaloric effect of Ni_(30)Cu)8Co_(12)Mn_(37)Ga_(13) alloy

The influence of heat treatment with different cooling rates on phase transition behaviors and magnetocaloric effect is systematically studied.Difference in atomic order is induced by changing cooling rates,where ordered phase is obtained in the furnace cooled（FC）sample while disordered phase is reserved in the water quenched（WQ）sample.The coupled magneto-structural transition is detected in both samples but the characteristic temperature significantly shifts to lower temperatures with increasing atomic order.Giant magnetic entropy change（ΔS_（mag））derived from magnetic field induced martensitic transformation is confirmed for both samples,and can be remarkably enhanced by the atomic ordering.The largestΔS_（mag） of 20.9J/（kg·K）is obtained at 307.5Kunder 5Tin the FC sample.