Abstract:
The disclosure describes a method of producing iron nitride magnets using Zn-doped iron oxide precursors. The iron oxide precursors are reduced and nitrided to produce a powder containing iron nitride in the Fe 16 N 2  phase. The inclusion of Zn in the iron oxide precursor enhances the magnetic properties of the iron nitride powder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    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. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    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. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    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. 
         [0004]    Iron nitride magnets based on the Fe 16 N 2  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 Fe 16 N 2  phase, which is the ordered version of Fe 8 N, is widely reported to have the largest magnetization of any compound, but is also difficult to manufacture. 
         [0005]    The reduction of Fe-based oxide and hydroxide nanopowder precursors and conversion to Fe 16 N 2  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 Fe 16 N 2  manufacture where an iron oxide starting material (or precursor) is reduced in a fluidized bed reactor with a reducing agent, such as H 2 , in a temperature range of 200-500° C. Subsequent nitridation of the reduced iron occurs through exposure in the fluidized bed reactor to pure NH 3  or NH 3 —N 2 —H 2  gases at 100-200° C. to form the Fe 16 N 2  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 Fe 16 N 2  phase yield, as well as improve resultant magnetic properties, such as magnetization and coercivity. While this method produces suitably high yields of Fe 16 N 2 , 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. 
         [0006]    As with the Sankar method and other methods, a key factor for a commercially viable Fe 16 N 2  magnet is using low cost precursors that yield high quality Fe 16 N 2 , as the precursor cost dominates the cost of the resultant Fe 16 N 2 . Just as importantly, the size, consistency, and quality of the precursor ultimately affect the quality of the Fe—N magnet. 
         [0007]    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 Fe 16 N 2  magnets. 
         [0008]    It would therefore be advantageous to develop an improved process for the synthesis of Fe 16 N 2  nanopowders using low-cost precursors, where the nanopowders exhibit improved suitability for use in Fe—N magnets. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed to the use of doped bacteria fermented Fe-oxide nanoparticles as an improved, low-cost precursor for synthesis of Fe 16 N 2  phase compounds. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]      FIGS. 1A-1C  are TEM images of undoped and Co- and Mn-doped Fe 3 O 4  precursor powders, according to one embodiment, synthesized by bacteria fermentation. 
           [0011]      FIGS. 2A-2C  are TEM images of Zn-doped Fe 3 O 4  precursor powders, according to one embodiment, synthesized by bacteria fermentation. 
           [0012]      FIG. 3  is a graph of VSM magnetic hysteresis for a Zn-doped Fe 3 O 4  precursor powder before and after reduction and nitridation treatment. 
           [0013]      FIG. 4  is a TEM image of the Zn-doped Fe 3 O 4  precursor powder analyzed in  FIG. 3  after reduction and nitridation treatment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Bacteria-fermentation derived Fe-oxide nanoparticles show good potential to form Fe 16 N 2  using conventional reduction and nitridation approaches. In one embodiment, low-cost Zn-doped Fe oxide precursors  101  yield high coercivity (&gt;1500 Oe) Fe 16 N 2  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 Fe 16 N 2  formation. A further advantage of a Zn-doped precursor  101  is that the Zn helps protect the resultant Fe 16 N 2  containing powders  200  from corrosion, both when stored prior to consolidation in bulk form and once consolidated. 
         [0015]    Transmission electron microscopy (TEM) images of bacteria-fermented Fe 3 O 4  precursors  101 , synthesized according to one method known in the art, are shown in  FIGS. 1A-1C .  FIG. 1A  shows undoped Fe 3 O 4  precursors  101 , which consists of relatively large, nonuniform particles  201  having a diameter of about 0.1 to 1 microns.  FIG. 1B  and  FIG. 1C  are images of the precursor  101  doped with Mn and Co, respectively, with the dopants  102  well distributed throughout the precursor  101 . In both  FIGS. 1B and 1C , doping the precursor  101  resulted in a finer particle size, generally in the 50-100 nm range. 
         [0016]    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% Fe 16 N 2 . Similarly, coercivity values were in the 400-800 Oe range (see Table 1). These levels are unsuitable for viable commercial production of Fe 16 N 2  powders  200 . Further optimization of reduction and nitridation conditions can certainly be identified to increase Fe 16 N 2  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. 
         [0017]      FIGS. 2A-2B  show TEM images for several Zn-doped Fe 3 O 4  precursors  101  synthesized using bacterial fermentation, with  FIGS. 2B-2C  showing precursors  101  with Zn substituted for Fe at the 1-10 at. % range. Zn dopants  102  in precursors  101  have not previously been utilized to enhance the magnetic properties of Fe 16 N 2  containing ferromagnetic materials, unlike dopants  102  such as Mn and Co which exhibit paramagnetic/ferromagnetic behavior, because Zn is diamagnetic. However, the use of Zn dopants  102  in the bacteria fermentation process results in a very fine and uniform Fe 3 O 4  precursor particle size, typically in the range of 10-40 nm and frequently centered in the 15-25 nm range. 
         [0018]    Smaller particle size, which aids more rapid and uniform nitriding, and the presence of Zn can improve the qualities of the Fe 16 N 2  powder  200  synthesized from the precursor  101 . For example, the inclusion of Zn increases the coercivity of the final Fe 16 N 2  product and is attributed, in part, to the fine, uniform, nanoscale precursor size imparted for Fe 3 O 4  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 Fe 16 N 2 . The incorporation of Zn/Zn oxide into the final product may also induce defects which favor increased levels of coercivity. 
         [0019]    As shown in Table 1, unlike the precursors doped with Mn and Co, the Zn-doped Fe 3 O 4  precursors  101  resulted in high Fe 16 N 2  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 Fe 16 N 2  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 Fe 16 N 2  powder  200 . Although reduction and nitridation of the Zn-doped Fe 3 O 4  precursors  101  produced relatively low yield Fe 16 N 2  powder  200  (about 8-40%) in initial trials, Zn-doped Fe 3 O 4  precursors  101  permit the creation of Fe 16 N 2  powders  200  with excellent magnetic properties, particularly high coercivity. 
         [0020]      FIG. 3  shows magnetic hysteresis curves from a vibrating sample magnetometer (VSM) study of a 10 atomic percent Zn-doped Fe 3 O 4  precursor  101 , shown before and after reduction and nitridation. The coercivity of the untreated 10 atomic percent Zn-doped Fe 3 O 4  precursor  101  is near zero, but reaches about 1500 Oe after reduction and nitridation. The high coercivity is despite only a ˜40% Fe 16 N 2  phase yield. As indicated in Table 1, the measurements are conducted at ambient temperature (e.g. about 15-25 degrees Celcius). 
         [0021]    An X-ray diffraction analysis of the resultant powder  200  indicates 41% Fe 16 N 2 , 17% Fe metal, and 42% incompletely reduced Zn—Fe—O. Unlike the powder  200  analyzed in  FIG. 3 , in the preferred embodiment, the Fe 16 N 2  yield is about 60-95%, which can be accomplished through adjustment and optimization of the reduction and nitridation process time and temperature conditions to achieve more complete reduction of the Zn-doped Fe-oxide precursor  101  prior to nitridation. Such process optimization is widely reported in the literature via reduction/nitridation treatments using more conventional (non Zn-doped) types of Fe 3 O 4  precursors  101 . For example, in one process reduction occurs in H 2  in a temperature range of 200−500° C. and subsequent nitridation is accomplished through exposure in a fluidized bed reactor to pure NH 3  or NH 3 —N 2 —H 2  gases at 100-200° C. to yield high percentages of the Fe 16 N 2  phase. 
         [0022]    With optimization, in the preferred embodiment a Fe 16 N 2  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. 
         [0023]      FIG. 4  shows high angle annular dark field TEM images of a Fe 16 N 2  powder  200  synthesized from an iron oxide precursor  101  that was subjected to the reduction and nitridation process. The image in  FIG. 4  confirms the formation of ordered Fe 16 N 2 . Elemental mapping suggests a core of Fe 16 N 2 , possibly surrounded by a Zn-containing oxide. That is, a coating  103  on the surface of the nanosized particle  201  contains one or more of the following: Zn, Zn—Fe—O, Zn—O, or Zn—Fe—N—O. From the image shown in  FIG. 4 , it is not possible to ascertain if any Zn was also incorporated directly into the Fe 16 N 2  phase. The presence of intermixed Zn/Zn—Fe-0/Zn-0/Zn—Fe—N—O in the final product is anticipated to be beneficial from a stability and corrosion viewpoint, as Zn galvanization coatings are well established to protect steel from corrosion. 
         [0024]    A high coercivity Fe 16 N 2 -containing powder  200  derived from low-cost Zn-doped bacteria fermented Fe 3 O 4  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 Fe 16 N 2  powders  200  necessitate storage and consolidation strategies that minimize air exposure, and may result in higher production costs. 
         [0025]    Table 1 is a summary of exploratory reduction and nitridation conversion reactions using bacterial fermented Fe 3 O 4  precursors  101 . Reduction is accomplished at 400-440° C. for up to 5 h in H 2 , followed by nitridation in NH 3  at 160° C. and up to 20 h. The measured values are at ambient temperature (i.e. about 15-25 degrees Celcius). 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Magnetic Properties after 
                   
                 Air Passivated Magnetic 
               
               
                   
                 nitridation, no passivation 
                   
                 Properties 
               
             
          
           
               
                   
                 Magnetization 
                 Coercivity 
                   
                 Magnetization 
                 Coercivity 
               
               
                   
                 at 16.5 kOe 
                 H c   
                 Estimated % 
                 at 16.5 kOe 
                 H c   
               
               
                 Precursor 
                 (emu/g) 
                 (Oe) 
                 Fe 16 N 2   
                 (emu/g) 
                 (Oe) 
               
               
                   
               
             
          
           
               
                 Undoped 
                 183.8 
                 395 
                 NA 
                 NA 
                 NA 
               
               
                 Fe 3 O 4   
               
               
                 Undoped 
                 193.3 
                 504 
                 NA 
                 168.4 
                 463 
               
               
                 Fe 3 O 4   
               
               
                 20 at % Mn 
                 123.8 
                 621 
                 29 
                 104 
                 660 
               
               
                 doped Fe 3 O 4   
               
               
                 [US-5] 
               
               
                 10 at % Co 
                 212.2 
                 485 
                 12 
                 195 
                 499 
               
               
                 doped Fe 3 O 4   
               
               
                 [US-47] 
               
               
                 10 at % Co 
                 212.3 
                 789 
                 23 
                 181.5 
                 824 
               
               
                 doped Fe 3 O 4   
               
               
                 [US-12] 
               
               
                 10 at % Zn 
                 100.8 
                 1459 
                 41 
                 88.3 
                 1548 
               
               
                 doped Fe 3 O 4   
               
               
                 [US-7] 
               
               
                 10 at % Zn 
                 104.8 
                 836 
                 46 
                 92.2 
                 838 
               
               
                 doped Fe 3 O 4   
               
               
                 [US-53] 
               
               
                 1 at % Zn 
                 185.7 
                 1092 
                 13 
                 135.4 
                 1089 
               
               
                 Fe 3 O 4   
               
               
                 1 at % Zn 
                 194.3 
                 1094 
                 21 
                 155.4 
                 1090 
               
               
                 Fe 3 O 4   
               
               
                 5 at % Zn 
                 186.9 
                 1029 
                 14 
                 150.1 
                 1066 
               
               
                 Fe 3 O 4   
               
               
                 10 at % Zn 
                 150.4 
                 820 
                 8 
                 115.7 
                 870 
               
               
                 Fe 3 O 4   
               
               
                   
               
             
          
         
       
     
         [0026]    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.