Y. Q. Wu

High thermal stability of carbon-coated L10-FePt nanoparticles prepared by salt-matrix annealing

Department of Physics, University of Texas at Arlington. Ames Laboratory and Department of Materials Science and Engineering, Iowa State University. NanoTech Institute, University of Texas at Dallas.

Structural phase transition and ferromagnetism in monodisperse 3 nm FePt particles

Department of Physics, The University of Texas at Arlington. Center for Materials Research and Analysis, and Department of Physics and Astronomy, University of Nebraska. Ames Laboratory and Department of Materials Science and Engineering, Iowa State University

Bulk FePt∕Fe3Pt nanocomposite magnets prepared by spark plasma sintering

Department of Physics, University of Texas at Arlington. Department of Mechanical Science and Engineering, Chiba Institute of Technology. Ames Laboratory, Iowa State University. Department of Physics and Astronomy, Iowa State University.

Studies of new YDy-based R/sub 2/Fe/sub 14/B magnets for high temperature performance (R=Y+Dy+Nd)

The effect of Nd substitution on microstructure and magnetic properties in [Nd/sub x/(YDy)/sub 0.5(1-x)/]/sub 2.2/Fe/sub 14/B ribbons melt-spun at 22 m/s has been systematically studied. As-spun ribbons with low Nd content consist of 2 : 17 and 2 : 14 : 1 phases in an amorphous matrix, while as-spun ribbons with high Nd contain 2 : 14 : 1 and Fe phases in the amorphous matrix. After annealing at 700/spl deg/C for 15 min, all of the ribbons exhibit only a single 2 : 14 : 1 phase in their X-ray diffraction patterns. Nd substitution can improve the maximum energy product of annealed ribbons but deteriorate the temperature stability of the ribbons. Increasing Nd (x) from 0 to 0.8, decreases coercivity from 22 to 13.5 kOe, but increases the maximum energy product from 5.87 to 11.2 MGOe. The temperature coefficients for remanence and coercivity increase from -0.045/spl deg/C to -0.106 %//spl deg/C, and -0.306 to -0.38 %//spl deg/C, respectively for the same substitution range. Transmission electron microscope microstructures show that the samples with less Nd content exhibit a more uniform distribution of grains. Their average grain size is about 40 nm. The studied results show that the YDy-based R/sub 2/Fe/sub 14/B magnets are very promising for high-temperature performance.

Formation of metastable Pr/sub 2/Fe/sub 23/B/sub 3/ phase and its effect on magnetic properties in rapidly quenched Pr/sub 9/Fe/sub 91-x/B/sub x/ nanocomposites

The formation of metastable Pr/sub 2/Fe/sub 23/B/sub 3/ phase and its effect on magnetic properties of melt-spun Pr/sub 9/Fe/sub 91-x/B/sub x/ (x = 4-12) have been investigated. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) studies show that the samples consist of Pr/sub 2/Fe/sub 14/B and /spl alpha/-Fe phases at low B content, and Pr/sub 2/Fe/sub 14/B, /spl alpha/-Fe and Fe/sub 3/B at high B content. The metastable Pr/sub 2/Fe/sub 23/B/sub 3/ phase appears in the intermediate B content (8-12 at%). This metastable Pr/sub 2/Fe/sub 23/B/sub 3/ phase results in a poor squareness in the demagnetization curve and is detrimental to the hard magnetic properties. Transmission electron microscopy (TEM) studies reveal larger average grain sizes when the B content is at 8.5 at% or above. It is further found that this metastable phase can be transformed into Pr/sub 2/Fe/sub 14/B, /spl alpha/-Fe, and Fe/sub 3/B by annealing the sample at temperature of 750/spl deg/C or above, leading to a more square demagnetization curve. However, B/sub r/ of the nanocomposite decreases slightly due to the excessive grain growth during high-temperature annealing.

Microstructural control of Nb addition in nanocrystalline hard magnets with different Nd content

In this paper, nanocrystalline alloys with Nd content near 10 at % are investigated, and their magnetic properties and microstructure are examined to obtain better understanding of the effect of Nb addition on microstructure control in the nanocrystalline magnets.

Effect of TiC addition on microstructure and magnetic properties for MRE2(Fe,Co)14B melt-spun ribbons (MRE=Nd+Y+Dy)

Effects of a TiC addition on microstructure and magnetic properties in [MRE2.2Fe14B](100−2x)∕17.2+TixCx(MRE=Nd+Y+Dy,x=1–5) ribbons, melt spun at a wheel speed of 16m∕s, were systematically studied. X-ray diffraction and differential thermal analysis data revealed that the addition of TiC improves the glass formability in the mixed rare earth alloys without Co, resulting in partially amorphous alloys. TEM observations showed that the average grain size in the as spun samples decreases from 200 to 20 nm with increasing x from 1 to 5, confirming that the addition of TiC can significantly improve microstructure. For an optimized [MRE2(Fe,Co)14B](100−2x)∕17.2+TixCx sample with x=2, spun at 25m∕s and annealed at 750 °C for 15 min, the room-temperature magnetic properties of Hcj=11.8kOe, Mr=7.2kGs, and (BH)max=11.3MGOe were obtained. Temperature coefficients for Mr and Hcj of −0.06 and −0.37%∕°C, respectively, also were measured in the temperature range of 27–100 °C. The new magnet alloy exhibits more uniform ma...

Behavior of Nb atoms in Nb substituted Nd/sub 2/Fe/sub 14/B nanocrystalline alloys investigated by atom probe tomography

The behavior of Nb atoms in Nd/sub 12/Fe/sub 82-x/B/sub 6/Nb/sub x/ (x=0 to 3) nanocrystalline alloys has been investigated by using atom probe tomography (APT) technique on near-atomic scale in the present paper. Three-dimensional atom probe (3DAP) analyses on the Nb substituted alloys clearly reveal that Nb atoms are enriched at grain boundaries with a peak concentration about 4 times higher than the average for Nd/sub 2/Fe/sub 14/B grains. A grain boundary region with a chemical composition near the Nb : Fe : B stoichiometry of 3 : 3 : 5 is also measured for the x=3 alloy. The results provide direct evidence of microstructural refinement due to solute drag of Nb atoms during solidification, resulting in Nb enrichment at grain boundaries and possibly the formation of a Nb-rich interfacial phase which give rise to enhanced magnetic properties.

Effect of pressure loading rate on the crystallographic texture of NdFeB nanocrystalline magnets

Plastic deformation of NdFeB nanocrystalline magnets has been performed at relatively low temperatures but high pressures compared to conventional hot deformation. The effect of deformation conditions on the crystallography texture of the anisotropic NdFeB magnets has been studied. It was observed that both deformation temperature and pressure loading rate significantly affect final crystallographic texture and thus magnetic properties. Energy product increases from 31 to 44 MGOe by simply reducing pressure loading rate from 300 MPa/min to 50 MPa/min. Microstructure observations confirm that better crystallographic texture has been obtained in the deformed magnets with a lower pressure loading rate.

Improved energy product in grained aligned and sintered MRE2Fe14B magnets (MRE=Y+Dy+Nd)

Sintered [Nd0.45(Y3Dy1)0.25∗0.55]2.8Fe14B magnets were prepared for the first time. Magnetic properties and microstructures of the magnets were investigated by magnetic measurements and electron microprobe analysis. The microstructure consists of a MRE2Fe14B (2-14-1) phase matrix having a grain size of ∼10 μm and a RE-rich grain boundary phase. However, sintering resulted in segregation of Y to the inner and Nd to the outside of the 2-14-1 grains. The magnet has a room temperature (BH)max of 25.4 MGOe, which is two times higher than that of the isotropic melt spun ribbons with similar compositions. The temperature coefficients of Br (α) and Hcj (β) for the magnet are −0.150 and −0.632%/°C from 27 to 127 °C, respectively. These temperature coefficients, especially for β, are also much higher than those of melt spun ribbons. The composition segregation in the 2-14-1 grains is believed to be responsible for the higher temperature coefficients.