Patent Application
20160379741
This invention relates generally to iron-based magnets. More specifically, the invention relates to precursors used in the synthesis of iron nitride (Fe—N) magnets.
Iron nitride magnets based on the Fe16N2 phase are of great interest as a magnetic material for applications ranging from data storage to electrical motors for vehicles, wind turbines, and other power generation equipment. This is because the component base elements, iron (Fe) and nitrogen (N), are inexpensive and widely available, in contrast to rare earth element based magnets which are costly and subject to supply availability risks. The Fe16N2 phase, which is the ordered version of Fe8N, is widely reported to have the largest magnetization of any compound, but is also difficult to manufacture.
The reduction of Fe-based oxide and hydroxide nanopowder precursors and conversion to Fe16N2 by nitridation is generally known in the art. See, for example, Sankar et al. (US 2011/0059005) or Ogawa et al. (EP 2 492 927). Sankar discloses one method of Fe16N2 manufacture where an iron oxide starting material (or precursor) is reduced in a fluidized bed reactor with a reducing agent, such as H2, in a temperature range of 200-500° C. Subsequent nitridation of the reduced iron occurs through exposure in the fluidized bed reactor to pure NH3 or NH3—N2—H2 gases at 100-200° C. to form the Fe16N2 phase. Oxide coatings (e.g. alumina or silica) and/or metal dopants (Co, Cr, Mn, Ni, Ti, other transition metals, and rare earths) have been used to improve Fe16N2 phase yield, as well as improve resultant magnetic properties, such as magnetization and coercivity. While this method produces suitably high yields of Fe16N2, it has proven challenging to also achieve high levels of magnetization and coercivity, particularly when using sufficiently low cost iron-based precursor powders, which compose a major portion of the production costs.
As with the Sankar method and other methods, a key factor for a commercially viable Fe16N2 magnet is using low cost precursors that yield high quality Fe16N2, as the precursor cost dominates the cost of the resultant Fe16N2. Just as importantly, the size, consistency, and quality of the precursor ultimately affect the quality of the Fe—N magnet.
Potentially low cost processes for creating metal oxide nanoparticles have been previously developed by Oak Ridge National Laboratory (U.S. Pat. No. 6,444,453, Lauf et al., “Mixed oxide nanoparticles and method of making”; U.S. Pat. No. 7,060,473, Phelps et al., “Fermentative process for making inorganic nanoparticles”; and US Pub. 2010/0184179, Rondinone et al., “Microbial-mediated method for metal oxide nanoparticle formation”). However, the nanoparticles produced by the fermentative processes described in these references are not limited to iron oxides and have not been optimized for subsequent nitridation and incorporation into Fe16N2 magnets.
It would therefore be advantageous to develop an improved process for the synthesis of Fe16N2 nanopowders using low-cost precursors, where the nanopowders exhibit improved suitability for use in Fe—N magnets.
The present invention is directed to the use of doped bacteria fermented Fe-oxide nanoparticles as an improved, low-cost precursor for synthesis of Fe16N2 phase compounds.
Bacteria-fermentation derived Fe-oxide nanoparticles show good potential to form Fe16N2 using conventional reduction and nitridation approaches. In one embodiment, low-cost Zn-doped Fe oxide precursors 101 yield high coercivity (>1500 Oe) Fe16N2 containing powder 200 comprised of a plurality of nanosized particles 201. The high coercivity is unexpected because Zn is diamagnetic and has not previously been considered a candidate dopant 102 to enhance Fe16N2 formation. A further advantage of a Zn-doped precursor 101 is that the Zn helps protect the resultant Fe16N2 containing powders 200 from corrosion, both when stored prior to consolidation in bulk form and once consolidated.
Transmission electron microscopy (TEM) images of bacteria-fermented Fe3O4 precursors 101, synthesized according to one method known in the art, are shown in
Despite their smaller particle size, initial reduction/nitridation trials using Mn- and Co-doped precursors 101 resulted in relatively low phase yields of about 12-30% Fe16N2. Similarly, coercivity values were in the 400-800 Oe range (see Table 1). These levels are unsuitable for viable commercial production of Fe16N2 powders 200. Further optimization of reduction and nitridation conditions can certainly be identified to increase Fe16N2 phase yield to more viable levels of 60-95% (with the possible exception of the 10 at. % Co-doped US-12 batch precursor 101, which exhibits a moderately attractive combination of coercivity over 780 Oe and magnetization over 210 emu/gram), but the screenings do not indicate sufficiently promising magnetic properties from these precursors 101 to warrant further development work with these dopants alone.
Smaller particle size, which aids more rapid and uniform nitriding, and the presence of Zn can improve the qualities of the Fe16N2 powder 200 synthesized from the precursor 101. For example, the inclusion of Zn increases the coercivity of the final Fe16N2 product and is attributed, in part, to the fine, uniform, nanoscale precursor size imparted for Fe3O4 when doped with Zn. More specifically, the presence of Zn reduces sintering during reduction and nitridation due to a Zn-rich surface on the particles and Zn modifies the inter-/intra-particle magnetic interactions of the resultant Fe16N2. The incorporation of Zn/Zn oxide into the final product may also induce defects which favor increased levels of coercivity.
As shown in Table 1, unlike the precursors doped with Mn and Co, the Zn-doped Fe3O4 precursors 101 resulted in high Fe16N2 coercivity values, ranging from about 1100-1550 Oe. In the examples shown in Table 1, Zn is present in the precursor 101 in a range of 1-10 atomic percent (substitution of Zn for Fe). However, Zn can be present in the precursor 101 at different percentages if the resultant Fe16N2 powder 200 maintains acceptable magnetic properties. As such, a person having skill in the art will appreciate that the percentage of Zn can be adjusted based on the intended application of the Fe16N2 powder 200. Although reduction and nitridation of the Zn-doped Fe3O4 precursors 101 produced relatively low yield Fe16N2 powder 200 (about 8-40%) in initial trials, Zn-doped Fe3O4 precursors 101 permit the creation of Fe16N2 powders 200 with excellent magnetic properties, particularly high coercivity.
An X-ray diffraction analysis of the resultant powder 200 indicates 41% Fe16N2, 17% Fe metal, and 42% incompletely reduced Zn—Fe—O. Unlike the powder 200 analyzed in
With optimization, in the preferred embodiment a Fe16N2 powder 200 created from low-cost Zn-doped precursors 101 has a coercivity level of about 2000-3000 Oe and magnetizations greater than 180 emu/g. Precursors 101 used in the preferred embodiment have Zn in the range of about 0.01 to 20 atomic percent substituted for Fe in the iron oxide precursor 101, with 1-10 atomic percent Zn preferred. In an alternative embodiment, co-doping of the precursor 101 can be performed to tailor and optimize magnetic properties with additions of Zn and at least one additional element from the group consisting of Al, B, C, Co, Cr, Hf, Mn, Nb, Ni, Si, Ta, Ti, V, Zr, and rare earths including Ce, La, Nd, Y, Dy, Sm at the 0.01-20 at. % level, with 1-10 at. % preferred.
A high coercivity Fe16N2-containing powder 200 derived from low-cost Zn-doped bacteria fermented Fe3O4 precursors 101 with enhanced stability and corrosion resistance would be very attractive from a commercial scale processing approach standpoint, as well as for consolidation to bulk magnets. Limited stability and poor corrosion resistance considerations of current Fe16N2 powders 200 necessitate storage and consolidation strategies that minimize air exposure, and may result in higher production costs.
Table 1 is a summary of exploratory reduction and nitridation conversion reactions using bacterial fermented Fe3O4 precursors 101. Reduction is accomplished at 400-440° C. for up to 5 h in H2, followed by nitridation in NH3 at 160° C. and up to 20 h. The measured values are at ambient temperature (i.e. about 15-25 degrees Celcius).
While the disclosure has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modification can be made therein without departing from the spirit and scope of the embodiments. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.
This application claims the benefit under 35 U.S.C. §119 of Provisional Ser. No. 62/185,057, filed Jun. 26, 2015, which is incorporated herein by reference.
This invention was made with Government support under DE-AC05-00OR22725 and DE-AR00-000645 awarded by the U.S. Department of Energy. The Government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 62185057 | Jun 2015 | US |
