B. M. Ma

Metal‐bonded Sm2Fe17‐N‐type magnets

A number of metal‐bonded Sm2Fe17‐N magnets have been fabricated. These magnets exhibit iHc = 5.1–17.0 kOe, Br = 6.4–8.4 kG, (BH)max=5.0–10.8 MGOe, Tc = 757 K, and ρ=6.2–6.7 g/cm3. Powder metallurgical techniques have been employed with a mixture of powdered Sm2Fe17‐N and Zn, Sn, or In. Heat treatment is carried out in the temperature range of 160–450 °C in a N2 atmosphere at pressures ranging from 0–900 psi. The effects of Zn, Sn, and In contents and heat treatment conditions on the magnetic properties have been studied. Zn as the binder significantly enhances the coercivity iHc from 1.8–2.5 kOe for Zn‐free magnets to 5–17 kOe for 9–20‐wt. % Zn‐containing magnets. The Fe‐Zn phase, FeZn4, and/or Fe3Zn7, formed during heat treatment, may play an important role in producing a high coercivity. Sn‐bonded magnets exhibit significant coercivity, whereas the In‐bonded materials do not. The coercivity behavior is discussed in terms of the chemistry of the system.

A study on the exchange coupling of NdFeB-type nanocomposites using Henkel plots

The phase transformation and magnetic properties of (Nd0.95La0.05)9.5FebalCo5Nb2B10.5 nanocomposite have been investigated systematically via thermomagnetic analysis, vibration sample magnetometer, x-ray diffraction, and conventional transmission electron microscopy. The Henkel plot was employed to quantify the strength of the exchange coupling between the hard and soft magnetic phases in the as-spun and the thermally treated samples. It was found that remanence Br, coercivity Hci, and maximum energy product BHmax obtained were affected by the magnetic phases present as well as the grain size of constituent phases and their distribution. The Henkel plot successfully interpreted the effect of the exchange coupling on Br, Hci, and BHmax obtained for samples treated below 750 °C. However, it became inadequate for samples treated above 750 °C. Although similar shapes of ΔM–H curves were obtained in the Henkel plot, severe degradation in Br, Hci, and BHmax was found when the thermal treatment temperature was i...

Structure and magnetic properties of RCo7−xZrx (R=Pr or Er, x=0–0.8)

Alloys of composition RCo7−xZrx (R=Pr or Er and x=0−0.8) were synthesized and characterized in the temperature range of 10–1273 K in fields up to 5 T. As with the SmCo7−xZrx system studied earlier in our laboratory, the effects of Zr doping on the stability of the TbCu7 phase and the increase in the anisotropy field HA are also observed in the systems of PrCo7−xZrx and ErCo7−xZrx. Nearly single phase TbCu7 materials were formed in as-cast alloys when x=0.1–0.2. In the case of R=Pr, HA changes from almost planar for x=0 to uniaxial with Ha∼100 kOe for x⩾0.2 at room temperature (RT). In the case of R=Er, HA for the x=0.1 composition is almost two times larger than that of the Zr-free alloys, which shows strong uniaxial anisotropy at both RT and 10 K. Spin reorientation behavior (when R=Pr) and R–M antiparallel coupling (when R=Er) were also observed.

Comparison on the magnetic and structural properties of Sm(Co0.67−xFe0.25Cu0.06Zr0.02Cx)8.0, where x=0–0.15, melt spun ribbons and cast alloys

The effects of C additions on the phase transformation and magnetic properties of Sm(Co0.67−xFe0.25Cu0.06Zr0.02Cx)8.0, where x ranged from 0 to 0.15, melt spun ribbons and cast alloys have been studied by x-ray diffraction (XRD), differential thermal analysis (DTA), and vibrating sample magnetometer. In addition to the Th2Zn17 structure, two additional compounds, namely, the ZrC and SmCoC2, were detected by XRD after a thermal treatment over 700–1160 °C. The as-spun ribbons were highly crystalline at x=0 and became mostly amorphous at x=0.10. An intrinsic coercivity, Hci, of 3.0 kOe was obtained for the as-spun ribbons with x=0.05. After an optimum heat treatment, the Hci of the ribbons with x=0.01 was increased to 8 kOe. Cast alloys of identical chemical compositions were also solution treated and precipitation hardened. At x=0 for the cast alloy, a Br of 10.8 kG, Hci of 24 kOe, Hc of 9.8 kOe, and (BH)max of 27 MGOe, were obtained after an optimum heat treatment.The effects of C additions on the phase transformation and magnetic properties of Sm(Co0.67−xFe0.25Cu0.06Zr0.02Cx)8.0, where x ranged from 0 to 0.15, melt spun ribbons and cast alloys have been studied by x-ray diffraction (XRD), differential thermal analysis (DTA), and vibrating sample magnetometer. In addition to the Th2Zn17 structure, two additional compounds, namely, the ZrC and SmCoC2, were detected by XRD after a thermal treatment over 700–1160 °C. The as-spun ribbons were highly crystalline at x=0 and became mostly amorphous at x=0.10. An intrinsic coercivity, Hci, of 3.0 kOe was obtained for the as-spun ribbons with x=0.05. After an optimum heat treatment, the Hci of the ribbons with x=0.01 was increased to 8 kOe. Cast alloys of identical chemical compositions were also solution treated and precipitation hardened. At x=0 for the cast alloy, a Br of 10.8 kG, Hci of 24 kOe, Hc of 9.8 kOe, and (BH)max of 27 MGOe, were obtained after an optimum heat treatment.

Magnetic properties and microstructure of nanocomposite R2(Fe,Co,Nb)14B/(Fe,Co) (R=Nd, Pr) magnets

Nanocomposite R2(Fe,Co,Nb)14B/(Fe,Co) (R=Nd, Pr) magnets prepared by crystallizing the as-made R8(Fe,Co,Nb)86B6 amorphous melt-spun ribbons have been studied. The coercivity is found to depend mainly on the grain size of the soft phase which is very sensitive to the sample composition. The average grain size is about 30 nm in R8Fe86B6, but the microstructure is not homogeneous and there are several large α-Fe grains with sizes up to 50–100 nm. The coercivities are 3.3 kOe in Nd8Fe86B6 and 4.9 kOe in Pr8Fe86B6 samples. Nb substitution significantly reduces the grain size of α-Fe and increases the coercivity. The highest coercivities obtained are 5.5 kOe in Nd8(Fe0.97Nb0.03)86B6 and 9.3 kOe in Pr8(Fe0.92Nb0.08)86B6 samples. Co substitution for Fe increases the grain size of both the 2:14:1 phase and α-Fe and dramatically decreases the coercivity. Increasing the B content in Co substituted samples leads to the formation of a more homogeneous and finer microstructure and thus to a partial recovery of the coercivity from 2.3 kOe in Nd8((Fe0.5Co0.5)0.97Nb0.03)86B6 to 4.3 kOe in Nd8((Fe0.5Co0.5)0.97Nb0.03)82B10 and from 2.1 kOe in Pr8((Fe0.5Co0.5)0.94Nb0.06)86B6 to 6.5 kOe in Pr8((Fe0.5Co0.5)0.94Nb0.06)82B10. It is further found that Co substitution improves the temperature dependence of the saturation magnetization.Nanocomposite R2(Fe,Co,Nb)14B/(Fe,Co) (R=Nd, Pr) magnets prepared by crystallizing the as-made R8(Fe,Co,Nb)86B6 amorphous melt-spun ribbons have been studied. The coercivity is found to depend mainly on the grain size of the soft phase which is very sensitive to the sample composition. The average grain size is about 30 nm in R8Fe86B6, but the microstructure is not homogeneous and there are several large α-Fe grains with sizes up to 50–100 nm. The coercivities are 3.3 kOe in Nd8Fe86B6 and 4.9 kOe in Pr8Fe86B6 samples. Nb substitution significantly reduces the grain size of α-Fe and increases the coercivity. The highest coercivities obtained are 5.5 kOe in Nd8(Fe0.97Nb0.03)86B6 and 9.3 kOe in Pr8(Fe0.92Nb0.08)86B6 samples. Co substitution for Fe increases the grain size of both the 2:14:1 phase and α-Fe and dramatically decreases the coercivity. Increasing the B content in Co substituted samples leads to the formation of a more homogeneous and finer microstructure and thus to a partial recovery of the coer...

The effect of boron and rare earth contents on the magnetic properties of La and Cr substituted α-Fe/R2Fe14B-type nanocomposites

The effect of phase transformations on the magnetic properties of rare earth lean (Nd0.95La0.05)9.5Fe82.5−xCr2B6+x (x=0 to 4.5) and (Nd0.95La0.05)7.5+yFe80.5−yCr2B10 (y=0 to 4) melt spun ribbons has been investigated. The phase mixture, after optimum thermal processing, was found to be strongly dependent upon the rare earth and boron contents. Two magnetic phases, namely α-Fe and R2Fe14B, were found in (Nd0.95La0.05)9.5Fe82.5−xCr2B6+x alloy ribbons with x ranging from 0 to 4.5. For a fixed rare earth content, increases in the boron concentration resulted in a higher volume fraction of the R2Fe14B phase, which led to an increase in the intrinsic coercive force from 7.1 kOe for x=0 to 12.6 kOe for x=4.2. A Br=9.6 kG, iHc=9.5 kOe, and (BH)max=15.5 MGOe have been obtained in the alloy ribbons with x=4.5. On the other hand, the increase in the total rare earth content, or y, was found to suppress the formation of the metastable Fe3B and/or R2Fe23B3 phases and to yield an α-Fe/R2Fe14B mixture for y>1. This incr...

Structure and magnetic properties of RCo7−xZrx(R=Y, Gd, Nd, or Ho, x=0–0.8)

Alloys with the composition RCo7−xZrx (R=Y, Gd, Nd, or Ho, x=0–0.8) were synthesized and characterized in the temperature range from 10 to 1273 K and in fields up to 5 T. As with the R=Sm, Pr, or Er systems studied earlier in our laboratory, the effects of Zr doping on the stability of TbCu7 structure phase and changes in the magnetocrystalline anisotropy HA are observed in the systems in which the rare earth is Y, Gd, Nd, or Ho. Nearly single phase TbCu7 structure materials were formed in the as-cast alloys when x=0.1 and 0.2. In the case of R=Y or Gd, a large increase in HA (which is mainly contributed by Co sublattice) by Zr doping was observed. For R=Y, HA increases from 18 kOe for x=0 to 74 kOe for x=0.2 at 300 K and from 20 kOe for x=0 to 82 kOe for x=0.2 at 10 K. For R=Gd, HA increases from 35 kOe for x=0 to 140 kOe for x=0.2 at 300 K and to 182 kOe for x=0.2 at 10 K. In the case of R=Ho and Nd, the R-sublattice favors a planar anisotropy at room temperature, presumably due to the negative second o...

Exchange coupled R2(Fe,Co,Nb)14B/(Fe,Co) (R=Nd,Pr) and Sm2(Fe,Co,Cr)17C2/(Fe,Co) nanocomposite magnets

Abstract Fe-rich exchange coupled nanocomposite R 8 (Fe,Co,Nb) 86 B 6 (R=Nd, Pr) (consisting of hard 2:14:1 and soft α-Fe(Co) phases) and Sm 2 (Fe,Co) 15 Cr 2 C 2 magnets (consisting of hard 2:17 and soft α-Fe(Co) grains) have been studied. In both systems the nanocomposite hard/soft microstructure is obtained by crystallizing the as-made amorphous melt-spun ribbons. The coercivity is found to depend mainly on the grain size of the soft phase which is very sensitive to the sample composition and annealing conditions. In the R–(Fe,Co,Nb)–B system, it is found that substitution of Nb for Fe significantly decreases the grain size of the α-Fe(Co) and increases the coercivity of the magnets. Substitution of Co for Fe increases the grain size and results in a dramatic decrease of coercivity. Increasing the B content of Co-substituted samples leads to a partial recovery of the coercivity. Magnetic properties M r / M s =0.7 and H c =6.5 kOe have been obtained for Pr 8 ((Fe 0.5 Co 0.5 ) 0.94 Nb 0.06 ) 82 B 10 . In the Sm 2 (Fe,Co) 15 Cr 2 C 2 system, it is found that a higher annealing temperature combined with a shorter annealing time leads to a higher coercivity. The optimum annealing condition is at 900°C for 1 min where the highest coercivity of 12.1 kOe is obtained. The grain size of α-Fe in annealed ribbons becomes much smaller with Co substitution, leading to a stronger exchange coupling between the 2:17 and α-Fe phase. A significant enhancement in the Curie temperature of the 2:17 phase is observed in these magnets as compared to their parent alloys due to the exchange coupling effect between the 2:17 phase and the Fe(Co) phase.

The effect of the coupling agent on the packing density and corrosion behavior of NdFeB and SmCo bonded magnets

The effects of the coupling agent addition on the specific density and corrosion behavior of NdFeB and SmCo bonded magnets have been investigated. The application of LICA44, a titanate coupling agent, improved the filled volume of the epoxy-based bonded magnets, especially at high compression pressures. When molded at 10.5 T/cm2, specific densities of 6.2 and 6.5 g/cm3 have been obtained for the NdFeB and SmCo bonded magnets, respectively. Incorporating a coupling agent was found to eliminate visible cracks on the SmCo bonded magnets, possibly due to the improvements in the wettability and compatibility of the system. The percentage of the filled volume was increased by 2%–7%. Incorporating LICA44 also improved corrosion resistance of the bonded magnets. For NdFeB, LICA44 reduced the steady state corrosion current Icorr from 69 to 38 μA/cm2. No significant changes in the Icorr were observed for SmCo due to the stable nature of SmCo.

The effects of particle size and distribution on the magnetic properties of coercive Sm(Co,Fe,Cu,Zr)z alloy powders for bonded magnet applications

The microstructure of Sm(CobalFexCu0.08Zr0.03)8.2, where x=0.23, 0.26, and 0.28, in the as-cast state and after various processing stages has been examined by optical microscopy. The size of the 2:17 matrix phase was found to be approximately 100 μm in the as-cast state. A slight increase in the size of the 2:17 matrix was observed after thermal processing. Subgrains of 10 to 20 μm are present in the 2:17 matrix of the fully processed ingots. Powders with a mean particle size range of 3–300 μm were found to exhibit a Gaussian distribution. A slight increase in intrinsic coercivity (Hci) was observed when the mean particle size was decreased from 300 to 200 μm and remained nearly constant for sizes ranging between 10 and 200 μm. A significant decrease in Hci was observed when powders were further reduced below 10 μm. Similar trends were also observed for remanence (Br), maximum energy product (BHmax), and squareness of the second quadrant demagnetization curve. The size of the subgrains was found to be cri...