Zhongmin Chen

Magnetic properties and microstructure of mechanically milled Sm2(Co,M)17-based powders with M=Zr, Hf, Nb, V, Ti, Cr, Cu and Fe

The structure, microstructure, and magnetic properties of nanostructured Sm2Co17-based powders synthesized by mechanically milling and subsequent annealing have been systematically studied. It has been found that a nanoscale 2:17 phase with an average grain size of about 30 nm is developed within the powders, which have an average particle size of about 5 μm. Optimum magnetic properties of Ms=110.5 emu/g, Mr=66.2 emu/g, Mr/Ms=0.60, Hc=9.6 kOe, and (BH)m=10.8 MGOe have been obtained in stoichiometric Sm2Co17 powders milled for 6 h and annealed at 800 °C for 30 min. The observed magnetic hardening is believed to arise from the high anisotropy of the Sm2Co17 phase and its nanoscale grain size. A small amount of Zr substitution for Co significantly increases the coercivity by increasing the anisotropy field of the Sm2Co17 phase. Cu substitution in Zr-contained samples further increases the coercivity by introducing a nanoscale 1:5 phase which forms a uniform mixture with the 2:17 nanograins. The highest coerc...

Enhancement of magnetic properties of nanocomposite Pr2Fe14B/α-Fe magnets by small substitution of Dy for Pr

Nanocomposite (Pr,Dy)2Fe14B/α-Fe magnets with compositions Pr8−xDyxFe86B6 (x=0, 0.5, 1, 1.5, and 2) have been synthesized by melt spinning using low wheel speeds in the range from 18 to 21.7 m/s. It has been found that the coercivity is significantly increased by Dy substitution. An optimum coercivity of 6.6 kOe is obtained in the Pr7Dy1Fe86B6 magnet as compared to 4.3 kOe in the Pr8Fe86B6 magnet. As a result, the energy product is increased from 9.1 MGOe in the Pr8Fe86B6 magnet to 16.9 MGOe in the Pr7Dy1Fe86B6 magnet. The reason for this is the finer and more uniform 2:14:1/α-Fe nanoscale microstructure developed in the magnets with Dy substitution. Because of the low wheel speed spinning, the majority of the nanoscale microstructure is crystallized directly out of the melt. The enhancement of magnetic properties by Dy substitution is mainly due to the microstructure refinement which leads to an enhanced exchange coupling between the Pr2Fe14B and α-Fe, whereas the anisotropy increase by the Dy substituti...

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...

Microstructure refinement and magnetic property enhancement of nanocomposite Pr2Fe14B/α-Fe magnets by small substitution of M for Fe (M=Cr, Nb, Ti and Zr)

Abstract A comprehensive study on the effect of various substitutions of M on the magnetic and structural properties of melt-spun nanocomposite Pr8Fe84M2B6 (M=Cr, Nb, Ti and Zr) magnets has been performed with the aim of refining the microstructure and therefore enhancing the hard magnetic properties. It has been found that magnetic properties are improved by all the substitutions. The largest enhancement is obtained in Nb substituted Pr8Fe84Nb2B6 magnets where a coercivity of 6.5 kOe and a maximum energy product of 17.9 MGOe have been obtained, as compared to the coercivity of 4.3 kOe and the energy product of 9.1 MGOe in the Pr8Fe86B6 magnet. Microstructure studies revealed a finer and more uniform 2:14:1/α-Fe nanoscale microstructure with M substitutions. The most uniform microstructure with the smallest average grain size of 10–20 nm is developed in the Nb substituted magnets. The enhancement of magnetic properties by M substitution is believed to be due to the microstructure refinement which leads to an enhanced exchange coupling between Pr2Fe14B and α-Fe.

Coercivity in mechanically milled Pr–Co–Zr powders with TbCu7 structure

The structural, microstructural, and magnetic properties of the PrCo7−xZrx (x=0, 0.2, 0.3, and 0.4) powders produced by mechanical milling have been studied with the aim of developing fine microstructure and high coercivity in these powders. X-ray diffraction, transmission electron microscope, and magnetic measurements show that a single phase with TbCu7-type structure and with high anisotropy of up to 100 kOe is obtained in as-cast PrCo6.7Zr0.3 alloy. Magnetic hardening is developed in the mechanically milled powders. An optimum coercivity of 5.3 kOe has been obtained in PrCo6.7Zr0.3 powders milled for 2 h and annealed at 800 °C for 1 min, which shows a uniform microstructure of the 1:7 phase with an average grain size of about 10–15 nm. The observed magnetic hardening is believed to arise from the high anisotropy field of the Pr(Co, Zr)7 phase and the uniform nanoscale microstructure developed by mechanical milling and subsequent annealing.

Studies on the formation, structure and magnetic properties of Cr substituted Sm2(Fe, Cr)17Cx compounds

Abstract The effects of Cr substitution on the formation, structure and magnetic properties of Sm 2 (Fe, Cr) 17 C x compounds have been systematically studied on arc-melted and melt-spun samples using XRD, TEM and magnetic measurements. It has been found that Cr substitution is an effective way to form the Sm 2 (Fe, Cr) 17 C x carbides. The minimum Cr amount for Sm 2 Fe 17− y Cr y C 2 to form an almost single 2 : 17 phase is around y = 2 and the maximum carbon amount for Sm 2 Fe 15 Cr 2 C x to remain in a single 2 : 17 structure is around x = 2. Magnetic measurements reveal that the Cr substituted Sm 2 (Fe, Cr) 17 C x carbides have a strong uniaxial magnetocrystalline anisotropy with anisotropy fields higher than 90 kOe obtained in Sm 2 Fe 15 Cr 2 C 2 and Sm 2 Fe 14 Cr 3 C 2 carbides. However, both the Curie temperature and the magnetization are found to decrease with Cr substitution. High coercivity can be obtained in Sm 2 (Fe, Cr) 17 C x ribbons produced by the melt-spinning technique. The highest coercivity of 13.2 kOe has been obtained in Sm 2 Fe 15 Cr 2 C 2 ribbons spun at 52 m/s and annealed at 750°C for 20 min.

Effects of substitutions M on the formation, structure and magnetic properties of Sm2Fe15M2C2 (M = V, Cr, Ti, Nb, Zr, Mn and Mo) compounds

Abstract The effects of substitutions M on the formation, structure and magnetic properties of Sm 2 Fe 15 M 2 C 2 (M = V, Cr, Ti, Nb, Zr, Mn and Mo) compounds have been systematically studied using XRD, magnetic and Mossbauer measurements. It has been found that V, Cr, Ti, Nb and Zr can help to form the 2 : 17 carbides, with a certain amount of MC (TiC, NbC or ZrC) carbides present in the Ti, Nb or Zr substituted samples, whereas Mn and Mo cannot help to form the 2 : 17 carbides. Magnetic measurements reveal that Cr and Nb substituted carbides have a strong c -axis magnetocrystalline anisotropy with anisotropy fields of 93.5 kOe for M = Cr and 72.5 kOe for M = Nb. Mossbauer studies reveal that Cr occupies mainly the 6c and 18h sites whereas Nb occupies only the 18f site.

Magnetic hardening in nanograin Sm-Co 2:17 magnets

The magnetic hardening behavior of high coercivity nanograin Sm-Co magnets has been examined through a detailed investigation into their magnetic and microstructural properties. We examined the effects of various substitutions like Zr, Fe and Cu, on the magnetic behavior. Initial magnetization and remanence curves are characteristic of domain wall pinning type or single domain particle behavior. A correlation of results indicates that the reversal is controlled by the pinning of interaction domain walls at local anisotropy minima.

Microstructure and magnetic properties of mechanically milled nanograined PrxCo100−x (x=15.4−20.5) powders

The structure, microstructure, and magnetic properties of high-coercivity PrCo5-based PrxCo100−x (x=15.4–20.5) powders synthesized by mechanical milling and subsequent annealing were systematically studied as a function of Pr content. While the magnetization decreases monotonously with the Pr content, the coercivity increases, reaching a maximum of 24.1 kOe in Pr19Co81, and then decreases for higher Pr content. As a result, the maximum energy product goes through a broad peak of about 11.3–11.8 MGOe at the 16.7–18.0 at. % Pr content. Microstructural studies reveal that a uniform nanoscale PrCo5/Pr2Co17 microstructure with an average grain size of about 15–30 nm is developed in powders with Pr content up to 19 at. %. The volume fraction of the Pr2Co17 decreases with Pr content and a nearly single PrCo5 structure is obtained in Pr19Co81 powders. Further increase in the Pr content leads to the presence of the less-hard Pr2Co7 phase in the form of large grains, resulting in lower coercivities. Evidence of int...

Magnetic properties of Pr/sub 2/Fe/sub 6.8/Co/sub 10.2-x/M/sub x/ (M=Si, Ga and V)

Structural and magnetic properties of the Pr/sub 2/Fe/sub 6.8/Co/sub 10.2-x/M/sub x/ intermetallic compounds with M=Si, Ga and V, and x=0, 0.5, 1.0, 1.5 and 2.0 have been investigated. Substitution of M for Co leads to a decrease of the saturation magnetization, Curie temperature, and anisotropy field. It is noteworthy that the V substitution has different influence on the magnetic properties of the Pr/sub 2/Fe/sub 6.8/Co/sub 10.2/ compounds from the Si and Ca substitutions, The spin-reorientation temperature decreases with increasing M for M=Si and Ga. This phenomena disappeared when M=V, and the saturation magnetization of the V substituted compounds decreased much faster than those of the Si and Ga substituted compounds.