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Title: Processing and Protection of Rare Earth Permanent Magnet Particulate for Bonded Magnet Applications

Abstract

Rapid solidification of novel mixed rare earth-iron-boron, MRE 2Fe 14B (MRE = Nd, Y, Dy; currently), magnet alloys via high pressure gas atomization (HPGA) have produced similar properties and structures as closely related alloys produced by melt spinning (MS) at low wheel speeds. Recent additions of titanium carbide and zirconium to the permanent magnet (PM) alloy design in HPGA powder (using He atomization gas) have made it possible to achieve highly refined microstructures with magnetic properties approaching melt spun particulate at cooling rates of 10 5-10 6K/s. By producing HPGA powders with the desirable qualities of melt spun ribbon, the need for crushing ribbon was eliminated in bonded magnet fabrication. The spherical geometry of HPGA powders is more ideal for processing of bonded permanent magnets since higher loading fractions can be obtained during compression and injection molding. This increased volume loading of spherical PM powder can be predicted to yield a higher maximum energy product (BH) max for bonded magnets in high performance applications. Passivation of RE-containing powder is warranted for the large-scale manufacturing of bonded magnets in applications with increased temperature and exposure to humidity. Irreversible magnetic losses due to oxidation and corrosion of particulates is a known drawbackmore » of RE-Fe-B based alloys during further processing, e.g. injection molding, as well as during use as a bonded magnet. To counteract these effects, a modified gas atomization chamber allowed for a novel approach to in situ passivation of solidified particle surfaces through injection of a reactive gas, nitrogen trifluoride (NF 3). The ability to control surface chemistry during atomization processing of fine spherical RE-Fe-B powders produced advantages over current processing methodologies. In particular, the capability to coat particles while 'in flight' may eliminate the need for post atomization treatment, otherwise a necessary step for oxidation and corrosion resistance. Stability of these thin films was attributed to the reduction of each RE's respective oxide during processing; recognizing that fluoride compounds exhibit a slightly higher (negative) free energy driving force for formation. Formation of RE-type fluorides on the surface was evidenced through x-ray photoelectron spectroscopy (XPS). Concurrent research with auger electron spectroscopy has been attempted to accurately quantify the depth of fluoride formation in order to grasp the extent of fluorination reactions with spherical and flake particulate. Gas fusion analysis on coated powders (dia. <45 μm) from an optimized experiment indicated an as-atomized oxygen concentration of 343ppm, where typical, nonpassivated RE atomized alloys exhibit an average of 1800ppm oxygen. Thermogravimetric analysis (TGA) on the same powder revealed a decreased rate of oxidation at elevated temperatures up to 300 C, compared to similar uncoated powder.« less

Authors:
 [1]
  1. Iowa State Univ., Ames, IA (United States)
Publication Date:
Research Org.:
Ames Lab., Ames, IA (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
933032
Report Number(s):
IS-T 2887
TRN: US200814%%819
DOE Contract Number:
AC02-07CH11358
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALLOYS; ATOMIZATION; AUGER ELECTRON SPECTROSCOPY; CORROSION RESISTANCE; FREE ENERGY; MAGNETIC PROPERTIES; MAGNETS; OXIDATION; PARTICULATES; PERMANENT MAGNETS; PROCESSING; RARE EARTHS; THERMAL GRAVIMETRIC ANALYSIS; THIN FILMS; TITANIUM CARBIDES; X-RAY PHOTOELECTRON SPECTROSCOPY

Citation Formats

Sokolowski, Peter Kelly. Processing and Protection of Rare Earth Permanent Magnet Particulate for Bonded Magnet Applications. United States: N. p., 2007. Web. doi:10.2172/933032.
Sokolowski, Peter Kelly. Processing and Protection of Rare Earth Permanent Magnet Particulate for Bonded Magnet Applications. United States. doi:10.2172/933032.
Sokolowski, Peter Kelly. Mon . "Processing and Protection of Rare Earth Permanent Magnet Particulate for Bonded Magnet Applications". United States. doi:10.2172/933032. https://www.osti.gov/servlets/purl/933032.
@article{osti_933032,
title = {Processing and Protection of Rare Earth Permanent Magnet Particulate for Bonded Magnet Applications},
author = {Sokolowski, Peter Kelly},
abstractNote = {Rapid solidification of novel mixed rare earth-iron-boron, MRE2Fe14B (MRE = Nd, Y, Dy; currently), magnet alloys via high pressure gas atomization (HPGA) have produced similar properties and structures as closely related alloys produced by melt spinning (MS) at low wheel speeds. Recent additions of titanium carbide and zirconium to the permanent magnet (PM) alloy design in HPGA powder (using He atomization gas) have made it possible to achieve highly refined microstructures with magnetic properties approaching melt spun particulate at cooling rates of 105-106K/s. By producing HPGA powders with the desirable qualities of melt spun ribbon, the need for crushing ribbon was eliminated in bonded magnet fabrication. The spherical geometry of HPGA powders is more ideal for processing of bonded permanent magnets since higher loading fractions can be obtained during compression and injection molding. This increased volume loading of spherical PM powder can be predicted to yield a higher maximum energy product (BH)max for bonded magnets in high performance applications. Passivation of RE-containing powder is warranted for the large-scale manufacturing of bonded magnets in applications with increased temperature and exposure to humidity. Irreversible magnetic losses due to oxidation and corrosion of particulates is a known drawback of RE-Fe-B based alloys during further processing, e.g. injection molding, as well as during use as a bonded magnet. To counteract these effects, a modified gas atomization chamber allowed for a novel approach to in situ passivation of solidified particle surfaces through injection of a reactive gas, nitrogen trifluoride (NF3). The ability to control surface chemistry during atomization processing of fine spherical RE-Fe-B powders produced advantages over current processing methodologies. In particular, the capability to coat particles while 'in flight' may eliminate the need for post atomization treatment, otherwise a necessary step for oxidation and corrosion resistance. Stability of these thin films was attributed to the reduction of each RE's respective oxide during processing; recognizing that fluoride compounds exhibit a slightly higher (negative) free energy driving force for formation. Formation of RE-type fluorides on the surface was evidenced through x-ray photoelectron spectroscopy (XPS). Concurrent research with auger electron spectroscopy has been attempted to accurately quantify the depth of fluoride formation in order to grasp the extent of fluorination reactions with spherical and flake particulate. Gas fusion analysis on coated powders (dia. <45 μm) from an optimized experiment indicated an as-atomized oxygen concentration of 343ppm, where typical, nonpassivated RE atomized alloys exhibit an average of 1800ppm oxygen. Thermogravimetric analysis (TGA) on the same powder revealed a decreased rate of oxidation at elevated temperatures up to 300 C, compared to similar uncoated powder.},
doi = {10.2172/933032},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

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  • Studies of the anomaly in the magnetization of PrCo/sub 5/ near 100K show the cause to be a switch from conical to axial anisotropy with rising temperature. Effects on magnetic properties rendered by partial replacement of Pr in PrCo/sub 5/ by La, Ce, and Nd were investigated. While Ce and La substitutions enhance the anisotropy, they diminish the magnetic moment and the T/sup c/. Nd substitution caused an elevation in T/sup c/ and M/sup s/ but diminished the anisotropy. Effects of the introduction of small amounts of M = Ti, Zr, and Hf for Pr in Pr/sub (1-x)/M/sub x/Co/sub 5/more » were studied; a drop in moment and anisotropy but a surprising elevation in T/sup c/ were observed. While Ti, and to a lesser extent Hf, could partially replace Co in Pr/sub 2/Co/sub (17-x)/M/sub x/, it was found the Zr can only enter the crystal structure by substituting for Pr, as in Pr/sub (2-x)/Zr/sub x/Co/sub 17/. In the Ti- and Hf-doped alloys, spin-reorientation effects were observed, but they were not apparent in the Zr-doped alloys. In the Cu-containing alloys with the formula Pr/sub 2/ (Co/sub 16/Cu)/sub (17-x)/17M/sub x/, it was found that Ti, Hf, and Zr could be introduced up to x = 1.0. Substitution by Zr showed the greatest effects upon spin reorientation temperature and anisotropy. Fundamental magnetic properties of the R/sub 2/Fe/sub 14/B compounds with R = Y, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Yb and Th were studied« less
  • A solid solution of three rare earths (RE) in the RE 2Fe 14B structure have been combined to create the novel mixed rare earth iron boron (MRE 2Fe 14B) alloy family. MRE 2Fe 14B exhibits reduced temperature dependent magnetic properties; remanence and coercivity. The desired form of MRE 2Fe 14B is a powder that can be blended with a polymer binder and compression or injection molded to form an isotropic polymer bonded permanent magnet (PBM). Commercially, Nd 2Fe 14B is the alloy of choice for PBMs. Powders of Nd 2Fe 14B are made via melt-spinning as can be MRE 2Femore » 14B which allows for direct comparisons. MRE 2Fe 14B made using melt-spinning at high wheel speeds is overquenched and must be annealed to an optimal hard magnetic state. Due to the rare earth content in the MRE 2Fe 14B powders, they must be protected from the environment in which they operate. This protection is accomplished by using a modified fluidized bed process to grow a protective fluoride coating nominally 15nm thick, to reduce air oxidation. MRE 2Fe 14B has demonstrated reduced temperature dependent magnetic properties in ribbon and PBM form. The real challenge has been modifying alloy designs that were successfully melt-spun to be compatible with high-pressure gas-atomization (HPGA). The cooling rates in HPGA are lower than melt-spinning, as the powders are quenched via convective cooling, compared to melt-spinning, which quenches initially by conductive cooling. Early alloy designs, in gas atomized and melt-spun form, did not have similar phase compositions or microstructures. Alloy additions, such as the addition of zirconium as a nucleation catalyst, were successful in creating similar phases and microstructures in the HPGA powders and melt-spun ribbon of the same MRE 2Fe 14B composition.« less
  • Investigation of the microstructure and details of magnetization reversal of the new class of permanent magnets was undertaken to understand the inter-relationship between the microstructure and magnetic properties of the magnets. The overall microstructure consists of almost defect-free grains of the matrix Nd{sub 2}Fe{sub 14}B surrounded by a Nd-rich, oxygen-stabilized, fcc phase. Through examination of the virgin magnetization curve, the applied-field dependence of the intrinsic coercivity, and remanence, it was shown that the magnet behaves as a nucleation-controlled magnet. Since the interiors of the matrix grains are defect-free, domain-wall nucleation occurs due to defects at the grain boundaries and two-phasemore » interfaces. One possible defect is oxygen which can be a cause for the large difference between the anisotropy field and the intrinsic coercivity, even in the case of an optimally treated magnet. The effect of post-sintering heat treatments upon the intrinsic coercivity and energy product was rationalized, based upon the above observations. Increase in the cooling rate after sintering leads to a decrease in the intrinsic coercivity due to the effect of quenching stresses, caused by thermal-expansion mismatch at the fcc phase-matrix interface.« less
  • The magnetic properties of (Co,Fe)/sub 3n + 5/R/sub n + 1/B/sub 2n/(R = Y, Ce, CeMM, Pe, and Gd) (n = 1,2) and the ternary boride of Fe-R-B, nominally Fe/sub 14/R/sub 2/B, were investigated. For the (Co,Fe)/sub 3n + 5/R/sub n + 1/B/sub 2n/ homologous series (R = Y, Ce, CeMM, Pr, Sm, Gd) (n = 1,2,3), the results of previous work and the present investigation can be summarized as follows: (1) the magnetic moments and Curie temperatures decrease with increasing boron content in the compound series. (2) For Co-Sm-B and Co-Gd-B compound series the coercivity increases with increasing boronmore » content in the compound, but this was not observed in other rare earth compound series. The coercive forces obtained with out special treatment have the highest values for the compounds with Sm and Gd. (3) The moment and the Curie temperature vary according to the type of rare earth element in the compound as well as its elemental moment and its coupling in the structure. The compounds with Sm have the highest curie temperatures. (4) The Fe replacement for Co atoms increases both the values of magnetic moments and Curie temperatures as the Fe content increases in the structure. The presence of Fe atoms does not increase the coercivity except for compounds based on Sm and Pr. (5) The maximum inclusion of Fe in each homologue decreases systematically with increasing series index n. (6) The addition of Zr increases the coercivity of the compound series.« less
  • Effects of composition and processing variables on the magnetic properties of rapidly solidified Fe-RE-B were explored using a vibrating sample magnetometer. Additionally, supporting microstructural analyses are given to help explain the observed variations. This work has concentrated on producing large intrinsic coercivities, i.e., greater than about 10 kOe, directly at controlled solidification rates in melt spinning. Intrinsic coercivities of only 100-1000 Oe are obtained at the lowest (0-4 m/s) and highest (40-80 m/s) wheel surface speeds in all alloys investigated. Intrinsic coercivities over 20 kOe were obtained at intermediate wheel surface speeds (20-30 m/s) in alloys of Fe/sub 76/R/sub 16/B/submore » 8/, where R is Pr or Nd. Typical hysteresis loops are also given. Substituting Sm and Ce for Pr and Nd does not lead to large intrinsic coercivities at all wheel surface speeds. Replacements of boron with the metalloids carbon or silicon are also ineffective in producing large intrinsic coercivities. The phase responsible for the large coercivities was identified as a tetragonal phase with a = 0.881 nm and c = 1.178 nm. The Curie temperature of the phase is shown to be about 285/sup 0/C. Moderate substitutions of Co for Fe in alloys based on (Fe/sub 100-x/Co/sub x/)/sub 76/Pr/sub 16/B/sub 8/ are shown to significantly raise the Curie temperature of the tetragonal phase and produce corresponding increases in the temperature coefficients of remanence and coercivity.« less